Abstract:
Methods, devices and systems are provided for transmitting and receiving MIMO signals. In one embodiment, transmitting of the MIMO signals involves pre-coding each of at least two data symbols using a respective pre-coding codeword to preclude a corresponding plurality of pre-coded data symbols. A respective signal is transmitted from each of a plurality of antennas, the respective signal including one of the pre-coded signals and at least one pilot for use in channel estimation. The signals collectively further include at least one beacon pilot vector consisting of a respective beacon pilot per antenna, the beacon pilot vector containing contents known to a receiver for use by the receiver in determining the codeword used to pre-code the at least one data signal.

Description:
RELATED APPLICATION 
       [0001]    This application claims the benefit of prior U.S. Provisional Application No. 60/783,711 filed Mar. 17, 2006, hereby incorporated by reference in its entirety. 
     
    
     FIELD OF INVENTION 
       [0002]    The present invention relates to closed loop MIMO (Multiple Input Multiple Output) techniques, more specifically closed loop MIMO techniques involving pre-coding. 
       BACKGROUND OF THE INVENTION 
       [0003]    Closed-loop MIMO aims to significantly improve throughput and reliability of nomadic user equipment (UEs). 
         [0004]    Pre-coding is proposed in UMTS (Universal Mobile Telecommunications System) LTE (Long Term Evolution) as one of the main approaches for closed-loop MIMO. Systems that employ pre-coding multiply a data symbol vector s containing one element per transmit antenna by a pre-coding matrix F prior to transmission. The received signal is defined by r=HFs+n, where H is the channel matrix, and n noise. For a system with two transmit antennas and two receive antennas, r, s, n are vectors with two elements each, H is a 2×2 matrix, and F is a 2×2 matrix. The pre-coding matrix F is chosen from a group of predefined matrixes that is called a codebook {F}. In some cases the receiver tells the transmitter which pre-coding matrix to use. For FDD (frequency division duplex) air interfaces the information identifying a pre-coding matrix may be fed back through either channel sounding approaches or codebook index approaches. TDD (time division systems) may also use the codebook based approach. A detailed example of an approach to pre-coding for MIMO transmission is described in D. J. Love, et al, “Limited Feedback Unitary Pre-coding for Spatial Multiplexing Systems”, IEEE Trans. Inf. Theory, vol. 51, no. 8, pp. 2967-2976, August 2005. 
         [0005]    Codebook index feedback involves the receiver signalling to the transmitter an index of which pre-coding matrix to use (so-called codeword index). There are a plurality of indexes each corresponding to a respective pre-coding matrix. One problem, however, is that codebook index feedback approaches use a large amount of uplink radio resources. 
         [0006]    Another problem occurs for common pilot based pre-coding schemes. In a common pilot based scheme, the pilots are not pre-coded, and hence have different channel matrices from the data. If the receiver knows the pre-coding matrices used by the transmitter in such scenarios, it can reconstruct the channel matrix with pre-coding effects. When the data channel matrix can be correctly reconstructed, data can be correctly decoded. If, however, the receiver uses a different pre-coding matrix from the transmitter, the constructed effective data channel matrix will be wrong. As a result detected data will be useless due to incorrect channel references. In this case the incorrectly detected data cannot be used for H-ARQ purposes either. 
         [0007]    An example of the common pilot approach is shown in  FIG. 1  for a two transmit antenna case. In  FIG. 1  (and other Figures described below), the horizontal axis  210  is frequency (OFDM (Orthogonal Frequency Division Multiplexing) sub-carriers) and the vertical axis  212  is time (OFDM symbols). Each small circle represents a transmission on a particular sub-carrier over a particular OFDM symbol duration. In locations  214 , pilots are transmitted by a first transmit antenna Tx-1, and in locations  216 , pilots are transmitted by a second transmit antenna Tx-2. Remaining locations are available for data transmission by both antennas. In the illustrated example, data includes pre-coded data  218  for a first UE (UE-1), and pre-coded data  220  for a second UE (UE-2). Typically, the pre-coding applied for pre-coded data  218  will be different from that applied for pre-coded data  220 . With the common pilot approach, the same pilots are used for both UEs and hence they cannot be pre-coded. 
         [0008]    In some existing approaches, the codeword index is fed back in a differential manner. In other words the difference between a current index and a previous index is fed back rather than the codeword index per se. Another problem with common pilot based closed-loop MIMO schemes relates to errors that occur in the transmission of the differential codeword index. Closed-loop MIMO is typically intended for nomadic UEs for whom channel conditions will not change very quickly. For each given currently used pre-coding matrix, a small subset of possible next codewords is defined, for example those that would most likely be selected from the full set due to slow channel variation. This subset of codewords can be determined in several ways, such as spatial correlation and matrix correlation. If an index associated with this subset is fed back rather than an index from the entire set of codewords, fewer bits are needed. However, if a feedback error occurs the transmitter will keep using the wrong subsets for subsequent pre-coding updates. That is to say feedback error propagates in differential codeword index feedback. Consider the following example. Suppose that the transmitter and receiver are synchronized at the beginning, and both use codeword V 0 . Now with the differential codebook index feedback, the new codeword would be V 1 , which is in the small differential codebook associated with V 0 . Since an error occurred, the transmitter thinks that it should use V 0 , which is also from the small codebook associated with V 0 . For the next feedback, the receiver will feed back a differential index associated with V 1 , but the transmitter will get the new codeword from the differential codebook associated with V 2 , and this goes on and on. This is how error gets propagated. 
