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Abstract

Spread spectrum communication using direct sequences that approximate a swept frequency waveform in which successive square waves are formed by the chips making up the sequence, and in which the durations of the square waves extend over a plurality of chips, and in which the frequency of the square waves varies across at least a portion of the sequence. Information is encoded by varying the direction of the direct sequence, so that the sequence is transmitted as either a forward or a reverse frequency-swept sequence. The spectrum of the direct sequences is in a passband. The direct sequence biphase modulates a carrier (e.g., RF) to produce a spread-spectrum signal with two passbands centered about the frequency of the modulated carrier. Received signals are fed to a correlator having forward and reverse sequence outputs, the outputs of the correlator are independently tracked, and generally different sampling times are used for taking samples of the forward and reverse correlator outputs for purposes of making decoding decisions. The receiver uses a local oscillator for demodulation that is not synchronized with the local oscillator used for modulation in the transmitter. The overall phase of successive direct sequences is pseudorandomly varied to produce a more uniform spreading of energy across the frequency band. Dual level coding is used, in which a quality and value outputs from the lower level decoding are used to provide, in effect, a correlator for the higher level decoding. Both 0 and 90 degree matched filter sections are provided to make the correlator insensitive to phase shifts.

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