Abstract:
A receiving apparatus in an OFDM communication system, in which the receiving apparatus includes a serial-to-parallel converter that converts a serial signal received through an antenna to parallel signals. A pre-processor processes an nth symbol converted in the serial-to-parallel converter using an (n−1)th symbol and an (n+1)th symbol. A Fourier transformer Fourier-transforms the output of the pre-processor and an equalizer equalizes a Fourier-transformed signal. A deinterleaver deinterleaves an equalized signal, a decoder decodes a deinterleaved signal, and a parallel-to-serial converter converts parallel decoded signal to a signal stream.

Description:
PRIORITY  
       [0001]     This application claims priority under 35 U.S.C. § 119 to an application entitled “Apparatus and Method for Receiving Signals in an OFDM Communication System” filed in the Korean Intellectual Property Office on Nov. 20, 2003 and assigned Serial No. 2003-82592, the contents of which are incorporated herein by reference.  
       BACKGROUND OF THE INVENTION  
       [0002]     1. Field of the Invention  
         [0003]     The present invention relates generally to an OFDM (Orthogonal Frequency Division Multiplexing) communication system, and in particular, to a receiving apparatus and method for efficiently recovering cyclicity between symbols in an OFDM communication system.  
         [0004]     2. Description of the Related Art  
         [0005]     To support data rates required for future-generation mobile communication services, OFDM has recently been considered as a fundamental technology for the future-generation mobile communication network.  
         [0006]      FIG. 1  is a block diagram of a transmitter in a conventional OFDM system. Referring to  FIG. 1 , a channel coder  101  encodes input data d(k) and an interleaver  102  interleaves the coded data. A signal mapper  103  converts the interleaved signal c(i) to signal vectors X(n, 0:N−1). An IFFT (Inverse Fast Fourier Transformer)  104  outputs transmission signal vectors x(n, 0:N−1) for the input of X(n, 0:N−1). A CP (Cyclic Prefix) inserter  105  inserts a guard interval into x(n, 0:N−1). The resulting signal is transmitted through a parallel to serial (P/S) converter  106  and finally an antenna.  
         [0007]     The OFDM system inserts a CP between every adjacent symbol pair in the time domain in order to handle multipath fading. Further, in order to completely eliminate inter-symbol interference (ISI) and inter-channel interference (ICI) caused by the multipath fading, the length of the CP must be longer than a channel impulse response (CIR).  
         [0008]      FIG. 2  illustrates a structure of an nth symbol when a total number of sub-channels is 8 (N=8) and the CP length is 4.  FIGS. 3 and 4  illustrate signal receptions when the CP is as long as the CIR and shorter than the CIR, respectively.  
         [0009]     If a channel with a CIR length of 4 is defined as h(D)=h 0 +h 1 D+h 2 D 2 +h 3 D 3 +h 4 D 4 , then an nth signal is received as illustrated in  FIG. 3 .  
         [0010]     In  FIG. 3 , the CP length is equal to the CIR length. r(r, 0:7) except for a CP, r(n, −4:−1) in the received symbol is a circular convolution of x(n, 0:7) and h(D) That is, r(n, −4:−1) is CP, such that it is removed from the received symbol and the remained part r(r, 0:7) becomes a circular convolution of x(n, 0:7) and h(D). Therefore, orthogonality is maintained between sub-channels, thereby avoiding ISI and ICI.  
         [0011]      FIG. 4  illustrates the structure of a received symbol when the CIR length is 4 and the CP length is 2. Referring to  FIG. 4 , as many samples r(n, 0) and r(n, 1) as the difference between the CIR length and the CP length contain (n−1)th symbol components, thereby causing ISI, which is illustrated in the shaded squares of  FIG. 4 .  
         [0012]     Because using a CP decreases the frequency efficiency of the OFDM system, many studies have been conducted on methods of efficiently eliminating ISI and ICI, while minimizing the use of the CP. As a result, iterative cancellation methods have been proposed such as residual ISI cancellation (RISIC) for canceling insufficient CP-caused interference.  
         [0013]     According to the RISIC, recovery of the defective samples involves elimination of the ISI component and recovery of the CP. In this case, recovered samples r′(n, 0) and r′(n, 1) can be expressed as shown below in Equations (1) and (2).
 
