Device paring for MU-MIMO communication
Patent Information
- Application Number
- PCT/CN2025/084011
- Authority / Receiving Office
- WO · WO
- Patent Type
- Applications
- Current Assignee / Owner
- Filing Date
- 2025-03-21
- Publication Date
- 2026-09-24
Smart Images
Figure CN2025084011_24092026_PF_FP_ABST
Abstract
Description
DEVICE PARING FOR MU-MIMO COMMUNICATIONFIELD
[0001] Various example embodiments relate to the field of communications and in particular, to a device, a method, an apparatus and a computer readable storage medium for device paring in multiple user-multiple input multiple output (MU-MIMO) communication.BACKGROUND
[0002] A communication network can be seen as a facility that enables communications between two or more communication devices, or provides communication devices access to a data network. A mobile or wireless communication network is one example of a communication network.
[0003] Such communication networks operate in accordance with standards, such as those promulgated by 3GPP (Third Generation Partnership Project) or ETSI (European Telecommunications Standards Institute) . Examples of such standards include the so-called 5G (5th Generation) standard or other standards promulgated by 3GPP.SUMMARY
[0004] In general, example embodiments of the present disclosure provide a solution for paring terminal devices for MU-MIMO communication.
[0005] In a first aspect, there is provided a network device. The network device comprises at least one processor and at least one memory including computer program codes. The at least one memory and the computer program codes are configured to, with the at least one processor, cause the network device to: determine that a first correlation between a first terminal device and a second terminal device is above or equal to a first threshold. The network device is further caused to select a first precoding matrix for the first terminal device and a second precoding matrix for the second terminal device. A second correlation between the first precoding matrix and the second precoding matrix is below a second threshold. The network device is further caused to determine, for multiple user-multiple input multiple output (MU-MIMO) communication, a terminal device pair at least comprising the first terminal device and the second terminal device.
[0006] In a second aspect, there is provided a method implemented at a network device. The method comprises: determining, at a network device, that a first correlation between a first terminal device and a second terminal device is above or equal to a first threshold; selecting a first precoding matrix for the first terminal device and a second precoding matrix for the second terminal device, wherein a second correlation between the first precoding matrix and the second precoding matrix is below a second threshold; and determining, for multiple user-multiple input multiple output (MU-MIMO) communication, a terminal device pair at least comprising the first terminal device and the second terminal device.
[0007] In a third aspect, there is provided an apparatus comprising: means for determining that a first correlation between a first terminal device and a second terminal device is above or equal to a first threshold; means for selecting a first precoding matrix for the first terminal device and a second precoding matrix for the second terminal device, wherein a second correlation between the first precoding matrix and the second precoding matrix is below a second threshold; and means for determining, for multiple user-multiple input multiple output (MU-MIMO) communication, a terminal device pair at least comprising the first terminal device and the second terminal device.
[0008] In a fourth aspect, there is provided a computer program comprising instructions, which, when executed by an apparatus, cause the apparatus at least to: determine that a first correlation between a first terminal device and a second terminal device is above or equal to a first threshold. The apparatus is further caused to select a first precoding matrix for the first terminal device and a second precoding matrix for the second terminal device. A second correlation between the first precoding matrix and the second precoding matrix is below a second threshold. The apparatus is further caused to determine, for multiple user-multiple input multiple output (MU-MIMO) communication, a terminal device pair at least comprising the first terminal device and the second terminal device.
[0009] In a fifth aspect, there is provided a network device. The network device comprises: first determining circuitry configured to determine that a first correlation between a first terminal device and a second terminal device is above or equal to a first threshold; selecting circuitry configured to select a first precoding matrix for the first terminal device and a second precoding matrix for the second terminal device, wherein a second correlation between the first precoding matrix and the second precoding matrix is below a second threshold; and second determining circuitry configured to determine, for multiple user-multiple input multiple output (MU-MIMO) communication, a terminal device pair at least comprising the first terminal device and the second terminal device.
[0010] In a sixth aspect, there is provided a non-transitory computer readable medium comprising program instructions for causing an apparatus to perform at least the method according to the second aspect.
[0011] 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
[0012] Some example embodiments will now be described with reference to the accompanying drawings, in which:
[0013] FIG. 1A illustrates an example communication network in which embodiments of the present disclosure may be implemented;
[0014] FIG. 1B illustrates an example of an uplink (UL) MU-MIMO paring scheme;
[0015] FIG. 2 illustrates an example of a pairing method for terminal devices in MU-MIMO communication which are high correlated according to example embodiments of the present disclosure;
[0016] FIG. 3 illustrates an example flow related to calculating the correlation between terminal devices according to example embodiments of the present disclosure;
[0017] FIG. 4 illustrates an example signaling process of the device pairing for MU-MIMO communication according to example embodiments of the present disclosure;
[0018] FIG. 5 illustrates an example flow related to selecting a precoding matrix for terminal devices in high correlation group (s) according to example embodiments of the present disclosure;
[0019] FIG. 6 illustrates an example of comparing system performance simulation result related to jointly using precoding matrix selection and correlation according to example embodiments of the present disclosure;
[0020] FIG. 7 illustrates a flowchart of a method implemented at a network device according to some embodiments of the present disclosure;
[0021] FIG. 8 illustrates a simplified block diagram of an apparatus that is suitable for implementing embodiments of the present disclosure; and
[0022] FIG. 9 illustrates a block diagram of an example computer readable medium in accordance with some embodiments of the present disclosure.
[0023] Throughout the drawings, the same or similar reference numerals represent the same or similar element.DETAILED DESCRIPTION
[0024] 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.
[0025] 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.
[0026] 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 submitted 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.
[0027] 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.
[0028] 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. As used herein, “at least one of the following: <a list of two or more elements>” and “at least one of <a list of two or more elements>” and similar wording, where the list of two or more elements are joined by “and” or “or” , mean at least any one of the elements, or at least any two or more of the elements, or at least all the elements.
