Methods, devices, apparatuses and computer-readable medium for channel estimation for a reconfigurable intelligent surface (RIS) assisted channel

Abstract

Example embodiments of the present disclosure relate to methods, devices, apparatuses and a computer-readable medium for communication. In an example method, a reconfigurable intelligent surface (RIS) performs, together with a transmitter (TX) and a receiver (RX), a first number of channel measurements associated with a first set of TX elements of the TX and a second set of RIS elements of the RIS and performs, together with the TX and the RX, a second number of channel measurements associated with a third subset of a third set of TX elements of the TX and a fourth subset of a fourth set of RIS elements of the RIS. In this way, the number of measurements required for channel estimation in RIS assisted communication system can be reduced. Therefore, complexity can be reduced and accuracy is improved for channel estimation by using RIS.

Description

METHODS, DEVICES, APPARATUSES AND COMPUTER-READABLE MEDIUM FOR CHANNEL ESTIMATION FOR A RECONFIGURABLE INTELLIGENT SURFACE (RIS) ASSISTED CHANNELFIELD

[0001] Example embodiments of the present disclosure generally relate to the field of telecommunication, and in particular, to methods, devices, apparatuses, and a computer-readable medium for channel estimation for a reconfigurable intelligent surface (RIS) assisted channel.BACKGROUND

[0002] Reconfigurable Intelligent Surface (RIS) is popular concept to improve coverage in mobile wireless network without installing costly base stations (BS). For channel estimation of RIS, conventional full least square (LS) solution is used as basic technique. For example, a decomposition-aided channel estimation approach has been proposed to perform channel estimation of RIS (Z. Zhou, N. Ge, Z. Wang and L. Hanzo, "Joint Transmit Precoding and Reconfigurable Intelligent Surface Phase Adjustment: A Decomposition-Aided Channel Estimation Approach", in IEEE Transactions on Communications, vol. 69, no. 2, pp. 1228-1243, Feb. 2021). However, this technique requires huge number of time instances for measurements. More specifically, this technique requires a full combination of the transmitter elements and RIS elements for channel estimation, which indicates a huge number of measurements, thus a big computation complexity.

[0003] So-called “fast” techniques, for example, those disclosed in WO2022182264A1 and “Anchor- Assisted Intelligent Reflecting Surface Channel Estimation for Multiuser Communications” (Xinrong Guan, Qingqing Wu, Rui Zhang, Global Communications Conference of Computer Science, 3 August 2020), are also proposed for channel estimation. However, Such techniques require knowledge of one of transmitter (TX)-RIS and RIS-RX (receiver) channel matrices, and may also require matrix inversion with high condition number, which may lead to low numerical accuracy of calculations. Channel conversion from angular representation model to channel representation may also be involved, which may lead to high complexity.SUMMARY1SUBSTITUTE SHEET (RULE 26)

[0004] In general, example embodiments of the present disclosure provide methods, devices, and a computer-readable medium for communication, for example, to reduce complexity and improve accuracy of channel estimation by using a reconfigurable intelligent surface which comprises a plurality of element sets in same geometry and different positions.

[0005] In a first aspect, there is provided a method. The method comprises: performing, at a reconfigurable intelligent surface (RIS) together with a transmitter (TX) and a receiver (RX), a first number of channel measurements associated with a first set of TX elements of the TX and a second set of RIS elements of the RIS; and performing, together with the TX and the RX, a second number of channel measurements associated with a third subset of states of a third set of TX elements of the TX and a fourth subset of states of a fourth set of RIS elements of the RIS, wherein the second number is less than the first number, wherein the third subset of states is determined based on a first subset of states of the first set, and the fourth subset is determined based on a second subset of states of the second set, and wherein the third subset of states and the fourth subset of states are determined based on power sorting of received powers at the RX during the first number of channel measurements. Number of states in third subset is less than number of elements in third set, number of states in fourth subset is less than number of elements in fourth set. In this way, the number of measurements required for channel estimation in RIS assisted communication system can be reduced. Therefore, complexity can be reduced and accuracy is improved for channel estimation by using RIS.

[0006] In some example embodiments, the TX comprises NTXTX elements, the RIS comprises NRISRIS elements, the first set comprises αNTXTX elements, 0 < α < 1, the second set comprises βNRISRIS elements, 0 < β < 1, and wherein performing the first number of channel measurements comprises: reflecting, using the βNRISRIS elements in the second set, αNTXpilot signals transmitted by the TX using the αNTXTX elements in the first set to the RX; and performing at least αNTXx βNRISchannel measurements together with the TX and the RX for performing LS estimation of cascaded channel associated with first set of TX elements and second set of RIS elements and RX.

[0007] In some example embodiments, the third subset comprises αNTXTX elements, the fourth subset comprises βNRISRIS elements, and performing the second number of channel measurements comprises: reflecting, using the βσNRISRIS states in the fourth subset, αNTXpilot signals of αγNTXlength transmitted by the αNTXTX elements in the third set, 0 < γ < 1,2SUBSTITUTE SHEET (RULE 26)0 < σ < 1 ; and performing ay Nyx x βσNRISchannel measurements together with the TX and the RX. In this way, the number of measurements required for channel estimation in RIS assisted communication system can be reduced. Therefore, complexity can be reduced and accuracy is improved for channel estimation by using RIS.

[0008] In some example embodiments, the power sorting comprises: measuring a set of received powers {Pij} at the RX, wherein Pijrepresents a received power at the RX associated with an i-th pilot signal and a j-th RIS elements state; determining a first set of total powers for the αNTXTX elements {PTX, i}, wherein PTX, i= ; and determiningαγNTXTX states to top αγNTXtotal powers in the first set of total powers as the first subset of the TX elements.

[0009] In some example embodiments, the power sorting further comprises: determining a second set of total powers for the βNRISRIS elements {PRIS, j}, PRIS, j= ; anddetermining βσNRISRIS elements set states corresponding to top βσNRIStotal powers in the second subset of states of the RIS elements. In this way, the number of measurements required for channel estimation in RIS assisted communication system can be reduced. Therefore, complexity can be reduced and accuracy is improved for channel estimation by using RIS.

[0010] In some example embodiments, at least one the following: relative positions of the third subset of TX elements in the third set are same as relative positions of the first subset of TX elements in the first set; relative positions of the fourth subset of RIS elements in the fourth set are same as relative positions of the second subset of RIS elements in the second set; phase and magnitude states of TX elements in the third subset are same as phase and magnitude states of TX elements in the first subset; or phase and magnitude states of RIS elements in the fourth subset are same as phase and magnitude states of RIS elements in the second subset. In this way, the number of measurements required for channel estimation in RIS assisted communication system can be reduced. Therefore, complexity can be reduced and accuracy is improved for channel estimation by using RIS.

[0011] In some example embodiments, the first set and third set are two of a first plurality of sets of TX elements in the TX; the second set and fourth set are two of a second plurality of sets of RIS elements in the RIS; and the number of the first plurality of sets is equal to the number of the second plurality of sets. In this way, the number of measurements required for channel estimation in RIS assisted communication system can be reduced.3SUBSTITUTE SHEET (RULE 26)Therefore, complexity can be reduced and accuracy is improved for channel estimation by using RIS.

[0012] In some example embodiments, at least one of the following: at least two sets among the first plurality of sets have an intersection; or at least two sets among the second plurality of sets have an intersection. In this way, the number of measurements required for channel estimation in RIS assisted communication system can be reduced. Therefore, complexity can be reduced and accuracy is improved for channel estimation by using RIS.

[0013] In some example embodiments, at least one of the following: a union of the first plurality of sets is a universal set of TX elements of the TX; or a union of the first plurality of sets is a universal set of RIS elements of the RIS. Here, the universal set of TX elements of the TX include (almost) all TX elements of the TX, and the universal set of RIS elements of the RIS include (almost) all RIS elements in the RIS. In this way, the number of measurements required for channel estimation in RIS assisted communication system can be reduced. Therefore, complexity can be reduced and accuracy is improved for channel estimation by using RIS.

[0014] In some example embodiments, a first order of the first plurality of sets in which channel measurements for a channel from the TX to the RX via the RIS are performed is different from a second order of the second plurality of sets in which the channel measurements are performed. In this way, the number of measurements required for channel estimation in RIS assisted communication system can be reduced. Therefore, complexity can be reduced and accuracy is improved for channel estimation by using RIS.

[0015] In some example embodiments, an unused RIS element which is in the fourth set but not in the fourth subset has a constant phase during the second number of channel measurements. In this way, the number of measurements required for channel estimation in RIS assisted communication system can be reduced. Therefore, complexity can be reduced and accuracy is improved for channel estimation by using RIS.

[0016] In some example embodiments, a channel from the TX to the RX via the unused RIS element is determined as a direct channel from the TX to the RX. In this way, the number of measurements required for channel estimation in RIS assisted communication system can be reduced. Therefore, complexity can be reduced and accuracy is improved for channel estimation by using RIS.

[0017] In some example embodiments, the first set and the third set have a first geometry; and the second set and the fourth set have a second geometry. In this way, the number of measurements required for channel estimation in RIS assisted communication system can4SUBSTITUTE SHEET (RULE 26)be reduced. Therefore, complexity can be reduced and accuracy is improved for channel estimation by using RIS.

[0018] In some example embodiments, the first geometry and the second geometry are the same or different. In this way, the number of measurements required for channel estimation in RIS assisted communication system can be reduced. Therefore, complexity can be reduced and accuracy is improved for channel estimation by using RIS.

[0019] In some example embodiments, at least one of the first geometry and the second geometry is one of the following: a rectangle; or a rectangle with at least one element being removed from at least one edge of the rectangle. In this way, the number of measurements required for channel estimation in RIS assisted communication system can be reduced. Therefore, complexity can be reduced and accuracy is improved for channel estimation by using RIS.

[0020] In some example embodiments, a matrix completion process is performed at least based on the first number of channel measurements and the second number of channel measurements to obtain a channel matrix of a channel from the TX to the RX via the RIS, and during the matrix completion process, a value in the channel matrix which is not measured in channel measurements for the channel is inferred from values in the channel matrix which are measured in the channel measurements. In this way, the number of measurements required for channel estimation in RIS assisted communication system can be reduced. Therefore, complexity can be reduced and accuracy is improved for channel estimation by using RIS.

[0021] In this way, according to the first aspect and its example embodiments, the number of measurements required for channel estimation in RIS assisted communication system can be reduced. Therefore, complexity can be reduced and accuracy is improved for channel estimation by using RIS.

[0022] In a second aspect, there is provided a method. The method comprises: performing, at a transmitter (TX) together with a reconfigurable intelligent surface (RIS) and a receiver (RX), a first number of channel measurements associated with a first set of TX elements of the TX and a second set of RIS elements of the RIS; and performing, together with the RIS and the RX, a second number of channel measurements associated with a third subset of a third set of TX elements of the TX and a fourth subset of a fourth set of RIS elements of the RIS, wherein the second number is less than the first number, wherein the third subset is determined based on a first subset of the first set, and the fourth5SUBSTITUTE SHEET (RULE 26)subset is determined based on a second subset of the second set, and wherein the first subset and the second subset are determined based on power sorting of received powers at the RX during the first number of channel measurements. In this way, the number of measurements required for channel estimation in RIS assisted communication system can be reduced. Therefore, complexity can be reduced and accuracy is improved for channel estimation by using RIS.

[0023] In some example embodiments, the TX comprises NTXTX elements, the RIS comprises NRISRIS elements, the first set comprises αNTXTX elements, 0 < α < 1, the second set comprises βNRISRIS elements, 0 < β < 1, and wherein performing the first number of channel measurements comprises: transmitting, using the αNTXTX elements in the first set, αNTXpilot signals to be reflected by the RIS using the βNRISRIS elements in the second set to the RX; and performing αNTXx βNRISchannel measurements together with the RIS and the RX. In this way, the number of measurements required for channel estimation in RIS assisted communication system can be reduced. Therefore, complexity can be reduced and accuracy is improved for channel estimation by using RIS.

[0024] In some example embodiments, the third subset comprises αNTXTX elements, the fourth subset comprises βNRISRIS elements, and performing the second number of channel measurements comprises: transmitting, using the αNTXTX elements in the third subset, αNTXpilot signals with αγNTXlength to be reflected by the RIS using the βNRISRIS elements in βσNRISstates in the fourth subset to the RX, 0 < γ < 1, 0 < σ < 1; and performing αγNTXx βσNRISchannel measurements together with the RIS and the RX. In this way, the number of measurements required for channel estimation in RIS assisted communication system can be reduced. Therefore, complexity can be reduced and accuracy is improved for channel estimation by using RIS.

[0025] In some example embodiments, the power sorting comprises: measuring a set of received powers {Pij} at the RX, wherein Pijrepresents a received power at the RX associated with an i-th pilot signal and a j-th RIS element; determining a first set of total powers for the αNTXTX elements {PTX, i}, wherein PTX, i= ; and determiningαγNTXTX elements corresponding to top αγNTXtotal powers in the first set of total powers as the first subset of the TX elements. In this way, the number of measurements required for channel estimation in RIS assisted communication system can be reduced. Therefore, complexity can be reduced and accuracy is improved for channel estimation by using RIS.6SUBSTITUTE SHEET (RULE 26)

[0026] In some example embodiments, the power sorting further comprises: determining a second set of total powers for the βNRISRIS elements {PRIS, j}, PRIS, j= ; anddetermining βσNRISRIS elements corresponding to top βσNRIStotal powers in the second set of total powers as the second subset of the RIS elements. In this way, the number of measurements required for channel estimation in RIS assisted communication system can be reduced. Therefore, complexity can be reduced and accuracy is improved for channel estimation by using RIS.

[0027] In some example embodiments, at least one the following: relative positions of the third subset of TX elements in the third set are same as relative positions of the first subset of TX elements in the first set; relative positions of the fourth subset of RIS elements in the fourth set are same as relative positions of the second subset of RIS elements in the second set; phase and magnitude states of TX elements in the third subset are same as phase and magnitude states of TX elements in the first subset; or phase and magnitude states of RIS elements in the fourth subset are same as phase and magnitude states of RIS elements in the second subset. In this way, the number of measurements required for channel estimation in RIS assisted communication system can be reduced. Therefore, complexity can be reduced and accuracy is improved for channel estimation by using RIS.

[0028] In some example embodiments, the first set and third set are two of a first plurality of sets of TX elements in the TX; the second set and fourth set are two of a second plurality of sets of RIS elements in the RIS; and the number of the first plurality of sets is equal to the number of the second plurality of sets. In this way, the number of measurements required for channel estimation in RIS assisted communication system can be reduced. Therefore, complexity can be reduced and accuracy is improved for channel estimation by using RIS.

[0029] In some example embodiments, at least one of the following: at least two sets among the first plurality of sets have an intersection; or at least two sets among the second plurality of sets have an intersection. In this way, the number of measurements required for channel estimation in RIS assisted communication system can be reduced. Therefore, complexity can be reduced and accuracy is improved for channel estimation by using RIS.

[0030] In some example embodiments, at least one of the following: a union of the first plurality of sets is a universal set of TX elements of the TX; or a union of the first plurality of sets is a universal set of RIS elements of the RIS. In this way, the number of measurements required for channel estimation in RIS assisted communication system can7SUBSTITUTE SHEET (RULE 26)be reduced. Therefore, complexity can be reduced and accuracy is improved for channel estimation by using RIS.

[0031] In some example embodiments, a first order of the first plurality of sets in which channel measurements for a channel from the TX to the RX via the RIS are performed is different from a second order of the second plurality of sets in which the channel measurements are performed. In this way, the number of measurements required for channel estimation in RIS assisted communication system can be reduced. Therefore, complexity can be reduced and accuracy is improved for channel estimation by using RIS.

[0032] In some example embodiments, an unused RIS element which is in the fourth set but not in the fourth subset has a constant phase during the second number of channel measurements. In this way, the number of measurements required for channel estimation in RIS assisted communication system can be reduced. Therefore, complexity can be reduced and accuracy is improved for channel estimation by using RIS.

[0033] In some example embodiments, a channel from the TX to the RX via the unused RIS element is determined as a direct channel from the TX to the RX. In this way, the number of measurements required for channel estimation in RIS assisted communication system can be reduced. Therefore, complexity can be reduced and accuracy is improved for channel estimation by using RIS.

[0034] In some example embodiments, the first set and the third set have a first geometry; and the second set and the fourth set have a second geometry. In this way, the number of measurements required for channel estimation in RIS assisted communication system can be reduced. Therefore, complexity can be reduced and accuracy is improved for channel estimation by using RIS.

[0035] In some example embodiments, the first geometry and the second geometry are the same or different. In this way, the number of measurements required for channel estimation in RIS assisted communication system can be reduced. Therefore, complexity can be reduced and accuracy is improved for channel estimation by using RIS.

[0036] In some example embodiments, at least one of the first geometry and the second geometry is one of the following: a rectangle; or a rectangle with at least one element being removed from at least one edge of the rectangle. In this way, the number of measurements required for channel estimation in RIS assisted communication system can be reduced. Therefore, complexity can be reduced and accuracy is improved for channel estimation by using RIS.8SUBSTITUTE SHEET (RULE 26)

[0037] In this way, according to the second aspect and its example embodiments, the number of measurements required for channel estimation in RIS assisted communication system can be reduced. Therefore, complexity can be reduced and accuracy is improved for channel estimation by using RIS.

[0038] In a third aspect, there is provided a method. The method comprises: performing, at a receiver (RX) together with a transmitter (TX) and a reconfigurable intelligent surface (RIS), a first number of channel measurements associated with a first set of TX elements of the TX and a second set of RIS elements of the RIS; and performing, together with the TX and the RIS, a second number of channel measurements associated with a third subset of a third set of TX elements of the TX and a fourth subset of a fourth set of RIS elements of the RIS, wherein the second number is less than the first number, wherein the third subset is determined based on a first subset of the first set, and the fourth subset is determined based on a second subset of the second set, and wherein the first subset and the second subset are determined based on power sorting of received powers at the RX during the first number of channel measurements. In this way, the number of measurements required for channel estimation in RIS assisted communication system can be reduced. Therefore, complexity can be reduced and accuracy is improved for channel estimation by using RIS.

[0039] In some example embodiments, the TX comprises NTXTX elements, the RIS comprises NRISRIS elements, the first set comprises αNTXTX elements, 0 < α < 1, the second set comprises βNRISRIS elements, 0 < β < 1, and wherein performing the first number of channel measurements comprises: receiving, αNTXpilot signals transmitted by the TX using the αNTXTX elements in the first set and reflected by the RIS using the βNRISRIS elements in the second set; and performing αNTXx βNRISchannel measurements together with the TX and the RIS. In this way, the number of measurements required for channel estimation in RIS assisted communication system can be reduced. Therefore, complexity can be reduced and accuracy is improved for channel estimation by using RIS.

[0040] In some example embodiments, the third subset comprises aNr-x TX elements, the fourth subset comprises βNRISRIS elements, and performing the second number of channel measurements comprises: receiving, αNTXpilot signals with αγNTXlength transmitted by the TX using the αNTXTX elements in the third subset and reflected by the RIS using the βNRISRIS elements in βσNRISstates in the fourth subset, 0 < γ < 1, 0 < σ < 1; and performing αγNTXx βσNRISchannel measurements together with the TX and the RIS. In this way, the number of measurements required for channel estimation in RIS assisted9SUBSTITUTE SHEET (RULE 26)communication system can be reduced. Therefore, complexity can be reduced and accuracy is improved for channel estimation by using RIS.

