Devices and methods for uplink channel sounding in a cell-free wireless network

Abstract

A network device (110a,b), for instance, a base station, of a plurality of distributed network devices (110a,b) is disclosed for providing network access to a plurality of user equipment, UEs, (120a,b) in a cell-free wireless network (100). The network device (110a,b) is configured to obtain a plurality of uncorrelated sounding signals and transmit the plurality of uncorrelated sounding signals to the plurality of UEs (120a,b) for allowing each UE (120a,b) to determine a precoder using a covariance matrix based on a superposition of the plurality of sounding signals received from the network device (110a,b) and a plurality of further uncorrelated sounding signals received by the respective UE (120a,b) from one or more further network devices of the plurality of network devices (110a,b).

Description

[0001] DEVICES AND METHODS FOR UPLINK CHANNEL SOUNDING IN A CELL-FREE WIRELESS NETWORK

[0002] TECHNICAL FIELD

[0003] The present disclosure relates to wireless communications. More specifically, the present disclosure relates to devices and methods for uplink channel sounding in a cell-free wireless network, in particular a 3GPP network.

[0004] BACKGROUND

[0005] The increase of Artificial Intelligence (Al) and Machine Learning (ML) applications provided via mobile networks is one of the main reasons that the uplink (UL) throughput in mobile networks has to be improved. Multi-antenna devices will play an increasingly important role in the next generation of MIMO evolution, including precoding at the user equipment (UE). Channel ageing is a problem for MIMO over fast changing channels. UL channel sounding needs to be realized in a distributed “cell-free” manner. The channel ageing problem will be even more pronounced with multi-antenna precoding at the UE, because of high mobility. Thus, there is a need for improved devices and methods for providing an uplink noncodebook transmission scheme, which minimizes channel ageing while enabling optimal distributed MU-MIMO.

[0006] SUMMARY

[0007] It is an objective of the present disclosure to provide improved devices and methods for channel sounding in a cell-free wireless network, in particular a 3GPP network, and providing an uplink non-codebook transmission scheme, which minimizes channel ageing while enabling optimal distributed MU-MIMO.

[0008] The foregoing and other objectives are achieved by the subject matter of the independent claims. Further implementation forms are apparent from the dependent claims, the description and the figures. In the following some or more of the following abbreviations and acronyms will be used:

[0009] 5G 5th generation mobile wireless communications

[0010] BS Base Station

[0011] CSI Channel State Information

[0012] CSI-RS CSI Reference Signal

[0013] DCI Downlink Control Information

[0014] DL Downlink

[0015] DU Distributed Unit

[0016] DFT Discrete Fourier Transformation

[0017] LEE Long Term Evolution

[0018] MIMO Multiple-Input Multiple-Output (system or channel)

[0019] MU Multiuser

[0020] MU-MIMO Multiuser MIMO

[0021] NW Network

[0022] OFDM Orthogonal Frequency Division Multiplexing

[0023] PUSCH Physical Uplink Shared Channel

[0024] RACH Random Access Channel

[0025] RAN Radio Access Network RRC Radio Resource Control

[0026] SRS Sounding Reference Signal

[0027] SRI SRS resource indicator

[0028] SSB Synchronization Signal Block

[0029] TPMI Transmit Precoding Indicator

[0030] TRP Transmission Reception Point

[0031] UE User Equipment

[0032] UL Uplink

[0033] As used herein, a MIMO channel (sometimes also referred to as MIMO system) is a wireless channel with multiple inputs and outputs, i.e., transmit antennas and receive antennas.

[0034] As used herein, an antenna port (or port) may refer to a physical antenna or a complex-weighted sum of multiple antennas, where the weighting coefficients are known as a precoder or precoding vector. Each antenna port may have a specific spatial transmission / reception characteristic over which a signal is transmitted and received.

[0035] As used herein, a Cell-Free Massive MIMO (also referred to as Distributed MIMO) is a system where a massive number of access points distributed over a large area coherently serve a massive number of user terminals, i.e. UE.

[0036] As used herein, network node or network device may have an inclusive meaning, comprising the wireless MIMO transceivers with the antennas plus other network components, like processors and memory. A network node or network device may represent the infrastructure side of the MIMO radio channel. It is made up of individual components which can be co-located or distributed. A network node or network device may equivalently be referred to as access point (AP), transmissionreception point (TRP), wireless transmission / reception unit (WTRU), base station (BS), gNB, eNB, base transceiver system (BTS), remote radio head (RRH), radio unit (RU), remote radio unit (RRU), and the like.

[0037] As used herein, channel sounding is a technique that evaluates the radio environment for wireless communication, especially MIMO systems.

[0038] As used herein, a precoder refers to a (usually linear) spatial transmit filter, which maps amplitude-scaled and phase-shifted copies of the transmit signal to multiple antennas.

[0039] As used herein, a spatial layer (or layer) is the part of a spatially multiplexed signal that is sent via a precoder or beam. Spatially multiplexing may be achieved by using multiple precoders in parallel, thus multiplexing the signals over different spatial channels.

[0040] As used herein, the “network side” refers to the infrastructure part of a wireless communication network, while the “UE side” refers to the user equipment.

