GB2495168A

GB2495168A - Rank considerations when determining a joint precoding matrix for use in a distributed antenna MIMO system

Info

Publication number: GB2495168A
Application number: GB201211156A
Authority: GB
Inventors: Tommi T Koivisto; Mihai Enescu; Timo E Roman; Karol Schober
Original assignee: Renesas Mobile Corp
Current assignee: Renesas Electronics Corp
Priority date: 2011-09-27
Filing date: 2011-09-27
Publication date: 2013-04-03
Anticipated expiration: 2031-09-27
Also published as: GB201211156D0; GB2495168B

Abstract

The present invention is concerned with transmissions from multiple distributed transmission points to a user terminal. The transmit points and the user terminal each have a multiple-input-multiple-output (MIMO) capability. For example, the present invention could be applied to coordinated multi-point (CoMP) transmissions. The user terminal determines a transmission rank and a precoding matrix for each of at least two transmission points of a plurality of transmission points. A joint transmission rank for a joint transmission involving each of the at least two transmission points is determined. Next, the user terminal selects a joint precoding matrix which provides the joint transmission rank and is based on the precoding matrices of respective transmission points. Furthermore, a sub-matrix of the joint precoding matrix is obtained by applying column permutations to one of the transmission point precoding matrices (see the third equation on page 20). The user terminals transmit channel state information (CSI) including a description of the selected joint precoding matrix.

Description

METHOD AND APPARATUS FOR

MULTIPLE INPUT MULTIPLE OUTPUT DISTRIBUTED ANTENNA ARRAYS

Technical Field

The present invention relates in one aspect to a method and apparatus for selecting a joint transmission precoding matrix. The present invention relates to communications technology generally and, in specific embodiments, more particularly to high rank multiple input multiple output "MIMO" for distributed antenna arrays.

Background

Release 10 of the 3rd Generation Partnership Project (3GPP) Long Term Evolution (LTE) specification includes several features related to downlink (DL) and uplink (UL) MIMO ("multiple input multiple output"), relays, bandwidth extension via carrier aggregation and enhanced inter-cell interference coordination (eICIC). In DL MIMO, in order to meet the peak spectral efficiency requirements of up to 30 bit/s/Hz, Release 10 extends Release 8/9 DL MIMO features by providing support for up to 8 stream transmissions, and hence up to 8x8 MIMO. Furthermore, enhanced support of multi-user (MU) MIMO is enabled in Release 10 and seamless switching between single-and multi-user operations is supported.

Release 10 further provides a double codebook for 8 transmit (Tx) antennas.

The double codebook for 8 Tx antennas is based on a modular design (or multi-granular), combining two feedback components from distinct codebooks: one feedback component represents the long-term (e.g. wideband) radio channel properties while the other one targets the short term (e.g. frequency selective) channel properties.

Non-uniform network deployments, also known as heterogeneous networks (HetNet), are potential scenarios considered in Long Term Evolution Release 11 for DL MIMO and coordinated multi-point transmission (C0MP). HetNet was considered during Releasc 10 within thc contcxt of enhanced inter-cell interference coordination/cancellation (eICIC) discussions, but Release 10 focused on mainly macro cells and pico cells deployed inside the macro cells, and hence only relatively light interaction between the macro cells and the pico cells in the form of time-domain resource partitioning with the exchange of scheduling information and patterns over the backhaul link. One difference between the macro cell and the remote radio head (RRH) lies in the utilised transmit powers, since the macro ccli may operate in the range of 46/49 dBm in a 10/20 MHz carrier while the RRHs could operate for example with 30/37 dBm. The HetNct scenarios in Release 11 arc considered for both coordinated multipoint transmission and for single cell MIMO enhancements.

For CoMP operation, detailed simulation assumptions have been described in RI-I 11125, CoMP simulation assumptions, which arc hcreby incorporatcd by reference.

For example, a macro ccli may consist of an array of antennas while a low power RRH may have one or an array of transmit antennas. Each antenna or array of antennas is understood to be a transmission point; hence the macro cell is a transmission point while the RRHs are also transmission points. The RRH and the macro cell are generally connected through optical fibre, and hence the feedback delays and capacity over the connection arc considered as ideal and unlimited in this case. The RRHs placement may be indoor or outdoor. A particular cxample may include an instance in which a macro cell is not present while RRHs are connected to a central unit which perfoims radio resource management (RRM). In a first example, it is assumcd that each transmit point has its own physical cell identifier (cell ID) and in a second example it is assumed that all transmit points have the same ccli ID. Tn both example cases there is a central unit which performs scheduling of the radio resources and is located for example at the macro cell. The RRHs, in this example, are arrays of antennas which are typically used in order to improve the spectral efficiency of the cell. Hence they can be also seen as simple radio frequency front ends pulled away from the macro cell and without RRM capability.

In the second example, where all transmit points have the same cell ID, the transmit points may be equipped with various numbers of transmit antennas. From a simulation assumption perspective, 3GPP is currently assuming 2, 4 and 8 transmission points for the macro cell and 1, 2 and 4 transmission points for the low power RR}-Is. In the simulations being carried out in 3GPP, both co-polarised and cross-polarised types of antennas are considered with the restriction that the same type of antennas are used for all transmit points in a given configuration. 1-lowever, specification-wise it is likely there will be no restriction in terms of number or type of transmit antennas, hence 8 transmission points could be considered also for RRHs.

In this example, mobile terminals are located in a cell formed by the macro cell and lie under the coverage of the RRHs. In a traditional macro cell-only scenario, these mobile terminals would be configured to determine the number of transmit antennas existing at the macro cell and report the channel state information (CSI) based on the common reference symbol (CR8) or channel state information reference symbol (CSI-RS) ports. CSI-RS provides support for 1, 2 4, and 8 transmission points. CSI-RS parameters, like the periodicity and pattem, are signalled as mobile terminal-specific information. In this scenario the mobile terminal may hear the RRH (or several RRHs) and the macro cell. In such cases, the mobde terminal signals the specific antenna ports associated with the transmission points on which it performs CSI estimation. For example, if the mobile terminal hears two RRHs and the macro cell, it receives the CSI-RS patterns and parameters of these three transmit points for which to compute the CSI. Once channel estimation is performed, and CSI feedback may be computed and reported to a central scheduling unit, such as the macro cell.

Both single user (SU) and MU MIMO may be supported; hence the computed feedback for these three transmit points enables closed-loop MIMO operation.

