Methods, baseband unit system and radio unit of a distributed base station having cascade-coupled radio units

Disclosed is a method performed by a first radio unit, RU, of a distributed base station system. The distributed base station system further comprises a Baseband Unit, BBU, connected to the first RU over a fronthaul link and a second RU connected to the first RU over an RU link. The first RU determines first part of beamforming weights that it uses for performing a first part of the beamforming of uplink signals received at its antennas into intermediate signals. The first RU further determines intermediate beamforming weights to be used for a second part of beamforming at the BBU. The first RU further receives, from the second RU, intermediate signals and intermediate beamforming weights determined by the second RU. The first RU combines the intermediate signals and sends them to the BBU. The first RU further combines the intermediate beamforming weights and sends them to the BBU.

This application is a 35 U.S.C. § 371 national phase filing of International Application No. PCT/SE2021/050302, filed Apr. 1, 2021, the disclosure of which is incorporated herein by reference in its entirety.

TECHNICAL FIELD

The present disclosure relates generally to methods, baseband unit (BBU) system and radio unit (RU) of a distributed base station having cascade-coupled RUs. More specifically, the present disclosure relates to a first RU of a distributed base station system, wherein the distributed base station system further comprises a BBU connected to the first RU over a fronthaul link and a second RU connected to the first RU over an RU link. The present disclosure also relates to a BBU system associated with the distributed base station system. The present disclosure further relates to computer programs and carriers corresponding to the above methods, RUs and systems.

BACKGROUND

In a centralized radio access network (C-RAN), also called a distributed base station system, radio access network (RAN), processing is conducted by two separate units: a radio unit (RU), and a base band unit (BBU). The BBU is connected to the RU via a fronthaul link. The RU may also be called remote radio unit (RRU). The base band unit may also be called base unit (BU) or digital unit or distributed unit (DU). The RU is connected to one or more antennas through which the RU wirelessly communicates with at least one user equipment (UE). The BBU is in its turn connected to other base station systems or base stations, and to a core network of a wireless communication system. The BBU is centralized and there is normally more than one RU connected to each BBU. Traditionally, the BBU performs advanced radio coordination features such as joint detection, joint decoding, coordinated multi-point transmission (CoMP), to increase the spectrum efficiency and network capacity, as well as baseband processing, whereas the RUs perform radio frequency (RF) processing and transmission/reception of the RF processed signals.

Originally, the RU was designed to reduce the cable loss of the coax cables between an antenna tower top where the actual antennas are situated and the bottom of the antenna tower where the base station functionality is hosted. Therefore, before 5thGeneration of mobile communication (5G), i.e. at 4G, e.g. Long Term Evolution (LTE), the RU was rather simple and was mainly doing RF processing with limited baseband processing, if any.

When going from 4G to 5G, there was a need to increase the wireless communication capacity towards the UEs in order to be able to deliver requested data amounts per time period in 5G. One enabler of the mobile evolution towards 5G is massive Multiple Input Multiple Output (MIMO) in which each RU has a plurality of antennas. Massive MIMO exploits spatial multiplexing to improve spectrum efficiency by using antenna arrays at the RU, which antenna array is equipped with N antennas simultaneously serving K user-layers in the same time-frequency resource. The typical scenario is N>>K e.g., N is 64, 128 or 256 while K is 8 or 16. As shown, the number of antennas is quite large. Massive MIMO is often referred to as massive beamforming, which can form narrow beams and focus on different directions, mitigating against the increased path loss of higher frequency bands. It also benefits multi-user MIMO, which allows the transmissions from/to multiple UEs simultaneously over separate spatial channels resolved by the massive MIMO technologies, while keeping high capacity for each UE. Therefore, it can significantly increase the spectrum efficiency and cell capacity.

In 5G evolution and future 6thGeneration of mobile communication (6G), massive MIMO is expected to support even more antennas, given that the cost per transceiver chain would decrease over time. To address this trend, the MIMO processing is foreseen to be more distributed and scalable, where a larger MIMO system is processed by multiple RUs, each of which only processes a subset of antennas. With such a scalable design, the MIMO system can easily scale with respect to the number of antennas.

To support such massive MIMO solutions, the required fronthaul link capacity needs to increase in proportion to the increase of number of antennas, at least when using current PHY-RF split between functionality of the BBU and the RU. This will dramatically drive up the fronthaul link costs.

To reduce the required fronthaul (FH) capacity, functional splits within the physical layer (PHY) are discussed and proposed. Basically, some baseband PHY functions will be moved to the RU, which mainly performs RF-related operations in the Common Public Radio Interface (CPRI) based implementation.

Specifically, emerging lower-layer split (LLS) options have the beamforming function placed in the RU to reduce the number of FH streams from the number of antennas to the number of user layers. As the number of user layers is much fewer than the number of antennas in the massive MIMO, the required FH capacity and thereby the FH costs are significantly reduced. For example, if the system is N=64 antennas and K=8 user layers, there are only 8 FH streams going through the FH link. This can reduce the required FH capacity, which is also proportional to traffic load. However, moving the beamforming processing, especially the beamforming calculation, to the RU increases significantly the complexity of the RU. Further, the system is not scalable to support more antennas. It needs to replace the existing RU to support more antennas. It also limits the joint-MIMO-processing possibility for coordinating multiple RUs at different places.

In “Functional Split of Zero-Forcing Based Massive MIMO for Fronthaul Load Reduction,” by Y. Huang, C. Lu, M. Berg and P. Odling, published in IEEE Access, vol. 6, pp. 6350-6359, 2018, an intra-PHY functional split scheme between the BBU and the RU regarding uplink is proposed. Instead of moving all MIMO/beamforming processing to the RU, the MIMO processing is decomposed to two parts. The first part requiring lower complexity is implemented in the RU, while the second part requiring higher complexity is implemented in the BBU. The proposed intra-PHY functional split takes advantage of a formation feature of a known Zero-forcing (ZF) method and separates the MIMO processing into a maximum ratio combining (MRC) part and an interference-cancellation part. The MRC processing only carries out Hermitian transpose of the estimated channel. This is computationally light and therefore the MRC processing is moved to the RU. The interference cancellation part contains matrix inversion, which is computationally heavy. This is instead carried out in the BBU. This scheme reduces the number of FH streams to the number of layers and achieves the same performance as an original ZF-based approach when it is implemented totally in RU. However, the proposed intra-PHY functional split using the ZF-based method does not consider inter-cell interference, i.e. interference from UEs connected to other base stations. Consequently, the performance degrades when strong inter-cell interferences are present.

WO2020/130895 of the present applicant describes a method for offloading some RU complexity to the BBU and in at least some embodiments to make a base station system scalable to support more antennas at the RUs, while keeping a moderate FH traffic load. Parts of this disclosure present a method in which the mathematical formulation of an Interference Rejection Combining (IRC) method is reformulated as a ZF (zero-forcing) method of an extended “channel” including interference aspects, and then the ZF process is decomposed into two parts. The first part performs MRC, which is much simpler than the full IRC and is therefore implemented in the RU. The second part takes remaining calculations, such as matrix inversion, with high complexity and is implemented in the BBU.

Hereby, the number of required FH streams can be reduced to the number of desired user layers plus the number of the interfering user layers of co-channel interfering UEs from other cells. This is theoretically sound as the number of the degrees of freedom are reserved to the BBU to mitigate both intra-cell and inter-cell interferences, i.e. the interferences between desired user layers in the same cell and the interferences from the interfering user layers from other cells.

