Abstract:
An improved interleaver design to fully explore the diversity of the MIMO OFDM systems provides higher diversity gain than usual. A method for wireless data communication using such interleaver design implements parsing a bit stream into multiple spatial data streams, interleaving the bits in each spatial data stream by performing bit circulation and column swapping to increase diversity of the wireless system, and transmitting the bits of each spatial data stream.

Description:
FIELD OF THE INVENTION  
       [0001]     The present invention relates generally to data communication, and more particularly, to data communication with transmission diversity using Multiple Input Multiple Output (MIMO) Orthogonal Frequency Division Multiplexing (OFDM) in multiple antenna channels.  
       BACKGROUND OF THE INVENTION  
       [0002]     In wireless communication systems, antenna diversity plays an important role in increasing the system link robustness. OFDM is used as a modulation technique for transmitting digital data using radio frequency signals (RF). In OFDM, a radio signal is divided into multiple sub-signals that are transmitted simultaneously at different frequencies to a receiver. Each sub-signal travels within its own unique frequency range (sub-channel), which is modulated by the data. OFDM distributes the data over multiple channels, spaced apart at different frequencies.  
         [0003]     OFDM modulation is typically performed using a transform such as Fast Fourier Transform (FFT) process wherein bits of data are encoded in the frequency-domain onto sub-channels. As such, in the transmitter, an Inverse FFT (IFFT) is performed on the set of frequency channels to generate a time-domain OFDM symbol for transmission over a communication channel. The IFFT process converts the frequency-domain phase and amplitude data for each sub-channel into a block of time-domain samples which are converted to an analogue modulating signal for an RF modulator. In the receiver, the OFDM signals are processed by performing an FFT process on each symbol to convert the time-domain data into frequency-domain data, and the data is then decoded by examining the phase and amplitude of the sub-channels. Therefore, at the receiver the reverse process of the transmitter is implemented. Further, transmit antenna diversity schemes are used to improve the OFDM system reliability. Such transmit diversity schemes in OFDM systems are encoded in the frequency-domain as described.  
         [0004]     MIMO has been selected as the basis for the high speed wireless local area network (WLAN) standards by the IEEE standardization group.  FIG. 1  shows a MIMO system splits the data before convolutional encoding. The system in  FIG. 1  includes a OFDM MIMO transmitter  100  implementing WLAN, comprising a source of data bits  102 , a spatial parser  104 , and multiple data stream processing paths  106 . Each data stream processing path  106  comprises: a channel encoder &amp; puncturer  108 , a frequency interleaver  110 , a constellation mapper  112 , an IFFT function  114 , a guard-band insertion GI window  116  and an RF modulator  118 .  
         [0005]     The system diagram in  FIG. 1  represents a MIMO OFDM structure for 20 MHz channelization, and uses two independent convolutional-code encoders for the two data paths. Further, two IEEE 802.11a interleavers are used independently, each interleaver  110  corresponding to each encoder. An interleaver  110  in  FIG. 1  provides an optimal design for single antenna systems by fully exploring the frequency diversity. However, for multiple antenna systems, this design does not explore the spatial diversity brought in by the multiple antennas. Thus, there is a need for an interleaver design to fully explore the diversity of the MIMO OFDM systems.  
       BRIEF SUMMARY OF THE INVENTION  
       [0006]     The present invention provides an improved interleaver design to fully explore the diversity of the MIMO OFDM systems. An interleaver according to the present invention provides higher diversity gain than usual. Such an interleaver provides column swap and bit circulation for multiple forward error code encoder MIMO OFDM systems. Accordingly, in one embodiment the present invention provides a system and method for wireless data communication, implementing the steps of: parsing a bit stream into multiple spatial data streams; interleaving the bits in each spatial data stream by performing bit circulation to increase diversity of the wireless system; and transmitting the bits of each spatial data stream. The steps of interleaving the bits in each spatial data stream further include the steps of performing column swapping.  
         [0007]     In one example, the steps of interleaving the bits include the steps of splitting the bits in each data stream into multiple groups corresponding to subcarriers in a transmission symbol, performing a column swap operation on the subcarriers, circulating the bits among the groups, and combining the bits for the different data streams to form a new bit sequence for transmission.  
         [0008]     In another embodiment, the steps of interleaving the bits in each spatial data stream further includes the steps of performing column swapping within an interleaving array of that spatial data stream, to increase diversity of the wireless system. The steps of interleaving the bits can further include the steps of splitting the bits in each data stream into multiple groups corresponding to subcarriers in a transmission symbol, performing column swapping within an interleaving array of that spatial data stream, circulating the bits among the groups, and combining the bits for the different data streams to form a new bit sequence for transmission. The steps of interleaving the bits in each spatial data stream includes the steps of, before circulation, performing a first interleaving permutation for column swapping wherein the stream data bits are written in by row, read out by column.  
         [0009]     These and other features, aspects and advantages of the present invention will become understood with reference to the following description, appended claims and accompanying figures. 
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0010]      FIG. 1  shows a block diagram of a MIMO OFDM transmitter.  
         [0011]      FIG. 2  shows an example block diagram of an embodiment of a MIMO OFDM transmitter according to an embodiment of the present invention.  
         [0012]      FIG. 3  shows an example block diagram of details of interleaving in  FIG. 2  FIGS.  4 A-C show example simulation results in a 20 MHz channel using a transmitter according to  FIG. 2 .  
     
