WO2014177211A1

WO2014177211A1 - User selection in communications

Info

Publication number: WO2014177211A1
Application number: PCT/EP2013/059126
Authority: WO
Inventors: Ganesh Venkatraman; Tuomo HÄNNINEN; Antti Tölli; Markku Juntti
Original assignee: Nokia Solutions And Networks Oy
Priority date: 2013-05-02
Filing date: 2013-05-02
Publication date: 2014-11-06

Abstract

A method for scheduling user terminals in a communications system comprises approximating, in a network apparatus, a null space projection by using a series of independent projections over terminals in a transmission set; obtaining, by using the approximated null space projection, a vertical displacement of a channel vector from a subspace formed by the terminals in the transmission set. A channel vector of a terminal is used to obtain a unit vector in the same direction. The unit vector is stacked to form a matrix U. A projection gain is obtained, and the obtained projection gain is subtracted from a channel norm to obtain a vertical displacement of the unit vector. The obtained vertical displacement is multiplied to achieve an equivalent null space projection metric. The achieved equivalent null space projection metric is used to select the terminals for the transmission set.

Description

USER SELECTION IN COMMUNICATIONS

FIELD OF THE INVENTION

The exemplary and non-limiting embodiments of this invention relate generally to wireless communications networks, and more particularly to scheduling of user terminals.

BACKGROUND ART

The following description of background art may include insights, discoveries, understandings or disclosures, or associations together with disclosures not known to the relevant art prior to the present invention but provided by the invention. Some such contributions of the invention may be specifically pointed out below, whereas other such contributions of the invention will be apparent from their context.

Multi-user MIMO (MU-MIMO) is an enhanced form of MIMO technology. MU-MIMO enables multiple independent user terminals to access a system enhancing the communication capabilities of individual terminals. MU-MIMO exploits a maximum system capacity by scheduling multiple users to be able to simultaneously access a same channel using spatial degrees of freedom offered by MIMO. MU-MIMO enables spatial sharing of channels. In MU-MIMO, the interference between different users on the same channel may be accommodated by using additional antennas and additional processing which enables the spatial separation of the different users. MU-MIMO systems enable a level of direct gain to be obtained in a multiple access capacity arising from the multi-user multiplexing schemes. This is proportional to the number of base station antennas employed. MU-MIMO allows spatial multiplexing gain to be achieved at the base station without the need for multiple antennas at the user terminal.

SUMMARY

The following presents a simplified summary of the invention in order to provide a basic understanding of some aspects of the invention. This summary is not an extensive overview of the invention. It is not intended to identify key/critical elements of the invention or to delineate the scope of the invention. Its sole purpose is to present some concepts of the invention in a simplified form as a prelude to the more detailed description that is presented later. Various aspects of the invention comprise a method, apparatus, computer program product, and a computer-readable storage medium as defined in the independent claims. Further embodiments of the invention are disclosed in the dependent claims.

An aspect of the invention relates to a method for scheduling user terminals in a communications system, the method comprising approximating, in a network apparatus, a null space projection by using a series of independent projections over user terminals in a transmission set cS^"; obtaining, by using the approximated null space projection, a vertical displacement of a channel vector from a subspace formed by the user terminals in the set S, wherein a channel vector ft, of a user terminal i is used to obtain a unit vector Mj in a same direction as ι·^Γ/ II ft,- II; stacking the unit vector it, to form a matrix U such that:

where g_l represents a projection of the channel vector h_L onto the matrix U; obtaining a projection gain g_jl where j represents the unit vector and I represents the user terminal; subtracting the obtained projection gain g_jt from a channel norm j j ι_£ | j to obtain a vertical displacement of the unit vector j; multiplying the obtained vertical displacement of the unit vector to achieve an equivalent null space projection metric rn_l such that: m_l = n¾II - \_gjl \) , V l E X\S (3); using the achieved equivalent null space projection metric m_l to select the user terminals for the transmission set S from user terminals in a set X; updating the transmission set S with user terminals having the largest m_l among the user terminals in the set X.

A further aspect of the invention relates to an apparatus comprising at least one processor, and at least one memory including a computer program code, wherein the at least one memory and the computer program code are configured to, with the at least one processor, cause the apparatus to perform any of the method steps. A stil! further aspect of the invention re!ates to a computer program product comprising program code means configured to perform any of the method steps when the program is run on a computer.