[0041] In some example embodiments, the power sorting comprises: measuring a set of received powers {Pij} at the RX, wherein Pijrepresents a received power at the RX associated with an i-th pilot signal and a j-th RIS element; determining a first set of total powers for the αNTXTX elements {PTX, i}, wherein PTX, i=P;?; and determining αγNTXTX elements corresponding to top αγNTXtotal powers in the first set of total powers as the first subset of the TX elements. In this way, the number of measurements required for channel estimation in RIS assisted communication system can be reduced. Therefore, complexity can be reduced and accuracy is improved for channel estimation by using RIS.

[0042] In some example embodiments, the power sorting further comprises: determining a second set of total powers for the βNRISRIS elements {PRIS, j}, PRIS, j=and determining βσNRISRIS elements corresponding to top βσNRIStotal powers in the second set of total powers as the second subset of the RIS elements. In this way, the number of measurements required for channel estimation in RIS assisted communication system can be reduced. Therefore, complexity can be reduced and accuracy is improved for channel estimation by using RIS.

[0043] In some example embodiments, received powers at the RX associated with TX elements in the third set but not in the third subset of are set to zero; and received powers at the RX associated with RIS elements in the fourth set but not in the fourth subset of are set to zero. In this way, the number of measurements required for channel estimation in RIS assisted communication system can be reduced. Therefore, complexity can be reduced and accuracy is improved for channel estimation by using RIS.

[0044] In some example embodiments, at least one the following: relative positions of the third subset of TX elements in the third set are same as relative positions of the first subset of TX elements in the first set; relative positions of the fourth subset of RIS elements in the fourth set are same as relative positions of the second subset of RIS elements in the second set; phase and magnitude states of TX elements in the third subset are same as phase and magnitude states of TX elements in the first subset; or phase and magnitude states of RIS elements in the fourth subset are same as phase and magnitude states of RIS elements in the second subset. In this way, the number of measurements required for channel estimation10SUBSTITUTE SHEET (RULE 26)in RIS assisted communication system can be reduced. Therefore, complexity can be reduced and accuracy is improved for channel estimation by using RIS.

[0045] In some example embodiments, the first set and third set are two of a first plurality of sets of TX elements in the TX; the second set and fourth set are two of a second plurality of sets of RIS elements in the RIS; and the number of the first plurality of sets is equal to the number of the second plurality of sets. In this way, the number of measurements required for channel estimation in RIS assisted communication system can be reduced. Therefore, complexity can be reduced and accuracy is improved for channel estimation by using RIS.

[0046] In some example embodiments, at least one of the following: at least two sets among the first plurality of sets have an intersection; or at least two sets among the second plurality of sets have an intersection. In this way, the number of measurements required for channel estimation in RIS assisted communication system can be reduced. Therefore, complexity can be reduced and accuracy is improved for channel estimation by using RIS.

[0047] In some example embodiments, at least one of the following: a union of the first plurality of sets is a universal set of TX elements of the TX; or a union of the first plurality of sets is a universal set of RIS elements of the RIS. In this way, the number of measurements required for channel estimation in RIS assisted communication system can be reduced. Therefore, complexity can be reduced and accuracy is improved for channel estimation by using RIS.

[0048] In some example embodiments, a first order of the first plurality of sets in which channel measurements for a channel from the TX to the RX via the RIS are performed is different from a second order of the second plurality of sets in which the channel measurements are performed. In this way, the number of measurements required for channel estimation in RIS assisted communication system can be reduced. Therefore, complexity can be reduced and accuracy is improved for channel estimation by using RIS.

[0049] In some example embodiments, an unused RIS element which is in the fourth set but not in the fourth subset has a constant phase during the second number of channel measurements. In this way, the number of measurements required for channel estimation in RIS assisted communication system can be reduced. Therefore, complexity can be reduced and accuracy is improved for channel estimation by using RIS.

[0050] In some example embodiments, a channel from the TX to the RX via the unused RIS element is determined as a direct channel from the TX to the RX. In this way, the number of measurements required for channel estimation in RIS assisted communication11SUBSTITUTE SHEET (RULE 26)system can be reduced. Therefore, complexity can be reduced and accuracy is improved for channel estimation by using RIS.

[0051] In some example embodiments, the first set and the third set have a first geometry; and the second set and the fourth set have a second geometry. In this way, the number of measurements required for channel estimation in RIS assisted communication system can be reduced. Therefore, complexity can be reduced and accuracy is improved for channel estimation by using RIS.

[0052] In some example embodiments, the first geometry and the second geometry are the same or different. In this way, the number of measurements required for channel estimation in RIS assisted communication system can be reduced. Therefore, complexity can be reduced and accuracy is improved for channel estimation by using RIS.

[0053] In some example embodiments, at least one of the first geometry and the second geometry is one of the following: a rectangle; or a rectangle with at least one element being removed from at least one edge of the rectangle. In this way, the number of measurements required for channel estimation in RIS assisted communication system can be reduced. Therefore, complexity can be reduced and accuracy is improved for channel estimation by using RIS.

[0054] In some example embodiments, a matrix completion process is performed at least based on the first number of channel measurements and the second number of channel measurements to obtain a channel matrix of a channel from the TX to the RX via the RIS, and during the matrix completion process, a value in the channel matrix which is not measured in channel measurements for the channel is inferred from values in the channel matrix which are measured in the channel measurements. In this way, the number of measurements required for channel estimation in RIS assisted communication system can be reduced. Therefore, complexity can be reduced and accuracy is improved for channel estimation by using RIS.

[0055] In this way, according to the third aspect and its example embodiments, the number of measurements required for channel estimation in RIS assisted communication system can be reduced. Therefore, complexity can be reduced and accuracy is improved for channel estimation by using RIS.

[0056] In a fourth aspect, there is provided a first apparatus according to the first aspect. The apparatus comprises: at least one processor; and at least one memory storing instructions that, when executed by the at least one processor, cause the first apparatus at12SUBSTITUTE SHEET (RULE 26)least to: perform, together with a transmitter (TX) and a receiver (RX), a first number of channel measurements associated with a first set of TX elements of the TX and a second set of RIS elements of the RIS; and perform, together with the TX and the RX, a second number of channel measurements associated with a third subset of a third set of TX elements of the TX and a fourth subset of a fourth set of RIS elements of the RIS, wherein the second number is less than the first number, wherein the third subset is determined based on a first subset of the first set, and the fourth subset is determined based on a second subset of the second set, and wherein the first subset and the second subset are determined based on power sorting of received powers at the RX during the first number of channel measurements.

[0057] In this way, according to the fourth aspect, the number of measurements required for channel estimation in RIS assisted communication system can be reduced. Therefore, complexity can be reduced and accuracy is improved for channel estimation by using RIS.

[0058] In a fifth aspect, there is provided a second apparatus according to the second aspect. The apparatus comprises: at least one processor; and at least one memory storing instructions that, when executed by the at least one processor, cause the second apparatus at least to: perform, together with a reconfigurable intelligent surface (RIS) and a receiver (RX), a first number of channel measurements associated with a first set of TX elements of the TX and a second set of RIS elements of the RIS; and perform, together with the RIS and the RX, a second number of channel measurements associated with a third subset of a third set of TX elements of the TX and a fourth subset of a fourth set of RIS elements of the RIS, wherein the second number is less than the first number, wherein the third subset is determined based on a first subset of the first set, and the fourth subset is determined based on a second subset of the second set, and wherein the first subset and the second subset are determined based on power sorting of received powers at the RX during the first number of channel measurements.

[0059] In this way, according to the fifth aspect, the number of measurements required for channel estimation in RIS assisted communication system can be reduced. Therefore, complexity can be reduced and accuracy is improved for channel estimation by using RIS.

[0060] In a sixth aspect, there is provided a third apparatus according to the third aspect. The apparatus comprises: at least one processor; and at least one memory storing instructions that, when executed by the at least one processor, cause the third apparatus at13SUBSTITUTE SHEET (RULE 26)least to: perform, at a receiver (RX) together with a transmitter (TX) and a reconfigurable intelligent surface (RIS), a first number of channel measurements associated with a first set of TX elements of the TX and a second set of RIS elements of the RIS; and perform, together with the TX and the RIS, a second number of channel measurements associated with a third subset of a third set of TX elements of the TX and a fourth subset of a fourth set of RIS elements of the RIS, wherein the second number is less than the first number, wherein the third subset is determined based on a first subset of the first set, and the fourth subset is determined based on a second subset of the second set, and wherein the first subset and the second subset are determined based on power sorting of received powers at the RX during the first number of channel measurements.

[0061] In this way, according to the sixth aspect, the number of measurements required for channel estimation in RIS assisted communication system can be reduced. Therefore, complexity can be reduced and accuracy is improved for channel estimation by using RIS.

[0062] In a seventh aspect, there is provided a communication system comprising a first apparatus according to the fourth aspect, a second apparatus according to the fifth aspect and a third apparatus according to the sixth aspect, wherein the communication system is configured to perform the method of any of the first, second or third aspect using the first apparatus, the second apparatus and the third apparatus. In this way, according to the seventh aspect, the number of measurements required for channel estimation in RIS assisted communication system can be reduced. Therefore, complexity can be reduced and accuracy is improved for channel estimation by using RIS.

[0063] In an eighth aspect, there is provided a non-transitory computer-readable storage medium comprising computer programs stored thereon. The computer programs, when executed on at least one processor, cause the at least one processor to perform the method of any of the first, second or third aspect. In this way, according to the third aspect, the number of measurements required for channel estimation in RIS assisted communication system can be reduced. Therefore, complexity can be reduced and accuracy is improved for channel estimation by using RIS.

[0064] In a ninth aspect, there is provided a chip comprising at least one processing circuit configured to perform the method of any the first, second or third aspect. In this way, according to the third aspect, the number of measurements required for channel estimation14SUBSTITUTE SHEET (RULE 26)in RIS assisted communication system can be reduced. Therefore, complexity can be reduced and accuracy is improved for channel estimation by using RIS.

[0065] In a tenth aspect, there is provided a computer program product tangibly stored on a computer-readable medium and comprising computer-executable instructions which, when executed, cause an apparatus to perform a method of any of the first, second or third aspect. In this way, according to the third aspect, the number of measurements required for channel estimation in RIS assisted communication system can be reduced. Therefore, complexity can be reduced and accuracy is improved for channel estimation by using RIS.

[0066] It is to be understood that the summary section is not intended to identify key or essential features of embodiments of the present disclosure, nor is it intended to be used to limit the scope of the present disclosure. Other features of the present disclosure will become easily comprehensible through the following description.BRIEF DESCRIPTION OF THE DRAWINGS

[0067] Some example embodiments will now be described with reference to the accompanying drawings, in which:

[0068] FIG. 1A illustrates an example of a network environment in which some example embodiments of the present disclosure may be implemented;

[0069] FIG. IB illustrates a schematic diagram of a part of a cascaded channel according to some embodiments of the present disclosure;

[0070] FIG. 1C illustrates a schematic diagram of another part of a cascaded channel according to some embodiments of the present disclosure;

[0071] FIG. 2 illustrates a signaling chart illustrating an example communication process in accordance with some example embodiments of the present disclosure;

[0072] FIG. 3A illustrates a schematic diagram illustrating RIS sets in accordance with some example embodiments of the present disclosure;

[0073] FIG. 3B illustrates a schematic diagram illustrating TX sets in accordance with some example embodiments of the present disclosure;

[0074] FIG. 4A illustrates another schematic diagram illustrating TX sets in accordance with some example embodiments of the present disclosure;

[0075] FIG. 4B illustrates another schematic diagram illustrating RIS sets in accordance15SUBSTITUTE SHEET (RULE 26)with some example embodiments of the present disclosure;

[0076] FIG. 5A illustrates a schematic diagram of a measurement process of a terminal device in accordance with some embodiments of the present disclosure;

[0077] FIG. 5B illustrates a schematic diagram of pilots of the transmitter in accordance with some embodiments of the present disclosure;

[0078] FIG. 5C illustrates a schematic diagram of a power sorting process in accordance with some embodiments of the present disclosure;

[0079] FIG. 5D illustrates another schematic diagram of a power sorting process in accordance with some embodiments of the present disclosure;

[0080] FIG. 5E illustrates a schematic diagram of matrix completion in accordance with some embodiments of the present disclosure;

[0081] FIG. 6A illustrates a schematic diagram of a first geometry of TX or RIS in accordance with some embodiments of the present disclosure;

[0082] FIG. 6B illustrates a schematic diagram of TX or RIS in accordance with some embodiments of the present disclosure;

[0083] FIG. 7A illustrates a schematic diagram of a second geometry of TX or RIS in accordance with some embodiments of the present disclosure;

[0084] FIG. 7B illustrates a schematic diagram of TX or RIS in accordance with some embodiments of the present disclosure;

[0085] FIG. 8A illustrates a schematic diagram of a dense geometry of TX or RIS in accordance with some embodiments of the present disclosure;

[0086] FIG. 8B illustrates a schematic diagram of a sparse geometry of TX or RIS in accordance with some embodiments of the present disclosure;

[0087] FIG. 9A illustrates a schematic diagram of an example sequence of sets shift in accordance with some embodiments of the present disclosure;

[0088] FIG. 9B illustrates a schematic diagram of an example sequence of sets shift in accordance with some embodiments of the present disclosure;

[0089] FIG. 10 illustrates a flowchart of an example method implemented at a reconfigurable intelligent surface in accordance with some embodiments of the present disclosure;16SUBSTITUTE SHEET (RULE 26)

[0090] FIG. 11 illustrates another flowchart of an example method implemented at a transmitter in accordance with some embodiments of the present disclosure;

[0091] FIG. 12 illustrates another flowchart of an example method implemented at a receiver in accordance with some embodiments of the present disclosure;

[0092] FIG. 13 illustrates a simplified block diagram of a first apparatus according to some example embodiments of the present disclosure;

[0093] FIG. 14 illustrates a simplified block diagram of a second apparatus according to some example embodiments of the present disclosure;

[0094] FIG. 15 illustrates a simplified block diagram of a third apparatus according to some example embodiments of the present disclosure; and

[0095] FIG. 16 illustrates a simplified block diagram of a device that is suitable for implementing some example embodiments of the present disclosure.

[0096] Throughout the drawings, the same or similar reference numerals represent the same or similar elements.DETAILED DESCRIPTION

[0097] Principles of the present disclosure will now be described with reference to some example embodiments. It is to be understood that these embodiments are described only for the purpose of illustration and help those skilled in the art to understand and implement the present disclosure, without suggesting any limitation as to the scope of the disclosure. The disclosure described herein can be implemented in various manners other than the ones described below.

[0098] In the following description and claims, unless defined otherwise, all technical and scientific terms used herein have the same meaning as commonly understood by one of ordinary skills in the art to which this disclosure belongs.

[0099] References in the present disclosure to “one embodiment,” “an embodiment,” “an example embodiment,” and the like indicate that the embodiment described may include a particular feature, structure, or characteristic, but it is not necessary that every embodiment includes the particular feature, structure, or characteristic. Moreover, such phrases are not necessarily referring to the same embodiment. Further, when a particular feature, structure, or characteristic is described in connection with an embodiment, it is submitted17SUBSTITUTE SHEET (RULE 26)that it is within the knowledge of one skilled in the art to affect such feature, structure, or characteristic in connection with other embodiments whether or not explicitly described.

[0100] It shall be understood that although the terms “first” and “second” etc. may be used herein to describe various elements, these elements should not be limited by these terms. These terms are only used to distinguish one element from another. For example, a first element could be termed a second element, and similarly, a second element could be termed a first element, without departing from the scope of example embodiments. As used herein, the term “and / or” includes any and all combinations of one or more of the listed terms.

[0101] The terminology used herein is for the purpose of describing particular embodiments only and is not intended to be limiting of example embodiments. As used herein, the singular forms “a”, “an” and “the” are intended to include the plural forms as well, unless the context clearly indicates otherwise. It will be further understood that the terms “comprises”, “comprising”, “has”, “having”, “includes” and / or “including”, when used herein, specify the presence of stated features, elements, and / or components etc., but do not preclude the presence or addition of one or more other features, elements, components and / or combinations thereof.

[0102] As used in this application, the term “circuitry” may refer to one or more or all of the following:(a) hardware-only circuit implementations (such as implementations in only analog and / or digital circuitry) and(b) combinations of hardware circuits and software, such as (as applicable):(i) a combination of analog and / or digital hardware circuit(s) with software / firmware and(ii) any portions of hardware processor(s) with software (including digital signal processor(s)), software, and memory(ies) that work together to cause an apparatus, such as a mobile phone or server, to perform various functions) and(c) hardware circuit(s) and or processor(s), such as a microprocessor s) or a portion of a microprocessor(s), that requires software (for example, firmware) for operation, but the software may not be present when it is not needed for operation.18SUBSTITUTE SHEET (RULE 26)

[0103] This definition of circuitry applies to all uses of this term in this application, including in any claims. As a further example, as used in this application, the term circuitry also covers an implementation of merely a hardware circuit or processor (or multiple processors) or portion of a hardware circuit or processor and its (or their) accompanying software and / or firmware. The term circuitry also covers, for example and if applicable to the particular claim element, a baseband integrated circuit or processor integrated circuit for a mobile device or a similar integrated circuit in server, a cellular network device, or other computing or network device.

[0104] As used herein, the term “communication network” refers to a network following any suitable communication standards, such as Long Term Evolution (LTE), LTE-Advanced (LTE-A), Wideband Code Division Multiple Access (WCDMA), High-Speed Packet Access (HSPA), Narrow Band Internet of Things (NB-IoT), Wireless Fidelity (WiFi) and so on. Furthermore, the communications between a terminal device and a network device in the communication network may be performed according to any suitable generation communication protocols, including, but not limited to, the fourth generation (4G), 4.5G, the future fifth generation (5G), IEEE 802.11 communication protocols, and / or any other protocols either currently known or to be developed in the future. Embodiments of the present disclosure may be applied in various communication systems. Given the rapid development in communications, there will of course also be future type communication technologies and systems with which the present disclosure may be embodied. It should not be seen as limiting the scope of the present disclosure to only the aforementioned system.

