Tradeoffs of source coding, channel coding and spreading in CDMA systems

We consider a CDMA system consisting of an image source coder, a convolutional channel coder, an interleaver, and a direct sequence spreading module. With different allocations of bandwidth to source coding, channel coding and spreading, the system is analyzed over a frequency selective Rayleigh fading channel. The performance of the system is evaluated using the cumulative distribution function of peak signal-to-noise ratio. We show that, among other things, given a fixed channel coding rate, allocating more bandwidth to source coding allows higher maximum image quality. At the same time, the probability of achieving this high quality is small. Allocating more bandwidth to spreading decreases the number of source information bits transmitted, thus limiting the achievable image quality, but the probability of achieving this maximum quality is high.