However, currently mobile terminal feedback operates on a single-transmission point basis or single-cell feedback. Hence, with such feedback, the maximum supported rank in the system is limited by the maximum of the individually supported ranks for each of the transmission points. Therefore the mobile terminal would typically report the rank based on the maximum rank supported by one individual transmission point, whereas the rank could in fact be higher if more transmission points were used for transmission and considered in finding the optimal rank.

Summary

According to a first aspect of the present invention, there is provided a method comprising determining a transmission rank and a precoding matrix for each of at lcast two transmission points of a plurality of transmission points; dctcrmining a joint transmission rank based on at least two of thc plurality of transmission points; selecting a joint transmission precoding matrix based on the determined precoding matrix for each of the at least two transmission points and the determincd joint transmission rank; and causing CSI to be transmitted to an access point, wherein the CSI describes the selected joint transmission prccoding matrix.

According to a second aspect of the present invention, there is provided apparatus comprising a processing system constructed and arranged to cause the apparatus at least to determine a transmission rank and a precoding matrix for each of at least two transmission points of a plurality of transmission points; determine a joint transmission rank based on at least two of the plurality of transmission points; select a joint transmission prccoding matrix bascd on the determined prccoding matrix for each of the at least two transmission points and the determined joint transmission rank; and cause CSI to be transmitted to an access point, wherein the CSI describes thc selected joint transmission prccoding matrix.

According to a third aspect of the present invention, there is provided apparatus comprising a processing system constructed and arranged to cause the apparatus to at least: determine a transmission strategy based on a received report that comprises at least one of transmission ranks for each of the at least two transmission points, prccoding matrices for each of the at least two transmission points, a joint transmission rank or a joint transmission precoding matrix; and cause a transmission using at least one of the precoding matrices for each of the at least two transmission points or the joint transmission precoding matrix.

According to a fourth aspect of the present invention, there is provided a S method comprising receiving a report comprising at least one of transmission ranks for each of the at least two transmission points, precoding matrices for each of the at least two transmission points, a joint transmission rank or a joint transmission precoding matrix; determining a transmission strategy based on thc received report; and causing a transmission using at least one of the precoding matrices for each of the at least two transmission points or the joint transmission precoding matrix.

A method, apparatus and computer program product are therefore provided according to an example embodiment to provide reporting for low rank feedback information for each transmission point up to a maximum rank per transmission point.

Based upon feedback information, an embodiment may enable higher rank transmission in instances in which laycrs for high rank transmission are provided through multiple transmission points. In one embodiment, the method, apparatus and computer program product may enable multiple transmit point higher rank transmissions even if the per transmit point partial channels do not allow sustaining transmission at ftlI rank.

In a further embodiment, a computer program product is provided that includes at least one computer readable non-transitory memory having program code storcd thereon with the program codc which when executed by an apparatus causing the apparatus at least to determine a transmission rank and a precoding matrix for each of at least two transmission points of a plurality of transmission points. A computer program product also includes program code that is further configured to determine a joint transmission rank based on at least two of the plurality of transmission points. A computer program product also includes program code that is further configured to select a joint transmission precoding matrix based on the determined precoding matrix for each of the at least two transmission points and the determined joint transmission rank. A computer program product also includes program code that is further configured to cause CSI to be transmitted to an access point, wherein the CSI describes the selected joint transmission precoding matrix, In yet another embodiment, an apparatus is provided that includes means for determining a transmission rank and a precoding matrix for each of at least two transmission points of a plurality of transmission points. An apparatus further comprises means for determining a joint transmission rank based on at least two of the plurality of transmission points. An apparatus frirther comprises means for selecting a joint transmission precoding matrix based on the determined precoding matrix for each of the at least two transmission points and the determined joint transmission rank. An apparatus further comprises means for causing CSI to be transmitted to an access point, wherein the CSI describes the selected joint transmission precoding matrix.

In a further embodiment, a computer program product is provided that includes at least one computer readable non-transitory memory having program code stored thereon with the program code which when executed by an apparatus causing the apparatus at least to receive a report comprising at least one of transmission ranks for each of the at least two transmission points, precoding matrices for each of the at least two transmission points, a joint transmission rank or a joint transmission precoding matrix. A computer program product also includes program code that is further configured to determine a transmission strategy based on the received report.

A computer program product also includes program code that is further configured to cause a transmission using at least one of the precoding matrices for each of the at least two transmission points or the joint transmission precoding matrix.

In yet another embodiment, an apparatus is provided that includes means for receiving a report comprising at least one of transmission ranks for each of the at least two transmission points, precoding matrices for each of the at least two transmission points, a joint transmission rank or a joint transmission precoding matrix. An apparatus further comprises means for determining a transmission strategy based on the received report. An apparatus further comprises means for causing a transmission using at least one of the precoding matrices for each of the at least two transmission points or the joint transmission preeoding matrix.

In the aspects described above, the processing system may comprise a processor and a memory including software.

Further features and advantages of the invention will become apparent from the following description of preferred embodiments of the invention, given by way of example only, which is made with reference to the accompanying drawings.

Brief Description of the Drawings

Figure 1 is a schematic representation of a system having a mobile terminal that may provide feedback and that may benefit from an embodiment of the present invention; Figure 2 is a block diagram of an apparatus that may be embodied by a mobile terminal in accordance with one embodiment of the present invention; Figure 3 is a flow chart illustrating operations performed by an example mobile terminal in accordance with one embodiment of the present invention; and Figure 4 is a flow chart illustrating operations performed by an example access point in accordance with one embodiment of the present invention.

Detailed Description

The present invention now will be described more filly hereinafter with reference to the accompanying drawings, in which some, but not all embodiments of the invention are shown. indeed, the invention may be embodied in many different forms and should not be construed as limited to the embodiments set forth herein;

S

rather, these embodiments are provided so that this disclosure will satisfy applicable legal requirements. Like numbers refer to like elements throughout.

As used in this specification, the term "circuitry" refers to all of the following: (a) hardware-only circuit implementations (such as implementations in only analog and/or digital circuitry) and (b) to combinations of circuits and software (and/or firmware), such as (as applicable): (i) to a combination of processor(s) or (ii) to portions of processor(s)/software (including digital signal processor(s)), software, and memory(ies) that work together to cause an apparatus, such as a mobile phone or scrver, to perform various functions) and (c) to circuits, such as a microprocessor(s) or a portion of a microprocessor(s), that require software or firmware for operation, even if the software or firmware is not physically present.

This definition of "circuitry" applies to all uses of this term in this application, including in any claims. As a further example, as used in this application, the term "circuitry" would also cover an implementation of merely a processor (or multiple processors) or portion of a processor and its (or their) accompanying software and/or firmware. The term "circuitry" would also cover, for example and if applicable to the particular claim element, a bascband integrated circuit or application specific integrated circuit for a mobile phone or a similar integrated circuit in server, a cellular nctwork device, or other nctwork device.