[0105] As used herein, the term “network device” refers to a node in a communication network via which a terminal device accesses the network and receives services therefrom. The network device may refer to a base station (BS), a transmitter (TX) or an access point (AP), for example, a node B (NodeB or NB), an evolved NodeB (eNodeB or eNB), a NR NB (also referred to as a gNB), a Remote Radio Unit (RRU), a radio header (RH), a remote radio head (RRH), a WiFi device, a relay, a low power node such as a femto, a pico, and so forth, depending on the applied terminology and technology. In the following description, the terms “network device”, “AP device”, “AP” and “access point” may be used interchangeably. In some circumstances, the term “network device” may also be referred to as “transmitter” (TX) which transmits signal(s) to a terminal device directly and / or via a reconfigurable intelligent surface.19SUBSTITUTE SHEET (RULE 26)

[0106] The term “terminal device” refers to any end device that may be capable of wireless communication. By way of example rather than limitation, a terminal device may also be referred to as a communication device, user equipment (UE), a receiver (RX), a Subscriber Station (SS), a Portable Subscriber Station, a Mobile Station (MS), a station (STA) or station device, or an Access Terminal (AT). The terminal device may include, but not limited to, a mobile phone, a cellular phone, a smart phone, voice over IP (VoIP) phones, wireless local loop phones, a tablet, a wearable terminal device, a personal digital assistant (PDA), portable computers, desktop computer, image capture terminal devices such as digital cameras, gaming terminal devices, music storage and playback appliances, vehicle-mounted wireless terminal devices, wireless endpoints, mobile stations, laptop-embedded equipment (LEE), laptop-mounted equipment (LME), USB dongles, smart devices, wireless customer-premises equipment (CPE), an Internet of Things (loT) device, a watch or other wearable, a VR (virtual reality) device, an XR (extended reality) device, a head-mounted display (HMD), a vehicle, a drone, a medical device and applications (for example, remote surgery), an industrial device and applications (for example, a robot and / or other wireless devices operating in an industrial and / or an automated processing chain contexts), a consumer electronics device, a device operating on commercial and / or industrial wireless networks, and the like. In the following description, the terms “station”, “station device”, “STA”, “terminal device”, “communication device”, “terminal”, “user equipment” and “UE” may be used interchangeably. In some circumstances, the term “terminal device” may also be referred to as “receiver” (RX) which receives signal(s) from a network device directly and / or via a reconfigurable intelligent surface.

[0107] The term “reconfigurable intelligent surface” (RIS) refers to a surface that consists of antenna array (or meta surface) where each element of the RIS is equipped with phase shifting electrical circuit to control phase of reflected signal(s). RIS is used to improve coverage in mobile wireless network without installation costly base stations. RIS assisted channels are used to compensate low quality coverage without new Base Station (BS) installation. For RIS assisted channels, channel estimation procedure need to estimate both BS-RIS and RIS-UE channels. The RIS solution is simple in structure and cheap in cost, and makes sense when number of elements is huge (by channel path loss reason). However, due to the huge number of elements, channel estimation procedure for RIS becomes a critical issue.20SUBSTITUTE SHEET (RULE 26)

[0108] However, RIS may bring about negative effects. For example, Knowledge of one of TX-RIS and RIS-RX channel matrices is required for channel estimation. Matrix inversion with high condition number is also required for channel estimation, which may lead to low numerical accuracy of calculations. Channel conversion from angular representation model to channel representation may also be required for channel estimation, which involves high complexity and potentially “bad” matrix may exist which cannot be normally inverted.

[0109] In view of such a situation, according to this disclosure, a method for communication is proposed. The method comprises performing, at a reconfigurable intelligent surface (RIS) together with a transmitter (TX) and a receiver (RX), a first number of channel measurements associated with a first set of TX elements of the TX and a second set of RIS elements of the RIS. The method also comprises performing, at the RIS and together with the TX and the RX, a second number of channel measurements associated with a third subset of a third set of TX elements of the TX and a fourth subset of a fourth set of RIS elements of the RIS. Here, the second number is less than the first number, the third subset is determined based on a first subset of the first set, and the fourth subset is determined based on a second subset of the second set, and the first subset and the second subset are determined based on power sorting of received powers at the RX during the first number of channel measurements.

[0110] In this way, the number of measurements required for channel estimation in RIS assisted communication system can be reduced. Therefore, complexity can be reduced and accuracy is improved for channel estimation by using RIS.

[0111] FIG. 1A illustrates an example communication system 100 in which some embodiments of the present disclosure can be implemented. The communication system 100, which is a part of a communication network, includes a reconfigurable intelligent surface (RIS) 110, a transmitter (TX) 120, and a receiver (RX) 130. The transmitter 120 plays the role of a network device, like a base station. The receiver 130 plays the role of a terminal device, like a UE. As illustrated in FIG. 1A, the RIS-assisted channel (TX-RIS-RX channel) between the transmitter 120 and the receiver 130 comprises two components, namely, the HA-HB sub-channel which involves a TX set and a RIS set, and the H0A-H0B sub-channel which involves TX elements other than the TX set and RIS elements other than the RIS set. Here, the RIS-assisted channel (TX-RIS-RX channel) may also be referred to as “cascaded channel”, because it consists of the TX-RIS channel21SUBSTITUTE SHEET (RULE 26)and RIS-RX channel. As described above, the TX-RIS channel consists of the HA sub-channel and the HO A sub-channel, and the RIS-RX channel consists of the HB sub-channel and the HOB sub-channel.

[0112] The configuration as illustrated in FIG. 1A may be used at a given stage of a measurement process for the RX 130 to perform a channel estimation from the TX 120 to the RX 130 directly or indirectly. It is noted that the direct channel from TX 120 to RX 130 includes the direct sub-channel HO and the RIS-assisted H0A-H0B sub-channel. The direct sub-channel HO denotes a sub-channel from TX 120 to RX 130 directly, without by way of RIS 110. In other words, all RIS elements that are not included in the RIS set at the given stage of the measurement process are included into the direct channel from TX 120 to RX 130. Also, all TX elements that are not included in the TX set at the given stage of the measurement process are included into the direct channel from TX 120 to RX 130.

[0113] All RIS elements that are not included in the RIS set at the given stage of the measurement process have constant state during this stage. All TX elements that are not included in the TX set at the given stage have constant state during this stage. At a given stage of the measurement process, the constant state of all RIS elements that are not included in the RIS set may be different from the constant state of all TX elements that are not included in the TX set.

[0114] The direct sub-channel HO without RIS reflection may be measured separately. The H0A and HOB may be different from stage to stage.

[0115] The communications between the TX 120 and the RX 130 with and / or without RIS reflection may conform to any suitable standards including, but not limited to, Long Term Evolution (LTE), LTE-Evolution, LTE-Advanced (LTE-A), Wideband Code Division Multiple Access (WCDMA), Code Division Multiple Access (CDMA) and Global System for Mobile Communications (GSM), Wireless Fidelity (WiFi) and the like. Furthermore, the communications may be performed according to any generation communication protocols either currently known or to be developed in the future. Examples of the communication protocols include, but not limited to, the first generation (1G), the second generation (2G), 2.5G, 2.75G, the third generation (3G), the fourth generation (4G), 4.5G, the fifth generation (5G), 5.5G, 5G- Advanced networks, the sixth generation (6G), or IEEE 802.11 communication protocols.

[0116] It is to be understood that the number of devices and their connection relationships22SUBSTITUTE SHEET (RULE 26)and types shown in FIG. 1A are for illustrative purposes only without suggesting any limitation. The communication system 100 may comprise any suitable number of devices adapted for implementing embodiments of the present disclosure.

[0117] FIG. IB illustrates a schematic diagram of a part of a cascaded channel 100B according to some embodiments of the present disclosure. 100B represents the sub-channel HA in FIG. 1 A. As illustrated in FIG. IB, the sub-channel HA is represented by an array. For example, the array may have a size of NTX x NRIS. NTX represents a geometry of the TX set involved in the HA sub-channel, and NRIS represents a geometry of the RIS set involved in the HA sub-channel. If the traditional LS method is adopted, number of measurements should be NTX x NRIS, which is equal to the size of HA.

[0118] In the example illustrated in FIG. IB, NTX is 19 and NRIS is 4. The sub-channel HA is represented by a 19 x 4 array, each element in the array is represented as a little square. These squares are filled with different color to indicate different TX sets. For example, in the example illustrated in FIG. IB, squares filled in the same manner belong to a same group. For example, all squares without filling (i.e., blank squares) belong to a same set, squares filled with left-right straight lines belong to a same set, squares filled with top-bottom straight lines belong to a same set, etc.

[0119] FIG. 1C illustrates a schematic diagram of a part of a cascaded channel 100C according to some embodiments of the present disclosure. The reference numeral 100C represents the sub-channel HB in FIG. 1A. As illustrated in FIG. 1C, the sub-channel HB is represented by an array, each element in the array is represented as a little square. These squares are filled with different color to indicate different RIS sets. For example, in the example illustrated in FIG. 1C, squares filled in the same manner belong to a same group. For example, squares filled with left-right straight lines belong to a same set, squares filled with top-bottom straight lines belong to a same set, etc.

[0120] FIG. 2 illustrates a signaling chart illustrating an example communication process 200 in accordance with some example embodiments of the present disclosure. Only for the purpose of discussion, the communication process 200 will be described with reference to FIGS. 1A-1C. The communication process 200 may involve the RIS 110, TX 120 and RX 130.

[0121] At block 210, the RIS 110 performs, together with TX 120 and RX 130, a first number of channel measurements associated with a first set of TX elements of the TX 12023SUBSTITUTE SHEET (RULE 26)and a second set of RIS elements of the RIS 110. At block 220, the RIS 110 performs, together with the TX 120 and the RX 130, a second number of channel measurements associated with a third subset of a third set of TX elements of the TX 120 and a fourth subset of a fourth set of RIS elements of the RIS 110. Here, the second number is less than the first number, the third subset is determined based on a first subset of the first set, and the fourth subset is determined based on a second subset of the second set, and the first subset and the second subset are determined based on power sorting of received powers at the RX during the first number of channel measurements. In this way, the number of measurements required for channel estimation in RIS assisted communication system (for example, communication system 100 A as illustrated in FIG. 1A) can be reduced. Therefore, complexity can be reduced and accuracy is improved for channel estimation by using RIS 110.

[0122] In some example embodiments, the TX 120 comprises NTXTX elements, the RIS 110 comprises NRISRIS elements, the first set comprises αNTXTX elements, 0 < α < 1, the second set comprises βNRISRIS elements, 0 < β < 1. In order to perform the first number of channel measurements, the RIS 110 reflects, using the βNRISRIS elements in the second set, αNTXpilot signals transmitted by the TX using the αNTXTX elements in the first set to the RX 130, and performs αNTXx βNRISchannel measurements together with the TX 120 and the RX 130. In this way, the number of measurements required for channel estimation in RIS assisted communication system (for example, communication system 100 A as illustrated in FIG. 1A) can be reduced. Therefore, complexity can be reduced and accuracy is improved for channel estimation by using RIS 110.

[0123] In some example embodiments, the third subset comprises αNTXTX elements, the fourth subset comprises βNRISRIS elements. In order to perform the second number of channel measurements, the RIS 110 reflects, using the βNRISRIS elements in βσNRISstates in the fourth subset, aN rx pilot signals with αγNTXlength transmitted by the TX 120 using the αNTXTX elements in the third subset, and performs αγNTXx βσNRISchannel measurements together with the TX 120 and the RX 130, 0 < γ < 1,0 < σ < l. In this way, the number of measurements required for channel estimation in RIS assisted communication system (for example, communication system 100 A as illustrated in FIG. 1A) can be reduced. Therefore, complexity can be reduced and accuracy is improved for channel estimation by using RIS 110.

[0124] In some example embodiments, in order to perform the power sorting, the RX 130 measures a set of received powers {Pij} at the RX 130, here Pijrepresents a received power24SUBSTITUTE SHEET (RULE 26)at the RX 130 associated with an i-th pilot signal and a j-th RIS element. In order to perform the power sorting, the RX 130 further determines a first set of total powers for the αNTXTX elements {PTX, i} (PTX, i= determines αγNTXTX elementscorresponding to top αγNTXtotal powers in the first set of total powers as the first subset of the TX elements. In this way, the number of measurements required for channel estimation in RIS assisted communication system (for example, communication system 100 A as illustrated in FIG. 1A) can be reduced. Therefore, complexity can be reduced and accuracy is improved for channel estimation by using RIS 110.

[0125] In some example embodiments, in order to perform the power sorting, the RX 130 further determines a second set of total powers for the βNRISRIS elements {PRIS, j} (PRIS, j= and determines βσNRISRIS elements corresponding to top βσNRIStotal powers inthe second set of total powers as the second subset of the RIS elements. In this way, the number of measurements required for channel estimation in RIS assisted communication system (for example, communication system 100A as illustrated in FIG. 1A) can be reduced. Therefore, complexity can be reduced and accuracy is improved for channel estimation by using RIS 110.

[0126] In some example embodiments, relative positions of the third subset of TX elements in the third set are same as relative positions of the first subset of TX elements in the first set. Alternatively or additionally, relative positions of the fourth subset of RIS elements in the fourth set are same as relative positions of the second subset of RIS elements in the second set. Alternatively or additionally, phase and magnitude states of TX elements in the third subset are same as phase and magnitude states of TX elements in the first subset. Alternatively or additionally, phase and magnitude states of RIS elements in the fourth subset are same as phase and magnitude states of RIS elements in the second subset. In this way, the number of measurements required for channel estimation in RIS assisted communication system (for example, communication system 100A as illustrated in FIG. 1 A) can be reduced. Therefore, complexity can be reduced and accuracy is improved for channel estimation by using RIS 110.

[0127] In some example embodiments, the first set and third set are two of a first plurality of sets of TX elements in the TX 120, the second set and fourth set are two of a second plurality of sets of RIS elements in the RIS 110, and the number of the first plurality of sets is equal to the number of the second plurality of sets. In this way, the number of25SUBSTITUTE SHEET (RULE 26)measurements required for channel estimation in RIS assisted communication system (for example, communication system 100 A as illustrated in FIG. 1A) can be reduced. Therefore, complexity can be reduced and accuracy is improved for channel estimation by using RIS 110.

[0128] In some example embodiments, at least two sets among the first plurality of sets have an intersection. Alternatively or additionally, at least two sets among the second plurality of sets have an intersection. In some example embodiments, a union of the first plurality of sets is a universal set of TX elements of the TX. Alternatively or additionally, a union of the first plurality of sets is a universal set of RIS elements of the RIS 110. In this way, the number of measurements required for channel estimation in RIS assisted communication system (for example, communication system 100 A as illustrated in FIG. 1A) can be reduced. Therefore, complexity can be reduced and accuracy is improved for channel estimation by using RIS 110.

[0129] In some example embodiments, a first order of the first plurality of sets in which channel measurements for a channel from the TX 120 to the RX 130 via the RIS 110 are performed is different from a second order of the second plurality of sets in which the channel measurements are performed. In this way, the number of measurements required for channel estimation in RIS assisted communication system (for example, communication system 100 A as illustrated in FIG. 1A) can be reduced. Therefore, complexity can be reduced and accuracy is improved for channel estimation by using RIS 110.

[0130] In some example embodiments, an unused RIS element which is in the fourth set but not in the fourth subset has a constant phase during the second number of channel measurements. In some example embodiments, a channel from the TX 120 to the RX 130 via the unused RIS element is determined as a direct channel from the TX 120 to the RX 130. In this way, the number of measurements required for channel estimation in RIS assisted communication system (for example, communication system 100A as illustrated in FIG. 1 A) can be reduced. Therefore, complexity can be reduced and accuracy is improved for channel estimation by using RIS 110.

[0131] In some example embodiments, the first set and the third set have a first geometry, and the second set and the fourth set have a second geometry. In some example embodiments, the first geometry and the second geometry are the same or different. In some example embodiments, at least one of the first geometry and the second geometry is a rectangle. Alternatively or additionally, at least one of the first geometry and the second geometry is a rectangle with at least one element being removed from at least one edge of26SUBSTITUTE SHEET (RULE 26)the rectangle. In this way, the number of measurements required for channel estimation in RIS assisted communication system (for example, communication system 100A as illustrated in FIG. 1A) can be reduced. Therefore, complexity can be reduced and accuracy is improved for channel estimation by using RIS 110.

[0132] In some example embodiments, a matrix completion process is performed at least based on the first number of channel measurements and the second number of channel measurements to obtain a channel matrix of a channel from the TX 120 to the RX 130 via the RIS 110. During the matrix completion process, a value in the channel matrix which is not measured in channel measurements for the channel is inferred from values in the channel matrix which are measured in the channel measurements. In this way, the number of measurements required for channel estimation in RIS assisted communication system (for example, communication system 100 A as illustrated in FIG. 1A) can be reduced. Therefore, complexity can be reduced and accuracy is improved for channel estimation by using RIS 110.

[0133] In this way, according to the first aspect and its example embodiments, the number of measurements required for channel estimation in RIS assisted communication system can be reduced. Therefore, complexity can be reduced and accuracy is improved for channel estimation by using RIS 110.

[0134] FIG. 3A illustrates a schematic diagram illustrating RIS 300A used in channel estimation in accordance with some example embodiments of the present disclosure. Only for the purpose of discussion, the RIS 300A will be described with reference to FIGS. 1A-1C.

[0135] As illustrated in FIG. 3 A, four RIS elements sets, namely, RIS element set 1, RIS element set 2, RIS element set 3 and RIS element set 4 (each referred to as “set 1”, “set 2”, “set 3” and “set 4" respectively for simplicity in FIG. 3A) have a same geometry. It is assumed that set l is a reference set, and set 2, set 3 and set 4 are all can be considered sets “shifted” relative to set 1. In other words, the difference between set 2 (or set 3 or set 4) and set 1 is that the positions of the two sets (namely, set 1 and set 2); the geometry of the two sets are the same, and state of an element in set 2 is the same as the state of the corresponding element in set 1. Here, “the corresponding element” means position of the element relative to set 2 is the same as the position of “the corresponding element” relative to set 1.27SUBSTITUTE SHEET (RULE 26)

[0136] It is to be noted that only four RIS element sets are illustrated in FIG. 3A. However, the number of RIS element sets is not limited to four. The number of RIS element sets may be greater or lower than 4. For example, the number of RIS element sets used in the channel estimation may be 2, 3, 5 or even greater.

[0137] In some example embodiments, at least two of the RIS sets are intersected. For example, as illustrated in FIG. 3 A, set 1 and set 2 are intersected.

[0138] In some example embodiments, RIS 110 is (almost) fully covered by RIS element sets.

[0139] FIG. 3B illustrates a schematic diagram illustrating a RIS 300B used in channel estimation in accordance with some example embodiments of the present disclosure. Only for the purpose of discussion, the RIS element sets 300B will be described with reference to FIGS. 1A-1C.

[0140] As illustrated in FIG. 3B, four RIS elements sets, namely, RIS element set 1, RIS element set 2, RIS element set 3 and RIS element set 4 (each referred to as “RIS set 1”, “RIS set 2”, “RIS set 3” and “RIS set 4" respectively for simplicity in FIG. 3B) have a same geometry. It is assumed that RIS set l is a reference set, and RIS set 2, RIS set 3 and RIS set 4 are all can be considered RIS sets “shifted” relative to RIS set 1. In other words, the difference between RIS set 2 (or RIS set 3 or RIS set 4) and RIS set 1 is that the positions of the two RIS sets (namely, RIS set 1 and RIS set 2); the geometry of the two RIS sets are the same, and state of an element in RIS set 2 is the same as the state of the corresponding element in RIS set 1. Here, “the corresponding element” has the same meaning as described before. Namely, “the corresponding element” means position of the element relative to RIS set 2 is the same as the position of “the corresponding element” relative to RIS set 1.

[0141] It is to be noted that only four RIS element sets are illustrated in FIG. 3A. However, the number of RIS element sets is not limited to four. The number of RIS element sets may be greater or lower than 4. For example, as described before in connection with FIG. 3 A, the number of RIS element sets used in the channel estimation may be 2, 3, 5 or even greater.

