WO2013140782A1

WO2013140782A1 - Channel quality indicator feedback method and user equipment

Info

Publication number: WO2013140782A1
Application number: PCT/JP2013/001845
Authority: WO
Inventors: Ziyuan GUO; Ming Ding; Lei Huang; Renmao Liu
Original assignee: Sharp Corp
Current assignee: Sharp Corp
Priority date: 2012-03-20
Filing date: 2013-03-18
Publication date: 2013-09-26
Anticipated expiration: 2014-09-20
Also published as: CN103326762A; CN103326762B

Description

CHANNEL QUALITY INDICATOR FEEDBACK METHOD AND USER EQUIPMENT

The invention relates to communication technology field, and more particularly, to a channel quality indicator feedback method in a Coordinated MultiPoint(CoMP) and a User Equipment (UE) applying the same.

Multi-antenna wireless transmission technique, or Multiple Input Multiple Output (MIMO), can achieve spatial multiplex gain and spatial diversity gain by deploying a plurality of antennas at both the transmitter and the receiver and utilizing the spatial resources in wireless transmission. Researches on information theory have shown that the capacity of a MIMO system grows linearly with the minimum of the number of transmitting antennas and the number of receiving antennas. Fig. 1 shows a schematic diagram of a MIMO system. As shown in Fig. 1, a plurality of antennas at the transmitter and a plurality of antennas at each of the receivers constitute a multi-antenna wireless channel containing spatial domain information. Further, Orthogonal Frequency Division Multiplexing (OFDM) technique has a strong anti-fading capability and high frequency utilization and is thus suitable for high-rate data transmission in a multi-path and fading environment. The MIMO-OFDM technique, in which MIMO and OFDM are combined, has become a core technique for a new generation of mobile communication.

For instance, the 3^rd Generation Partnership Project (3GPP) organization is an international organization in mobile communication field and plays an important role in standardization of 3G cellular communication technologies. Since the second half of the year 2004, the 3GPP organization has initiated a so-called Long Term Evolution (LTE) project for designing Evolved Universal Terrestrial Radio Access (EUTRA) and Evolved Universal Terrestrial Radio Access Network (EUTRAN). The MIMO-OFDM technique is employed in the downlink of the LTE system. In a conference held in Shenzhen, China in April 2008, the 3GPP organization started a discussion on the standardization of 4G cellular communication systems (currently referred to as LTE-A systems). In this conference, a concept known as "multi-antenna multi-BS coordination" gets extensive attention and support. Its core idea is that multiple BSs can provide communication services for one or more UEs simultaneously, so as to improve data transmission rate for a UE located at the edge of a cell.

With regard to the multi-antenna multi-BS coordination, fundamental agreements are mainly available from the following Non Patent Literature 1 (standard document) by March, 2010: 3GPP TR 36.814 V9.0.0 (2010-03), "Further advancements for E-UTRA physical layer aspects (Release 9)", which can be outlined as follows:

(1) In a multi-antenna multi-BS service, a UE needs to report channel state/statistical information of a link between the UE and each BS/cell in a set of cells. This set of cells is referred to as a measurement set for multi-antenna multi-BS transmission.

(2) The set of BSs/cells for which the UE actually perform information feedback can be a subset of the measurement set and is referred to as a coordination set for multi-antenna multi-BS transmission. Here, the coordination set for multi-antenna multi-BS transmission can be the same as the measurement set for multi-antenna multi-BS transmission.

(3) A BS/cell in the coordination set for multi-antenna multi-BS transmission participates in Physical Downlink Shared Channel (PDSCH) transmission for the UE, either directly or indirectly.

(4) The scheme in which multiple BSs directly participate in coordinated transmission is referred to as Joint Processing (JP). The JP scheme needs to share PDSCH signal of the UE among the multiple BSs participating the coordination and can be divided into two approaches. One is referred to as Joint Transmission (JT) in which the multiples BSs transmit their PDSCH signals to the UE simultaneously. The other one is referred to as Dynamic Cell Selection (DCS) in which at any time instance, only one of the BSs which has the strongest signal link is selected to transmit its PDSCH signal to the UE.

(5) The scheme in which multiple BSs indirectly participate in coordinated transmission is referred to as Coordinated Beamforming/Coordinated Scheduling (CB/CS). In this CB/CS scheme, instead of sharing PDSCH signal of the UE among the multiple BSs participating in the coordination, the beams/resources for transmission of PDSCHs for different UEs are coordinated among the multiple BSs to suppress the interference between each other.

(6) For a UE operating in the multi-antenna multi-BS coordinated transmission environment, information feedback is mainly carried out separately for each BS and is transmitted over the uplink resources of the serving BS.

As used herein, the term "information feedback" refers to a process in which a UE needs to feed back CSI to a BS such that the BS can perform corresponding operations such as radio resource management. There are primarily the following three CSI feedback approaches in the prior art documents.

(Complete CSI Feedback): The UE quantizes all elements in a transceiver channel matrix and feeds back each of the elements to the BS. Alternatively, the UE can analog modulate all elements in the transceiver channel matrix and feeds back them to the BS. Alternatively, the UE can obtain an instantaneous covariance matrix for the transceiver channel matrix, quantizes all elements in the covariance matrix and feeds back each of the elements to the BS. Thus, the BS can reconstruct an accurate channel from the channel quantization information fed back from the UE. This approach is described in Non Patent Literature 2: 3GPP R1-093720, "CoMP email summary", Qualcomm and its implementation is illustrated in Fig. 2.

(Statistic-Based CSI Feedback): The UE applies a statistical process on a transceiver channel matrix, e.g., calculating a covariance matrix thereof, quantizes the statistical information and then feeds back it to the BS. Thus, the BS can obtain statistical state information of the channel based on the feedback from the UE. This approach is described in Non Patent Literature 2: 3GPP R1-093720, "CoMP email summary", Qualcomm and its implementation is illustrated in Fig. 3.

(CSI Feedback Based on Codebook Space Search): A finite set of CSI is predefined by the UE and the BS (i.e., codebook space, a typical codebook space includes channel rank and/or pre-coding matrix and/or channel quality indication, etc.). Upon detection of a transceiver channel matrix, the UE searches in the codebook space for an element best matching the CSI of the current channel matrix and feeds back the index of the element to the BS. Thus, the BS looks up the predefined codebook space based on the index to obtain rough CSI. This approach is described in Non Patent Literature 3: 3GPP, R1-083546, "Per-cell precoding methods for downlink joint processing CoMP", ETRI, and its implementation is illustrated in Fig. 4.

