Abstract:
Methods and apparatus are described for implementing multiple user, multiple input, multiple output (MU-MIMO) communications involving the use of beamforming where signals transmitted from a transmitter are received at multiple receivers. The method includes schemes for the calculation of interference implemented at the transmitting end, and in some embodiments receiving ends, of the channel.

Description:
RELATED APPLICATIONS  
       [0001]    The present patent application is a continuation of U.S. patent application Ser. No. 12/266,983 filed on Nov. 7, 2008, which is a continuation in part of U.S. patent application Ser. No. 12/202,901 filed on Sep. 2, 2008, the U.S. patent application Ser. No. 12/266,983 claims the benefit of U.S. Provisional Patent Application Ser. No. 60/986,808, filed on Nov. 9, 2007, U.S. patent application Ser. No. 12/202,901 claims the benefit of U.S. Provisional Patent Application Ser. No. 60/969,022 filed on Aug. 30, 2007, the entire contents of the foregoing applications are incorporated herein by reference. 
     
    
     FIELD OF THE INVENTION   
       [0002]    The present invention relates to systems and methods for Multi-User Multiple Input, Multiple Output communication systems. 
       BACKGROUND  
       [0003]    Multiple input, multiple output (“MIMO”) systems use a plurality of transmission antennas and a plurality of reception antennas to provide for spatial multiplexing of signals such that signals transmitted via multiple transmission antennas are independent of one another. This may be advantageous, for these transmitted signals are subject to a host of distortions, including shadowing, fading, and multipath interference. Such distortion can impact the amplitude and/or phase of the signal, which can inhibit high-speed data communication. 
         [0004]    A multi-user MIMO (MU-MIMO) system may use beamforming to spatially multiplex mobile stations. Beams are formed by a precoder at the transmitter which precodes users&#39; data with different precoder vectors, which are also known as codewords. A precoder vector contains weights on the transmit antennas that linearly combine the transmit data. In a MIMO system, the weights may be obtained from the singular value decomposition (SVD) of the MIMO channel matrix. In a MU-MIMO system that employs Eigen-beamforming, data streams to different users are multiplexed in Eigen-beams on the same time-frequency resource. Multiplexing different users at different time slots may cause intra-cell interference variability, even in a low speed environment. 
         [0005]    In any communication system, the quality and capacity of a communication channel are affected by such factors as interference, allocation of communication resources, the communication schemes or algorithms used on the communication channel, and the particular communication equipment implemented at transmitting and receiving ends of the channel. Reliability, throughput, and capacity gain depend on channel quality information, such as the carrier signal to interference ratio (C/I) fed back from mobile stations, used by a base station. In order to fully take advantage of adaptive coding and modulation, the channel quality feedback needs to track the changes in channel condition. With interference variability due to spatial multiplexing, the degradation in link adaptation may severely limit the spectral efficiency of multi-user MIMO. In Eigen-beamforming spatial division multiple access (“SDMA”), multiple users are scheduled on the same time-frequency resource separated by Eigen-beams. 
       SUMMARY OF THE INVENTION 
       [0006]    In one broad aspect, there is provided in a wireless system including a base station having multiple antennas operable to transmit signals to a plurality of mobile stations, each of said mobile stations having multiple antennas, a method for implementing multiple user, multiple input, multiple output (MU-MIMO) communications, the method comprising: transmitting from the base station at least one precoder vector to at least one mobile station. 
         [0007]    In another broad aspect, there is provided a receiver comprising: a processor, and a plurality of antennas connected to the processor, each antenna configured to receive at least one precoder vector from a base station. 
         [0008]    In another broad aspect, there is provided a multiple input, multiple output wireless station operable to transmit signals to a subscriber terminal, the wireless station comprising: a signal generator operable to generate a signal having a signal portion indicative of an interfering precoder vector that may affect a subscriber terminal in communication with the wireless station. 
         [0009]    Other aspects and features of the present invention will become apparent, to those ordinarily skilled in the art, upon review of the following description of the specific embodiments of the invention. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0010]    The invention will now be described in greater detail with reference to the accompanying diagrams, in which: 
           [0011]      FIG. 1  is a block diagram of a prior art MU-MIMO system. 
           [0012]      FIG. 2  is a block diagram of a prior art MU-MIMO system which illustrates inter-user interference. 
           [0013]      FIG. 3  illustrates a prior art MU-MIMO system in communication with a mobile station where that mobile station has very limited knowledge of the inter-user interference. 
           [0014]      FIG. 4  illustrates a MU-MIMO system in communication with a mobile station according to some embodiments where mobile stations has knowledge of interfering precoder vectors. 
           [0015]      FIG. 5  shows MU-MIMO system according to some embodiments involving signalling interfering precoder parameters to at least one mobile station. 
           [0016]      FIG. 6A  shows a MU-MIMO system according to some embodiments involving defining a number of multiplexed mobile stations. 
