EP4690513A1 - Methods and devices for characterization of a coverage enhancing device - Google Patents

Abstract

A method for characterization of a coverage enhancement device which is capable of beamforming by individually applying one of a finite number of phase shifts to respective elements of an array to forward an electromagnetic wave signal, the method comprising: receiving (404), from a control node, a request indicative of a tuning pattern to apply to the respective elements of the array for remote determination of gain characteristics of the elements; and applying (407), in accordance with said tuning pattern, a change of the phase shift of the respective element to forward a reference signal from a transmitter towards a receiver.

Description

METHODS AND DEVICES FOR CHARACTERIZATION OF A COVERAGE ENHANCING DEVICE Technical field This disclosure relates to the field of panel stations which are used for forwarding a wireless channel by applying configurable phase shifts in an array of elements, to obtain beamforming towards an intended receiver station. Specifically, solutions are proposed for a configuration process usable for optimizing the beamforming. Background In wireless communication, a wireless channel is used to transfer information and data between different nodes acting as transmitter and receiver, using an electromagnetic wave signal. It is therefore beneficial if the wireless channel is constructed to ensure that the signal successfully reaches the receiver. Besides applying sufficient transmit power, an advantageous technology is so-called beamforming, whereby transmitted energy may be focused and directed towards the receiver. Beamforming and beam management has been more frequently considered since the development of the so-called 5G version of wireless communication under supervision of the 3^rd Generation Partnership Project (3GPP), and particularly for use in the mm wave spectrum. A reconfigurable antenna panel comprising an array of a plurality of antenna elements may e.g. be configured at a node, wherein the antenna elements may be suitably fed such that a combined bearer is obtained with suitable directional properties. Another type of device of similar technology is a panel station or forwarding station, herein referred to as a coverage-enhancing device (CED), configured to forward a signal from a transmitter towards a receiver. By way of example, such a CED is sometimes referred to as a Reconfigurable Intelligent Surface (RIS), alternatively a Large Intelligent Surface (LIS). Such CEDs using reconfigurable panels aim to influence the wireless channel in a passive or active way, wherein its elements, arranged in an array, reflect electromagnetic waves with a digitally configurable phase shift, and possibly with gain. Such CEDs are typically designed to reflect impinging electromagnetic waves, though they can also be designed to transmit impinging electromagnetic waves or be transmissive to deflect impinging electromagnetic waves passing through the panel of the CED. In order to relay the signal from the transmitter to the receiver, the CED applies a beamforming pattern. Ideally, this pattern can take any desired form, but in reality, severe restrictions exist. Hardware constraints limit the amount of phase shifts that can be configured into each element of the array in the CED. This limitation implies a finite number of phase shifts of each individual element. Moreover, even if the combined elements of the array are configured to provide beamforming towards an intended direction, variations or imperfections in hardware or variations due to temperature or ageing, such as in individual phase shifters for the elements, may lead to amplitude and/or beam shape variations in the forwarded signal. This may result in varying gain or loss in terms of contribution to the combined forwarded signal. Such amplitude variations may cause performance restriction of the CED since the elements may not provide the signal contribution as intended by the applied phase shift. Therefore, there is a general need for improvement in the field of configurable panels (CEDs) in which the elements are configurable in a finite number of phase shifts to obtain beamforming. Summary The general object outlined above is overcome by providing a solution for analysis and characterization of a CED to obtain knowledge of unequal power loss over its array of elements. The proposed solution is defined by the terms of the independent claims, whereas various additional features are set out in the dependent claims. According to a first aspect, a method is provided for characterization of a coverage enhancement device which is capable of beamforming by individually applying one of a finite number of phase shifts to respective elements of an array to forward an electromagnetic wave signal, the method comprising: receiving, from a control node, a request indicative of a tuning pattern to apply to the respective elements of the array for remote determination of gain characteristics of the elements; and applying, in accordance with said tuning pattern, a change of the phase shift of the respective element to forward a reference signal from a transmitter towards a receiver. According to a second aspect, a CED is provided which is usable for forwarding an electromagnetic wave signal from a transmitter to a receiver. The CED comprises: an array comprising a plurality of elements, wherein each element is capable of selectively applying one of a finite number of phase shifts; a control unit configured to set a phase pattern to the array, wherein the elements individually apply one of said phase shifts to accomplish beamforming; an interface for obtaining a control information from a control node; wherein said control unit is configured to: obtain, from control node using the interface, a request indicative of a tuning pattern to apply to the respective elements of the array for remote determination of gain characteristics of the elements; and apply, in accordance with said tuning pattern, a change of the phase shift of the respective element to forward a reference signal from a transmitter towards a receiver. According