Among the above three approaches, the complete CSI feedback has the best performance, but is impractical to be applied to actual systems due to the highest feedback overhead. In particular, in the multi-antenna multi-BS coordination system, its feedback overhead grows in proportional to the increase of the number of BSs and it is even more impractical. The CSI feedback based on codebook space search has the lowest feedback overhead, but is worst in terms of performance since it cannot accurately describe the channel state such that the transmitter cannot make full use of channel characteristics and cannot perform the transmission accordingly. However, it is extremely simple to implement and can typically accomplish feedback with a few bits. Hence, it is widely applied in actual systems. The statistic-based CSI feedback achieves a good tradeoff between these two approaches. When the channel state has significant statistical information, this approach can accurately describe the channel state with a relatively small amount of feedback, thereby achieving a relatively ideal performance.

Currently, in the LTE and the LTE-A systems, in consideration of factors for practical system implementation, the CSI feedback based on codebook space search is employed in a single cell transmission mode. In the multi-BS/cell coordination mode in the LTE-A system, it is expected that this CSI feedback based on codebook space search will continue to be used.

For the channel state information feedback method based on codebook space search, there are two feedback channels in the LTE system, a Physical Uplink Control CHannel (PUCCH) and a Physical Uplink Shared CHannel (PUSCH). In general, the PUCCH is configured for transmission of synchronized, basic CSI with low payload; while PUSCH is configured for transmission of bursty, extended CSI with high payload. For the PUCCH, a complete CSI is composed of different feedback contents which are transmitted in different sub-frames. For the PUSCH, on the other hand, a complete CSI is transmitted within one sub-frame. Such design principles remain applicable in the LET-A system.

The feedback contents can be divided into three categories: Channel Quality Indicator (CQI), Pre-coding Matrix Index (PMI) and Rand Index (RI), all of which are bit quantized feedbacks. The CQI typically corresponds to a transmission format having a packet error rate no more than 0.1.

In the LTE system, the following eight types of MIMO Transmission Mode (TM) for downlink data are defined:

(1) Single antenna transmission. This is used for signal transmission at a single antenna BS. This approach is a special instance of MIMO system and can only transmit a single layer of data.

(2) Transmission diversity. In a MIMO system, diversity effects of time and/or frequency can be utilized to transmit signals, so as to improve the reception quality of the signals. This approach can only transmit a single layer of data.

(3) Open-loop space division multiplexing. This is a space division multiplexing without the need for PMI feedback from UE.

(4) Closed-loop space division multiplexing. This is a space division multiplexing in which PMI feedback from UE is required.

(5) Multi-user MIMO. There are multiple UEs simultaneously participating in the downlink communication of the MIMO system.

(6) Closed-loop single layer pre-coding. Only one single layer of data is transmitted using the MIMO system. The PMI feedback from UE is required.

(7) Beam forming transmission. The beam forming technique is employed in the MIMO system. A dedicated reference signal is used for data demodulation at UE. Only one single layer of data is transmitted using the MIMO system. The PMI feedback from UE is not required.

(8) Two-layer beam forming transmission. The UE can be configured to feed back PMI and RI, or not to feed back PMI and RI.

In the LTE-A system, the above eight types of transmission modes may be retained and/or canceled, and/or a new transmission mode, dynamic MIMO switching, can be added, by which the BS can dynamically adjust the MIMO mode in which the UE operates.

In order to support the above MIMO transmission modes, a variety of CSI feedback modes are defined in the LTE system. Each MIMO transmission mode corresponds to a number of CSI feedback modes, as detailed in the following.

There are four CSI feedback modes for the PUCCH, Mode 1-0, Mode 1-1, Mode 2-0 and Mode 2-1. These modes are combination of four types of feedbacks, including:

(1) Type 1: one preferred sub-band location in a Band Part (BP, which is a subset of the Set S and has its size dependent on the size of the Set S) and a CQI for the sub-band. The respective overheads are L bits for the sub-band location, 4 bits for the CQI of the first codeword and 3 bits for the CQI of the possible second codeword which is differentially coded with respect to the CQI of the first codeword.

(2) Type 2: broadband CQI and PMI. The respective overheads are 4 bits for the CQI of the first codeword, 3 bits for the CQI of the possible second codeword which is differentially coded with respect to the CQI of the first codeword and 1, 2 or 4 bits for PMI depending on the antenna configuration at BS.

(3) Type 3: RI. The overhead for RI is 1 bit for two antennas, or 2 bits for four antennas, depending on the antenna configuration at BS.

(4) Type 4: broadband CQI. The overhead is constantly 4 bits.

The UE feeds back different information to the BS in correspondence with the above different types.

The Mode 1-0 is a combination of Type 3 and Type 4. That is, the feedbacks of Type 3 and Type 4 are carried out at different periods and/or with different sub-frame offsets. In the Mode 1-0, the broadband CQI of the first codeword in the Set S and possibly the RI information are fed back.

The Mode 1-1 is a combination of Type 3 and Type 2. That is, the feedbacks of Type 3 and Type 2 are carried out at different periods and/or with different sub-frame offsets. In the Mode 1-1, the broadband PMI of the Set S, the broadband CQIs for the individual code words and possibly the RI information are fed back.

The Mode 2-0 is a combination of Type 3, Type 4 and Type 1. That is, the feedbacks of Type 3, Type 4 and Type 1 are carried out at different periods and/or with different sub-frame offsets. In the Mode 2-0, the broadband CQI of the first codeword in the Set S, possibly the RI information as well as one preferred sub-band location in the BP and the CQI for the sub-band are fed back.

The Mode 2-1 is a combination of Type 3, Type 2 and Type 1. That is, the feedbacks of Type 3, Type 2 and Type1 are carried out at different periods and/or with different sub-frame offsets. In the Mode 2-1, the broadband PMI of the Set S, the broadband CQIs for the individual codewords and possibly the RI information, as well as one preferred sub-band location in the BP and the CQI for the sub-band are fed back.

There are thus the following correspondences between the MIMO transmission modes and the CSI feedback modes:
MIMO TM 1): Mode 1-0 and Mode 2-0;
MIMO TM 2): Mode 1-0 and Mode 2-0;
MIMO TM 3): Mode 1-0 and Mode 2-0;
MIMO TM 4): Mode 1-1 and Mode 2-1;
MIMO TM 5): Mode 1-1 and Mode 2-1;
MIMO TM 6): Mode 1-1 and Mode 2-1;
MIMO TM 7): Mode 1-0 and Mode 2-0;
MIMO TM 8): Mode 1-1 and Mode 2-1, with PMI/RI feedback from UE; or Mode 1-0 and Mode 2-0, without PMI/RI feedback from UE.