         [0009]    To address this error propagation problem, it has been proposed to periodically reset by transmitting a whole codeword index to correct possible feedback errors. However, this raises more problems than it solves. To begin, the whole codeword can be wrong, and hence error propagates. The resetting is a random process: the reset can begin despite no errors occurring, and may not be done when an error does occur. To, make the probability of error propagation low, a large number of resets is required and this quickly diminishes the benefit of differential codeword index feedback. To completely eliminate error propagation, a reset is performed each time, in which case it will cease to be a differential index feedback approach. 
         [0010]    Dedicated pilot based schemes suffer shortcomings as well. In a dedicated pilot based scheme, pilots can be pre-coded, and hence have the same channel matrices as data. One problem, however, is that since each UE trying to communicate with a base transceiver station (BTS) does not know what pre-coding matrix is being used by other UEs, the UE is unable to monitor the channel. More specifically, they do not know which pre-coding matrix is being used, do not know the rank of the current channel, cannot estimate per-layer based signal to interference noise ratio (SINR), and are unable to do channel dependent scheduling, to name a few examples. 
         [0011]    An example of the dedicated pilot approach is shown in  FIG. 2  for a two transmit antenna case. In locations  222 , 224 , dedicated pilots specific to the first UE are transmitted by a first transmit antenna Tx-1 and second antenna Tx-2 respectively. In locations  226 , 228 , dedicated pilots specific to the second UE are transmitted by a first transmit antenna Tx-1 and second antenna Tx-2 respectively. Remaining locations are available for data transmission by both antennas. In the illustrated example, data includes pre-coded data  230  for a first UE (UE-1), and pre-coded data  232  for a second UE (UE-2). Typically, the pre-coding applied for pre-coded data  230  will be different from that applied for pre-coded data  230 . With the dedicated pilot approach, different pilots are used for each UE in the sense that they are pre-coded using the same pre-coding matrix as used for the data for each user. 
         [0012]    Since both pilot and data go through the same channel, a dedicated pilot scheme is more resilient to codeword index feedback error than common pilot based schemes. Feedback causes performance degradation, but the decoded data can still be used for H-ARQ. 
         [0013]    As indicated above, for dedicated pilot based approaches differential feedback error propagates, but the consequences are less severe than in the common pilot case for the UE of interest. However, some of the benefit of pre-coding with all the uplink feedback is lost. Since now the pre-coding matrix is wrong, which is equivalent to a random matrix, HF will have the same properties as H. This means pre-coding is equivalent to no pre-coding, and hence the benefit is lost. When the pre-coding matrix becomes random, the system behaves like an open loop system. Finally, a wrong per-layer based power allocation can further hamper system performance. 
       SUMMARY OF INVENTION 
       [0014]    According to an aspect of the invention, there is provided a method of transmitting comprising: pre-coding each of at least two data symbols using a respective pre-coding codeword to produce a corresponding plurality of pre-coded data symbols; transmitting a respective signal from each of a plurality of antennas, the respective signal comprising one of the pre-coded signals and at least one pilot for use in channel estimation; the signals collectively further comprising at least one beacon pilot vector consisting of a respective beacon pilot per antenna, the beacon pilot vector containing contents known to a receiver for use by the receiver in determining the codeword used to pre-code the at least one data signal. 
         [0015]    In some embodiments, each transmitted signal is an OFDM signal, and wherein for each antenna the respective signal comprises a null in each sub-carrier and time location used to transmit the at least one pilot in the respective signal of each other antenna. 
         [0016]    In some embodiments, the method further comprises pre-coding the pilots for use in channel estimation for transmission; wherein the at least one beacon pilot vector is transmitted without pre-coding. 
         [0017]    In some embodiments, the pilots for use in channel estimation are transmitted without pre-coding; the at least one beacon pilot vector is transmitted with pre-coding. 
         [0018]    In some embodiments, the method further comprises receiving feedback indicating which pre-coding codeword to use. 
         [0019]    In some embodiments, the feedback comprises a differential codeword index. 
         [0020]    In some embodiments, the method comprises transmitting to a plurality of receivers with frequency division duplex (FDD) or time division duplex (TDD) separation between content of different receivers; wherein pre-coding comprises using a respective pre-coding codeword for each receiver; the at least one pilot comprises a respective at least one pilot dedicated to each receiver that is pre-coded using the same codeword as the data for that receiver. 
         [0021]    In some embodiments, the method comprises transmitting to a plurality of receivers with FDD or TDD separation between content of different receivers; wherein pre-coding comprises using a respective pre-coding codeword for each receiver; the at least one pilot comprise pilots that are for use by all receivers. 
         [0022]    In some embodiments, the method is performed for each of a plurality of FDD or TDD MIMO radio resources. 
         [0023]    In some embodiments, the FDD or TDD MIMO radio resources are OFDM resources. 
         [0024]    In some embodiments, said FDD or TDD MIMO radio resource is single carrier based. 
         [0025]    According to another aspect of the invention, there is provided a method of receiving comprising: receiving a MIMO signal containing data symbols pre-coded with a codeword, the MIMO signal including pilots, and including at least one beacon pilot vector containing contents known to a receiver, each beacon pilot vector containing one symbol from each transmit antenna; processing the at least one beacon pilot vector to determine which codeword was used to pre-code the data symbols. 
         [0026]    In some embodiments, the method further comprises determining if the determined codeword was a codeword expected to be used. 