[0029] 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 (e.g., firmware) for operation, but the software may not be present when it is not needed for operation.
[0030] 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.
[0031] As used herein, the term “communication network” refers to a network following any suitable communication standards, such as 5G New Radio (NR) , 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) 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 first generation (1G) , the second generation (2G) , 2.5G, 2.75G, the third generation (3G) , the fourth generation (4G) , 4.5G, the future fifth generation (5G) 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 systems.
[0032] 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) 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 base station of a 5G system) , a Remote Radio Unit (RRU) , a radio header (RH) , a remote radio head (RRH) , a relay, a low power node such as a femto, a pico, and so forth, depending on the applied terminology and technology.
[0033] 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 Subscriber Station (SS) , a Portable Subscriber Station, a Mobile Station (MS) , 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 head-mounted display (HMD) , a vehicle, a drone, a medical device and applications (e.g., remote surgery) , an industrial device and applications (e.g., 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 “terminal device” , “communication device” , “terminal” , “user equipment” and “UE” may be used interchangeably.
[0034] Multi-User Multiple Input Multiple Output (MU-MIMO) is a wireless communication technology that allows the network (e.g., the network device) communicate with multiple users (e.g., terminal devices) simultaneously. It enhances network efficiency, increases capacity, and reduces latency in high-density environments by enabling parallel data transmission to multiple users. Specifically, the MU-MIMO uses a spatial diversity technique (e.g., the beamforming technical) which exponentially improves the capacity and spectrum utilization of communication systems without increasing system bandwidth. In an example, for MU-MIMO, different terminal devices use the same time-frequency resources for uplink transmission. From the receiving side, these data streams may be considered as to different antennas from a user terminal, which forms a virtual MIMO system.
[0035] Generally, the capacity improvement of the MU-MIMO communication system highly depends on a good pairing scheme. As an example, the UL MU-MIMO pairing scheme may be implemented in spatial domain multiplexing (SD) module. The network selects the terminal devices output from a time domain multiplexing (TD) module whose correlation is smaller than a pre-defined threshold to guarantee that the terminal devices with good isolation can be paired. That is, the terminal devices with high correlation can’ t be paired and it will impact the total capacity of network. In turn, the paired terminal devices may be scheduled with the same time-frequency resources. In addition, for discussion purposes, the pairing scheme is further discussed with reference to FIG. 1B below.
[0036] However, there may be a limitation in the above paring scheme. For example, in the scenarios where terminal devices are located very close to each other, particularly when their mutual correlation exceeds a predefined threshold, these terminal devices cannot be paired in most cases. The reasons are that, under the above scenarios, the channel properties for these terminal are nearly identical, which causes decoding issues at the network. The network cannot correctly separate the signals from highly correlated terminal devices, leading to significant interference and deteriorated performance.
[0037] As such, this pairing scheme excludes the terminal devices that are highly correlated, which seems necessary to prevent decoding errors. However, this exclusion of terminal devices with high correlation leads to reduced network capacity. Specifically, with the number of terminal device increases, a larger subset of terminal devices (which that are close to each other) may be excluded from the pairing list. This may diminish the overall capacity of the communication system. This issue is particularly problematic in dense network environments where many users are located near each other.
[0038] In view of these analyses and considerations, embodiments of the disclosure provide a solution for the device pairing for MU-MIMO communication. Specifically, a joint pairing scheme that optimizes the pairing process for rank-1 terminal devices, irrespective of the locations of the terminal devices within the network is proposed. In some embodiments of the disclosure, a rank-1 user in MU-MIMO communication refers to a terminal device whose wireless channel supports only a single spatial data stream transmission. In the joint pairing scheme, the device pairing is not only based on the correlation between terminal devices, but also a precoding matrix selection, for example, the selection of orthogonal precoding matrixes.
[0039] In an aspect, a network device determines that a first correlation between a first terminal device and a second terminal device is above or equal to a first threshold. For example, the first terminal device and the second terminal device may be determined as highly correlated in the sense of the pairing scheme. The network device selects a first precoding matrix for the first terminal device and a second precoding matrix for the second terminal device. A second correlation between the first precoding matrix and the second precoding matrix is below a second threshold. As an example, the first precoding matrix and the second precoding matrix may be orthogonal. Then, the network device determines a terminal device pair at least comprising the first terminal device and the second terminal device for MU-MIMO communication.
[0040] In this way, the pairing possibility of the terminal devices may be increased in the MU-MIMO communication. Specifically, by incorporating these two factors (i.e., the correlation and the precoding selection) into the pairing decision, the UL pairing ratio for Rank-1 users who typically have lower spatial diversity compared to higher rank users, may be increased. As such, this pairing scheme may achieve the pairing of the terminal devices which are highly correlated, by minimizing interference while maintaining or improving the system throughput.
[0041] Principles and embodiments of the present disclosure will be described in detail below with reference to the accompanying drawings. Reference is first made to FIG. 1A, which illustrates an example communication system 100 (or referred to as communication network) in which embodiments of the present disclosure may be implemented. The system 100, for example, a communication network, includes a network device 110, a first terminal device 120, a second terminal device 130, a third terminal device 140 and a fourth terminal device 150. In some embodiments, the first terminal device 120 and second terminal device 130 may be located close with each other, or have the similar channel characteristics which are difficult to be distinguished in the spatial domain. In addition, the network device 110 is capable to perform MU-MIMO communication with one or more of the above terminal devices 120, 130, 140 and 150. It is to be understood that the number of network devices and terminal devices is only for the purpose of illustration without suggesting any limitations. The system 100 may include any suitable number of network devices and terminal devices adapted for implementing embodiments of the present disclosure.
