WO2013068835A2

WO2013068835A2 - Method and apparatus for channel state information feedback

Info

Publication number: WO2013068835A2
Application number: PCT/IB2012/002426
Authority: WO
Inventors: Zhilan A XIONG; Hongwei SH YANG; Matthew Baker
Original assignee: Alcatel Lucent
Priority date: 2011-11-07
Filing date: 2012-10-31
Publication date: 2013-05-16
Also published as: WO2013068835A3; TW201325132A; CN103095401B; CN103095401A

Abstract

The present invention generally relates to a method and apparatus for channel state information feedback in wireless communications. According to the embodiments of the present invention, there is provided a two-stage feedback mechanism which enables transmission of different types of channel feedback in each stage. Particularly, during a first stage, first channel state information (for example, an absolute CQl associated with a long-term and/or wideband characteristic of a channel) of a reference point in a measurement set is fed back; during a second stage, second channel state information (for example, a differential CQl associated with a short-term and/or subband characteristic of the channel) of each point in the measurement set is fed back. The information as fed back at the first stage and the second stage is used to reconstruct the channel state information for each point in the measurement set at the base station. In this way, feedback overheads regarding channel state information can be effectively reduced. Further, with limited feedback overheads, the accuracy of CQl feedback can be enhanced. Furthermore, single-point transmission and CoMP transmission are both enabled.

Description

METHOD AND APPARATUS FOR CHANNEL STATE INFORMATION FEEDBACK

FIELD OF THE INVENTION [0001] Embodiments of the present invention generally relate to the field of wireless communication, and more specifically, relate to a method and apparatus for channel state information feedback in wireless communications.

BACKGROUND OF THE INVENTION

[0002] Long term evolved (LTE)/ LTE-Advanced (LTE-A) standards and systems in 3 GPP are regarded as an important development trend of wireless communication systems. In such a wireless communication system, in order to achieve higher frequency spectrum utilization, the system adopts the scheme of same frequency networking such that a user equipment (UE) at a cell edge will receive a same frequency interference from a neighboring cell, which seriously dampens the quality of service and throughput of an edge user. In order to satisfy the LTE-A's requirements on aspects of system capacity, transient peak data rate, frequency spectrum, cell edge user throughput and delay, etc., a Coordinated Multi-Point (CoMP) technology has been proposed to improve system performance.

[0003] The CoMP technology transforms original interference from a neighboring cell into useful information through coordination between base stations across cells based on sharing of channel state information (CSI) and data information between respective coordinated base stations. A core idea of CoMP is that when a user equipment is located at a cell edge region, it may receive signals from multiple cells (or base stations) simultaneously, and meanwhile, the transmission of the user equipment per se may also be received simultaneously by multiple cells (or base stations).

[0004] Depending on whether the base station sides share user data, the CoMP transmission technology is mainly divided into two cases. One case is joint processing (JP), and the other case is coordinated scheduling/ beamforming (CS/CB).

[0005] Joint processing is also called "interference utilization," which means that the user is served jointly by multiple coordinated base stations; at the coordinated base station sides, an interference signal is utilized as a useful signal through joint processing to cancel inter-user interference. The coordinated scheduling/ beamforming is also called "interference avoidance," which means that the user is only served by a single base station, and through effective allocation of system resources, the temporal, frequency or spatial conflicts of the resources used at an edge area of neighboring cells are reduced.

[0006] In the LTE/LTE-A systems, the CoMP transmission can be effectively utilized to reduce inter-cell interference (ICI) and improve the coverage area of data rate, cell edge throughput, and/or improve overall system throughput. It is known that CoMP transmission (particularly joint CoMP transmission) requires a great amount of channel state information feedback from each UE so as to perform collective scheduling and collective precoding among all coordinated base stations (also called eNodeB or eNB in 3 GPP).

[0007] In 3GPP RANI, an important issue for a new task of CoMP is whether the definition of a channel quality indicator (CQl) feedback should be modified with respect to an FDD downlink CoMP. The CQl feedback depends on a plurality of parameters, including, but not limited to RI (Rank Indicator), PMI (Precoding Matrix Indicator), and the PDSCH transmission mode in hypothetical CQl reference resources. Although it is indefinite yet whether it is necessary to introduce a new PDSCH transmission mode for the CoMP, in any case, any CQl value as derived by the UE depends on the hypothesis regarding whether it is a single-point transmission or a CoMP transmission (regardless of whether it is CS/CB or JP).

[0008] It is unrealistic to expect that the UE feeds back a great amount of CQl values each corresponding to different combinations of these hypothetical parameters, because it means a considerable CQl computational complexity and uplink feedback overheads at the UE side.

[0009] Further, any mismatch between the hypothetical parameters at the UE side and the actual parameters as finally used at the eNB side would also affect the applicability of the reported CQL

[0010] Despite the above, in the case of not modifying the CQl definition in Release 10 (Rel-10), it is still possible for the eNB to adaptive ly adjust the received CQl feedback through a certain open loop so as to match the actual transmission mode. However, interference uncertainty in the real world also makes it hard to obtain an accurate CQL

[0011] Besides, in order to achieve attractive performance gain from CoMP, it is still possible to perform dynamic switching between single-point transmission and CoMP. Therefore, it is required that the CQl feedback design should enable the eNB to reconstruct a single-point CQl and multi-point CQL SUMMARY OF THE INVENTION

[0012] Based on the above situation, it is desirable to further reduce feedback overheads for channel state information. Further, it is desirable to improve the CQl feedback accuracy in the case of limited feedback overheads. Still further, it is desirable that the CQl feedback mechanism can support both single-point transmission and CoMP transmission.

