Effect of imperfect signal recovery on performance of QPSK communication system

Summary form only given. The contribution of this paper is in determining the bit error probability in detecting a quaternary phase-modulated signal Gray coded after transmission over the Gaussian communication channel in the presence of cochannel interference when the phase locked loop (PLL) is used for the synchronization reference signal extraction, which is imperfect. The numerical results are determined on the basis of the nonlinear PLL model. This nonlinear PLL model is much more complex to analyze than the linear one, but frequently a PLL system must operate in such conditions where the external fluctuations due to the additive noise are so intense that classical linear PLL theory neither characterises adequately the loop performance nor explains the loop behaviour. On the basis of the obtained results the influences of the signal-to-noise ratio in the channel, the signal-to-noise ratio in the loop bandwidth, the interference-to-signal ratio and the loop parameter values to the performance of the communication system are evident. From the obtained results it is possible to calculate the needed signal power, the interference-to-signal ratio and the loop parameter values in order to achieve the desired error probability. We give an exact comparison between the performance of a BPSK system in the presence of noise, imperfect carrier signal recovery and the interference determined in Draca et al. (1990), and the performance of the QPSK system in the presence of the same negative influences determined in this paper.