    
     DETAILED DESCRIPTION OF THE INVENTION  
       [0013]     In one embodiment, the present invention provides an improved method interleaving for a MIMO system that implements the IEEE WLAN standard. The interleaving method improves exploration of the diversity of an MIMO OFDM system, providing higher diversity gain than usual.  
         [0014]      FIG. 2  shows a block diagram of an example OFDM MIMO transmitter  200  of a MIMO system, wherein the transmitter  200  implements an embodiment of the improved interleaving method according to the present invention. The transmitter  200  comprises: a source of data bits  202 , a bitwise spatial parser  204 , and multiple data stream processing paths  206  (e.g., two paths for two antennas  203 ). Each data stream processing path  206  corresponds to a transmit antenna  203 , and comprises: a channel encoder &amp; puncturer  208 , a frequency interleaver  210 , a constellation mapper  212 , an IFFT function  214 , a guard-band insertion GI window  216  and an RF modulator  218 . Each data stream processing path  206  further includes a bit circulation function  211 , connected between the interleaver  210  and the constellation mapper  212 , described further below.  
         [0015]      FIG. 2  further shows a receiver  150  corresponding to the transmitter  200 , forming a MIMO system. The receiver  150  includes a bit de-circulation unit  151  that performs the reverse operation of bit circulation unit  211 , and deinterleavers  152  that perform the reverse operation of the interleavers  210  in the transmitter  200 .  
         [0016]     In this embodiment, the interleavers  210  provide column swap and the bit circulation unit  211  provides bit circulation for bits circulation/rotation among different spatial streams to incorporate the spatial diversity into one data stream.  
         [0017]      FIG. 3  shows an example block diagram of an embodiment of interleaving by column swap (i.e., column skip) and bit circulation using the interleaver  210  and the bit circulation unit  211 , respectively. In this embodiment, the interleaving method incorporates a column skip operation, as follows. In each interleaver  210 , in a first permutation  210   a , the bits are written in by row, read out by column. This includes a column skip operation. After the data bits are written in block, instead of reading out the bits from column  0   1   2   3  . . , the bits from columns  0 , k,  1 , k+1,  2 , k+2, . . . or k,  0 , k+1,  1 , k+2,  2 , . . . , are read out, where k is a number selected as the column-skip (i.e., columns swap operation  310   b ).  
         [0018]     In the following example, k is set to 8, which is the middle column of the block interleaver. On both transmit (Tx) data path streams the write-in input bit indices are:  
                                                                                               0   1   2   3   4   5   6   7   8   9   10   11   12   13   14   15       16   17   18   19   20   21   22   23   24   25   26   27   28   29   30   31       32   33   34   35   36   37   38   39   40   41   42   43   44   45   46   47                  
 
         [0019]     The read-out bits indices are:  
                                                                             0 16 32   8 24 40   1 17 33   9 25 42                7 23 39   15 31 47                      
 
         [0020]     In a second permutation  210   c , PAM (Pulse Amplitude Modulation) order rotation as described in IEEE 802.11a standard is performed. PAM is a one dimensional modulation with the change of amplitude. A QAM modulation can be viewed as two PAM modulations. One is in-phase (I), the other is quadrature(Q).  
         [0021]     The bit circulation unit  211  includes, for each data stream path  206 : a splitter  220 , a bit circulator  222 , and a combiner  224 . In the bit circulation unit  211  of  FIG. 3 , in each splitter  220  the output bits of the corresponding IEEE 802.11a interleaver  210  are split into two groups. One group (Group  1 ) corresponds to the bits in the odd index subcarriers in an OFDM symbol. The other group (Group  2 ) corresponds to the bits in the even index subcarriers in an OFDM symbol. For example, in a BPSK modulated OFDM system, each subcarrier carries 1 bit and the bit-splitting will look like the following:  
         [0022]     Group  1 : 1 3 5 7 9 . . . .47  
         [0023]     Group  2 : 2 4 6 8 10 . . . .48  
         [0024]     Further, in a 64 QAM modulated OFDM system, where each subcarrier carries 6 bits, the bit-splitting will look like the following:  
                                                                                     Group 1: 1 2 3 4 5 6; 13 14 15 16 17 18; . . .   277                278 279 280 281 282                Group 2: 7 8 9 10 11 12; 19 20 21 22 23 24; . . . 283 284                285 286 287 288                      
 