           [0017]      FIG. 6B  shows a defined a number of multiplexed mobile stations over a period of time according to some embodiments. 
           [0018]      FIG. 7A  shows a MU-MIMO system according to some embodiments involving codebook MU-MIMO. 
           [0019]      FIG. 7B  is a flowchart of steps in some embodiments of MU-MIMO communication. 
           [0020]      FIG. 7C  shows interference over a period of time according to some embodiments. 
           [0021]      FIG. 8A  is a flowchart of steps in some embodiments of MU-MIMO communication. 
           [0022]      FIG. 8B  is a flowchart of steps in some embodiments of MU-MIMO communication. 
           [0023]      FIG. 9  is a flowchart of steps in some embodiments of MU-MIMO communication. 
           [0024]      FIG. 10  shows MU-MIMO system according to some embodiments involving multicast communication with mobile stations. 
           [0025]      FIG. 11  illustrates a method according to some embodiments where interfering precoder vectors are indicated in a separate field from the message indicating a mobile station&#39;s assigned resources. 
           [0026]      FIG. 12A  illustrates a method according to some embodiments where interfering precoder vectors are indicated in an existing field intended for another purpose or multiple purposes. 
           [0027]      FIG. 12B  illustrates a method according to some embodiments where interfering precoder vectors are indicated in an existing field intended for another purpose or multiple purposes. 
           [0028]      FIG. 13  illustrates a method according to some embodiments where interfering precoder vectors are indicated by the message type. 
           [0029]      FIG. 14  illustrates a time-frequency resource zone according to some embodiments where the SDMA zone is defined by the control channel in time/frequency. 
           [0030]      FIG. 15  is a flowchart of steps in some embodiments of MU-MIMO communication. 
           [0031]      FIG. 16  is a flowchart of steps in some embodiments of MU-MIMO communication. 
           [0032]      FIG. 17A  illustrates a method according to some embodiments involving dynamic assignment of multiplexed mobile stations to SDMA levels. 
           [0033]      FIG. 17B  illustrates a method according to some embodiments involving semi-static assignment of multiplexed mobile stations to SDMA levels. 
           [0034]      FIG. 17C  illustrates a method according to some embodiments involving assignment of multiplexed mobile stations following a hopping pattern. 
           [0035]      FIG. 18  is a flowchart of steps in some embodiments of MU-MIMO communication. 
           [0036]      FIG. 19  is a flowchart of steps in some embodiments of MU-MIMO communication. 
           [0037]      FIG. 20  is a flowchart of steps in some embodiments of MU-MIMO communication. 
       
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
       [0038]    In a MIMO system, a base station (BS) provides communication services for a coverage area or cell in a wireless communication system. The term “base station” can refer to any access point providing coverage to an area, such as a wireless station. The base station transmits communication signals to mobile stations (MS) via multiple antennas. Mobile stations are also commonly referred to as user terminals, user equipment, subscriber terminals, and communication devices, for instance. The term “mobile station” can refer to any receiving device (stationary or mobile). At a mobile station side, multiple receive antennas are employed for each mobile station. 
         [0039]      FIG. 1  is a block diagram of a prior art MIMO system  100 , which includes a base station  102  having a plurality of antennas  104 , a group of mobile stations MS N    124  consisting of MS 1    118 , MS 2    120 , and MS 3    122 , with antenna arrays  112 ,  114 , and  116  respectively, and communications signals  106 ,  108 , and  110 . In operation, communication signal  106  is transmitted from the base station  102  via antenna arrays  104  to MS 1    118 , and is received by antenna array  112 . Communication signal  108  is transmitted from the base station  102  via antenna arrays  104  to MS 2    120 , and is received by antenna array  114 . Communication signal  110  is transmitted from the base station  102  via antenna arrays  104  to MS 3    122 , and is received by antenna array  116 . 
         [0040]    It should be appreciated that the system of  FIG. 1  is intended for illustrative purposes only. As will be apparent to those skilled in the art to which the present invention pertains, base station  102  includes further components in addition to the antenna array  104 , such as components to generate the signals  106 ,  108 , and  110  for instance. Similarly, the mobile stations MS 1    118 , MS 2    120 , and MS 3    122  include components to process received signals, such as a MIMO decoder. Also, the base station  102  and the mobile stations MS 1    118 , MS 2    120 , and MS 3    122  normally support both transmit and receive operations. It will also be apparent to those skilled in the art that the number of antennas in arrays  104 ,  112 ,  114 , and  116  may be more or less than that shown in  FIG. 1 . Finally, it will be apparent to those skilled in the art that the number of mobile stations in group MS N    124  is not restricted to three but may be more or less than three. 