A still further aspect of the invention relates to a computer-readable storage medium comprising program code means configured to perform any of the method steps when executed on a computer.

Although the various aspects, embodiments and features of the invention are recited independently, it should be appreciated that all combinations of the various aspects, embodiments and features of the invention are possible and within the scope of the present invention as claimed.

BRIEF DESCRIPTION OF THE DRAWINGS

In the following the invention will be described in greater detail by means of exemplary embodiments with reference to the attached drawings, in which

Figure 1a shows a comparison of scheduling schemes;

Figure 1 b shows a comparison of a sum rate achieved by varying users in the system;

Figure 2 shows a comparison of the complexity involved in the calculation of user metrics;

Figure 3 shows a simplified block diagram illustrating exemplary system architecture;

Figure 4 shows a simplified block diagram illustrating exemplary apparatuses;

Figure 5 shows a messaging diagram illustrating an exemplary messaging event according to an embodiment of the invention;

Figure 6 shows a schematic diagram of a flow chart according to an exemplary embodiment of the invention;

Figure 7 shows a schematic diagram of a flow chart according to another exemplary embodiment of the invention.

DETAILED DESCRIPTION OF SOME EMBODIMENTS

An exemplary embodiment relates to user selection for MU-MIMO based on reduced null space projections. An exemplary embodiment involves selecting the users for MU- MIMO transmission using the low complex null space equivalent metric without compromising much on the performance.

In an exemplary embodiment, an algorithm tries to find out a set of users whose channel vectors are uncorrelated in order to multiplex the user's data on the same tone. The complexity involved in the selection process is minimal in comparison with existing solutions. An exemplary embodiment may be implemented easily in dedicated HW platforms for scheduling, such as FPGA's.

Efficient utilization of the scarce wireless spectrum is an important concern in current wireless standards such as LTE and LTE-A. The spectral utilization is increased by multiplexing more users on the same tone with the use of precoding. The precoders may be ZF (zero-forcing) precoders designed based on the user's channel whose data are to be multiplexed on the same tone or by using any other precoding scheme which maximizes the received SNR.

In order to multiplex the data using precoders, the users are selected in a way to find a set of users whose channels are linearly independent in a spatial dimension. This requires intense search algorithms to find the set of users meant to be multiplexed.

Since the user selection requires an exhaustive search to find a best set of users, an exemplary embodiment proposes algorithms to carry out the search in an efficient way with reduced complexity. An exemplary algorithm provides one such way of selection with reduced complexity without sacrificing much on the achievable performance. This algorithm provides the complexity which increases linearly over the number of users in the system.

Multi-user multiple-input multiple-output (MU-MIMO) based transmission gained popularity with development of multiple antenna transmission which enables providing a huge throughput enhancement utilizing the spatial dimension. The user selection for MU-MIMO requires channel orthogonality so as to decouple transmitted data for each user utilizing the spatial dimension. An existing search algorithm is based on semi-orthogonal user selection (SUS) using Gram Schmidt QR. Existing algorithms based on block diagonalization (BD) of stacked channel vectors project successively on to the null space. An existing precoder design was proposed in an iterative manner for BD based user search, also addressing complexity issues involved in diagonalizing a large concatenated matrix. Existing user selection schemes for MU-MIMO based transmission scheme may aim at minimizing the average beamforming power. Scheduling schemes may assume an infinite buffer model which in reality is not usually true. The existing algorithms may involve queuing model assumptions and buffer limitations. Capacity achieving scheduling based on information theory may provide fairness among users thereby providing a stable queue length. The selection may be based on the volume of the enclosed vectors.

The existing user selection algorithms are thus based on projecting the user channel over the null space of the existing users channel vectors. The projection is formulated as a multiplication of a square matrix of the number of transmission antennas. This projection is used to calculate the metric based on which the users are selected for a MU-MIMO transmission set.

An algorithm according to the exemplary embodiment performs user selection for MU-MIMO by approximating the null space metric involved in the metric calculation. The null space of a vector is characterized by a perpendicular space to the given vector thereby providing a zero projection length for the given vector. The null space of the given vector h is given by

where h^" is the conjugate of h without a transpose. The matrix P provides the null space of the channel vector h_t which corresponds to the user i E S where s correspond to the transmission user set. In general, matrix P represents the null space of each user channel vector in the transmission set s which is formed with the matrix U which stacks each vector as:

p = I u(u^Hu) HJ"

The approximation of the projection metric of user VI e 3C is given by ||P^Hh || with P being the null space matrix for users in s.

