CN105917594A - Base station, mobile station and method thereof - Google Patents

Abstract

The embodiment of the invention discloses a base station, a mobile station and a method thereof. The communication method for a mobile station which is served by a base station having at least one transmission point each equipped with multiple transmit antennas includes determining a best subset of transmit antennas for each transmission point which is selected, based on channel gains, among the multiple transmit antennas of the transmission point; and reporting to the base station quantized channel state information CSI for the transmit antennas in the best subset. The embodiments of the invention can reduce the feedback overhead.

Description

Base station, mobile station and method thereof

Technical Field

The present invention relates to the field of mobile communication technology, and more particularly, to a base station, a mobile station, and methods thereof.

Background

Multi-antenna systems, such as massive MIMO (multiple input multiple output), are a promising technology in future 5G and other communication systems, which can achieve great improvements in spectral efficiency and simple TX/RX (transmit/receive) architecture. Ideally, the more TX antennas, the greater the benefit from perfect CSIT (transmit side channel state information) based antenna selection, which can result in significant feedback overhead and cost. In addition, the dedicated RF components for each antenna, such as a/D (analog to digital) converters, D/a (digital to analog) converters, and amplifiers, generate significant costs.

In multi-antenna systems with a limited number of RF (radio frequency) chains (or RF hardware), the resources for feedback are limited. Therefore, there is a need to investigate practical arrangements with limited feedback and RF chains.

Disclosure of Invention

The embodiment of the invention relates to a base station, a mobile station and a method thereof for reducing feedback overhead.

In a first aspect, a communication method for a mobile station is provided, the mobile station being served by a base station, the base station having at least one transmission point, each of the transmission points being equipped with a plurality of transmit antennas, the method comprising: determining an optimal subset of transmit antennas for each selected transmission point from among a plurality of transmit antennas for the transmission point based on channel gain; and reporting the quantized channel state information, CSI, of the transmit antennas in the best subset to the base station.

In a first possible implementation form of the method according to the first aspect, the determining an optimal subset of transmit antennas for each transmission point includes: obtaining the channel gains for the plurality of transmit antennas of the transmission point; and selecting the transmit antenna with the largest channel gain as the best subset.

In a second possible implementation form of the method according to the first aspect as such or according to any of the preceding implementation forms of the first aspect, the determining the optimal subset of transmit antennas for each transmission point comprises: obtaining the channel gains for the plurality of transmit antennas of the transmission point; and selecting the transmitting antenna with the channel gain larger than a threshold value as the optimal subset.

In a third possible implementation form of the method according to the first aspect as such or according to any of the preceding implementation forms of the first aspect, the method further comprises: reporting the index of the transmit antenna in the best subset to the base station.

In a fourth possible implementation form of the method according to the first aspect as such or according to any of the preceding implementation forms of the first aspect, the determining the optimal subset of transmit antennas for each transmission point comprises: determining the best subset based on indication information received from the base station, the indication information indicating an index of a transmit antenna in the best subset selected by the base station according to a channel gain.

In a fifth possible implementation form of the method according to the first aspect as such or according to any of the preceding implementation forms of the first aspect, the reporting quantized channel state information, CSI, of the transmit antennas in the best subset to the base station comprises: reporting the quantized CSI using a codebook, a size of the codebook depending on a size of the optimal subset.

In a sixth possible implementation form of the method according to the first aspect as such or according to any of the preceding implementation forms of the first aspect, the method further comprises: receiving size information indicating a size of the best subset from the base station; or selecting the size of the best subset at the mobile station.

In a seventh possible implementation form of the method according to the first aspect as such or according to any of the preceding implementation forms of the first aspect, the method further comprises: reporting, to the base station, quantized CSI for transmit antennas other than the transmit antennas in the best subset.

In a second aspect, a communication method of a base station is provided, the base station having at least one transmission point, each of the transmission points being equipped with a plurality of transmit antennas, the method comprising: receiving a report from a mobile station reporting quantized channel state information, CSI, of a best subset of transmit antennas determined for each selected transmission point from a plurality of transmit antennas for the transmission point based on channel gain; and transmitting data based on the quantized CSI.

In a first possible implementation form of the method according to the second aspect, the method further comprises: obtaining the channel gains for the plurality of transmit antennas of the transmission point; and selecting the transmit antenna with the largest channel gain as the best subset.

In a second possible implementation form of the method according to the second aspect as such or according to any of the preceding implementation forms of the second aspect, the method further comprises: obtaining the channel gains for the plurality of transmit antennas of the transmission point; and selecting the transmitting antenna with the channel gain larger than a threshold value as the optimal subset.

In a third possible implementation form of the method according to the second aspect as such or according to any of the preceding implementation forms of the second aspect, the method further comprises sending indication information to the mobile station, the indication information being used to indicate to the mobile station the index of the transmit antenna in the best subset.

In a fourth possible implementation form of the method according to the second aspect as such or according to any of the preceding implementation forms of the second aspect, the method further comprises receiving indication information from a mobile station, the indication information indicating an index of a transmit antenna in the best subset selected by the mobile station according to a channel gain.

In a fifth possible implementation form of the method according to the second aspect as such or according to any of the preceding implementation forms of the second aspect, the method further comprises the reporting of the quantized CSI using a codebook, the size of the codebook depending on the size of the best subset.

In a sixth possible implementation form of the method according to the second aspect as such or according to any of the preceding implementation forms of the second aspect, size information indicating a size of the best subset is received from the mobile station; or selecting the size of the best subset at the base station.

In a seventh possible implementation form of the method according to the second aspect as such or according to any of the preceding implementation forms of the second aspect, the method further comprises receiving a report from the mobile station, the report reporting quantized CSI for transmit antennas other than the transmit antennas in the best subset.

In a third aspect, there is provided a mobile station served by a base station having at least one transmission point, each of the transmission points being equipped with a plurality of transmit antennas, the mobile station comprising: a determining unit for determining an optimal subset of transmit antennas for each selected transmission point from the plurality of transmit antennas for the transmission point based on the channel gain; and a reporting unit for reporting the quantized channel state information CSI of the transmit antennas in the best subset to a base station.

In a first possible implementation form of the mobile station according to the third aspect, the determining unit is configured to obtain the channel gains of the multiple transmit antennas of the transmission point, and select the transmit antenna with the largest channel gain as the optimal subset.

In a second possible implementation form of the mobile station according to the third aspect or according to any of the preceding implementation forms of the third aspect, the determining unit is configured to obtain the channel gains of the multiple transmit antennas of the transmission point, and select, as the optimal subset, the transmit antenna for which the channel gain is greater than a threshold.

In a third possible implementation form of the mobile station according to the third aspect as such or according to any of the preceding implementation forms of the third aspect, the reporting unit is further configured to report the index of the transmit antenna in the best subset to the base station.

In a fourth possible implementation form of the mobile station according to the third aspect as such or according to any of the preceding implementation forms of the third aspect, the determining unit is configured to determine the best subset based on indication information received from the base station, the indication information indicating an index of a transmit antenna in the best subset selected by the base station according to a channel gain.

