System Design and Modeling of Optical Scattering Communications

Abstract

Demand for optical wireless communications (OWCs) is growing at a rapid pace. There is a growing interest from the commercial and the military in having secure, efficient, and high bandwidth OWC systems for tactical applications. An OWC system relies on optical wavelength, ranging from infrared to ultraviolet (UV) to convey a piece of information. With recent advances in the design and manufacturing of UV sources and detectors, optical scattering communication has attracted much interest for diverse applications. Atmospheric scattering in the deep UV band is much higher than other optical bands. This phenomenon enables establishing of a non-line-of-sight (NLOS) link. This thesis is devoted to the system design and modeling of various aspects of optical scattering communications employing ultraviolet wavelength. It provides the most comprehensive NLOS optical scattering communication systems design and modeling.

The focus of the studies discussed in the second chapter is to investigate the spatial diversity techniques for optical scattering communications. With the received signal characterized by a continuous waveform detector, the switch and stay combining (SSC) diversity technique is implemented at the receiver over correlated turbulence fading channel. Next, to avoid the requirement of an adaptive threshold in demodulating the on-off keying (OOK) symbol, the SSC diversity technique equipped with M-ary phase-shift keying (MPSK) is presented. The chapter also presents a maximal selection transmit diversity for optical scattering communications.

The third chapter presents techniques for optical spectrum sensing for distributed optical scattering communications. First, we develop an energy detection-based optical spectrum sensing for a UV network where each user is equipped with a continuous waveform detector. Second, we propose a new generic blind optical spectrum sensing technique. The proposed technique is based on finding the statistical ratio of the received continuous waveform signal. It does not require prior knowledge of the signal and noise power. Next, we propose two novel cooperative spectrum sensing techniques for optical scattering communication networks with randomly distributed collaborative users pointing in arbitrary directions. In particular, we present centralized- and decentralized-based cooperative spectrum sensing with hard decision fusion. We assume multiple scattering and characterize the received signal by the photon-counting receiver. To obtain the distribution of the received irradiance over the multi-scattering channel, a novel method based on the Mellin transform is proposed. In the centralized technique, all the raw data (in the form of statistically dependent random variables) available at the collaborative users are combined using AND rule for decision, whereas, in the decentralized-based spectrum sensing, instead of sending all the raw data, only one-bit information is required to be sent to the fusion center to make the decision.

In the fourth chapter, we present an artificial neural network (ANN) based algorithm to estimate the data. In addition, analytical frameworks are presented to estimate the channel model parameters under different fading distributions.

In the fifth chapter, we present a novel state-of-the-art UV-based indoor optical communications. The channel model is developed and analyzed. To ensure that the UV radiations are under allowable safety exposure limits, strict peak and average power constraints are imposed. Based on the power budget, the necessary conditions for the input distribution of the intensity-modulated direct detection (IM/DD) OOK modulated signal are derived. The link gain analysis is presented to investigate the directed line-of-sight (LOS) component and diffused links with multiple reflections and NLOS scattered components. The time-delay statistics for various paths are analyzed. Next, we present a novel multiuser indoor communication over a power-constrained Poisson channel. To reject the interference from multiple users, we present a minimum mean square error (MMSE) receiver. Moreover, a downlink beamforming optimization problem is formulated using second-order cone programming to maximize the received signal-to-interference-plus-noise ratio (SINR). The proposed multiuser system is also realized in the experiment. The influence of the control parameter, that is, the average-to-peak power ratio, on the performance is investigated.

The next chapter is dedicated to implementing orbital angular momentum (OAM) with UV communications. As a method to improvise the reliability of the UV communication system, we develop an OAM-based UV communication. Our work presents a significant increase in the UV channel capacity.

The seventh chapter presents a novel decentralized and self-organizing non-line-of-sight (NLOS) UV-based V2V communication (IVC). Experiments for proof-of-principle purposes were conducted with two vehicles in both stationary and time-varying UV channels. It is demonstrated that the proposed UV-IVC is capable of providing low-cost, low-power, and NLOS capable V2V communications with acceptable performance.

The last chapter concludes and summarizes the optical scattering communication systems developed in the previous chapters. In conclusion, the results and analysis presented in this thesis show the feasibility of employing optical scattering communication technologies in next-generation free-space optical communications. The future scope of optical scattering communications is also elaborated in the conclusion.