to a third aspect, a method carried out in a control station is provided for remote characterization of a coverage enhancing device usable for forwarding an electromagnetic wave signal from a transmitter station to a receiver station by individually applying one of a finite number of phase shifts to respective elements of an array. The method comprises: configuring (403) the coverage enhancing device with a tuning pattern for the array for applying a change to the phase shift; controlling transmission (408) of a reference signal from the transmitter while the coverage enhancing device applies the tuning pattern; obtaining (411) measurements of reception of the forwarded reference signal by the receiver. Upon applying the tuning pattern, forwarded reference signals may be received in a receiver where signal strength can be measured. Based on the tuning pattern, knowledge of gain difference between phase shift states may be obtained, which may be used for computing optimum beamforming coefficient for a codebook to apply to the CED. Brief description of the drawings Various examples will be described with reference to the drawings, in which: Fig.1 schematically illustrates a radio network and a CED usable for conveying electromagnetic wave signals between an access node of the wireless network and further stations, such as a wireless device; Fig.2 schematically illustrates functional elements of a CED comprising an array of elements in accordance with various examples of the proposed solution; Fig.3 schematically illustrates an access node of a wireless network, configured to operate as control node for remote characterization of a CED, in accordance with various examples of the proposed solution; and Fig.4 shows a signaling diagram, illustrating various steps carried out in different examples of methods according to the proposed solution. Detailed description In the following description, for purposes of explanation and not limitation, details are set forth herein related to various examples. However, it will be apparent to those skilled in the art that the present disclosure may be practiced in other examples that depart from these specific details. In some instances, detailed descriptions of well- known devices, circuits, and methods are omitted so as not to obscure the description of the present disclosure with unnecessary detail. The functions of the various elements including functional blocks, including but not limited to those labeled or described as “computer”, “processor” or “controller”, may be provided through the use of hardware such as circuit hardware and/or hardware capable of executing software in the form of coded instructions stored on computer readable medium. Thus, such functions and illustrated functional blocks are to be understood as being either hardware-implemented and/or computer-implemented and are thus machine-implemented. In terms of hardware implementation, the functional blocks may include or encompass, without limitation, digital signal processor (DSP) hardware, reduced instruction set processor, hardware (e.g., digital or analog) circuitry including but not limited to application specific integrated circuit(s) (ASIC), and (where appropriate) state machines capable of performing such functions. In terms of computer implementation, a computer is generally understood to comprise one or more processors or one or more controllers, and the terms computer and processor and controller may be employed interchangeably herein. When provided by a computer or processor or controller, the functions may be provided by a single dedicated computer or processor or controller, by a single shared computer or processor or controller, or by a plurality of individual computers or processors or controllers, some of which may be shared or distributed. Moreover, use of the term “processor” or “controller” shall also be construed to refer to other hardware capable of performing such functions and/or executing software, such as the example hardware recited above. The drawings provide performance plots and are otherwise to be regarded as being schematic, where representations and elements illustrated in the drawings are not necessarily shown to scale. Rather, the various elements are represented such that their function and general purpose become apparent to a person skilled in the art. Any connection or coupling between functional blocks, devices, components, or other physical or functional units shown in the drawings or described herein may also be implemented by an indirect connection or coupling. A coupling between components may also be established over a wireless connection. Functional blocks may be implemented in hardware, firmware, software, or a combination thereof. Generally speaking, the configurable panel according to the proposed solution may be employed for deflecting an incoming electromagnetic wave signal, or signal for short, or for transmitting a signal. In various examples, the configurable panel may also add gain to the signal that is relayed. These types of operation are collectively referred to herein as forwarding, or conveying, the signal between a transmitter and a receiver. Fig.1 schematically illustrates a wireless communication scenario, providing an example of a scene in which the solutions provided herein may be incorporated. A wireless network 100 comprises an access network 120, such as a 5G NR access network, usable for communication over an air interface with further stations, such as the wireless device 1. Such wireless devices are commonly referred to as User Equipment (UE). The access network may comprise a plurality of access nodes or base stations 121, 122, configured to provide a wireless interface for connection to, inter alia, the UE 1. For an NR implementation, the base station may be referred to as a gNB. Each base station comprises a point of transmission and reception, referred to as a Transmission and Reception Point (TRP), which coincides with an antenna of the respective base station. Logic for operating the base station may be configured at the TRP or at another physical location. The wireless network 100 further includes a core network 