[0041] Downlink data may be transmitted through channels such as the Physical Downlink Shared Channel (PDSCH). Downlink control signals may be transmitted through channels such as the Physical Downlink Control Channel (PDCCH).

[0042] As used herein, a sounding resource may refer to a certain combination of time / frequency allocation resources (e.g. sub-band or component carrier) and antenna port (spatial layer).

[0043] As used herein, a sub-band may refer to several consecutive resource blocks. According to a first aspect a multi-antenna network device, e.g. a base station, of a plurality of distributed multi-antenna network devices is provided for providing network access to a plurality of multi-antenna user equipment, UEs, in a cell-free wireless network. The multi-antenna network device according to the first aspect is configured to obtain a plurality of uncorrelated sounding signals s;. Moreover, the multi-antenna network device according to the first aspect is configured to transmit the plurality of uncorrelated sounding signals s;using the plurality of antennas of the network device to the plurality of UEs within a channel coherence time for allowing each UE to determine a precoder using a covariance matrix based on a superposition of the plurality of sounding signals received from the network device and a plurality of further uncorrelated sounding signals received by the respective UE from one or more further network devices of the plurality of network devices. Thus, the network device according to the first aspect allows supporting channel sounding in a cell-free wireless network, in particular a 3GPP network, and providing an uplink non-codebook transmission scheme, which minimizes channel ageing while enabling optimal distributed MU-MIMO. As will be appreciated, the network device according to the first aspect makes use of the duality principle by sounding the DL channel in order to estimate the UL channel.

[0044] In a further possible implementation form, the multi-antenna network device is configured to generate the plurality of uncorrelated sounding signals s;.

[0045] In a further possible implementation form, the multi-antenna network device is configured to receive a plurality of UL sounding signals from the plurality of UEs and to generate the plurality of uncorrelated sounding signals s;based on the plurality of UL sounding signals received from the plurality of UEs.

[0046] In a further possible implementation form, the multi-antenna network device is configured to generate the plurality of uncorrelated sounding signals s;and apply a weighting factor to each sounding signal s;, wherein each weighting factor is based on a joint optimization of a plurality of MIMO spatial filters and a plurality of transmission powers of the plurality of UEs.

[0047] In a further possible implementation form, one or more of the plurality of uncorrelated sounding signals s;comprises one or more random signal portions, i.e. random signal components.

[0048] In a further possible implementation form, the multi-antenna network device is configured to transmit the plurality of uncorrelated sounding signals s;to the plurality of UEs using a plurality of pre-defmed time and / or frequency resources, in particular frequency sub-carriers and / or time slots.

[0049] In a further possible implementation form, the multi-antenna network device is configured to receive one or more precoded UL transmissions from one or more of the plurality of UEs, wherein the one or more precoded UL transmissions are based on the respective precoder determined by the respective UE using a covariance matrix based on a superposition of the plurality of sounding signals received from the network device and a plurality of further uncorrelated sounding signals received by the respective UE from one or more further network devices of the plurality of network devices.

[0050] In a further possible implementation form, the network device is further configured to provide a respective regularization matrix Qkto each of the plurality of UEs for allowing each UE to determine a respective precoder using a respective covariance matrix and a respective regularization matrix Qk.

[0051] According to a second aspect a method is provided for operating a multi-antenna network device, e.g. a base station, of a plurality of distributed multi-antenna network devices for providing network access to a plurality of multi-antenna user equipment, UEs, in a cell-free wireless network. Ehe method according to the second aspect comprises the steps of: obtaining a plurality of uncorrelated sounding signals s;: and transmitting the plurality of uncorrelated sounding signals s;using the plurality of antennas of the network device to the plurality of UEs within a channel coherence time for allowing each UE to determine a precoder using a covariance matrix based on a superposition of the plurality of sounding signals received from the network device and a plurality of further uncorrelated sounding signals received by the respective UE from one or more further network devices of the plurality of network devices.

[0052] The method according to the second aspect can be performed by the network device according to the first aspect. Thus, further features of the method according to the second aspect result directly from the functionality of the network device according to the first aspect as well as its different implementation forms described above and below.

[0053] According to a third aspect a multi-antenna user equipment, UE, is provided for network access provided by a plurality of multi-antenna network devices in a cell-free wireless network. The multi-antenna UE according to the third aspect is configured to receive a plurality of uncorrelated sounding signals s;using the plurality of antennas of the UE from the plurality of network devices within a channel coherence time. Moreover, the multi-antenna UE according to the third aspect is configured to determine a precoder using a covariance matrix based on a superposition of the plurality of uncorrelated sounding signals received from the plurality of network devices. Thus, the UE according to the third aspect allows supporting channel sounding in a cell-free wireless network, in particular a 3GPP network, and providing an uplink non-codebook transmission scheme, which minimizes channel ageing while enabling optimal distributed MU-MIMO. As will be appreciated, the UE according to the third aspect makes use of the duality principle by sounding the DL channel in order to estimate the UL channel.

