Short-range communications using diffusely scattered infrared radiation

Short range optical communication using diffusely scattered infrared radiation is examined. Aspects which relate to providing multichannel and duplex capability when one end of the link is not fixed, are considered. A link comparison is made between optical and microwave systems of similar complexity. Although it is shown that the optical system is not as sensitive as its microwave counterpart for typical ambient infrared backgrounds, it is advantageous with regard to spectrum availability and reuse, immunity from multipath distortion, and the maturity of the technology. Characteristics of the optical scatter propagation path are investigated using a static analytical model. Measurements of dynamic path loss provide data for the portable application. A signal to noise ratio enhancement is obtained by the use of short high power optical pulses. Selection of a digital modulation format permits the application of time division multiplex techniques to achieve the desired duplex and multichannel capability. System capabilities are demonstrated to be strongly influenced by device limitations, especially the light emitting diode (led) sources. A method of obtained receiver gain using detector optics is described and explained. Finally, the analysis and measured data are used to guide the design of a prototype portable telephone system providing two full duplex audio channels using digitally encoded speech is described.