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

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

When going from <NUM> to <NUM>, there is a need to increase the wireless communication capacity towards the UEs in order to be able to deliver requested data amounts per time period in <NUM>. One enabler of the mobile evolution towards <NUM> is massive Multiple Input Multiple Output, MIMO, in which each RRU has a plurality of antennas. In other words, massive MIMO exploits spatial multiplexing to improve spectrum efficiency by using antenna arrays at the RRU, which antenna array is equipped with N antennas simultaneously serving K user-layers in the same time-frequency resource. The typical scenario is N » K. e.g., N is <NUM>, <NUM> or <NUM> while K is <NUM> or <NUM>. As shown, the number of antennas are quite large. To support such massive MIMO solutions, the required fronthaul link capacity needs to increase in proportion to the increase of number of antennas, when using the current PHY-RF split between functionality of BBU and RRU. This will dramatically drive up the fronthaul link costs.

To address this, new split options regarding fronthaul functionality have been discussed in 3GPP, see <NPL>. Some of the discussed split options (option <NUM>-<NUM>, <NUM>-<NUM>), as well as the current PHY-RF split (option <NUM>) are shown in <FIG>. A discussed split option is an Intra PHY split option called option <NUM>-<NUM> in <FIG>, in which Inverse Fast Fourier Transformation, IFFT, and mapping is performed in the RRU, as well as the digital to RF conversion. The <NUM>-<NUM> split option increases fronthaul efficiency and hereby reduces the need for fronthaul capacity due to reduced frequency redundancy and by adapting the fronthaul bit rate to traffic load, which is possible when IFFT and mapping is performed in the RRU instead of in the BBU. However, there is still a problem that there is one bit stream per antenna transmitted over the front haul, and for massive MIMO there will still be very much data needed to be sent over the fronthaul link.

In order to achieve a split between functionality of the RRU and the BBU so that the RRU becomes cost-efficient and also so that the fronthaul connection capacity can be kept on a reasonable level, the split option <NUM>-<NUM> has been suggested. Here the BBU encodes the data of MIMO user layers and sends coded data per user layer to the RRU, in which also modulation and precoding is performed at the RRU, in addition to the functionality of option <NUM>-<NUM>. This lowers the amount of bit streams to be sent over the fronthaul link down to the number of MIMO user layers and consequently reduces the necessary capacity of the fronthaul link, as well as reduces the needed number of bits per sample.

Precoding coefficients are usually calculated in the BBU and sent to the RRU via the fronthaul link, and the RRU performs the precoding based on the precoding coefficients. A reason for calculating the precoding coefficients in the BBU instead of in the RRU is that the coefficient calculation requires intensive computational operations and it is a request to make the RRU as simple as possible for cost efficiency reasons. However, the amount of precoding coefficients Np is proportional to the number of antennas N and the number of user layers K. For massive MIMO, the number of antennas N is large and therefore the amount of precoding coefficients Np increases dramatically. Consequently, a lot of the fronthaul capacity is taken for transporting precoding coefficients at the expense of transporting actual signals. When assuming <NUM> subcarriers, i.e. one resource block, RB, in LTE, per precoding coefficient, <NUM> antennas and <NUM> bits for each coefficient, the number of bits per subcarrier per layer becomes <NUM>*<NUM>/<NUM> = <NUM> bits, which is more than <NUM> times larger than <NUM> bits of <NUM> Quadrature Amplitude Modulation, QAM, for data. If the precoder needs to update every <NUM> symbols, which means every <NUM> in LTE, the number of bits for the precoding coefficients per subcarrier per layer per symbol that needs to be sent from the BBU to the precoder of the RRU over the fronthaul link becomes <NUM>*<NUM>/<NUM>/<NUM> = <NUM> bits. Regarding <NUM> QAM data symbols, <NUM>% of the fronthaul link traffic will then be precoding coefficients. This will increase further when the antenna size increases, e.g. to <NUM> and <NUM> etc. Therefore, with massive MIMO, the fronthaul link overhead for transporting the precoding coefficients is very large and can dominate the fronthaul link traffic for large configurations.

In "<NPL>, there is one method referred to as direction selection, DS or beam selection, BS, which is able to reduce the fronthaul link overhead. In this work, we refer to as DS method to facilitate the denotation. It is discussed in a hybrid beamforming scenario. The idea can be used to reduce the number of precoding coefficients through fronthaul link. In the DS method, the RRU generates a fixed number of directive beams towards many directions covering the service area. The fixed number of beams are formed with a set of fixed beamforming coefficients. For example, applying Discrete Fourier Transform, DFT, coefficients on the signals towards the antenna elements can generate the same number of beams as the number of antenna elements. To simplify the denotation, the directive beams are called "the directions". The BBU selects a subset of the directions according to the channel information and calculate the coefficients corresponding to the subset of the directions. Since the number of selected directions are smaller than the number of antennas, the number of the coefficients for the selected directions is reduced compared to the original case. The method is based on the fact that the propagation concentrates to some dominant directions, from the main reflections around. We show further down in this document in documentation of simulation results that the DS needs to select much more directions than the number of user layers and therefore the reduction of the coefficients number is limited.