[0042] Communications in the communication system 100 may be implemented according to any proper communication protocol (s) , comprising, but not limited to, cellular communication protocols of the first generation (1G) , the second generation (2G) , the third generation (3G) , the fourth generation (4G) and the fifth generation (5G) and on the like, wireless local network communication protocols such as Institute for Electrical and Electronics Engineers (IEEE) 802.11 and the like, and / or any other protocols currently known or to be developed in the future. Moreover, the communication may utilize any proper wireless communication technology, comprising but not limited to: Code Division Multiple Access (CDMA) , Frequency Division Multiple Access (FDMA) , Time Division Multiple Access (TDMA) , Frequency Division Duplex (FDD) , Time Division Duplex (TDD) , Multiple-Input Multiple-Output (MIMO) , Orthogonal Frequency Division Multiple (OFDM) , Discrete Fourier Transform spread OFDM (DFT-s-OFDM) and / or any other technologies currently known or to be developed in the future. Only for discussion purposes, an example of the MU-MIMO paring flow is discussed below.
[0043] FIG. 1B illustrates an example of an uplink (UL) MU-MIMO paring scheme. In the example of FIG. 1B, the pairing of the terminal device is determined by the spatial domain multiplexing (SD) module or scheduler 160. The SD module 160 obtain information related to UEs to be paired from TD module, e.g., UEs in timing domain multiplexing list. Then, the SD 160 starts the paring. The SD 160 checks whether all UEs in the list are processed. If it is yes, the SD 160 may send a list of spatial domain multiplexed UEs to the FD module (or scheduler) . If it is no, SD 160 may select one UEs from the UEs to be paired as a root UE. Then, SD 160 determines whether the selected UE is eligible for root UE. If the selected UE is not eligible for root UE, the flow returns back to determine whether all UE are processed. If the selected UE is eligible for root UE, SD 160 goes through all non-paired UEs in the list from the TD module, and find ones which can be paired with this root UE based on that the ones are not highly correlated with the root UE. If there is no UE that can be paired with the root UE, the flow returns back to check whether all UEs in the list are processed. Otherwise, a virtual UE which include root UE and the paired ones is created. In some embodiments, the SD scheduler 160 may set each of the UEs as the root UE to perform the above flow.
[0044] As such, in the above example, the UEs which are highly correlated cannot be paired into a UE pair. In some embodiments of the disclosure, the high correlated UEs
[0045] Reference is now made to FIG. 2, which illustrates an example signaling process 0200 of the device pairing for MU-MIMO communication according to example embodiments of the present disclosure. For the purpose of discussions, the signaling process 200 will be described with reference to FIG. 1A. It would be appreciated that although the signaling process 200 has been described in the communication environment 100 of FIG. 1A, this signaling process 200 may be likewise applied to other communication scenarios.
[0046] In the signaling process 200, the network device 110 determines (210) that a first correlation between a first terminal device 120 and a second terminal device 130 is above or equal to a first threshold.
[0047] In some embodiments, in calculating mutual correlation among the terminal devices to be paired, the network device 110 may determine the first correlation between the first terminal device 120 and second terminal device 130. For example, the first terminal device 120 to the fourth terminal device 150 are the terminal devices to be paired for the MU-MIMO communication, and the network device 110 can determine the correlations among these terminal devices for pairing. Taking the first correlation between the terminal devices 120 and 130 as an example, the network device 110 may determine the first correlation based on two “correlation or interference directions” between the terminal devices 120 and 130 , in some embodiments.
[0048] For example, the network device 110 may determine a third correlation from the first terminal device to the second terminal device and a fourth correlation from the second terminal device to the first terminal device. Then, the network device 110 may determine a larger one among the third correlation and the fourth correlation as the first correlation. For illustration purposes, the correlation calculation is further discussed with reference to FIG. 3.
[0049] FIG. 3 illustrates an example flow related to calculating the correlation between terminal devices according to example embodiments of the present disclosure.
[0050] In the example of FIG. 3, the network device 110 aims to calculate the mutual correlation between terminal device devices. The correlation between the terminal devices may be computed by using the correlation of their dominant beamforming weights and the covariance matrix derived from the channel estimation.
[0051] In the example, the sounding reference signal (SRS) data from all transmit receive points (TRX) of the network device 110 may be sent from the radio frequency unit (RFU) to layer 1 (L1) of the network device 110. In some embodiments of the disclosure, the SRS is an uplink reference signal transmitted by terminal device (s) to enable the network to estimate uplink channel quality of the corresponding terminal device (s) . This information may be used to optimize resource scheduling, beamforming, MIMO precoding, and link adaptation, especially critical for multi-antenna transmissions and time-division duplex (TDD) systems leveraging channel reciprocity.
[0052] Still referring to FIG. 3, regarding the raw SRS channel data transmission, the channel estimation is performed in L1 of the network device 110 based on pre-combined SRS received with cell-level beams. As shown in the example, the SRS is pre-combined to perform SRS channel estimation and to generate a covariance matrix (CoMa) for a terminal device. Then, at L2, the network device 110 performs the beam selection using the generated covariance matrix. For example, the network device 110 selects the best beam (s) for terminal device (s) from the beam set based on the covariance matrix derived from the channel estimation. Once the beam beam (s) are selected, the network device 110 may calculate the correlation between terminal devices as below.
[0053] As mentioned above, the network device 110 may determine the first correlation based on two “correlation or interference directions” between the terminal devices 120 and 130. Only for representation simplicity, assuming that UE (i) is an example the first terminal device 120, and UE (j) is an example the second terminal device 130. In this case, the UL beamforming gain with the kth best beam for UE (i) (i.e., the first terminal device 120) is given by equation (1) : where wi, k represents the beamforming weight vector for UE (i) at its kth best beam (size: N×1, where N is the number of antennas per polarization) , hi represents the channel coefficient vector of UE (i) , measured over periodic SRS (size : Nx1) and Ri represents long-term covariance matrix (CoMa) of UE (i) .
