A spectral domain approach to design spreading sequences for DS-CDMA systems in frequency selective fading channels

In this paper, we propose a frequency domain approach to design a set of spreading sequences for a DS-CDMA system in the presence of frequency selective fading. We show that the number of parameters involved in the design of optimal spreading sequences can be made smaller when they are analyzed in the frequency domain, than when they are analyzed in the time domain. Further, it is shown that for multipath channels, designing a set of spreading sequences in the frequency domain reduces to assigning a spreading sequence for each user with a spectrum whose spectral lines are located at the strongest spectral components of the user´s channel´s spectrum while keeping the cross correlation at a very low value for spreading sequences with overlapping spectra. We show that by designing spreading sequences in the frequency domain, the performance of some existing algorithms for the design of spreading sequences can be improved and/or the computational complexities can be reduced. We further show that the proposed idea can be extended to the design of spreading sequences where the chips of the spreading sequences are derived from a constant amplitude alphabet