Still, CQI, PMI and RI are primary feedback contents in the single BS TM of the LTE-A system. In order that the feedback modes for a UE are consistent with those corresponding to the TM 4) and 8) and that the new TM 9) may be supported, the Mode 1-1 and Mode 2-1 in the LTE-A system are optimized for a scenario where a BS is equipped with 8 transmission antennas. That is, a PMI is collectively determined from two channel pre-coding matrix indices, W1 and W2, where W1 represents broadband/long-term channel characteristics and W2 represents sub-band/short-term channel characteristics. For transmission of W1 and W2 over PUCCH, Mode 1-1 can be sub-divided into two sub-modes: sub-mode 1 of sub-mode 2 of Mode 1-1. Some improvements are also performed on the initial mode 2-1.

In order to support the newly defined feedback mode, the following feedback types is newly introduced in the LTE-A system:

(1) Type 1a: one preferred sub-band location in a Band Part (BP, which is a subset of the Set S and has its size dependent on the size of the Set S) and a CQI for the sub-band, plus a W2 for another sub-band. The overhead for the sub-band location is L bits. The total overhead for the CQI and the W2 is 8 bits when RI=1, 9 bits when 1<RI<5, and 7 bits when RI>4.

(2) Type 2a: W1. The overhead for W1 is 4 bits when RI<3, 2 bits when 2<RI<8, and 0 bit when RI=8.

(3) Type 2b: broadband W2 and broadband CQI. The total overhead of the broadband W2 and the broadband CQI is 8 bits when RI=1, 11 bits when 1<RI<4, 10 bits when RI=4, and 7 bits when RI>4.

(4) Type 2c: broadband CQI, W1 and broadband W2. The total overhead of the broadband CQI, the W1 and the broadband W2 is 8 bits when RI=1, 11 bits when 1<RI<4, 9 bits when RI=4, and 7 bits when RI>4. It is to be noted that, in order to control feedback overhead, the value range of the W1 and the broadband W2 here is obtained by down-sampling the full value range of the W1 and the broadband W2.

(5) Type 5: RI and W1. The total overhead for the RI and the W1 is 4 bits for 8 antennas and 2-layer data multiplexing and 5 bits for 8 antennas and 4 or 8-layer data multiplexing. It is to be noted that, in order to control feedback overhead, the value range of the W1 here is obtained by down-sampling the full value range of the W1.

(6) Type 6: RI and PTI. PTI stands for Pre-coding Type Indicator and has an overhead of 1 bit for representing information on pre-coding type. The total overhead for the RI and the PTI is 2 bits for 8 antennas and 2-layer data multiplexing, 3 bits for 8 antennas and 4-layer data multiplexing, and 4 bits for 8 antennas and 8-layer data multiplexing.

In the description, "W1" and "W2" when used alone refer to "sub-band W1" and "sub-band W2" respectively, while "wideband W1" and "wideband W2" are referred to by their full expressions.

The Sub-Mode 1 of Mode 1-1, Sub-Mode 2 of Mode 1-1 and the new Mode 2-1 have the following relationship with the existing types and these new types:

(1) The Sub-Mode 1 of Mode 1-1 is a combination of Type 5 and Type 2b. That is, the feedbacks of Type 5 and Type 2b are carried out at different periods and/or with different sub-frame offsets.

(2) The Sub-Mode 2 of Mode 1-1 is a combination of Type 3 and Type 2/2c.

(2.1) For TM 4) or 8), the Sub-Mode 2 of Mode 1-1 is a combination of Type 3 and Type 2. That is, the feedbacks of Type 3 and Type 2 are carried out at different periods and/or with different sub-frame offsets.

(2.2) For TM 9), the Sub-Mode 2 of Mode 1-1 is a combination of Type 3 and Type 2c. That is, the feedbacks of Type 3 and Type 2c are carried out at different periods and/or with different sub-frame offsets.

(3) The new Mode 2-1 relates to TM 9) only and is a combination of Type 6, Type 2b and Type 2a/1a.

(3.1) When the PTI of Type 6 is 0, the new Mode 2-1 is a combination of Type 6, Type 2b and Type 2a. That is, the feedbacks of Type 6, Type 2b and Type 2a are carried out at different periods and/or with different sub-frame offsets.

(3.2) When the PTI of Type 6 is 1, the new Mode 2-1 is a combination of Type 6, Type 2b and Type 1a. That is, the feedbacks of Type 6, Type 2b and Type 1a are carried out at different periods and/or with different sub-frame offsets.

In the multi-antenna multi-BS coordination of LTE-A, for CQI being one of the feedback content, it is needed to standardize its definition and feedback schemes. Currently, there are some existing schemes, e.g., Non Patent Literature 4: 3GPP R1-113276, LGE, "CQI calculation for CoMP" (a 3GPP document numbered as R1-113276, "CQI calculation for CoMP", LGE corporation). Assume there are M BSs participating in the multi-antenna multi-BS coordination, a signal power of a i^th BS obtained by the UE measuring a reference signal is S_i, the measured noise power of the UE is

and a CQI fed back to the i^th BS is CQI_i, then there are several definitions as follows:

As to CSI feedback in the multi-antenna multi-BS coordination in the LTE-A system, the 66^th conference of 3GPP TSG RAN WG1 held in Zhuhai, China in October 2011 (Non Patent Literature 5: RAN1 Chairman's Note, 3GPP TSG RAN WG1 Meeting #66bis, Zhuhai, China, Oct, 2011) discussed and made the following preliminary decisions: the feedback content is based on a form of separately feeding back CSI to each BS and is supplemented by feeding back relative CSI (e.g., phase information and the like) between BSs, so as to dynamically support JT, DPS, CS/CB and the like operations in a unified framework of CSI feedback. The feedback schemes need to contain at least one of the first three approaches as follows:

aggregated CSI feedback based on a plurality of CSI reference signals (multiple CSI-RS resources);

CSI feedback per CSI reference signal, which also feeds back relative CSI among different CSI reference signals;
CSI feedback per CSI reference signal;
CSI feedback based on Release 8 CRS reference signal per cell.

In summary, for the CSI feedback in the multi-antenna multi-BS coordination in the LTE-A system, feedback contents may involve CSI based on codebook space search, such as CQI, PMI, RI. Studies are still needed for the definition and feedback schemes of CQI.

