Method and device for selecting spreading parameters for an ofdm-cdma system

A device for selecting frequency (SF) or time (ST) spreading parameters or both for a communication system including a transmitter and a receiver and using a quadrature amplitude modulation and implementing a code division multiple access (CDMA) technique and an orthogonal frequency division multiplexing (OFDM), in which the frequency domain is divided into F=N/SF sub-bands of SF subcarriers, wherein N is the size of the fast Fourier transform used by the receiver and SF is the frequency spreading parameter, and the time domain is divided into T=NS/ST blocks of ST OFDM symbols, wherein NS is the number of OFDM symbols used to code a data frame and ST is the time spreading parameter, the device further comprising:

• means for estimating the attenuation coefficients of the propagation channel;
• means for determining, for all the F sub-bands of SF subcarriers and the T blocks of ST OFDM symbols, at least one asymptotic signal-to-interference+noise ratio (SINR) from the coefficients of the attenuation of the propagation channel and the type of equalizer used by the receiver;
• calculation means for deducing from this, for each coded information block obtained from the modulation, the compressed asymptotic signal-to-interference+noise ratio, which is expressed by an analytical formula dependent on said frequency (SF) and time (ST) spreading parameters;
• and
• means for selecting the frequency (SF) and time (ST) spreading parameters which maximize the value of the compressed asymptotic signal-to-interference+noise ratio.