[0013] In order to realize one or more of the above objectives, embodiments of the present invention provide a method and apparatus for feeding back channel state information in a wireless communication system.

[0014] In one aspect of the present invention, there is provided a method for feeding back channel state information (CSI) in a user equipment, comprising: feeding back first channel state information of a reference point in a measurement set during a first stage; and feeding back second channel state information of each point in the measurement set during a second stage. [0015] In the embodiments of the present invention, the first channel state information comprises an absolute channel quality indicator (CQI) of the reference point; and the second channel state information comprises a differential CQI of each point in the measurement set. Further, the differential CQI of each point may be subjected to differential encoding with respect to the absolute CQI of the reference point or subjected to differential encoding with respect to the absolute CQI of that point.

[0016] In a second aspect of the present invention, there is provided a method for obtaining channel state information (CSI) at a base station, comprising: receiving first channel state information of a reference point in a measurement set; receiving second channel state information of each point in the measurement set; and reconstructing channel state information of each point in the measurement set based on the first channel state information and the second channel state information. In the embodiments of the present invention, the base station is any one of the following: a serving base station, a primary transmission base station, a base station having a best channel quality, or a base station having a strong computing power.

[0017] In a third aspect of the present invention, there is provided an apparatus for feeding back channel state information in a user equipment, comprising: a first feedback module configured to feed back first channel state information of a reference point in a measurement set during a first stage, and a second feedback module configured to feed back second channel state information of each point in the measurement set during a second stage.

[0018] In a fourth aspect of the present invention, there is provided an apparatus for obtaining channel state information at a base station, comprising: a first receiving module configured to receive first channel state information of a reference point in a measurement set; a second receiving module configured to receive second channel state information of each point in the measurement set; and a reconstructing module configured to reconstruct channel state information of each point in the measurement set based on the first channel state information and the second channel state information.

BRIEF DESCRIPTION OF THE DRAWINGS

[0019] Through reading the following detailed description with reference to the accompanying drawings, the above and other objectives, features and advantages of the embodiments of the present invention will become more comprehensible. In the drawings, a plurality of embodiments of the present invention will be illustrated in an exemplary and non-limiting manner, wherein:

[0020] Fig. 1 shows an exemplary diagram of a wireless communication system using CoMP transmission;

[0021] Fig. 2 generally illustrates the principle of channel state information feedback according to the exemplary embodiments of the present invention;

[0022] Fig. 3 schematically illustrates a flowchart of a method 300 for feeding back channel state information in a user equipment according to the exemplary embodiments of the present invention; [0023] Fig. 4 schematically illustrates a flowchart of a method 400 for obtaining channel state information at a base station according to the exemplary embodiments of the present invention;

[0024] Fig. 5 schematically illustrates a block diagram of an apparatus 500 for feeding back channel state information in a user equipment according to the exemplary embodiments of the present invention; and

[0025] Fig. 6 schematically illustrates a block diagram of an apparatus 600 for obtaining channel state information at a base station according to the exemplary embodiments of the present invention;

[0026] In the drawings, like or corresponding numerical signs indicate the same or corresponding contents or parts.

DETAILED DESCRIPTION OF EMBODIMENTS

[0027] Hereinafter, embodiments of the present invention will be described in detail with reference to the drawings. First, with reference to Fig. 1, Fig. 1 shows an exemplary diagram of a wireless communication system 100 using CoMP transmission.

[0028] As illustrated in Fig. 1 , it exemplarily shows three base stations (eNB) and two user equipments (UE). In the system 100 using CoMP transmission, when the user equipment UE is located at a cell edge, multiple points (i.e., base stations eNBl, eNB2, and eNB3) share the channel state information and data information to a certain extent, thereby converting original interference from neighboring cells into useful information through coordination among inter-cell base stations.

[0029] Although Fig. 1 merely illustrates points in a form of base station eNB, they may also be remote radio heads RRH of the base stations. Here, these involved points are generally called a measurement set. Generally, the UE needs to measure a reference signal for each point in the measurement set so as to obtain channel state information.

[0030] Fig. 2 generally illustrates the principle of channel state information feedback according to the exemplary embodiments of the present invention.

[0031] As shown in Fig. 2, a core idea of the various embodiments of the present invention is that in a wireless communication system, the channel state information (CSI) feedback of the UE is divided into two stages, and different channel state information is fed back at each stage. More specifically, during the first stage 201 or the first period of time, first channel state information of a reference point in a measurement set is fed back; during the second stage 202 or the second period of time, second channel state information of each point in the measurement set is fed back. The first channel state information may be an absolute CQI, while the second channel state information may be differential CQIs. In some embodiments, the absolute CQI may be a wideband and/or long-term CQI, and the differential CQI may be a subband and/or short-term CQI.