110, to which the access network 120 is connected. The core network 110 is in turn connected to other communication networks 130, such as the Internet. The UE 1 may be any device operable to wirelessly communicate with the network 100 through the base stations 121, 122, such as a mobile telephone, computer, tablet, a machine to machine (M2M) device, an IoT (Internet of Things) device or other. The UE 1 may be stationary or mobile. A CED 140, such as a RIS, is usable for conveying signals between an access node 121 of the access network 120 and further stations, such as the UE 1. The CED 140 may in this context be used in uplink (UL), wherein the UE 1 acts as transmitter Tx and the access node 121 acts as receiver Rx, and/or in downlink (DL), wherein the access node 121 acts as Tx and the UE 1 acts as Rx. While the drawing of Fig.1 illustrates a scenario where the CED 140 is operated to convey a signal between the access node 121 and the UE 1, it shall be noted that the CED 140 may alternatively be employed for conveying a signal between two UEs, i.e., in a device-to-device (D2D) setup. Fig.2 schematically illustrates various functional elements of the CED 140, which is usable for embodying the proposed solution. The CED 140 comprises at least one array 141 of panel elements 142, wherein each panel element 142 is capable of selectively applying one of a finite number of phase shifts, by means of a connected phase shifter 143. In various examples, the array may further comprise an amplifier circuit 144. A control unit 145 in the CED 140 is connected to set a phase pattern b to the array 141, such that the panel elements 142 individually apply a phase shift by means of the phase shifter 143, so as to obtain beamforming. In this context, beamforming is intended to mean that a signal impinging on the array 141 is selectively steered to a desired direction and shape/width. The control unit 145 comprises logic to operate the phase setting of the array. The logic may include a processing device 146, including one or multiple processors, microprocessors, data processors, co-processors, and/or some other type of component that interprets and/or executes instructions and/or data. The processing device 146 may be implemented as hardware (e.g., a microprocessor, etc.) or a combination of hardware and software (e.g., a system-on-chip (SoC), an application-specific integrated circuit (ASIC), etc.). The processing device 146 may be configured to perform one or multiple operations based on an operating system and/or various applications or programs. The control unit 145 may further include memory storage 147, which may include one or multiple memories and/or one or multiple other types of storage media. For example, the memory storage 147 may include a random access memory (RAM), a dynamic random access memory (DRAM), a cache, a read only memory (ROM), a programmable read only memory (PROM), flash memory, and/or some other type of memory. The memory storage 147 is configured for holding computer program code, which may be executed by the processing device 146, wherein the control unit 145 is configured to control the CED 140 to carry out any of the method steps as provided herein. Software defined by said computer program code may include an application or a program that provides a function and/or a process. The software may include device firmware, an operating system (OS), or a variety of applications that may execute in the logic. The control unit 145 further comprises an interface 148 for obtaining input from a control node. The interface may be a wireless input for communicating with the access node 121, configured as the control node, with which the CED 140 is configured to operate. In another example, the control node may be a separate network node of the wireless network 100. Alternatively, or additionally, the input may comprise a connector for wired input to the control node. Such input may be used to configure the control unit 145 to determine and apply a certain phase pattern b to the array of elements 142. Moreover, as will be explained, the interface 148 may be configured for receiving and reacting to a request for applying a tuning pattern for remote characterization of the CED 140. This may be referred to as operating a characterization mode for the CED 140. Fig.3 schematically illustrates a radio node in the form of an access node 121 of the wireless network 100 as presented herein, and for carrying out various method steps as outlined. In various examples, the access node 121 is a radio base station for operation in the radio communication network 100, to serve one or more radio UEs, such as the UE 1. The access node 121 may comprise a wireless transceiver 313, such as a radio transceiver for communicating with other entities of the radio communication network 100, such as the UE 1. The transceiver 313 may thus include a radio receiver and transmitter for communicating through at least an air interface. The transceiver may comprise a radio modem. The access node 121 may further comprise, or be connected to, an antenna 315, which may include an antenna array. The antenna is connected to the transceiver 313. The access node 121 may, inter alia, operate as a transmitter Tx for transmitting signals to be forwarded by the CED 140, and/or as a receiver Rx for receiving signals forwarded by the CED 140. The access node 121 may have one or more transmission and reception point(s) TRP(s), making use of the antenna 315. The antenna 315 may thus be configured for beamforming, so as to selectively transmit and receive radio signals in the direction of the CED 140. The access node 121 further comprises logic circuitry 310 configured to control the access node 121 to communicate with the UE 10 via the radio transceiver 313 on the physical channel. The logic circuitry 310 may realize a scheduler for scheduling communication of a data set according to the solutions proposed herein, and for configuring the UE to operate according to the scheduling. The logic circuitry 310 may include a processing device 311, including one or multiple processors, microprocessors, data processors, co-processors, and/or some other