Method adapted for beamforming, baseband unit system, and radio unit of a distributed base station system - Patents.com

Abstract

A method is disclosed that is implemented by a BBU system that includes a distributed base station system including a baseband unit (BBU), a first radio unit (RU) connected to the BBU through a fronthaul link, and a second RU connected to the first RU through an RU link. The method includes: determining first and second partitions of beamforming weights (BFWs) used to beamform K user layer DL signals to be sent to a number of UEs based on DL channel estimates of the first respective second RUs; and determining a first partition of the BFW based on the DL channel estimates of the first RU and the second RU to perform interference cancellation between the user layer signals. The first partition of the BFW and the first and second partitions of the BFW are sent to the first RU, and the first RU forwards the received BFW to the second RU.

Description

[Technical field]

[0001] The present disclosure generally relates to a method, a baseband unit (BBU) system, and a radio unit (RU) of a distributed base station system adapted for beamforming. More specifically, the present disclosure deals with such a method, system, and unit when the RU has multiple antennas for sending downlink (DL) signals and receiving uplink (UL) signals, such as in a multiple-input multiple-output (MIMO) system. The present disclosure further relates to a computer program and carrier corresponding to the above method, system, and unit. [Background technology]

[0002] A centralized radio access network (C-RAN) is also called a radio access network (RAN) of a distributed base station system, and processing is performed by two separate units: a radio unit (RU) and a baseband unit (BBU). The BBU is connected to the RU via a fronthaul link. The RU may also be called a remote radio unit (RRU). The baseband unit may also be called a base unit (BU) or a digital unit or a distributed unit (DU). The RU is connected to one or more antennas, through which the RU communicates wirelessly with at least one user equipment (UE). The BBU is then connected to other base station systems or base stations, and to a core network of the wireless communication system. The BBU is centralized, and typically two or more RUs are connected to each BBU. Traditionally, the BBU performs advanced wireless coordination functions such as joint detection, joint decoding, and coordinated multipoint transmission (CoMP), which increase spectrum efficiency and network capacity, as well as baseband processing, and the RU performs radio frequency (RF) processing and transmission / reception of RF processed signals.

[0003] Originally, the RU was designed to reduce cable losses in the coaxial cable between the top of the antenna tower, where the actual antennas are located, and the bottom of the antenna tower, where the base station functions are hosted. Thus, in pre-5G mobile communications, i.e. 4G, e.g., Long Term Evolution (LTE), the RU was rather simple and mainly performed RF processing, with baseband processing being the only option.

[0004] When moving from 4G to 5G, there was a need to increase the wireless communication capacity towards UEs to be able to deliver the amount of data required per period in 5G. One enabling factor that enabled the mobile evolution towards 5G is massive multiple-input multiple-output (MIMO) where each RU has multiple antennas. In other words, massive MIMO leverages spatial multiplexing to improve the spectral efficiency by using an antenna array at the RU with N antennas simultaneously serving K user layers in the same time-frequency resource. A typical scenario is N>>K, e.g., N is 64, 128, or 256, and K is 8 or 16. As shown, the number of antennas is very large. Massive MIMO, often referred to as massive beamforming, can form narrow beams and focus them in different directions to mitigate the increased path loss in high frequency bands. It also lends itself to multi-user MIMO, which allows simultaneous transmission to and from multiple UEs through separate spatial channels decomposed by massive MIMO techniques while still maintaining high capacity for each UE. Thus, the spectral efficiency and cell capacity can be increased significantly.

[0005] In the 5G evolution and future sixth generation (6G) of mobile communications, Massive MIMO is expected to support a larger number of antennas, with the assumption that the cost per transceiver chain will decrease over time. To address this trend, it is foreseen that MIMO processing will become more distributed and scalable, with larger MIMO systems being processed by multiple RUs, with each RU processing only a subset of the antennas. Such a scalable design allows MIMO systems to easily scale with the number of antennas.

[0006] In the legacy common public radio interface type (CPRI type) of time domain fronthaul (FH), the BBU performs beamforming and determines the time domain IQ samples per antenna branch. The time domain IQ samples per antenna branch are then conveyed in DL to the RU through the fronthaul link. Since a massive MIMO system has many antennas, i.e., antenna branches, the required FH link capacity increases dramatically as the number of antenna branches increases, which significantly increases the FH cost. To address this challenge, different lower layer splitting (LLS) options have been applied. The basic idea is to move the beamforming function to the RU and convey the user layer samples or data in the frequency domain through the FH link. In this way, the number of FH streams is reduced from the number of antennas, N, to the number of user layers, K. It is noted here that in a typical scenario, N>>K. One LLS option is called 3GPP Option 7-3 and is presented in 3GPP TR 38.801 V14.0.0, Section 11.1.2.7. In 3GPP option 7-3, channel estimation of the radio channel between the RU and the UE is performed in the BBU, for example, based on the UL reference signal and calculation of beamforming weights (BFW) in the frequency domain. The BFW in the frequency domain is then sent to the RU over the FH. The BBU also performs forward error correction (FEC) encoding and sends the coded bits of the user layer to the RU over the FH. The RU modulates the received user layer signal and beamforms the user layer signal into an antenna signal using the BFW received from the BBU. The RU further transforms the antenna signal into the time domain, for example, by using an inverse fast Fourier transform (IFFT), and performs the necessary RF processing before the antenna signal is transmitted over the air to the UE through the respective antennas of the antenna branch. Another LLS option is called O-RAN LLS and is presented in O-RAN.WG4.CUS.0-v04.00, O-RAN Fronthaul Working Group, Control, User and Synchronization Plane Standard. This option is very similar to 3GPP option 7-3.The difference is that modulation is performed in the BBU instead of the RU in the case of O-RAN, so that the RU obtains the modulated user layer signal from the BBU. O-RAN specifies two types of RUs, namely, CAT-A RU and CAT-B RU, which are called CAT-A O-RU and CAT-B O-RU, respectively, in O-RAN. In the case of CAT-A O-RU, frequency domain beamforming is realized in the BBU, which is called O-DU in O-RAN. In the case of CAT-B O-RU, frequency domain beamforming is realized in the CAT-B O-RU. In O-RAN, frequency domain beamforming is called precoding and beamforming in CAT-B O-RU. In the case of CAT-A O-RU, frequency domain beamforming is called precoding. Note that the BBU and RU are called O-DU and O-RU, respectively, in O-RAN. In eCPRI terminology, the BBU and RU are called eREC (eCPRI Radio Equipment Control) and eRE (eCPRI Radio Equipment), respectively. In other terminology, the BBU and RU may be called LLS-CU and LLS-DU, respectively.

[0007] Although the LLS option mentioned above reduces the amount of DL user plane data sent over the FH, the BFW that needs to be sent over the control plane creates high burstiness over the FH, which in turn is the main driver of increasing the FH peak rate. This is because the transmission window carrying the BFW is very short. Hence, high burstiness is created to carry many BFWs in such a short time.

[0008] In the commonly owned international patent application WO2020 / 256609, a scheme was proposed to reduce the number of BFWs carried over the FH. The idea described therein is to split the BFW into two parts. The BFW of the second part is significantly reduced by performing layer-specific beam selection. Then, based on the reduced BFW of the second part, the BFW of the first part is calculated. The amount of data in the two parts is much less than without reduction, but the performance degradation is small. In WO2020 / 256609, the BBU sends the BFW of both the first and second parts to the RU. The RU then calculates, i.e. combines, the BFW based on the received first and second parts and then performs beamforming using the combined BFW.

[0009] However, WO2020 / 256609 only addresses a point-to-point FH topology, where each RU has a dedicated FH link to a BBU, as shown in FIG. 1. In the example of a point-to-point FH topology of a distributed base station system 10 as shown in FIG. 1, a first RU 30 is connected to the BBU 20 through a first FH link 25, a second RU 40 is connected to the BBU 20 through a second FH link 35, and a third RU 50 is connected to the BBU 20 through a third FH link 45. The first, second, and third RUs 30, 40, 50 are configured to transmit and receive user plane data as antenna signals to and from the UEs 31, 32, 33. Such a point-to-point FH requires many fiber connections and the same number of BBU ports, even if the RUs are configured as a larger joint MIMO system.

[0010] On the other hand, the present disclosure addresses a cascaded topology of RUs, as shown in FIG. 2. In the cascaded topology, the first RU 120 is connected to the BBU 110 through a fronthaul link 140, just like the point-to-point topology. However, the second RU 160 is connected to the first RU 120 through a separate RU-RU link 165, and the third RU 170 is connected to the second RU 160 through another separate RU-RU link 175. Furthermore, if there are more RUs, they are connected to the RUs in a row in sequence. In FIG. 2, only three RUs are shown, but there may be more RUs in such a cascaded topology. The cascaded RU deployment will reduce the amount of FH fiber links and reduce the number of BBU ports to one. This will help reduce the deployment cost, i.e., fiber connections, and the system complexity, i.e., BBU ports.

[0011] However, in a cascaded topology, especially when using the method described in WO2020 / 256609, all BFWs sent by the BBUs for all cascaded RUs are sent through one and the same FH link 140, resulting in a larger BFW burst through the FH link 140. This further increases the FH peak rate in the cascaded FH chain, especially in the FH link 140 between the BBU 110 and the first RU 120. As a result, a solution is needed to handle a distributed base station system with RUs cascaded to BBUs. Such a solution should preferably keep the amount of data sent through the FH links low in general, but also manage the bursts of BFWs sent through the FH links to keep them at a manageable level. Summary of the Invention

[0012] The object of the present invention is to address at least some of the problems and challenges mentioned above. These and other objects can be achieved by using the methods, network nodes and wireless devices as defined in the accompanying independent claims.

[0013] According to an aspect, a method is provided that is implemented by a BBU system of a wireless communication network, the wireless communication network comprising a distributed base station system. The distributed base station system comprises a BBU, a first RU connected to the BBU through a fronthaul link, the first RU having N1 antennas, and a second RU connected to the first RU through an RU link, the second RU having N2 antennas. The method includes determining a first partition of a BFW in a frequency domain based on a DL channel estimation of the first RU, the BFW being used to beamform K user layer DL signals sent to a number of UEs, the first partition of the BFW being determined in the frequency domain for spreading the K user layer signals to antenna signals of the N1 antennas of the first RU. The method further includes determining a second partition of a BFW in the frequency domain based on a DL channel estimation of the second RU, the second partition of the BFW being determined in the frequency domain for spreading the K user layer signals to antenna signals of the N2 antennas of the second RU. The method further includes determining a first portion of a BFW in the frequency domain based on DL channel estimates of the first RU and the second RU, where the first portion of the BFW is determined to perform interference cancellation between the user layer signals. Further, the method includes triggering sending the first portion of the BFW and the first and second partitions of the BFW to the first RU and triggering sending the K user layer signals in the frequency domain to the first RU.

[0014] According to another aspect, a method is provided, which is implemented 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 through a fronthaul link and a second RU connected to the first RU through a RU link, the second RU comprising N2 antennas. The method includes receiving, from the BBU, K user layer DL signals in a frequency domain, which are sent to a number of UEs. The method further includes receiving, from the BBU, a first portion of a BFW and first and second partitions of the BFW for beamforming the K user layer signals in the frequency domain. The first partition is based on a determined DL channel estimate of the first RU, the first partition being determined for spreading the user layer signals to antenna signals of the N1 antennas of the first RU in the frequency domain. The second partition is based on a determined DL channel estimate of the second RU, the second partition being determined for spreading the user layer signals to antenna signals of the N2 antennas of the second RU in the frequency domain. The first portion of the BFW is based on the determined DL channel estimates of the first RU and the second RU, and the first portion of the BFW is determined to perform interference cancellation between the user layer signals. The method further includes determining a BFW for the first RU based on the received first portion and the first partition of the BFW, sending the received first portion and the second partition of the BFW to the second RU over the RU link, and sending the received K user layer DL signals in the frequency domain to the second RU over the RU link. The method further includes beamforming the received K user layer signals into antenna signals using the determined BFW for the first RU, and sending the antenna signals to the number of UEs via the N1 antennas.