[0032] It should be noted that the arrows as shown in Fig. 2 merely represent the relations between data and respective points in the measurement set, which do not represent actual transmission directions. In the first stage, it is allowed that the first channel state information is only transmitted to a designated point in the measurement set, i.e., it is only required to transmit one absolute CQL In the second stage, respective differential CQIs may be fed back to respective points in the measurement set, respectively. Alternatively, in the second stage, it is allowed that the second channel state information is still merely transmitted to the designated point, i.e., M differential CQIs, wherein M denotes the number of points in the measurement set.

[0033] As known to those skilled in the art, the CQI may be reported periodically or non-periodically. Therefore, the first-stage feedback and the second-stage feedback may be periodically repeated or initiated based on event triggering. In case of periodical repetition, their cycles may be either identical or different. The information as fed back at the first stage and the second stage is used to reconstruct the CQI of each point in the measurement set at the base station. In this way, feedback overheads regarding channel state information can be effectively reduced.

[0034] It should be noted that, sometimes, the terms "stage" and "period of time" can be used interchangeable. However, the skilled in the art should understand that the term "stage"is not limited to a time period.

[0035] With reference to Fig. 3, Fig. 3 schematically illustrates a flowchart of a method 300 for feeding back channel state information in a user equipment according to the exemplary embodiments of the present invention. It should be noted that according to the embodiments of the present invention, the involved wireless communication system generally comprises at least one user equipment (UE). The method 300 may be executed at each of the UEs, for example, executed by a UE itself or its component.

[0036] After the method 300 starts, in step S301 , the UE feeds back first channel state information of a reference point in a measurement set during a first stage.

[0037] According to the embodiments of the present invention, the first channel state information may comprise an absolute CQI of a channel between the UE and the reference point in the measurement set. The absolute CQI may be a wideband and/or long-term CQI. In other words, the first channel state information may mainly involve cell-specific channel information. The definition of the absolute CQI may re-use the definition of a single-point CQI in Rel-10 so as to obtain channel quality from the reference point to the UE.

[0038] The long-term wideband CQI differs dependent on a specific implementation. Hereinafter, it will be illustrated how to derive a long-term wideband CQI. Those skilled in the art would appreciate that the following depiction is merely exemplary, not limitative, and those skilled in the art would adopt any known or future developed appropriate technical means to measure it. The scope of the present invention is not limited thereto.

[0039] The UE may for example update the long-term wideband CQI of its current subframe by weighting the long-term wideband CQI of a previous subframe and the short-term wideband CQI of the current subframe, which, for example, may be expressed as:

CQI _c n) = a CQI _c n - l)+ (l - a )CQI_{Cj J}

CQI _c An)

[0040] In the above equation, ' indicates the long-term wideband CQI from the th

CQI_C

point to the UE at the n subframe (current subframe); ^ ^Cj'^J indicates the short-term th

wideband CQI from the ^J point to the j^th UE at the current subframe; ^a is a weighting factor and differs dependent on a specific implementation; and ⁷ indicates a serial number of a reference point for the j^th UE.

CQI

[0041] Herein, the short-term wideband CQI, i.e., ^Cj'^J may be calculated in a plurality of manners. For example, the equations (2) or (3) below provide two alternative manners. It should be noted that in the equations (2) and (3), the receive beamformer, PMI and eigen channel are all wideband. )

th

H C ,

[0042] In the above equations, ^C''^J denotes the channel from the ^j point to the j

UE; ³ denotes a receive beamformer as predicted at the j UE; ^Cj'^J denotes a feedback PMI for the th 2

³ point from the j^th UE; ^σ° denotes the noise plus interference outside of the CoMP measurement set, σ ■ j"¹

and ^J '' is the noise plus total interference of the ^J UE.

[0043] In option 1 , the short-term wideband CQI of the reference point may be derived under an assumption that the interference merely comes from outside of the CoMP measurement set. In option 2, the short-term wideband CQI of the reference point may be calculated through regarding the total interference power from all other points as interference.

[0044] Option 1 may easily derive the CoMP-specific CQI based on the PMI feedback and the per-point CQI. Besides, through an appropriate compensation, option 1 may also derive the CQIs of SU-MIMO and MU-MMO. Option 2 can well support SU-MIMO and MU-MIMO. Further, the CoMP-specific CQI may be derived based on the PMI feedback and the per-point CQI feedback. It is seen that both option 1 and option 2 provide a flexible switching capability among SU-MIMO, MU-MIMO, and CoMP.

CQI_C .

[0045] If the long-term wideband CQI is not reported in the current subframe, then ' is the latest reported long-term wideband CQI; otherwise, ^; equals to the latest updated long-term wideband CQI, which will be reported in the current subframe.

[0046] According to the embodiments of the present invention, the reference point in the measurement set may be any one of the following: a serving point, a primary transmission point, a point with best channel quality, or a point with middle channel quality. Further, the reference point may be selected by the serving point or recommended by the UE.

[0047] Specifically, for CoMP CS/CB, the reference point may be the serving point or the primary transmission point of the UE. For CoMP JP, the reference point may be the serving point of the UE, the point with best channel quality, or the point with middle channel quality.

