Patent Publication Number: US-2009232087-A1

Title: User Equipment Multiplexing In Downlink Multiuser, Multiple Input Multiple Output Orthogonal Frequency Division Multiple Access

Description:
CLAIM OF PRIORITY 
     This application claims priority under 35 U.S.C. 119(e)(1) to U.S. Provisional Application No. 61/034,851 filed Mar. 7, 2008. 
    
    
     TECHNICAL FIELD OF THE INVENTION 
     The technical field of this invention is wireless communication. 
     BACKGROUND OF THE INVENTION 
       FIG. 1  shows an exemplary wireless telecommunications network  100 . The illustrative telecommunications network includes base stations  101 ,  102  and  103 , though in operation, a telecommunications network necessarily includes many more base stations. Each of base stations  101 ,  102  and  103  are operable over corresponding coverage areas  104 ,  105  and  106 . Each base station&#39;s coverage area is further divided into cells. In the illustrated network, each base station&#39;s coverage area is divided into three cells. Handset or other user equipment  109  is shown in Cell A  108 . Cell A  108  is within coverage area  104  of base station  101 . Base station  101  transmits to and receives transmissions from user equipment  109 . As user equipment  109  moves out of Cell A  108  and into Cell B  107 , user equipment  109  may be handed over to base station  102 . Because user equipment  109  is synchronized with base station  101 , user equipment  109  can employ non-synchronized random access to initiate handover to base station  102 . 
     Non-synchronized user equipment  109  also employs non-synchronous random access to request allocation of up-link  111  time or frequency or code resources. If user equipment  109  has data ready for transmission, which may be traffic data, measurements report, tracking area update, user equipment  109  can transmit a random access signal on up-link  111 . The random access signal notifies base station  101  that user equipment  109  requires up-link resources to transmit the user equipment&#39;s data. Base station  101  responds by transmitting to user equipment  109  via down-link  110 , a message containing the parameters of the resources allocated for user equipment  109  up-link transmission along with a possible timing error correction. After receiving the resource allocation and a possible timing advance message transmitted on down-link  110  by base station  101 , user equipment  109  optionally adjusts its transmit timing and transmits the data on up-link  111  employing the allotted resources during the prescribed time interval. 
     A downlink multiuser, multiple input, multiple output MIMO (DL MU-MIMO) communication system involves a single base station transmitting to multiple UEs at the same time over the same frequency bandwidth. One example of a DL MU-MIMO scheme is the dirty-paper coding technique. From an information theory perspective this dirty-paper coding technique is the optimal MU-MIMO scheme in terms of achieving the maximum sum capacity. An alternative and more practical MU-MIMO technique is transmit preceding. In transmit preceding the data to each UEs is multiplied to a UE-specific preceding matrix and then transmitted at the base station antenna array simultaneously. 
     In an orthogonal frequency division multiple access (OFDMA) system, system bandwidth is divided into a number of sub-bands each consisting of a set of subcarriers. The number and indices of UEs supported by DL MU-MIMO on a particular sub-band can be different. 
     SUMMARY OF THE INVENTION 
     This invention is a method of downlink, multiuser, multiple input, multiple output communication between a single base station and plural user equipment. The time/frequency available to the base station is divided into a plurality of resource blocks. These resource buffers are distributed into sub-bands. Each user equipment is assigned to a sub-band with at least one sub-band assigned plural user equipment. The base station transmits user equipment according to assigned sub-bands. 
     A single user equipment is assigned one or multiple sub-band, where these sub-bands can be consecutive or non-adjacent in frequency domain. Plural user equipment is assigned to another sub-band. It is possible to assign a single user equipment on one sub-band, and multiple user equipment on another sub-band. For example, a first user equipment solely to a first sub-band and the first user equipment and at least one other is assigned a second sub-band. 
     In one embodiment the base station has N transmit antennas. N user equipment is assigned to a first sub-band. A single codeword/data stream is assigned to each of these user equipment. These are operated with rank 1, where rank denotes the number of data layers in a codeword. Less than N user equipment is assigned to a second sub-band. The user equipment assigned to the second sub-band includes operating with rank greater than 1 including multiple data layers for each user equipment assigned to the second sub-band. The sum of the operating rank of each user equipment assigned to the second sub-band is less than or equal to N. 
     In another embodiment a first second user equipment is assigned a first sub-band and the first and a third user equipment are assigned to a second sub-band. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       These and other aspects of this invention are illustrated in the drawings, in which: 
         FIG. 1  is a diagram of a communication system of the present invention having three cells; 
         FIG. 2  illustrates the total operating bandwidth divided into a number of resource blocks (sub-bands) according to the prior art; and 
         FIG. 3  is an example of user equipment multiplexing in downlink multiuser, multiple input multiple output according to this invention. 
     