[0054] In some embodiments, the interference from UE (j) to UE (i) may be calculated as follows:
[0055] That is, the interference from the UE (j) to UE (i) (which may be also referred to as “a second interference level” in some embodiments) is determined based on a beamforming (weight) vector for UE (i) and a channel (coefficient) vector of UE (j) . In turn, the interference from the UE (i) to UE (j) (which may be also referred to as “afirst interference level” in some embodiments) is determined based on a beamforming vector for UE (j) and a channel vector of UE (i) .
[0056] Then, the correlation from UE (j) to UE (i) (which may be also referred to as “afourth correlation from the second terminal device to the first terminal device” in some embodiments) is given by equation (3) as below.
[0057] In addition, the correlation from UE (i) to UE (j) (which may be also referred to as “athird correlation from the first terminal device to the second terminal device” may be determined in the same way. Then, in some embodiments, the network device 110 may determine a larger one among the third correlation and the fourth correlation as the first correlation between the UE (i) (i.e., the first terminal device) and the UE (j) (i.e., the second terminal device) . For example, the network device 110 may determine the first correlation as below: corr= max (corrj, i, corri, j) (4) In some embodiments, the correlation values range from 0 to 1, where the value 0 indicates no correlation, and the value 1 indicates totally correlated. As such, this ensures that the maximum interference effect is considered when determining the correlation between terminal devices.
[0058] Referring back to FIG. 2, the network device 110 selects (220) a first precoding matrix for the first terminal device 120 and a second precoding matrix for the second terminal device 130. A second correlation between the first precoding matrix and the second precoding matrix is below a second threshold. In some embodiments, the first precoding matrix and the second precoding matrix may be orthogonal.
[0059] In other words, in some embodiments of the disclosure, to ensure that the high correlated terminal devices can be paired, the network device 110 selects precoding matrixes for these terminal device based on a criterion. The criterion may include that the precoding matrixes selected for the high correlated terminal devices are low correlated, e.g., orthogonal.
[0060] In some embodiments, the network device 110 may generate high correlation groups based on the correlation calculation between the terminal devices. For example, the first terminal device 120 and the second terminal device 130 may be grouped into the same high correlation groups. In addition, the terminal devices with low correlation still can be directly paired, thus improving the overall grouping efficiency. For example, the terminal devices 120 –150 are the plurality of terminal devices to be paired. A third correlation between the third terminal device 140 and fourth terminal device 150 is below the first correlation threshold, the network device 110 may determine the terminal device pair comprising the third terminal device and the fourth terminal device directly. In addition, in some embodiments, the network device 110 may generate a high correlation group including the first terminal device and the second terminal device, since the first correlation is above or equal to the first correlation threshold. That is, the network device 110 may generate high correlation groups to ensure that terminal devices with high correlation are assigned to the same group, while other terminal devices with low correlation can be directly paired.
[0061] In a specific example, once the correlation between the terminal devices is calculated, the network device 110 may proceed with the following grouping rules. For example, a grouping rule may be if the correlation between two terminal devices is not greater than the threshold T, they can be directly paired without additional grouping. Another grouping rule may be if the correlation between two terminal devices is greater than or equal to the threshold T, these terminal devices may be assigned to the same high-correlation group (HCG) . In this way, multiple terminal devices can form HCG (s) if their pairwise correlation values all exceed the threshold T. In some embodiments, two rank-1 terminal devices may form one HCG. In addition, all terminal devices that are not assigned to an HCG may be directly paired (e.g., based on the flow discussed in FIG. 1B) and will not be forcibly excluded.
[0062] To better illustrate the process, consider the following correlation table for 8 UEs. Table 1 shows the calculated correlations among these UEs. Table 1
[0063] In addition, assuming that the first correlation threshold T =0.85. In this case, the grouping process may be as follow. Regarding the high correlation group (s) , since UE1 and UE8 have a correlation of 0.9 > 0.85, they are grouped together as Group A: {UE1, UE8} . Since UE2, UE3 and UE5 have the following correlations: UE5 &UE2: 0.9 > 0.85, and UE5 &UE3: 0.95 > 0.85. Therefore, they are grouped with the maximum correlation between Corr (UE5, UE3) and Corr (UE5, UE2) as Group B: {UE5, UE3} . In some embodiments, the UE2 may be exclude from the future terminal device pair.
[0064] In addition, the remaining UEs have correlation values lower than T with each other. Thus, they remain as separate pairs without being forced into a group.
[0065] In the method discussed in FIG. 1B, if certain UEs have high correlation, they might have been excluded from the MU-MIMO process. For instance, UE1 and UE8 might have been excluded because their correlation distribution is high. However, in the new grouping strategy, the network device 110 may allow high-correlation terminal devices to form groups and apply the orthogonal precoding matrix (which will be discussed in detail below) instead of being excluded. This will be helpful to improve overall system efficiency.
[0066] In some embodiments of the disclosure, a novel precoding matrix (indicator) selection method is introduced for terminal devices (for example, rank-1 terminal devices) in the generated high correlation group (HCG) . The selection method ensures that terminal devices in the high-correlation group can be in the same terminal device pair by selecting the orthogonal precoding matrix to reduce interference and improve system performance.
