Abstract:
A method implemented in a user equipment configured to be used in a multi-user (MU) multiple-input multiple-output (MIMO) wireless communications system is disclosed. The method includes transmitting to a base station a first channel state information (CSI) report determined according to a single-user (SU) MIMO rule, and transmitting to the base station a second CSI report determined according to an MU-MIMO rule. Other methods, apparatuses, and systems also are disclosed.

Description:
This application is a divisional of co-pending U.S. patent application Ser. No. 13/456,881, entitled, “ENHANCEMENT OF DOWNLOAD MULTI-USER MULTIPLE-INPUT MULTIPLE-OUTPUT WIRELESS COMMUNICATIONS,” filed on Apr. 26, 2012, which in turn claims the benefit of U.S. Provisional Application No. 61/480,690, entitled, “Enhancements to DL MU-MIMO,” filed Apr. 29, 2011, U.S. Provisional Application No. 61/543,591, entitled, “Enhancements to DL MU-MIMO,” filed Oct. 5, 2011, and U.S. Provisional Application No. 61/556,560, entitled, “DL MU-MIMO Enhancement via Residual Error Norm Feedback,” filed Nov. 7, 2011, of which the contents of all are incorporated herein by reference. 
    
    
     BACKGROUND OF THE INVENTION 
     The present invention relates to wireless communications system and more particularly to multi-user (MU) multiple-input multiple-output (MIMO) wireless communications system. 
     The present invention considers the problem of designing efficient channel state information (CSI) feedback schemes in order to allow improved multi-user multi-input multi-output resource allocation at a base-station (BS), resulting in increased system spectral efficiency. A cell in which multiple users feedback CSI and the BS performs MU-MIMO resource allocation is depicted in  FIG. 1 . 
     Referring to  FIG. 1 , user terminals  110 , e.g. users 1 ( 111 ) to K ( 119 ), send quantized channel feedbacks  120  to base station  130 . At base station  130 , DL (downlink) MU-MIMO resource allocation  140  is performed according to quantized channel feedbacks  120  and streams, e.g. user 1 stream  151  to user K stream  159 , are subjected to RB (resource block) and/or MCS (modulation and coding scheme) allocation and transmit precoding  160 . Signals are transmitted via n T  antennas from base station  130  and received by n R  antennas, for example, at user 1 ( 111 ). 
     Note that the quality of resource allocation done by the BS depends on the accuracy of each user&#39;s CSI report. On the other hand, allowing a very accurate CSI feedback can result in a large signaling overhead. The key challenges that need to be overcome before spectral efficiency gains from MU-MIMO can realized are, for example, as follows:
         Improving CSI accuracy without a large signaling overhead, or   Exploiting the enhanced CSI reports at the BS in an efficient manner.       

     In order to solve the above problem, others have proposed various solutions, such as increasing CSI feedback overhead; CSI feedback under assumptions on BS scheduling; and complex algorithms for joint scheduling. 
     CQI (Channel Quality Indicator) PMI (Precoding Matrix Indicator) reporting enhancements targeting DL MU-MIMO operations on PUSCH 3-1 as well as PUSCH 3-2 were considered by several companies [ 1 ]. The proposed enhancement to PUSCH 3-2 comprised enabling sub-band PMI reporting in addition to the sub-band CQI reporting. On the other hand, enhancements to PUSCH 3-1 that were considered suggested that in addition to 3rd Generation Partnership Project (3GPP) Release (Rel-) 8 Mode 3-1 feedback, a user equipment (UE) can be configured via higher layer signalling to report as follows:
         A wideband PMI calculated assuming restricted rank equal to one, along with a per subband CQI targeting MU-MIMO operation.   The MU-MIMO CQI is computed assuming the interfering PMIs are orthogonal to the single-user (SU) MIMO rank 1 PMI and for 4 TX, the total number of co-scheduled layers is assumed to be 4 at the time of MU CQI computation [1].       

