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

Although there are attractive sequence design algorithms for code division multiple access (CDMA) systems in frequency selective channels, unfortunately in most of these algorithms, the designing has been done with the assumption that the chips of the sequences have unconstrained amplitudes and phases. This increases the required feedback bandwidth and the peak to average power ratio (PAPR) at the transmitter. In this paper, we observe that the spectrum of the spreading sequences designed according to existing algorithms are narrowband and are located at the strongest spectral components of the users' channels. The narrowbandness of spreading sequences allows one to represent each user's sequence with fewer number of parameters and, hence, reduces the required number of feedback bits to represent those sequences. Further, the narrowbandness of the spectra allows one to break the global optimization of the users' cross correlation matrix into simple sub optimal local (grouped) optimizations. In this paper, we capitalize the aforementioned observations to design constrained amplitude sequences, for frequency selective channels