  Statistical mechanics approach is useful not only in analyzing macroscopic
system performance of wireless communication systems, but also in discussing
design problems of wireless communication systems. In this paper, we discuss a
design problem of spreading sequences in code-division multiple-access (CDMA)
systems, as an example demonstrating the usefulness of statistical mechanics
approach. We analyze, via replica method, the average mutual information
between inputs and outputs of a randomly-spread CDMA channel, and discuss the
optimization problem with the average mutual information as a measure of
optimization. It has been shown that the average mutual information is
maximized by orthogonally-invariant random Welch bound equality (WBE) spreading
sequences.