[0142] In some example embodiments, at least two of the RIS sets are intersected. For example, as illustrated in FIG. 3B, RIS set 1 and RIS set 2 may be intersected. Alternatively or additionally, RIS set 2 and RIS set 3 may be intersected. Alternatively or28SUBSTITUTE SHEET (RULE 26)additionally, RIS set 3 and RIS set 4 may be intersected.

[0143] In some example embodiments, RIS 110 is (almost) fully covered by RIS element sets. For example, as illustrated in FIG. 3B, RIS 400B is fully covered by RIS set 1-4.

[0144] FIG. 4A illustrates a schematic diagram illustrating TX 400A used in channel estimation in accordance with some example embodiments of the present disclosure. Only for the purpose of discussion, the RIS element sets 300B will be described with reference to FIGS. 1A-1C and 3 A.

[0145] As illustrated in FIG. 4 A, four TX elements sets, namely, TX element set 1, TX element set 2, TX element set 3 and TX element set 4 (each referred to as “set 1”, “set 2”, “set 3” and “set 4" respectively for simplicity in FIG. 4A) have a same geometry. It is assumed that set l is a reference set, and set 2, set 3 and set 4 are all can be considered sets “shifted” relative to set 1. In other words, the difference between set 2 (or set 3 or set 4) and set 1 is that the positions of the two sets (namely, set 1 and set 2); the geometry of the two sets are the same, and state of an element in set 2 is the same as the state of the corresponding element in set 1. Here, “the corresponding element” means position of the element relative to set 2 is the same as the position of “the corresponding element” relative to set 1.

[0146] It is to be noted that only four TX element sets are illustrated in FIG. 4A. However, the number of TX element sets is not limited to four. The number of TX element sets may be greater or lower than 4. For example, the number of TX element sets used in the channel estimation may be 2, 3, 5 or even greater.

[0147] In some example embodiments, at least two of the TX sets are intersected. For example, as illustrated in FIG. 4A, set 1 and set 2 are intersected.

[0148] In some example embodiments, TX 120 is (almost) fully covered by TX element sets.

[0149] FIG. 4B illustrates a schematic diagram illustrating TX 400B in accordance with some example embodiments of the present disclosure. Only for the purpose of discussion, the RIS element sets 300B will be described with reference to FIGS. 1 A-1C and 3B.

[0150] As illustrated in FIG. 4B, four TX elements sets, namely, TX element set 1, TX element set 2, TX element set 3 and TX element set 4 (each referred to as “TX set 1”, “TX set 2”, “TX set 3” and “TX set 4" respectively for simplicity in FIG. 4B) have a same29SUBSTITUTE SHEET (RULE 26)geometry. It is assumed that TX set 1 is a reference set, and TX set 2, TX set 3 and TX set 4 are all can be considered TX sets “shifted” relative to TX set 1. In other words, the difference between TX set 2 (or TX set 3 or TX set 4) and TX set 1 is that the positions of the two TX sets (namely, TX set 1 and TX set 2); the geometry of the two TX sets are the same, and state of an element in TX set 2 is the same as the state of the corresponding element in TX set 1. Here, “the corresponding element” has the same meaning as described before. Namely, “the corresponding element” means position of the element relative to TX set 2 is the same as the position of “the corresponding element” relative to TX set 1.

[0151] It is to be noted that only four TX element sets are illustrated in FIG. 4B. However, the number of TX element sets is not limited to four. The number of TX element sets may be greater or lower than 4. For example, as described before in connection with FIG. 4A, the number of TX element sets used in the channel estimation may be 2, 3, 5 or even greater.

[0152] In some example embodiments, at least two of the TX sets are intersected. For example, as illustrated in FIG. 4B, TX set 1 and TX set 2 may be intersected. Alternatively or additionally, RIS set 2 and RIS set 4 may be intersected. Alternatively or additionally, RIS set 3 and RIS set 4 may be intersected. Alternatively or additionally, RIS set 3 and RIS set 1 may be intersected.

[0153] In some example embodiments, RIS 110 is (almost) fully covered by RIS element sets. For example, as illustrated in FIG. 3B, RIS 400B is fully covered by RIS set 1-4.

[0154] In some example embodiments, at least one of TX 120 or RIS 110 is covered with intersection between different sets. More specifically, at least two of TX element sets are intersected and / or at least two of RIS element sets are intersected.

[0155] In some example embodiments, (almost) every ( / all) element of TX 120 and RIS 110 is contained in at least one set. More specifically, every ( / all) TX element is contained in at least one TX element set. In other words, TX surface is (almost) fully covered by TX element sets. Meanwhile, every ( / all) RIS element is contained in at least one RIS element set. In other words, RIS is (almost) fully covered by RIS element sets.

[0156] FIG. 5A illustrates a schematic diagram of a measurement process 500A of a terminal device in accordance with some other embodiments of the present disclosure.30SUBSTITUTE SHEET (RULE 26)For the purpose of discussion, the measurement process 500A will be described from the perspective of the terminal device 130 with reference to FIGS. 1 A-4B.

[0157] For channel estimation of an RIS-assisted channel, measurements are realized by stages. Every stage performs measurement of cascaded channel between a given TX element set (for example, set 1 or set 2 or set 3 or set 4 of TX 400A as illustrated in FIG. 4 A) and a given RIS element set (for example, set 1 or set 2 or set 3 or set 4 of RIS 300 A as illustrated in FIG. 3A). RX 130 is supposed to have a receiver at every element. Rest RIS elements that are not included in a given set at given stage have constant state of phase shifter (PS). Measurement may be realized by least square (LS) method that enables to exclude influence of unused elements.

[0158] During first stage of measurements for channel estimation for the RIS-assisted HA-HB sub-channel, statistics of received power at RX 130 enables to find directions (phase / magnitude states of elements in TX 120 and RIS 110) where received power at RX 130 is high, for example, higher than a threshold. During rest stages of measurements for channel estimation for the RIS-assisted HA-HB sub-channel, measurements are obtained only in states of high power at the first stage. Received power at low power directions is supposed to be zero without physical measurement.

[0159] At block 510, set 1 measurement is performed. For example, TX set 1 as illustrated in FIG. 4A and RIS set 1 as illustrated in FIG. 3A may perform set 1 measurement together with RX 130.

[0160] At block 520, power sorting is performed at the RX 130 to find directions (phase / magnitude states of elements in TX 120 and RIS 110) where received power at RX 130 is high. In other words, TX directions (phase / magnitude states of elements in TX 120) can be detected (determined, singled out, selected) based on received power at TX 130 which are above a threshold. Change it in another way, elements (a sub set) in TX set 1 with the TX directions associated with a high received power at RX 130 can be determined by the RX 130 through the power sorting procedure. This also applies to RIS 110. More specifically, RIS directions (phase / magnitude states of elements in RIS 110) can be detected (determined, singled out, selected) based on received power at TX 130 which are above the threshold. Change it in another way, elements (a sub set) in RIS set 1 with the RIS directions associated with a high received power at RX 130 also can be determined by the RX 130 through the power sorting procedure. As to how to find directions in a TX set31SUBSTITUTE SHEET (RULE 26)of TX 120 and an RIS set of RIS 110 where received power at RX 130 is high, it will be described in more detail with reference to FIGS. 5B-5D.

[0161] At a given measurement stage after the first stage, only elements with the high-power directions as found in the power sorting procedure are used, and elements with low-power directions are not used. Here, it is assumed that, at a given measurement stage after the first stage, elements (a sub set) in TX sets having TX directions associated with high received power at RX 130 determined at block 520 and elements (a sub set) in RIS sets having RIS directions associated with high received power at RX 130 determined at block 520 are used to “reproduce” high received power at RX 130.

[0162] Such an assumption is based on that, the distance between RIS 110 and TX 120 is much larger than the dimension of either RIS 110 or TX 120. For example, the distance between RIS 110 and TX 120 maybe larger than 100 times of the length and width of either RIS 110 or TX 120. In this case, a TX set shifted from TX set 1 and a RIS set shifted from RIS set 1 may be regarded as resembling the TX set 1 and RIS set 1, and the TX elements (TX sub set) and RIS elements (RIS sub set) which produce the high received power at RX 130 may “reproduce” high received power at RX 130 after being shifted to different positions. Here, “shifted” means being subjected to a “shifting” movement in a surface (for example, in the reconfiguration intelligent surface 110), however, no other movement (like rotation, spinning, etc.) is involved.

[0163] Preferably, the position of TX set 1 relative to TX 120 may be the same as or similar to the position of RIS set 1 relative to RIS 110. For example, TX set 1 may be at the left bottom of TX 120, and RIS set 1 is also at the left bottom of RIS 110. In this case, more preferably, the ratio of the width of TX set 1 to TX 120 may be the same as or similar to the ratio of the width of RIS set 1 to RIS 110, and the ratio of the length of TX set 1 to TX 120 may be the same as or similar to the ratio of the length of RIS set 1 to RIS 110. Other TX sets in TX 120 may be shifted versions of the TX set 1 in the same or similar “shifting” manner as other RIS sets in RIS 110 which are shifted versions of the RIS set 1. For example, TX set 4 may be at the top right of TX 120, and RIS set 4 is also at the top right of RIS 110.

[0164] More specifically, at block 530, a measurement for set 2 is performed. For example, TX set 2 as illustrated in FIG. 4A and RIS set 2 as illustrated in FIG. 3A may perform the measurement for set 2 together with RX 130. During set 2 measurement, only32SUBSTITUTE SHEET (RULE 26)TX elements with the TX directions associated with high received power at RX 130 as found in the power sorting procedure 520 are used, and TX elements with other directions than the TX directions associated with high received power at RX 130 are not actually (physically) used. Meanwhile, only RIS elements with the RIS directions associated with high received power at RX 130 as found in the power sorting procedure 520 are used, and RIS elements with other directions than the RIS directions associated with high received power at RX 130 are not actually (physically) used.

[0165] Processing as indicated in block 530 is performed set by set. For example, when there are N (N is the number of RIS element sets, N is 4 in the example illustrated in FIG. 3 A) sets in the RIS 110 and N sets in the TX 120, after TX directions and RIS directions associated with high received power at RX 130 are determined at block 520, TX sets other than TX set 1 and RIS sets other than RIS set 1 are used in later stages to perform measurements, like set 2 measurement indicated in block 530.

[0166] At block 540, set N measurement is performed. For example, TX set N and RIS set N may perform set N measurement together with RX 130. During set N measurement, as in set 2 measurement illustrated in block 530, only TX elements with the TX directions associated with high received power at RX 130 as found in the power sorting procedure 520 are used, and TX elements with other directions than the TX directions associated with high received power at RX 130 are not actually (physically) used. Meanwhile, only RIS elements with the RIS directions associated with high received power at RX 130 as found in the power sorting procedure 520 are used, and RIS elements with other directions than the RIS directions associated with high received power at RX 130 are not actually (physically) used.

[0167] At block 550. after the RIS element sets used in the measurement fully cover the RIS 110, and the TX element sets used in the measurement fully cover the TX 120, a matrix completion (MC) procedure is applied to restore low rank full TX-RIS channel matrix from knowledge only part of matrix elements, that is, measurements obtained in states of high power at all stages. RIS-RX matrix is fully measured during described procedure.

[0168] Total number of measurements required by described procedure with LS at every stage is less than LS measurement of full system. This will be described in detail later.

[0169] FIG. 5B illustrates a schematic diagram of pilots 500A of the transmitter 120 in33SUBSTITUTE SHEET (RULE 26)accordance with some other embodiments of the present disclosure. “Pilot” here means “reference signal” used for channel estimation and measured at RX 130. For the purpose of discussion, the pilots 500A will be described from the perspective of the transmitter 120 with reference to FIGS. 1 A and 5 A.

[0170] As illustrated in FIG. 5B, pilots are transmitted from TX 120 to RIS 110 via scatterers S1, S2 and S3. Here, “pilot” may also be referred to as “reference signal” for measurement, and “scatterers” are used to refer to objects that will change the direction of an incident signal, namely a pilot, such that the pilot will incident onto the RIS 110, otherwise the pilot may miss RIS 110 (cannot incident onto RIS 110), which can be seen from FIG. 5B.

[0171] Different pilots may result in different received power at RX 130. Therefore, during power sorting procedure indicated at block 520 in the example illustrated in FIG. 5 A, when TX directions of elements in TX set 1 associated high received power at RX 130 and RIS directions of elements in RIS set 1 associated high received power at RX 130 are determined, TX pilots may be determined (selected, found, singled out, detected) at the same time. This means, a portion of the full pilot sets is determined at block 520.

[0172] FIG. 5C illustrates a schematic diagram of a power sorting process 500C in accordance with some other embodiments of the present disclosure. For the purpose of discussion, the power sorting process 500C will be described from the perspective of the transmitter 120 with reference to FIGS. 1 A and 5 A.

[0173] As illustrated in FIG. 5C, during the power sorting process 500C (which is also indicated in block 520 in the example illustrated in FIG. 5A), statistics of received power at RX 130 are sorted, and TX directions (phase / magnitude states of elements in TX 120) where received power at RX 130 is high, for example, higher than a threshold are selected (determined). During rest stages of measurements for channel estimation for the TX-RIS indirect HA sub-channel, measurements are obtained only in states of high power at the first stage.

[0174] FIG. 5D illustrates another schematic diagram of a power sorting process 500D in accordance with some other embodiments of the present disclosure. For the purpose of discussion, the power sorting process 500D will be described from the perspective of the transmitter 120 with reference to FIGS. 1 A and 5 A.

[0175] As illustrated in FIG. 5D, during the power sorting process 500D (which is also34SUBSTITUTE SHEET (RULE 26)indicated in block 520 in the example illustrated in FIG. 5A), statistics of received power at RX 130 are sorted, and RIS directions (phase / magnitude states of elements in RIS 110) where received power at RX 130 is high, for example, higher than a threshold are selected (determined). During rest stages of measurements for channel estimation for the TX-RIS indirect HA sub-channel, measurements are obtained only in states of high power at the first stage.

[0176] FIG. 5E illustrates another schematic diagram of a matrix completion process 500E in accordance with some other embodiments of the present disclosure. For the purpose of discussion, the power sorting process 500E will be described from the perspective of the transmitter 120 with reference to FIGS. 1 A and 5 A.

[0177] As illustrated in FIG. 5E, after the RIS element sets used in the measurement fully cover the RIS 110, and the TX element sets used in the measurement fully cover the TX 120, a matrix completion (MC) procedure is applied to restore low rank full TX-RIS channel matrix from knowledge only part of matrix elements, that is, measurements obtained in states of high power at all stages. This is indicated at block 550 in the example illustrated in FIG. 5 A.

[0178] As mentioned above, total number of measurements required by described procedure with LS at every stage is less than LS measurement of full system. On one hand, assume NIRS=1024, NTX=32, where NIRS is the number of elements contained in RIS 110, and NTX is the number of elements contained in TX 120. In this case, NFULL=NTX*NRIS = 32768 measurements, where NFULL denotes full matrix using the traditional LS method.

[0179] On the other hand, according to the RIS-assisted indirect HA sub channel, the number of the actual measurements may be represented as NMC = αNTXx β NRIS + (N-1) x αγNTX x βσ NRIS, where αNTXx β NRIS denotes the number of measurements at the first stage as illustrated at block 510 before power sorting (at block 520), and (N-1) x αγNTX x βσ NRIS denotes the number of measurements at the following stage after power sorting, namely set 2 measurement at block 530, . . ., set N measurement as illustrated from block 530 till block 540, where N denotes the number of sets of TX 120 and the number of sets of RIS 110, a denotes a subset (a part of TX elements) in each TX element set used in the measurements for TX-RIS indirect HA sub-channel, β denotes a subset (a part of IRS elements) in each RIS element set used in the measurements for TX-RIS indirect HA sub-channel, γ denotes used part of TX pilots in measurement, a denotes used part of IRS35SUBSTITUTE SHEET (RULE 26)combinations in measurement. Taken N=4, α =18 / 32, β =1 / 4, y=8 / 18, σ =1 / 2, NMC is computed to be 7680, only about 23% of NFULL, which is 32768, as mentioned above.

[0180] As illustrated in FIG. 5E, M represents the channel matrix between RIS 110 and TX 120, Ncol and Nrow represents the number of column and rows of the channel matrix, and a box in the channel matrix M denotes an element in the channel matrix M. A box in the channel matrix M filled with slash indicates that the corresponding measurement result of the element corresponding element in the channel matrix denoted by the box is obtained through physical measurement, for example, via a TX set and a RIS set, and a blank box without filling indicates that the corresponding measurement value of the corresponding element in the channel matrix denoted by the box is actually not obtained through physical measurement, so its measurement result should be inferred from those whose measurement result is obtained through physical measurement. In other words, measurement results for all blank boxes in the left table should be inferred from measurement results of the gray boxes, which is done by a matrix completion (MC) procedure. In the example as illustrated in FIG. 5A, a channel matrix with 32768 elements can be inferred from physical measurement results of 7680 elements.

[0181] In the matrix completion (MC) procedure, when the rank of the channel matrix, i.e., M, is much smaller than both Ncol and Nrow, the channel matrix can be recovered (in other words, measurement results for all blank boxes in the left table should be inferred with high accuracy from measurement results of the gray boxes) through the MC procedure.

[0182] Geometry of the TX and RIS sets may be different. For example, at least one of TX sets and RIS sets may be of rectangular geometry. Alternatively, at least one of TX sets and RIS sets may be of elliptical geometry. Alternatively, at least one of TX sets and RIS sets may be of linear geometry. In fact, TX sets and RIS sets may be of every type of geometry. Consequently, intersected regions of the TX or the RIS may also have different geometry.

[0183] FIG. 6A illustrates a schematic diagram of a first geometry of TX sets or RIS sets in accordance with some embodiments of the present disclosure. As illustrated in FIG. 6A, a TX set or RIS set may be of a rectangular geometry. In this case, Discrete Fourier Transform (DFT) vectors exactly correspond to the spatial directions. Maximum number of elements in intersection can be obtained if TX set and RIS set both are of a rectangular geometry.

[0184] FIG. 6B illustrates a schematic diagram of TX or RIS in accordance with some36SUBSTITUTE SHEET (RULE 26)embodiments of the present disclosure. More specifically, FIG. 6B illustrated intersections of TX sets or RIS sets when the TX sets or RIS sets take a geometry as illustrated in FIG. 6A.

[0185] FIG. 7A illustrates a schematic diagram of a second geometry of TX sets or RIS sets in accordance with some embodiments of the present disclosure. As illustrated in FIG. 7 A, a TX set or RIS set may be of a rectangular geometry with small intersections. In this case, DFT vectors do not exactly correspond to the spatial directions, and number of elements in intersection is smaller than the first geometry as illustrated in FIG. 6A, so number of measurements can be reduced compared with the first geometry as illustrated in FIG. 6A.

[0186] FIG. 7B illustrates a schematic diagram of TX or RIS in accordance with some embodiments of the present disclosure. More specifically, FIG. 7B illustrated intersections of TX sets or RIS sets when the TX sets or RIS sets take a geometry as illustrated in FIG. 7A.

