Performance of symbol timing recovery, frame synchronization and equalization for continuous-phase modulation using limiter-discriminator detection.

摘要

Continuous-phase modulation (CPM) is a nonlinear modulation that has a good performance in error probability and bandwidth efficiency. CPM signals also have constant envelope which makes the use of very efficient, but nonlinear power amplifiers attractive.;The applications of CPM have been limited to simple schemes such as binary, full response schemes, due to complexity in implementing the optimum coherent receiver which is followed by a maximum likelihood sequence estimator (MLSE). The implementation complexity comes from the number of states needed to describe a CPM signal using a multilevel, partial response scheme. Consequently, the computation of the demodulator metrics requires a large number of filter banks. An alternative to a coherent receiver is a non coherent receiver, such as the limiter-discriminator detector (LDD) which is followed by a symbol-by-symbol demodulator, which is simpler to implement even though its performance is not as good as that of the coherent receiver.;This research investigates the problems in estimating the synchronization parameters, in particular the symbol timing recovery and the frame synchronization in a CPM system that uses the LDD. Since the output of the LDD resembles a pulse amplitude modulated (PAM) signal, we propose synchronization algorithms designed for such linear modulation schemes (PAM, QAM, etc) to be applied to the LDD system. However, these algorithms were designed with an assumption that the noise process at the input of the synchronizer has a Gaussian distribution, while the noise process at the output of the LDD does not have a Gaussian distribution. Furthermore, the noise component and the signal component might be correlated. We analyze the performance of these synchronization algorithms when applied to the LDD system and compare their performance to the CPM synchronization algorithms.;Another topic that is also investigated is an application of an equalizer after the LDD to perform a symbol-by-symbol demodulation. The equalizer tap coefficients are designed assuming that the transmitted symbols and the noise samples at its input are white and are statistically independent of each other, even though in practice that might not be the case. The performance of a T/2 fractionally-spaced equalizer (FSE) and a symbol-spaced equalizer (SSE) have been analyzed as the parameters of the CPM signal are varied.