  Signal to Noise Ratio (SNR) is an important index for wireless
communications. In CDMA systems, spreading sequences are utilized. This series
of papers show the method to derive spreading sequences as the solutions of the
non-linear programming: maximize SNR. In this paper, we consider a
frequency-selective wide-sense-stationary uncorrelated-scattering (WSSUS)
channel and evaluate the worst case of SNR. Then, we derive the new expression
of SNR whose main term consists of the periodic correlation terms and the
aperiodic correlation terms. In general, there is a relation between SNR and
mean-square correlations, which are indices for performance of spreading
sequences. Then, we show the relation between our expression and them. With
this expression, we can maximize SNR with the Lagrange multiplier method. In
Part II, with this expression, we construct two types optimization problems and
evaluate them.