The overall complexity of the base station system of WO2020/130895 scales linearly with the number of antennas, instead of cubic scaling. However, WO2020/130895 addresses only a point-to-point FH topology, where each RU has a dedicated FH link to the BBU, as shown inFIG.1. In the point-to-point FH topology example of a distributed base station system10as shown inFIG.1, a first RU30is connected to a BBU20over a first FH link25, a second RU40is connected to the BBU20over a second FH link35and third RU50is connected to the BBU20over a third FH link45. The first, second and third RUs30,40,50are arranged to transmit and receive user-plane data as antenna signals to/from UEs31,32,33. Such a point-to-point FH would need many fiber connections and the same number of BBU ports, even if those RUs are configured as a joint larger MIMO system.

In this disclosure on the other hand, a cascaded topology of RUs is addressed, as shown inFIG.2. In a cascaded topology, a first RU120is connected to the BBU110over a fronthaul link140as in the point-to-point topology. However, the second RU160is then connected to the first RU120via a separate RU-RU link165, and a third RU170is connected to the second RU160via another separate RU-RU link175. Further, if there are any more RUs, they are in their turn connected to an RU, one after the other as in a line. InFIG.2, there are only three RUs illustrated but there may be many more RUs in such a cascade-coupled topology. The cascaded RU deployment would reduce the amount of FH fiber links and the number of BBU ports to 1. This would help reduce the deployment costs, i.e. fiber connections and system complexity, i.e. BBU ports.

However, in the cascade-coupled topology, especially when using the methods described in WO2020/130895, each RU will send a separate data flow including both fronthaul user-plane data and control plane data over the same cascaded RU-RU links and towards the BBU eventually via the FH link140between the BBU110and the first RU120. This increases the FH bit rate at each RU-RU link. Especially, the FH link140between the BBU110and the first RU120aggregates all RU data, where the total traffic increases proportionally to the number of RUs. In this case, much more expensive optical transceivers are required to handle the increased FH traffic to reduce the number of fibers. Also, a port of the BBU connected to first RU120would need to process much more data which would increase the BBU complexity. Consequently, there is a need for a solution to handle distributed base station systems having RUs cascade-coupled to the BBU. Such a solution should preferably manage to keep the total amount of data of both FH user-plane and control plane sent over the FH link low, preferably on the same level of the individual FH link in the star topology, and also require minimum changes in the BBU processing.

SUMMARY

It is an object of the invention to address at least some of the problems and issues outlined above. It is possible to achieve these objects and others by using methods, network nodes and wireless devices as defined in the attached independent claims.

According to one aspect, a method is provided that is performed by a first RU of a distributed base station system, the first RU comprising N1 antennas. The distributed base station system further comprises a BBU connected to the first RU over a fronthaul link and a second RU connected to the first RU over an RU link, the second RU comprising N2 antennas. The method comprises obtaining first uplink signals y1in frequency domain as received at the N1 antennas from a number of UEs, the first uplink signals y1comprising K user-layer signals in frequency domain overlaid with interference signals and noise. The method further comprises obtaining a first channel estimate Ĥ1of wireless communication channels H1in frequency domain between the number of UEs and the N1 antennas, and determining a first part of BFW)for the first RU based on the first channel estimate Ĥ1and on reference signals received at the N1 antennas together with the first uplink signals, the first part of BFW being used to reduce the N1 first uplink signals to K+M first intermediate signals {tilde over (y)}1, where K+M is lower than N1. The method further comprises determining the K+M first intermediate signals {tilde over (y)}1based on the N1 first uplink signals y1and on the first part of the BFW for the first RU, and determining a first part of intermediate BFW C1for interference cancellation at the BBU based on the first channel estimate Ĥ1and on the reference signals received at the N1 antennas together with the first uplink signals. Further, the method comprises receiving, from the second RU, K+M second intermediate signals {tilde over (y)}2, the second intermediate signals being determined by the second RU based on second uplink signals y2in frequency domain as received at the N2 antennas of the second RU from the number of UEs and on a first part of BFW for the second RU determined by the second RU based on a second channel estimate Ĥ2of wireless communication channels H2in frequency domain between the number and the N2 antennas and on reference signals received at the N2 antennas together with the second uplink signals, the first part of BFW of the second RU being used to reduce the N2 second uplink signals to the K+M second intermediate signals {tilde over (y)}2, where K+M is lower than N2. The method further comprises receiving, from the second RU, a second part of intermediate BFW C2for interference cancellation, determined by the second RU based on the second channel estimate Ĥ2and on the reference signals received at the N2 antennas together with the second uplink signals. The method further comprises combining the first intermediate signals {tilde over (y)}1and the second intermediate signals {tilde over (y)}2into combined intermediate signals {tilde over (y)}1+{tilde over (y)}2, combining the first part of intermediate BFW C1and the second part of intermediate BFW C2into combined intermediate BFW C1+C2, and sending, to the BBU over the fronthaul link, the combined intermediate signals {tilde over (y)}1+{tilde over (y)}2and the combined intermediate BFW C1+C2.

According to another aspect, a method is provided that is performed by a BBU system of a wireless communication network. The wireless communication network comprises a distributed base station system having a BBU, a first RU connected to the BBU over a fronthaul link, and a second RU connected to the first RU over an RU link. The first RU comprises N1 antennas and the second RU comprises N2 antennas. The method comprises receiving, from the first RU, combined intermediate signals {tilde over (y)}1+{tilde over (y)}2in frequency domain comprising first intermediate signals {tilde over (y)}1and second intermediate signals {tilde over (y)}2. The first intermediate signals {tilde over (y)}1are determined by the first RU based on first uplink signals y1as received at the N1 antennas of the first RU from a number of UEs. The first uplink signals y1comprise K user-layer signals overlaid with interference signals and noise. The first intermediate signals are further determined based on a first part of BFW for the first RU, determined by the first RU based on a first channel estimate Ĥ1of wireless communication channels H1in frequency domain between the number UEs and the N1 antennas, and on reference signals received at the N1 antennas together with the first uplink signals. The first part of BFW is used to reduce the N1 first uplink signals to K+M first intermediate signals {tilde over (y)}1, where K+M is lower than N1. The second intermediate signals {tilde over (y)}2are determined by the second RU based on second uplink signals y2as received at the N2 antennas of the second RU from the number of UEs. The second uplink signals y2comprise K user-layer signals overlaid with interference signals and noise. The second intermediate signals are further determined based on first part of BFW for the second RU, determined by the second RU based on a second channel estimate Ĥ2of wireless communication channels H2in frequency domain between the number of UEs and the N2 antennas, and on reference signals received at the N2 antennas together with the second uplink signals. The method further comprises receiving from the first RU, combined intermediate BFW C1+C2comprising a first part of intermediate BFW C1and a second part of intermediate BFW C2. The first part of intermediate BFW C1is determined by the first RU based on the first channel estimate Ĥ1and on the reference signals received at the N1 antennas together with the first uplink signals. The second part of intermediate BFW C2is determined by the second RU based on the second channel estimate Ĥ2and on the reference signals received at the N2 antennas together with the second uplink signals. The method further comprises determining a second part of BFW based on the combined intermediate BFW C1+C2, and determining an estimation r of the K user-layer signals based on the combined intermediate signals {tilde over (y)}1+{tilde over (y)}2and the second part of BFW.

