Channel estimation in the presence of repeaters

Abstract

There is provided techniques for configuring repeaters for channel estimation. A method is performed by a first radio transceiver device. The method comprises configuring at least a first repeater and a second repeater with a configuration comprising complex scale factors to be applied by the repeaters when forwarding pilot signals. The method comprises communicating the pilot signals with a second radio transceiver device over a wireless direct path between the first radio transceiver device and the second radio transceiver device and over wireless indirect paths between the first radio transceiver device and the second radio transceiver device via the wireless repeaters.

Description

[0001]CHANNEL ESTIMATION IN THE PRESENCE OF REPEATERS The project leading to this application has received funding from the European Union’s Horizon 2020 research and innovation programme under grant agreement No 101013425. TECHNICAL FIELD Embodiments presented herein relate to methods, a radio transceiver device, a system comprising repeaters, computer programs, and a computer program product for configuring the repeaters for channel estimation. BACKGROUND In general terms, (cellular) massive multiple-input multiple-output (MIMO) systems, also known as co-located massive MIMO systems can be used to increase connectivity and rate performance in wireless communication systems. Some MIMO system are configured for time-division duplexing (TDD) operation. Some MIMO system are configured to rely on channel reciprocity to obtain downlink (DL) channel estimates at network nodes at the network-side from uplink pilots transmitted by user equipment (UEs) at the user-side. Channel reciprocity is key to harvest most performance gains, since training the channel in the UL requires a pilot overhead that is proportional to the number of antenna ports at the UE side of the link, which is much smaller compared to training the channel in the DL which requires a pilot overhead that is proportional to the number of antenna ports at the network nodes. Moreover, in such reciprocity-based operation, there is no need for feedback of the measured DL channels to the network-side, as is required in systems that perform explicit DL channel estimation such as non-reciprocal frequency-division duplex (FDD) systems. Throughout this disclosure, reciprocity-based massive MIMO systems with TDD operation are considered. One challenge for co-located massive MIMO systems is that of efficiently serving UEs in unfavorable locations. One non-limiting example considers UEs involved in outdoor-to-indoor communications, where indoor UEs are served from a network node located outdoors. Another non-limiting example considers UEs located in rural environments, where a reduced number of channel scatters exits, and where the path loss can be very high. Firstly, there may exist a degradation in terms of coverage. E.g. in outdoor-to-indoor communications, due to an additional path loss from signal propagation through walls and windows, and in rural environments due to long propagation distances and / or electromagnetic shadowing from a large objects (plus lack of alternative scatterers). The former is especially an issue in buildings that have thick walls containing sheet metal and / or concrete reinforcement bars, or windows with energy- saving coatings, or both. Secondly, the richness of the electromagnetic channel may be compromised. E.g. in outdoor-to-indoor communications, if the only significant propagation path from an outdoor network node to an indoor UE is through a window, the resulting channel will be effectively “rank-one”, also known as a key-hole channel, irrespective of how many antennas the network node and UE have, and irrespective of how much local scattering there is around the network node and UE. The issue can be aggravated in a multi-user scenario, where many UEs are served through such a keyhole channel. In the rural environments case, one issue is that there are only few reliable scatterers between the network node and UE. Some technologies that aim at improving coverage (not so much the channel richness / rank) specifically include installation and use of active repeaters that amplify the signal between network nodes and UEs. Some technologies that aim at improving coverage (not so much the channel richness / rank) specifically include installation and use of reflective intelligent surfaces (RIS) that essentially function as repeaters but without amplification (though with some “beam steering" capability). In the case of outdoor-to-indoor communications, some traditional technologies also include using slicing of the energy-coating of the windows to allow radiofrequency waves to pass through without substantial loss. Further, some traditional technologies also include installing waveguides that go through the wall, with an antenna at the outside and at the inside of the building. The herein disclosed embodiments are concerned with the use of repeaters. In general terms, a repeater is a device that amplifies the incoming radio-frequency (RF) field and re-transmits it instantaneously. Different types of repeaters exist. Single-antenna repeaters use the same antenna for reception and transmission. For this purpose, single-antenna repeaters use, e.g., a circulator or similar device to isolate the outgoing wave from the incoming. Some of the advantages associated with single-antenna repeaters are that they are energy-efficient and have small form factors, can be manufactured at low cost, and can installed at low cost with the availability of an electric grid connection as the only requirement. Dual-antenna repeaters, in contrast, have two antenna ports (e.g., two antenna elements or two panels of antennas). At a given point in time, one antenna port is used is used for reception whilst the other is used for transmission; the roles may shift over time to accommodate signal forwarding in both DL and UL directions. While dual-antenna repeaters have certain advantages, they require installation of antennas with sufficient isolation distance (for example, one outdoors and another indoors, in an indoor-to-outdoor coverage application). Consider a single repeater which instantaneously amplifies-and-forwards a signal in the same frequency it has received it. A network receives such a signal from a UE, the signal will have propagated over an effective channel that is the sum of (i) the propagation associated with the direct link from the UE to the network node and (ii) the propagation from UE to the repeater times the repeater’s response times the propagation from the repeater to the network node. In the remainder of this disclosure, this second channel contribution will be referred to as the repeater’s additional channel contribution. According to a first example, this information can be used to beamform away from such a repeater (by placing a null in the direction associated with the repeater using a zero-forcing precoder) in order to avoid creation of undesirably strong signals re- radiated by the repeater (or avoid the additional propagation paths created by such a repeater). For example, assume that the repeater and the UE are located close to a cell border. Then, if it is possible to communicate with the UE without using repeater, then there is no need to use repeater (since the repeater might create inter-cell interference). According to a second example, this information can be used for fault detection and / or to diagnose whether a repeater is working properly. Explained briefly, in order to diagnose whether a repeater works properly, the channel estimate can be tracked over time and analyzed to identify if it fluctuates unduly. In particular, if the repeater is installed in a stationary location in line-of-sight to the network node, its channel should remain constant over time. Anomalous fluctuations can then be discovered by applying a change detection algorithm to the channel response. According to a third example, this information can be used to detect which repeaters contribute substantially towards favorable link conditions to one or more UEs. Here, the physical directions associated with a UE-to-network node and repeater’s associated channels can be compared to determine how different these directions are (if sufficiently different, the repeater contributes to an increased channel rank). According to a fourth example, this information can be used to adaptively configure and, for a given time interval, select some