Patent Publication Number: US-7583766-B2

Title: System, and associated method, for utilizing block BLAST with PPIC in a MIMO multicode MC-CDMA system

Description:
TECHNICAL FIELD 
   The invention relates generally to wireless telecommunications and, more particularly, to MIMO MultiCode MC-CDMA wireless telecommunications. Still more particularly, the invention relates to a system, and associated method, for utilizing block BLAST with Partially Parallel Interference Cancellation (PPIC) algorithm to reduce interference in a MIMO MultiCode MC-CDMA System. 
   BACKGROUND 
   Substantial research is being conducted in connection with MIMO OFDM to combine MIMO techniques with multicarrier (MC) schemes, it being understood that OFDM is but a special form of MC-CDMA. MIMO MC-CDMA is also being considered for the “4G” radio access scheme to provide the target data rate of the 4G system. In order to improve the system throughput in MIMO MC-CDMA, multicode transmission techniques are preferably incorporated into the system to provide the same information data rate as MIMO OFDM. 
   In order to provide the target data rate of the “4G” system, multicode transmission with MIMO technique is preferably combined simultaneously with MC-CDMA. However, under a multipath fading channel, MIMO Multicode MC-CDMA is problematic, because inherent in it are two interferences, namely, one from inter-code interference between the multicode under the multipath fading channel, and a second from inter-antenna interference caused from an independent stream of different antennas. 
   In mitigating the inter-code interference and inter-antenna interference, it is first noted that a relatively simple Bell Labs Space Time (BLAST) algorithm may be used in a MIMO (non-multicode) MC-CDMA system to distinguish the different TX-antenna streams, chip-by-chip. The aforementioned two interferences then provide the error floor performance. BLAST is described in further detail in U.S. Pat. No. 6,097,771 filed on Jul. 1, 1996, on behalf of Gerard J. Foschini and entitled “Wireless communications system having a layered space-time architecture employing multi-element antennas”, in an article published in Bell Labs Tech. J., pages 41-59, Autumn 1996 by Gerard J. Foschini which was entitled “Layered space-time architecture for wireless communication in a fading environment when using multi-element antennas”, in EP 0 817 401 A2, filed on Jul. 1, 1996, on behalf of Gerard J. Foschini and entitled “Wireless communications system having a layered space-time rchitecture employing multi-element antennas”, and in EP 0 951 091 A2, filed on Apr. 15, 1998, on behalf of Gerard J. Foschini and Glenn D. Golden, and entitled “Wireless communications system having a space-time architecture employing multi-element antennas at both the transmitter and the receiver”, all of which are incorporated herein by reference in their respective entireties. 
   Second, it is noted that, in contrast to MIMO MC-CDMA, in MIMO OFDM systems, there is only one interference from the independent stream of the different antennas, and the simple BLAST algorithm is effective for mitigating this inter-antenna interference. 
   Third, because MIMO OFDM has no spreading code, multicode is used, and as a consequence, there is no Multiple Access Interference (MAI) existing in MIMO OFDM systems. However, in a MIMO MC-CDMA system, additional interference is provided by the MAI caused by inter-code non-orthogonality. 
   In light of the foregoing, it is apparent that there is a need for a system and method which may be utilized to enhance the performance of the MIMO Multicode MC-CDMA system under a multipath fading channel. Such a system and method should, among other things, simultaneously mitigate both inter-code interference and inter-antenna interference. 
   SUMMARY 
   The present invention, accordingly, provides a block BLAST-like algorithm which includes a Partially Parallel Interference Cancellation (PPIC) algorithm effective for simultaneously mitigating the inter-code interference and inter-antenna interference of MIMO Multicode MC-CDMA systems. The BLAST algorithm and the PPIC algorithm are both used to differentiate the information stream between the inter-codes and inter-antennas, and ensure better performance of MIMO Multicode MC-CDMA systems under the multipath fading channel. Additionally, the block BLAST-like algorithm and PPIC techniques are combined to be used over one spreading-length block symbols. 
   For the multipath fading channel, MIMO Multicode MC-CDMA will have an error floor performance when a simple BLAST algorithm is used, chip-by-chip, to obtain a MIMO de-multiplexed symbol, and then the de-multiplexed symbols are despread and demodulated. 
   It is noted that the algorithm of the present invention has also taken into account the case of different channel information distributed in the different chips/subcarriers on MC-CDMA system, which is different from the general downlink CDMA case. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
     For a more complete understanding of the present invention, and the advantages thereof, reference is now made to the following descriptions taken in conjunction with the accompanying drawings, in which: 
       FIG. 1  presents a single transmitter antenna of a MIMO Multicode MC-CDMA system embodying features of the present invention; 
       FIG. 2  presents a MIMO Multicode MC-CDMA transmitter system embodying features of the present invention; 
       FIG. 3  presents a MIMO Multicode MC-CDMA receiver system utilizing a BLAST-PPIC algorithm in accordance with principles of the present invention; 
       FIG. 4  exemplifies a spreading code matrix; 
       FIG. 5  exemplifies a channel matrix; 
       FIG. 6  depicts a high level flow chart illustrating control logic embodying features of the present invention for performing the BLAST-PPIC algorithm of  FIG. 3 ; 
       FIG. 7  depicts a flow chart illustrating in greater detail the control logic of  FIG. 6 ; and 
       FIG. 8  presents a performance chart of MIMO Multicode MC-CDMA system. 
   

