Link Analysis and Optics Design

Before we can address such issues as electrical receiver design and modulation design, we need a solid understanding of the optical components that underlie a wireless infrared communication system. The purpose of this chapter is to present a link budget analysis for non-directed line-of-sight optical communications, and to present design strategies for optimizing the transmitter and receiver optical components. We examine the signal-to-noise ratio (SNR) improvement of a hemispherical concentrator as an optical antenna. We propose a hemispherical thin-film optical filter, and compare its performance to that of a traditional planar thin-film filter. For both filter types, we jointly optimize the transmitter radiation pattern, the filter orientation, and the filter bandwidth. The results of this chapter indicate that a 269-mW transmitter and a 1-cm2photodetector are sufficient to achieve a 100-Mb/s non-directed link over a range of 4 m, even in the presence of intense background light. These numerical results are based on a sphere radius of 2 cm; in principle, an arbitrary performance improvement can be achieved by an arbitrarily large sphere radius, implying a direct trade-off between performance and size.