As shown, there is a need for an improved solution for handling transmission of downlink data in a distributed base station system comprising a BBU and a RRU. Also, there is a need for a solution that more efficiently uses fronthaul link capacity and still achieves good air interface performance in a distributed base station system. <CIT> discloses a scheme where cross channels are measured, precoders for interference alignment are determined in a central unit and sent to base stations for use to align interference.

It is an object of the invention to address at least some of the problems and issues outlined above. It is possible to achieve these objects and others by using a method and an apparatus as defined in the attached independent claims.

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

According to an embodiment, in order to make a more efficient split of RAN processing functionality between the RRU and the BBU when the RRU has a plurality of antennas, the present invention suggests to decompose the precoding coefficients into two parts, wherein a first part is determined in the RRU and a smaller second part is determined in the BBU and sent from the BBU to the RRU. Then at the RRU, the first and the second parts of the precoding coefficients are used for precoding signals to be sent from the RRU to the UEs via the multiple antennas. By such a solution, a cost-efficient usage of fronthaul link capacity is achieved at the same time as good air interface performance and a cost-efficient RRU.

<FIG> illustrates a wireless communication network in which the present invention may be used. The wireless communication network comprises a distributed base station system <NUM>, which in turn comprises a BBU <NUM> and a RRU <NUM>. The BBU <NUM> has connections to other base station nodes or other RAN nodes and further to a core network (symbolized with <NUM> in <FIG>) so that the distributed base station system can communicate to other nodes of the communication network. The BBU is connected with the RRU via a fronthaul link <NUM>. The fronthaul link <NUM> may be any kind of connection, such as a dedicated wireline or wireless connection or a connection via a network, as long as the connection fulfils fronthaul requirements, e.g. in capacity and latency. The RRU further has a plurality of antennas <NUM>, <NUM>, <NUM> through which wireless signals are communicated towards and from one or more UEs <NUM>, <NUM>, <NUM>. The wireless signals comprises data to be communicated from or to the UEs <NUM>, <NUM>, <NUM>. The BBU <NUM> and the RRU <NUM> comprise RAN functionality for handling the data and signals to be communicated between the RRU <NUM> and the UEs <NUM>, <NUM>, <NUM>. The RAN functionality is distributed between the BBU and the RRU as will be described further down in this disclosure.

<FIG>, in conjunction with <FIG>, describes a method performed by a RRU <NUM> of a distributed base station system <NUM> of a wireless communication network. The distributed base station system <NUM> further comprising a BBU <NUM> connected to the RRU. The RRU <NUM> is connected to a plurality of antennas <NUM>, <NUM>, <NUM> through which the RRU wirelessly communicates with at least one UE <NUM>, <NUM>, <NUM>. The method comprises receiving <NUM> uplink signals from the at least one UE, determining <NUM> a downlink channel estimate from the received uplink signals, e.g. based on channel reciprocity between uplink and downlink signals, determining <NUM> a first part of precoding coefficients to be used for precoding signals to be sent downlink to the at least one UE <NUM>, <NUM>, <NUM> based on the determined downlink channel estimate, and sending <NUM> information related to the received uplink signals to the BBU. The method further comprises receiving <NUM> a second part of the precoding coefficients from the BBU, the second part of the precoding coefficients being determined based on the sent information, precoding <NUM> downlink signals of data received from the BBU using the first and the second part of the precoding coefficients, and sending <NUM> the precoded downlink signals to the at least one UE via the plurality of antennas.

By determining a first part of the precoding coefficients at the RRU and by receiving a second part of the precoding coefficients from the BBU, a good balance can be achieved between a low complex cost-efficient RRU and a front haul interface having a limited transmission capacity. As a result, a cost-efficient distributed base station can be achieved that at the same time provides a high quality air interface.

The uplink signals may be uplink reference signals such as Sounding Reference Signals, SRS, De-Modulation Reference Signal, DMRS or any other signal that may be defined for the purpose of assisting downlink channel estimation. An uplink channel may be estimated from the uplink signals and the downlink channel estimate may be determined from the estimated uplink channel, for example by removing frontend difference from the estimated uplink channel and the UE transmit power. Alternatively, the downlink channel estimate may be determined directly from the received uplink signals. The invention is applicable to any kind of wireless communication technology, such as 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 invention is especially advantageous in TDD-based technologies, as in TDD the downlink and uplink channels are reciprocal over the air. In TDD, any difference between uplink and downlink channel is mainly from the radio frontend difference in uplink and downlink, which difference can be calibrated out. The information related to the received uplink signals, sent to the BBU, is|the actual uplink signal or a channel estimate H of the uplink signal or any information on which the channel estimate can be determined. According to an embodiment, the BBU is informed of what beamforming operation the RRU will take, and consequently, which part of the preceding coefficients the RRU will determine. This may be set at configuration so that the BBU is informed from the configuration which beamforming operation the RRU will take and therefore which beamforming operation the BBU should take.

