Abstract:
Systems and methods are disclosed for use in a communications network that includes transmitting a set of known precodes on a plurality of subbands and storing a correlation of transmitted precodes with a time of transmission of the precodes. These systems and methods also include receiving a set of channel quality indicators (CQI) that correspond to the time of transmission of the precodes and determining which precodes may be used in communication based upon the received CQIs and the correlation of the transmitted precodes with the time of transmission of the precodes.

Description:
CROSS-REFERENCE TO RELATED APPLICATION(S) AND CLAIM OF PRIORITY 
       [0001]    The present application is related to U.S. Provisional Patent Application No. 61/196,015, filed Oct. 14, 2008, entitled “LOW OVERHEAD MIMO SCHEME”. Provisional Patent Application No. 61/196,015 is assigned to the assignee of the present application and is hereby incorporated by reference into the present application as if fully set forth herein. The present application hereby claims priority under 35 U.S.C. §119(e) to U.S. Provisional Patent Application No. 61/196,015. 
     
    
     TECHNICAL FIELD OF THE INVENTION 
       [0002]    The present application relates generally to wireless communications and, more specifically, to reducing the overhead required by various communication schemes. 
       BACKGROUND OF THE INVENTION 
       [0003]    Network communication between two communication nodes can comprise overhead traffic and data traffic. Overhead traffic refers to traffic that is used to facilitate network communication. Examples of overhead traffic in wireless communication include, but are not limited to, reference signal overhead and feedback overhead. Overhead traffic and data traffic generally each consume a part of the network communication bandwidth. As the amount of overhead traffic increases, there may be a corresponding decrease in bandwidth available for data traffic. 
         [0004]    Therefore, reducing required overhead traffic results in a corresponding increase of bandwidth for data traffic. Therefore, there is a need in the art for an improved transmission scheme. In particular, there is a need for a low overhead multiple in, multiple out (MIMO) transmission scheme that is capable of maximizing user available resources. 
       SUMMARY OF THE INVENTION 
       [0005]    In one embodiment, a method is disclosed for use in a communications network that includes transmitting a set of known precodes on a plurality of subbands and storing a correlation of transmitted precodes with a time of transmission of the precodes. This method also includes receiving a set of channel quality indicators (CQI) that correspond to a time of transmission of the precodes and determining which precodes may be used in communication based upon the received CQI and the correlation of the transmitted precodes with the time of transmission of the precodes. 
         [0006]    In another embodiment, a base station is disclosed that includes a transmitter that transmits a set of signals with known precodes on at least two subbands. This base station also includes a storage device to store a time when the set of known precodes are transmitted and a receiver to receive a set of channel quality indicators (CQI) that correspond to the time of transmission of the precodes. In addition, this base station includes a processor to determine which precodes may be used in communication based upon the received CQI and the correlation of the transmitted precodes and with time of transmission of the precodes. 
         [0007]    In yet another embodiment, a mobile device is disclosed that comprises a receiver that receives a precoded signal, wherein the precoded signal is transmitted using a multiple in multiple out (MIMO) scheme, a processor that interprets the precoded signal, and a transmitter that transmits a channel quality indicator based upon the precoded signal, wherein the transmitter transmits the CQI determined using the precoded signal without providing feedback related to the precoded signal. 
         [0008]    To address the above-discussed deficiencies of the prior art, it is a primary object to provide, for use in a wireless network, systems and methods of reducing overhead traffic in a wireless network. 
         [0009]    Before undertaking the DETAILED DESCRIPTION OF THE INVENTION below, it may be advantageous to set forth definitions of certain words and phrases used throughout this patent document: the terms “include” and “comprise,” as well as derivatives thereof, mean inclusion without limitation; the term “or,” is inclusive, meaning and/or; the phrases “associated with” and “associated therewith,” as well as derivatives thereof, may mean to include, be included within, interconnect with, contain, be contained within, connect to or with, couple to or with, be communicable with, cooperate with, interleave, juxtapose, be proximate to, be bound to or with, have, have a property of, or the like; and the term “controller” means any device, system or part thereof that controls at least one operation, such a device may be implemented in hardware, firmware or software, or some combination of at least two of the same. It should be noted that the functionality associated with any particular controller may be centralized or distributed, whether locally or remotely. Definitions for certain words and phrases are provided throughout this patent document, those of ordinary skill in the art should understand that in many, if not most instances, such definitions apply to prior, as well as future uses of such defined words and phrases. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0010]    For a more complete understanding of the present disclosure and its advantages, reference is now made to the following description taken in conjunction with the accompanying drawings, in which like reference numerals represent like parts: 
           [0011]      FIG. 1  illustrates an exemplary wireless network that transmits ACK/NACK messages in the uplink according to the principles of the present disclosure; 
           [0012]      FIG. 2A  is a high-level diagram of an OFDMA transmitter according to one embodiment of the present disclosure; 
           [0013]      FIG. 2B  is a high-level diagram of an OFDMA receiver according to one embodiment of the present disclosure; 
           [0014]      FIG. 3  illustrates an example of a single-code word MIMO transmission scheme according to an exemplary embodiment of the disclosure; 
           [0015]      FIG. 4  illustrates an example of a multi-code word MIMO transmission scheme according to an exemplary embodiment of the disclosure; 
           [0016]      FIG. 5  illustrates an example of a multi-code word MIMO scheme for 2-layers transmission in the 3GPP LTE (Third Generation Partnership Project Long Term Evolution) system according to an exemplary embodiment of the disclosure; 
           [0017]      FIG. 6  illustrates an example of a multi-code word MIMO scheme for 3-layers transmission in the 3GPP LTE (Third Generation Partnership Project Long Term Evolution) system according to an exemplary embodiment of the disclosure; 
           [0018]      FIG. 7  illustrates an example of a multi-code word MIMO scheme for 4-layers transmission in the 3GPP LTE (Third Generation Partnership Project Long Term Evolution) system according to an exemplary embodiment of the disclosure; 
           [0019]      FIG. 8  illustrates an example of a single-user MIMO system according to an exemplary embodiment of the disclosure; 
           [0020]      FIG. 9  illustrates an example of a multi-user MIMO system according to an exemplary embodiment of the disclosure; 
           [0021]      FIG. 10  illustrates MIMO feedback based precoding according to an exemplary embodiment of the disclosure; 
           [0022]      FIG. 11  illustrates MIMO precoding according to an exemplary embodiment of the disclosure; 
           [0023]      FIG. 12  is a table of a codebook used in the 3GPP LTE system used in several of the exemplary embodiments of the disclosure; 
           [0024]      FIG. 13  is an example of MIMO precoding on different subbands according to an exemplary embodiment of the disclosure; 
           [0025]      FIG. 14  is an example of mapping of downlink reference signals in the 3GPP LTE system according to an exemplary embodiment of the disclosure; 
           [0026]      FIG. 15  is an example of precoding vector cycling for initialization and reset of precoding according to an exemplary embodiment of the disclosure; 
           [0027]      FIG. 16  is an example of MIMO precoding on different subbands according to an exemplary embodiment of the disclosure; 
           [0028]      FIG. 17  is an example of precoding vector cycling for initialization and reset of precoding using a subset of rank-1 precoders according to an exemplary embodiment of the disclosure; 
           [0029]      FIG. 18  is another example of precoding vector cycling for initialization and reset of precoding using a subset of rank-1 precoders according to an exemplary embodiment of the disclosure; 
           [0030]      FIG. 19  is an example of a mapping of downlink reference signals for rank-1 transmissions according to an exemplary embodiment of the disclosure; 
           [0031]      FIG. 20  is an example of precoding vector cycling for initialization and reset of precoding using a subset of rank-1 and rank-2 precoders according to an exemplary embodiment of the disclosure; 
           [0032]      FIG. 21  is an example of mapping of downlink reference signals for rank-2 transmissions according to an exemplary embodiment of the disclosure; 
           [0033]      FIG. 22  is an example of MIMO precoding for rank-1, rank-2 and rank-3 transmissions according to an exemplary embodiment of the disclosure; 
           [0034]      FIG. 23  is an example of mapping of downlink reference signals for rank-3 transmissions according to an exemplary embodiment of the disclosure; 
           [0035]      FIG. 24  is an example of MIMO precoding for rank-1, rank-2, rank-3 and rank-4 transmissions according to an exemplary embodiment of the disclosure; 
           [0036]      FIG. 25  is an example of mapping of downlink reference signals for rank-4 transmissions according to an exemplary embodiment of the disclosure; 
           [0037]      FIG. 26  is a flow diagram illustrating the detection of transmission rank using the reference signals and calculation of CQI assuming the detected rank according to an exemplary embodiment of the disclosure; 
           [0038]      FIG. 27  is an example of Spatial-Division Medium Access (SDMA) or Multi-user MIMO according to an exemplary embodiment of the disclosure; 
           [0039]      FIG. 28  is an example of SDMA or multi-user MIMO precoding transmissions according to an exemplary embodiment of the disclosure; 
           [0040]      FIG. 29  is an example of the mapping of reference signals for transmissions on beam- 1  in SDMA according to an exemplary embodiment of the disclosure; 
           [0041]      FIG. 30  is an example of the mapping of reference signals for transmissions on beam- 2  in SDMA according to an exemplary embodiment of the disclosure; and 
           [0042]      FIG. 31  is a flow diagram illustrating the detection of the preferred beam and CQI reporting according to an exemplary embodiment of the disclosure. 
       