[0054] In a further possible implementation form, the multi-antenna UE according to the third aspect is configured to transmit a plurality of UL sounding signals to the plurality of network devices for allowing the plurality of network devices to generate the plurality of uncorrelated sounding signals s;based on the plurality of UL sounding signals.

[0055] In a further possible implementation form, each sounding signal s;is weighted with a weighting factor and wherein each weighting factor is based on a joint optimization of a plurality of MEMO spatial filters and a plurality of transmission powers of the plurality of UEs.

[0056] In a further possible implementation form, one or more of the plurality of uncorrelated sounding signals s;may comprise one or more random signal portions.

[0057] In a further possible implementation form, the multi-antenna UE according to the third aspect is configured to receive the plurality of uncorrelated sounding signals s;from the plurality of network devices using a plurality of pre-defined time and / or frequency resources, in particular frequency sub-carriers and / or time slots.

[0058] In a further possible implementation form, the multi-antenna UE according to the third aspect is configured to transmit one or more precoded UL transmissions to one or more of the plurality of network devices, wherein the one or more precoded UL transmissions are based on the respective precoder determined by the UE according to the third aspect using the covariance matrix based on the superposition of the plurality of sounding signals received from the plurality of network devices.

[0059] In a further possible implementation form, the UE according to the third aspect is configured to determine the covariance matrix Zkon the basis of the following equation: Zk= E[ykyk'] = hklihk'l2l1=l...L l2= l...L wherein E[ ] denotes the expectation value, ykdenotes the superposition of the plurality of sounding signals received by the UE according to the third aspect, and hklik'l2denote the channel vectors, i.e. the effective channels (including possible precoders) experienced by a sounding signal, when being transmitted to the k-th UE.

[0060] In a further possible implementation form, the UE according to the third aspect is configured to determine the precoder using the covariance matrix and a regularization matrix Qk.

[0061] In a further possible implementation form, the UE according to the third aspect is configured to receive the regularization matrix Qkfrom one or more of the plurality of multi-antenna network devices.

[0062] In a further possible implementation form, the UE according to the third aspect is configured to determine the precoder lVkbased on the following equation: wherein Hkdenotes the channel matrix for the UE according to the third aspect.

[0063] According to a fourth aspect a method is provided for operating a multi-antenna user equipment, UE, for network access provided by a plurality of multi-antenna network devices in a cell-free wireless network. The method according to the fourth aspect comprises the steps of: receiving a plurality of uncorrelated sounding signals s;using the plurality of antennas of the UE from the plurality of network devices within a channel coherence time; and determining a precoder using a covariance matrix based on a superposition of the plurality of sounding signals received from the plurality of network devices.

[0064] The method according to the fourth aspect can be performed by the UE according to the third aspect. Thus, further features of the method according to the fourth aspect result directly from the functionality of the UE according to the third aspect as well as its different implementation forms described above and below.

[0065] According to a fifth aspect, a computer program product is provided, comprising a computer-readable storage medium for storing program code which causes a computer or a processor to perform the method according to the second aspect or the method according to the fourth aspect, when the program code is executed by the computer or the processor.

[0066] Details of one or more embodiments are set forth in the accompanying drawings and the description below. Other features, objects, and advantages will be apparent from the description, drawings, and claims. BRIEF DESCRIPTION OF THE DRAWINGS

[0067] In the following, embodiments of the present disclosure are described in more detail with reference to the attached figures and drawings, in which:

[0068] Fig. la shows a schematic diagram illustrating a cell-free wireless network including a plurality of network devices according to an embodiment in the form of base stations for providing network access to a plurality of UEs according to an embodiment;

[0069] Fig. lb shows a schematic diagram illustrating a cell-free wireless network including a plurality of network devices according to an embodiment in the form of RUs and a DU for providing network access to a plurality of UEs according to an embodiment;

[0070] Fig. 2a shows a signalling diagram illustrating a conventional non-codebook transmission scheme between a base station and a UE;

[0071] Fig. 2b shows a schematic diagram illustrating the cross-DU interference problem;

[0072] Fig. 3 shows a schematic diagram illustrating different aspects of a non-codebook transmission scheme implemented by a plurality of network devices and a plurality of UEs according to an embodiment;

[0073] Fig. 4 shows a signalling diagram illustrating a non-codebook transmission scheme between a base station and a UE according to an embodiment;

[0074] Fig. 5 shows examples of uncorrelated random bit sequences used for generating sounding signals transmitted by a network device according to an embodiment;

[0075] Fig. 6 shows a flow diagram illustrating a method of operating a network device according to an embodiment; and

[0076] Fig. 7 shows a flow diagram illustrating a method of operating an UE according to an embodiment.

[0077] In the following, identical reference signs refer to identical or at least functionally equivalent features.

[0078] DETAILED DESCRIPTION OF THE EMBODIMENTS

[0079] In the following description, reference is made to the accompanying figures, which form part of the disclosure, and which show, by way of illustration, specific aspects of embodiments of the present disclosure or specific aspects in which embodiments of the present disclosure may be used. It is understood that embodiments of the present disclosure may be used in other aspects and comprise structural or logical changes not depicted in the figures. The following detailed description, therefore, is not to be taken in a limiting sense, and the scope of the present disclosure is defined by the appended claims.

