Application of Smart Antennas to Cognitive Radio Systems
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Date
2013-02
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Addis Ababa University
Abstract
This dissertation studies three interrelated topics: design of non-uniform antenna arrays
using genetic algorithm (GA), application of smart antennas to cognitive radio (CR) and
determination of the distributions of signal-to-interference ratio (SIR) in Rayleigh fading.
The performance of a single-element antenna is limited. To obtain high directivity,
narrow beamwidth, low sidelobes, point-to-point and preferred-coverage pattern characteristics,
etc., antenna arrays are used. Nowadays, antenna arrays appear in wireless
terminals and smart antennas, so robust and e cient array design is increasingly becoming
necessary. In array design, it is frequently desirable to achieve both a narrow
beamwidth and a low sidelobe level. A uniform array yields the smallest beamwidth and
hence the highest directivity. It is followed, in order, by the Dolph-Chebyshev and Binomial
arrays. In contrast, Binomial arrays usually possess the smallest sidelobes followed,
in order, by the Dolph-Chebyshev and uniform arrays. In the dissertation, GA is used to
design a non-uniform linear array that approximates the beamwidth of a uniform array
and having smaller sidelobe level than the Dolph-Chebyshev array. The designed array
generally exhibits the largest directivity as compared to the uniform, Binomial and DolphChebyshev
arrays. The result can be used in applications where narrow beamwidth and
low sidelobes levels are preferred such as in switched-beam smart antennas.
The second topic addressed by the dissertation is application of smart antennas to
cognitive radios. Conventional CRs exploit the licensed spectrum by opportunistically
seeking the underutilized radio resource in time, frequency and space (geographic) domains.
However, there are other dimensions that need to be explored further for spectrum
opportunity. The angle dimension is a typical example. The literatures surveyed point
to the fact that little work has been done in this dimension. The dissertation particulary
investigates the performance of CRs equipped with smart antennas in exploiting the
angle opportunity in Rayleigh fading channel. The proposed cognitive transmitter keeps
the interference to the primary receiver below a given threshold while at the same time
ensuring high enough signal-to-interference-plus-noise ratio (SINR) at the cognitive receiver.
It was shown that the angle spectrum can be signi cantly exploited for spectrum
opportunity using smart antennas. It was also shown that the angle spectral e ciency
for Rayleigh channel is lower as compared to a non-fading channel with the same power
levels and radio terminal locations. Moreover it has been shown that increasing the cognitive
transmitter power beyond a certain level has negligible e ect on the angle spectral
e ciency.
The third topic of the dissertation extends the determination of the distribution of
SIN in Rayleigh fading channel when N interferers are present. In fading channels the
SINR and SIR are random variables and their distributions have to be known for a correct
description of the receiver. The distributions of SIR in Rayleigh fading have been
described by deriving a closed form expression for the probability density function, cumulative
distribution and outage probabilities.
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Keywords
Antenna Arrays, Uniform Arrays, Non-Uniform Arrays, Binomial Arrays, Dolph-Chebyshev Arrays, Genetic Algorithm, Cognitive Radio, Smart Antenna, Angle Spectrum, Rayleigh Fading, SINR, SIR