repeaters to operate and select some repeaters to sleep. The regular way of estimating the repeater’s additional channel contribution is to perform a double reference signal (RS) measurement within the coherence time / frequency of the channel. Namely, a measurement with the repeater on (i.e., operating) and a subsequent measurement with the repeater off (i.e., not operating). Here, the measurement taken with repeater off is for estimating the direct UL channel from UE to network node. The result after subtracting one of the estimated channels from the other, corresponds to the desired repeater’s additional channel contribution. If two or more repeaters are deployed in the communication network, the described double measurement method can be applied per repeater whilst having the remaining repeaters of the network switched off. This requires 2^ measurements in total, where ^ is the number of repeaters. Alternatively, one measurement can be performed with all repeaters switched off, and then ^ subsequent measurements, each measurement being associated with one distinct repeater activated. This method requires ^ ^ 1 measurements. One issue with existing methodologies for channel estimation is that the estimation quality of a repeater’s additional channel contribution is limited by the signal-to- noise ratio (SNR) of its respective double-RS measurement. For example, the estimation quality may vary depending on where the repeaters are located in the cell (e.g., cell-edge or not) and depending on he UL power used by the UE for the double measurement. Another issue with existing methodologies is that the repeaters need to be repeatedly switched off and on every time the repeater’s channels need to be estimated. SUMMARY An object of embodiments herein is to address the above issues. A particular object is to provide channel estimation in the presence of repeaters without experiencing the above issues. A particular object is to provide channel estimation in the presence of repeaters yielding better estimation quality whilst using the same measurement overhead. A particular object is to provide channel estimation in the presence of repeaters without requiring the repeaters to be repeatedly switched off and on every time the repeater’s channels need to be estimated. According to a first aspect there is presented a first radio transceiver device for configuring repeaters for channel estimation. The first radio transceiver device is configured to configure at least a first repeater and a second repeater with a configuration comprising complex scale factors to be applied by the repeaters when forwarding pilot signals. According to the configuration, the first repeater and the second repeater are to apply mutually different complex scale factors at a first instance at which the pilot signals are to be forwarded, and / or the first repeater is to apply mutually different complex scale factors at a first instance and a second instance at which the pilot signals are to be forwarded. The first radio transceiver device is configured to communicate the pilot signals with a second radio transceiver device over a wireless direct path between the first radio transceiver device and the second radio transceiver device and over wireless indirect paths between the first radio transceiver device and the second radio transceiver device via the wireless repeaters. According to a second aspect there is presented a method for configuring repeaters for channel estimation. The method is performed by a first radio transceiver device. The method comprises configuring at least a first repeater and a second repeater with a configuration comprising complex scale factors to be applied by the repeaters when forwarding pilot signals. According to the configuration, the first repeater and the second repeater are to apply mutually different complex scale factors at a first instance at which the pilot signals are to be forwarded, and / or the first repeater is to apply mutually different complex scale factors at a first instance and a second instance at which the pilot signals are to be forwarded. The method comprises communicating the pilot signals with a second radio transceiver device over a wireless direct path between the first radio transceiver device and the second radio transceiver device and over wireless indirect paths between the first radio transceiver device and the second radio transceiver device via the wireless repeaters. According to a third aspect there is presented a computer program for configuring repeaters for channel estimation. The computer program comprises computer code which, when run on processing circuitry of a radio transceiver device, causes the radio transceiver device to perform actions. One action comprises the radio transceiver device to configure at least a first repeater and a second repeater with a configuration comprising complex scale factors to be applied by the repeaters when forwarding pilot signals. According to the configuration, the first repeater and the second repeater are to apply mutually different complex scale factors at a first instance at which the pilot signals are to be forwarded, and / or the first repeater is to apply mutually different complex scale factors at a first instance and a second instance at which the pilot signals are to be forwarded. One action comprises the radio transceiver device to communicate the pilot signals with a second radio transceiver device over a wireless direct path between the first radio transceiver device and the second radio transceiver device and over wireless indirect paths between the first radio transceiver device and the second radio transceiver device via the wireless repeaters. According to a fourth aspect there is presented a system comprising at least a first repeater and a second repeater for channel estimation. Each of the repeaters is configured to receive, from a first radio transceiver device, a configuration comprising complex scale factors to be applied by the repeaters when forwarding pilot signals. According to the configuration, the first repeater and the second repeater are to apply mutually different complex scale factors at a first instance at which the pilot signals are to be forwarded, and / or the first repeater is to apply mutually different complex scale factors at a first instance and a second instance at which the pilot signals are to be forwarded. Each of the repeaters is configured to forward the pilot signals between the first radio transceiver device and a second radio transceiver device whilst applying the scale factors in accordance with the configuration. According to a fifth aspect there is presented a method for configuring repeaters for channel estimation. The method is performed by a system comprising at least a first repeater and a second repeater. The method comprises receiving, by each of the repeaters and from a first radio transceiver device, a configuration comprising complex scale factors to be applied by the repeaters when forwarding pilot signals. According to the configuration, the first repeater and the second repeater are to apply mutually different complex scale factors at a first instance at which the pilot signals are to be forwarded, and / or the first repeater is to apply mutually different complex scale factors at a first instance and a second instance at which the pilot signals are to be forwarded. The method comprises forwarding, by each of the repeaters, the pilot signals between the first radio transceiver device and a second radio transceiver device whilst applying the scale factors in accordance with the configuration. According to a sixth aspect there is presented a computer program for configuring repeaters for channel estimation. The computer program comprises computer code which, when run on processing circuitry of a system comprising repeaters, causes the repeaters to perform actions. One action comprises each of the repeaters to receive, from a first radio transceiver device, a configuration comprising complex scale factors to be applied by the repeaters when forwarding pilot signals. According to the configuration, the first repeater and the second repeater are to apply mutually different complex scale factors at a first instance at which the pilot signals are to be forwarded, and / or the first repeater is to apply mutually different complex scale factors at a first instance and a second instance at which the pilot signals are to be forwarded. One action comprises each of the repeaters to forward the pilot signals between