[0048] According to the embodiments of the present invention, the first channel state information may be transmitted to a designated point in the measurement set. This designated point may be any one of the following: a serving point, a primary transmission point, a point with best channel quality, or a point with a strong computing power. Further, the designated point may be selected by the serving point or recommended by the UE.

[0049] Next, the method 300 proceeds to step S302. Here, during the second stage or period of time, the UE feeds back the second channel state information of each point in the measurement set.

[0050] According to the embodiments of the present invention, the second channel state information may comprise a differential CQI of each point in the measurement set. The differential CQI may indicate a subband and/or short-term CQI. Or in other words, the differential CQI may primarily involve channel state information regarding inter-cell coordination.

[0051] The differential CQIs may be subjected to differential encoding in multiple manners. According to one embodiment of the present invention, the differential CQIs may be subj ected to differential encoding with respect to the absolute CQI (long-term wideband CQI) of the reference point in the measurement set (hereinafter referred to "differential alternative I"). According to another embodiment of the present invention, the differential CQIs may be subjected to differential encoding with respect to the absolute CQIs (long-term wideband CQI) of respective points in the measurement set (hereinafter referred to "differential alternative Π"). The two differential alternatives may be expressed as follows, respectively:

Differential Alternative I

Differential Alternative II: ^C® w ^C^ ' wherein, ^^'·⁷ , and ' are the short-term differential CQI, short-term CQI, and long-term wideband CQI for the channel from the i^th point to the j* UE, respectively. For a non-reference point, i.e., i^'≠c ¹, , i ·₊ts , long- ₊term wi -dAeband A ^ CmQIf( CQ! t '^Ji ^ ) i-s equa il _* to CQ ^,JM ' . CQI , ^J, (n) ' and , CQI^{, J} can be calculated by replacing ^J by ' in the equations (1), (2) and (3).

[0052] It should be noted that the short-term CQI involved during the second stage may be measured by any currently known or future developed appropriate technical means. The scope of the present invention is not limited thereto.

[0053] In the differential alternative I, all short-term differential CQIs for respective points in the measurement set are determined based on the same long-term wideband CQI (i.e., the absolute CQI of the reference point). Therefore, according to this alternative, the per-point short-term CQI can be easily re-constructed at the base station. However, due to the power imbalance issue in the CoMP measurement set, the short-term CQI for a neighboring point may be greatly discrepant with the absolute CQI of the reference point, such that the obtained differential result may have a larger dynamic range. In one embodiment of the present invention, by selecting the reference point to be a point with middle channel quality, the dynamic range of the differential result can be effectively narrowed, which further reduces feedback overheads.

[0054] In the differential alternative II, the per-point short-term differential CQI in the measurement set is determined based on the per-point absolute CQI (long-term wideband CQI). Therefore, the feedback overheads induced by the abovementioned power imbalance issue can be controlled. However, since the long-term wideband CQI for each non-reference point is not directly fed back to the base station, the base station has to estimate it, which might cause a certain error and some mismatch.

[0055] According to the embodiments of the present invention, the per-point second channel state information can be transmitted to respective points in the measurement set. According to this embodiment, the uplink feedback overheads may be dispersed to respective points in the measurement set. The respective points may further collect the received channel state information to a designated point, such that the designated point may reconstruct the CQIs.

[0056] According to another embodiment of the present invention, the second channel state information can be still transmitted to the designated point in the measurement set. In this way, the designated point (for example, a point with a strong computing power) may reconstruct the per-point CQI and then transmits the reconstructed result to respective points in the measurement set.

[0057] Those skilled in the art would appreciate that the communication between respective points can be easily implemented, which is generally not limited by bandwidth, rate, and the like.

[0058] The method 300 ends after the end of step S302. Through performing the method 300 at the UE, it may be seen that the per-point long-term and/or wideband channel state information and the short-term and/or narrowband channel state information in the measurement set is fed back to the base station during different stages (e.g., periods of time). Specifically, in the first stage, it is allowed that the first channel state information is only transmitted to a designated point in the measurement set, i.e., it is only required to transmit one absolute CQI. In this way, feedback overheads regarding channel state information can be effectively reduced. Further, through feeding back the differential CQI for each point in the measurement set during the second stage, feedback overheads can be further reduced, and meanwhile single-point CQI and multi-point CQI are effectively enabled, i.e., the switching among SU-MIMO, MU-MIMO, and CoMP can be enabled.

[0059] It should be particularly noted that although the method 300 ends after step S302 in Fig. 3, it is only for illustrative and schematic purposes. In practice, the UE can periodically perform step S301 and S302. In other words, the method 300 or the steps S301 and S303 can be periodically performed.

[0060] Particularly, according to some embodiments of the present invention, the cycles for performing S301 and S302 may be different. For example, the above described feedback cycle of the first period of time for feeding back the first channel state information may be larger than or equal to the feedback cycle of the second period of time for feeding back the second channel state information. In other words, transmission of the first channel state information is not as frequent as transmission of the second channel state information. This embodiment is based on the following consideration: compared with the short-term and/or narrowband channel characteristics, the change of the long-term and/or wideband channel characteristics is less frequent. Through discriminatively setting the feedback cycles for the first stage and the second stage, feedback overheads can be effectively reduced. Of course, the relationship between cycles of the two periods of time is not limitative.