[0015] According to another aspect, a method is provided that is implemented by a BBU system of a wireless communication network, the wireless communication network comprising a distributed base station system comprising a BBU and a first RU connected to the BBU through a fronthaul link, the first RU comprising N1 antennas. The distributed base station system further comprises a second RU connected to the first RU through a RU link, the second RU comprising N2 antennas. The method includes determining a first partition of a BFW in the frequency domain based on an UL channel estimate of the first RU, the first partition of the BFW being used to combine in the frequency domain antenna signals received at the N1 antennas of the first RU from a number of UEs into K user layer signals of the first RU. The method further includes determining a second partition of a BFW in the frequency domain based on an UL channel estimate of the second RU, the second partition of the BFW being used to combine in the frequency domain antenna signals received at the N2 antennas of the second RU from a number of UEs into K user layer signals of the second RU. The method further includes determining, based on UL channel estimates of the first RU and the second RU, a first portion of a BFW in the frequency domain, determined to perform interference cancellation between user layer signals of the first and second RUs, and triggering sending the first portion of the BFW and the first and second partitions of the BFW to the first RU.

[0016] According to another aspect, a method is provided, implemented 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 through a fronthaul link and a second RU connected to the first RU through a RU link, the second RU comprising N2 antennas. The method includes receiving UL antenna signals on N1 antennas from a number of UEs, and receiving a first portion of a BFW and first and second partitions of the BFW from a BBU, where the first partition is based on a determined UL channel estimate of a first RU, the first partition being determined in the frequency domain for combining the UL antenna signals received on the N1 antennas into a user layer signal, the second partition being based on a determined UL channel estimate of a second RU, the second partition being determined in the frequency domain for combining the antenna signals received on the N2 antennas of the second RU into a user layer signal, the first portion of the BFW is based on the determined UL channel estimates of the first RU and the second RU, and the first portion of the BFW is determined to perform interference cancellation between the user layer signals. The method further includes determining a BFW for the first RU based on the received first portion and the first partition of the BFW, sending the received first portion and the second partition of the BFW to the second RU over the RU link, and combining UL antenna signals received on the N1 antennas into a first K user layer signals in the frequency domain using the BFW determined for the first RU. The method further includes receiving, from the second RU, second K user layer signals combined by the second RU from antenna signals received on the N2 antennas of the second RU from a number of UEs, the combining by the second RU being based on the sent first portion and the second partition of the BFW sent by the first RU to the second RU. The method further includes combining the first and second K user layer signals into K combined user layer signals, and sending the combined K user layer signals to the BBU.

[0017] According to another aspect, a BBU system configured to operate in a wireless communication network is provided. The wireless communication network includes a distributed base station system including a BBU and a first RU connected to the BBU through a fronthaul link, the first RU having N1 antennas. The distributed base station system further includes a second RU connected to the first RU through a RU link, the second RU having N2 antennas. The BBU system also includes a processing circuit and a memory. The memory includes instructions executable by the processing circuit, whereby the BBU system is operable to determine a first partition of a BFW in the frequency domain based on a DL channel estimate of the first RU, the BFW being used to beamform K user layer DL signals sent to a number of UEs, the first partition of the BFW being determined in the frequency domain to extend the K user layer signals to antenna signals of the N1 antennas of the first RU. The BBU system is further operable to determine, based on the DL channel estimation of the second RU, a second partition of the BFW in the frequency domain, where the second partition of the BFW is determined in the frequency domain for spreading the K user layer signals into antenna signals of the N2 antennas of the second RU, and to determine, based on the DL channel estimation of the first RU and the second RU, a first portion of the BFW in the frequency domain, where the first portion of the BFW is determined for performing interference cancellation between the user layer signals. The BBU system is further operable to trigger sending the first portion of the BFW and the first and second partitions of the BFW to the first RU, and to trigger sending the K user layer signals in the frequency domain to the first RU.

[0018] According to another aspect, a first RU is provided configured to operate in 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 via a fronthaul link and a second RU connected to the first RU via a RU link, the second RU comprising N2 antennas. The first RU comprises a processing circuit and a memory. The memory includes instructions executable by the processing circuit, whereby the first RU is operable to receive, from a BBU, K user layer DL signals in a frequency domain to be sent to a number of UEs, and to receive, from the BBU, a first portion of a BFW and first and second partitions of a BFW for beamforming the K user layer signals in the frequency domain, where the first partition is based on a determined DL channel estimate of the first RU, and the first partition is determined for spreading, in the frequency domain, the user layer signals to antenna signals of the N1 antennas of the first RU, the second partition is based on a determined DL channel estimate of the second RU, and the second partition is determined for spreading, in the frequency domain, the user layer signals to antenna signals of the N2 antennas of the second RU, and the first portion of the BFW is based on the determined DL channel estimates of the first RU and the second RU, and the first portion of the BFW is determined for performing interference cancellation between the user layer signals. The first RU is further operable to determine a BFW for the first RU based on the received first portion and the first partition of the BFW, send the received first portion and the second partition of the BFW to the second RU via the RU link, send the K user layer DL signals received in the frequency domain to the second RU via the RU link, beamform the K user layer signals received into antenna signals using the BFW determined for the first RU, and send the antenna signals to a number of UEs via the N1 antennas.

[0019] According to another aspect, a BBU system configured to operate in a wireless communication network is provided. The wireless communication network includes a distributed base station system including a BBU and a first RU connected to the BBU through a fronthaul link, the first RU equipped with N1 antennas. The distributed base station system further includes a second RU connected to the first RU through a RU link, the second RU equipped with N2 antennas. The BBU system also includes a processing circuit and a memory. The memory includes instructions executable by the processing circuit, such that the BBU system is operable to determine, based on a UL channel estimate of the first RU, a first partition of a BFW in the frequency domain used to combine antenna signals received at the N1 antennas of the first RU from a number of UEs into K user layer signals of the first RU in the frequency domain, and to determine, based on a UL channel estimate of the second RU, a second partition of a BFW in the frequency domain used to combine frequency domain signals received at the N2 antennas of the second RU from a number of UEs into K user layer signals of the second RU. The BBU system is further operable to determine, based on the UL channel estimates of the first RU and the second RU, a first portion of a BFW in the frequency domain, which is determined to perform interference cancellation between user layer signals of the first and second RUs, and trigger sending the first portion of the BFW and the first and second partitions of the BFW to the first RU.

[0020] According to another aspect, a first RU is provided configured to operate in 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 via a fronthaul link and a second RU connected to the first RU via a RU link, the second RU comprising N2 antennas. The first RU comprises a processing circuit and a memory. The memory includes instructions executable by the processing circuit, whereby the first RU is operable to receive UL antenna signals on the N1 antennas from a number of UEs and receive a first portion of a BFW and first and second partitions of the BFW from the BBU, the first partition being based on a determined UL channel estimate of the first RU, the first partition being determined in the frequency domain for combining the UL antenna signals received on the N1 antennas into a user layer signal, the second partition being based on a determined UL channel estimate of the second RU, the second partition being determined in the frequency domain for combining the antenna signals received on the N2 antennas of the second RU into a user layer signal, the first portion of the BFW being based on the determined UL channel estimates of the first RU and the second RU, and the first portion of the BFW being determined to perform interference cancellation between the user layer signals. The first RU is also operable to determine a BFW for the first RU based on the received first portion and first partition of the BFW, send the received first portion and second partition of the BFW to the second RU over the RU link, and combine UL antenna signals received on the N1 antennas into a first K user layer signals in the frequency domain using the BFW determined for the first RU. The first RU is also operable to receive second K combined user layer signals by the second RU from antenna signals received on the N2 antennas of the second RU from a number of UEs based on the first portion and second partition of the BFW sent by the first RU to the second RU, combine the first and second K user layer signals into K combined user layer signals, and send the combined K user layer signals to the BBU.

[0021] According to other aspects, there is also provided a computer program and a carrier, the details of which are set out in the claims and the detailed description.

[0022] Further possible features and advantages of this solution will become apparent from the following detailed description.

[0023] The solution is described in more detail below by means of exemplary embodiments and with reference to the accompanying drawings. [Brief description of the drawings]

[0024] [Figure 1] FIG. 2 is a block diagram illustrating a point-to-point topology of a distributed base station system with multiple RUs connected to a BBU. [Diagram 2] FIG. 2 is a block diagram illustrating a cascaded topology of a distributed base station system with multiple RUs connected to a BBU. [Diagram 3] 1 is a flow chart illustrating a method implemented by a BBU system for DL ​​communication according to a possible embodiment. [Figure 4] 11 is a flowchart illustrating a method implemented by a first RU for DL ​​communication according to a possible embodiment. [Diagram 5] 4 is a flow chart illustrating a method implemented by a BBU system for UL communication, according to a possible embodiment. [Figure 6] 11 is a flowchart illustrating a method implemented by a first RU for UL communication according to a possible embodiment. [Figure 7] FIG. 2 is a block diagram illustrating in further detail a possible implementation of a distributed base station system for DL ​​communication according to the present invention. [Figure 8] FIG. 13 is a schematic block diagram illustrating a BBU system in further detail according to a further possible embodiment; [Figure 9] FIG. 11 is a schematic block diagram showing the first RU in further detail according to a further possible embodiment; DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0025] FIG. 2 illustrates a wireless communication network in which the present invention may be used. The wireless communication network comprises a distributed base station system 100, which comprises a BBU 110 and a first RU 120. The BBU 110 has connections to other base stations or other RAN nodes, and also to a core network (represented as 150 in FIG. 2), so that the distributed base station system 100 can communicate with other nodes of the communication network. The BBU 110 is connected to the first RU 120 via a fronthaul link 140. The fronthaul link 140 may be any type of connection, such as a dedicated wired or wireless connection, or a connection through a network, as long as the connection meets the fronthaul requirements, for example in terms of capacity and latency. The first RU 120 further comprises multiple antennas 121, 122, 123, through which wireless signals are communicated to and from one or more UEs 131, 132, 133. The wireless signals comprise data communicated to and from the UEs 131, 132, 133. The distributed base station system 100 further comprises a second RU 160 connected to the first RU 120 through a RU link 165. It is observed that the second RU 160 does not have a direct connection to the BBU 110, but is connected to the BBU through the RU link 165, the first RU 120, and the fronthaul link 140. The second RU 160 further comprises a plurality of antennas 161, 162, 163 through which radio signals are communicated to and from one or more UEs 131, 132, 133. The distributed base station system 100 further comprises a third RU 170 connected to the second RU 160 through a second RU link 175. It is observed that the third RU 170 does not have a direct connection to the BBU 110, but is connected to the BBU through the second RU link 175, the second RU 160, the RU link 165, the first RU 120, and the fronthaul link 140. The third RU 170 further has multiple antennas 171, 172, 173 through which radio signals are communicated to and from one or more UEs 131, 132, 133.The distributed base station system 100 may further include RUs cascaded onto the third RU 170 in a similar manner.

[0026] The BBU 110, and the first RU 120, second RU 160, third RU 170, and possible other RUs each comprise a RAN function that handles data and signals communicated between the BBU 110, the RUs 120, 160, 170, and the UEs 131, 132, 133. The RAN function is distributed between the BBU 110 and the RUs, as described further below in this disclosure. It may be noted that in 3GPP, the BBU may be further divided into two units called a distributed unit (DU) and a 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, e.g., L3 and above, of the BBU.

[0027] The wireless communication network 100 may be any type of wireless communication network capable of providing wireless access to wireless devices. Examples of such wireless communication networks are networks based on Global System for Mobile Communications (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, and fifth generation (5G) wireless communication networks based on technologies such as New Radio (NR), and any possible future sixth generation (6G) wireless communication networks.

[0028] The UEs 131, 132, 133 may be any type of communication device capable of wirelessly communicating with the RUs 120, 160, 170 using radio signals. For example, the UEs may be machine-type UEs or UEs capable of machine-to-machine (M2M) communication, sensors, tablets, mobile terminals, smartphones, laptop embedded equipment (LEE), laptop mounted equipment (LME), USB dongles, customer premises equipment (CPE), etc.