[0080] For instance, it is to be understood that a disclosure in connection with a described method may also hold true for a corresponding device or system configured to perform the method and vice versa. For example, if one or a plurality of specific method steps are described, a corresponding device may include one or a plurality of units, e.g. functional units, to perform the described one or plurality of method steps (e.g. one unit performing the one or plurality of steps, or a plurality of units each performing one or more of the plurality of steps), even if such one or more units are not explicitly described or illustrated in the figures. On the other hand, for example, if a specific apparatus is described based on one or a plurality of units, e.g. functional units, a corresponding method may include one step to perform the functionality of the one or plurality of units (e.g. one step performing the functionality of the one or plurality of units, or a plurality of steps each performing the functionality of one or more of the plurality of units), even if such one or plurality of steps are not explicitly described or illustrated in the figures. Further, it is understood that the features of the various exemplary embodiments and / or aspects described herein may be combined with each other, unless specifically noted otherwise.

[0081] Fig. la shows a schematic diagram illustrating a wireless network 100, in particular a cell-free wireless network 100 according to an embodiment configured to provide mobile communication services. In an embodiment, the wireless network 100 may be a current or future 3rd Generation Partnership Project (3GPP) wireless network 100, for instance, a 5G or a post 5G network. The wireless network 100 comprises a plurality of network devices 110a, b in the form of base stations 110a, b, e.g. gNBs 110a, b for providing network access to a plurality of UEs 120a, b.

[0082] As illustrated in Fig. la, each network device 110a, b, e.g. base station 110a, b may comprise processing circuitry 111 and a communication interface 113, for instance, a transceiver unit 113 including a plurality of antennas. The processing circuitry 111 may be implemented in hardware and / or software. The hardware may comprise digital circuitry, or both analog and digital circuitry. Digital circuitry may comprise components such as application-specific integrated circuits (ASICs), field- programmable gate arrays (FPGAs), digital signal processors (DSPs), or one or more general-purpose processors. Moreover, each network device 110a, b, e.g. base station 110a, b may comprise a memory 115 configured to store executable program code which, when executed by the processing circuitry 111, causes the network device 110a, b, e.g. base station 110a, b to perform the functions and operations described herein.

[0083] Likewise, each UE 120a, b may comprise processing circuitry 121 and a communication interface 123, for instance, a transceiver unit 123 including a plurality of antennas. The processing circuitry 121 may be implemented in hardware and / or software. The hardware may comprise digital circuitry, or both analog and digital circuitry. Digital circuitry may comprise components such as application-specific integrated circuits (ASICs), field-programmable gate arrays (FPGAs), digital signal processors (DSPs), or one or more general-purpose processors. Moreover, each UE 120a, b may comprise a memory 125 configured to store executable program code which, when executed by the processing circuitry 121, causes the UE 120a, b to perform the functions and operations described herein.

[0084] Fig. lb shows a further embodiment of the cell-free wireless network 100, wherein the network devices are implemented in the form of a plurality of Radio Units, RUs, 11 Oa-d and / or one or more Distributed Units, DUs, 130a in communication with a Central Unit, CU, 140.

[0085] According to the current 3GPP framework of standards a device may be configured in two different modes for PUSCH multiantenna precoding, referred to as codebook-based transmission and non-codebook-based transmission respectively.

[0086] Codebook-based transmission is based on a limited set of predefined precoders (beamformers) and, therefore, not suitable for MU-MIMO. In the non-codebook-based transmission according to embodiments disclosed herein the UE determines its transmission strategy without relying on a predefined codebook. The precoder may be determined directly from measurements by the UE rather than from an indicated codebook / TPMI index. This approach exploits reciprocity between UL and DL channels. The scenario under consideration (distributed multiantenna TRPs, nearfield and multiuser MIMO) requires a non-codebook-type approach because interference control and SDMA, with distributed panels cannot be achieved by a simple codebook based on DFT beams or similar. For such scenarios, the non-codebook-based transmission has been introduced. Fig. 2a shows a signalling diagram illustrating a conventional non-codebook transmission scheme between a base station 10a and a UE 20a. As will be appreciated, the conventional non-codebook transmission scheme illustrated in Fig. 2a involves a 3-step approach for computing the UL precoders. The first step is the transmission of the CSI-RS (downlink channel sounding). However, this alone is not sufficient. Device selection of a precoder based on downlink measurements (CSI-RS) may not necessarily be the best precoder from a network point of view. Thus, the NR non-codebook-based precoding includes additional steps (SRS / SRI) where the network can modify the device-selected precoder, i.e., remove some beams / layers or equivalently some columns, from the selected precoder at the UE.

