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Tesi etd-03312008-220839

Thesis type
Tesi di dottorato di ricerca
Synchronization Issues for Continuous Phase Modulations
Settore scientifico disciplinare
Corso di studi
Relatore Prof. Luise, Marco
Parole chiave
  • soft-based receiver
  • cyclostationarity
  • cpm
  • synchronization
Data inizio appello
Data di rilascio
Riassunto analitico
Continuous-phase modulations (CPMs) have been studied for many years. Mainly<br>because of the attractive property of constant envelope, that make this family the most<br>suitable modulations in the satellite field, they found their first relevant practical application<br>in mobile communications, where they represent the standard signal format for GSM. In<br>spite of the huge literature that is available on the subject, more elaborate CPM schemes<br>have found no application until now because of the implementation complexity of the<br>detector, and of synchronization problems. In the last few years several methods have<br>been proposed to solve synchronization issues related to Continuous Phase Modulations<br>(CPMs). Unfortunately most of these techniques are ad-hoc algorithms, developed to meet<br>synchronization problems for a particular subclass of signals.<br>In this thesis, we tackle the issue of both cyclostationary-based and soft-based<br>synchronization for continuous phase modulations (CPMs), by extending to such signals<br>(including the multi-h variant) the corresponding methods that were already proposed for<br>linearly-modulated data signals.<br>As first, we will examine the timing synchronization algorithm. The problem of blind<br>timing recovery with linear modulations has several efficient solutions, irrespective of the<br>complexity of the signal constellation. The workhorse in this respect is Gardner’s timing<br>error detector, that is a closed loop estimator. Another approach is represented by the wellknown<br>Oerder-Meyr’s (O&amp;M) estimator, that is on the contrary an open loop method.<br>Gini and Giannakis (G&amp;G) showed that the O&amp;M estimator is a particular sample of a<br>larger class of estimators that exploit the cyclostationarity of a data signal. We will show in<br>the following that the G&amp;G approach can be extended to CPM signals, and we will perform<br>an approximate analytical performance evaluation.<br>The second part of this work is about another general synchronization method that can<br>be applied to CPM signals, that exploit the trellis structure inherent in the modulator. In<br>particular we will focus on phase estimation, being the phase the parameter that shows the<br>lower implementation complexity. Our approach is the so-called code-aided<br>synchronization, that take benefits from the a priori information available from the code<br>structure applied to a linear modulation scheme. We try to extend this approach to CPMs,<br>since this signals share with a coded system the trellis structure, due to the inherent memory<br>of the CPM modulator. The algorithm starts with a first step, called Expectation-<br>Maximisation. It is an iterative method which enables to solve ML optimisation<br>problem.<br>In the special case of the sole phase, being CPM a phase modulation, the computational<br>complexity decreases and it is possible to find a closed form for the estimator, that is data<br>aided and makes use of the state transition probabilities given by a symbol-based BCJR, developed in an ad-hoc way for an uncoded CPM signal.