WO2014012206A1

WO2014012206A1 - Method and apparatus for transmission mode switching in a wireless communication system

Info

Publication number: WO2014012206A1
Application number: PCT/CN2012/078701
Authority: WO
Inventors: Lei Jiang; Ming Lei; Shi Jin; Yongyu Dai; Yongming Huang
Original assignee: Nec (China) Co., Ltd.
Priority date: 2012-07-16
Filing date: 2012-07-16
Publication date: 2014-01-23
Also published as: CN104272798A; CN104272798B

Abstract

The present invention relates to method and apparatus for transmission mode switching in a wireless communication system. According to an embodiment of the present invention, a method for transmission mode switching in a wireless communication system is provided. The method comprises: receiving information indicative of statistical channel state information of a user equipment with respect to a set of m base stations including one reference base station and m-1 candidate cooperative base stations; calculating first sum capacity C₁ for a cooperation transmission mode with respect to said set of m base stations and second sum capacity C_2,k for a non-cooperation transmission mode with respect to each base station of said set of m base stations, where k=1,... m; determining downlink transmission mode based on said first sum capacity C₁ and said second sum capacity C_2,k.The embodiments of the present invention further relates to a corresponding base station device and computer program product used to performing corresponding functionalities.

Description

METHOD AND APPARATUS FOR TRANSMISSION MODE SWITCHING IN A WIRELESS COMMUNICATION SYSTEM

TECHNICAL FIELD

The present invention generally relates to a communication system, particularly to a method and apparatus for transmission mode switching in a wireless communication system.

DESCRIPTION OF THE RELATED ART

In wireless communications, multiuser multiple-input multiple-output (MIMO) transmission technologies have received considerable attention in recent years, due to their ability for providing significantly enhanced spectral efficiency and link reliability compared with conventional single antenna systems . It becomes a key technology used in the Third Generation Partnership Project (3GPP) Standard for Long Term Evolution (LTE) /LTE-Advanced (LTE-A) Long Term Evolutio .

To increase capacity of the network and reducing sever inter-cell interference between macro base stations in higher node deployment density, a network that consists of a mix of macro-cells and low-power and low-cost nodes or wireless assess points has emerged, which is called heterogeneous network. There are many deployment scenarios for a HetNet . For example, for a indoor scenario, femtocells can be used with transmission power lower than lOOmW and connected with macro cells via digital subscriber lines (DSLs) ; indoor relays can be used with transmission lower than lOOmW; indoor remote radio head (RRH) /Hotzone such as indoor picos are used with transmission power lower than lOOmW and connected with macro cells via high speed backhaul such as optical fiber. In an outdoor scenario, outdoor relays can be used with 250mW-2W transmission power; outdoor RRH/Hotzones such as outdoor picos can be used with 250mW-2W transmission power and connected with macro cells via high-speed backhaul such as optical fiber. In general, such small, low-power base stations can be called low-power base stations, which are in contrast to macro base stations in a HetNet.

In a HetNet, a macro base station and low-power base station (s) deployed in the edge of the macro base station's coverage can operate coordinately to act as an equivalent virtual MIMO system. Cooperation has been recognized as a promising technique for mitigating the problem of interference between macro base stations and low-power base stations. However, for a user equipment that is close to the macro base station or one of low-power base stations, such as a RRH node, non-cooperation transmission mode usually exhibits a higher capacity than cooperation transmission mode .

It is desired to provide a switching mechanism for alternating transmission mode between cooperation and non-cooperation to enhance the system capacity.

SUMMARY OF THE INVENTION

To solve the problems in the prior art, one or more method and apparatus embodiments according to the present invention aim to provide an improved solution for transmission mode switching in a wireless communication system.

