Matra Communication

Xavier Lasne

Fabrice Belveze

Albert Roseiro

A receiving device performs N separate demodulations (N.gtoreq.2) each supplying respective estimates of successive binary symbols resulting from differential coding of a sequence of bits sent by a transmitting device, the differential coding being of the form a.sub.k c.sub.k .sym.a.sub.f(k) where a.sub.k and c.sub.k designate the binary symbol at position k and the bit at position k, f(k) designates an integer equal to k-1 at most and .sym. designates the exclusive-OR operation. Each estimate of a binary symbol at position k is a real number s.sub.k.sup.(i) (1=i=N) the sign of which represents the most probable value of the symbol and the modulus of which measures the likelihood of the most probable value. The value of a bit at position k of the sequence is estimated on the basis of the number: ##EQU1##

Method of digital demodulation

Based on the sequence of the information bits to be transmitted, the coder forms a second sequence of bits c.sub.n, which includes a redundant subset. A differential coding of the form d.sub.n =c.sub.n .sym.d.sub.f(n) with f(n)&lt;n produces a third sequence of bits d.sub.n. The signal sent represents the bits of the third sequence in a specified order. Having obtained likelihood data r.sub.n associated with the bits d.sub.n, the receiver calculates estimates of the bits c.sub.n of the redundant subset as a function of the sign of r.sub.n .multidot.r.sub.f(n). By exploiting the redundancy, the receiver can detect the bits c.sub.n of the subset having erroneous estimates, and rectify the sign of the less reliable of the two associated likelihood data items r.sub.n and r.sub.f(n). The other bits c.sub.n ' of the second sequence then benefit from additional protection if there is at least one bit c.sub.n of the redundant subset such that n'=f(n) or f(n')=n or f(n).

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Beynes, France

Xavier Lasne