[NPL 1] 3GPP TR 36.814 V9.0.0 (2010-03), "Further advancements for E-UTRA physical layer aspects (Release 9) ".
[NPL 2] 3GPP R1-093720, "CoMP email summary", Qualcomm.
[NPL 3] 3GPP, R1-083546, "Per-cell precoding methods for downlink joint processing CoMP", ETRI.
[NPL 4] 3GPP R1-113276, LGE, "CQI calculation for CoMP".
[NPL 5] RAN1 Chairman's Note, 3GPP TSG RAN WG1 Meeting #66bis, Zhuhai, China, Oct, 2011.

It is an object of the present invention to solve the problem of insufficient and inoperable or unreasonable CQI feedback in multi-antenna multi-BS coordination by providing a novel CQI feedback approach.

According to the first aspect of the present invention, a User Equipment (UE) is provided. The UE comprises: a coordinated Base Station (BS) set determining unit, configured to determine a set of coordinated BSs participating in multi-BS coordination, the set of coordinated BSs containing a serving BS and non-serving BSs; and a Channel Quality Indicator (CQI) feedback unit, configured to feed back, for one predefined sub-band or each predefined sub-band of multiple predefined sub-bands, a difference between a CQI of non-coherent joint transmission and a common CQI value obtained by the UE to the serving BS.

According to the second solution of the present invention, a Channel Quality Indicator (CQI) feedback method is provided. The CQI feedback method comprises: determining a set of coordinated BSs participating in multi-BS coordination, the set of coordinated BSs containing a serving BS and non-serving BSs; and feeding back, for one predefined sub-band or each predefined sub-band of multiple predefined sub-bands, a difference between a CQI of non-coherent joint transmission and a common CQI value obtained by a User Equipment (UE) to the serving BS.

As such, the CQI feedback method and the UE provided for the multi-BS coordination mode according to the present invention have the advantages of simple implementation and low signaling overhead.

The above and other objects, features and advantages of the present invention will be more apparent from the following preferred embodiments illustrated with reference to the figures, in which:
Fig. 1 is a schematic diagram of a MIMO system; Fig. 2 is a schematic diagram of complete CSI feedback; Fig. 3 is a schematic diagram of statistic-based CSI feedback; Fig. 4 is a schematic diagram of CSI feedback based on codebook space search; Fig. 5 is a schematic diagram of a multi-cell cellular communication system; Fig. 6 is a flowchart illustrating the CQI feedback method according to the present invention; and Fig. 7 is a schematic block diagram of a UE according to the present invention.

Preferred embodiments of the present invention will be detailed with reference to the drawings. In the following description, details and functions unnecessary to the present invention are omitted so as not to obscure the concept of the invention. For clear and detailed explanation of the implementation steps of the present invention, some specific examples applicable to the LTE-A cellular communication system are given below. Herein, it is to be noted that the present invention is not limited to the application exemplified in the embodiments. Rather, it is applicable to other communication systems, such as the future 5G system.

Fig. 5 is a schematic diagram of a multi-cell cellular communication system. The cellular system divides a service coverage area into a number of adjacent wireless coverage areas, i.e., cells. In Fig. 5, the entire service area is formed by

cells

100, 102 and 104, each being illustratively shown as a hexagon. Base Stations (BSs) 200, 202 and 204 are associated with the

cells

100, 102 and 104, respectively. As known to those skilled in the art, each of the BSs 200-204 includes at least a transmitter and a receiver. Herein, it is to be noted that a BS, which is generally a serving node in a cell, can be an independent BS having a function of resource scheduling, a transmitting node belonging to an independent BS, a relay node (which is generally configured for further enlarging the coverage of a cell), or the like. As illustratively shown in Fig. 5, each of the BSs 200-204 is located in a particular area of the corresponding one of the cells 100-104 and is equipped with an omni-directional antenna. However, in a cell arrangement for the cellular communication system, each of the BSs 200-204 can also be equipped with a directional antenna for directionally covering a partial area of the corresponding one of the cells 100-104, which is commonly referred to as a sector. Thus, the diagram of the multi-cell cellular communication system as shown in Fig. 5 is illustrative only and does not imply that the implementation of the cellular system according to the present invention is limited to the above particular constraints.

As shown in Fig. 5, the BSs 200-204 are connected with each other via X2 interfaces 300, 302 and 304. In a LTE-A system, a three-layer node network architecture including base station, radio network control unit and core network is simplified into a two-layer node architecture in which the function of the radio network control unit is assigned to the base station and a wired interface named "X2" is defined for coordination and communication between base stations.

In Fig. 5, the BSs 200-204 are also connected with each other via air interfaces, A1 interfaces, 310, 312 and 314. In a future communication system, it is possible to introduce a concept of relay node. Relay nodes are connected with each other via wireless interfaces and a base station can be considered as a special relay node. Thus, a wireless interface named "A1" can then be used for coordination and communication between base stations.

Additionally, an upper layer entity 220 of the BSs 200-204 is also shown in Fig. 5, which can be a gateway or another network entity such as mobility management entity. The upper layer entity 220 is connected to the BSs 200-204 via S1 interfaces 320, 322 and 324, respectively. In a LTE system, a wired interface named "S1" is defined for coordination and communication between the upper layer entity and the base station.

A number of User Equipments (UEs) 400-430 are distributed over the cells 100-104, as shown in Fig. 5. As known to those skilled in the art, each of the UEs 400-430 includes a transmitter, a receiver and a mobile terminal control unit. Each of the UEs 400-430 can access the cellular communication system via its serving BS (one of the BSs 200-204). It should be understood that while only 16 UEs are illustratively shown in Fig. 5, there may be a large number of UEs in practice. In this sense, the description of the UEs in Fig. 5 is also for illustrative purpose only. Each of the UEs 400-430 can access the cellular communication network via its serving BS. The BS directly providing communication service to a certain UE is referred to as the serving BS of that UE, while other BSs are referred to non-serving BSs of that UE. The non-serving BSs can function as coordinated BSs of the serving BS and provide communication service to the UE along with the serving BS.

For explanation of this embodiment, the UE 416 is considered. The UE 416 operates in a multi-BS coordination mode, has BS 202 as its serving BS and has

BSs

200 and 204 as its coordinated BSs. It is to be noted that this embodiment focuses on the UE 416, which does not imply that the present invention is only applicable to one UE scenario. Rather, the present invention is fully applicable to multi-UE scenario. For example, the inventive method can be applied to the

UEs

408, 410, 430 and the like as shown in Fig. 5.In an exemplary scenario, there is one serving BS and two coordinated BSs. However, the present invention is not limited to this. In fact, the present invention is not limited to any specific number of serving BS(s) or coordinated BS(s).