         [0027]    In some embodiments, the method further comprises comparing the determined codeword with an expected codeword; if there is a match between the determined codeword and the expected codeword, determining there is no error in codeword feedback, and performing decoding. 
         [0028]    In some embodiments, the pilots are not pre-coded and the at least one beacon pilot vector is pre-coded, and wherein processing the at least one beacon pilot vector to determine which codeword was used to pre-code the data comprises: performing channel estimation using the pilots to produce channel estimates; using the known contents of the at least one beacon pilot vector, and the channel estimates to determine which codeword was used. 
         [0029]    In some embodiments, the pilots are also pre-coded and the at least one beacon pilot vector is not pre-coded, and wherein processing at least one beacon pilot vector to determine which codeword was used to pre-code the data comprises: performing channel estimation using the pre-coded pilots to produce channel estimates; using the known contents of the at least one beacon pilot vector, and the channel estimates to determine which codeword was used. 
         [0030]    In some embodiments, the method further comprises transmitting feedback indicating which codeword to use. 
         [0031]    In some embodiments, the method further comprises comparing the determined codeword with the codeword indicated by the feedback to determine if there has been a pre-coding codeword feedback error. 
         [0032]    In some embodiments, the feedback comprises a differential codeword index. 
         [0033]    In some embodiments, the method further comprises tracking a channel of other receivers by processing the at least one un-precoded beacon pilot vector and pre-coded pilots of other receivers. 
         [0034]    In some embodiments, the method further comprises upon detecting that there is no pre-coding codeword feedback error, using received data for H-ARQ purposes; upon detecting that there is a pre-coding codeword feedback error, using the determined codeword in place of the codeword indicated by the feedback. 
         [0035]    In some embodiments, transmitting the differential codeword feedback based on the codeword determined using the at least one beacon pilot vector. 
         [0036]    In some embodiments, the method comprises in a transmitter, executing the method of transmitting as described in embodiments above; in a receiver: receiving a MIMO signal containing data symbols pre-coded with a codeword, the MIMO signal including pilots, and including at least one beacon pilot vector containing contents known to a receiver, each beacon pilot vector containing one symbol from each transmit antenna; processing the at least one beacon pilot vector to determine which codeword was used to pre-code the data symbols. 
         [0037]    According to yet another aspect of the invention, there is provided a transmitter that executes methods of transmitting a described above. 
         [0038]    According to yet a further aspect of the invention, there is provided a receiver that executes methods of receiving as described above. 
         [0039]    According to yet another aspect of the invention, there is provided a method comprising: provisioning a frequency division duplex MIMO radio resource for facilitating detection of feedback errors; where said resource includes at least one pilot for each transmit antenna; and where said provisioning includes pre-coding a known signal vector when said pilots are not pre-coded. 
         [0040]    According to another aspect of the invention, there is provided a system comprising: a controller operable to: provision a frequency division duplex MIMO radio resource for facilitating detection of feedback errors; where said resource includes at least one pilot for each transmit antenna; and where said controller is further operable to pre-code a known signal vector when said pilots are not pre-coded. 
         [0041]    According to another aspect of the invention, there is provided a system comprising: a controller operable to: provision a frequency division duplex MIMO radio resource for facilitating detection of feedback errors; where said resource includes at least one pilot for each transmit antenna; and where said controller is further operable to provision a non-pre-coded known signal vectors when said pilots are pre-coded. 
         [0042]    In some embodiments, said FDD MIMO radio resource is OFDM based. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0043]    Embodiments of the invention will now be described with reference to the attached drawings in which: 
           [0044]      FIG. 1  is an OFDM signal layout diagram for transmit signals that include common pilots; 
           [0045]      FIG. 2  is an OFDM signal layout diagram for OFDM signals containing dedicated pilots; 
           [0046]      FIG. 3  is an OFDM signal layout diagram for an OFDM signal containing common pilots and pre-coded beacon pilots; 
           [0047]      FIG. 4  is an example of equations for performing a pre-coding check; 
           [0048]      FIG. 5  is an OFDM signal layout diagram showing an OFDM signal with dedicated pilots and non-pre-coded beacon pilots; 
           [0049]      FIG. 6  is an example of equations for performing a pre-coding codeword check; 
           [0050]      FIG. 7  is a block diagram of a cellular communication system; 
           [0051]      FIG. 8  is a block diagram of an example base station that might be used to implement some embodiments of the present invention; 
           [0052]      FIG. 9  is a block diagram of an example wireless terminal that might be used to implement some embodiments of the present invention; 
           [0053]      FIGS. 10A and 10B  are block diagrams of a logical breakdown of example OFDM transmitter architectures that might be used to implement some embodiments of the present invention; and 
           [0054]      FIG. 11  is a block diagram of a logical breakdown of an example OFDM receiver architecture that might be used to implement some embodiments of the present invention. 
           [0055]      FIG. 12  is a block diagram of an example OFDM receiver architecture. 
       
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
       [0056]    In accordance with embodiments of the invention various closed-loop MIMO systems and methods that may involve pre-coding index feedback are described. Specifically the embodiments presented below are intended for use in future 3GPP, 3GPP2 and IEEE 802.16 based wireless standards. The broader inventions set out in the summary, however, are not limited in this regard. Furthermore, while embodiments of the invention are described in the context of an OFDM air interface, the broader concepts are not limited in this regard and are equally applicable to other air interfaces such as the single carrier uplink air interface used for UMTS LTE, or any other FDD air interface adopting a frequency domain MIMO detection approach. 
