Abstract:
Disclosed is a method for increasing a data transfer rate without an increase in the whole bandwidth using intrinsic spreading codes and orthogonal codes. The method uses interleaving, OFDM modulation/demodulation, and maximum likelihood detection (MLD) to overcome the effects of multipath fading or signal interference, determines grouped optimal values by a grouping method of dividing the intrinsic spreading codes in series, and calculates an integrated optimal value for all the intrinsic spreading codes using the grouped optimal values, thereby reducing the complexity of MLD according to the length of the intrinsic spreading code and acquiring an improved performance.

Description:
CROSS REFERENCE TO RELATED APPLICATION  
         [0001]    This application is based on Korea Patent Application No. 2002-83746 filed on Dec. 24, 2002 in the Korean Intellectual Property Office, the content of which is incorporated herein by reference.  
         BACKGROUND OF THE INVENTION  
         [0002]    (a) Field of the Invention  
           [0003]    The present invention relates to a system and method for reducing the effects of multi-path fading and signal interference in a system using orthogonal codes and binary signal values. More specifically, the present invention relates to a maximum likelihood detection (MLD) system and method that reduces the complexity of MLD and improves performance in the system using orthogonal codes and binary signal values.  
           [0004]    (b) Description of the Related Art  
           [0005]    The modulation/demodulation methods for supporting a data transfer rate increasing in a confined frequency band include the quadrature amplitude modulation (QAM) method. The QAM modulation method may enhance the data transfer rate because the amount of information increases with an increase in the constellation, but it has a problem in regard to its mobility and application for 16-QAM or greater with a separation of more than a predetermined value. Namely, the QAM method is susceptible to the effect of distortion because the redundancy of noise decreases with an increase in the constellation. That is, the QAM modulation method has a trade-off relationship between information and noise.  
           [0006]    It is known that the channel capacity of channels having a rich scattering characteristic is proportional to the number of transceiver antennas in the same bandwidth. Hence, studies have been done on a method for detecting received signals using a multiple input/multiple output (MIMO) antenna system with a plurality of antennas so as to increase channel capacity. But this method is known to have a problem in regard to its implementation, because the mobile terminal concerned is required to have a plurality of antennas and a rich scattering characteristic for channels.  
           [0007]    In addition, there has been recently suggested a method for increasing data transfer rate without an increase in the entire bandwidth for users by using an intrinsic spreading code and orthogonal codes. But this method, which utilizes binary values, has problems in regard to overcoming the effects of multipath fading or signal interference and the complexity of MLD calculations according to the length of the spreading code.  
         SUMMARY OF THE INVENTION  
         [0008]    It is an advantage of the present invention to solve the problems with prior art and to provide a system and method for overcoming the effects of multipath fading or signal interference that is taken into consideration in the multipath environment.  
           [0009]    It is another advantage of the present invention to reduce the complexity of calculations according to the length of the intrinsic spreading code in MLD and to improve the performance, thereby reducing the complexity in the system implementation.  
           [0010]    In one aspect of the present invention, there is provided a wireless communication system that includes: a transmitter including an orthogonal encoder for converting serially input binary signals to parallel binary signals and orthogonally encoding the parallel binary signals, a first multiplier for multiplying the orthogonally encoded binary signals by an intrinsic spreading code to spread the orthogonally encoded binary signals, and an OFDM (Orthogonal Frequency Division Multiplexing) modulator for OFDM-modulating the spread signals; and a receiver including an OFDM demodulator for demodulating the OFDM-modulated signals, and a maximum likelihood detector for performing a maximum likelihood detection of the demodulated spread signals.  
           [0011]    The maximum likelihood detector groups the OFDM-demodulated signals into a predetermined number of blocks to perform the maximum likelihood detection, and uses the grouped maximum likelihood detection values to perform a whole maximum likelihood detection.  
