Patent Publication Number: US-2006013330-A1

Title: MIMO transmission system and method of OFDM-based wireless LAN systems

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS  
      This application claims benefit under 35 U.S.C. § 119 from Korean Patent Application No. 2004-55623, filed on Jul. 16, 2004, the entire content of which is incorporated herein by reference.  
     BACKGROUND OF THE INVENTION  
      1. Field of the Invention  
      The present general inventive concept relates to a wireless local area network (WLAN) system, and more particularly, to a multiple input multiple output (MIMO) transmission system and method of an orthogonal frequency division multiplexing (OFDM)-based wireless LAN system to enhance a diversity effect of the MIMO transmission system of the OFDM-based wireless LAN system under flat-fading environments.  
      2. Description of the Related Art  
      The existing standard IEEE 802.11 for a wireless LAN has supported a transmission rate of 2 Mbps in industrial, scientific medical (ISM) bands of 2.4 GHz using Direct Sequence Spread Spectrum (DSSS), Frequency Hopping Spread Spectrum (FHSS), or Infrared (IR) technique. However, the standard IEEE 802.11 can not satisfy increasing demands of high transmission rates, so substandards IEEE 802.11a and IEEE 802.11b for new physical layers were established.  
      The substandard IEEE 802.11a has chosen an orthogonal frequency division multiplexing (OFDM) transmission system using an OFDM modulation method in order to overcome the limitation of the DSSS system in a 5 GHz unlicensed band of Unlicensed National Information Infrastructure (U-NII) and obtain a higher transmission rate. Error corrections are carried out by convolution encoders having encoding rates of ½, ⅔, and ¾ and a ½ Viterbi decoder, and sub-carrier modulations are carried out through Binary Phase Shift Keying (BPSK), Quadrature Phase Shift Keying (QPSK), Quadrature Amplitude modulation (QAM), or 64-QAM scheme.  
      Thus, the substandard IEEE 802.11a supports a high-speed variable transmission rate from 6 Mbps to 54 Mbps by combining encoders and modulators depending on channel states. Further, the substandard IEEE 802. 11a has an advantage of a simple structure of 52 sub-carriers, a short training period of time and a simple equalization by use of the OFDM transmission system, and robustness to interferences among multiple paths, since its goal is Ethernet-based services in room environments.  
      Further, the existing wireless communication systems are for high-quality and large-volume multimedia data transmissions over limited frequencies, and increase a transmission data rate with a MIMO transmission system using plural antennas in order to send a large volume of data over the limited frequencies.  
      An inter symbol interference (ISI) and a frequency selectively fading in the MIMO transmission system can be almost completely removed when the OFDM-based system is used together with the MIMO transmission system.  
      A conventional MIMO OFDM-based wireless LAN transmission system converts a signal encoded through convolutional encoders, puncturers, and interleavers into a modulation signal of BPSK, QPSK, QAM, and so on, according to a symbol mapper, and inserts guard intervals during modulations by applying an inverse fast Fourier transform. IN addition, the conventional MIMO OFDM-based wireless LAN transmission system converts a modulated digital signal into an analog signal and sends the converted analog signal through an antenna.  
      A problem occurs when a channel&#39;s root mean square (RMS) delay spread is likely to have a value less than about 100 nS, which indicates a standard deviation or the root mean square of a delay of reflection waves, since room environments in which the wireless LAN is primarily placed provide narrow frequency bands compared to outdoor environments. In the wireless LAN system using a band of about 20 MHz, the channels have flat-fading frequency characteristics regardless of frequencies due to the RMS delay spread, and all sub-carriers are affected by the similar fading.  
      Such fading environments degrade a signal-receiving function due to no effect of the diversity of the channel encoding, and such a problem occurs in the same way as in the MIMO transmission system using plural antennas.  
     SUMMARY OF THE INVENTION  
      In order to solve the foregoing and/or other problems, the present general inventive concept provides a MIMO OFDM-based wireless LAN system and method to enhance an effect of diversity of transmission systems thereof under flat fading environments.  
      Additional aspects and advantages of the present general inventive concept will be set forth in part in the description which follows and, in part, will be obvious from the description, or may be learned by practice of the general inventive concept.  