the first radio transceiver device and a second radio transceiver device whilst applying the scale factors in accordance with the configuration. According to a seventh aspect there is presented a computer program product comprising a computer program according to at least one of the third aspect and the sixth aspect and a computer readable storage medium on which the computer program is stored. The computer readable storage medium could be a non-transitory computer readable storage medium. Advantageously, these aspects address the above issues. Advantageously, these aspects provide channel estimation in the presence of repeaters without experiencing the above issues, or where the above issues at least are mitigated or reduced. Advantageously, these aspects provide channel estimation in the presence of repeaters yielding better estimation quality whilst using the same measurement overhead. Advantageously, these aspects provide channel estimation in the presence of repeaters without requiring the repeaters to be repeatedly switched off and on every time the repeater’s channels need to be estimated. Since each repeater might be activated more than once during the measurement procedure, and due to the disclosed repeater configurations during the measurement procedure, the procedure allows for better estimation quality of the repeater’s channels with the same (or less) overhead than the above disclosed methodologies. Other objectives, features and advantages of the enclosed embodiments will be apparent from the following detailed disclosure, from the attached dependent claims as well as from the drawings. Generally, all terms used in the claims are to be interpreted according to their ordinary meaning in the technical field, unless explicitly defined otherwise herein. All references to "a / an / the element, apparatus, component, means, module, step, etc." are to be interpreted openly as referring to at least one instance of the element, apparatus, component, means, module, step, etc., unless explicitly stated otherwise. The steps of any method disclosed herein do not have to be performed in the exact order disclosed, unless explicitly stated. BRIEF DESCRIPTION OF THE DRAWINGS The inventive concept is now described, by way of example, with reference to the accompanying drawings, in which: Fig.1 is a schematic diagram illustrating a communication system according to embodiments; Figs.2 and 3 are flowcharts of methods according to embodiments; Fig.4 is a schematic diagram showing functional units of a radio transceiver device according to an embodiment; Fig.5 is a schematic diagram showing functional modules of a radio transceiver device according to an embodiment; Fig.6 is a schematic diagram showing functional units of a repeater according to an embodiment; Fig.7 is a schematic diagram showing functional modules of a repeater according to an embodiment; and Fig.8 shows one example of a computer program product comprising computer readable means according to an embodiment. DETAILED DESCRIPTION The inventive concept will now be described more fully hereinafter with reference to the accompanying drawings, in which certain embodiments of the inventive concept are shown. This inventive concept may, however, be embodied in many different forms and should not be construed as limited to the embodiments set forth herein; rather, these embodiments are provided by way of example so that this disclosure will be thorough and complete, and will fully convey the scope of the inventive concept to those skilled in the art. Like numbers refer to like elements throughout the description. Any step or feature illustrated by dashed lines should be regarded as optional. For conciseness consider the exemplary communication system 100 in Fig.1 comprising a single-antenna UE 200b, a network node 200a (such as a base station, gNB, access point, access node, integrated access and backhaul node, or the like) with ^ antennas configured for (D-)MIMO operation, and three single-antenna repeaters 300a, 300b, 300c. It is here noted that the role, or functionality, of the UE 200b in this disclosure can also be performed by another network node. The network node 200a can therefore be referred to as a first radio transceiver device 200a whereas the UE 200b can be referred to as a second radio transceiver device 200b. However, this does not necessarily imply that there is any hierarchical relation between the first radio transceiver device 200a and the second radio transceiver device 200b. Let α^be the (complex-valued) response of repeater ^ and let ^ be the signal sent by the UE. The signal ^ here may be the amplitude of a sinusoid, or the (complex-valued) amplitude of a symbol modulated using any modulation scheme known in the art. For example, in an orthogonal frequency-division multiplexing (OFDM) system, the signal can be the amplitude corresponding a particular subcarrier in an OFDM symbol. If the repeaters 300a:300c are located far enough away from each other, such that no significant loopback effects occur, then the noise-free signal received at the network node 200a is ^ ൌ ^^^^ ൫α^^^ோభ^ோభ^^ αଶ^^ோమ^ோమ^^ αଷ^^ோయ^ோయ^൯^ where the ^ ൈ are off (e.g. α^ൌ 0^∀^^), ^ோ^^is the channel from the ^:th repeater to the network node 200a, and ^^ோ^is the channel from the UE 200b to the ^:th repeater. Suppose ^ measurements are taken with different configurations of the repeater gains. The repeaters 300a:300c are configured by the network node 200a with these configurations via, e.g. a control channel. For each measurement, the UE 200b transmits a predetermined (reference) signal, and the network node 200a measures its received signal and estimates the respective channel. For measurement ^, where ^ ൌ 1,… ,^, let ^α௧^^ be the corresponding values of the repeater gains ^α^^. To exemplify the process, let the transmitted signal be ^ ൌ 1 for all t. The noise-free measurements are then, for ^ ൌ 1,… ,^,^൯ Each compone o^^a^ோ^ ோ^^co espo g o a ay a e a at the network node) can be separately estimated. To estimate the ^:th component (corresponding to the ^:th base station antenna), it is sufficient to consider the ^:th component of ^௧, say ^௧^, for t = 1, ..., T. On vector form, In the presence of a least-squares solution that minimizes^‖^^^െ ^^^‖ with be used. If the matrix ^ has linearly independent columns (which requires T ≥ 4), this least-squares solution is ^^^ൌ ^^ு^^ି^^^^^where ^ுdenotes the post processing approaches, e.g., based on the minimum-mean squared error (MMSE) criterium, are possible. In order to be able to estimate ^^the entries in the first column of the repeaters’ gain matrix ^ do not necessarily need to equal 1, but this simplification will be used when deriving a solution to the problem since it does not compromise the optimality of the solution. It is here noted that the methodology of using ^ ^ 1 measurements can also be described under the current matrix formulation, but constrained to have repeaters’ gain matrices ^ of the form1^Next, designs of ^^^^or op ma per ormance w e sc ose . The embodiments disclosed herein in particular relate to techniques for configuring repeaters 300a:300c for channel estimation. In order to obtain such techniques, there is provided a radio transceiver device 200a, a method performed by the radio transceiver device 200a, a computer program product comprising code, for example in the form of a computer program, that when run on processing circuitry of the radio transceiver device 200a, causes the radio transceiver device 200a to perform the method. In order to obtain such techniques, there is further provided a system comprising repeaters 300a:300c, a method performed by the repeaters 300a:300c, and a computer program product comprising code, for example in the form of a computer program, that when run on processing circuitry of the repeaters 300a:300c, causes the repeaters 300a:300c to perform the method. Reference is now made to Fig.2 illustrating a method for configuring repeaters 300a:300c for channel estimation as performed by the radio transceiver device 200a according to an embodiment. S102: The first radio transceiver device 200a configures at least a first repeater 300a and a second repeater 300b with a configuration. The configuration comprises complex scale factors to be applied by the repeaters 300a:300c when forwarding pilot signals. According to the configuration, the first repeater 300a and the second repeater 300b are to apply mutually different complex scale factors at a first instance at which the pilot signals are to