Method, system and unit for a distributed base station system for processing downlink communications

Abstract

A method is disclosed which is performed by an RRU (120) of a distributed base station system (100) of a wireless communication network, the distributed base station system (100) further comprising a BBU (110) connected to the RRU, the RRU (120) being connected to a plurality of antennas (121, 122, 123) through which the RRU transmits a user layer signal to a plurality of UEs (161, 162, 163). The method comprises: transmitting information related to a received uplink signal to the BBU so that the BBU can obtain a first part of a precoding coefficient to be used for precoding a digital user layer signal. The method further comprises: obtaining a second part of the precoding coefficient and receiving a digital user layer signal precoded using the first part of the precoding coefficient from the BBU. The method further comprises: also precoding the received digital user layer signal precoded using the first part of the precoding coefficient using the second part of the precoding coefficient, and transmitting the user layer signal to the plurality of UEs (161, 162, 163) via the plurality of antennas (121, 122, 123).

Description

Technical Field

[0001] The present disclosure generally relates to methods, systems and units of a distributed base station system for processing downlink communications. More specifically, the present disclosure relates to a remote radio unit RRU and a method performed by such an RRU of a distributed base station system of a wireless communication network. The distributed base station system also includes a baseband unit BBU connected to the RRU, and the RRU is connected to multiple antennas, and the RRU wirelessly communicates with at least one UE through the multiple antennas. The present disclosure also relates to a corresponding BBU system and a method performed by such a BBU system. The present disclosure also relates to a computer program and a carrier corresponding to the above method, BBU system and RRU. Background Art

[0002] In a centralized radio access network C-RAN (also known as a distributed base station system, radio access network RAN), processing is performed by two independent units: a remote radio unit RRU and a baseband unit BBU. The BBU is connected to the RRU via a fronthaul link. The RRU is connected to one or more antennas, through which the RRU communicates wirelessly with at least one user equipment UE. The BBU is in turn connected to other base station systems or base stations, as well as the core network of the wireless communication system. The BBU is centralized, and there may be more than one RRU connected to each BBU. Traditionally, the BBU performs advanced radio coordination functions (such as joint detection, joint decoding, coordinated multi-point transmission CoMP), thereby improving spectral efficiency and network capacity, as well as baseband processing, while the RRU performs radio frequency RF processing and transmission / reception of RF-processed signals. This division of base station functions between the BBU and the RRU is called PHY-RF division.

[0003] The RRU was originally designed to reduce the cable losses in the coaxial cables between the top of the tower where the actual antennas are located and the bottom of the tower where the base station functions are hosted. Therefore, before the fifth generation of mobile communications 5G, that is, at the time of 4G (such as Long Term Evolution LTE), the RRU was quite simple, mainly doing RF processing with limited baseband processing (if any).

[0004] When evolving from 4G to 5G, the wireless communication capacity for UEs needs to be increased so that the requested amount of data can be delivered in each time period in 5G. One driving force for the mobile evolution towards 5G is massive multiple-input multiple-output MIMO, where each RRU has multiple antennas. In other words, massive MIMO exploits spatial multiplexing to improve spectral efficiency by using an antenna array at the RRU, which is equipped with N antennas that simultaneously serve K user layers in the same time-frequency resources. The typical scenario is N>>K, for example, N is 64 / 128 or 256, and K is 8 or 16. As shown, the number of antennas is quite large. In order to support such a massive MIMO solution, when using the current PHY-RF division between the functions of the BBU and RRU, the required fronthaul link capacity needs to increase proportionally with the increase in the number of antennas. This will greatly increase the fronthaul link cost.

[0005] To address this, new partitioning options for fronthaul functionality have been discussed in 3GPP, see 3GPP TR 38.801, V14.0.0, published in March 2017: “Study on new radio access technology: Radio access architecture and interfaces”. Figure 1 Some of the partitioning options discussed (options 7-1, 7-3) and the current PHY-RF partitioning (option 8) are shown in FIG. Figure 1 The internal PHY partitioning option called option 7-1 in the RRU is performed in the RRU, where the inverse fast Fourier transform IFFT and cyclic prefix CP addition are performed in the RRU, and the digital to RF conversion is also performed. The 7-1 partitioning option increases the fronthaul efficiency and therefore reduces the need for fronthaul capacity due to reduced frequency redundancy and by adapting the fronthaul bit rate to the traffic load, which is feasible when the IFFT is performed in the RRU (rather than in the BBU). However, there is still a problem that each antenna sends a bit stream over the fronthaul, and for massive MIMO, there is still a lot of data that needs to be sent over the fronthaul link.

[0006] In order to achieve the division between the functions of the RRU and the BBU so that the RRU becomes cost-effective and also so that the fronthaul connection capacity can be maintained at a reasonable level, division option 7-3 has been proposed. Here, the BBU encodes the data of the MIMO user layer and sends the encoded data of each user layer to the RRU. Then, in addition to the functions of option 7-1, the RRU also performs modulation, resource mapping, and precoding of the modulated signal. This reduces the amount of bit streams to be sent over the fronthaul link to the number of MIMO user layers, thereby reducing the required capacity of the fronthaul link and also reducing the number of bits required for each sample.

[0007] In option 7-3, the precoding coefficients are usually calculated in the BBU and sent to the RRU via the fronthaul link, and the RRU performs precoding based on the precoding coefficients. The reason for calculating the precoding coefficients in the BBU rather than in the RRU is that the calculation of the coefficients requires a large number of computing operations, and, for cost efficiency reasons, it is required to make the RRU as simple as possible. However, the number of precoding coefficients N p Proportional to the number of antennas N and the number of user layers K. For massive MIMO, the number of antennas N is large, so the number of precoding coefficients N p Increases dramatically. Therefore, a large amount of fronthaul capacity is spent on transmitting precoding coefficients at the expense of transmitting the actual signal. When assuming 12 subcarriers per precoding coefficient (i.e., one resource block RB in LTE), 64 antennas, and 16 bits per coefficient, the number of bits per subcarrier per layer is 16*64 / 12=85.3 bits, which is 10 times larger than the 8 bits of 256 quadrature amplitude modulation QAM for data. If the precoder needs to be updated every 14 symbols (which means every 1 millisecond in LTE), then the number of bits of precoding coefficients per symbol per layer per subcarrier for the precoder that needs to be sent from the BBU to the RRU over the fronthaul link becomes 16*64 / 12 / 14=6.1 bits. Therefore, for 256 QAM data symbols, 43% of the fronthaul link traffic will become precoding coefficients. This will increase further when the antenna size increases (for example, to 128 and 256, etc.). Therefore, in the case of massive MIMO, the fronthaul link overhead for transmitting the precoding coefficients is very large and can account for a major portion of the fronthaul link traffic for large configurations. If the RRU is left to determine the precoding coefficients itself instead, the RRU will become computationally complex, resulting in a distributed base station system that is not cost-effective.

[0008] In "Low RF-Complexity Millimeter-Wave Beamspace-MIMO Systems by Beam Selection" published by Amadori et al. in IEEE Transactions on Communications, Volume 63, Issue 6, Pages 2212-2223, June 2015, there is a method called direction selection DS or beam selection BS, which can reduce the fronthaul link overhead by sending precoding coefficients from BBU to RRU. For ease of representation, this method is referred to as the DS method below. It is discussed in the hybrid beamforming scenario. This concept can be used to reduce the number of precoding coefficients through the fronthaul link. In the DS method, the RRU generates a fixed number of directional beams in many directions covering the service area. This fixed number of beams is formed by a set of fixed beamforming coefficients. For example, by applying discrete Fourier transform DFT coefficients to the signal toward the antenna element, the same number of beams as the number of antenna elements can be generated. To simplify the representation, the directional beam is called a "direction". The BBU selects a subset of directions based on the channel information and calculates coefficients corresponding to the subset of directions. Since the number of selected directions is less than the number of antennas, the number of coefficients for the selected directions is reduced compared to the original case. This method is based on the fact that propagation is concentrated to some main directions from the main reflections around. However, the DS method requires selecting much more directions than the number of user layers, and therefore the reduction in the number of beamforming coefficients is limited, and therefore the reduction in the fronthaul overhead due to the transmission of the fronthaul coefficients is also limited.

