Optical Wireless Communications: Theory and Applications

Open Access
Author:
Aminikashani, Mohammadreza
Graduate Program:
Electrical Engineering
Degree:
Doctor of Philosophy
Document Type:
Dissertation
Date of Defense:
March 18, 2016
Committee Members:
  • Mohsen Kavehrad, Dissertation Advisor
  • Timothy Joseph Kane, Committee Chair
  • Julio Urbina, Committee Member
  • Jamal Rostami, Committee Member
Keywords:
  • Optical wireless communications
  • free-space optical communication
  • visible light communication
  • OFDM
  • indoor navigation
Abstract:
This dissertation focuses on optical communications having recently attracted significant attentions as a promising complementary technique for radio frequency (RF) in both short- and long-range communications. These systems offer significant technical and operational advantages such as higher capacity, virtually unlimited reuse, unregulated spectrum and robustness to electromagnetic interference. Optical wireless communication (OWC) can be used both indoors and outdoors. Part of the dissertation contains novel results on terrestrial free-space optical (FSO) communications. FSO communication is a line-of sight technique that uses lasers for high rate wireless communication over distances up to several kilometers. In comparison to RF counterparts, a FSO link has a very high optical bandwidth available, allowing aggregate data rates on the order of Tera bits per second (1 Tera bits per second is 1000 Giga bites per second). However, FSO suffers limitations. The major limitation of the terrestrial FSO communication systems is the atmospheric turbulence, which produces fluctuations in the irradiance of the transmitted optical beam, as a result of random variations in the refractive index through the link. The existence of atmospheric-induced turbulence degrades the performance of FSO links particularly with a transmission distance longer than 1 kilometer. The identification of a tractable probability density function (pdf) to describe atmospheric turbulence under all irradiance fluctuation regimes is crucial in order to study the reliability of a terrestrial FSO system. This dissertation addresses this daunting problem and proposes a novel statistical model that accurately describes turbulence-induced fading under all irradiance conditions and unifies most of the proposed statistical models derived until now in the literature. The proposed model is important for the research community working on FSO communications because it allows them to fully capitalize on the potentials of currently used FSO systems. Furthermore, utilizing this new statistical channel model, closed-form expressions for the diversity gain and the error rate performance of FSO links with spatial diversity are derived. In addition to addressing ways to improve outdoor FSO communication systems, this dissertation addresses some major challenges in indoor visible light communication (VLC). VLC is an advantageous technique that is proposed for wireless indoor communications. In VLC systems, the existence of multiple paths between the transmitter and receiver causes multipath distortion, particularly in links using non-directional transmitters and receivers, or in links relying upon non-line of-sight propagation. This multipath distortion can lead to intersymbol interference (ISI) at high bit rates. Multicarrier modulation usually implemented by orthogonal frequency division multiplexing (OFDM) can be used to mitigate ISI and multipath dispersion. Nevertheless, the performance of VLC systems employing OFDM modulation is significantly affected by nonlinear characteristic of light-emitting diode (LED) due to the large peak-to-average power ratio (PAPR) of OFDM signal. In other words, signal amplitudes below the LED turn-on-voltage and above the LED saturation point are clipped. This dissertation targets these important issues and successfully addresses them by developing some techniques to reduce high PAPR of optical OFDM signal and determining the optimum operating characteristics of LEDs for combined lighting and communications applications. VLC can also provide a practical solution for indoor positioning as global positioning system (GPS) does not provide an accurate and rapid indoor positioning since GPS radio signals are attenuated and scattered by walls of large buildings and other objects. A practical VLC system would be likely to deploy the same configuration for both positioning and communication purposes where high speed data rates are desired. This dissertation also proposes a novel OFDM VLC system that provides a high data rate transmission and can be used for both indoor positioning and communications where the multipath reflections are taken into account. Description of an experimental demonstration is also part of the dissertation where a software defined radio (SDR) was employed as the primary hardware and software interface to verify some of the results of the topics discussed earlier.