   DETAILED DESCRIPTION 
   In the following discussion, numerous specific details are set forth to provide a thorough understanding of the present invention. However, it will be obvious to those skilled in the art that the present invention may be practiced without such specific details. In other instances, well-known elements have been illustrated in schematic or block diagram form in order not to obscure the present invention in unnecessary detail. Additionally, for the most part, details concerning BLAST, MIMO, MC-CDMA, and the like, have been omitted inasmuch as such details are not considered necessary to obtain a complete understanding of the present invention, and are considered to be within the skills of persons of ordinary skill in the relevant art. 
   It is noted that, unless indicated otherwise, all functions described herein may be performed by a processor such as a microprocessor, a controller, a microcontroller, an application-specific integrated circuit (ASIC), an electronic data processor, a computer, or the like, in accordance with code, such as program code, software, integrated circuits, and/or the like that are coded to perform such functions. Furthermore, it is considered that the design, development, and implementation details of all such code would be apparent to a person having ordinary skill in the art based upon a review of the present description of the invention. 
   Referring to  FIG. 1  of the drawings, the reference numeral  100  generally designates a MIMO Multicode MC-CDMA system for a single transmitter antenna embodying features of the present invention. The system  100  includes a converter  102  configured for receiving modulated serial symbol streams of all users, and for converting the serial streams of symbols to K blocks of P streams of symbols. The converter  102  is coupled for transmitting the K blocks of P streams of symbols to K×P spreaders  104 . Each spreader  104  is configured for spreading the streams using Walsh-Hadamard codes of length J. The length of the Walsh-Hadamard codes by vectors S 1 , . . . , S p  is denoted as J and the length of symbol streams at the inputs of the kth block by vectors C k,l , . . . , C k,P  is Q. Then the spread stream at the output of the spreaders  104  will be a vector of length QJ. 
   The spreaders  104  are coupled to K summers  106 , each of which is configured for summing the vectors of length QJ to form a single spread stream. The summers  106  are each coupled to a serial-to-parallel converter  108  configured for converting and transmitting the serial stream to a parallel stream for an OFDM modulator  110 . The OFDM modulator  110  inserts N p  equally spaced pilot symbols into the stream to form the MC-CDMA block of N b  symbols in the frequency domain, which is then converted to a time-domain MC-CDMA block using IFFT transformation. 
   The modulator  110  is preferably coupled via a line  111  to a cyclic prefix (CP) module  112  configured for using an IFFT to add a CP of appropriate length to the time-domain signal at the output of the OFDM modulator  110  to prevent ISI (Inter-Symbol Interference) and Inter-Channel Interference (ICI). The CP module  112  is coupled to a multiplexer  114  effective for modulating the stream output from the CP module  112  to IF (Intermediate Frequency) by a waveform of cos(2nf c t). The signal is then transmitted over a multipath fading channel  208 . 
   It may be appreciated that the modulated symbols in the branches  107  from the summers  106  to  108  are converted from serial to parallel, and then spread by different spreading codes and added and passed through one branch  111  for OFDM modulation, and then sent to a TX antenna, which constitutes a Multicode MC-CDMA system. In order to improve the data rate multiple antenna technique in accordance with principles of the present invention, MIMO is employed. The different information streams C 1,p , . . . , C N,p , are transmitted via different transmitter (TX) antennas ANT (1-N)  206 , which may use the same or different spreading codes between TX-antennas, as discussed further below with reference to  FIG. 2 . After multicode spreading summation is performed for one TX-antenna, the data stream for each TX-antenna is converted from serial to parallel, OFDM-modulated, and then passed to TX-antenna for RF transmission. 