[0009] In addition, the 7-3 option of performing precoding at the RRU limits the coordination possibilities of multiple RRUs connected to a centralized BBU. For example, the 7-3 option cannot support the BBU to jointly process user layer signals directed to multiple RRUs in order to achieve mutual benefit of the user layer signals, because the BBU only works on the coded bits. Instead, the user layer signals of different RRUs are processed independently of each other. The 7-3 option also limits the system scalability for larger MIMO support. In order to upgrade to a larger MIMO system, the existing RRU must be replaced with a new RRU with more antennas. If this can be achieved by stacking multiple RRUs together to expand the number of antennas, it will be good. For example, two RRUs (RRU1 and RRU2, each with 64 antennas, where both RRUs are connected to the same BBU) form a 128-antenna MIMO system. In this case, if RRU 1 and RRU 2 are not cascaded (i.e., deployed at two adjacent sites), this can form a distributed MIMO system with 128 antennas. With option 7-3, each RRU can only be used individually for, for example, interference pre-mitigation (e.g., using a method based on interference rejection combining (IRC)). In this case, the degrees of freedom in the system are not increased, i.e., the number of MIMO layers cannot be increased. On the other hand, it is possible to distribute the MIMO layers among the RRUs. Compared to a system using option 8 or 7-1, option 7-3 cannot achieve full MIMO gains when multiple RRUs are used. Returning to options 8 and 7-1, the signals of the RRUs can be coordinated when the precoding is performed in the BBU. Then, for example, interference at the UE caused by transmissions from different RRUs can be pre-eliminated in the BBU. All degrees of freedom can be maintained by using more RRUs, and therefore more layers can be sent. However, as mentioned earlier, for options 7-1 and 8 in massive MIMO, the number of fronthaul streams that need to be sent via the fronthaul is too high.

[0010] As shown, an improved solution is needed to handle the transmission of downlink data in a distributed base station system including BBUs and RRUs. Moreover, a solution is also needed that more efficiently utilizes the fronthaul link capacity and still achieves full MIMO gain in air interface performance in a distributed base station system where one BBU is connected to multiple RRUs, while achieving user layer signal coordination in the BBU. Summary of the invention

[0011] It is an object of the present invention to solve at least some of the problems and issues outlined above. These and other objects are achieved by using the method and apparatus defined in the attached independent claims.

[0012] According to one aspect, a method performed by an RRU of a distributed base station system of a wireless communication network is provided. The distributed base station system also includes a BBU connected to the RRU. The RRU is connected to multiple antennas, and the RRU sends user layer signals to multiple UEs through the multiple antennas, using the same time-frequency resources to send the user layer signals. The method includes: receiving uplink signals from multiple UEs; sending information related to the received uplink signals to the BBU so that the BBU can obtain a first part of the precoding coefficients for precoding digital user layer signals in the frequency domain; and obtaining a second part of the precoding coefficients. The method also includes: receiving a digital user layer signal precoded using the first part of the precoding coefficients from the BBU; precoding the received digital user layer signal with the second part of the precoding coefficients; and converting the digital user layer signal precoded with the first part and the second part of the precoding coefficients into an analog user layer signal. Thereafter, the analog user layer signal is sent to multiple UEs via multiple antennas.

[0013] According to another aspect, a method performed by a BBU system of a wireless communication network is provided. The wireless communication network includes a distributed base station system having a BBU and at least one RRU connected to the BBU. The method includes: receiving information from at least one RRU, the information being related to uplink signals received by the at least one RRU from a plurality of UEs wirelessly connected to the at least one RRU. The method also includes: obtaining a first part of a precoding coefficient for precoding a digital user layer signal based on the received information, the precoded user layer signal to be sent by at least one RRU to a plurality of UEs, determining the first part of the precoding coefficient so as to pre-mitigate interference between user layer signals sent to a plurality of UEs, the precoding coefficient including a first part and a second part. The method also includes: precoding the digital user layer signal using only the first part of the precoding coefficient; and sending the precoded digital user layer signal to at least one RRU to be further precoded with the second part of the precoding coefficient before sending the user layer signal to the UE.

[0014] According to another aspect, an RRU capable of operating in a distributed base station system of a wireless communication network is provided. The distributed base station system also includes a BBU connected to the RRU. The RRU is connected to multiple antennas, and the RRU is arranged to send user layer signals to multiple UEs using the same time-frequency resources through the multiple antennas. The RRU includes a processing circuit and a memory. The memory contains instructions executable by the processing circuit, so that the RRU can operate to: receive uplink signals from multiple UEs; send information related to the received uplink signals to the BBU so that the BBU can obtain a first part of the precoding coefficients for precoding digital user layer signals in the frequency domain; and obtain a second part of the precoding coefficients. The RRU can also operate to: receive a digital user layer signal precoded using the first part of the precoding coefficients from the BBU; and precode the received digital user layer signal with the second part of the precoding coefficients. The RRU can also operate to: convert the digital user layer signal precoded with the first part and the second part of the precoding coefficients into an analog user layer signal; and send the analog user layer signal to multiple UEs via multiple antennas.

[0015] According to another aspect, a BBU system capable of operating in a wireless communication network is provided, the wireless communication network including a distributed base station system having a BBU and at least one RRU connected to the BBU. The BBU system includes a processing circuit and a memory. The memory contains instructions executable by the processing circuit, so that the BBU system is operable to: receive information from at least one RRU, the information being related to uplink signals received by the at least one RRU from a plurality of UEs wirelessly connected to the at least one RRU. The BBU system is also operable to: obtain a first part of a precoding coefficient for precoding a digital user layer signal based on the received information, the precoded user layer signal to be sent by the at least one RRU to a plurality of UEs, the first part of the precoding coefficient being determined so as to pre-mitigate interference between the user layer signals sent to the plurality of UEs, the precoding coefficient comprising a first part and a second part. The BBU system is also operable to: precode the digital user layer signal using only the first part of the precoding coefficient; and before sending the user layer signal to the UE, send the precoded digital user layer signal to the at least one RRU to be further precoded with the second part of the precoding coefficient.

[0016] According to other aspects, a computer program and a carrier are also provided, the details of which will be described in the claims and the detailed description.

[0017] Other possible features and benefits of the solution will become apparent from the following detailed description. BRIEF DESCRIPTION OF THE DRAWINGS

[0018] The solution will now be described in more detail by means of exemplary embodiments and with reference to the accompanying drawings, in which:

[0019] Figure 1 is a flow chart illustrating different known functional partitionings of a distributed base station system.

[0020] Figure 2 is a block diagram illustrating a communication scenario in which the present invention may be used.

[0021] Figure 3 is a flow chart illustrating a method performed by an RRU according to a possible embodiment of the present invention.

[0022] Figure 4 is a flow chart illustrating a method performed by a BBU system according to a possible embodiment of the present invention.

[0023] Figure 5 is a block diagram illustrating a distributed base station system according to other possible embodiments in more detail.

[0024] Figure 6 to Figure 7 is a block diagram illustrating an RRU according to other possible embodiments in more detail.

[0025] Figures 8 to 9 is a block diagram illustrating a BBU system according to other possible embodiments in more detail. DETAILED DESCRIPTION

[0026] According to an embodiment, in order to achieve a more efficient division of RAN processing functions between one or more RRUs and a BBU in a distributed base station system, when one or more RRUs each have multiple antennas, the present invention proposes to decompose precoding into two parts: a first part and a second part. The BBU performs the first part of precoding using the first part of the precoding coefficients. The first part of the precoding is performed so as to pre-mitigate interference between user layer signals. On the other hand, one or more RRUs perform the second part of precoding using the second part of the precoding coefficients. The second part of the precoding may be performed so as to spatially focus the energy of each user layer toward its UE. When a digital user layer signal is to be sent from the BBU to a UE via one or more RRUs, the digital user layer signal is first precoded using the first part of the precoding coefficients. Then, the digital user layer signal precoded with the first part of the precoding coefficients is sent to at least one RRU. Then, before the precoded digital user layer signals received at one or more RRUs are sent from the multiple antennas of the one or more RRUs to their corresponding UEs, the received precoded digital user layer signals are precoded again using the second part of the precoding coefficients. Such an embodiment effectively utilizes the fronthaul connection between the RRU and the BBU because a limited number of user layer signals are sent via the fronthaul compared to options 7-1 and 8. Moreover, interference between signals sent from different RRUs can be mitigated because the first part of the precoding is performed by the BBU for interference mitigation, so when two or more RRUs send user layer signals at least partially to the same UE, the BBU can coordinate interference mitigation for the two or more RRUs. This is not possible with the prior art option 7-3.

