In this thesis work, we propose a methode to improve the performance of the π/4-DQPSK modem by applying the passband carrier recovery to the conventional baseband processing of the π/4-DQPSK signal. Usually, the conventional π/4-DQPSK wireless modem compensates a frequency drift only in baseband. When a baseband signal downconverted from received signal suffers a severe signal degradation due to the excessive frequency drift, baseband signal processing, i. e. frequency drift compensation in baseband, encounters difficulty in handling the degraded signal and results in poor performance. Thus, in this thesis work, first, we process the signal in passband to compensate the instability of the local oscillator. We use a balanced quadricorrelator (BQ) at the receiver for passband carrier recovery. With BQ, we can minimize the signal degradation in passband and obtain a far better performance with additional frequency drift compensation in baseband using the signal from passband carrier recovery. When we use the passband carrier recovery alone, the carrier frequency tracking at the receiver is accomplished in a 150 symbol time for the frequency offset of 1000Hz. With the frequency compensation in passband as well as in baseband, the modem recovers data from the received sigal more rapidly within a 50 symbol time.
We simulate the π/4-DQPSK modem with a frequency error reflecting the instability of the local oscillator. To compensate the frequency error, we simulate the carrier recovery algorithms in passband as well as in baseband. Simulation results show that the modem recovers data more rapidly and the SNR gain of 4dB or above, is obtained.