The null space metric is approximated by the product of perpendicular distance from each channel vector of users in the transmission set s. The U matrix holds the normalized unit vector in the direction of the respective channel vector of users in S as given by u = = [«i u \s\\

'!.·>'! ί (1 )

The projection on U by the user 1 ε X channel vector h_t is denoted by g] where each element in g| corresponds to the projection of h on ii.

(2)

The vertical distance is given by the difference between the channel vector norm ||h]|| and the corresponding projection gain | _j| | . The difference is then multiplied to obtain the null space projection equivalent of the metric ||ρ^Ηη?Ί| as given by the following: m_l = nfJiOl -

(3)

The metric m, is used to find the next user for the set S by selecting the user with a maximum null space projection gain n | . The performance of this reduced null space projection is illustrated in Figure 1 a and 1 b which compare an exemplary embodiment with existing algorithms.

Figure 1 a considers 50 users system with path loss varying uniformly over [-30, 0]. The transmit precoder is based on zero forcing and water filling power allocation to maximize the achievable capacity. Figure 1 a compares the performance of basic scheduling schemes such as round robin, proportional fair and norm based scheduling. Apart from those, Figure 1 a also illustrates the performance of QR based greedy user selection and the proposed reduced null space user selection. Figure 1 b compares the performance of the proposed scheme with the existing algorithms by varying the number of users in the system for the selection procedure.

An exemplary embodiment enables a way to find the transmission set using the metric which provides a measure of linearly independent user channel vectors. One such measure is obtained by finding the null space of the vectors in which holds the set of users already selected. The calculation of null space requires the inversion of the Gramm matrix which requires complex calculations which are implemented either by using Householder transformation or Givens rotation. Since scheduling is performed over each scheduling block in the TTI, the overall capacity will be limited if the complexity per scheduling block is increased in the MU-MIMO user selection.

An exemplary embodiment provides an alternative for estimating the null space by the product of the vertical distance between the unit vectors of the user channel which are already chosen. This only requires vector multiplication which is of complexity instead of complexity. An exemplary embodiment provides the same performance of QR based selection when the number of antennas is 2 which are quite common in current standards. The use of memory in storing the already performed calculations may reduce the complexity even further as shown by a complexity plot given in Figure 2.

Figure 2 illustrates a complexity analysis according to an exemplary embodiment. In an exemplary embodiment, the complexity involved in the calculation of user metrics are illustrated in Figure 2 for an exemplary algorithm and the QR based scheduling. Figure 2 shows the complexity of an exhaustive search as well which increases exponentially with the users. The existing QR based scheduling and the proposed one is of linear complexity with the user count. Figure 2 compares the number of complex additions and multiplications involved in the calculation of user metric which is used for the selection. The reduced null space selection may be improved further by storing the earlier calculations for each user in the memory.

In an exemplary embodiment, since each user metric may be evaluated separately once the U matrix has been evaluated, the metric calculations may be parallelized greatly with available processing power.

An exemplary algorithm aims at selecting the users for MU-MIMO transmission by addressing the complexity. Since the metric used to select the users may be based on the projection of the channel vector onto the set of unit vectors, the complexity involved in the metric calculation is greatly reduced as may be seen from Figure 2. An exemplary algorithm may be implemented on FPGA which has a dedicated circuitry to carry out the metric calculation. This reduces the complexity involved in the metric calculation and the previous calculations for the same users may be saved in the memory in order to reduce the complexity further.

The number of complex FLOPS (multiplication) required in performing user selection with N_T transmit antennas is given below for each scheduling scheme.

QR based approach:

JV_T - 1

\X \ (/V| (/V_T - 1) + /V ) + ^ 2 i N¾ + i² (i + JV_T)

1

Reduced null space approach:

Exemplary embodiments will now be described more fully herein after 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; rather, these embodiments are provided so thai this disclosure will satisfy applicable legal requirements. Although the specification may refer to "an", "one", or "some" embodiment(s) in several locations, this does not necessarily mean that each such reference is to the same embodiment(s), or that the feature only applies to a single embodiment. Single features of different embodiments may also be combined to provide other embodiments. Like reference numerals refer to like elements throughout.