[0027] Figure 2A wireless communication network in which the present invention can be used is shown. The wireless communication network comprises a distributed base station system 100, which in turn comprises a BBU 110 and a first RRU 120. The BBU 110 has connections to other base station nodes or other RAN nodes and further to a core network (denoted by 150 in the figure), so that the distributed base station system can communicate with other nodes of the communication network. The BBU 110 is connected to the first RRU via a fronthaul link 140. The fronthaul link 140 can be any type of connection, such as a dedicated wired or wireless connection, or a connection via a network, as long as the connection (e.g., in terms of capacity and latency) meets the fronthaul requirements. The first RRU 120 also has multiple antennas 121, 122, 123, through which wireless signals are transmitted to and from one or more UEs 161, 162, 163. The wireless signal includes data to be transmitted from or to the UE 161, 162, 163. The distributed base station system 100 may also include a second RRU 130. The BBU 110 is connected to the second RRU 130 via another fronthaul link 141. The other fronthaul link 141 may be any type of connection, for example, a dedicated wired or wireless connection, or a connection via a network, as long as the connection (for example, in terms of capacity and latency) meets the fronthaul requirements. The other fronthaul link 141 may or may not share the same connection as the fronthaul link 140. The second RRU 130 also has multiple antennas 131, 132, 133, through which wireless signals are transmitted toward and from one or more UEs 161, 162, 163. The BBU 110 and the first and second RRUs 120 and 130 include RAN functions for processing data and signals to be transmitted between the first and second RRUs 120 and 130 and the UEs 161, 162, 163. The RAN functions are distributed between the BBU 110 and the RRUs 120, 130 as will be further described below in this disclosure.

[0028] Combination Figure 2 , Figure 3A method performed by an RRU 120 of a distributed base station system 100 of a wireless communication network is described. The distributed base station system 100 also includes a BBU 110 connected to the RRU. The RRU 120 is connected to a plurality of antennas 121, 122, 123, through which the RRU 120 transmits user layer signals to a plurality of UEs 161, 162, 163, the user layer signals being transmitted using the same time-frequency resources. The method comprises: receiving 202 uplink signals from a plurality of UEs; transmitting 204 information about the received uplink signals to the BBU 110 so that the BBU can obtain a first part of precoding coefficients for precoding digital user layer signals in the frequency domain; and obtaining 206 a second part of the precoding coefficients. The method further comprises: receiving 208 a digital user layer signal precoded using a first portion of precoding coefficients from the BBU 110; precoding 210 the received digital user layer signal using a second portion of the precoding coefficients; and converting 212 the digital user layer signal precoded using the first portion and the second portion of the precoding coefficients into an analog user layer signal. Thereafter, the analog user layer signal is transmitted 214 to the plurality of UEs 161, 162, 163 via the plurality of antennas 121, 122, 123. The analog user layer signal is also converted into a radio frequency domain before being wirelessly transmitted to the plurality of UEs.

[0029] Thus, the user layer signal is partially precoded at the BBU and partially precoded at the RRU. Since part of the precoding is performed at the BBU and part at the RRU, the computational complexity at the RRU can be reduced compared to having all computations performed at the RRU, making the RRU less complex and therefore more cost-effective. Furthermore, in the case where the BBU is connected to more than one RRU, since part of the precoding is performed at the BBU, the BBU can coordinate the precoding of the user layer signals for the more than one RRU.

[0030] The uplink signal may be an uplink reference signal, such as a sounding reference signal SRS, a demodulation reference signal DMRS, or any other signal that may be defined for the purpose of assisting downlink channel estimation. The uplink channel may be estimated from the uplink signal, and the downlink channel estimate may be determined from the estimated uplink channel, for example by removing the front-end difference from the estimated uplink channel and the UE transmit power. Alternatively, the downlink channel estimate may be determined directly from the received uplink signal. Alternatively, the downlink channel may be estimated by the UE based on a downlink signal received and measured by the UE from the RRU. The UE then sends the downlink channel estimate to the RRU as an uplink signal. The present invention is applicable to any kind of wireless communication technology, for example, technologies based on: time division multiple access TDMA, frequency division multiple access FDMA, code division multiple access CDMA, orthogonal frequency division multiple access OFDMA, time division duplex TDD, or frequency division duplex FDD. The present invention is particularly advantageous in TDD-based technologies because in TDD, the downlink channel and the uplink channel are reciprocal in the air. In TDD, any difference between the uplink channel and the downlink channel mainly comes from the radio front-end difference in the uplink and downlink, which can be calibrated. The information sent to the BBU about the received uplink signal can be the actual uplink signal, or the uplink channel estimate determined by the RRU based on the uplink signal, or the downlink channel estimate determined by the RRU based on the uplink signal. When the BBU system has determined the second part of the precoding coefficients, the second part of the precoding coefficients can be obtained by receiving from the BBU, or the second part of the precoding coefficients can be obtained by determining by the RRU based on the received uplink signal. According to an embodiment, the BBU is informed of what beamforming coefficients the RRU will use for precoding (i.e. which coefficients are the "second part of the coefficients"), and therefore informed of which part of the precoding coefficients the BBU will use for precoding (i.e. which coefficients are the "first part of the coefficients"). According to another embodiment, the BBU is informed of what precoding scheme the RRU uses, for example according to the configuration. This can be set in the configuration so that the BBU is informed of which precoding scheme the RRU will use to determine its precoding coefficients according to the configuration.

[0031] "Precoding" means spatial coding, which defines how the downlink signals of different user layers are distributed from the various antennas 121, 122, 123 of the RRU 120 when transmitted from the antennas towards the UEs 161, 162, 163. For multiple user layers, the transmitted signal of each antenna is a linear combination of the signals of the different user layers, where the signal of each user layer is multiplied by the precoding coefficients and the multiplication results of all the signals of the user layers are added together. In the special case of only one user layer, the transmitted signal of each antenna is the signal of the user layer multiplied by the precoding coefficients. Precoding works in the signal domain, thereby achieving array gain, spatial diversity gain, spatial multiplexing gain and interference mitigation in space. It should not be confused with the encoding and decoding of data, which involves, for example, adding redundancy in the bit domain to make the transmitted data more robust to interference, or compressing the data so that fewer bits can be sent than the original data. User layer signals refer to the transmitted signals mapped to different MIMO layers of single-user multiple-input multiple-output MIMO or multi-user MIMO. In LTE and NR, in addition to scheduling in MIMO layers in the spatial domain, they are also scheduled in a frequency-time grid. In the present disclosure, the user layer can be equivalent to the MIMO layer. In MIMO transmission, multiple antennas on the transmitter side and / or the receiver side are used to spatially separate the multipath radio channel into multiple MIMO layers so that each MIMO layer can carry an independent data stream, and the interference between the independent data streams is mitigated between the MIMO layers. This is generally referred to as the spatial multiplexing gain of MIMO. For example, in the downlink DL, each user layer signal carries an independent data stream for a specific user. User layer signals from different MIMO layers are precoded to mitigate interference. The precoded user layer signal is sent from the antenna as an analog signal. The digital user layer signal is a frequency domain signal. The analog user layer signal is a time domain signal. Converting the precoded digital user layer signal to the precoded analog user layer signal means, for example, transforming the signal from the frequency domain to the time domain using an inverse fast Fourier transform IFFT.

[0032] According to an embodiment, the obtained 206 second part of the precoding coefficients is determined such that when the simulated user layer signals are transmitted 214, the energy of each transmitted user layer signal is spatially focused towards its corresponding UE 161, 162, 163. This determination of the second part of the precoding coefficients may be performed by the RRU, or the determination may be performed by the BBU system, where the second part of the coefficients is received by the RRU from the BBU system. The determination of the precoding coefficients for spatially focusing the transmitted energy is less complex than the determination of the precoding coefficients for mitigating interference between user layer signals at the UE, where the precoding is performed in the BBU. By performing the lower complexity coefficient determination for spatially focusing the transmitted energy at the RRU and the higher complexity coefficient determination for mitigating interference at the BBU, the RRU may be made less complex and therefore more cost effective.