"Pre-coding" signifies a spatial coding defining how the downlink signals of different user layers are to be distributed from the individual antennas <NUM>, <NUM>, <NUM> when transmitted from the antennas towards the UEs <NUM>, <NUM>, <NUM>. For multiple user layers, the transmit signal of each antenna is a linear combination of the signal of different user layers, in which the signal of each user layer is multiplied with a precoding coefficient and then the multiplication results of all signal of user layers are added together. In a special case of only one user layer, the transmitted signal of each antenna is the signal of the user layer multiplied with a precoding coefficient. The precoding works in the signal domain, to achieve the array gain, spatial diversity gain, spatial multiplexing gain and interference mitigation spatially. This is not to be mixed with encoding and decoding of data, which deals with e.g. adding redundancy in the bit domain in order to make the transmitted data more robust to interference, or to compress data in order to be able to send less bits than the original data.

According to an embodiment, the method further comprises combining <NUM> the first part of the precoding coefficients with the second part of the precoding coefficients before the precoding of downlink signals with the combined first and second parts of the precoding coefficients. By combining the first and second parts of the precoding coefficients before precoding the downlink signals, the number of multiplications with the downlink signals is saved. Otherwise, the downlink signals need to be multiplied twice, first with the second part of the precoding coefficient, and then with the first part of the precoding coefficients. The precoding coefficients may be described in a matrix format, consequently, a first matrix for the first part of the precoding coefficients and a second matrix for the second part of the precoding coefficients. In such an embodiment, the downlink signal is described in a vector format where each vector element denotes a signal of a user layer. A combination of the first and second matrix would then be multiplied with the downlink signal. By combining the first and second matrices and storing them as a combined matrix may also save memory, as a matrix with the first and second matrix combined has smaller number of coefficients than that of two separate matrices.

According to another embodiment, the first part of the precoding coefficients contains more information than the second part of the precoding coefficients. By determining precoding coefficient having more information, e.g. a larger part of the precoding coefficients, at the RRU and a smaller part at the BBU, an even smaller part of coefficients is needed to be sent over the fronthaul link, whereby fronthaul capacity is further saved.

According to another embodiment, the first part of the precoding coefficients is determined <NUM> so as to spatially concentrate energy transmitted from the antennas in directions towards the UEs, and the second part of the precoding coefficients is determined so as to pre-mitigate interference. Normally, it is less computing-intensive to determine precoding coefficients for spatially concentrating energy than for mitigating interference. Therefore, the RRU would only need to increase computing power moderately, compared to computing power needed in the RRU when computing all coefficients in the BBU, when the determining of the precoding coefficients for spatially concentrating energy is moved to the RRU. The second part of the precoding coefficients may be determined so as to pre-mitigate interference in order to further improve Signal to Interference Ratio, SIR. The wording pre-mitigate is used because there is no interference to mitigate yet at the transmitter side. The beamforming scheme pre-mitigates the interferences which otherwise will happen at the receiver side.

According to another embodiment, the first part of the precoding coefficients is determined <NUM> based on a Maximum Ratio Transmission, MRT, operation on the downlink channel estimate. As shown further down in this document, an MRT-based method results in better air interface performance compared to the DS method, while fewer precoding coefficients need to be sent through the fronthaul link.

According to another embodiment, the downlink channel estimate is determined <NUM> in a matrix format H, and wherein the first part of the precoding coefficients is determined <NUM> as the Hermitian transpose H* of the channel estimate matrix H. An inverse matrix operation is a complex operation whereas a Hermitian transpose operation is a simple operation in relation to the inverse matrix operation. The complex inverse matrix operation is here performed by the BBU, as can be seen in relation to <FIG> describing the steps performed by the BBU. By making the complex operation at the BBU and an inverse-free, i.e. simpler operation at the RRU, the complexity increase of RRU is moderate compared to performing all computing of precoding coefficients at the BBU. Hereby, a cost-efficient RRU can still be achieved.

<FIG>, in conjunction with <FIG>, describes a method performed by a system of a wireless communication network, the wireless communication network comprising a distributed base station system <NUM> having a BBU <NUM> and a RRU <NUM> connected to the BBU. The method comprises receiving <NUM>, from the RRU <NUM>, information related to uplink signals received by the RRU from at least one UE <NUM>, <NUM>, <NUM> wirelessly connected to the RRU, determining <NUM>, based on the received information, only a second part of precoding coefficients for precoding downlink signals to be sent by the RRU to the UEs, the precoding coefficients comprising a first part and the second part, and sending <NUM> the determined second part of the precoding coefficients to the RRU, so that the RRU can use the second part of the precoding coefficients together with the first part of the precoding coefficients for precoding downlink signals to be sent to the UEs.