     FIG. 2  depicts a MIMO Multicode MC-CDMA transmitter system utilizing N transmit (TX) antenna systems  100 . N multipliers  202  are configured for receiving modulated symbols, and multiplying the symbols by a spreading code. Each multiplier  202  is coupled to a respective summer  204  for summing the symbols from a respective antenna, and then transmitting the summed symbols to a TX antenna system  100 , described above with respect to  FIG. 1 , including an antenna  206 , coupled for transmitting the modulated symbols over an RF channel  208 . As shown in  FIG. 2  and described in further detail with respect to  FIG. 3 , M RX antennas  210  are configured for receiving the modulated symbols transmitted from the TX antennas  206 , and are coupled for transmitting the received symbols to a receiver  212 . 
     FIG. 3  depicts details of the MIMO Multicode MC-CDMA receiver system  212  of  FIG. 2 . Each RX antenna  210  is coupled to a CP/FFT deletion module  302  configured for deleting the CP and FFT modulation from the data symbols received in each antenna  210 . Each CP/FFT deletion module  302  is coupled to a spreading code matrix module  304 , which in turn is coupled to a channel matrix module  306 , configured for correlating the demodulated symbols with a spreading code matrix S m  and a channel matrix H m  to obtain a correlation reception vector y m . The channel matrix modules  306  are coupled to summers  308  effective for adding the correlation reception vectors Y m  from all antennas  210  to generate a whole correlation reception vector y. The summers  308  are coupled to a module  310  effective for implementing, in accordance with principles of the present invention, a BLAST-PPIC algorithm over the correlation reception vector y, to thereby recover the original transmission bits over the different antennas. It is noted that in the BLAST-PPIC algorithm, multicode interference is also regarded as being equivalent to inter-antenna interferences. 
   While the BLAST-PPIC algorithm is discussed in further detail below with reference to  FIG. 7 , it is summarized in  FIG. 6  by a flow chart  600 . Accordingly, in step  602 , the BLAST algorithm is implemented over one block of correlation reception data y i  and its estimation channel values H to generate a group of temporary decision symbols C, which may constitute information bits between inter-code distribution and inter-antenna distribution. Step  602  is described in further detail below with respect to steps  704  and  706  of  FIG. 7 . In step  604 , one symbol, C k   i , is selected having a maximum SNR value from the temporary decision values generated in step  602 . By combining other temporary decision symbols, C, with the estimated channel value H, a group of temporary interference signals are recovered over the pre-selected symbol, C k   i , and those interference signals are subtracted from the correlation reception signal vector, y i . Then, from the corrected reception signal vector y i+1  and original estimation channel values, a more accurate decision symbol C k   i+1  is reached. In step  606 , steps  602  and  604  are repeated for other symbols C {circumflex over (k)}   i , which may constitute the information bits between the inter-code distribution and inter-antenna distribution. In order to improve the symbol decision accuracy, additional iterations may be used. Steps  604  and  606  are described in further detail below with respect to steps  708 - 728  of  FIG. 7 . 
   A suitable reception signal vector y, a multicode spreading code matrix S, and an estimated channel value matrix H must be constructed where there is multipath fading, and also when different channel values are distributed on different chips within a single spreading code. In the block-like BLAST algorithm with PPIC, the two interferences, between inter-code and inter-antenna, are regarded as the same case. 
   In the MIMO Multicode MC-CDMA system it is assumed that the received signal at the receiver antenna m in the t-th chip is r m,t . In the following equation (1), n designates the transmitter (TX) antenna index (N is maximum TX-antenna number), m designates the receiver (RX) antenna index (M is the maximum RX-antenna number), t denotes the chip index, J is the spreading code length, and P is the multicode number. 
                       r     m   ,   t       =         ∑     n   =   1     N     ⁢       h   nmt     ⁢       ∑     p   =   1     P     ⁢       C   np     ⁢     s   pt             +     η     m   ,   t           ⁢     
     ⁢     t   =     1   ⁢   Λ   ⁢           ⁢   J       ⁢     
     ⁢   m   =     1   ⁢   Λ   ⁢           ⁢   M       ⁢     
             (   1   )               
where s pt  is the t-th chip of the p-th spreading code; h nmt  is the estimated channel information in the t-th chip at the transmitter antenna n and receiver antenna m and C np  is the information bit transmitted at the n-th antenna and spread by the p-th spreading code and η m,t  is AWGN noise on the t-th chip of m-th receiver antenna.
 