[0029] 3 in conjunction with FIG. 2 describes a method implemented by a BBU system of a wireless communication network, the wireless communication network comprising a distributed base station system 100. The distributed base station system 100 comprises a BBU 110, a first RU 120 connected to the BBU 110 via a fronthaul link 140, the first RU 120 comprising N1 antennas 121, 122, 123, and a second RU 160 connected to the first RU 120 via a RU link 165, the second RU 160 comprising N2 antennas 161, 162, 163. The method includes determining 204 a first partition of a BFW in the frequency domain based on DL channel estimates of the first RU 120, the BFW to be used for beamforming K user layer DL signals sent to a number of UEs 131, 132, 133, the first partition of the BFW being determined in the frequency domain for spreading the K user layer signals to antenna signals of N1 antennas 121, 122, 123 of the first RU 120. The method further includes determining 206 a second partition of the BFW in the frequency domain based on DL channel estimates of the second RU 160, the second partition of the BFW being determined in the frequency domain for spreading the K user layer signals to antenna signals of N2 antennas 161, 162, 163 of the second RU 160. The method further includes determining (212) a first portion of a BFW in the frequency domain based on DL channel estimates of the first RU 120 and the second RU 160, where the first portion of the BFW is determined to perform interference cancellation between the user layer signals. Furthermore, the method includes triggering (214) sending the first portion of the BFW and the first and second partitions of the BFW to the first RU 120, and triggering (216) sending the K user layer signals in the frequency domain to the first RU 120.

[0030] The RU link 165 is a communication link or connection between two RUs, with only the first of the RUs having a direct link to the BBU. The RU link may also be referred to as an RU-RU link or an RU-RU fronthaul link. The DL channel estimate of the first RU is an estimate of the communication channel between the UE 131, 132, 133 and the first RU 120, and the DL channel estimate of the second RU is an estimate of the communication channel between the UE 131, 132, 133 and the second RU 160. The UEs 131, 132, 133 may all be considered to be connected to their respective UEs even if one or more of the UEs visible to the first RU are not visible to the second RU or vice versa, in which case the channel estimates of the UEs not visible to one RU are set to zero. The respective DL channel estimates are determined based on channel measurements performed by the UE on the actual DL signals sent by the respective RUs, or more commonly, the BBU determines uplink (UL) channel estimates from UL signals sent from the UE to the respective RUs, using the known reciprocity of UL and DL.

[0031] As claimed, a first portion of the BFW is determined (212) to perform interference mitigation / cancellation between user layer signals in beamforming at the first and second RUs, and the first and second partitions of the BFW, which may be collectively referred to as a second portion of the BFW, are determined (204, 206) to extend the user layer signals to the antenna signals of the respective first and second RUs. Beamforming weights (BFWs) may also be referred to as precoding coefficients, and beamforming may also be referred to as precoding.

[0032] The BFW may be determined as a beam area BFW, which is all in the frequency domain, or as an antenna area BFW. The antenna area or beam area is on the frequency domain, i.e., the antenna area or beam area BFW is applied to the user layer signal in the frequency domain. If the BBU system determines the beam area BFW, the BBU system may send the determined beam area BFW to the RU, which converts the beam area BFW back to the antenna area BFW and then applies the antenna area BFW to the user layer signal on one or group of subcarriers for the N1 antennas. Alternatively, the RU receives the beam area BFW from the BBU system, applies the beam area BFW to the user layer signal, and then converts the beamformed beam area signal back to the antenna area signal on one or group of subcarriers for the N1 antennas. Further alternatively, the BBU system determines the beam area BFW, converts the determined beam area BFW to the antenna area BFW, and then sends the antenna area BFW to the RU. The RU applies the antenna domain BFW to the user layer signals on one or group of subcarriers for the N1 antennas.

[0033] As mentioned above, when using a cascaded topology to connect RUs to a BBU, the BFWs for all RUs (here, the first RU 120 and the second RU 160) and user data for all RUs are sent to the first RU 120 through the fronthaul link 140, which causes a high load on the fronthaul link 140 between the BBU 110 and the first RU 120 of the cascade. According to the present invention, the BFW for a RU is decomposed into a first part that is common to the cascaded RUs and a second part that is determined separately for each cascaded RU. The first part is sent only once from the BBU 110 to the first RU 120. The first RU 120 copies the common first part of the BFW and sends the copy further to the subsequent RU 160. On the other hand, the second part having a UR-specific partition is sent to the first RU 120 through the fronthaul link 140 so that the first RU 120 can receive its own partition, called the first partition, and can further send other partitions of the second part to the subsequent RU 160. By separating the BFW into such a first part of the BFW that is common to both the first RU 120 and the second RU 160 (as well as any other subsequent cascaded RUs), the first part of the BFW only needs to be sent once through the fronthaul link. As a result, compared to WO2020 / 256609, less data is sent through the fronthaul link compared to sending a separate BFW for each RU.

[0034] The BBU system of the wireless communication network implementing the method may be the BBU 110, a unit within the BBU, or a unit within a distributed base station system. Alternatively, the BBU system implementing the method may be located within or at any other network node of the communication network, such as a node remote from the UE, e.g., another network element within or close to the RAN, or another RAN node. In this alternative, and in a cloud solution embodiment described below, the BBU 110 receives uplink signals, e.g., reference signals such as SRS or DMRS that the RUs are receiving from the UE, from each of the first and second RUs 120, 160, or information related to these uplink signals, and communicates the uplink signals / information to another network node, which determines DL channel estimates for each RU and, from the respective DL channel estimates, determines the first and second portions of the BFW as shown above. The other network node then transmits the determined first and second portions of the BFW back to the BBU 110 for further distribution to the first RU 120. Alternatively, the system of a wireless communication network implementing the method may be a group of network nodes, the functionality for implementing the method being distributed across different physical or virtual nodes of the network, the latter sometimes being called a "cloud solution".

[0035] According to one embodiment, the method further includes compressing (208) the first and second partitions of the BFW to a higher degree than any compression of the first portion of the BFW. The compression 208 is performed before the first and second partitions of the BFW are sent to the first RU 120 through the fronthaul link 140. It has been found that the first portion of the beamforming for interference mitigation or cancellation is rather sensitive to the accuracy of the BFW, while the first and second partitions, also referred to as the second portion of the BFW for extending the user layer signal to the antenna signal, are relatively robust. In other words, the second portion of the BFW is less susceptible to errors, e.g., due to lossy compression, compared to the first portion of the BFW. Based on this understanding, in order to save fronthaul link capacity and without significantly compromising the accuracy of the beamformed signal due to the robustness of the second portion, the second portion of the BFW (i.e., the first and second partitions) is compressed more than the first portion of the BFW before being sent to the first RU. This allows even less data to be sent over the fronthaul link 140 compared to the above-described embodiment without significant loss of performance. The first and second partitions of the BFW being more highly compressed than any compression of the first portion of the BFW means that the first and second partitions of the BFW are more strongly compressed than the first portion of the BFW. The term "higher compression" is intended to encompass "compression with fewer bits" as well as "compression by selecting only a subset of the BFW." "Higher compression" also refers to "lossier compression."

[0036] According to another embodiment, the first portion of the BFW is determined 212 based on the DL channel estimate of the first RU 120, the DL channel estimate of the second RU 160, and based on the compressed first and second partitions of the BFW. This achieves a better determination of the first portion of the BFW, since the first portion of the BFW can also mitigate interference caused by any inaccuracies due to the compression of the first and second partitions of the BFW. At the same time, the amount of data sent over the fronthaul link is kept at a low level.

[0037] According to another embodiment, the first and second partitions of the BFW are compressed (208) by selecting only a subset of the determined (204, 206) first and second partitions of the BFW, the subset being several BFWs with the largest magnitude for each user layer signal. Furthermore, triggering (214) sending the compressed first and second partitions of the BFW to the first RU 120 suggests sending the selected subsets of the first and second partitions of the BFW to the first RU. The selected subsets can be sent only as sending a bit mask indicating the value of the selected subset as well as the selected entries in the original matrix of the second part of the BFW. When the first and second partitions are combined into one matrix, the bit mask has the first and second partitions arranged similarly. For example, the bit mask maps to beams or antenna branches / elements for different user layers. For each user layer and N1 / N2 antennas / beams, the bitmask is N1 / N2 bits per layer, with a "1" representing the selected beam / antenna and a "0" representing the unselected beam / antenna. The number of BFWs with the largest magnitude may correspond to one or more of the strongest channel components or coefficients. Often, the energy for one user layer is concentrated on some channel coefficients more than others. For example, such compression would be rather efficient, especially when the channel estimation is in the beam domain. The channel coefficients are complex numbers representing the channel attenuation and phase shift. In the antenna element domain, it means the channel attenuation and phase shift from one transmit antenna to one receive antenna. In the beam domain, it means the channel attenuation and phase shift from one transmit beam, i.e. from one transmit beam to one receive antenna. The beams are usually predefined by the number of directional beams pointing in different directions within the covered angle. Such beams can be obtained by using Discrete Fourier Transform (DFT) based functions.

[0038] According to an alternative embodiment of the above, the first and second partitions of the BFW are compressed (208) by using fewer bits to represent the individual BFWs than the number of bits used for the first and second partitions before compression (208). The first and second partitions of the BFW are able to be compressed in this manner because they are less susceptible to errors than the first portion of the BFW. Fewer bits per beamforming weight may allow fewer quantization levels to be used to describe one beamforming weight sample. Additionally, the first and second partitions of the BFW may be represented by block floating point or block scaling, which have proven to be efficient quantization schemes that use fewer bits.

[0039] According to one embodiment, the method further includes sending (209) to the first RU 120 first compression information regarding the compression (208) of the first partition of the BFW and second compression information regarding the compression (208) of the second partition of the BFW, the first and second compression information being for decompression in the first and second RUs, respectively. This compression information includes information on how the compression was performed so that the first and second RUs can decompress / reconstruct the compressed first and second partitions of the BFW, respectively. The compression information may be a bit mask as mentioned above, indicating which bits were sent when only a subset of the determined (204, 206) first and second partitions of the BFW was selected. If the first and second partitions of the BFW were compressed by using fewer bits, the compression information may or may not be required to be sent through the FH.

[0040] According to another embodiment, the method further includes combining (210) the first and second partitions of the BFW into a second portion of the BFW. The compression (208) can be performed on the first partition, i.e., the first RU, and on the second partition, i.e., the second RU, of the second portion of the BFW separately, i.e., before combining, or the compression (208) can be performed after the first and second partitions are combined (210). "Combining" here refers to putting the first and second partitions of the BFW into a matrix larger than the respective matrices describing the first and second partitions of the second portion of the BFW, respectively. The larger matrix represents the entire second portion of the BFW. The formulation of the larger matrix facilitates the calculation of the first portion of the BFW when based on the first and second partitions.

[0041] According to yet another embodiment, the method further includes determining (202) a DL channel estimate for the first RU 120 based on UL signals originating from several UEs 131, 132, 133 and received at antennas 121, 122, 123 of the first RU 120, and determining (203) a DL channel estimate for the second RU 160 based on UL signals originating from several UEs 131, 132, 133 and received at antennas 161, 162, 163 of the second RU 160. For wireless communication networks based on air-propagating technologies, such as time division duplex (TDD) based technologies, the DL channel estimate can be determined fairly accurately from the UL signal. When the channel estimate is accurate, e.g., when the SINR is high, such an approach works even better than a codebook-based approach. In TDD, for example, any difference between the UL and DL channels is mainly due to differences in the radio front-ends in the UL and DL, which can be eliminated by calibration. The UL signals may be reference signals originating from several UEs, such as Sounding Reference Signals (SRS) and Demodulation Reference Signals (DMRS). The reference signals may be the same reference signal or different signals from the UEs. The reference signals may be sent simultaneously or at different times.

[0042] According to yet another embodiment, the method further includes normalizing the first portion of the BFW and the first and second partitions, and sending to the first RU 120 information regarding a per-user layer scaling factor associated with the normalization.

[0043] In yet another embodiment, the distributed base station system 100 further comprises a third RU 170 connected to the second RU 160 via a second RU link 175, the third RU 170 comprising N3 antennas 171, 172, 173. The method further comprises determining (207) a third partition of the BFW in the frequency domain based on the determined DL channel estimate of the third RU 170, the third partition of the BFW being determined to extend, in the frequency domain, the K user layer signals to antenna signals of the N3 antennas 171, 172, 173 of the third RU 170. Furthermore, triggering (214) sending the first portion of the BFW and the first and second partitions of the BFW to the first RU 120 further comprises triggering sending the third partition of the BFW to the first RU 120.

