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Receive Diversity 1 1 Introduction

Summary: Receive Diversity 1
1 Introduction
When discussing beamforming we used the resources of an antenna array to service multiple users
simultaneously. The interference cancellation schemes were formulated independent of whether the
channels were line-of-sight (LOS) or fading. Diversity combining devotes the entire resources of
the array to service a single user. Specifically, diversity schemes enhance reliability by minimizing
the channel fluctuations due to fading. The central idea in diversity2 is that different antennas
receive different versions of the same signal. The chances of all these copies being in a deep fade
is small. These schemes therefore make most sense when the fading is independent from element
to element and are of limited use (beyond increasing the SNR) if perfectly correlated (such as in
LOS conditions). Independent fading would arise in a dense urban environment where the several
multipath components add up very differently at each element.3.
Early in this course we saw that fading has three components: path loss, large-scale and small-
scale fading. Over fairly long periods the first two components are approximately constant and can
be dealt with using power control. Furthermore, these components of fading are very close to being
constant across all elements of the array (perfectly correlated). Diversity combining is specifically
targeted to counteract small scale fading. We will therefore use slow, flat, Rayleigh fading as our
model for the signal fluctuation. It must be emphasized that the Rayleigh model is the easiest and
most tractable. However, this model is not valid in all situations. The results we present here will
therefore be only "ballpark" figures, to illustrate the workings of diversity combining.


Source: Adve, Raviraj - Department of Electrical and Computer Engineering, University of Toronto


Collections: Engineering