According to an aspect of the present invention, an embodiment of the present invention provides a method for transmission mode switching in a wireless communication system. The method comprises: receiving information indicative of statistical channel state information with respect to a set of m base stations including one serving base station and m-1 candidate cooperative base stations; calculating first sum capacity C_x for a cooperation transmission mode with respect to the set of m base stations and second sum capacity C₂,_k for a non- cooperation transmission mode with respect to each base station of the set of m base stations, where k=l,... m; determining downlink transmission mode based on the first sum capacity Ci and the second sum capacity C₂,_k-

According to another aspect of the present invention, an embodiment of the present invention provides an apparatus for transmission mode switching in a wireless communication system. The apparatus comprises: receiving unit configured to receive information indicative of statistical channel state information with respect to a set of m base stations including one serving base station and m-1 candidate cooperative base stations; calculating unit configured to calculate first sum capacity Ci for a cooperation transmission mode with respect to the set of m base stations and second sum capacity C₂,_k for a non- cooperation transmission mode with respect to each base station of the set of m base stations, where k=l,... m,- exterminating unit configured to determine downlink transmission mode based on the first sum capacity x and the second sum capacity C₂,k-

According to further aspect of the present invention, an embodiment of the present invention provides a base station in a wireless communication system. The base station comprises an apparatus for transmission mode switching in a wireless communication system according to any of various embodiments of the present invention. According to further aspect of the present invention, an embodiment of the present invention provides an apparatus for transmission mode switching in a wireless communication system. The apparatus comprises: means for controlling to receive information indicative of statistical channel state information with respect to a set of m base stations including one serving base station and m-1 candidate cooperative base stations; means for calculating first sum capacity d for a cooperation transmission mode with respect to said set of m base stations and second sum capacity C₂,_k for a non-cooperation transmission mode with respect to each base station of said set of m base stations, where k=l,„. m; means for determining downlink transmission mode based on said first sum capacity Ci and said second sum capacity C₂,k-

BRIEF DESCRIPTION OF THE DRAWINGS

Inventive features regarded as the characteristics of the present invention are set forth in the appended claims. However, the present invention, its implementation mode, other objectives, features and advantages will be better understood through reading the following detailed description on the exemplary embodiments with reference to the accompanying drawings, where in the drawings:

Fig. 1 schematically illustrates an example of a wireless communication system in which embodiments according to an embodiment of the present invention can be implemented; Fig.2 schematically illustrates a flowchart of operating procedure of a user equipment in a wireless communication system according to an embodiment of the present invention;

Fig.3 schematically shows a flow chart of a method for transmission mode switching in a wireless communication system according to an embodiment of the present invention;

Fig.4 schematically illustrates a flowchart of switching procedure of downlink transmission mode;

Fig.5 schematically shows a block diagram of a user equipment according to an embodiment of the present invention;

Fig.6 schematically shows a block diagram of a base station according to an embodiment of the present invention.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT

Hereinafter, embodiments of the present invention will be described with reference to the accompanying drawings. In the following description, many specific details are illustrated so as to understand the present invention more comprehensively. However, it is apparent to the skilled in the art that implementation of the present invention may not have these details . Additionally, it should be understood that the present invention is not limited to the particular embodiments as introduced here . On the contrary, any arbitrary combination of the following features and elements may be considered to implement and practice the present invention, regardless of whether they involve different embodiments. Thus, the following aspects, features, embodiments and advantages are only for illustrative purposes, and should not be understood as elements or limitations of the appended claims, unless otherwise explicitly specified in the claims.

Fig.1 schematically illustrates an example of a wireless communication system in which embodiments according to an embodiment of the present invention can be implemented. Referring to Fig.l, a HetNet 100 as shown consists of a macro base station 110 and at least one low-power base station, for example, RRH nodes 120-1, 120-2, 120-3, 120-4, 120-5. The macro base station 110 and the RRH nodes 120-1, 120-2, 120-3, 120-4, 120-5 can cooperate with one another and serve user equipments in the cooperation transmission mode . The RRH nodes 120-1, 120-2, 120-3, 120-4, 120-5 are connected with the macro base station 110 via backhaul, for example optical fiber. It should be noted that although taking the RRH node as an example of low-power base stations, the low-power base station according to one or more embodiments of the present invention is not limited to RRH nodes, but includes any suitable low-power base stations such as femto base stations, relay stations, pico base stations, etc.

A scenario is considered without loss any generality where a user equipment 130 is locally served by the RRH node 120-1. A set of candidate cooperative base stations can be determined by the user equipment 130, which includes the macro base station 110, the RRH nodes 120-2 and 120-3. Downlink transmission in the cooperation transmission mode is shown in Fig.1. With movement of the user equipment 130 in the HetNet 100, transmission mode can be switched adaptively to obtain a relative high capacity.