 r ′( n, 0)= r ( n, 0) − r       3     ( n− 1,7)− r       4     ( n− 1,6)+ r       3     ( n, 5)+r     4     ( n, 4)   (1)
 
 r ′( n, 1)= r ( n, 1) − r       4     ( n− 1,7)+ r       4     ( n, 5)   (2)
 
         [0014]     The subtraction of r 3 (n−1, 7) and r 4 (n−1, 6) from the received signal r(n, 0) in Equation (1) and the subtraction of r 4 (n−1, 7) from the received signal r(n, 1) in Equation (2) are equivalent to ISI cancellation. The addition of r 3 (n, 5) and r 4 (n, 4) to r(n, 0) and the addition of r 4 (n, 5) to r(n, 1) are equivalent to CP recovery. The CP recovery is repeated along with detection of x(n, 0:7).  
         [0015]     However, the conventional ISI cancellation method, such as the RISIC, effectively recovers a CP only if a CIR is shorter than an OFDM symbol period, that is, when interference power is much less than signal power, an effective CP recovery is possible.  
         [0016]     Another shortcoming of the conventional ISI cancellation method is that because a current symbol is estimated and a CP is recovered using the symbol estimate, when a long channel delay leads to a high interference power, reduction of interference power by CP recovery cannot be expected due to errors in the symbol estimation.  
         [0017]     While various methods have been proposed using techniques of SISO (Soft-Input Soft-Output) channel decoding, optimal detection filtering, and leaked signal energy spread to the next symbol to overcome the above shortcomings, a SISO channel decoder demonstrates a very slight performance improvement under an SER (Symbol Error Rate) and the optimal detection filtering requires a complex process of inversion of a channel transmission function matrix in an initial stage. Additionally, an ISI combiner using the leaked signal energy spread needs estimation of the next transmitted symbol.  
       SUMMARY OF THE INVENTION  
       [0018]     An object of the present invention is to substantially solve at least the above problems and/or disadvantages and to provide at least the advantages below. Accordingly, an object of the present invention is to provide a receiving apparatus and a cyclicity recovering method for recovering a CP by pre-iteration processing an ISI-removed signal and a next received signal, thereby increasing ISI cancellation performance.  
         [0019]     Another object of the present invention is to provide a receiving apparatus and a cyclicity recovering method for efficiently recovering a CP when a CIR is shorter than an OFDM symbol period in a system that does not use the CP.  
         [0020]     The above and other objects are achieved by providing a receiving apparatus and method in an OFDM communication system. In the receiving apparatus, a serial-to-parallel converter converts a serial signal received through an antenna to parallel signals. A pre-processor processes an nth symbol converted in the serial-to-parallel converter using an (n−1)th symbol and an (n+1)th symbol. A Fourier transformer Fourier-transforms the output of the pre-processor and an equalizer equalizes a Fourier-transformed signal. A deinterleaver deinterleaves an equalized signal, a decoder decodes a deinterleaved signal, and a parallel-to-serial converter converts parallel decoded signal to a signal stream. 
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0021]     The above and other objects, features, and advantages of the present invention will become more apparent from the following detailed description when taken in conjunction with the accompanying drawings in which:  
         [0022]      FIG. 1  is a block diagram of a transmitter in a conventional OFDM system;  
         [0023]      FIG. 2  illustrates a structure of a transmission symbol when the total number of sub-channels is 8 (N=8) and a CP length is 4;  
         [0024]      FIG. 3  illustrates a structure of a received symbol of the transmission symbol illustrated in  FIG. 2  on a channel with a CIR length of 4;  
         [0025]      FIG. 4  illustrates a structure of a received symbol of the transmission symbol illustrated in  FIG. 2  on a channel with a CIR length of 2;  
         [0026]      FIG. 5  is a block diagram of a receiving apparatus in an OFDM communication system according to a preferred embodiment of the present invention;  
         [0027]      FIG. 6  illustrates a structure of a signal received in the receiving apparatus illustrated in  FIG. 5 ;  
         [0028]      FIG. 7  is a flowchart illustrating a data receiving operation in the OFDM communication system according to the preferred embodiment of the present invention;  
         [0029]      FIGS. 8 and 9  are graphs illustrating channel environments under which simulations are performed to assess the performance of the receiving apparatus using a CP recovery method of the present invention; and  
         [0030]      FIGS. 10 and 11  are graphs illustrating a performance of the receiving apparatus of the present invention under the channel environments illustrated in  FIGS. 8 and 9 . 
     