be forwarded, and / or the first repeater 300a is to apply mutually different complex scale factors at a first instance and a second instance at which the pilot signals are to be forwarded. That is, the complex scale factors are different from each other. Examples of in what way the complex scale factors can be different from each other will be disclosed below. S104: The first radio transceiver device 2 0a communicates the pilot signals with a second radio transceiver device 200b over a wireless direct path between the first radio transceiver device 200a and the second radio transceiver device 200b and over wireless indirect paths between the first radio transceiver device 200a and the second radio transceiver device 200b via the wireless repeaters 300a:300c. Embodiments relating to further details of configuring repeaters 300a:300c for channel estimation as performed by the radio transceiver device 200a will now be disclosed with continued reference to Fig.2. In general terms, there are ^ repeaters 300a:300c. The configuration might thencomprise ^ ^ 1 different complex scale factors per each repeater 300a:300c, and ^ ^1 pilot signals might be communicated with the second radio transceiver device200b. According to a first example, for a first instance repeater R1 is to use a first configuration and a second repeater R2 is to use a second configuration. That is, in some embodiments, according to the configuration the first repeater 300a is to apply a first complex scale factor at the first instance at which the pilot signals are forwarded, and wherein according to the configuration the second repeater 300b is to apply a second complex scale factor at the first instance at which the pilot signals are forwarded. According to a second example, for a first instance a repeater R1 is to use a first configuration and for a second instance the repeater R1 is to use a second configuration. That is, in some embodiments, according to the configuration the first repeater 300a is to apply a first complex scale factor at the first instance at which the pilot signals are communicated, and wherein according to the configuration the first repeater 300a is to apply a second complex scale factor at the second instance at which the pilot signals are communicated. In some examples, the matrix ^ thus either has at least two non-zero ^:s in one of its columns (as in the first example), or at least two non-zero ^:s in one of its rows (as in the second example). Therefore, in some aspects, each of the complex scale factors has an instance index and a repeater index. The instance index represents the instance at which the complex scale factor is to be applied. The repeater index represents for which repeater the comple scale factor is to be applied. According to some embodiments, according to the configuration at least two complex scale factors have one instance index and / or repeater index in common. However, just because the least two complex scale factors have one instance index and / or repeater index in common does not mean that their values are the same; the condition that the complex scale factors are different from each other still applies. Further aspects of designing ^^௧^^ for optimal performance will be disclosed next. If ^^contains additive, white, measurement noise, then the sum of the variances of the components of ^^^is equal to ସ ^ var^^^^^^^^ ൌ Tr^^^ு^^ି^^ Suppose the norm of each gain matrix ^ is to be constrained such that ‖^ଶ‖ଶ^ ^ ^ ^^, ‖^ସ‖ଶ^ ^ for some ^ , , ^ ^ ^^ ^ ^that correspond to gain at the three repeaters 300a:300c. The first column has norm ^ by construction. The UE’s 200b power control is included in ^.Under this constraint, it can be shows that Tr^^^ு^^ି^^is minimized when thecolumns of ^ are mutually orthogonal. In embodiments, the complex scale factors impose a signed amplitude scaling at the repeaters 300a:300c. One special case of this is where the repeaters’ 300a:300c gain matrix ^ is taken to be the four first columns of a Hadamard matrix, and scaling these columns appropriately. Hence, in some embodiments, the complex scale factors for each configuration equals a respective column of a Hadamard matrix. This has the advantage of all three repeaters 300a:300c being always active during the measurement procedure and always operating with their nominal amplitude gain, only shifting the phase by 180 degrees. Thus, in some embodiments, the complex scale factors impose a phase shift at the repeaters 300a:300c. For example, for ^^ ൌ ^4, and for T = 8, In some aspects, the are equal to the columns of a unitary matrix. One example of a unitary matrix is a Discrete Fourier Transform matrix. Therefore, in some embodiments, the complex scale factors for each configuration equals a respective column of a Discrete Fourier Transform matrix. Hence, the matrix ^ might be taken to be the four first columns of a discrete Fourier- transform matrix. In some embodiments, for ^ repeaters 300a:300c and ^ instances, the complex scale factors for the repeaters 300a:300c are arrangeable in a matrix ^ according to: ^^^ ^^ଶ ^^^ۍ1⋯ ^ଶ^ ^ଶଶ ⋯ ^ ېۑ ۑ ۑ where ^^^is the complex ^ at instance ^. In the columns of the matrix ^ are mutually orthogonal. In some embodiments, the instance is a time instance. For example, the different measurements might be taken at different points in time, for example, during different OFDM symbols (but within estimated coherence time of channel). Hence, in some embodiments, the time instance corresponds to at least one OFDM symbol within a coherence time of the wireless indirect path. The repeaters 300a:300c might then need to be reconfigured prior to eac measurement (e.g., within the cyclic-prefix interval) in order to change their amplification gains. In some aspects, if the repeaters 300a:300c are capable of configuring their gains to be frequency-dependent (for example, if the repeaters 300a:300c are capable of sampling, down-converting, performing digital signal processing, upconverting and forwarding the signals), the measurements may be taken at different points in the frequency domain, such that the measurement index ^ corresponds to different frequency points (for example, different subcarriers). Therefore, in some embodiments, the instance is a frequency instance, such as a subcarrier. In some aspects, the multiple estimates ^^^(for the ^:th antenna array) are taken over some predetermined period of time, and the component-wise mean-square value of these measurements are computed (one component for the direct link and one for each repeater 300a:300c). The so-obtained mean-square values (one for the direct link and one for each repeater 300a:300c) can be further averaged over the antenna index ^ to obtain the average (power) path gain between each individual repeater 300a:300c and the antenna array at the network node 200a. Once the pilot signals have been communicated, the network node 200a might perform the channel estimation. In particularly, in some embodiments, the first radio transceiver device 200a further is configured to perform (optional) step S106. S106: The first radio transceiver device 200a performs channel estimation for the wireless indirect paths based on measurements made on the pilot signals communicated over the wireless direct path and the wireless indirect path. Alternatively, the channel estimation is delegated to another network entity, such as the UE 200b, depending on the direction in which the pilot signals are communicated. In some embodiments, performing the channel estimation involves estimating complex-valued channel vectors of the wireless indirect paths. In some aspects, the magnitude of the en ries of the involved channel vectors are estimated, namely |^^^| andห^^ோ^^ோ^^ห. Hence, in some embodiments, performing the channel estimation involves estimating amplitudes of channel vectors of the wireless indirect paths. The value of such channel magnitudes might be averaged over a certain frequency band, for example 1^ ேand 1^ ே^ห^^ோ^^^^^ோ^^^^^หcan be estimated, where ^ index. Whilst the magnitudes of these channel estimates do not provide as much channel information as the complex-valued channel estimates ^^^and ^^ோ^^ோ^^, they are still useful for many of the above listed applications (e.g., repeater fault detection, etc). Here, the network node 200a may instead take into account the magnitude of the measurements, namely |^^^|, … , |^்^|, to conduct the post-processing. In this case, the entries of the repeaters’ gain matrix ^ are required to be real and positive, i.e., only repeater