With reference to Figs. 2-6, various embodiments of the present invention will be described in details. Fig.2 schematically illustrates a flow chart of operating procedure of a user equipment in a wireless communication system according to an embodiment of the present invention.

In step S210, the received signal-to-noise ratios (SNRs) from all base stations are estimated by a user equipment and a reference base station is determined based on the estimated SNR.

According to an exemplary embodiment of the present invention, as shown in Fig.l, all the user equipments in the macro-cell can be served by the same macro base station 100. Generally, each user may be closer to a certain RRH node and relatively far away from the other. Therefore, it would be advantageous that the RRH nodes that are far away from the user equipment are switched off and a certain number of useful RRH nodes are selected to serve the user equipment. Let ©denotes the set of all base stations including the macro base station 110 and all the RRH nodes. The received SNRs, SNR Vi e Θ is estimated . The i^*th base station with the maximum received SNR is selected as the reference base station, wherein i* =argmaxSNR,. \ In step S220, candidate cooperative base stations are obtained based on the estimated SNRs.

According to an exemplary embodiment of the present invention, cooperative factors can be calculated to determine the candidate cooperative base stations. The cooperative factors can be defined as follows:

OF _{f =} ^__il vi e 0\ } 2) ' SNR,

Having pre-defined a threshold δ , the set of candidate cooperative base stations can be obtained by

In step S230, information of statistical channel state information is determined by the user equipment and then can be reported to the side of base stations, for example, via the determined reference base station.

Assuming that the long-term statistical channel state information is updated every N time slots, the transmit correlation matrix of the user equipment can be obtained by

where hfⁿ⁾ is the channel matrix in the nth time slot.

According to an exemplary embodiment of the present invention, the information indicative of statistical channel state information includes U,_f and Λ,. , where R . = ϋ,,Λ,,ϋ^ ,

U_(f is a unitary matrix of respective eigenvector and Λ,, is a diagonal matrix with eigenvalues as its diagonal elements a transmit correlation matrix R_fi..

The information indicative of statistical channel state information includes U, and A₍ . is then reported to the side of base stations via the reference base station for further handling.

Fig.3 schematically shows a flow chart of a method for transmission mode switching in a wireless communication system according to an embodiment of the present invention.

In step S310, information indicative of statistical channel state information of a user equipment with respect to a set of m base stations is received.

According to an embodiment of the present invention, the set of m base stations has been determined by the respective user equipment according to received signals from all the base stations and may include one reference base station, such as the i^*th base station, and m-1 candidate cooperative base stations, such as the obtained set of candidate cooperative base stations Φ . The user equipment calculates the information indicative of statistical channel state information with respect to the determined set of m base stations can be calculated and reported to the side of base station, for example, via its reference base station. According to an exemplary embodiment of the present invention, the information indicative of statistical channel state information includes U_r. and A_(j , where a transmit correlation matrix R_{f f} = U, ,.Λ, .V" , is a unitary matrix of respective eigenvector and A,_f is a diagonal matrix with eigenvalues as its diagonal elements.

In some implementations of the present invention, all the involved base stations are interconnected with each other via backhaul such as a fiber to exchange information. So it may assumed that all the calculations in the side of base stations described here are performed by a virtual central processing unit, in which at least part of computational capabilities of all the involved base stations maybe integrated. Hereafter, the term "central processing unit" is used to refer to the computational capabilities of the base station.

In step S320, first sum capacity Ci for a cooperation transmission mode with respect to the set of m base stations and second sum capacity C₂,_k for a non-cooperation transmission mode with respect to each base station of said set of m base stations, where k=l,... m, are calculated.

For the cooperation transmission mode, the set of m base stations works together and transit data symbols to the user equipment cooperatively. Generally, the user equipment can get perfect channel state information, while the base stations are only able to obtain the information indicative of statistical channel state information and the exchanged information among those base stations.

The ergodic sum capacity based on instantaneous channel state information in the downlink system can be expressed as:

where F_f denotes large scale fading, P_f denotes transmit power of the ith base station, h, eD ^lxM' denotes the small-scall MIMO channel fading vector from the ith base station to the user equipment, w₍. ed ^'^*1 denotes the selected beamforming precoding vector, E{«} denotes the expectation with respect to all random variables within the bracket, |«j denotes the magnitude of a scalar .

