Abstract:
In a digital communication system such as a mobile radio system, fading distortion is compensated with a raised precision. In a transmitter, pilot signals are regularly inserted in the information signals. The amplitude of the pilot signals are set larger than the maximum possible amplitude of the information signals. The modulation scheme of the pilot signals may be different from that of the information signals. In a receiver, the fading distortion of each of the pilot signals is determined. The fading distortions of the information signals are estimated by interpolation using the determined fading distortion of the pilot signal, and then compensated. The frequency band of each information signal is preferably limited with a roll-off filter with a roll-off coefficient ranging from 0.1 to 0.4.

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
       [0001]    This application is a Continuation of application Ser. No. 10/601,591 filed on Jun. 24, 2003, which is a Divisional of application Ser. No. 09/292,398 filed on Apr. 15, 1999, now U.S. Pat. No. 6,608,843, and for which priority is claimed under 35 U.S.C. § 120; and this application claims priority of application Ser. No. 10-105990 filed in Japan on Apr. 16, 1998 under 35 U.S.C. § 119; the entire contents of all are hereby incorporated by reference. 
     
    
     BACKGROUND OF THE INVENTION 
       [0002]    1. Field of the Invention 
         [0003]    The invention relates to a digital communications system and more specifically to modulation methods facilitating fading distortion compensation that performs a quasi-synchronous detection using a pilot signal. 
         [0004]    2. Description of the Prior Art 
         [0005]    In a digital communications system, especially in a digital mobile radio system, the envelope of information signals or symbols is distorted (i.e., the phases and the amplitudes of information signals involves phase errors and amplitude errors) due to fading. The phase error of the received signal causes an error in the frequency of a local oscillator in the receiver. The error of the local oscillation frequency with respect to the carrier frequency is hereinafter referred to as a “frequency offset”. The phase error (or frequency offset) and the amplitude error of the received signal have to be estimated and compensated for in the receiver. 
         [0006]    A fading distortion compensation scheme using a pilot signal is described by S. Sampei, “Rayleigh Fading Compensation Method for 16-QAM MODEM in Digital Land Mobile Radio Systems,” Trans. IEICE (The Institute of Electronics, Information and Communication Engineers) Japan, Vol. J72-B-II, No. 1, January 1989, pp. 7-15 (which is hereby incorporated by reference).  FIG. 1  is a diagram showing a signal constellation used in this 16-QAM system. In  FIG. 1 , small black-filled circles indicate 16 signal points in an in-phase (I) and quadrature-phase (Q) plane. One of the signal points with the maximum amplitude, that is, any of the signal points A, B, C and D is assigned to a pilot signal. (Since one of the 16 signal points in the signal constellation is used for a pilot signal, the remaining 15 points are available for the information signals.) A pilot signal is inserted in every frame or every N−1 information symbols (assuming that N symbols constitute one frame) in a transmitter. The estimation and compensation of distortions (due to fading) of information signals or symbols are achieved by interpolation using the pilot signals. 
         [0007]    In such quasi-synchronous detection as just described, larger-amplitude pilot symbols yields a higher precision in estimation of the frequency offset and the amplitude error of the information signals, which results in an improvement of the bit error rate, which is a function of the ratio of the carrier signal power to the noise power density per one symbol. However, enlarging the amplitude of the pilot signals without taking any measure lowers the power efficiency of the power amplifier of the transmitter system due to an increase in the ratio of the peak to the average transmission power. 
         [0008]    It is therefore an object of the invention to provide a method of and a system for compensating for fading distortion of the received signal with a raised precision and thereby to provide a digital communication system that permits a reception of a reduced bit error rate. 
       SUMMARY OF THE INVENTION 
       [0009]    In accordance with the principles of the invention, a point that differs in phase from any of the signal points for possible information symbols and that is larger in amplitude than any of the signal points is selected for a pilot signal point in a signal constellation (or a signal point map plotted on a in-phase and quadrature-phase plane). A pilot signal is inserted in every frame or every predetermined number of information signals. 
         [0010]    In a receiver, the fading distortion of each of the pilot signals regularly inserted in the received signals is determined. The fading distortions of the information signals are estimated by interpolation using the determined fading distortion of the pilot signal, and then compensated for. 
         [0011]    In a preferred embodiment, the amplitude of the pilot signal is set not larger than 1.6 times a maximum possible amplitude of the information signals. 
         [0012]    The frequency band of each information signal is preferably limited with a roll-off filter with a roll-off coefficient ranging from 0.1 to 0.4. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWING 
         [0013]    The features and advantages of the present invention will be apparent from the following description of an exemplary embodiment of the invention and the accompanying drawing, in which: 
           [0014]      FIG. 1  is a diagram showing a signal constellation used in this 16-QAM system; 
           [0015]      FIG. 2  is a schematic block diagram showing a part of an illustrative embodiment of a mobile telephone terminal that incorporates a fading distortion compensation system in accordance with the principles of the invention; 
           [0016]      FIG. 3  is a diagram of an exemplary signal constellation for 16-APSK (amplitude phase shift keying) used in a first example; 
           [0017]      FIG. 4  is a diagram showing a symbol stream transmitted in a digital communication system that serves the mobile telephone terminal  1  of  FIG. 1 ; and 
           [0018]      FIGS. 5 through 8  are signal constellations for  m -QAM, 16-QAM, 8-PSK and QPSK according to the principles of the invention. 
       
