Analysis of Overlapped Code Division Multiple Access System in Gaussian Multiple Access Channel

This paper proposes an overlapped code division multiple access (OVCDMA) system where each user is assigned a unique convolutional spreading code and all the users access the channel simultaneously. Node error probability and symbol error probability upper bounds of OVCDMA in Gaussian multiple access channel are derived based on the random coding arguments. The former one ensures the existence of convolutional spreading code dedicated to M-ary (M>2) modulation and the later one is tight for moderate signal-to-noise ratio. In addition, the random coding simulation results show that the symbol error rate gets lower by increasing the convolutional spreading code length. Finally, several convolutional spreading codes dedicated to QPSK are found by random coding search and free Euclidean distance calculation. They are compared with conventional modulation scheme under the condition of same spectral efficiency. Simulations for these fixed convolutional spreading codes show that notable coding gain are achieved in Gaussian multiple access channel.