     We propose a broad framework for enhanced CSI reporting by the users in order to obtain an improvement in MU-MIMO performance. We also illustrate mechanisms using which the eNodeB (eNB) can exploit such enhanced CSI feedback. System level simulations show that a simple form of enhanced feedback results in substantial system throughput improvements in homogenous networks and more modest improvements over heterogeneous networks. 
     [1] Alcatel-Lucent, Alcatel-Lucent Shanghai Bell, AT&amp;T, ETRI, Icera Inc., LG Electronics, Marvell, NEC, New Postcom, Pantech, Qualcomm, RIM, Samsung, Texas Instruments, “Way Forward on CQI/PMI reporting enhancement on PUSCH 3-1 for 2, 4 and 8 TX,” 3GPP TSG RAN WG1 R1-105801 62bis, Xian, China, October 2010. 
     BRIEF SUMMARY OF THE INVENTION 
     An objective of the present invention is to achieve a high spectral efficiency, for example, even around a cell edge in an MU-MIMO wireless communications system. 
     An aspect of the present invention includes a method implemented in a user equipment configured to be used in a multi-user (MU) multiple-input multiple-output (MIMO) wireless communications system, comprising: transmitting to a base station a first channel state information (CSI) report determined according to a single-user (SU) MIMO rule; and transmitting to the base station a second CSI report determined according to an MU-MIMO rule. 
     Another aspect of the present invention includes a method implemented in a base station configured to be used in a multi-user (MU) multiple-input multiple-output (MIMO) wireless communications system, comprising: receiving from a user equipment a first channel state information (CSI) report determined according to a single-user (SU) MIMO rule; and receiving from the user equipment a second CSI report determined according to an MU-MIMO rule. 
     Still another aspect of the present invention includes a multi-user (MU) multiple-input multiple-output (MIMO) wireless communications system, comprising: a base station; and a user equipment, wherein the user equipment transmits to the base station a first channel state information (CSI) report determined according to a single-user (SU) MIMO rule, and a second CSI report determined according to an MU-MIMO rule. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  depicts an illustrative diagram for CSI feedback. 
         FIG. 2  depicts an illustrative diagram for multiplexing SU-CSI and enhanced feedback. 
         FIG. 3  depicts an illustrative diagram for combining SU-CSI and enhanced feedback. 
         FIG. 4  depicts an illustrative diagram for multiplexing SU-CSI and combined CSI feedback. 
     
    
    
     DETAILED DESCRIPTION 
     We consider a downlink comprising K users and multiple orthogonal RBs that are available in each scheduling interval. We first model the actual received signal vector that the user will see on a representative resource element in an RB, if it is scheduled on that RB, as
 
 y   1   =H   1   *U   1   s   1   +H   1   *U     1     s     1   +η 1   (1)
 
where y 1  represents the N×1 received signal vector on an RB (N being the number of receive antennas) and H 1  represents the M×N channel matrix (M being the number of transmit antennas) with H 1 * denoting its Hermitian. U 1  and U   1   represent the transmit precoding matrices used by the BS to transmit data to user-1 and the other co-scheduled users (or user equipments), respectively, and s 1  and s   1   represent the transmit symbol vectors intended for user-1 and the other co-scheduled users, respectively. Finally η 1 , represents the additive noise vector. Note that under MU-MIMO transmission on that RB U   1   will be a non-zero matrix whereas under SU-MIMO transmission on that RB U   1   will be a zero matrix.
 
     The model in equation (1) is the model in the aftermath of scheduling. The scheduling which involves RB, MCS and transmit precoder allocation by the BS is done by the BS scheduler whose input is the quantized CSI (referred to henceforth as just CSI) fed back by the users. 
     The conventional procedure employed by the users to report CSI is to compute a rank indicator (RI), precoding matrix indicator (PMI), which together determine a precoder from a quantization codebook, along with up-to 2 channel quality indicators or indices (CQI(s)). Note that the columns of the selected precoder represent a set of preferred channel directions and the CQI(s) represent quantized SINRs (signal to interference plus noise ratios). Further, for a rank R precoder, R SINRs (one for each column) can be recovered from the up-to 2 CQI(s). More importantly, this CSI is computed by the user using SU-MIMO rules, i.e., after assuming that it alone will be scheduled on an RB. Such CSI is referred to here as SU-CSI. 
     Clearly, if the BS wants to do MU-MIMO transmissions on an RB then it may, modify the SU-CSI reported by the users in order to do proper MCS assignment and RB allocation. However, even after such modifications MU-MIMO performance is degraded due to a large mismatch between UE reported SU-CSI and the actual channel conditions that UE will see on an RB with MU-MIMO transmissions. 
     In order to address this problem we propose enhanced CSI feedback along with a finer model that can exploit the enhanced CSI feedback report and can be used for better MU-MIMO resource allocation at the BS. The finer model, a post scheduling model, can be given by, but not restricted to,
 
 y   1   ={circumflex over (D)}   1   1/2   {circumflex over (V)}   1   †   U   1   s   1   +{circumflex over (D)}   1   1/2 ( {circumflex over (V)}   1   †   +R   1   †   Q   1   † ) U   1   s     1   +η 1   (2)
 