           [0012]    The transmitter of the wireless communication system further includes: a first serial-to-parallel converter for serial-to-parallel converting the signals spread with the intrinsic spreading code; and an interleaver for interleaving the serial-to-parallel converted signals and sending the interleaved signals to the OFDM modulator. The receiver further includes: a deinterleaver for deinterleaving the OFDM-demodulated signals; and a first parallel-to-serial converter for parallel-to-serial converting the deinterleaved signals and sending the parallel-to-serial converted signals to the maximum likelihood detector.  
           [0013]    The maximum likelihood detector includes: a second multiplier for multiplying the OFDM-demodulated signals by the intrinsic spreading code; a grouping section for grouping the multiplied signals into blocks; a grouping maximum approximation detector for performing a maximum likelihood detection of the grouped blocks; an integrated maximum approximation detector for performing a whole maximum likelihood detection based on the grouped maximum approximation values; an orthogonal despreader for orthogonally despreading a sequence having a maximum approximation value to output parallel signals; and a second parallel-to-serial converter for converting the parallel output signals to serial signals. 
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0014]    The accompanying drawings, which are incorporated in and constitute a part of the specification, illustrate an embodiment of the invention, and, together with the description, serve to explain the principles of the invention:  
         [0015]    [0015]FIG. 1 is a block diagram of a wireless communication system according to an embodiment of the present invention;  
         [0016]    [0016]FIG. 2 is a detailed block diagram of a grouping maximum likelihood detector according to an embodiment of the present invention; and  
         [0017]    [0017]FIG. 3 is a signal diagram showing the multiplication of orthogonal codes and intrinsic spreading codes. 
     
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS  
       [0018]    In the following detailed description, only the preferred embodiment of the invention has been shown and described, simply by way of illustration of the best mode contemplated by the inventor(s) of carrying out the invention. As will be realized, the invention is capable of modification in various obvious respects, all without departing from the invention. Accordingly, the drawings and description are to be regarded as illustrative in nature, and not restrictive.  
         [0019]    For an evident description of the present invention, the parts not related to the description are omitted in the illustrations. The same reference numerals are assigned to the same parts all through the specification.  
         [0020]    Hereinafter, the embodiment of the present invention will be described in detail with reference to the accompanying drawings.  
         [0021]    [0021]FIG. 1 is a block diagram of a wireless communication system according to the embodiment of the present invention.  
         [0022]    The wireless communication system according to the embodiment of the present invention comprises a transmitter that includes a single orthogonal code (hereinafter referred to as “TOC”) block  100  including a serial-to-parallel converter  101 , and an orthogonalizer using first to fourth orthogonal codes (hereinafter expressed as “Sub-w(1,2,3,4)”), a serial-to-parallel converter  110 , an interleaver  120 , and an OFDM modulator  130 ; and a receiver that includes an OFDM demodulator  200 , a deinterleaver  210 , a parallel-to-serial converter  220 , and a grouping maximum likelihood detector  230 . Here, the configuration shown in the TOC block corresponds to the basic structure of the prior art for increasing channel capacity using orthogonal codes and binary signals.  
         [0023]    The TOC block  100  orthogonalizes serially input binary signals using the first to fourth Sub-w(1,2,3,4). The orthogonalized binary signals are summated, multiplied by an intrinsic spreading code W 1 , and sent to the serial-to-parallel converter  110 . The parallel bit streams output from the serial-to-parallel converter  110  are interleaved from the interleaver  120 . The interleaved bit streams are fed into the OFDM modulator  130  for OFDM modulation. The modulated signals are sent through OFDM channels. The individual signal waveforms of the OFDM channels are present at a point where the power of the channels of a different channel center frequency approaches zero, so they cause no interference even when they are overlapped.  
         [0024]    The OFDM modulated signals transferred on a plurality of carriers through the OFDM channels are demodulated from the OFDM demodulator  200 . The demodulated signals are deinterleaved from the deinterleaver  21 , converted to serial bit streams via the parallel-to-serial converter  220 , and sent to the grouping maximum likelihood detector  230 . The grouping maximum likelihood detector  230  performs MLD in the manner of the method that will be described later according to the embodiment of the present invention.  