[0044] 4, in conjunction with FIG. 2, describes a method implemented by a first RU 120 of a distributed base station system 100, the first RU 120 comprising N1 antennas 121, 122, 123. The distributed base station system 100 further comprises a BBU 110 connected to the first RU 120 via a fronthaul link 140 and a second RU 160 connected to the first RU 120 via a RU link 165, the second RU comprising N2 antennas. The method includes receiving (302) from the BBU 110 K user layer DL signals in the frequency domain to be sent to a number of UEs 131, 132, 133. The method further includes receiving (304) from the BBU 110 a first portion of a BFW and first and second partitions of the BFW for beamforming the K user layer signals in the frequency domain. The first partition is based on the determined DL channel estimate of the first RU 120, and the first partition is determined for spreading, in the frequency domain, the user layer signal into antenna signals of N1 antennas 121, 122, 123 of the first RU 120. The second partition is based on the determined DL channel estimate of the second RU 160, and the second partition is determined for spreading, in the frequency domain, the user layer signal into antenna signals of N2 antennas 161, 162, 163 of the second RU 160. The first part of the BFW is based on the determined DL channel estimates of the first RU 120 and the second RU 160, and the first part of the BFW is determined for performing interference cancellation between the user layer signals. The method further includes determining a BFW for the first RU based on the received first portion and the first partition of the BFW (310), sending the received first portion and the second partition of the BFW to the second RU 160 via the RU link 165 (312), and sending the received K user layer DL signals in the frequency domain to the second RU 160 via the RU link 165 (314).The method further includes beamforming (316) the received K user layer signals into antenna signals using the determined BFW for the first RU 160, and transmitting (318) the antenna signals to a number of UEs 131, 132, 133 via N1 antennas 121, 122, 123.

[0045] Such a method allows the first RU to receive and use the BFW adapted for the first RU and send the BFW adapted for the subsequent cascaded RU, i.e., RU2, to the subsequent cascaded RU. The first part of the BFW is used for all RUs connected to the BBU, but is sent only once through the fronthaul link, thus saving the capacity of the FH link. The determination (310) of the BFW for the first RU is typically performed by multiplying the first part of the BFW by the first section of the BFW. If the first part of the BFW and the first section are matrices, the multiplication is a matrix multiplication. The first and second RUs may be realized as separate RUs on different printed circuit boards (PCBs). Alternatively, the first and second RUs may be realized as different radio processors on a single PCB.

[0046] According to one embodiment, the method further includes extracting (305) the first partition and the first portion of the BFW from the received first portion and the first and second partitions of the BFW. The extraction (305) may be performed such that, if the first and second partitions are combined into a common matrix, the first RU extracts the portion of the matrix that belongs to the first partition. For example, the common matrix is ​​a four-column matrix, with the first two columns belonging to the first partition of the BFW and the last two columns belonging to the second partition. The first RU then uses or extracts the first two columns and sends the remaining two columns to the second RU. In another alternative, the extraction may be performed such that, if the first and second partitions are sent in different messages, the first RU receives a message including the first partition of the BFW and further sends the message with the second partition to the second RU. This allows the first RU to use only the BFW determined for it, and send, i.e., forward, to the second RU, the unused second partition and the first portion used by both the first and second RUs.

[0047] According to one embodiment, the method further includes recovering (308) the first partition of the BFW if the first partition of the BFW was compressed at the BBU. This step is performed if the BBU compressed the first partition of the BFW to a higher degree than any compression of the first part of the BFW. Recovery may be dequantization or reconstruction by zeroing out a matrix according to the received bitmask (see below). In the case of dequantization, the first RU requires information about the format used, such as the compression scheme and any configuration parameters. The format used is typically preconfigured so that the BBU and the first RU know the format in advance.

[0048] According to another embodiment, the method further includes receiving (303) from the BBU 110 first compression information related to compression of a first section of the BFW and second compression information related to compression of a second section of the BFW, the first and second sections of the BFW being compressed by the BBU to a higher degree than any compression of the first portion of the BFW, and the decompressing (308) of the first section of the BFW being performed according to the first compression information. The method further includes sending (313) the second compression information to the second RU 160 through the RU link 165. The first and second compression information may be a bit mask. This is applicable when the first and second sections of the BFW are compressed at the BBU, for example, by selecting only a subset of the determined first and second sections of the BFW, the subset being some strongest BFWs for each user layer signal.

[0049] According to another embodiment in which the received (304) first portion and the first and second partitions of the BFW are normalized, the method further includes receiving (306) information from the BBU regarding a per-user layer scaling factor associated with the normalization, and scaling (307) the received normalized first portion and the first partition of the BFW according to the information regarding the scaling factor prior to beamforming (316). The received first portion and the first and second partitions of the BFW are normalized by the BBU according to the per-user layer scaling factor, which may be different from the scaling factor used by the first RU for scaling (307).

[0050] According to an alternative example, the method further includes sending information regarding the per-user-layer scaling factor to the second RU 160, which enables the second RU to also scale the first portion and the second partition of the BFW.

[0051] According to yet another embodiment, the distributed base station system 100 further comprises a third RU 170 connected to the second RU 160 through a second RU link 175, the third RU 170 comprising N3 antennas 171, 172, 173. Receiving 304 the BFW further comprises receiving a third section of the BFW determined based on DL channel estimation for the third RU 170 to extend the user layer signals in the frequency domain to antenna signals of the N3 antennas 171, 172, 173 of the third RU 170, the first portion of the BFW being based on DL channel estimation for the first RU 120, the second RU 160, and the third RU 170. The method further includes sending the third section of the BFW for further transmission to the second RU 160 through the RU link 165 and to the third RU 170 through the second RU link 175.

[0052] 5 in conjunction with FIG. 2 describes a method implemented by a BBU system of a wireless communication network, the wireless communication network comprising a distributed base station system 100 comprising a BBU 110 and a first RU 120 connected to the BBU 110 through a fronthaul link 140, the first RU comprising N1 antennas 121, 122, 123. The distributed base station system 100 further comprises a second RU 160 connected to the first RU 120 through a RU link 165, the second RU 160 comprising N2 antennas 161, 162, 163. The method includes determining (402) a first partition of a BFW in the frequency domain based on a UL channel estimate of the first RU 120, the first partition of the BFW being used to combine in the frequency domain antenna signals received at the N1 antennas of the first RU 120 from several UEs 131, 132, 133 into K user layer signals of the first RU. The method further includes determining (404) a second partition of the BFW in the frequency domain based on the UL channel estimates of the second RU 160, where the second partition of the BFW is used to combine frequency domain signals received at the N antennas of the second RU 160 from several UEs 131, 132, 133 into K user layer signals of the second RU. The method further includes determining (406) a first portion of the BFW in the frequency domain based on the UL channel estimates of the first RU 120 and the second RU 160, where the first portion of the BFW is determined to perform interference cancellation between the user layer signals of the first and second RUs, and triggering (408) sending the first portion of the BFW and the first and second partitions of the BFW to the first RU 120.

[0053] This method in the BBU is similar to the method in the BBU of the above embodiment, however, this method is adapted to UL communication of signals, whereas the above embodiment was adapted to DL communication of signals. The advantages etc. are similar to those for DL ​​communication of signals. Moreover, the embodiment described above in this disclosure in relation to FIG. 3 is also applicable to this scenario of UL communication of signals.

[0054] 6, in conjunction with FIG. 2, describes a method performed by a first RU 120 of the distributed base station system 100, the first RU 120 equipped with N1 antennas 121, 122, 123. The distributed base station system 100 further includes a BBU 110 connected to the first RU 120 via a fronthaul link 140 and a second RU 160 connected to the first RU 120 via a RU link 165, the second RU equipped with N2 antennas 161, 162, 163. The method includes receiving UL antenna signals from a number of UEs 131, 132, 133 on N1 antennas (452); and receiving from the BBU 110 a first portion of a BFW and first and second partitions of the BFW (454), where the first partition is based on a determined UL channel estimate of the first RU 120, the first partition being determined in the frequency domain for combining the UL antenna signals received on the N1 antennas into a user layer signal, the second partition being based on a determined UL channel estimate of the second RU 160, the second partition being determined in the frequency domain for combining the antenna signals received on the N2 antennas of the second RU 160 into a user layer signal, the first portion of the BFW is based on the determined UL channel estimates of the first RU 120 and the second RU 160, and the first portion of the BFW is determined to perform interference cancellation between the user layer signals. The method further includes determining a BFW for the first RU based on the received first portion and the first partition of the BFW (456); sending the received first portion and the second partition of the BFW to the second RU 160 via the RU link 165 (458); and combining (460), in the frequency domain, the UL antenna signals received on the N1 antennas into the first K user layer signals using the BFW determined for the first RU 120.The method further includes receiving (462) from the second RU 160 a second K user layer signal combined by the second RU from antenna signals received at the N antennas of the second RU 160 from the several UEs 131, 132, 133, where the combining by the second RU is based on the sent first and second partitions of the BFW sent by the first RU to the second RU. The method further includes combining (464) the first and second K user layer signals into K combined user layer signals and sending (466) the combined K user layer signals to the BBU 120.

[0055] The method implemented by the first RU 120 is adapted for UL communication of signals, while the embodiment described in relation to FIG. 4 is adapted for DL ​​communication of signals. The combining performed on the user layer signals by the first division of the beamforming weights of the antenna signals may be performed by pre-equalizing the antenna signals that actually perform the combining. This combining / pre-equalization may perform coherent combining, e.g., maximum ratio combining. Furthermore, by combining the already combined or pre-equalized first and second K user layer signals into K combined user layer signals (464), only one set of combined signals is sent through the base station system to the BBU 110 through the fronthaul link 140 for all cascaded RUs. This allows the capacity usage of the fronthaul connection to be kept low. Furthermore, the embodiments described above in this disclosure in relation to FIG. 4 are also applicable to this scenario of UL communication of signals.

[0056] In the following, a system model for DL ​​reciprocity-based beamforming according to one embodiment is described. For DL ​​in both 3GPP Option 7-3 and Cat-BO-RU (where beamforming is performed in RU) in O-RAN architecture, channel estimation and BFW calculation are performed in the BBU system, and beamforming is performed in the RU. For cascaded RUs addressed in this invention, the calculated BFW needs to be conveyed from the BBU system to the first RU via the fronthaul link. The first RU extracts the corresponding BFW and then forwards the remaining BFW to the next RU, i.e., the second RU. This process is repeated for each RU in the cascaded chain until the BFW reaches the last RU. This is usually done in the control plane of the fronthaul interface / link. Meanwhile, multiple layers of user layer data, e.g., user layer coded bits in 3GPP Option 7-3 or compressed modulation symbols in O-RAN, are usually conveyed in the data plane from the BBU to each RU in the same way, i.e., through the cascade via the first RU. Each RU in the cascaded chain converts the received user plane data into modulation symbols, applies the received BFW targeted to that specific RU on the modulation symbols of multiple layers, and then converts the beamformed symbols to analog signals to transmit over the air on different antennas towards the UE.

[0057] Now consider a scenario where there are K user layers (i.e., UEs receiving user data) in a desired cell. There are L RUs in a local area cascaded in a daisy chain, effectively forming a large antenna array. The first RU (RU 1) is the one with the fronthaul interface that connects to the BBU. RU l (with l=1, ..., L) transmits DL channels towards the K UEs. TIFF0007602058000001.tif5170, where N l is the number of antennas installed on RU l. Therefore, the effective large antenna array has a total of The antenna elements are TIFF0007602058000002.tif10170. The corresponding effective DL channel estimate with L RUs is: Considering that reciprocity-assisted transmission (RAT) is performed for an effective antenna array, the BFW for the zero-forcing (ZF) based method can be expressed as: TIFF0007602058000004.tif14170On the other hand, the BFW for the minimum mean squared error (MMSE) based method can be expressed as follows: TIFF0007602058000005.tif12170, where σ 2 is a measure of interference and noise (or error term), and I is a K × K identity matrix. The BFW is then split into a first part P1 and a second part P2, and the BFW is commonly written as P = P2P1, where the first part P1 is the common part for all L RUs, and the second part TIFF0007602058000006.tif5170 includes respective coefficient parts that are specific to each of the L RUs. The first part P1 performs pre-mitigation of interference between user layers and is composed of K×K complex values. The second part TIFF0007602058000007.tif7170 contains individual coefficients for each RU, and the coefficients TIFF0007602058000008.tif5170 expands the signals of different user layers pre-combined by P1 to transmit the signals in the antenna elements or beam directions of RU l. Therefore, the BFW of RU l is P RU l =P 2,RU l P1. Therefore, the transmitted signal of RU l after beamforming can be expressed as y l =P RU l x=P 2,RU l P1x where x=[x1x2… x K ]T For each element x i Let be the signal vector representing the modulation symbols of layer i, TIFF0007602058000009.tif9170 is each element TIFF0007602058000010.tif5170 shows the transmit signal vector of RU l, which represents the transmit symbols of antenna i or beam i of RU l.

