Abstract:
A digital multi-level multi-phase modulation system utilizes quaternary differential encoding and decoding of only the first two of N digital signal trains. A decision circuit is used to examine the frame pulses in one of the first two signal trains and in at least one of the remaining signal trains and generates output signals which can be used in a gate circuit to resolve the phase-lock ambiguity of the recovered carrier and thereby reproduce the original N signal trains.

Description:
BACKGROUND OF THE INVENTION 
     This invention relates to a digital multi-level multi-phase modulation communication system. 
     Conventionally, digital multi-level multi-phase modulation communication systems use coherent detection for phase demodulation at the receiving end because of its advantage in respect of the carrier to noise ratio (C/N ratio) required. However, due to the phase ambiguity in phase-locking that occurs during the reproduction of the reference carrier, the coherent detection system sometimes produces a demodulated pulse pattern differing from the pulse pattern supplied. To prevent this phenomenon, the prior art has used the differential encoder/decoder circuitry. For instance, according to the 2 m  -phase PSK modulation system (m is 1, 2, 3 . . . ), the pulse pattern fed is subjected to 2 m  -ary differential encoding, and the pulse train demodulated at the receiving end is subjected to 2 m  -ary differential decoding to thereby reproduce the original signal. 
     The 16 QAM modulation communication system is most popular as the 2 m  -phase PSK modulation system. Since the system has 12 phases, it requires theoretically duodenary differential encoding, but it actually needs only quaternary differential encoding because, in order to provide 4-level modulated signals, the phase ambiguity in phase-locking that occurs to the reference carrier reproduced must be limited to 4 phases at π/2 radians. This feature is embodied in one conventional type of 16 QAM communication system that uses two quaternary differential encoder circuits at the transmitting end and uses two quaternary differential decoder circuits at the receiving end to demodulate the received signal into the original signal. The system uses a very simple differential coding (encoding/decoding) operation, but it has a bit error rate twice as high as the other systems because all four trains of the baseband digital input signal undergo the quaternary differential encoding/decoding. For details of the above described 16 QAM communication system, reference is made to: 
     (1) I. Horikawa et al., &#34;Characteristics of a High Capacity 16 QAM Digital Radio System on a Multi-path Fading Channel,&#34; ICC &#39;79 Conference Record, Volume 3 or 4, pp. 48.4.1-48.4.6, June 10-14, 1979; 
     (2) Japanese patent application Disclosure No. 109811/77 (disclosing the technique reported in reference (1)) 
     SUMMARY OF THE INVENTION 
     Therefore, the primary object of this invention is to provide a digital multi-level multi-phase modulation/demodulation system which is free from the above defect of the conventional system and has a low bit error rate. 
     This invention provides a digital multi-level multi-phase modulation communication system having a transmitting section and a receiving section: wherein the transmitting section includes first means (8) for performing quaternary differential encoding on the first two trains (S 1 , S 2 ) of N trains (N is an integer of 4 or more) of digital signal, a frame pulse being included in (N-1) of said digital signal trains, to generate a first pair of digital signals (S 1  &#39;, S 2  &#39;); and second means (2-7, 10, 11) for generating a modulated wave by performing multi-level multi-phase modulation on the carrier with a first set of (N-2) digital signals comprising the first (N-2) trains of digital signal (S 3  -S n ) the first two trains of digital signal, and with the second two trains of digital signal; and wherein the receiving section includes third means (12-14) for performing coherent detection and multi-level decision on the modulated wave to generate a second set of (N-2) digital signals (S 3  &#34;-S N  &#34;) and a second pair of digital signals (S 1  &#34;,S 2  &#34;) corresponding to the first set of (N-2) digital signals and the first pair of digital signals, respectively, fourth means (15) for performing quaternary differential decoding on the second pair of digital signals (S 1  &#34;,S 2  &#34;) to generate the first two trains of digital signal, fifth means (17) for establishing frame synchronism by means of the output signal (S 2 ) from the fourth means, sixth means (21-23) responsive to the output of the fifth means for extracting a frame pulse corresponding to the frame pulse included in the second set of (N-2) trains of digital signal and in the output signal of the fourth means, seventh means (24, 25) responsive to the output of the sixth means for generating a control signal (G 1 , G 2 ) representing the phase-locking state of the carrier recovered in said third means, and eighth means (19) responsive to the control signal for converting the second set of (N-2) digital signals into the same trains as the first set of (N-2) digital signals. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     This invention is now described in detail by reference to the accompanying drawings, in which: 
