Method and apparatus for demodulation of relative phase modulated binary data

• US 4,007,330 A
• Filed: 08/13/1975
• Issued: 02/08/1977
• Est. Priority Date: 08/13/1975
• Status: Expired due to Term

• Current Assignee: Allied Corp. (Honeywell International Inc.)
• Original Assignee: Bunker Ramo Corporation (Honeywell International Inc.)
• Inventors: Winters, Michael G.
• Primary Examiner(s): Safourek, Benedict V.
• Assistant Examiner(s): Masinick, Michael A.
• Application Number:
  US05/604,085
• Time in Patent Office: 545 Days
• Field of Search: 178/66 R, 178/67, 178/88, 325/344, 325/345, 325/349, 325/45, 325/47-49, 325/63, 325/65, 325/323, 325/324, 325/474-476, 329/112, 329/126, 329/142
• US Class Current: 375/330
• CPC Class Codes: H04L 27/2331 wherein the received signal is demodulated using one or more delayed versions of itself

Abstract

A phase modulated carrier wave, in which a phase reversal represents one of two binary values is demodulated by sampling at a rate f_s greater than the frequency of the carrier, comparing the phase of each sample with a sample delayed a nominal bit period in one channel, a sample delayed less than the nominal bit period in a second channel, and a sample delayed more than the nominal bit period in a third channel to compensate for positive and negative Doppler shift of the carrier from its nominal frequency in the second and third channels, respectively. A code word is detected in each of the three channels by comparing each demodulated sample bit with a replica of the code word, and correlating the results separately for each channel. The channel with the highest correlation is selected as the one most closely tuned to the carrier frequency received.

Claims

1. In a communication system employing a carrier on which binary signals have been modulated by reversing the phase of the carrier 180°

for representation of one binary value and not reversing phase for representation of the other binary value, a method of demodulating binary digits modulated on said carrier comprising

conditioning the modulated carrier signal received in preparation for demodulation by phase comparison of each data bit period of said carrier with its succeeding data bit period,

delaying the conditioned carrier signal for a plurality of distinct periods with at least one delay period less than the nominal period of a data bit modulated on said carrier, one delay period equal to said nominal data bit period, and one delay period greater than said nominal data bit period,

phase comparing the conditioned carrier signal as delayed by said distinct periods in separate channels with the conditioned carrier signal undelayed, thereby demodulating said conditioned carrier signal with compensation for positive Doppler shift, no Doppler shift and negative Doppler shift, and

selecting the channel producing a signal having the most detectable binary signals in a first group of a number, N, of successive data bit periods.

2. A method as defined by claim 1 wherein said first group of successive data bit periods is comprised of an N-bit code and the selecting process is comprised of the steps of

continually code comparing the output of each demodulation channel with a static replica of said N-bit code, each demodulated bit being compared in sequence with every replica bit to produce a comparison bit 1 each time a demodulated bit compares with a replica bit,

summing separately for each demodulation channel all comparison bits produced by the comparison of demodulated bits with replica bits at any one time to produce a value the magnitude of which represents the degree of correlation between demodulated bits and said replica bits, and

determining which channel reaches the highest correlation.

3. A method as defined in claim 2 wherein

the step of conditioning the modulated carrier signal includes converting the modulated carrier signal to a square wave of the same frequency and phase as said carrier signal, periodically sampling said square wave at a constant rate to produce a number, S, of samples during each nominal data period,

wherein the step of phase comparing the conditioned carrier signal in the separate demodulation channels is comprised of comparing S-Δ

S, S and S+Δ

S samples of the delayed conditioned carrier signal with a corresponding number of undelayed samples, where -Δ

S is a number selected for compensation of positive Doppler shift and +Δ

S is a number selected for compensation of negative Doppler shift such that the respective delays in the delaying process are for periods of S-Δ

S, S and S+Δ

S sample periods, the step of code comparing is carried out for every sample of every demodulation channel, and

where correlation values of the respective demodulation channels are summed over the last N(S-Δ

S), N.sup.. S and N(S+Δ

S) samples demodulated and compared with the replica bits.