       Common Pilot Embodiment 
       [0057]    In accordance with an embodiment of the invention, a closed-loop MI MO scheme is provided that uses common pilots in which “beacon pilot vectors” are introduced to enable a receiver to determine a pre-coding codeword used at the transmitter, for example to enable a pre-coding codeword check. Details of such a pre-coding check are provided below. Practically speaking this check can occur nearly instantaneously. For purposes of the common pilot scheme, the beacon pilot vectors are pre-coded whereas the common pilots are not. 
         [0058]    Referring now to the detailed example of  FIG. 3 , the frequency axis is indicated at  210  and the time axis is indicated at  212 . The figure uses a shorthand notation to show what is transmitted on two different antennas. In locations  240 , common pilots are transmitted by a first transmit antenna Tx-1, and in locations  241 , common pilots are transmitted by a second transmit antenna Tx-2. In the locations used for pilots by one antenna, nulls are transmitted on the other antenna. In each location  246 , a beacon pilot vector for UE-1 is transmitted, and in each location  248 , a beacon pilot vector for UE-2 is transmitted. Each beacon pilot vector location includes a vector with one element per antenna transmitted simultaneously on the same sub-carrier frequency. This is contrast to the other pilot locations, in which one pilot signal is transmitted at a given time on a given sub-carrier frequency on one antenna with other antennas transmitting nulls. Remaining locations are available for data to be transmitted by both antennas. In the illustrated example, data includes pre-coded data  242  for a first UE (UE-1), and pre-coded data  244  for a second UE (UE-2). Typically, the pre-coding applied for pre-coded data  242  will be different from that applied for pre-coded data  244 . With the common pilot approach, the same pilots are used for both UEs and hence they cannot be pre-coded. 
         [0059]    In the illustrated example, the beacon pilot vectors are embedded within the area used to transmit data for each of two users. The beacon pilot vectors within area  243  are pre-coded with the same pre-coding used for the pre-coded data  242  for UE-1, and the beacon pilot vectors within area  245  are pre-coded with the same pre-coding used for the pre-coded data  244  for UE-2. 
         [0060]      FIG. 3  shows a very specific example in which OFDM resources are used for two different UEs. Of course the number of UEs that can be handled in this manner is implementation specific. The number and location of the pilots for each of the UEs are implementation specific. While the example has focused on a two transmit antenna case, more generally, this is extendable to an N transmit antenna case. In some such embodiments, the pilots include respective pilots for each of the antennas. While groups of two pilots (one for each transmit chain) are shown in  FIG. 3 , those of ordinary skill in the art will recognize that more than two pilots could be used without departing from the scope of the broader concepts. Similarly, the number and location of beacon pilot vectors shown in  FIG. 3  are not limiting. If more beacon pilot vectors are used, additional resources are required though better results may be achieved. 
         [0061]      FIG. 4  shows an example of equations that can be used to determine what codeword was used in the transmitter, for example so as to perform a pre-coding codeword check. The equations pertain to each beacon pilot vector location, where it is assumed that the receiver knows which pre-coding codebook it is expecting to be used for its data. In  FIG. 4 , H c  represents the channel matrix corresponding to the location of a particular beacon pilot vector. An estimate of this can be derived from common pilots. s is the transmitted beacon pilot vector that includes one component per antenna. V p  is the pre-coding matrix, and n is additive noise. V k =,k=1, . . . L is the pre-coding codebook. An expression for the received beacon pilots F beacon     —     pilot  is given by the first equation  250 . The second equation  252  is used to determine which pre-coding matrix was used in the transmitter from a set of L possible pre-coding matrices. Specifically, the expression inside the “argmin” operator in the right hand side of Equation  252  is evaluated for each of the L pre-coding matrices and the pre-coding matrix V p  that results in the minimum of this value is identified. Assuming this matches what the receiver is expecting, then the pre-coding codeword check succeeds. If this does not match what the receiver is expecting, then there is an error. 
         [0062]    The approach described above is applied for each of the UE, to independently verify the respective pre-coding matrix used. Each UE needs to verify all the pre-coding matrices intended for it. Note that a TIE can have several pre-coding matrices, due to channel differences in its occupied bandwidth. 
         [0063]    Multiple beacon pilot vectors within a feedback period can be used jointly for the pre-coding codeword check purposes. However, a minimum of one beacon pilot in a given feedback period is needed. 
         [0064]    Where content for multiple users is contained in received signals, the pre-coding codeword check described above is performed at each receiver. 
       Dedicated Pilot Embodiment 
       [0065]    In accordance with another embodiment of the invention, a closed-loop MIMO scheme using dedicated pilots is provided in which beacon pilot vectors are introduced to enable a pre-coding codeword check. For purposes of the dedicated pilot scenario, the beacon pilot vectors are not pre-coded and the pilots are pre-coded. This enables discernment of the pre-coding being used by the transmitter. 