[0067] In some embodiments, to select the first matrix and the second matrix, the network device 110 may select, from a first plurality of matrixes, a third precoding matrix associated with the first and second terminal devices in the high correlation group. In some embodiments, the first plurality of matrixes are multiple matrixes generated and maintained at the network device 110 side. For discussion purposes, the network device 110 may generate an example new transmit power precoding matrix indicator (TPMI) below. Table 2
[0068] In some embodiments, the sub-matrixes (or column vectors) of a precoding matrix among the first plurality of matrixes may be mutually orthogonal. For example, the column vectors of the Wsub, 0 are orthogonal with each other. In MU-MIMO systems, orthogonality is crucial for reducing inter-user interference, enhancing signal decoupling capability, and optimizing signal detection. As an example, when the two columns of a matrix are orthogonal, and the two columns are determined as the precoding matrix for different users, interference between different user data streams may be minimized, allowing the receiver to distinguish multiple user signals more effectively. In addition, a first sub-matrix (e.g., a first column vector) of the newly generated third precoding matrix, e.g., Wsub, 0, may correspond to a first precoding matrix, and a second sub-matrix (e.g., a second column vector) of the newly generated third precoding matrix may correspond to a second precoding matrix which is orthogonal with the first precoding matrix. In an example, the first precoding matrix and second precoding matrix may be the precoding matrixes for single-layer transmission using two antenna ports. Then, the first precoding matrix and the orthogonal second precoding matrix may be selected by selecting one matrix from the new generated transmit power precoding matrix table.
[0069] In this way, a new precoding matrix structure similar to the precoding matrix for Rank-2 UE may be adopted. For example, in Rank-2 (i.e., two-layer spatial multiplexing) user equipment (UE) , orthogonal precoding matrices are typically used to ensure that signals are well distributed across different antenna ports. Correspondingly, the network device 110 aims for the PMI matrix in the newly designed table 2 to have a structure similar to Rank-2 UE, thereby improving data stream transmission efficiency, particularly in high-correlation user groups.
[0070] In some embodiments, the network device 110 may determine, for the first terminal device and the second terminal device, a sum of quality metrics of received signals associated with a matrix in the first plurality of matrixes. Then, the network device 110 may select the third precoding matrix from the first plurality of matrixes (e.g., the table 1) by maximizing the sum of quality metrics.
[0071] In some embodiments, the quality metrics of received signal may include the signal-to-interference-plus-Noise Ratio (SINR) , reference signal received power (RSRP) , reference signal received quality (RSRQ) and so on. In some embodiments, the sum of quality metrics may be determined based on a channel gain for at least one of the first terminal device and a second terminal device. Moreover, the channel gain may be determined based on a matrix in the first plurality of matrixes (e.g., the Table 2) and channel estimation matrix.
[0072] Only for discussion purposes, taking the SINR as an example. In this case, the SINR associated with user i (e.g., the first terminal device 120) may be calculated as: where Gi, Subc (nr) represents the channel gain between user i and receive antenna port nr at subcarrier Subc, is the estimated noise power, obtained using SRS, and Pboost is the power boosting factor for SRS relative to Physical Uplink Shared Channel (PUSCH) .
[0073] In addition, as mentioned above, the channel gain Gi, Subc may be determined by the channel estimation matrix and the precoding matrix. Specifically, the channel gain Gi, Subc (nr) may be calculated as below: where is the channel estimation matrix for user u at subcarrier Subc, obtained via SRS, and Wsub, i (: , u) is a new generated precoding matrix (e.g., a precoding matrix in Table 2) , which is used to adjust the transmitted signals for different users.
[0074] As such, the sum of the quality metrics of the received signal associated with the first terminal device and second terminal device in the high correlation group may be calculated as ∑users∑subcarriersSINRi (Subc) . As mentioned above, the network device 110 may select the third precoding matrix from the first plurality of matrixes (e.g., the table 1) by maximizing the sum of quality metrics. That is, the goal is to find the optimal precoding matrix (i.e., the third precoding matrix) in the first plurality of matrixes which maximizes the sum of SINR across all users (nuser) and all subcarriers (Subc) . In an example, the third precoding matrix (Pi*) may be calculated below: where the SINRi (Subc) is obtained based on the equation (5) .
[0075] In addition, in some embodiments, the selection of third precoding matrix may be related to coherent property of the terminal device (s) . In some embodiments of the disclosure, the coherent terminal device may refer to the terminal device which antennas are coherent with each other. For example, if at least one of the first terminal device and the second terminal device are non-coherent terminal device, the network device 110 may select the third matrix from a portion of the first plurality of matrix. As an example, if the terminal devices are non-coherent, only index 0 and 1 in Table 2 can may be used in the precoding matrix selection process. Alternatively, in some embodiments, if the first terminal device and the second terminal device are coherent terminal devices, the network device 110 may select the third matrix from the first plurality of matrix. As an example, if terminal devices are coherent, all available precoding matrix in Table 2 may be considered in the above calculation.
[0076] For discussion purposes, an example of the selection of third precoding matrix is further discussed with reference to FIG. 4. FIG. 4 illustrates an example signaling process of the device pairing for MU-MIMO communication according to example embodiments of the present disclosure.
[0077] In the example of FIG. 4, at 410, the network device 110 may generate a new precoding matrix indicator (PMI) selection table (e.g., table 2) which includes Wsub, 0-Wsub, 5. At 420, the network device 110 may calculate the channel gain with channel estimation matrix and the precoding matrix in Table 2. At 430, the network device 110 may calculate SINR with the calculated channel gain. At 440, the network device 110 may find the optimal precoding matrix based on:
[0078] Referring back to FIG. 2, after obtaining the optimal precoding matrix (i.e., the third precoding matrix) from the new generated plurality of matrixes (e.g., the Table 2) , the network device 110 may further select the corresponding first matrix and second matrix from the predefined multiple precoding matrixes. For discussion purposes, the predefined multiple precoding matrixes may be also referred to as a second plurality of matrixes. In some embodiments, the second plurality of matrixes may be precoding matrixes for MU-MIMO communication, for instance, precoding matrixes for single-layer transmission using two antenna ports. As an example, the second plurality of matrixes may be as below. Table 3
[0079] In some embodiments, the network device 110 may select, among the second plurality of matrixes, the first precoding matrix corresponding a first sub-matrix (e.g., a first column vector) of the selected third precoding matrix, and the second precoding matrix corresponding a second sub-matrix (e.g., a second column vector) of the selected third precoding matrix. As an example, if the network device 110 selects Wsub, 0 from Table 2 which maximizes the sum of received metrics, then the network device may select W (0) as the first precoding matrix and select the W (1) as the second precoding matrix.