         [0025]    First, the entire signal processing procedures of the present invention system will be described in detail by way of the following example.  
         [0026]    Input data are sent to the TOC block  100  using orthogonal codes to increase channel capacity, and to the serial-to-parallel converter  110  through an intrinsic spreading code.  
         [0027]    It is assumed that the binary signals output from the serial-to-parallel converter  110  are d(1)=(d, −d, d, d), where d represents the minimum distance of the constellation. The orthogonal codes, the intrinsic spreading code and the binary signal data can be used in the following description, where the binary signal data 0 and 1 are respectively denoted as “−” and “+” in the orthogonal codes and the intrinsic spreading code.  
         [0028]    Hence, the first to fourth orthogonal codes are expressed as follows:  
         [0029]    Sub-w(1)=(1 1 1 1)→(+ + + +)  
         [0030]    Sub-w(2)=(1 0 1 0)→(+ − + −)  
         [0031]    Sub-w(3)=(1 1 0 0)→(+ + − −)  
         [0032]    Sub-w(4)=(1 0 0 1)→(+ − − +)  
         [0033]    The intrinsic spreading code, W 1 =(0 1 0 1 0 1 0 1) is expressed by (− + − + − + − +).  
         [0034]    If considering d as a constant, the input data binary signals d(1)=(d, −d, d, d) from the serial-to-parallel converter  110  can be expressed by d(1)=(+1 −1 +1 +1).  
         [0035]    Each of the first to fourth orthogonal codes Sub-w(1), Sub-w(2), Sub-w(3) and Sub-w(4) is multiplied by the binary signal d(1) to produce output code C as follows:  
         [0036]    C(1)=(+1 +1 +1 +1)  
         [0037]    C(2)=(−1 +1 −1 +1)  
         [0038]    C(3)=(+1 +1 −1 −1)  
         [0039]    C(4)=(+1 −1 −1 +1).  
         [0040]    These output codes are summated to give S=(+2 +2 −2 +2).  
         [0041]    The summated value S is multiplied by the intrinsic spreading code W 1  to give spread data SD=(−2 +2 −2 +2 +2 −2 −2 +2).  
         [0042]    The spread code values are the output values before the serial-to-parallel converter  110 . The serial-to-parallel converted signals are fed into the interleaver  120 , and the output of the interleaver  120  through the spread data SD is transferred via the OFDM modulator  130 . The OFDM modulator  130  generally comprises an IFFT (Inverse Fast Fourier Transform) section, a parallel-to-serial converter, a guard interval inserter, and a carrier (RF) section, none of which are shown. The input binary spread code value SC can be output through the internal processing of the OFDM modulator  130 .  
         [0043]    Now, a process for demodulating the channel-passed input data and demapping the demodulated data into the original signals will be described in detail.  
         [0044]    The data received through the OFDM channels are fed into the deinterleaver  210  via the OFDM demodulator  200 . The OFDM demodulator  200  generally includes the functions of multiplying the noise-mixed input signals by the carrier (RF), removing an RF component via a low-pass filter (LPF), performing an A/D conversion, removing the guard interval, and performing an FFT, which functions are not shown.  
         [0045]    The deinterleaver  210  performs the reverse process of the interleaver of the transmitter and outputs the deinterleaved data to the parallel-to-serial converter  230 . After the parallel-to-serial conversion, the signal values are output as optimized values through a grouping MLD.  
         [0046]    The spread data SD=(−2 +2 −2 +2 +2 −2 −2 +2) are multiplied by the intrinsic spreading code W 1  through a multiplier. The intrinsic spreading code W 1  has the same value of the intrinsic spreading code W 1 =(0 1 0 1 0 1 0 1) of the transmitter and is expressed by (− + − + − + − +).  
