Abstract:
Methods and apparatus for detecting multiple-level phase shift keying (MPSK) signals based on detectors that have nonlinear dynamics transfer characteristics are disclosed. The receiver circuit can be implemented easily using devices such as the op-amps to provide the required dynamic characteristics. The performance is enhanced by transmitting multiple cycles of the PSK signals and gating of the received waveforms.

Description:
CROSS-REFERENCES TO RELATED APPLICATIONS  
       [0001]    This application claims the benefit of U.S. Provisional Application No. 60/336,198, filed Dec. 4, 2001, entitled “METHOD AND APPARATUS FOR MULTI-LEVEL PHASE SHIFT KEYING COMMUNICATIONS.” 
     
    
     STATEMENT AS TO RIGHTS TO INVENTIONS MADE UNDER FEDERALLY SPONSORED RESEARCH AND DEVELOPMENT  
       [0002] NOT APPLICABLE 
     
    
     
       REFERENCE TO A “SEQUENCE LISTING,” A TABLE, OR A COMPUTER PROGRAM LISTING APPENDIX SUBMITTED ON A COMPACT DISK  
         [0003]    NOT APPLICABLE  
         BACKGROUND OF THE INVENTION  
         [0004]    This invention relates generally to techniques for generating pulses and more specifically to techniques for converting arbitrary analog waveforms to produce sequences of pulses.  
           [0005]    Phase Shift Keying (PSK) is a well-known modulation scheme and is used in much communication equipment. It has the best performance in an additive white Gaussian noise (AWGN) channel as compared to other modulation techniques, such as Frequency Shift Keying (FSK) and On Off Keying (OOK). For typical communication equipment that uses the PSK scheme, a coherent detector is used to recover the encoded digital information from a PSK modulated carrier. As many carrier cycles are required to recover the encoded symbol, the carrier frequency is usually very high as compared to the modulating signal.  
           [0006]    In commonly owned, co-pending U.S. Patent Application No. 09/850,713, filed May 7, 2001, entitled “Method &amp; Apparatus for Generating Pulses from Phase Shift Keying Analog Waveforms,” it discloses a receiver that is developed based on nonlinear circuits (commonly owned U.S. Pat. No. 6,259,390, incorporated herein for all purposes) that generate pulses from analog waveforms. The receiver configuration is capable of decoding one cycle of analog waveform to produce a group of pulses. Though the receiver system is efficient, further enhancement in performance is needed in the pulse processing subsystem.  
         BRIEF SUMMARY OF THE INVENTION  
         [0007]    A method and apparatus for detecting a received PSK modulated signal is disclosed. In one embodiment of the invention, the transmitted signal is an information waveform representative of one or more symbols to be communicated. The received signal is processed to produce a pulse waveform comprising groups of pulses. A detection waveform is used to mask out extraneous pulses that do not correspond to the information waveform. The remaining groups of pulses are then decoded by a pulse processing system to reproduce the original symbols. 
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0008]    The teaching of the present invention can be readily understood by considering the following detailed description in conjunction with the accompanying drawings:  
         [0009]    [0009]FIG. 1 shows a simplified block diagram of the transmitter in an illustrative embodiment of the present invention;  
         [0010]    [0010]FIG. 2 shows a simplified block diagram of the receiver in an illustrative embodiment of the present invention;  
         [0011]    [0011]FIG. 3 illustrates a typical transfer curve which characterizes the circuitry of the present invention;  
         [0012]    [0012]FIG. 4 shows the ideal received waveform and the gating signals for the BPSK modulation scheme;  
         [0013]    [0013]FIG. 5 illustrates a receiver circuit having two detectors for the communication system, according to an embodiment of the present invention;  
         [0014]    [0014]FIG. 6 shows the waveforms of the transmission and detection process based on BPSK modulation scheme;  
         [0015]    [0015]FIG. 7 shows the waveforms of the transmission and detection process based on QPSK modulation scheme; and  
         [0016]    [0016]FIG. 8 shows the waveforms of the transmission and detection process based on multiple cycle per symbol transmission.  
     
