Abstract:
A MIMO communication system implements an interleaver design with multiple encoders for more than two transmit antennas for high throughput WLAN communication systems. Multiple encoders are utilized in the transmitter and multiple decoders are utilized in the receiver, wherein each encoder operates at lower clock speed than would be necessary with a single encoder. In conjunction with using multiple encoders, a modified interleaving function for each spatial stream processing allows fully exploring the diversity gains. The provided interleaving function is suitable for transmitter architectures with multiple encoders. Similarly, a modified de-interleaving function is provided that is suitable for receiver architectures with multiple decoders.

Description:
FIELD OF THE INVENTION  
       [0001]     The present invention relates generally to data communication, and more particularly, to data communication with transmission diversity using 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 by an encoder 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 a decoder 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]     With the increase in transmission rates, higher operational speeds in the encoder and decoder are resulting in difficulties in implementing such channel encoders/decoders. The transmitter architecture with only one encoder is adopted in current IEEE 802.11n (high throughput WLAN) proposals.  FIG. 1  shows one of the examples for such designs implemented in a transmitter  100 . The transmitter  100  includes an FEC encoder  102 , a puncture unit  104 , a spatial stream parser  106 , and multiple stream processing paths. Each stream processing path performs the functions of: frequency interleaving  108 , QAM mapping  110 , antenna mapping  112 , IFFT operation  114 , inserting guard interval (GI)  116 , analog RF modulation  118  and antenna  120 . In the transmitter  100 , a data stream is first encoded using the FEC encoder before it is split into multiple spatial streams by the spatial stream parser. Multiple interleaving functions with different frequency rotation values are applied after the spatial parsing. As there is only one encoder in the coding chain, there is only one decoder in the decoding chain at a receiver (not shown).  
         [0005]     The interleaver design in the high throughput wireless local area network (WLAN) systems is an important issue for MIMO-OFDM transmission. In the current approaches in IEEE 802.11n standards (S.A. Mujtaba, “TGn Sync Proposal Technical Specification,” a contribution to IEEE 802.11, 11-04/0889r4, March 2005, and C. Kose and B. Edwards, “WWiSE Proposal: High throughput extension to the 802.11 Standard,” a contribution to IEEE 802.11, 11-05-0149r2, March 2005, incorporated herein by reference), the transmitter architecture with single channel encoder is considered regardless of the number of the transmit antennas. When the data rate or the number of the data streams increases, the encoder/decoder must operate with very high speed, causing circuit design and timing implementation difficulties.  
       BRIEF SUMMARY OF THE INVENTION  
       [0006]     According to an embodiment of the present invention a solution to overcoming the aforementioned implementation difficulties for high rate transmissions is to utilize multiple encoders in the transmitter, and thus multiple decoders at the receiver, wherein each encoder operates at lower clock speed than would be necessary with a single encoder. In conjunction with using multiple encoders, a modified interleaving function for each spatial stream processing allows fully exploring the diversity gains. The provided interleaving function is suitable for transmitter architectures with multiple encoders. Similarly, a modified de-interleaving function is provided that is suitable for receiver architectures with multiple decoders.  
         [0007]     Accordingly, in one implementation, the present invention provides a method of data communication in a wireless system, comprising the steps of: parsing a bit stream into multiple spatial data streams; encoding the multiple spatial data streams via multiple encoders, wherein each spatial data stream is encoded by a corresponding encoder to generate an encoded stream; interleaving the bits in all encoded streams by performing bit circulation to increase diversity of the wireless system; and transmitting the bits of each spatial data stream.  
         [0008]     An additional step includes further parsing each encoded stream into a plurality of encoded spatial data streams, wherein the steps of interleaving further includes the steps of interleaving the bits in all encoded spatial data streams by performing bit circulation to increase diversity of the wireless system.  
         [0009]     The present invention further includes a wireless communication system that implements the method of the present invention.  
         [0010]     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  
       [0011]      FIG. 1  shows a block diagram of a conventional wireless transmitter architecture with a single encoder.  
         [0012]      FIG. 2  shows a functional block diagram of a wireless transmitter implementing interleaving architecture according to an embodiment of the present invention.  
         [0013]      FIG. 3  shows a functional block diagram of an example implementation of the frequency interleaving architecture of  FIG. 2  with multiple output streams.  
         [0014]      FIG. 4  shows a functional block diagram of an example implementation of the bit circulation block for a stream in each frequency interleaving output of  FIG. 3 . 
     
