On the benefits of non-uniform signaling in chip-asynchronous CDMA

This paper explores chip-asynchronous CDMA transmission when single-user demodulation and decoding are used at the receiver. First, a chip-level channel model is presented to account for chip-asynchronism between different users. Using the model, the symmetric capacity of chip-asynchronous CDMA is investigated. Numerical results indicate that a sum rate of 2 bits per channel use is achievable when chip symbols {-1, 1} are utilized in appropriate non-equiprobable fashion. This represents over a 2.5 times capacity gain over traditional uniform chip signaling in chip-synchronous CDMA. Consequently, the symmetric sum capacity is further investigated when the zero chip symbol is also utilized for CDMA transmission. The obtained capacity results indicate that nonuniform signaling with {-1,0,+1} chip symbols can lead to an energy efficient CDMA transmission scheme which could achieve energy savings of up to 3 dB over the traditional CDMA with equiprobable {-1, +1} chip symbols.