[0187] FIG. 8A illustrates a schematic diagram of a dense geometry of TX set or RIS set in accordance with some embodiments of the present disclosure. As illustrated in FIG. 8A, a TX set or RIS set may be of a dense geometry.

[0188] FIG. 8B illustrates a schematic diagram of a sparse geometry of TX set or RIS set in accordance with some embodiments of the present disclosure. As illustrated in FIG. 8A, a TX set or RIS set may be of a dense geometry. Here, “sparse geometry” is in contrary to “dense geometry” as illustrated in FIG. 8A. As illustrated in FIG. 8B, a TX set or RIS set may not necessarily comprise continuous elements; a TX set or RIS set may comprise “sparse elements” which are separated with a distance.

[0189] Intersections may be absent at RIS 110 or TX 120, but at least one of TX 120 or RIS 110 has intersections in order to perform the above-mentioned MC procedure. When RIS sets have intersection(s), additional algorithm should be implemented before application of the matrix completion.

[0190] Here, HB1means part of HBchannel connected with RIS1 set of RIS elements.37SUBSTITUTE SHEET (RULE 26)denotes set of RIS elements that involved into measurement during SET1measurement.

[0191] During measurements of cascaded channelmeasured up to arbitrary diagonal matrix with non-zero elements i.e. measured value is equal tomultiplied at left to some inversion of diagonal matrixthen measured, so total cascaded channel iswhich doesn’t depend onmatrix.is short for =is an arbitrary diagonal matrix with non-zerodiagonal elements, size is equal to number of elements in RIS1 set, and R1 =

[0194] Here, denotes set of RIS elements that involved into measurement duringSET2 measurement. c / d2is arbitrary diagonal matrix (asbut for RIS2 set of elements). When RIS sets of elements have no intersections this is independent matrices, but when RIS sets intersect therebetween, matrices are dependent. In considered example supposed sets intersected in one element, and possible to find complex valued constant c, so partial channels connected with RIS elements of intersection will have minimal difference.which is an arbitrary diagonal matrix with non-zero diagonal elements, size is equal to number of elements in set. R2c is complex valued constant to provide closestpartial channels connected with RIS set intersections. is short for38SUBSTITUTE SHEET (RULE 26)

[0196] Here,means intersection of sets, i.e., commonelement(s) of For example, if there is a single element set, then |x|=l .

[0197] is desired, for example, to minimize inconsistency in signal strenghth when a receiver is served by the RIS1 or RIS2. Given example is for 1 element intersection. Desired parameter is c ,that minimizes distance between measured vectors of partial channel from intersected element of RIS to RX. By this ambiguity in diagonal element of matrix D is removed, where D is arbitrary diagonal matrix with non-zero diagonal elements and cascaded channel.

[0198] Therefore,is determined.thus can be brought into to compute the exact value of c .

[0199] FIG. 9A illustrates a schematic diagram of an example sequence of sets shift in accordance with some embodiments of the present disclosure. A TX (for example, TX 120 as illustrated in FIG. 1A) with four TX sets, i.e., TX setl, TX set2, TX set3 and TX set4, is illustrated in the left half of FIG. 9 A.

[0200] In such a case, the sequence of sets shift may be, as illustrated in FIG. 9 A, from the left four TX sets, which are in a top-down and left-right arrangement, into a linear arrangement of the right four RIS sets, which is arranged from RIS setl to RIS set2, RIS set3 and RIS set4 sequentially from left to right, as illustrated in the right half of FIG. 9 A.

[0201] FIG. 9B illustrates a schematic diagram of another example sequence of sets shift in accordance with some embodiments of the present disclosure. A TX (for example, TX 120 as illustrated in FIG. 1A) with four TX sets, i.e., TX setl, TX set2, TX set3 and TX set4, is illustrated in the left half of FIG. 9B.

[0202] In such a case, the sequence of sets shift may be, as illustrated in FIG. 9B, from the left four TX sets, which are in a top-down and left-right arrangement, into a linear arrangement of the right four RIS sets, which is arranged from RIS set4 to RIS set3, RIS setl and RIS set2 sequentially from left to right, as illustrated in the right half of FIG. 9B.

[0203] FIG. 10 illustrates a flowchart of an example method 1000 implemented at a reconfigurable intelligent surface 110 in accordance with some other embodiments of the39SUBSTITUTE SHEET (RULE 26)present disclosure. For the purpose of discussion, the method 1000 will be described from the perspective of the reconfigurable intelligent surface 110 with reference to FIGS. 1 A, IB, 2 and 5.

[0204] At block 1010, the RIS 110 performs, together with TX 120 and RX 130, a first number of channel measurements associated with a first set of TX elements of the TX 120 and a second set of RIS elements of the RIS 110. At block 1020, the RIS 110 performs, together with the TX 120 and the RX 130, a second number of channel measurements associated with a third subset of a third set of TX elements of the TX 120 and a fourth subset of a fourth set of RIS elements of the RIS 110. Here, the second number is less than the first number, the third subset is determined based on a first subset of the first set, and the fourth subset is determined based on a second subset of the second set, and the first subset and the second subset are determined based on power sorting of received powers at the RX during the first number of channel measurements.

[0205] In some example embodiments, the TX 120 comprises NTXTX elements, the RIS 110 comprises NRISRIS elements, the first set comprises αNTXTX elements, 0 < α < 1, the second set comprises βNRISRIS elements, 0 < β < 1. In order to perform the first number of channel measurements, the RIS 110 reflects, using the βNRISRIS elements in the second set, αNTXpilot signals transmitted by the TX using the αNTXTX elements in the first set to the RX 130, and performs αNTXXx βNRISchannel measurements together with the TX 120 and the RX 130. In this way, the number of measurements required for channel estimation in RIS assisted communication system (for example, communication system 100 A as illustrated in FIG. 1A) can be reduced. Therefore, complexity can be reduced and accuracy is improved for channel estimation by using RIS 110.

[0206] In some example embodiments, the third subset comprises αNTXXTX elements, the fourth subset comprises βNRISRIS elements. In order to perform the second number of channel measurements, the RIS 110 reflects, using the βNRISRIS elements in βσNRISstates in the fourth subset, aN rx pilot signals with αγNTXlength transmitted by the TX 120 using theαNTXTX elements in the third subset, and performs αγNTXx βσNRISchannel measurements together with the TX 120 and the RX 130, 0 < γ < 1,0 < σ < l. In this way, the number of measurements required for channel estimation in RIS assisted communication system (for example, communication system 100 A as illustrated in FIG. 1A) can be reduced. Therefore, complexity can be reduced and accuracy is improved for channel estimation by using RIS 110.40SUBSTITUTE SHEET (RULE 26)

[0207] In some example embodiments, in order to perform the power sorting, the RX 130 measures a set of received powers {Pij} at the RX 130, here Pijrepresents a received power at the RX 130 associated with an i-th pilot signal and a j-th RIS element. In order to perform the power sorting, the RX 130 further determines a first set of total powers for the αNTXTX elements {PTX, i} (PTX, i=and determines αγNTXTX elements corresponding to top αγNTXtotal powers in the first set of total powers as the first subset of the TX elements. In this way, the number of measurements required for channel estimation in RIS assisted communication system (for example, communication system 100 A as illustrated in FIG. 1A) can be reduced. Therefore, complexity can be reduced and accuracy is improved for channel estimation by using RIS 110.

[0208] In some example embodiments, in order to perform the power sorting, the RX 130 further determines a second set of total powers for the βNRISRIS elements {PRIS, j} (PRIS, j=Py); and determines βσNRISRIS elements corresponding to top βσNRIStotal powers in the second set of total powers as the second subset of the RIS elements. In this way, the number of measurements required for channel estimation in RIS assisted communication system (for example, communication system 100A as illustrated in FIG. 1A) can be reduced. Therefore, complexity can be reduced and accuracy is improved for channel estimation by using RIS 110.

[0209] In some example embodiments, relative positions of the third subset of TX elements in the third set are same as relative positions of the first subset of TX elements in the first set. Alternatively or additionally, relative positions of the fourth subset of RIS elements in the fourth set are same as relative positions of the second subset of RIS elements in the second set. Alternatively or additionally, phase and magnitude states of TX elements in the third subset are same as phase and magnitude states of TX elements in the first subset. Alternatively or additionally, phase and magnitude states of RIS elements in the fourth subset are same as phase and magnitude states of RIS elements in the second subset. In this way, the number of measurements required for channel estimation in RIS assisted communication system (for example, communication system 100A as illustrated in FIG. 1 A) can be reduced. Therefore, complexity can be reduced and accuracy is improved for channel estimation by using RIS 110.

[0210] In some example embodiments, the first set and third set are two of a first plurality of sets of TX elements in the TX 120, the second set and fourth set are two of a second41SUBSTITUTE SHEET (RULE 26)plurality of sets of RIS elements in the RIS 110, and the number of the first plurality of sets is equal to the number of the second plurality of sets. In this way, the number of measurements required for channel estimation in RIS assisted communication system (for example, communication system 100 A as illustrated in FIG. 1A) can be reduced. Therefore, complexity can be reduced and accuracy is improved for channel estimation by using RIS 110.

[0211] In some example embodiments, at least two sets among the first plurality of sets have an intersection. Alternatively or additionally, at least two sets among the second plurality of sets have an intersection. In some example embodiments, a union of the first plurality of sets is a universal set of TX elements of the TX. Alternatively or additionally, a union of the first plurality of sets is a universal set of RIS elements of the RIS 110. In this way, the number of measurements required for channel estimation in RIS assisted communication system (for example, communication system 100 A as illustrated in FIG. 1A) can be reduced. Therefore, complexity can be reduced and accuracy is improved for channel estimation by using RIS 110.

[0212] In some example embodiments, a first order of the first plurality of sets in which channel measurements for a channel from the TX 120 to the RX 130 via the RIS 110 are performed is different from a second order of the second plurality of sets in which the channel measurements are performed. In this way, the number of measurements required for channel estimation in RIS assisted communication system (for example, communication system 100 A as illustrated in FIG. 1A) can be reduced. Therefore, complexity can be reduced and accuracy is improved for channel estimation by using RIS 110.

[0213] In some example embodiments, an unused RIS element which is in the fourth set but not in the fourth subset has a constant phase during the second number of channel measurements. In some example embodiments, a channel from the TX 120 to the RX 130 via the unused RIS element is determined as a direct channel from the TX 120 to the RX 130. In this way, the number of measurements required for channel estimation in RIS assisted communication system (for example, communication system 100A as illustrated in FIG. 1 A) can be reduced. Therefore, complexity can be reduced and accuracy is improved for channel estimation by using RIS 110.

[0214] In some example embodiments, the first set and the third set have a first geometry, and the second set and the fourth set have a second geometry. In some example embodiments, the first geometry and the second geometry are the same or different. In some example embodiments, at least one of the first geometry and the second geometry is a42SUBSTITUTE SHEET (RULE 26)rectangle. Alternatively or additionally, at least one of the first geometry and the second geometry is a rectangle with at least one element being removed from at least one edge of the rectangle. In this way, the number of measurements required for channel estimation in RIS assisted communication system (for example, communication system 100A as illustrated in FIG. 1A) can be reduced. Therefore, complexity can be reduced and accuracy is improved for channel estimation by using RIS 110.

[0215] In some example embodiments, a matrix completion process is performed at least based on the first number of channel measurements and the second number of channel measurements to obtain a channel matrix of a channel from the TX 120 to the RX 130 via the RIS 110. During the matrix completion process, a value in the channel matrix which is not measured in channel measurements for the channel is inferred from values in the channel matrix which are measured in the channel measurements. In this way, the number of measurements required for channel estimation in RIS assisted communication system (for example, communication system 100 A as illustrated in FIG. 1A) can be reduced. Therefore, complexity can be reduced and accuracy is improved for channel estimation by using RIS 110.

[0216] In this way, according to method 1000, the number of measurements required for channel estimation in RIS assisted communication system can be reduced. Therefore, complexity can be reduced and accuracy is improved for channel estimation by using RIS.

[0217] FIG. 11 illustrates another flowchart of an example method 1100 implemented at a transmitter (for example, transmitter 120 as illustrated in FIG. 1A0 in accordance with some other embodiments of the present disclosure. For the purpose of discussion, the method 1100 will be described from the perspective of the transmitter 120 with reference to FIGS. 1A, IB, 2 and 5.

[0218] At block 1110, the transmitter 120 performs, together with the RIS 110 and RX 130, a first number of channel measurements associated with a first set of TX elements of the TX 120 and a second set of RIS elements of the RIS110, and performs, together with the RIS 110 and the RX 130, a second number of channel measurements associated with a third subset of a third set of TX elements of the TX 120 and a fourth subset of a fourth set of RIS elements of the RIS 110. Here, the second number is less than the first number, the third subset is determined based on a first subset of the first set, and the fourth subset is determined based on a second subset of the second set, and the first subset and the second43SUBSTITUTE SHEET (RULE 26)subset are determined based on power sorting of received powers at the RX during the first number of channel measurements.

[0219] In some example embodiments, the TX 120 comprises NTXTX elements, the RIS 110 comprises NRISRIS elements, the first set comprises αNTXTX elements, 0 < α < 1, the second set comprises βNRISRIS elements, 0 < β < 1. In order to performing the first number of channel measurements, the TX 120 transmits, using the αNTXTX elements in the first set, αNTXpilot signals to be reflected by the RIS using the βNRISRIS elements in the second set to the RX 130, and performs αNTXx βNRISchannel measurements together with the RIS 110 and the RX 130. In this way, the number of measurements required for channel estimation in RIS assisted communication system (for example, communication system 100 A as illustrated in FIG. 1A) can be reduced. Therefore, complexity can be reduced and accuracy is improved for channel estimation by using RIS 110.

[0220] In some example embodiments, the third subset comprises αNTXTX elements, the fourth subset comprises βNRISRIS elements. In order to perform the second number of channel measurements, the TX 120 transmits, using the aNyx TX elements in the third subset, αNTXpilot signals with αγNTXlength to be reflected by the RIS 110 using the βNRISRIS elements in βσNRISstates in the fourth subset to the RX 130, and performs αγNTXx βσNRISchannel measurements together with the RIS 110 and the RX 130, 0 < γ < 1, 0 < o < 1. In this way, the number of measurements required for channel estimation in RIS assisted communication system (for example, communication system 100A as illustrated in FIG. 1 A) can be reduced. Therefore, complexity can be reduced and accuracy is improved for channel estimation by using RIS 110.

[0221] In some example embodiments, in order to perform the power sorting, the RX 130 measures a set of received powers {Pij} at the RX 130 (here, Pijrepresents a received power at the RX 130 associated with an i-th pilot signal and a j-th RIS element. In order to perform the power sorting, the RX 130 also determines a first set of total powers for the αNTXTX elements {PTX, i} (PTX, i=and determines αγNTXTX elements corresponding to top αγNTXtotal powers in the first set of total powers as the first subset of the TX elements. In this way, the number of measurements required for channel estimation in RIS assisted communication system (for example, communication system 100 A as illustrated in FIG. 1A) can be reduced. Therefore, complexity can be reduced and accuracy is improved for channel estimation by using RIS 110.44SUBSTITUTE SHEET (RULE 26)

[0222] In some example embodiments, in order to perform the power sorting, the RX 130 further determines a second set of total powers for the βNRISRIS elements {PRIS, j} (PRIS, j=and determines βσNRISRIS elements corresponding to top βσNRIStotal powers in the second set of total powers as the second subset of the RIS elements. In this way, the number of measurements required for channel estimation in RIS assisted communication system (for example, communication system 100A as illustrated in FIG. 1A) can be reduced. Therefore, complexity can be reduced and accuracy is improved for channel estimation by using RIS 110.

[0223] In some example embodiments, relative positions of the third subset of TX elements in the third set are same as relative positions of the first subset of TX elements in the first set. Alternatively or additionally, relative positions of the fourth subset of RIS elements in the fourth set are same as relative positions of the second subset of RIS elements in the second set. Alternatively or additionally, phase and magnitude states of TX elements in the third subset are same as phase and magnitude states of TX elements in the first subset. Alternatively or additionally, phase and magnitude states of RIS elements in the fourth subset are same as phase and magnitude states of RIS elements in the second subset. In this way, the number of measurements required for channel estimation in RIS assisted communication system (for example, communication system 100A as illustrated in FIG. 1 A) can be reduced. Therefore, complexity can be reduced and accuracy is improved for channel estimation by using RIS 110.

[0224] In some example embodiments, the first set and third set are two of a first plurality of sets of TX elements in the TX, the second set and fourth set are two of a second plurality of sets of RIS elements in the RIS, and the number of the first plurality of sets is equal to the number of the second plurality of sets. In this way, the number of measurements required for channel estimation in RIS assisted communication system (for example, communication system 100 A as illustrated in FIG. 1A) can be reduced. Therefore, complexity can be reduced and accuracy is improved for channel estimation by using RIS 110.

[0225] In some example embodiments, at least two sets among the first plurality of sets have an intersection. Alternatively or additionally, at least two sets among the second plurality of sets have an intersection. In some example embodiments, a union of the first plurality of sets is a universal set of TX elements of the TX. Alternatively or additionally, a union of the first plurality of sets is a universal set of RIS elements of the RIS. In this45SUBSTITUTE SHEET (RULE 26)way, the number of measurements required for channel estimation in RIS assisted communication system (for example, communication system 100 A as illustrated in FIG. 1A) can be reduced. Therefore, complexity can be reduced and accuracy is improved for channel estimation by using RIS 110.

[0226] In some example embodiments, a first order of the first plurality of sets in which channel measurements for a channel from the TX to the RX via the RIS are performed is different from a second order of the second plurality of sets in which the channel measurements are performed. In this way, the number of measurements required for channel estimation in RIS assisted communication system (for example, communication system 100 A as illustrated in FIG. 1A) can be reduced. Therefore, complexity can be reduced and accuracy is improved for channel estimation by using RIS 110.

[0227] In some example embodiments, an unused RIS element which is in the fourth set but not in the fourth subset has a constant phase during the second number of channel measurements. In some example embodiments, a channel from the TX to the RX via the unused RIS element is determined as a direct channel from the TX 120 to the RX 130. In this way, the number of measurements required for channel estimation in RIS assisted communication system (for example, communication system 100 A as illustrated in FIG. 1A) can be reduced. Therefore, complexity can be reduced and accuracy is improved for channel estimation by using RIS 110.

[0228] In some example embodiments, the first set and the third set have a first geometry; and the second set and the fourth set have a second geometry. In some example embodiments, the first geometry and the second geometry are the same or different. In some example embodiments, at least one of the first geometry and the second geometry is a rectangle. Alternatively or additionally, at least one of the first geometry and the second geometry is a rectangle with at least one element being removed from at least one edge of the rectangle. In this way, the number of measurements required for channel estimation in RIS assisted communication system (for example, communication system 100A as illustrated in FIG. 1A) can be reduced. Therefore, complexity can be reduced and accuracy is improved for channel estimation by using RIS 110.