The present invention is applicable to any user terminal, network node, server, corresponding component, and/or to any communication system or any combination of different communication systems that support a MU- MIMO transmission scheme. The communication system may be a fixed communication system or a wireless communication system or a communication system utilizing both fixed networks and wireless networks. The protocols used the specifications of communication systems, servers and user terminals, especially in wireless communication, develop rapidly. Such development may require extra changes to an embodiment. Therefore, all words and expressions should be interpreted broadly and they are intended to illustrate, not to restrict, the embodiment.

In the following, different embodiments will be described using, as an example of a system architecture whereto the embodiments may be applied, an architecture based on LTE (long term evolution) or LTE-A (long term evolution advanced) network elements, without restricting the embodiment to such an architecture, however. The embodiments described in these examples are not limited to the LTE/LTA-A radio systems but can also be implemented in other radio systems, such as UMTS (universal mobile telecommunications system), GSM, EDGE, WCDMA, Bluetooth network, WLAN or other fixed, mobile or wireless network. In an embodiment, the presented solution may be applied between elements belonging to different but compatible systems such as LTE, LTE-A and UMTS.

A general architecture of a communication system is illustrated in Figure 3. Figure 3 is a simplified system architecture only showing some elements and functional entities, all being logical units whose implementation may differ from what is shown. The connections shown in Figure 3 are logical connections; the actual physical connections may be different. It is apparent to a person skilled in the art that the systems also comprise other functions and structures. It should be appreciated that the functions, structures, elements and the protocols used in or for user terminal scheduling, are irrelevant to the actual invention. Therefore, they need not to be discussed in more detail here.

The exemplary radio system of Figure 3 comprises a network node

301 of a network operator. The network node 301 may include e.g. an LTE/LTE-A base station (eNB), radio network controller (RNC), or any other network element, and/or a combination of network elements. The network node 301 may be connected to one or more core network (CN) elements (not shown in Figure 3) such as a mobile switching centre (MSC), MSG server (MSS), mobility management entity (MME), gateway GPRS support node (GGSN), serving GPRS support node (SGSN), home location register (HLR), home subscriber server (HSS), visitor location register (VLR). In Figure 3, the radio network node 301 that may also be called eNB (enhanced node-B, evolved node-B) or network apparatus of the radio system, hosts the functions for radio resource management in a public land mobile network. Figure 3 shows one or more user equipment 302 located in the service area of the radio network node 301 . The user equipment refers to a portable computing device, and it may also be referred to as a user terminal. Such computing devices include wireless mobile communication devices operating with or without a subscriber identification module (SIM) in hardware or in software, including, but not limited to, the following types of devices: mobile phone, smart-phone, personal digital assistant (PDA), handset, laptop computer. In the example situation of Figure 3, the user equipment 302 is capable of connecting to the radio network node 301 via a connection 303.

Figure 4 is a block diagram of an apparatus according to an embodiment of the invention. Figure 4 shows user equipment 302 located in the area of a radio network node 301 . The user equipment 302 is configured to be in connection with the radio network node 301 . The user equipment or UE

302 comprises a controller 401 operationally connected to a memory 402 and a transceiver 403. The controller 401 controls the operation of the user equipment 302. The memory 402 is configured to store software and data. The transceiver 403 is configured to set up and maintain a wireless connection 303 to the radio network node 301 . The transceiver is operationally connected to a set of antenna ports 404 connected to an antenna arrangement 405. The antenna arrangement 405 may comprise a set of antennas. The number of antennas may be one to four, for example. The number of antennas is not limited to any particular number. The user equipment 302 may also comprise various other components, such as a user interface, camera, and media player. They are not displayed in the figure due to simplicity. The radio network node 301 , such as an LTE-A base station (eNode-B, eNB) or LTE-LAN access point (AP), comprises a controller 406 operationally connected to a memory 407, and a transceiver 408. The controller 406 controls the operation of the radio network node 301 . The memory 407 is configured to store software and data. The transceiver 408 is configured to set up and maintain a wireless connection to the user equipment 302 within the service area of the radio network node 301 . The transceiver 408 is operationally connected to an antenna arrangement 409. The antenna arrangement 409 may comprise a set of antennas. The number of antennas may be two to four, for example. The number of antennas is not limited to any particular number. The radio network node 301 may be operationally connected (directly or indirectly) to another network element (not shown in Figure 4) of the communication system, such as a radio network controller (RNC), a mobility management entity (MME), an MSG server (MSS), a mobile switching centre (MSG), a radio resource management (RRM) node, a gateway GPRS support node, an operations, administrations and maintenance (OAM) node, a home location register (HLR), a visitor location register (VLR), a serving GPRS support node, a gateway, and/or a server, via an interface. The embodiments are not, however, restricted to the network given above as an example, but a person skilled in the art may apply the solution to other communication networks provided with the necessary properties. For example, the connections between different network elements may be realized with internet protocol (IP) connections.