Fig. 6 is a flowchart illustrating the CQI feedback method 600 according to the present invention. In description of the embodiment, the following scenario of multi-BS coordination is assumed.

(Exemplary Scenario): Take the UE 416 as an example, which operates in a multi-BS coordination mode, has BS 202 as its serving BS, and has

BSs

200 and 204 as its coordinated BSs (non-serving BSs). The UE 416 can be a single antenna or multi-antenna device.

As for other UEs operable in the multi-BS coordination mode (e.g., any one of UEs 400-430), its serving BS and coordinated BSs may be assumed in a similar way.

At step S610, a set of coordinated BSs participating in multi-antenna multi-BS coordination is determined.

In an embodiment, the UE (e.g., UE 416) can periodically report to the serving BS (e.g., the serving BS 202) path loss information from the UE to its adjacent BSs. Accordingly, the serving BS can estimate the geographic location of the UE from the report, then determine the non-serving BSs participating in multi-BS coordination for the UE (e.g., BSs 200 and 204) and notify the UE of the non-serving BSs. The UE can obtain the set of coordinated BSs based on the serving BS and the notified non-serving BSs. Alternatively, the UE can determine by itself the non-serving BSs to participate in multi-BS coordination for the UE from the measured path loss information, thereby determining the set of coordinated BSs.

As step S630, in accordance with the feedback design in the existing system, for one predefined sub-band or each predefined sub-band of multiple predefined sub-bands, a difference between a CQI of non-coherent joint transmission and a common CQI value measured by the UE is fed back to the serving BS (hereinafter, referred to as a difference CQI in the sense of aggregation).

By feeding back the difference between the CQI of non-coherent joint transmission and the common CQI value to the serving BS, it is possible to make the serving BS know the CQI of non-coherent joint transmission with low overhead.

Alternatively, the method 600 further includes: determining, for one predefined sub-band or each predefined sub-band of multiple predefined sub-bands, a CQI of each BS within the set of coordinated BSs in the predefined sub-band. The method 600 further includes one or more of the following steps of: calculating, for one predefined sub-band or each predefined sub-band of multiple predefined sub-bands, a common CQI value in the predefined sub-band based on the CQI of each BS within the set of coordinated BSs in the predefined sub-band; feeding back, for one predefined sub-band or each predefined sub-band of multiple predefined sub-bands, the CQI of each BS within the set of coordinated BSs in the predefined sub-band to the serving BS; feeding back a sequence number(s) of the one predefined sub-band or multiple predefined sub-bands to the serving BS; and selecting, for each bandwidth part(BP) in a system bandwidth, a sub-band providing the maximum CQI of non-coherent joint transmission as a preferred sub-band of the corresponding bandwidth part.

As a non-limiting example, the one predefined sub-band or multiple predefined sub-bands are all the preferred sub-bands in the system bandwidth, or sub-bands capable of providing the maximum CQI of non-coherent joint transmission among all the preferred sub-bands in the system bandwidth.

As a non-limiting example, a common CQI value in one predefined sub-band or each predefined sub-band of multiple predefined sub-bands is a sum of all CQIs of all BSs within the set of coordinated BSs in the predefined sub-band. As another non-limiting example, a common CQI value in one predefined sub-band or each predefined sub-band of multiple predefined sub-bands is the maximum CQI among CQIs of all BSs within the set of coordinated BSs in the predefined sub-band. It should be noted that the common CQI value in the present invention is not limited to these two examples, but may be a result of a linear function or a non-linear function having CQIs of all the BSs within the set of coordinated BSs as variants. This will be explained below in detail.

Detailed explanations of the CQI feedback operation from the UE to the serving BS will be given below by referring to the following specific instance.

In this instance, the feedback mode of the UE 416 is configured by the serving BS 202 as Mode 2-0.

The system bandwidth of the serving BS is of N_RB resource blocks, and each k resource blocks constitute one sub-band. If

there are

sub-bands in total over the entire system bandwidth. If

there is additionally one sub-band that includes

blocks of resource, and there are

sub-bands in total over the entire system bandwidth. Sequence numbers are allocated to the sub-bands from low to high in terms of frequency.

The system bandwidth of the serving BS contains J bandwidth parts (BP). The value of J may be determined by the system. The values that can be taken may refer to table 7.2.2-2 in the document "Physical layer procedures(Release 10)", 3GPP TS 36.213 v10.4.0 (2011-12). A bandwidth part numbered as j includes N_j sub-bands. The range of j is

and may increases as the frequency increases. The bandwidth parts are connected end to end in the frequency domain and there is no interval between any two BPs.

In Mode 2-0:

1. The UE 416 may select one preferred sub-band for each bandwidth part. The preferred sub-band refers to a sub-band that is capable of providing the maximum CQI of non-coherent joint transmission under joint transmission.

2. In the preferred sub-band of the UE in each bandwidth part, the UE 416 may obtain a CQI of each BS within the set of coordinated BSs in the sub-band by measurement and calculation. The UE may also calculate a CQI under joint transmission in the sub-band, i.e., CQI_JT. The CQI here only reflects a channel quality of the first code word, even if RI>1.

3. The UE may traverse preferred sub-bands in all the bandwidth parts, so as to calculate a difference CQI in the sense of aggregation when using the sub-band.

4. The UE may:

a) select a sub-band capable of providing the maximum CQI of non-coherent joint transmission from all the preferred sub-bands as the optimal sub-band, and feed back a CQI of each BS in the optimal sub-band, a sequence number of the optimal sub-band and a difference CQI in the sense of aggregation to the serving BS 202; or

b) feed back CQIs of each BS in all the preferred sub-bands, a sequence number of each preferred sub-band and a difference CQI in the sense of aggregation to the serving BS 202;

5. CQI may be denoted using 4 bits, and the number of bits for sequence numbers of one or more sub-bands and the difference CQI in the sense of aggregation may be determined depending on actual system implementations. Assume that there are 16 bandwidth parts in this instance, and each bandwidth part contains one preferred sub-band. That is, there are 16 preferred sub-bands in total for the UE.