In a fifth possible implementation form of the mobile station according to the third aspect as such or according to any of the preceding implementation forms of the third aspect, the reporting unit is configured to report the quantized CSI using a codebook, a size of the codebook depending on a size of the best subset.

In a sixth possible implementation form of the mobile station according to the third aspect as such or according to any of the preceding implementation forms of the third aspect, the determining unit is further configured to determine the size of the best subset based on size information received from the base station or to select the size of the best subset at the mobile station.

In a seventh possible implementation form of the mobile station according to the third aspect as such or according to any of the preceding implementation forms of the third aspect, the reporting unit is further configured to report, to the base station, quantized CSI for transmit antennas other than the transmit antennas in the best subset.

In a fourth aspect, there is provided a base station having at least one transmission point, each of the transmission points being equipped with a plurality of transmit antennas, the base station comprising: a receiver for receiving a report from a mobile station, the report reporting quantized channel state information, CSI, of a best subset of transmit antennas determined for each selected transmission point from among a plurality of transmit antennas for the transmission point based on channel gain; and a transmitter for transmitting data based on the quantized CSI.

In a first possible implementation form of the base station according to the fourth aspect, the base station further comprises a processor configured to obtain the channel gains of the plurality of transmit antennas of the transmission point, and to select the transmit antenna with the largest channel gain as the best subset.

In a second possible implementation form of the base station according to the fourth aspect as such or according to any of the preceding implementation forms of the fourth aspect, the base station further comprises a processor configured to obtain the channel gains of the plurality of transmit antennas of the transmission point and to select as the optimal subset the transmit antenna for which the channel gain is greater than a threshold.

In a third possible implementation form of the base station according to the fourth aspect as such or according to any of the preceding implementation forms of the fourth aspect, the transmitter is further configured to send indication information to the mobile station, the indication information being used to indicate to the mobile station the index of the transmit antenna in the best subset.

In a fourth possible implementation form of the base station according to the fourth aspect as such or according to any of the preceding implementation forms of the fourth aspect, the receiver is further configured to receive indication information from a mobile station, the indication information indicating an index of a transmit antenna in the best subset selected by the mobile station according to a channel gain.

In a fifth possible implementation form of the base station according to the fourth aspect as such or according to any of the preceding implementation forms of the fourth aspect, the reporting of the quantized CSI uses a codebook, the size of which depends on the size of the best subset.

In a sixth possible implementation form of the base station according to the fourth aspect as such or according to any of the preceding implementation forms of the fourth aspect, the receiver is further configured to receive size information from the mobile station indicating a size of the best subset; or the base station further comprises a processor for selecting the size of the best subset at the base station.

In a seventh possible implementation form of the base station according to the fourth aspect as such or according to any of the preceding implementation forms of the fourth aspect, the receiver is further configured to receive a report from the mobile station, the report being used for reporting quantized CSI for transmit antennas other than the transmit antennas in the best subset.

Therefore, the embodiment of the invention selects the optimal subset of the transmitting antennas based on the channel gain and feeds back the selected subset, thereby reducing the feedback overhead.

Drawings

In order to more clearly illustrate the embodiments of the present invention or the technical solutions in the prior art, the drawings used in the description of the embodiments or the prior art will be briefly described below, it is obvious that the drawings in the following description are only some embodiments of the present invention, and for those skilled in the art, other drawings can be obtained according to these drawings without creative efforts.

Fig. 1 shows an example of a massive MIMO system to which the present invention can be applied.

Figure 2 illustrates a method of one embodiment of the invention.

Fig. 3 shows an example of the gain of the antenna selection according to one embodiment of the invention.

Fig. 4 shows an example of the gain of antenna selection according to another embodiment of the present invention.

Fig. 5 shows an example of the gain of antenna selection according to another embodiment of the present invention.

Fig. 6 shows an example of the gain of antenna selection according to another embodiment of the present invention.

Fig. 7 illustrates a method of another embodiment of the present invention.

Fig. 8 shows a block diagram of a mobile station of one embodiment of the invention.

Fig. 9 shows a block diagram of a mobile station of another embodiment of the present invention.

Fig. 10 shows a block diagram of a base station of an embodiment of the invention.

Detailed Description

The technical solution in the embodiments of the present invention will be clearly and completely described below with reference to the drawings of the embodiments. It is to be understood that the described embodiments are merely a few embodiments of the invention, and not all embodiments. All other embodiments, which can be derived by a person skilled in the art from these embodiments without making any inventive step, fall within the scope of protection of the present invention.

Switching a subset of antennas to an RF chain can significantly improve the overall capacity of the system. This indicates that the more antennas are available than RF chains, the greater the antenna subset selection gain. The selection of antennas according to the proposed efficient search algorithm achieves almost optimal performance with exhaustive search. However, the search algorithm assumes perfect CSIT, which is not particularly feasible in massive MIMO systems because the feedback overhead is too large to be affordable. On the other hand, under the condition of limited feedback, the more transmit antennas, the better sum rate performance does not necessarily occur because the quantization quality is poor. As the number of transmit antennas increases, the quality of the quantized CSI degrades, and multi-user interference can limit system performance.

Fig. 1 shows an example of a massive MIMO system to which the present invention can be applied, the system comprising a BS (base station) serving K MSs (mobile stations): MS 1 to MS K.

The BS is equipped with a large antenna (N)_T) And a limited number of RF chains (N)_RF) I.e. N_T>＝N_RF. To fully understand how system performance is related to the number of RF chains, two different cases (N) will be considered_T>N_RFOr N_T＝N_RF). To maximize system (rate) performance, a subset of transmit antennas is selected and switched to the RF chains. The selection process may be done at the receiver side and the corresponding CSI is reported over limited backhaul resources. We assume independent identically distributed (i.i.d.) rayleigh channels.

For the SU MISO (single user, multiple input single output) case, the optimal beamforming strategy to maximize the rate is Matched Beamforming (MBF) which requires accurate CSIT. However, reporting all CSI with limited feedback results in large quantization loss. On the other hand, higher accuracy can be achieved by feeding back only partial CSI. Naturally, the idea is to select and report the strongest channel (with the largest amplitude). Since the beamforming gain is proportional to the number of antennas selected, this necessarily breaks the optimized trade-off between quantization quality and beamforming gain. In practice, the method can be interpreted as a hybrid selection/MBF with limited feedback.

For the MU MISO (multi-user, multiple input single output) case, each MS reports the most important CSI (with the largest channel gain), and the transmitter treats the unreported CSI as zero. A Zero Forcing (ZF) beamforming strategy is considered based on the partial channel information. With large scale transmit antennas, the probability of MS reporting mutually non-overlapping antenna subsets is high. The ZF beamformer used to cancel the multi-user interference is then also the matched beamforming of the desired signal. Indeed, there is also residual multi-user interference due to imperfections in the reported CSI and unreported CSI. However, if the available feedback bits can be responsible for the size of the selected subset of antennas, the first factor can be overcome. The second factor is negligible since the channel gain for the unreported channel is small. In summary, when we obtain gain that exceeds that of the conventional MU MISO system, multiplexing gain can be achieved.

