On the differences between uplink and downlink transmission in code-spread CDMA systems

The bit error rate (BER) of a code spread CDMA system, using low rate convolutional codes for spreading, is analyzed. The system is assumed to operate over a Rayleigh flat fading channel, and white Gaussian noise is added at the receiver front end. Based on a Gaussian approximation, expressions for the pdf of the signal-to-noise ratios are derived for both uplink and downlink transmission. Computer simulations of the system BER, as well as analytical results based on a union bound, are presented. The analytical results are in close agreement with the computer simulations.