A method, apparatus and computer program product of an example embodimcnt of the present invcntion are conflgurcd to cnablc a mobile terminal to exploit low rank feedback for higher rank transmissions by diagonalizing or semi-diagonalizing reported per cell feedback received from multiple points. In example embodiments, diagonalization and/or semi-diagonali zat ion of per-cell feedback al lows for higher rank transmissioa for SU-MIMO.

In an embodiment, assuming a total number K transmission points, a joint precoding matrix W for low transmission rank R may be used for all transmission points and is formed as:

WI W2 W =

K K \

where the precoder W is of dimension x R and each matrb Wk, k1 K is of dimension N x R,with N being the number of transmit antennas at the k-th transmission point and Rbeing a transmission rank common to all K transmission points. In an embodiment, the rank R ffilfills the relationship R max Rk, where Rk is the reported rank for the k-th transmission point. Jo

In an embodiment, the matrices W", Id, K, are configured to contain a codeword/precoder specific to the k-th transmission point, intra-transmission point precoders and/or inter-transmission point amplitude/phase combiners. For example, for rank U each of the precoders \Vk can be expressed as \V _ckW where Ct-is the inter-transmission point amplitude/phase combiner (a complex-valued scalar) and Wt-is the intra-transmission point precoder. In an embodiment, intra-traasmission point precoders are configured to target separate transmissions from each transmission point. Inter-transmission point combiners are further configured to support joint transmission from the K transmission points to the mobile terminal. For example, an intra-transmission point precoder can be selected from a single codebook e.g. like 2-Tx and 4-Tx 3GPP LTE Rd-S codebooks or from a double codebook e.g. like 3GPP LTE Rel-lO codcbook for 8-transmission points, all of which are hereby incorporated by reference. Alternatively or additionally, the inter-transmission point combiners may be used to coherently combine the precoders associated with each of the K transmission points in order to obtain a resuhing preeoder W. For example, inter-transmission point combiners target coherent combining between the beams of each transmission point formed by above described intra-transmission point precoders.

Alternatively or additionally, the lifter-transmission point combiners may comprise an amplitude term, which may improve performance in cases in which beams from different transmission points are received with substantial power imbalance.

In an embodiment, assuming that k-th transmission point may comprisc N transmit antennas, k=1 K, and a mobile terminal is equipped with iV receive antennas, and in an instance in which N,c <N, the k-th transmission point may therefore transmit at most Nf layers at a time. Alternatively or additionally, in an instance in which a transmission is carried simultaneously from K transmission points, the associated joint transmission channel is of dimension Nr x and thus k-I

K

for example allows for transmission up to mJn Ac, yk layers at a time.

However, in some embodiments, mobile terminal feedback is provided on a per transmission point basis of rank Rk «= N for the k-th transmission point.

Alternativcly or additionally, in a configuration where thc K transmission points simultaneously transmit to a mobile terminal, there is potential for a total transmission rank RT suchthat R1 «= min[Nr:N:9.

Although the method, apparatus and computer program product may be implcmented in a variety of diffcrcnt systems, one example of such a system is shown in Figure I, which includes a first communication device (e.g. mobile terminal 10) that is capable of communication via an access point 12, such as a base station, a macro cell, a Node B, an evolved Node B (eN B), a coordination unit, a macro basc station or other access point, with a network 14 (e.g. a core network). While the network may be configured in accordance with LTE or LU-Advanced (LTE-A), other networks may support the method, apparatus and computer program product of embodiments of the present invention, including those configured in accordance with wideband code division multiple access (W-CDMA), CDMA2000, global system for mobile communications (GSM), general packet radio service (GPRS) and/or the like.

The network 14 may include a collection of various different nodes, devices or ifinctions that may be in communication with each other via corresponding wired and/or wireless interfaces. For example, the network may include one or more cells, including access point 12 and which may serve a respective coverage area. The access point could be, for example, part of one or more cellular or mobile networks or public land mobile nctworks (PLMNs). In turn, other devices such as processing dcvices (e.g. personal computers, server computers or the like) may bc coupled to the mobile terminal 10 and/or other communication devices via the network.

A communication device, such as the mobile terminal 10 (also known as user equipment (UE)), may be in communication with other communication devices or other devices via the access point 12 and, in turn, the network 14. In some cases, the communication device may include an antenna for transmitting signals to and for receiving signals from an access point.

In some example embodiments, the mobile terminal 10 may be a mobile communication device such as, for example, a mobile telephone, portable digital assistant (PDA), pagcr, laptop computer, or any of numerous other hand held or portable communication devices, computation devices, con tent generation devices, content consumption devices, or combinations thereof As such, the mobile terminal may include one or more processors that may define processing circuitry either alone or in combination with one or more memories. The processing circuitry may utilise instructions stored in the memory to cause the mobile terminal 1 0 to operate in a particular way or execute specific functionality when the instructions are executed by the one or more processors. The mobile terminal 10 may also include communication circuitry and corresponding hardware/software to enable communication with other devices and/or the network 14.

In one embodiment, for example, the mobile terminal 10 and/or the access point 12 may be embodied as or otherwise include an apparatus 20 as generically represented by the block diagram of Figure 2. While the apparatus 20 may be employed, for example, by a mobile terminal 10 or an access point 12, it should be noted that the components, devices or elements described below may not be mandatory and thus some may be omitted in certain embodiments. Additionally, some embodiments may include further or different components, devices or elements beyond those shown and described herein.

As shown in Figure 2, the apparatus 20 may include or otherwise be in communication with processing circuitry 22 that is configurable to perform actions in accordance with example embodiments described hcrcin. The processing circuitry may be configured to perform data processing, application execution and/or other processing and management services according to an example embodiment of the present invention. In some embodiments, the apparatus or the processing circuitry may be embodied as a chip or chip set. In other words, the apparatus or the processing circuitry may comprise one or more physical packages (e.g. chips) including materials, components and/or wires on a structural assembly (e.g. a baseboard). The structural assembly may provide physical strength, conservation of size, and/or limitation of electrical interaction for component circuitry included thereon. The apparatus or the processing circuitry may therefore, in some cases, be configured to implement an embodiment of the present invention on a single chip or as a single "system-on-a-chip." As such, in some cases, a chip or chipset may constitutc means for performing one or more operations for providing thc flinctionaiities described herein.