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
       [0043]      FIGS. 1 through 31 , discussed below, and the various embodiments used to describe the principles of the present disclosure in this patent document are by way of illustration only and should not be construed in any way to limit the scope of the disclosure. Those skilled in the art will understand that the principles of the present disclosure may be implemented in any suitably arranged wireless communication system to reduce the amount of overhead traffic required in a communications scheme, including a wireless communications scheme. 
         [0044]      FIGS. 1 through 31 , discussed below, and the various embodiments used to describe the principles of the present disclosure in this patent document are by way of illustration only and should not be construed in any way to limit the scope of the disclosure. Those skilled in the art will understand that the principles of the present disclosure may be implemented in any suitably arranged wireless communication system. 
         [0045]      FIG. 1  illustrates exemplary wireless network  100 , which transmits control messages according to the principles of the present disclosure. In the illustrated embodiment, wireless network  100  includes base station (BS)  101 , base station (BS)  102 , base station (BS)  103 , and other similar base stations (not shown). Base station  101  is in communication with base station  102  and base station  103 . Base station  101  is also in communication with Internet  130  or a similar IP-based network (not shown). Any type or configuration of base stations, including, but not limited to E-node B base stations used in third generation wireless standards, maybe used with the present systems and methods. 
         [0046]    Base station  102  provides wireless broadband access (via base station  101 ) to Internet  130  to a first plurality of subscriber stations within coverage area  120  of base station  102 . The first plurality of subscriber stations includes subscriber station  111 , which may be located in a small business (SB), subscriber station  112 , which may be located in an enterprise (E), subscriber station  113 , which may be located in a WiFi hotspot (HS), subscriber station  114 , which may be located in a first residence (R), subscriber station  115 , which may be located in a second residence (R), and subscriber station  116 , which may be a mobile device (M), such as a cell phone, a wireless laptop, a wireless PDA, or the like. 
         [0047]    Base station  103  provides wireless broadband access (via base station  101 ) to Internet  130  to a second plurality of subscriber stations within coverage area  125  of base station  103 . The second plurality of subscriber stations includes subscriber station  115  and subscriber station  116 . In an exemplary embodiment, base stations  101 - 103  may communicate with each other and with subscriber stations  111 - 116  using OFDM or OFDMA techniques. 
         [0048]    Base station  101  may be in communication with either a greater number or a lesser number of base stations. Furthermore, while only six subscriber stations are depicted in  FIG. 1 , it is understood that wireless network  100  may provide wireless broadband access to additional subscriber stations. It is noted that subscriber station  115  and subscriber station  116  are located on the edges of both coverage area  120  and coverage area  125 . Subscriber station  115  and subscriber station  116  each communicate with both base station  102  and base station  103  and may be said to be operating in handoff mode, as known to those of skill in the art. 
         [0049]    Subscriber stations  111 - 116  may access voice, data, video, video conferencing, and/or other broadband services via Internet  130 . In an exemplary embodiment, one or more of subscriber stations  111 - 116  may be associated with an access point (AP) of a WiFi WLAN. Subscriber station  116  may be any of a number of mobile devices, including a wireless-enabled laptop computer, personal data assistant, notebook, handheld device, or other wireless-enabled device. Subscriber stations  114  and  115  may be, for example, a wireless-enabled personal computer (PC), a laptop computer, a gateway, or another device. 
         [0050]      FIG. 2A  is a high-level diagram of an orthogonal frequency division multiple access (OFDMA) transmit path.  FIG. 2B  is a high-level diagram of an orthogonal frequency division multiple access (OFDMA) receive path. In  FIGS. 2A and 2B , the OFDMA transmit path is implemented in base station (BS)  102  and the OFDMA receive path is implemented in subscriber station (SS)  116  for the purposes of illustration and explanation only. However, it will be understood by those skilled in the art that the OFDMA receive path may also be implemented in BS  102  and the OFDMA transmit path may be implemented in SS  116 . 
         [0051]    The transmit path in BS  102  comprises channel coding and modulation block  205 , serial-to-parallel (S-to-P) block  210 , Size N Inverse Fast Fourier Transform (IFFT) block  215 , parallel-to-serial (P-to-S) block  220 , add cyclic prefix block  225 , up-converter (UC)  230 . The receive path in SS  116  comprises down-converter (DC)  255 , remove cyclic prefix block  260 , serial-to-parallel (S-to-P) block  265 , Size N Fast Fourier Transform (FFT) block  270 , parallel-to-serial (P-to-S) block  275 , channel decoding and demodulation block  280 . 
         [0052]    At least some of the components in  FIGS. 2A and 2B  may be implemented in software while other components may be implemented by configurable hardware or a mixture of software and configurable hardware. In particular, it is noted that the FFT blocks and the IFFT blocks described in this disclosure document may be implemented as configurable software algorithms, where the value of Size N may be modified according to the implementation. 
         [0053]    Furthermore, although this disclosure is directed to an embodiment that implements the Fast Fourier Transform and the Inverse Fast Fourier Transform, this is by way of illustration only and should not be construed to limit the scope of the disclosure. It will be appreciated that in an alternate embodiment of the disclosure, the Fast Fourier Transform functions and the Inverse Fast Fourier Transform functions may easily be replaced by Discrete Fourier Transform (DFT) functions and Inverse Discrete Fourier Transform (IDFT) functions, respectively. It will be appreciated that for DFT and IDFT functions, the value of the N variable may be any integer number (i.e., 1, 2, 3, 4, etc.), while for FFT and IFFT functions, the value of the N variable may be any integer number that is a power of two (i.e., 1, 2, 4, 8, 16, etc.). 
         [0054]    In BS  102 , channel coding and modulation block  205  receives a set of information bits, applies coding (e.g., Turbo coding) and modulates (e.g., QPSK, QAM) the input bits to produce a sequence of frequency-domain modulation symbols. Serial-to-parallel block  210  converts (i.e., de-multiplexes) the serial modulated symbols to parallel data to produce N parallel symbol streams where N is the IFFT/FFT size used in BS  102  and SS  116 . Size N IFFT block  215  then performs an IFFT operation on the N parallel symbol streams to produce time-domain output signals. Parallel-to-serial block  220  converts (i.e., multiplexes) the parallel time-domain output symbols from Size N IFFT block  215  to produce a serial time-domain signal. Add cyclic prefix block  225  then inserts a cyclic prefix to the time-domain signal. Finally, up-converter  230  modulates (i.e., up-converts) the output of add cyclic prefix block  225  to RF frequency for transmission via a wireless channel. The signal may also be filtered at baseband before conversion to RF frequency. 
         [0055]    The transmitted RF signal arrives at SS  116  after passing through the wireless channel and reverse operations to those at BS  102  are performed. Down-converter  255  down-converts the received signal to baseband frequency and remove cyclic prefix block  260  removes the cyclic prefix to produce the serial time-domain baseband signal. Serial-to-parallel block  265  converts the time-domain baseband signal to parallel time domain signals. Size N FFT block  270  then performs an FFT algorithm to produce N parallel frequency-domain signals. Parallel-to-serial block  275  converts the parallel frequency-domain signals to a sequence of modulated data symbols. Channel decoding and demodulation block  280  demodulates and then decodes the modulated symbols to recover the original input data stream. 
         [0056]    Each of base stations  101 - 103  may implement a transmit path that is analogous to transmitting in the downlink to subscriber stations  111 - 116  and may implement a receive path that is analogous to receiving in the uplink from subscriber stations  111 - 116 . Similarly, each one of subscriber stations  111 - 116  may implement a transmit path corresponding to the architecture for transmitting in the uplink to base stations  101 - 103  and may implement a receive path corresponding to the architecture for receiving in the downlink from base stations  101 - 103 . 
         [0057]    Multiple Input Multiple Output (MIMO) schemes use multiple transmit antennas and multiple receive antennas to improve the capacity and reliability of a wireless communication channel. A MIMO system promises linear increase in capacity with K where K is the minimum of number of transmit (M) and receive antennas (N) (i.e. K=min(M,N)). 
         [0058]    In one example, four different data streams are transmitted separately from the four transmit antennas. The transmitted signals are received at the four receive antennas. Some form of spatial signal processing is performed on the received signals in order to recover the four data streams. An example of spatial signal processing is V-BLAST which uses the successive interference cancellation principle to recover the transmitted data streams. Other variants of MIMO schemes include schemes that perform some kind of space-time coding across the transmit antennas (e.g. D-BLAST) and also beamforming schemes such as SDMA (Spatial Division Multiple Access). 
         [0059]    The MIMO channel estimation consists of estimating the channel gain and phase information for links from each of the transmit antennas to each of the receive antennas. Therefore, the channel for M×N MIMO system consists of an N×M matrix, as shown in Equation 1 below: 
         [0000]    
       
