Time-Selective Signaling and Reception for Multipath Fading Channels

| Diversity techniques provide a powerful approach to combat fading. Due to the presence of multipath and Doppler shifts, the mobile wireless channel inherently provides diversity that can be exploited using appropriate signaling and reception. While existing CDMA systems exploit only multipath diversity using spread-spectrum signaling, we develop signaling and reception schemes that exploit joint multpath-Doppler diversity. Signi cant performance gain can be obtained even for the small Doppler spreads encountered in practice. Additionally, the time-selective signaling scheme allows for substantially higher level of diversity and thereby brings the fading channel closer to an additive white Gaussian noise channel. This facilitates the use of error control codes developed for the Gaussian channel. I. Time-Selective Signaling and Reception The signaling and reception schemes are developed based on a time-frequency representation of the channel derived in [2]. This representation decomposes the channel into a series of almost independent fading channels corresponding to the different multipath-Doppler-shifted signal components. The signaling waveforms are spread both in time and in frequency to achieve resolvable Doppler and multipath diversity at the receiver. A similar signaling scheme is developed in [3]. However, diversity is not explicitly used in this case. Although the symbol duration is increased, the data rate is maintained by allowing the symbols to overlap in time. This introduces inter-symbol interference (ISI) which is dealt with at the receiver. Since we focus on the e ects of the channel, we restrict ourselves to the single-user case. The received signal, r(t), can be represented as