         [0041]      FIG. 2  is a block diagram of a prior art MIMO system  200 , and illustrates inter-user interference.  FIG. 2  includes four signals  202 ,  204 ,  206 , and  208  denoted by s 1   (1) , s 2   (1) , s 1   (2) , and s 2   (2)  respectively, base station  230  having two pairs of antennas  210  and  212 , two mobile stations MS 1    220  and MS 2    222 , each having two antennas  214 / 216  and  224 / 226  respectively and a MIMO decoder  218  and  228  respectively. In operation, the signals  202 ,  204 ,  206 , and  208 , are transmitted from the base station  230  via respective ones of each pair of the antennas  210  and  212  to MS 1    220  and MS 2    222 . Signals received by the antennas  214 / 216  and  224 / 226  in MS 1    220  and MS 2    222  are processed by the MIMO decoders  218  and  228 . Interference in the MIMO system of  FIG. 2  is indicated at  232 . As shown, any communication signals that are received at one of the mobile stations MS 1    220  or MS 2    222  but intended for the other of the mobile station  220  or  222  represent interference at that mobile station. For example, versions of s 1   (2)    206  and s 2   (2)    208  received at MS 1    220  represent interference. It will be apparent to those skilled in the art that although  FIG. 2  only shows two mobile stations MS 1    220  and MS 2    222 , the number of mobile stations in a MU-MIMO system is not limited to two, and the embodiments described herein may function with different numbers of mobile stations. 
         [0042]      FIG. 3  is a block diagram that illustrates communication with one of a plurality of mobile stations in a prior art MU-MIMO system  300 .  FIG. 3  shows mobile stations MS 1    316 , MS 2    318 , and MS 3    320 , x 1  which denotes data  302  to be transmitted to MS 1    316 , precoder  304 , V 1  which denotes precoder vector  306  to precode data  302  to be transmitted to MS 1    316 , H which denotes MIMO channel matrix  308  between a base station and MS 1    316 , y 1  which denotes received signal  314  of MS 1    316 , I which denotes inter-user interference  310 , and “n” which denotes other cell interference and noise  312 . In operation, data x 1   302  is fed to precoder  304  which precodes the data  302  with precoder vector V 1    306  for MS 1    316 . The MIMO channel matrix H  308  transforms the precoded signal into received signal y 1   314  of MS 1    316 . Inter-user interference I  310  and other cell interference n  312  is added to received signal  314 . Therefore, in the multi-user MIMO system of  FIG. 3 , the received signal  314  of MS 1    316  can be expressed as: 
         [0000]        y+HV   1   x   1   +I+n    (1)
 
         [0000]      where 
         [0000]        I=HV   2   x   2   +HV   3   x   3    (2)
 
         [0000]    and V 2  to V 3  (not shown) are the precoders used to precode data x 2  and x 3  (not shown) to be transmitted to MS 2  and MS 3 . 
         [0043]    As will be apparent to those skilled in the art, although not shown in  FIG. 3 , a base station (not shown) also communicates with MS 2    318  and MS 3    320 . For communications with MS 2    318 , MS 1    316  and MS 3    320  are interfering mobile stations, and for communications with MS 3    320 , MS 1    316  and MS 2    318  are interfering mobile stations. It will also be apparent to those skilled in the art that the number of mobile stations in a MU-MIMO system is not limited to three, and the embodiments described herein may function with more or less than three mobile stations. 
         [0044]    Currently, the C/I feedback may assume a certain margin that represents the degradation in C/I due to spatial division multiple access (“SDMA”). This results in an unnecessary forfeiture of otherwise available system capacity. 
         [0045]    Furthermore, in conventional MU-MIMO systems, a different number of users can be multiplexed at different times. Therefore, the number of mobile stations multiplexed in the current received signal has to be estimated. Since an estimate of interference based on an estimated number of mobile stations can affect the C/I computation which is subsequently fed back to the transmitter for link adaptation, a worst case scenario (e.g., assuming a maximum number of users are multiplexed) is necessarily assumed. As a consequence, an unduly conservative margin can be applied to the C/I, which can compromise network capacity and/or performance. Selecting a conservative modulation and coding scheme (“MCS”) can degrade the overall performance of MU-MIMO. 
         [0046]      FIG. 4  is a block diagram that illustrates communication with one of a plurality of mobile stations in a MU-MIMO system  400 , according to some embodiments, where the mobile stations have knowledge of the interfering precoder vectors. In  FIG. 4 , data x 1    302 , precoder  304 , precoder vector V 1    306 , MIMO channel matrix H  308 , received signal y 1    314 , inter-user interference I  310  and other cell interference and noise  312  operate the same as in  FIG. 3 .  FIG. 4  shows further elements V 2  and V 3  which denote interfering precoder vectors  324  and  326  used to precode data transmitted to MS 2    318  and MS 3    320 . In operation MS 1    316  has knowledge of the interfering precoder vectors V 2    324  and V 3    326 . 
