WO2018149477A1

WO2018149477A1 - Codebook design for user-specific precoding

Info

Publication number: WO2018149477A1
Application number: PCT/EP2017/053255
Authority: WO
Inventors: Henrik Asplund; Niklas JALDÈN; David Astely; Martin Johansson
Original assignee: Telefonaktiebolaget Lm Ericsson (Publ)
Priority date: 2017-02-14
Filing date: 2017-02-14
Publication date: 2018-08-23

Abstract

There is provided mechanisms for determining precoding weight vectors in a codebook for user-specific precoding. A method is performed by a radio transceiver device. The method comprises obtaining information about which directions are intended to be served by the radio transceiver device and directions in which interference from transmission by the radio transceiver device should be limited. The method comprises determining one precoding weight vector for each respective one of the directions to be served based on expected power in this respective one of the directions in relation to expected power in the directions in which interference should be limited

Description

CODEBOOK DESIGN FOR USER-SPECIFIC PRECODING

TECHNICAL FIELD

Embodiments presented herein relate to a method, a radio transceiver device, a computer program, and a computer program product for determining precoding weight vectors in a codebook for user-specific precoding.

BACKGROUND

In general terms, achieving ubiquitous high data-rate coverage requires an efficient use of available resources. With multiple antennas at the transmitter and/or the receiver, it could be possible to exploit the spatial degrees of freedom offered by multipath fading inside the wireless radio propagation channel in order to provide a substantial increase in the data rates and reliability of wireless transmission. In the downlink (transmission from an access node in the network to a user), there are three basic approaches for utilizing the antenna: diversity, multiplexing and beamforming. With beamforming, the radiation pattern of the antennas may be controlled by transmitting a signal from a plurality of antenna elements with an antenna element specific gain and phase. In this way, radiation patterns with different pointing directions and beam widths in both elevation and azimuth

directions may be created. The gains from adjusting the beam shapes used for transmissions come from both increased received power (as e.g. defined by increased signal to noise ratio, SNR) as well as a possibly lower interference (e.g. defined by increased signal to interference plus noise ratio, SINR) in a multi cell scenario.

However, how much of these gains that could be realized generally depends on how well the antenna system at the transmitter can direct the energy to the receivers of the target users, and how well it avoids emitting energy to the interfered users. The area of beamforming is commonly divided in two parts; user specific beamforming (UE-BF) and cell specific beamforming (CS-BF). With user specific beamforming, the transmit beam used is chosen to optimize the radio propagation channel between the network and a single user. This is the method to use when transmitting user specific data. Cell specific beamforming targets a large set of simultaneous users.

Currently, user specific beamforming is typically implemented through the use of codebooks. The codebooks could be proprietary or standardized. When using codebook based transmissions, each user (which knows the codebook prior to transmission) may estimate what the gain would be for each code word and then feed back information of this to the network. The network uses the feedback information to determine what precoders to use when transmitting the data. The user selects the precoder that optimizes the channel for transmission on the given link, disregarding the interference generated to the rest of the system.

Another method for user specific precoding is to utilize the reciprocity of the radio propagation channel. The network may collect information and characterize the radio propagation channel based on uplink (transmission from a user to an access node in the network) measurements. This information is later used in the downlink when determining how to pre-code the signal.

In general terms, the interference situation experienced at an access node (as seen on the uplink) and the possible directions in which an access node may interfere (on the downlink) is cell specific. This means that that the interference characteristics may differ from cell to cell, both in intensity as well as spatial distribution. However, current codebooks are designed based on assumptions of the array geometry and uniform directions to cover but without respect to the interference it may generate. In other words, the codebook contains spatially distributed beams designed to maximize the transmitted power in a given angular (spatial) domain. Further, the codebooks are designed to suit all possible cells and deployments.

However, there is still a need for improved codebooks. SUMMARY

An object of embodiments herein is to provide codebooks that not only are designed to optimize the received power at the user.

According to a first aspect there is presented a method for determining precoding weight vectors in a codebook for user-specific precoding. The method is performed by a radio transceiver device. The method comprises obtaining information about which directions n = 1,2, ... , N are intended to be served by the radio transceiver device and directions m = 1,2, ... , M in which interference from transmission by the radio transceiver device should be limited. The method comprises determining one precoding weight vector for each respective one of the directions to be served based on expected power in this respective one of the directions in relation to expected power in the directions in which interference should be limited.

Advantageously this method enables codebooks to be designed not only based on optimizing the received power at the user

Advantageously this method uses knowledge of the spatial characteristics of the radio propagation channel in order to design appropriate cell specific codebooks for user-specific precoding to users within the cell that will improve the SINR and not only the SNR, thereby improving the SINR in a multi-cellular network and not only the SNR for each link.