According to another aspect, a first RU is provided that is operable in a distributed base station system of a wireless communication network, the first RU comprising N1 antennas. The distributed base station system further comprises a BBU connected to the first RU over a fronthaul link and a second RU connected to the first RU over an RU link. The second RU comprises N2 antennas. The first RU comprises a processing circuitry and a memory. Said memory contains instructions executable by said processing circuitry, whereby the first RU is operative for obtaining first uplink signals y1in frequency domain as received at the N1 antennas from a number of UEs, the first uplink signals y1comprising K user-layer signals in frequency domain overlaid with interference signals and noise, and obtaining a first channel estimate Ĥ1of wireless communication channels H1in frequency domain between the number of UEs and the N1 antennas. The first RU is further operative for determining a first part of BFW for the first RU based on the first channel estimate Ĥ1and on reference signals received at the N1 antennas together with the first uplink signals, the first part of BFW being used to reduce the N1 first uplink signals to K+M first intermediate signals {tilde over (y)}1, where K+M is lower than N1, and determining the K+M first intermediate signals {tilde over (y)}1based on the N1 first uplink signals y1and on the first part of the BFW for the first RU. The first RU is further operative for determining a first part of intermediate BFW C1for interference cancellation at the BBU based on the first channel estimate Ĥ1and on the reference signals received at the N1 antennas together with the first uplink signals. The first RU is further operative for receiving, from the second RU, K+M second intermediate signals {tilde over (y)}2, the second intermediate signals being determined by the second RU based on second uplink signals y2in frequency domain as received at the N2 antennas of the second RU from the number of UEs and on a first part of BFW for the second RU determined by the second RU based on a second channel estimate Ĥ2of wireless communication channels H2in frequency domain between the number UEs and the N2 antennas and on reference signals received at the N2 antennas together with the second uplink signals, the first part of BFW of the second RU being used to reduce the N2 second uplink signals to the K+M second intermediate signals {tilde over (y)}2, where K+M is lower than N2. The first RU is further operative for receiving, from the second RU, a second part of intermediate BFW C2for interference cancellation, determined by the second RU based on the second channel estimate Ĥ2and on the reference signals received at the N2 antennas together with the second uplink signals. The first RU is further operative for combining the first intermediate signals {tilde over (y)}1and the second intermediate signals {tilde over (y)}2into combined intermediate signals {tilde over (y)}1+{tilde over (y)}2, combining the first part of intermediate BFW C1and the second part of intermediate BFW C2into combined intermediate BFW C1+C2, and sending, to the BBU110over the fronthaul link, the combined intermediate signals {tilde over (y)}1+{tilde over (y)}2and the combined intermediate BFW C1+C2.

According to another aspect, a BBU system is provided that is operable in a wireless communication network. The wireless communication network comprises a distributed base station system having a BBU, a first RU connected to the BBU over a fronthaul link, the first RU comprising N1 antennas, and a second RU connected to the first RU over an RU link, the second RU comprising N2 antennas. The BBU system comprises a processing circuitry and a memory. Said memory contains instructions executable by said processing circuitry, whereby the BBU system is operative for receiving, from the first RU, combined intermediate signals {tilde over (y)}1+{tilde over (y)}2in frequency domain comprising first intermediate signals {tilde over (y)}1and second intermediate signals {tilde over (y)}2. The first intermediate signals {tilde over (y)}1are determined by the first RU based on first uplink signals y1as received at the N1 antennas of the first RU from a number of UEs, the first uplink signals y1comprising K user-layer signals overlaid with interference signals and noise, and on first part of BFW for the first RU, determined by the first RU based on a first channel estimate Ĥ1of wireless communication channels H1in frequency domain between the number UEs and the N1 antennas and on reference signals received at the N1 antennas together with the first uplink signals, the first part of BFW being used to reduce the N1 first uplink signals to K+M first intermediate signals {tilde over (y)}1, where K+M is lower than N1. The second intermediate signals {tilde over (y)}2are determined by the second RU based on second uplink signals y2as received at the N2 antennas of the second RU from the number of UEs, the second uplink signals y2comprising K user-layer signals overlaid with interference signals and noise, and on first part of BFW for the second RU, determined by the second RU based on a second channel estimate Ĥ2of wireless communication channels H2in frequency domain between the number of UEs and the N2 antennas, and on reference signals received at the N2 antennas together with the second uplink signals. The BBU system is further operative for receiving from the first RU, combined intermediate BFW C1+C2comprising a first part of intermediate BFW C1and a second part of intermediate BFW C2, the first part of intermediate BFW C1being determined by the first RU based on the first channel estimate Ĥ1and on the reference signals received at the N1 antennas together with the first uplink signals, the second part of intermediate BFW C2being determined by the second RU based on the second channel estimate Ĥ2and on the reference signals received at the N2 antennas together with the second uplink signals. The BBU system is further operative for determining a second part of BFW based on the combined intermediate BFW C1+C2, and for determining an estimation r of the K user-layer signals based on the combined intermediate signals {tilde over (y)}1+{tilde over (y)}2and the second part of BFW.

According to other aspects, computer programs and carriers are also provided, the details of which will be described in the claims and the detailed description.

Further possible features and benefits of this solution will become apparent from the detailed description below.

DETAILED DESCRIPTION

FIG.2illustrates a wireless communication network in which the present invention may be used. The wireless communication network comprises a distributed base station system100, which in turn comprises a BBU110and a first RU120. The BBU110has connections to other base station nodes or other RAN nodes and further to a core network (symbolized with150inFIG.2) so that the distributed base station system100can communicate with other nodes of the communication network. The BBU110is connected to the first RU120via a fronthaul link140. The fronthaul link140may be any kind of connection, such as a dedicated wireline or wireless connection or a connection via a network, as long as the connection fulfils fronthaul requirements, e.g. in capacity and latency. The first RU120further has a plurality of antennas121,122,123through which wireless signals are communicated towards and from one or more UEs131,132,133. The wireless signals comprise data to be communicated from or to the UEs131,132,133. The distributed base station system100further comprises a second RU160that is connected to the first RU120over an RU link165. Observe that the second RU160has no direct connection to the BBU110but is connected to the BBU via the RU link165, the first RU120and the fronthaul link140. The second RU160further has a plurality of antennas161,162,163through which wireless signals are communicated towards and from the one or more UEs131,132,133. The distributed base station system100may further comprise a third RU170that is connected to the second RU160over a second RU link175. Observe that the third RU170has no direct connection to the BBU110but is connected to the BBU via the second RU link175, the second RU160, the RU link165, the first RU120and the fronthaul link140. The third RU170further has a plurality of antennas171,172,173through which wireless signals are communicated towards and from the one or more UEs131,132,133. The distributed base station system100may comprise further RUs cascade-coupled onto the third RU170in a similar way.

The BBU110and the first RU120, second RU160and third RU170and any possible other RUs each comprise RAN functionality for handling the data and signals to be communicated between the BBU110, the RUs120,160,170and the UEs131,132,133. The RAN functionality is distributed between the BBU110and the RUs as will be described further down in this disclosure. It can be noted that in 3GPP, the BBU can be further split to two units called Distributed Unit (DU) and Central Unit (CU), where the DU is arranged to perform lower layer processing, e.g. L1 and L2 of the BBU, and the CU is arranged to perform higher layer processing of the BBU, e.g. L3 and higher. Note that the BBU and the RU are referred to as O-DU and O-RU, respectively, in O-RAN. In eCPRI terminologies, the BBU and the RU are referred to as eREC (eCPRI Radio Equipment Control) and eRE (eCPRI Radio Equipment), respectively. In another terminology, the BBU and the RU may be referred to as LLS-CU and LLS-DU, respectively.

The wireless communication network100may be any kind of wireless communication network that can provide radio access to wireless devices. Example of such wireless communication networks are networks based on Global System for Mobile communication (GSM), Enhanced Data Rates for GSM Evolution (EDGE), Universal Mobile Telecommunications System (UMTS), Code Division Multiple Access 2000 (CDMA 2000), Long Term Evolution (LTE), LTE Advanced, Wireless Local Area Networks (WLAN), Worldwide Interoperability for Microwave Access (WiMAX), WiMAX Advanced, as well as fifth generation (5G) wireless communication networks based on technology such as New Radio (NR), and any possible future sixth generation (6G) wireless communication network.