         [0047]    As will be apparent to those skilled in the art, MU-MIMO system  400  includes further components in addition to the precoder  304 , such as a signal generator operable to generate a signal having a signal portion indicative of precoder vectors. It will also be apparent to those skilled in the art, although not shown in  FIG. 4 , a base station also communicates with MS 2    318  and MS 3    320 . Accordingly, MS 2    318  and MS 3    320  may have knowledge of respective interfering precoder vectors  306 / 326  for MS 2    318  and  306 / 324  for MS 3 , as shown in  FIG. 4 . As will also be apparent to those skilled in the art, interfering precoder vectors may also arise from mobile stations outside of multiplexed mobile stations MS 1    316 , MS 2    318 , and MS 3    320 , and possibly from a coverage area not serviced by the base station. When a mobile station MS 1    316 , MS 2    318  or MS 3    320  obtains knowledge of its respective interfering precoder vectors, C/I may be more accurately estimated for that mobile station. Moreover, the number of users multiplexed in the transmission may be deduced in some embodiments. 
         [0048]    A more accurate C/I computation may lead to improved link adaptation performance. For example, in some embodiments, if a minimum mean square error (“MMSE”) receiver is used, the instantaneous receiver weights may be computed as 
         [0000]        w =( HH   H   +R   i ) −i   H    (2)
 
         [0000]    for which R H  is the Hermitian transposition of the channel matrix  308 . R i  is the instantaneous correlation of the interference  310  plus noise  312 . R i  can be estimated more accurately with the knowledge of the interfering precoder vectors. The weights obtained can better suppress the interference  310 . In other embodiments, mobile stations MS 1    316 , MS 2    318  and MS 3    320  may make use of the knowledge of interfering precoder vectors to perform interference cancellation. 
         [0049]      FIG. 5  shows a MU-MIMO communications system  500 , according to some embodiments, including a base station  502 , a plurality of mobile stations MS N    516  consisting of MS 1    510 , MS 2    512  and MS 3    514 , and v 1 , v 2 , v 3 , and v 0  which denote precoder parameters  504 ,  506 ,  508 , and  518  respectively. Precoder parameters v 1    504 , v 2    506 , and v 3    508  are indicative of precoder vectors V 1 , V 2 , and V 3  (not shown) used to precode data for transmission to MS 1    510 , MS 2    512 , and MS 3    514  respectively. Precoder parameters v 0    518  are indicative of precoder vector(s) arising from mobile station(s) outside of group MS N    516  and possibly from a coverage area not serviced by the base station  502 . Precoding parameters  504 ,  506 ,  508 , and  518  may be the precoder vectors, indices, bitmaps, or other information relating to precoder vectors as discussed in relation to other embodiments. In operation, base station  502  multiplexes mobile stations MS 1    510 , MS 2    512  and MS 3    514  using known MIMO or SDMA methods. When communicating with MS 1    510 , base station  502  signals interfering precoder parameters v 2    506  and v 3    508  to MS 1    510 . Similarly, for communications with MS 2    512 , V 1  and V 3  are interfering precoder vectors, and for communications with MS 3 , V 1  and V 3  are interfering precoder vectors. Therefore parameters v 1    504  and v 3    508  are signalled to MS 2    512  and parameters v 1    504  and v 2    506  are signalled to MS 3    514 . The base station  502  may also signal interfering precoder parameters v 0    518  to the mobile stations MS N    516  as shown. Although  FIG. 5  shows group MS N    516  having three mobile stations, N is not restricted to be equal to three. The number of mobile stations in MS N    516  is likewise not restricted in other embodiments to be discussed. 
         [0050]    As will be apparent to those skilled in the art, base station  502  includes components such as a signal generator operable to generate a signal having a signal portion indicative of precoder vectors. As will also be apparent to those skilled in the art, it may not be necessary for all mobile stations to have knowledge of interfering precoder vectors as shown in  FIG. 5  in order to improve C/I calculation for one or more mobile stations. 
         [0051]      FIG. 6A  shows a MU-MIMO system  600  according to some embodiments including a base station  602 , a defined group of mobile stations MS N   616  consisting of MS 1    610 , MS 2    612  and MS 3    614 . In operation, base station  602  configures a MU-MIMO zone such that the number of mobile stations in group MS N    616  remains constant over a period of time. In  FIG. 6B , N denotes the number  618  of mobile stations in group MS N    616 , and T denotes a period of time  620 . In operation, N  618  remains constant over T  620 . The base station may determine N based on several factors including environment, system capability, etc. Period of time T  620  may be equal to a superframe, so that N  618  is configured every superframe. Mobile stations MS 1    610 , MS 2    612  and MS 3    614  can report a more accurate C/I with a constant number N  618  of mobile stations  616  over period of time  620 . The number N  618  of mobile stations in Group MS N    616  may be signalled or otherwise determined by mobile stations MS 1    610 , MS 2    612  and MS 3    614 . Therefore, the worst case scenario (maximum multiplexing of users) may not need to be assumed in C/I estimation. 
         [0052]    In codebook based MU-MIMO, sets of predefined precoder vectors are used. These predefined sets of precoder vectors form a codebook. Precoder vectors in the codebook are indexed, and each precoder vector correspond to an index or bitmap value. 