[0058] Then, according to one embodiment, the DL BFW is compressed. The BFW in P is If each complex value is represented by Q bits, then P can be directly carried on the fronthaul link using TIFF0007602058000012.tif requires 10170 bits. The present invention transports P1 and P2 from the BBU to the first RU, respectively. Two different embodiments are provided to compress P2 at the BBU, so that the total bits required to transport P1 and P2 can be significantly reduced compared to transporting P directly.

[0059] A block diagram of a distributed base station having one BBU 510 and l RUs 520, 560, 570 cascaded to the BBU 510 according to one embodiment is shown in Figure 7 for 3GPP Option 7-3 architecture. Further details regarding the operation of the BBU 510 and the cascaded RUs 520, 560, 570, respectively, are described.

[0060] The BBU 510 comprises a forward error correction (FEC) coding unit that codes user layer signals carried in DL towards the RUs 520, 560, 570 over the fronthaul link 540. The BBU 510 further comprises a BFW calculation unit 514 that determines or calculates a BFW for each RU. The BFW calculation unit 514 may alternatively be located outside the BBU or within the BBU system, such as implemented as a cloud solution.

[0061] A first embodiment of compressing the second part P2 of the BFW in the BBU is a so-called layer-specific selection of the BFW. In this embodiment, before calculating the first and second parts of the BFW, i.e. P1 and P2, the channel coefficients may be transformed to the beam / direction domain if the channel has not been estimated in the beam / direction domain. Then, the second part P2 of the BFW is determined as a separate part for each RU. TIFF0007602058000013.tif8170, where Compressed second part of TIFF0007602058000014.tif6170BFW TIFF0007602058000015.tif5170 is determined as follows: TIFF0007602058000016.tif5170 is the data for each column of P2. It is obtained by keeping the selected entries of TIFF0007602058000017.tif5170 and setting the unselected entries to 0. The selected The entry in TIFF0007602058000018.tif5170 is the entry with the largest amplitude (absolute value) and corresponds to the strongest channel coefficient in this layer. For TIFF0007602058000019.tif7170, the corresponding M l non-zero entries, the remainder of the entries are zero, where TIFF0007602058000020.tif6170. Compressed TIFF0007602058000021.tif5170 can be expressed by the following formula. The common first part P1 of TIFF0007602058000022.tif8170BFW is then calculated based on equation (1) or (2), depending on whether the ZF method or the MMSE method is used. Calculated according to TIFF0007602058000023.tif5170. For ZF-based methods, TIFF0007602058000024.tif5170. For the MMSE-based method, TIFF0007602058000025.tif5170. It is observed that the ZF-based method and the MMSE-based method are merely example methods, and any other possible method may be used. Then the first part of BFW The non-zero values ​​in TIFF0007602058000026.tif6170 are The selected entry in TIFF0007602058000027.tif5170 is sent to the first RU 520 along with a bitmask indicating the selected entry in TIFF0007602058000027.tif5170. The bitmask size for all RUs is TIFF0007602058000028.tif7170 and can be shared across multiple resource blocks (RBs), thereby reducing overhead. Here, the number of bits required to carry the BFW, not considering the bitmask overhead, is (M+K)KQ. TIFF0007602058000029.tif5170To ensure a well-tuned matrix inversion for the calculation, TIFF0007602058000030.tif5170 is proposed. To obtain good performance, the selected entry should capture most of the channel energy of the DL channel estimate H.

[0062] As an example, if L RUs each have N antennas and each selects K out of N beams, the number of BFWs carried is (L+1)K. 2 In the uncompressed case, the number of BFWs is LNK. Therefore, the compression ratio is TIFF0007602058000031.tif10170. For large L, it approaches K / N. For L=4, N=32, and K=16, compression reduces the BFW to 62.5% of the uncompressed case. For K=8, it is further reduced to 31.25%. Thus, the link capacity required to carry the BFW can be significantly reduced. In terms of performance, our studies have shown that the performance of K beam selection for a K-layer system is very close to that of a perfect system without beam selection.

[0063] The second embodiment of compressing the second part P2 of the BFW in the BBU is compressing P2 using fewer bits. Originally, when the BFW of P is carried, it needs to be quantized with high precision because large quantization error reduces the inter-layer interference cancellation performance. In the present invention, for the same reason, the BFW of P1 must also be quantized to the same precision level as the BFW of P. However, P2 can tolerate some precision loss with fewer quantization bits. Therefore, carrying P2 over the FH link with fewer bits can also reduce the required FH capacity. Note that this embodiment does not require the BFW to be calculated based on the beam / direction domain channel.

[0064] The second portion of the BFW may be determined as follows: TIFF0007602058000032.tif8170, where TIFF0007602058000033.tif5170, i.e. in the same manner as in the first embodiment. Then, the second part P2 of the BFW is compressed with fewer bits per BFW, resulting in This embodiment can be implemented in preference to the first embodiment, i.e., to represent non-zero weights with fewer bits. TIFF0007602058000035.tif5170 can be expressed by the following formula. The common first part P1 of TIFF0007602058000036.tif8170BFW is then calculated based on equation (1) or (2), depending on whether the ZF method or the MMSE method is used. Calculated according to TIFF0007602058000037.tif5170. For ZF-based methods, TIFF0007602058000038.tif5170. For the MMSE-based method, TIFF0007602058000039.tif5170. It is observed that the ZF-based method and the MMSE-based method are merely exemplary methods, and any other possible method may be used. Then, the first part of the BFW TIFF0007602058000040.tif5170 and the compressed second part of BFW TIFF0007602058000041.tif5170 is sent to the first RU 520.

[0065] Returning to Figure 7, we now look at what happens in each of the RUs 520, 560, and 570 when they receive the FEC coded user layer signal and the calculated first and second portions of the BFW from the BBU 510. The first RU 520 comprises a modulation unit 522 that modulates the FEC coded user layer signal received over the FH link 540. The first RU 520 further comprises an extraction unit 524 that extracts a first RU specific partition of the second portion of the BFW received over the FH link 540, which is referred to as the first partition of the BFW and is shown in Figure 7. The extraction unit 524 further extracts the second portion of the remaining BFW. The first RU sends TIFF0007602058000043.tif5170, i.e., without the first segment, to the second RU 560. The first RU also sends the first portion of the BFW after copying it to the second RU 560. The first RU also sends the FEC coded user layer signal received from the BBU to the second RU 560.

[0066] The first RU 520 then TIFF0007602058000044.tif6170, i.e., based on the first portion of the BFW and the compressed first partition of the BFW. Furthermore, the first RU 520 transmits the modulated user layer signal x to the first RU P RU-1 The beamforming is performed with the BFW of antenna signal y1 = P RU-1 The first RU 520 further comprises a beamforming unit 526 that performs an inverse fast Fourier transform (IFFT) on the beamformed user layer signal in the frequency domain, thereby converting the signal into the time domain. The time domain signal is then converted from digital to analog by a digital-to-analog converter and then frequency up-converted to an RF signal before being transmitted over the air from the N antennas of the first RU to the UEs 531, 532.

[0067] Further, with respect to the first embodiment of the compression of the second part P2 of the BFW implemented in the BBU, the first RU 520 compresses the first part P2 of the BFW. TIFF0007602058000045.tif5170, and the non-zero weight of the second part TIFF0007602058000046.tif5170, 5170 from the BBU along with a bitmask of TIFF0007602058000047.tif5170. In this embodiment, the extraction unit 524 not only extracts the first partition of the second portion of the BFW but also extracts the bitmask for the first partition of the BFW. Similarly, the extraction unit 524 not only sends the remaining partition of the second portion of the BFW to the second RU 560 but also sends the remaining bitmask for the remaining partition of the second portion of the BFW. Furthermore, the first partition of the BFW is reconstructed to zero out a matrix using the extracted bitmask for the first partition of the BFW.

[0068] For the second embodiment, the first partition of the BFW is reconstructed by inverse quantization.

[0069] Note that in 3GPP Option 7-3, the coded bits are received by the first RU 520 from the BBU 510 along with the BFW through the fronthaul interface. The modulation symbol x is modulated on the received coded bits. In O-RAN, the modulation symbol x may be compressed. In this case, the first RU recovers the compressed symbols to obtain the modulation symbol x.

[0070] The second RU 560 has the same units as the first RU, namely, a modulation unit 562, an extraction unit 564, a beamforming unit 566, an IFFT and RF related circuitry 568, and N2 antennas. The modulation unit 562 modulates the FEC coded user layer signal received from the first RU. The second RU 560 receives the remaining section of the second part of the BFW from the first RU 520, i.e., the second part of the BFW that does not include the first section. The extraction unit 564 of the second RU extracts the section of the second part of the BFW that is specific to the first RU, which is referred to as the second section of the BFW and is shown in FIG. 7. The extraction unit 564 further extracts the second portion of the remaining BFW. The second RU 560 also sends the FEC coded user layer signal received from the first RU 510 to RU L 570. The second RU 560 also sends the FEC coded user layer signal received from the first RU 510 to the remaining RUs, here RU L 570. The second RU 560 also sends the FEC coded user layer signal received from the first RU 510 to RU L 570.

[0071] The second RU 560 then TIFF0007602058000050.tif6170, i.e., based on the first portion of the BFW and the compressed second partition of the BFW. Further, the beamforming unit 566 transmits the modulated user layer signal x to the second RUP RU-2 The beamforming is performed with the BFW of the antenna signal y2 = P RU-2 The IFFT and RF related circuitry 568 performs an IFFT on the antenna signals in the frequency domain so that the signals are transformed to the time domain and then transmitted over the air from the N antennas of the second RU to the UEs 531, 532.

[0072] Further, with respect to the first embodiment of the compression of the second part P2 of the BFW implemented in the BBU, the second RU 560 compresses the first part P2 of the BFW. TIFF0007602058000051.tif5170, and the non-zero weights of the remaining segments of the second part TIFF0007602058000052.tif5170, 5170 from the first RU along with the remaining bitmask of TIFF0007602058000053.tif5170. In this embodiment, the extraction unit 564 not only extracts the second partition of the second part of the BFW, but also extracts the bitmask for the second partition of the BFW. Similarly, the extraction unit 564 not only sends the remaining partition of the second part of the BFW to the RU L 570, but also sends the remaining bitmask for the remaining partition of the second part of the BFW. Furthermore, the second partition of the BFW is reconstructed to zero out a matrix using the extracted bitmask for the second partition of the BFW.

[0073] For the second embodiment, the second partition of the BFW is reconstructed by inverse quantization.

[0074] RU L 570 has similar units as the first and second RUs, namely a modulation unit 572, a beamforming unit 576, IFFT and RF related circuits 578, and N L antennas with roles similar to those of the first and second RUs. However, since RU L is the last of the cascade, it does not need to have an extraction unit. Similarly, since the received data is used by the last RU of the cascade, it does not need to extract any data and send other data further.