Tn an example embodiment, the processing circuitry 22 may include a processor 24 and memory 28 that may be in communication with or otherwise control a communication interface 26 and, in some cases, a user interface 29. As such, the processing circuitry may be embodied as a circuit chip (e.g. an integrated circuit chip) configured (e.g. with hardware, software or a combination of hardware and software) to perform operations described herein. However, in some embodiments taken in the context of the mobile terminal 10, the processing circuitry may be embodied as a portion of a mobile computing device or other mobile terminal.

The user interface 29 (if implemented) may be in communication with the processing circuitry 22 to receive an indication of a user input at the user interface andior to provide an audible, visual, mechanical or other output to the user. As such, the user interface may include, for example, a keyboard, a mousc, a joystick, a display, a touch screen, a microphone, a speaker, and/or othcr input/output mechanisms. Thc apparatus 20 need not always include a user interface. For example, in instances in which the apparatus is embodied as an access point 12, the apparatus may not include a user interface. As such, the user interface is shown in dashed lines in Figure 2.

The communication interface 26 may include one or more interface mechanisms for enabling communication with other devices and'or networks. In some cases, the communication interface may be any means such as a device or circuitry embodied in either hardware, or a combination of hardware and software that is configured to receive and/or transmit data front7to a network 14 and/or any other device or module in communication with the processing circuitry 22, such as between the mobile terminal 10 and the access point 12. In this regard, the communication interface may include, for example, an antenna (or multiple antennas) and supporting hardware and/or software for enabling communications with a wireless communication network and/or a communication modem or other hardwarc/sofiware for supporting communication via cable, digital subscriber line (DSL), universal serial bus (USB), Ethemet or other methods.

In an example embodiment, the memory 28 may include one or more non-transitory memory devices such as, for example, volatile and/or non-volatile memory that may be either fixed or removable. The memory 28 may be configured to store information, data, applications, instructions or the like for enabling the apparatus 20 to carry out various functions in accordance with example embodiments of the present invention. For example, the memory 28 could be configured to buffer input data for processing by the processor 24. Additionally or alternatively, the memory 28 could be configured to store instructions for execution by the processor 24. As yet another alternative, the memory 28 may include one of a plurality of databases that may store a variety of tiles, contents or data sets. Among the contents of the memory 28, applications may be stored for execution by the processor 24 in order to carry out the firnctionality associated with each respective application. In some cases, the memory 28 may be in communication with the proccssor 24 via a bus for passing information among components of the apparatus 20.

The processor 24 may be embodied in a number of different ways. For example, the processor 24 may be embodied as various processing means such as one or more of a microprocessor or other processing element, a coproeessor, a controller or various other computing or processing devices including integrated circuits such as, for example, an ASIC (application specific integrated circuit), an FPGA (field proammable gate array), or the like. In an example embodiment, the processor 24 may be configured to execute instructions stored in the memory 28 or otherwise accessible to the processor 24. As such, whether configured by hardware or by a combination of hardware and software, the processor 24 may represent an entity (e.g. physically embodicd in circuitry -in the form of processing circuitry 22) capable of performing operations according to embodiments of the present invention while configured accordingly. Thus, for example, when the processor 24 is embodied as an ASIC, FPGA or the like, the processor 24 may be specifically configured hardware for conducting the operations described herein. Alternatively, as another example, when the processor 24 is embodied as an executor of sofiware instructions, the instructions may specifically configure the processor 24 to perform the operations described herein.

A method, apparatus and computer program product of an example embodiment of the present invention is configured to enable the apparatus 20 embodied, for example, by a mobile terminal 10, which may include means, such as the processing circuitry 22, the processor 24, communications interface 26 or the like, to report ranks up to R, thus advantageously providing the example mobile terminal with the ability to exploit low rank feedback for higher rank transmission by diagonalizing or semi-diagonalizing the reported per point feedback consisting of W matrices. For example the reported per point feedback matrices Wk, k=1,. . .K, may be diagonalized: WI 0 0 0 o w2 0 0 w= o o.0 o o o WK where, rn one embodiment, each sub-matrix Wc is of dimension N x R and the matrb W is thus of dimension1 9 x R with RT = Rk. In an embodiment, a joint transmission precoding matrix may be diagonalized such that each of its associated spatial layers is transmitted by one of the K transmission points.

In an embodiment, each transmit point may contribute by Rfr to the total transmission rank R. In other words, different layers for higher ranks are transmitted from different transmission points. For example, the reported per point feedback matrices W' , k=1,. . .K, may be semi-diagonalized: W' 0 0 %v2 0 0 W= 0. 0 0 0 W" where, in one embodiment, the matrix Wis of size [E1v)xRi R,. =R,2+tR1 with 2bSg the transmission rank over the first two transmission points. In an embodiment, a joint transmission precoding matrix may be semi-diagonalized such that at least one of its associated spatial layers is transmitted by one of the K transmission points and at least another one of its associated layers is transmitted jointly by at least two of the K transmission points.

For example, the first two transmit points (indexed by k=l, 2) may be configured to perform joint transmission and contribute to the total rankl4with a contribution equal to iç2 while each of remaining transmit points (indexed by k=3,..., K) may contribute to the total rank with a contribution equal to R. Alternatively or additionally, an optimal rank i (with either diagonalization/semi-diagonalization) may be sclcctcd to maximise a pcrtbrmance metric, such as throughput, of the communication system. For example, ranks between R «= max and 1t «=mifl[Nr,EN,UJ may be handled by taking a subset of columns from full matrix W. !br example the first R0, columns in an instance in which the transmission points are ordered according to fbr example received power in W. hi an exampic considering K=3 transmission points, a first macro cell that comprises N,' =4 transmit antennas, a second and third RRH that each comprise N? =2, N? =2 transmit antennas, assuming that per transmission point ranks are R=2, 14=2, R3=i andthenumberofreceiveantennasatthemobileterminalis N, =4, a mobile terminal is configured to support transmission rants 1-4 in downlink.

By way of ifirther example, reported feedback may be a maximum rank feedback per point and may include one or more corresponding combiners, and is the following: Vx w=w2 c,

-

where is thc k-tb transmission point rcported codeword from an associated = [c (12)1 codebook, Ck is an inter-cell combiner diagonal matrix and Rk is the transmission rank corresponding to the k-tb transmission point By way of further example, the k-tb transmission points contributes by Rk to the overall transmission rank which is the sum of Rk, k1 K. The received reported per point feedback may be used forjoint transmission in an embodiment with Rankl: "4 v2 (:,1) = "2xI 3.1) descriptive feedback construction according to a matrix 1 0 0 0 where the joint transmission layers arc defined along the horizontal axis and the transmission points are defined along the vertical axis.