[0033] According to another embodiment, the second part of the precoding coefficients is determined by determining the maximum ratio transmission MRT coefficients of each user layer signal based on the uplink signal. The RRU can determine the MRT coefficients directly based on the uplink signal, alternatively, the RRU can determine the downlink channel estimate based on the uplink signal, and determine the MRT coefficients based on the downlink channel estimate. Compared with the direction selection or beam selection method, the MRT-based method leads to better air interface performance.

[0034] According to another embodiment, the method further comprises: selecting 209 a certain number of spatial directions from a larger number of possible spatial directions for each digital user layer signal based on the received uplink signal, the selected number of spatial directions being less than the number of antennas but greater than or equal to the number of digital user layer signals, and wherein precoding 210 is performed for the selected number of spatial directions. The selection is completed for each digital user layer signal. That is, a separate selection of spatial directions is performed for each user layer signal based on the uplink signal. In addition, precoding is also performed based on the received uplink signal. Therefore, by precoding the selected number of spatial directions, each digital user layer signal can be gathered toward its UE. In addition, since the number of spatial directions is less than the number of antennas and less than the number of possible spatial directions, the complexity of the precoding step is reduced. The spatial direction here refers to a directional beam from the corresponding antenna of the RRU toward different directions. The possible spatial direction may be a predefined direction. For example, the possible spatial directions are predefined by performing a spatial digital Fourier transform DFT operation. The selection is performed based on the received uplink signal. As mentioned above, the uplink signal is used to determine the downlink channel estimate, and the downlink channel estimate describes the channel gain in different possible spatial directions. The selection is to select a subset from these predefined possible spatial directions. For an N-antenna system (e.g., a uniform linear array (ULA)), the N-point DFT operation effectively forms N directional beams in different directions at the same time. Each input port of the DFT represents such a directional beam. If a signal is sent on one input port of the DFT, the other input points can be zero, and the signal will be transformed to all antennas and cause the signal to be sent in the direction corresponding to the input point. Basically, the spatial DFT transforms the channel into a directional domain. If a planar array is used, a two-dimensional DFT can be used. The selection of directions for each user layer signal is better than the DS method of making one selection for all user layers. The DS method will not be so accurate, and therefore cannot achieve the same high signal level as this method when received at the UE, because the selected direction will not be particularly suitable for each user layer signal, that is, it will not be particularly suitable for each UE. Furthermore, for this reason, in order not to make the signal level achieved at the UE too low, the DS method requires selecting more directions, and more directions means that more data must be sent via the fronthaul link.

[0035] According to a variation of this embodiment, the selected number of spatial directions has a higher channel gain or better signal quality according to the received uplink signal compared to non-selected spatial directions of the possible spatial directions. This outperforms the DS method of making a common selection for all user layers. The signal quality can be measured as SNR or SINR.

[0036] Combination Figure 2, Figure 4 A method performed by a BBU system of a wireless communication network is described. The wireless communication network comprises a distributed base station system 100 having a BBU 110 and at least one RRU 120, 130 connected to the BBU. The method comprises: receiving 302 information from at least one RRU 120, 130, the information relating to uplink signals received by the at least one RRU from a plurality of UEs 161, 162, 163 wirelessly connected to the at least one RRU 120, 130; and obtaining 304 a first part of precoding coefficients for precoding digital user layer signals based on the received information, the precoded user layer signals to be sent by the at least one RRU 120, 130 to the plurality of UEs 161, 162, 163, the first part of the precoding coefficients being determined so as to pre-mitigate interference between the user layer signals sent to the plurality of UEs, the precoding coefficients comprising a first part and a second part. The method further comprises precoding 306 the digital user layer signal using only the first part of the precoding coefficients and sending 308 the precoded digital user layer signal to at least one RRU 120, 130 for further precoding with the second part of the precoding coefficients before sending the user layer signal to the UE 161, 162, 163.

[0037] The BBU system of the wireless communication network that performs the method may be the BBU 110. Alternatively, the BBU system may be any other network node of the communication network, for example, a node that is farther from the UE, such as another network element or another RAN node in or close to the RAN. In this alternative, the BBU 110 receives information about the uplink signal received by the RRU 120 from the UE from the RRU 120, and transmits the information to another network node for performing the determination 404, and then the other network node sends the determined second part of the precoding coefficients back to the BBU 110 for further distribution to the RRU 120. Alternatively, the BBU system may be a group of network nodes, in which the functions for performing the method are distributed on different physical or virtual nodes of the network. The latter may be referred to as a "cloud solution".

[0038] The precoded digital user layer signal sent to at least one RRU 120, 130 will be precoded again at the RRU, this time using the second part of the precoding coefficients. The second part of the precoding coefficients can be determined so that the energy sent from multiple antennas is spatially concentrated in the direction towards multiple UEs. For example, the precoded user layer signals sent from different antennas can be coherently received at the UE so that the received energy is enhanced for each user layer signal. The second part of the precoding coefficients can be determined by the RRU, or determined by the BBU and sent to the RRU. The number of precoded digital user layer signals can be at least as many as the number of user layers. According to a preferred embodiment, the number of precoded digital user layer signals is the same as the number of user layers. This method is particularly advantageous when more than one RRU is connected to the BBU, because the BBU receives information about uplink signals from more than one RRU, and can calculate precoding coefficients to pre-mitigate interference between user layer signals for all RRUs, and use the calculated precoding coefficients to precode user layer signals for the RRUs. Therefore, in terms of mitigating interference of signals from different RRUs, coordination can be achieved for precoding and thereby achieving full MIMO gain through the number of antennas of all connected RRUs. For example, two 64-antenna RRUs together with one BBU form a 128-antenna MIMO system.

[0039] According to an embodiment, the method further comprises: determining 303 a second part of the precoding coefficients based on the received information; and sending 305 the determined second part of the second precoding coefficients to the at least one RRU 120 , 130 .

[0040] According to another embodiment, at least one RRU 120, 130 includes a first RRU 120 and a second RRU 130. In addition, for at least one UE of the plurality of UEs 161, 162, 163, a precoded digital user layer signal of the UE is sent 308 to the first RRU 120 and the second RRU 130. As a result, the at least one UE can receive its user layer signal from the first RRU 120 and from the second RRU 130, thereby receiving diversity from the antennas 121, 122, 123 of the first RRU 120 and the antennas 131, 132, 133 of the second RRU 130. Therefore, a more robust signal transmission is received compared to using only one RRU per UE. In addition, since the precoding coefficients for pre-mitigating interference of the user layer signal are calculated and this part of the precoding is performed at the BBU 110, the precoding can be coordinated to mitigate interference between the wireless signals transmitted from both the first RRU and the second RRU.

[0041] Figure 5An exemplary block diagram of an embodiment of a distributed base station system for DL ​​communication when one RRU 420 is connected to one BBU 410 is shown. For the sake of simplicity of the drawing, in this embodiment, only one RRU is connected to the BBU. However, the base station system can be easily expanded to include multiple RRUs connected to one BBU. In this example, there are sixty-four antenna elements 4301 ... 430 on the distributed base station side. 64 , and there are two UEs 461 and 462, each of which has an antenna element. The processing steps of this embodiment are described below.

[0042] User data in the frequency domain arrives at the input 409 of the BBU 410 from other parts of the wireless communication network, and the destination of the user data is two UEs 461, 462. The user data is processed separately in the first processor unit 411 of the BBU 410. The first processing may include, for example, forward error correction (FEC) coding, mapping to the user layer, and modulation. For example, the user data is first encoded by FEC coding and then mapped to the user layer. Thereafter, the encoded user layer data is modulated into a digital user layer signal. The BBU also includes a first precoder 412, which is used to perform a first precoding using a first precoding coefficient to pre-compensate for potential interference between user layer signals, which would occur at each UE 461, 462 if no precoding was performed. The first precoding takes into account an effective channel, which includes a radio channel toward the corresponding UE and processing of the user layer signal in the RRU, for example, a second precoding in the RRU. In other words, when determining the first precoding coefficient, the second precoding to be performed in the RRU is also taken into account. In this example, the output of the first precoder 412 is two precoded digital user layer signals, each intended for one user layer. The partially precoded digital user layer signals are then transmitted to the RRU 420 via the fronthaul interface 440 .