[0229] In this way, according to the second aspect and its example embodiments, the number of measurements required for channel estimation in RIS assisted communication system can be reduced. Therefore, complexity can be reduced and accuracy is improved for channel estimation by using RIS.46SUBSTITUTE SHEET (RULE 26)

[0230] FIG. 12 illustrates another flowchart of an example method 1200 implemented at a receiver (for example, receiver 130 as illustrated in FIG. 1 A) in accordance with some other embodiments of the present disclosure. For the purpose of discussion, the method 1200 will be described from the perspective of the receiver 130 with reference to FIGS. 1A, IB, 2 and 5.

[0231] At block 1210, the RX 130 performs, together with the TX 120 and the RIS 110, a first number of channel measurements associated with a first set of TX elements of the TX 120 and a second set of RIS elements of the RIS 110. At block 1220, the RX 130 performs, together with the TX 120 and the RIS 110, a second number of channel measurements associated with a third subset of a third set of TX elements of the TX 120 and a fourth subset of a fourth set of RIS elements of the RIS 110. Here, the second number is less than the first number, the third subset is determined based on a first subset of the first set, and the fourth subset is determined based on a second subset of the second set, and the first subset and the second subset are determined based on power sorting of received powers at the RX during the first number of channel measurements.

[0232] In some example embodiments, the TX 120 comprises NTXTX elements, the RIS 110 comprises NRISRIS elements, the first set comprises αNTXTX elements, 0 < α < 1, the second set comprises βNRISRIS elements, 0 < β < 1. In order to performing the first number of channel measurements, the RX 130 receives, αNTXpilot signals transmitted by the TX 120 using the αNTXTX elements in the first set and reflected by the RIS 110 using the βNRISRIS elements in the second set, and performs αNTXx βNRISchannel measurements together with the TX 120 and the RIS 110. In this way, the number of measurements required for channel estimation in RIS assisted communication system (for example, communication system 100 A as illustrated in FIG. 1A) can be reduced. Therefore, complexity can be reduced and accuracy is improved for channel estimation by using RIS 110.

[0233] In some example embodiments, the third subset comprises αNTXTX elements, the fourth subset comprises βNRISRIS elements. In order to perform the second number of channel measurements, the RX 130 receives, αNTXpilot signals with αγ NTXlength transmitted by the TX 120 using the αNTXTX elements in the third subset and reflected by the RIS 110 using the βNRISRIS elements in βσNRISstates in the fourth subset, and performs αγ NTXx βσNRISchannel measurements together with the TX 120 and the RIS 110, 0 < γ < 1, 0 < σ < 1. In this way, the number of measurements required for channel47SUBSTITUTE SHEET (RULE 26)estimation in RIS assisted communication system (for example, communication system 100 A as illustrated in FIG. 1A) can be reduced. Therefore, complexity can be reduced and accuracy is improved for channel estimation by using RIS 110.

[0234] In some example embodiments, in order to perform the power sorting, the RX 130 measures a set of received powers {Pij} at the RX, here, Pijrepresents a received power at the RX associated with an i-th pilot signal and a j-th RIS element. In order to perform the power sorting, the RX 130 also determines a first set of total powers for theαNTXTX elements {PTX, i} (PTX, i=and determines αγNTXTX elements corresponding to top αγNTXtotal powers in the first set of total powers as the first subset of the TX elements. In this way, the number of measurements required for channel estimation in RIS assisted communication system (for example, communication system 100 A as illustrated in FIG. 1A) can be reduced. Therefore, complexity can be reduced and accuracy is improved for channel estimation by using RIS 110.

[0235] In some example embodiments, in order to perform the power sorting, the RX 130 determines a second set of total powers for the βNRISRIS elements {PRIS, j} (PRIS, j=and determines βσNRISRIS elements corresponding to top βσNRIStotal powers in the second set of total powers as the second subset of the RIS elements. In this way, the number of measurements required for channel estimation in RIS assisted communication system (for example, communication system 100A as illustrated in FIG. 1A) can be reduced. Therefore, complexity can be reduced and accuracy is improved for channel estimation by using RIS 110.

[0236] In some example embodiments, received powers at the RX 130 associated with TX elements in the third set but not in the third subset of are set to zero, and received powers at the RX 130 associated with RIS elements in the fourth set but not in the fourth subset of are set to zero. In this way, the number of measurements required for channel estimation in RIS assisted communication system (for example, communication system 100A as illustrated in FIG. 1A) can be reduced. Therefore, complexity can be reduced and accuracy is improved for channel estimation by using RIS 110.

[0237] In some example embodiments, relative positions of the third subset of TX elements in the third set are same as relative positions of the first subset of TX elements in the first set. Alternatively or additionally, relative positions of the fourth subset of RIS elements in the fourth set are same as relative positions of the second subset of RIS48SUBSTITUTE SHEET (RULE 26)elements in the second set. Alternatively or additionally, phase and magnitude states of TX elements in the third subset are same as phase and magnitude states of TX elements in the first subset. Alternatively or additionally, phase and magnitude states of RIS elements in the fourth subset are same as phase and magnitude states of RIS elements in the second subset. In this way, the number of measurements required for channel estimation in RIS assisted communication system (for example, communication system 100A as illustrated in FIG. 1 A) can be reduced. Therefore, complexity can be reduced and accuracy is improved for channel estimation by using RIS 110.

[0238] In some example embodiments, the first set and third set are two of a first plurality of sets of TX elements in the TX 120, the second set and fourth set are two of a second plurality of sets of RIS elements in the RIS 110, and the number of the first plurality of sets is equal to the number of the second plurality of sets. In this way, the number of measurements required for channel estimation in RIS assisted communication system (for example, communication system 100 A as illustrated in FIG. 1A) can be reduced. Therefore, complexity can be reduced and accuracy is improved for channel estimation by using RIS 110.

[0239] In some example embodiments, at least two sets among the first plurality of sets have an intersection. Alternatively or additionally, at least two sets among the second plurality of sets have an intersection. In some example embodiments, a union of the first plurality of sets is a universal set of TX elements of the TX 120. Alternatively or additionally, a union of the first plurality of sets is a universal set of RIS elements of the RIS 110. In this way, the number of measurements required for channel estimation in RIS assisted communication system (for example, communication system 100A as illustrated in FIG. 1 A) can be reduced. Therefore, complexity can be reduced and accuracy is improved for channel estimation by using RIS 110.

[0240] In some example embodiments, a first order of the first plurality of sets in which channel measurements for a channel from the TX 120 to the RX 130 via the RIS 110 are performed is different from a second order of the second plurality of sets in which the channel measurements are performed. In this way, the number of measurements required for channel estimation in RIS assisted communication system (for example, communication system 100 A as illustrated in FIG. 1A) can be reduced. Therefore, complexity can be reduced and accuracy is improved for channel estimation by using RIS 110.

[0241] In some example embodiments, an unused RIS element which is in the fourth set but not in the fourth subset has a constant phase during the second number of channel49SUBSTITUTE SHEET (RULE 26)measurements. In some example embodiments, a channel from the TX 120 to the RX 130 via the unused RIS element is determined as a direct channel from the TX 120 to the RX 130. In this way, the number of measurements required for channel estimation in RIS assisted communication system (for example, communication system 100A as illustrated in FIG. 1 A) can be reduced. Therefore, complexity can be reduced and accuracy is improved for channel estimation by using RIS 110.

[0242] In some example embodiments, the first set and the third set have a first geometry, and the second set and the fourth set have a second geometry. In some example embodiments, the first geometry and the second geometry are the same or different. In some example embodiments, at least one of the first geometry and the second geometry is a rectangle. Alternatively or additionally, at least one of the first geometry and the second geometry is a rectangle with at least one element being removed from at least one edge of the rectangle. In this way, the number of measurements required for channel estimation in RIS assisted communication system (for example, communication system 100A as illustrated in FIG. 1A) can be reduced. Therefore, complexity can be reduced and accuracy is improved for channel estimation by using RIS 110.

[0243] In some example embodiments, a matrix completion process is performed at least based on the first number of channel measurements and the second number of channel measurements to obtain a channel matrix of a channel from the TX 120 to the RX 130 via the RIS 110, and during the matrix completion process, a value in the channel matrix which is not measured in channel measurements for the channel is inferred from values in the channel matrix which are measured in the channel measurements. In this way, the number of measurements required for channel estimation in RIS assisted communication system (for example, communication system 100 A as illustrated in FIG. 1A) can be reduced. Therefore, complexity can be reduced and accuracy is improved for channel estimation by using RIS 110.

[0244] In this way, according to the third aspect and its example embodiments, the number of measurements required for channel estimation in RIS assisted communication system can be reduced. Therefore, complexity can be reduced and accuracy is improved for channel estimation by using RIS.

[0245] FIG. 13 illustrates a simplified block diagram of a first apparatus 1300 according to some example embodiments of the present disclosure. The first apparatus 1300 may be implemented as a device or a chip in the device, and the scope of the present application is50SUBSTITUTE SHEET (RULE 26)not limited in this respect. The first apparatus 1300 may include multiple modules for performing corresponding processes in the method 1000 as discussed in FIG. 10. The first apparatus 1300 may be implemented as the RIS 110 as shown in FIG. 1A or a part of the RIS 110. FIG. 13 will be described below with reference to FIGS. 1 A, 2, 5 and 10.

[0246] As illustrated in FIG. 13, the first apparatus 1300 comprises a performing module 1310. In some embodiments, the first apparatus 1300 may further comprise a determining module 1320 and a reflecting module 1330. The performing module 1310 is used to perform operations, the determining module 1320 is used to determine data, and the reflecting module 1330 is used to reflect signals. For example, the performing module 1310 is configured to perform, together with a transmitter (TX, for example, TX 120 as illustrated in FIG. 1A) and a receiver (RX, for example, RX 130 as illustrated in FIG. 1A), a first number of channel measurements associated with a first set of TX elements of the TX and a second set of RIS elements of the RIS, and perform, together with the TX and the RX, a second number of channel measurements associated with a third subset of a third set of TX elements of the TX and a fourth subset of a fourth set of RIS elements of the RIS. Here, the second number is less than the first number, the third subset is determined based on a first subset of the first set, and the fourth subset is determined based on a second subset of the second set, and the first subset and the second subset are determined based on power sorting of received powers at the RX during the first number of channel measurements.

[0247] In some example embodiments, the TX comprises NTXTX elements, the first apparatus 1300 comprises NRISRIS elements, the first set comprises αNTXTX elements, 0 < α < 1, the second set comprises βNRISRIS elements, 0 < β < 1. In order to perform the first number of channel measurements, the first apparatus 1300 reflects, using the βNRISRIS elements in the second set, αNTXpilot signals transmitted by the TX using the αNTXTX elements in the first set to the RX, and performs αNTXx βNRISchannel measurements together with the TX and the RX. In this way, the number of measurements required for channel estimation in RIS assisted communication system (for example, communication system 100 A as illustrated in FIG. 1A) can be reduced. Therefore, complexity can be reduced and accuracy is improved for channel estimation by using RIS 110.

[0248] In some example embodiments, the third subset comprises αNTXTX elements, the fourth subset comprises βNRISRIS elements. In order to perform the second number of channel measurements, the first apparatus 1300 reflects, using the βNRISRIS elements in51SUBSTITUTE SHEET (RULE 26)P^NRISstates in the fourth subset, αNTXpilot signals with αγNTXlength transmitted by the TX using the αNTXTX elements in the third subset, and performs αγNTXx βσNRISchannel measurements together with the TX and the RX, 0 < γ < 1, 0 < σ < l. In this way, the number of measurements required for channel estimation in RIS assisted communication system (for example, communication system 100A as illustrated in FIG. 1A) can be reduced. Therefore, complexity can be reduced and accuracy is improved for channel estimation by using RIS 110.

[0249] In some example embodiments, in order to perform the power sorting, the RX comprises a measuring module configured to measure a set of received powers {Pij} at the RX, here Pijrepresents a received power at the RX associated with an i-th pilot signal and a j-th RIS element. The RX also comprises a determining module configured to determine a first set of total powers for the αNTXTX elements {PTX, i} (PTX, i=and determine αγNTXTX elements corresponding to top αγNTXtotal powers in the first set of total powers as the first subset of the TX elements. In this way, the number of measurements required for channel estimation in RIS assisted communication system (for example, communication system 100 A as illustrated in FIG. 1A) can be reduced. Therefore, complexity can be reduced and accuracy is improved for channel estimation by using RIS 110.

[0250] In some example embodiments, the RX further comprise a determining module configured to determine a second set of total powers for the βNRISRIS elements {PRIS, j}(PRIS, j= , and determine βσNRISRIS elements corresponding to top βσNRIStotalpowers in the second set of total powers as the second subset of the RIS elements. In this way, the number of measurements required for channel estimation in RIS assisted communication system (for example, communication system 100 A as illustrated in FIG. 1A) can be reduced. Therefore, complexity can be reduced and accuracy is improved for channel estimation by using RIS 110.

[0251] In some example embodiments, relative positions of the third subset of TX elements in the third set are same as relative positions of the first subset of TX elements in the first set. Alternatively or additionally, relative positions of the fourth subset of RIS elements in the fourth set are same as relative positions of the second subset of RIS elements in the second set. Alternatively or additionally, phase and magnitude states of TX elements in the third subset are same as phase and magnitude states of TX elements in52SUBSTITUTE SHEET (RULE 26)the first subset. Alternatively or additionally, phase and magnitude states of RIS elements in the fourth subset are same as phase and magnitude states of RIS elements in the second subset. In this way, the number of measurements required for channel estimation in RIS assisted communication system (for example, communication system 100A as illustrated in FIG. 1 A) can be reduced. Therefore, complexity can be reduced and accuracy is improved for channel estimation by using RIS 110.

[0252] In some example embodiments, the first set and third set are two of a first plurality of sets of TX elements in the TX, the second set and fourth set are two of a second plurality of sets of RIS elements in the first apparatus 1300, and the number of the first plurality of sets is equal to the number of the second plurality of sets. In this way, the number of measurements required for channel estimation in RIS assisted communication system (for example, communication system 100 A as illustrated in FIG. 1A) can be reduced. Therefore, complexity can be reduced and accuracy is improved for channel estimation by using RIS 110.

[0253] In some example embodiments, at least two sets among the first plurality of sets have an intersection. Alternatively or additionally, at least two sets among the second plurality of sets have an intersection. In some example embodiments, a union of the first plurality of sets is a universal set of TX elements of the TX. Alternatively or additionally, a union of the first plurality of sets is a universal set of RIS elements of the first apparatus 1300. In this way, the number of measurements required for channel estimation in RIS assisted communication system (for example, communication system 100A as illustrated in FIG. 1 A) can be reduced. Therefore, complexity can be reduced and accuracy is improved for channel estimation by using RIS 110.

[0254] In some example embodiments, a first order of the first plurality of sets in which channel measurements for a channel from the TX to the RX via the first apparatus 1300 are performed is different from a second order of the second plurality of sets in which the channel measurements are performed. In this way, the number of measurements required for channel estimation in RIS assisted communication system (for example, communication system 100 A as illustrated in FIG. 1A) can be reduced. Therefore, complexity can be reduced and accuracy is improved for channel estimation by using RIS 110.

[0255] In some example embodiments, an unused RIS element which is in the fourth set but not in the fourth subset has a constant phase during the second number of channel measurements. In some example embodiments, a channel from the TX to the RX via the unused RIS element is determined as a direct channel from the TX to the RX. In this way,53SUBSTITUTE SHEET (RULE 26)the number of measurements required for channel estimation in RIS assisted communication system (for example, communication system 100 A as illustrated in FIG. 1A) can be reduced. Therefore, complexity can be reduced and accuracy is improved for channel estimation by using RIS 110. In some example embodiments, the first set and the third set have a first geometry, and the second set and the fourth set have a second geometry. In some example embodiments, the first geometry and the second geometry are the same or different. In some example embodiments, at least one of the first geometry and the second geometry is a rectangle. Alternatively or additionally, at least one of the first geometry and the second geometry is a rectangle with at least one element being removed from at least one edge of the rectangle. In this way, the number of measurements required for channel estimation in RIS assisted communication system (for example, communication system 100 A as illustrated in FIG. 1A) can be reduced. Therefore, complexity can be reduced and accuracy is improved for channel estimation by using RIS 110.

[0256] In some example embodiments, a matrix completion process is performed at least based on the first number of channel measurements and the second number of channel measurements to obtain a channel matrix of a channel from the TX to the RX via the first apparatus 1300. During the matrix completion process, a value in the channel matrix which is not measured in channel measurements for the channel is inferred from values in the channel matrix which are measured in the channel measurements. In this way, the number of measurements required for channel estimation in RIS assisted communication system (for example, communication system 100A as illustrated in FIG. 1A) can be reduced. Therefore, complexity can be reduced and accuracy is improved for channel estimation by using RIS 110.

[0257] In some embodiments, the first apparatus 1300 further comprises module(s) for performing other steps in some embodiments of the method 1000. In some embodiments, the module(s) may comprise at least one processor and at least one memory including computer program code, the at least one memory and computer program code are configured to, with the at least one processor, cause the performance of the fist apparatus 1300.

[0258] In this way, the number of measurements required for channel estimation in RIS assisted communication system can be reduced. Therefore, complexity can be reduced and accuracy is improved for channel estimation by using RIS.54SUBSTITUTE SHEET (RULE 26)

[0259] FIG. 14 illustrates a simplified block diagram of a second apparatus 1400 according to some example embodiments of the present disclosure. The second apparatus 1400 may be implemented as a device or a chip in the device, and the scope of the present application is not limited in this respect. The second apparatus 1400 may include multiple modules for performing corresponding processes in the method 1100 as discussed in FIG. 11. The second apparatus 1400 may be implemented as the TX 120 as shown in FIG. 1 A or a part of the TX 120. FIG. 14 will be described below with reference to FIGS. 1 A, 2, 5 and 11.

[0260] As illustrated in FIG. 14, the second apparatus 1400 comprises a performing module 1410. In some embodiments, the second apparatus 1400 may further comprise a determining module 1420 and a transmitting module 1430. The performing module 1410 is used to perform operations, the determining module 1420 is used to determine data, and the transmitting module 1430 is used to transmit data. For example, the performing module 1410 is configured to perform, together with a reconfigurable intelligent surface (RIS, for example, RIS 110 as illustrated in FIG. 1 A) and a receiver (RX, for example, RX as illustrated in FIG. 1A), a first number of channel measurements associated with a first set of TX elements of the TX and a second set of RIS elements of the RIS, and perform, together with the RIS and the RX, a second number of channel measurements associated with a third subset of a third set of TX elements of the TX and a fourth subset of a fourth set of RIS elements of the RIS. Here, the second number is less than the first number, the third subset is determined based on a first subset of the first set, and the fourth subset is determined based on a second subset of the second set, and the first subset and the second subset are determined based on power sorting of received powers at the RX during the first number of channel measurements.

[0261] In some example embodiments, the second apparatus 1400 comprises NTXTX elements, the RIS comprises NRISRIS elements, the first set comprises αNTXTX elements, 0 < α < 1, the second set comprises βNRISRIS elements, 0 < β < 1. The performing module 1420 may comprise a transmitting module configured to transmit, using the αNTXTX elements in the first set, αNTXpilot signals to be reflected by the RIS using the βNRISRIS elements in the second set to the RX, and a sub performing module configured to perform αNTXx βNRISchannel measurements together with the RIS and the RX. In this way, the number of measurements required for channel estimation in RIS assisted communication system (for example, communication system 100 A as illustrated in FIG. 1A)55SUBSTITUTE SHEET (RULE 26)can be reduced. Therefore, complexity can be reduced and accuracy is improved for channel estimation by using RIS 110.

