Multiuser coding based on detecting matrices for synchronous-CDMA systems

Abstract

A multiuser coding approach to the design of spreading sequences for synchronous CDMA systems in which each receiver makes use of the knowledge of the spreading sequences and idle status of all users was recently introduced. For interuser-interference-free communications using sequences of length L, the system capacity (i.e. maximum number of simultaneous users) of the resultant system is about L log2(L)/2 while that of conventional system is only L. In this work, it is shown that in this approach the synchronous CDMA system is equivalent to the classical multiuser binary adder channel. Consequently, the multiuser codes based on ternary detecting matrices can be used as the spreading sequences. A new recursive construction of ternary detecting matrices which generalizes and/or improves the best previously known constructions is presented. Comparing with previous works, a remarkable improvement on the system capacity is obtained.