The UEs131,132,133may be any type of communication device capable of wirelessly communicating with the RUs120,160,170using radio signals. For example, the UEs may be a machine type UE or a UE capable of machine to machine (M2M) communication, a sensor, a tablet, a mobile terminal, a smart phone, a laptop embedded equipped (LEE), a laptop mounted equipment (LME), a USB dongle, a Customer Premises Equipment (CPE) etc. The UE may also be called a wireless communication device or wireless device.

FIG.3, in conjunction withFIG.2, describes a method performed by a first RU120of a distributed base station system100, the first RU120comprising N1 antennas121,122,123. The distributed base station system100further comprises a BBU110connected to the first RU120over a fronthaul link140and a second RU160connected to the first RU120over an RU link165, the second RU comprising N2 antennas161,162,163. The method comprises obtaining202first uplink signals y1in frequency domain as received at the N1 antennas121,122,123from a number of UEs131,132,133, the first uplink signals y1comprising K user-layer signals in frequency domain overlaid with interference signals and noise. The method further comprises obtaining204a first channel estimate Ĥ1of wireless communication channels H1in frequency domain between the number of UEs131,132,133and the N1 antennas121,122,123, and determining208a first part of beamforming weights (BFW) for the first RU based on the first channel estimate Ĥ1and on reference signals received at the N1 antennas together with the first uplink signals, the first part of BFW being used to reduce the N1 first uplink signals to K+M first intermediate signals {tilde over (y)}1, where K+M is lower than N1. The method further comprises determining210the K+M first intermediate signals {tilde over (y)}1based on the N1 first uplink signals y1and on the first part of the BFW for the first RU, and determining212a first part of intermediate BFW C1for interference cancellation at the BBU based on the first channel estimate Ĥ1and on the reference signals received at the N1 antennas together with the first uplink signals. Further, the method comprises receiving214, from the second RU160, K+M second intermediate signals {tilde over (y)}2, the second intermediate signals being determined by the second RU based on second uplink signals y2in frequency domain as received at the N2 antennas161,162,163of the second RU from the number of UEs131,132,133and on a first part of BFW for the second RU determined by the second RU based on a second channel estimate Ĥ2of wireless communication channels H2in frequency domain between the number UEs131,132,133and the N2 antennas161,162,163and on reference signals received at the N2 antennas together with the second uplink signals, the first part of BFW of the second RU being used to reduce the N2 second uplink signals to the K+M second intermediate signals {tilde over (y)}2, where K+M is lower than N2. The method further comprises receiving216, from the second RU160, a second part of intermediate BFW C2for interference cancellation, determined by the second RU based on the second channel estimate Ĥ2and on the reference signals received at the N2 antennas together with the second uplink signals. The method further comprises combining218the first intermediate signals {tilde over (y)}1and the second intermediate signals {tilde over (y)}2into combined intermediate signals {tilde over (y)}1+{tilde over (y)}2, combining220the first part of intermediate BFW C1and the second part of intermediate BFW C2into combined intermediate BFW C1+C2, and sending222, to the BBU110over the fronthaul link140, the combined intermediate signals {tilde over (y)}1+{tilde over (y)}2and the combined intermediate BFW C1+C2.

When such a method and a cascaded topology is used, the required fronthaul capacity over the fronthaul link can be kept on a level that is the same as the required fronthaul capacity for each link in a star topology using the method presented in WO2020/130895. In other words, due to the combining of the first and second intermediate signals, the combined intermediate signals keep the same dimension as a single intermediate signal. Further due to the combining of the first and second intermediate BFW, the combined intermediate BFW keep the same dimension as intermediate BFW from one RU. Also, the dimension of the combined intermediate signal and the combined intermediate BFW keep the same dimension even if the number of cascaded RUs increase.

The “interference signals” are signals originating from UEs wirelessly connected to other base stations or base station systems than this base station system. The K user-layer signals use the same time-frequency resource when transmitted wirelessly from the UEs to the respective RU. N1 and N2 in “N1/N2 antennas” are integers larger than or equal to 2. The first and second channel estimate are based on reference signals sent by the UEs131,132,133towards the respective first and second RU, and how the sent reference signals are received at the respective first and second RU compared to what they looked like when they were sent. The first uplink signals y1, as well as the second uplink signals y2, can be modelled as a vector where each vector element represents the received signal at each antenna among the N1 or N2 antennas. One channel estimate Ĥ is determined for a limited time period and frequency range. According to an embodiment, the channel is estimated for each resource block (RB). In this embodiment there are many channel estimates determined, for example, the LTE 20 MHz range has 100 RBs and the NR 100 MHz has 273 RBs with subcarrier spacing of 30 kHz.

The main purpose for the determining of first intermediate signals using the first part BFWs is to reduce the number of streams from N1 to K+M, where K is the number of layers and M is a number representing the additional degrees of freedom kept for handling a number of co-channel interferences from other cells, i.e. signals from UEs connected to other base stations. Basically, K degrees of freedom is used to cancel out the intra interferences between the number of UEs and M degrees of freedom is used to mitigate the inter-cell interferences from other cells. To achieve the best performance, M is a design parameter that should be larger than the number of dominant inter-cell interferences.

The combining218signifies adding the first and second intermediate signals elementwise so that there will be the same amount of elements in the combined signal as in each of the intermediate signals. The combining220signifies adding the first and second part of intermediate BFW elementwise so that there will be the same amount of elements in the combined intermediate BFW as in each of the first and second intermediate BFW.

According to an embodiment, the method further comprises determining206a first error estimate G1based on the obtained first channel estimate Ĥ1and on the reference signals yref,m,1received at the N1 antennas from the number of UEs131,132,133together with the first uplink signals, the received reference signals having {tilde over (M)} symbols for m=1, . . . , {tilde over (M)}, where {tilde over (M)}≥M, and wherein the first part of the BFW for the first RU as well as the first part of the intermediate BFW are determined208,212based also on the first error estimate G1.

In this embodiment, the first part of the BFW for the first RU as well as the first part of the intermediate BFW are determined not only based on the first channel estimate Ĥ1but also on the first error estimate G1. By such an error estimate, the interferences from other UEs connected other base stations than the described base station system is better handled since the error estimate contains the information of the interferences and noises which are utilized here to mitigate the interferences. Further, the received second intermediate signals {tilde over (y)}2, and the received second part of the intermediate BFW for the second RU were determined by the second RU based also on a second error estimate G2based on the second channel estimate Ĥ2and on the reference signals yref,m,2as received at the N2 antennas from the number of UEs131,132,133together with the second uplink signals.

According to an embodiment, the first error estimate G1is determined206as

According to another embodiment, the determining210of the first intermediate signals {tilde over (y)}1comprises multiplying the first uplink signals y1with the transpose and conjugate A1* of a first extended channel estimate A1, which is obtained based on the first channel estimate Ĥ1and the first error estimate G1. Therefore, the extend channel represents both the channel information and the information regarding interferences. In other words, the first part of the BFW makes the uplink signals from the N1 antennas co-phased with relation to the extended channel estimate before the combining of the intermediate signals, so hereby the uplink signals from the N1 antennas are combined coherently in phase. Further, in the second RU, the second intermediate signals {tilde over (y)}2were determined by the second RU by multiplying the second uplink signals y2with the transpose and conjugate A2* of a second extended channel estimate A2, which is obtained based on the second channel estimate Ĥ2and the second error estimate G2.