type of component that interprets and/or executes instructions and/or data. Processing device 311 may be implemented as hardware (e.g., a microprocessor, etc.) or a combination of hardware and software (e.g., a system-on-chip (SoC), an application- specific integrated circuit (ASIC), etc.). The processing device 311 may be configured to perform one or multiple operations based on an operating system and/or various applications or programs. The logic circuitry 310 may further include memory storage 312, which may include one or multiple memories and/or one or multiple other types of storage mediums. For example, memory storage 312 may include a random access memory (RAM), a dynamic random access memory (DRAM), a cache, a read only memory (ROM), a programmable read only memory (PROM), flash memory, and/or some other type of memory. Memory storage 312 may include a hard disk (e.g., a magnetic disk, an optical disk, a magneto-optic disk, a solid state disk, etc.). The memory storage 312 is configured for holding computer program code, which may be executed by the processing device 311, wherein the logic 310 is configured to control the access node 121 to carry out any of the method steps as provided herein. Software defined by said computer program code may include an application or a program that provides a function and/or a process. The software may include device firmware, an operating system (OS), or a variety of applications that may execute in the logic 310. The access node may comprise a scheduler, or scheduling function, which as such may be realized by operation of program code in the logic circuitry 310. The scheduler is operated for allocating resources for communication using the transceiver 313 over radio, such as with the UE 10. Each resource may in this context be indicative of a unit of time and/or frequency of a radio frame structure, according to the established art. The access node 121 may further comprise an interface 316, configured for communication with the core network 110. Methods and devices are proposed herein, which are usable for characterizing CED gain variation between the elements, resulting in amplitude variation of forwarded signals, which may be caused by HW variation for the different elements, such as varying phase shifter performance. At the CED 140, the method includes receiving, from a control node, a request indicative of a tuning pattern for the array for applying a change to the phase shift for the elements 142 for remote characterization to determine gain characteristics of the elements 142. The CED 140 applies the change of the phase shift at respective elements 142 in accordance with said tuning pattern to forward a reference signal from a transmitter station. At a control station, e.g., the access node 121, a method is proposed for remote characterization of the CED 140 which is configured to forward an electromagnetic wave signal from a transmitter Tx to a receiver Rx. The method comprises: configuring the CED 140 with a tuning pattern for the array for applying a change to the phase shift, i.e., to some or all of the elements 142; controlling transmission of a reference signal from the transmitter Tx while the coverage enhancing device applies the tuning pattern; and obtaining measurements of reception of the forwarded reference signal by the receiver Rx. The method thus involves applying a tuning pattern while forwarding of a reference signal from a transmitter Tx, such as the access node 121, to a receiver Rx, such as the UE 1. In an alternative approach, the same station may be configured as both Tx and Rx, such as the access node 121. In this context, the CED 140 is configured to be retrodirective, and this further requires full duplex operation of the Tx/Rx station. By means of the tuning pattern being configured to apply a change to the phase shift for the elements 142 of the array 141, characterization of the elements 142 may be obtained based on measured signal strength obtained in the receiver Rx. An advantage with the proposed solution is that it is a deterministic sequence, where a set of CED states, with different tuning based on the tuning pattern, associated with iterated transmission of reference signals are used. The measurement made of received forwarded signals may be used to determine an optimal CED configuration, such as a codebook for optimized beamforming. Various aspects of the proposed solution are described below with reference to Fig.4, which provides a signaling diagram according to different examples of the proposed methods. The CED 140 is configured to apply one of a finite number of phase shifts to each element 142 of the array 141. As an example, each phase shifter 143 may be quantized with 2 bits, i.e., capable of assuming 4 different phase shift settings, for example, 0°, 90°,180° and 270°. Conceptually, the simplest way to characterize amplitude variation, dependent on phase setting, would be to configure the CED 140 to forward four repeated signals, e.g., Channel State Information Reference Signal (CSI-RS), where all elements are configured to, in sequence, 0°, 90°, 180°, and 270°. The received amplitudes during the four measurements would then directly correspond to the amplitude per phase state. Unfortunately, this simple approach fails in general. The reason being that if all elements are configured with the same phase, then the CED 140 beamforms reflects according to Snell’s law. But if the Rx is not located along that direction, it receives essentially no power, and the estimation fails. To make such a procedure work, it would thus have to be ensured that the Rx is located in an appropriate location, i.e., direction with regard to the Tx, e.g., by co-location of the Tx and the Rx. In contrast, the proposed solution suggests the use of a tuning pattern, which configures the CED 140 for iterated forwarding of the reference signal with CED 140 configurations having the same, or substantially the same, directional properties and center of origin. Fig.4 illustrates a transmitter Tx, such as the access node 121, and a receiver Rx, which e.g., may be the UE 1 or the access node 121. The CED 140 is configured to forward