Dpsk communications with doppler compensation

Abstract

A data receiver receives differential phase-shift keyed (DPSK) signals, and filters the signal by a process including frequency conversion, under the control of an estimated carrier frequency (f.sub.re), to produce a filtered output signal which is applied to a DPSK demodulator. The filtering compensates for the Doppler frequency errors, and reduces the phase error. The estimated frequency is determined by second and third controllable filters, which filter the DPSK carrier signal at frequencies above and below the estimated carrier frequency by an offset frequency which depends on the data rate. A first frequency error estimate is made in a processor coupled to the second and third filters, from the ratio of the amplitudes of the first and second filter output signals. A second-order tracking loop is coupled to the processor for averaging the frequency error estimate over a predetermined number of bits, to generate the estimate of the carrier frequency. The Doppler frequency compensation loop tends to correct the phase error, but may leave residual phase errors. In an embodiment of the invention, an estimate of the phase change per bit is generated by a differential phase tracker coupled to the output of the first filter, and applied to the demodulator, in which it is used to aid in demodulation. The estimate of the phase change per bit may also be applied to the Doppler frequency compensation loop for aiding in the Doppler frequency compensation.