According to another embodiment, the first part of the intermediate BFW C1are determined212by multiplying a transpose and conjugate A1* of a first extended channel estimate A1, which is obtained based on the first channel estimate Ĥ1and the first error estimate G1, with the first extended channel estimate A1. Further, in the second RU, the second part of the intermediate BFW C2are determined by multiplying a transpose and conjugate A2* of a second extended channel estimate A2, which is obtained based on the second channel estimate Ĥ2and the second error estimate G2, with the second extended channel estimate A2.

FIG.4, in conjunction withFIG.2, describes a method performed by a BBU system700of a wireless communication network, the wireless communication network comprising a distributed base station system100having a BBU110, a first RU120connected to the BBU110over a fronthaul link140, and a second RU160connected to the first RU120over an RU link165. The first RU comprises N1 antennas121,122,123and the second RU comprises N2 antennas161,162,163. The method comprises receiving302, from the first RU120, combined intermediate signals {tilde over (y)}1+{tilde over (y)}2in frequency domain comprising first intermediate signals {tilde over (y)}1and second intermediate signals {tilde over (y)}2. The first intermediate signals {tilde over (y)}1are determined by the first RU120based on first uplink signals y1as received at the N1 antennas121,122,123of the first RU from a number of UEs131,132,133. The first uplink signals y1comprise K user-layer signals overlaid with interference signals and noise. The first intermediate signals are further determined based on a first part of BFW for the first RU, determined by the first RU based on a first channel estimate Ĥ1of wireless communication channels H1in frequency domain between the number UEs131,132,133and the N1 antennas121,122,123, and on reference signals received at the N1 antennas together with the first uplink signals. The first part of BFW is used to reduce the N1 first uplink signals to K+M first intermediate signals {tilde over (y)}1, where K+M is lower than N1. The second intermediate signals {tilde over (y)}2are determined by the second RU160based on second uplink signals y2as received at the N2 antennas161,162,163of the second RU from the number of UEs131,132,133. The second uplink signals y2comprise K user-layer signals overlaid with interference signals and noise. The second intermediate signals are further determined based on first part of BFW for the second RU, determined by the second RU based on a second channel estimate Ĥ2of wireless communication channels H2in frequency domain between the number of UEs131,132,133and the N2 antennas161,162,163, and on reference signals received at the N2 antennas together with the second uplink signals. The method further comprises receiving304from the first RU120, combined intermediate BFW C1+C2comprising a first part of intermediate BFW C1and a second part of intermediate BFW C2. The first part of intermediate BFW C1is determined by the first RU120based on the first channel estimate Ĥ1and on the reference signals received at the N1 antennas together with the first uplink signals. The second part of intermediate BFW C2is determined by the second RU160based on the second channel estimate Ĥ2and on the reference signals received at the N2 antennas together with the second uplink signals. The method further comprises determining306a second part of BFW based on the combined intermediate BFW C1+C2, and determining308an estimation r of the K user-layer signals based on the combined intermediate signals {tilde over (y)}1+{tilde over (y)}2and the second part of BFW.

The BBU system700of the wireless communication network that performs the method may be the BBU110, a unit in the BBU or in the distributed base station system100. Alternatively, the BBU system700that performs the method may be arranged in or at any other network node of the communication network, such as a node further away from the UEs, e.g. another network element in the RAN or close to the RAN or another RAN node. In this alternative, and in the cloud-solution embodiment discussed below, the BBU110receives, from the first RU120the combined intermediate signals {tilde over (y)}1+{tilde over (y)}2and the combined intermediate BFW C1+C2, and communicates the combined intermediate signals {tilde over (y)}1+{tilde over (y)}2and the combined intermediate BFW C1+C2to the other network node that determines306,308the second part of the BFW as well as the estimation r of the K user-layer signals. Alternatively, the BBU system700that performs the method may be a group of network nodes, wherein functionality for performing the method is spread out over different physical, or virtual, nodes of the network. The latter may be called a “cloud-solution”.

According to an embodiment, the determining306of the second part of the BFW comprises determining the second part of BFW based on the inverse of the combined intermediate BFW C1+C2.

According to an embodiment, the second part of BFW is determined306as the first K rows of the inverse of the combined intermediate BFW C1+C2.

According to another embodiment, the estimation r of the K user-layer signals is determined308by multiplying the combined intermediate signals {tilde over (y)}1+{tilde over (y)}2with the second part of BFW.

In the following, a model of an embodiment of a cascaded base station system such as the one shown inFIG.2is described. In this system, consider an exemplified scenario with K user layers, where K UEs are served in a cell and each UE has one antenna and one user layer, and L RUs in the local area cascade-coupled to a BBU in a daisy chain serving the K UEs in the cell. Note that one UE can have more than one antenna and more than one user layer. The RUs may effectively form a large antenna array serving one cell. The first RU in the chain, i.e. RU 1 is the one with fronthaul interface connecting to the BBU. In antenna-element domain or beam/direction domain, the uplink channel between the antennas of the K UEs and the Nlantennas or beams of RU l (for k=1, . . . , L) is denoted as Hl∈Nl×K. The effective large antenna array has therefore Σl=1LNlantenna elements in total. The effective uplink channel between the antennas of the K UEs and the combined effective antenna array comprising L RUs will be

H=[H1⋮HL]∈ℂ(∑l=1L⁢Nl)×K
If interference-rejection combining (IRC) is conducted regarding the effective antenna array, the equalizer can be expressed as
WIRC=H*(HH*+R)−1(1)
where R is the estimated covariance matrix of interference-and-noise and H* is the transpose and conjugate of the effective uplink channel matrix H. One way to estimate R is to use the interference-and-noise signals. As shown in WO2020/130895, the equalizer, aka IRC matrix, can be transformed into
WIRC=ΛK(A*A)−1A*
where A=[H G], A is an extended channel estimate, A* is the transpose and conjugate of the extended channel matrix A, and G is the error estimate. G can be composed in at least three different ways based on an error matrix obtained from reference signals such as demodulation reference signals (DMRS). The three different ways will be detailed further down in this disclosure. The ΛKdenotes the first K rows of a (K+M)×(K+M) identity matrix, where M is the number of column vectors in G. Regarding the L RUs that form the effective antenna array, the extended channel estimate A can be described as

Therefore, the overall IRC beamforming weights (BFWs) can be decomposed as

WIRC=ΛK(∑l=1L⁢Al*⁢Al)-1︸BBU⁢[A1*⋯AL*]︸RUs(2)
Here the first part of the BFWs, [A1* . . . AL*], represents the beamforming weights used in the RUs where each individual RU l uses the corresponding submatrix of Al* as the first part of BFWs used by RU l for the first beamforming of the UL received signal. The second part of the BFWs ΛK(Σl=1LAl*Al)−1are determined in the BBU based on combined intermediate BFWs Σl=1LAl*Alto perform interference cancellation of the UL signals received from all RUs. After these two steps of the beamforming using two parts of the BFWs in the RUs and the BBU, respectively, the interferences between user-layer signals are mitigated according to the derived IRC criteria. Furthermore, to reduce the amount of data in the cascaded RU chain sent over the fronthaul link FH (140inFIG.2), each RU in the cascaded chain combines its own beamformed signal, which is also referred to as intermediate signal, with the combined beamformed signals of all previous RUs in the chain, which are received from the previous RU in the chain, and then forwards the combined signal further to the next RU in the chain. Each RU also combines its own intermediate BFWs of Al*Alwith the combined intermediate BFWs of all previous RUs in the chain, which are received from the previous RU in the chain, and then forwards the combined intermediate BFWs to the next RU. After receiving the combined coefficients from the first RU in the chain connected to BBU, the BBU calculates the second part of BFWs, based on the received combined intermediate BFWs as ΛK(Σl=1LAl*Al)−1, and apply the second part of the BFWs to the combined signal received from the first RU.FIG.5illustrates the case of 2 cascaded RUs andFIG.6the case of L cascaded RUs.