In addition, if we can eliminate the antenna selection overhead, the system performance can be further improved. Since the MS needs to inform the BS of the selected subset of antennas, overhead caused in a massive MIMO system may be very large. In the literature, studies have been made on the predictability from downlink (uplink) to uplink (downlink) in FDD systems. This indicates that to some extent, the Channel Ordering Information (COI) is known to the BS. In this mode, all feedback bits are available to quantize the channel. To reveal the potential gain, we will investigate the case with/without antenna selection overhead.

Assuming that csi (csir) is preferred at the receiver, there are "B" feedback bits available at each MS. At each time instant, the receiver of the MS first selects the strongest channel with the largest amplitude based on the instantaneous CSI and reports the index of the selected combination and the corresponding CSI to the transmitter of the BS using Random Vector Quantization (RVQ). For "B"A feedback bit, legacy/reference scheme defined as N_T＝N_RF. The present invention seeks to investigate when the proposed scheme would outperform the reference scheme and how the gain varies proportionally with NT and B. That is, the present invention proposes how to maximally obtain antenna selection gain under practical settings (limited feedback and RF chains) by dynamic antenna selection.

Fig. 2 illustrates a communication method of an embodiment of the present invention. The method of fig. 2 is performed by an MS, e.g., one of MS 1 through MS K as shown in fig. 1, the MS being served by a BS having at least one transmission point, each transmission point being equipped with multiple transmit antennas.

201. The MS determines an optimal subset of transmit antennas for each selected transmission point from among the plurality of transmit antennas for the transmission point based on the channel gain.

A BS may include one or more transmission points such as an antenna array, an antenna group, DA (distributed antenna) elements, and the like. As shown in fig. 1, each transmission point is provided with a plurality of (N)_T) Transmitting antennas, wherein N is for different transmission points_TMay be different or fixed.

202. The MS reports the quantized CSI of the transmit antennas in the best subset to the BS.

Therefore, the embodiment of the invention selects the optimal subset of the transmitting antennas based on the channel gain and feeds back the selected subset, thereby reducing the feedback overhead.

In particular, the best subset has the most significant CSI among all transmit antennas. The CSI of other transmit antennas outside the optimal subset may not be reported, so that the feedback overhead is reduced, and the negative impact caused by such imperfect CSI reporting can be controlled within an acceptable range.

The subset selection may be performed at the MS. As a proposed non-limiting scheme, in step 201, the MS may acquire channel gains of all of the multiple transmit antennas of the transmission point and select the transmit antenna with the largest channel gain as the optimal subset.

The proposed scheme is compared below with a reference scheme, wherein N is chosen randomly_TN in one antenna_RFAnd the other one is sent.

SU case 1: no RF chain constraints, with selective overhead

Consider the proposed scheme, where from N_TM out of the antennas are selected, and the average output SNR is expressed as:

wherein,

and input SNR is P/sigma²Enhanced by beamforming and attenuated by quantization. The beamforming gain consists of two terms: the former term represents the array gain and the latter term represents the antenna selection gain. To inform the transmitter (BS) which antennas were selected, the receiver (MS) sends the indices of the antenna subsets using certain bits (selection overhead) while quantizing the selected CSI using the valid bits.

By using N_TThe average output SNR for the reference scheme for each antenna is expressed as:

${\mathrm{SNR}}_{ref}=\frac{P}{{\mathrm{\σ}}^2}\·N_T\·(1-2^{-\frac{B}{N_T-1}})---\left(3\right)$

it can be seen that (1)<This indicates that the proposed scheme does not provide gain in this case. Using all N_TOne antenna is the better choice.

SU case 2: with RF chain constraints and selective overhead

Consider the proposed scheme, where from N_TSelecting N from each antenna_RFThe average output SNR is expressed as:

$SNR\_cand=\frac{P}{{\mathrm{\&sigma;}}^2}\&CenterDot;(N_{RF}+N_{RF}\&CenterDot;\underset{n=N_{RF}+1}{\overset{N_T}{\&Sigma;}}\frac{1}{n})\&CenterDot;(1-2^{-\frac{B_{eff}}{N_{RF}-1}})---\left(4\right)$

wherein,

from N to N_TRandomly selecting N from each antenna_RFThe average output SNR of the reference scheme of individuals is expressed as:

$SNR\_ref=\frac{P}{{\mathrm{\&sigma;}}^2}\&CenterDot;N_{RF}\&CenterDot;(1-2^{-\frac{B}{N_{RF}-1}})---\left(6\right)$

note that (6) is a relative to N_RFEventually saturating at the following fixed point:

$\underset{N_{RF}\&RightArrow;\mathrm{\&infin;}}{\lim }{\mathrm{SNR}}_{ref}=\frac{P}{{\mathrm{\&sigma;}}^2}\&CenterDot;B\&CenterDot;ln\left(2\right)---\left(7\right)$

this is a function of the input SNR and B only, which indicates that we prefer to utilize as many RF chains as possible with limited feedback. However, the marginal gain achieved by the RF chain increase becomes smaller and smaller. On the other hand, for a fixed number of RF chains, B and N are identified for which the proposed scheme has a gain over the reference scheme_TThe critical point of (c) and the gain of (c) are also related to (B) and (N)_TChange of occurrenceThe study was carried out.

a) Fixed N_RFAnd N_T

By means of harmonic series (harmonic series) and binomial coefficients,

$\begin{array}{c}SNR\_cand\&GreaterEqual;SNR\_ref,\mathrm{\&lambda;}={\log }_2\left(\left(\begin{array}{c}{N}_T\\ N_{RF}\end{array}\right)\right)/(N_{RF}-1)\\ \&DoubleRightArrow;B\&GreaterEqual;(N_{BF}-1){\log }_2\{\&lsqb;(1+\log (\frac{N_T}{N_{RF}}\left)\right)2^{\mathrm{\&lambda;}}-1\&rsqb;/\log \left(\frac{N_T}{N_{RF}}\right)\}\end{array}---\left(8\right)$

to achieve some gain, the number of feedback bits should satisfy (4). Gain Δ SNR ═

SNR _ cand-SNR _ ref varies proportionally as follows

$\mathrm{\&Delta;SNR}~(\mathrm{\&alpha;}-2^{{-\mathrm{\&gamma;}}^B})---\left(9\right)$

Where α and γ are constants independent of B. When B tends to be infinite, it is,

${\mathrm{\&Delta;SNR}}_{dB}=10{\log }_{10}\frac{\left(4\right)}{\left(6\right)}=10{\log }_{10}\&lsqb;1+log\left(\frac{N_T}{N_{RF}}\right)\&rsqb;---\left(10\right)$

e.g. N_T/N_RF＝50,ΔSNR_dB7 dB. Gain log (N)_T) And proportionally changed.