         [0053]      FIG. 7A  shows a MU-MIMO system  700  according to some embodiments including a base station  502 , a plurality of mobile stations MS N    516  consisting of MS 1    510 , MS 2    512  and MS 3    514 , codebook  718 , and i 1 , i 2 , and i 3  which denote precoder vector parameters  704 ,  706 , and  708  respectively. The elements of  FIG. 7A  operate in a similar fashion as those in  FIG. 5 . In addition, base station  502  and mobile stations  516  store codebook  718 . Precoding parameters i 1    704 , i 2    706  and i 3    708  are the index or bitmap value of the corresponding precoder vectors V 1 , V 2 , and V 3  (not shown) which are used to precode data to be transmitted to MS 1    510 , MS 2    512 , and MS 3    514  respectively, and which are contained in the codebook  718 . Base station  502  signals the index i 1    704 , i 2    706  or i 3    708  of interfering precoder vectors to mobile stations MS 1    510 , MS 2    512  or MS 3    514 , and mobile stations MS 1    510 , MS 2    512  or MS 3    514  then retrieve interfering precoder vectors from the codebook  718 . 
         [0054]      FIG. 7B  is a flowchart of steps in some embodiments involving involving codebook MU-MIMO as it may be implemented by the elements of  FIG. 7A . At step  720 , the codebook  718  is known to base station  502  and the mobile stations  516 . At step  722 , the base station  502  signals index or bitmap values  706 / 708 ,  704 / 708 , or  704 / 706  corresponding to the interfering precoder vectors. At step  724 , mobile station MS 1    510 , MS 2    512 , and MS 3    514  use the index or bitmap values  706 / 708 ,  704 / 708 , or  704 / 706  to retrieve their respective interfering precoder vectors. 
         [0055]    Codebook MU-MIMO may or may not be implemented in conjunction with other embodiments described herein. When configuring the number of users in a MU-MIMO system, as shown in  FIGS. 6A and 6B , a mobile station MS 1    510 , MS 2    512 , or MS 3    514  may predict the inter-user interference by averaging the interference caused by using different precoder vectors in the codebook  718 .  FIG. 7C  shows an average inter-user interference I ave    722 , actual interference I(t)  724 , and period of time T  620 . Interference I(t)  724  varies around the average inter-user interference I ave    722  over the time period T  620  wherein the number of users in the MU-MIMO system is constant. As explained above, I ave  is the average of interference caused by using different precoder vectors in the codebook. It should be appreciated that  FIG. 7C  is intended for illustrative purposes only in order to show how average interference may relate to the number of users in a codebook MU-MIMO communication system. 
         [0056]    In some embodiments, with reference to the elements of  FIG. 5 , precoder vectors are computed on the fly by the base station  502  based on the channel state information (CSI) at the base station  502 , and the interfering precoder vectors are quantized and signalled to the mobile stations MS 1    510 , MS 2    512 , or MS 3    514 . Since a precoder vector may be obtained by the singular value decomposition of the MIMO channel, the following embodiments shown in  FIGS. 8A ,  8 B, and  9  may be realized. 
         [0057]      FIG. 8A  is a flowchart of a method for a MU-MIMO system according to some embodiments, with reference to the elements shown in  FIG. 5 , involving channel sounding. At step  802 , the channel in one direction (e.g., the forward link) is estimated at the base station  502  based on pilots transmitted from the other direction (e.g., the reverse link). At step  804 , the precoder vectors for mobile stations MS N    516  are calculated at the base station  502 . At step  806 , the interfering precoder vector coefficients used in the multi-user transmission are quantized. At step  808 , quantized precoder coefficients are signalled to the respective mobile stations MS 1    510 , MS 2    512 , or MS 3    514 . In the embodiments shown in  FIG. 8A , the quantized precoder coefficients are the interfering precoder parameters  506 / 508 ,  504 / 508 , or  504 / 506  shown in  FIG. 5 . 
         [0058]      FIG. 8B  is a flowchart of an example of an embodiment of the method shown in  FIG. 8A . Steps  802  and  804  are carried out in the same manner as discussed above in connection with  FIG. 8A . At step  810 , a base station scheduler may calculate the correlation of different precoder vectors and, at step  812 , multiplex mobile stations MS 1    510 , MS 2    512 , or MS 3    514 , whose precoder vectors have the lowest correlation to minimize the inter-user interference. At step  814 , the interfering precoder vector coefficients used in the multi-user transmission are quantized. At step  816 , quantized precoder coefficients are signalled to the respective mobile stations MS 1    510 , MS 2    512 , or MS 3    514 . In the embodiments shown in  FIG. 8A , the quantized precoder coefficients are the interfering precoder parameters  506 / 508 ,  504 / 508 , or  504 / 506  shown in  FIG. 5 . 