         
           
             
               
                 
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         [0060]    In Equation 1, h ij  represents the channel gain from transmit antenna j to receive antenna i. In order to enable the estimations of the elements of the MIMO channel matrix, separate pilots are transmitted from each of the transmit antennas. 
         [0061]    An example of a single-code word MIMO scheme  300  is given in  FIG. 3 . In the case of single-code word MIMO transmission, the transmission begins with the selection of a codeword (CW) in codework block  302 . A cyclic redundancy check (CRC) is attached to the codeword selected in block  302  in CRC block  304 . Also in scheme  300 , the output from the CRC block  304  is coded using a turbo or low-density parity-check (LDPC) block  306 . The output from the turbo/LDPC block  306  is then modulated in modulation block  308  using a modulation scheme known to one skilled in the art. Examples of known modulation schemes include, but are not limited to QPSK and 16-QAM. The modulated output from the modulation block  308  is demuxed in demux block  310  into a plurality of data layers. These data layers formed at the demux block  310  are then transmitted to a precoding block  312  prior to transmission. Optional precoding is applied to map K layers to M transmit antennas in precoding block  312 . 
         [0062]    In the case of a multiple codeword MIMO transmission  400 , shown in  FIG. 4 , a information block  402  is de-multiplexed into smaller information blocks. In the example illustrated in  FIG. 4 , the information is broken up into three blocks. Codewords  406 ,  408 , and  410  are selected for each smaller information block. 
         [0063]    Individual CRCs are attached to these smaller information blocks in CRC blocks  412 ,  414 , and  416  and then separate coding in blocks  418 ,  420 , and  422  and modulation in blocks  424 ,  426 , and  428  are performed on these smaller blocks. It should be noted that in the case of multi-code word MIMO transmissions, different modulation and coding can be used on each of the individual streams resulting in a so called PARC (per antenna rate control) scheme. Also, multi-code word transmission allows for more efficient post-decoding interference cancellation because, a CRC check can be performed on each of the code words before the code word is cancelled from the overall signal. In this way, only correctly received code words are cancelled avoiding any interference propagation in the cancellation process. The output from the modulation blocks  424 ,  426 , and  428  are then transmitted to optional precoding block  430  prior to transmission. 
         [0064]    One of the applications of the currently disclosed systems and methods is in a 3GPP LTE system. In a 3GPP LTE system, a maximum of two codewords are used for transmission of 2, 3 or 4 MIMO layers as shown in  FIGS. 5 ,  6 , and  7 . 
         [0065]    As shown in  FIG. 5 , for a rank-2 or 2 layers transmission  500 , codeword- 1  (CW 1 ) is transmitted from Layer- 0  while a codeword- 2  (CW 2 ) is transmitted from Layer- 1 . In  FIG. 5 , information  502  is placed into demux block  504 . A first codeword (CW 1 )  506  and a second codeword (CW 2 )  508  are selected. CW 1   506  is merged with a first output from the demux block  504  and placed into the CRC block  510 . CW 2   508  is merged with a second output from the demux block  504  and placed into the CRC block  512 . The output from CRC block  510  is coded in Turbo/LDPC coding block  514  and modulated in modulation block  518 . The output from CRC block  512  is coded in Turbo/LDPC coding block  516  and modulated in modulation block  520 . In this rank-2 embodiment, the output from modulation block  518  is assigned to layer  0 , and the output from the modulation block  520  is assigned to layer  1 . The output from the modulation blocks  518 ,  520  is then precoded in precoding block  522  prior to being transmitted using an antenna. 
         [0066]    As shown in  FIG. 6 , for a rank-3 or 3 layers transmission  600 , a codeword- 1  (CW 1 ) is transmitted from Layer- 0  only while a codeword- 2  CW 2  is transmitted from Layer- 1  and Layer- 2 . In  FIG. 6 , information  602  is placed into demux block  604 . A first codeword (CW 1 )  606  and a second codeword (CW 2 )  608  are selected. CW 1   606  is merged with a first output from the demux block  604  and placed into the CRC block  610 . CW 2   608  is merged with a second output from the demux block  604  and placed into the CRC block  612 . The output from CRC block  610  is coded in Turbo/LDPC coding block  614  and modulated in modulation block  618 . The output from CRC block  612  is coded in Turbo/LDPC coding block  616  and modulated in modulation block  620 . In this rank-3 embodiment, the output from modulation block  618  is assigned to layer  0 , and the output from the modulation block  620  broken up in demux block  630 . A first output from demux block  630  is assigned to Layer  1 , and a second output from demux block  630  is assigned to Layer  2 . The output from the modulation block  618  and demux block  630  are then precoded in precoding block  622  prior to being transmitted using an antenna. 
         [0067]    As shown in  FIG. 7 , for a 4 layers transmission  700 , codeword- 1  (CW 1 ) is transmitted from Layer- 0  and Layer- 1  while a codeword- 2  (CW 2 ) is transmitted from Layer- 2  and Layer- 3 . In  FIG. 7 , information  702  is placed into demux block  704 . A first codeword (CW 1 )  706  and a second codeword (CW 2 )  708  are selected. CW 1   706  is merged with a first output from the demux block  704  and placed into the CRC block  710 . CW 2   708  is merged with a second output from the demux block  704  and placed into the CRC block  712 . The output from CRC block  710  is coded in Turbo/LDPC coding block  714  and modulated in modulation block  718 . The output from CRC block  712  is coded in Turbo/LDPC coding block  716  and modulated in modulation block  720 . In this rank-2 embodiment, the output from modulation block  718  is broken up in demux block  730 . A first output from demux block  730  is assigned to layer  0 , and a second output from demux block  630  is assigned to layer  1 . The output from the modulation block  720  broken up in demux block  732 . A first output from demux block  732  is assigned to layer  2 , and a second output from demux block  732  is assigned to layer  3 . The output from the demux blocks  730 ,  732  is then precoded in precoding block  722  prior to being transmitted using an antenna. 
         [0068]      FIG. 8  is an example  800  of a single-user MIMO system. In the case of single-user MIMO, all the MIMO layers are transmitted from a BS  806  to a second user equipment (UE- 2 )  804 . None of the data is transmitted to the first user equipment (UE- 1 )  802 . The embodiment illustrated in  FIG. 8  illustrates that through MIMO communications, a plurality of layers may be transmitted to a single UE devices. It is understood that the disclosed systems and methods may be used to reduce the overhead traffic required to maintain communications between the BS  806  and the UE- 2   804 . 
         [0069]      FIG. 9  is an example  900  of a multi-user MIMO system. As shown in  FIG. 9 , the MIMO layers are shared among multiple UEs. In the example illustrated in  FIG. 9 , BS  906  transmits a first layer to UE- 1   902  and transmits a second layer to UE- 2   904 . The embodiment illustrated in  FIG. 9  illustrates that through MIMO communications, a plurality of layers may be transmitted to a plurality of UE devices. It is understood that the disclosed systems and methods may be used to reduce the overhead traffic required to maintain communications between the BS  906  and the UE  902 ,  904 . While two UE devices are illustrated in  FIG. 9 , it is explicitly understood that any number of UE devices may be present in  FIG. 9 . It is further understood that a plurality of layers may be transmitted to a plurality of UE devices. For instance, in one alternative embodiment, a plurality of layers may be transmitted to UE  902  while a single layer is transmitted to UE  904 . In other alternative embodiments, a plurality of layers may be transmitted to UE  902  and UE  904 . In yet other embodiments, a plurality of UE devices may be present each of which is in communication with a BS, with each of the plurality of UE devices using at least one communication layer with the BS  906 . 
         [0070]    In a closed-loop MIMO precoding system, for each transmit antenna size, there is a set of precoding matrices that are constructed and known to both the BS and the UE. This set of known precoding matrices may be referred to as the “codebook” and denoted as P={P 1 , . . . , PL}. Here L=2q denotes the size of the codebook and q is the number of (feedback) bits needed to index the codebook. Once the codebook is specified for a MIMO system, the receiver observes a channel realization, selects the best precoding matrix (codeword) to be used at the moment, and feedback the index of the codeword to the transmitter. 
         [0071]    One idea of the limited feedback precoding MIMO system is illustrated in  FIG. 10 . In the example  1000  shown in  FIG. 10 , a transmitter  1002  uses a precoding matrix over a MIMO channel  1004  to transmit data to a receiver  1006 . The receiver  1006  transmits a feedback codeword index to the transmitter  1002 . This feedback is overhead traffic. Reducing the feedback traffic would result in a corresponding increase of available bandwidth for communication between the transmitter  1002  and the receiver  1006 . 
         [0072]    An optional pre-coding employs a unitary pre-coding before mapping the data streams to physical antennas as is shown in  FIG. 11 . This creates a set of virtual antennas (VA) or MIMO layers before the pre-coding. In this case, each of the codewords is potentially transmitted from all the physical transmit antennas. Two examples of unitary precoding matrices, P 1  and P 2  for the case of two transmit antenna can be illustrated as shown in Equation 2 below: 
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                     2 
                   