   The received signal can be written in the following matrix form:
 
 r   m   =S   m   H   m   C+η   m   m =1 ΛM   (2)
 
where r m  is the reception signal vector at the receiver antenna m; S m  is the spreading code matrix and H m  is the estimated channel matrix; C is the transmission information data and η m  is receiver noise.
 
   In the MIMO multicode MC-CDMA system the spreading code matrix can be written as depicted in  FIG. 4  under a multipath fading channel.  FIG. 4  depicts how the spreading code matrix may be written, or how the channel matrix may be written. 
   The transmission information bit is:
 
C=[c 11 Λc N1 c 12 Λc N2 Λc 1P Λc NP ] T   (3)
 
   A channel correlation combination matrix for m-th receiver antenna may be constructed as: 
   
     
       
         
           
             
               
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   By multiplying the received signal vector r m  by spreading code matrix S m  and channel matrix H m , a new reception vector is reached: 
   
     
       
         
           
             
               
                 
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     FIG. 7  depicts a flow chart of control logic that may be utilized to implement the block BLAST-like algorithm with PPIC for the reception vector y and channel correlation combination matrix R. 
   Beginning in step  702  and proceeding to step  704 , after OFDM demodulation by the OFDM module  110 , the demodulated data r m  is multiplied by the multicode spreading code matrix S m  and estimated channel value matrix H m  ( FIGS. 4-5 ). 
   In step  706 , the correlation reception vector y can be calculated by combining the reception correlation vector over all the receiver antennas (formula (6)). The reception vector y contains the inter-code interference and inter-antennas interference. 
   In steps  708 - 728 , by using the correlation reception vector y, and channel combination matrix R , a block-like BLAST algorithm with partially parallel interference cancellation (PPIC) may recover the data information C between antennas and multicode information at the same time. 
   More specifically, in step  708 , the pseudo-inverse value of the channel correlation matrix R is calculated, which value constitutes the multiplication summation between the spreading code matrix S m  and the channel matrix H m  and its conjugation over all receiver antennas. 
   In step  710 , by using the correlation reception vector y and channel correlation inverse matrix G=R −1  the temporary hard-decision information C=de mod(G·y) between the antennas and multicode can be worked out, where the information between multicode is also regarded as the same as that between antennas. The channel correlation inverse matrix G is then sorted by the ascending index of diagonal value of the channel correlation matrix inverse G, the index vector k can be obtained. Steps  712 - 728  depict the temporary symbol recovery and interference cancellation from the reception vector. 
   In a first iteration of step  714 , according to the index vector k, the temporary decision in this minimum index k n   i  (or maximum SNR value) is multiplied with this symbol&#39;s channel vector and a suitable coefficient a to get a first vector; in addition the other symbols are multiplied with those symbol&#39;s channel value and a coefficient (1−a) to get a second vector. By combining the foregoing first and second vectors, the interference signal can be recovered by the combination vector. 
   In step  716 , the correlation reception vector y k     n     i  is reduced by the temporary recovered interference signal caused by other symbols on selecting the suitable coefficient a. 
   In step  718 , demodulating this corrected reception vector y k     n     i+1  with the (k n   i , k n   i ) diagonal position value of channel correlation inverse matrix G, a new decision symbol C k     n     i+1  for this symbol is reached. Other indexed symbols are determined similarly in a first iteration of steps  714 - 718 . 