Rank2: -Pc" 0 = %V2\2L (22) Ew21 O1IcR 01 descriptive feedback construction according to: 1 0 0 0 Rank3: -ru.1) 0 1 W3 = 2 (2.2) 0 0 c descriptivc fccdback construction according to i 200 Rank4: = 0 0 [w9; 0{c") ..

semi-diagonalization 0 1 21 descriptive feedback construction according to. 0 0 1 0 The multi-point feedback matrices are constructed based on the per point precoders and appropriate combiners. The degree of diagonalization of these per point combiners in the final multi-point feedback depends on the achievable throughput.

By way of an additiona' example, in which K=2 transmission points, a first example transmission point includes a macro cell comprising N] = 4 transmit antennas, a second transmission point includes an example RRH comprising N = 2 transmit antennas, assuming that per transmission point ranks are = 2, R, = 2 and the number of rcccivc antennas at the mobilc terminal is Air = 4; the example mobile terminal is configured to support transmission ranks 1-4 in downlink. Alternatively or additionally, reported feedback may include maximum rank feedback per transmission point and corresponding combiners, and may include the following: = -\VN. ri?1 where WfrR is the k-th transmission point reported codeword from an associated codebook, Ck = (22) is an inter-cell combiner diagonal matrix and Rk is the L° J transmission rank corresponding to the k-tb transmission point. For example, the k-th transmission points contributes by Rk to the overall transmission rank which is the sum of Rk, k=1 K. The received reported feedback can be used for joint transmission with Rank 1: = 4x (:,1) 6x1 Rank2: "14x2 6*2 =[w2x22rc1 c2)Il iv' -(22) 6x2 -[W222e2) £ "layer permutation" descriptive feedback construction according to: L2 I Rank3: =r2 -0 L 0 we,1) Rank4: w[W4 __° r2 "full diagonalization" In alternative embodiments, there may be multiple possibilities on how the per transmission point precoders are arranged for higher ranks, and the mobile terminal may alternatively or additionally report the index of the chosen matrix arrangement with additional required combiners as pad of the feedback. The embodiments described herein may also be used with any joint feedback containing per point feedbacks and combiners. Alternatively or additionally, the per point feedback as well as combiner may be a codeword from a codebook.

Figures 3 and 4 illustrate example operations performed by a method, apparatus and computer program product, such as apparatus 20 of Figure 2 in accordance with one embodiment of the present invention. It will be understood that each block of the flowcharts, and combinations of blocks in the flowcharts, may be implemented by various means, such as hardware, firmware, processor, circuitry and/or other device associated with execution of software including one or more computer program instructions. For example, one or more of the procedures described above may be embodied by computer program instructions. In this regard, the computer program instructions which embody the procedures described above may be stored by a memory device 28 of an apparatus employing an embodiment of the present invention and executed by a processor 24 in the apparatus. As will be appreciated, any such computer program instructions may be loaded onto a computer or other programmable apparatus (e.g. hardware) to produce a machine, such that the resulting computer or other programmable apparatus provides for implementation of the functions specified in the flowcharts' block(s). These computer program instructions may also be stored in a non-transitory computer-readable storage memory that may direct a computer or other programmable apparatus to function in a particular manner, such that the instructions stored in the computer-readable storage memory produce an article of manufacture, the execution of which implements the function specified in the flowcharts' block(s). The computer program instructions may also be loaded onto a computer or other programmable apparatus to cause a series of operations to be performed on the computer or other programmable apparatus to produce a computer-implemented process such that the instructions which execute on the computer or other programmable apparatus provide operations for implementing the functions specified in the flowcharts' block(s). As such, the operations of Figures 3 and 4, when executed, convert a computer or processing circuitry into a particular machine configured to perform an example embodiment of the present invention. Accordingly, the operations of Figures 3 and 4 define an algorithm for configuring a computer or processing circuitry 22, e.g. processor, to perform an example embodiment. In some cases, a general purpose computer may be provided with an instance of the processor which performs the algorithm of Figures 3 and 4 to transform the general purpose computer into a particular machine configured to perform an example embodiment.

Accordingly, blocks of the flowcharts support combinations of means for performing the spccificd functions and combinations of operations for pcrforming the specified functions. It will also be understood that one or morc blocks of the flowchart, and combinations of blocks in the flowcharts, can be implemented by special purpose hardware-bascd computer systems which perform the spccificd flrnctions, or combinations of special purpose hardware and computer instructions.

In some embodiments, certain ones of the operations above may be modified or further amplified as described below. Moreover, in some embodiments additional optional operations may also be included (some examples of which are shown in dashed lines in Figure 3). It should be appreciated that each of the modifications, optional additions or amplifications below may be included with the operations above either alone or in combination with any others among the features described herein.

Figure 3 is a flow chart illustrating mobile terminal feedback performed in accordance with one embodiment of the present invention. In an embodiment, the apparatus 20 cmbodied, for example, by a mobile terminal 10, may include means, such as the processing circuitry 22, the processor 24 or the like, for estimating a MIMO channel corresponding to each transmit point. In embodiments using LTE, a MIMO channel may be estimated from CSI-RS.

Referring now to Figure 3. as shown in operation 30, the apparatus 20 embodied, for example, by a mobile terminal 10, may include means, such as the processing circuitry 22, the processor 24 or the like, for determining a transmission rank and a precoding matrix for each of at least two transmission points of a plurality of transmission points. In an embodiment, for each transmit point k=1,.., K, the mobile terminal may be configured to compute an optimum rank and a corresponding optimum precoder WPR. These are computed, for example using means such as the processing circuitry 22, the processor 24 or the like, by sampling each of the possible matrices in a (specified) codebook for each rank, computing a selected performance metric such as the throughput and selecting the rank-precoder combination. For example a rank-prccoder combination may bc chosen that maximises throughput.

In an embodiment, based on the computed precoders for each transmit point, the apparatus 20 embodied, for example, by a mobile terminal 10, may include means, such as the processing circuitry 22, the processor 24 or the like, for computing combiners Cfr, for example, by sampling each of the possible combiners in the eodebook and forming a joint precoder, such as:

WI W2 W=

where W' = Wkch. In an embodiment, a combiner may be chosen that maximises the joint transmission throughput.