Following Jensen's inequality, the equation 5) can be expressed as

+∑r_i/iE{(_iw_i.)(h,.w_i)^/i} 6) m (=1

As the term h_i w_j. is a scalar, the equation 6) can be further expressed as

Under the condition that the base stations only know statistical channel state information, the optimal beamformer of the user equipment for the transmission is the principal eigenvector of R_ti , i.e., w,. =u_fl, i = \,2,---,m 8) where u_fj, denotes the eigenvector corresponding to the maximum eigenvalue of the transmit correlation matrix R₍. , which is the column of the unitary matrix U_(i corresponding to the maximum eigenvalue X_t. .

The ergodic sum capacity can be simplified by substituting the equation 8) into 7) , that is

As the columns of the unitary matrix U_(;i are mutually orthogonal, according to an embodiment of the present invention, the first sum capacity Ci is calculated by

where i e {l,2,...m}denotes the ith base station of said set of m base stations, Γ, denotes large scale fading, P_i denotes transmit power of the ith base station, X_{i; l} denotes a maximum eigenvalues of a transmit correlation matrix R_;(. , which can be obtained from the received information indicative of statistical channel state information.

Therefore, it can be seen that for the cooperation transmission mode, the ergodic sum capacity of the downlink system can be approximated utilizing only the information indicative of statistical channel state information.

For the non- cooperation transmission mode, the set of m base stations serve its own user equipments respectively, and interferences exit among the base stations. Generally, the user equipment can get perfect channel state information, while the base stations are only able to obtain the information indicative of statistical channel state information. When the user equipment is served by the kth base station, the ergodic sum capacity based on instantaneous channel state information in the downlink system can be expressed as

11)

where k = \,---,m , Γ_; denotes large scale fading, P_i denotes transmit power of the ith base station, h_f eO ^ly'N' denotes the small-scale MIMO channel fading vector from the ith base station to the user equipment, w₍. eD"' denotes the selected beamforming precoding vector, E{»} denotes the expectation with respect to all random variables within the bracket, |·| denotes the magnitude of a scalar.

X E(X

Following Jensen' s inequality and E{—}≥ ~^ ~ ' ^wh©^re X and Y are both real-valued continuous random variables, the equation 11) can be expressed approximately as

As the term h,.w₍. is a scalar, the equation 12) can further expressed as

The ergodic sum capacity can be simplified by substituting the equation 8) into 13) , that is

As the columns of the unitary matrix U,,. are mutually orthogonal, according to an embodiment of the present invention, the second sum capacity C₂,_k is calculated by

where i e {l,2,...m} denotes the ith base station of said set of m base stations, Γ, denotes large scale fading, P. denotes transmit power of the ith base station, λ_ι> denotes a maximum eigenvalues of a transmit correlation matrix Ι¾_ί , which can be obtained from the received information indicative of statistical channel state information.

Therefore, it can be seen that for the non-cooperation transmission mode, the ergodic sum capacity of the downlink system can be approximated utilizing only the information indicative of statistical channel state information.

In step S33 0 , downlink transmission mode of the user equipment is determined based on the first sum capacity Ci and the second sum capacity C_2>k. A detailed procedure of determining downlink transmission mode for the user equipment will set forth with reference to Fig. 4.

Fig .4 schematically illustrates a flow chart of switching procedure of downlink transmission mode.

In step S400, the switching procedure of downlink transmission mode starts responsive to obtain the first sum capacity C_x and the second sum capacity C₂,_k based on the statistical channel state information. In step S410, the first sum capacity C_x is compared with the maximum of the second sum capacity C_2(k.

In step S420, it is determined if the first sum capacity Ci is greater than the maximum of the second sum capacity C₂,_k.

If the result of the determination in step S420 is "yes", i.e., the first sum capacity Ci is greater than the maximum of the second sum capacity C₂,k, then the processing of switching procedure proceeds with steps S430-S433. Otherwise, the processing of switching procedure proceeds with steps S440-S443. In step S430, the central processing unit in the side of base stations determines to utilize cooperation transmission mode .

In step S431, it is determined if the current transmission mode is cooperation transmission mode. If the result of the determination in step S431 is "yes", then the current transmission mode is remained unchanged and the cooperative base stations and the user equipment continue performing the cooperative downlink transmission.