[0087] The conventional non-codebook scheme disclosed in 3GPP TS38.214, 6.1.1.2 includes the SRS / SRI feedback mechanism illustrated in Fig. 2a. As already mentioned above, this mechanism includes 3 steps (CSI-RS, SRS, SRI). This has the disadvantage that the latency between SRS and PUSCH transmission translates into performance loss caused by channel ageing. Thus, to avoid channel ageing, a 1-step UL sounding approach without additional feedback / reporting step, as implemented by embodiments disclosed herein, is beneficial. In addition to channel ageing, there is another disadvantage of the conventional approach in that for estimating the interference contributions between UE / TRP links where the TRP and UE are associated with different DUs, it is necessary that the DUs exchange information (e.g. CSI or codes), which increases the overhead. This is illustrated by the example shown in Fig. 2b, where the UE 20a would not know the codes used by DU2. In such a distributed system, interference measurements would require either massive exchange of codes (prohibitive) or other artificial concepts, like ZP-CSI-RS (wasting resources).

[0088] As will be described in more detail in the following, to address the shortcomings of the conventional approaches described above each of the network devices 110a, b, e.g. base stations 110a, b of Fig. la and / or the network devices 1 lOa-d, e.g. RUs 11 Oa-d and DU 130a ofFig. Ib is configured to obtain a plurality of uncorrelated sounding signals s;and to transmit the plurality of uncorrelated sounding signals s;to the plurality of UEs 120a,b for allowing each UE 120a,b to determine a precoder using a covariance matrix based on a superposition of the plurality of sounding signals received from the plurality of network devices 1 1 Oa-d: 130a. Complementary therefore, each UE 120a, b is configured to receive the plurality of uncorrelated sounding signals s;from the plurality of network devices 1 1 Oa-d: 130a, b and to determine a precoder using a covariance matrix based on a superposition of the plurality of uncorrelated sounding signals received from the plurality of network devices 11 Oa-d; 130a,b.

[0089] Thus, embodiments disclosed herein address the shortcomings of the conventional approaches described above. More specifically embodiments disclosed herein provide a UL transmission mode fulfilling one or more of the following requirements.

[0090] Firstly, a single-step scheme is provided for minimizing the latency by performing direct NW-to-UE sounding by exploiting the duality between UL and DL multiuser channels.

[0091] Secondly, optimal multiuser cell-free nearfield precoding operating on the boundary of the MU-MIMO throughput region for linear transceivers is enabled. That is, in an embodiment described in more detail further below, the precoders used by the UEs 120a, b are representable in the following form, where the dual parameters q±... qLand Qkare computed at the NW side in the following form, as illustrated in Fig. 4:

[0092] Thirdly, to keep the overhead in cell-free systems at a reasonable level, the excessive exchange of reference codes between DUs and other concepts for estimation cross-DU interference, like ZP-CSI-RS, is avoided.

[0093] Fourthly, the regularization matrix Qkis (most generally) a Hermitian positive definite matrix, which serves the purpose of regularizing the matrix inversion. The intuition, from a practical perspective, is that we want to control the transmission power radiated from the UE antennas. The Tx power is subject to a power constraint (e.g. sum power, per-antenna power). By properly selecting the matrix Qk, various power constraints may be taken into account in an optimal manner. In an embodiment the regularization matrix Qkmay be obtained as the outcome of a joint optimization problem, together with q±... qL, where, for instance, a multiuser utility, e.g. the UL sum-rate, may be optimized subject to power constraints. This optimization may be performed at the NW side. Then, the UE 120a, b may be informed about the regularization matrix Qk. According to an embodiment Qkmay be transmitted from the NW 110a, b to the UE 120a, b, either via an explicit signal or by embedding this information into the sounding signals. Furthermore, the UE 120a, b may be configured to determine the regularization by itself. The simplest example is a regularization via a scaled identity matrix. This is generally suboptimal but might suffice for certain scenarios.

[0094] Thus, as illustrated in Fig. 3, according to embodiments disclosed herein each UE 120a,b may determine an optimized uplink precoder wt The embodiment illustrated in Fig. 3 is based on a model, where L sounding signals si are sent simultaneously over M antennas by the network devices, linear spatial processing is performed at the NW side with a complex combining matrix V, and the A-Lh UE 120a, b is equipped with antennas. Embodiments disclosed herein emulate the uplink channel by sending properly adjusted sounding signals over the downlink. Each UE 120a, b may derive its optimal UL precoders by observing the -dimensional signal yk, respectively. This has the advantage that the individual per-layer interference channels hu do not have to be estimated.

[0095] In the conventional approach signals intended for other UEs potentially lead to pilot contamination, as illustrated in Fig. 2b. However, this interference contribution (referred to as “signals to other UEs” in Fig. 2b) actually contains valuable information about the multi-user interference structure of the channel. Embodiments disclosed herein take advantage of this additional information, by using the covariance Zk, containing signal contributions from all layers 1, 2, diagfqq ... qL} Hk11'

[0096] As already described above, the regularization matrix Qkmay be transmitted from the NW to the A-Lh UE 120a, b so that each UE 120a, b may determine an optimized MMSE precoder in the following form:

[0097] As will be appreciated and as already mentioned above, embodiments disclosed herein beneficially make use of the duality between uplink and downlink MEMO channels. Duality means that for optimizing MU-MIMO precoders it is often easier to solve the Lagrangian dual problem instead. Often, this has the advantage that certain optimization variables decouple, which in turn enables more efficient algorithms. The dual problem involves a transposed channel and reversed roles of transmitters and receivers.