[0080] Still referring to FIG. 2, since the orthogonal first and second precoding matrixes are selected, the network device 110 determines, for multiple user-multiple input multiple output (MU-MIMO) communication, a terminal device pair at least comprising the first terminal device and the second terminal device. In addition, in some embodiments, the network device 110 may transmit a first indication of the first precoding matrix to the first terminal device. The network device may transmit a second indication of the second precoding matrix to the second terminal device.
[0081] For discussion purposes, a specific example of the above embodiments is further discussed with reference to FIG. 5. FIG. 5 illustrates an example flow related to selecting a precoding matrix for terminal devices in high correlation group (s) according to example embodiments of the present disclosure.
[0082] In the example of FIG. 5, at 510, the network device 110 calculates mutual correlation among multiple UEs to be paired. At 520, the network device 110 may generate a high correlation group (s) based on the correlation calculation. At 530, the network device 110 may perform the novel precoding matrix selection for rank-1 terminal devices in the high correlation group (s) , e.g., the embodiments discussed in FIG. 4.
[0083] FIG. 6 illustrates an example of comparing system performance simulation result related to jointly using precoding matrix selection and correlation according to example embodiments of the present disclosure.
[0084] In the simulation shown in FIG. 6, the curve 610 represents the simulation result in SisEigenGoB beamforming method without considering both the precoding matrix selection and terminal device correlation, and the curve 620 represents simulation result in SisEigenGoB beamforming method in which both precoding matrix selection and terminal device correlation are jointly considered. The situation of curve 620 may be also represented as “SrsEigenGoB-On” . “On” indicates that the algorithm takes both PMI selection and correlation information into account, leading to improved interference management and better overall performance.
[0085] The curve 630 represents the simulation result in DmrsBasedEBB beamforming method without considering both the precoding matrix selection and terminal device correlation, and the curve 640 represents simulation result in DmrsBasedEBB beamforming method in which both precoding matrix selection and terminal device correlation are jointly considered. The situation of curve 640 may be also represented as “DmrsBasedEBB -On” .
[0086] As shown in FIG. 6, in the case of curves 610 and 630, the correlation information is not utilized to select different PMIs, leading to higher interference between the terminal devices in the same beam.
[0087] The benefits of jointly using precoding matrix selection and correlation (i.e., SrsEigenGoB-On &DmrsBasedEBB-On) at least includes reduced interference, higher SINR and better resource utilization. For example, by considering the high correlation between UEs and selecting orthogonal PMIs, the interference is reduced, and the system can efficiently utilize the spectrum. Since the interference is reduced, the SINR improves for both terminal devices, leading to higher throughput and better system capacity. In addition, the algorithm ensures that both terminal devices can operate effectively within the same frequency and time resources by selecting optimal precoding matrix.
[0088] However, only using the correlation between terminal devices to determine the terminal device pair may cause some potential issues at least including increased interference, lower SINR and reduced throughput. For example, using the non-orthogonal PMI index for highly correlated terminal devices leads to higher mutual interference, which degrades the performance. The SINR may drop because the signals from the terminal devices are not effectively separated, leading to mutual interference in the beamforming process. In addition, the interference results in a lower data rate and less efficient use of the channel.
[0089] In view of the above, jointly considering the precoding matrix selection and terminal device correlation leads to substantial performance improvements, particularly in scenarios where the terminal devices are highly correlated (e.g., located close to each other in a line of sight, LoS, environment) . By using the embodiments of the disclosure, the system can dynamically select orthogonal precoding matrix which will reduce interference and boosting SINR, which in turn increases overall system throughput.
[0090] FIG. 7 illustrates a flowchart 700 of a method implemented at a network device according to some embodiments of the present disclosure. The network device performing the method 700 may be an example of the network device 110 above.
[0091] At 710, the network device 110 determines that a first correlation between a first terminal device and a second terminal device is above or equal to a first threshold. At 720, the network device 110 selects a first precoding matrix for the first terminal device and a second precoding matrix for the second terminal device. A second correlation between the first precoding matrix and the second precoding matrix is below a second threshold. At 730, the network device 110 determines, for multiple user-multiple input multiple output (MU-MIMO) communication, a terminal device pair at least comprising the first terminal device and the second terminal device.
[0092] In some embodiments, the first precoding matrix and the second precoding matrix are orthogonal.
[0093] In some embodiments, the network device may further determine the first correlation by the following: determining a third correlation from the first terminal device to the second terminal device; determining a fourth correlation from the second terminal device to the first terminal device; and determining a larger one among the third correlation and the fourth correlation as the first correlation.
[0094] In some embodiments, at least one of the third correlation and the fourth correlation is determined based on the following: a first interference level from the first terminal device to the second terminal device; and a second interference level from the second terminal device to the first terminal device.
[0095] In some embodiments, the first interference level is determined based on a beamforming vector for the second terminal device and a channel vector of the first terminal device; and the second interference level is determined based on a beamforming vector for the first terminal device and a channel vector of the second terminal device.
[0096] In some embodiments, the first and second terminal devices are from a plurality terminal devices to be paired, and the network device may select the first and second precoding matrixes by the following: determining the terminal device pair comprising more than one terminal device from the plurality of terminal devices, wherein a third correlation between a third terminal device and a fourth terminal device, among the more than one terminal devices, is below the first threshold; and generating a high correlation group including the first terminal device and the second terminal device, based on determining that the first correlation is above or equal to the first threshold.
[0097] In some embodiments, the network device may select, from a first plurality of matrixes, a third precoding matrix associated with the first and second terminal devices in the high correlation group. The third precoding matrix comprises a first sub-matrix and a second sub-matrix which are orthogonal.