According to a second aspect there is presented a radio transceiver device for determining precoding weight vectors in a codebook for user-specific precoding. The radio transceiver device comprises processing circuitry. The processing circuitry is configured to cause the radio transceiver device to obtain information about which directions n = 1,2, ... , N are intended to be served by the radio transceiver device and directions m = 1,2, ... , M in which interference from transmission by the radio transceiver device should be limited. The processing circuitry is configured to cause the radio transceiver device to determine one precoding weight vector for each respective one of the directions to be served based on expected power in this respective one of the directions in relation to expected power in the directions in which interference should be limited.

According to a third aspect there is presented a radio transceiver device for determining precoding weight vectors in a codebook for user-specific precoding. The radio transceiver device comprises processing circuitry and a storage medium. The storage medium stores instructions that, when executed by the processing circuitry, cause the radio transceiver device to perform operations, or steps. The operations, or steps, cause the radio transceiver device to obtain information about which directions n = 1,2, ... , N are intended to be served by the radio transceiver device and directions m = 1,2, ... , M in which interference from transmission by the radio transceiver device should be limited. The operations, or steps, cause the radio transceiver device to determine one precoding weight vector for each respective one of the directions to be served based on expected power in this respective one of the directions in relation to expected power in the directions in which interference should be limited.

According to a fourth aspect there is presented a radio transceiver device for determining precoding weight vectors in a codebook for user-specific precoding. The radio transceiver device comprises an obtain module configured to obtain information about which directions n = 1,2, ... , N are intended to be served by the radio transceiver device and directions m = 1,2, ... , M in which interference from transmission by the radio transceiver device should be limited. The radio transceiver device comprises a determine module configured to determine one precoding weight vector for each respective one of the directions to be served based on expected power in this respective one of the directions in relation to expected power in the directions in which interference should be limited.

According to an embodiment the radio transceiver device is an access node.

According to a fifth aspect there is presented a computer program for determining precoding weight vectors in a codebook for user-specific precoding, the computer program comprising computer program code which, when run on a radio transceiver device, causes the radio transceiver device to perform a method according to the first aspect.

According to a sixth aspect there is presented a computer program product comprising a computer program according to the fifth aspect and a computer readable storage medium on which the computer program is stored. The computer readable storage medium could be a non-transitory computer readable storage medium.

It is to be noted that any feature of the first, second, third, fourth, fifth and sixth aspects may be applied to any other aspect, wherever appropriate.

Likewise, any advantage of the first aspect may equally apply to the second, third, fourth, fifth and/or sixth aspect, respectively, and vice versa. Other objectives, features and advantages of the enclosed embodiments will be apparent from the following detailed disclosure, from the attached dependent claims as well as from the drawings.

Generally, all terms used in the claims are to be interpreted according to their ordinary meaning in the technical field, unless explicitly defined otherwise herein. All references to "a/an/the element, apparatus, component, means, module, step, etc." are to be interpreted openly as referring to at least one instance of the element, apparatus, component, means, module, step, etc., unless explicitly stated otherwise. The steps of any method disclosed herein do not have to be performed in the exact order disclosed, unless explicitly stated.

BRIEF DESCRIPTION OF THE DRAWINGS

The inventive concept is now described, by way of example, with reference to the accompanying drawings, in which:

Fig. 1 is a schematic diagram illustrating a communications system according to embodiments; Figs. 2, 3, 7, 8, and 9 schematically illustrate beam patterns according to embodiments;

Figs. 4, 5, and 6 are flowcharts of methods according to embodiments;

Fig. 10 is a schematic diagram showing functional units of a radio transceiver device according to an embodiment;

Fig. 11 is a schematic diagram showing functional modules of a radio transceiver device according to an embodiment; and

Fig. 12 shows one example of a computer program product comprising computer readable storage medium according to an embodiment. DETAILED DESCRIPTION