                   ] 
                 
               
             
           
         
       
     
         [0073]    Assuming modulation symbols S 1  and S 2  are transmitted at a given time from stream  1  and stream  2 , respectively. Then the modulation symbols after precoding with matrix P 1  and P 2  can be written as equations 3 and 4 respectively, below: 
         [0000]    
       
         
           
             
               
                 
                   
                     
                       
                         
                           T 
                           1 
                         
                         = 
                           
                          
                         
                           
                             P 
                             1 
                           
                            
                           
                             [ 
                             
                               
                                 
                                   
                                     S 
                                     1 
                                   
                                 
                               
                               
                                 
                                   
                                     S 
                                     2 
                                   
                                 
                               
                             
                             ] 
                           
                         
                       
                     
                   
                   
                     
                       
                         = 
                           
                          
                         
                           
                             
                               1 
                               
                                 2 
                               
                             
                              
                             
                               [ 
                               
                                 
                                   
                                     1 
                                   
                                   
                                     1 
                                   
                                 
                                 
                                   
                                     1 
                                   
                                   
                                     
                                       - 
                                       1 
                                     
                                   
                                 
                               
                               ] 
                             
                           
                           × 
                           
                             [ 
                             
                               
                                 
                                   
                                     S 
                                     1 
                                   
                                 
                               
                               
                                 
                                   
                                     S 
                                     2 
                                   
                                 
                               
                             
                             ] 
                           
                         
                       
                     
                   
                   
                     
                       
                         = 
                           
                          
                         
                           
                             1 
                             
                               2 
                             
                           
                            
                           
                             [ 
                             
                               
                                 
                                   
                                     
                                       S 
                                       1 
                                     
                                     + 
                                     
                                       S 
                                       2 
                                     
                                   
                                 
                               
                               
                                 
                                   
                                     
                                         
                                     
                                      
                                     
                                       
                                         S 
                                         1 
                                       
                                       - 
                                       
                                           
                                       
                                        
                                       
                                         S 
                                         2 
                                       
                                     
                                   
                                 
                               
                             
                             ] 
                           
                         
                       
                     
                   
                 
               
               
                 
                   [ 
                   
                     Eqn 
                     . 
                     
                         
                     
                      
                     3 
                   
                   ] 
                 
               
             
             
               
                 
                   
                     
                       
                         
                           T 
                           2 
                         
                         = 
                           
                          
                         
                           
                             P 
                             2 
                           
                            
                           
                             [ 
                             
                               
                                 
                                   
                                     S 
                                     1 
                                   
                                 
                               
                               
                                 
                                   
                                     S 
                                     2 
                                   
                                 
                               
                             
                             ] 
                           
                         
                       
                     
                   
                   
                     
                       
                         = 
                           
                          
                         
                           
                             
                               1 
                               
                                 2 
                               
                             
                              
                             
                               [ 
                               
                                 
                                   
                                     1 
                                   
                                   
                                     1 
                                   
                                 
                                 
                                   
                                     j 
                                   
                                   
                                     
                                       - 
                                       j 
                                     
                                   
                                 
                               
                               ] 
                             
                           
                           × 
                           
                             [ 
                             
                               
                                 
                                   
                                     S 
                                     1 
                                   
                                 
                               
                               
                                 
                                   
                                     S 
                                     2 
                                   
                                 
                               
                             
                             ] 
                           
                         
                       
                     
                   
                   
                     
                       
                         = 
                           
                          
                         
                           
                             1 
                             
                               2 
                             
                           
                            
                           
                             [ 
                             
                               
                                 
                                   
                                     
                                       S 
                                       1 
                                     
                                     + 
                                     
                                       S 
                                       2 
                                     
                                   
                                 
                               
                               
                                 
                                   
                                     
                                       j 
                                        
                                       
                                           
                                       
                                        
                                       
                                         S 
                                         1 
                                       
                                     
                                     - 
                                     
                                       j 
                                        
                                       
                                           
                                       
                                        
                                       
                                         S 
                                         2 
                                       
                                     
                                   
                                 
                               
                             
                             ] 
                           
                         
                       
                     
                   
                 
               
               
                 
                   [ 
                   
                     Eqn 
                     . 
                     
