Adaptable bandwidth system for reliable wireless transmission

Abstract

The concept of a simple adaptable bandwidth system is investigated which can be implemented to maintain reliability of the communication link in a non-directed diffused indoor optical wireless environment under varying channel conditions. The proposed system is found to be more sensitive to any variations in signal-tonoise ratio (SNR) at higher noise magnitudes compared to SNR variations at lower noise magnitudes. INTRODUCTIONIn an optical wireless communication system with non-directed diffused radiation, user mobility requires the communication link to operate reliably under widely varying channel conditions which are determined by the ambient light level, the distance between transmitter and receiver, the degree to which the transmission path is obstructed and the strength of inter-symbol interference induced due to multipath dispersion. The variations in channel conditions are reflected in the signal-to-noise ratio (SNR) at the receiver end. Under adverse conditions, instead of incurring link failure, it is advisable to downgrade the performance and maintain the link at lower data rates and bandwidth. Here, an adaptable bandwidth filter is proposed along with SNR improvement system in order to impart reliability of transmission under varying noise/SNR conditions in an optical wireless system.Since the optical wireless systems operate at frequencies of several hundred Megahertz to several Gigahertz, the use of an analogue filtering system is justified as it offers an economical solution compared to digital systems which require ultra fast sampling circuits and hence can become extremely expensive. Also, the signal sampling and A/D conversion process results in quantisation noise which is a form of distortion at frequencies dependent on the signal and the sampling rate. In order to minimise the quantisation noise, a large number of bits needs to be used which further complicates the design of A/D and D/A converters.