In an embodiment, once transmission point-specific precoders w have been selected, the apparatus 20 embodied, for example, by a mobile terminal 10, may include means, such as the processing circuitry 22, the processor 24 or the like, that may be configured to compute a higher rank optimum per-point feedback arrangement in an instance in which the maximum rank supported by a distributed antenna system is higher than a chosen per-transmission point rank. As shown in operation 32, the apparatus 20 embodied, for example, by a mobile terminal 10, may include means, such as the processing circuitry 22, the processor 24 or the like, for determining at least one rank and at least one precoder for a plurality of transmission points. In an embodiment, each rank from the transmission point -specific rank to the maximum supported rank R «= miN, NJ. may be analysed, such as by the processor 24.

As shown in operation 34, the apparatus 20 embodied, for example, by a mobile terminal 10, may include means, such as the processing circuitry 22, the processor 24 or thc like, for determining at least one precodcr for a determined a maximizing rank supported for a transmission point. In an embodiment, for each rank a corresponding precoder or precoders, as described herein, is formed by diagonalizing or semi-diagonalizing the full matrix W. As shown in operation 36, the apparatus 20 embodied, for example, by a mobile terminal 10, may include means, such as the processing circuitry 22, the processor 24 or the like, for selecting a rank and precoder from at least one of the determined rank and preeodcr or the determined maximising rank and precoder. In an embodiment, the apparatus 20 embodied, for example, by a mobile terminal 10, may include means, such as the processing circuitry 22, the processor 24 or the like, for determining a rank-precoder combination (feedback arrangement), such as a rank-precoder combination that maximises throughput.

As shown in operation 38, the apparatus 20 embodied, for example, by a mobile terminal 10, may include means, such as the processing circuitty 22, the processor 24 or the like, for determining a joint transmission rank based on at least two of the plurality of transmission points. In an embodiment, a joint transmission rank is determined under the assumption that joint transmission would occur as determined by the mobile terminal. As shown in operation 40, the apparatus 4 embodied, for example, by a mobile terminal 10, may include means, such as the processing circuitry 22, the processor 24 or the like, for generating a joint transmission precoding matrix based on the determined precoding matrix for each of the at least two transmission points and the determined joint transmission rank. In an embodiment, the precoder may be fixed for each rank or there may be alternate ways S of arranging (diagonalizing/semi-diagonalizing) the precoders into a joint transmission precoding matrix, such as W. As shown in operation 42, the apparatus 20 embodied, for example, by a mobile terminal 10, may include means, such as the processing circuitry 22, the processor 24, the communications interface 26 or the like, for causing channel state information "CSI" to be transmitted to an access point. The CSI may include the joint transmission precoding matrix, an index, and/or a pointer to the joint transmission precoding matrix. CSI feedback may, for example, include means and/or signalling that describes a joint transmission precoding matrix in a manner such that a mobile IS terminal, an access point, or the like may receive and/or access an unambiguous description of the joint transmission precoding matrix.

In an embodiment, transmission point-specific matrices with or without combiner, e.g., or Wk respectively, along with the corresponding rank may be included with the transmitted CSI. Furthermore, in an embodiment, at least one transmission point -specific rank indicator is reported. Multiple indicators may be reported in an instance in which the same rank is not assumed for all transmission points. Alternatively or additionally, the apparatus 20 embodied, for example, by a mobile teniiinal 10, may include means, such as the processing circuitry 22, the processor 24, the communications interFace 26 or the like, for causing the optimum total rank which corresponds to the at least semi-diagonalized precoding matrix to be reported.

Alternatively or additionally, the apparatus 20 embodied, for example, by a mobile terminal 10, may include means, such as the processing circuitry 22, the processor 24, the communications interface 26 or the like, for causing an index to be reported corresponding to one of multiple possible matrix arrangements, e.g an index to a codebook of different alternatives to (semi-) diagonalize the matrix W Alternatively or additionally, the apparatus 20 embodied, for example, by a mobile terminal 10, may include means, such as the processing circuitry 22, the processor 24, the communications interface 26 or the like, for causing CQIs to be reported corresponding to a transmission with at least one transmission point-specific precoder, CQI(s) corresponding to low rank R13,joint transmission from all transmission points, and/or CQI(s) corresponding to high rank R0,joint transmission from all transmission points. Alternatively or additionally, the CQI may be wideband, e.g. only one value is reported for the whole band for each quantity, or sub band, e.g. multiple values may be reported for at least a portion of the quantities.

As shown in operation 50 of Figure 4, the apparatus 20 embodied, for example, by an access point 12, may include means, such as the processing circuitry 22, the processor 24, the communications interface 26 or the like, for receiving a report comprising at least one of transmission ranks for each of the at least two transmission points, precoding matrices for each of the at least two transmission points, a joint transmission rank or a joint transmission preeoding matrix. The report, such as a CSI report is further explained with reference to operation 42 of Figure 3.

As shown in operation 52, the apparatus 20 embodied, for example, by an access point 12, may include means, such as the processing circuitry 22, the processor 24 or the like, for determining a transmission strategy based on the received report.

For example, the apparatus 20 embodied, for example, by an access point 12, may include means, such as the processing circuitry 22, the processor 24 or the like, for comparing resulting estimated per-point transmission throughput to joint transmission throughput to determine an optimum transmission strategy.

The apparatus 20 embodied, for example, by an access point 12, may include means, such as the processing circuitry 22, the processor 24 or the like, for determining corresponding antenna weights based on the received index of transmission point specific precoders and the index of the high rank precoder.

Alternatively or additionally the apparatus 20 embodied, for example, by an access point 12, may include means, such as the processing circuitry 22, the processor 24 or the like, for arranging a frill precoding matrix W for the high rank precoder.

As shown in operation 54, the apparatus 20 embodied, for example, by an access point 12, may include means, such as the processing circuitry 22, the processor 24 or the like, for determining whether SLJ-MIMO will be used. In an embodiment, the apparatus 20 embodied, for example, by an access point 12, may include means, such as the processing circuitry 22, the processor 24 or the like, for determining a transmission rank based on a received Cs! report and in some embodiments on IS scheduling decisions.

For example, the apparatus 20 embodied, for example, by an access point 12, may include means, such as the processing circuitry 22, the processor 24 or the like, for determining whether MU-MIMO should be utilised. An example access point may utihse the lower reported rank or another lower rank. In an instance in which the apparatus 20 is embodied, for example, by an access point 12, determines that SIJ-MIMO should be utiliscd, such as by the processor 24, the access point 12, may utilise the high reported rank.