signals at least from the transmitter Tx to the receiver Rx, and typically also in the reverse direction. At 401, the CED 140 may be configured to transmit a capability message for receipt 402 in a control node. The capability message may be transmitted in response to a capability information request, transmitted by the control node. The control node may be configured in or in connection with the transmitter Tx. The capability message is indicative of capability information, either explicitly or implicitly such that the capability information may be obtained in the control node based on the capability message. The capability information may be indicative of the finite number of phase shifts the array is capable of applying to each element 142. Alternatively, or optionally, the capability information may be indicative of the number of elements 142 in the array. The capability information may be used in the control node for selecting or configuring the tuning pattern to apply. At 403, the control node provides control signaling for reception 404 in the CED 140. The control signaling may be indicative of control information, related to activation of a characterization mode. At 405, the CED 140 may transmit an acknowledgement of the activation for receipt 406 in the control node. The control information may be indicative of the tuning pattern to apply. In some examples, the tuning pattern may be enclosed in control information conveyed by the control signaling. In other examples, the tuning pattern may be prestored in the CED 140. In some examples, the control signaling provides a request or trigger for the CED 140 to apply the tuning pattern, such as to enter the characterization mode. In other examples, the control signaling is indicative of a timer for activating the tuning pattern of the CED 140. Said timer may e.g., identify periodic activation of the tuning pattern. In some examples, control signaling to configure the CED 140 to apply the tuning pattern, e.g., to enter the characterization mode, may be triggered by the UE 1 reported RSRP, e.g., when it is below a threshold level. In some examples, the tuning pattern may define subset configurations, identifying subsets of the total number of elements 142 in the array 141, as will be explained. The tuning pattern thus identifies application of a phase change to all elements 142 of one subset for one iteration of forwarding a reference signal, and successive configurations to apply the phase change to different subsets for different iterations. In some examples, the tuning pattern is indicative of a number of tuning iterations dependent on the finite number of phase shifts, as will be exemplified below. In another example, the tuning pattern may be indicative of a number of tuning iterations dependent on the number of elements. The tuning pattern may define a sequence of application of the phase shift to the elements of the array. In some examples, as outlined below, each iteration comprises application of the phase change to elements having a common initial phase shift for forwarding from the transmitter to the receiver. As indicated in the drawing, the CED 140 may apply an initial phase pattern, or beamforming pattern, b0, i.e., a combination of phase shifts to apply to respective elements 142 of the array 141, for forwarding a signal from the Tx to the Rx. In the initial phase pattern b0, each element 142 thus has a configured initial phase shift. The initial phase pattern b0 may be indicated by the control signaling 403. The initial phase pattern b₀ may as such be a presently used configuration of the CED 140 for forwarding between the Tx and Rx. Steps 407 (407-1, 407-2, …, 407-n) indicate tuning of the array 141 according to the tuning pattern, which may be referred to as operating the characterization mode. At each tuning, a change is applied to the initial phase shift of one subset of elements 142. The complementary elements of the array, not forming part of the subset to which the change is applied, applies the initial phase shift. The tuning pattern may thus be indicative of a plurality of tuning iterations 407-1, 407-2, …, 407-n which each provide a phase pattern with the same directional properties of the array 401. In some examples, the phase change is thus applied to each of the elements 142, respectively, in at least one of said tuning iterations, such as only once. In this context, subgrouping may be configured such that each element 142 is comprised in only one of a plurality of subgroups, or subsets. In some examples, the tuning pattern is indicative of a time pattern for applying the phase change to different elements 142, or subsets of elements, of the array 141. In this context, the tuning pattern may be indicative of timing of reference signal (RS) transmission 408 (408-1, 408-2, …, 408-n), for which tuning shall be configured. Since the CED 140 in itself is not a receiver of the reference signals, it will not be “aware” of the forwarding it carries out. In some examples, the tuning pattern is indicative of a number (n) of the tuning iterations 407-1, 407-2, …, 407-n. The time pattern and/or the obtained number n of tuning iterations may configure the CED 140 to conclude termination of use of the tuning pattern, e.g., termination of the activated characterization mode. The forwarded reference signal is received 409-1, 409-2, …, 409-n in the Rx, wherein measurement of signal strength of the received signal is carried out, such as Reference Signal Received Power (RSRP). At 410, the Rx provides a measurement report for receipt 411 in the control node. This measurement report may be transmitted from the Rx to the Tx, optionally conveyed via the CED 140. Where the same device, e.g., the access node 121, is configured as Tx and Rx, the obtainment may be completed by the establishment of the signal strength measurements. At 412 the control node may configure 413 the CED 140 with a codebook for the array 141 based on the obtained measurements. This may e.g., be the case when the measurement report identifies that HW properties of the array 141 indicate that an updated codebook is optimal. By way of example, the drawing indicates that