[0043] The RRU 420 includes a second precoder 421 that performs a second precoding on the received partially precoded digital user layer signal. The digital user layer signal, now precoded by both the first precoder and the second precoder, is then converted from the digital domain to the analog domain in a digital-to-analog (D / A) converter 429 and the frequency is converted from baseband to radio frequency. Thereafter, the digital user layer signal is transmitted to the receiver via sixty-four antennas 4301…430 64 The resulting precoded signal (now analog and at radio frequency) is sent wirelessly towards the UE.

[0044] The second precoder 421 precodes each partially precoded digital user layer signal so as to gather the energy of each user layer signal toward its intended UE. For example, the precoded user layer signals transmitted from different antennas are then coherently received at the UE so that the received energy is enhanced for each user layer signal. The second precoding coefficient for the second precoding is calculated based on the measurement of the uplink signal from the UE performed by the RRU. Based on the knowledge of the relationship between the uplink channel and the downlink channel of the wireless communication link between the RRU and the corresponding UE 461, 462, the channel coefficient defining the downlink channel is estimated using the measurement of the uplink signal. The second precoding coefficient is then determined using the channel coefficient so as to gather the energy of each user layer signal toward its intended UE. The channel coefficient can be used in a certain domain, such as an element domain or a directional domain. More details are given below. In order to simplify the description, a directional domain implementation is assumed here. The element domain implementation will be similar to the directional domain implementation.

[0045] More specifically, in the second precoder 421, the first user layer signal destined for the first UE 461 is processed by the first mapping unit 422 and the first calculation unit 423, and the second user layer signal destined for the second UE 462 is processed by the second mapping unit 424 and the second calculation unit 425. In the respective first mapping units 422 and second mapping units 424, the user layer signals intended for the respective first UE 461 and second UE 462 are then mapped to R directions out of N possible directions, where R<=N. Basically, each mapping unit 422, 424 selects R directions from the N possible directions and extends the respective user layer signal to these R directions. Therefore, the signal in each selected direction R is the same user layer signal when leaving the mapping unit 422, and the signal in each selected direction R is the same user layer signal when leaving the mapping unit 424. It should be noted that, due to channel differences, the selection of direction R is usually different for different user layer signals. The selection of the direction R for the corresponding user layer signal depends on the location of the corresponding UE to receive the corresponding signal, and the propagation characteristics, i.e., the channel characteristics. The advantage of performing such a direction selection is that the complexity of performing the next step, i.e., the complexity of precoding with the second part of the coefficients in the RRU, can be reduced due to the reduction in the number of precoding coefficients applied for each user layer. Moreover, when some possible directions with large channel estimation errors are excluded for certain user layers, the performance can also be improved. According to an embodiment, for each user layer, a direction selection is implemented, wherein a) the number of selected spatial directions is greater than or equal to the number of all user layers; b) the selected direction of each user layer should contain the most representative (e.g., the strongest or highest SNR) channel components of the user layer. Criterion a) ensures that the matrix of the effective channel (including the second precoding in the RRU and the air channel) will be full rank, which ensures that the matrix is ​​reversible and is therefore beneficial for precoding in the BBU, while criterion b) ensures that the signal is sent through the strongest channel component.

[0046] Then, the mapping signals of each user layer are multiplied with the precoding coefficients of the second precoder 421 in their respective computations 423, 425, and the precoding coefficients are calculated as described above, for example. In an embodiment, the coefficients of the second precoder are equal to the MRT coefficients of the selected directions, in order to maximize the energy of each user layer signal received at each UE. Thereafter, for each selected direction, the precoded signals of all the involved user layers (two in this case) are added together and sent to the D / A converter 429. In the direction domain implementation, before the signals precoded in the direction domain are D / A converted, they are transformed to the antenna element domain by a spatial discrete Fourier transform (DFT) unit 428, such that the signals to be sent by each antenna element are sorted to their correct antenna elements. For example, a 64-point DFT (i.e., a fast Fourier transform (FFT) unit) can transfer sixty-four antenna elements to sixty-four directions. In the case of directly performing the mapping to the antenna element domain, the DFT unit 428 can be removed.

[0047] Hereinafter, different steps of embodiments of the present invention are theoretically described. Let H denote the downlink channel estimation matrix in the antenna element domain or direction domain with size K×N (where K << N). Zero-forcing (ZF) is a well-known precoding scheme, especially in the case of not considering interference from adjacent cells, which achieves near-optimal performance for large-scale MIMO systems. Here, it is used to derive the method according to embodiments of the present invention, thus showing that even though the present invention is completed by two parts of precoding coefficients in the BBU and RRU, it can still achieve the same performance as ZF precoding. Basically, ZF precoding will completely pre-eliminate the interference between user layers. In the ideal case of assuming ideal channel information, through ZF precoding, the receiver at the UE receives interference-free user layer signals. The ZF precoding coefficients can be calculated as the pseudo inverse of the DL channel estimation matrix. The pseudo inverse of the rectangular channel matrix H can be expressed as

[0048] H + =H * (HH * ) -1

[0049] where H * denotes the Hermitian transpose of matrix H. According to the ZF formula, the signal wirelessly transmitted from the antenna after precoding can be expressed as

[0050] x b =H + x=H * ((HH * )-1 x)=H * x p

[0051] Among them, x p =(HH * ) -1 x is the input signal precoded with the first part of the coefficients, i.e. in the BBU. The transmitted signal x can be b Considered as consisting of two steps. * Perform MRT operation related to channel H. And the term (HH * ) -1 Acts as a cover for air channel H and MRT operation H * The effective channel HH * The original user layer signal x at the input of the first precoder is a K-dimensional vector. The precoded signal x p is also K-dimensional and is transmitted via the fronthaul link. Then, the precoded signal x received at the RRU is p Through the MRT matrix H * is mapped to N antenna elements / directions, becoming x b This justifies the invention from a theoretical point of view, i.e. it achieves the same performance as if the precoding was implemented only in the BBU (as in functional partitioning option 7-1). Note that other pre-cancellation techniques such as minimum mean square error (MMSE) can still be applied in this way. Note that, as mentioned before, signal mapping can be done selectively by selecting R elements / directions for mapping.

[0052] According to another embodiment, for the downlink direction, the BBU is connected to an RRU, which is equipped with N antennas, for example in a uniform linear array (ULA), which communicates with K single-antenna UEs. The channel estimation in the RRU is performed in the directional domain. The data symbols to each user layer constitute the K-dimensional input signal x in the BBU. The BBU uses the estimate H of the effective downlink channel seen from its FH interface b =HH * This means that H b Covering the second precoding performed in the RRU and the downlink air channel. The BBU pre-mitigates the signal via ZF precoding for interference between two UEs as

[0053]

[0054] The operation in RRU is implemented as follows: b is a full rank K×K matrix.

[0055] In the RRU, the second precoding is performed to convert x pMapped to N directions / antenna elements

[0056] x b =Bx p

[0057] Where B represents the beamforming coefficients of the second precoding performed in the RRU based on the estimated downlink channel in some domain (e.g., antenna element domain or directional domain). If the signal is now in the directional domain, an additional step is taken to transform it to the antenna element domain. In this work, we implement spatial DFT to visualize the over-the-air channel in N fixed azimuth directions for the ULA example. In this case, the transmitted signal becomes

[0058] x e =Fx b

[0059] Where F represents the normalized N×N DFT matrix. After passing through the air channel H, the received signal at the user side can be expressed as

[0060]

[0061] Ideally, we estimate H in the BBU b =HFB, where H b is defined to cover the effective channel in the air channel and the precoding in the RRU including spatial DFT. And for MRT precoding in the RRU, B = (HF) * , where HF is defined as the channel in the directional domain (e.g. using DFT). The above example shows an element domain implementation for the second precoder in the RRU, but is based on directional domain channel estimation. In this case, no explicit spatial DFT module is required in the hardware design. Instead, the second precoding coefficients are calculated as B = (HF) * , which implicitly includes the DFT.

[0062] Consider Figure 2 The scenario shown in FIG. 1 is a scenario in which two RRUs 120, 130 are connected to the same BBU 110 and the two RRUs communicate with K UEs (in Figure 2 In the example, K=3). Each RRU is equipped with N antennas (in Figure 2 The air channels sensed by RRU1 and RRU2 are denoted as H RRU1 and H RRU2 , where both are K×N matrices. In the BBU, the precoded signal x is generated as described above. p In the case of two RRUs, H bThe information of the second precoding performed in both RRU1 and RRU2, and the channel H sensed by them respectively RRU1 and H RRU2 Therefore, the coordinated processing at the BBU of multiple RRUs is achieved. Then, x is transmitted through the connected fronthaul link. p is transmitted to the two RRUs 120 and 130. For example, the BBU may transmit the precoded user layer signal x p Multicast to multiple RRUs involved.

