Performance of multicarrier DS/SSMA over fast Rayleigh fading channels with Doppler diversity

The significant loss in performance of multicarrier direct-sequence spread-spectrum multiple-access systems over fast-fading channels is investigated. First, the channel model for individual subchannels is obtained, using a canonical decomposition to a wide-sense stationary (WSS), uncorrelated scattering channel. Next, a receiver structure, which features both Doppler diversity and frequency diversity, is presented and analyzed. It is found that large Doppler spreads cause the average magnitude of the desired signals to diminish dramatically. Expressions for the bit-error rate are derived for both uncoded and coded systems by applying the standard Gaussian approximation. Numerical results, which show that Doppler diversity is preferable over frequency diversity in fast-fading channels, are provided for different combinations of the diversity orders. Numerical results, which show that the intersymbol and intersubchannel interference can be effectively suppressed, and the multiple-access interference tends to degenerate into a WSS process with the statistical periodicity destroyed by the fast-fading channels, are also provided. Index Terms--Doppler diversity, Doppler spread, doubly selective fading channels, frequency diversity, multicarrier direct-sequence spread-spectrum multiple access (MC/DS/SSMA).