The inventive concept will now be described more fully hereinafter with reference to the accompanying drawings, in which certain embodiments of the inventive concept are shown. This inventive concept may, however, be embodied in many different forms and should not be construed as limited to the embodiments set forth herein; rather, these embodiments are provided by way of example so that this disclosure will be thorough and complete, and will fully convey the scope of the inventive concept to those skilled in the art. Like numbers refer to like elements throughout the description. Any step or feature illustrated by dashed lines should be regarded as optional. Fig. 1 is a schematic diagram illustrating a communications system 100 where embodiments presented herein can be applied. The communications system 100 comprises a first set of radio transceiver devices 200a, 200b, and a second set of radio transceiver devices 300a, 300b. In some aspects the radio transceiver devices 200a, 200b represent access nodes and the radio transceiver devices 300a, 300b represent users served by the access nodes. Hereinafter radio transceiver devices 300a, 300b may thus be referred to as users. Examples of access nodes include but are not limited to radio access network nodes, radio base stations, base transceiver stations, node Bs, evolved node Bs, and access points. Examples of users include but are not limited to wireless devices, mobile stations, mobile phones, handsets, wireless local loop phones, user equipment (UE), smartphones, laptop computers, tablet computers, network equipped sensors, wireless modems, and network equipped vehicles. Each radio transceiver device 200a, 200b is configured to transmit in one or more beams 400a, 400b. In the illustrative example of Fig. 1 where the radio transceiver devices 200a, 200b represent access nodes and the radio transceiver devices 300a, 300b represent users served by the access nodes, radio transceiver device 200a serves users in a (imagined) building, where the users are located on low floors and high floors but not on middle floors in the (imagined) building, and radio transceiver device 200b serves other users in another (imagined) building, where the users are located on the upper half of the floors but not on the lower half of the floors in the further (imagined) building. Hence, there could be a potential gain in restricting the beam shapes and the supported beam directions according to the respective beams 400a, 400b in order to avoid interference. But as disclosed above, current codebooks are designed to optimize the received power at the user (as exemplified by the radio transceiver devices 300a, 300b) and not necessarily taking interference into account. The embodiments disclosed herein therefore relate to mechanisms for determining precoding weight vectors in a codebook for user-specific precoding. In order to obtain such mechanisms there is provided a radio transceiver device 200a, 200b, a method performed by the radio transceiver device 200a, 200b, a computer program product comprising code, for example in the form of a computer program, that when run on a radio transceiver device 200a, 200b, causes the radio transceiver device 200a, 200b to perform the method.

The concept of cell specific (or downloadable) codebooks has not been standardized yet. Cell specific codebooks may improve the system

performance by taking cell specific aspects, such as spatial characteristics of interference, into account during the design procedure. Further, evolving communications systems, such as the fifth generation telecommunications system (5G), are likely to be dependent on user specific beamforming in order to increase data rates, and interference aware beamforming may be needed to enable this. When considering optimal beam patterns to use for transmission (to a single user), different metrics may be considered. In general, when the interference created to the rest of the system is not considered, a good metric is typically to maximize the received power at the user, i.e., to perform SNR-maximizing precoding. One way to achieve interference-aware precoding when having knowledge of the spatial information of the radio propagation channel between the access node and the user, for example the covariance information at the transmitter at the access node, is to use a beamforming (or precoding) vector

corresponding to the eigenvector corresponding to the strongest (or largest) eigenvalue of the covariance matrix for the radio channel. For terminology, let the spatial covariance matrix for a given transmit direction n = 1,2, ... , N , that is intended to be served by a cell, be denoted by R_n. Further, let the spatial covariance matrix to a direction (user /location) m not intended to be served by the cell, i.e., an interfered direction (user/location), be denoted by Q_m for m = 1,2, ... , M. An optimization goal when designing the codebook could be to maximize the received power s = w^HRw at the user whilst minimizing the interference q = w^HQw to the other users (assuming that the spatial information to the interfered users (directions) is represented by the interference covariance matrix Q = a₀I +∑_n a_nQ_n, where / is the identity matrix and a_n for n = 0,2,1, N are scaling factors) . For this optimization goal the signal to be transmitted to the user could be beam-formed using a weight vector v corresponding to the eigenvector w (for corresponding eigenvalue X) maximizing the generalized eigenvalue problem:

Rw = Qw The eigenvector w_£ corresponding to the largest eigenvalue Λ_£ will represent a beam that maximizes the ratio s/q.

Examples of the beam patterns for corresponding weight vectors (in a 16 element antenna array) are shown in Figs. 2 and 3. Fig. 2 shows the beam pattern 150 for an intended user when disregarding interference and only maximizing received power, and Fig. 3 shows the beam pattern 160 intended for the user whilst minimizing the interference to a secondary user. The angular directions to the intended user and the secondary users are indicated at lines 140 and 130, respectively. When calculating the beam pattern to optimize the received power at the intended user the beam pattern 150 is obtained. When optimizing the beam pattern to the user, while minimizing the interference, the beam pattern 160 is obtained. Assumptions on angular spread and ratio of SINR to the intended user and the interfered user are left out. Figs. 4 and 5 are flowcharts illustrating embodiments of methods for determining precoding weight vectors in a codebook for user-specific precoding. The methods are performed by the radio transceiver device 200a, 200b. The methods are advantageously provided as computer programs 1220. Reference is now made to Fig. 4 illustrating a method for determining precoding weight vectors in a codebook for user-specific precoding as performed by the radio transceiver device 200a, 200b according to an embodiment.