The system of the wireless communication network that performs the method may be the BBU <NUM>. Alternatively, the system that performs the method may be any other network node of the communication network, such as a node further away from the UE, e.g. another network element in the RAN or close to the RAN or another RAN node. In this alternative, the BBU <NUM> receives from the RRU <NUM>, the information related to uplink signals received by the RRU <NUM> from the UEs, and communicates the information to the other network node for performing the determination <NUM>, where after the other network node sends the determined second part of precoding coefficients back to the BBU <NUM> for further distribution to the RRU <NUM>. Alternatively, the system of the communication network that performs the method may be a group of network nodes, wherein functionality for performing the method are spread out over different physical, or virtual, nodes of the network. The latter may be called a "cloud-solution".

The first part of the precoding coefficients are determined by the RRU. The RRU will use the second part of the precoding coefficients received from the system of the communication network in combination with the first part of the precoding coefficients that it has computed itself when precoding signals to be sent from the RRU towards the UEs. The system determines only a second part of the coefficients, and not all coefficients, including the first part as well as the second part of the coefficients. Consequently, only a smaller part, i.e. the second part of the coefficients have to be sent over the fronthaul link to the RRU compared to sending all precoding coefficients over the fronthaul link. Thereby, fronthaul link capacity is saved.

According to an embodiment, the second part of the precoding coefficients is determined <NUM> so as to pre-mitigate interference. Then the first part of the precoding coefficients are determined by the RRU so as to spatially concentrate energy transmitted from the antennas in directions towards the UEs.

According to another embodiment, the received information <NUM> is a downlink channel estimate of the uplink signals received by the RRU from the UEs. Alternatively, the received information <NUM> is measurements on the received uplink signals, and then the method comprises determining <NUM> a downlink channel estimate on the received measurements. Then, the determining <NUM> of the second part of the precoding coefficients is performed based on the determined downlink channel estimate.

According to another embodiment, the second part of the precoding coefficients is determined based on a Zero-forcing pre-cancellation operation on the downlink channel estimate. The Zero-forcing pre-cancellation operation may be performed according the following: The channel estimate is in a matrix format H, and the second part of the precoding coefficients is determined <NUM> as (HH*)-<NUM>, i.e. the Hermitian transpose of the channel estimate matrix H vector multiplied with the channel estimate matrix H, and taking the inverse of the vector multiplication. The Zero-forcing-based approach performs quite well when the interferences from neighboring cells are not considered.

According to another embodiment, the second part of the precoding coefficients may be determined based on Interference Rejection Combining, IRC or based on Minimum Mean square error, MMSE with the downlink channel estimate, when considering interferences from neighboring cells. When the interference is strong, IRC and MMSE can achieve even better performance than Zero-forcing, depending on the existing amount of information about noise and interference for the downlink channel. MMSE requires information of the noise/interference level. IRC needs the covariance matrix of the interferences and noise.

Basically, the proposed solution decomposes the determination of precoding coefficients into two parts, where a second smaller part is calculated by the BBU and sent to the RRU and the first part is calculated locally by the RRU. Therefore, the number of precoding coefficients that needs to be sent through the fronthaul link is reduced compared to sending all precoding coefficients. The first part is calculated by the RRU utilizing the fact that the RRU can estimate the channel by itself. The RRU then combines the first part and the second part of the precoding coefficient and precodes the transmit user layer signal accordingly. According to an embodiment, a solution is provided based on maximum ratio transmission, MRT, where the first part of the precoding coefficients are determined to spatially focusing the energy to the UEs by MRT, while the second part of the precoding coefficients performs interference pre-cancellation. This approach achieves less coefficients sent over the fronthaul link and better air interface performance than the prior art DS method descried in the Background.

In the following, a model is used for describing some embodiments of the invention. In this model, the precoding coefficients are described in a matrix format and the downlink signals is described in a vector format. In the model, a massive MIMO system is assumed where the RRU has N antenna elements and there are K MIMO user layers. A precoder is set up that precodes the incoming signals of the K user layers so that the signals are spread to the N antenna elements. The precoder <NUM> has a precoding function with precoding coefficients that can be described in a matrix format P, as an NxK matrix per a group of subcarriers sharing the same precoding coefficients. According to the invention, the precoding coefficient matrix P is decomposed into a first multiplicative terms P<NUM>, which is an NxR matrix symbolizing a first part of the precoding coefficients, and P<NUM>, which is an RxK matrix symbolizing a second part of the precoding coefficients. Then P<NUM> is determined locally at the RRU and P<NUM> is determined by the BBU and sent from the BBU to the RRU over the fronthaul link. As R is set R≤N, the dimension of P<NUM> is smaller than P. Therefore, transporting P<NUM> over the fronthaul link instead of transporting all precoding coefficients P reduces the overhead over the fronthaul link by N/R times. For example, if N = <NUM> and R = <NUM>, the number of precoding coefficients transported over the fronthaul ink is reduced by <NUM> times. <FIG> describes a RRU <NUM> having a function as described in the present embodiment. The RRU has a modulator <NUM> that modulates incoming coded data of each user layer to a modulated signal, based on e.g. QAM modulation. The modulated signals of all user layers can be modeled as x, in which each element represents a signal of a user layer. The RRU further has a precoder <NUM> that multiplies the incoming signals x with the precoding coefficients of the precoding matrix P to generate the transmit signals from each antenna, y. The preceding matrix P is in its turn achieved by combining a first part P<NUM> of the precoding coefficients, determined at the RRU, and a second part P<NUM> of the precoding coefficients, obtained over the fronthaul link from the BBU.