    
     DETAILED DESCRIPTION OF THE INVENTION  
       [0015]     According to an embodiment of the present invention a solution to overcoming the aforementioned implementation difficulties for high rate transmissions is to utilize multiple encoders in the transmitter, and thus multiple decoders at the receiver, wherein each encoder operates at lower clock speed. In conjunction with using multiple encoders, the interleaving function for each spatial stream processing path is modified according to the present invention to fully explore the diversity gains. The present invention provides modified interleaving functions suitable for transmitter architectures with multiple encoders. Similarly, the present invention provides modified de-interleaving functions suitable for receiver architectures with multiple decoders.  
         [0016]     An implementation of a modified interleaving function according to the present invention includes two stages of operations: frequency interleaving, and bit circulation.  FIG. 2  shows a functional block diagram of a communication system  130  including a transmitter  200  and a receiver  150 . The transmitter  200  with multiple encoders  204 , which implements a modified interleaving function according to an embodiment of the present invention. The transmitter  200  comprises a source of data bit stream  202 , a parser  204  (e.g., a Parser 1  unit), channel encoders/puncturers  206 , parsers  208  (e.g., multiple Parser 2  units), frequency interleavers  210 , a bit circulation unit  212 , constellation mappers  214 , antenna mapping Q  215 , inverse Fast Fourier Transform (iFFT) units  216 , GI insertion units  218 , RF modulators  220  and antennas  222 . Commonly assigned patent application Ser. No. 11/253,855, filed Oct. 18, 2005, entitled “A method of designing interleavers for multiple-encoder MIMO OFDM systems”, provides an example of interleaver for multiple encoders, and commonly assigned patent application docket SAM2B.PAU.25, entitled “An Interleaver Design With Column Swap And Bit Circulation For Multiple Convolutional Encoder MIMO OFDM System”, provides an example of bit circulation, both of which patent applications are incorporated herein by reference.  
         [0017]     The receiver  150  corresponds 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  212 , and deinterleavers  152  that perform the reverse operation of the interleavers  210  in the transmitter  200 .  
         [0018]     In the transmitter  200 , there are N ss  data streams, wherein  
           N   ss     =       ∑     i   =   1       N   e       ⁢     M   i         ,       
 
 where N e  is the number of encoders  206  and M i  is the number of output streams for the i th  encoder. As shown in  FIG. 2 , the information bits are first parsed into N e  streams by the parser  204 , wherein channel encoding applied to each of the N e  bit streams by a corresponding encoder  204 . Each coded and punctured bit stream is then further parsed by a respective parser  208  (there are N e  number of parsers  208 , and M i  number of streams are output from each parser  208 ). The parsed stream from the parsers  208  are then processed in corresponding M i  number of frequency interleavers  210 . The outputs of the frequency interleavers  210  are then processed in the bit circulation unit  212 . 
 
         [0019]      FIG. 3  shows a functional block diagram of an example implementation of the frequency interleaver  210  in conjunction with the parser  208  (e.g., Parser  2 ). The frequency interleaver  210  comprises first permutation functions  302  and second permutation functions  304 . For the i th  encoder output, there are M i  output streams after the interleaver operations. The frequency interleaver  210  further includes frequency rotation units  306 .  
         [0020]     Three stages of operations are involved with interleaver operation: (1) first permutation operations by first permutation functions  302 , (2) second permutation operations by second permutation functions  304 , and (3) frequency rotations by frequency rotation units  306 . In order to fully explore the diversity gains when multiple encoders (e.g., encoders  206 ,  FIG. 1 ) are present, bit circulation operation is followed after the interleaving functions.  
         [0021]     For each spatial stream processed within the interleaver  210 , the first two stages of interleaving (i.e., first and second permutation operations) are identical among the different spatial streams, which in this example are the same as in a conventional IEEE 802.11a interleaving. Alternatively, instead of the IEEE 802.11a interleaving, other interleaving examples are possible and anticipated by the present invention.  
         [0022]     However, for the third operation (i.e., frequency rotations), the amount of frequency rotation by the frequency rotation units  306  varies among the spatial streams. In general, the frequency rotation amount can be a variable in each spatial stream, although fixed rotation amount may also be used.  
         [0023]     The bit circulation operations can be an extension of an interleaver design in said commonly assigned patent application Ser. No. 11/253,855, filed Oct. 18, 2005, entitled “A method of designing interleavers for multiple-encoder MIMO OFDM systems” (incorporated herein by reference). An example implementation of the bit circulation according to the present invention is described below.  
         [0024]      FIG. 4  shows example bit circulation architecture  400  for multiple encoders, comprising two stages of operations: splitting units  402 , and combining unit  404 . For example, the i th  output stream from each encoder is first split into N e  sub-streams by splitting units  402  in bit-by-bit fashion, and the j th  sub-streams from all splitting units  402  are then combined by the combining units  404  to form the j th  (out of the N e ) output stream of the bit circulation function  400 .  
         [0025]     Alternative frequency rotations may be used for frequency interleaver design (an example frequency rotation is described in S. A. Mujtaba, “TGn Sync Proposal Technical Specification,” a contribution to IEEE 802.11, 11-04/0889r4, March 2005, incorporated herein by reference). Other bit circulation rules can also be applied. Although a certain bit may end up in an antenna different from its original one before bit circulation, similar system performance can be obtained as long as a certain bit is on the same position of the data groups before a splitting unit  402  and after a combining unit  404 .  
         [0026]     As such, the bit circulator  400  for each processing path comrises a splitter  402  and a combiner  404 . In the example shown in  FIG. 4 , it is assumeed that M i  are equal for all i=1, . . . , N e . The splitter  402  splits the output bits of the corresponding frequeny interleaver  210  into N e  sub-streams. The combiner  404  combines the bits from the corresponding frequency interleaver  210  in each encoder to form a new bit sequence for transmission. For example, as shown in  FIG. 4 , output of first combiner  404  for ith stream is the first output of the splitter in stream i of encoder  1  combined with the second output of the splitter in stream i of encoder  2 , and so on.  
         [0027]     In case of non-equal Mi, the splitter can split the output bits of the corresponding frequency interleaver into N ss  groups. The combiner combines the bits from the corresponding frequency interleaver in each encoder to form a new bit sequence for transmission, in a similar manner as the step described above.  
         [0028]     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.