Signal to Noise Ratio (SNR) is an important index for wireless communications. There are many methods for increasing SNR. In CDMA systems, spreading sequences are used. We consider the frequency-selective wide-sense-stationary uncorrelated-scattering (WSSUS) channel and evaluate the worst case of SNR. We construct the non-linear programing for maximizing the lower bound of the average of SNR. This problem becomes the convex programming if we did not take into account the norm constraint. We derive necessary conditions for optimal spreading sequences for the problem.