Space Diversity in Indoor Broadband Optical Wireless Communications

Open Access
Author:
Alqudah, Yazan A.
Graduate Program:
Electrical Engineering
Degree:
Doctor of Philosophy
Document Type:
Dissertation
Date of Defense:
May 29, 2003
Committee Members:
  • Mohsen Kavehrad, Committee Chair
  • John Metzner, Committee Member
  • Shizhuo Yin, Committee Member
  • Natarajan Gautam, Committee Member
Keywords:
  • Optical Wireless Communications
Abstract:
Wireless optical (infrared) link provides a secure and a promising alternative to radio for wireless indoor connectivity, be it for terminals or sensors. The large spectrum of unregulated band enables a link to provide broadband access needed for multimedia and other bandwidth demanding applications. The spatial confinement of infrared light provides interference-free bandwidth-reuse in adjacent rooms. The ability to create spatially independent channels in a small physical space holds the promise of large link capacity. The main challenges in the design of an infrared link include: susceptibility to shadowing, multipath dispersion, and limited range resulting from noise generated by ambient light. Shadowing caused by benign objects blocking signal path results in service degradation, if not complete interruption. Configurations employing wide beam transmitter to service many receiver locations suffer multipath. Noise at receiver is generated by ambient light. Even in a uniformly lit environment, noise generated by natural and artificial light varies depending on receiver location and orientation. To combat the adverse effects of temporal dispersion in high-speed applications, an accurate channel impulse response is needed. The impulse response is used to analyze and to compensate for the effects of multipath dispersion. In this work, a new approach for obtaining the channel impulse response is presented resulting in tremendous savings in calculation time and bringing insight into the channel behavior. The ability to create spatially independent channels has motivated a new configuration called Multi-Spot Diffusing (MSD) configuration. In which, a transmitter acts as an array antenna, with each element transmitting data over an independent channel. A multibranch receiver is employed to receive independent copies of transmitted data through each of its branches. In our research, we analyze MSD link with the objective of determining the optimal number of branches that results in maximum signal-to-noise ratio, minimum probability of error and minimum outage probability. The MSD configuration increases link capacity and reliability by providing a multi-input multi-output channel between transceivers. The availability of N spatial channels implies the possibility of increasing data rate by N folds compared to a single channel. In order to improve link reliability, our research considers novel spatial diversity coding techniques. In orthogonal spatial coding, each channel is responsible for carrying one of N symbols. The receiver decides on a transmitted symbol by comparing received signals on its branches and selects the branch corresponding to maximum signal. In another proposed scheme, a symbol is represented by N bit code word. Each bit is transmitted through a separate channel. The diversity receiver decides on a symbol that corresponds to the highest correlation with a received code word. Thus, information is transmitted not only through signal shapes, but also through branches that receive them. Traditionally, a non-directed non-line-of sight link configuration uses a ceiling as crossover between a transmitter and a receiver. In many practical environments, the ceiling is not available, either due to environment structure or poor reflectivity of ceiling surface. In this work, we propose and assess the feasibility of using a new configuration that relies on a wall’s upper region as crossover. Both configuration are characterized and compared based on path loss and delay spread.