configurations in the power domain are desirable (since phase changesare not possible to detect in |^^^|, … , |^்^|). Based on the measurements |^^^|, … ,|^்^|, similar post processing schemes at the ones previously described can beperformed to estimate |^^^| and ห^^ோ^^ோ^^ห. One advantage of this example is that the entire measurement procedure does not need to be executed within the coherent time of the channel (which basically corresponds to the time it takes for the phase of the channel to change significantly) but rather on a slower time basis, e.g., proportional to the large-scale parameters of the channel. As disclosed above, pilot signals are in S104 communicated between the first radio transceiver device 200a and the second radio transceiver device 200b. In some aspects the first radio transceiver device 200a transmits the pilot signals towards the second transceiver device 200b and the repeaters 300a:300c. That is, in some embodiments, the pilot signals are communicated by being transmitted by the first radio transceiver device 200a towards the second radio transceiver device 200b and the repeaters (300a:300c). Aspects of estimating the repeater channels from downlink measurements at the UE 200b will be disclosed next. In some aspects the first radio transceiver device 200a receives the pilot signals from the second transceiver device 200b and the repeaters 300a:300c. That is, in some embodiments, the pilot signals are communicated by being received by the first radio transceiver device 200a from the second radio transceiver device 200b and the repeaters 300a:300c. Instead of having the UE 200b transmit predetermined signals and estimating the channels at the network node 200a, the network node 200a could instead transmit pilot signals and the UE 200b could estimate the channels. Advantageously, the network node 200a can then transmit orthogonal pilot signals from each antenna. The UE 200b may then feed back the estimated channels to the network node 200a via a CSI feedback report. Reference is now made to Fig.3 illustrating a method for repeaters 300a:300c to be configured for channel estimation as performed by a system comprising the repeaters 300a:300c according to an embodiment. S202: Each of the repeaters 300a:300c receives, from the first radio transceiver device 200a, a configuration comprising complex scale factors to be applied by the repeaters 300a:300c when forwarding pilot signals. As disclosed above, according to the configuration, the first repeater 300a and the second repeater 300b are to apply mutually different complex scale factors at a first instance at which the pilot signals are to be forwarded, and / or the first repeater 300a is to apply mutually different complex scale factors at a first instance and a second instance at which the pilot signals are to be forwarded. S204: Each of the repeaters 300a:300c forwards the pilot signals between the first radio transceiver device 200a and a second radio transceiver device 200b whilst applying the scale factors in accordance with the configuration. Embodiments relating to further details o the repeaters 300a:300c to be configured for channel estimation will now be disclosed. In general terms, the embodiments, aspects, and examples as disclosed above with respect to the method as performed by the first radio transceiver device 200a apply also for the methods performed by each of the repeaters 300a:300c. As disclosed above, in some embodiments, according to the configuration the first repeater 300a is to apply a first complex scale factor at the first instance at which the pilot signals are forwarded, and according to the configuration the second repeater 300b is to apply a second complex scale factor at the first instance at which the pilot signals are forwarded. As disclosed above, in some embodiments, according to the configuration the first repeater 300a is to apply a first complex scale factor at the first instance at which the pilot signals are communicated, and according to the configuration the first repeater 300a is to apply a second complex scale factor at the second instance at which the pilot signals are communicated. As disclosed above, in some embodiments, each of the complex scale factors has an instance index and a repeater index, wherein the instance index represents the instance at which the complex scale factor is to be applied, wherein the repeater index represents for which repeater the complex scale factor is to be applied, and wherein according to the configuration at least two complex scale factors have one instance index and / or repeater index in common. As disclosed above, in some embodiments, the complex scale factors impose a signed amplitude scaling at the repeaters 300a:300c. As disclosed above, in some embodiments, the complex scale factors for each configuration equals a respective column of a Hadamard matrix. As disclosed above, in some embodiments, the complex scale factors impose a phase shift at the repeaters 300a:300c. As disclosed above, in some embodiments, the complex scale factors for each configuration equals a respective column of a Discrete Fourier Transform matrix. As disclosed above, in some embodiments, for ^ repeaters 300a:300c and ^ instances, the complex scale factors for the repeaters 300a:300c are arrangeable in a matrix ^ according to: ^^^ ^^ଶ ^^^ۍ1⋯ ^ଶ^ ^ ⋯ ^ۑ ۑ ۑ where ^^^is the complex ^ at instance ^. As disclosed above, in some embodiments, the columns of the matrix ^ are mutually orthogonal. Fig.4 schematically illustrates, in terms of a number of functional units, the components of a radio transceiver device 200a according to an embodiment. Processing circuitry 210 is provided using any combination of one or more of a suitable central processing unit (CPU), multiprocessor, microcontroller, digital signal processor (DSP), etc., capable of executing software instructions stored in a computer program product 810a (as in Fig.8), e.g. in the form of a storage medium 230. The processing circuitry 210 may further be provided as at least one application specific integrated circuit (ASIC), or field programmable gate array (FPGA). Particularly, the processing circuitry 210 is configured to cause the radio transceiver device 200a to perform a set of operations, or steps, as disclosed above. For example, the storage medium 230 may store the set of operations, and the processing circuitry 210 may be configured to retrieve the set of operations from the storage medium 230 to cause the radio transceiver device 200a to perform the set of operations. The set of operations may be provided as a set of executable instructions. Thus the processing circuitry 210 is thereby arranged to execute methods as herein disclosed. The storage medium 230 may also comprise persistent storage, which, for example, can be any single one or combination of magnetic memory, optical memory, solid state memory or even remotely mounted memory. The radio transceiver device 200a may further comprise a communications (comm.) interface 220 for communications with other entities, functions, nodes, and devices, in the communication system 100. As such the communications interface 220 may comprise one or more transmitters and receivers, comprising analogue and digital components. The processing circuitry 210 controls the general operation of the radio transceiver device 200a e.g. by sending data and control signals to the communications interface 220 and the storage medium 230, by receiving data and reports from the communications interface 220, and by retrieving data and instructions from the storage medium 230. Other components, as well as the related functionality, of the radio transceiver device 200a are omitted in order not to obscure the concepts presented herein. Fig.5 schematically illustrates, in terms of a number of functional modules, the components of a radio transceiver device 200a according to an embodiment. The radio transceiver device 200a of Fig.5 comprises a number of functional modules; a configure module 210a configured to perform step S102, and a communicate module 210b configured to perform step S104. radio transceiver device 200a of Fig.5 may further comprise a number of optional functional modules, such as any an estimate module 210c configured to perform step S106. In general terms, each functional module 210a:210c may be implemented in hardware or in software. Preferably, one or more or all functional modules 210a:210c may be implemented by the processing circuitry 210, possibly in cooperation with the communications interface 