Assume that there are M BSs in total in the set of coordinated BSs participating in multi-antenna multi-BS coordination, where a signal power value corresponding to the serving BS obtained by the UE measuring a reference signal is S₀, and a signal power value corresponding to a non-serving BS obtained by the UE measuring a reference signal is denoted as

A sum of signal power values of M BSs in the set of coordinated BSs calculated by the UE is

A noise power value of the UE is

A CQI value corresponding to the serving BS calculated by the UE is CQI₀. A CQI value corresponding to a non-serving BS calculated by the UE is

A difference CQI in the sense of aggregation is denoted as

The UE's actual CQI of non-coherent joint transmission is CQI_JT.

CQI_JT may be defined as:

Here, the functions f₁ and f₂ are the generalized weighted summation functions. Specifically,

g_i(x₁,x₂,...,x_n) may be either a linear function or a non-linear function. The typical linear function may include, for example, the summation function and the weighted summation function, and the non-typical linear functions may include, for example, the exponential function and the logarithmic function.

The CQI may be defined as either

It should be noted that the definition of CQI is not limited to this, but may employ various other appropriate forms.

Depending on different functions f₁, f₂, and g_i, the difference CQI in the sense of aggregation may be, but not limited to the following forms:

Form 1:

Form 2:

Form 3:

It should be noted that the difference CQI in the sense of aggregation is not limited to the above three forms, but may employ all forms conforming with the following function:

In this instance, there are three BSs in the set of coordinated BSs participating in multi-antenna and multi-BS:

BSs

200, 202 and 204. A CQI measurement value of the serving BS 202 is denoted as CQI₀, and CQI measurement values of the

non-serving BSs

200 and 204 may be denoted as CQI₁ and CQI₂ respectively. The UE searches all the possible preferred sub-bands for the UE and measures and calculates a CQI of non-coherent joint transmission in a corresponding sub-band b, i.e.,

For example, definitions for the function f and CQI in Form 2 are assumed, then in the b^th preferred sub-band:

Assume that CQI_JT on the 16^th sub-band, denoted as

is maximum among all the preferred sub-bands.

After calculating

it is needed to quantize

based on a quantization table. When designing quantization values in the quantization table, signs may be not considered. This is because in the practical system, depending on the used definitions of different functions f and

the UE may not be scheduled to use non-coherent joint transmission no matter

is negative or positive. Moreover, when

is lower than a certain threshold quantization value, the UE may not be scheduled to use non-coherent joint transmission. Therefore,

below a certain threshold quantization value may be uniformly represented by using one quantization index, so as to compress the quantization table and save bit overhead of feedback. Quantization step in the quantization table is variable. That is, it is unnecessarily uniformly quantized.

One example quantization table applicable for defining

in the above Form 1 is shown in Table 1. This quantization table considers cases where

may be either positive or negative. The actually used quantization table may uniformly quantize

below a certain threshold quantization value as one quantization index. For example, all

below -1 in Table 1 may be quantized as "000". The number of bits for

is 3 in Table 1. However, the number of bits for the quantization index of

in the practical system is not limited to 3, but may be of any other number. In Table 1, intervals between different quantization values are the same and are all equal to 1. That is, the quantization step is constant.

However, in the practical system, the quantization step is not necessarily constant.

One example quantization table applicable for defining

in the above Form 2 is shown in Table 2. This quantization table considers cases where

may be negative but is impossible positive. The actually used quantization table may uniformly quantize

below -7 in Table 2 may be quantized as "00". The number of bits for

is 2 in Table 2. However, the number of bits for the quantization index of

in the practical system is not limited to 2, but may be of any other number. In Table 2, intervals between different quantization values are different. In the practical system, the quantization step may be or may be not the same.

As for the feedback to the serving BS 202, the UE may employ the following two kinds of feedback contents:

1. The UE feeds back a CQI of each BS in the optimal sub-band, a sequence number of the optimal sub-band, and a difference CQI in the sense of aggregation. The CQI of the serving BS 202 in the optimal sub-band is indicated using 4 bits. The sequence number of the optimal sub-band is indicated using p bits. The CQI of the non-serving BS 200 in the optimal sub-band is indicated using 4 bits. The CQI of the non-serving BS 204 in the optimal sub-band is indicated using 4 bits. The difference CQI in the sense of aggregation is indicated using q bits. In this example, let the parameter p=4, that is, the UE indicates that the sequence number of the optimal sub-band is 16 by using 4 bits "1111". Alternatively, p may be not equal to 4 by in other manners, and then the sequence number of the optimal sub-band may be indicated by a sequence number of a bandwidth part plus a sequence number of the sub-band within the bandwidth part. Correspondingly, the parameter p may be equal to the number of bits for the sequence number of the bandwidth part plus the number of bits for the sequence number of the sub-band within the bandwidth part. For example, if the optimal sub-band is the second sub-band in the 16^thbandwidth part, and the 16^th bandwidth part contains 5 sub-bands,

bits are needed to indicate the sequence number of the bandwidth part,

bits are needed to indicate the sequence number of the sub-band within the bandwidth part, the parameter p=4+3=7, and the sequence number of the optimal sub-band is formed by "1111" and "010". If the quantization table 1 is used in this example, q=3. If the quantization table 2 is used in this example, q=2. In the practical system, the value of q is specified by the system.

2. The UE feeds back a CQI of each BS in preferred sub-bands in all the bandwidth parts, a sequence number of each preferred sub-band, and a difference CQI in the sense of aggregation in each preferred sub-band. The CQI of the serving BS 202 in each preferred sub-band is indicated using 4 bits. The sequence number of each preferred sub-band is indicated using p bits. The CQI of the non-serving BS 200 in each preferred sub-band is indicated using 4 bits. The CQI of the non-serving BS 204 in each preferred sub-band is indicated using 4 bits. The difference CQI in the sense of aggregation in each preferred sub-band is indicated using q bits. A sequence number of a preferred sub-band is formed by two parts: a sequence number of a bandwidth part and a sequence number of the sub-band within the corresponding bandwidth part. Correspondingly, the parameter p may be equal to the number of bits for the sequence number of the bandwidth part plus the number of bits for the sequence number of the sub-band within the bandwidth part. If the quantization table 1 is used in this example, q=3. If the quantization table 2 is used in this example, q=2. In the practical system, the value of q is specified by the system.

In order to implement the above CSI feedback method, the present invention further provides a UE. Fig. 7 shows a schematic block diagram of a UE 700 according to the present invention. As shown in Fig. 7, the UE 700 according to the present invention includes a coordinated BS set determining unit 710, a CQI feedback unit 730, a CQI determining unit 750 and a common CQI value calculating unit 770. The CQI determining unit 750 and the common CQI value calculating unit 770 are optional and are illustrated in dash lines in Fig. 7.