[0061] Now, referring to Fig. 4, Fig. 4 schematically illustrates a flowchart of a method 400 for obtaining channel state information at a base station according to the exemplary embodiments of the present invention. According to the embodiments of the present invention, the method 400 may be performed at the base station of the wireless communication system, for example, performed by the base station itself or its components.

[0062] After the method 400 starts, in step S401 , first channel state information of a reference point in a measurement set is received. It should be understood that step S401 corresponds to step S301 in the method 300. Therefore, the description on the first channel state information as provided with reference to step S301 of the method 300 is likewise applicable to step S401. Particularly, according to the embodiments of the present invention, the first channel state information may involve an absolute CQI for a channel between a UE and a reference point in the measurement set, and the absolute CQI may indicate the long-term and/or wideband CQI of the channel, i.e., a cell-specific channel characteristic.

[0063] Next, the method 400 proceeds to step S402, where the base station receives second channel state information of each point in the measurement set. It should be understood that step S402 corresponds to step S302 in the method 300. Therefore, the description on the second channel state information as provided with reference to step S302 of the method 300 is likewise applicable to step S402. Particularly, according to the embodiments of the present invention, the second channel state information may involve a differential CQI of the channel between the UE and the each point in the measurement set, the differential CQI indicating a short-term and/or narrowband channel characteristic, i.e., channel characteristic related to inter-cell coordination.

[0064] It should be understood that as above mentioned, the first and second channel state information is not necessarily directly received by the base station from the UE; the base station may also receive the above information from other coordinated base stations in the measurement set.

[0065] The method 400 then proceeds to step S403. Here, the base station reconstructs channel state information of each point in the measurement set based on the received first channel state information and second channel state information. Based on different differential encoding schemes for the second channel state information, the base station performs different processing correspondingly.

[0066] If the second channel state information is encoded based on the differential scheme I, i.e., performing differential encoding with respect to the absolute CQI of the reference point, the base station may easily reconstruct the CQI of each point. For example, it may be calculated as:

Differential scheme I: ^{, C}''^{J ,} (6)

CQI_C

[0067] It may be seen that equation (6) corresponds to equation (4), where ' and can be obtained by the two-stage feedback of the UE; therefore, for differential scheme I, the base station needs no additional information to reconstruct the CQI of each point.

[0068] If the second channel state information is encoded based on the differential scheme II, i.e., performing differential encoding with respect to the absolute CQI of respective point, the base station may reconstruct based on the following equation:

Differential Scheme II: ^€& '·^■> ^{( <}~ ^{Ι V} (7)

It may be seen that equation (7) corresponds to equation (5). As previously mentioned, because the absolute CQI (long-term wideband CQI, i.e., ⁱ ) of each non-reference point is not directly fed back to the base station, the base station has to estimate it. The base station may estimate based on the reciprocity of the uplink and downlink channels. For example, the long-term wideband CQI of the non-reference point may be reconstructed according to the following equation:

CQI. . = 5. CQI _C ,.,i≠c ,.

<5 i^th where, ^J denotes the compensated ratio of the signal powers received on the uplink by the ' point c '^h δ i^th and the ^J point. In order to get ''->^' , the considered signal powers should come from the ^J UE and be corrected based on the reciprocity of FDD downlink and uplink.

[0070] Those skilled in the art would appreciate that the above scheme of estimating the absolute CQI of a non-reference point is merely exemplary, and those skilled in the art may estimate by any currently known or future developed appropriate technical means. The scope of the present invention is not limited thereto.

[0071] According to the embodiments of the present invention, the base station for reconstructing the CQI of each point may be any one of the following: a serving base station, a primary transmission base station, a base station having a best channel quality, or a base station having a strong computing power.

[0072] Through the above depiction on the method 300 and method 400, those skilled in the art would appreciate that according to the embodiments of the present invention, a new channel state information feedback mechanism is constructed between a base station and a UE. Different from the prior art, according to this mechanism, the channel state information feedback is completed in two stages, where different types of channel state information are transmitted in each stage. Besides, in the first stage, it is only needed to feed back an absolute CQI to a designated point in the measurement set, for example, the long-term and/or wideband CQI of the reference point. And in the second stage, a differential CQI for each point in the measurement set can be fed back, for example, the differential CQI indicating the short-term and/or subband CQI of each point. The feedback information as respectively transmitted in the two stages is collected at the base station and is collectively used to generate complete feedback information reflecting the overall channel condition.

[0073] Hereinafter, referring to Fig. 5, Fig. 5 schematically illustrates a block diagram of an apparatus 500 for feeding back channel state information in a user equipment according to the exemplary embodiments of the present invention. According to the embodiments of the present invention, the apparatus 500 may reside at one or more user equipments UEs of a wireless communication system, or be associated with the UE in other manner. It would be appreciated that the apparatus 500 is operable to perform the above described method 300.

[0074] As shown in Fig. 5, according to the embodiments of the present invention, the apparatus 500 comprises a first feedback module 501 configured to feed back first channel state information of a reference point in a measurement set during a first stage; and a second feedback module 502 configured to feed back second channel state information of each point in the measurement set during a second stage.