S102: The radio transceiver device 200a, 200b obtains information about which directions n = 1,2, ... , N are intended to be served by the radio transceiver device 200a, 200b and directions m = 1,2, ... , M in which interference from transmission by the radio transceiver device 200a, 200b should be limited. The directions m = 1,2, ... , M in which interference from transmission by the radio transceiver device 200a, 200b should be limited could here be interpreted as those directions that at least are intended not to be interfered in by the radio transceiver device 200a, 200b. Hence, the wording "should be limited" could, in some embodiments, be interpreted as "is to be limited".

The radio transceiver device 200a, 200b designs the codebook such that the received signal power at the radio transceiver device 300a (such as served wireless devices) intended to receive transmission from the radio transceiver device 200a, 200b is maximized whilst minimizing the interference generated to radio transceiver device 300b not intended to receive

transmission from the radio transceiver device 200a, 200b (such as non- served wireless devices). Particularly, the radio transceiver device 200a, 200b is configured to perform step S104:

S104: The radio transceiver device 200a, 200b determines one precoding weight vector for each respective one of the directions to be served based on expected power in this respective one of the directions in relation to expected power in the directions in which interference should be limited.

Steps S102 and S104 represent a tractable method for how to create codebooks to be used for user specific beamforming and that minimizes the interference generated whilst maximizing the received power for target receivers.

The beam shapes can be generated with a criterion to optimize the received signal power at the served receivers while minimizing the interference generated to non-served receivers. Hence, according to an embodiment the precoding weight vectors are determined with a criterion to minimize interference generated in the directions intended not to be interfered in and to maximize received power in the directions intended to be served.

Alternatively, precoding weight vectors are determined with a criterion to maximize the ratio between the received power in the directions intended to be served and the sum of the interference generated in other directions. Embodiments relating to further details of determining precoding weight vectors in a codebook for user-specific precoding as performed by the radio transceiver device 200a, 200b will now be disclosed.

In some embodiments the precoding weight vector for direction n is based on spatial channel characteristics for directions m and n. In some embodiments the expected power is estimated using spatial channel characteristics in the directions m = 1,2, ... , M in which interference should be limited.

In some aspects the information about which directions n = 1,2, ... , N are intended to be served by the radio transceiver device 200a, 200b is based on e.g. uplink measurements of served users 300a, 300b (such as wireless device 300a for radio transceiver device 200a acting as an access point). Hence, according to an embodiment the spatial channel characteristics for direction m is based on uplink measurements of at least one of currently and

previously served users 300a in direction m. In this way the codebook could contain a higher sampling density and finer granularity in areas where users typically are located, or in scenarios/locations where small gain differences may make larger differences in user/system throughput. Further, there could be lower sampling densities in areas where there are only a few users or areas there the extra beamforming gain does not affect the overall throughput substantially.

The information about which directions n = 1,2, ... , N are intended to be served by the radio transceiver device 200a, 200b could be based on long- term statistics or short-term statistics. As an example, it could be possible to use information based on long-term statistics about which directions n = 1,2, ... , N are intended to be served by the radio transceiver device 200a, 200b together with short-term statistics of the radio propagation channel when designing the codebook, so as to design the codebook based on reciprocity.

In further aspects, the spatial channel characteristics could be obtained from uplink measurements, from a database of stored spatial channel

characteristics, from another radio transceiver device 200a, 200b, or be configured in the radio transceiver device 200a, 200b, the latter allowing the spatial channel characteristics to be determined during system design.

There could be different types of spatial channel characteristics. According to an embodiment the spatial channel characteristics are represented by interference characteristics. According to other embodiments the spatial channel characteristics comprise aggregated weighted spatial covariance information Q for radio propagation channels in the directions in which interference should be limited. The aggregated weighted spatial covariance information Q could represent spatial covariance information for directions not to be interfered in and be expressed as

where b_mis a weighting factor, where Q_m represents spatial covariance information for direction m and the summation is taken over all directions m = 1,2, ... , M in which interference should be limited. The weighting factor b_m could be design-specific or a configured weighting. In this respect, the weighting factor b_m may depend on aspects such as time, load, etc. of the cell. That is, according to an embodiment the spatial covariance information for direction m is weighted with a weighting factor b_m.