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT  
       [0031]     Preferred embodiments of the present invention will be described in detail herein below with reference to the accompanying drawings. In the following description, well-known functions or constructions are not described in detail because they would obscure the invention in unnecessary detail.  
         [0032]      FIG. 5  is a block diagram of a receiving apparatus in an OFDM communication system according to a preferred embodiment of the present invention. Referring to  FIG. 5 , the receiving apparatus includes a serial-to-parallel (S/P) converter  501  for converting a serial signal received through an antenna to parallel signals, a first delay  502  for delaying the parallel signals by one symbol period, a CP remover  503  for removing a CP from the delayed signals, an ISI remover  504  for canceling ISI from an OFDM symbol received from the CP remover  503 , an ICI remover  505  for canceling ICI from the ISI-removed OFDM symbol, an FFT (Fast Fourier Transformer)  606  for fast-Fourier-transforming the ISI- and ICI-free OFDM symbol, a one-tap equalizer  507  for equalizing the FFT signal, a demapper  508  for demapping the equalized signal, a deinterleaver  509  for deinterleaving the demapped signal, a SISO decoder  510  for decoding the deinterleaved signal, and a P/S converter  511  for converting the decoded parallel signals to a signal sequence.  
         [0033]     The receiving apparatus further includes an interference canceling unit  550  for generating an ISI duplicate and an ICI duplicate from the output of the SISO decoder  510  to cancel the ISI and the ICI. Further, the interference canceling unit  550  outputs the ISI duplicate and the ICI duplicate to the ISI remover  504  and the ICI remover  505 , respectively.  
         [0034]     The interference canceling unit  550  includes an interleaver  521  for interleaving the output signal of the SISO decoder  510 , a soft-symbol mapper  522  for modulating the interleaved signal, an IFFT  523  for inverse-fast-Fourier-transforming the modulated symbol, a second delay  524  for delaying the IFFT signal by one symbol period, an ISI duplicate generator  525  for generating an ISI duplicate from the delayed signal and outputting the ISI duplicate to the ISI remover  504 , and an ICI duplicate generator  526  for generating an ICI duplicate from the IFFT signal and outputting the ICI duplicate to the ICI remover  505 .  
         [0035]     Also, the receiving apparatus further includes a pre-iteration processor  530  for pre-iteration processing the output signal of the ISI remover  504  and the output signal of the S/P converter  501  and outputting the pre-processed signal to the FFT  506 .  
         [0036]     The pre-iteration processor  530  recovers a CP by applying a signal component of an nth symbol period included in a signal r(n+1, −G:N−1) received during an (n+1)th symbol period after the S/P conversion to the output of the ISI remover  504  and provides a signal for the nth symbol period with the recovered CP to the FFT  506 .  
         [0037]     In an embodiment of the present invention, the receiving apparatus further includes a switch  515  for selectively switching the outputs of the ICI remover  505  and the pre-iteration processor  530  to the FFT  506 .  
         [0038]     When the number of pre-iteration processes in the pre-iteration processor  530  is 1, the switch  515  switches the output of the pre-iteration processor  530  to the FFT  560 . When the number of pre-iteration processes is larger than 1 and less than a predetermined number, the switch  515  switches the output of the ICI remover  505  to the FFT  506 . When the number of pre-iteration processes is equal to or greater than the predetermined number, the pre-iteration process is terminated.  
         [0039]      FIG. 6  illustrates the structure of a signal received at the receiving apparatus according to a preferred embodiment of the present invention. It is assumed herein that a CIR length is 2 (L=2), a CP length is 0 (G=0), and the number of sub-channels is 8 (N=8). The CP recovery is based on the idea that signal components of an nth symbol period required for CP recovery are found in (L−G) samples of a signal received during an (n+1)th symbol period.  
         [0040]     Referring to  FIG. 6 , supposing that samples affected by ISI are r(n, o) and r(n, 1), perfect channel knowledge is acquired, and no errors occur in coding of the previous symbol, ISI cancellation is represented as shown below in Equations (3) and (4).
 