                         
                     
                      
                     4 
                   
                   ] 
                 
               
             
           
         
       
     
         [0074]    To illustrate the use of Equations 3 and 4,  FIG. 11  shows the transmission of a plurality of data streams from a plurality of virtual antennas.  FIG. 11  illustrates a system  1100  where two precoding blocks  1102 ,  1104  accept input from a first and second virtual antenna. The two precoding blocks  1102 .  1104  each prepare a separate layer that is encoded using an inverse fast Fourier transform (IFFT). For instance, first precoding block  1102  may use a first matrix P 1 , and transmit data to a first IFFT block  1106  and a second IFFT block  1108 . The second precoding block  1104  may use a second matrix P 2  and transmit data to a third IFFT block  1110  and a fourth IFFT block  1112 . First IFFT block  1106  and third IFFT block  1110  may transmit data using a first antenna, and second IFFT block  1108  and fourth IFFT block  1112  may transmit data using a second antenna. 
         [0075]    In the example shown in  FIG. 11 , the symbol 
         [0000]    
       
         
           
             
               T 
               11 
             
             = 
             
               
                 ( 
                 
                   
                     S 
                     1 
                   
                   + 
                   
                     S 
                     2 
                   
                 
                 ) 
               
               
                 2 
               
             
           
         
       
       
         
           and 
         
       
       
         
           
             
               T 
               12 
             
             = 
             
               
                 ( 
                 
                   
                     S 
                     1 
                   
                   + 
                   
                     S 
                     2 
                   
                 
                 ) 
               
               
                 2 
               
             
           
         
       
     
         [0000]    may be transmitted from antenna  1  and antenna  2 , respectively, when precoding is done using precoding matrix P 1 . Similarly, the symbol 
         [0000]    
       
         
           
             
               T 
               21 
             
             = 
             
               
                 ( 
                 
                   
                     S 
                     1 
                   
                   + 
                   
                     S 
                     2 
                   
                 
                 ) 
               
               
                 2 
               
             
           
         
       
       
         
           and 
         
       
       
         
           
             
               T 
               22 
             
             = 
             
               
                 ( 
                 
                   
                     j 
                      
                     
                         
                     
                      
                     
                       S 
                       1 
                     
                   
                   - 
                   
                     j 
                      
                     
                         
                     
                      
                     
                       S 
                       2 
                     
                   
                 
                 ) 
               
               
                 2 
               
             
           
         
       
     
         [0000]    will respectively be transmitted from antenna  1  and antenna  2  when precoding is done using precoding matrix P 2  as shown in  FIG. 11 . It should be noted that precoding is done on an OFDM subcarrier level before the IFFT operation as illustrated in  FIG. 11 . 
         [0076]    An example of precoding is discrete Fourier transform (DFT) based on Fourier precoding. A Fourier matrix is a N×N square matrix with entries given by Equation 5 below: 
         [0000]    
       
         
           
             
               
                 
                   
                     
                       P 
                       mn 
                     
                     = 
                     
                       
                         1 
                         
                           N 
                         
                       
                       * 
                       
                          
                         
                           j2π 
                            
                           
                               
                           
                            
                           
                             mn 
                             / 
                             N 
                           
                         
                       
                        
                       
                           
                       
                        
                       m 
                     
                   
                   , 
                   
                     n 
                     = 
                     0 
                   
                   , 
                   1 
                   , 
                   
                     … 
                      
                     
                         
                     
                      
                     
                       ( 
                       
                         N 
                         - 
                         1 
                       
                       ) 
                     
                   
                 
               
               
                 
                   [ 
                   
                     Eqn 
                     . 
                     
                         
                     
                      
                     5 
                   
                   ] 
                 
               
             
           
         
       
     
         [0077]    A 2×2 Fourier matrix can be expressed as shown in Equation 6 below: 
         [0000]    
       
         
           
             
               
                 
                   
                     P 
                     2 
                   
                   = 
                   
                     
                       
                         1 
                         
                           2 
                         
                       
                       * 
                       
                         [ 
                         
                           
                             
                               1 
                             
                             
                               1 
                             
                           
                           
                             
                               1 
                             
                             
                               
                                  
                                 
                                   j 
                                    
                                   
                                       
                                   
                                    
                                   π 
                                 
                               
                             
                           
                         
                         ] 
                       
                     
                     = 
                     
                       
                         1 
                         
                           2 
                         
                       
                       * 
                       
                         [ 
                         
                           
                             
                               1 
                             
                             
                               1 
                             
                           
                           
                             
                               1 
                             
                             
                               
                                 - 
                                 1 
                               
                             
                           
                         
                         ] 
                       
                     
                   
                 
               
               
                 
                   [ 
                   
                     Eqn 
                     . 
                     
                         
                     
                      
                     6 
                   
                   ] 
                 
               
             
           
         
       
     
         [0078]    Similarly, a 4×4 Fourier matrix can be expressed as Equation 7 below: 
         [0000]    
       
         
           
             
               
                 
                   
                     
                       
                         
                           P 
                           4 
                         
                         = 
                           
                          
                         
                           
                             1 
                             
                               4 
                             
                           
                           * 
                           
                             [ 
                             
                               
                                 
                                   1 
                                 
                                 
                                   1 
                                 
                                 
                                   1 
                                 
                                 
                                   1 
                                 
                               
                               
                                 
                                   1 
                                 
                                 
                                   
                                      
                                     
                                       j 
                                        
                                       
                                           
                                       
                                        
                                       
                                         π 
                                         / 
                                         2 
                                       
                                     
                                   
                                 
                                 
                                   
                                      
                                     
                                       j 
                                        
                                       
                                           
                                       
                                        
                                       π 
                                     
                                   
                                 
                                 
                                   
                                      
                                     
                                       j 
                                        
                                       
                                           
                                       
                                        
                                       3 
                                        
                                       
                                           
                                       
                                        
                                       
                                         π 
                                         / 
                                         2 
                                       
                                     
                                   
                                 
                               
                               
                                 
                                   1 
                                 
                                 
                                   
                                      
                                     
                                       j 
                                        
                                       
                                           
                                       
                                        
                                       π 
                                     
                                   
                                 
                                 
                                   
                                      
                                     
                                       j 
                                        
                                       
                                           
                                       
                                        
                                       2 
                                        
                                       π 
                                     
                                   
                                 
                                 
                                   
                                      
                                     
                                       j 
                                        
                                       
                                           
                                       
                                        
                                       3 
                                        
                                       π 
                                     
                                   
                                 
                               
                               
                                 
                                   1 
                                 
                                 
                                   
                                      
                                     
                                       j 
                                        
                                       
                                           
                                       
                                        
                                       3 
                                        
                                       
                                         π 
                                         / 
                                         2 
                                       
                                     
                                   
                                 
                                 
                                   
                                      
                                     
                                       j 
                                        
                                       
                                           
                                       
                                        
                                       3 
                                        
                                       π 
                                     
                                   
                                 
                                 
                                   
                                      
                                     
                                       j9 
                                        
                                       
                                           
                                       
                                        
                                       
                                         π 
                                         / 
                                         2 
                                       
