Book Description
Because of low-cost optical devices and virtually unlimited bandwidth, optical wireless communications (OWC) for indoor wireless local area networks (WLANs) have recently become an attractive alternative to radio frequency systems. Since optical signals cannot penetrate through walls or other opaque barriers, the security of infrared WLANs is very high and there is no interference between rooms. Subsequently, cell planning is simple and easy, and the potential capacity of an optical-based network in a building is extremely high. However, the system link is susceptible to path loss and multipath dispersion. In addition, the average transmit power is constrained by eye-safety regulations and power consumption concerns. Hence, most recent research deals with the physical layer aspects such as modulation, equalization and error-control coding in order to cope with these draw-backs, especially the effects of multipath dispersion. The objective of this thesis is to study practical signaling techniques capable of eliminating the effects of intersymbol interference (ISI). Regarding the drawbacks of OWC, modulation schemes which are power and band-width efficient are considered. Pulse-position modulation (PPM) has been employed for IrDA and IEEE802.11 standards because it offers high power efficiency. However, it re-quires high bandwidth so that its performance is considerably degraded when the channel is more corrupted by ISI. A number of modified PPM techniques have been proposed to improve bandwidth efficiency. This thesis introduces a hybrid between pulse-amplitude modulation (PAM) and differential pulse-position modulation (DPPM), named differential amplitude pulse-position modulation (DAPPM), in order to gain a better compromise between power and bandwidth efficiency. It yields better bandwidth and/or power efficiency than PAM, PPM and DPPM depending on the number of amplitude levels (A), and the maximum length (L) of a symbol. The channel capacity of PPM, DPPM and DAP.