The coordinated BS set determining unit 710 may determine a set of coordinated BSs for the UE 700 based on the non-serving BSs notified by the serving BS. Alternatively, the coordinated BS set determining unit 710 may by itself determine non-serving BSs to participate in the multi-BS coordination for the UE based on path loss information measured by the UE 700, so as to determine the set of coordinated BSs. The set of coordinated BSs is formed by the serving BS and the non-serving BSs.

The CQI feedback unit 730 may feed back, for one predefined sub-band or each predefined sub-band of multiple predefined sub-bands, a difference between a CQI of non-coherent joint transmission and a common CQI value (i.e., a difference CQI in the sense of aggregation) measured by the UE to the serving BS, in accordance with feedback designs of the existing system. The detailed description of the difference fed back by the CQI feedback unit 730 to the non-serving BS may refer to the foregoing detailed explanations. For the purpose of clarity and conciseness, details of difference to be fed back are omitted here.

The CQI determining unit 750 may determine, for one predefined sub-band or each predefined sub-band of multiple predefined sub-bands, a CQI of each BS within the set of coordinated BSs in the predefined sub-band by using the existing CQI calculation techniques.

The common CQI value calculating unit 770 may, for one predefined sub-band or each predefined sub-band of multiple predefined sub-bands, a common CQI value based on the CQI of each BS within the set of coordinated BSs in the predefined sub-band.

As a non-limiting example, the CQI feedback unit 730 may further feed back, for one predefined sub-band or each predefined sub-band of multiple predefined sub-bands, CQI of each BS within the set of coordinated BSs in the predefined sub-band to the serving BS. As another non-limiting example, the CQI feedback unit 730 may further feed back a sequence number(s) of the one predefined sub-band or multiple predefined sub-bands to the serving BS.

As a non-limiting example, a common CQI value in one predefined sub-band or each predefined sub-band of multiple predefined sub-bands is a sum of all CQIs of all BSs in the set of coordinated BSs in the predefined sub-band. As another non-limiting example, a common CQI value in one predefined sub-band or each predefined sub-band of multiple predefined sub-bands is the maximum CQI among CQIs of all BSs in the set of coordinated BSs in the predefined sub-band. It should be noted that the definitions of the common CQI value is not limited to these, but may employ various forms determined by the following function:

Detailed description for this formula may refer to the description in the foregoing examples. For the purpose of clarity and conciseness, the detailed description is omitted here.

Alternatively, the UE 700 may further include a selecting unit (not shown), configured to select, for each bandwidth part in a system bandwidth, a sub-band providing the maximum CQI of non-coherent joint transmission as a preferred sub-band for the corresponding bandwidth part. As a non-limiting example, the UE 700 may traverse all the preferred sub-bands in the system bandwidth, to calculate and feed back a difference CQI using these sub-bands. As another non-limiting example, the UE 700 may select a sub-band capable of providing the maximum CQI of non-coherent joint transmission from all the preferred sub-bands as the optimal sub-band, and feed back a CQI of each BS in the optimal sub-band, a sequence number of the optimal sub-band and a difference CQI when using the optimal sub-band to the serving BS.

It should be noted that two or more different units in the present invention may be logically or physically combined. For example, the CQI determining unit 750 and the common CQI value calculating unit 770 may be combined as one single unit.

In the present invention, the UE 700 may feed back a difference between a CQI of non-coherent joint transmission and a CQI common value (i.e., a difference CQI in the sense of aggregation) to the serving BS, so as to implement an effect of feeding back CQI of non-coherent joint transmission to the serving BS with small overhead.

The present invention can also be expressed as follows.

According to the first aspect of the present invention, a User Equipment (UE) is provided. The UE comprises: a coordinated Base Station (BS) set determining unit, configured to determine a set of coordinated BSs participating in multi-BS coordination, the set of coordinated BSs containing a serving BS and a non-serving BS; and a Channel Quality Indicator (CQI) feedback unit, configured to feed back, for one predefined sub-band or each predefined sub-band of multiple predefined sub-bands, a difference between a CQI of non-coherent joint transmission and a common CQI value obtained by the UE to the serving BS.

Preferably, the UE further comprises a CQI determining unit, configured to determine, for one predefined sub-band or each predefined sub-band of multiple predefined sub-bands, a CQI of each BS within the set of coordinated BSs in the predefined sub-band.

Preferably, the UE further comprises a common CQI value calculating unit, configured to calculate, for one predefined sub-band or each predefined sub-band of multiple predefined sub-bands, a common CQI value in the predefined sub-band based on the CQI of each BS within the set of coordinated BSs in the predefined sub-band.

Preferably, wherein the CQI feedback unit is further configured to feed back, for one predefined sub-band or each predefined sub-band of multiple predefined sub-bands, the CQI of each BS within the set of coordinated BSs in the predefined sub-band to the serving BS.

Preferably, the CQI feedback unit is further configured to feed back a sequence number(s) of the one predefined sub-band or multiple predefined sub-bands to the serving BS.

Preferably, the UE further comprises a selecting unit, configured to select, for each bandwidth part in a system bandwidth, a sub-band providing the maximum CQI of non-coherent joint transmission as a preferred sub-band for the corresponding bandwidth part.

Preferably, the one predefined sub-band or multiple predefined sub-bands are all the preferred sub-bands in the system bandwidth.

Preferably, the one predefined sub-band or multiple predefined sub-bands are sub-bands capable of providing the maximum CQI of non-coherent joint transmission among all the preferred sub-bands in the system bandwidth.

Preferably, a common CQI value in one predefined sub-band or each predefined sub-band of multiple predefined sub-bands is a sum of all CQIs of all BSs within the set of coordinated BSs in the predefined sub-band.

Preferably, a common CQI value in one predefined sub-band or each predefined sub-band of multiple predefined sub-bands is the maximum CQI among CQIs of all BSs within the set of coordinated BSs in the predefined sub-band.

According to the second solution of the present invention, a Channel Quality Indicator (CQI) feedback method is provided. The CQI feedback method comprises: determining a set of coordinated BSs participating in multi-BS coordination, the set of coordinated BSs containing a serving BS and a non-serving BS; and feeding back, for one predefined sub-band or each predefined sub-band of multiple predefined sub-bands, a difference between a CQI of non-coherent joint transmission and a common CQI value obtained by a User Equipment (UE) to the serving BS.