         [0066]    Referring now to the detailed example of  FIG. 5 , in locations  260 , 262 , dedicated pilots are transmitted by a first transmit antenna Tx-1 and a second antenna Tx-2 respectively that are specific to a first UE. In locations  264 , 266 , dedicated pilots are transmitted by the first transmit antenna Tx-1 and the second antenna Tx-2 respectively that are specific to a second UE. In locations  272 , beacon pilot vectors are transmitted. As in the previous example, the beacon pilot vectors in a given location consist of a vector with one element per antenna. Remaining locations are available for data to be transmitted by both antennas. In the illustrated example, data includes pre-coded data  268  for the first UE, and pre-coded data  270  for the second UE. Typically, the pre-coding applied for pre-coded data  268  will be different from that applied for pre-coded data  270 . With the dedicated pilot approach, different pilots are used for each UE in the sense that they are pre-coded using the same pre-coding matrix as used for the data for each user. More specifically, the dedicated pilots  260 , 262  that are embedded within the area  261  used to transmit pre-coded data  268  for UE 1 are pre-coded with the same pre-coding as was used for the pre-coded data  268 . Similarly, the dedicated pilots  264 , 266  for the second UE are located within the area  265  used to transmit pre-coded data  270  to the second UE, and the same pre-coding is applied to both the dedicated pilots and the pre-coded data. No pre-coding is applied to the beacon pilot vectors  272 . 
         [0067]      FIG. 5  shows a very specific example in which OFDM resources are used for two different UEs. Of course the number of UEs that can be handled in this manner is implementation specific. The number and location of the pilots for each of the UEs are implementation specific. While the example has focused on a two transmit antenna case, more generally, this is extendable to an N transmit antenna case. In some such embodiments, the pilots include respective pilots for each of the antennas. While groups of two pilots (one for each transmit chain) are shown in  FIG. 5 , those of ordinary skill in the art will recognize that more than two pilots could be used without departing from the scope of the broader concepts. Similarly, the number and location of beacon pilot vectors shown in  FIG. 5  are not limiting. If more beacon pilot vectors are used, additional resources are required though better results may be achieved. 
         [0068]      FIG. 6  shows an example of a set of equations that can be used to perform a pre-coding check for the example of  FIG. 5 , all of which pertain to a particular beacon pilot location. In  FIG. 6 , H c  represents the channel matrix without pre-coding. “G” is an effective channel matrix including both the effects of the channel (estimated from dedicated pilots) and the pre-coding matrix, and s beacon     —     pilot  is the transmitted beacon pilot vector. The effective channel matrix will thus be different for each user given that different pre-coding has been applied. V p  is the pre-coding matrix and n is additive noise. An expression for the received beacon pilot vectors r beacon     —     pilot  is given by the first equation  280 . The second equation  282  gives G as a function of H c  and V p  and the fourth equation  286  gives H c  as a function of V p′  and G. The third equation  284  is used to determine which pre-coding matrix was used in the transmitter from a set of L possible pre-coding matrixes. More specifically, the expression inside the “argmin” operator in Equation  284  is evaluated for each of the possible pre-coding matrices V k , for k=1 to L and the expression that results in the minimum value is selected as {circumflex over (V)} p . Assuming this matches what the receiver is expecting, then the pre-coding codeword check succeeds. If this does not match what the receiver is expecting, then there is an error. 
         [0069]    The approach described above is applied for each of the UE, to independently verify the respective pre-coding matrix used. Each UE needs to verify all the pre-coding matrices intended for it. Note that a UE can have several pre-coding matrices, due to channel differences in its occupied bandwidth. 
         [0070]    Once again multiple beacon pilot vectors within a feedback period can be used jointly for the pre-coding codeword check purposes. However, a minimum of one beacon pilot vector in a given feedback period is needed. 
         [0071]    When estimation noise power is larger than the codeword quantization distances, an estimation error can occur. The larger the distance between codebook entries the smaller the probability of error. Several methods can be employed to make detection more reliable:
       use more than one beacon pilot vector for each user (for each sub-band for the example of  FIG. 5 );   track the codeword error—if the same wrong codeword is detected twice, then it is safe to assume that this specific codeword was used as an earlier pre-coding codeword; or   when in doubt, use the whole index feedback approach.       
 
         [0075]    With the dedicated pilot approach, other UEs in the system can track the channel by looking at the dedicated pilots and the unpre-coded beacons so that they can use proper closed-loop schemes and be scheduled properly. Since other UEs do not know V p , they cannot track the channel used by pre-coding. This makes scheduling difficult, because a scheduler has no way to know in advance which UE this resource should be allocated to. To solve this problem, the other UEs can examine the non-pre-coded beacon pilot vectors. Since the number of codewords is limited, this provides an efficient way for channel tracking. 
         [0076]    Denoting s pilot  as a known pilot vector, then the received signal on that particular pilot tone is given by 
         [0000]    
       
      
       
         r 
       
       pilot 
       =H 
       c 
       
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       pilot 
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         [0000]    From dedicated pilots, we have G=H c V p . Let {V k ,k=1, . . . , L} be the pre-coding codebook; then the pre-coding codeword V p  used by the BTS can be estimated as 
         [0000]    
       
         
           
             
                 
             
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         [0000]    where we assume that a channel matrix is U c D c V c′  in its SVD form. After knowing V p , a UE will be able to estimate the current channel easily by computing 
         [0000]    
       
      
       Ĥ 
       c 
       =Ĝ{circumflex over (V)} 
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         [0077]    With both the dedicated pilot and common pilot embodiments described above feedback errors can be detected very quickly. If a received packet is still in error but the packet data error was not caused by pre-coding codeword feedback error, the received data can be used for H-ARQ purposes. In the dedicated pilot case, even when pre-coding feedback is wrong, the received data can still be used for H-ARQ purpose. The reason is that with dedicated pilots, the pilot channel is the same as the data channel, and hence the reference is still correct for coherent detection. In the common pilot case, when pre-coding feedback is wrong, and if the receiver does not know what pre-coding matrix is being used by the transmitter, the data channel cannot be correctly reconstructed. In this case the received data will need to be discarded. However, when a feedback error occurs, if the pre-coding matrix used by the transmitter can be detected successfully notwithstanding the error, then the data channel can still be correctly reconstructed. In other words, if the receiver knows what pre-coding matrix is being used by the transmitter, regardless of whether the feedback is correct or wrong, the received data can still be used. Of course, in this case, as explained above, the benefit of pre-coding is reduced. 