[0098] In some embodiments, the network device may select the third precoding matrix by the following: determine, for the first terminal device and the second terminal device, a sum of quality metrics of received signals associated with a matrix in the first plurality of matrixes; and select the third precoding matrix from the first plurality of matrixes by maximizing the sum of quality metrics.
[0099] In some embodiments, the network device may select the third matrix by the following: determining that at least one of the first terminal device and the second terminal device are non-coherent terminal device; and selecting the third matrix from a portion of the first plurality of matrix.
[0100] In some embodiments, the network device may select the third matrix by the following: determining that the first terminal device and the second terminal device are coherent terminal devices; and selecting the third matrix from the first plurality of matrix.
[0101] In some embodiments, the sum of quality metrics is determined based on a channel gain for at least one of the first terminal device and a second terminal device, and the channel gain is determined based on a matrix in the first plurality of matrixes and channel estimation matrix.
[0102] In some embodiments, the network device may select the first and second precoding matrixes by the following: selecting, from a second plurality of precoding matrixes, the first precoding matrix corresponding to the first sub-matrix in the third precoding matrix, and the second precoding matrix corresponding to the second sub-matrix in the third precoding matrix, wherein the second plurality of precoding matrix is predefined.
[0103] In some embodiments, the network device may further transmit, to the first terminal device, a first indication of the first precoding matrix; and transmit, to the second terminal device, a second indication of the second precoding matrix.
[0104] In some embodiments, an apparatus capable of performing any of the method 700 (for example, the terminal device) may comprise means for determining that a first correlation between a first terminal device and a second terminal device is above or equal to a first threshold; means for selecting a first precoding matrix for the first terminal device and a second precoding matrix for the second terminal device, wherein a second correlation between the first precoding matrix and the second precoding matrix is below a second threshold; and means for determining, for multiple user-multiple input multiple output (MU-MIMO) communication, a terminal device pair at least comprising the first terminal device and the second terminal device.
[0105] In some embodiments, the first precoding matrix and the second precoding matrix are orthogonal.
[0106] In some embodiments, the apparatus may further include means for determining the first correlation which includes: means for determining a third correlation from the first terminal device to the second terminal device; means for determining a fourth correlation from the second terminal device to the first terminal device; and means for determining a larger one among the third correlation and the fourth correlation as the first correlation.
[0107] In some embodiments, at least one of the third correlation and the fourth correlation is determined based on the following: a first interference level from the first terminal device to the second terminal device; and a second interference level from the second terminal device to the first terminal device.
[0108] In some embodiments, the first interference level is determined based on a beamforming vector for the second terminal device and a channel vector of the first terminal device; and the second interference level is determined based on a beamforming vector for the first terminal device and a channel vector of the second terminal device.
[0109] In some embodiments, the first and second terminal devices are from a plurality terminal devices to be paired, and the means for selecting the first and second precoding matrixes may include: means for determining the terminal device pair comprising more than one terminal device from the plurality of terminal devices, wherein a third correlation between a third terminal device and a fourth terminal device, among the more than one terminal devices, is below the first threshold; and means for generating a high correlation group including the first terminal device and the second terminal device, based on determining that the first correlation is above or equal to the first threshold.
[0110] In some embodiments, the apparatus may further include means for selecting, from a first plurality of matrixes, a third precoding matrix associated with the first and second terminal devices in the high correlation group. The third precoding matrix comprises a first sub-matrix and a second sub-matrix which are orthogonal.
[0111] In some embodiments, the means for selecting the third precoding matrix may include: means for determining, for the first terminal device and the second terminal device, a sum of quality metrics of received signals associated with a matrix in the first plurality of matrixes; and means for selecting the third precoding matrix from the first plurality of matrixes by maximizing the sum of quality metrics.
[0112] In some embodiments, the means for selecting the third matrix may include: means for determining that at least one of the first terminal device and the second terminal device are non-coherent terminal device; and means for selecting the third matrix from a portion of the first plurality of matrix.
[0113] In some embodiments, the means for selecting the third matrix may include: means for determining that the first terminal device and the second terminal device are coherent terminal devices; and means for selecting the third matrix from the first plurality of matrix.
[0114] In some embodiments, the sum of quality metrics is determined based on a channel gain for at least one of the first terminal device and a second terminal device, and the channel gain is determined based on a matrix in the first plurality of matrixes and channel estimation matrix.
[0115] In some embodiments, the means for selecting the first and second precoding matrixes may include: means for selecting, from a second plurality of precoding matrixes, the first precoding matrix corresponding to the first sub-matrix in the third precoding matrix, and the second precoding matrix corresponding to the second sub-matrix in the third precoding matrix, wherein the second plurality of precoding matrix is predefined.
[0116] In some embodiments, the apparatus may further include means for transmitting, to the first terminal device, a first indication of the first precoding matrix; and means for transmit, to the second terminal device, a second indication of the second precoding matrix.
[0117] In some embodiments, the apparatus further comprises means for performing other steps in some embodiments of the method 700. In some embodiments, the means comprises 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 apparatus.
[0118] FIG. 8 is a simplified block diagram of a device 800 that is suitable for implementing embodiments of the present disclosure. The device 800 may be provided to implement the communication device, for example the terminal device, the first network device, or the second network device. As shown, the device 800 includes one or more processors 810, one or more memories 820 coupled to the processor 810, and one or more communication modules 840 coupled to the processor 810.
[0119] The communication modules 840 is for bidirectional communications. The communication modules 840 has at least one antenna to facilitate communication. The communication interface may represent any interface that is necessary for communication with other network elements.
[0120] The processor 810 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 multicore processor architecture, as non-limiting examples. The device 800 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.
[0121] The memory 820 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) 824, 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) 822 and other volatile memories that will not last in the power-down duration.
