Abstract:
Methods and systems are disclosed for space interleaved communication of data in a multiple antenna communication system. Data is interleaved over space by interleaving the data over a plurality of transmit branches in the multiple antenna system. The disclosed spatial interleaving increases the performance gains resulting from the spatial transmit diversity of the multiple antenna communication system. Spatial interleaving can be performed, for example, over the bits before the bits are interleaved over frequency. In another variation, spatial interleaving is performed over the symbols after the bits are mapped to symbols.

Description:
FIELD OF THE INVENTION  
       [0001]     The present invention relates generally to multiple antenna communication systems, and more particularly, to methods and apparatus for interleaving data in a Multiple Input Multiple Output Orthogonal Frequency Division Multiplexing (MIMO-OFDM) system.  
       BACKGROUND OF THE INVENTION  
       [0002]     A MIMO-OFDM system increases the data bandwidth of a traditional OFDM system by transmitting separate data streams on multiple transmit antennas. Each receiver in the system receives a combination of these data streams, thereby increasing the achievable throughput. A well known MIMO-OFDM transmit scheme is called the joint encoding (JC) MIMO-OFDM transmit scheme, since the encoder jointly encodes data over all the transmit branches. In other words, the encoding is performed before the data is split over the multiple transmit branches. For a discussion of an exemplary joint encoding MIMO-OFDM transmission scheme, see, for example, Xiaodong Li et al., “Effect of Iterative Detection and Decoding on the Performance of BLAST,” Proc. of the IEEE Global Telecommunications Conference (GLOBECOM) 2000, vol. 2, 1061-1066 (2000). In the disclosed joint encoding architecture, the incoming binary data is scrambled in order to randomize the binary data, and the scrambled binary data is encoded. The encoded data is then demultiplexed over the multiple transmit branches. The joint encoding MIMO-OFDM transmission scheme requires the encoder and decoder to operate at a multiple of the speed of a traditional IEEE 11a/g system.  
         [0003]     In another exemplary MIMO-OFDM system, separate encoders are placed in each transmit branch after the data is demultiplexed. This technique is referred to as per antenna coding (PAC). See, A. van Zelst, “Per-Antenna-Coded Schemes for MIMO OFDM”, Proc. of the IEEE International Conference on Communications (ICC) 2003, vol. 4, 2832-2836 (May, 2003), incorporated by reference herein. Per antenna coding does not require the encoder and decoder to operate at a multiple of the speed of a traditional IEEE 11a/g system. Rather, multiple single speed decoders can be used. In addition, the error correcting capability of the encoder and decoder do not need to be shared across multiple transmit branches and, therefore, the performance of the error correction does not decrease with an increasing number of transmit branches.  
         [0004]     When convolutional encoding is performed, the MIMO-OFDM system can be scaled back to a simple single transmit branch system (compliant with the IEEE 11a/g standard), without the need for a different encoder and, more importantly, without the need for a different decoder at the receiver. An advantage of joint convolutional encoding is that the code uses spatial diversity (achieved when multiple transmit branches are used) in an optimal way. While per antenna coding techniques sustain the error correcting capabilities per branch and inherently improve the performance of multiple antenna communication systems with respect to the joint coding, a number of performance issues remain, related to spatial diversity. A need therefore exists for methods and systems that increase the performance gains resulting from the spatial transmit diversity of the MIMO-OFDM system.  
       SUMMARY OF THE INVENTION  
       [0005]     Generally, methods and systems are disclosed for space interleaved communication of data in a multiple antenna communication system. Data is interleaved over space by interleaving the data, such as bits or symbols, over a plurality of transmit branches in the multiple antenna system. The disclosed spatial interleaving increases the performance gains resulting from the spatial transmit diversity of the multiple antenna communication system. In one exemplary embodiment, spatial interleaving is performed over the bits before the bits are interleaved over frequency. In another exemplary embodiment, spatial interleaving is performed over the symbols after the bits are mapped to symbols.  
         [0006]     A more complete understanding of the present invention, as well as further features and advantages of the present invention, will be obtained by reference to the following detailed description and drawings. 
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0007]      FIG. 1  is a schematic block diagram of an exemplary conventional MIMO-OFDM system;  
         [0008]      FIG. 2  is a schematic block diagram of an exemplary conventional joint coding MIMO-OFDM transmit system;  
         [0009]      FIG. 3  is a schematic block diagram of an exemplary MIMO-OFDM transmit system incorporating the features of the present invention for providing spatial interleaving over bits;  
         [0010]      FIG. 4  is a schematic block diagram of an alternative embodiment of the present invention for providing spatial interleaving over symbols; and  
         [0011]      FIGS. 5A and 5B  illustrate a suitable exemplary technique for performing spatial interleaving. 
     
    
     DETAILED DESCRIPTION  
       [0012]      FIG. 1  is a schematic block diagram of an exemplary conventional MIMO-OFDM system  100 . The system comprises a number of transmitters  110 , transmit antennas  120 , receive antennas  130 , and receivers  140 . Source signals S 1  to S Nt  are encoded, modulated, and transmitted by the associated transmitter  110  over the channel matrix  160 . Each receiver  130  is capable of receiving a combination of the transmitted data streams on the receive antennas  130 , thereby increasing the achievable throughput.  