         [0078]    In addition, in some embodiments, differential feedback is employed, and the subsequent differential codeword index feedback is based on the codeword currently used by the transmitter (as verified by the check), and this eliminates any error propagation instantly. That is to say, when a codeword feedback error has been determined, the index that was used by the transmitter is known from the check, and the next differential codeword feedback will be based on this index. 
         [0079]    With reference to  FIG. 7 , a base station controller (BSC)  10  controls wireless communications within multiple cells  12 , which are served by corresponding base stations (BS)  14 . In general, each base station  14  facilitates communications using OFDM with mobile terminals  16 , which are within the cell  12  associated with the corresponding base station  14 . The movement of the mobile terminals  16  in relation to the base stations  14  results in significant fluctuation in channel conditions. As illustrated, the base stations  14  and mobile terminals  16  may include multiple antennas to provide spatial diversity for communications. 
         [0080]    A high level overview of the mobile terminals  16  and base stations  14  of the present invention is provided prior to delving into the structural and functional details. With reference to  FIG. 8 , a base station  14  configured according to one embodiment of the present invention is illustrated. The base station  14  generally includes a control system  20 , a baseband processor  22 , transmit circuitry  24 , receive circuitry  26 , multiple antennas  28 , and a network interface  30 . The receive circuitry  26  receives radio frequency signals bearing information from one or more remote transmitters provided by mobile terminals  16  (illustrated in  FIG. 9 ). A low noise amplifier and a filter (not shown) may be provided that cooperate to amplify and remove out-of-band interference from the signal for processing. Down conversion and digitization circuitry (not shown) will then down convert the filtered, received signal to an intermediate or baseband frequency signal, which is then digitized into one or more digital streams. 
         [0081]    The baseband processor  22  processes the digitized received signal to extract the information or data bits conveyed in the received signal. This processing typically comprises demodulation, decoding, and error correction operations. As such, the baseband processor  22  is generally implemented in one or more digital signal processors (DSPs) or application-specific integrated circuits (ASICs). The received information is then sent across a wireless network via the network interface  30  or transmitted to another mobile terminal  16  serviced by the base station  14 . 
         [0082]    On the transmit side, the baseband processor  22  receives digitized data, which may represent voice, data, or control information, from the network interface  30  under the control of control system  20 , and encodes the data for transmission. The encoded data is output to the transmit circuitry  24 , where it is modulated by a carrier signal having a desired transmit frequency or frequencies. A power amplifier (not shown) will amplify the modulated carrier signal to a level appropriate for transmission, and deliver the modulated carrier signal to the antennas  28  through a matching network not shown). Modulation and processing details are described in greater detail below. 
         [0083]    With reference to  FIG. 9 , a mobile terminal  16  configured according to one embodiment of the present invention is illustrated. Similarly to the base station  14 , the mobile terminal  16  will include a control system  32 , a baseband processor  34 , transmit circuitry  36 , receive circuitry  38 , multiple antennas  40 , and user interface circuitry  42 . The receive circuitry  38  receives radio frequency signals bearing information from one or more base stations  14 . Preferably, a low noise amplifier and a filter (not shown) cooperate to amplify and remove out-of-band interference from the signal for processing. Down conversion and digitization circuitry (not shown) will then down convert the filtered, received signal to an intermediate or baseband frequency signal, which, is then digitized into one or more digital streams. 
         [0084]    The baseband processor  34  processes the digitized received signal to extract the information or data bits conveyed in the received signal. This processing typically comprises demodulation, decoding, and error correction operations, as will be discussed on greater detail below. The baseband processor  34  is generally implemented in one or more digital signal processors (DSPs) and application specific integrated circuits (ASICs). 
         [0085]    For transmission, the baseband processor  34  receives digitized data, which may represent voice, data, or control information, from the control system  32 , which it encodes for transmission. The encoded data is output to the transmit circuitry  36 , where it is used by a modulator to modulate a carrier signal that is at a desired transmit frequency or frequencies. A power amplifier (not shown) will amplify the modulated carrier signal to a level appropriate for transmission, and deliver the modulated carrier signal to the antennas  40  through a matching network (not shown). Various modulation and processing techniques available to those skilled in the art are applicable to the present invention. 
         [0086]    In OFDM modulation, the transmission band is divided into multiple, orthogonal carrier waves. Each carrier wave is modulated according to the digital data to be transmitted. Because OFDM divides the transmission band into multiple carriers, the bandwidth per carrier decreases and the modulation time per carrier increases. Since the multiple carriers are transmitted in parallel, the transmission rate for the digital data, or symbols, on any given carrier is lower than when a single carrier is used. 
