Why design spreading codes for multiuser CDMA channels?

Abstract

We examine the behaviour of the information theoretic capacity of jointly detected (multiuser) symbol synchronous DS-SSMA systems, when randomly selected spreading sequences are used. We find upper and lower bounds on capacity for a certain cross-correlation measure, and show that if the number of users is larger than the sequence length, that the lower bound tends to the maximum capacity with increasing sequence length. This implies that for large systems, random spreading sequences are optimal in terms of capacity. We also examine more realistic cases for the number of users and sequence length, and find that in certain cases that the use of random spreading sequences results only in a very small decrease in capacity.