[0075] According to the embodiments of the present invention, the first channel state information may comprise an absolute CQI of a channel between the UE and the reference point in the measurement set. The absolute CQI may be a wideband and/or long-term CQI. Besides, the second channel state information may comprise a differential CQI for each point in the measurement set. The differential CQI may indicate a subband and/or short-term CQI.

[0076] The second feedback module 502 may be configured to adopt multiple manners to perform differential encoding to the differential CQI. According to one embodiment of the present invention, the differential CQI may be encoded using the differential scheme I, i.e., subjected to differential encoding with respect to the absolute CQI (long-term wideband CQI) of a reference point in the measurement set. According to another embodiment of the present invention, the differential CQI may be encoded using the differential scheme II, i.e., subjected to differential encoding with respect to the absolute CQI (long-term wideband CQI) of respective point in the measurement set. [0077] The first feedback module 501 may be configured to feed back first channel state information to a designated point in a measurement set, and the second feedback module 502 may be configured to feed back respective second channel state information to respective points in the measurement set. Alternatively, the second feedback module 502 may be configured to feed back the second channel state information to the designated point in the measurement set.

[0078] The reference point in the measurement set may be any one of the following: a serving point, a primary transmission point, a point with best channel quality, or a point with middle channel quality. The designated point may be any one of the following: a serving point, a primary transmission point, a point with best channel quality, or a point with a strong computing power. Both the reference point and the designated point can be selected by the serving point or recommended by the UE.

[0079] As above mentioned, the apparatus 500 shown in Fig. 5 may act as an executive entity for the above mentioned method 300. Thus, various features as above described with reference to Fig. 3 are all suitable for the apparatus 500, which will not be detailed here.

[0080] Hereinafter, referring to Fig. 6, Fig. 6 schematically illustrates a block diagram of an apparatus 600 for obtaining channel state information at a base station according to the exemplary embodiments of the present invention. According to the embodiments of the present invention, the apparatus 600 may reside at a base station of a wireless communication system, or be associated with the base station in other manner. It would be appreciated that the apparatus 600 is operable to perform the above described method 400.

[0081] As shown in Fig. 6, according to the embodiments of the present invention, the apparatus 600 comprises a first receiving module 601 configured to receive first channel state information of a reference point in a measurement set; a second receiving module 602 configured to receive second channel state information of each point in the measurement set; and a reconstructing module 603 configured to reconstruct the channel state information of each point in the measurement set based on the first channel state information and the second channel state information.

[0082] According to the embodiments of the present invention, the first channel state information may involve an absolute CQI for a channel between a UE and the reference point in the measurement set, and the absolute CQI may indicate the long-term and/or wideband CQI of the channel, i.e., a cell-specific channel characteristic. And the second channel state information may involve a differential CQI of a channel between the UE and the each point in the measurement set, the differential CQI indicating a short-term and/or narrowband channel characteristic, i.e., channel characteristic related to inter-cell coordination.

[0083] It should be understood that as above mentioned, the first and second channel state information is not necessarily directly received by the base station from the UE; the base station may also receive the above information from other coordinated base stations in the measurement set.

[0084] Based on different differential encoding schemes for the second channel state information, the reconstructing module 603 performs different processing correspondingly. For example, for differential scheme I and differential scheme II, the reconstructing module 603 may perform reconstruction according to the above described step S403, which will not be detailed here.

[0085] The base station for reconstructing channel state information may be any one of the following: a serving base station, a primary transmission base station, a base station having the best channel quality, or a base station having a strong computing power. The base station then transmits the reconstructed result to each point in the measurement set.

[0086] As above mentioned, the apparatus 600 shown in Fig. 6 may act as an executive entity for the above mentioned method 400. Thus, various features as above described with reference to Fig. 4 are all suitable for the apparatus 600, which will not be detailed here.

[0087] It should be understood that the partitioning of respective modules in apparatuses 500 and 600 is merely exemplary. For example, the function of a single module as above mentioned may be implemented by multiple modules. In other words, the above multiple modules may also be implemented by a single module. The scope of the present invention is not limited thereto.

[0088] It should be understood that respective modules comprised in apparatuses 500 and

600 may be implemented in various manners, comprising software, hardware, firmware, or any combination thereof. For example, in some embodiments, respective modules in apparatuses 500 and 600 may be implemented using software and/or firmware blocks. Alternatively or additionally, respective modules of apparatus 500 and apparatus 600 may also be implemented by hardware blocks. For example, respective modules of apparatuses 500 and 600 may be implemented as an integrated circuit (IC) chip or dedicated integrated circuit (ASIC). Respective modules in apparatuses 500 and 600 may be implemented as a system-on-chip (SOC). Other manners that are currently known or developed in the future are also feasible, and the scope of the present invention is not limited thereto.

[0089] The spirit and principle of the present invention has been illustrated above with reference to a plurality of exemplary embodiments. Experiments show that the methods and apparatuses according to the embodiments of the present invention can effectively reduce feedback overheads of a wireless communication system. Further, with limited feedback overheads, the accuracy of CQI feedback can be enhanced. Still further, the CQI feedback mechanism according to the embodiments of the present invention can support both single-point transmission and CoMP transmission.