4. In a communication system employing a carrier on which binary signals have been modulated by reversing the phase of the carrier 180°

for representation of one binary value and not reversing phase for representation of the other binary value, a method of demodulating binary digits modulated on said carrier comprising

converting the modulated carrier signal to a square wave of the same frequency and phase as said carrier signal,

sampling said square wave at a constant rate to produce a number, S, of samples during each data bit period,

delaying said samples for a data bit period,

comparing the phase of said delayed samples with undelayed samples, thereby demodulating said square-wave signal samples to produce demodulated data bit samples

continually code comparing said demodulated data bit samples with a static replica of an N-bit code modulated on said carrier, each demodulated data bit sample being compared in sequence with each replica bit to produce a bit 1 each time a demodulated data bit sample compares with a replica code bit,

summing separately all comparison bits produced during each sample period to produce a value the magnitude of which represents the degree of correlation between demodulated bits and said replica bits, and

accumulating said values to produce a correlation sum over the last N.sup.. S samples.

5. The method of claim 4 including the step of comparing said correlation sum during each sample period to determine if it exceeds a predetermined threshold value.

6. In a communication system employing a carrier on which binary signals have been modulated by reversing the phase of the carrier 180°

for representation of one binary value and not reversing phase for representation of the other binary value, apparatus for demodulating binary digits modulated on said carrier comprising

means for conditioning the modulated carrier signal received in preparation for demodulation by phase comparison of each data bit period of said carrier with its succeeding data bit period,

a plurality of delay means for delaying the conditioned carrier signal output of said conditioning means for a plurality of distinct periods, one delay means for a period less than the nominal period of a data bit modulated on said carrier, one delay means for a period equal to said nominal data bit period, and one delay means for a period greater than said nominal data bit period,

plurality of phase comparing means for comparing the output of each delay means with said conditioned carrier undelayed, thereby demodulating said conditioned carrier signal with compensation for positive Doppler shift, no Doppler shift and negative Doppler shift in respective comparing means, and

means connected to each of said plurality of phase comparing means for selecting the phase comparing means producing a signal having the most detectable binary signals in a first group of a number, N, of successive data bit periods.

7. Apparatus as defined in claim 6 wherein said first group of successive data bit periods is comprised of an N-bit code, and said selecting means is comprised of

a plurality of code comparing means, a separate one for continually comparing the output of a different one of said phase comparing means with a static replica of said N-bit code, each demodulated bit being compared in sequence with every replica bit to produce a comparison bit 1 each time a demodulated bit compares with a replica bit,

a plurality of summing means, a separate one for summing the comparison bits from a different one of said code comparing means produced by the comparison of demodulated bits with replica bits at any one time to produce a value the magnitude of which represents the degree of correlation between demodulated bits and said replica bits, and

means for determining which summing means reaches the highest correlation.

8. Apparatus as defined in claim 7 wherein said signal conditioning means includes means for converting the modulated carrier signal to a square wave of the same frequency and phase as said carrier signal and means for periodically sampling said square wave at a constant rate to produce a number, S, of samples during each nominal data period, whereby phase comparisons by said phase comparing means is comprised of comparing S-Δ

S, S and S+Δ

S samples of the delayed samples with a corresponding number of undelayed samples, where -Δ

S is a number selected for compensation of positive Doppler shift and +Δ

S is a number selected for compensation of negative Doppler shift such that the respective delays in said plurality of delay means are for periods of S-Δ

S, S and S+Δ

S sample periods, and wherein each of said code comparing means operates on every sample, and including a plurality of accumulating means for accumulating separately said correlation values of said separate summing means over the last N(S-Δ

S), N.sup.. S and N(S+Δ

S) sample periods.

9. In a communication system employing a carrier on which binary signals have been modulated by reversing the phase of the carrier 180°

for representation of one binary value and not reversing phase for representation of the other binary value, apparatus for demodulating binary digits modulated on said carrier comprising

means for converting the modulated carrier signal to a square wave of the same frequency and phase as said carrier signal,

means for sampling said square wave at a constant rate to produce a number, S, of samples during each data bit period,

means for delaying said samples for a data bit period,

means for comparing the phase of said delayed samples with undelayed samples, thereby demodulating said square-wave signal samples to produce demodulated data bit samples,

means for continually code comparing said demodulated data bit samples with a static replica of an N-bit code modulated on said carrier, each modulated data bit sample being compared in sequence with each replica bit to produce a bit 1 each time a demodulated data bit sample compares with a replica code bit,

means for summing separately all comparison bits produced during each sample period to produce a value the magnitude of which represents the degree of correlation between demodulated bits and said replica bits, and

means for accumulating said values to produce a correlation sum over the last N.sup.. S samples.

10. Apparatus as defined in claim 9 including means for comparing said correlation sum druing each sample period to determine if it exceeds a predetermined threshold value.