Performance of Multicarrier DS/SSMA Over Fast Rayleigh Fading Channels With Doppler Diversity

Abstract

Differential encoding is known to simplify receiver implemen- tation because it bypasses channel estimation.However, over rapidly fading wireless channels, extra transceiver modules are necessary to enable differ- ential transmission.Relying on a basis-expansion model for time- and fre- quency-selective (doubly selective) channels, we derive such a generalized block-differential (BD) codex and prove that it achieves maximum Doppler and multipath diversity gains, while affording low-complexity maximum- likelihood decoding.We further show that existing BD systems over fre- quency-selective or time-selective channels follow as special cases of our novel system.Simulations using the widely accepted Jakes model corrobo- rate our theoretical analysis. Abstract—The significant loss in performance of multicarrier di- rect-sequence spread-spectrum multiple-access (DS/SSMA) 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, 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, are also pro- vided.It is show that the multiple-access interference tends to degenerate into a wide-sense stationary process with a statistical periodicity destroyed by the fast-fading channel. Abstract—We present a setting for decoding of low-density parity-check (LDPC) codes jointly with channel estimation, suitable for transmission over memoryless compound channels.We show that the performance of the combined scheme can be rigorously evaluated by means of density evo- lution, and focus on density evolution as a tool for designing a channel esti- mator that matches not only to the channel, but also to the LDPC ensemble, as well.The utility of the concept is exemplified for a compound binary sym- metric channel and an unknown-phase additive white Gaussian channel.