      The foregoing and/or other aspects and advantages of the present general inventive concept may be achieved by providing a MIMO transmission system of an OFDM-based wireless LAN system, the MIMO transmission comprising a de-multiplexer to split an input bit stream into at least one bit stream to distribute the input bit stream to at least one path, at least one channel encoder to encode the bit stream outputted from the de-multiplexer, at least one puncturer to control an encoding rate of the channel encoder, a spatial bit exchanger to switch bits of the bit stream to arrange the bits of the bit stream output from the puncturer to the path, at least one interleaver to distribute errors that can occur in a transmission channel with respect to the switched bit stream outputted from the spatial bit exchanger, at least one symbol mapper to modulate the bit stream outputted from the interleaver for subcarriers, at least one IFFT/GI insertion unit to apply an inverse fast Fourier transform to the bit stream modulated by the symbol mapper, and inserting a guard interval, at least one DAC/RF unit to convert a digital signal of the IFFT/GI insertion unit into an analog signal for wireless transmissions, and at least one antenna for at least one channel which is connected to the DAC/RF unit.  
      The spatial bit exchanger may includes a switching unit to switch to different paths individual bits of the bit stream from the puncturer, and a routing control unit to control sequential switching operations of the switching unit according to a pre-set algorithm in order for the bit stream of one path to be sequentially sent to the different paths. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
      These and/or other aspects and advantages of the present general inventive concept will become apparent and more readily appreciated from the following description of the embodiments, taken in conjunction with the accompanying drawings of which:  
       FIG. 1  is a block diagram showing a transmission system of an OFDM-based wireless LAN system according to an embodiment of the present general inventive concept;  
       FIG. 2  is a block diagram showing a spatial bit exchanger of the transmission system of  FIG. 1 ;  
       FIG. 3  is a view showing operations of the spatial bit exchanger of  FIG. 2  according to an embodiment of the present general inventive concept; and  
       FIG. 4  is a view showing bit stream exchanges of the spatial bit exchanger of  FIG. 2 . 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS  
      Reference will now be made in detail to the embodiments of the present general inventive concept, examples of which are illustrated in the accompanying drawings, wherein like reference numerals refer to the like elements throughout. The embodiments are described below in order to explain the present general inventive concept while referring to the figures.  
       FIG. 1  is a block diagram showing a transmission system  100  for an OFDM-based wireless LAN system according to an embodiment of the present general inventive concept.  
      In  FIG. 1 , the transmission system  100  includes a de-multiplexer  101 , channel encoders  103   a  to  103   n,  puncturers  105   a  to  105   n,  a spatial bit exchanger  107 , interleavers  109   a  to  109   n,  symbol mappers  111   a  to  111   n,  IFFT/GI insertion units  113   a  to  113   n,  DAC/RF units  115   a  to  115   n,  and antennas  117   a  to  117   n.    
      The individual components of the transmission system  100  carry out a process of channel encoding and modulations for wireless transmission of input bit streams. The transmission system  100  can be a multiple-input/multiple-output (MIMO) transmission system having multiple transmission paths through multiple antennas  117   a  to  117   n.    
      The input bit stream inputted to the MIMO transmission system  100  are split into plural bit streams through the de-multiplexer  101 , and each of the split bit streams is encoded, OFDM-modulated, and transmitted through a corresponding one of the multiple transmission paths, wherein the OFDM stands for ‘Orthogonal Frequency Division Multiplexing.’ The split bit stream may be a unit bit stream to be transmitted through a corresponding channel to a corresponding one of the antennas  117   a  to  117   n.    
      Further, when the unit bit streams split in the de-multiplexer  101  are transmitted to the same transmission antenna, all bits of the unit bit streams are affected by the same fading due to flat-fading environments, so the spatial bit exchanger  107  is used to sequentially exchange the antennas  117   a  to  117   n  to transmit the bits of the unit bit streams through the sequentially exchanged antennas  117   a  to  117   n.  Thus, the bits are transmitted to different transmission antennas  117   a  to  117   n,  and their fading characteristics become different, so that diversity gains can be obtained.  
      The de-multiplexer  101  splits an inputted single bit stream into plural bit streams to be distributed through multiple paths to the individual antennas  117   a  to  117   n.    
      The channel encoders  103   a  to  103   n  each implement channel encoding using a convolutional code or the like.  
      The puncturers  105   a  to  105   n  control an encoding rate of the channel encoders  103   a  to  103   n.  The puncturers  105   a  to  105   n  delete predetermined selected bits to reduce coding overhead of the channel encoders  103   a  to  103   n.    