[0090] It should be noted that the flowcharts and block diagrams in the figures illustrate the system architecture, functions and operations potentially implemented by the method, system and apparatus according to various embodiments of the present invention. In this regard, each block in the flowcharts or block diagrams may represent a module, a program segment, or a part of code, which contains one or more executable instructions for performing specified logic functions. It should also be noted that, in some alternative implementations, the functions noted in the blocks may also occur in a sequence different from what is noted in the drawings. For example, two blocks shown consecutively may be performed in parallel substantially or in an inverse order, depending on the involved functions.

[0091] It should also be noted that each block in the block diagrams and/or flowcharts and a combination of blocks in block diagrams and/or flow charts may be implemented by a dedicated hardware-based system for executing a prescribed function or operation or may be implemented by a combination of dedicated hardware and computer instructions. It should be understood that the figures and embodiments of the present invention are merely for exemplary purposes, not intended for limiting the protection scope of the present invention.

[0092] The method according to the present invention may be implemented in software, hardware, or a combination of software and hardware. The hardware part may be implemented with a dedicated logic; the software part may be stored in a memory and executed by an appropriate instruction execution system, for example a microprocessor, a personal computer or a mainframe. In a preferred embodiment, the present invention is implemented as software, including, without limitation to, firmware, resident software, micro-code, etc.

[0093] It should be noted that in order to make the embodiments of the present invention more comprehensible, the above description omits some more specific technical details which are known to the skilled in the art and may be essential to implement the present invention. The purpose for providing the description of the present invention is to explain and describe, not to exhaust or limit the present invention within the disclosed form. To a person of normal skill in the art, various modifications and alternations are obvious.

[0094] Though a plurality of embodiments of the present invention have been described above, those skilled in the art should understand that these depictions are only exemplary and illustrative. Based on the teachings and inspirations from the specification, modifications and alterations may be made to the respective embodiments of the present invention without departing from the true spirit of the present invention. Thus, the features in the specification should not be regarded as limitative. The scope of the present invention is only limited by the appended claims.

Claims

What Is Claimed Is:

1. A method for feeding back channel state information (CSI) in a user equipment, comprising: feeding back first channel state information of a reference point in a measurement set during a first stage; and

feeding back second channel state information of each point in the measurement set during a second stage.

2. The method according to claim 1 , wherein:

the first channel state information comprises an absolute channel quality indicator (CQl) of the reference point; and

the second channel state information comprises a differential CQl of each point in the measurement set.

3. The method according to claim 2, wherein the absolute CQl is a long-term wideband CQl, which is determined by weighting a long-term wideband CQl of a previous subframe and a short-term wideband CQl of a current subframe.

4. The method according to claim 3, wherein the short-term wideband CQl is calculated as:

U_jH

CQI_C

(2), or

th

wherein ~" from the ^1- point to the j^th UE at the current subframe; ^Cj denotes a channel from the ^J point to the UE; ^J denotes a predicted receive beamformer at the UE; ^Vc' denotes a feedback Precoding Matrix Indicator (PMI) for the th

^J point from the UE; ^σ° denotes noise plus interference outside of the CoMP measurement set, and σ ■ j"¹

J '' denotes noise plus total interference power of the ^J UE.

5. The method according to claim 2, wherein the differential CQl of each point is subjected to differential encoding with respect to the absolute CQl of the reference point, the differential CQl being calculated as: wherein and ^Q^ J denote the short-term differential CQl and short-term CQl, for a

CQl

channel from the i^th point to the j^th UE, respectively; and ' denotes the long-term wideband CQl th

from the ^J point to the j^th UE.

6. The method according to claim 2, wherein the differential CQl of each point is subjected to differential encoding with respect to the absolute CQl of said point, the differential CQl being calculated as:

= CQI - CQI _t

(5) wherein , ^!< and ' are the short-term differential CQl, short-term CQl, and long-term wideband CQl for a channel from the 1^th point to the UE, respectively.

7. The method according to any one of claims 1-6, wherein the reference point is any one of the following: a serving point, a primary transmission point, a point with best channel quality, or a point with middle channel quality, and the reference point is selected by the serving point or recommended by the user equipment.

8. The method according to any one of claims 1-6, wherein the first channel state information is fed back to a designated point in the measurement set, and the second channel state information of each point is fed back to said point in the measurement set, respectively.

9. The method according to any one of claims 1-6, wherein the first channel state information and the second channel state information are fed back to a designated point in the measurement set.

10. The method according to claim 8 or 9, wherein the designated point is any one of the following: a serving point, a primary transmission point, a point with best channel quality, or a point with a strong computing power, and the designated point is selected by the serving point or recommended by the user equipment.

11. A method for obtaining channel state information (CSI) at a base station, comprising:

receiving first channel state information of a reference point in a measurement set;

receiving second channel state information of each point in the measurement set; and reconstructing channel state information of each point in the measurement set based on the first channel state information and the second channel state information.

12. The method according to claim 11 , wherein:

13. The method according to claim 12, wherein the differential CQl of each point is subjected to differential encoding with respect to the absolute CQl of the reference point, and the reconstructing channel state information of each point in the measurement set is computed as:

CQ , = CQl , + Δ . . wherein and ^Q^ J denote the short-term differential CQl and short-term CQl, for a

CQl

channel from the i^th point to the j^th UE, respectively; and ' denotes the long-term wideband CQl

th

from the ^Cj point to the j^th UE.