                                     
                                   
                                 
                               
                             
                             ] 
                           
                         
                       
                     
                   
                   
                     
                       
                         = 
                           
                          
                         
                           
                             1 
                             
                               4 
                             
                           
                           * 
                           
                             [ 
                             
                               
                                 
                                   1 
                                 
                                 
                                   1 
                                 
                                 
                                   1 
                                 
                                 
                                   1 
                                 
                               
                               
                                 
                                   1 
                                 
                                 
                                   j 
                                 
                                 
                                   
                                     - 
                                     1 
                                   
                                 
                                 
                                   
                                     - 
                                     j 
                                   
                                 
                               
                               
                                 
                                   1 
                                 
                                 
                                   
                                     - 
                                     1 
                                   
                                 
                                 
                                   1 
                                 
                                 
                                   
                                     - 
                                     1 
                                   
                                 
                               
                               
                                 
                                   1 
                                 
                                 
                                   
                                     - 
                                     j 
                                   
                                 
                                 
                                   
                                     - 
                                     1 
                                   
                                 
                                 
                                   j 
                                 
                               
                             
                             ] 
                           
                         
                       
                     
                   
                 
               
               
                 
                   [ 
                   
                     Eqn 
                     . 
                     
                         
                     
                      
                     7 
                   
                   ] 
                 
               
             
           
         
       
     
         [0079]    Other forms of precoding include matrices obtained using Householder equation. An example of a 4×4 Householder matrix is given below in Equation 8: 
         [0000]    
       
         
           
             
               
                 
                   
                     M 
                     1 
                   
                   = 
                   
                     
                       
                         I 
                         4 
                       
                       - 
                       
                         2 
                          
                         
                           u 
                           0 
                         
                          
                         
                           
                             u 
                             1 
                             H 
                           
                           / 
                           
                             
                                
                               
                                 u 
                                 0 
                               
                                
                             
                             2 
                           
                         
                       
                     
                     = 
                     
                       
                         1 
                         
                           4 
                         
                       
                       * 
                       
                         [ 
                         
                           
                             
                               1 
                             
                             
                               1 
                             
                             
                               1 
                             
                             
                               1 
                             
                           
                           
                             
                               1 
                             
                             
                               1 
                             
                             
                               
                                 - 
                                 1 
                               
                             
                             
                               
                                 - 
                                 1 
                               
                             
                           
                           
                             
                               1 
                             
                             
                               
                                 - 
                                 1 
                               
                             
                             
                               1 
                             
                             
                               
                                 - 
                                 1 
                               
                             
                           
                           
                             
                               1 
                             
                             
                               
                                 - 
                                 1 
                               
                             
                             
                               
                                 - 
                                 1 
                               
                             
                             
                               1 
                             
                           
                         
                         ] 
                       
                        
                       
                         ( 
                         0.1 
                         ) 
                       
                     
                   
                 
               
               
                 
                   [ 
                   
                     Eqn 
                     . 
                     
                         
                     
                      
                     8 
                   
                   ] 
                 
               
             
           
         
       
     
         [0080]    In Equation 8, the following equation may be used: 
         [0000]        u   0   T =[1−1−1−1]  [EQN. 9] 
         [0081]    An example of HH 4-Tx antennas MIMO precoding used in the 3GPP LTE system is given in table  1200  shown in  FIG. 12 . This example is intended to be exemplary, as modifications may be made to this table consistent with systems and methods disclosed herein. 
         [0082]    The precoding used for MIMO transmission needs to be fedback by the UE to the BS. The precoding feedback information consists of precoding-matrix or column identity. Moreover, due to frequency-selective fading in an OFDM system, the optimal precoding over different subbands can be different as shown in  FIG. 13 . Therefore, the precoding information can be sent on a subband basis. In the example of a scheme  1300  as shown in  FIG. 13 , the 300 used subcarriers are divided into 5 subbands of 60 subcarriers each. In this embodiment, rank-1 transmission is assumed over all the subbands. The precoders used for rank-1 transmission in SB 1 ,  2 ,  3 ,  4  and  5  are W 4   {1} , W 1   {1} , W 1   {1} , W 9   {1}  and W 15   {1} , respectively, in  FIG. 13 . 
         [0083]    When a system can support 4×4 MIMO, rank-4 (4 MIMO layers) transmissions are not always desirable. The MIMO channel experienced by the UE generally limits the maximum rank that can be used for transmission. In general for weak users in the system, a lower rank transmission is preferred over a high rank transmission from a throughput perspective. Moreover, due to frequency-selective fading, optimal rank may be different on different subbands. Therefore, for optimal performance, a UE need to feedback the rank information on a subband basis as shown in a scheme  1340  also shown in  FIG. 13 . In the example of scheme  1340 , the transmission on SB 1 ,  2 ,  3 ,  4  and  5  use a rank-1, 2, 2, 1 and 3, respectively. 
         [0084]    The precoding matrix indication (PMI) and rank feedback on a subband basis can result in significant feedback overhead. For example, and assuming 4-bits per subband for PMI and 2-bits per subband for rank, the total overhead for feedback on 5 subbands is 30 bits. For larger system bandwidths, the system needs to support a larger number of subbands resulting in even larger feedback overhead. Also, for finer granularity of PMI/rank feedback in frequency, the overhead also increases. Therefore, there is a need to improve the PMI and rank feedback mechanisms that reduces the overhead. 
         [0085]    The downlink reference signals mapping for 4-Tx antennas ports in a 3GPP (3rd Generation Partnership Project) LTE (Long Term Evolution) system is shown in  FIG. 14  chart  1400 . The notation R p  is used to denote a resource element used for reference signal transmission on antenna port p. It can be noted that the density on antenna ports  2  and  3  is half the density on antenna ports  0  and  1 . 
         [0086]    The reference signal overhead per antenna port for ports  0  and  1  is 4.76% while the density is 2.38% for antenna ports  2  and  3 . The total reference signal overhead for the four ports is 14.28%. This represents a significant overhead in the system. When the number of antenna ports increases beyond 4, the reference signal overhead also increases correspondingly. Moreover, the receiver needs to provide feedback on the preferred precoding matrix (PM) on a subband basis to the transmitter. This result in significant signaling overhead from the receiver to the transmitter. Therefore, there is a need to reduce both the reference signal and feedback overhead in a MIMO system. 
         [0087]    A scheme that reduces both the reference signal overhead as well as the feedback overhead for MIMO communications is disclosed herein. In a first embodiment, the precoding used for transmission is initialized and reset by cycling through all or a subset of precoders. In the example of  FIG. 15 , the transmitter cycle through all the rank-1 precoders from table  1200  of  FIG. 12 . The cycling continues until all or a selected subset of precoders are used on all or a subset of the subbands (SBs). The reference signal is also precoded. Therefore, even with 4-TX antennas transmission, a single precoded reference signal is transmitted for rank-1 transmissions. The receivers can make channel quality (CQI) measurements on different subbands and report back to the transmitter. 
         [0088]    The CQI feedback timing is such that transmitter can link the received CQI to the precoders used by the transmitter. For example in  FIG. 16 , the transmitter schedules a receiver in the subband# 1  in subframe (k+1), where k is the subframe, based on the CQI this receiver reported for subband# 1  and SF# 5 . Since the transmitter used W 4   {1}  precoder in subband# 1  and SF# 5  based on which the receiver reported CQI, the transmitter uses the same precoder W 4   {1}  for transmission to this receiver in SB# 1  in subframe (k+1). Note that the CQI is valid for the precoder that was used in measuring the CQI. Using the same principle, the transmitter schedules transmissions to other receivers in the other subbands. In some embodiments, it is understood that k may be an integer greater than 6. 
         [0089]    It is understood that a single receiver can be allocated more than one subband using different precoders. In this case, when there is no transmission within a given subband of a subframe, the transmitter transmits only the reference signal using a precoder that is not used for transmission in the current subframe. This is to provide more precoders choice for receivers making measurements on different subbands as shown in  FIG. 16 . For example, in SB# 3  and subframe#(k+1), there is no data transmission. 
         [0090]    It is also understood that the transmitter uses a precoder W 6   {1}  for reference signal transmission only which is not used for transmission to any receiver within the subframe#(k+1). Similarly, precoder W 3   {1}  which is not used for transmission to any receiver within the subframe#(k+2), is used for reference signal only transmission within SB# 2  in subframe#(k+2). In subframe#(k+3), two subbands, SB# 3  and SB# 4  are used for reference signal transmission only and so on. 
         [0091]    Referring to  FIG. 16 , a transmission is performed to a first receiver using precoder W 1   {1}  on SB# 2  in subframe#(k+1). A second receiver makes CQI measurements on SB# 2  in subframe#(k+1) and reports back the CQI to the transmitter. The transmitter then schedules the second receiver in SB# 2  in subframe#(k+5) using the same precoder W 1   {1}  that the receiver assumed in CQI measurements. Note that the second receiver may not be aware of the exact precoder used in SB# 2  in subframe#(k+1) when the second receiver made the CQI measurement. However, transmitter keeps track of the precoders used in different subbands and different subframe. When a CQI is reported by a receiver, the transmitter can link the CQI to the precoder used in a given subband and subframe. 
         [0092]    A third receiver makes CQI measurement in SB# 3  in subframe#(k+1) and reports back the CQI to the transmitter. The transmitter transmitted reference signal only in SB# 3  in subframe#(k+1) using precoder W 6   {1} . The transmitter then schedules the third receiver in SB# 3  in subframe#(k+6) using the same precoder W 6   {1}  that the receiver assumed in CQI measurements. 
         [0093]    In one embodiment of the present disclosure, a BS keeps track of precoders that are used at any particular time using a table stored in a computer readable medium. The BS receives CQI measurements that the BS can correlate to the CQI measurements by accessing the computer readable medium. In some embodiments, the BS can create a table of CQI measurements, SB, and precoder used. This table can then be ranked to determine the best CQI for a particular UE device. A table  1600  can be used to schedule receivers is illustrated in  FIG. 16 .  FIG. 16  shows the location of reference and data transmissions  1602  as well as the location of the reference signal  1604  in use according to results created by the cycling of precoders. 
         [0094]    An example of precoding vector cycling for initialization and reset of precoding using a subset of rank-1 precoders {W 0   {1} , W 4   {1} , W 8   {1} , W 12   {1} } is shown in a table  1700  illustrated in  FIG. 17 . In a frequency-selective channel, the optimal precoder can be different for different frequency subbands. The cycling shown in  FIG. 17  assures that each of the four precoders in the set {W 0   {1} , W 4   {1} , W 8   {1} , W 12   {1} } is transmitted from all the subbands. 
         [0095]    In another embodiment shown in  FIG. 18 , the precoders are first cycled in frequency and then in time as illustrated by A table  1800 .  FIG. 18  is intended to illustrate an example of precoding vector cycling for initialization and reset of precoding using a subset of rank-1 precoders {W 0   {1} , W 4   {1} , W 8   {1} W 12   {1} }. 
         [0096]    The mapping of downlink reference signals for rank-1 transmissions according to the principles of the current disclosure is shown in a table  1900  shown in  FIG. 19 . It should be noted that a single reference signal is transmitted irrespective of the number of transmit antennas used for rank-1 transmission. 
         [0097]    In another embodiment shown in  FIG. 20  a table  2000 , a subset of precoders consisting both rank-1 and rank-2 precoders 
         [0000]    
       