Although the apparatus 301 , 302 has been depicted as one entity, different modules and memory may be implemented in one or more physical or logical entities. The apparatus may also be a user terminal which is a piece of equipment or a device that associates, or is arranged to associate, the user terminal and its user with a subscription and allows a user to interact with a communications system. The user terminal presents information to the user and allows the user to input information. In other words, the user terminal may be any terminal capable of receiving information from and/or transmitting information to the network, connectable to the network wirelessly or via a fixed connection. Examples of the user terminals include a personal computer, a game console, a laptop (a notebook), a personal digital assistant, a mobile station (mobile phone), a smart phone, and a line telephone. The apparatus 301 , 302 may generally include a processor, controller, control unit or the like connected to a memory and to various interfaces of the apparatus. Generally the processor is a central processing unit, but the processor may be an additional operation processor. The processor may comprise a computer processor, application-specific integrated circuit (ASIC), field-programmable gate array (FPGA), and/or other hardware components that have been programmed in such a way to carry out one or more functions of an embodiment.

The memory 402, 407 may include volatile and/or non-volatile memory and typically stores content, data, or the like. For example, the memory 402, 407 may store computer program code such as software applications or operating systems, information, data, content, or the like for a processor to perform steps associated with operation of the apparatus in accordance with embodiments. The memory may be, for example, random access memory (RAM), a hard drive, or other fixed data memory or storage device. Further, the memory, or part of it, may be removable memory detachably connected to the apparatus.

The techniques described herein may be implemented by various means so that an apparatus implementing one or more functions of a corresponding mobile entity described with an embodiment comprises not only prior art means, but also means for implementing the one or more functions of a corresponding apparatus described with an embodiment and it may comprise separate means for each separate function, or means may be configured to perform two or more functions. For example, these techniques may be implemented in hardware (one or more apparatuses), firmware (one or more apparatuses), software (one or more modules), or combinations thereof. For a firmware or software, implementation can be through modules (e.g. procedures, functions, and so on) that perform the functions described herein. The software codes may be stored in any suitable, processor/computer- readable data storage medium(s) or memory unit(s) or article(s) of manufacture and executed by one or more processors/computers. The data storage medium or the memory unit may be implemented within the processor/computer or external to the processor/computer, in which case it can be communicatively coupled to the processor/computer via various means as is known in the art. The signalling chart of Figure 5 illustrates the required signalling according to an exemplary embodiment. In the example of Figure 5, an apparatus, such as a base station 301 , may calculate, in item 501 , a transmission set, i.e. the base station 301 may select 501 a set of user terminals for MU-MIMO transmission. This means that the base station approximates the null space projection by using a series of independent projections over normalized channel vectors of users in the transmission set s. The projection over the unit vectors is given by the vector g_y with 1 being the user from the set JC. The normalized vectors in the direction of the channel vectors of users in the set s is given by the U. The unit vector in the direction of the channel vector hi of a user terminal i is given by h-VUhJI. The matrix U and the projection of the channel of user 1 on to the directional matrix 11 represented as g_j are given by: u =

JIM ^"' iiVii = [«1 u \s\ (1 )

The elements of the vector gj represented by g_j, holds the projection of the channel vector of user 1 denoted by h_j on to unit vector

U with j E S. The projection gain of the channel h, is subtracted from ||h_j || to provide the estimate of the vertical displacement from the unit vector u The vertical displacements of the channel vector hj over all j ε S are multiplied to obtain the approximation of the null space of channel vectors formed by the users in the set c . The null space equivalent metric m, for the user 1 is given by