[0075] According to another embodiment, there may now be a scaling factor between MIMO user layers. Once the BFW is calculated, the DL implementation may need to adjust the power for the number of co-scheduled UEs or MIMO layers in order to allocate power to all UEs or MIMO layers. Let D denote a K×K diagonal matrix, with each diagonal element representing a power allocation coefficient for the corresponding column P. The power allocation coefficients not only control the power allocation between MIMO layers, but also normalize the BFW, thereby reducing the dynamic range of the BFW and facilitating efficient quantization. Including the scaling matrix D, the transmit signal can be expressed as follows: y=PDx According to the above embodiment, the BFW includes a first and a second part. TIFF0007602058000054.tif5170. The two parts of the BFW of TIFF0007602058000055.tif6170 also need to be normalized so that the dynamic range of the BFW is reduced when conveying through the fronthaul link 540. To achieve the same transmitted signal y as described above, the normalized first and second parts of the BFW can be constructed as follows: TIFF0007602058000056.tif12170Here, D1 is a K×K diagonal matrix, and each diagonal element is represents the scaling factor for TIFF0007602058000057.tif6170. D2 is a K × K diagonal matrix, and each diagonal element is a represents the scaling factor for TIFF0007602058000058.tif5170. To reconstruct y, the BBU 510 also needs to send to the first RU 520 the scaling factor: TIFF0007602058000059.tif6170, where TIFF0007602058000060.tif4170 is a diagonal matrix containing K real values. The first RU 520 is Build TIFF0007602058000061.tif5170, 5170 to the second RU 560, together with the remaining portion of TIFF0007602058000062.tif5170, and then to the remaining RUs, ie, RU L, as described above. Upon receiving TIFF0007602058000063.tif5170, the transmitted signal from the effective antenna array can be obtained by the first RU as follows: TIFF0007602058000064.tif6170The result is the originally intended transmitted signal, y.

[0076] A similar method as described above for DL ​​transmission of user layer signals can also be used for UL transmission of user layer signals, provided the architecture is one in which channel estimation and BFW computation is performed in the BBU and beamforming is performed in each of the cascaded RUs. In a similar manner to the DL, the BBU sends the BFW to the cascaded RUs via the first RU, then the second RU, etc. Each RU receives the BFW, extracts its own BFW, and forwards the remainder of the BFW to the next RU.

[0077] Let TIFF0007602058000065.tif7170 denote a valid UL channel with L RU. When ZF-based beamforming is used, the BFW can be expressed as TIFF0007602058000066.tif11170When MMFE-based beamforming is used, the BFW can be expressed as follows: TIFF0007602058000067.tif11170, where σ 2 is a measure of interference and noise (or error terms), and I is a K×K identity matrix. In a similar manner to the DL BFW case, W1 denotes the first part of the UL BFW and W2 denotes the second part of the UL BFW. Both W1 and W2 are calculated in the BBU. According to the invention, the second part of the BFW, W2, is calculated using one of the embodiments described above for the DL. It can be compressed as TIFF0007602058000068.tif5170. TIFF0007602058000069.tif7170 while TIFF0007602058000070.tif7170, and therefore, The selection of the TIFF0007602058000071.tif4170 entry is This should be done as a selection of TIFF0007602058000072.tif4170 columns. The BBU is calculated based on TIFF0007602058000073.tif6170. In the first compression embodiment, Compressed with respect to the non-zero portion of TIFF0007602058000074.tif5170 TIFF0007602058000075.tif5170 plus a bitmask, or the full size in the case of the second compression embodiment. TIFF0007602058000076.tif5170, TIFF0007602058000077.tif5170 is sent to the first RU via the fronthaul link.

[0078] In the first RU, the first section of the second part of the BFW TIFF0007602058000078.tif6170 is reconstructed based on the first partition extracted from the received BFW and, if the first compression embodiment is used, based on the bitmask. The first RU is the remaining part of the second part of the BFW. TIFF0007602058000079.tif5170, and the rest of the bitmask (for the first compression embodiment) into the first part of the BFW The first RU forwards the received data to the second RU along with TIFF0007602058000080.tif5170. The first RU then sets the BFW for the first RU. TIFF0007602058000081.tif6170 and perform beamforming accordingly. The procedure is repeated for each of the cascaded RUs until the BFW reaches RU L. Using the calculated BFW for the first RU to perform beamforming on the UL signals received on the antennas from the UE, the first RU combines the UL antenna signals received on the N1 antennas into a first K user layer signals. The first RU then receives from the second RU the second K user layer signals that the second RU beamformed, i.e. combined in a similar manner, from the antenna signals received from the UE on the N2 antennas of the second RU, and the combination by the second RU is based on the BFW calculated by the second RU from the first and second partitions of the BFW forwarded by the first RU when receiving the BFW from the BBU. The first RU then combines the first and second K user layer signals into K combined user layer signals and sends the K combined user layer signals to the BBU. If there are more than two cascaded RUs, subsequent RUs in the chain will beamform the antenna signals received at their antennas in a similar manner, i.e., using a specific division of the first portion of the BFW and the second portion of the BFW, into combined user layer signals and send the combined user layer signals up the cascade towards the BBU to combine with user layer signals of RUs located closer to the BBU on the way to the BBU.

[0079] Besides being used in a distributed base station with separate RUs, each RU located on a separate printed circuit board (PCU), the above-mentioned embodiment may also be used in a single large RU design with multiple radio processors, where each radio processor plays the role of a RU in the above-mentioned embodiment. In this case, the radio processors / RUs may be located on one and the same PCB. Therefore, the radio processors are implemented in a cascaded topology, i.e., as described in FIG. 2. This can significantly reduce the number of SerDes lanes on the PCB between the radio processor and the fronthaul interface connected to the BBU, compared to a star topology design. The overall required fronthaul link capacity is also significantly reduced.

[0080] 8, in conjunction with FIG. 2, describes a BBU system 600 configured to operate in a wireless communication network. The wireless communication network includes a first distributed base station system 100 including a BBU 110 and a first RU 120 connected to the BBU 110 through a fronthaul link 140, the first RU including N1 antennas 121, 122, 123. The distributed base station system further includes a second RU 160 connected to the first RU 120 through a RU link 165, the second RU 160 including N2 antennas 161, 162, 163. The BBU system 600 also includes a processing circuit 603 and a memory 604. The memory includes instructions executable by the processing circuitry such that the BBU system 600 is operable to determine, based on a DL channel estimation of the first RU 120, a first partition of a BFW in the frequency domain to be used for beamforming K user layer DL signals to be sent to several UEs 131, 132, 133, and the first partition of the BFW is determined in the frequency domain to extend the K user layer signals to antenna signals of N1 antennas 121, 122, 123 of the first RU 120. The BBU system 600 is further operable to determine, based on the DL channel estimation of the second RU 160, a second partition of the BFW in the frequency domain, where the second partition of the BFW is determined in the frequency domain for spreading the K user layer signals onto antenna signals of the N antennas 161, 162, 163 of the second RU 160, and to determine, based on the DL channel estimation of the first RU 120 and the second RU 160, a first portion of the BFW in the frequency domain, where the first portion of the BFW is determined for performing interference cancellation between the user layer signals. The BBU system 600 is further operable to trigger sending the first portion of the BFW and the first and second partitions of the BFW to the first RU 120, and to trigger sending the K user layer signals in the frequency domain to the first RU 120.

[0081] The BBU system 600 may be the actual BBU 110, a unit within the BBU 110, or a unit within the distributed base station system 100. Alternatively, the BBU system 600 may be located within or at any other network node of the communication network, such as a node remote from the UE, e.g., another network element within or close to the RAN, or another RAN node. In this alternative, and in a cloud solution embodiment described below, the BBU 110 is arranged to receive uplink signals, e.g., reference signals such as SRS or DMRS that the RUs are receiving from the UE, from each of the first and second RUs 120, 160, or information related to these uplink signals, and communicate the uplink signals / information to another network node, which is arranged to determine DL channel estimates for each RU, and from the respective DL channel estimates, determine the first and second portions of the BFW as indicated above. The other network node is then arranged to transmit the determined first and second portions of the BFW back to the BBU 110 for further distribution to the first RU 120. Alternatively, the BBU system 600 may be implemented as a group of network nodes, where the functionality of the BBU system is distributed across different physical or virtual nodes of the network, the latter sometimes being called a "cloud solution".

[0082] According to one embodiment, the BBU system 600 is further operable to compress the first and second partitions of the BFW to a greater extent than any compression of the first portion of the BFW.

[0083] According to another embodiment, the BBU system 600 is operable to determine a first portion of the BFW based on a DL channel estimate of the first RU 120, a DL channel estimate of the second RU 160, and based on the compressed first and second partitions of the BFW.

[0084] According to another embodiment, the BBU system 600 is operable to compress the first and second sections of the BFW by selecting only a subset of the determined first and second sections of the BFW, which are several BFWs having the largest magnitude for each user layer signal, and is operable to trigger sending the compressed first and second sections of the BFW to the first RU 120 by sending the selected subset of the first and second sections of the BFW to the first RU.

[0085] According to another embodiment, the BBU system 600 is operable to compress the first and second partitions of the BFW by using fewer bits to represent the individual BFWs than the number of bits used for the first and second partitions before compression.

[0086] According to yet another embodiment, the BBU system 600 is further operable to send to the first RU 120 first compression information related to the compression of a first section of the BFW and second compression information related to the compression of a second portion of the BFW, the first and second compression information being for decompression in the first and second RUs, respectively.

[0087] According to yet another embodiment, the BBU system 600 is further operable to combine the first and second partitions of the BFW into a second portion of the BFW.

[0088] According to yet another embodiment, the BBU system 600 is further operable to determine a DL channel estimate for the first RU 120 based on UL signals originating from the several UEs 131, 132, 133 and received on the antennas 121, 122, 123 of the first RU 120, and to determine a DL channel estimate for the second RU 160 based on UL signals originating from the several UEs 131, 132, 133 and received on the antennas 161, 162, 163 of the second RU 160.

[0089] According to yet another embodiment, the BBU system 600 is further operable to normalize the first portion and the first and second partitions of the BFW and send to the first RU 120 information regarding a per-user layer scaling factor associated with the normalization.

[0090] According to another embodiment, the distributed base station system 100 further comprises a third RU 170 connected to the second RU 160 through a second RU link 175, the third RU 170 comprising N3 antennas 171, 172, 173. The BBU system 600 is further operable to determine a third partition of the BFW in the frequency domain based on the determined DL channel estimate of the third RU 170, the third partition of the BFW being determined to extend, in the frequency domain, the K user layer signals to antenna signals of the N3 antennas 171, 172, 173 of the third RU 170. The BBU system 600 is also operable to trigger sending the first portion of the BFW as well as the first and second partitions of the BFW to the first RU 120 by triggering sending the third partition of the BFW to the first RU 120.

[0091] According to an alternative aspect, the memory 604 of the BBU system of FIG. 8 includes instructions executable by the processing circuit 603 such that the BBU system 600 is operable to determine, based on a UL channel estimate of the first RU 120, a first partition of a BFW in the frequency domain used to combine, in the frequency domain, antenna signals received on the N1 antennas of the first RU 120 from several UEs 131, 132, 133 into K user layer signals of the first RU, and to determine, based on a UL channel estimate of the second RU 160, a second partition of a BFW in the frequency domain used to combine, in the frequency domain, frequency domain signals received on the N2 antennas of the second RU 160 from several UEs 131, 132, 133 into K user layer signals of the second RU. The BBU system 600 is further operable to determine, based on the UL channel estimates of the first RU 120 and the second RU 160, a first portion of a BFW in the frequency domain, which is determined to perform interference cancellation between the user layer signals of the first and second RUs, and trigger sending the first portion of the BFW and the first and second partitions of the BFW to the first RU 120. This alternative aspect handles user layer signals. The embodiment described above in connection with FIG. 8 is also applicable to this aspect.

[0092] According to other embodiments, the BBU system 600 may further comprise a communication unit 602, which may be considered as comprising conventional means of communicating with other network nodes of the wireless communication network 100, such as a BBU, and with the first RU 120, if the BBU system is not an actual BBU. The instructions executable by said processing circuit 603 may be arranged as a computer program 605, for example stored in said memory 604. The processing circuit 603 and the memory 604 may be arranged in a subarrangement 601. The subarrangement 601 may be a microprocessor and suitable software and storage, thus a programmable logic device (PLD), or other electronic component / processing circuit configured to implement the above-mentioned method. The processing circuit 603 may comprise one or more programmable processors, application specific integrated circuits, field programmable gate arrays, or combinations of the above adapted to execute instructions.

[0093] The computer program 605 may be arranged such that, when the instructions of the program are executed on the processing circuit, it causes the BBU system 600 to perform the steps described in any of the described embodiments of the BBU system 600 and its methods. The computer program 605 may be held by a computer program product connectable to the processing circuit 603. The computer program product may be the memory 604 or at least be arranged in the memory. The memory 604 may be realized as, for example, a RAM (random access memory), a ROM (read only memory), or an EEPROM (electrically erasable programmable read only memory). In some embodiments, a carrier may house the computer program 605. 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 a computer readable storage medium. The computer readable storage medium may be, for example, a CD, a DVD, or a flash memory, from which the program can be downloaded to the memory 604. Alternatively, the computer program may be stored on a server or any other entity to which the BBU system 600 accesses via the communication unit 602. The computer program 605 may then be downloaded into the memory 604 from the server.