[0098] According to embodiments disclosed herein, the duality mentioned above may be exploited in the following way. Irrespective of the MU-optimization strategy (e.g. max sum-rate, or the like), an optimized precoder at the A-Lh UE 120a, b has the following form in the dual downlink channel:

[0099] The vector- valued channel hk[is the effective propagation channel between the k-th UE 120a,b and the NW, including spatial processing at the NW side. All L channels are included in the matrix Hku= |hk l... hkL] e clV'f XZ' and the matrix Hkonly contains a subset of layers that are associated with the A-Lh UE 120a, b. Moreover, the regularization matrix Qkand the real- valued non-negative variables q±... qLdepend on the MU optimization at the NW side. As will be appreciated, embodiments of the network devices 1 lOa-d, 130a and the UEs 120a, b disclosed herein allow replacing the conventional multi-step sounding scheme (illustrated in Fig. 2a), by a l-step sounding scheme which minimizes the channel ageing effect. Consequently, the precoder mismatch is minimized, which in turn translates into performance gains. Moreover, according to embodiments disclosed herein, the cross-gNB 1 lOa-d interference need not be estimated, because this interference is implicitly included in the observations E[ykyk'] (superposition of all sounding components), and no explicit knowledge of individual components is required.

[0100] More specifically, Fig. 4 shows a signalling diagram illustrating a non-codebook transmission scheme between the network device 110a, e.g. base station 110a and the UE 120a according to an embodiment. In step 1 of Fig. 4, the network device 110a, e.g. base station or RU 110a broadcasts an SSB signal, which is received by the UE 120a. Based on this signal, the UE 120a performs synchronization and acquires system parameters. This is followed by the RACH procedure, and the UE 120a may send Capability Information “dual sounding” to the network device 110a via an RRC message. In step 2 of Fig. 4, the UE 120a sends an SRS signal, which is received by the network device 110a. This allows the NW to acquire initial CSI, based on which the NW can perform cooperative interference and resource control for the UL, which includes computation of the spatial combiners vhthe sounding signals s;, as well as of the scaling factors , each belonging to a spatial layer I. In step 3 of Fig. 4, the network device 110a sends an RRC or DCI configuration message to the UE 120a, which tells the UE 120a to operate in “dual sounding” mode. In step 4 of Fig. 4, the network device 110a sends the dual sounding signals, as alreadv described above, preferably in a synchronized manner, which are received by the UE 120a. Based on the observed sample covariance Zk, the UE 120a determines its precoder as lVk= (Zk+ Qk')~1Hk. In step 5 of Fig. 4, the UE 120a uses the new precoders, e.g., for PUSCH transmission, which is received by the network device 110a. In step 6 of Fig. 4, the UE 120a sends SRS signals to the network device 110a. Based on the received SRS signals, the NW is updating the cooperative interference and resource control. Step 7 of Fig. 4 is repeating step 4, i.e., the network device 110a sends the dual sounding signals, which are received by the UE 120a. This allows the UE to update the covariance Zkin a simplified manner (rank-1 update), since previous knowledge of Zkhas been obtained from step 4 already. In step 8 of Fig. 4, the UE 120a uses the updated precoders for PUSCH transmission, which is received by the network device 110a.

[0101] In an embodiment, the sounding signals may be (pseudo )random with no codebook required. The uncorrelated sounding signals s;are sent within the coherence time of the channel. Fig. 5 shows examples of uncorrelated random bit sequences which may be used for generating sounding sequences according to an embodiment. The sounding signals may be sent on predefined resource / slot over the physical DL channel. At the A-Lh UE 120a, b the observed signal is:

[0102] The signals are uncorrelated, i.e., E[s;isl] = 0, 7, + l2. This provides the required covariance matrix at the UE 120a, b, which enables computation of the optimal precoding matrix:

[0103] The / c-th UEs 120a,b own channel Hkmay be estimated by conventional channel estimation, if the sounding signals associated with this UE 120a, b are known at the receiver (e.g. from a predefined codebook).

[0104] In an embodiment, the computation of the UE precoders may involve the inversion of the covariance matrix Zk. During a tracking phase illustrated in Fig. 4, the inverse may be updated only gradually (small rank update). This update is enabled by the Sherman Morrison Woodbury formula:

[0105] In an embodiment, a Recursive Least Squares (RLS) algorithm may be used for the tracking phase.

[0106] In an embodiment, at least some of the operations described above, in particular the precoding, may be performed individually for different sub-bands of an operating frequency range.

[0107] In an embodiment, the UL transmission mode described above may be indicated by one or more RRC / DCI parameters. Moreover, as not all UE types may be able to support the UL transmission mode described above, for instance, because of higher requirements with respect to the computational capability of the UE, a UE capability may be defined for indicating the capability to support the UL transmission mode described above.