    
    
       [0019]    Throughout the drawing, the same elements when shown in more than one figure are designated by the same reference numerals. 
       DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
       [0020]      FIG. 2  is a schematic block diagram showing a part of an illustrative embodiment of a mobile telephone terminal  1  that incorporates a fading distortion compensation system in accordance with the principles of the invention. A transmission system (shown in upper part of  FIG. 2 ) of the mobile telephone terminal  1  includes a serial-to-parallel (S/P) converter  10 ; a base band signal generator  20  having its input connected with a converter  10  output; a pilot (or frame) signal inserter  110  having its in-phase (I) input and quadrature-phase (Q) input connected with respective outputs of the base band signal generator  20 ; low pass filters (LPF)  30  having their inputs connected with pilot signal inserter  110  outputs; a radio transmitter portion  40  having its I and Q inputs connected with I and Q LPF  30  outputs, respectively; an antenna duplexer  50  having transmission input connected with the transmitter portion  40  output; and an antenna  60  used for both transmission and reception. A reception system (shown in lower part of  FIG. 2 ) of the mobile telephone terminal  1  includes a radio receiver portion  70  having its input connected with a duplexer  50  reception output; LPFs  80  having their input connected with radio receiver portion  70  I and Q outputs; a fading distortion compensator  100  having its I and Q inputs connected with respective LPF  80  outputs; and a decision maker  90 , having its input connected with a compensator  100  output, for providing received data. The fading distortion compensator  100  includes a phase error estimator  120 , an amplitude error estimator  130  and a phase and amplitude compensator  140 . The mobile telephone terminal  1  further includes a controller  170  for controlling overall operation of the terminal  1 . 
         [0021]    In transmission operation, binary data is supplied in the form of a bitstream to the S/P converter  10 . The S/P converter  10  converts the serial binary data into a parallel data of a predetermined number of bits. The base band signal generator  20  generates an in-phase (I) and quadrature-phase (Q) components or signals for a symbol associated with the parallel data. 
         [0022]      FIG. 3  is a diagram of an exemplary signal constellation for 16-APSK (amplitude phase shift keying) used in a first example.  FIG. 4  is a diagram showing an operation of the pilot signal inserter  110  of  FIG. 3 . In  FIGS. 3 and 4  and the following figures of signal communications, small black-filled circles indicate information symbols and small white-filled circles indicate pilot (or frame) symbols. 
         [0023]    In this specific example, the base band signal generator  20  generates I and Q components of one (ISi) of the 16 possible information symbols IS 1 , IS 2 , . . . , IS P  which is associated with each of the parallel data supplied to the generator  20 , where i=1, 2, . . . P, and P is the number of possible information symbols (16 in this specific example) in the modulation scheme. The information symbol stream supplied from the base band signal generator  20  is shown in the upper part of  FIG. 4 . 
         [0024]    The pilot signal inserter  110  inserts a pilot signal PS in every predetermined number of information symbols, say, N−1 symbols S 1 , . . . S N−1  to make a frame of N symbols S 0 , S 1 , . . . S N−1  as shown in  FIG. 4 . In each frame of the symbol stream output from the pilot signal inserter  110 , 
         [0000]    
       