      The interleavers  109   a  to  109   n  distribute for error corrections a localized error burst that may occur in transmission channels.  
      The symbol mappers  111   a  to  111   n  apply BPSK modulation, QPSK modulation, QAM modulation, or the like to the unit bit streams to be transmitted to the respective OFDM subcarriers.  
      The IFFT/GI insertion units  113   a  to  113   n  implement the OFDM modulation by applying the inverse fast Fourier transform (IFFT) to digital signals modulated by the symbol mappers  111   a  to  111   n  to transform the digital signals into time-domain data, and insert a guard interval (GI) into each frame of the time-domain data to minimize an inter-symbol interference.  
      The DAC/RF (digital-analog converter/radio frequency) units  115   a  to  115   n  convert the OFDM-modulated digital signals into analog signals such that the analog signals are transmitted through the antennas  117   a  to  117   n  for the individual channels. The analog signals may be an RF signal.  
      The spatial bit exchanger  107  exchanges transmission paths before inputting to the interleavers  109   a  to  109   n  the unit bit streams passing through the channel encoders  103   a  to  103   n  and puncturers  105   a  to  105   n  in the corresponding signal processing paths of the respective transmission antennas  117   a  to  117   n.    
      The spatial bit exchanger  107  exchanges the input bits among the antennas  117   a  to  117   n  in the spatial dimension so that one input bit stream is not transmitted to a single antenna but to the plural antennas  117   a  to  117   n.    
       FIG. 2  is a block diagram showing the spatial bit exchanger  107  of  FIG. 1 .  
      In  FIG. 2 , the spatial bit exchanger  107  includes a switching unit  201  and a routing control unit  203 .  
      The routing control unit  203  receives n rows of bit streams from n puncturers  105   a  to  105   n,  and controls the switching unit  201  to route the bit streams to different paths. The routing control unit  203  routes to different paths the bits received from individual paths at each point of time according a pre-set algorithm, e.g., switching algorithm.  
      The switching algorithm of the routing control unit  203  includes a variety of methods of distributing and outputting to plural paths the bits of the bit streams input through a path.  
      The switching unit  201  switches to the different paths the bit streams of the individual paths inputted according to controls of the routing control unit  203 .  
       FIG. 3  is a view explaining operations of the spatial bit exchanger  107  of  FIG. 2 .  
      The spatial bit exchanger  107  shown in  FIG. 3  is based on the MIMO transmission system  100  having three paths connected to three antennas, for example. The spatial bit exchanger  107  inputs three rows of bit streams, e.g., first, second, and third streams, to the switching unit  201 .  
      The routing control unit  203  controls the bits of the first stream to be transmitted to the path for the second antenna  117   b,  the bits of the second bit stream to the path for the third antenna  117   c,  and the bits of the third bit stream to the path for the first antenna  117   a,  and controls the bits thereafter to be transmitted to a different path in another different method.  
       FIG. 4  is a view explaining the bit stream exchange of the spatial bit exchanger  107  of  FIGS. 1 and 3 . Referring  FIGS. 3 and 4 , a reference numeral “a” indicates three rows of bit streams inputted to the spatial bit exchanger  107 , and a reference numeral “b” indicates bit streams output through the spatial bit exchanger  107 .  
      The spatial bit exchanger  107  switches the bits of input bit streams in order for the bits of the respective input bit streams to be arranged and sent to the three antennas  117   a,    117   b,  and  117   c.  That is, the bits of the first bit streams A 0 , A 1 , and A 2  are respectively transmitted to the first antenna  117   a,  the second antenna  117   b,  and the third antenna  117   c,  and the bits thereafter are arranged and sent to the antennas  117   a,    117   b,  and  117   c  in the same method.  
      Accordingly, since the bits of one bit stream are transmitted to the different antennas  117   a  to  117   n,  the fading characteristics become different so that diversity gains can be obtained.  
      The transmission system can carry out a MIMO transmission mode for the OFDM-based wireless LAN system.  
      As described above, the MIMO OFDM-based wireless LAN system according to the present general inventive concept can enhance the diversity gains by the channel encoding in the flat-fading environments having a small RMS delay spread through the bit exchanges among the transmission antennas.  
      Although a few embodiments of the present general inventive concept have been shown and described, it will be appreciated by those skilled in the art that changes may be made in these embodiments without departing from the principles and spirit of the general inventive concept, the scope of which is defined in the appended claims and their equivalents.