[0094] 9, in conjunction with FIG. 2, illustrates a first RU 120 with N1 antennas 121, 122, 123 configured to operate in a distributed base station system 100. The distributed base station system 100 further includes a BBU 110 connected to the first RU 120 via a fronthaul link 140 and a second RU 160 connected to the first RU 120 via a RU link 165, the second RU with N2 antennas. The first RU 120 includes a processing circuit 703 and a memory 704. The memory includes instructions executable by the processing circuit, whereby the first RU 120 is operable to receive, from the BBU 110, K user layer DL signals in the frequency domain to be sent to several UEs 131, 132, 133, and to receive, from the BBU 110, a first portion of a BFW and first and second partitions of a BFW for beamforming the K user layer signals in the frequency domain, the first partition being based on a determined DL channel estimate of the first RU 120, the first partition being determined for extending the user layer signals in the frequency domain to antenna signals of the N1 antennas 121, 122, 123 of the first RU 120, and the second partition being based on a determined DL channel estimate of the second RU 160, the second partition being determined for beamforming the user layer signals in the frequency domain to antenna signals of the N1 antennas 121, 122, 123 of the first RU 120. A first portion of the BFW is determined to extend the BFW to antenna signals of N2 antennas 161, 162, 163 of the first RU 120 and the second RU 160, the first portion of the BFW being based on the determined DL channel estimates of the first RU 120 and the second RU 160, and the first portion of the BFW is determined to perform interference cancellation between user layer signals.The first RU 120 is further operable to determine a BFW for the first RU 120 based on the received first portion and first partition of the BFW, send the received first portion and second partition of the BFW to the second RU 160 via the RU link 165, send the K user layer DL signals received in the frequency domain to the second RU 160 via the RU link 165, beamform the received K user layer signals into antenna signals using the BFW determined for the first RU 160, and send the antenna signals to several UEs 131, 132, 133 via N1 antennas 121, 122, 123.

[0095] According to one embodiment, the first RU 120 is further operable to extract a first partition and a first portion of the BFW from the received first portion and the first and second partitions of the BFW.

[0096] According to another embodiment, the first RU 120 is further operable to decompress the first section of the BFW if the first section of the BFW was compressed at the BBU.

[0097] According to another embodiment, the first RU 120 is further operable to receive from the BBU 110 first compression information related to the compression of a first section of the BFW and second compression information related to the compression of a second section of the BFW, the first and second sections of the BFW being compressed by the BBU to a higher degree than any compression of the first portion of the BFW, and the first RU is operable to perform decompression of the first section of the BFW according to the first compression information and transmit the second compression information to the second RU 160 via the RU link 165.

[0098] According to yet another embodiment, the received first portion and the first and second partitions of the BFW are normalized, and the first RU is also operable to receive information from the BBU 110 regarding a per-user layer scaling factor associated with the normalization, and to scale the received normalized first portion and the first partition of the BFW according to the information regarding the pre-beamforming scaling factor.

[0099] According to yet another embodiment, the first RU 120 is further operable to send information regarding the scaling factor per user layer to the second RU 160.

[0100] In yet another embodiment, the distributed base station system 100 further comprises a third RU 170 connected to the second RU 160 through a second RU link 175, the third RU 170 comprising N3 antennas 171, 172, 173. The first RU 120 is operable to receive a BFW determined based on DL channel estimates for the third RU 170 to extend the user layer signals to antenna signals of the N3 antennas 171, 172, 173 of the third RU 170 in the frequency domain by also receiving a third partition of the BFW, where a first portion of the BFW is based on DL channel estimates for the first RU 120, the second RU 160, and the third RU 170. First RU 120 is also operable to send a third section of the BFW to second RU 160 over RU link 165 for further transmission to third RU 170 over second RU link 175.

[0101] According to an alternative aspect, the memory 704 of the first RU 120 of FIG. 9 includes instructions executable by the processing circuit 703 such that the first RU is operable to receive UL antenna signals on N1 antennas from several UEs 131, 132, 133 and receive a first portion of a BFW and first and second partitions of a BFW from the BBU 110, the first partition being based on a determined UL channel estimate of the first RU 120, the first partition being determined in the frequency domain for combining the UL antenna signals received on the N1 antennas into a user layer signal, the second partition being based on a determined UL channel estimate of the second RU 160, the second partition being determined in the frequency domain for combining the antenna signals received on the N2 antennas of the second RU 160 into a user layer signal, the first portion of the BFW being determined by the first RU 120 and the second RU 160. Based on the determined UL channel estimate of 160, a first portion of a BFW is determined to perform interference cancellation between the user layer signals. The first RU 120 is also operable to determine a BFW for the first RU based on the received first portion and the first partition of the BFW, send the received first portion and the second partition of the BFW to the second RU 160 over RU link 165, and combine, in the frequency domain, the UL antenna signals received at the N1 antennas into the first K user layer signals using the BFW determined for the first RU 120. The first RU 120 is also operable to receive from the second RU second K combined user layer signals based on the first and second partitions of the BFW sent by the first RU to the second RU from antenna signals received at the N2 antennas of the second RU 160 from several UEs 131, 132, 133, combine the first and second K user layer signals into K combined user layer signals, and send the K combined user layer signals to the BBU 110. The embodiment described above in connection with FIG. 9 is also applicable to this aspect.

[0102] According to another embodiment, the first RU 120 may further comprise a communication unit 702, which may be considered as comprising conventional means of wirelessly communicating with the UEs 131, 132, 133, such as a transceiver for wireless transmission and reception of signals. The communication unit 702 may also comprise conventional means of communicating with the BBU 110 and with the second RU 160. The instructions executable by said processing circuitry 703 may be arranged as a computer program 705, for example stored in said memory 704. The processing circuitry 703 and the memory 704 may be arranged in a sub-arrangement 701. The sub-arrangement 701 may be a microprocessor and suitable software and storage, thus a programmable logic device (PLD), or other electronic component / processing circuitry configured to implement the above-mentioned method. The processing circuitry 703 may comprise one or more programmable processors, application specific integrated circuits, field programmable gate arrays, or combinations of the above adapted to execute instructions.

[0103] The computer program 705 may be arranged such that when the instructions of the program are executed on the processing circuitry, it causes the first RU 120 to perform the steps described in any of the described embodiments of the first RU 120 and its methods. The computer program 705 may be held by a computer program product connectable to the processing circuitry 703. The computer program product may be the memory 704 or at least be located in the memory. The memory 704 may be realized as, for example, a RAM (random access memory), a ROM (read only memory), or an EEPROM (electrically erasable programmable read only memory). In some embodiments, a carrier may carry the computer program 705. 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 a computer readable storage medium. The computer readable storage medium may be, for example, a CD, a DVD, or a flash memory, from which the program may be downloaded to the memory 704. Alternatively, the computer program may be stored on a server or any other entity that the first RU 120 accesses via the communication unit 702. The computer program 705 may then be downloaded into the memory 704 from the server.

[0104] Although the above description contains multiple specificities, these should not be construed as limiting the scope of the concepts described herein, but merely as providing illustrations of some exemplary embodiments of the concepts described. It will be understood that the scope of the concepts currently described fully encompasses other embodiments that may become apparent to those skilled in the art, and accordingly the scope of the concepts currently described is not limited. Reference to an element in the singular is not intended to mean "only one" unless specifically stated to be only one, but rather "one or more." All structural and functional equivalents to the elements of the above embodiments known to those skilled in the art are expressly incorporated herein by reference and are intended to be encompassed by this specification. Moreover, it is not necessary for a device or method to address each and every problem that the concepts currently described are attempting to solve in order to be encompassed herein. In the exemplary drawings, dashed lines typically mean that the features within the dashed lines are optional.

Claims

1. 1. A method implemented by a baseband unit (BBU) system of a wireless communication network, the wireless communication network comprising a distributed base station system (100) comprising: a BBU (110); a first radio unit (RU) (120) connected to the BBU (110) through a fronthaul link (140), the first RU having N1 antennas (121, 122, 123); and a second RU (160) connected to the first RU (120) through a RU link (165), the second RU (160) having N2 antennas (161, 162, 163), determining (204) a first partition of a beamforming weight (BFW) in a frequency domain based on a downlink (DL) channel estimate of the first RU (120), the BFW being used to beamform K user layer DL signals to be sent to a number of UEs (131, 132, 133), the first partition of the BFW being determined in the frequency domain to extend the K user layer signals to antenna signals of the N1 antennas (121, 122, 123) of the first RU (120); determining (206) a second partition of the BFW in a frequency domain based on DL channel estimation of the second RU (160), the second partition of the BFW being determined to extend, in the frequency domain, the K user layer signals to antenna signals of the N antennas (161, 162, 163) of the second RU (160); determining (212) a first portion of the BFW in a frequency domain based on the DL channel estimates of the first RU (120) and the second RU (160), where the first portion of the BFW is determined to perform interference cancellation between the user layer signals; triggering (214) sending the first portion of the BFW and the first and second partitions of the BFW to the first RU (120); triggering (216) sending the K user layer signals in the frequency domain to the first RU (120); A method comprising:

2. compressing the first and second partitions of the BFW to a greater degree than any compression of the first portion of the BFW (208); The method of claim 1 further comprising:

3. 3. The method of claim 2, wherein the first portion of the BFW is determined (212) based on the DL channel estimate of the first RU (120), the DL channel estimate of the second RU (160), and based on the compressed first and second partitions of the BFW.

4. 4. The method of claim 2 or 3, wherein the first and second partitions of the BFW are compressed (208) by selecting only a subset of the determined (204, 206) first and second partitions of the BFW, the subset being several BFWs having the largest magnitude for each user layer signal, and wherein the triggering (214) of sending the compressed first and second partitions of the BFW to the first RU (120) suggests sending the selected subset of the first and second partitions of the BFW to the first RU.

5. 4. The method of claim 2 or 3, wherein the first and second partitions of the BFW are compressed (208) by using fewer bits to represent each BFW than the number of bits used for the first and second partitions before the compression (208).

6. sending (209) to the first RU (120) first compression information regarding the compression (208) of the first section of the BFW and second compression information regarding the compression (208) of the second section of the BFW, the first and second compression information being for decompression in the first and second RUs, respectively; 6. The method according to any one of claims 2 to 5.

7. normalizing the first portion of the BFW and the first and second partitions; and sending to the first RU (120) information regarding a per-user layer scaling factor associated with the normalization; The method of claim 1 , further comprising:

8. The distributed base station system (100) further comprises a third RU (170) connected to the second RU (160) through a second RU link (175), the third RU (170) comprising N3 antennas (171, 172, 173), and the method further comprises: determining (207) a third partition of the BFW in a frequency domain based on the determined DL channel estimate of the third RU (170), wherein the third partition of the BFW is determined to extend, in the frequency domain, the K user layer signals to antenna signals of the N3 antennas (171, 172, 173) of the third RU (170); 8. The method of claim 1, wherein the triggering (214) of sending the first portion of the BFW and the first and second partitions of the BFW to the first RU (120) further comprises triggering sending the third partition of the BFW to the first RU (120).