b) Fixed N_RFAnd B

$SNR\_cand\&GreaterEqual;SNR\_ref\&DoubleRightArrow;x\&GreaterEqual;2^{\frac{-B}{N_{RF}-1}}\left(\right(1+x)2^{\frac{N_{RF}}{(N_{RF}-1)log\left(2\right)}\&CenterDot;x+\mathrm{\&mu;}}-1)---\left(11\right)$

Suppose N_T/N_RF≥e,

Note that (8) is a sufficient condition of (7). Then, we obtain N_TCritical point of

$\&DoubleRightArrow;N_{RF}\&le;N_T\&le;N_{RF}\&CenterDot;\exp \&lsqb;\frac{log\left(2\right)(B-(\mathrm{\&mu;}+1)(N_{RF}-1))}{N_{RF}}\&rsqb;---\left(13\right)$

$\mathrm{\&mu;}=\frac{N_{RF}}{N_{RF}-1}{\log }_2\left(e\right)-\frac{N_{RF}}{2(N_{RF}-1)}{\log }_2\left(2{\mathrm{\&pi;N}}_{RF}\right)---\left(14\right)$

This indicates that the ratio is bounded, i.e., to achieve gain from the proposed scheme of finite B, N_T/N_RF≤ρ(B,N_RF). With N_TIncreased, selection overhead approaches/exceeds the number of available bits, and the number of valid/residual bits is too small to be measured accuratelyThe channel is channelized. It is not always advantageous to increase the number of TX antennas compared to the prior art. When B is very small or N_TThe proposed solution is, in the larger part, less than the reference solution. We can increase B or decrease N_TTo obtain the gain.

c) Fixed N_RF、N_TAnd B

In this case, the proposed scheme may be inferior to the reference scheme. Gain can be obtained by using partial RF chains. In this way, the selection overhead is reduced and more feedback bits result in higher quantization accuracy. At the same time, the transmit array gain becomes lower and an optimization procedure should be taken to achieve the best balance between the advantages and disadvantages of using partial RF chains.

Expressing M as the number of RF chains used, finding the optimal M<＝N_RFTo maximize the average output SNR and to feed back the corresponding CSI subset. The generic optimization function is defined as follows:

$S_M=\arg \underset{M\&Element;\&lsqb;1,N_{RF}\&rsqb;}{\max }\underset{S_M\&Subset;\left\{\left(\begin{array}{c}{N}_T\\ M\end{array}\right)\right\}}{\max }f\left(SNR\right)---\left(15\right)$

this requires a complex exhaustive search algorithm to achieve the optimal solution. Fortunately, we use a low complexity selection method to maximize the SNR in our case. More specifically, the channels are sorted by magnitude and the largest ones are reported.

The average output SNR in this case is expressed as:

$SNR\_cand\left(M\right)=\frac{P}{{\mathrm{\&sigma;}}^2}\&CenterDot;(M+M\&CenterDot;\underset{n=M+1}{\overset{N_T}{\&Sigma;}}\frac{1}{n})\&CenterDot;(1-2^{-\frac{B_{eff}}{M-1}}),1\&le;M\&le;N_{RF}---\left(16\right)$

due to the coupled nature, it is difficult to obtain a closed-form solution for M that makes the proposed scheme better than the reference scheme. Need 1<M<＝N_RFThe numerical optimization process of (1). This indicates that the proposed scheme provides at least a lower bound for gain. When B is large, it is optimal to use all RF chains, (16) is summarized as (4). Otherwise, the value is optimized (16) and compared to a reference scheme, and then it is determined whether to increase B or decrease N_T。

The proposed scheme can be further improved provided that the transmitter already knows the Channel Ordering Information (COI) in advance. For FDD systems, we can predict the uplink channel from the downlink channel and vice versa. In short, it is possible to model frequency selective fading (energy) on the uplink/downlink using a finite-order Autoregressive (AR) process. When the relative deviation of the uplink/downlink carrier frequencies is small, it indicates that the two links share a common AR coefficient. Thus, both links experience similar trends in variation. For this reason, the assumption of COI at the transmitter is valid and the selection overhead is also eliminated. To reveal the potential gains, we consider again the two above cases without selection overhead.

SU case 3: no RF chain constraints, no selection overhead

Assuming that M antennas are selected, the average output SNR of the proposed scheme is expressed as:

$\begin{array}{cc}{\mathrm{SNR}}_{cand}=\frac{P}{{\mathrm{\&sigma;}}^2}\&CenterDot;(M+M\&CenterDot;\underset{n=M+1}{\overset{N_T}{\&Sigma;}}\frac{1}{n})\&CenterDot;(1-2^{-\frac{B}{M-1}})& M\&Element;\&lsqb;1,N_T\&rsqb;\end{array}---\left(17\right)$

while the reference scheme utilizes all N_TThe antenna comprises:

${\mathrm{SNR}}_{ref}=\frac{P}{{\mathrm{\&sigma;}}^2}\&CenterDot;N_T\&CenterDot;(1-2^{-\frac{B}{N_T-1}})---\left(18\right)$

it is difficult to derive a closed expression of M that makes the proposed solution better than the reference solution, i.e., (17) > (18). However, as is readily apparent from Monte Carlo (Monte-Carlo) simulations, gain can be achieved by selecting a portion of the transmit antennas. The gain depends on NT, NRF and B. In the example given in fig. 3, there is a lower bound on the advantage of the proposed scheme over the reference scheme. The maximum gain is achieved in the middle region by a trade-off between CSI accuracy and beamforming gain.

SU case 4: with RF chain constraints and no selection overhead

Assuming that M antennas are selected, the average output SNR of the proposed scheme is expressed as:

$\begin{array}{cc}{\mathrm{SNR}}_{cand}=\frac{P}{{\mathrm{\&sigma;}}^2}\&CenterDot;(M+M\&CenterDot;\underset{n=M+1}{\overset{N_T}{\&Sigma;}}\frac{1}{n})\&CenterDot;(1-2^{-\frac{B}{M-1}})& M\&Element;\&lsqb;1,N_{RF}\&rsqb;\end{array}---\left(19\right)$

and the reference scheme is from N_TRandomly selecting N from each antenna_RFThe method comprises the following steps:

${\mathrm{SNR}}_{ref}=\frac{P}{{\mathrm{\&sigma;}}^2}\&CenterDot;N_{RF}\&CenterDot;(1-2^{-\frac{B}{N_{RF}-1}})---\left(20\right)$

since the performance of the reference scheme is degraded by the limited RF chain, more gain is obtained compared to SU case 3, as shown in fig. 3, where curves "Pro" and "Ref" correspond to the proposed scheme and the reference scheme, respectively.

Next, a zero-forcing beamforming strategy is employed to obtain the multiplexing gain in view of the multi-user MISO Broadcast Channel (BC). Non-perfectly reported CSI as well as unreported CSI can lead to the presence of residual multi-user interference. The proposed scheme is the same as the SU case described above, while two reference schemes are provided.