InFIG.5, the uplink signals sent by the UEs431,432are received at the N1antennas of RU1420and at the N2antennas of RU2460. InFIG.5, without loss of generality, we directly model the system in frequency domain, where the conversion from time-domain signals to frequency-domain signals is not shown. An optional spatial discrete Fourier Transformation (DFT) unit422,462transforms the frequency-domain uplink signals from antenna-element domain to beam domain, which can be used to improve signal SNR of the strong beams. Basically, the signals received at N antennas are transformed to the signals of N beams. The i-th received uplink signal in frequency domain at the first RU420is denoted as yi,RU 1∈N1×1, and the i-th received uplink signal in frequency domain at the second RU460is denoted as yi,RU 2∈N2×1. Then, at the first RU, the channel estimation Ĥ1of the channel H1between the first RU420and the UEs431,432is obtained. Similarly, the channel estimation Ĥ2of the channel H2between the second RU460and the UEs431,432is obtained. In case of IRC, the error matrix is also estimated and the error estimate G1for the first RU and G2for the second RU are derived accordingly to compose the extended channel coefficients A1=[Ĥ1G1] and A2=[Ĥ2G2] for the respective first and second RU. This may be performed in a respective unit called beamforming (BF) control unit426,466of the respective first and the second RU. Further, at the respective BF control unit424,464of the first and the second RU, the respective transpose and conjugate A1* and A2* of the extended channel coefficient matrix A1and A2are determined. Then, at a coherent combining unit422of the first RU420, the received uplink signals yi,RU 1are multiplied with the transpose and conjugate A1* of the first extended channel coefficient matrix A1to implement coherent combining. Similarly, at a coherent combining unit462of the second RU460, the received uplink signals yi,RU 2are multiplied with the transpose and conjugate A2* of the second extended channel coefficient matrix A2to implement coherent combining. This is the first part of the beamforming and it is as shown performed in the respective RU. As a result, first intermediate signals {tilde over (y)}i,RU 1and second intermediate signals {tilde over (y)}i,RU 2are achieved at the first and the second RU, respectively.

Besides receiving uplink signals from its N1antenna elements, the first RU420receives the second intermediate signals {tilde over (y)}i,RU 2from the second RU460, which the second RU460sends over the RU link165(seeFIG.2). The first RU420combines in a combiner427, its first intermediate signals {tilde over (y)}i,RU lwith the second intermediate signals {tilde over (y)}i,RU 2received from the second RU460into combined intermediate signals {tilde over (z)}i,RU 1, as {tilde over (z)}i,RU 1={tilde over (y)}i,RU 2+{tilde over (y)}i,RU 1. The combined intermediate signals {tilde over (z)}i,RU 1are then forwarded over the fronthaul link to the BBU410.

The first RU420further determines in its BF control unit426first intermediate BFWs C1=A1*A1. Similarly, the second RU460determines in its BF control unit 466 second intermediate BFWs C2=A2*A2. The second RU460sends the second intermediate BFWs C2over the first RU link to the first RU, where they are combined in a combiner428with the first intermediate BFWs C1into combined intermediate BFWs Ccom=C1+C2. The combined intermediate BFWs are then sent over the fronthaul link to the BBU410. Note that the use of separate BF control units426,466in each RU that is in charge of calculating Al* and Al*Alis mainly for illustration purposes. The actual implementation can look different. For example, the BF control unit and Coherent Combining unit may be one and the same unit.

The BBU410then receives the combined intermediate signals {tilde over (z)}i,RU 1and the combined intermediate BFWs Ccom. A BF control unit414of the BBU determines a second part of BFW as ΛKCcom−1which means the first K rows of an inverse of the combined intermediate BFWs matrix. The first K rows of Ccom−1can be obtained efficiently by sub-block matrix inverse. Represent Ccomin four sub-block matrices as

Cc⁢o⁢m=[B1B3B2B4]
where B1is a K×K sub-block matrix, B2is a M×K sub-block matrix, B3is a K×M sub-block matrix and B4is a M×M sub-block matrix. According to the block matrix inversion property, the first K rows of Ccom−1can be derived as
ΛKCcom−1=[(B1−B3B4−1B2)−1−(B1−B3B4−1B2)−1B3B4−1]
To perform matrix inversion is rather computationally heavy, especially if there are many antennas. To leave this calculation to the BBU would save computational resources at the RUs. The IRC equalization can then be implemented easily in an IRC cancellation unit412in the BBU410in order to obtain a good estimation r of the K user layer signals as follows:
ri=ΛKCcom−1{tilde over (z)}i,RU 1
The estimation r of the K user layer signals is then post-treated e.g. by being demodulated in a demodulator416.

With reference toFIG.6, a more general embodiment is provided when having L cascade coupled RUs. At the RUs520,560,570ofFIG.6, the operations described below are performed. Same as inFIG.5, without loss of generality, inFIG.6, we directly model the system in frequency domain, where the conversion from time-domain signal to frequency-domain signal is not shown. The optional spatial discrete Fourier Transformation (DFT) unit422,462,472of the respective RU transforms the frequency-domain uplink signal from antenna-element domain to beam domain, which can be used to improve signal SNR of the strong beams. Basically, the signals received at N antennas are transformed to the signals of N beams. At the RU l for l=1, . . . , L, the i-th received uplink signal in frequency domain is denoted as yi,RU l∈Nl×1. The channel estimation Ĥlof the channel Hlbetween RU l520,560,570and the UEs431,432is obtained at the respective BF control unit526,566,576. In case of IRC, the error estimation Glis determined accordingly to compose the extended channel coefficients Al=[ĤlGl].

The transpose and conjugate of the extended channel coefficients Al* of the respective RU are determined in the BF control unit526,566,576of the respective RU520,560,570. Thereafter in a coherent combining unit524,564,574of the respective RU520,560,570, the respective received uplink signals of each RU yi,RU l∈Nl×1are multiplied with the transpose and conjugate of the extended channel coefficients Al* to implement coherent combining in the respective RU, i.e. to determine the first part of the BFW in the respective RU regarding the extended channel coefficients as
{tilde over (y)}i,RU l=Al*yi,RU l
Generally, yi,RU l∈(K+M)×1is the processed received uplink signal at RU l, called intermediate signal or signals.

Besides receiving uplink data signals from the antenna elements, RU l also receives forwarded data streams, i.e. intermediate signals and intermediate BFWs from RU (l+1) if l<L. In other words, RU l receives forwarded intermediate signals and intermediate BFWs from all RUs that are positioned further away from the BBU in the cascaded chain than RU l. Let {tilde over (z)}i,RU (l+1)∈(K+M)×1denote the forwarded intermediate signals where {tilde over (z)}i,RU L={tilde over (y)}i,RU L=AL*yi,RU L, and let Σi=l+1LCi∈(K+M)×(K+M)denote the forwarded intermediate BFWs where Ci=Ai*Aifrom RU (l+1). RU l will combine the forwarded intermediate signals together with its own processed intermediate signal {tilde over (y)}i,RU linto combined intermediate signals {tilde over (z)}i,RU las
{tilde over (z)}i,RU l={tilde over (z)}i,RU(l+1)+{tilde over (y)}i,RU l
Then RU l forwards the combined intermediate signals {tilde over (z)}i,RU lto RU (l−1) if l>1.

In addition, RU l will determine, in its BF control unit526,566,576, its own intermediate BFWs as Al*Al. The own intermediate BFW are then combined with the forwarded intermediate BFWs from the RUs further down the cascaded chain into updated aka combined intermediate BFWs as Σi=lLCi. The combined intermediate BFWs Σi=lLCiat RU l are also forwarded to RU (l−1) if l>1. The above process repeats at every RU in the cascade until the data stream and coefficients reach RU 1520. RU 1 combines the intermediate data streams and intermediate BFWs one more time and sends {tilde over (z)}i,RU 1and Ccom=Σi=1LCiover the fronthaul interface to the BBU510.