<FIG> describes another RRU having a function as in the prior art DS method. As in <FIG>, the RRU comprises a modulator <NUM> and a precoder <NUM>. In the DS method, P<NUM> can be modeled as a submatrix of a full fixed beamforming matrix F with the dimension of NxM, where M denotes the number of directions the beamforming is configured to point to. For example, for an N-element uniform linear array, the full fixed beamforming matrix can be an NxN DFT matrix forming N directions. The DS method selects R columns as P<NUM>=[F]R, where [F]R denotes the submatrix of F with R selected columns. The information regarding which R columns are selected are also provided by the BBU together with P<NUM>. Specifically, for a zeroforcing (ZF) precoder, P<NUM> = (H[F]R)+ where A+ denotes the pseudo inverse of a matrix A.

In contrast to the DS method, the inventive method exploits the fact that the RRU has access to uplink channel information from which it can determine the channel downlink estimate itself. For example in a TDD system, the downlink channel information can be derived from the uplink channel information, from the reciprocity of the downlink and uplink channels. The RRU can estimate the uplink channel from e.g. received reference symbols transmitted by the UE, and then estimate the downlink channel accordingly. As discussed in relation to <FIG>, embodiments of the present invention proposes to let the RRU calculate the first part of the precoding coefficients P<NUM> according to the channel information estimated in the RRU, instead of merely relying on the fixed beamforming matrix F in the DS method. As described before, a smaller matrix of the second part of the precoding coefficients P<NUM> is obtained from the BBU.

As the first part of the precoding coefficients P<NUM> is calculated by the RRU, and it is preferable to have a cost-efficient RRU, it is preferable to avoid high computational complexity in the RRU, e.g. avoiding matrix inversion operations for P<NUM> calculation. According to an embodiment, P<NUM> is determined by the RRU using an MRT-based operation on the downlink channel estimate in order to spatially concentrating the energy to the intended UEs. The MRT operation is as such a rather simple operation, i.e. not especially computationally complex. Further, the BBU determines the second part of the precoding coefficients P<NUM> in order to achieve interference mitigation for the downlink channel estimate, e.g. pre-cancellation of estimated interferences between user layers and/or mitigating estimated interferences to UEs in other cells, e.g. pre-nulling. In this embodiment, P<NUM> can be a KxK matrix. Thus it is possible to reduce the number of coefficients sent over the fronthaul link from the BBU to the RRU down to be only proportional to the number of user layers. More details describing embodiments for determining P<NUM> in order to spatially concentrate the transmit energy to the intended UEs and for determining P<NUM> in order to achieve interference mitigation are given in the following.

According to an embodiment, the method for determining the first part of the precoding coefficients P<NUM> and the second part of the precoding coefficients P<NUM> is derived from a zero-forcing, ZF, -based pre-cancellation scheme. Let H denote the downlink channel matrix in antenna-element/direction domain of size K × N, where N is the number of antenna elements and K is the number of MIMO user layer and K « N. A ZF precoding matrix P, wherein P= P<NUM>P2, can be formulated as the pseudo inverse of the channel matrix H as
<MAT>
where H+ denotes the pseudo inverse of H, H- denotes the Hermitian transpose of H and A-<NUM> denotes the inverse of any square matrix A. Then the transmit signals after precoding can be expressed as
<MAT>.

In the above expression, we let P<NUM> = H* and P<NUM> = (HH*)-<NUM>. Then the transmit signal can be written as
<MAT>.

In this way, the Hermitian transpose by P<NUM> represents a MRT operation, while P<NUM> represents a ZF pre-cancellation operation. Also P<NUM> is reduced to a KxK matrix and thereby the number of coefficients that needs to be sent through the fronthaul link is reduced to the number of user layers. The scheme can be extended to a direction domain implementation, where a fixed number of beams pointing to different directions covering a range of space of interest are generated simultaneously and the beamforming operations are performed before transmitting on these beams. In such a way, the channel is transferred to a direction domain based on these fixed beams. In the RRU, the precoded signals are transmitted to a fixed number of directions, modeled as a fixed beamforming matrix F. Then we define the direction domain-channel matrix as
<MAT>
where H denotes the original element-domain channel matrix and F denotes the fixed beamforming matrix. In this case,
<MAT>
and
<MAT>.

The direction domain channel can be quite sparse, as the propagation concentrates to the directions towards the UEs and the main reflection points for the UEs. We can simplify P<NUM> to perform MRT on selected directions for each user-layer. This can reduce the complexity for calculating. In this case, P<NUM> with selective MRT can be expressed as
<MAT>
where Hz denotes a sparse matrix of Hd by keeping the selected R elements per row in Hd and zeroing out the rest of the elements on each row. In this case,
<MAT>.