[0262] In some example embodiments, the third subset comprises αNTXTX elements, the fourth subset comprises βNRISRIS elements. The performing module may comprise a transmitting module configured to transmit, using the αNTXTX elements in the third subset, αNTXpilot signals with αγNTXlength to be reflected by the RIS using the βNRISRIS elements in βσNRISstates in the fourth subset to the RX, and a sub performing module configured to perform ay αγ NTXx βσNRISchannel measurements together with the RIS and the RX, 0 < γ < 1,0 < σ < 1. In this way, the number of measurements required for channel estimation in RIS assisted communication system (for example, communication system 100 A as illustrated in FIG. 1A) can be reduced. Therefore, complexity can be reduced and accuracy is improved for channel estimation by using RIS 110.

[0263] In some example embodiments, the RX may comprise a measuring module configured to measure a set of received powers {Pij} at the RX (here, Pijrepresents a received power at the RX associated with an i-th pilot signal and a j-th RIS element and a determining module configured to determine a first set of total powers for the αNTXTX elements {PTX, i} (Pyx, i =and determine αγNTXTX elements corresponding to top αγNTXtotal powers in the first set of total powers as the first subset of the TX elements.

[0264] In some example embodiments, the RX may further comprise a determining module configured to determine a second set of total powers for the βNRISRIS elements{PRIS, j} (PRIS, j= , and determine βσNRISRIS elements corresponding to topβσNRIStotal powers in the second set of total powers as the second subset of the RIS elements. In this way, the number of measurements required for channel estimation in RIS assisted communication system (for example, communication system 100A as illustrated in FIG. 1A) can be reduced. Therefore, complexity can be reduced and accuracy is improved for channel estimation by using RIS 110.

[0265] In some example embodiments, relative positions of the third subset of TX elements in the third set are same as relative positions of the first subset of TX elements in the first set. Alternatively or additionally, relative positions of the fourth subset of RIS elements in the fourth set are same as relative positions of the second subset of RIS elements in the second set. Alternatively or additionally, phase and magnitude states of TX elements in the third subset are same as phase and magnitude states of TX elements in56SUBSTITUTE SHEET (RULE 26)the first subset. Alternatively or additionally, phase and magnitude states of RIS elements in the fourth subset are same as phase and magnitude states of RIS elements in the second subset. In this way, the number of measurements required for channel estimation in RIS assisted communication system (for example, communication system 100A as illustrated in FIG. 1 A) can be reduced. Therefore, complexity can be reduced and accuracy is improved for channel estimation by using RIS 110.

[0266] In some example embodiments, the first set and third set are two of a first plurality of sets of TX elements in the TX, the second set and fourth set are two of a second plurality of sets of RIS elements in the RIS, and the number of the first plurality of sets is equal to the number of the second plurality of sets. In this way, the number of measurements required for channel estimation in RIS assisted communication system (for example, communication system 100 A as illustrated in FIG. 1A) can be reduced. Therefore, complexity can be reduced and accuracy is improved for channel estimation by using RIS 110.

[0267] In some example embodiments, at least two sets among the first plurality of sets have an intersection. Alternatively or additionally, at least two sets among the second plurality of sets have an intersection. In some example embodiments, a union of the first plurality of sets is a universal set of TX elements of the TX. Alternatively or additionally, a union of the first plurality of sets is a universal set of RIS elements of the RIS. In this way, the number of measurements required for channel estimation in RIS assisted communication system (for example, communication system 100 A as illustrated in FIG. 1A) can be reduced. Therefore, complexity can be reduced and accuracy is improved for channel estimation by using RIS 110.

[0268] In some example embodiments, a first order of the first plurality of sets in which channel measurements for a channel from the TX to the RX via the RIS are performed is different from a second order of the second plurality of sets in which the channel measurements are performed. In this way, the number of measurements required for channel estimation in RIS assisted communication system (for example, communication system 100 A as illustrated in FIG. 1A) can be reduced. Therefore, complexity can be reduced and accuracy is improved for channel estimation by using RIS 110.

[0269] In some example embodiments, an unused RIS element which is in the fourth set but not in the fourth subset has a constant phase during the second number of channel measurements. In some example embodiments, a channel from the TX to the RX via the unused RIS element is determined as a direct channel from the second apparatus 1400 to57SUBSTITUTE SHEET (RULE 26)the RX. In this way, the number of measurements required for channel estimation in RIS assisted communication system (for example, communication system 100A as illustrated in FIG. 1 A) can be reduced. Therefore, complexity can be reduced and accuracy is improved for channel estimation by using RIS 110.

[0270] In some example embodiments, the first set and the third set have a first geometry; and the second set and the fourth set have a second geometry. In some example embodiments, the first geometry and the second geometry are the same or different. In some example embodiments, at least one of the first geometry and the second geometry is a rectangle. Alternatively or additionally, at least one of the first geometry and the second geometry is a rectangle with at least one element being removed from at least one edge of the rectangle. In this way, the number of measurements required for channel estimation in RIS assisted communication system (for example, communication system 100A as illustrated in FIG. 1A) can be reduced. Therefore, complexity can be reduced and accuracy is improved for channel estimation by using RIS 110.

[0271] In some embodiments, the second apparatus 1400 further comprises module(s) for performing other steps in some embodiments of the method 1100. In some embodiments, the module(s) may comprise at least one processor and at least one memory including computer program code, the at least one memory and computer program code configured to, with the at least one processor, cause the performance of the second apparatus 1400. In this way, the number of measurements required for channel estimation in RIS assisted communication system (for example, communication system 100 A as illustrated in FIG. 1A) can be reduced. Therefore, complexity can be reduced and accuracy is improved for channel estimation by using RIS 110.

[0272] In this way, the number of measurements required for channel estimation in RIS assisted communication system can be reduced. Therefore, complexity can be reduced and accuracy is improved for channel estimation by using RIS.

[0273] FIG. 15 illustrates a simplified block diagram of a third apparatus 1500 according to some example embodiments of the present disclosure. The third apparatus 1500 may be implemented as a device or a chip in the device, and the scope of the present application is not limited in this respect. The third apparatus 1500 may include multiple modules for performing corresponding processes in the method 1200 as discussed in FIG. 12. The third apparatus 1500 may be implemented as the RX 130 as shown in FIG. 1A or a part of the RX 130. FIG. 15 will be described below with reference to FIGS. 1A, 2, 5 and 12.58SUBSTITUTE SHEET (RULE 26)

[0274] As illustrated in FIG. 15, the third apparatus 1500 comprises a performing module 1510. In some embodiments, the third apparatus 1500 may further comprise a determining module 1520 and a receiving module 1530. The performing module 1510 is used to perform operations, the determining module 1520 is used to determine data, and the receiving module 1530 is used to receive data. For example, the performing module 1510 is configured to perform, together with a transmitter (TX, for example, TX 120 as illustrated in FIG. 1A) and a reconfigurable intelligent surface (RIS, for example, RIS 110 as illustrated in FIG. 1A), a first number of channel measurements associated with a first set of TX elements of the TX and a second set of RIS elements of the RIS, and perform, together with the TX and the RIS, a second number of channel measurements associated with a third subset of a third set of TX elements of the TX and a fourth subset of a fourth set of RIS elements of the RIS. Here, the second number is less than the first number, the third subset is determined based on a first subset of the first set, and the fourth subset is determined based on a second subset of the second set, and the first subset and the second subset are determined based on power sorting of received powers at the RX during the first number of channel measurements.

[0275] In some example embodiments, the TX comprises NTXTX elements, the RIS comprises NRISRIS elements, the first set comprises αNTXTX elements, 0 < α < 1, the second set comprises βNRISRIS elements, 0 < β < 1. The performing module 1510 may comprise a receiving module configured to receive, αNTXpilot signals transmitted by the TX using the αNTXTX elements in the first set and reflected by the RIS using the βNRISRIS elements in the second set, and a sub performing module configured to perform αNTXx βNRISchannel measurements together with the TX and the RIS. In this way, the number of measurements required for channel estimation in RIS assisted communication system (for example, communication system 100 A as illustrated in FIG. 1A) can be reduced. Therefore, complexity can be reduced and accuracy is improved for channel estimation by using RIS 110.

[0276] In some example embodiments, the third subset comprises αNTXTX elements, the fourth subset comprises βNRISRIS elements. The perform module 1510 may comprise a receiving module configured to receive, αNTXpilot signals with αγNTXlength transmitted by the TX using the αNTXTX elements in the third subset and reflected by the RIS using the βNRISRIS elements in βσNRISstates in the fourth subset, 0 < γ < 1,0 < σ < l, and a sub performing module configured to perform αγNTXx βσNRISchannel measurements together with the TX and the RIS. In this way, the number of measurements required for channel59SUBSTITUTE SHEET (RULE 26)estimation in RIS assisted communication system (for example, communication system 100 A as illustrated in FIG. 1A) can be reduced. Therefore, complexity can be reduced and accuracy is improved for channel estimation by using RIS 110.

[0277] In some example embodiments, in order to perform the power sorting, the third apparatus 1500 may comprise a measuring module configured to measure a set of received powers {Pij} at the RX, here, Pijrepresents a received power at the RX associated with an i-th pilot signal and a j-th RIS element, and a determining module configured to determine a first set of total powers for the αNTXTX elements {PTX, i} (PTX, i=1};), and determine αγNTXTX elements corresponding to top αγNTXtotal powers in the first set of total powers as the first subset of the TX elements. In this way, the number of measurements required for channel estimation in RIS assisted communication system (for example, communication system 100 A as illustrated in FIG. 1A) can be reduced. Therefore, complexity can be reduced and accuracy is improved for channel estimation by using RIS 110.

[0278] In some example embodiments, in order to perform the power sorting, the third apparatus 1500 may comprise a determining module configured to determine a second set of total powers for the βNRISRIS elements {PRIS, j} (PRIS, j=and determine βσNRISRIS elements corresponding to top βσNRIStotal powers in the second set of total powers as the second subset of the RIS elements. In this way, the number of measurements required for channel estimation in RIS assisted communication system (for example, communication system 100 A as illustrated in FIG. 1A) can be reduced. Therefore, complexity can be reduced and accuracy is improved for channel estimation by using RIS 110.

[0279] In some example embodiments, received powers at the third apparatus 1500 associated with TX elements in the third set but not in the third subset of are set to zero, and received powers at the third apparatus 1500 associated with RIS elements in the fourth set but not in the fourth subset of are set to zero. In this way, the number of measurements required for channel estimation in RIS assisted communication system (for example, communication system 100 A as illustrated in FIG. 1A) can be reduced. Therefore, complexity can be reduced and accuracy is improved for channel estimation by using RIS 110.60SUBSTITUTE SHEET (RULE 26)

[0280] In some example embodiments, relative positions of the third subset of TX elements in the third set are same as relative positions of the first subset of TX elements in the first set. Alternatively or additionally, relative positions of the fourth subset of RIS elements in the fourth set are same as relative positions of the second subset of RIS elements in the second set. Alternatively or additionally, phase and magnitude states of TX elements in the third subset are same as phase and magnitude states of TX elements in the first subset. Alternatively or additionally, phase and magnitude states of RIS elements in the fourth subset are same as phase and magnitude states of RIS elements in the second subset. In this way, the number of measurements required for channel estimation in RIS assisted communication system (for example, communication system 100A as illustrated in FIG. 1 A) can be reduced. Therefore, complexity can be reduced and accuracy is improved for channel estimation by using RIS 110.

[0281] In some example embodiments, the first set and third set are two of a first plurality of sets of TX elements in the TX, the second set and fourth set are two of a second plurality of sets of RIS elements in the RIS, and the number of the first plurality of sets is equal to the number of the second plurality of sets. In this way, the number of measurements required for channel estimation in RIS assisted communication system (for example, communication system 100 A as illustrated in FIG. 1A) can be reduced. Therefore, complexity can be reduced and accuracy is improved for channel estimation by using RIS 110.

[0282] In some example embodiments, at least two sets among the first plurality of sets have an intersection. Alternatively or additionally, at least two sets among the second plurality of sets have an intersection. In some example embodiments, a union of the first plurality of sets is a universal set of TX elements of the TX. Alternatively or additionally, a union of the first plurality of sets is a universal set of RIS elements of the RIS. In this way, the number of measurements required for channel estimation in RIS assisted communication system (for example, communication system 100 A as illustrated in FIG. 1A) can be reduced. Therefore, complexity can be reduced and accuracy is improved for channel estimation by using RIS 110.

[0283] In some example embodiments, a first order of the first plurality of sets in which channel measurements for a channel from the TX to the third apparatus 1500 via the RIS are performed is different from a second order of the second plurality of sets in which the channel measurements are performed. In this way, the number of measurements required for channel estimation in RIS assisted communication system (for example, communication61SUBSTITUTE SHEET (RULE 26)system 100 A as illustrated in FIG. 1A) can be reduced. Therefore, complexity can be reduced and accuracy is improved for channel estimation by using RIS 110.

[0284] In some example embodiments, an unused RIS element which is in the fourth set but not in the fourth subset has a constant phase during the second number of channel measurements. In some example embodiments, a channel from the TX to the third apparatus 1500 via the unused RIS element is determined as a direct channel from the TX to the third apparatus 1500. In this way, the number of measurements required for channel estimation in RIS assisted communication system (for example, communication system 100 A as illustrated in FIG. 1A) can be reduced. Therefore, complexity can be reduced and accuracy is improved for channel estimation by using RIS 110.

[0285] In some example embodiments, the first set and the third set have a first geometry, and the second set and the fourth set have a second geometry. In some example embodiments, the first geometry and the second geometry are the same or different. In some example embodiments, at least one of the first geometry and the second geometry is a rectangle. Alternatively or additionally, at least one of the first geometry and the second geometry is a rectangle with at least one element being removed from at least one edge of the rectangle. In this way, the number of measurements required for channel estimation in RIS assisted communication system (for example, communication system 100A as illustrated in FIG. 1A) can be reduced. Therefore, complexity can be reduced and accuracy is improved for channel estimation by using RIS 110.

[0286] In some example embodiments, a matrix completion process is performed at least based on the first number of channel measurements and the second number of channel measurements to obtain a channel matrix of a channel from the TX to the third apparatus 1500 via the RIS, and during the matrix completion process, a value in the channel matrix which is not measured in channel measurements for the channel is inferred from values in the channel matrix which are measured in the channel measurements. In this way, the number of measurements required for channel estimation in RIS assisted communication system (for example, communication system 100A as illustrated in FIG. 1A) can be reduced. Therefore, complexity can be reduced and accuracy is improved for channel estimation by using RIS 110.

[0287] In some embodiments, the apparatus further comprises module(s) for performing other steps in some embodiments of the method 1200. In some embodiments, the module(s) may comprise at least one processor and at least one memory including62SUBSTITUTE SHEET (RULE 26)computer program code, the at least one memory and computer program code configured to, with the at least one processor, cause the performance of the apparatus.

[0288] In this way, embodiments, the number of measurements required for channel estimation in RIS assisted communication system can be reduced. Therefore, complexity can be reduced and accuracy is improved for channel estimation by using RIS.

[0289] FIG. 16 illustrates a simplified block diagram of a device 1600 that is suitable for implementing some example embodiments of the present disclosure. The device 1600 may be provided to implement a communication device, for example, the reconfigurable intelligent surface 110, the transmitter 120 or the receiver 130 as shown in FIG. 1A. As shown, the device 1600 includes one or more processors 1610, one or more memories 1620 coupled to the processor 1610, and one or more communication modules 1640 coupled to the processor 1610.

[0290] The communication module 1640 is for bidirectional communications. The communication module 1640 may include a transmitter 1641 for transmitting data and a receiver 1642 for receiving data. The communication module 1640 has at least one antenna to facilitate communication. The communication interface may represent any interface that is necessary for communication with other network elements.

[0291] The processor 1610 may be of any type suitable to the local technical network and may include one or more of the following: general purpose computers, special purpose computers, microprocessors, digital signal processors (DSPs) and processors based on multi core processor architecture, as non-limiting examples. The device 1600 may have multiple processors, such as an application specific integrated circuit chip that is slaved in time to a clock which synchronizes the main processor.

[0292] The memory 1620 may include one or more non-volatile memories and one or more volatile memories. Examples of the non-volatile memories include, but are not limited to, a Read Only Memory (ROM) 1624, an electrically programmable read only memory (EPROM), a flash memory, a hard disk, a compact disc (CD), a digital video disk (DVD), and other magnetic storage and / or optical storage. Examples of the volatile memories include, but are not limited to, a random access memory (RAM) 1622 and other volatile memories that will not last in the power-down duration.

[0293] A computer program 1630 includes computer executable instructions that are executed by the associated processor 1610. The program 1630 may be stored in the ROM63SUBSTITUTE SHEET (RULE 26)1624. The processor 1610 may perform any suitable actions and processing by loading the program 1630 into the RAM 1622.

[0294] The embodiments of the present disclosure may be implemented by means of the program 1630 so that the device 1600 may perform any process of the disclosure as discussed with reference to FIG. 2, 5 A and 10-12. The embodiments of the present disclosure may also be implemented by hardware or by a combination of software and hardware.

[0295] In some example embodiments, the program 1630 may be tangibly contained in a computer-readable medium which may be included in the device 1600 (such as in the memory 1620) or other storage devices that are accessible by the device 1600. The device 1600 may load the program 1630 from the computer-readable medium to the RAM 1622 for execution. The computer-readable medium may include any types of tangible non-volatile storage, such as ROM, EPROM, a flash memory, a hard disk, CD, DVD, and the like.

[0296] Generally, various embodiments of the present disclosure may be implemented in hardware or special purpose circuits, software, logic or any combination thereof. Some aspects may be implemented in hardware, while other aspects may be implemented in firmware or software which may be executed by a controller, microprocessor or other computing device. While various aspects of embodiments of the present disclosure are illustrated and described as block diagrams, flowcharts, or using some other pictorial representations, it is to be understood that the block, apparatus, system, technique or method described herein may be implemented in, as non-limiting examples, hardware, software, firmware, special purpose circuits or logic, general purpose hardware or controller or other computing devices, or some combination thereof.

[0297] The present disclosure also provides at least one computer program product tangibly stored on a non-transitory computer-readable storage medium. The computer program product includes computer-executable instructions, such as those included in program modules, being executed in a device on a target real or virtual processor, to carry out the method 1000 or 1100 or 1200 as described above with reference to FIG. 5 or 6. Generally, program modules include routines, programs, libraries, objects, classes, components, data structures, or the like that perform particular tasks or implement particular abstract data types. The functionality of the program modules may be combined64SUBSTITUTE SHEET (RULE 26)or split between program modules as desired in various embodiments. Machine-executable instructions for program modules may be executed within a local or distributed device. In a distributed device, program modules may be located in both local and remote storage media.

