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Coding for Multiple Access Communications

An idealized code-division multiple-access (CDMA) system is used as a vehicle to illustrate a new approach to coding for multiple-access channels that aims to achieve reasonable efficiency while avoiding the prohibitive complexity of optimum coding schemes. The new approach combines "partial demodulation" to separate users into roughly independent groups with the creation of "virtual channels" that allow the decoder for each user to act as if that user were the only user of the system. The virtual channels are created by means of simple uniquely decodable codes for the noisy binary adder channel. The approach is motivated and explained by means of examples.