Performance sensitivity of quasi-synchronous, multicarrier DS-CDMA systems due to carrier frequency disturbance

The multiple access interference (MAI) of a quasi-synchronous, multicarrier DS-CDMA system can be substantially reduced by using sequences having low crosscorrelation at small shifts around the origin. This paper shows that the time-frequency crosscorrelation rather than the usual (time-domain) crosscorrelation determines the MAI when the system is operated in the presence of carrier frequency offset (CFO) which arises due to the frequency-accuracy limit of the oscillator. Analysis on the time-frequency crosscorrelation properties reveals that (i) a system using Walsh codes or Suehiro-Hatori (1985) polyphase sequences can be driven into outage in the presence of CFO as a result of significant worst-case MAI, and (ii) it is possible to minimize the MAI for systems using preferentially phased Gold codes, cyclic-shift m-sequences or Lin-Chang (see IEEE Trans. Commun., vol.45, p.221-26, 1997) sequences only if the product of chip period and maximum frequency deviation is less than around 0.01