A duplex infra-red system for in-building communications

This paper discusses the application of optical links using indirect scatter to in-building portable communications. The design considerations for such systems are described, and the reasons for selecting a time division multiplexed approach with ON-OFF keying. Both analytical and experimental determination of the path loss for indirect scatter channels are discussed, and it is shown that about 85 dB of path loss can be expected for practical devices in representative office environments. A means for improving S/N in a pulsed system which is average power limited is described. An experimental system based on the above considerations was constructed consisting of a fixed base unit (and range extension repeaters), having essentially hemispheric field of view, and a portable unit having a radial transmitter and hemispheric receiver fields of view. The operation, construction, and performance of the prototype system is discussed.