Demod-Remod Coherent Tracking Receiver for QPSK and SQPSK

A demod-remod coherent tracking loop for QPSK and SQPSK is described and analyzed. This type of receiver is attractive since all of the available signal power is despread and used to generate the tracking error signal when the pulses reaching the nonlinearities are rectangular. The same demod-remod circuit is widely used for bandlimited pulses, for which less than full power is converted to the carrier phase error signal. All system parameters pertinent to system performance are enumerated. The effects of the IF and arm filters on the statistics of the noise are determined. With respect to the signal, the filters are considered only insofar as they reduce the delivered signal power. It is demonstrated how a tracking phase error converts some of the signal power into additional noise power. The results of performance computation are presented.