The proposed scheme: ranking the channel gains, selecting the largest M, and reporting the index of the selected subset and the CSI corresponding to the subset

Reference scheme 1: the number of fixed MSs is Nu and reports the first M antennas for transmission (equivalently, randomly from N_TSelecting M out of the antennas) of the CSI

Reference scheme 2: the number of fixed MSs, equal to the number of reported antennas (Nu ═ M), and reports the first M antennas for transmission (equivalently, randomly from N_TSelecting M out of the antennas) of the CSI

MU case 1: no RF chain constraints, with selective overhead

The average rate of the proposed scheme is expressed as:

$R_{cand}=M\&CenterDot;E_{H,W}\&lsqb;\log (1+\frac{P/M\&CenterDot;|h_i^H{v}_{\max ,i}{|}^2}{1+P/M\&CenterDot;\underset{j\&NotEqual;i}{\&Sigma;}|h_i^H{v}_{\max ,j}{|}^2})\&rsqb;---\left(21\right)$

where expected values are employed on the channel and RVQ codebooks. V_max,iRepresenting the beamforming vector calculated with the partial (maximum channel gain) CSI, considering the unreported CSI as zero. At the same time, the average rate of the two reference schemes is expressed as

$R_{ref1}=N_u\&CenterDot;E_{H,W}\&lsqb;\log (1+\frac{P/N_u\&CenterDot;|h_i^H{v}_i{|}^2}{1+P/N_u\&CenterDot;\underset{j\&NotEqual;i}{\&Sigma;}|h_i^H{v}_j{|}^2})\&rsqb;---\left(22\right)$

$R_{ref2}=M\&CenterDot;E_{H,W}\&lsqb;log(1+\frac{P/M\&CenterDot;|h_i^H{v}_i{|}^2}{1+P/M\&CenterDot;\underset{j\&NotEqual;i}{\&Sigma;}|h_i^H{v}_j{|}^2})\&rsqb;---\left(23\right)$

Closed-form approximations of (22) and (23) may then be derived, while the closed-form expressions remain open. However, from the simulations in fig. 4 (as shown by curves 401 to 403), we can see that the proposed scheme cannot provide a gain better than the reference scheme due to the large selection overhead, especially in large scale transmit antenna systems. Also, we can eliminate this overhead with COI at the transmitter.

MU case 2: no RF chain constraints, no selection overhead

In this case, the proposed scheme actually achieves a gain compared to the two reference schemes, since

1) It ignores the selection overhead and obtains the same CSI quality as reference schemes 1 and 2;

2) it retains most of the channel information by reporting the most prominent channels; and

3) if the MS selects mutually non-overlapping subsets, the simple matched beamforming on the reported subset is also the ZFBF of other MSs; the multi-user interference is small since it originates from an unreported channel (with relatively small channel gain).

Fig. 5 shows simulation results of the proposed scheme with gain over two reference schemes.

As can be seen from the curves 501 to 503, the middle M achieves the maximum gain. The reason is as follows: for small M, the quantization of CSI has a high quality, while the non-overlapping subsets of high probability bring high beamforming gain. However, since there is much unreported CSI, this corresponds to low multiplexing gain and large multi-user interference. On the other hand, large M (close to N)_T) The opposite is true. Thus, the middle M achieves the best balance between these factors.

It should be noted that the gain depends strongly on the non-overlapping subsets, which facilitates both ZF and matched beamforming. The number of reported partial CSI must be comparable to N_TOtherwise multi-user interference can degrade system performance. On the other hand, if the number of Nu is fixed and the number of Nt increases, we will eventually get non-overlapping subsets. But due to the larger amount of unreported CSI, multi-user interference may dominate.

However, with N_TAnd if the transmitter can dynamically decide the number of MSs, the optimal number of MSs will increase. In this way, inter-user interference can be small (we feed back a sufficient amount of significant CSI). Then we equivalently make Nu comparable to the SU-MISO case. For each MS we have shown that when N is_TThe marginal gain becomes smaller as it increases gradually. This can be seen in fig. 6.

The proposed scheme is easily combined with multi-user diversity when the number of MSs is very large, i.e. selecting MSs with non-overlapping subsets for doing soAnd (5) sending. The number of MSs and the number of reported CSI for each MS should be determined by the transmitter based on a closed/approximate value of the total rate. Given N_TAnd B, by maximizing the total rate expression, which is N_TB and M. An MS selection algorithm is also required.

Although it is seen by simulations that the gain of the proposed scheme is greater than that of the reference scheme, the analysis results will still help to understand the impact of these system parameters on the rate performance. Here we give a closed-form approximation of (22) and (23) and provide guidance for computing the closed-form expression (21).

$\begin{array}{c}{R}_{ref1}=Nu\&CenterDot;E_{H,W}\&lsqb;\log (1+\frac{P/Nu\&CenterDot;|H_i^H{v}_i{|}^2}{1+P/Nu\&CenterDot;\underset{j\&NotEqual;i}{\&Sigma;}|h_i^H{v}_j{|}^2})\&rsqb;\&ap;Nu\&CenterDot;\log (1+\frac{P/Nu\&CenterDot;E_{H,W}\left\{\right|h_i^H{v}_i{|}^2\}}{1+P/Nu\&CenterDot;\underset{j\&NotEqual;i}{\&Sigma;}{E}_{H,W}\left\{\right|h_i^H{v}_j{|}^2\}})\\ =Nu\&CenterDot;\log (1+\frac{\frac{P}{Nu}\&CenterDot;(M-Nu+1)}{1+\frac{P}{Nu}\&CenterDot;M\&CenterDot;\underset{j\&NotEqual;i}{\&Sigma;}\frac{1}{M-1}{2}^{\frac{-B}{M-1}}})=Nu\&CenterDot;\log (1+\frac{P\&CenterDot;(M-Nu+1)/Nu}{1+P\&CenterDot;\frac{Nu-1}{Nu}\&CenterDot;\frac{M}{M-1}\&CenterDot;2^{\frac{-B}{M-1}}})\end{array}---\left(24\right)$

$\begin{array}{c}{R}_{ref2}=M\&CenterDot;E_{H,W}\&lsqb;\log (1+\frac{P/M\&CenterDot;|h_i^H{v}_i{|}^2}{1+P/M\&CenterDot;\underset{j\&NotEqual;i}{\&Sigma;}|h_i^H{v}_j{|}^2})\&rsqb;\&ap;M\&CenterDot;\log (1+\frac{P/M\&CenterDot;E_{H,W}\left\{\right|h_i^H{v}_i{|}^2\}}{1+P/M\&CenterDot;\underset{j\&NotEqual;i}{\&Sigma;}{E}_{H,W}\left\{\right|h_i^H{v}_j{|}^2\}})\\ =M\&CenterDot;\log (1+\frac{\frac{P}{M}\&CenterDot;M\&CenterDot;\frac{1}{M}}{1+\frac{P}{M}\&CenterDot;M\&CenterDot;\underset{j\&NotEqual;i}{\&Sigma;}\frac{1}{M-1}{2}^{\frac{-B}{M-1}}})=M\&CenterDot;\log (1+\frac{P/M}{1+P\&CenterDot;2^{\frac{-B}{M-1}}})\end{array}---\left(25\right)$

$\begin{array}{c}{R}_{cand}=M\&CenterDot;E_{H,W}\&lsqb;\log (1+\frac{P/M\&CenterDot;|h_i^H{v}_{\max ,i}{|}^2}{1+P/M\&CenterDot;\underset{j\&NotEqual;i}{\&Sigma;}|h_i^H{v}_{\max ,j}{|}^2})\&rsqb;\\ \&ap;M\&CenterDot;\log (1+\frac{P/M\&CenterDot;E_{H,W}\left\{\right|h_i^H{v}_{\max ,i}{|}^2\}}{1+P/M\&CenterDot;E_{H,W}\left\{\underset{j\&NotEqual;i}{\&Sigma;}\right|h_i^H{v}_{\max ,j}{|}^2\}})\end{array}---\left(26\right)$

To compute the expected values in the numerator and denominator of (26), we need to build a probabilistic model to represent the average beamforming gain (mainly determined by the matched beamforming gain from the non-overlapping subsets).