         
           
             
               
                 
                   Sj 
                   = 
                   
                     { 
                     
                       
                         
                           PS 
                         
                         
                           
                             
                               for 
                                
                               
                                   
                               
                                
                               j 
                             
                             = 
                             0 
                           
                         
                       
                       
                         
                           
                             IS 
                             i 
                           
                         
                         
                           
                             
                               
                                 for 
                                  
                                 
                                     
                                 
                                  
                                 j 
                               
                               = 
                               
                                 1 
                                 , 
                                 2 
                               
                             
                             , 
                             … 
                              
                             
                                 
                             
                             , 
                             
                               N 
                               - 
                               1. 
                             
                           
                         
                       
                     
                   
                 
               
               
                 
                   ( 
                   1 
                   ) 
                 
               
             
           
         
       
     
         [0025]    It is seen from  FIG. 3  that the pilot signal PS (=S 0 ) is preferably disposed so as to make an angle of π/8 with adjacent information signal points. Thus, the pilot signal PS=(PS I , PS Q ) is preferably set to any of the following points: 
         [0000]    
       
         
           
             
               PS 
               I 
             
             = 
             
               R 
               * 
               
                 cos 
                  
                 
                   ( 
                   
                     
                       π 
                       4 
                     
                     + 
                     
                       π 
                       8 
                     
                   
                   ) 
                 
               
             
           
         
       
       
         
           
             
               
                 PS 
                 Q 
               
               = 
               
                 R 
                 * 
                 
                   sin 
                    
                   
                     ( 
                     
                       
                         π 
                         4 
                       
                       + 
                       
                         π 
                         8 
                       
                     
                     ) 
                   
                 
               
             
             , 
           
         
       
     
         [0026]    where i=1, 2, . . . , 16 and R is the amplitude of the pilot signal PS. 
         [0027]    According to the invention, the amplitude R of the pilot signal PS is set larger than that of any information signal Sj (=ISi), Ri, as shown in  FIG. 3 . Specifically, it is preferable to set the pilot signal amplitude R for a range larger than the maximum amplitude Rmax of the information symbols and not larger than 1.6 times the maximum amplitude Rmax, that is, 
         [0000]        R max&lt; R≦ 1.6 *R max.  (2) 
         [0028]    It is noted that each signal Sj is processed in the form of corresponding I and Q components Sj I  and Sj Q . The signals Sj from the pilot signal inserter  110  is limited in frequency band by the LPFs  30 . The LPFs  30  are preferably roll-off filters (or Nyquist filters) having the following characteristic: 
         [0000]    
       
         
           
             
               
                 
                   
                     H 
                      
                     
                       ( 
                       ω 
                       ) 
                     
                   
                   = 
                   
                     { 
                     
                       
                         
                           1 
                         
                         
                           
                             ω 
                             ≤ 
                             
                               
                                 ω 
                                 0 
                               
                                
                               
                                 ( 
                                 
                                   1 
                                   - 
                                   α 
                                 
                                 ) 
                               
                             
                           
                         
                       
                       
                         
                           
                             
                               
                                 1 
                                 2 
                               
                                
                               
                                 [ 
                                 
                                   1 
                                   - 
                                   
                                     sin 
                                      
                                     
                                       ( 
                                       
                                         
                                           π 
                                           
                                             2 
                                              
                                             
                                               αω 
                                               0 
                                             
                                           
                                         