As shown in operation 56, the apparatus 20 embodied, for example, by an access point!2, may include means, such as the processing circuitry 22, the processor 24, the communications interface 26 or the like, for causing a transmission using at least one of the precoding matrices for each of the at least two transmission points or the joint transmission precoding matrix. !n an embodiment, at least one of data (e.g. PDSCH) and/or control (e.g. PDCCH) is to be transmitted. The transmission may be based on the determined antenna weights. In an embodiment, the apparatus 20 embodied, for example, by an access point 12, may include means, such as the processing circuitry 22, the processor 24, the communications interface 26 or the like, for causing data/control to be transmitted to a mobile terminal utilising the determined antenna weights.

Advantageously, the apparatus 20, method and computer program product as described herein enables flexible SU MIMO/MU MIMO operation using a diagonalizing (semi-diagonalizing) technique. In a MU MIMO operation embodiment, a low rank of the precoding matrix can be readily available in the form of W8x1, or W8x2 (rcsp. W6x1, or W6x2) above. In a high rank ¶1 MIMO operation embodiment, the diagonalizing (semi-diagonalizing) method may be used to obtain higher rank precoders in the form of W8x3, or W8x4 (resp. W6x3, or W6x4).

Alternatively or additionally the apparatus 20, method and computer program product as described herein provides support for dynamic SU/MIJ operation with, for example, little additional complexity.

Many modifications and other embodiments of the inventions set forth herein will come to mind to one skilled in the art to which these inventions pertain having the benefit of the teachings presented in the foregoing descriptions and the associated drawings. Therefore, it is to be understood that the inventions arc not to be limited to the specific embodiments disclosed and that modifications and other embodiments are intended to be included within the scope of the appended claims. Moreover, although the foregoing descriptions and the associated drawings describe example embodiments in the context of certain example combinations of elements and/or functions, it should be appreciated that different combinations of elements and/or functions may be provided by alternative embodiments without departing from the scope of the appended claims. In this regard, for example, different combinations of elements and/or functions than those explicitly described above are also contemplated as may be set forth in some of the appended claims. Although specific terms are employed herein, they are used in a generic and descriptive sense only and not for purposes of limitation.

Description Page 29

The following numbered clauses on pages 29 to 34 of thc prcscnt description correspond to the claims of British patent application no. 1116623.8 as filed. The claims of the present application as filed, which is divided from British patent application no. 1116623.8, can bc found on the subsequent pages 35 to 38 of thc specification which begin with the heading "CLAIMS".

1. A method comprising: determining a transmission rank and a prccoding matrix for each of at least two transmission points of a plurality of transmission points; determining a joint transmission rank bascd on at least two of thc plurality of transmission points sclccting a joint transmission precoding matrix based on the dctcrmincd precoding matrix for cach of the at least two transmission points and the determined joint transmission rank; and causing channel statc information "CSI" to be transmitted to an acccss point, wherein the CSI dcscribcs the selected joint transmission precoding matrix.

2. A method according to claim 1, comprising causing at least one of the detcrmincd transmission ranks for cach of the at least two transmission points, the determined precoding matrices for each of thc at least two transmission points, the dctermined joint transmission rank or thc selected joint transmission prccoding matrix to be transmitted to the access point.

3. A mcthod according to claim I or claim 2, wherein the joint transmission preeoding matrix is diagonalized such that each of its associated spatial layers is transmitted by a transmission point of the plurality of transmission points.

4. A method according to claim I or claim 2, wherein the joint transmission precoding matrix is semi-diagonalized such that at least one of an associated spatial layers is transmitted by a transmission point of the plurality of transmission points and

Description Page 30

at least another associated spatial layer is transmitted jointly by at least two transmission points of the plurality of transmission points.

5. A method aeeordhig to any of claims 1 to 4, wherein the joint transmission precoding matrix is formed based on a column permutation of the determined precoding matrices for at least one of the at least two transmission points.

6. A method according to any of claims I to 5, wherein thc joint traasmission precoding matrix comprises sub-matrices based on the determined precoding matrices for each of the at least two transmission points.

7. A method according to any of claims I to 6, wherein the determined precoding matrices for each of the at least two transmission points comprises an intra-transmission point precoder and an inter-transmission point combiner.

8. A method according to any of claims I to 7, wherein dctermining preeoding matrices for each of the at least two transmission points comprises determining a preeoding matrix that is configured to optimise a performance metric.

9. A method according to any of claims to 8, wherein selecting a joint transmission precoding matrix comprises: generating at least one of a plurality of candidate joint transmission precoding matrices, each of the generated plurality of candidate joint transmission precoding matrices being based on the determined preeoding matrix for each of the at least two transmission points and the determined joint transmission rank; and selecting a generated candidate joint transmission precoding matrix from the generated candidate joint transmission precoding matrices based on a performance metric.

10. Apparatus comprising:

Description Page 31

a processing systcm constructed and arranged to causc thc apparatus to at I cast: determine a transmission rank and a precoding matrix for each of at least two transmission points of a plurality of transmission points; determine a joint transmission rank based on at least two of the plurality of transmission points; select a joint transmission precoding matrix bascd on the determined precoding matrix for cach of the at least two transmission points and thc determined joint transmission rank; and cause channel state information "CSI" to be transmitted to an access point, wherein the CSI describes the selected joint transmission precoding matrix.

11. Apparatus according to claim 10 wherein the processing systcm is constructed and arranged to causc the apparatus to causc at least onc of thc determined transmission ranks for each of the at least two transmission points, the determined precoding matriccs for cach of the at least two transmission points, thc determined joint transmission rank or the selected joint transmission precoding matrix to be transmitted.

12. Apparatus according to claim 10 or claim 11, wherein thc joint transmission precoding matrix is diagonalized such that each of its associated spatial layers is transmitted by a transmission point of the plurality of transmission points.

13. Apparatus according to claim 10 or claim 11, whcrcin the joint transmission precoding matrix is scmi-diagonalized such that at least one of an associated spatial layers is transmitted by a transmission point of the plurality of transmission points and at Icast another associated spatial layer is transmittcd jointly by at Icast two transmission points of thc plurality of transmission points.

Description Page 32

14. Apparatus according to any of claims 10 to 13, wherein the joint transmission precoding matrix is formed based on a column permutation of the determined precoding matrices for at least one of the at least two transmission points.

15. Apparatus according to any of claims 10 to 14, wherein the joint transmission precoding matrix comprises sub-matrices based on the determined precoding matrices for each of the at least two transmission points.

16. Apparatus according to any of claims 10 to 15, wherein thc determined precoding matrices for each of the at least two transmission points comprises an intra-transmission point precoder and an inter-transmission point combiner.

17. A method according to any of claims 10 to 16, wherein determining precoding matrices for each of the at least two transmission points comprises determining a precoding matrix that is configured to optimise a performance metric.