 {tilde over (r)}   (0) ( n 0)= r ( n, 0)− h   1   x ( n− 1,7)− h   2   x ( n− 1,6)= h   0   x ( n, 0)= r   0 ( n, 0)  (3)
 
and
 
 {tilde over (r)}   (0) ( n, 1)= r ( n, 1)− h   2   x ( n− 1,7)= h   0   x ( n, 1)+ h   1   x ( n, 0)= r   0 ( n, 1)+ r   1 ( n, 0)  (4)
 
         [0041]     To recover a CP after the ISI cancellation, h 1 x(n,7)+h 2 x(n,6)=r 1 (n,7)+r 2 (n,6) must be added to the ISI-removed received signal {tilde over (r)} (0) (n,0), and h 2 x(n,7)=r 2 (n,7) must be added to the ISI-removed received signal {tilde over (r)} (0) (n,1). The information is included in r(n+1,0) and r(n+1,1), respectively. Considering that r(n+1,0) and r(n+1,1) also include information about the (n+1)th symbol, r(n+1,0) and r(n+1,1) are added to {tilde over (r)} (0) (n,0) and {tilde over (r)} (0) (n,1), with appropriate weights, to thereby minimize an average interference power. This is shown below in Equations (5) and (6).
 
 {overscore (r)}   (0) ( n, 0)= {tilde over (r)}   (0) ( n, 0)+ w (0) xr ( n+ 1,0)  (5)
 
and
 
 {overscore (r)}   (0) ( n, 1)= {tilde over (r)}   (0) ( n, 1)+ w (1) xr ( n+ 1,1)  (6)
 
         [0042]     The process of minimizing the average interference power is called pre-iteration processing (PIP).  
         [0043]     Assuming the transmission samples are mutually independent, weights w(0) and w(1), which minimize the average interference power, are determined by Equations (7) and (8).  
               w   ⁡     (   0   )       =         ∑     i   =   1     2     ⁢            h   i          2           ∑     i   =   0     2     ⁢            h   i          2                 (   7   )                 w   ⁡     (   1   )       =         ∑     i   =   2     2     ⁢            h   i          2           ∑     i   =   0     2     ⁢            h   i          2                 (   8   )             
 