[0108] As described above, the sounding signals transmitted by the plurality of network devices 1 lOa-d, e.g. base stations or RUs 1 lOa-d should be uncorrelated among each other. This includes the signals being generated by the same DU, as well as the signals being sent from other DUs. As will be appreciated, this approach may have its limits, for instance, for a system extending over an extremely large area (e.g. an entire city). In this case it is only required that signals within a certain “neighborhood” are uncorrelated. Signals that are “very far” away are irrelevant because of the path loss. If the signal strength of a certain sounding signal measured at some RU is below a certain threshold (e.g. if it is below the noise), then uncorrelatedness is not required. Then this signal makes no meaningful contribution and may be neglected.

[0109] Fig. 6 shows a flow diagram illustrating a method 600 for operating a network device, such as the base stations or RUs 110a- d of figures la and lb and / or the DUs 130a,b of Fig. lb, of a plurality of distributed network devices HOa-d; 130a,b for providing network access to a plurality of user equipment, UEs, 120a, b in a cell-free wireless network 100. The method 600 comprises a step 601 of obtaining a plurality of uncorrelated sounding signals s;. Moreover, the method 600 comprises a step 603 of transmitting the plurality of uncorrelated sounding signals s;to the plurality of UEs 120a, b for allowing each UE 120a, b to determine a precoder using a covariance matrix based on a superposition of the plurality of sounding signals received from the network device 11 Oa-d; 130a, b and a plurality of further uncorrelated sounding signals received by the respective UE 120a, b from one or more further network devices of the plurality of network devices 11 Oa-d; 130a, b.

[0110] The method 600 can be performed by any one of the network devices 11 Oa-d; 130a, b. Thus, further features of the method 600 result directly from the functionality of the network devices 1 lOa-d; 130a, b as well as the different embodiments thereof described above and below.

[0111] Fig. 7 shows a flow diagram illustrating a method 700 for operating a user equipment, UE, 120a, b for network access provided by a plurality of network devices 1 lOa-d; 130a, b in a cell-free wireless network 100. The method 700 comprises a step 701 of receiving a plurality of uncorrelated sounding signals s;from the plurality of network devices 1 lOa-d; 130a,b. Moreover, the method 700 comprises a step 703 of determining a precoder using a covariance matrix based on a superposition of the plurality of uncorrelated sounding signals received from the plurality of network devices 1 lOa-d; 130a,b.

[0112] The method 700 can be performed by the UE 120a, b. Thus, further features of the method 700 result directly from the functionality of the UE 120a,b as well as the different embodiments thereof described above and below.

[0113] The person skilled in the art will understand that the "blocks" ("units") of the various figures (method and apparatus) represent or describe functionalities of embodiments of the present disclosure (rather than necessarily individual "units" in hardware or software) and thus describe equally functions or features of apparatus embodiments as well as method embodiments (unit = step). In the several embodiments provided in the present application, it should be understood that the disclosed system, apparatus, and method may be implemented in other manners. For example, the described embodiment of an apparatus is merely exemplary. For example, the unit division is merely a logical function division and may be another division in an actual implementation. For example, a plurality of units or components may be combined or integrated into another system, or some features may be ignored or not performed. In addition, the displayed or discussed mutual couplings or direct couplings or communication connections may be implemented by using some interfaces. The indirect couplings or communication connections between the apparatuses or units may be implemented in electronic, mechanical, or other forms.

[0114] The units described as separate parts may or may not be physically separate, and parts displayed as units may or may not be physical units, may be located in one position, or may be distributed on a plurality of network units. Some or all of the units may be selected according to actual needs to achieve the objectives of the solutions of the embodiments.

[0115] In addition, functional units in the embodiments of the disclosure may be integrated into one processing unit, or each of the units may exist alone physically, or two or more units may be integrated into one unit.

Claims

CLAIMS1. A network device (11 Oa-d; 130a, b) of a plurality of distributed network devices (11 Oa-d; 130a, b) for providing network access to a plurality of user equipment, UEs, (120a, b) in a cell-free wireless network (100), wherein the network device (1 lOa-d; 130a, b) is configured to: obtain a plurality of uncorrelated sounding signals s;: and transmit the plurality of uncorrelated sounding signals s;to the plurality of UEs (120a,b) for allowing each UE (120a, b) to determine a precoder using a covariance matrix based on a superposition of the plurality of sounding signals received from the network device l' I I Oa-d: 130a,b) and a plurality of further uncorrelated sounding signals received by the respective UE (120a, b) from one or more further network devices of the plurality of network devices (1 1 Oa-d: 130a, b).

2. The network device (1 1 Oa-d: 130a, b) of claim 1, wherein the network device (1 1 Oa-d: 130a, b) is configured to generate the plurality of uncorrelated sounding signals s;.

3. The network device (1 lOa-d: 130a, b) of claim 2, wherein the network device (1 1 Oa-d: 130a, b) is configured to receive a plurality of UL sounding signals from the plurality of UEs (120a, b) and to generate the plurality of uncorrelated sounding signals s;based on the plurality of UL sounding signals received from the plurality of UEs (120a,b).

4. The network device (1 1 Oa-d: 130a, b) of claim 1, wherein the network device (1 1 Oa-d: 130a, b) is configured to generate the plurality of uncorrelated sounding signals s;and apply a weighting factorto each sounding signal s;and wherein each weighting factoris based on a joint optimization of a plurality of spatial filters and a plurality of transmission powers of the plurality of UEs (120a,b).