where {circumflex over (D)} 1   1/2  is a diagonal matrix of effective channel gains, {circumflex over (V)} 1  denotes a semi-unitary matrix whose columns represent preferred channel directions, Q 1  is a semi-unitary matrix whose columns lie in the orthogonal complement of {circumflex over (V)} 1 , i.e. Q 1   † {circumflex over (V)} 1 =0, and R 1  is a matrix which satisfies the Frobenius-norm constraint ∥R 1 ∥ F   2 ≦ε 1   2  for some ε 1 &gt;0.
         MU-CQI reporting: The UE is configured to also report additional CQI computed using MU-MIMO rules and possibly an additional PMI. To compute MU-CQI corresponding to a precoder Ĝ 1 , the UE assumes a post-scheduling model as in equation (2) in which {circumflex over (D)} 1   1/2 , {circumflex over (V)} 1  are equal to the diagonal matrix of the dominant unquantized singular values and the dominant unquantized right singular vectors, respectively, of its downlink channel matrix. It sets U 1 =Ĝ 1  and assumes that the columns of U   1   are isotropically distributed in the subspace defined by I−Ĝ 1 Ĝ 1   †  (orthogonal complement of Ĝ 1 ). In addition it assumes Q 1 =0 which is reasonable in this case since {circumflex over (V)} 1  is taken to contain all the unquantized dominant singular vectors so no significant interference can be received from signals in its orthogonal complement. Then, to compute MU-SINRs the UE can be configured to assume a particular number of columns in U   1   and either an equal power per scheduled stream or a non-uniform power allocation in which a certain fraction of energy per resource element energy per resource element (EPRE) is shared equally among columns of U   1    with another fraction (possibly the remaining fraction) being shared equally among columns in U   1   .   Enhanced CSI Reporting (SU-MIMO CSI and Residual Error): The UE can be configured for enhanced CSI reporting. Suppose that using SU-MIMO rules the UE determined a precoder Ĝ 1  of a preferred rank r 1  and the corresponding quantized SINRs {SI{circumflex over (N)}R 1   i } i=1   r     1   . In order to determine the residual error, the UE assumes a post-scheduling model as in equation (2) in which       

                 D   ^     1     =         r   1       ρ   1       ⁢   diag   ⁢     {       SI   ⁢     N   ^     ⁢     R   1   1       ,   …   ⁢           ,     SI   ⁢     N   ^     ⁢     R   1     r   1           }             
and {circumflex over (V)} 1 =Ĝ 1 . Then let
 
               P   1   ⊥     =     I   -         G   ^     1     ⁢       G   ^     1   †               
denote the projection matrix whose range is the orthogonal complement of Ĝ 1 . Let us refer to the matrix
 
               E   1     ⁢     =   Δ     ⁢       Q   1     ⁢     R   1             
as the (normalized) residual error matrix and the matrix C 1 =E 1   † E 1  as the residual error correlation matrix and note that C 1 =D 1   −1/2 F 1 H 1   † P 1   ⊥ H 1 F 1   † D 1   −1/2 . The UE can be configured to report some approximation of either the residual error matrix or the residual error correlation matrix. These include:
         Quantizing and reporting the dominant diagonal values of R1 along with the corresponding columns in Q 1 .   Quantizing and reporting the diagonal values of C 1      Quantizing and reporting the trace of       

               C   1     ,       ε   1   2     =       tr   ⁡     (     C   1     )       =     tr   ⁡     (       F   1     ⁢     H   1   †     ⁢     P   1   ⊥     ⁢     H   1     ⁢     F   1   †     ⁢       D   ~     1     -   1         )                 
which can be thought of as the normalized total residual error.
 
     The BS can configure the user to report a particular enhanced feedback form. A simple example of the enhanced feedback form is the residual error norm,
 
ε 1 =√{square root over ( tr ( F   1   H   1   †   P   1   H   1   F   1   †   {tilde over (D)}   1   −1 ))}   (3)
 
where tr(.) denotes the trace operation, F 1 H 1   †  denotes the filtered user channel, and P 1 =(I−{circumflex over (V)} 1 {circumflex over (V)} 1   † ) is a projection matrix. PMI V 1  of some rank r 1  and r 1  quantized SINRs {SI{circumflex over (N)}R 1   i } i=1   r     1    are determined using SU-MIMO rules {tilde over (D)}=diag {SI{circumflex over (N)}R 1   1 , . . . , SI{circumflex over (N)}R 1   r     1   }. Various other forms for the enhanced feedback and various other norms for the residual error can apply to the enhanced feedback.
 