    
     DETAILED DESCRIPTION OF THE INVENTION  
       [0017]    [0017]FIG. 1 shows a block diagram of the transmitter, according to an embodiment of the present invention.  
         [0018]    The digital information source is shown as the block  10 . The MPSK modulator  11  modulates the digital source waveform to the desired MPSK signals (e.g., BPSK, QPSK, 8PSK, 16PSK etc) for transmission. To improve the BER performance, we can also use multiple cycles per symbol (e.g., 4 cycles per symbol etc) to encode each symbol. The modulated signal  12  is then amplified and/or wave shaped or up-converted to suitable wave  14  before being sent to the communication channel. The channel can be wire-line or wireless. In FIG. 1, an antenna  15  is shown for the case of a wireless channel.  
         [0019]    [0019]FIG. 2 shows the receiver system of the invention. For the case of a wireless channel, the system comprises an antenna  210  which receives the MPSK modulated transmitted signal. The received signals may pass through an optional amplifier and/or wave shaper circuit, or down-converter  200  to condition the incoming signal to make it suitable for optimum detection by the subsequent circuit. The conditioned signal  201  from the circuit  200  is then fed to a nonlinear circuit combination  206 , comprising an inductor  203  connected in series to a circuit  204 . The circuit  204  has an N-shaped I-V characteristic as shown in FIG. 3 with the impasse points positioned as shown. The lower impasse point is located at a small positive voltage.  
         [0020]    The output  202  from the circuit  204  comprises groups of pulses or periods of silence depending on the received signals. A gating circuit  209  and a pulse processing circuit  207  then determine the appropriate decoded digital signal  208  based on the received groups of pulses. The gating circuit sets suitable timing windows which are temporally aligned with the information waveform at the transmitting end of a communication system.  
         [0021]    The gating function serves to mask out those pulses which do not correspond to the pulses in the original information waveform, while leaving the remaining groups of pulses which correspond to the information waveform intact. By detecting the number of pulses in each group, we can reproduce the symbols represented by the information waveform. This approach improves the receiver BER performance substantially. In the case of BPSK signals, two gating circuits are used, each with a different gating window as shown in FIG. 4.  
         [0022]    The characteristic curve of the circuit  204  is shown in FIG. 3. The transfer curve has two impasse points P 1 =(V v,  i v ) and P 3 =(V p,  i p ). Here, i v  and i p  represent the valley and the peak current of the N curve. In general, we do not require that the curves be piecewise linear. The only requirement is that the characteristic curve consists of three distinct regions such that the middle region is having negative impedance slope, while the two external regions are having positive impedance slopes. Under the condition that the input signal is operating at the line segment P 1 -P 3  of the characteristic curve, pulses will be generated which traveled along the state trajectory P 4  P 3  P 2  P 1  P 4 . The number of pulses being generated depends on the available time (i.e., the duration that the input signal is operating on the line segment P 1 -P 3 ) and the speed of the trajectory.  
         [0023]    Referring now to FIG. 5, we show another receiver configuration that is used in the form of dual detector mode.  
         [0024]    In this illustrative embodiment of the invention, a duo detector configuration is shown and it can also be extended to multiple detector configurations. The I-V characteristics of each N-type circuit may be constructed to have different set of impasse points, so that it responds to the input signals differently than another of the N-type circuits, which is characterized by its own set of impasse points.  
         [0025]    Similar to the single detector system, the second detector circuit  512  also consists of an inductor  509  and another nonlinear circuit  510  connected in series. The nonlinear circuit  5   10  also has an N-type I-V transfer characteristics. However, the transfer curve is positioned at different location by applying suitable voltage at the input  511 , and biasing etc. The input  504  and  511  can also be used to dynamically manipulate the transfer curves. The output from the circuit  512  also consists of a series of pulses or silences depending on the received signals. As the transfer curves of the circuits  505  and  512  are different, they responded to the same input signal  501  differently.  
         [0026]    The pulse processing circuit counts the number of pulses that occur in each gating circuit outputs and form a metric. Based on the values of the metric, it determines which is the most likely symbol being transmitted.  
         [0027]    Next, we describe the response of the system in FIG. 5. In the following, we first explain using M=2-ary BPSK modulation scheme.  
         [0028]    [0028]FIG. 6 illustrates a typical response of the receiver shown in FIG. 4 based on numerical simulation. The waveform  601  is the symbol to be transmitted. In this illustrative example, the signal that is being transmitted is the symbol { 1   2   1   1  }. The BPSK signal is shown as the waveform  602 . Due to the additive white Gaussian noise presence in the channel, the received signal is corrupted and is shown as the waveform  603 . The outputs from the two nonlinear circuits  505  and  512  comprise a series of pulses depending on the location of the signals as well as the level of the noises. This is shown as the waveform  604  and  605  for the positive and negative detectors in FIG. 5 respectively. Depending on the tuning of the nonlinear circuit, the presence of the digital signal can be set to generate a specified number of pulses. In this illustrative example, seven pulses are generated if a low noise signal is received. The waveform  606  shows the gating waveform for the symbol  1 . The gating waveform has two weighting values of ±1. The waveform  607  shows the signals after the gating function. Upon receiving these pulses, the pulse processing system determines the decoded digital signals. Essentially, the pulse processing system performs the following tasks: 1. For each half cycle, calculate the metric of each symbol δ i , 0≦i≦M−1, by summing the number of positive and negative pulses. 2. Compare the metrics of each symbol and decides that x m (t) is the most likely transmitted symbol if δ m  is the largest amongst all the δ i . In this illustrative example shown in FIG. 6, the decoded symbol is shown as  608  which is the same as the symbol sent.  
         [0029]    [0029]FIG. 7 illustrates another example for the case of QPSK modulation scheme. In this case, the symbol that is being sent is { 4   1   3   2 } which is shown as  701 . The transmitted signal is shown as waveform  702 . The received waveform is shown as  703 . The pulses that are generated from the two N-type circuits are shown as  704  and  705 . The gating signals for the symbol  1  is shown as  706 . The resultant signals after the gating function is illustrated as  707  and the recovered symbols are shown as  708 .  
         [0030]    The bit error rate performance of the receiver can be improved by employing multiple cycle per symbol for the transmission. FIG. 8 illustrates the response with four cycles per symbol based on the BPSK scheme. In the figure, the symbol that is being transmitted is the symbol set { 1   1   2   1  } shown as  801 . The BPSK signal is shown as  802  and the noisy received signal is  803 . Pulses are generated at the output of the nonlinear circuits and are shown as  804  and  805 . These pulses are passed through gating circuits and the waveform  806  shows a gating signal for the symbol  1 . The resultant signal is shown as  807  and recovered symbols are shown as  808 .