Signal Design to Optimize Trade-Off between Bandwidth Efficiency and Power Efficiency in Uplink CDMA Systems

Abstract

In this paper, we compare two signal designs for uplink quasi-synchronous code division multiple access (CDMA) channels in order to optimize the trade-off between bandwidth efficiency and power efficiency. The design we call band-limited DS/CDMA design, is based on the time-domain assignment of Gold sequences, just as in the ordinary DS/CDMA, but with band-constrained cyclic chip interpolation functions, which is unlike the ordinary DS/CDMA. The other design, MC/CDMA design, is based on frequency-domain assignment of the sequences, as in the ordinary MC/CDMA. In both designs, we assume insertion of guard intervals at the transmitter and frequency-domain processing in reception. Assuming quasi-synchronous arrival of CDMA signals at the CDMA base station and FFT in the effective symbol interval, the intersymbol interference is evaded in both designs. First we identified the signal parameters that optimize bandwidth efficiency in each of the band-limited DS design and MC design. Second, we clarified the signal parameters that optimize the power efficiency as functions of frequency efficiency in each of the two designs. Finally, we derived and compared the trade-off between the bandwidth efficiency and power efficiency of band-limited DS and MC designs. We found a superiority of band-limited DS design over MC design with respect to the optimized trade-off.