220 and / or the storage medium 230. The processing circuitry 210 may thus be arranged to from the storage medium 230 fetch instructions as provided by a functional module 210a:210c and to execute these instructions, thereby performing any steps of the radio transceiver device 200a as disclosed herein. The radio transceiver device 200a may be provided as a standalone device or as a part of at least one further device. For example, part of the functionality of the radio transceiver device 200a may be provided in a node of a radio access network or in a node of a core network. Alternatively, the functionality of the radio transceiver device 200a may be distributed between at least two devices, or nodes. These at least two nodes, or devices, may either be part of the same network part (such as the radio access network or the core network) or may be spread between at least two such network parts. In general terms, instructions that are required to be performed in real time may be performed in a device, or node, operatively closer to the cell than instructions that are not required to be performed in real time. Thus, a first portion of the instructions performed by the radio transceiver device 200a may be executed in a first device, and a second portion of the instructions performed by the radio transceiver device 200a may be executed in a second device; the herein disclosed embodiments are not limited to any particular number of devices on which the instructions performed by the radio transceiver device 200a may be executed. Hence, the methods according to the herein disclosed embodiments are suitable to be performed by a radio transceiver device 200a residing in a cloud computational environment. Therefore, although a single processing circuitry 210 is illustrated in Fig.4 the processing circuitry 210 may be distributed among a plurality of devices, or nodes. The same applies to the functional modules 210a:210c of Fig.5 and the computer program 820a of Fig.8. Some (radio) access network architectures define network nodes (or gNBs) comprising multiple component parts or nodes: a central unit (CU), one or more distributed units (DUs), and one or more radio units (RUs). The protocol layer stack of the network node is divided between the CU, the DUs and the RUs, with one or more lower layers of the stack implemented in the RUs, and one or more higher layers of the stack implemented in the CU and / or DUs. The CU is coupled to the DUs via a fronthaul higher layer split (HLS) network; the CU / DUs are connected to the RUs via a fronthaul lower-layer split (LLS) network. The DU may be combined with the CU in some embodiments, where a combined DU / CU may be referred to as a CU or simply a baseband unit. A communication link for communication of user data messages or packets between the RU and the baseband unit, CU, or DU is referred to as a fronthaul network or interface. Messages or packets may be transmitted from the network node 200 in the downlink (i.e., from the CU to the RU) or received by the network node 200 in the uplink (i.e., from the RU to the CU). Fig.6 schematically illustrates, in terms of a number of functional units, the components of a repeater 300a:300c according to an embodiment. Processing circuitry 310 is provided using any combination of one or more of a suitable central processing unit (CPU), multiprocessor, microcontroller, digital signal processor (DSP), etc., capable of executing software instructions stored in a computer program product 810b (as in Fig.8), e.g. in the form of a storage medium 330. The processing circuitry 310 may further be provided as at least one application specific integrated circuit (ASIC), or field programmable gate array (FPGA). Particularly, the processing circuitry 310 is configured to cause the repeater 300a:300c to perform a set of operations, or steps, as disclosed above. For example, the storage medium 330 may store the set of operations, and the processing circuitry 310 may be configured to retrieve the set of operations from the storage medium 330 to cause the repeater 300a:300c to perform the set of operations. The set of operations may be provided as a set of executable instructions. Thus the processing circuitry 310 is thereby arranged to execute methods as herein disclosed. The storage medium 330 may also comprise persistent storage, which, for example, can be any single one or combination of magnetic memory, optical memory, solid state memory or even remotely mounted memory. The repeater 300a:300c may further comprise a communications interface 320 for communications with other entities, functions, nodes, and devices, in the communication system 100. As such the communications interface 320 may comprise one or more transmitters and receivers, comprising analogue and digital components. The processing circuitry 310 controls the general operation of the repeater 300a:300c e.g. by sending data and control signals to the communications interface 320 and the storage medium 330, by receiving data and reports from the communications interface 320, and by retrieving data and instructions from the storage medium 330. Other components, as well as the related functionality, of the repeater 300a:300c are omitted in order not to obscure the concepts presented herein. Fig.7 schematically illustrates, in terms of a number of functional modules, the components of a repeater 300a:300c according to an embodiment. The repeater 300a:300c of Fig.7 comprises a number of functional modules; a receive module 310a configured to perform step S202, and a forward module 310b configured to perform step S204. The repeater 300a:300c of Fig.7 may further comprise a number of optional functional modules, as represented by functional module 310c. In general terms, each functional module 310a:310c may be implemented in hardware or in software. Preferably, one or more or all functional modules 210a:210c may be implemented by the processing circuitry 10, possibly in cooperation with the communications interface 320 and / or the storage medium 330. The processing circuitry 310 may thus be arranged to from the storage medium 330 fetch instructions as provided by a functional module 310a:310c and to execute these instructions, thereby performing any steps of the repeater 300a:300c as disclosed herein. Fig.8 shows one example of a computer program product 810a, 810b comprising computer readable means 830. On this computer readable means 830, a computer program 820a can be stored, which computer program 820a can cause the processing circuitry 210 and thereto operatively coupled entities and devices, such as the communications interface 220 and the storage medium 230, to execute methods according to embodiments described herein. The computer program 820a and / or computer program product 810a may thus provide means for performing any steps of the radio transceiver device 200a as herein disclosed. On this computer readable means 830, a computer program 820b can be stored, which computer program 820b can cause the processing circuitry 310 and thereto operatively coupled entities and devices, such as the communications interface 320 and the storage medium 330, to execute methods according to embodiments described herein. The computer program 820b and / or computer program product 810b may thus provide means for performing any steps of the repeater 300a:300c as herein disclosed. In the example of Fig.8, the computer program product 810a, 810b is illustrated as an optical disc, such as a CD (compact disc) or a DVD (digital versatile disc) or a Blu- Ray disc. The computer program product 810a, 810b could also be embodied as a memory, such as a random access memory (RAM), a read-only memory (ROM), an erasable programmable read-only memory (EPROM), or an electrically erasable programmable read-only memory (EEPROM) and more particularly as a non-volatile storage medium of a device in an external memory such as a USB (Universal Serial Bus) memory or a Flash memory, such as a compact Flash memory. Thus, while the computer program 820a, 820b is here schematically shown as a track on the depicted optical disk, the computer program 820a, 820b can be stored in any way which is suitable for the computer program product 810a, 810b. The inventive concept has mainly been described above with reference to a few embodiments. However, as is readily appreciated by a person skilled in the art, other embodiments than the ones disclosed above are equally possible within the scope of the inventive concept, as defined by the appended patent claims.

