Adaptation techniques in optical wireless communications

The need for high-speed local area networks to meet the recent developments
in multimedia and video transmission applications has recently focused interest
on optical wireless communication. Optical wireless systems boast some
advantages over radio frequency (RF) systems, including a large unregulated
spectrum, freedom from fading, confidentiality and immunity against
interference from electrical devices. They can satisfy the dual need for mobility
and broadband networking. However, optical wireless links are not without
flaws. They are affected by background noise (artificial and natural light
sources) and suffer from multipath dispersion. The former can degrade the
signal-to-noise ratio, while the latter restricts the maximum transmission rate
available.
The aim of this thesis is to investigate a number of techniques to overcome
these drawbacks and design a robust high-speed indoor optical wireless system
with full mobility. Beam delay and power adaptation in a multi-spot diffusing
system is proposed in order to increase the received optical signal, reduce the
delay spread and enable the system to operate at higher data rates. The thesis
proposes employing angle diversity receivers and imaging diversity receivers as
in order to reduce background noise components. Moreover, the work
introduces and designs a high-speed fully adaptive optical wireless system that
employs beam delay, angle and power adaptation in a multi-spot diffusing
configuration and investigates the robustness of the link design in a realistic
indoor office. Furthermore, a new adaptive optical wireless system based on a
finite vocabulary of stored holograms is introduced. This method can effectively
optimise the spots’ locations and reduce the design complexity of an adaptive
optical wireless system. A fast adaptation approach based on a divide-andconquer
methodology is proposed and integrated with the system to reduce the
time required to identify the optimum hologram. The trade-off between complexity and performance enhancement of the adaptive finite holograms
methods compared with the original beam power and angle adaptation is
investigated.