MU case 3: with RF chain constraints and no selection overhead

In this case, it is very important how to switch the RF chain to a partial transmit antenna. RF switching needs to be optimized at the transmitter by exhaustive search. It should be noted that MU case 3 gives a lower bound for MU case 2. Simulation results show that the proposed scheme is less than MU case 2, but the gain is still greater than that of the reference scheme.

Alternatively, as another embodiment, the subset selection may be based on thresholds decided and configured by a controller of the communication system (e.g. a control function in the base station), instead of or in addition to the selection based on the gain ranking.

Specifically, in step 201, the MS may obtain channel gains of multiple transmit antennas of the transmission point and select the transmit antenna with the channel gain greater than the threshold as the best subset.

Optionally, as another embodiment, CSI of the transmit antennas other than the best subset may be further reported to the BS. That is, some or all of the transmit antennas outside of the best subset may constitute a weaker subset for which the MS may report CSI. The selection of the weaker subset may be similar to the selection of the best subset, e.g., based on the ordering of the channel gains and selecting the one with the smallest channel gain, or based on another threshold to select the transmit antenna with a channel gain below the threshold. As another example, the weaker subset may simply be the remaining antennas after the best subset is selected.

Alternatively, the subset selection may be performed at the BS side in a manner similar to that performed at the MS side described above, in which case, in step 201, the MS may determine the best subset based on indication information received from the base station, the indication information indicating the index of the transmit antenna in the best subset selected by the base station according to the channel gain.

Further, when the MS determines the best subset and/or the weaker subset, the MS may report the index of the transmit antenna in these subsets to the BS. On the other hand, when the BS determines the best subset and/or the weaker subset, the BS may indicate to the MS the indices of the transmit antennas in these subsets.

Alternatively, the MS may report the quantized CSI using a codebook, the size of which depends on the size of the best subset.

The size of the best subset may be determined by the BS, in which case the MS may receive size information indicating the size of the best subset from the base station. Or the size of the best subset may be determined by the MS itself, e.g., the MS may select the size of the best subset.

In summary, the MS may follow the following behavior:

1) if the number of RF chains of a BS or transmission point is equal to the number of transmit antennas, the MS reports the quantized CSI for all transmit antennas, i.e., the best subset is equal to the entire set of available antennas.

2) If the number of RF chains of the BS or transmission point is less than the number of transmit antennas, and if the MS is requested to report a subset of antennas of size equal to the number of RF chains, the MS selects and reports the best subset of antennas and the quantized CSI of this subset as long as the number of available feedback bits is greater than a certain threshold (which is a function of the number of transmit antennas and the number of RF chains), otherwise, the MS reports the quantized CSI of a predefined set of antennas of which the number is equal to the number of RF chains.

3) If the number of RF chains of the BS or transmission point is less than the number of transmit antennas, and if for a given number of feedback bits the MS is requested to report a subset of antennas of size equal to the number of RF chains, the MS reports the best subset of antennas and the quantized CSI of this subset as long as the number of transmit antennas is less than a certain threshold (a function of the number of feedback bits and the number of RF chains), otherwise the MS reports the quantized CSI of a predefined set of antennas (the number of which is equal to the number of RF chains).

4) If the number of RF chains of a BS or transmission point is less than the number of transmit antennas, and if the MS is not constrained to report any particular antenna subset size, the MS selects the best antenna subset and reports its index and corresponding quantized CSI. In this case, the antenna subset size may be strictly smaller than the number of RF chains.

In summary, increasing the number of TX antennas is beneficial in SU/MU MISO scenarios with dynamic and user-specific antenna selection under limited feedback and/or RF chain constraints.

Fig. 7 shows a communication method of another embodiment of the present invention. The method of fig. 7 is performed by a BS, such as the BS shown in fig. 1, having at least one transmission point, each transmission point equipped with multiple transmit antennas.

701. The BS receives a report from the MS for reporting quantized channel state information, CSI, of a best subset of transmit antennas determined for each selected transmission point from among a plurality of transmit antennas for the transmission point based on channel gain.

702. The BS transmits data based on the quantized CSI.

Therefore, the embodiment of the invention selects the optimal subset of the transmitting antennas based on the channel gain and feeds back the selected subset, thereby reducing the feedback overhead.

The method process of fig. 7 corresponds or is similar to the method process of fig. 2 and, therefore, will not be described in detail herein.

Alternatively, the subset selection may be performed at the BS, in which case the BS may acquire channel gains of a plurality of transmit antennas of the transmission point and select the transmit antenna having the largest channel gain as the optimal subset in step 701.

Alternatively, in step 701, the BS may acquire channel gains of a plurality of transmission antennas of the transmission point and select a transmission antenna having the channel gain greater than a threshold as the optimal subset. Alternatively, as another embodiment, the subset selection may be performed at the MS, in which case, in step 701, the BS may receive indication information from the mobile station indicating the index of the transmit antenna in the best subset selected by the mobile station according to the channel gain.

Optionally, as another embodiment, CSI of the transmit antennas other than the best subset may be further reported to the BS. That is, some or all of the transmit antennas outside of the best subset may constitute a weaker subset for which the MS may report CSI. The selection of the weaker subset may be similar to the selection of the best subset, e.g., the one with the smallest channel gain may be selected based on the ordering of the channel gains, or based on another threshold to select the transmit antenna with a channel gain less than the threshold. As another example, the weaker subset may simply be the remaining antennas after the best subset is selected.

Further, when the MS determines the best subset and/or the weaker subset, the MS may report the indices of the transmit antennas in these subsets to the BS. On the other hand, when the BS determines the best subset and/or the weaker subset, the BS may indicate to the MS the indices of the transmit antennas in these subsets, e.g., by sending the MS corresponding indication information.

Alternatively, the MS may report quantized CSI using a codebook, the size of which depends on the size of the best subset.

The size of the best subset may be decided by the BS, in which case the MS may receive size information indicating the size of the best subset from the base station; or the size of the best subset may be determined by the MS itself, e.g., the MS may select the size of the best subset.