14. The method according to claim 12, wherein the differential CQl of each point is subjected to differential encoding with respect to the absolute CQl of said point, and the reconstructing channel state information of each point in the measurement set is computed as:

C O/ _, CO/ Λ

(7) wherein and ' J are the short-term differential CQl, short-term CQl, and long-term wideband CQl for a channel from the i point to the j UE, respectively.

15. The method according to any one of claims 1 1 -14, wherein the base station is any one of the following: a serving point, a primary transmission point, a point with best channel quality, or a point with a strong computing power.

16. An apparatus for feeding back channel state information (CSI) in a user equipment, comprising:

a first feedback module configured to feed back first channel state information of a reference point in a measurement set during a first stage; and

a second feedback module configured to feed back second channel state information of each point in the measurement set during a second stage.

17. The apparatus according to claim 16, wherein: the first channel state information comprises an absolute channel quality indicator (CQl) of the reference point; and

18. The apparatus according to claim 17, wherein the absolute CQl is a long-term wideband CQl that is determined by weighting a long-term wideband CQl of a previous subframe and a short-term wideband CQl of a current subframe.

19. The apparatus according to claim 18, wherein the short-term wideband CQl is calculated

U_jH

CQI_C

(2), or u _;H„ f (3 )

CQI_C

wherein ^ ^CjJ denotes the short-term wideband CQl from the ¹ point to the j^th UE at the current subframe; ^Cj'^J denotes a channel from ^J point to the j^th UE; ^J denotes a predicted receive beamformer at the UE; ^c> '^} denotes a feedback Precoding Matrix Indicator (PMI) for the

J '' denotes noise plus total interference power of the ^J UE.

20. The apparatus according to claim 17, wherein the differential CQl of each point is subjected to differential encoding with respect to the absolute CQl of the reference point, the differential CQl being calculated as:

wherein and ^Q^ denote the short-term differential CQl and short-term CQl, for a

CQl

channel from the i^th point to the j^th UE, respectively; and ^Cj'^J denotes the long-term wideband CQl from the ^Cj point to the j^th UE.

21. The apparatus according to claim 17, wherein the differential CQl of each point is subjected to differential encoding with respect to the absolute CQl of said point, the differential CQl being calculated as: wherein ^' , _{^ anc}[ ^QI _{i arg} ^_{g 8}^_ΟΓ^.^_6Γηι differential CQI, short-term CQI, and long-term wideband CQI for a channel from the 1^th point to the UE, respectively.

22. The apparatus according to any one of claims 16-21, wherein the reference point is any one of a serving point, a primary transmission point, a point with best channel quality, or a point with middle channel quality, and the reference point is selected by the serving point or recommended by the user equipment.

23. The apparatus according to any one of claims 16-21, wherein the first feedback module is configured to feed back the first channel state information to a designated point in the measurement set, and the second feedback module is configured to feed back the second channel state information of each point to said point in the measurement set.

24. The apparatus according to any one of claims 16-21, wherein the first feedback module is configured to feed back the first channel state information to a designated point in the measurement set, and the second feedback module is configured to feed back the second channel state information of each point to the designated point in the measurement set.

25. The apparatus according to claim 23 or 24, wherein the designated point is any one of the following: a serving point, a primary transmission point, a point with best channel quality, or a point with a strong computing power, and the designated point is selected by the serving point or recommended by the user equipment.

26. An apparatus for obtaining channel state information (CSI) at a base station, comprising: a first receiving module configured to receive first channel state information of a reference point in a measurement set;

a second receiving module configured to receive second channel state information of each point in the measurement set;

a reconstructing module configured to reconstruct channel state information of each point in the measurement set based on the first channel state information and the second channel state information.

27. The apparatus according to claim 26, wherein:

the first channel state information comprises an absolute channel quality indicator (CQI) of the reference point; and

the second channel state information comprises a differential CQI of each point in the measurement set.

28. The apparatus according to claim 27, wherein the differential CQI of each point is subjected to differential encoding with respect to the absolute CQI of the reference point, and the reconstructing module is configured to reconstruct channel state information of each point in the measurement set according to the following equation:

CQI, , = CQI _C , + Δ . . wherein and ^Q^ J denote the short-term differential CQI and short-term CQI, for a channel from the i^th point to the j^th UE, respectively; and ^C' denotes the long-term wideband CQI th

from the ^Cj point to the j^th UE.

29. The apparatus according to claim 27, wherein the differential CQI of each point is subjected to differential encoding with respect to the absolute CQI of said point, and the reconstructing module is configured to reconstruct channel state information of each point in the measurement set according to the following equation:

( 01 , ( 01 Λ

(7) wherein and ' J are the short-term differential CQI, short-term CQI, and long-term wideband CQI for a channel from the 1^th point to the j* UE, respectively.

30. The apparatus according to any one of claims 26-29, wherein the base station is any one of the following: a serving point, a primary transmission point, a point with best channel quality, or a point with a strong computing power.