Note that inFIG.6, there is a BF control block in each RU that is in charge of calculating Al* and Al*Al. This is only for illustration purposes. The actual implementation can look different.

The BBU510then receives the combined intermediate signals {tilde over (z)}i,RU 1and the combined intermediate BFWs Ccomfrom the first RU520over the fronthaul link. The process of the BBU510and its sub-units IRC cancellation512, BF control514and demodulator516are the same or substantially the same as in the BBU410ofFIG.5and the corresponding subunits of the BBU ofFIG.5.

For determining the error estimate Gl, at the RU l for l=1, . . . , L, there are several different methods that can be used. Below are three methods presented as three possible embodiments. According to a first embodiment, once the desired channel Hlis estimated using a reference signal, e.g. DMRS-signal, xref,m, with M known reference symbols for m=1, . . . , M, sent by the UEs, an Nl×M matrix of the error estimate Glcan be composed as
Gl=1/√{square root over (M)}[{circumflex over (g)}1,l. . . ĝM,l]
where ĝm,l=yref,m,l−Ĥlxref,m, for m=1, . . . , M, and yref,m,RU lare the reference signals as received at the antennas of RU l.

According to a second embodiment, the error estimate is determined like in the first embodiment as {tilde over (G)}l=1/√{square root over ({tilde over (M)})}[ĝ1,l. . . ĝ{tilde over (M)},l] where M>M. Then singular value decomposition (SVD), or a principal component analysis (PCA)-based method, or similar method is conducted to obtain singular values and left singular vectors of {tilde over (G)}l. The M largest singular values are then used to compose a diagonal matrix ΣMand the left singular vectors associating with the M largest eigenvalues are used to compose an Nl×M matrix UM. Then Glis obtained as Gl=UMΣM.

According to a third embodiment, the estimated covariance matrix of interference-and-noise R mentioned in Equation (1) can be estimated by the RU l in various ways under different estimation criteria, e.g. lease square (LS), minimum mean square error (MMSE), linear minimum mean square error (LMMSE) etc., based on, for example, reference signals like sounding reference signal (SRS), DMRS, and other information like signal-to-interference-and-noise power ratio (SINR) estimate and UE feedback on channel conditions. Then let {circumflex over (R)}ldenote the estimation of the interference-covariance matrix at RU l. The eigenvalue decomposition (EVD) of {circumflex over (R)}lis then expressed as
{circumflex over (R)}l=QΣQ−1=QΣ1/2Σ1/2Q*
where Q is the Nl×Nleigenvector matrix andΣis the diagonal matrix whose diagonal elements are the eigenvalues. Only the M strongest eigenvalues and the corresponding eigenvectors are used, while the rest of the elements in the matrix are removed. In this way, a dimension-reduced matrix is obtained:
Gl=QMΣM1/2
whereΣMdenotes an M×M diagonal matrix composed by the largest M eigenvalues, and QMis composed by M eigenvectors corresponding to the M largest eigenvalues. If the dominant eigenvalues are included, the approximation is valid that {circumflex over (R)}l≈GlGl*.

Except for being used for a distributed base station system with separately located RUs, where the RUs are arranged on separate Printed Circuit Boards (PCBs), the above-described embodiments may also be used in a single large RU design with multiple radio processors, where each radio processor would take the role of an RU in the embodiments above. In this case, the radio processors/RUs of the distributed base station system may be arranged on one and the same PCB. The radio processors are then implemented in a cascaded topology, i.e. as in the describedFIG.2. This can significantly reduce the number of SerDes lanes on the PCB between the radio processors and a fronthaul interface that is to be connected to the BBU, comparing to a star-topology design. The overall required fronthaul link capacity is also significantly reduced.

FIG.7, in conjunction withFIG.2, describes a first RU120operable in a distributed base station system100of a wireless communication network, the first RU120comprising N1 antennas121,122,123. The distributed base station system100further comprises a BBU110connected to the first RU120over a fronthaul link140and a second RU160connected to the first RU120over an RU link165. The second RU comprises N2 antennas161,162,163. The first RU120comprises a processing circuitry603and a memory604. Said memory contains instructions executable by said processing circuitry, whereby the first RU120is operative for obtaining first uplink signals y1in frequency domain as received at the N1 antennas121,122,123from a number of UEs131,132,133, the first uplink signals y1comprising K user-layer signals in frequency domain overlaid with interference signals and noise, and obtaining a first channel estimate Ĥ1of wireless communication channels H1in frequency domain between the number of UEs131,132,133and the N1 antennas121,122,123. The first RU120is further operative for determining a first part of BFW for the first RU based on the first channel estimate Ĥ1and on reference signals received at the N1 antennas together with the first uplink signals, the first part of BFW being used to reduce the N1 first uplink signals to K+M first intermediate signals {tilde over (y)}1, where K+M is lower than N1, and determining the K+M first intermediate signals {tilde over (y)}1based on the N1 first uplink signals y1and on the first part of the BFW for the first RU. The first RU is further operative for determining a first part of intermediate BFW C1for interference cancellation at the BBU based on the first channel estimate Ĥ1and on the reference signals received at the N1 antennas together with the first uplink signals. The first RU is further operative for receiving, from the second RU160, K+M second intermediate signals {tilde over (y)}2, the second intermediate signals being determined by the second RU based on second uplink signals y2in frequency domain as received at the N2 antennas161,162,163of the second RU from the number of UEs131,132,133and on a first part of BFW for the second RU determined by the second RU based on a second channel estimate Ĥ2of wireless communication channels H2in frequency domain between the number UEs131,132,133and the N2 antennas161,162,163and on reference signals received at the N2 antennas together with the second uplink signals, the first part of BFW of the second RU being used to reduce the N2 second uplink signals to the K+M second intermediate signals {tilde over (y)}2, where K+M is lower than N2. The first RU is further operative for receiving, from the second RU160, a second part of intermediate BFW C2for interference cancellation, determined by the second RU based on the second channel estimate Ĥ2and on the reference signals received at the N2 antennas together with the second uplink signals. The first RU is further operative for combining the first intermediate signals {tilde over (y)}1and the second intermediate signals {tilde over (y)}2into combined intermediate signals {tilde over (y)}1+{tilde over (y)}2, combining the first part of intermediate BFW C1and the second part of intermediate BFW C2into combined intermediate BFW C1+C2, and sending, to the BBU110over the fronthaul link140, the combined intermediate signals {tilde over (y)}1+{tilde over (y)}2and the combined intermediate BFW C1+C2.

According to an embodiment, the first RU120is further operative for determining a first error estimate G1based on the obtained first channel estimate Ĥ1and on the reference signals yref,m,1received at the N1 antennas from the number of UEs131,132,133together with the first uplink signals, the received reference signals having {tilde over (M)} symbols for m=1, . . . , {tilde over (M)}, where {tilde over (M)}≥M, and wherein the first RU is operative for determining the first part of the BFW as well as the first part of the intermediate BFW based also on the first error estimate G1.

According to another embodiment, the first RU120is operative for determining the first error estimate G1as

According to another embodiment, the first RU120is operative for determining the first intermediate signals {tilde over (y)}1by multiplying the first uplink signals y1with the transpose and conjugate A1* of a first extended channel estimate A1, which is obtained based on the first channel estimate Ĥ1and the first error estimate G1.