In order to determine the improvements achieved by embodiments of the present invention, simulations have been performed wherein a full MRT scheme is compared to a selective MRT scheme. P<NUM> is determined by the BBU and transmitted over the fronthaul link to the RRU and P<NUM> is determined by the RRU. A "full MRT scheme" signifies that the MRT is performed on all antenna elements or in all directions, this means at least theoretically all elements in P<NUM> could be non-zero. A "selective MRT scheme" signifies that for each user layer a subset of the elements or directions are selected for doing MRT, which means that some elements of P<NUM> are set to <NUM>. The results are shown in <FIG>. The full MRT is called "MRT" and the selective MRT is called "sMRT" in <FIG>. Further, the results are compared in <FIG> to simulations when the DS scheme has been used. <FIG> show Signal to Interference Ratio, SINR, for simulated signals sent from a RRU to UEs, signals that have been precoded using the different schemes full MRT, selective MRT and DS, for R=<NUM> and for R=<NUM>. For the DS method, RxK coefficients are to be determined at the BBU and sent over the fronthaul link to the RRU, the higher R the more coefficients are determined at the BBU. For both MRT and sMRT methods, KxK coefficients are to be determined at the BBU and sent over the fronthaul link to the RRU. For sMRT, R relates to the RRU complexity only. The lower R, the fewer MRT operations need to be done in RRU, which reduces the number of multiplications. R is the number of directions or antennas selected.

<FIG> shows SINR results when it is assumed that perfect channel information is accessible for calculating P<NUM> and P<NUM>. The results show that the MRT scheme performs the best and theoretically achieves ZF preceding performance, in which P<NUM> only needs 8x8=<NUM> coefficients. When using <NUM> selected directions, the DS scheme performs about <NUM> dB worse than the MRT at the <NUM>th percentile. Even if <NUM> selected directions is used, in which 16x8=<NUM> coefficients is needed, the DS scheme performs about <NUM> dB worse than the MRT schemes at the <NUM>th percentile. It means that the DS scheme needs to select even more directions to approach the MRT scheme, which means increasing the FH overhead. The sMRT scheme performs very close to the MRT scheme, even for R=<NUM>. Consequently, the sMRT schemes are performing clearly better than the DS scheme.

<FIG> shows SINR results when it is assumed that there are channel estimation errors. In this case, the channel estimation is performed in the direction domain to improve the estimation accuracy, i.e. increasing the estimation SNR for strong directions. The channel estimation SNR per transmit antenna element is set to be <NUM> dB. As can be seen in <FIG>, and as expected, the performances of all schemes degrade compared to <FIG>, due to the presence of the channel estimation errors. The MRT schemes still perform clearly better than the DS scheme. For example, for DS and <NUM> directions (R=<NUM>), the result is still about <NUM> dB worse than the full MRT scheme. Also, the full MRT scheme is not the best any more. It performs slightly worse than the sMRT schemes, especially for the region of lower achieved SINR. It is because of the fact that the weak directions may negatively contribute to the performance due to large channel estimation errors on these directions. It is beneficial to exclude them from the MRT process.

To conclude, the simulations presented in <FIG> show that for perfect channel information as well as for simulated channel estimation errors, the MRT-based schemes are much better than the DS scheme. The DS scheme needs more than doubled number of coefficients to approach the MRT-based schemes. The MRT-based schemes are able to reduce the number of coefficients through the fronthaul link to the number of user layers, while achieving high performance at the air interface. Comparing to the original case of transporting 64x8=<NUM> coefficients in the simulation, the sMRT and the MRT-based scheme only needs to transport 8x8=<NUM> coefficients. Consequently, eight times reduction is achieved.

<FIG>, in conjunction with <FIG>, shows a RRU <NUM> operable in a distributed base station system <NUM> of a wireless communication network. The distributed base station system <NUM> further comprises a BBU <NUM> connected to the RRU. The RRU <NUM> is operative for wireless communication with at least one UE <NUM>, <NUM>, <NUM> through a plurality of antennas <NUM>, <NUM>, <NUM>. The RRU <NUM> comprises a processor <NUM> and a memory <NUM>. Said memory contains instructions executable by said processor, whereby the RRU <NUM> is operative for receiving uplink signals from the at least one UE, determining a downlink channel estimate from the received uplink signals, and determining a first part of precoding coefficients to be used for precoding signals to be sent downlink to the at least one UE based on the determined downlink channel estimate. The RRU <NUM> is further operative for sending information related to the received uplink signals to the BBU, receiving a second part of the precoding coefficients from the BBU, the second part of the precoding coefficients being determined based on the sent information, precoding downlink signals of data received from the BBU using the first and the second part of the precoding coefficients, and sending the precoded downlink signals to the at least one UE via the plurality of antennas.

According to an embodiment, the RRU is further operative for combining the first part of the precoding coefficients with the second part of the precoding coefficients before the precoding of the downlink signals with the combined first and second parts of the precoding coefficients.

