Exploiting Doppler Diversity in Mobile Wireless Communications

Signal fading due to variations in channel characteristics is one of the major factors limiting the performance of modern mobile wireless communication systems. To overcome fading, various diversity techniques have been developed that eeectively transmit multiple copies of the signal over the channel. The state-of-the-art receiver used in code-division multiple access systems is the RAKE receiver that exploits multipath diversity. However, the RAKE receiver assumes slow channel variations over time and the noncoherent RAKE is often used in practice for robustness to such variations. Temporal channel variations are caused by relative motion between the transmitter and receiver and often become signiicant in mobile communications scenarios. Even though the performance of the RAKE receiver substantially degrades in such situations, we show that the Doppler induced by rapid channel variations in fact provides another means for diversity that can be further exploited to combat fading. We develop the concept of Doppler diversity and propose a new framework that exploits joint mul-tipath-Doppler diversity in an optimal fashion. The framework is applicable in several existing and proposed mobile wireless multiple access systems, and can provide substantial performance improvement over the RAKE receiver.