[0298] Program code for carrying out methods of the present disclosure may be written in any combination of one or more programming languages. These program codes may be provided to a processor or controller of a general purpose computer, special purpose computer, or other programmable data processing apparatus, such that the program codes, when executed by the processor or controller, cause the functions / operations specified in the flowcharts and / or block diagrams to be implemented. The program code may execute entirely on a machine, partly on the machine, as a stand-alone software package, partly on the machine and partly on a remote machine or entirely on the remote machine or server.

[0299] In the context of the present disclosure, the computer program codes or related data may be carried by any suitable carrier to enable the device, apparatus or processor to perform various processes and operations as described above. Examples of the carrier include a signal, computer-readable medium, and the like.

[0300] The computer-readable medium may be a computer-readable signal medium or a computer-readable storage medium. A computer-readable medium may include but not limited to an electronic, magnetic, optical, electromagnetic, infrared, or semiconductor system, apparatus, or device, or any suitable combination of the foregoing. More specific examples of the computer-readable storage medium would include an electrical connection having one or more wires, a portable computer diskette, a hard disk, a random access memory (RAM), a read-only memory (ROM), an erasable programmable read-only memory (EPROM or Flash memory), an optical fiber, a portable compact disc read-only memory (CD-ROM), an optical storage device, a magnetic storage device, or any suitable combination of the foregoing.

[0301] Further, while operations are depicted in a particular order, this should not be understood as requiring that such operations be performed in the particular order shown or in sequential order, or that all illustrated operations be performed, to achieve desirable results. In certain circumstances, multitasking and parallel processing may be advantageous. Likewise, while several specific implementation details are contained in the above discussions, these should not be construed as limitations on the scope of the65SUBSTITUTE SHEET (RULE 26)present disclosure, but rather as descriptions of features that may be specific to particular embodiments. Certain features that are described in the context of separate embodiments may also be implemented in combination in a single embodiment. Conversely, various features that are described in the context of a single embodiment may also be implemented in multiple embodiments separately or in any suitable sub-combination.

[0302] Although the present disclosure has been described in languages specific to structural features and / or methodological acts, it is to be understood that the present disclosure defined in the appended claims is not necessarily limited to the specific features or acts described above. Rather, the specific features and acts described above are disclosed as example forms of implementing the claims.66SUBSTITUTE SHEET (RULE 26)

Claims

WHAT IS CLAIMED IS:

1. A method for communication, comprising: performing, at a reconfigurable intelligent surface (RIS) together with a transmitter (TX) and a receiver (RX), a first number of channel measurements associated with a first set of TX elements of the TX and a second set of RIS elements of the RIS; and performing, together with the TX and the RX, a second number of channel measurements associated with a third subset of a third set of TX elements of the TX and a fourth subset of a fourth set of RIS elements of the RIS, wherein the second number is less than the first number, wherein the third subset is determined based on a first subset of the first set, and the fourth subset is determined based on a second subset of the second set, and wherein the first subset and the second subset are determined based on power sorting of received powers at the RX during the first number of channel measurements.

2. The method of claim 1, wherein the TX comprises NTXTX elements, the RIS comprises NRISRIS elements, the first set comprises αNTXTX elements, 0 < α < 1, the second set comprises βNRISRIS elements, 0 < β < 1, and wherein performing the first number of channel measurements comprises: reflecting, using the βNRISRIS elements in the second set, αNTXpilot signals at least αNTXlength transmitted by the TX using theαNTXTX elements in the first set to the RX; and performing at least αNTXx βNRISchannel measurements together with the TX and the RX.

3. The method of claim 2, wherein the third set comprises αNTXTX elements, the fourth subset comprises βNRISRIS elements, and performing the second number of channel measurements comprises: reflecting, using the βσNRISRIS elements states in the fourth subset, αNTXpilot signals of αγNTXlength transmitted by the TX using the αNTXTX elements in the third set, 0 < γ < 1, 0 < σ < 1; and performing αγNTXx βσNRISchannel measurements together with the TX and theRX.67SUBSTITUTE SHEET (RULE 26)4. The method of claim 2, wherein the power sorting comprises: measuring a set of received powers {Pij} at the RX, wherein Pijrepresents a received power at the RX associated with an i-th pilot signal and a j-th RIS elements state; determining a first set of total powers for the αNTXTX elements {PTX, i}, whereindetermining αγNTXTX elements corresponding to top αγNTXtotal powers in the first set of total powers as the first subset of the TX elements states.

5. The method of claim 4, wherein the power sorting further comprises: determining a second set of total powers for the βNRISRIS elements {PRIS, j}, PRIS, j=Ejf ^ and determining βσNRISRIS elements corresponding to top βσNRIStotal powers in the second set of total powers as the second subset of the RIS elements states.

6. The method of any of claims 1-5, wherein at least one the following: relative positions of the third set of TX elements are same as relative positions of the first set of TX elements; relative positions of the fourth set of RIS elements are same as relative positions of the second set of RIS elements; phase and magnitude states of TX elements in the third subset of states are same as phase and magnitude states of TX elements in the first subset of states; or phase and magnitude states of RIS elements in the fourth subset of states are same as phase and magnitude states of RIS elements in the second subset of states.

7. The method of any of claim 1-6, wherein: the first set and third set are two of a first plurality of sets of TX elements in the TX; the second set and fourth set are two of a second plurality of sets of RIS elements in the RIS; and the number of the first plurality of sets is equal to the number of the second plurality of sets.

8. The method of claim 7, wherein at least one of the following:68SUBSTITUTE SHEET (RULE 26)at least two sets among the first plurality of sets have an intersection; or at least two sets among the second plurality of sets have an intersection.

9. The method of claim 7 or 8, wherein at least one of the following: a union of the first plurality of sets is a universal set of TX elements of the TX; or a union of the first plurality of sets is a universal set of RIS elements of the RIS.

10. The method of any of claims 7-9, wherein a first order of the first plurality of sets in which channel measurements for a channel from the TX to the RX via the RIS are performed is different from a second order of the second plurality of sets in which the channel measurements are performed.

11. The method of any of claims 1-10, wherein an RIS elements unused during given set of measurements have a constant phase during given set of channel measurements.

12. The method of claim 11, wherein a channel from the TX to the RX via the unused RIS element is determined as a direct channel from the TX to the RX.

13. The method of any of claims 1-12, wherein: the first set and the third set have a first geometry; and the second set and the fourth set have a second geometry.

14. The method of claim 13, wherein the first geometry and the second geometry are the same or different.

15. The method of claim 13 or 14, wherein at least one of the first geometry and the second geometry is one of the following: a rectangle; or a rectangle with at least one element being removed from at least one edge of the rectangle.

16. The method of any of claims 1-15, wherein:69SUBSTITUTE SHEET (RULE 26)a matrix completion process is performed at least based on the first number of channel measurements and the second number of channel measurements to obtain a channel matrix of a channel from the TX to the RX via the RIS, and during the matrix completion process, a value in the channel matrix which is not measured in channel measurements for the channel is inferred from values in the channel matrix which are measured in the channel measurements.

17. A method for communication, comprising: performing, at a transmitter (TX) together with a reconfigurable intelligent surface (RIS) and a receiver (RX), a first number of channel measurements associated with a first set of TX elements of the TX and a second set of RIS elements of the RIS; and performing, together with the RIS and the RX, a second number of channel measurements associated with a third subset of a third set of TX elements of the TX and a fourth subset of a fourth set of RIS elements of the RIS, wherein the second number is less than the first number, wherein the third subset is determined based on a first subset of the first set, and the fourth subset is determined based on a second subset of the second set, and wherein the first subset and the second subset are determined based on power sorting of received powers at the RX during the first number of channel measurements.

18. The method of claim 17, wherein the TX comprises NTXTX elements, the RIS comprises NRISRIS elements, the first set comprises αNTXTX elements, 0 < α < 1, the second set comprises βNRISRIS elements, 0 < β < 1, and wherein performing the first number of channel measurements comprises: transmitting, using the αNTXTX elements in the first set, αNTXpilot signals to be reflected by the RIS using the βNRISRIS elements in the second set to the RX; and performing αNTXx βNRISchannel measurements together with the RIS and the RX.

19. The method of claim 18, wherein the third subset comprises αNTXTX elements, the fourth subset comprises βNRISRIS elements, and performing the second number of channel measurements comprises: transmitting, using the αNTXTX elements in the third subset, αNTXpilot signals with αγNTXlength to be reflected by the RIS using the βNRISRIS elements in βσNRISstates in the fourth subset to the RX, 0 < γ < 1, 0 < o < 1; and70SUBSTITUTE SHEET (RULE 26)performing αγNTXx βσNRISchannel measurements together with the RIS and theRX.

20. The method of claim 18, wherein the power sorting comprises: measuring a set of received powers {Pij} at the RX, wherein Pijrepresents a received power at the RX associated with an i-th pilot signal and a j-th RIS element; determining a first set of total powers for the αNTXTX elements {PTX, i}, whereindetermining αγNTXTX elements corresponding to top αγNTXtotal powers in the first set of total powers as the first subset of the TX elements.

21. The method of claim 20, wherein the power sorting further comprises: determining a second set of total powers for the βNRISRIS elements {PRIS, j}, PRIS, j=determining βσNRISRIS elements corresponding to top βσNRIStotal powers in the second set of total powers as the second subset of the RIS elements.

22. The method of any of claims 17-21, wherein at least one the following: relative positions of the third subset of TX elements in the third set are same as relative positions of the first subset of TX elements in the first set; relative positions of the fourth subset of RIS elements in the fourth set are same as relative positions of the second subset of RIS elements in the second set; phase and magnitude states of TX elements in the third subset are same as phase and magnitude states of TX elements in the first subset; or phase and magnitude states of RIS elements in the fourth subset are same as phase and magnitude states of RIS elements in the second subset.

23. The method of any of claim 17-22, wherein: the first set and third set are two of a first plurality of sets of TX elements in the TX; the second set and fourth set are two of a second plurality of sets of RIS elements in the RIS; and71SUBSTITUTE SHEET (RULE 26)the number of the first plurality of sets is equal to the number of the second plurality of sets.

24. The method of claim 23, wherein at least one of the following: at least two sets among the first plurality of sets have an intersection; or at least two sets among the second plurality of sets have an intersection.

25. The method of claim 23 or 24, wherein at least one of the following: a union of the first plurality of sets is a universal set of TX elements of the TX; or a union of the first plurality of sets is a universal set of RIS elements of the RIS.

26. The method of any of claims 23-25, wherein a first order of the first plurality of sets in which channel measurements for a channel from the TX to the RX via the RIS are performed is different from a second order of the second plurality of sets in which the channel measurements are performed.

27. The method of any of claims 17-26, wherein an unused RIS element which is in the fourth set but not in the fourth subset has a constant phase during the second number of channel measurements.

28. The method of claim 27, wherein a channel from the TX to the RX via the unused RIS element is determined as a direct channel from the TX to the RX.

29. The method of any of claims 17-28, wherein: the first set and the third set have a first geometry; and the second set and the fourth set have a second geometry.

30. The method of claim 29, wherein the first geometry and the second geometry are the same or different.

31. The method of claim 29 or 30, wherein at least one of the first geometry and the second geometry is one of the following: a rectangle; or72SUBSTITUTE SHEET (RULE 26)a rectangle with at least one element being removed from at least one edge of the rectangle.

32. The method of any of claims 17-31, wherein: a matrix completion process is performed at least based on the first number of channel measurements and the second number of channel measurements to obtain a channel matrix of a channel from the TX to the RX via the RIS, and during the matrix completion process, a value in the channel matrix which is not measured in channel measurements for the channel is inferred from values in the channel matrix which are measured in the channel measurements.

33. A method for communication, comprising: performing, at a receiver (RX) together with a transmitter (TX) and a reconfigurable intelligent surface (RIS), a first number of channel measurements associated with a first set of TX elements of the TX and a second set of RIS elements of the RIS; and performing, together with the TX and the RIS, a second number of channel measurements associated with a third subset of a third set of TX elements of the TX and a fourth subset of a fourth set of RIS elements of the RIS, wherein the second number is less than the first number, wherein the third subset is determined based on a first subset of the first set, and the fourth subset is determined based on a second subset of the second set, and wherein the first subset and the second subset are determined based on power sorting of received powers at the RX during the first number of channel measurements.

34. The method of claim 33, wherein the TX comprises NTXTX elements, the RIS comprises NRISRIS elements, the first set comprises αNTXTX elements, 0 < α < 1, the second set comprises βNRISRIS elements, 0 < β < 1, and wherein performing the first number of channel measurements comprises: receiving, αNTXpilot signals transmitted by the TX using the αNTXTX elements in the first set and reflected by the RIS using the βNRISRIS elements in the second set; and performing αNTXx βNRISchannel measurements together with the TX and the RIS.73SUBSTITUTE SHEET (RULE 26)35. The method of claim 34, wherein the third subset comprises αNTXTX elements, the fourth subset comprises βNRISRIS elements, and performing the second number of channel measurements comprises: receiving, αNTXpilot signals with αγNTXlength transmitted by the TX using the αNTXTX elements in the third subset and reflected by the RIS using the βNRISRIS elements in βσNRISstates in the fourth subset, 0 < γ < 1,0 < σ <l; and performing αγNTXx βσNRISchannel measurements together with the TX and the RIS.

36. The method of claim 34, wherein the power sorting comprises: measuring a set of received powers {Pij} at the RX, wherein Pijrepresents a received power at the RX associated with an i-th pilot signal and a j-th RIS element; determining a first set of total powers for the aNyx TX elements {PTX, i}, whereindetermining αγNTXTX elements corresponding to top αγNTXtotal powers in the first set of total powers as the first subset of the TX elements.

37. The method of claim 36, wherein the power sorting further comprises: determining a second set of total powers for the βNRISRIS elements {PRIS, j}, PRIS, j=determining βσNRISRIS elements corresponding to top βσNRIStotal powers in the second set of total powers as the second subset of the RIS elements.

38. The method of any of claims 33-37, wherein: received powers at the RX associated with TX elements in the third set but not in the third subset of are set to zero; and received powers at the RX associated with RIS elements in the fourth set but not in the fourth subset of are set to zero.

39. The method of any of claims 33-38, wherein at least one the following: relative positions of the third subset of TX elements in the third set are same as relative positions of the first subset of TX elements in the first set;74SUBSTITUTE SHEET (RULE 26)relative positions of the fourth subset of RIS elements in the fourth set are same as relative positions of the second subset of RIS elements in the second set; phase and magnitude states of TX elements in the third subset are same as phase and magnitude states of TX elements in the first subset; or phase and magnitude states of RIS elements in the fourth subset are same as phase and magnitude states of RIS elements in the second subset.

40. The method of any of claim 33-39, wherein: the first set and third set are two of a first plurality of sets of TX elements in the TX; the second set and fourth set are two of a second plurality of sets of RIS elements in the RIS; and the number of the first plurality of sets is equal to the number of the second plurality of sets.

41. The method of claim 40, wherein at least one of the following: at least two sets among the first plurality of sets have an intersection; or at least two sets among the second plurality of sets have an intersection.

42. The method of claim 40 or 41, wherein at least one of the following: a union of the first plurality of sets is a universal set of TX elements of the TX; or a union of the first plurality of sets is a universal set of RIS elements of the RIS.

43. The method of any of claims 40-42, wherein a first order of the first plurality of sets in which channel measurements for a channel from the TX to the RX via the RIS are performed is different from a second order of the second plurality of sets in which the channel measurements are performed.

44. The method of any of claims 33-43, wherein an unused RIS element which is in the fourth set but not in the fourth subset has a constant phase during the second number of channel measurements.

45. The method of claim 44, wherein a channel from the TX to the RX via the unused RIS element is determined as a direct channel from the TX to the RX.75SUBSTITUTE SHEET (RULE 26)46. The method of any of claims 33-45, wherein: the first set and the third set have a first geometry; and the second set and the fourth set have a second geometry.

47. The method of claim 46, wherein the first geometry and the second geometry are the same or different.

48. The method of claim 46 or 47, wherein at least one of the first geometry and the second geometry is one of the following: a rectangle; or a rectangle with at least one element being removed from at least one edge of the rectangle.

49. The method of any of claims 33-48, wherein: a matrix completion process is performed at least based on the first number of channel measurements and the second number of channel measurements to obtain a channel matrix of a channel from the TX to the RX via the RIS, and during the matrix completion process, a value in the channel matrix which is not measured in channel measurements for the channel is inferred from values in the channel matrix which are measured in the channel measurements.

50. A first apparatus, comprising: at least one processor; and at least one memory storing instructions that, when executed by the at least one processor, cause the first apparatus at least to: perform, together with a transmitter (TX) and a receiver (RX), a first number of channel measurements associated with a first set of TX elements of the TX and a second set of RIS elements of the RIS; and perform, together with the TX and the RX, a second number of channel measurements associated with a third subset of a third set of TX elements of the TX and a fourth subset of a fourth set of RIS elements of the RIS, wherein the second number is less than the first number, wherein the third subset is determined based on a first subset of the first set, and the fourth subset is determined based on a second subset of the second set, and76SUBSTITUTE SHEET (RULE 26)wherein the first subset and the second subset are determined based on power sorting of received powers at the RX during the first number of channel measurements.

51. A second apparatus, comprising: at least one processor; and at least one memory storing instructions that, when executed by the at least one processor, cause the second apparatus at least to: perform, together with a reconfigurable intelligent surface (RIS) and a receiver (RX), a first number of channel measurements associated with a first set of TX elements of the TX and a second set of RIS elements of the RIS; and perform, together with the RIS and the RX, a second number of channel measurements associated with a third subset of a third set of TX elements of the TX and a fourth subset of a fourth set of RIS elements of the RIS, wherein the second number is less than the first number, wherein the third subset is determined based on a first subset of the first set, and the fourth subset is determined based on a second subset of the second set, and wherein the first subset and the second subset are determined based on power sorting of received powers at the RX during the first number of channel measurements.

52. A third apparatus, comprising: at least one processor; and at least one memory storing instructions that, when executed by the at least one processor, cause the third apparatus at least to: perform, at a receiver (RX) together with a transmitter (TX) and a reconfigurable intelligent surface (RIS), a first number of channel measurements associated with a first set of TX elements of the TX and a second set of RIS elements of the RIS; and perform, together with the TX and the RIS, a second number of channel measurements associated with a third subset of a third set of TX elements of the TX and a fourth subset of a fourth set of RIS elements of the RIS, wherein the second number is less than the first number,77SUBSTITUTE SHEET (RULE 26)wherein the third subset is determined based on a first subset of the first set, and the fourth subset is determined based on a second subset of the second set, and wherein the first subset and the second subset are determined based on power sorting of received powers at the RX during the first number of channel measurements.

53. A communication system, comprising a first apparatus according to claim 50, a second apparatus according to claim 51 and a third apparatus according to claim 52, wherein the communication system is configured to perform the method of any of claims 1-49 using the first apparatus, the second apparatus and the third apparatus.

54. A non-transitory computer readable medium comprising computer program stored thereon, the computer program, when executed on at least one processor, causing the at least one processor to perform the method of any of claims 1-49.

55. A chip comprising at least one processing circuit configured to perform the method of any of claims 1-49.

56. A computer program product tangibly stored on a computer-readable medium and comprising computer-executable instructions which, when executed, cause an apparatus to perform the method of any of claims 1-49.78SUBSTITUTE SHEET (RULE 26)

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