         [0059]      FIG. 9  is a flowchart of a method for a MU-MIMO system according to some embodiments, with reference to the elements shown in  FIG. 5 , involving calculating precoder vectors based on quantized channel coefficients. At step  902 , quantized channel coefficients are fed back from mobile stations MS N    516  to the base station  502 . Similar to channel sounding, at step  904 , the precoder vectors are obtained at the base station  502  based on the quantized channel coefficients. At step  906 , the interfering precoder vector coefficients used in the multi-user transmission are quantized. At step  908 , the quantized precoder coefficients are signalled to the mobile stations MS 1    510 , MS 2    512 , or MS 3    514 . In the embodiments shown in  FIG. 9 , the quantized precoder coefficients are the interfering precoder parameters  506 / 508 ,  504 / 508 , or  504 / 506  shown in  FIG. 5 . 
         [0060]    In some embodiments, the signalling of interfering precoder parameters to mobile stations can be purely unicast.  FIG. 5  illustrates unicast signalling of the interfering precoder parameters  506 / 508 ,  504 / 508 , or  504 / 506  and possibly  518  to the respective mobile station MS 1    510 , MS 2    512 , or MS 3    514 . Unicast signalling may be beneficial if the multiplexed mobile stations are in very different geometry. In this manner, the unicast signalling can be adapted by power control or resource assignment to geometry, or channel conditions, of each mobile station MS 1    510 , MS 2    512 , or MS 3    514  independently. 
         [0061]      FIG. 10  illustrates MU-MIMO system  1000  in which precoding parameters are signalled in a multicast fashion.  FIG. 10  shows a base station  502 , a group of mobile stations MS N    516  which consists of mobile stations MS 1    510  MS 2    512 , and MS 3    514 , and v 1 , v 2 , and v 3  which denote precoder parameters  504 ,  506 , and  508 . Precoder parameters v 1    504 , v 2    506 , and v 3    508  are indicative of precoder vectors V 1 , V 2 , and V 3  (not shown) used to precode data for transmission to MS 1    510 , MS 2    512 , and MS 3    514  respectively. In operation, the base station  502  signals all or part of the precoder information  504 ,  506 , and  508  to all of the spatially multiplexed mobile stations  516 . Each SDMA mobile station  516  determines the interfering precoder vectors in the set by deleting its own precoder from the set. Multicast signalling may be beneficial if the multiplexed mobile stations  516  are in similar geometery. In this manner, the multicast message is received by several mobile stations  516  preventing the need for several unicast messages, and hence, reducing signalling resources. Multicast signalling may save on overhead bandwidth and may avoid the need of duplicating information. Although other embodiments described herein have been described with reference to a unicast system, a multicast system may also be used in conjunction with other embodiments of the invention. 
         [0062]    In some embodiments, the interfering precoder parameters may be signalled (i.e., indicated) to mobile stations via the message indicating the station&#39;s assigned resources.  FIGS. 11 to 13  provide detail regarding how a base station may signal interfering precoder parameters to mobile stations.  FIG. 11  shows a total assignment message  1100  consisting of a portion of the assignment message  1102  and a separate field  1104 . In operation, interfering precoder parameters are indicated in the separate field  1104  indicating the interfering precoder parameters to the mobile station. 
         [0063]    In other embodiments, interfering precoder parameters may be indicated in an existing field intended for another purpose or multiple purposes. 
         [0064]    In some cases, a mobile station may be notified that a field now contains the interfering precoder parameter(s) by a bit indicator in the message, such as illustrated in  FIGS. 12A and 12B .  FIGS. 12A and 12B  show a total assignment message  1200  consisting of a portion of the assignment message  1202 , an indicator bit  1204 , and a desired precoder parameter field or other field  1206 . In operation, the indicator bit  1204  indicates whether or not an interfering precoder parameter is being signalled in the existing field  1206 . The indicator bit  1204  in  FIG. 12A  indicates that an interfering precoder parameter is not signalled. In  FIG. 12B , the indicator bit  1204  shows that an interfering precoder parameter is signalled and is then followed by the interfering precoder parameter. The interfering precoder parameter may simply be an interfering precoder vector&#39;s index, such as in codebook MU-MIMO, and more than one interfering precoder parameter can be indicated. 
         [0065]    Alternatively, a mobile station may be notified that a field now contains interfering precoder parameter(s) by the assignment message type.  FIG. 13  shows a total assignment message  1300  consisting of a message type indicator  1304 , a portion of the assignment message  1302 , and an interfering precoder parameter field  1306 . In FIG.  13 , the message type indicator  1304  indicates that the interfering precoder parameter(s) is signalled. 
         [0066]    Multi-user MIMO and single-user MIMO can be switched dynamically based on many factors such as user channel conditions, quality of service (“QoS”) etc. In the dynamic case, at any given time and/or time-frequency resource, users may or may not be multiplexed. Another way is to define a zone in time or time-frequency resource whereby transmissions can be restricted to SDMA. 