According to another embodiment, the first part of the precoding coefficients contains more information than the second part of the precoding coefficients.

According to another embodiment, the RRU is operative for determining the first part of the precoding coefficients so as to spatially concentrate energy transmitted from the antennas in directions towards the UEs. Further, the second part of the precoding coefficients is determined so as to pre-mitigate interference.

According to another embodiment, the RRU is operative for determining the first part of the precoding coefficients based on a MRT operation on the downlink channel estimate.

According to another embodiment, the RRU is operative for determining the downlink channel estimate in a matrix format H, and for determining the first part of the precoding coefficients as H*, i.e. the Hermitian transpose of the downlink channel estimate matrix H.

According to other embodiments, the RRU <NUM> may further comprise a communication unit <NUM>, which may be considered to comprise conventional means for communication with a BBU <NUM> over the fronthaul link <NUM> (<FIG>), as well as conventional means for wireless communication with the UEs <NUM>, <NUM>, <NUM>, such as transceiver for wireless communication. The instructions executable by said processor <NUM> may be arranged as a computer program <NUM> stored e.g. in said memory <NUM>. The processor <NUM> and the memory <NUM> may be arranged in a sub-arrangement <NUM>. The sub-arrangement <NUM> may be a micro-processor and adequate software and storage therefore, a Programmable Logic Device, PLD, or other electronic component(s)/processing circuit(s) configured to perform the methods mentioned above.

The computer program <NUM> may be arranged such that when its instructions are run in the processor <NUM>, they cause the RRU <NUM> to perform the steps described in any of the described embodiments of the RRU <NUM>. The computer program <NUM> may be carried by a computer program product connectable to the processor <NUM>. The computer program product may be the memory <NUM>, or at least arranged in the memory. The memory <NUM> may be realized as for example a RAM (Random-access memory), ROM (Read-Only Memory) or an EEPROM (Electrical Erasable Programmable ROM). Further, the computer program <NUM> may be carried by a separate computer-readable medium, such as a CD, DVD or flash memory, from which the program could be downloaded into the memory <NUM>. Alternatively, the computer program may be stored on a server or any other entity connected to the wireless communication network to which the RRU <NUM> has access via the communication unit <NUM>. The computer program <NUM> may then be downloaded from the server into the memory <NUM>.

<FIG>, in conjunction with <FIG>, describes another embodiment of a RRU <NUM> operable in a distributed base station system <NUM> of a wireless communication network. The distributed base station system <NUM> further comprises a BBU <NUM> connected to the RRU. The RRU <NUM> is operative for wireless communication with at least one UE <NUM>, <NUM>, <NUM> through a plurality of antennas <NUM>, <NUM>, <NUM>. The RRU <NUM> comprises a first receiving module <NUM> for receiving uplink signals from the at least one UE, a first determining module <NUM> for determining a downlink channel estimate from the received uplink signals, and a second determining module <NUM> for determining a first part of precoding coefficients to be used for precoding signals to be sent downlink to the at least one UE based on the determined downlink channel estimate. The RRU <NUM> further comprises a first sending module <NUM> for sending information related to the received uplink signals to the BBU, a second receiving module <NUM> for receiving a second part of the precoding coefficients from the BBU, the second part of the precoding coefficients being determined based on the sent information, a precoding module <NUM> for precoding downlink signals of data received from the BBU using the first and the second part of the precoding coefficients, and a second sending module <NUM> for sending the precoded downlink signals to the at least one UE via the plurality of antennas. The RRU <NUM> may further comprise a communication unit <NUM> similar to the communication unit described in <FIG>.

<FIG>, in conjunction with <FIG>, shows a system <NUM> operable in a wireless communication network. The wireless communication network comprises a distributed base station system <NUM> having a BBU <NUM> and a RRU <NUM> connected to the BBU. The system <NUM> comprises a processor <NUM> and a memory <NUM>. Said memory contains instructions executable by said processor, whereby the system <NUM> is operative for receiving, from the RRU <NUM>, information related to uplink signals received by the RRU from at least one UE <NUM>, <NUM>, <NUM> wirelessly connected to the RRU. The system <NUM> is further operative for determining, based on the received information, only a second part of precoding coefficients for precoding downlink signals to be sent by the RRU to the UEs, the precoding coefficients comprising a first part and the second part, and sending the determined second part of the precoding coefficients to the RRU, so that the RRU can use the second part of the precoding coefficients together with the first part of the precoding coefficients for precoding downlink signals to be sent to the UEs.

The system <NUM> operable in the wireless communication network may be arranged in the BBU <NUM>. Alternatively, the system <NUM> may be arranged in any other network node of the wireless communication network, such as a node further away from the UE, e.g. another network element in the RAN or close to the RAN or another RAN node. In this alternative, the BBU <NUM> receives from the RRU <NUM>, the information related to uplink signals received by the RRU <NUM> from the UEs, and communicates the information to the other network node where the system <NUM> is arranged, for performing the determination <NUM>, where after the other network node sends the determined second part of precoding coefficients back to the BBU <NUM> for further distribution to the RRU <NUM>. Alternatively, the system <NUM> may be arranged spread out over a group of network nodes, wherein functionality of the system is spread out over different physical, or virtual, nodes of the network. The latter may be called a "cloud-solution".

