Optimal time-frequency signaling for rapidly time-varying channels

We introduce a new signaling scheme for time- and frequency-selective channels that is a generalization of multi-carrier code division multiple access (MC-CDMA) signaling used in slowly time-varying channels. The Fourier basis functions used in conventional MC-CDMA systems encounter temporal distortion in rapidly time-varying channels resulting in degraded performance. The proposed scheme transmits the data over a set of orthonormal time-frequency basis functions whose time-frequency support is matched to the coherence time and bandwidth of the channel. The time-frequency signaling scheme approximately diagonalizes the time-varying multipath channel and each basis function encounters flat Rayleigh fading. We derive an optimal choice of basis parameters that yield the most accurate diagonalization for given multipath and Doppler spreads of the channel. The proposed system fully exploits the diversity afforded by the channel and delivers improved performance in rapidly time-varying channels in contrast to degraded performance of existing systems under such conditions