Direct differential detection of phase-shift-keyed signals: a local-oscillatorless DPSK receiver

The phase-diversity mixing concept, originally applied in communication receivers to local-oscillator-aided reception, is here applied to the direct (i.e. local-oscillatorless) differential detection (D³) of phase-shift-keyed (PSK) transmissions. The received signal and its delay replica are first compared by means of a quadrature mixer, directly at the receiver front end, at the optical or microwave signal stage. The baseband phase-diversity signals thus obtained are linearly transformed in a baseband phase equaliser, and thus cancel out undesired, detrimental phase shifts which invariably occur in the wave-generating and wave-conveying components. The resulting D³ PSK receiver is robust under variations in carrier frequency and in differential delay. Its development establishes coherent modulation methods, such as PSK and narrowband FSK, as viable alternatives to on/off keying, which at present is exclusively used in direct detection systems