In some aspects the precoding weight vector in direction n is defined by the eigenvector w_n corresponding to the largest eigenvalue λ_η such that R_nw_n = h_nQ^wn- That is_> according to an embodiment the precoding weight vector for each direction n = 1,2, ... , N are to be served is determined finding the largest eigenvalue that solves a generalized eigenvalue problem given as n where R_n represents spatial covariance information in direction n, where w_nrepresents precoding weight vector n, where Q represents spatial covariance information for directions not to be interfered in, and where Λ. eigenvalue n. In another embodiment the precoding weight vector for each direction to be served is determined such that

(w^R_nw_n)/(w^Qw_n) is maximized, where R_n represents spatial covariance information in direction n, where w_n represents precoding weight vector n, and where Q represents spatial covariance information for directions not to be interfered in.

In some aspects where at least two interference directions Q_m are known, scaling factors a_m where m = 1,2, ... , M could be used to scale/prioritize certain directions in which it is extra crucial to avoid interference. Hence, according to an embodiment the spatial covariance information for direction m is scaled with an individual scaling factor a_m.

In some aspects a scaling factor a₀ is used to scale the overall account taken to the interference. Hence, according to an embodiment the spatial covariance information is scaled with an overall scaling factor a₀. For low values of a₀ more of the interference directions is accounted for. This may be extra beneficial for an interference limited scenario such as systems working at very high loads. By setting a large value to a₀ less of the interference is accounted for and more of the noise is accounted for, which would result in a codebook designed with less interference suppression and is hence more suited for a noise limited system or a low load system. Particularly, according to an embodiment Q is given as

where / is an identity matrix. Further, Q could be based on both cell-specific weighting factors b_m and scaling factors a_m, where m = 0,1,2, ... , M. According to an embodiment the overall scaling factor a₀ is based on a ratio between interference and noise in the directions m = 1,2, ... , M intended not to be interfered in by the radio transceiver device 200a, 200b. This enables an interference-agnostic codebook to be determined for situations with low load or coverage-challenged users. This further enables an interference-aware codebook to be determined for scenarios with high load. This further enables the codebook to be determined based on information exchanged with, or obtained from, a further radio transceiver device 200a, 200b (see step S112 below). Reference is now made to Fig. 5 illustrating methods for determining precoding weight vectors in a codebook for user-specific precoding as performed by the radio transceiver device 200a, 200b according to further embodiments. It is assumed that steps S102, S104 are performed as described above with reference to Fig. 4 and a thus repeated description thereof is therefore omitted.

According to some aspects the determined precoding weight vectors w_n are normalized such that 11 w_n \ \ = 1 for all n = 1,2, ... , N. Hence, according to an embodiment the radio transceiver device 200a, 200b is configured to perform step S106: S106: The radio transceiver device 200a, 200b normalizes each of the determined precoding weight vectors.

According to some aspects the determined weight vectors are added to the cell-specific codebook. Hence, according to an embodiment the radio transceiver device 200a, 200b is configured to perform step S108: S108: The radio transceiver device 200a, 200b adds the determined precoding weight vectors to the codebook.

According to some aspects the radio transceiver device 200a, 200b transmits signals using the codebook. Hence, according to an embodiment the radio transceiver device 200a, 200b is configured to perform step S110: S110: The radio transceiver device 200a, 200b applies the determined precoding weight vectors to signals to be transmitted from the radio transceiver device 200a, 200b to users 300a served by the radio transceiver device 200a, 200b. According to some aspects the radio transceiver device 200a, 200b exchanges information related to the precoding weight vectors with another radio transceiver device 200a, 200b. This information could be any information that is used when determining the precoding weight vectors, such as load levels etc. Such information could be used the radio transceiver devices 200a, 200b to perform coordination, possibly coordinating the determination of the precoding weight vectors between the radio transceiver devices 200a, 200b. Hence, according to an embodiment the radio transceiver device 200a, 200b is configured to perform step S112:

S112: The radio transceiver device 200a, 200b exchanges information relating to the precoding weight vectors with a further radio transceiver device 200a, 200b.

One particular embodiment for determining precoding weight vectors in a codebook for user-specific precoding as performed by the radio transceiver device 200a, 200b based on at least some of the above disclosed

embodiments will now be disclosed with reference to the flowchart of Fig. 6.

S201: The radio transceiver device 200a, 200b obtains information of directions not intended to be served, i.e., interfering/interfered directions. Information of interfering/interfered directions, and hence Q_m, may be measured in uplink or obtained during system design of from a stored database of spatial information.

S202: The radio transceiver device 200a, 200b determines an aggregated (summed) interference covariance matrix Q = ∑b_mQ_m where b_m may be a design-specific or configured weighting, which may be used to prioritize certain directions where interference may be extra harmful. S203: The radio transceiver device 200a, 200b checks if Q has changed substantially since a previous Q was determined, and/or if the codebook is to be re-determined or re-optimized. If so, step S204 is entered. Otherwise step S201 is re-entered and the radio transceiver device 200a, 200b keeps logging interference characteristics. In this respect, steps 201-203 may run continuously and independently of steps 204-207.