According to another embodiment, the first RU120is operative for determining the first part of the intermediate BFW C1by multiplying a transpose and conjugate A1* of a first extended channel estimate A1, which is obtained based on the first channel estimate Ĥ1and the first error estimate G1, with the first extended channel estimate A1. According to other embodiments, the first RU120may further comprise a communication unit602, which may be considered to comprise conventional means for wireless communication with the wireless devices131,132,133, such as a transceiver for wireless transmission and reception of signals in the communication network. The communication unit602may also comprise conventional means for communication with the BBU110. The instructions executable by said processing circuitry603may be arranged as a computer program605stored e.g. in said memory604. The processing circuitry603and the memory604may be arranged in a sub-arrangement601. The sub-arrangement601may be a micro-processor and adequate software and storage therefore, a Programmable Logic Device, PLD, or other electronic component(s)/processing circuit(s) configured to perform the methods mentioned above. The processing circuitry603may comprise one or more programmable processor, application-specific integrated circuits, field programmable gate arrays or combinations of these adapted to execute instructions.

The computer program605may be arranged such that when its instructions are run in the processing circuitry, they cause the first RU120to perform the steps described in any of the described embodiments of the first RU120and its method. The computer program605may be carried by a computer program product connectable to the processing circuitry603. The computer program product may be the memory604, or at least arranged in the memory. The memory604may be realized as for example a RAM (Random-access memory), ROM (Read-Only Memory) or an EEPROM (Electrical Erasable Programmable ROM). In some embodiments, a carrier may contain the computer program605. The carrier may be one of an electronic signal, an optical signal, an electromagnetic signal, a magnetic signal, an electric signal, a radio signal, a microwave signal, or computer readable storage medium. The computer-readable storage medium may be e.g. a CD, DVD or flash memory, from which the program could be downloaded into the memory604. Alternatively, the computer program may be stored on a server or any other entity to which the first RU120has access via the communication unit602. The computer program605may then be downloaded from the server into the memory604.

FIG.8, in conjunction withFIG.2, describes a BBU system700operable in a wireless communication network. The wireless communication network comprises a distributed base station system100having a BBU110, a first RU120connected to the BBU110over a fronthaul link140, the first RU comprising N1 antennas121,122,123, and a second RU160connected to the first RU120over an RU link165, the second RU comprising N2 antennas161,162,163. The BBU system700comprises a processing circuitry703and a memory704. Said memory contains instructions executable by said processing circuitry, whereby the BBU system700is operative for receiving, from the first RU120, combined intermediate signals {tilde over (y)}1+{tilde over (y)}2in frequency domain comprising first intermediate signals {tilde over (y)}1and second intermediate signals {tilde over (y)}2. The first intermediate signals {tilde over (y)}1are determined by the first RU120based on first uplink signals y1as received at the N1 antennas121,122,123of the first RU from a number of UEs131,132,133, the first uplink signals y1comprising K user-layer signals overlaid with interference signals and noise, and on first part of BFW for the first RU, determined by the first RU based on a first channel estimate Ĥ1of wireless communication channels H1in frequency domain between the number UEs131,132,133and the N1 antennas121,122,123, and on reference signals received at the N1 antennas together with the first uplink signals, the first part of BFW being used to reduce the N1 first uplink signals to K+M first intermediate signals {tilde over (y)}1, where K+M is lower than N1. The second intermediate signals {tilde over (y)}2are determined by the second RU160based on second uplink signals y2as received at the N2 antennas161,162,163of the second RU from the number of UEs131,132,133, the second uplink signals y2comprising K user-layer signals overlaid with interference signals and noise, and on first part of BFW for the second RU, determined by the second RU based on a second channel estimate Ĥ2of wireless communication channels H2in frequency domain between the number of UEs131,132,133and the N2 antennas161,162,163, and on reference signals received at the N2 antennas together with the second uplink signals. The BBU system700is further operative for receiving from the first RU120, combined intermediate BFW C1+C2comprising a first part of intermediate BFW C1and a second part of intermediate BFW C2, the first part of intermediate BFW C1being determined by the first RU120based on the first channel estimate Ĥ1and on the reference signals received at the N1 antennas together with the first uplink signals, the second part of intermediate BFW C2being determined by the second RU160based on the second channel estimate Ĥ2and on the reference signals received at the N2 antennas together with the second uplink signals. The BBU system700is further operative for determining a second part of BFW based on the combined intermediate BFW C1+C2, and for determining an estimation r of the K user-layer signals based on the combined intermediate signals {tilde over (y)}1+{tilde over (y)}2and the second part of BFW.

The BBU system700may be realized as the actual BBU110, as a unit in the BBU or as a unit somewhere in the distributed base station system100. Alternatively, the BBU system700may be arranged in or at any other network node of the communication network, such as a node further away from the UEs, e.g. another network element in the RAN or close to the RAN or another RAN node. In this alternative, and in the cloud-solution embodiment discussed below, the BBU110is arranged to receive, from the first RU120, the combined intermediate signals {tilde over (y)}1+{tilde over (y)}2and the combined intermediate BFW C1+C2, and to communicate the combined intermediate signals {tilde over (y)}1+{tilde over (y)}2and the combined intermediate BFW C1+C2to the other network node and the BBU system of the other network node is arranged to determine306,308the second part of the BFW as well as the estimation r of the K user-layer signals. Alternatively, the BBU system700may be realized as a group of network nodes, wherein functionality for performing of the BBU system700is spread out over the group of network nodes. The group of network nodes may be different physical, or virtual, nodes of the network. This alternative BBU system realization may be called a cloud-solution.

According to an embodiment, the BBU system700is operative for determining the second part of the BFW by determining the second part of BFW based on the inverse of the combined intermediate BFW C1+C2.

According to an embodiment, the BBU system700is operative for determining the second part of the BFW as the first K rows of the inverse of the combined intermediate BFW C1+C2.

According to another embodiment, the BBU system700is operative for determining the estimation r of the K user-layer signals by multiplying the combined intermediate signals {tilde over (y)}1+{tilde over (y)}2with the second part of the BFW.

According to other embodiments, the first BBU system700may further comprise a communication unit702, which may be considered to comprise conventional means for communication with other nodes of the communication network. In case the BBU system is realized as the actual BBU110, the communication unit702may comprise conventional means for communicating with the first RU120. In case the BBU system is realized as another unit or node than the BBU110, the communication unit702may comprise conventional means for communicating with the BBU110. The instructions executable by said processing circuitry703may be arranged as a computer program705stored e.g. in said memory704. The processing circuitry703and the memory704may be arranged in a sub-arrangement701. The sub-arrangement701may be a micro-processor and adequate software and storage therefore, a Programmable Logic Device, PLD, or other electronic component(s)/processing circuit(s) configured to perform the methods mentioned above. The processing circuitry703may comprise one or more programmable processor, application-specific integrated circuits, field programmable gate arrays or combinations of these adapted to execute instructions.

The computer program705may be arranged such that when its instructions are run in the processing circuitry, they cause the BBU system700to perform the steps described in any of the described embodiments of the BBU system700and its method. The computer program705may be carried by a computer program product connectable to the processing circuitry703. The computer program product may be the memory704, or at least arranged in the memory. The memory704may be realized as for example a RAM (Random-access memory), ROM (Read-Only Memory) or an EEPROM (Electrical Erasable Programmable ROM). In some embodiments, a carrier may contain the computer program705. The carrier may be one of an electronic signal, an optical signal, an electromagnetic signal, a magnetic signal, an electric signal, a radio signal, a microwave signal, or computer readable storage medium. The computer-readable storage medium may be e.g. a CD, DVD or flash memory, from which the program could be downloaded into the memory704. Alternatively, the computer program may be stored on a server or any other entity to which the BBU system700has access via the communication unit702. The computer program705may then be downloaded from the server into the memory704.