         [0067]    An SDMA zone is a defined time-frequency region that may be used for the purpose of MU-MIMO transmissions. For the purpose of MU-MIMO transmissions, an SDMA zone may also be referred to as a MU-MIMO zone. The region can consist of one or more logical channels. The logical channels may or may not be physically contiguous. MU-MIMO assignments can be made in this resource space. In some cases, within this zone certain rules can be defined to facilitate operation for either the base station, or for the mobile station, or both. 
         [0068]      FIG. 14  shows a time-frequency resource zone  1400  according to some embodiments including an SDMA zone  1404  and a control channel signalling zone  1402 . In operation, SDMA zone  1404  is defined by control channel signalling zone  1402  in time/frequency. The defined SDMA zone  1404  may include rules or constraints as described below. In  FIG. 14 , the control channel signalling zone  1402  is not part of the SDMA zone  1404 . SDMA zone  1404  may also be referred to as a MU-MIMO zone. 
         [0069]    The signalling requirement for the dynamic and non-dynamic cases may be different. In the dynamic case, since mobile stations do not know whether they will be receiving data in SDMA or not, they most likely will report all their preferred precoder vectors. Therefore, a base station needs to signal to a user if only a subset of their preferred precoder vectors is used due to SDMA, in additional to the interfering precoder vectors. 
         [0070]    On the other hand, if it is known beforehand, via initial configuration or upper layer signalling, that an SDMA zone  1404  exists, as shown in  FIG. 14 , certain rules, constraints, or zone parameters can be defined such as the maximum number of precoder vectors to report per user, or a fixed number of multiplexed mobile stations. For example, in order to multiplex more users, it can be configured that only one precoder per user is reported in the SDMA zone  1404 . In this case, the base station may only need to signal the interfering precoder vectors to the users. The desired precoder may not need to be signalled if dedicated pilots are used or if a timing relationship exists between feedback and transmission. Specifically, a mobile station can assume that if a precoded transmission is received, the precoder used is based on the precoder reported a certain number of slots before. Defining the SDMA zone in this way may be useful in non-dynamic switching. 
         [0071]    In some embodiments, interfering precoder vectors may be determined by mobile stations automatically without the base station explicitly signalling precoder parameters to the mobile stations as shown in  FIGS. 15 and 16 . 
         [0072]      FIG. 15  is a flowchart of steps in some embodiments, involving mobile stations determining interfering precoder vectors respectively. At step  1502 , precoder vectors are grouped into sets with low correlation or into orthogonal sets. At step  1504 , this grouping is pre-computed and stored at the base station and mobile stations for different numbers of SDMA layers. Each multiplexed transmission data stream constitutes an SDMA layer. SDMA layers are transmission channels occupying the same time-frequency resources that can be separated using spatial techniques. At step  1506 , the base station signals mobile station(s) the total number of SDMA layers and a respective desired precoder. At step  1508 , the mobile station(s) refers to the orthogonal, or low correlation, group that its precoder belongs to and deduces its respective interfering precoder vectors. This is because, apart from the desired precoder, the other precoder vectors used for a certain number of SMDA layers are the interference. The total number of SDMA layers may not need to be signalled in every MU-MIMO transmission if the number of layers is configured every time T  620  as shown in  FIGS. 6A and 6B , and as explained above. For example, the base station may signal to mobile station(s) the total number of SDMA layers and the desired precoder only one time per superframe. 
         [0073]    In another embodiment, the precoder vectors can be cycled in a pre-defined pattern.  FIG. 16  is a flowchart showing steps in some embodiments involving determining interfering precoder vectors when precoder vectors are cycled in a predefined pattern. At step  1602 , the precoder vectors are cycled in a pre-defined pattern known to a base station and mobile stations. At step  1604 , the base station chooses the best mobile station(s) to use a particular precoder. At step  1606 , with the knowledge of which layer the mobile station(s) is scheduled on, both the interfering precoder vectors and the desired precoder vectors are deduced by the mobile station(s). 
         [0074]    In some further embodiments, a large number of assignments of resources over which data is transmitted may be needed for multi-user and single-user MIMO. For example, VoIP or gaming users may take advantage of the parallel transmissions offered by MIMO. 