         [0047]    The individual values of the spread data are multiplied by the intrinsic spreading code W 1  to output a value of (+2 +2 +2 +2 −2 −2 +2 +2).  
         [0048]    The output value is revised through the grouping MLD and then fed into an orthogonal despreader  234 .  
         [0049]    The data fed into the orthogonal despreader  234  are multiplied by the first to fourth orthogonal codes Sub-w(1) to (4) to give the following values:  
             (       +   2     +   2   +   2   +   2   -   2   -   2   +   2   +   2     )               (       -   2     -   2   +   2   +   2   -   2   -   2   -   2   -   2     )               (       +   2     +   2   +   2   +   2   +   2   +   2   -   2   -   2     )               (       +   2     +   2   -   2   -   2   +   2   +   2   +   2   +   2     )                               
 
         [0050]    The individual values are integrated for one period and multiplied by the value of one period (e.g., one period of W 1  is 8) to give (1, −1, 1, 1), which is then multiplied by d. The restored values (d, −d, d, d) are parallel-to-serial converted into the original values.  
         [0051]    Next, the grouping MLD process according to the embodiment of the present invention will be described in detail with reference to FIG. 2.  
         [0052]    [0052]FIG. 2 is a block diagram of the grouping maximum likelihood detector. The grouping maximum likelihood detector comprises, as shown in FIG. 2, a multiplier for multiplying an input signal by the intrinsic spreading code W 1 , a grouping maximum approximation detection processor  232 , an integrated maximum approximation detection processor  233 , a deorthogonal spreader  234 , and a parallel-to-serial converter  235 .  
         [0053]    Next, the operations of the grouping maximum approximation detection processor  232  and the integrated maximum approximation detection processor  233  shown in FIG. 2 will be described by way of the specific equations.  
         [0054]    The input signals from the OFDM demodulator  200 , the deinterleaver  210 , and the parallel-to-serial converter  220  are fed into the grouping maximum approximation detection processor  232 . The input signals are multiplied by the intrinsic spreading code W 1  and then grouped by a grouping section  231 .  
         [0055]    In the grouping step, the individual bit interval information of the first to fourth orthogonal codes Sub-w(1) to (4) are summated and then multiplied by the intrinsic spreading code W 1 . The multiplication value is then divided by the bit interval of Sub-w(1,2,3,4) to determine the interval of the intrinsic spreading code. When the interval of the intrinsic spreading code is 8 and the Sub-w(1,2,3,4) is a 4-bit interval, each group after grouping has an interval of 2.  
         [0056]    After grouping, the received signals R k1 , R k2 , . . . , R kl-1  and R kl  are subjected to noise and Rayleigh fading. Thus the signal R kl  represented by k vectors can be given by the following equation: 
           R   kl   =H   k1   S   kl   T   +N   [Equation 1] 
         [0057]    where the diagonal matrix H k1  is the Rayleigh fading constant of a sub carrier allocated to the grouped block; S kl   T  is the transposed value of the transmitted sequence; and N is the noise vector.  
         [0058]    The groping maximum approximation detection processor  233  performs MLD for R kl . In the MLD, the transmitted sequence is selected that minimizes the Euclidean distance e j   2  among all the available transmitted and received sequences.  
         [0059]    Let the set of all the transmittable sequences be V jl  (j=1, . . . , 2 Nw/Sw ), then the Euclidean distance e j   2  can be calculated from the following equation: 
           e   j   2   =min|R   kl   −H   k   V   jl   T | 2   [Equation 2] 
         [0060]    The most approximating transmitted sequence V jl =S′ kl  is selected when the Euclidean distance is at a minimum. Following the maximum approximation detection processing for k blocks, where k is the bit length of each Sub-w(1,2,3,4), the maximum likelihood value processed from the integrated maximum approximation processor  233  is used to perform the optimized MLD for integrated R kl  blocks.  
