Time - Selective Signaling and Reception for Multipath Fading

| 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. Signiicant 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 sig-naling 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 eeects of the channel, we restrict ourselves to the single-user case. The received signal, r(t), can be represented as r(t) n;k (t) + n(t) where b i is the i th transmitted bit (1), u i n;k (t) is the time and frequency shifted version of the spreading code q(t) (time-shifted by nTc where Tc is the chip duration, and frequency-shifted by k=Te where Te is the symbol duration), h i n;k is the corresponding fading coeecient and n(t) is the additive white Gaussian noise. The number of multipath and Doppler components at the receiver are dTm=Tce + 1 and dBdTee + 1 respectively , where Tm and Bd are the multipath and Doppler spread of the channel. The receiver performs joint multipath-Doppler diversity combining and ISI cancellation assuming perfect knowledge of the channel coeecients. The optimal 1 Srikrishna Bhashyam and Behnaam Aazhang are with the maximum likelihood sequence detector is developed. Due to its computational complexity, sub-optimal linear detectors are developed. The two linear detectors studied are the decorre-lating …