If the result of the determination in step S431 is "No", then in step S433, the transmission mode is switched to the cooperation transmission mode, in which the set of m base stations work together to transmit cooperatively downlink data symbols to the user equipment.

In step S 440, the central processing unit in the side of base stations determines to utilize non- cooperation transmission mode.

In step S441, it is determined if the current transmission mode is non-cooperation transmission mode.

If the result of the determination in step S441 is "yes", then the current transmission mode is remained unchanged and the processing of switching procedure goes to step S442.

If the result of the determination in step S441 is "No", then in step S443, the transmission mode is switched to the non-cooperation transmission mode and the processing of switching procedure goes to step S442.

In step S442, a k^*th proper serving base station is selected based on the second sum capacity C₂,_k-

According to an embodiment of the present invention, the k^*th serving base station can be selected from the set of m base stations by

&* = arg max C_2k 15) ke{l,—,m)

According to another embodiment of the present invention, the k^*th serving base station can be directly selected as the determined i^*th reference base station. In the non-cooperation transmission mode, the selected k^*th base station acts as the serving base station of the user equipment to perform downlink transmission.

According to the above one or more embodiments of the present invention, the transmission mode can be switched adaptively between two transmission modes i.e., the cooperation transmission mode and the non-cooperation transmission mode according to the results of a capacity analysis based on information indicative of statistical channel state information reported from respective user equipments . The adaptive switching scheme according to the various embodiments of the present invention may have a comparable performance of system capacity with the exact switching method based on instantaneous channel state information, but have relatively high availability and efficiency.

The processing according to one or more embodiments of the present invention has been depicted in detail with reference to Figs. 2-4. It should be noted that the above depiction is only exemplary, not intended for limiting the present invention. In other embodiments of the present invention, this method may have more, or less, or different steps, and numbering the steps is only for making the depiction more concise and much clearer, but not for stringently limiting the sequence between each steps while the sequence of steps may be different from the depiction. For example, in some embodiments, the above one or more optional steps may be omitted. Specific embodiment of each step may be different from the depiction. All these variations fall within the spirit and scope of the present invention.

Fig. 5 schematically shows a simplified block diagram of a user equipment according to an embodiment of the present invention.

In general, the various embodiments of the UE 500 can include, but are not limited to, cellular phones, PDAs having wireless communication capabilities, portable computers having wireless communication capabilities, image capture devices such as digital cameras having wireless communication capabilities, gaming devices having wireless communication capabilities, as well as portable -units or terminals that incorporate combinations of such functions. The UE 500 is adapted for communication with one or more base stations in the wireless communication system via its antenna array 550.

The UE 500 includes a data processor (DP) 510, a memory (MEM) 520 coupled to/embedded in the DP 510, and suitable RF transmitter TX/receiver RX module 540 coupling the antenna array 550 to the DP 510. The RF TX/RX module 540 is for bidirectional wireless communications with at least one base station. The MEM 520 stores a program (PROG) 530.

The PROG 530 is assumed to include program instructions that, when executed by the DP 510, enable the UE 500 to operate in accordance with the exemplary embodiments of this invention, as discussed herein with the operating procedure of a user equipment as shown in Fig.2.

The MEM 520 may be of any type suitable to the local technical environment and may be implemented using any suitable data storage technology, such as semiconductor based memory devices, magnetic memory devices and systems, optical memory devices and systems, fixed memory and removable memory, as non-limiting examples. While only one MEM is shown in the UE 500, there may be several physically distinct memory units in the UE 500.

The DP 510 performs any required calculation as described with reference to Fig.2. The DP 510 may be of any type suitable to the local technical environment, and may include one or more of general purpose computers, special purpose computers, microprocessors, DSPs and processors based on multi-core processor architecture, as non-limiting examples.

The user equipment 500 comprises a reporting unit (not shown in Fig. 5) . The report unit is configured to report to a base station which serves said user equipment information indicative of statistical channel information, wherein the feedback information. It can be appreciated that the functionalities of the reporting unit can be implemented by- one or more suitable modules of the user equipment 500 as described above. Fig.6 schematically shows a simplified block diagram of a base station according to an embodiment of the present invention.

