Performance of an infrared wireless CDMA system

Infrared (IR) is a new medium for wideband wireless applications especially at indoors. Infrared has abundant bandwidth and does not require FCC approval enabling rapid deployment. Intensity modulated systems do not experience multipath fading. However, health regulations limit the laser output power [IEC 825]. Photodetectors with large photosensitive areas have high sensitivity but they also have high junction capacitance that limits the bandwidth. High ambient noise is another concern in infrared systems. Spreading the spectrum, which enables low power transmission, can be used to overcome these limitations. In this paper, we evaluate the performance of a direct spread CDMA type infrared wireless system in a multiuser environment considering: multi-user interference and inter-symbol-interference assuming different channel models. According to the simulation a BER of 10-7 can be achieved with a moderate bit rate when, the channel impulse response exponentially decays. The maximum bit rate, however, depends on the mean square value of the channel dispersion. Lab prototype agrees with the theory and shows ambient light is the biggest concern in a pico cellular environment with few users.