Patent ID: 7974357

Claim:
A method for soft demodulation of data modulated according to a square quadrature amplitude modulation of order greater than or equal to 4, in a communication system implementing a multi-carrier code division multiple access or OFDM-CDMA technique, using non-binary spreading codes, comprising: determining a simplified expression for a logarithm of a likelihood ratio LLR(b l,q ) for an in-phase pathway for a user k according to an equation: LLR ⁡ ( b I , q ) = ( 4 ⁢ p k σ 2 ⁢ 1 N ⁢ ∑ l = 0 N - 1 ⁢  c k , l  2 ⁢  h ^ l  ) · M I , q where M l,q =y k,l for q=1 M l,q =−|M l,q−1 |+m l,q for q> 1 where m l,q corresponds to half of a distance between boundaries of a partition relating to b l,q and where: p k is a parameter representative of a power applied to a k th user, σ 2 is a variance of a noise, N is a number of sub-carriers, l is an index representative of a sub-carrier (1≦l≦N), where the index is a positive integer, c k,l is a value of a spreading code for a l th sub-carrier and for the user k, ĥ l is an estimate of a coefficient of a transmission channel for the l th sub-carrier, y k,l corresponds to a real part of a complex symbol after equalization and despreading intended to be demodulated, and determining a simplified expression for a logarithm of a likelihood ratio LLR(b Q,q ) for a quadrature pathway for the user k in accordance with an equation: LLR ⁡ ( b Q , q ) = ( 4 ⁢ p k σ 2 ⁢ 1 N ⁢ ∑ l = 0 N - 1 ⁢  c k , l  2 ⁢  h ^ l  ) · M Q , q where M Q,q =y k,Q for q=1 M Q,q =−|M Q,q−1 |+m Q,q for q> 1 where m Q,q corresponds to half of a distance between boundaries of a partition relating to b Q,q and where: y k,Q corresponds to an imaginary part of the complex symbol after equalization and despreading intended to be demodulated.