         [0044]      FIG. 7  is a flowchart illustrating a CP recovering method according to a preferred embodiment of the present invention. Referring to  FIG. 7 , the receiving apparatus receives an nth OFDM symbol, delays the nth OFDM symbol by one symbol period, and then receives an (n+1)th OFDM symbol in step S 701 . In step S 702 , the receiving apparatus cancels ISI from the nth OFDM symbol using the estimates and channel information of the nth and (n−1)th OFDM symbols. The ISI-removed signal is expressed as shown below in Equation (9).  
                   r   ~       (   0   )       ⁡     (     n   ,   k     )       =     {             r   ⁡     (     n   ,   k     )       -       ∑     i   =     G   +   k   +   1       L     ⁢       h   i     ⁢       x   ^     ⁡     (       n   -   1     ,     N   +   G   +   k   -   i       )                   0   ≤   k   &lt;     L   -   G                 r   ⁡     (     n   ,   k     )               L   -   G     ≤   k   &lt;   N                     (   9   )               
         [0045]     After the ISI cancellation from the nth OFDM symbol, the receiving apparatus subtracts the product of an nth OFDM symbol component in an (n+1)th OFDM symbol and a weight w(k) from the nth OFDM symbol, thereby recovering the cyclicity in step S 703 . The cyclicity-recovered signal is obtained as shown in Equation (10),  
                   r   ~       (   0   )       ⁡     (     n   ,   k     )       =     {                     r   ~       (   0   )       ⁡     (     n   ,   k     )       -       w   ⁡     (   k   )       ×     r   ⁡     (       n   +   1     ,   k     )                 0   ≤   k   &lt;     L   -   G                 r   ⁡     (     n   ,   k     )               L   -   G     ≤   k   &lt;   N           ⁢     
     ⁢   where   ⁢           ⁢     w   ⁡     (   k   )         =           ∑     i   =     G   +   k   +   1       L     ⁢            h   i          2           ∑     i   =   0     L     ⁢            h   i          2         .                 (   10   )             
 
         [0046]     The cyclicity recovery is a PIP, as stated earlier. Each time the PIP is performed, a PIP indicator I is incremented by one in step S 704 .  
         [0047]     In step S 705 , the receiving apparatus determines if I is 1. If I=1, the receiving apparatus performs FFT, equalization, deinterleaving, and decoding on the PIP output symbol {overscore (r)} (iter) (n,0: N−1) in step S 706  and estimates a transmission signal {circumflex over (x)} (iter) (n,0: N−1) from the decoded signal in step S 707 .  
         [0048]     However, if I≠1, the receiving apparatus determines whether I is a predetermined iteration number I th  in step S 708 . If I≠I th , the receiving apparatus performs FFT, equalization, deinterleaving, and decoding on the ISI-removed {tilde over (r)} (iter) (n,0: N−1) in step S 709  and estimates a transmission signal {circumflex over (x)} (iter) (n,0: N−1) from the decoded signal in step S 707 .  
         [0049]     If I=I th , the receiving apparatus terminates the CP recovery algorithm.  
         [0050]      FIGS. 8 and 9  are graphs illustrating channel environments under which simulations are performed to assess the performance of the receiving apparatus when the CP recovery method is used according to the present invention.  FIGS. 10 and 11  are graphs illustrating the performance of the receiving apparatus of the present invention under the channel environments illustrated in  FIGS. 8 and 9 . The simulations were performed using the conventional CP recovery method and the inventive CP recovery method in a coded-OFDM system under the conditions of N=64, G=0, and a coding rate ½-convolutional code with K=7.  
         [0051]     Referring to  FIGS. 10 and 11 , it is noted that the inventive CP recovery offers better SER performance than the conventional CP recovery under the channel environments having delay characteristics illustrated in  FIGS. 8 and 9 .  
         [0052]     In accordance with the present invention as described above, the CP of an nth received symbol is recovered using an estimate of an (n−1)th received symbol and an nth symbol component included in an (n+1)th received symbol. Therefore, the inventive CP recovery method enables reliable CP recovery.  
         [0053]     Additionally, efficient recovery of the cyclicity of a symbol through PIP, irrespective of a CP length, maximizes channel capacity and effectively removes ISI in the inventive CP recovery method.  
         [0054]     While the present invention has been shown and described with reference to certain preferred embodiments thereof, it will be understood by those skilled in the art that various changes in form and details may be made therein without departing from the spirit and scope of the present invention as defined by the appended claims.