9. 1. A method, implemented by a first RU (120) of a distributed base station system (100), the first RU (120) comprising N1 antennas (121, 122, 123), the distributed base station system (100) further comprising a BBU (110) connected to the first RU (120) through a fronthaul link (140), and a second RU (160) connected to the first RU (120) through an RU link (165), the second RU comprising N2 antennas (161, 162, 163), receiving (302) K user layer DL signals in the frequency domain from said BBU (110) to be sent to several UEs (131, 132, 133); receiving from the BBU (110) a first portion of beamforming weights (BFWs) and first and second partitions of BFWs for beamforming the K user layer signals in a frequency domain, the first partition being determined based on a determined DL channel estimate of the first RU (120), the first partition being determined for extending the user layer signals in the frequency domain to antenna signals of the N1 antennas (121, 122, 123) of the first RU (120), and the second partition being determined based on a determined DL channel estimate of the second RU (160); receiving (304) the first portion of the BFW and the first and second partitions of the BFW, where the second partition is determined for extending the user layer signals to antenna signals of the N2 antennas (161, 162, 163) of the second RU (160) in a frequency domain based on the determined DL channel estimates of the first RU (120) and the second RU (160), and the first portion of the BFW is determined for performing interference cancellation between the user layer signals; determining (310) a BFW for the first RU based on the received first portion and a first partition of the BFW; sending (312) the received first portion and a second section of the BFW to the second RU (160) over the RU link (165); sending (314) the K user layer DL signals received in the frequency domain to the second RU (160) via the RU link (165); beamforming (316) the received K user layer signals into antenna signals using the BFW determined for the first RU; Sending (318) the antenna signals to the number of UEs (131, 132, 133) via the N1 antennas (121, 122, 123); A method comprising:

10. Extracting the first partition and the first portion of the BFW from the received first portion of the BFW and the first and second partitions of the BFW (305). The method of claim 9 further comprising:

11. If the first portion of the BFW was compressed in the BBU, decompressing the first portion of the BFW (308). The method of claim 9 or 10, further comprising:

12. receiving (303) from the BBU (110) first compression information relating to a compression of the first section of the BFW and second compression information relating to a compression of the second section of the BFW, the first and second sections of the BFW being compressed by the BBU to a higher degree than any compression of the first portion of the BFW, and the decompressing (308) of the first section of the BFW being performed in accordance with the first compression information; sending (313) the second compressed information to the second RU (160) over the RU link (165); The method of claim 11 further comprising:

13. The received (304) first portion of the BFW and the first and second partitions are normalized, the method comprising: receiving from the BBU information regarding a per-user layer scaling factor associated with the normalization (306); Scaling (307) the received normalized first portion and a first partition of the BFW according to the information regarding the scaling factor before the beamforming (316); 13. The method of any one of claims 9 to 12, further comprising:

14. The method of claim 13, further comprising sending the information regarding the scaling factor per user layer to the second RU (160).

15. The distributed base station system (100) further comprises a third RU (170) connected to the second RU (160) through a second RU link (175), the third RU (170) comprising N3 antennas (171, 172, 173), and the receiving (304) of the BFW is determined based on DL channel estimation for the third RU (170) to extend the user layer signal into antenna signals of the N3 antennas (171, 172, 173) of the third RU (170) in a frequency domain.

15. The method of claim 9, further comprising receiving a third section of the BFW determined based on DL channel estimates for the first RU (120), the second RU (160), and the third RU (170), the method further comprising sending the third section of the BFW to the second RU (160) over the RU link (165) and further transmitting the third section of the BFW to the third RU (170) over the second RU link (175).

16. 1. A method implemented by a BBU system of a wireless communication network, the wireless communication network comprising a distributed base station system (100) comprising: a BBU (110); a first RU (120) connected to the BBU (110) through a fronthaul link (140), the first RU having N1 antennas (121, 122, 123); and a second RU (160) connected to the first RU (120) through an RU link (165), the second RU having N2 antennas (161, 162, 163), comprising: determining (402) a first partition of a BFW in a frequency domain based on an UL channel estimate of the first RU (120), the first partition of a BFW being used to combine, in the frequency domain, antenna signals received at the N1 antennas of the first RU (120) from a number of UEs (131, 132, 133) into K user layer signals of the first RU; determining (404) a second partition of the BFW in a frequency domain based on an UL channel estimate of the second RU (160), the second partition of the BFW being used to combine frequency domain signals received at the N antennas of the second RU (160) from the several UEs (131, 132, 133) into K user layer signals of the second RU; determining (406) a first portion of the BFW in a frequency domain based on the UL channel estimates of the first RU (120) and the second RU (160), where the first portion of the BFW is determined to perform interference cancellation between the user layer signals at the first and second RUs; triggering (408) sending the first portion of the BFW and the first and second partitions of the BFW to the first RU (120); A method comprising:

17. 1. A method, implemented by a first RU (120) of a distributed base station system (100), the first RU (120) comprising N1 antennas (121, 122, 123), the distributed base station system (100) further comprising a BBU (110) connected to the first RU (120) through a fronthaul link (140), and a second RU (160) connected to the first RU (120) through an RU link (165), the second RU comprising N2 antennas, receiving (452) UL antenna signals from a number of UEs (131, 132, 133) on said N1 antennas; receiving (454) a first portion of a BFW and first and second partitions of the BFW from the BBU (110), the first partition being based on a determined UL channel estimate of the first RU (120), the first partition being determined for combining the UL antenna signals received at the N1 antennas into a user layer signal in a frequency domain, the second partition being based on a determined UL channel estimate of the second RU (160), the second partition being determined for combining the UL antenna signals received at the N1 antennas into a user layer signal in a frequency domain, receiving (454) a first portion of a BFW and first and second partitions of the BFW, the first portion of the BFW being determined for combining antenna signals received at the N2 antennas of the second RU (160) into a user layer signal in a wavenumber domain, the first portion of the BFW being based on the determined UL channel estimates of the first RU (120) and the second RU (160), the first portion of the BFW being determined for performing interference cancellation between the user layer signals; determining a BFW for the first RU based on the received first portion and a first partition of the BFW (456); sending (458) the received first portion and a second section of the BFW to the second RU (160) over the RU link (165); combining (460) the received UL antenna signals on the N1 antennas into first K user layer signals in the frequency domain using the BFW determined for the first RU (120); receiving (462) from the second RU (160) a second K number of user layer signals combined by the second RU from antenna signals received at the N2 antennas of the second RU (160) from the several UEs (131, 132, 133), the combining by the second RU being based on the first portion and a second partition of the BFW sent by the first RU to the second RU; combining (464) the first and second K user layer signals into K combined user layer signals; sending (466) the combined K user layer signals to the BBU (120); A method comprising:

18. 1. A BBU system (600) configured to operate in a wireless communication network, the wireless communication network comprising a distributed base station system (100) comprising: a BBU (110); a first RU (120) connected to the BBU (110) through a fronthaul link (140), the first RU having N1 antennas (121, 122, 123); and a second RU (160) connected to the first RU (120) through an RU link (165), the second RU having N2 antennas (161, 162, 163), the BBU system (600) comprising a processing circuit (603) and a memory (604), the memory comprising instructions executable by the processing circuit; and the BBU system (600) comprising: determining a first partition of a BFW in a frequency domain based on DL channel estimation of the first RU (120), the BFW being used to beamform K user layer DL signals to be sent to several UEs (131, 132, 133), the first partition of the BFW being determined in the frequency domain to extend the K user layer signals to antenna signals of the N1 antennas (121, 122, 123) of the first RU (120); determining a second partition of the BFW in a frequency domain based on the DL channel estimation of the second RU (160), the second partition of the BFW being determined in the frequency domain to extend the K user layer signals to antenna signals of the N2 antennas (161, 162, 163) of the second RU (160); determining a first portion of the BFW in a frequency domain based on the DL channel estimates of the first RU (120) and the second RU (160), the first portion of the BFW being determined to perform interference cancellation between the user layer signals; triggering sending of the first portion of the BFW and the first and second partitions of the BFW to the first RU (120); Triggering the K user layer signals in the frequency domain to the first RU (120). The BBU system (600) is operable as follows.

19. The BBU system (600) of claim 18, wherein the BBU system (600) is further operable to perform a method according to any one of claims 2 to 8.

20. When executed by at least one processing circuit of a BBU system (600) of a wireless communication network, the BBU system (600) includes a distributed base station system (100) including a BBU (110), a first RU (120) connected to the BBU (110) through a fronthaul link (140), the first RU having N1 antennas (121, 122, 123), and a second RU (160) connected to the first RU (120) through an RU link (165), the second RU (160) having N2 antennas (161, 162, 163), the ... includes a BBU (110), determining a first partition of a BFW in a frequency domain based on DL channel estimation of the first RU (120), the BFW being used to beamform K user layer DL signals sent to several UEs (131, 132, 133), the first partition of the BFW being determined in order to extend the K user layer signals to antenna signals of the N1 antennas (121, 122, 123) of the first RU (120) in the frequency domain; determining a second partition of the BFW in a frequency domain based on DL channel estimation of the second RU (160), the second partition of the BFW being determined to extend the K user layer signals to antenna signals of the N2 antennas (161, 162, 163) of the second RU (160) in the frequency domain; determining a first portion of the BFW in a frequency domain based on the DL channel estimates of the first RU (120) and the second RU (160), wherein the first portion of the BFW is determined to perform interference cancellation between the user layer signals; triggering sending of the first portion of the BFW and the first and second partitions of the BFW to the first RU (120); triggering the K user layer signals in the frequency domain to the first RU (120); A computer program product (605) comprising instructions for causing the computer to carry out the steps of:

21. A first RU (120) configured to operate in a distributed base station system (100), the first RU (120) comprising N1 antennas (121, 122, 123), the distributed base station system (100) further comprising: a BBU (110) connected to the first RU (120) through a fronthaul link (140); and a second RU (160) connected to the first RU (120) through an RU link (165), the second RU comprising N2 antennas (161, 162, 163), the first RU (120) comprising a processing circuit (703) and a memory (704), the memory comprising instructions executable by the processing circuit, the first RU (120) comprising: receiving K user layer DL signals in the frequency domain from the BBU (110) to be sent to several UEs (131, 132, 133); receiving from the BBU (110) a first portion of a BFW and first and second partitions of a BFW for beamforming the K user layer signals in a frequency domain, the first partition being determined based on a determined DL channel estimate of the first RU (120), the first partition being determined for extending the user layer signals in a frequency domain to antenna signals of the N1 antennas (121, 122, 123) of the first RU (120); and based on the determined DL channel estimates of the first RU (120) and the second RU (160), the second partition is determined to extend the user layer signals to antenna signals of the N2 antennas (161, 162, 163) of the second RU (160) in a frequency domain; and based on the determined DL channel estimates of the first RU (120) and the second RU (160), the first portion of the BFW is determined to perform interference cancellation between the user layer signals; determining a BFW for the first RU (120) based on the received first portion and a first partition of the BFW; sending the received first portion and a second section of the BFW to the second RU (160) over the RU link (165); Sending the K received user layer DL signals in the frequency domain to the second RU (160) via the RU link (165); beamforming the received K user layer signals into antenna signals using the BFW determined for the first RU (160); Sending the antenna signals to the number of UEs (131, 132, 133) via the N1 antennas (121, 122, 123). A first RU (120).

22. The first RU (120) of claim 21, wherein the first RU (120) is further operable to execute a method according to any one of claims 12 to 17.

23. A first RU (120) operable in a distributed base station system (100), the first RU (120) having N1 antennas (121, 122, 123), the distributed base station system (100) further comprising a BBU (110) connected to the first RU (120) through a fronthaul link (140), and a second RU (160) connected to the first RU (120) through an RU link (165), the second RU (160) having N2 antennas, when executed by at least one processing circuit of the first RU (120), receiving, from said BBU (110), K user layer DL signals in frequency domain to be sent to several UEs (131, 132, 133); Receive a first portion of a BFW and first and second partitions of a BFW from the BBU (110) for beamforming the K user layer signals in a frequency domain, the first partition being determined based on a determined DL channel estimate of the first RU (120), the first partition being determined for extending the user layer signals in a frequency domain to antenna signals of the N1 antennas (121, 122, 123) of the first RU (120), and the second partition being determined based on a determined DL channel estimate of the second RU (120). Based on the determined DL channel estimates of the second RU (160), the second partition is determined in a frequency domain to extend the user layer signals to antenna signals of the N2 antennas (161, 162, 163) of the second RU (160), and the first part of the BFW is determined based on the determined DL channel estimates of the first RU (120) and the second RU (160), and the first part of the BFW is determined to perform interference cancellation between the user layer signals; determining a BFW for the first RU (120) based on the received first portion and a first partition of the BFW; sending the received first portion and a second section of the BFW to the second RU (160) over the RU link (165); sending the K received user layer DL signals in the frequency domain to the second RU (160) via the RU link (165); beamforming the received K user layer signals into antenna signals using the BFW determined for the first RU (160); sending said antenna signals to said number of UEs (131, 132, 133) via said N1 antennas (121, 122, 123); A computer program (705) comprising instructions for causing the computer to carry out the steps of:

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