         [0087]    OFDM modulation requires the performance of an Inverse Fast Fourier Transform (IFFT) on the information to be transmitted. For demodulation, the performance of a Fast Fourier Transform (FFT) on the received signal is required to recover the transmitted information. In practice, the IFFT and FFT are provided by digital signal processing carrying out an Inverse Discrete Fourier Transform (IDFT) and Discrete Fourier Transform (DFT), respectively. Accordingly, the characterizing feature of OFDM modulation is that orthogonal carrier waves are generated for multiple bands within a transmission channel. The modulated signals are digital signals having a relatively low transmission rate and capable of staying within their respective bands. The individual carrier waves are not modulated directly by the digital signals. Instead, all carrier waves are modulated at once by IFFT processing. 
         [0088]    In some embodiments, OFDM is used for at least the downlink transmission from the base stations  14  to the mobile terminals  16 . Each base station  14  is equipped with n transmit antennas  28 , and each mobile terminal  16  is equipped with m receive antennas  40 . Notably, the respective antennas can be used for reception and transmission using appropriate duplexers or switches and are so labeled only for clarity. 
         [0089]    With reference to  FIGS. 10A and 10B , a logical OFDM transmission architecture is provided according to one embodiment.  FIG. 10A  is an example of a dedicated pilot embodiment.  FIG. 10B  is an example of a common pilot embodiment. In both cases, initially, the base station controller  10  ( FIG. 7 ) will send data to be transmitted to various mobile terminals  16  to the base station  14 . The base station  14  may use the channel quality indicators (CQIs) associated with the mobile terminals to schedule the data for transmission as well as select appropriate coding and modulation for transmitting the scheduled data. The CQIs may be directly from the mobile terminals  16  or determined at the base station  14  based on information provided by the mobile terminals  16 . In either case, the CQI for each mobile terminal  16  is a function of the degree to which the channel amplitude (or response) varies across the OFDM frequency band. 
         [0090]    The scheduled data  44 , which is a stream of bits, is scrambled in a manner reducing the peak-to-average power ratio associated with the data using data scrambling logic  46 . A cyclic redundancy check (CRC) for the scrambled data is determined and appended to the scrambled data using CRC adding logic  48 . Next, channel coding is performed using channel encoder logic  50  to effectively add redundancy to the data to facilitate recovery and error correction at the mobile terminal  16 . Again, the channel coding for a particular mobiles terminal  16  is based on the CQI. The channel encoder logic  50  uses known Turbo encoding techniques in one embodiment. The encoded data is then processed by rate matching logic  52  to compensate for the data expansion associated with encoding. 
         [0091]    Bit interleaver logic  54  systematically reorders the bits in the encoded data to minimize the loss of consecutive data bits. The resultant data bits are systematically mapped into corresponding symbols depending on the chosen baseband modulation by mapping logic  56 . Preferably, Quadrature Amplitude Modulation (QAM) or Quadrature Phase Shift Key (QPSK) modulation is used. The degree of modulation is preferably chosen based on the CQI for the particular mobile terminal. The symbols may be systematically reordered to further bolster the immunity of the transmitted signal to periodic data loss caused by frequency selective fading using symbol interleaver logic  58 . 
         [0092]    At this point, groups of bits have been mapped into symbols representing locations in an amplitude and phase constellation. 
         [0093]    Now referring specifically to  FIG. 10A , for the dedicated pilot embodiment, the symbols are processed by S/P, SM layer mapping function  63  which performs serial to parallel (S/P) conversion, and spatial multiplexing (SM) layer mapping. The output of this process is multiplied by the pre-coding matrix multiplier  65 . A pilot sequence  69  is also multiplied by the pre-coding matrix multiplier  65  using the same pre-coding matrix. The output of the pre-coding matrix multiplier  65  is input to pilot and beacon pilot vectors insertion function  67 . Non-pre-coded beacon pilot vectors  71  are also input to the pilot and beacon pilot vectors insertion function  67 . The pilots and beacon pilot vectors and data are then organized into two output streams, one per antenna. 
         [0094]    Now referring specifically to  FIG. 10B , for the common pilot embodiment, the symbols are processed by S/P, SM layer mapping function  63  which performs serial to parallel conversion, and spatial multiplexing layer mapping. The output of this process is multiplied by the pre-coding matrix multiplier  73 . Beacon pilot vectors  77  are also multiplied by the pre-coding matrix multiplier  73  using the same pre-coding matrix. The output of the pre-coding matrix multiplier  73  is input to pilot and beacon pilot vectors insertion function  75 . A non-pre-coded pilot sequence  79  is also input to the pilot and beacon pilot vectors insertion function  75 . The pilots and beacon pilot vectors and data are then organized into two output streams, one per antenna. 
         [0095]    Referring again to both  FIGS. 10A and 10B , each output stream is sent to a corresponding IFFT processor  62 , illustrated separately for ease of understanding. Those skilled in the art will recognize that one or more processors may be used to provide such digital signal processing, alone or in combination with other processing described herein. The IFFT processors  62  will operate on the respective symbols to provide an inverse Fourier Transform. The output of the IFFT processors  62  provides symbols in the time domain. The time domain symbols are grouped into frames, which are associated with a prefix by prefix insertion logic  64 . Each of the resultant signals is up-converted in the digital domain to an intermediate frequency and converted to an analog signal via the corresponding digital up-conversion (DUC) and digital-to-analog (D/A) conversion circuitry  66 . The resultant (analog) signals are then simultaneously modulated at the desired RF frequency, amplified, and transmitted via the RF circuitry  68  and antennas  28 . Notably, pilot signals known by the intended mobile terminal  16  are scattered among the sub-carriers. The mobile terminal  16 , which is discussed in detail below, will use the pilot signals for channel estimation. 
