6
Conclusions
Continuous-phase modulations (CPMs) have been studied for many years, mainly because of the attractive properties of constant envelope and bandwidth efficiency, that make this family the most suitable modulation in the satellite field. In spite of these favourable features and the huge literature that is available on the subject, the CPM family have found no application until now, except for very simple schemes as MSK, because of the implementation complexity of the detector, and of synchronization problems.
In this thesis, we provided new synchronization algorithms, a cyclostationary-based timing estimator and a soft-based phase recovery method for continuous phase modulations (CPMs), by extending to such signals (including the multi-h variant) the corresponding methods that were already proposed for linearly-modulated data signals.
We demonstrated that the derived algorithms can be applied to all the CPM formats, without restrictions, so both are general and give good performance.
As first, we examined the timing synchronization algorithm, exploiting Gini and Giannakis cyclostationarity-based method.
Our cyclostationarity-based estimator for symbol timing of a CPM signal has the following features:
1) the algorithm is blind, feedforward and insensitive to carrier phase;
2) the optimization can easily be performed to suit the algorithm to the particular signal that is considered by finding the optimum values of the autocorrelation lag and of the higher moments order;
3) the estimator suffers from self-noise at high SNR and noise enhancement at low SNR, but it is applicable to all practical CPM schemes and to multi-h formats as well;
4) a residual frequency offset brings a bias in the timing deviation evaluation.
In the second part of this work we derived another general synchronization method, exploiting the trellis structure inherent in the modulator of the CPM signals. In particular
Conclusions 76
we focused on phase estimation, being the phase the parameter that showed the lower implementation complexity. This algorithm takes benefits from the information available from the trellis structure, a sort of soft information for coded systems.
Our soft-based estimator for phase error has the following features: 1) the algorithm is blind;
2) it is applicable to all the modulations of the CPM family;
3) the estimator shows performance comparable to MCRB for all the examined formats, that cover the partial and full response binary and multilevel signals.
The computational complexity of both algorithms can be handled thanks to the development in computer science.
In Chapter 5 we showed a second soft-based algorithm, that make use of a closed loop, but this algorithm has not been simulated yet. The soft algorithms for the other nuisance parameters have not been investigated.