[0063] In each RRU, the second precoding is performed according to its sensed channel to convert x p Mapped to the N directions / antenna elements of the RRU. In this example, the beamforming coefficient matrices implemented in each RRU are represented as B RRU1 and B RRU2 When MRT precoding is performed in each RRU, B RRU1 =(H RRU1 F) * And B RRU2 =(H RRU2 F) * For the theoretical analysis, an ideal estimate in the BBU is assumed:

[0064]

[0065] The signal received by the user side is y=y RRU1 +y RRU2 ,in

[0066]

[0067]

[0068] This formula is equivalent to having a single RRU equipped with 2N antennas. Therefore, this example shows that the method fully doubles the MIMO system by having two RRUs. The same process applies to more than two RRUs.

[0069] Combination Figure 2 , Figure 6An RRU 120 is shown that can operate in a distributed base station system 100 of a wireless communication network. The distributed base station system 100 also includes a BBU 110 connected to the RRU. The RRU 120 is connected to a plurality of antennas 121, 122, 123, and the RRU is arranged to send user layer signals to a plurality of UEs 161, 162, 163 through the plurality of antennas using the same time-frequency resources. The RRU 120 includes a processing circuit 603 and a memory 604. The memory contains instructions executable by the processing circuit so that the RRU 120 is operable to: receive uplink signals from a plurality of UEs; send information related to the received uplink signals to the BBU so that the BBU can obtain a first part of precoding coefficients to be used for precoding digital user layer signals in the frequency domain; and obtain a second part of the precoding coefficients. The RRU 120 is also operable to: receive a digital user layer signal precoded using the first part of the precoding coefficients from the BBU; and precode the received digital user layer signal with the second part of the precoding coefficients. The RRU 120 is further operable to: convert digital user layer signals precoded with the first and second parts of precoding coefficients into analog user layer signals; and transmit the analog user layer signals to the plurality of UEs 161 , 162 , 163 via the plurality of antennas 121 , 122 , 123 .

[0070] According to an embodiment, the obtained second part of the precoding coefficients is arranged to be determined such that: when transmitting the simulated user layer signals, the energy of each transmitted user layer signal is spatially concentrated towards its corresponding UE 161, 162, 163. The RRU may be configured to determine the second part of the precoding coefficients, or alternatively, the RRU may be configured to receive the precoding coefficients from a BBU, wherein the BBU is configured to determine the second part of the precoding coefficients.

[0071] According to another embodiment, the second part of the precoding coefficients is arranged to be determined by determining the MRT coefficients for each user layer signal based on the uplink signal.

[0072] According to another embodiment, the RRU 120 is further operable to: select a certain number of spatial directions from a larger number of possible spatial directions based on the received uplink signal and for each digital user layer signal, the selected number of spatial directions being less than the plurality of antennas but greater than or equal to the number of digital user layer signals. In addition, the RRU is operable to: precode the received digital user layer signal for the selected number of spatial directions. According to another variation of this embodiment, the selected number of spatial directions has a higher channel gain or better signal quality according to the received uplink signal compared to unselected spatial directions of the possible spatial directions.

[0073] According to other embodiments, the RRU 120 may further include a communication unit 602, which may be considered to include conventional means, such as a wireless transceiver, for communicating with the BBU 110 via the fronthaul link 140 and for wirelessly communicating with the UEs 161-163. Instructions executable by the processing circuit 603 may be arranged as, for example, a computer program 605 stored in the memory 604. The processing circuit 603 and the memory 604 may be arranged in a sub-device 601. The sub-device 601 may be a microprocessor configured to perform the above method and is therefore a suitable software and storage device, a programmable logic device PLD or other electronic component / processing circuit. The processing circuit 603 may include one or more programmable processors, application specific integrated circuits, field programmable gate arrays or a combination thereof suitable for executing instructions.

[0074] The computer program 605 may be arranged so that when its instructions are executed in the processing circuit, these instructions cause the RRU 120 to perform the steps described in any of the described embodiments of the RRU 120. The computer program 605 may be carried by a computer program product connectable to the processing circuit 603. The computer program product may be a memory 604, or at least arranged in a memory. The memory 604 may be implemented as, for example, a RAM (random access memory), a ROM (read only memory) or an EEPROM (electrically erasable programmable ROM). In addition, the computer program 605 may be carried by a separate computer readable medium such as a CD, a DVD or a flash memory, from which the program may be downloaded into the memory 604. Alternatively, the computer program may be stored on a server or any other entity connected to the wireless communication network 100, which the RRU 120 accesses via the communication unit 602. Then, the computer program 605 may be downloaded from the server into the memory 604.

[0075] Combination Figure 2 , Figure 7Another embodiment of an RRU 120 is shown, the RRU 120 being operable in a distributed base station system 100 of a wireless communication network. The distributed base station system 100 further comprises a BBU 110 connected to the RRU. The RRU 120 is further connected to a plurality of antennas 121, 122, 123, the RRU being arranged to transmit user layer signals to a plurality of UEs 161, 162, 163 via the plurality of antennas 121, 122, 123 using the same time-frequency resources. The RRU 120 comprises: a first receiving module 702 for receiving uplink signals from a plurality of UEs; and a transmitting module 704 for transmitting information relating to the received uplink signals to the BBU, so that the BBU can obtain a first part of precoding coefficients to be used for precoding digital user layer signals in the frequency domain. The RRU 120 further includes: an acquisition module 706 for obtaining a second part of the precoding coefficients; a second receiving module 708 for receiving a digital user layer signal precoded using the first part of the precoding coefficients from the BBU; and a precoding module 710 for precoding the received digital user layer signal using the second part of the precoding coefficients. The RRU 120 further includes: a conversion module 712 for converting the digital user layer signal precoded using the first part and the second part of the precoding coefficients into an analog user layer signal; and a transmitting module 714 for transmitting the analog user layer signal to the multiple UEs 161, 162, 163 via the multiple antennas 121, 122, 123. The RRU 120 may also include Figure 6 A communication unit 602 similar to the communication unit described in . In an embodiment, Figure 7 The module is implemented as Figure 6 A computer program running on a processing circuit such as processing circuit 603 as shown.

[0076] Combination Figure 2 , Figure 8A BBU system 800 is shown that can operate in a wireless communication network, the wireless communication network including a distributed base station system 100 having a BBU 110 and at least one RRU 120, 130 connected to the BBU 110. The BBU system 800 includes a processing circuit 803 and a memory 804. The memory contains instructions executable by the processing circuit so that the BBU system can operate to: receive information from at least one RRU 120, 130, the information being related to uplink signals received by the at least one RRU from a plurality of UEs 161, 162, 163 wirelessly connected to the at least one RRU 120, 130. The BBU system 800 is further operable to: obtain, based on the received information, a first portion of precoding coefficients for precoding a digital user layer signal, the precoded user layer signal to be transmitted by at least one RRU 120, 130 to a plurality of UEs 161, 162, 163, determine the first portion of the precoding coefficients so as to pre-mitigate interference between the user layer signals transmitted to the plurality of UEs, the precoding coefficients comprising a first portion and a second portion. The BBU system 800 is further operable to: precode the digital user layer signal using only the first portion of the precoding coefficients; and transmit the precoded digital user layer signal to at least one RRU for further precoding with the second portion of the precoding coefficients before transmitting the user layer signal to the UEs 161, 162, 163.

[0077] The BBU system 800 that performs the method may be arranged in the BBU 110, in fact, it may be the BBU 110. Alternatively, the BBU system may be any other network node of the communication network, for example, a node that is further away from the UE, such as another network element in the RAN or close to the RAN, or another RAN node. In this alternative, the BBU 110 receives information about the uplink signal received by the RRU 120 from the UE from the RRU 120, and transmits the information to another network node in order to perform the determination 404, which then sends the determined second part of the precoding coefficients back to the BBU 110 for further distribution to the RRU 120. Alternatively, the BBU system may be a group of network nodes, in which the functions for performing the method are distributed over different physical or virtual nodes of the network. The latter may be referred to as a "cloud solution".

