Patent ID: 8155230

Claim:
A method comprising: soft demapping, by Logarithm of the Likelihood Ratio calculation of complex-valued symbols (y( 1 ), y( 2 ), . . . , y(k), . . . , y(K)) corresponding to respective transmitted complex symbols (a( 1 ), a( 2 ), . . . , a(k), . . . , a(K)) contained in a signal transmitted by code division multiple access multi-carrier transmitter and constituted by binary data coded by convolutional coding and quadrature-modulated according to an M-QAM modulation, M being an integer equal to 2 2a and a being an integer greater than or equal to 2, 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, bQ, 1 , bQ, 2 , . . . , bQ,p, . . . , bQ,s}, wherein the bit bI,q is the q th coded bit of an in-phase part of the transmitted signal and the bit bQ,p is the p th coded bit of a quadrature part of the transmitted signal, wherein the soft demapping includes obtaining a calculation of a Logarithm of Likelihood Ratio LogLR(bI,q) of the bit bI,q and a calculation of a Logarithm of Likelihood Ratio LogLR(bQ,p) of the bit bQ,p so that ⁢ Log ⁢ ⁢ LR ⁡ ( bI , q ) = ( η / V ) ⁢ Z I , q , with ⁢ η = p k ⁢ ∫ 0 1 ⁢  h ⁡ ( ⅇ 2 ⁢ ⅈπ ⁢ ⁢ f )  2 ⁢ ⅆ f , and V = α ⁢ p _ ⁡ ( ∫ 0 1 ⁢  h ⁡ ( ⅇ 2 ⁢ ⅈπ ⁢ ⁢ f )  4 ⁢ ⅆ f - ( ∫ 0 1 ⁢  h ⁡ ( ⅇ 2 ⁢ ⅈπ ⁢ ⁢ f )  2 ⁢ ⅆ f ) 2 ) + σ 2 ⁢ ∫ 0 1 ⁢  h ⁡ ( ⅇ 2 ⁢ ⅈπ ⁢ ⁢ f )  2 ⁢ ⅆ f ⁢ wherein h(e 2iπf ) is a channel frequency response, p k is a power associated with the spreading code of the user of rank k, α is equal to the total number K of spreading codes divided by the length N of the spreading codes, p is a mean power associated with all the K spreading codes, σ 2 is a thermal noise variance, and Z I,q =y I for q=1, wherein y I is the in-phase component of the complex-valued signal y(k), and Z I,q =−|Z I,q−1 |+η×m I,q for q>1, wherein m I,q is half the distance between the partitions of the complex plane S (1) I,q and S (0) I,q such that: S (1) 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), and S (0) 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), and Log LR ( bQ,p )=(η/ V ) Z Q,p , with Z Q,p =y Q for p=1, wherein y Q is the quadrature component of the complex-valued symbol y(k), and Z Q,p =−|Z Q,p−1 |+η×m Q,p for p>1, wherein m Q,p is half the distance between partitions of the complex plane S (1) Q,p and S (0) Q,p such that: S (1) 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), and S (0) 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).