         
           
             { 
             
               
                 W 
                 0 
                 
                   { 
                   1 
                   } 
                 
               
               , 
               
                 
                   W 
                   0 
                   
                     { 
                     14 
                     } 
                   
                 
                 
                   2 
                 
               
               , 
               
                 W 
                 8 
                 
                   { 
                   1 
                   } 
                 
               
               , 
               
                 
                   W 
                   0 
                   
                     { 
                     12 
                     } 
                   
                 
                 
                   2 
                 
               
             
             } 
           
         
       
     
         [0000]    are used for transmission. The receivers make CQI measurements assuming the used precoders and report back the CQI to the transmitter. The transmitter then schedules the receivers based on the received CQI. In general, for some receivers, a given combination of rank and precoder will turn out to be good on certain subbands and these receivers will report a higher CQI on these subbands. In the case of a proportional fair scheduler, these receivers will likely be scheduled on subbands where they report a higher CQI. 
         [0098]    The mapping of downlink reference signals for rank-2 transmissions according to the principles of the current disclosure is shown in table  2100  shown in  FIG. 21 . Note that for rank-1 transmissions such as transmissions on SB# 1 ,  3  and  5  in subframe# 1  in  FIG. 20 , the reference signal mapping of rank-1 shown in  FIG. 19  is used. 
         [0099]    In another embodiment shown in  FIG. 22  table  2200 , a subset of precoders consisting rank-1, rank-2 and rank-3 precoders 
         [0000]    
       
         
           
             { 
             
               
                 W 
                 0 
                 
                   { 
                   1 
                   } 
                 
               
               , 
               
                 
                   W 
                   0 
                   
                     { 
                     14 
                     } 
                   
                 
                 
                   2 
                 
               
               , 
               
                 
                   W 
                   0 
                   
                     { 
                     124 
                     } 
                   
                 
                 
                   3 
                 
               
               , 
               
                 W 
                 8 
                 
                   { 
                   1 
                   } 
                 
               
               , 
               
                 
                   W 
                   8 
                   
                     { 
                     12 
                     } 
                   
                 
                 
                   2 
                 
               
               , 
               
                 
                   W 
                   8 
                   
                     { 
                     124 
                     } 
                   
                 
                 
                   3 
                 
               
             
             } 
           
         
       
     
         [0000]    is used for transmission. The receivers make CQI measurements assuming the used precoders and report back the CQI to the transmitter. The transmitter then schedules the receivers based on the received CQI. Note that the transmitter keeps track of the rank and precoder used in each subband and subframe and, therefore, can relate the received CQI to the rank and precoder used. In general, for some receivers, a given combination of rank and precoder will turn out to be good on certain subbands and these receivers will report a higher CQI on these subbands. In the case of a proportional fair scheduler, these receivers will be likely to be scheduled on subbands where the receivers report a higher CQI. In the example of  FIG. 22 , the receivers scheduled in subframe#(k+n) are based on CQI measurements in subframe#(k+1) and subframe#(k+2), where n is any integer. 
         [0100]    The mapping of downlink reference signals for rank-3 transmissions according to the principles of the disclosure is shown in a table  2300  in  FIG. 23 . Note that for rank-1 transmissions such as transmissions on SB# 2  in subframe#(k+1) in  FIG. 22 , the reference signal mapping of rank-1 shown in Figure is used. Similarly, for rank-2 transmissions such as transmissions on SB# 5  in subframe#(k+1) in  FIG. 22 , the reference signal mapping of rank-2 shown in  FIG. 21  is used. 
         [0101]    In another embodiment of the disclosure shown in a table  2400  shown in  FIG. 24 , a subset of precoders consisting rank-1, rank-2, rank-3 and rank-4 precoders 
         [0000]    
       
         
           
             { 
             
               
                 W 
                 0 
                 
                   { 
                   1 
                   } 
                 
               
               , 
               
                 
                   W 
                   0 
                   
                     { 
                     14 
                     } 
                   
                 
                 
                   2 
                 
               
               , 
               
                 
                   W 
                   0 
                   
                     { 
                     124 
                     } 
                   
                 
                 
                   3 
                 
               
               , 
               
                 
                   W 
                   0 
                   
                     { 
                     1234 
                     } 
                   
                 
                 
                   4 
                 
               
               , 
               
                 W 
                 8 
                 
                   { 
                   1 
                   } 
                 
               
               , 
               
                 
                   W 
                   8 
                   
                     { 
                     12 
                     } 
                   
                 
                 
                   2 
                 
               
               , 
               
                 
                   W 
                   8 
                   
                     { 
                     124 
                     } 
                   
                 
                 
                   3 
                 
               
               , 
               
                 
                   W 
                   0 
                   
                     { 
                     1234 
                     } 
                   
                 
                 
                   4 
                 
               
             
             } 
           
         
       
     
         [0000]    is used for transmission.
 