[0078] According to an embodiment, the BBU system 800 is further operable to determine a second portion of precoding coefficients based on the received information, and send the determined second portion of precoding coefficients to at least one RRU.

[0079] According to another embodiment, at least one RRU 120, 130 includes a first RRU 120 and a second RRU 130. For at least one UE among the plurality of UEs 161, 162, 163, the BBU system 800 is operable to send a precoded digital user layer signal of the UE to the first RRU 120 and the second RRU 130.

[0080] According to other embodiments, in the case where the BBU system 800 is arranged at a place other than the actual BBU 110, the BBU system 800 may further include a communication unit 802, which may be considered to include a conventional device for communicating with the RRU 120 and the BBU 110. Instructions executable by the processing circuit 803 may be arranged as, for example, a computer program 805 stored in the memory 804. The processing circuit 803 and the memory 804 may be arranged in a sub-device 801. The sub-device 801 may be a microprocessor configured to perform the above-mentioned method and is therefore a suitable software and storage device, a programmable logic device PLD, or other electronic component / processing circuit. The processing circuit 803 may include one or more programmable processors, application specific integrated circuits, field programmable gate arrays, or a combination thereof suitable for executing instructions.

[0081] The computer program 805 may be arranged such that when its instructions are executed in the processing circuit, these instructions cause the BBU system 800 to perform the steps described in any of the described embodiments of the BBU system 800. The computer program 805 may be carried by a computer program product connectable to the processing circuit 803. The computer program product may be a memory 804, or at least arranged in the memory. The memory 804 may be implemented as, for example, a RAM (random access memory), a ROM (read only memory) or an EEPROM (electrically erasable programmable ROM). In addition, the computer program 805 may be carried by a separate computer-readable medium such as a CD, a DVD or a flash memory, from which the program may be downloaded into the memory 804. Alternatively, the computer program may be stored on a server or any other entity connected to the wireless communication network 100, which the BBU system 800 accesses via the communication unit 802. Then, the computer program 805 may be downloaded from the server into the memory 804.

[0082] Combination Figure 2 , Fig. 9An alternative embodiment of a BBU system 800 capable of operating in a wireless communication network is described, the wireless communication network including a distributed base station system 100 having a BBU 110 and at least one RRU 120, 130 connected to the BBU 110. The BBU system 800 includes a receiving module 902 for receiving information from the at least one RRU 120, 130, the information being related to uplink signals received by the at least one RRU from a plurality of UEs 161, 162, 163 wirelessly connected to the at least one RRU 120, 130. The BBU system 800 also includes an obtaining module 904 for obtaining, based on the received information, a first part of a precoding coefficient for precoding a digital user layer signal to be transmitted by the at least one RRU 120, 130 to the plurality of UEs 161, 162, 163, the first part of the precoding coefficient being determined so as to pre-mitigate interference between the user layer signals transmitted to the plurality of UEs, the precoding coefficient comprising a first part and a second part. The BBU system 800 also includes: a precoding module 906 for precoding a digital user layer signal using only a first portion of the precoding coefficients; and a sending module 908 for sending the precoded digital user layer signal to at least one RRU before sending the user layer signal to the UE 161, 162, 163 for further precoding using a second portion of the precoding coefficients. The BBU system 800 may also include Figure 8 A communication unit 602 similar to the communication unit described in . In an embodiment, Fig. 9 The module is implemented as Figure 8 A computer program running on a processing circuit such as processing circuit 803 as shown.

[0083] Although the above description contains many particularities, these particularities should not be interpreted as limiting the scope of the concepts described herein, but only providing an illustration of some exemplary embodiments of the concepts described. It will be understood that the scope of the current concept fully encompasses other embodiments that are obvious to those skilled in the art, and therefore, the scope of the current concept is not limited. Unless such a description is explicitly made, reference to an element in the singular is not intended to mean "one and only one", but rather "one or more". All structural and functional equivalents of the elements of the above-described embodiments known to those of ordinary skill in the art are expressly incorporated herein by reference and are intended to be included therein. In addition, it is not necessary for a device or method to solve every problem sought to be solved by the current concept because it is included herein. In the exemplary accompanying drawings 2, the dotted line generally indicates that the features within the dotted line are optional.

Claims

1. A method performed by a remote radio unit RRU (120) of a distributed base station system (100) of a wireless communication network, the distributed base station system (100) further comprising a baseband unit BBU (110) connected to the RRU, the RRU (120) being connected to a plurality of antennas (121, 122, 123), the RRU sending user layer signals to a plurality of UEs (161, 162, 163) via the plurality of antennas, the user layer signals being sent using the same time-frequency resources, the method comprising: receiving (202) uplink signals from the plurality of UEs; sending (204) information about the received uplink signal to the BBU to enable the BBU to obtain a first portion of precoding coefficients to be used for precoding a digital user layer signal in the frequency domain; Obtaining (206) a second portion of the precoding coefficients; receiving (208) from the BBU a digital user layer signal precoded by the first portion using the precoding coefficients; Precoding a received digital user layer signal using the second portion of the precoding coefficients in the frequency domain (210); converting (212) a digital user layer signal precoded with the first and second portions of the precoding coefficients into an analog user layer signal; and The simulated user layer signal is transmitted (214) to the plurality of UEs (161, 162, 163) via the plurality of antennas (121, 122, 123).

2. The method according to claim 1, wherein the obtained (206) second part of the precoding coefficients is determined so that when sending (214) the simulated user layer signals, the energy of each user layer signal sent is spatially concentrated towards its corresponding UE (161, 162, 163).

3. The method according to claim 1 or 2, wherein the second part of the precoding coefficients is determined by determining a maximum ratio transmission (MRT) coefficient of each user layer signal based on the uplink signal.

4. The method according to claim 1 or 2, further comprising: Based on the received uplink signal and for each digital user layer signal, a second number of spatial directions is selected (209) from a first number of possible spatial directions, the second number of spatial directions being less than the plurality of antennas but greater than or equal to the number of digital user layer signals, and wherein the precoding (210) is performed on the second number of spatial directions, wherein the first number is greater than the second number.

5. The method of claim 4, wherein the second number of spatial directions according to the received uplink signal have a higher channel gain or a better signal quality compared to unselected spatial directions among the possible spatial directions.

6. A method performed by a baseband unit (BBU) system (800) of a wireless communication network, the wireless communication network comprising a distributed base station system (100) having a BBU (110) and at least one remote radio unit (RRU) (120, 130) connected to the BBU, the method comprising: receiving (302) information from the at least one RRU (120, 130), the information relating to uplink signals received by the at least one RRU from a plurality of UEs (161, 162, 163) wirelessly connected to the at least one RRU (120, 130); Based on the received information, obtaining (304) a first part of precoding coefficients for precoding digital user layer signals to be transmitted by the at least one RRU (120, 130) to the plurality of UEs (161, 162, 163), the first part of the precoding coefficients being determined so as to pre-mitigate interference between the user layer signals transmitted to the plurality of UEs, the precoding coefficients comprising the first part and the second part; precoding the digital user layer signal using only the first portion of the precoding coefficients (306); and The precoded digital user layer signal is sent (308) to the at least one RRU for further precoding with the second portion of the precoding coefficients in the frequency domain before sending the user layer signal to the plurality of UEs (161, 162, 163).

7. The method according to claim 6, further comprising: determining (303) the second part of the precoding coefficients based on the received information, and The determined second portion of the precoding coefficients is sent (305) to the at least one RRU (120, 130).

8. The method according to claim 6 or 7, wherein the at least one RRU (120, 130) comprises a first RRU (120) and a second RRU (130), wherein for at least one UE among the plurality of UEs (161, 162, 163), a precoded digital user layer signal of the UE is transmitted (308) to the first RRU (120) and the second RRU (130).

9. A remote radio unit (RRU) (120) operable in a distributed base station system (100) of a wireless communication network, the distributed base station system (100) further comprising a baseband unit (BBU) (110) connected to the RRU, the RRU (120) being connected to a plurality of antennas (121, 122, 123), the RRU being arranged to transmit user layer signals to a plurality of UEs (161, 162, 163) via the plurality of antennas using the same time-frequency resources, the RRU (120) comprising a processing circuit (603) and a memory (604), the memory containing instructions executable by the processing circuit, whereby the RRU (120) is operable to: receiving uplink signals from the plurality of UEs; sending information about the received uplink signal to the BBU to enable the BBU to obtain a first portion of precoding coefficients to be used for precoding a digital user layer signal in the frequency domain; Obtaining a second portion of the precoding coefficients; receiving, from the BBU, a digital user layer signal precoded by the first portion using the precoding coefficients; precoding a received digital user layer signal using said second portion of said precoding coefficients in the frequency domain; converting a digital user layer signal precoded with the first part and the second part of the precoding coefficients into an analog user layer signal, and The simulated user layer signal is transmitted to the plurality of UEs (161, 162, 163) via the plurality of antennas (121, 122, 123).