[0122] A computer program 830 includes computer executable instructions that are executed by the associated processor 810. The program 830 may be stored in the ROM 824. The processor 1710 may perform any suitable actions and processing by loading the program 1730 into the RAM 822.
[0123] The embodiments of the present disclosure may be implemented by means of the program 830 so that the device 800 may perform any process of the disclosure as discussed with reference to FIGS. 2 to 7. The embodiments of the present disclosure may also be implemented by hardware or by a combination of software and hardware.
[0124] In some embodiments, the program 830 may be tangibly contained in a computer readable medium which may be included in the device 800 (such as in the memory 820) or other storage devices that are accessible by the device 800. The device 800 may load the program 830 from the computer readable medium to the RAM 822 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. FIG. 9 shows an example of the computer readable medium 900 in form of CD or DVD. The computer readable medium has the program 830 stored thereon.
[0125] 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.
[0126] 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 700 as described above with reference to FIG. 7. 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 combined 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.
[0127] 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.
[0128] 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.
[0129] 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. The term “non-transitory, ” as used herein, is a limitation of the medium itself (i.e., tangible, not a signal) as opposed to a limitation on data storage persistency (e.g., RAM vs. ROM) .
[0130] 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 the 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.
[0131] 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.
Claims
1.A network device comprising:at least one processor; andat least one memory storing instructions that, when executed by the at least one processor, cause the network device at least to:determine that a first correlation between a first terminal device and a second terminal device is above or equal to a first threshold;select a first precoding matrix for the first terminal device and a second precoding matrix for the second terminal device, wherein a second correlation between the first precoding matrix and the second precoding matrix is below a second threshold; anddetermine, for multiple user-multiple input multiple output (MU-MIMO) communication, a terminal device pair at least comprising the first terminal device and the second terminal device.2.The network device of claim 1, wherein the first precoding matrix and the second precoding matrix are orthogonal.3.The network device of claim 1 or 2, wherein the network device is further caused to determine the first correlation by the following:determining a third correlation from the first terminal device to the second terminal device;determining a fourth correlation from the second terminal device to the first terminal device; anddetermining a larger one among the third correlation and the fourth correlation as the first correlation.4.The network device of claim 3, wherein at least one of the third correlation and the fourth correlation is determined based on the following:a first interference level from the first terminal device to the second terminal device; anda second interference level from the second terminal device to the first terminal device.5.The network device of claim 4, wherein:the first interference level is determined based on a beamforming vector for the second terminal device and a channel vector of the first terminal device; andthe second interference level is determined based on a beamforming vector for the first terminal device and a channel vector of the second terminal device.6.The network device of any of claims 1 to 5, wherein the first and second terminal devices are from a plurality terminal devices to be paired, and the network device is caused to select the first and second precoding matrixes by the following:determining the terminal device pair comprising more than one terminal device from the plurality of terminal devices, wherein a third correlation between a third terminal device and a fourth terminal device, among the more than one terminal devices, is below the first threshold; andgenerating a high correlation group including the first terminal device and the second terminal device, based on determining that the first correlation is above or equal to the first threshold.7.The network device of claim 6, wherein the network device is further caused to:select, from a first plurality of matrixes, a third precoding matrix associated with the first and second terminal devices in the high correlation group,wherein the third precoding matrix comprises a first sub-matrix and a second sub-matrix which are orthogonal.8.The network device of claim 7, wherein the network device is caused to select the third precoding matrix by the following:determine, for the first terminal device and the second terminal device, a sum of quality metrics of received signals associated with a matrix in the first plurality of matrixes; andselect the third precoding matrix from the first plurality of matrixes by maximizing the sum of quality metrics.9.The network device of claim 8, wherein the network device is caused to select the third matrix by the following:determining that at least one of the first terminal device and the second terminal device are non-coherent terminal device; andselecting the third matrix from a portion of the first plurality of matrix.10.The network device of claim 8, wherein the network device is caused to select the third matrix by the following:determining that the first terminal device and the second terminal device are coherent terminal devices; andselecting the third matrix from the first plurality of matrix.11.The network device of any of claims 8 to 10, wherein:the sum of quality metrics is determined based on a channel gain for at least one of the first terminal device and a second terminal device, andthe channel gain is determined based on a matrix in the first plurality of matrixes and channel estimation matrix.12.The network device of any of claims 7 to 11, wherein the network device is caused to select the first and second precoding matrixes by the following:selecting, from a second plurality of precoding matrixes, the first precoding matrix corresponding to the first sub-matrix in the third precoding matrix, and the second precoding matrix corresponding to the second sub-matrix in the third precoding matrix,wherein the second plurality of precoding matrix is predefined.13.The network device of any of claims 1 to 12, wherein the network device is further caused to:transmit, to the first terminal device, a first indication of the first precoding matrix; andtransmit, to the second terminal device, a second indication of the second precoding matrix.14.A method comprising:determining, at a network device, that a first correlation between a first terminal device and a second terminal device is above or equal to a first threshold;selecting a first precoding matrix for the first terminal device and a second precoding matrix for the second terminal device, wherein a second correlation between the first precoding matrix and the second precoding matrix is below a second threshold; anddetermining, for multiple user-multiple input multiple output (MU-MIMO) communication, a terminal device pair at least comprising the first terminal device and the second terminal device.15.An apparatus comprising:means for determining that a first correlation between a first terminal device and a second terminal device is above or equal to a first threshold;means for selecting a first precoding matrix for the first terminal device and a second precoding matrix for the second terminal device, wherein a second correlation between the first precoding matrix and the second precoding matrix is below a second threshold; andmeans for determining, for multiple user-multiple input multiple output (MU-MIMO) communication, a terminal device pair at least comprising the first terminal device and the second terminal device.16.A computer readable medium comprising program instructions stored thereon for performing at least the method of claim 14.