     We list several flow diagrams that describe aspects of the invention. In each figure, the flow diagram describes the operations that are conducted at a user-terminal. The operations are enabled by signaling from the eNB (or base-station) certain parameters on a downlink (feed-forward) control channel that are then received as inputs by the user. The feed-back is sent by the user on an uplink (feed-back) control channel and is received by the eNB. The parameters signaled by the base-station to a user may be interpreted by that user in a particular way that is described in detail in the further system details. Moreover, wherever applicable, the feedback sent by the user may allow the eNB to unambiguously determine the portion of the feedback determined by the user as SU-CSI and the portion determined as per the enhanced feedback form. 
     In each channel state information (CSI) reporting interval the user reports its CSI. The BS (or eNB) can configure a user for periodic CSI reporting and fix the periodicity and offset which together determine the exact sequence of intervals for which the user may report its CSI. This sequence will be henceforth referred to as the sequence for CSI reporting. 
     The user equipment can transmit to the base station an SU-CSI feedback and an enhanced CSI feedback, which are received by the base station. The transmission and the reception can be performed in a various ways as follows: 
     1. Multiplexing SU-CSI and Enhanced Feedback 
     In order to obtain the benefits of accurate MU-MIMO resource allocation without excessive feedback overhead, the eNB can multiplex intervals in which the user reports enhanced feedback with the ones in which it reports its SU-CSI feedback without enhanced feedback. The periodicity and offset of the sub-sequence formed by intervals designated for enhanced feedback within the sequence for CSI reporting can be configured by the eNB, based on factors such as user mobility. 
     As shown in  FIG. 2 , at step  201 , a UE receives residual error form configuration from a BS and receives also sequence and sub-sequence configuration information. Next, at step  202 , the UE determines SU-CSI in each interval configured for SU-CSI report or determines enhanced CSI in each interval configured for enhanced CSI report. Then, at step  203 , the UE feeds back the CSI to the BS. 
     Several ways of further reducing enhanced CSI feedback are described in the further system details. These include, for instance, letting the precoder used for computing the enhanced CSI be a function of previously reported precoder(s) contained in SU-CSI reports and/or reporting one or more components in the enhanced CSI feedback in a wideband fashion and/or reporting one or more components in the enhanced CSI feedback in a differential fashion. 
     2. Combining SU-CSI and Enhanced Feedback 
     In the second class of feedback schemes, the user combines SU-MIMO CSI report and enhanced CSI report and feeds them back in each interval. 
     As shown in  FIG. 3 , at step  301 , a UE receives residual error form configuration from a BS and receives also sequence and sub-sequence configuration information. Next, at step  302 , the UE determines in each interval configured for CSI report SU-CSI and enhanced CSI. Then, at step  303 , the UE feeds back combined CSI to the BS. 
     Methods of further reducing enhanced CSI feedback overhead are described in the further system details. These include, for instance, letting the precoder used for computing the enhanced CSI be a function of the precoder computed for SU-CSI report and/or reporting one or more components in the enhanced CSI feedback in a wideband fashion and/or reporting one or more components in the enhanced CSI feedback in a differential fashion. 
     3. Multiplexing SU-CSI and Combined CSI Feedback 
       FIG. 4  shows another method of CSI reporting. At step  401 , a UE receives residual error form configuration from a BS and receives also sequence and sub-sequence configuration information. Next, at step  402 , the UE determines SU-CSI in each interval configured for SU-CSI report or determines combined CSI for combined CSI reporting. Then, at step  403 , the UE feeds back CSI to the BS. 
     In  FIGS. 2, 3, and 4 , the sequence information includes, for example, periodicity and offset for the SU CSI reporting and the sub-sequence configuration information includes, for example, periodicity and offset for the enhanced CSI reporting. For example, the enhanced CSI report includes any indication, such as a quantized value, of the residual error matrix or the residual error correlation matrix. 
       FIGS. 2, 3, and 4  may apply to MU-CQI reporting as well. 
     In conclusion, we considered enhancements to the MU-MIMO operation by enhancing the user CSI reporting which enables more accurate MU-MIMO SINR computation at the eNB and by a finer modeling of the received output seen by a user in the aftermath of scheduling. Our results using a simple form of enhanced feedback show substantial system throughput improvements in homogenous networks and improvements also in heterogeneous networks. One important feature of the gains obtained is that they are quite robust in the sense that they are not dependent on an effective outer loop link adaptation (OLLA) implementation. 
     The foregoing is to be understood as being in every respect illustrative and exemplary, but not restrictive, and the scope of the invention disclosed herein is not to be determined from the Detailed Description, but rather from the claims as interpreted according to the full breadth permitted by the patent laws. It is to be understood that the embodiments shown and described herein are only illustrative of the principles of the present invention and that those skilled in the art may implement various modifications without departing from the scope and spirit of the invention. Those skilled in the art could implement various other feature combinations without departing from the scope and spirit of the invention.