                                          
                                         
                                           ( 
                                           
                                             ω 
                                             - 
                                             
                                               ω 
                                               0 
                                             
                                           
                                           ) 
                                         
                                       
                                       ) 
                                     
                                   
                                 
                                 ] 
                               
                             
                           
                         
                         
                           
                             
                               
                                 ω 
                                 0 
                               
                                
                               
                                 ( 
                                 
                                   1 
                                   - 
                                   α 
                                 
                                 ) 
                               
                             
                             ≤ 
                             ω 
                             ≤ 
                             
                               
                                 ω 
                                 0 
                               
                                
                               
                                 ( 
                                 
                                   1 
                                   + 
                                   α 
                                 
                                 ) 
                               
                             
                           
                         
                       
                       
                         
                           0 
                         
                         
                           
                             ω 
                             ≥ 
                             
                               
                                 ω 
                                 0 
                               
                                
                               
                                 ( 
                                 
                                   1 
                                   + 
                                   α 
                                 
                                 ) 
                               
                             
                           
                         
                       
                     
                   
                 
               
               
                 
                   ( 
                   3 
                   ) 
                 
               
             
           
         
       
     
         [0029]    where H(ω) is a amplitude characteristic of the roll-off filters  30 , ω is an angular frequency, ω 0  is a Nyquist angular frequency and α is a roll-off coefficient. It is preferable to set the roll-off coefficient α for a range from 0.1 to 0.4. 
         [0030]    The filtered signals are modulated and amplified by the radio transmitter portion  40 , and eventually transmitted via the duplexer  50  and the antenna  60 . 
         [0031]    In reception operation, the I and Q components of the received signals received by the antenna  50 , the duplexer  60  and the radio receiver portion  70  is filtered by the LPFs  80  and supplied to the phase error estimator  120 , the amplitude error estimator  130  and the phase and amplitude compensator  140 . The phase error estimator  120  provides an estimated phase error signal to the compensator  140 . The amplitude error estimator  130  provides, to the compensator  140 , an estimated amplitude error signal for each information signal Sj (j=1, 2, . . . N−1) through interpolation using a pilot signal S 0 . The phase and amplitude compensator  140  responsively compensates each information signal by using the estimated phase and amplitude error signals to provide compensated I and Q components. The decision maker  90  provides data associated with the compensated I and Q components. 
         [0032]    In this way, the invention enables the bit error rate to be reduced without influencing on the ratio of peak to average power at the amplifier (AMP)  44  in the radio transmission portion  40  because the precision in estimation of frequency and amplitude errors of the information signals is enhanced. 
         [0033]    Modification 
         [0034]    Though the embodiment has been described in conjunction with the 16-APSK the invention is applicable to any more-than-7-signal-point modulation scheme. Examples are presented for 2 m -QAM (quadrature amplitude modulation) (m≧3), 16-QAM, 8-PSK (phase shift keying) and QPSK (quadrature phase shift keying) in the following. 
         [0035]      FIGS. 5 through 8  are signal constellations for 2 m -QAM (m≧3), 16-QAM, 8-PSK and QPSK according to the principles of the invention. 
         [0036]    As for 2 m -QAM as shown in  FIG. 5 , if the information signal points ISi (i=1, 2, . . . , 2 m ) are written as (ISi I , ISi Q ) in I-Q coordinates, the points are expressed as follows: 
         [0000]      IS i   I   =s (2 m-1   a   1 +2 m-2   a   2 + . . . +2 0   a   m ) 
         [0000]      IS i   Q   =s (2 m-1   b   1 +2 m-2   b   2 + . . . +2 0   b   m )  (4) 
         [0037]    where s is a constant and each of a k  and b k  (k=1, 2, . . . m) represents 1 and −1,i.e., (a k , b k ) represents four points (1, 1), (1, −1), (−1, 1) and (−1, −1). In this case, the pilot signal PS is disposed on either of the I and Q axes such that the amplitude (R) of PS is larger than that (Ri) of any possible symbol points. In the specific example of  FIG. 5 , the pilot signal is disposed on the positive range of the I axis. 
         [0038]    In case of 16-QAM, the possible symbol points are expressed as follows: 
         [0000]      IS i   I   =s (2 1   a   1 +2 0   a   2 ) 
         [0000]      IS i   Q   =s (2 1   b   1 +2 0   b   2 ).  (5) 
         [0039]    In this case, the pilot signal PS is preferably disposed on either of the I and Q axes such that the amplitude (R) of PS is larger than that (Ri) of any possible symbol points ISi as shown in  FIG. 6   
         [0040]    In case of 8-PSK as shown in  FIG. 7 , the possible signal points are expressed as: 
         [0000]      IS i   I   =r *cos( iπ/ 4) 
         [0000]      IS i   Q   =r *sin( iπ/ 4).  (6) 
         [0041]    In this case, since the pilot signal PS is preferably disposed so as to make an angle of π/8 with adjacent information points, the pilot signal PS=(PS I , PS Q ) is preferably set to any of the following points: 
         [0000]    
       