The receivers make CQI measurements assuming the used precoders and report back the CQI to the transmitter. The transmitter then schedules the receivers based on the received CQI. Note that the transmitter keeps track of the rank and precoder used in each subband and subframe and, therefore, can relate the received CQI to the rank and precoder it used. The receivers need not be aware of the actual precoders used. This is because the reference signals used for both CQI measurements and data demodulation are precoded. In general, for some receivers, a given combination of rank and precoder will turn out to be good on certain subbands and these receivers will report a higher CQI on these subbands. In the case of a proportional fair scheduler, these receivers will be likely to be scheduled on subbands where the receivers report a higher CQI. In the example of  FIG. 24 , the receivers scheduled in subframe#(k+n) are based on CQI measurements in subframe#(k+1) and subframe#(k+2).
 
         [0102]    The mapping of downlink reference signals for rank-4 transmissions according to the principles shown in  FIG. 25 . It is understood that for rank-1 transmissions such as transmissions on SB# 2  in subframe# 1  in  FIG. 24 , the reference signal mapping of rank-1 shown in  FIG. 19  is used. Similarly, for rank-2 transmissions such as transmissions on SB# 1  in subframe#(k+2) in  FIG. 24 , the reference signal mapping of rank-2 shown in  FIG. 21  is used. Moreover, for rank-3 transmissions such as transmissions on SB# 1  in subframe#(k+1) in  FIG. 24 , the reference signal mapping of rank-3 shown in  FIG. 23  is used. 
         [0103]    A flow chart  2600  showing detection of transmission rank using the reference signals and calculation of CQI assuming the detected rank is given in  FIG. 26 . The transmission rank is determined by detection of the presence or absence of the reference signal for the corresponding rank. This can be achieved by using certain sequences for reference signal transmission such as a Pseudo-Noise (PN) sequence. Also, the reference signals for different layers transmitted are orthogonal in time-frequency. Therefore, the receiver can try to detect these pre-known patterns at the corresponding location and compare the result of detection against a threshold. Also, when a rank greater than 1 is detected, the receiver may send a CQI for each layer separately. 
         [0104]    In the flowchart  2600  shown in  FIG. 2608 , there is an attempt to detect R 3  in block  2602 . In decision block  2604 , if R 3  is detected, it can be assumed that rank  4  is present in block  2614 . If there is no R 3  detected, then there is an attempt to detect R 2  in block  2606 . In decision block  2608 , if R 2  is detected, it can be assumed that rank  3  is present in block  2616 . If there is no R 2  detected, then there is an attempt to detect R 1  in block  2610 . In decision block  2612 , if R 1  is detected it can be assumed that rank  2  is present in block  2618 . If there is no R 1  detected, then rank  1  can be determined to be present in block  2620 . 
         [0105]    In another embodiment of the current invention shown in  FIG. 27 , simultaneous transmission is performed to more than one receiver using the same time-frequency resources. The orthogonality of signals is enabled by using different precoders also referred to as different beams for transmission to multiple users on the same resources. This type of transmissions is referred to as spatial division multiple access (SDMA) or multi-user MIMO. In the example shown in  FIG. 27 , a receiver- 1   2704  is served using precoder W 0   {1}  while a receiver- 2   2706  is served using precoder W 8   {1}  by a transmitter  2702 . These two precoders create two quasi-orthogonal beams for simultaneous transmissions to the two receivers. 
         [0106]    An example of SDMA or multi-user MIMO using a subset codebook {W 0   {1} , W 4   {1} , W 8   {1} , W 12   {1} } is shown in  FIG. 28 . Transmission over two beams or simultaneous transmission to two receivers in the same resources is assumed. In the case of SDMA or multi-user MIMO, the transmission rank from the receiver perspective is assumed as 1, (i.e., single rank reception). However, the principles of the disclosure also apply to the case where the rank of each receiver in SDMA or multi-user MIMO can be greater than 1. In the embodiment of  FIG. 28 , scheduling of receivers in subframe#(k+n) based on CQI measurements in subframe#(k+1). Table  2800  of  FIG. 28  shows both the reference signal and data transmission  2802  and the reference signal only  2804 . 
         [0107]    In the case of SDMA or multi-user MIMO with rank-1 receptions, the reference signals for beam- 1  and beam- 2  are transmitted as rank-1 transmissions as shown in table  2900  of  FIG. 29  and table  3000  of  FIG. 30 . Tables  2900  and  3000  show reference signals transmission for beam- 1  and beam- 2  respectively. In order for the receivers to help determine the preferred beam, the reference signals can be scrambled by a beam-specific PN-sequence. The receivers can then make CQI measurements on each of the beams received by descrambling the reference signals by the beam specific PN-sequence. The receivers then feed the CQI back to the transmitter on one or more received beams. Note that a PN-sequence is used as an example here. Other sequences such as Zadoff-Chu (ZC) sequences, Generalized Chirp Like (GCL) sequences or computer generated sequences can be used as reference signals for different beams. 
         [0108]    An example of a flow chart  3100  showing detection of the preferred beam and CQI reporting is shown in  FIG. 31 . As the reference signals for different beams use different sequences, the receiver can detect the different beams transmitted and can also calculate the CQI on the transmitted beams. According to the flow-chart  3100  of  FIG. 31 , the receiver can also determine its preferred beam. The information on the preferred beam, (i.e., that is, beam identity along with the corresponding CQI) can be feedback to the transmitter. 
         [0109]    In the flowchart  3100  illustrated by  FIG. 31 , R 0  is received in block  3102 . In decision block  3104 , there is a determination if Beam- 1  has been detected. If Beam- 1  has been detected, the CQI is calculated in block  3112  and then Beam- 2  is attempted to be detected in decision block  3108 . If Beam- 1  has not been detected, Beam- 2  is attempted to be detected in decision block  3108  without calculating the CQI of beam- 1 . If Beam- 2  is not detected in decision block  3108 , the CQI of Beam- 1  is reported in block  3110 . If beam- 2  is detected in decision block  3108 , the CQI for beam- 2  is calculated in block  3114 . The CQI of Beam- 1  and Beam- 2  is then compared in block  3116 , and the highest CQI and the ID of the selected beam is then determined and reported in block  3118 . It is understood that if Beam- 2  is detected and Beam- 1  is not, the CQI for Beam- 2  will be reported in block  3118  without the need for a comparison in block  3116 . 
         [0110]    It is expressly understood that the systems and methods disclosed herein may be used for both asynchronous and synchronous communications. In some embodiments of asynchronous mode, a time component may be transmitted in conjunction with other information, such as CQI. In some embodiments of synchronous mode, only information, such as CQI information, may be sent. 
         [0111]    Although the present disclosure has been described with an exemplary embodiment, various changes and modifications may be suggested to one skilled in the art. It is intended that the present disclosure encompass such changes and modifications as fall within the scope of the appended claims.