Method for the soft demapping, by Logarithm of the Likelihood Ratio calculation, of complex-valued symbols (y, y,..., y(k),..., y) corresponding to transmitted complex symbols (a, a,..., a(k),..., a) contained in a signal transmitted by a code division multiple access multi-carrier transmitter and constituted by binary data coded by convolutional coding and quadrature modulated according to a M-QAM modulation, where M is an integer equal to 2 m and m is an integer greater than or equal to 1, a complex symbol transmitted a(k) of rank k, k=1, 2,..., K being spread by a spreading code associated with a user of rank k, the binary data coded by convolutional coding constituting, for the user of rank k, a sequence of bits {bI, 1; bI, 2;..., bI,q;..., bI,r; by, 1; bQ,2;..., bQ, p;..., bQ, s}, wherein the bit bI, q is the q th bit used to modulate an in-phase part of the transmitted signal and the bit bQ,p is the p th bit used to modulate a quadrature part of the transmitted signal, characterised in that it includes a Logarithm of the Likelihood Ratio LRV (bI,q) calculation of the bit bI,q and a Logarithm of the Likelihood Ratio LRV (bQ,p) calculation of the bit bQ,p which use the respective following formulas: LRV bI q = ln ⁢ ∑ λ ⁢ ∈ S I, q 1 exp - 1 2 ⁢ β y ̃ k - λ 2 ∑ λ ⁢ ∈ S I, q 0 ⁢ exp - 1 2 ⁢ β y ̃ k - λ 2 and LRV bQ p = ln ⁢ ∑ λ ⁢ ∈ S Q, p 1 exp - 1 2 ⁢ β y ̃ k - λ 2 ∑ λ ⁢ ∈ S Q, p 0 ⁢ exp - 1 2 ⁢ β y ̃ k - λ 2 with: - β = η 2 /V, where - η is the finite limit, when the total number K of spreading codes and the length N of the spreading codes tend towards infinity and the quantity α=K/N is less than or equal to 1, of a random variable η k such that: y k = η k a k + τ k, where y(k) is the complex-valued symbol of rank k which corresponds to the transmitted complex symbol a(k) and τ k is a random variable representing a noise value produced by multiple access noise and thermal noise filtering, - V is the finite limit of the variance of the random variable τ k, β being given by the equation: β = p k ⁢ ∫ 0 1 h e 2 ⁢ iπ f 2 ⁢ d f α ⁢ p ̅ ⁢ ∫ 0 1 h e 2 ⁢ iπ f 4 ⁢ d f ∫ 0 1 h e 2 ⁢ iπ f 2 ⁢ d f - ∫ 0 1 h e 2 ⁢ iπ f 2 + d f + σ 2 where - h(e 2iΠf) is the channel frequency response - p is the mean power associated with all the K spreading codes - p k is the power associated with the spreading code of the user of rank k - σ 2 is the thermal noise variance. - ỹ (k) is a standardised complex-valued symbol such that ỹ (k) = y(k)/η, - λ is a reference complex symbol of QAM constellation associated with the M-QAM modulation, - S I,q is a partition of the complex plane which brings together all the reference complex symbols having a "1" at the position (I, q), - S I,q is a partition of the complex plane which brings together all the reference complex symbols having a "0" at the position (I, q), - S Q,p is a partition of the complex plane which brings together all the reference complex symbols having a "1" at the position (Q, p), - S Q,p is a partition of the complex plane which brings together all the reference complex symbols having a "0" at the position (Q, p). - β = η 2 /V, where - η is the finite limit, when the total number K of spreading codes and the length N of the spreading codes tend towards infinity and the quantity α=K/N is less than or equal to 1, of a random variable η k such that: y k = η k a k + τ k, where y(k) is the complex-valued symbol of rank k which corresponds to the transmitted complex symbol a(k) and τ k is a random variable representing a noise value produced by multiple access noise and thermal noise filtering, - V is the finite limit of the variance of the random variable τ k, β being given by the equation: β = p k ⁢ ∫ 0 1 h e 2 ⁢ iπ f 2 ⁢ d f α ⁢ p ̅ ⁢ ∫ 0 1 h e 2 ⁢ iπ f 4 ⁢ d f ∫ 0 1 h e 2 ⁢ iπ f 2 ⁢ d f - ∫ 0 1 h e 2 ⁢ iπ f 2 + d f + σ 2 where - h(e 2iΠf) is the channel frequency response - p is the mean power associated with all the K spreading codes - p k is the power associated with the spreading code of the user of rank k - σ 2 is the thermal noise variance. - ỹ (k) is a standardised complex-valued symbol such that ỹ (k) = y(k)/η, - λ is a reference complex symbol of QAM constellation associated with the M-QAM modulation, - S I,q is a partition of the complex plane which brings together all the reference complex symbols having a "1" at the position (I, q), - S I,q is a partition of the complex plane which brings together all the reference complex symbols having a "0" at the position (I, q), - S Q,p is a partition of the complex plane which brings together all the reference complex symbols having a "1" at the position (Q, p), - S Q,p is a partition of the complex plane which brings together all the reference complex symbols having a "0" at the position (Q, p). - h(e 2iΠf) is the channel frequency response - p is the mean power associated with all the K spreading codes - p k is the power associated with the spreading code of the user of rank k - σ 2 is the thermal noise variance. - ỹ (k) is a standardised complex-valued symbol such that ỹ (k) = y(k)/η, - λ is a reference complex symbol of QAM constellation associated with the M-QAM modulation, - S I,q is a partition of the complex plane which brings together all the reference complex symbols having a "1" at the position (I, q), - S I,q is a partition of the complex plane which brings together all the reference complex symbols having a "0" at the position (I, q), - S Q,p is a partition of the complex plane which brings together all the reference complex symbols having a "1" at the position (Q, p), - S Q,p is a partition of the complex plane which brings together all the reference complex symbols having a "0" at the position (Q, p).