5. The network device ( 1 1 Oa-d: 130a,b) of any one of the preceding claims, wherein one or more of the plurality of uncorrelated sounding signals s;comprises one or more random signal portions.

6. The network device (1 1 Oa-d: 130a, b) of any one of the preceding claims, wherein the network device (1 1 Oa-d: 130a, b) is configured to transmit the plurality of uncorrelated sounding signals s;to the plurality of UEs (120a, b) using a plurality of pre-defined time and / or frequency resources.

7. The network device (1 1 Oa-d: 130a, b) of any one of the preceding claims, wherein the network device (1 1 Oa-d: 130a,b) is configured to receive one or more precoded UL transmissions from one or more of the plurality of UEs (120a, b), wherein the one or more precoded UL transmissions are based on the respective precoder determined by the respective UE (120a, b) using a covariance matrix based on a superposition of the plurality of sounding signals received from the network device (1 1 Oa-d: 130a, b) and a plurality of further uncorrelated sounding signals received by the respective UE (120a, b) from one or more further network devices of the plurality of network devices (1 1 Oa-d: 130a, b).

8. The network device (1 1 Oa-d: 130a, b) of any one of the preceding claims, wherein the network device (1 1 Oa-d: 130a, b) is further configured to provide a respective regularization matrix Qkto each of the plurality of UEs (120a, b) for allowing each UE (120a,b) to determine the respective precoder using the respective covariance matrix and the respective regularization matrix Qk.

9. A method (600) for operating a network device l' I I Oa-d: 130a, b) of a plurality of distributed network devicesI' I I Oa-d: 130a, b) for providing network access to a plurality of user equipment, UEs, (120a, b) in a cell-free wireless network (100), wherein the method (600) comprises: obtaining (601) a plurality of uncorrelated sounding signals s;: and transmitting (603) the plurality of uncorrelated sounding signals s;to the plurality of UEs (120a, b) for allowing each UE (120a, b) to determine a precoder using a covariance matrix based on a superposition of the plurality of sounding signals received from the network device 1' 1 l Oa-d: 130a,b) and a plurality of further uncorrelated sounding signals received by the respective UE (120a, b) from one or more further network devices of the plurality of network devices 1' 1 l Oa-d: 130a,b).

10. A user equipment, UE, (120a, b) for network access provided by a plurality of network devices (1 1 Oa-d: 130a, b) in a cell-free wireless network (100), wherein the UE (120a, b) is configured to: receive a plurality of uncorrelated sounding signals s;from the plurality of network devices (1 1 Oa-d: 130a, b ): and determine a precoder using a covariance matrix based on a superposition of the plurality of uncorrelated sounding signals received from the plurality of network devices (1 1 Oa-d: 130a, b).

11. The UE (120a,b) of claim 10, wherein the UE (120a,b) is configured to transmit a plurality of UL sounding signals to the plurality of network devices ( 1 1 Oa-d: 130a,b) for allowing the plurality of network devices ( 1 1 Oa-d: 130a,b) to generate the plurality of uncorrelated sounding signals s;based on the plurality of UL sounding signals.

12. The UE (120a,b) of claim 10 or 11, wherein each sounding signal s;is weighted with a weighting factorand wherein each weighting factoris based on a joint optimization of a plurality of spatial filters and a plurality of transmission powers of the plurality of UEs (120a, b) using a covariance matrix based on the superposition of the plurality of sounding signals received from the plurality of network devices (1 1 Oa-d: 130a, b).

13. The UE (120a,b) of any one of claims 10 to 12, wherein the UE (120a,b) is configured to determine the covariance matrix Zkon the basis of the following equation:wherein E[ ] denotes the expectation value, ykdenotes the superposition of the plurality of sounding signals received by the UE (120a, b), and hklihk'ldenote the channel vectors.

14. The UE (120a, b) of any one of claims 10 to 13, wherein the UE (120a, b) is configured to determine the precoder using the covariance matrix and a regularization matrix Qk.

15. The UE (120a, b) of claim 14, wherein the UE (120a, b) is configured to receive the regularization matrix Qkfrom one or more of the plurality of network devices (1 1 Oa-d: 130a, b).

16. The UE (120a, b) of claim 14 or 15, wherein the UE (120a, b) is configured to determine the precoder IVj. based on the following equation:Wk= (Zk+ QklHk, wherein Hkdenotes the channel matrix for the UE (120a, b).

17. A method (700) of operating a user equipment, UE, (120a, b) for network access provided by a plurality of network devices (1 lOa-d; 130a, b) in a cell-free wireless network (100), wherein the method (700) comprises: receiving (701) a plurality of uncorrelated sounding signals s;from the plurality of network devices (1 lOa-d:130a.b): and determining (703) a precoder using a covariance matrix based on a superposition of the plurality of sounding signals received from the plurality of network devices (1 1 Oa-d: 130a, b).

18. A computer program product comprising a computer-readable storage medium for storing program code which causes a computer or a processor to perform the method (600) of claim 9 or the method (700) of claim 17 when the program code is executed by the computer or the processor.

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