The base station 600 is adapted for communication with user equipments in the wireless communication system. As discussed previously, the base station 600 can be a macro base station (e.g., macro eNB) or a low-power base station (e.g., RRH node, pico base station, relay station, femto base station, etc. )

The base station 600 includes a data processor (DP) 610, a memory (MEM) 620 coupled to/embedded in the DP 610, and suitable RF transmitter TX/receiver RX module 640 coupling antenna array 650 to the DP 610. The RF TX/RX module 640 is for bidirectional wireless communications with at least one UE. The MEM 620 stores a program (PROG) 630. The PROG 630 is assumed to include program instructions that, when executed by the DP 610, enable the base station 600 to operate in accordance with the exemplary embodiments of this invention, as discussed herein with the operating procedures as shown in Figs. 3 and 4. The MEM 620 may be of any type suitable to the local technical environment and may be implemented using any suitable data storage technology, such as semiconductor based memory devices, magnetic memory devices and systems, optical memory devices and systems , fixed memory and removable memory, as non-limiting examples. While only one MEM is shown in the base station 600, there may be several physically distinct memory units in the base station 600.

The DP 610 performs any required calculation as described with reference to Figs .3 and 4. The DP 610 may be of any type suitable to the local technical environment, and may include one or more of general purpose computers, special purpose computers, microprocessors, DSPs and processors based on multi-core processor architecture, as non- limiting examples.

The base station 600 comprises a receiving unit, a calculating unit and a determining unit (not shown in Fig.6) . The receiving unit is configured to receive, from either a user equipment or another base station in the wireless communication system, information indicative of statistical channel state information of a user equipment with respect to a set of m base stations including one reference base station and m-1 candidate cooperative base stations. The calculating unit is configured to calculate first sum capacity Ci for a cooperation transmission mode with respect to the set of m base stations and second sum capacity C₂,_k for a non-cooperation transmission mode with respect to each base station of the set of m base stations, where k=l,... m. The determining unit is configured to determine downlink transmission mode based on the first sum capacity Ci and the second sum capacity C₂,_k- t can be appreciated that the functionalities of the receiving unit, the calculating unit and the determining unit can be implemented by one or more suitable modules of the base station 600 as described above.

In general, the various exemplary embodiments may be implemented in hardware or special purpose circuits, software, logic or any combination thereof. For example, some aspects may be implemented in hardware, while other aspects may be implemented in firmware or software which may be executed by a controller, microprocessor or other computing device, although the invention is not limited thereto. While various aspects of the exemplary embodiments of this invention may be illustrated and described as block and signaling diagrams, it is well understood that these blocks, apparatus, systems, techniques or methods described herein may be implemented in, as non-limiting examples, hardware, software, firmware, special purpose circuits or logic, general purpose hardware or controller or other computing devices, or some combination thereof .

As such, it should be appreciated that at least some aspects of the exemplary embodiments of the inventions may be practiced in various components such as integrated circuit chips and modules. As well known in the art, the design of integrated circuits is by and large a highly automated process.

The present invention may also be embodied in the computer program product which comprises all features capable of implementing the method as depicted herein and may implement the method when loaded to the computer system. The present invention has been specifically illustrated and explained with reference to the preferred embodiments. The skilled in the art should understand various changes thereto in form and details may be made without departing from the spirit and scope of the present invention.

Claims

WHAT IS CLAIMED

1. A method for transmission mode switching in a wireless communication system, comprising: receiving information indicative of statistical channel state information of a user equipment with respect to a set of m base stations including one reference base station and m-1 candidate cooperative base stations; calculating first sum capacity Ci for a cooperation transmission mode with respect to said set of m base stations and second sum capacity C₂,k or a non-cooperation transmission mode with respect to each base station of said set of m base stations, where k=l_f... m; determining downlink transmission mode based on said first sum capacity C_x and said second sum capacity C₂,_k-

2. The method according to Claim 1, wherein said first sum capacity Ci is calculated by

where i e {l,2,...m] denotes the th base station of said set of m base stations, Γ₍. denotes large scale fading, P_f denotes transmit power of the ith base station, λ^_λ denotes a maximum eigenvalues of a transmit correlation matrix ₍₁..

3. The method according to Claim 1, wherein said second sum capacity C₂,_k is calculated by

where ie j.,2,...m] denotes the ith base station of said set of m base stations, Γ denotes large scale fading, P. denotes transmit power of the ith base station, λ_ιί denotes a maximum eigenvalues of a transmit correlation matrix R_i; .