         [0075]    When multiplexing users on different layers, the assignment may be dynamic or semi-static.  FIGS. 17A ,  17 B, and  17 C show two mobile stations  1702  and  1704  and SDMA layers L 1   1706 , L 2   1708 , L 3   1710 , and L 4   1712 , periods of time ι 1    1714  and ι 2    1716 . In operation, mobile stations  1702  and  1704  are assigned to SDMA layer(s) L 1   1706 , L 2   1708 , L 3   1710 , or L 4   1712 . In the dynamic case, as shown in  FIG. 17A , different mobile stations  1702  and  1704  may be scheduled on different layers L 1   1706 , L 2   1708 , L 3   1710 , or L 4   1712  over time. In the semi-static case, as shown in  FIG. 17B , mobile stations  1702  and  1704  occupy the same layer(s) L 1   1706 , L 2   1708 , L 3   1710 , or L 4   1712  for a period of time ι 1    1714 . The signalling overhead for the semi-static case may thus be reduced. In some embodiments, assignment of mobile stations  1702  and  1704  to SDMA layers L 1   1706 , L 2   1708 , L 3   1710 , or L 4   1712  may follow a hopping pattern known to a base station and the mobile stations  1702  and  1704 .  FIG. 17C  shows an example of mobile stations  1702  and  1704  being assigned to SDMA layers L 1   1706 , L 2   1708 , L 3   1710 , or L 4   1712  following a hopping pattern with a pattern duration of ι 2    1716 . In this manner, the mobile stations  1702  and  1704  may occupy each layer L 1   1706 , L 2   1708 , L 3   1710 , and L 4   1712  for some time for diversity, and may require no additional signalling. As with other embodiments described herein, it should be appreciated that  FIGS. 17A ,  17 B, and  17 C are intended for illustrative purposes only. As will be apparent to those skilled in the art to which the present invention pertains, the number of mobile stations  1702  and  1704  is not restricted to two. SDMA layers L 1   1706 , L 2   1708 , L 3   1710 , and L 4   1712  are illustrative examples and do not restrict the number of layers, or the type of assignments to SDMA layers that may be made in accordance with some embodiments. 
         [0076]      FIG. 18  is a flowchart showing steps in some embodiments, involving grouping mobile stations according to MIMO mode. At step  1802 , a base station groups mobile stations with the same MIMO mode. At step  1804 , the base station signals the MIMO mode only once per scheduling event rather than once per mobile station. Grouping mobile stations according to MIMO mode may lead to further reduction of the signalling with large numbers of mobile stations. 
         [0077]    In order to reduce the potentially large signalling overhead, a bitmap-like signalling structure can be used to signal MSs, and signal the MIMO modes (e.g., spatial multiplexing, STTD, etc.)either by indicating at least one MIMO mode in the bitmap signalling, or by associating a bitmap with a MIMO mode for a period of time. Suitable examples for such signalling are described in, but not limited to, the signalling methods published as international patent application WO 2007/045101 published Apr. 26, 2007 and entitled “Multiplexing Schemes for OFDMA” owned by Nortel Networks Limited, the assignee of the subject application (Attorney Docket No. 18022ROWO04W). A bitmap is a set of bits, where the position and value of each bit is significant. For example, a different mobile station may be assigned to each bit position and the value of the bit may indicate whether or not the mobile station has been assigned a resource. In some cases, further reduction of signalling the MIMO mode may also be possible. The bitmap signalling approach may work well in conjunction with MIMO mode grouping shown in  FIG. 18 . For example, it is usually the case that a number of assignments use the same MIMO mode. Therefore, the MIMO mode can be signalled only once for the group of assignments. 
         [0078]    With a more accurate C/I estimation based on the information of multi-user interference, a base station may also multiplex mobile stations intelligently to reduce the interference variability of subsequent transmissions.  FIG. 19  is a flowchart showing steps in some embodiments involving intelligent multiplexing. At step  1902 , mobile stations with the same or similar set of precoder vectors are multiplexed. At step  1904 , if more than two mobile stations are multiplexed (and thus more than two precoder vectors are used), proceed to step  1906 . At step  1906 , the change in scheduled mobiles (or precoder vectors) is limited to a subset of precoders used in each scheduling interval at any instance such that the interference can be somewhat regulated. Limiting the subset of precoders in this manner may minimize the changes in the interference variability. 
         [0079]    If the channel is known at a base station, for example, using the channel sounding method, it is possible that the desired precoder needs not be signalled.  FIG. 20  is a flowchart showing steps in some embodiments when the channel may be known. At step  2002 , if the channel is known at the base station, then proceed to step  2004 . At step  2004 , a base station and mobile station run the same precoder selection or calculation algorithm with the channel matrix as an input. The selected precoder can then be known automatically at both the base station and mobile station. 
         [0080]    In some cases, where the base station signals the interfering precoder vectors, the base station may not need to signal the selected precoder using the above method(s), and may signal the interfering precoder vectors used by the interfering mobile stations to a mobile station. 
         [0081]    While Eigen beamforming has been primarily considered here, techniques and methods described are applicable to other beamforming techniques including systems with array beamforming, fixed beamforming, or those using angle of arrival. For example, in fixed beamforming systems, the mobile station can be notified of the interfering beamforming vectors&#39; indices rather than precoder vectors. 
         [0082]    While the invention has been shown and described with reference to certain preferred embodiments, it is to be understood and appreciated by those skilled in the art that various changes in form and detail may be made herein without departing from the scope and spirit of the invention as defined by the appended claims. 
         [0083]    What has been described is merely illustrative of the application of the principles of the invention. Other arrangements and methods can be implemented by those skilled in the art without departing from the spirit and scope of the present invention.