         
           
             
               PS 
               I 
             
             = 
             
               R 
               * 
               
                 cos 
                  
                 
                   ( 
                   
                     
                       π 
                       4 
                     
                     + 
                     
                       π 
                       8 
                     
                   
                   ) 
                 
               
             
           
         
       
       
         
           
             
               PS 
               Q 
             
             = 
             
               R 
               * 
               
                 sin 
                  
                 
                   ( 
                   
                     
                       π 
                       4 
                     
                     + 
                     
                       π 
                       8 
                     
                   
                   ) 
                 
               
             
           
         
       
     
         [0042]    where k=1, 2, . . . , 8 and R is the amplitude of the pilot symbol that satisfy: 
         [0000]        r&lt;R≦ 1.6 *r.    
         [0043]    In case of QPSK as shown in  FIG. 8 , the possible signal points are expressed as: 
         [0000]    
       
         
           
             
               
                 
                   
                     
                       ISi 
                       I 
                     
                     = 
                     
                       r 
                       * 
                       
                         cos 
                          
                         
                           ( 
                           
                             
                               π 
                               4 
                             
                             + 
                             
                               π 
                               2 
                             
                           
                           ) 
                         
                       
                     
                   
                    
                   
                     
 
                   
                    
                   
                     
                       ISi 
                       Q 
                     
                     = 
                     
                       r 
                       * 
                       
                         
                           sin 
                            
                           
                             ( 
                             
                               
                                 π 
                                 4 
                               
                               + 
                               
                                 π 
                                 2 
                               
                             
                             ) 
                           
                         
                         . 
                       
                     
                   
                 
               
               
                 
                   ( 
                   7 
                   ) 
                 
               
             
           
         
       
     
         [0044]    In this case, since the pilot signal PS is preferably disposed so as to make an angle of π/4 with adjacent information points, the pilot signal PS=(PS I , PS Q ) is preferably set to any of the following points: 
         [0000]    
       
         
           
             
               PS 
               I 
             
             = 
             
               R 
               * 
               
                 cos 
                  
                 
                   ( 
                   
                     
                       π 
                       2 
                     
                      
                     
                       ( 
                       
                         1 
                         + 
                         i 
                       
                       ) 
                     
                   
                   ) 
                 
               
             
           
         
       
       
         
           
             
               PS 
               Q 
             
             = 
             
               R 
               * 
               
                 
                   sin 
                    
                   
                     ( 
                     
                       
                         π 
                         2 
                       
                        
                       
                         ( 
                         
                           1 
                           + 
                           i 
                         
                         ) 
                       
                     
                     ) 
                   
                 
                 . 
               
             
           
         
       
     
         [0045]    Many widely different embodiments of the present invention may be constructed without departing from the spirit and scope of the present invention. It should be understood that the present invention is not limited to the specific embodiments described in the specification, except as defined in the appended claims.