4. The method according to any of Claims 1-3, wherein said information indicative of statistical channel state information includes U_tJ and A,₍. , where R_{i ;}. = U, ,A_fJ.U" , U₍, is a unitary matrix of respective eigenvector and Λ,. is a diagonal matrix with eigenvalues as its diagonal elements.

5. The method according to any of Claims 1-3, comprising determining, responsive to determination that Ci is greater than a maximum of C₂,_k, where k=l,...,m, that downlink transmission for said user equipment is performed cooperatively by said set of m base stations.

6. The method according to any of Claims 1-3, comprising determining, responsive to determination that Ci is less than C₂,_k, 3A: ε {l,2,...,m} , that downlink transmission for said user equipment is performed by a k^*th base station that satisfies k* ~ arg max C, _k or

ke(\,—jn) downlink transmission for said user equipment is performed by said reference base station.

7. The method according to any of Claims 1-3, wherein said wireless communication system is a heterogeneous network including at least one macro base station and at least one remote radio head nodes .

8. An apparatus for transmission mode switching in a wireless communication system, comprising: receiving unit configured to receive information indicative of statistical channel state information of a user equipment with respect to a set of m base stations including one reference base station andtn-1 candidate cooperative base stations ; calculating unit configured to calculate first sum capacity d for a cooperation transmission mode with respect to said set of m base stations and second sum capacity C₂,_k for a non- cooperation transmission mode with respect to each base station of said set of m base stations, where k=l,... m; determining unit configured to determine downlink transmission mode based on said first sum capacity Ci and said second sum capacity C₂,_k-

9. The apparatus according to Claim 8 , wherein said calculating unit is configured to calculate said first sum capacity Ci by

where i e {l,2,... m)denotes the ith base station of said set of m base stations, Γ,. denotes large scale fading, P. denotes transmit power of the ith base station, denotes a maximum eigenvalues of a transmit correlation matrix R, , .

10. The apparatus according to Claim 8, wherein said calculating unit is configured to calculate said second sum capacity C₂,k by

where i ε {l,2,...m} denotes the ith base station of said set of m base stations, Γ, denotes large scale fading, P_s denotes transmit power of the ith base station, X_{l i} denotes a maximum eigenvalues of a transmit correlation matrix R_{i (} .

11. The apparatus according to any of Claims 8-10, wherein said information indicative of statistical channel state information includes U_{f j} and A_{( i} , where R_{; i} = U, ,-A_t .U^ , X_t- is a unitary matrix of respective eigenvector and A_{( i} is a diagonal matrix with eigenvalues as its diagonal elements.

12. The apparatus according to any of Claims 8-10, wherein said determining unit is configured to determine, responsive to determination that Ci is greater than a maximum of C₂,_k/ where k=l,.,.,m, that downlink transmission for said user equipment is performed cooperatively by said set of m base stations.

13. The apparatus according to any of Claims 8-10, wherein said determining unit is configured to determine, responsive to determination that C_x is less than C₂,_k, 3k e {l,2,...,m} , that downlink transmission for said user equipment is performed by a k^*th base station that satisfies k* = arg max C_{2 k} ; or

ke{\,—,m) downlink transmission for said user equipment is performed by said reference base station.

14. The apparatus according to any of Claims 8-10, wherein said wireless communication system is a heterogeneous network including at least one macro base station and at least one remote radio head nodes .

15. A base station in a wireless communication system, comprising an apparatus for transmission mode switching in a wireless communication system according to any of Claims 8-14.

16. The base station according to Claim 15, where said base station is a macro base station.

17. The base station according to Claim 15, where said base station is a remote radio head node.

18. An apparatus for transmission mode switching in a wireless communication system, comprising: means for controlling to receive information indicative of statistical channel state information of a user equipment with respect to a set of m base stations including one reference base station and m-1 candidate cooperative base stations; means for calculating first sum capacity Ci for a cooperation transmission mode with respect to said set of m base stations and second sum capacity C_2/¾; for a non- cooperation transmission mode with respect to each base station of said set of m base stations, where k=l,... m; means for determining downlink transmission mode based on said first sum capacity Ci and said second sum capacity C_2jk.