Traditional fixed-rate error correction codes fail to exploit the time varying nature of the cellular communication channels. These channels are suitable most of the times, and, only occasionally, undergo deep fading. The design criterion for fixed-rate codes is base on the average or the worst channel's conditions, which for these channels does not lead to an optimum design. On the other hand, Variable rate adaptive systems, can adapt to the changing channel conditions. During good channel conditions, more information can be sent with less error protection, but when the channel conditions become worse, these systems use more powerful codes for higher error protection.
In this paper, an adaptive coding and modulation scheme is proposed for a DS/CDMA system used in cellular radio channels. This system uses coherent M-ary orthogonal modulation with RAKE receiver. Both Rayleigh fading and a combination of Rayleigh fading plus shadowing are discussed. Analytical bounds and simulations are used to evaluate the performance of the system. It is found there are significant improvement in bit error rate and the average throughput of the adaptive system. More important, it is found that adaptive coding scheme is relatively robust in shadowing environment
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