         [0096]    Reference is now made to  FIG. 11  to illustrate reception of the transmitted signals by a mobile terminal  16 . Upon arrival of the transmitted signals at each of the antennas  40  of the mobile terminal  16 , the respective signals are demodulated and amplified by corresponding RF circuitry  70 . For the sake of conciseness and clarity, only one of the two receive paths is described and illustrated in detail. Analog-to-digital (A/D) converter and down-conversion circuitry  72  digitizes and downconverts the analog signal for digital processing. The resultant digitized signal may be used by automatic gain control circuitry (AGC)  74  to control thee gain of the amplifiers in the RF circuitry  70  based on the received signal level. 
         [0097]    Initially, the digitized signal is provided to synchronization logic  76 , which includes coarse synchronization logic  78 , which buffers several OFDM symbols and calculates an auto-correlation between the two successive OFDM symbols. A resultant time index corresponding to the maximum of the correlation result determines a fine synchronization search window, which is used by fine synchronization logic  80  to determine a precise framing starting position based on the headers. The output of the fine synchronization logic  80  facilitates frame acquisition by frame alignment logic  84 . Proper framing alignment is important so that subsequent FFT processing provides an accurate conversion from the time to the frequency domain. The fine synchronization algorithm is based on the correlation between the received pilot signals carried by the headers and a local copy of the known pilot data. Once frame alignment acquisition occurs, the prefix of the OFDM symbol is removed with prefix removal logic  86  and resultant samples are sent to frequency offset correction logic  88 , which compensates for the system frequency offset caused by the unmatched local oscillators in the transmitter and the receiver. Preferably, the synchronization logic  76  includes frequency offset and clock estimation logic  82 , which is based on the headers to help estimate such effects on the transmitted signal and provide those estimations to the correction logic  88  to properly process OFDM symbols. 
         [0098]    At this point, the OFDM symbols in the time domain are ready for conversion to the frequency domain using FFT processing logic  90 . The results are frequency domain symbols, which are sent to processing logic  92 . The processing logic  92  extracts the scattered pilot signal using scattered pilot extraction logic  94 , determines a channel estimate based on the extracted pilot signal using channel estimation logic  96 , and provides channel responses for all sub-carriers using channel reconstruction logic  98 . In order to determine a channel response for each of the sub-carriers, the pilot signal is essentially multiple pilot symbols that are scattered among the data symbols throughout the OFDM sub-carriers in a known pattern in both time and frequency.  FIG. 12  illustrates an exemplary scattering of pilot symbols among available sub-carriers over a given time and frequency plot in an OFDM environment. Continuing with  FIG. 11 , the processing logic compares the received pilot symbols with the pilot symbols that are expected in certain sub-carriers at certain times to determine a channel response for the sub-carriers in which pilot symbols were transmitted. The results are interpolated to estimate a channel response for most, if not all, of the remaining sub-carriers for which pilot symbols were not provided. The actual and interpolated channel responses are used to estimate an overall channel response, which includes the channel responses for most, if not all, of the sub-carriers in the OFDM channel. 
         [0099]    The frequency domain symbols and channel reconstruction information, which are derived from the channel responses for each receive path are provided to an STC decoder  100 , which provides STC decoding on both received paths to recover the transmitted symbols. The channel reconstruction information provides equalization information to the STC decoder  100  sufficient to remove the effects of the transmission channel when processing the respective frequency domain symbols. 
         [0100]    The recovered symbols are placed back in order using symbol de-interleaver logic  102 , which corresponds to the symbol interleaver logic  58  of the transmitter. The de-interleaved symbols are then demodulated or de-mapped to a corresponding bitstream using de-mapping logic  104 . The bits are then de-interleaved using bit de-interleaver logic  106 , which corresponds to the bit interleaver logic  54  of the transmitter architecture. The de-interleaved bits are then processed by rate de-matching logic  108  and presented to channel decoder logic  110  to recover the initially scrambled data and the CRC checksum. Accordingly, CRC logic  112  removes the CRC checksum, checks the scrambled data in traditional fashion, and provides it to the de-scrambling logic  114  for de-scrambling using the known base station de-scrambling code to recover the originally transmitted data  116 . 
         [0101]      FIG. 12  shows a block diagram of receiver components for performing beacon pilot extraction, and pre-coding matrix detection. These components can be added to a receiver such as shown in  FIG. 11 , and in fact some components are shown in  FIG. 12  that are in common with those of  FIG. 11 . The output of FFT  90  is processed by scattered pilot extraction  94 , channel estimation and channel reconstruction  98 . The output of FFT  90  is also processed by beacon pilot extraction  400 . Based on the extracted beacon pilots, the pre-coding codeword matrix detection is performed at  402 . For the common pilots embodiment, this is fed back to the channel reconstruction function  98 . Outputs of the channel estimation  96  and the pre-coding matrix detection  402  are input to a differential codebook index search  404  which generates feedback that is sent back to the transmitter. 
         [0102]    What has been described is merely illustrative of the application of the principles of the invention. Other arrangements and methods can be implemented by those skilled in the art without departing from the spirit and scope of the present invention. 
         [0103]    Numerous modifications and variations of the present invention are possible in light of the above teachings. It is therefore to be understood that within the scope of the appended claims, the invention may be practiced otherwise than as specifically described herein.