Fig. 8 shows a block diagram of a mobile station in an embodiment of the invention. The mobile station 80 is served by a base station having at least one transmission point, each equipped with multiple transmit antennas. As shown in fig. 8, the mobile station includes a determination unit 81 and a reporting unit 82.

A determining unit 81 is configured to determine an optimal subset of transmit antennas for each selected transmission point from the plurality of transmit antennas for the transmission point based on the channel gain.

A reporting unit 82, configured to report the quantized channel state information CSI of the transmit antennas in the best subset to the base station.

Therefore, the embodiment of the invention selects the optimal subset of the transmitting antennas based on the channel gain and feeds back the selected subset, thereby reducing the feedback overhead.

The components of the mobile station 80 may implement corresponding procedures related to the MS as described in the embodiments of fig. 1 to 6, and for brevity, will not be described again here.

Optionally, as an embodiment, the determining unit 81 is configured to obtain channel gains of multiple transmitting antennas of the transmission point, and select the transmitting antenna with the largest channel gain as the optimal subset.

Optionally, as another embodiment, the determining unit 81 is configured to obtain channel gains of multiple transmitting antennas of the transmission point, and select the transmitting antenna with the channel gain greater than the threshold as the optimal subset.

Optionally, as another embodiment, the reporting unit 82 is further configured to report the index of the transmitting antenna in the best subset to the base station.

Alternatively, as another embodiment, the determining unit 81 is configured to determine the optimal subset based on indication information received from the base station, where the indication information indicates the index of the transmitting antenna in the optimal subset selected by the base station according to the channel gain.

Optionally, as another embodiment, the reporting unit 82 is configured to report the quantized CSI by using a codebook, wherein the size of the codebook depends on the size of the optimal subset.

Optionally, as another embodiment, the determining unit 81 is further configured to determine the size of the best subset based on size information received from the base station, or to select the size of the best subset at the mobile station.

Optionally, as another embodiment, the reporting unit 82 is further configured to report the quantized CSI of the transmitting antennas other than the transmitting antennas in the best subset to the base station.

Fig. 9 shows a block diagram of a mobile station in another embodiment of the invention.

As shown in fig. 9, the mobile station 90 includes a processor 91, a transmitter 92, and a receiver 93.

The processor 91 is configured to determine an optimal subset of transmit antennas for each selected transmission point from the plurality of transmit antennas for the transmission point based on the channel gain.

The transmitter 92 is configured to report the quantized channel state information CSI of the transmit antennas in the best subset to the base station.

Therefore, the embodiment of the invention selects the optimal subset of the transmitting antennas based on the channel gain and feeds back the selected subset, thereby reducing the feedback overhead.

The components of the mobile station 90 may implement corresponding procedures related to the MS as described in the embodiments of fig. 1 to 6, and for simplicity, the details are not repeated here.

Optionally, as an embodiment, the processor 91 is configured to obtain channel gains of multiple transmit antennas of the transmission point, and select a transmit antenna with the largest channel gain as the optimal subset.

Optionally, as another embodiment, the processor 91 is configured to obtain channel gains of multiple transmit antennas of the transmission point, and select the transmit antenna with the channel gain greater than a threshold as the optimal subset.

Optionally, as another embodiment, the transmitter 92 is further configured to report the index of the transmitting antenna in the best subset to the base station.

Alternatively, as another embodiment, the processor 91 is configured to determine the best subset based on indication information received by the receiver 93 from a base station, the indication information indicating an index of a transmit antenna in the best subset selected by the base station according to a channel gain.

Optionally, as another embodiment, the transmitter 92 is configured to report the quantized CSI using a codebook, the size of which depends on the size of the optimal subset.

Optionally, as another embodiment, the processor 91 is further configured to determine the size of the best subset based on size information received from the base station, or to select the size of the best subset at the mobile station.

Optionally, as another embodiment, the transmitter 92 is further configured to report the quantized CSI of the transmit antennas other than the transmit antennas in the best subset to the base station.

Fig. 10 shows a block diagram of a base station in an embodiment of the invention. The base station 100 may have at least one transmission point, each equipped with multiple transmit antennas. As shown in fig. 10, the base station 100 includes a receiver 101 and a transmitter 102.

The receiver 101 is configured to receive a report from the mobile station, the report being based on the quantized channel state information CSI of the best subset of transmit antennas determined for each selected transmission point from the plurality of transmit antennas for that transmission point.

The transmitter 102 is configured to transmit data based on the quantized CSI.

Therefore, the embodiment of the invention selects the optimal subset of the transmitting antennas based on the channel gain and feeds back the selected subset, thereby reducing the feedback overhead.

The components of the base station 100 may implement corresponding processes related to the BS described in the embodiments of fig. 1 to fig. 6, and for the sake of simplicity, the details are not repeated here.

Optionally, as an embodiment, the base station 100 may further include a processor 103, configured to obtain channel gains of multiple transmit antennas of the transmission point, and select the transmit antenna with the largest channel gain as the optimal subset.

Optionally, as another embodiment, the base station 100 may further include a processor 103, configured to obtain channel gains of multiple transmit antennas of the transmission point, and select the transmit antenna with the channel gain greater than a threshold as the optimal subset.

Optionally, as another embodiment, the transmitter 102 is further configured to send indication information to the mobile station, where the indication information is used to indicate to the mobile station the index of the transmitting antenna in the best subset.

Optionally, as another embodiment, the receiver 101 is further configured to receive indication information from the mobile station, where the indication information indicates an index of a transmit antenna in the best subset selected by the mobile station according to the channel gain.

Optionally, as another embodiment, the reporting of quantized CSI uses a codebook, the size of which depends on the size of the optimal subset.

Optionally, as another embodiment, the receiver 101 is further configured to receive size information indicating a size of the best subset from the mobile station.

Optionally, as another embodiment, the base station 100 may further include a processor 103 for selecting the size of the best subset at the base station.

Optionally, as another embodiment, the receiver 101 is further configured to receive a report from the mobile station, the report being used for reporting the quantized CSI of the transmit antennas other than the transmit antennas in the best subset.

In embodiments of the present invention, the MS may be any of the following, and may be fixed or mobile, and examples of fixed MSs may include user equipment, terminals, mobile stations, subscriber units or stations, and so on. Examples of a mobile MS may include a cellular telephone, a Personal Digital Assistant (PDA), a wireless modem, a wireless communication device, a handheld device, a laptop computer, a cordless telephone, or a Wireless Local Loop (WLL) station, among others.

It should be noted that terms such as "first, second, etc. used in this context are only used for distinguishing one entity or operation from another entity or operation, and do not represent an actual relationship or order between these entities or operations. Also, the terms "comprises," "comprising," or any other variation thereof, are intended to be inclusive and not exclusive, such that a process, method, article, or apparatus that comprises a single element, does not include only that element, but may include other elements not expressly listed or inherent to such process, method, article, or apparatus. Unless otherwise defined, an element defined by the term "comprising an …" does not exclude the presence of other identical elements from the process, method, object or device that comprises the element.