         [0061]    The conventional MLD algorithm, which uses the fading constant matrix H kl , requires channel information for assigning a weight to all the transmitted sequences V jl , so its complexity increases at a ratio of geometrical progression with an increase in the length of the sequence and a decrease in the Euclidean distance. Contrarily, the embodiment of the present invention groups all the user signals from the transmitter into blocks in the units of bit of the orthogonal code Sub-w(1,2,3,4) multiplied by the spreading code W 1  allocated to the users. According to the embodiment of the present invention, 2 Nw/Sw  available sequences are detected from the MLD rather than k sequences, where Nw is the interval length of the intrinsic spreading code W 1 ; and Sw is the bit interval length of each orthogonal code. Namely, as many transmitted signals as the length of the intrinsic spreading code, Nw, are grouped into blocks in the units of bit of each Sub-w(1,2,3,4) so as to reduce the complexity of MLD. Let the code length of each Sub-w(1,2,3,4) be Sw, then the transmitted signals are grouped into Sw blocks. The partially spread block in each unit bit has the maximum approximations of 2 Nw/Sw  partially spread code bits for each partially spread code bit. During transmission, the data bits d il , each of which is spread by the orthogonal codes Sub-w(1,2,3,4), are spread by 2 Nw/Sw  partially spread code vectors.  
         [0062]    The codes used for spreading are orthogonal Walsh-Hadamard codes and have the cross-correlation of zero. For the spread chip stream of the k-th block Sk having a length of Nw/Sw can be expressed by the following equation:  
         [0063]    [Equation 3]  
         S   k     =         ∑     i   =         Nw   Sw        k     +   1           Nw   Sw          (     k   +   1     )                           Sub       (     1   ,   2   ,   3   ,   4     )     ,   k            C   i         =     [       s     0   ,   k       ,     s     1   ,   k       ,              …              ,     s       (       Nw   Sw     -   1     )     ,   k         ]                             
 
         [0064]    where C is the intrinsic spreading code.  
         [0065]    The orthogonal despreader  235  uses the Sub-w(1,2,3,4) codes to despread the received signals spread with the Sub-w(1,2,3,4) codes and subjected to the grouping MLD process, and divides the resulting value of the despreading by the length of the intrinsic spreading code, Nw to obtain data values received from the transmitter. The data values are processed at the parallel-to-serial converter  235  into finally received data.  
         [0066]    [0066]FIG. 3 shows an example of the multiplication of the orthogonal code by the intrinsic spreading code.  
         [0067]    To simplify the description of the grouping process that involves the multiplication of the orthogonal code Sub-w(1,2,3,4) by the intrinsic spreading code W 1 , the lengths of the intrinsic spreading code W 1  and the orthogonal code are set to 8 and 4, respectively. Actually, the intrinsic spreading code has a considerably large length.  
         [0068]    In FIG. 3, signal  3 A shows the 2-fold expanded bit interval of one code of Sub-w(1,2,3,4); signal  3 B is the intrinsic spreading code W 1 ; and signal  3 C shows the multiplication of the intrinsic spreading code and the Sub-w(1,2,3,4), where Nw is 8, Sw is 4, and Nw/Sw is 2. The bit interval length of Sub-w(1,2,3,4) is 4, so the length of the intrinsic spreading code Nw is grouped into 4 blocks. Namely, the interval length of the grouped blocks is 2. Expediently, a short code length has been exemplified in this description, but it must be taken into consideration that the length of the intrinsic spreading code is considerably large.  
         [0069]    While this invention has been described in connection with what is presently considered to be the most practical and preferred embodiment, it is to be understood that the invention is not limited to the disclosed embodiments, but, on the contrary, is intended to cover various modifications and equivalent arrangements included within the spirit and scope of the appended claims.  
         [0070]    The present invention uses the intrinsic spreading code and the orthogonal codes to increase the data transfer rate without an increase in the whole bandwidth allocated for users, and performs a grouping MLD according to the configuration of the present invention to remarkably reduce the complexity of the MLD system and improve the performance.