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Mitigating the Degradation Effects of Fading Channels

Apr 5, 2002

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We are generally concerned with dispersive fading, meaning that received energy is not lost or absorbed. But it arrives at the receiver at an inappropriate time (superimposed on other symbols). How do we learn to live with such fading? This article outlines the tools that are available for dealing with the deleterious effect of fading, and how to mitigate these effects so that the channel approaches AWGN performance.

This article is excerpted from Digital Communications: Fundamentals and Applications, Second Edition (Prentice-Hall, 2001, ISBN 0-13-084788-7).

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Introduction

This article emphasizes so-called Rayleigh fading, primarily in the UHF band, that affects mobile systems such as cellular and personal communication systems (PCS). The major elements that contribute to fading and their degradation effects in a communication channel are briefly summarized. Emphasis is then placed on methods for mitigating the degradation. Two examples of specific mitigation techniques are examined: the Viterbi equalizer implemented in the Global System for Mobile (GSM) Communications, and the Rake receiver used in CDMA systems built to meet Interim Standard-95 (IS-95).

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Editor's Note: This article is offered in PDF in order to provide consistency and clarity to the in-text equations.

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