From the description of the embodiments of the invention, it will be clear to a person skilled in the art that the invention can be implemented by software in combination with necessary general hardware, but also by hardware only, the former being preferred. Based on this understanding, the solution of the invention itself or parts of it that contribute to the prior art may be implemented in the form of a software product, and the software product may be stored in a storage medium, such as a ROM/RAM, hard disk, compact disk, etc., containing instructions that enable a computer device (personal computer, server or network device, etc.) to perform the methods described in the embodiments or parts of the embodiments.

Although the present invention has been described in terms of preferred embodiments, it should be noted that various modifications or alterations may be made to the embodiments by those skilled in the art, and these modifications or alterations should fall within the scope of the present invention.

Claims (32)

1. A communication method of a mobile station, the mobile station being served by a base station, the base station having at least one transmission point, each of the transmission points being equipped with a plurality of transmit antennas, the method comprising:

determining an optimal subset of transmit antennas for each selected transmission point from among a plurality of transmit antennas for the transmission point based on channel gain; and

reporting the quantized channel state information, CSI, of the transmit antennas in the best subset to the base station.

2. The method of claim 1, wherein the determining a best subset of transmit antennas for each transmission point comprises:

obtaining the channel gains for the plurality of transmit antennas of the transmission point; and

selecting the transmit antenna with the largest channel gain as the best subset.

3. The method of claim 1, wherein the determining a best subset of transmit antennas for each transmission point comprises:

obtaining the channel gains for the plurality of transmit antennas of the transmission point; and

selecting the transmit antenna with the channel gain greater than a threshold as the best subset.

4. The method of claim 2 or 3, further comprising:

reporting the index of the transmit antenna in the best subset to the base station.

5. The method of claim 1, wherein the determining a best subset of transmit antennas for each transmission point comprises:

determining the best subset based on indication information received from the base station, the indication information indicating an index of a transmit antenna in the best subset selected by the base station according to a channel gain.

6. The method according to any of claims 1 to 5, wherein the reporting of quantized channel state information, CSI, of the transmit antennas in the best subset to the base station comprises:

reporting the quantized CSI using a codebook, a size of the codebook depending on a size of the optimal subset.

7. The method of claim 5, further comprising:

receiving size information indicating a size of the best subset from the base station; or

Selecting, at the mobile station, a size of the best subset.

8. The method of any of claims 1 to 7, further comprising:

reporting, to the base station, quantized CSI for transmit antennas other than the transmit antennas in the best subset.

9. A communication method of a base station having at least one transmission point, each of the transmission points being equipped with a plurality of transmit antennas, the method comprising:

receiving a report from a mobile station reporting quantized channel state information, CSI, of a best subset of transmit antennas determined for each selected transmission point from a plurality of transmit antennas for the transmission point based on channel gain; and

transmitting data based on the quantized CSI.

10. The method of claim 9, further comprising:

obtaining the channel gains for the plurality of transmit antennas of the transmission point; and

selecting the transmit antenna with the largest channel gain as the best subset.

11. The method of claim 9, further comprising:

obtaining the channel gains for the plurality of transmit antennas of the transmission point; and

selecting the transmit antenna with the channel gain greater than a threshold as the best subset.

12. The method of claim 10 or 11, further comprising:

and sending indication information to the mobile station, wherein the indication information is used for indicating the indexes of the transmitting antennas in the optimal subset to the mobile station.

13. The method of claim 9, further comprising:

receiving indication information from a mobile station, the indication information indicating an index of a transmit antenna in the best subset selected by the mobile station according to a channel gain.

14. The method according to any of claims 9-13, wherein the reporting of the quantized CSI uses a codebook, the size of which depends on the size of the best subset.

15. The method of claim 14, further comprising:

receiving size information indicating a size of the optimal subset from the mobile station; or

Selecting, at the base station, a size of the best subset.

16. The method of any of claims 9 to 15, further comprising:

receiving a report from a mobile station reporting quantized CSI for transmit antennas other than the transmit antennas in the best subset.

17. A mobile station, the mobile station being served by a base station, the base station having at least one transmission point, each of the transmission points being equipped with a plurality of transmit antennas, the mobile station comprising:

a determining unit for determining an optimal subset of transmit antennas for each selected transmission point from the plurality of transmit antennas for the transmission point based on the channel gain; and

and a reporting unit, configured to report, to the base station, the quantized channel state information CSI of the transmit antennas in the optimal subset.

18. The mobile station of claim 17, wherein the determining unit is configured to obtain the channel gains for the plurality of transmit antennas of the transmission point and to select the transmit antenna with the largest channel gain as the best subset.

19. The mobile station of claim 17, wherein the determining unit is configured to obtain the channel gains for the plurality of transmit antennas of the transmission point, and to select the transmit antenna for which the channel gain is greater than a threshold as the best subset.

20. The mobile station according to claim 18 or 19, said reporting unit further being configured to report to a base station an index of a transmit antenna in said best subset.

21. The mobile station according to claim 17, wherein the determining unit is configured to determine the best subset based on indication information received from the base station, the indication information indicating an index of a transmit antenna in the best subset selected by the base station according to a channel gain.

22. The mobile station according to any of claims 17 to 21, wherein the reporting unit is configured to report quantized CSI using a codebook, the size of which depends on the size of the best subset.

23. The mobile station of claim 22, wherein the determining unit is further configured to determine the size of the best subset based on size information received from the base station or to select the size of the best subset at the mobile station.

24. The mobile station of any of claims 1 to 7, wherein the reporting unit is further configured to report the quantized CSI for the transmit antennas other than the transmit antennas in the best subset to the base station.

25. A base station having at least one transmission point, each of the transmission points equipped with a plurality of transmit antennas, the base station comprising:

a receiver for receiving a report from a mobile station, the report reporting quantized channel state information, CSI, of a best subset of transmit antennas determined for each selected transmission point from among a plurality of transmit antennas for the transmission point based on channel gain; and

a transmitter for transmitting data based on the quantized CSI.

26. The base station of claim 25, further comprising a processor configured to obtain the channel gains for the plurality of transmit antennas of the transmission point and to select the transmit antenna with the largest channel gain as the best subset.

27. The base station of claim 25, further comprising a processor configured to obtain the channel gains for the plurality of transmit antennas of the transmission point and to select the transmit antenna with the channel gain greater than a threshold as the best subset.

28. The base station according to claim 26 or 27, wherein the transmitter is further configured to send indication information to the mobile station, the indication information indicating to the mobile station the index of the transmit antenna in the best subset.

29. The base station of claim 25, wherein the receiver is further configured to receive indication information from a mobile station, the indication information indicating an index of a transmit antenna in the best subset selected by the mobile station according to a channel gain.

30. The base station according to any of claims 25 to 29, wherein the reporting of the quantized CSI uses a codebook, the size of which depends on the size of the best subset.

31. The base station of claim 30, wherein

The receiver is further configured to receive size information from the mobile station indicating a size of the best subset; or

The base station further includes a processor for selecting, at the base station, a size of the best subset.

32. The method of any of claims 25-31, wherein the receiver is further configured to receive a report from a mobile station reporting quantized CSI for transmit antennas other than the transmit antennas in the best subset.