the CED 140, by its control unit 415, is reconfigured based on the obtained updated codebook to apply a different phase pattern, or beamforming pattern. b'₀ to forward signals from the Tx to the Rx. A detailed example of how to operate remote characterization which is activated according to the proposed solution will now be provided with reference to Fig.4. In this example, the tuning pattern identifies ^ reference signaling occasions, where ^ is the n^{umber of phase shift states per element, and where we assume ^ to be even. Let ℘ =} _{denote the ^ different available phase shifts (in degrees). For the specific} example where the phase shifters 143 in the CED 140 are quantized with 2 bits, this vector may be ℘ = {0°, 90°,180°, 270°}. In this context, each reference signaling occasion, or iteration, is correlated with one of the available phase shifts. Let the current phase setting be denoted by ^_^, i.e., the ^:th entry in ^_^, i.e., ^_^,^ , is an entry in ℘. In other words, ^_^ is the matrix/vector of phase setting for the elements 142 in the array 141, such as for example ^_^,^ – ^_^,^^^ for a panel of 100 elements, where each ^_^,^ is one of {0°, 90°,180°, 270°}. In this example there are thus as many iterations (reference signal occasions) as there are available phase shifts. For the sake of simplicity, we can denote each iteration by q, i.e., q is one of 1, 2, 3 or 4 in the example where ℘ = {0°, 90°,180°, 270°}. In this example, tuning is thus made for a number of iterations according to the tuning pattern, which number is equal to the available finite number of phase shifts. During the ^:th reference signal, the tuning pattern provides that the phase setting vector ^_^ is configured in the following way: where ^_^,^ is the ^:th element of ^_^ , and ^_^,^ is the ^:th element of ^_^, i.e., the initial phase pattern. This also provides that all elements which are not configured with phase shift ^_^ will maintain the configuration according to the initial phase pattern ^_^. The subset, for iteration q, may thus be defined as elements 142 having a common initial phase shift according to the initial phase pattern ^_^, e.g., all or substantially all such elements. In an alternative example, subset grouping may be configured such that each j^th element 142 is comprised in the same subset. The number of subsets, and iterations, will then be dependent on the total number of elements 142 in the array 141. The applied phase change is, here, a 180 degree change of the initial phase shift according to an initial phase pattern ^_^. By way of example: when RS2 is transmitted 408-2, all elements 142 that would have had a configured phase shift of 90 degrees according to the initial phase pattern ^_^, are instead configured to 270 degrees. In this context, relative phase shift of the elements of each subset is maintained through the iterations. Since ^ is assumed even, it is guaranteed that ^_^,^ + 180° ∈ ℘ provided that the phase values in ℘ are uniformly spaced. Let ^_^ denote the fraction of elements in ^ that are configured with phase shift ^_^; for reasonably large arrays, these numbers are roughly given by 1/^. Finally, let _^ be the associated amplitude of phase state ^; these are the variables we seek to estimate. We have that the received signal !_^ (409-q) during reference signal RSq (in the absence of noise): where " is absorbing various constants, e.g., path-loss, loss due to quantized beamforming, number of antennas, etc. This specifies a set of ^ linear equations in ^ unknowns, wherefore solvable (in the variables (_^ = " _^). By receiving the reported RSRP of those CSI-RS from the UEs, the gNB can obtain the knowledge of the gain difference between different phase shifter states. Based on this information, the gNB can compute the optimal beamforming coefficients for the CED devices. This computation may be carried out in the control node, which may reside in the access node 121 or in any other part in the wireless network 100. In one example, the beamforming coefficients are computed for a specific direction; said direction can be represented by an ) × 1 steering vector + where ) equals the number of antennas at the CED. Some preliminary computations are needed be the index of the phase shift applied by the .:th antenna, the phase shift applied by the .:th antenna is ∈ ℘ with ^{a corresponding amplitude 01. Altogether, the response in direction + for the selection} By optimal beamforming coefficients, we refer to the solution to the optimization problem: a_{rg max |! |} ^E 2 6_{2; +}8 _. This is a combinatorial optimization problem whose domain has cardinality ^⁴ - a number which quickly becomes infeasible to exhaust whenever ^ and/or ) grows. However, the problem can be solved with complexity F6)^E^8 and we provide an algorithm next. Partition the closed interval [0,2G] into ^ subsets, defined by: H_{^ = {I: arg m^ax ^cos6^^ − I8 = ^}.} Viewed on the unit circle, the sets H_^, … , H_^ are contiguous, wherefore they implicitly define ^ points on the unit circle where the sets intersect. Let these points be d^{enoted by N^ , ^ = 1 … ^. Next, define the set O = {N^ + arg69-8 mod 2G, ^ =} 1 _{… ^, . = 1 … )}. Let Q^, … , Q^4 be the elements in O sorted in ascending order.} With that, we are now ready to formally state the algorithm that solves the above optimization problem. Algorithm 1. Solving arg m ^E 2ax |!⁶2; +^8| . O^{utputs: 2TUV = arg m} ₂ ^{ax |!62; +8|E , W = m} ₂ ^{ax |!62; +8|E} W _{= 0} For X = 1 … )^ do If X < )^ Select an arbitrary number I satisfying Q_Z ≤ I < Q_Z\^ else Select an arbitrary number I satisfying Q_Z ≤ I < 2G end For . = 1 … ) do arg⁶9-⁸ ∈ H_^ end compute _ = |!62; +8|^E If _ > W W = _ end end. We remark that in the inner for-loop “Select = ], where I − arg⁶9-⁸ ∈ H_^" there is only a single index . for which needs to be updated compared with the previous iteration. Thus, the algorithm can be made to run in F6)^8 time, but this has not been done here. Various features and explanatory basis for the proposed solution have been outlined in the foregoing. The proposed solution may take any shape as provided herein, including any combination of the appended claims.