18. Apparatus according to any of claims 10 to 17, wherein the processing system is constructed and arranged to cause the apparatus to: generate at least one of a plurality of candidate joint transmission precoding matrices, each of the generated plurality of candidate joint transmission precoding matrices being based on the determined precoding matrix for each of the at least two transmission points and the determined joint transmission rank; and select a generated candidate joint transmission precoding matrix from the generated candidate joint transmission preeoding matrices based on a performance metric.

19. Apparatus comprising: a processing system constructed and arranged to cause the apparatus to at least: determine a transmission strategy based on a received report that comprises at least one of transmission ranks for each of the at least two transmission points,

Description Page 33

precoding matriccs for cach of the at least two transmission points, a joint transmission rank or a joint transmission precoding matrix; and cause a transmission using at least one of the precoding matrices for each of thc at least two transmission points or the joint transmission precoding matrix.

20. Apparatus according to claim 19, wherein the processing system is constructed and arranged to cause the apparatus to determine whethcr at Icast one of multipic uscr multiple input multipic output or single user multiple input multiple output will be used based on the received report.

21. A method comprising: receiving a report comprising at least one of transmission ranks for each of the at least two transmission points, precoding matrices for each of the at least two transmission points, a joint transmission rank or a joint transmission precoding matrix; determining a transmission strategy based on the received report; and causing a transmission using at least one of the preeoding matrices for each of the at least two transmission points or the joint transmission precoding matrix.

22. A method according to claim 21, comprising determining whether at least one of multiple user multiple input multiple output or single user multiple input multiple output will be used based on the received report.

23. A computer program comprising code such that when the computer program is executed on a computing device, the computing device is arranged to carry out a method according to any of claims I to 9 or claims 21 or 22.

24. A method of controlling multiple input multiple output "MIMO" for distributed antenna arrays substantially in accordance with any of the examples as described herein with reference to the accompanying drawings.

Description Page 34

25. Apparatus fbr controlling multiple input multiple output "MEMO" fbr distributed antenna arrays substantially in accordance with any of the examples as described herein with refrrence to the accompanying drawings.

Claims

<claim-text>CLAIMS1. A method comprising: determining a transmission rank and a precoding matrix for each of at least two transmission points of a plurality of transmission points; S determining a joint transmission rank based on at least two of the plurality of transmission points; selecting a joint transmission precoding matrix based on the determined precoding matrix for each of the at least two transmission points and the determined joint transmission rank; and causing channel state information "CSI" to be transmitted to an access point, wherein the CSI describes the selected joint transmission precoding matrix, wherein the joint transmission precoding matrix is formed based on a column permutation of the determined precoding matrices for at least one of the at least two transmission points.</claim-text> <claim-text>2. A method according to claim 1, comprising causing at least one of the determined transmission ranks for each of the at least two transmission points, the determined precoding matrices for each of the at least two transmission points, the determined joint transmission rank or the selected joint transmission precoding matrix to be transmitted to the access point.</claim-text> <claim-text>3. A method according to claim I or claim 2, wherein the joint transmission precoding matrix is diagonalized such that each of its associated spatial layers is transmitted by a transmission point of the plurality of transmission points.</claim-text> <claim-text>4. A method according to any of claims I to 3, wherein the joint transmission precoding matrix comprises sub-matrices based on the determined precoding matrices for each of the at least two transmission points.</claim-text> <claim-text>5. A method according to any of claims I to 4, wherein the dctermined precoding matrices for cach of the at least two transmission points comprises an intra-transmission point precoder and an inter-transmission point combiner.</claim-text> <claim-text>6. A method according to any of claims 1 to 5, wherein determining precoding matrices for each of the at least two transmission points comprises determining a precoding matrix that is configured to optimise a performance metric.</claim-text> <claim-text>7. A method according to any of claims 1 to 6, wherein selecting a joint transmission precoding matrix comprises: generating at least one of a plurality of candidate joint transmission precoding matrices, each of the generated plurality of candidate joint transmission preeoding matrices being based on the determined precoding matrix for each of the at least two transmission points and the determined joint transmission rank; and selecting a generated candidate joint transmission precoding matrix from the generated candidate joint transmission precoding matrices based on a performance metric.</claim-text> <claim-text>8. Apparatus comprising: a processing system constructed and arranged to cause the apparatus to at least: determine a transmission rank and a precoding matrix for each of at least two transmission points of a plurality of transmission points; determine a joint transmission rank based on at least two of the plurality of transmission points: select a joint transmission precoding matrix based on the determined precoding matrix for each of the at least two transmission points and the determined joint transmission rank; and cause channel state information "CSI" to be transmitted to an access point, wherein the CSI describes the selected joint transmission precoding matrix, wherein the joint transmission precoding matrix is formed based on a column permutation of the determined precoding matrices for at least one of the at least two transmission points.</claim-text> <claim-text>9. Apparatus according to claim 8 wherein the processing system is constructed and arranged to cause the apparatus to cause at least one of the determined transmission ranks for each of the at least two transmission points, the determined preeoding matrices for each of the at least two transmission points, the determined joint transmission rank or the selected joint transmission precoding matrix to be transmitted.</claim-text> <claim-text>10. Apparatus according to claim S or claim 9, wherein the joint transmission precoding matrix is diagonalizcd such that each of its associated spatial laycrs is transmitted by a transmission point of the plurality of transmission points.</claim-text> <claim-text>11. Apparatus according to any of claims S to 10, wherein the joint transmission precoding matrix comprises sub-matrices based on the determined precoding matrices for each of the at least two transmission points.</claim-text> <claim-text>12. Apparatus according to any of claims S to II, wherein the determined precoding matrices for each of the at least two transmission points comprises an intra-transmission point prccodcr and an inter-transmission point combiner.</claim-text> <claim-text>13. Apparatus according to any of claims S to 12, wherein determining precoding matriccs for each of the at least two transmission points comprises determining a precoding matrix that is configured to optimise a performance metric.</claim-text> <claim-text>14. Apparatus according to any of claims 8 to 13, wherein the processing system is constructed and arranged to cause the apparatus to: generate at least one of a plurality of candidate joint transmission precoding matrices, each of the generated plurality of candidate joint transmission preeoding matrices being based on the determined precoding matrix for each of the at least two transmission points and the determined joint transmission rank; and select a generated candidate joint transmission precoding matrix from the generated candidate joint transmission precoding matrices based on a performance metric.</claim-text> <claim-text>15. A computer program comprising code such that when the computer program is executed on a computing device, the computing device is arranged to carry out a method according to any of claims Ito 7.</claim-text>