S204: The radio transceiver device 200a, 200b defines the direction in space that transmission from the radio transceiver device 200a, 200b is supposed to support and determines a channel covariance matrix R_n given some assumption on the angular spread.

S205: The radio transceiver device 200a, 200b for a given direction n in space determines the eigenvector w_n corresponding to the largest eigenvalue λ_η such that R_nw_n = A_nQw_n, where Q is as mentioned above the aggregated interference covariance. S206: The radio transceiver device 200a, 200b normalizes w_n such that

I | | = I.

S207: The radio transceiver device 200a, 200b adds the weight vector to the cell specific codebook.

Step S205 could then be entered for the next value of n and steps S205-S206 could thus be repeatedly performed until all values of n = 1,2, ... , N have been looped through.

Figs. 7, 8, and 9 provide examples of how the beams in interference aware codebooks (a) as designed according to the herein disclosed embodiments compare to classical codebooks (b) as designed based on the Discrete Fourier Transform and not considering interference. Figs. 7, 8, and 9 show only one realization (and more sparsely sampled than codebooks commonly are) with one level of interference versus noise scaling. Shorter vertical lines 130 indicate directions intended to be served by the codebook. Longer vertical lines 140 indicate directions where interfered user could be located, thus representing directions in which the radiated power should be suppressed.

Fig. 7 represents a beam shape realization of a first codebook example. This codebook is (conceptually at least) an attractive codebook for a scenario where there are two groups of served users and three groups of non-served users.

Fig. 8 represents a beam shape realization of a second codebook example. This codebook is (conceptually at least) an attractive codebook for the radio transceiver device 200a when transmitting in beam 400a in Fig. 1. Fig. 9 represents a beam shape realization of a third codebook example. This codebook is (conceptually at least) an attractive codebook for the radio transceiver device 200b when transmitting in beam 400b in Fig. 1.

Fig. 10 schematically illustrates, in terms of a number of functional units, the components of a radio transceiver device 200a, 200b according to an embodiment. Processing circuitry 210 is provided using any combination of one or more of a suitable central processing unit (CPU), multiprocessor, microcontroller, digital signal processor (DSP), etc., capable of executing software instructions stored in a computer program product 1210 (as in Fig. 12), e.g. in the form of a storage medium 230. The processing circuitry 210 may further be provided as at least one application specific integrated circuit (ASIC), or field programmable gate array (FPGA).

Particularly, the processing circuitry 210 is configured to cause the radio transceiver device 200a, 200b to perform a set of operations, or steps, S102- S112, as disclosed above. For example, the storage medium 230 may store the set of operations, and the processing circuitry 210 may be configured to retrieve the set of operations from the storage medium 230 to cause the radio transceiver device 200a, 200b to perform the set of operations. The set of operations may be provided as a set of executable instructions. l8

Thus the processing circuitry 210 is thereby arranged to execute methods as herein disclosed. The storage medium 230 may also comprise persistent storage, which, for example, can be any single one or combination of magnetic memory, optical memory, solid state memory or even remotely mounted memory. The radio transceiver device 200a, 200b may further comprise a communications interface 220 at least configured for

communications with other entities, nodes, and devices in the

communications system 100. As such the communications interface 220 may comprise one or more transmitters and receivers, comprising analogue and digital components. The processing circuitry 210 controls the general operation of the radio transceiver device 200a, 200b e.g. by sending data and control signals to the communications interface 220 and the storage medium 230, by receiving data and reports from the communications interface 220, and by retrieving data and instructions from the storage medium 230. Other components, as well as the related functionality, of the radio transceiver device 200a, 200b are omitted in order not to obscure the concepts presented herein.

Fig. 11 schematically illustrates, in terms of a number of functional modules, the components of a radio transceiver device 200a, 200b according to an embodiment. The radio transceiver device 200a, 200b of Fig. 11 comprises a number of functional modules; an obtain module 210a configured to perform step S102 and a determine module 210b configured to perform step S104. The radio transceiver device 200a, 200b of Fig. 11 may further comprise a number of optional functional modules, such as any of a normalize module 210c configured to perform step S106, an add module 2iod configured to perform step S108, an apply module 2ioe configured to perform step S110, and an exchange module 2iof configured to perform step S112. In general terms, each functional module 2ioa-2iof may in one embodiment be implemented only in hardware and in another embodiment with the help of software, i.e., the latter embodiment having computer program instructions stored on the storage medium 230 which when run on the processing circuitry makes the radio transceiver device 200a, 200b perform the corresponding steps mentioned above in conjunction with Fig 11. It should also be mentioned that even though the modules correspond to parts of a computer program, they do not need to be separate modules therein, but the way in which they are implemented in software is dependent on the programming language used. Preferably, one or more or all functional modules 2ioa-2iof may be implemented by the processing circuitry 210, possibly in cooperation with the communications interface 220 and/or the storage medium 230. The processing circuitry 210 may thus be configured to from the storage medium 230 fetch instructions as provided by a functional module 2ioa-2iof and to execute these instructions, thereby performing any steps as disclosed herein.