According to an embodiment, the system <NUM> is operative for determining the second part of the precoding coefficients so as to pre-mitigate interference.

According to another embodiment, the received information is a downlink channel estimate of the uplink signals received by the RRU <NUM> from the UEs <NUM>-<NUM>, or the received information is measurements on the received uplink signals and the system <NUM> is further operative for determining a downlink channel estimate on the received measurements. Further, the system is operative for determining the second part of the precoding coefficients based on the determined downlink channel estimate.

According to another embodiment, the system <NUM> is further operative for determining the second part of the precoding coefficients based on a Zero-forcing pre-cancellation operation on the downlink channel estimate.

According to another embodiment, the system <NUM> is operative determining the second part of the precoding coefficients based on IRC or on MMSE on the downlink channel estimate.

According to other embodiments, the system <NUM> may further comprise a communication unit <NUM>, which may be considered to comprise conventional means for communication with a RRU <NUM> over the fronthaul link <NUM> (<FIG>). The instructions executable by said processor <NUM> may be arranged as a computer program <NUM> stored e.g. in said memory <NUM>. The processor <NUM> and the memory <NUM> may be arranged in a sub-arrangement <NUM>. The sub-arrangement <NUM> may be a micro-processor and adequate software and storage therefore, a Programmable Logic Device, PLD, or other electronic component(s)/processing circuit(s) configured to perform the methods mentioned above.

The computer program <NUM> may be arranged such that when its instructions are run in the processor <NUM>, they cause the system <NUM> to perform the steps described in any of the described embodiments of the system <NUM>. The computer program <NUM> may be carried by a computer program product connectable to the processor <NUM>. The computer program product may be the memory <NUM>, or at least arranged in the memory. The memory <NUM> may be realized as for example a RAM (Random-access memory), ROM (Read-Only Memory) or an EEPROM (Electrical Erasable Programmable ROM). Further, the computer program <NUM> may be carried by a separate computer-readable medium, such as a CD, DVD or flash memory, from which the program could be downloaded into the memory <NUM>. Alternatively, the computer program may be stored on a server or any other entity connected to the wireless communication network to which the system <NUM> has access via the communication unit <NUM>. The computer program <NUM> may then be downloaded from the server into the memory <NUM>.

<FIG>, in conjunction with <FIG>, describes another embodiment of a system <NUM> operable in a wireless communication network. The wireless communication network comprises a distributed base station system <NUM> having a BBU <NUM> and a RRU <NUM> connected to the BBU. The system <NUM> comprises a receiving module <NUM> for receiving, from the RRU <NUM>, information related to uplink signals received by the RRU from at least one UE <NUM>, <NUM>, <NUM> wirelessly connected to the RRU, and a determining module <NUM> for determining, based on the received information, only a second part of precoding coefficients for precoding downlink signals to be sent by the RRU to the UEs, the precoding coefficients comprising a first part and the second part. The system <NUM> further comprises a sending module <NUM> for sending the determined second part of the precoding coefficients to the RRU, so that the RRU can use the second part of the precoding coefficients together with the first part of the precoding coefficients for precoding downlink signals to be sent to the UEs. The system <NUM> may further comprise a communication unit <NUM> similar to the communication unit described in <FIG>.

Claim 1:
A method performed by a remote radio unit, RRU, (<NUM>) of a distributed base station system (<NUM>) of a wireless communication network, the distributed base station system (<NUM>) further comprising a base band unit, BBU, (<NUM>) connected to the RRU, the RRU (<NUM>) being connected to a plurality of antennas (<NUM>, <NUM>, <NUM>) through which the RRU wirelessly communicates signals with at least one user equipment, UE, (<NUM>, <NUM>, <NUM>), the signals comprising data to be communicated towards and from the at least one UE, the method comprising:
receiving (<NUM>) uplink signals from the at least one UE;
determining (<NUM>) a downlink channel estimate from the received uplink signals;
determining (<NUM>) a first part of precoding coefficients to be used for precoding signals to be sent downlink to the at least one UE based on the determined downlink channel estimate;
sending (<NUM>) information related to the received uplink signals to the BBU, said information being the actual uplink signal or a channel estimate H of the uplink signal or any information on which a channel estimate of the uplink signals can be determined,
receiving (<NUM>) a second part of the precoding coefficients from the BBU, the second part of the precoding coefficients being determined based on the sent information,
precoding (<NUM>) downlink signals of data received from the BBU using the first and the second part of the precoding coefficients,
sending (<NUM>) the precoded downlink signals to the at least one UE via the plurality of antennas,
wherein the first part of the precoding coefficients is determined (<NUM>) so as to spatially concentrate energy transmitted from the antennas in directions towards the UEs, and wherein the second part of the precoding coefficients is determined so as to pre-mitigate interference.