    
    
     DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS 
     This invention concerns a MU-MIMO system in the multi-carrier OFDMA context. The operating bandwidth is divided into non-overlapping resource blocks (RB). 
       FIG. 2  illustrates division of operating bandwidth in non-overlapping RBs. In  FIG. 2  the total resources available to the base station is divided in frequency into RB bandwidths and in time into sub-frames. This divides the operating time/bandwidth into M RBs. A resource block is set of adjacent sub-carriers (tones). In the LTE (Long Term Evolution) Rel-8, a 5 MHz bandwidth has 25500 RBs of 180 KHz each. The total operating bandwidth is 4.5 MHz after allowing 0.5 MHz bandwidth to be used for band edge interference protection. For scheduling purposes, RBs may be concatenated into larger blocks. Such an entity is defined as a sub-band. One sub-band consists of n RBs, where n is a positive integer. Assume the system bandwidth provides N sub-bands. While all the N sub-bands may include the same number of RBs, it is also possible for different sub-bands to have different numbers of RBs. This is particularly relevant when M is not a multiple of N. 
     At a particular time instant and on a particular sub-band, the serving base station can simultaneously transmit to multiple UEs. These multiple UEs are multiplexed in the spatial domain. Data to each UE will be modulated by a preferred preceding matrix from a codebook known a priori to both the base station and the UEs. A particular UE can be scheduled on a number of consecutive or adjacent sub-bands. 
     In accordance with this invention for the n-th sub-band, the number of UEs multiplexed is denoted by Nu (n). The indices of the multiplexed UE is represented by U=[X(n, 1), X(n, 2) . . . X(n, Nu(n))]. 
       FIG. 3  illustrates an example of a possible logical organization according to this invention. The number of UEs multiplexed on a sub-band can be sub-band specific. A different number of UEs can be multiplexed on different sub-bands.  FIG. 3  illustrates sub-band  1   301 , sub-band  2   302 , sub-band  3   303 , sub-band  4   304 , sub-band  5   305 , sub-band  6   306  and sub-band  6   306 . In  FIG. 3  the base station can transmit to: three UEs (UE 1   321 , UE 2   321  and UE 3   323 ) on sub-band  1   301  via multiplexer  311 ; to two UEs (UE 1   321  and UE 3   323 ) on sub-bands  2   302  via multiplexer  312 ; to two UEs (UE 1   321  and UE 3   323 ) on sub-band  303  via multiplexer  313 ; to two UEs (UE 2   322  and UE 3   323 ) via multiplexer  314 ; to only one UE (UE 1   321 ) on sub-band  5   305  via multiplexer  315 ; to only one UE (UE 3   323 ) on sub-band  6   306  via multiplexer  316 ; and to only one UE (UE 4   324 ) on sub-band  7   307  via multiplexer  317 . This invention does permits the same number of UEs to be multiplexed on different sub-bands, such as UE 1   321  and UE 2   322  on sub-band  3   303  and UE 2   322  and UE 3   323  on sub-band  4   304 . 
     The base station may transmit to a single UE on one sub-band making Nu=1.  FIG. 3  illustrates this for sub-band  5   305 , sub-band  6   306  and sub-band  7   307 . The base station may transmit to multiple UEs on a different sub-band where Nu&gt;1.  FIG. 3  illustrates this for sub-band  1   301 , sub-band  2   302 , sub-band  3   303  and sub-band  4   304 . The system may operate in the single-user MIMO (SU-MIMO) mode on certain sub-bands supporting only one UE and also operate in MU-MIMO mode on other sub-bands supporting multiple UEs. 
     A UE may operate in SU-MIMO and MU-MIMO mode simultaneously in different sub-bands. In the example illustrated in  FIG. 3 , UE 1   321  is in MU-MIMO mode in sub-band  1   301  multiplexed via multiplexer  311  with UE 2   322  and UE 3   303  and in SU-MIMO model in sub-band  5   305 .  FIG. 3  illustrates the example of UE 4   324  operating only in SU-MIMO mode in sub-band  307 .  FIG. 3  also illustrates the example of UE 2   322  operating only in MU-MIMO mode in sub-band  1   301 , sub-band  3   303  and sub-band  4   304 . 
     The maximum number of UEs that can be multiplexed on a sub-band is equal to the number of transmit antennas N t  at the serving base station. For example, a MU-MIMO system with  4  transmit antennas may communicate with a maximum of  4  UEs simultaneously in the downlink on a given sub-band. In this case, each UE operates in rank-1 transmission where a single codeword codeword/data sub-stream is targeted for transmission to this UE. On a sub-band where the number of multiplexed UEs is less than N t , the UE rank may be equal to or greater than  1 . The summation of the ranks of all UEs multiplexed on a sub-band should be equal to or less than N t . 
     For a particular UE, the indices of other UEs that share the same sub-band could be sub-band specific. This means that a UE can be grouped with different UEs on different sub-bands. In the example illustrated in  FIG. 3  assume UE 1   321  is target UE.  FIG. 3  illustrates that UE 1   321  can be paired with UE 3   323  in sub-band  2   302  and paired with UE 2   322  in sub-band  3   303 .