The radio transceiver device 200a, 200b may be provided as a standalone device or as a part of at least one further device. For example, the radio transceiver device 200a, 200b may be provided in a node of the radio access network (such as in an access node) or in a node of the core network.

Alternatively, functionality of the radio transceiver device 200a, 200b may be distributed between at least two devices, or nodes. These at least two nodes, or devices, may either be part of the same network part (such as the radio access network or the core network) or may be spread between at least two such network parts.

Thus, a first portion of the instructions performed by the radio transceiver device 200a, 200b may be executed in a first device, and a second portion of the of the instructions performed by the radio transceiver device 200a, 200b may be executed in a second device; the herein disclosed embodiments are not limited to any particular number of devices on which the instructions performed by the radio transceiver device 200a, 200b may be executed. Hence, the methods according to the herein disclosed embodiments are suitable to be performed by a radio transceiver device 200a, 200b residing in a cloud computational environment. Therefore, although a single processing circuitry 210 is illustrated in Fig. 10 the processing circuitry 210 may be distributed among a plurality of devices, or nodes. The same applies to the functional modules 2ioa-2iof of Fig. 11 and the computer program 1220 of Fig. 12 (see below).

Yet alternatively, the herein disclosed functionality of the radio transceiver device 200a, 200b (particularly as defined by steps S102-S112 and S201- S207) is implemented in a radio transceiver device 300a, 300b acting as a wireless device, thereby enabling each wireless device to design its own codebooks. Such codebooks could by the wireless device be used during communications with an access node or with another wireless device.

Fig. 12 shows one example of a computer program product 1210 comprising computer readable storage medium 1230. On this computer readable storage medium 1230, a computer program 1220 can be stored, which computer program 1220 can cause the processing circuitry 210 and thereto operatively coupled entities and devices, such as the communications interface 220 and the storage medium 230, to execute methods according to embodiments described herein. The computer program 1220 and/or computer program product 1210 may thus provide means for performing any steps as herein disclosed.

In the example of Fig. 12, the computer program product 1210 is illustrated as an optical disc, such as a CD (compact disc) or a DVD (digital versatile disc) or a Blu-Ray disc. The computer program product 1210 could also be embodied as a memory, such as a random access memory (RAM), a read-only memory (ROM), an erasable programmable read-only memory (EPROM), or an electrically erasable programmable read-only memory (EEPROM) and more particularly as a non-volatile storage medium of a device in an external memory such as a USB (Universal Serial Bus) memory or a Flash memory, such as a compact Flash memory. Thus, while the computer program 1220 is here schematically shown as a track on the depicted optical disk, the computer program 1220 can be stored in any way which is suitable for the computer program product 1210. The inventive concept has mainly been described above with reference to a few embodiments. However, as is readily appreciated by a person skilled in the art, other embodiments than the ones disclosed above are equally possible within the scope of the inventive concept, as defined by the appended patent claims.

Claims

1. A method for determining precoding weight vectors in a codebook for user-specific precoding, the method being performed by a radio transceiver device (200a, 200b), the method comprising:

obtaining (S102) information about which directions n = 1,2, ... , N are intended to be served by the radio transceiver device (200a, 200b) and directions m = 1,2, ... , M in which interference from transmission by the radio transceiver device (200a, 200b) should be limited; and

determining (S104) one precoding weight vector for each respective one of the directions to be served based on expected power in this respective one of the directions in relation to expected power in the directions in which interference should be limited.

2. The method according to claim 1, further comprising:

normalizing (S106) each of the determined precoding weight vectors.

3. The method according to claim 1 or 2, further comprising:

adding (S108) the determined precoding weight vectors to the codebook.

4. The method according to any of the preceding claims, further comprising:

applying (S110) the determined precoding weight vectors to signals to be transmitted from the radio transceiver device (200a, 200b) to users (300a) served by the radio transceiver device (200a, 200b).

5. The method according to any of the preceding claims, further comprising:

exchanging (S112) information relating to the precoding weight vectors with a further radio transceiver device (200a, 200b).

6. The method according to any of the preceding claims, wherein the precoding weight vector for direction n is based on spatial channel characteristics for directions m and n.

