Slow frequency and time hopping in wireless communication

Abstract

This thesis provides an analytical treatment of the operation of diversity, possible ways of maximizing the diversity, and performance tradeoffs that limit the achievable diversity in the scheme called SFH/TDMA (Slow Frequency Hopping/Time Division Multiple Access). Comparing the performance of SFH/TDMA with that of CDMA (Code Division Multiple Access) is a problem of both practical and theoretical interest. We contribute to the understanding of the problem by comparing a simplified generic CDMA system with an equivalent simplified generic SFH/TDMA system. We show that CDMA inherently has more interferer diversity. We then suggest time hopping which is a way of increasing interferer diversity in SFH/TDMA by exploiting bursty transmission. Later in the thesis, fading diversity is addressed. Previous researchers have observed that there seems to be a optimum diversity level in SFH/TDMA beyond which diversity hurts performance. We find, for the finite-state block-fading channel model, that when the receiver (but not the transmitter) has perfect side information on the channel state, diversity can only improve performance. In the absence of such side information, channel capacity decreases with diversity because of degrading channel estimation. We conclude that it is this tradeoff between decreasing capacity and increasing diversity that gives rise to the existence of an optimum diversity level.