     FIG. 1 is an illustrative block diagram of the conventional 16 QAM communication system; 
     FIG. 2 is an illustrative signal vector mapping for 16 QAM; 
     FIG. 3 is an illustrative block diagram of the 16 QAM communication system according to this invention; 
     FIG. 4 is a specific circuit diagram of the decision circuit of FIG. 3; 
     FIG. 5 is a specific circuit diagram of the gate circuit of FIG. 3; 
     FIG. 6 is an illustrative signal vector mapping for 16 QAM of this invention; 
     FIG. 7 is another specific circuit diagram of the decision circuit of FIG. 3; 
     FIG. 8 is another specific circuit diagram of the gate circuit of FIG. 3; and 
     FIG. 9 is an illustrative block diagram of a 64 (8×8) QAM communication system according to this invention. 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     In FIG. 1, reference numeral 101 designates a transmitting section; 102, a receiving section; 1, a transmit data processing unit; 2, a transmit local oscillator; 3, a divider; 4, a π/2 phase shifter; 5 and 6, each an amplitude modulator; 7, a combiner; 8 and 9, each a summing logic circuit; 10 and 11, each a D/A converter circuit; 12, a QAM demodulator circuit; 13 and 14, each an A/D converter circuit; 15 and 16, each a subtracting logic circuit; 17, a frame synchronizer circuit; and 18, a receive data processing unit. In the transmitting section 101, input data signals DATA 1 and 2 are supplied to the transmit data processing unit 1 where the symbol rates of DATA 1 and 2 are changed for adding frame pulses for frame synchronization, stuff pulses for providing synchronism between input signals, and pulses for order-wire channel to the DATA 1 and 2 which are then deserialized to generate four signal trains S 1  to S 4 . While the above description assumes the supply of two input DATA 1 and 2, it is to be understood that any number can be selected for the number of input signal trains. 
     The pair of signals S 1  and S 2  and that of S 3  and S 4  are supplied to the summing logic circuits 8 and 9, respectively, where they are subjected to quaternary differential encoding to generate a pair of signals S 1  &#39; and S 2  &#39; and that of S 3  &#39; and S 4  &#39;. The signals S 1  &#39; and S 3  &#39; are supplied to the D/A converter circuit 10 to be converted to a 4-level signal P, and the signals S 2  &#39; and S 4  &#39; supplied to the D/A converter 11 to be converted to a 4-level signal Q. The signals P and Q are supplied to the amplitude modulators 5 and 6, respectively. To the other input of the modulator 5 is supplied a carrier obtained by branching in the divider 3 the carrier from the transmit local oscillator 2, and to the other input of the modulator 6 is supplied a carrier which is in quadrature with the carrier supplied to the modulator 5 and which is obtained by being passed through the π/2 phase shifter 4 downstream of the divider 3. These carriers are modulated with the signals Q and P, combined together in the combiner 7 to achieve 16 QAM as illustrated in FIG. 2, which is then supplied to the receiving section 102. In the receiving section 102, the QAM demodulator circuit 12 performs coherent detection to demodulate the 4-level signals P and Q which are fed to the A/D converter circuits 13 and 14 for 4-level decision to generate two pairs of 2-level digital signals S 1  &#34; and S 3  &#34; as well as S 2  &#34; and S 4  &#34;. These signal pairs correspond to the signal pairs S 1  &#39; and S 3  &#39; as well as S 2  &#39; and S 4  &#39;, respectively, generated in the transmitting section 101, but due to the phase ambiguity in phase-locking with the reference carrier recovered at the QAM demodulator circuit 12, their pattern arrays may or may not agree with digital signals S 1  &#34; and S 4  &#39;. Therefore, the pair of signals S 1  &#34; and S 2  &#34; as well as the pair of signals S 3  &#34; and S 4  &#34; are subjected to quaternary differential decoding in the subtracting logic circuits 15 and 16, respectively, to provide signals S 1  to S 4  that agree with the signals fed S 1  to S 4 . The signal S 4  is branched and supplied to the frame synchronizer circuit 17 (for a possible embodiment of the circuit, see U.S. Pat. No. 3,978,285) for achieving frame synchronization to reproduce a frame timing pulse FS. The signal FS is supplied to the receive data processing unit 18. In the unit 18, through the conversion process which is the reverse of the process performed in the transmit data processing unit 1, the signal FS removes from the input signals S 1  to S 4  all additional signals that have been inserted into said signals at the transmit data processing unit 1, thus reproducing signals in agreement with the original signals DATA 1 and 2. As described above, the conventional 16 QAM communication system of FIG. 1 has the two quaternary differential coding circuits at the transmitting and receiving sections, respectively. Accordingly, a error rate in this system is about twice as high as that in the system without any differential coding circuit. 