Preferably, the CQI feedback method further comprises: determining, for one predefined sub-band or each predefined sub-band of multiple predefined sub-bands, a CQI of each BS within the set of coordinated BSs in the predefined sub-band.

Preferably, the CQI feedback method further comprises: calculating, for one predefined sub-band or each predefined sub-band of multiple predefined sub-bands, a common CQI value in the predefined sub-band based on the CQI of each BS within the set of coordinated BSs in the predefined sub-band.

Preferably, the CQI feedback method further comprises: feeding back, for one predefined sub-band or each predefined sub-band of multiple predefined sub-bands, the CQI of each BS within the set of coordinated BSs in the predefined sub-band to the serving BS.

Preferably, the CQI feedback method further comprises: feeding back a sequence number(s) of the one predefined sub-band or multiple predefined sub-bands to the serving BS.

Preferably, the CQI feedback method further comprises: selecting, for each bandwidth part in a system bandwidth, a sub-band providing the maximum CQI of non-coherent joint transmission as a preferred sub-band of the corresponding bandwidth part.

It should be noted that the solution of the present invention has been described above by a way of example only. However, the present invention is not limited to the above steps and element structures. It is possible to adjust, add and remove the steps and elements structures depending on actual requirements. Thus, some of the steps and elements are not essential for achieving the general inventive concept of the present invention. Therefore, the features necessary for the present invention is only limited to a minimum requirement for achieving the general inventive concept of the present invention, rather than the above specific examples.

The present invention has been described above with reference to the preferred embodiments thereof. It should be understood that various modifications, alternations and additions can be made by those skilled in the art without departing from the spirits and scope of the present invention. Therefore, the scope of the present invention is not limited to the above particular embodiments but only defined by the claims as attached.

Claims

A User Equipment (UE), comprising:
a coordinated Base Station (BS) set determining unit, configured to determine a set of coordinated BSs participating in multi-BS coordination, the set of coordinated BSs containing a serving BS and a non-serving BS; and
a Channel Quality Indicator (CQI) feedback unit, configured to feed back, for one predefined sub-band or each predefined sub-band of multiple predefined sub-bands, a difference between a CQI of non-coherent joint transmission and a common CQI value obtained by the UE to the serving BS.
The UE according to claim 1, further comprising:
a CQI determining unit, configured to determine, for one predefined sub-band or each predefined sub-band of multiple predefined sub-bands, a CQI of each BS within the set of coordinated BSs in the predefined sub-band.
The UE according to claim 2, further comprising:
a common CQI value calculating unit, configured to calculate, for one predefined sub-band or each predefined sub-band of multiple predefined sub-bands, a common CQI value in the predefined sub-band based on the CQI of each BS within the set of coordinated BSs in the predefined sub-band.
The UE according to claim 2, wherein the CQI feedback unit is further configured to feed back, for one predefined sub-band or each predefined sub-band of multiple predefined sub-bands, the CQI of each BS within the set of coordinated BSs in the predefined sub-band to the serving BS.
The UE according to claim 2, wherein the CQI feedback unit is further configured to feed back a sequence number(s) of the one predefined sub-band or multiple predefined sub-bands to the serving BS.
The UE according to any one of claims 1-5, further comprising:
a selecting unit, configured to select, for each bandwidth part in a system bandwidth, a sub-band providing the maximum CQI of non-coherent joint transmission as a preferred sub-band for the corresponding bandwidth part.
The UE according to claim 6, wherein the one predefined sub-band or multiple predefined sub-bands are all the preferred sub-bands in the system bandwidth.
The UE according to claim 6, wherein the one predefined sub-band or multiple predefined sub-bands are sub-bands capable of providing the maximum CQI of non-coherent joint transmission among all the preferred sub-bands in the system bandwidth.
The UE according to claim 3, wherein a common CQI value in one predefined sub-band or each predefined sub-band of multiple predefined sub-bands is a sum of all CQIs of all BSs within the set of coordinated BSs in the predefined sub-band.
The UE according to claim 3, wherein a common CQI value in one predefined sub-band or each predefined sub-band of multiple predefined sub-bands is the maximum CQI among CQIs of all BSs within the set of coordinated BSs in the predefined sub-band.
A Channel Quality Indicator (CQI) feedback method, comprising:
determining a set of coordinated BSs participating in multi-BS coordination, the set of coordinated BSs containing a serving BS and a non-serving BS; and
feeding back, for one predefined sub-band or each predefined sub-band of multiple predefined sub-bands, a difference between a CQI of non-coherent joint transmission and a common CQI value obtained by a User Equipment (UE) to the serving BS.
The CQI feedback method according to claim 11, further comprising:
determining, for one predefined sub-band or each predefined sub-band of multiple predefined sub-bands, a CQI of each BS within the set of coordinated BSs in the predefined sub-band.
The CQI feedback method according to claim 12, further comprising:
calculating, for one predefined sub-band or each predefined sub-band of multiple predefined sub-bands, a common CQI value in the predefined sub-band based on the CQI of each BS within the set of coordinated BSs in the predefined sub-band.
The CQI feedback method according to claim 12, further comprising:
feeding back, for one predefined sub-band or each predefined sub-band of multiple predefined sub-bands, the CQI of each BS within the set of coordinated BSs in the predefined sub-band to the serving BS.
The CQI feedback method according to claim 12, further comprising:
feeding back a sequence number(s) of the one predefined sub-band or multiple predefined sub-bands to the serving BS.
The CQI feedback method according to any one of claims 11-15, further comprising:
selecting, for each bandwidth part in a system bandwidth, a sub-band providing the maximum CQI of non-coherent joint transmission as a preferred sub-band of the corresponding bandwidth part.
The CQI feedback method according to claim 16, wherein the one predefined sub-band or multiple predefined sub-bands are all the preferred sub-bands in the system bandwidth.
The CQI feedback method according to claim 16, wherein the one predefined sub-band or multiple predefined sub-bands are sub-bands capable of providing the maximum CQI of non-coherent joint transmission among all the preferred sub-bands in the system bandwidth.
The CQI feedback method according to claim 13, wherein a common CQI value in one predefined sub-band or each predefined sub-band of multiple predefined sub-bands is a sum of all CQIs of all BSs within the set of coordinated BSs in the predefined sub-band.
The CQI feedback method according to claim 13, wherein a common CQI value in one predefined sub-band or each predefined sub-band of multiple predefined sub-bands is the maximum CQI among CQIs of all BSs within the set of coordinated BSs in the predefined sub-band.