   In steps  720 - 726 , one or more determinations are made whether to execute additional iterations of steps  714 - 718  to improve the data detection correction. 
   In order to test the performance the BLAST-PPIC algorithm for MIMO Multicode MC-CDMA, a simulation has been performed. In this simulation, 8 Walsh code for Multicode was used, and the spreading length of each code is also 8. In the system, there are 2 TX-antennas and 2 RX-antennas for the MIMO transmission. There are 1536 subcarriers for the data transmission, and 100 subcarriers for pilot transmission in the system. The IFFT/FFT transformation point is 2048. The symbol modulation is QPSK. The channel condition is a METRA Pedestrian A 3 km/hr multipath fading channel. 
     FIG. 8  depicts results of the simulation of the BLAST algorithm for MIMO Multicode MC-CDMA and BLAST-PPIC for MIMO Multicode MC-CDMA. When BLAST is simply used for MIMO Multicode MC-CDMA, the system is not working normally and the interference caused by inter-code and inter-antenna&#39;s information will give the error floor performance. From this figure, the BLAST-PPIC algorithm can work well for MIMO Multicode MC-CDMA system and provide the better performance. 
   The system and method of the present invention results in a number of advantages over the prior art. For example, the BLAST-PPIC algorithm provides a solution for the joint use of multicode transmission and multiple transmitter and receiver antennas under a multipath fading channel; by using the algorithm disclosed herein, the error floor can be overcome. Furthermore, the invention considers the case of different channel information distributed on different chips, which is different from the general downlink CDMA case. Still further, the invention provides a representation of a spreading code matrix and a channel matrix, which two matrixes are considered the concept of chip-equalization. This is different from the case of a general CDMA system because, in general, a CDMA receiver will use a Rake reception which is a simplified case. This matrix formulation can provide the method for future work to continuously simplify and optimize (or sub-optimize) the MIMO Multicode MC-CDMA. In a further advantage, the BLAST-PPIC algorithm reduces the computation tasks of large numbers of the pseudo-inverse matrix in the simple BLAST algorithm; and there only exists one pseudo-inverse operation in the beginning of executing the algorithm, and others are only the multiplication and addition operation. 
   It is understood that the present invention may take many forms and embodiments. Accordingly, several variations may be made in the foregoing without departing from the spirit or the scope of the invention. For example, when the MIMO spatial multiplexing scheme is converted into a MIMO diversity scheme to be used in an MC-CDMA system, there are still multiple different interferences. The present invention may be employed to reduce such interferences, by first obtaining a single unified spreading channel matrix R and received correlated signal Y; and second by using the BLAST-PPIC algorithm to determine the original information bits. The invention may also be used to reduce interference induced from the use of multiple antennas over a large number of different multiple antenna schemes employed in an MC-CDMA system, to thereby increase the spectrum efficiency of the system. 
   Having thus described the present invention by reference to certain of its preferred embodiments, it is noted that the embodiments disclosed are illustrative rather than limiting in nature and that a wide range of variations, modifications, changes, and substitutions are contemplated in the foregoing disclosure and, in some instances, some features of the present invention may be employed without a corresponding use of the other features. Many such variations and modifications may be considered obvious and desirable by those skilled in the art based upon a review of the foregoing description of preferred embodiments. Accordingly, it is appropriate that the appended claims be construed broadly and in a manner consistent with the scope of the invention.