Claims

CLAIMS 1. A first radio transceiver device (200a) for configuring repeaters (300a:300c) for channel estimation, wherein the first radio transceiver device (200a) is configured to: configure at least a first repeater (300a) and a second repeater (300b) with a configuration comprising complex scale factors to be applied by the repeaters (300a:300c) when forwarding pilot signals, wherein according to the configuration, the first repeater (300a) and the second repeater (300b) are to apply mutually different complex scale factors at a first instance at which the pilot signals are to be forwarded, and / or the first repeater (300a) is to apply mutually different complex scale factors at a first instance and a second instance at which the pilot signals are to be forwarded; and communicate the pilot signals with a second radio transceiver device (200b) over a wireless direct path between the first radio transceiver device (200a) and the second radio transceiver device (200b) and over wireless indirect paths between the first radio transceiver device (200a) and the second radio transceiver device (200b) via the wireless repeaters (300a:300c).

2. The first radio transceiver device (200a) according to claim 1, wherein according to the configuration the first repeater (300a) is to apply a first complex scale factor at the first instance at which the pilot signals are forwarded, and wherein according to the configuration the second repeater (300b) is to apply a second complex scale factor at the first instance at which the pilot signals are forwarded.

3. The first radio transceiver device (200a) according to claim 1 or 2, wherein according to the configuration the first repeater (300a) is to apply a first complex scale factor at the first instance at which the pilot signals are communicated, and wherein according to the configuration the first repeater (300a) is to apply a second complex scale factor at the second instance at which the pilot signals are communicated.

4. The first radio transceiver device (200a) according to any preceding claim, wherein each of the complex scale factors has an instance index and a repeater index, wherein the instance index represents the instance at which the complex scale factoris to be applied, wherein the repeater index represents for which repeater the complex scale factor is to be applied, and wherein according to the configuration at least two complex scale factors have one instance index and / or repeater index in common.

5. The first radio transceiver device (200a) according to any preceding claim, wherein the complex scale factors impose a signed amplitude scaling at the repeaters (300a:300c).

6. The first radio transceiver device (200a) according to claim 5, wherein the complex scale factors for each configuration equals a respective column of a Hadamard matrix.

7. The first radio transceiver device (200a) according to any of claims 1 to 4, wherein the complex scale factors impose a phase shift at the repeaters (300a:300c).

8. The first radio transceiver device (200a) according to claim 7, wherein the complex scale factors for each configuration equals a respective column of a Discrete Fourier Transform matrix.

9. The first radio transceiver device (200a) according to any preceding claim, wherein for ^ repeaters (300a:300c) and ^ instances, the complex scale factors for the repeaters (300a:300c) are arrangeable in a matrix ^ according to: 1^^^ ^^ଶ⋯^^^ۍ1 ^ଶ^ ^ଶଶ ⋯ ^ଶ^ې^⋱ ଷ^ ۑ⋮ۑۑ ^்^ے where ^^^is the complexat repeater ^ at instance ^.first radio transceiver device (200a) according to claim 9, wherein columns of the matrix ^ are mutually orthogonal.

11. The first radio transceiver device (200a) according to any preceding claim, wherein the first radio transceiver device (200a) further is configured to:perform channel estimation for the wireless indirect paths based on measurements made on the pilot signals communicated over the wireless direct path and the wireless indirect path.

12. The first radio transceiver device (200a) according to claim 11, wherein performing the channel estimation involves estimating complex-valued channel vectors of the wireless indirect paths.

13. The first radio transceiver device (200a) according to claim 10, wherein performing the channel estimation involves estimating amplitudes of channel vectors of the wireless indirect paths.

14. The first radio transceiver device (200a) according to any preceding claim, wherein the instance is a time instance.

15. The first radio transceiver device (200a) according to claim 14, wherein the time instance corresponds to at least one OFDM symbol within a coherence time of the wireless indirect path.

16. The first radio transceiver device (200a) according to any of claims 1 to 13, wherein the instance is a frequency instance, such as a subcarrier.

17. The first radio transceiver device (200a) according to any preceding claim, wherein the pilot signals are communicated by being transmitted by the first radio transceiver device (200a) towards the second radio transceiver device (200b) and the repeaters (300a:300c).

18. The first radio transceiver device (200a) according to of claims 1 to 16, wherein the pilot signals are communicated by being received by the first radio transceiver device (200a) from the second radio transceiver device (200b) and the repeaters (300a:300c).

19. The first radio transceiver device (200a) according to of claims 1 to 16, wherein there are ^ repeaters (300a:300c), wherein the configuration comprises ^ ^ 1 different complex scale factors per each repeater (300a:300c), and wherein ^ ^ 1 pilot signals are communicated with the second radio transceiver device (200b).

