Spread spectrum techniques for indoor wireless IR communications

Multipath dispersion and fluorescent light interference are two major problems in indoor wireless infrared communications systems. Multipath dispersion introduces intersymhol interference at data rates above 10 Mb/s, while fluorescent light induces severe narrowband interference to baseband modulation schemes commonly used such as OOK and PPM. This article reviews the research into the application of direct sequence spread spectrum techniques to ameliorate these key channel impairments without having to resort to complex signal processing techniques. The inherent properties of a spreading sequence are exploited in order to combat the ISI and narrowband interference. In addition, to reduce the impact of these impairments, the DSSS modulation schemes have strived to be bandwidth-efficient and simple to implement. Three main DSSS waveform techniques have been developed and investigated. These are sequence inverse keying, complementary sequence inverse keying, and M-ary biorthogonal keying (MBOK). The operations of the three systems are explained; their performances were evaluated through simulations and experiments for a number of system parameters, including spreading sequence type and length. By comparison with OOK, our results show that SIK, CSIK, and MBOK are effective against multipath dispersion and fluorescent light interference becausc the penalties incurred on the DSSS schemes are between 0-7 dB, while the penalty on OOK in the same environment is more than 17 dB. The DSSS solution for IR wireless transmission demonstrates that a transmission waveform can he designed to remove the key channel impairments in a wireless IR system.