Coding method and system with enhanced security
Electric circuit for the production of a number of different codes
Method of coding and decoding messages Patent #: 4465153
ApplicationNo. 06/535923 filed on 09/26/1983
US Classes:375/150, Correlation-type receiver375/336, Minimum shift keying375/343, Correlative or matched filter380/42, Data stream/substitution enciphering380/46, Nonlinear (e.g., pseudorandom)380/47, Plural generators708/818Having tapped delay line
ExaminersPrimary: Safourek, Benedict V.
Attorney, Agent or Firm
International ClassesH04B 1/707 (20060101)
H04J 13/00 (20060101)
AbstractIn a spread spectrum communications system employing cyclic code shift keying as its primary modulation, the transmission waveform is spread for transmission security by modulo-2 adding a pseudo-noise sequence to the CCSK data symbols prior to phase modulating onto a carrier signal for transmission. If the transmission modulation is minimum shift keying (MSK) the two components of the data stream are applied to the carrier with a differential encoding step implicit in the modulation scheme. This differential encoding characteristic makes stripping of the PN spread function prior to CCSK demodulation difficult at the receiving end. In order to demodulate this waveform in an optimum manner, an array correlator, the adjacent correlator stages of which have one chip relative time displacements of their CCSK reference waveform, is employed. In effect the array correlator becomes a parallel array of matched filters matched to each cyclic shift of the incoming waveform. By modulo-2 addition of the PN spreading waveform with the time displaced CCSK reference waveform in each stage of the correlator, the correlator can be made to match all versions of the spread symbol being received.