Method and modem for phase synchronization and tracking

1. A method of demodulating a signal in a packet communications system, the signal including at least one burst representing data samples pertaining to one or more users, the method comprising the steps of:

(a) phase estimation and tracking during at least the duration of the burst by using a frequency assumption test;

(b) for a sample k of the received signal generating a correction and tracking signal Scp(i) by using a value of frequency deviation idF and a residual phase value dφ;

(c) correcting an input signal Rk by multiplying it by a correction and tracking signal Scp(i);

(d) transmitting the frequency-corrected and phase-corrected signal;

(e) at a step of computation of the residual phase value dφ, using a reference constellation signal Dk and the frequency-corrected and phase-corrected signal to estimate the value SNR(i) for index;

(f) integrating the residual phase value dφ and transmitting the residual phase value dφ during step (b);

(g) determining a variation in the value of index i and reiterating steps (a) to (f);

(h) for each assumption, taking the sum of the local SNR(i) values per sample, and determining a variation in the value of k; and

(i) extracting the frequency deviation value indexed i for which the value of the SNR is optimum; wherein the generation of correction and tracking signal is given by:

ψ k+1 =ψ k +G.F(φ k , ρ k ).dφ k +dF where F(φ,ρ) is a weighting function;

ψ k is an estimated phase;

G is the loop gain;

F is the weighting function;

dφ k is the phase error; and

dF is the frequency deviation assumption.

2. The method according to claim 1 , wherein the value of the SNR is obtained by minimizing the following expression:

SNR=Σ k |Rk−Dk| 2 .

3. The method according to claim 1 , wherein said extracting step is executed iteratively.

4. The method according to claim 1 , wherein the method is executed for the optimum frequency and the corresponding phase residual value is determined.

5. The method according to claim 4 , wherein a delay is introduced into a signal, sample by sample, and wherein the phase-corrected and frequency-corrected signal obtained for the optimum frequency value is applied.

6. The method according to claim 1 , wherein the phase-corrected and frequency-corrected signal is transmitted to a decoding step before the estimation of the value of the SNR on the decoded symbols.

7. The method according to claim 6 , further comprising applying a decision criterion to each signal output from the decoding step, the applied decision criterion being a quality indicator.

8. The method according to claim 7 , wherein the quality indicator is an indicator of speed of convergence of the decoding algorithm, a consistency check word (or CRC=Cyclic Redundancy Check) or any other means offered by the encoder (non-decoding for a REED SOLOMON code, an appreciation of error rate at input of the decoder, etc.).

9. A device to demodulate a signal in a packet communications system, the signal comprising at least one burst representing data samples relative to one or more users wherein the device comprises means to implement the method according to claim 1 .