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Beamforming techniques for wireless communications in low-rank channels: analytical models and synthesis algorithms.
Comisso, Massimiliano
2008-03-18
Contributor(s)
Vescovo, Roberto
Abstract
The objective of this thesis is discussing the application of
multiple antenna technology in some selected areas of wireless
networks and fourth-generation telecommunication systems. The
original contributions of this study involve, mainly, two research
fields in the context of the emerging solutions for high-speed
digital communications: the mathematical modeling of distributed
wireless networks adopting advanced antenna techniques and the
development of iterative algorithms for antenna array pattern
synthesis. The material presented in this dissertation is the
result of three-year studies performed within the
Telecommunication Group of the Department of Electronic
Engineering at the University of Trieste during the course of
Doctorate in Information Engineering.
In recent years, an enormous increase in traffic has been
experienced by wireless communication systems, due to a
significant growth in the number of users as well as to the
development of new high bit rate applications. It is foreseen that
in the near future this trend will be confirmed. This challenging
scenario involves not only the well established market of cellular
systems, but also the field of emerging wireless technologies,
such as WiMAX (Worldwide interoperability for Microwave Access)
for wireless metropolitan area networks, and Wi-Fi (Wireless
Fidelity) for wireless local area networks, mobile ad-hoc networks
and wireless mesh networks. The rapid diffusion of architectures
adopting an ad-hoc paradigm, in which the network infrastructure
is totally or partially absent and that can be deployed using
low-cost self-configuring devices, has further enlarged the number
of systems that have to coexist within a limited frequency
spectrum. In such evolving environment, the development of
interference mitigation methods to guarantee the communication
reliability, the implementation of proper radio resource
allocation schemes for managing the user mobility as well as for
supporting multimedia and high speed applications, represent the
most relevant topics. Classic approaches are focused on the use of
the time-frequency resources of the propagation channel. However,
to satisfy the increasing demand of network capacity, while
guaranteeing at the same time the necessary levels in the quality
of the offered services, operators and manufacturers must explore
new solutions.
In this scenario, the exploitation of the spatial domain of the
communication channel by means of multiple antenna systems can be
a key improvement for enhancing the spectral efficiency of the
wireless systems. In a rich scattering environment, the use of
multiple antennas enables the adoption of diversity and spatial
multiplexing techniques for mitigating and, respectively,
exploiting multipath fading effects. In propagation environments
characterized by small angular spreads, the combination of antenna
arrays and beamforming algorithms provides the possibility to
suppress the undesired sources and to receive the signals incoming
from the desired ones. This leads to an increase of the signal to
interference plus noise ratio at the receiver that can be
exploited to produce relevant benefits in terms of communication
reliability and/or capacity. A proper design of the medium access
control layer of the wireless network can enable the simultaneous
exchange of packets between different node pairs as well as the
simultaneous reception of packets from multiple transmitters at a
single node. Switched-beam antennas, adaptive antennas (also
referred to as smart antennas), and phased-antenna arrays
represent some of the available beamforming techniques that can be
applied to increase the overall system capacity and to mitigate
the interference, in a scenario where several different
technologies must share the same frequency
spectrum.
In the context of distributed wireless networks using multiple
antenna systems, the core of this thesis is the development of a
mathematical model to analyze the performance of the network in
presence of multipath fading, with particular reference to a
scenario in which the signal replicas incoming at the receiver are
confined within a small angle and are characterized by small
relative delays. This propagation environment, referred to as
low-rank, is the typical operating scenario of smart antennas,
which necessitate high spatial correlation channels to work
properly. The novel aspects of this study are represented by the
theoretical and numerical modeling of the sophisticated adaptive
antennas in conjunction with a detailed description of the channel
statistics and of the IEEE 802.11 medium access control scheme. A
theoretical model providing a more realistic perspective may be
desirable, considering that, at present, not only cost and
competition issues, but also too optimistic expectations, as
compared to the first measurements on the field, have induced the
wireless operators to delay the adoption of smart antenna
technology.
The presented analysis includes the most relevant elements that
can influence the network behavior: the spatial channel model, the
fading statistic, the network topology, the access scheme, the
beamforming algorithm and the antenna array geometry. This last
aspect is numerically investigated considering that the size of
the user terminal represents a strict constraint on the number of
antennas that can be deployed on the device, and so the
maximization of the performance becomes related to the geometrical
distribution of the radiators. In ad-hoc and mesh networks, the
typical communication devices, such as laptops, palmtops and
personal digital assistants require compact and cheap antenna
structures as well as beamforming algorithms easy to implement. In
particular, the low-cost characteristics have guaranteed a wide
popularity to wireless mesh technology, which have encouraged the
birth of a new social phenomenon, known as wireless community
networks, whose objective is the reduction of the Internet access
cost.