         [0013]      FIG. 2  is a schematic block diagram of an exemplary conventional joint coding MIMO-OFDM transmit system  200 . The system  200  features a single convolution encoder  210  operating at a speed equal to the sum of the data rates of the transmit branches  290 . The scrambler  205  scrambles the source data S x  to create a random data pattern. The convolutional encoder  210  performs a Viterbi encoding compliant with the IEEE 802.11 a/g standard. For a detailed description of convolutional encoding, see, for example, Shu Lin, Daniel J. Costello Jr, “Error Control Coding: Fundamentals and Applications.” The data rate at the output of the encoder is twice the rate of the input signal to the encoder  210 . If a data rate different from that of the encoder  210  is desired, the encoded data may be optionally punctured (block  215 ) to null (eliminate) redundant bits in the data stream. While eliminating the redundant bits creates errors in the data stream, the errors can be corrected by the receiver  140 .  
         [0014]     The punctured data is demultiplexed (serial-to-parallel converter  220 ) to create the individual data streams  225  associated with each transmit branch  290 . Each data stream  225  is then interleaved over frequency (block  230 ) to randomize the data. The interleaved data is then mapped to QAM symbols by mappers  235 , and transformed to the time-domain by IFFT  240 . A cyclic extender  245  then appends a cyclic extension to provide for multipath robustness at the receiver  140 . Preambles are also periodically inserted into each data stream (block  250 ) to provide for synchronization of the data at the receiver  140 , and transmitted via transmit antenna  255 . Each receiver  140  is architecturally the inverse of the transmitter system  200 .  
         [0015]      FIG. 3  is a schematic block diagram of an exemplary MIMO-OFDM transmit system  300  incorporating the features of the present invention for providing spatial interleaving over bits. In  FIG. 3 , blocks  305  through  355  perform similar functions to the corresponding blocks in  FIG. 2 . In this embodiment, however, the demultiplexing (serial-to-parallel conversion)  320  is performed before the convolutional encoding  310 . A separate convolution encoder  310  and puncturer  315  are therefore dedicated to each transmit branch  390 . Since this reduces the performance requirement of the encoder  310  and decoder (not shown), the implementation of both devices is simplified. In addition, the error correcting capability of the code does not need to be shared across multiple transmit branches  390  and, therefore, the performance of the error correcting code will not decrease as the number of transmit branches  390  increases. In addition, the MIMO-OFDM system  300  of  FIG. 3  still has the advantage of being able to be easily scaled back to a simple single transmit branch system compliant with the IEEE 802.11 a/g standard.  
         [0016]     In the MIMO-OFDM system  300  of  FIG. 3 , an interleaving over space  360  is performed prior to the interleaving over frequency  330 . In this case, the interleaving over space  360  is performed over bits in a round robin fashion, as will be described in more detail in conjunction with  FIG. 5 . The performance of interleaving over bits is superior to interleaving over symbols, although both interleaving over space techniques provide improved performance in comparison to a system without such interleaving.  
         [0017]      FIG. 4  is a schematic block diagram of an alternative embodiment of the MIMO-OFDM transmit system  300  of  FIG. 3  incorporating the features of the present invention for providing spatial interleaving over symbols. In  FIG. 4 , blocks  405  through  460  perform similar functions to the corresponding blocks in  FIGS. 2 and 3 . In this alternative embodiment, however, the interleaver over space  460  is placed after the interleaver over frequency  430  in the transmit branch  490 . The interleaving over space  460  is performed over symbols in order to prevent any negative impact on the results of the interleaving over frequency  430 . The interleaving over space  460  is also performed in a round robin fashion, as will be described in more detail in conjunction with  FIG. 5 , although other techniques are possible provided there is no negative impact on the interleaving over frequency  430 .  
         [0018]     It should be noted that, in the transmit systems  300 ,  400 , the scrambler  305 ,  405  was placed before the demultiplexer  320 ,  420  in the transmit branches  390 ,  490 . The scrambler  305 ,  405  may, alternatively, be placed before or after the demultiplexer  320 ,  420  in every transmit branch  390 ,  490  separately (depending on the design of the scrambler  305 ,  405 ). In this case, however, each transmit branch  390 ,  490  would require a scrambler intialization sequence.  
         [0019]      FIGS. 5A and 5B  illustrate a suitable exemplary technique  500  for performing spatial interleaving over three transmit antennas.  FIG. 5A  represents the bits or symbols prior to interleaving and  FIG. 5B  represents the bits or symbols after interleaving over space. In  FIGS. 5A and 5B , each chain of bits or symbols  520 ,  525 ,  530  represents the data associated with a transmit branch  390 ,  490 . During the first interleaving operation  570 , the bit or symbol from transmit branch A  520  is transferred to transmit branch B  525 , the bit or symbol from transmit branch B  525  is transferred to transmit branch C  530 , and the bit or symbol from transmit branch C  530  is transferred to transmit branch A  520 . During the second interleaving operation  580 , the bit or symbol from transmit branch A  520  is transferred to transmit branch C  530 , the bit or symbol from transmit branch C  530  is transferred to transmit branch B  525 , and the bit or symbol from transmit branch B  525  is transferred to transmit branch A  520 . The results of these operations  570 ,  580  are shown in  FIG. 5B . In other embodiments of the interleaver over space  360 ,  460 , the interleaving operation  570 ,  580  may be performed over every bit or symbol, or over every nth bit or symbol. In one aspect of the invention, during the spatial interleaving operation, a chain of bits or symbols associated with a frequency band of one transmit branch are moved to the same frequency band of a different transmit branch. For instance, during the spatial interleaving operation, the chain of bits or symbols being moved from branch A  520  to branch B  525  are associated with the same frequency band in branch A  520  and branch B  525 .  
         [0020]     It is to be understood that the embodiments and variations shown and described herein are merely illustrative of the principles of this invention and that various modifications may be implemented by those skilled in the art without departing from the scope and spirit of the invention.