         [0025]     The bit circulator  222  for each data stream processing path  206  exchanges the bits in Group  2  for the first spatial stream with Group  1  for the second spatial stream. The combiner  224  for each data stream processing path  206  combines the bits for different spatial streams to form a new bit sequence for transmission. In another example, the bits in group  2  of both streams are exchanged as well.  
         [0026]     Simulation has been conducted to verify the performance of the interleaving method of  FIG. 3  for 20 MHz channelization. Simulation results verify the improved performance of a MIMO system implementing an interleaving method described above (e.g.,  FIGS. 2-3  for 20 MHz channelization). The coding and modulation set (MCS) for an example simulation is listed in Table 1 below. MCS14 uses 64 QAM, rate 3/4 convolutional code (133, 171). (IEEE 802.11 document # 11-04-0889-02-000n, “TGn Sync Proposal Technical Specification,” January 2005, incorporated herein by reference.)  
                           TABLE 1                       Symbol   Number of spatial streams   Modulation   Coding rate                   MCS14   2   64-QAM   3/4       MCS13   2   64-QAM   2/3       MCS11   2   16-QAM   1/2                  
 
         [0027]     FIGS.  4 A-C shows example simulation results. All simulation settings and parameters are the same as in Table 1 above.  
         [0028]     Specifically  FIG. 4A  shows an example of the performance improvement with column swap and bit circulation. The simulations were conducted under IEEE 802.11n Channel model B. MCS11/13/14 were simulated. The example curves  401   a ,  401   b  and  401   c  correspond to MCS11, MCS 13 and MCS14 simulations, respectively, and represent the Packet Error Rate(PER) vs. SNR performance with the column swap and bit circulation operation of the present invention. The curves  402   a ,  402   b , and  402   c  also correspond to MCS11, MCS 13 and MCS14 simulations, respectively, and represent the PER vs. SNR performance of the system without the column swap and bit circulation operation. The curves in  FIG. 4A  illustrate that for different MCS modes, the performance improvement according to an embodiment of the present invention ranges from 0.5 to 1 dB at PER level of 10 −2 .  
         [0029]      FIG. 4B  shows another example of the performance improvement with column swap and bit circulation. The simulations were conducted under IEEE 802.11n Channel model D. MCS11/13/14 were simulated. The example curves  403   a ,  403   b  and  403   c  correspond to MCS11, MCS 13 and MCS14 simulations, respectively, and represent the Packet Error Rate(PER) vs. SNR performance with the column swap and bit circulation operation of the present invention. The curves  404   a ,  404   b , and  404   c  also correspond to MCS11, MCS 13 and MCS14 simulations, respectively, and represent the PER vs. SNR performance of the system without the column swap and bit circulation operation. The curves in  FIG. 4B  illustrate that for different MCS modes, the performance improvement according to an embodiment of the present invention ranges from 0.5 to 1 dB at PER level of 10 −2 .  
         [0030]      FIG. 4C  shows another example of the performance improvement with column swap and bit circulation. The simulations were conducted under IEEE 802.11n Channel model E. MCS11/13/14 were simulated. The example curves  405   a ,  405   b  and  405   c  correspond to MCS11, MCS 13 and MCS14 simulations, respectively, and represent the Packet Error Rate(PER) vs. SNR performance with the column swap and bit circulation operation of the present invention. The curves  406   a ,  406   b , and  406   c  also correspond to MCS11, MCS 13 and MCS14 simulations, respectively, and represent the PER vs. SNR performance of the system without the column swap and bit circulation operation. The curves in  FIG. 4C  illustrate that for different MCS modes, the performance improvement according to an embodiment of the present invention ranges from 0.5 to 1 dB at PER level of 10 −2 .  
         [0031]     The above example interleaving implementations according to the present invention provide e.g. about 0.5 to 1 dB gain over usual interleaving methods. Although the description herein is based on two data streams in a two-antenna system, as those skilled in the art will recognize, the present invention is not limited to a specific number of transmission data streams and transmission antennas. With N transmission data streams, each stream can be split into N sub-streams for bit circulation. The optimal flip method would depend on N, but using the same principle as described in the examples above. The optimal swap number also depends on N, but using the same principle as described in the examples above.  
         [0032]     The present invention has been described in considerable detail with reference to certain preferred versions thereof; however, other versions are possible. Therefore, the spirit and scope of the appended claims should not be limited to the description of the preferred versions contained herein.