The adoption of multi-antenna systems is the purpose of the IEEE
802.11n amendment, which, however, not considering modifications
of the medium access control layer, provides higher bit rates for
the single link, but does not allow simultaneous communications
between different couples of nodes. This aspect must be taken into
account together with the fact that, nowadays, IEEE 802.11x
represents the leading family of standards for wireless local
communications, and enhancement proposals have to pay careful
attention to the backward compatibility issues. The mathematical
model presented in this thesis discusses the suitable parameter
settings to exploit advanced antenna techniques in 802.11-based
networks when the access scheme supports multiple communications
at the same time, maintaining a realistic description for the
antenna patterns and the channel behavior.
The presentation of two new iterative algorithms for antenna array
pattern synthesis represents the core of the last part of this
dissertation. The proposed solutions are characterized by
implementation simplicity, low computational burden and do not
require the modification of the excitation amplitudes of the array
elements. These advantages make the presented algorithms suitable
for a wide range of communication systems, while matching also the
inexpensiveness of mesh and ad-hoc devices. In particular,
phase-only synthesis techniques allow the adoption of a cheaper
hardware, including only phase shifters, which are available at a
reasonable price, while avoiding the use of the more expensive
power dividers.
The first presented algorithm employs the spatial statistic of the
channel for properly placing the pattern nulls, in order to
suppress the undesired power incoming from a given angular
interval. This solution exploits the improved knowledge of the
spatial properties of the propagation environment for enhancing
the interference suppression capabilities at the transmitter and
receiver sides. The second algorithm is a phase-only technique
that is able to generate multiple nulls towards the undesired
directions and multiple main lobes towards the desired ones. This
method provides the possibility to perform spatial multiplexing
adopting low-cost electronic components.
The thesis is organized in three parts. The first one provides the
background material and represents the basics of the following
arguments, while the other two parts are dedicated to the original
results developed during the research activity. With reference to
the first part, the fundamentals of antenna array theory are
briefly summarized in the first chapter. The most relevant aspects
of the wireless propagation environment are described in the
second chapter, focusing on the characteristics of the spatial
domain in a low-rank scenario. The third chapter presents a
classification of the different multiple antenna techniques
according to the channel properties and provides an overview of
the most common beamforming algorithms. The fourth chapter
introduces the most significant aspects of the distributed
wireless networks, presenting the main open issues and the current
proposals for the exploitation of the potential offered by antenna
array systems.
The second part describes the original results obtained in the
mathematical modeling of ad-hoc and mesh networks adopting smart
antennas in realistic propagation scenarios. In particular, the
fifth chapter presents the theoretical analysis to evaluate the
number of simultaneous communications that can be sustained by a
distributed wireless network using adaptive antennas in presence
of multipath. The sixth chapter extends this model to
switched-beam antennas, while addressing the mobility aspects and
discussing the cost-benefit tradeoff that is related to the use of
multiple antenna techniques in today's wireless networks. A
detailed throughput-delay analysis is performed in the seventh
chapter, where the impact of advanced antenna systems on
802.11-based networks is investigated using a Markov chain model.
The influence of the antenna array geometry is examined in the
eighth chapter adopting a numerical approach based on a
discrete-time simulator, which is able to take into account the
details of the channel and of the antenna system behavior.
The third part describes the original results obtained in the
field of antenna array pattern synthesis. The ninth chapter
presents the technique developed to modify the excitation phases
of an antenna array in order to reject interferers spread over an
angular region according to a given spatial statistic. The tenth
chapter describes the iterative algorithm for phased arrays, which
is able to produce low side-lobe level patterns with multiple
prescribed main lobes and nulls. Finally, the eleventh chapter
summarizes the thesis contributions and remarks the most important
conclusions.
The intent of the work presented hereafter is to examine the
benefits that derive from the employment of smart antenna
techniques from a realistic perspective, as well as to provide
some useful solutions to improve the reliability of the
communications and to increase the network capacity.
Insegnamento
Publisher
Università degli studi di Trieste
Languages
en