Measured flat fading performance of Doppler corrected Nyquist filtered 4800 bps DQPSK modem

We consider the measured performance of three Doppler correction algorithms with asynchronous timing recovery for the differential detection of filtered DQPSK transmitted over a flat fading channel. The three algorithms all involve a combination of open and closed loop components. Their relative performance is found to be similar and, in addition, all have similar complexity when implemented in digital-signal processor (DSP) firmware. Some algorithms have burst communication application but herein we only do real time tests for serial communication links. One fading simulator is based on DSP firmware and realizes the Rician fading model with time variation based on the vehicle Doppler frequency. We also present results that include a commercial channel simulator that operates at an intermediate frequency of 70 MHz. Our experiments are all conducted at a transmission rate of 4800 bps and the results are expected to be applicable to DSP based, mobile satellite communications. Implementation losses are measured relative to a digital computer simulation of the communication process. Losses are found to be a small fraction of a dB, even in severe Doppler variation environment (i.e., a Doppler rate of 20 Hz/s)