     FIG. 3 is one embodiment of the 16 QAM communication system according to this invention which is free from the above described defect of the conventional system: In FIG. 3, a transmitting section is generally indicated at 201, and a receiving section at 202; 19 is a gate circuit and 20 is a decision circuit. In FIG. 3, like numbers identify like components of FIG. 1. Referring now to the transmitting section 201, signals S 1  to S 4  are produced in the transmit data processing unit 1. Signals S 1 , S 3  and S 4  or signals S 2 , S 3  and S 4  have a frame pulse inserted therein. The following description assumes that the frame pulse is not inserted in signal S 1 , that is, the pulse is inserted in signals S 2 , S 3  and S 4  (the frame pulse train is hereinafter referred to as signal M). The signals S 1  and S 2  are supplied to the summing logic circuit 8 where they are subjected to quaternary differential encoding to generate signals S 1  &#39; and S 2  &#39;. The signals S 3  and S 4  are not subjected to the differential encoding. The pair of signals S 1  &#39; and S 3  and that of S 2  &#39; and S 4  are supplied to the D/A converters 10 and 11, respectively. The D/A converter 10 generates a 4-level signal P&#39;, and the D/A converter 11 a 4-level signal Q&#39;. The signals Q&#39; and P&#39; are supplied to the amplitude modulators 5 and 6 where the signals modulate carriers which are in quadrature with each other as described above. The combiner 7 combines the modulated carriers and, as described above, produces 16 QAM vector signals as illustrated in FIG. 2. 
     Referring now to the receiving section 202, the 4-level signals P&#39; and Q&#39; demodulated in the QAM demodulator 12 are supplied to the A/D converter circuits 13 and 14, respectively, which perform 4-level decision to generate a pair of signals S 1  &#34; and S 3  &#34; and that of signals S 2  &#34; and S 4  &#34;. The signals S 1  &#34; and S 2  &#34; that correspond to the signals S 1  and S 2  are supplied to the subtracting logic circuit 15 where they are subjected to quaternary differential decoding to reproduce signals that agree with the signals S 1  and S 2 . The signal S 2  is branched into two, one of which is supplied to the frame synchronizer circuit 17 to establish frame synchronization for reproduction of the frame timing signal FS. The signals S 3  &#34;, S 4  &#34;, S 2  and FS are supplied to a decision circuit 20 which generates decision signals G 1  and G 2  representing phase-locking state of the carrier recovered in the QAM demodulator circuit 12. The decision signals G 1  and G 2  are supplied to the gate circuit 19. The gate circuit 19 uses the decision signals G 1  and G 2  to control the input signals S 3  &#34; and S 4  &#34; so that they agree with the signals S 3  and S 4  regardless of the phase-locking state of the recovered carrier in the QAM demodulator circuit 12, and as a result the circuit 19 generates signals S 3  and S 4 . The thus reproduced signals S 1  to S 4  are supplied to the receive data processing unit 18 which is responsive to the signal FS to produce signals DATA 1 and 2 by removing the additional bits inserted in the original signals in the transmit data processing unit 1. 
     The decision circuit 20 and gate circuit 19 of FIG. 3 are hereinafter described by reference to FIGS. 4 and 5 which illustrate an embodiment of the decision circuit and gate circuit, respectively, wherein reference numerals 21 to 23 designate D-type flip-flops; 24 and 25, Exclusive-OR (EX-OR) gates; 26, a channel selector; 27, 28 47 and 48, OR/NOR gates; 29 to 44, AND gates; and 45 and 46, OR gates. 
     Referring to FIG. 4, signals S 2 , S 3  &#34; and S 4  &#34; are supplied to the D-type flip-flops 21 to 23, respectively, where they are sampled with the frame timing pulse FS, and as a result, the frame pulse M is extracted from each signal. Table 1a below shows the change in the signals S 3  &#34; and S 4  &#34; depending on the phase-locking state of the recovered carrier at the QAM demodulator 12. Table 1b shows the condition of the frame pulse M extracted from the signals S 3  &#34; and S 4  &#34;, following the change indicated in Table 1a. The outputs of the flip-flops 21 and 23 are supplied to the Exclusive-OR gates 24 and 25 where an Exclusive-OR operation is applied to the outputs to generate decision signals G 1  and G 2 . respectively, as identified in Table 1c below. 
     
