Optimal allocation of bandwidth for source coding, channel coding, and spreading in CDMA systems

This paper investigates the tradeoffs between source coding, channel coding, and spreading in code-division multiple-access systems, operating under a fixed total bandwidth constraint. We consider two systems, each consisting of a uniform source with a uniform quantizer, a channel coder, an interleaver, and a direct-sequence spreading module. System A is quadrature phase-shift keyed modulated and has a linear block channel coder. A minimum mean-squared error receiver is also employed in this system. System B is binary phase-shift keyed modulated. Rate-compatible punctured convolutional codes and soft-decision Viterbi decoding are used for channel coding in system B. The two systems are analyzed for both an additive white Gaussian noise channel and a flat Rayleigh fading channel. The performances of the systems are evaluated using the end-to-end mean squared error. A tight upper bound for frame-error rate is derived for nonterminated convolutional codes for ease of analysis of system B. We show that, for a given bandwidth, an optimal allocation of that bandwidth can be found using the proposed method.