10. The RRU (120) according to claim 9, wherein the obtained second part of the precoding coefficients is arranged to be determined such that when transmitting the simulated user layer signals, the energy of each user layer signal transmitted is spatially concentrated towards its corresponding UE (161, 162, 163).

11. The RRU (120) according to claim 9, wherein the RRU is configurable to determine the second part of the precoding coefficients, or alternatively, the RRU is configurable to receive the precoding coefficients from the BBU, wherein the BBU is configured to determine the second part of the precoding coefficients.

12. The RRU (120) according to any of claims 9 to 11, wherein the second part of the precoding coefficients is arranged to be determined by determining a Maximum Ratio Transmission (MRT) coefficient for each user layer signal based on the uplink signal.

13. The RRU (120) according to any one of claims 9 to 11, further operable to: select a second number of spatial directions from the first number of possible spatial directions based on the received uplink signal and for each digital user layer signal, the second number of spatial directions being less than the plurality of antennas but greater than or equal to the number of digital user layer signals, and the RRU is operable to: precode the received digital user layer signal for the second number of spatial directions, wherein: The first number is greater than the second number.

14. The RRU (120) according to claim 13, wherein the second number of spatial directions according to the received uplink signal have a higher channel gain or a better signal quality than unselected spatial directions of the possible spatial directions.

15. A baseband unit (BBU) system (800) capable of operating in a wireless communication network, the wireless communication network comprising a distributed base station system (100), the distributed base station system (100) having a BBU (110) and at least one remote radio unit (RRU) (120, 130) connected to the BBU (110), the BBU system (800) comprising a processing circuit (803) and a memory (804), the memory containing instructions executable by the processing circuit, so that the BBU system (800) is operable to: receiving information from the at least one RRU (120, 130), the information relating to uplink signals received by the at least one RRU from a plurality of UEs (161, 162, 163) wirelessly connected to the at least one RRU (120, 130); Based on the received information, obtaining a first part of precoding coefficients for precoding digital user layer signals to be sent by the at least one RRU (120, 130) to the plurality of UEs (161, 162, 163), the first part of the precoding coefficients being determined so as to pre-mitigate interference between the user layer signals sent to the plurality of UEs, the precoding coefficients comprising the first part and the second part; precoding the digital user layer signal using only the first portion of the precoding coefficients; as well as The precoded digital user layer signal is sent to the at least one RRU to be further precoded with the second part of the precoding coefficients in the frequency domain before sending the user layer signal to the UE (161, 162, 163).

16. The BBU system (800) according to claim 15, further operable to: determining the second portion of the precoding coefficients based on the received information; and The determined second part of the precoding coefficients is sent to the at least one RRU (120, 130).

17. The BBU system (800) according to claim 15 or 16, wherein the at least one RRU (120, 130) comprises a first RRU (120) and a second RRU (130), wherein for at least one UE among the plurality of UEs (161, 162, 163), the BBU system (800) is operable to: send a precoded digital user layer signal of the UE to the first RRU (120) and the second RRU (130).

18. A non-transitory computer-readable storage medium storing a computer program (605), the computer program comprising instructions which, when executed by at least one processing circuit of a remote radio unit (RRU) (120) of a distributed base station system (100), cause the RRU (120) to perform the following steps, wherein the distributed base station system (100) further comprises a baseband unit (BBU) (110) connected to the RRU, and wherein the RRU (120) is connected to a plurality of antennas (121, 122, 123), the RRU being arranged to transmit user layer signals to a plurality of UEs (161, 162, 163) via the plurality of antennas using the same time-frequency resources, the steps comprising: receiving uplink signals from the plurality of UEs; sending information about the received uplink signal to the BBU to enable the BBU to obtain a first portion of precoding coefficients to be used for precoding a digital user layer signal in the frequency domain; Obtaining a second portion of the precoding coefficients; receiving, from the BBU, a digital user layer signal precoded by the first portion using the precoding coefficients; precoding a received digital user layer signal using said second portion of said precoding coefficients in the frequency domain; converting a digital user layer signal precoded by the first part and the second part of the precoding coefficients into an analog user layer signal; and The simulated user layer signal is transmitted to the plurality of UEs (161, 162, 163) via the plurality of antennas (121, 122, 123).

19. A non-transitory computer-readable storage medium storing a computer program (805), the computer program comprising instructions, which, when executed by at least one processing circuit of a baseband unit (BBU) system (800) of a wireless communication network, causes the BBU system (800) to perform the following steps, wherein the wireless communication network comprises a distributed base station system (100), the distributed base station system (100) having a BBU (110) and at least one remote radio unit (RRU) (120, 130) connected to the BBU (110), the steps comprising: receiving information from the at least one RRU (120, 130), the information relating to uplink signals received by the at least one RRU from a plurality of UEs (161, 162, 163) wirelessly connected to the at least one RRU (120, 130); Based on the received information, obtaining a first part of precoding coefficients for precoding digital user layer signals to be sent by the at least one RRU (120, 130) to the plurality of UEs (161, 162, 163), the first part of the precoding coefficients being determined so as to pre-mitigate interference between the user layer signals sent to the plurality of UEs, the precoding coefficients comprising the first part and the second part; precoding the digital user layer signal using only the first portion of the precoding coefficients; as well as The precoded digital user layer signal is sent to the at least one RRU to be further precoded with the second part of the precoding coefficients in the frequency domain before sending the user layer signal to the plurality of UEs (161, 162, 163).

20. A remote radio unit RRU (120) operable in a distributed base station system (100) of a wireless communication network, the distributed base station system (100) further comprising a baseband unit BBU (110) connected to the RRU, the RRU (120) being connected to a plurality of antennas (121, 122, 123), the RRU being arranged to transmit user layer signals to a plurality of UEs (161, 162, 163) via the plurality of antennas using the same time-frequency resources, the RRU (120) comprising: A first receiving module (702) is configured to receive uplink signals from the plurality of UEs; a sending module (704) for sending information related to the received uplink signal to the BBU so that the BBU can obtain a first part of precoding coefficients to be used for precoding a digital user layer signal in the frequency domain; An obtaining module (706) for obtaining a second part of the precoding coefficients; A second receiving module (708) is used to receive a digital user layer signal precoded by the first part using the precoding coefficient from the BBU; A precoding module (710) for precoding a received digital user layer signal using the second part of the precoding coefficients in the frequency domain; A conversion module (712) for converting a digital user layer signal precoded with the first part and the second part of the precoding coefficients into an analog user layer signal; as well as A transmitting module (714) is used to send the simulated user layer signal to the multiple UEs (161, 162, 163) via the multiple antennas (121, 122, 123).

21. A baseband unit (BBU) system (800) capable of operating in a wireless communication network, the wireless communication network comprising a distributed base station system (100), the distributed base station system (100) having a BBU (110) and at least one remote radio unit (RRU) (120, 130) connected to the BBU (110), the BBU system (800) comprising: a receiving module (902) for receiving information from the at least one RRU (120, 130), the information being related to uplink signals received by the at least one RRU from a plurality of UEs (161, 162, 163) wirelessly connected to the at least one RRU (120, 130); an obtaining module (904), configured to obtain, based on the received information, a first part of a precoding coefficient for precoding a digital user layer signal, the precoded user layer signal to be sent by the at least one RRU (120, 130) to the plurality of UEs (161, 162, 163), the first part of the precoding coefficient being determined so as to pre-mitigate interference between the user layer signals sent to the plurality of UEs, the precoding coefficient comprising the first part and a second part; A precoding module (906) configured to precode the digital user layer signal using only the first portion of the precoding coefficients; as well as A sending module (908) is used to send the precoded digital user layer signal to the at least one RRU to further precode the user layer signal with the second part of the precoding coefficients in the frequency domain before sending the user layer signal to the multiple UEs (161, 162, 163).

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