                       TABLE 1______________________________________Locked  a           b              cPhase   S.sub.3 &#34;           S.sub.4 &#34;                   S.sub.3 &#34;                         S.sub.4 &#34;                               S.sub.2                                    G.sub.1                                         G.sub.2______________________________________1       S.sub.3 S.sub.4 M     M     M    0    02       -S.sub.4           S.sub.3 --M   M     M    1    03       -S.sub.3           -S.sub.4                   M     M     M    1    14       S.sub.4 -S.sub.3                   --M   --M   M    0    1______________________________________ 
    
     The gate circuit 19 is such that is satisfies the relationships defined in Table 1a, 1b and 1c. 
     
                       TABLE 2______________________________________Locked Phase  S.sub.3                S.sub.4    G.sub.1                               G.sub.2______________________________________1             S.sub.3 &#34;                S.sub.4 &#34;  0   02             S.sub.4 &#34;                -S.sub.3 &#34; 1   03             -S.sub.3 &#34;                -S.sub.4 &#34; 1   14             -S.sub.4 &#34;                S.sub.3 &#34;  0   1______________________________________ 
    
     Table 2 is equivalent to Table 1a and 1c except that the change in the signals S 3  and S 4  is indicated. FIG. 5 is an illustrative circuit diagram embodying the relationships indicated in Table 2. It is to be noted that the size of the channel selector 26 of FIG. 5 can be reduced noticeably by using a commercially available integrated circuit containing 2 or 3 selectors in one package. The foregoing description concerns one embodiment of this invention as applied to the modulated signal vector mapping of FIG. 2, and a slight modification becomes necessary with a different modulated signal vector mapping. Therefore, another embodiment of this invention is now described in connection with its application to the modulated signal vector mapping of FIG. 6. Table 3a below shows the change in signals S 3  &#34; and S 4  &#34; depending on the phase-locking state at the QAM modulator 12. Let it be assumed that frame pulses M of opposite polarities are inserted in the S 3  and S 4  fed (M for S 3  , and M for S 4 ). Frame pulses as indicated in Table 3b are extracted from the signals S 3  &#34; and S 4  &#34;. Therefore, if the frame pulse extracted from one of the signals S 3  &#34; and S 4  &#34; (S 3  &#34; for the purpose of the present description) and that from the signal S 2  are supplied to an Exclusive-OR gate, and the circuit produces at the output a decision signal G 3  as indicated in Table 3c. 
     
                       TABLE 3______________________________________    a         b              cLocked Phase      S.sub.3 &#34;              S.sub.4 &#34;                      S.sub.3 &#34;                            S.sub.4 &#34;                                  S.sub.2                                       G.sub.3______________________________________1          S.sub.3 S.sub.4 M     --M   M    02          S.sub.4 S.sub.3 --M   M     M    13          S.sub.3 S.sub.4 M     --M   M    04          S.sub.4 S.sub.3 --M   M     M    1______________________________________ 
    
     FIG. 7 is circuit diagram of the decision circuit 20&#39; that embodies the relationships indicated in Table 3. In the Figure, reference numerals 49 and 50 designate D-type flip-flops; and 51, an Exclusive-OR gate. The gate circuit 19&#39; is such that it satisfies the relationships defined in Tables 3a and 3c. Table 4 below is the equivalent of Table 3a and 3c except that the change in signals S 3  and S 4  is shown. FIG. 8 is a circuit diagram of the gate circuit 19&#39; that embodies the relationships indicated in Table 4; in the Figure, reference numeral 52 designates an OR/NOR gate, 53 to 56, AND gates; 57 and 58, OR gates. 
     
                       TABLE 4______________________________________Locked Phase S.sub.3      S.sub.4                            G.sub.3______________________________________1            S.sub.3 &#34;    S.sub.4 &#34;                            02            S.sub.4 &#34;    S.sub.3 &#34;                            13            S.sub.3 &#34;    S.sub.4 &#34;                            04            S.sub.4 &#34;    S.sub.3 &#34;                            1______________________________________ 
    
     FIG. 9 is an embodiment of this invention as applied to the 8×8 QAM system, wherein: reference numeral 59 denotes a transmit data processing unit; 60 and 61, each a 2-level to 8-level converter circuit; 62 and 63, each an 8-level to 2-level converter circuit; 64, a gate circuit the same as the circuit 19; and 65, a receive data processing unit. The intended object of the invention can be achieved simply by adding the gate circuit 64 to the circuit components of FIG. 3, except that the 2-level/4-level D/A and A/D converter circuits of FIG. 3 are replaced by 2-level/8-level D/A and A/D converters. It is to be understood that the same circuit configuration can be applied to the 8×4 QAM system except that either signal S 5  or S 6  is deleted at the transmitting end. 
     As described in the foregoing, the system of this invention performs quaternary differential encoding/decoding on signals S 1  and S 2 , but no such encoding/decoding need be applied to signals S 3 , S 4  and higher-order signals. Therefore, the system is not only free from the potential increase in the bit error rate but it can be implemented with a simple circuit configuration. In addition, as will be readily understood from the description hereinabove, the system of this invention can be applied to 2 l  ×2 k  QAM (wherein l=2, 3, 4 . . . ; k=2, 3, 4 . . . ) system.