20. A system comprising at least a first epeater (300a) and a second repeater (300b) for channel estimation, wherein each of the repeaters (300a:300c) is configured to: receive, from a first radio transceiver device (200a), a configuration comprising complex scale factors to be applied by the repeaters (300a:300c) when forwarding pilot signals, wherein according to the configuration, the first repeater (300a) and the second repeater (300b) are to apply mutually different complex scale factors at a first instance at which the pilot signals are to be forwarded, and / or the first repeater (300a) is to apply mutually different complex scale factors at a first instance and a second instance at which the pilot signals are to be forwarded; and forward the pilot signals between the first radio transceiver device (200a) and a second radio transceiver device (200b) whilst applying the scale factors in accordance with the configuration.

21. The system according to claim 20, wherein according to the configuration the first repeater (300a) is to apply a first complex scale factor at the first instance at which the pilot signals are forwarded, and wherein according to the configuration the second repeater (300b) is to apply a second complex scale factor at the first instance at which the pilot signals are forwarded.

22. The system according to claim 20 or 21, wherein according to the configuration the first repeater (300a) is to apply a first complex scale factor at the first instance at which the pilot signals are communicated, and wherein according to the configuration the first repeater (300a) is to apply a second complex scale factor at the second instance at which the pilot signals are communicated.

23. The system according to any of claims 20 to 22, wherein each of the complex scale factors has an instance index and a repeater index, wherein the instance index represents the instance at which the complex scale factor is to be applied, wherein the repeater index represents for which repeater the complex scale factor is to be applied, and wherein according to the configuration at least two complex scale factors have one instance index and / or repeater index in common.

24. The system according to of claims to 22, wherein the complex scale factors impose a signed amplitude scaling at the repeaters (300a:300c).

25. The system according to claim 24, wherein the complex scale factors for each configuration equals a respective column of a Hadamard matrix.

26. The system according to any of claims 20 to 24, wherein the complex scale factors impose a phase shift at the repeaters (300a:300c).

27. The system according to claim 26, wherein the complex scale factors for each configuration equals a respective column of a Discrete Fourier Transform matrix.

28. The system according to any of claims 20 to 27, wherein for ^ repeaters (300a:300c) and ^ instances, the complex scale factors for the repeaters (300a:300c) are arrangeable in a matrix ^ according to: ^^^ ^^ଶ ^^^ۍ1⋯ 1^ଶ^ ^ଶଶ ⋯ ^ଶ^ېۑ ۑ ۑ where ^^^is the complex^ at instance ^.

29. The system according to claim 28, wherein columns of the matrix ^ are mutually orthogonal.

30. A method for configuring repeaters (300a:300c) for channel estimation, the method being performed by a first radio transceiver device (200a), the method comprising: configuring (S102) at least a first repeater (300a) and a second repeater (300b) with a configuration comprising complex scale factors to be applied by the repeaters (300a:300c) when forwarding pilot signals, wherein according to the configuration, the first repeater (300a) and the second repeater (300b) are to apply mutually different complex scale factors at a first instance at which the pilot signals are to be forwarded, and / or the first repeater(300a) is to apply mutually different complex scale factors at a first instance and a second instance at which the pilot signals are to be forwarded; and communicating (S104) the pilot signals with a second radio transceiver device (200b) over a wireless direct path between the first radio transceiver device (200a) and the second radio transceiver device (200b) and over wireless indirect paths between the first radio transceiver device (200a) and the second radio transceiver device (200b) via the wireless repeaters (300a:300c).

31. A method for configuring repeaters (300a:300c) for channel estimation, the method being performed by a system comprising at least a first repeater (300a) and a second repeater (300b), the method comprising: receiving (S202), by each of the repeaters (300a:300c) and from a first radio transceiver device (200a), a configuration comprising complex scale factors to be applied by the repeaters (300a:300c) when forwarding pilot signals, wherein according to the configuration, the first repeater (300a) and the second repeater (300b) are to apply mutually different complex scale factors at a first instance at which the pilot signals are to be forwarded, and / or the first repeater (300a) is to apply mutually different complex scale factors at a first instance and a second instance at which the pilot signals are to be forwarded; and forwarding (S204), by each of the repeaters (300a:300c), the pilot signals between the first radio transceiver device (200a) and a second radio transceiver device (200b) whilst applying the scale factors in accordance with the configuration.

32. A computer program (820a) for configuring repeaters (300a:300c) for channel estimation, the computer program comprising computer code which, when run on processing circuitry (210) of a first radio transceiver device (200a), causes the first radio transceiver device (200a) to: configure (S102) at least a first repeater (300a) and a second repeater (300b) with a configuration comprising complex scale factors to be applied by the repeaters (300a:300c) when forwarding pilot signals,wherein according to the configura on, the first repeater (300a) and the second repeater (300b) are to apply mutually different complex scale factors at a first instance at which the pilot signals are to be forwarded, and / or the first repeater (300a) is to apply mutually different complex scale factors at a first instance and a second instance at which the pilot signals are to be forwarded; and communicate (S104) the pilot signals with a second radio transceiver device (200b) over a wireless direct path between the first radio transceiver device (200a) and the second radio transceiver device (200b) and over wireless indirect paths between the first radio transceiver device (200a) and the second radio transceiver device (200b) via the wireless repeaters (300a:300c).

33. A computer program (820b) for configuring repeaters (300a:300c) for channel estimation, the computer program comprising computer code which, when run on processing circuitry (310) of a system comprising at least a first repeater (300a) and a second repeater (300b), causes each of the repeaters (300a:300c) to: receive (S202), from a first radio transceiver device (200a), a configuration comprising complex scale factors to be applied by the repeaters (300a:300c) when forwarding pilot signals, wherein according to the configuration, the first repeater (300a) and the second repeater (300b) are to apply mutually different complex scale factors at a first instance at which the pilot signals are to be forwarded, and / or the first repeater (300a) is to apply mutually different complex scale factors at a first instance and a second instance at which the pilot signals are to be forwarded; and forward (S204) the pilot signals between the first radio transceiver device (200a) and a second radio transceiver device (200b) whilst applying the scale factors in accordance with the configuration.

34. A computer program product (810a, 810b) comprising a computer program (820a, 820b) according to at least one of claims 32 and 33, and a computer readable storage medium (830) on which the computer program is stored.

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