The metric ni| is evaluated for each user in the set K\S to find the next user for the set cS. Once the metric for each user are obtained, the user with maximum null space projection metric ni| is selected as the next user for the set S. The base station 301 may then transmit, in item 502, a signal to the user terminals 302 in the calculated transmission set s, based on the calculation 501 . In item 503, the user terminals 302 in the calculated transmission set s may receive the signal 502 from the base station 301 . In item 504, the user terminal 302 may respond by transmitting a signal 504 to the base station 30 . In item 505, the base station may receive the signal 504 from the user terminal 302.

Figure 6 is a flow chart illustrating a non-limiting embodiment of the invention. In item 601 , the apparatus 301 , such as base station capable of MU- MIMO transmission, may calculate a transmission set, i.e. the base station 301 may select 601 a set of user terminals for MU-MIMO transmission. This means that the apparatus 301 approximates a null space projection by using a series of independent projections over the channel vectors of user terminals in a transmission set s. The approximation for the null space is obtained by multiplying the vertical displacement of a channel vector of user 1 from the unit norm vectors of the user channel hj Vi e c . The displacement metric is obtained by forming the directional matrix U given by: u = = i" i - ^u\s\] (1 )

!! i!i

where g| represents a projection of the channel vector h, onto the matrix U; obtains a projection gain g,, where j represents the unit vector and 1 represents the user terminal; subtracts the obtained projection gain from a channel norm HhJI to obtain the vertical displacement of the unit vector j; multiplies the vertical displacement of the vectors to achieve an equivalent null space projection metric ni| such that:

uses the achieved equivalent null space projection metric ni| to select the user terminals for the transmission set s from user terminals in the set X; and updates the transmission set s with user terminals with the largest ni] among the user terminals in the set X. The apparatus 301 may then transmit, in item 602, a signal 502 to the user terminals 302 in the calculated transmission set s, based on the calculation 601. In item 603, the apparatus 301 may receive a signal 504 from the user terminal 302.

The signalling chart of Figure 7 illustrates the required signalling according to an exemplary embodiment. In the example of Figure 7, an apparatus 302, such as a user terminal capable of receiving MU-MIMO transmission, may receive, in item 701 , the signal 502 from the base station 30 . In item 702, the user terminal may respond by transmitting the signal 504 to the base station 301 .

Thus, a method for scheduling user terminals in a communications system comprises approximating, in a network apparatus, a null space projection by using a series of independent projections over terminals in a transmission set; obtaining, by using the approximated null space projection, a vertical displacement of a channel vector from a subspace formed by the terminals in the transmission set. A channel vector of a terminal is used to obtain a unit vector in the same direction. The unit vector is stacked to form a matrix U. A projection gain is obtained, and the obtained projection gain is subtracted from a channel norm to obtain a vertical displacement of the unit vector. The obtained vertical displacement is multiplied to achieve an equivalent null space projection metric. The achieved equivalent null space projection metric is used to select the terminals for the transmission set.

The steps/points, signalling messages and related functions described above in Figures 1 to 7 are in no absolute chronological order, and some of the steps/points may be performed simultaneously or in an order differing from the given one. Other functions can also be executed between the steps/points or within the steps/points and other signalling messages sent between the illustrated messages. Some of the steps/points or part of the steps/points can also be left out or replaced by a corresponding step/point or part of the step/point. The apparatus operations illustrate a procedure that may be implemented in one or more physical or logical entities. The signalling messages are only exemplary and may even comprise several separate messages for transmitting the same information. In addition, the messages may also contain other information.

Thus in an exemplary embodiment, there is provided a method for scheduling user terminals in a communications system, the method comprising approximating, in a network apparatus, a null space projection by using a series of independent projections over user terminals in a transmission set c ; obtaining, by using the approximated null space projection, a vertical displacement of a channel vector from a subspace formed by the user terminals in the set S, wherein a channel vector h_t of a user terminal i is used to obtain a unit vector it, in a same direction as Λι·^Γ/Ι|Λ,ΊΙ ; stacking the unit vector ii_j to form a matrix U such that: u = τ£τ ... = ^" "ml (1 ),

l! ^ft!S! l

where ^ represents a projection of the channel vector ft, onto the matrix U; obtaining a projection gain g_jt where j represents the unit vector and I represents the user terminal; subtracting the obtained projection gain g_jl from a channel norm to obtain a vertical displacement of the unit vector j; multiplying the obtained vertical displacement of the unit vector to achieve an equivalent null space projection metric m_l such that: m_t = n iiOlM - \_gjl \) , V l E X\S (3); using the achieved equivalent null space projection metric m_l to select the user terminals for the transmission set S from user terminals in a set X; updating the transmission set S with user terminals having the largest m_l among the user terminals in the set X.

