Adaptive coding and modulation for spread-spectrum signals

We consider coding and edulation schemes for direct-sequence spread-spectrum (DS-SS) systems with multiple-access (MA) interference in a fading environment. Our goal is to be able to adapt the code to different environments. Communication in an environment with a large amount of interference will require a lower transmission (code) rate while small levels of interference will allow a high rate transmission. The following coded modulation systems are considered: a conventional 64-ary orthogonal (M₆₄) code, the trellis coded Nordstrom-Robinson (TCNR) code, and the trellis coded Reed-Muller (TCRM) code. Each of these is concatenated with a (63, k) Reed-Solomon (RS) outer code. The performance measure of interest is the error probability and throughput. These are evaluated by making a Gaussian approximation on the multiple-access interference. Our results show, given an error probability requirement, and a certain data rate, a maximum number of users for each of the three DS-SS schemes can be determined. This indicates the receiver can adjust the coding and modulation scheme to achieve the given requirement