Claims

CLAIMS 1. A method for characterization of a coverage enhancement device which is capable of beamforming by individually applying one of a finite number of phase shifts to respective elements of an array to forward an electromagnetic wave signal, the method comprising: receiving (404), from a control node, a request indicative of a tuning pattern to apply to the respective elements of the array for remote determination of gain characteristics of the elements; and applying (407), in accordance with said tuning pattern, a change of the phase shift of the respective element to forward a reference signal from a transmitter towards a receiver.

2. The method of claim 1, comprising: transmitting (401) a capability message for receipt in the control node, which capability message is indicative of said finite number of phase shifts and/or the number of elements in the array.

3. The method of claim 2, wherein said tuning pattern is indicative of a number of tuning iterations dependent on the finite number of phase shifts.

4. The method of claim 2, wherein said tuning pattern is indicative of a number of tuning iterations dependent on the number of elements.

5. The method of any preceding claim, comprising: applying the tuning pattern based on a timer obtained in said request.

6. The method of any preceding claim, comprising: receiving (413) a codebook configuration for configuring the array based on measurement on reference signals obtained in the receiver, which reference signals have been forwarded using the tuning pattern.

7. The method of any preceding claim, wherein said tuning pattern defines a sequence of application of said phase change to the elements of the array.

8. The method of any preceding claim, wherein the array is configured according to an initial phase pattern for forwarding from the transmitter to the receiver, in which each element has a configured initial phase shift, and wherein said change is applied to the initial phase shift of the respective element.

9. The method of any preceding claim, wherein the phase change comprises a 180 degree change of an initial phase shift according to an initial phase pattern.

10. The method of any preceding claim, wherein said tuning pattern is indicative of a time pattern for applying the phase change to different elements of the array.

11. The method of any preceding claim, wherein said tuning pattern is indicative of a plurality of tuning iterations which each provide a phase pattern with the same directional properties of the array.

12. The method of any preceding claim, wherein said tuning pattern is indicative of a number of tuning iterations.

13. The method of claim 12, wherein each iteration comprises application of the phase change to elements having a common initial phase shift for forwarding from the transmitter to the receiver.

14. The method of claim 12 or 13, wherein each of the elements is subjected to the phase change in at least one of said tuning iterations.

15. The method of any of claims 11-14, wherein the tuning pattern identifies subsets of said elements.

16. The method of claim 15, wherein relative phase shift of the elements within each subset is maintained through said iterations.

17. The method of claim 15 or 16, wherein each subset comprises all elements having a common initial phase shift according to an initial phase pattern for forwarding from the transmitter to the receiver.

18. The method of claim 13 or 17, wherein tuning is made for said finite number of iterations according to the tuning pattern.