In another exemplary embodiment, there is provided a method comprising performing metric calculations for user terminals in order to select user terminals for MU-MIMO transmission.

In yet another exemplary embodiment, there is provided a method comprising storing information on the metric calculations performed for the user terminals.

In yet another exemplary embodiment, there is provided a method comprising reducing complexity involved in the metric calculations to be performed for user terminals.

In yet another exemplary embodiment, there is provided a method comprising evaluating each user terminal separately based on the matrix U to obtain an updated transmission set.

In yet another exemplary embodiment, there is provided a method comprising searching for the user terminals the data transmission channels of which are spatially independent, by using a search algorithm; decoupling user data by using ZF precoders in order to design a precoder for multiplexed transmission. In yet another exemplary embodiment, there is provided a method comprising an apparatus comprising at least one processor, and at least one memory including a computer program code, wherein the at least one memory and the computer program code are configured to, with the at least one processor, cause the apparatus to perform any of the method steps.

In yet another exemplary embodiment, there is provided a computer program product comprising program code means configured to perform any of the method steps when the program is run on a computer.

In yet another exemplary embodiment, there is provided a computer-readable storage medium comprising program code means configured to perform any of the method steps when executed on a computer.

It will be obvious to a person skilled in the art that, as the technology advances, the inventive concept can be implemented in various ways. The invention and its embodiments are not limited to the examples described above but may vary within the scope of the claims.

Claims

1 . A method for scheduling user terminals in a communications system, the method comprising

approximating, in a network apparatus, a null space projection by using a series of independent projections over user terminals in a transmission set S;

obtaining, by using the approximated null space projection, a vertical displacement of a channel vector from a subspace formed by the user terminals in the set S, wherein a channel vector h_t of a user terminal i is used to obtain a unit vector u_t in a same direction as

\\ ;

stacking the unit vector rt, to form a matrix U such that:

where g_L represents a projection of the channel vector h_l onto the matrix U;

obtaining a projection gain g_jl where j represents the unit vector and I represents the user terminal;

subtracting the obtained projection gain g_jt from a channel norm ll/i_j ll to obtain a vertical displacement of the unit vector /;

multiplying the obtained vertical displacement of the unit vector to achieve an equivalent null space projection metric m_l such that: m_L = U Ull W - \_gjl \) , V l e X\S (3); using the achieved equivalent null space projection metric m_l to select the user terminals for the transmission set S from user terminals in a set X;

updating the transmission set S with user terminals having the largest m_l among the user terminals in the set X.

2. A method according to claim 1 , c h a r a c t e r i z e d by performing metric calculations for user terminals in order to select user terminals for MU-MIMO transmission.

3. A method according to claim 2, characterized by storing information on the metric calculations performed for the user terminals.

4. A method according to claim 2 or 3, characterized by reducing complexity involved in the metric calculations to be performed for user terminals.

5. A method according to any of claims 1 -4, characterized by evaluating each user terminal separately based on the matrix U to obtain an updated transmission set.

6. A method according to any of claims 1 -5, characterized by

searching for the user terminals the data transmission channels of which are spatially independent, by using a search algorithm;

decoupling user data by using ZF precoders in order to design a precoder for multiplexed transmission.

7. An apparatus comprising

at least one processor; and

at least one memory including a computer program code, characterized in that the at least one memory and the computer program code are configured to, with the at least one processor, cause the apparatus (30 ) to perform any of the method steps of claims 1 to 6.

8. A computer program product, characterized by comprising program code means configured to perform any of method steps of claims 1 to 6 when the program is run on a computer.

9. A computer-readable storage medium, characterized by comprising program code means configured to perform any of method steps of claims 1 to 6 when executed on a computer.