Moments and Channel Capacity of Single and Dual-Branch Receivers Over Weibull Fading
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Moments and Channel Capacity of Single and
Dual-Branch Receivers Over Weibull Fading Channels



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ABSTRACT

Diversity combining receivers is the well-known technique used to combat fad-
ing and improve system performance. There are several diversity reception methods
employed in digital communication receivers including maximal-ratio combining, se-
lection combining and switched diversity. Among the combining methods, switched
diversity offers one of the least complexity solution for fading mitigation and can be
used in conjunction with coherent, non-coherent and differentially coherent modu-
lation schemes.

Introduction

In typical wireless communication channels, the radio signals generally propagate
according to three mechanisms: reflection, diffraction, and scattering due to the
presence of various natural and manmade structures such as buildings, mountains
etc. which results in multipath signal propagation (see Figure 1.1). All these multi-
path signals which arrive at the receiver at slightly different times add constructively
or destructively resulting in the variation of the amplitude, phase and frequency of
the resultant received signal known as Fading. Unlike wired channels that are sta-
tionary and predictable, radio channels are extremely random and do not offer easy
analysis.

Fading in a Communication Channel

Due to multiple reflections from various objects, the electromagnetic waves travel
along different paths of varying lengths. Fading causes rapid fluctuations in the
signal, hence the receiver performance decreases. According to the variation of the
signal strengths and receiver capability, the transmitted signal undergo the effects
of path loss variance with distance, shadowing and multipath fading.

Small Scale Fading

As a mobile moves over very small distances, the instantaneous received signal
strength may fluctuate rapidly giving rise to small scale fading. It occurs because
the received signal is a sum of many contributions coming from different directions
which are random in phase and amplitude. To model the small scale fading, the signal variations are considered over an area of the order of signal wavelength. In
this type of fading, the mean powers of the received signal varies about the mean
value and is almost constant.

Large Scale Fading

The variation of the mean received signal strength for larger distance (several hun-
dreds or thousands of meters) between an arbitrary transmitter-receiver (T-R) is
called large scale fading. The variation in the received signal strength occurs be-
cause of shadowing of the signal by buildings, mountains etc. Propagation models
that characterize the mean signal strength are useful in estimating the radio coverage
area of a transmitter.
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