Adaptive data rate multi-carrier direct-sequence spread spectrum in Rayleigh fading

We propose an adaptive data rate multi-carrier direct sequence spread spectrum system for transmission over frequency selective, slowly fading channels. Adaptation is via the transformation used to distribute symbols across the subcarriers. Initially, the transformation is either a serial-to-parallel conversion of the incoming data into M parallel streams, or a split of each symbol across M subcarriers. By keeping the subcarrier symbol and chip rates constant, in split mode, we obtain a bit rate R_b equal to the symbol rate R_s, and frequency diversity through maximal ratio combining. Conversely, when the channel is not deeply fading, the S:P transformation is used, and we treat each subcarrier as a single-carrier one, obtaining a bit rate R_b=MR_s. The adaptive transformation is analyzed using order statistics, and we use these analytical and computer simulation results to show that our proposed system has a higher data rate than a pure "split" system, and better error performance than a pure S:P system, and can operate anywhere between these two extremes via selection of an appropriate threshold.