7. The method according to any of the preceding claims, wherein the expected power is estimated using spatial channel characteristics in the directions in which interference should be limited.

8. The method according to claim 7, wherein the spatial channel characteristics for direction m is based on uplink measurements of at least one of currently and previously served users (300a) in direction m.

9. The method according to claim 7, wherein the spatial channel characteristics are obtained from uplink measurements, obtained from a database of stored spatial channel characteristics, obtained from another radio transceiver device (200a, 200b), or are configured in the radio transceiver device (200a, 200b).

10. The method according to claim 7, wherein the spatial channel characteristics comprise aggregated weighted spatial covariance information for radio propagation channels in the directions in which interference should be limited.

11. The method according to claim 11, wherein the spatial covariance information for direction m is weighted with a weighting factor b_m.

12. The method according to any of the preceding claims, wherein the precoding weight vector for each direction to be served is determined by finding the largest eigenvalue that solves a generalized eigenvalue problem given as

R_nw_n = _nQw_n, where R_n represents spatial covariance information in direction n, where w_n represents precoding weight vector n, where Q represents spatial covariance information for directions not to be interfered in, and where λ_η is eigenvalue n.

13. The method according to any of the preceding claims, wherein the precoding weight vector for each direction to be served is determined such that

(w^R_nw_n)/(w^Qw_n), is maximized, where R_n represents spatial covariance information in direction n, where w_n represents precoding weight vector n, and where Q represents spatial covariance information for directions not to be interfered in.

14. The method according to any of the preceding claims, wherein the spatial covariance information for direction m is scaled with an individual scaling factor a_m.

15. The method according to claim 1, wherein the spatial covariance information is scaled with an overall scaling factor a₀.

16. The method according to claims 13, 14, and 15, wherein Q is given as

where / is an identity matrix.

17. The method according to claim 15 or 16, wherein the overall scaling factor a₀ is based on a ratio between interference and noise in the directions intended not to be interfered in by the radio transceiver device (200a, 200b). 18. The method according to any of the preceding claims, wherein the precoding weight vectors are determined with a criterion to minimize interference generated in the directions intended not to be interfered in and to maximize received power in the directions intended to be served.

19. A radio transceiver device (200a, 200b) for determining precoding weight vectors in a codebook for user-specific precoding, the radio

transceiver device (200a, 200b) comprising processing circuitry (210), the processing circuitry being configured to cause the radio transceiver device (200a, 200b) to:

obtain information about which directions n = 1,2, ... , N are intended to be served by the radio transceiver device (200a, 200b) and directions m = 1,2, ... , M in which interference from transmission by the radio transceiver device (200a, 200b) should be limited; and

determine one precoding weight vector for each respective one of the directions to be served based on expected power in this respective one of the directions in relation to expected power in the directions in which

interference should be limited.

20. A radio transceiver device (200a, 200b) for determining precoding weight vectors in a codebook for user-specific precoding, the radio

transceiver device (200a, 200b) comprising:

processing circuitry (210); and

a storage medium (230) storing instructions that, when executed by the processing circuitry (210), cause the radio transceiver device (200a, 200b) to:

determine one precoding weight vector for each respective one of the directions to be served based on expected power in this respective one of the directions in relation to expected power in the directions in which interference should be limited. 21. A radio transceiver device (200a, 200b) for determining precoding weight vectors in a codebook for user-specific precoding, the radio

transceiver device (200a, 200b) comprising:

an obtain module (210a) configured to obtain information about which directions n = 1,2, ... , N are intended to be served by the radio transceiver device (200a, 200b) and directions m = 1,2, ... , M in which interference from transmission by the radio transceiver device (200a, 200b) should be limited; and

a determine module (210b) configured to determine one precoding weight vector for each respective one of the directions to be served based on expected power in this respective one of the directions in relation to expected power in the directions in which interference should be limited.

22. A computer program (1220) for determining precoding weight vectors in a codebook for user-specific precoding, the computer program comprising computer code which, when run on processing circuitry (210) of a radio transceiver device (200a, 200b), causes the radio transceiver device (200a, 200b) to:

obtain (S102) information about which directions n = 1,2, ... , N are intended to be served by the radio transceiver device (200a, 200b) and directions m = 1,2, ... , M in which interference from transmission by the radio transceiver device (200a, 200b) should be limited; and

determine (S104) one precoding weight vector for each respective one of the directions to be served based on expected power in this respective one of the directions in relation to expected power in the directions in which interference should be limited.

23. A computer program product (1210) comprising a computer program (1220) according to claim 22, and a computer readable storage medium

(1230) on which the computer program is stored.