19. A coverage enhancement device (140) usable for forwarding an electromagnetic wave signal from a transmitter (Tx) to a receiver (Rx), the coverage enhancement device comprising: an array (141) comprising a plurality of elements (142), wherein each element is capable of selectively applying one of a finite number (Q) of phase shifts; a control unit (145) configured to set a phase pattern (b) to the array, wherein the elements individually apply one of said phase shifts to accomplish beamforming; an interface (148) for obtaining a control information from a control node; wherein said control unit is configured to: obtain, from control node using the interface, a request indicative of a tuning pattern to apply to the respective elements of the array for remote determination of gain characteristics of the elements; and apply, in accordance with said tuning pattern, a change of the phase shift of the respective element to forward a reference signal from a transmitter towards a receiver.

20. The coverage enhancement device of claim 19, wherein said control unit is configured to: transmit a capability message for receipt in the control node, which capability message is indicative of said finite number of phase shifts and/or the number of elements in the array.

21. The coverage enhancement device of claim 20, wherein said tuning pattern is indicative of a number of tuning iterations dependent on the finite number of phase shifts.

22. The coverage enhancement device of claim 20, wherein said tuning pattern is indicative of a number of tuning iterations dependent on the number of elements.

23. The coverage enhancement device of any of claims 19-22, wherein said control unit is configured to apply the tuning pattern based on a timer obtained in said request.

24. The coverage enhancement device of any of claims 19-23, wherein said control unit is configured to: receive a codebook configuration, which codebook is configured based on measurement on reference signals obtained in a receiver, which reference signals have been forwarded using the tuning pattern.

25. The coverage enhancement device of any of claims 19-24, wherein said tuning pattern defines a sequence of application of said phase change to the elements of the array.

26. The coverage enhancement device of any of claims 19-25, wherein the array is configured according to an initial phase pattern for forwarding from the transmitter to the, in which each element has a configured initial phase shift, and wherein said change is applied to the initial phase shift of the respective element.

27. The coverage enhancement device of any of claims 19-26, wherein the phase change comprises a 180 degree change of an initial phase shift according to an initial phase pattern.

28. The coverage enhancement device of any of claims 19-27, wherein said tuning pattern is indicative of a time pattern for applying the phase change to different elements of the array.

29. The coverage enhancement device of any of claims 19-28, wherein said tuning pattern is indicative of a plurality of tuning iterations which each provide a phase pattern with the same directional properties of the array.

30. The coverage enhancement device of any of claims 19-29, wherein said tuning pattern is indicative of a number of tuning iterations.

31. The coverage enhancement device of claim 30, wherein each iteration comprises application of the phase change to elements having a common initial phase shift for forwarding from the transmitter to the receiver.

32. The coverage enhancement device of any of claims 28-31, wherein each of the elements is subjected to the phase change in at least one of said tuning iterations.

33. The coverage enhancement device of claim 32, wherein said control unit is configured to apply the change of the phase shift to subsets of said elements according to said tuning pattern.

34. The coverage enhancement device of claim 33, wherein relative phase shift of the elements within each subset is maintained through said iterations.

35. The coverage enhancement device of claim 33 or 34, wherein said subset comprises all elements having a common initial phase shift according to an initial phase pattern for forwarding from the transmitter to the receiver.

36. The coverage enhancement device of claim 31 or 35, wherein the control unit is configured to tune the array for said finite number of iterations according to the tuning pattern.

37. A method carried out in a control station for remote characterization of a coverage enhancing device usable for forwarding an electromagnetic wave signal from a transmitter to a receiver by individually applying one of a finite number of phase shifts to respective elements of an array, the method comprising: configuring (403) the coverage enhancing device with a tuning pattern for the array for applying a change to the phase shift; controlling transmission (408) of a reference signal from the transmitter while the coverage enhancing device applies the tuning pattern; obtaining (411) measurements of reception of the forwarded reference signal by the receiver.

38. The method of claim 37, comprising: configuring (412) the coverage enhancing device with a codebook for the array based on the obtained measurements.

39. The method of claim 37 or 38, wherein the phase change comprises a 180 degree change of an initial phase shift according to an initial phase pattern for forwarding from the transmitter to the receiver.

40. The method of any of claims 37-39, wherein said tuning pattern is indicative of a time pattern for applying the phase change to different elements of the array, wherein transmission of the reference signal is controlled to be repeated according to said time pattern.

41. The method of any of claims 37-40, wherein transmission of the reference signal comprises a number of transmission occasions based on capability information associated with the coverage enhancing device.