User-separating demodulation for code-division multiple-access systems

A user-separating (US) demodulator for a multiple-access system with digital transmission is defined to be a demodulator that, without knowledge of the channel codes of the various users, provides the decoder for each user with a scalar-valued output for each symbol period that permits maximum-likelihood decoding of that user´s data. It is shown that such a US demodulator exists in general only for approximations to the true statistics of the modulator-input sequences of the interfering users. It is argued that approximating the interfering modulator-input sequences as independent, white Gaussian processes is the practical compromise between accuracy and simplicity in code-division multiple-access (CDMA) systems. The US demodulator for this approximation is shown to consist of a kind of matrix whitening fitter followed by a kind of matched-filter for the user in question. It is further shown that this US demodulator can often be well approximated as the symbol-by-symbol demodulator that makes the linear minimum mean-squared error estimate of each modulator-input symbol for the user in question. Simulation results are presented to confirm the theory of US demodulation and to illustrate its practical utility