Digital pulse interval modulation for indoor optical wireless communication systems
Over the past decade, infrared has attracted a considerable amount of interest as an alternative medium to radio for short-range indoor wireless local area networks. Infrared offers a number of significant advantages over its radio frequency counterpart, such as the abundance of bandwidth that is currently unregulated worldwide, the availability of low cost emitters and detectors, inherent security and resistance to multi path fading. The work presented in this thesis focuses on modulation techniques, the fundamental aim being to assess the suitability of digital pulse interval modulation (DPIM) for use in indoor optical wireless communication systems. Infrared transceivers are subject to eye safety regulations, and consequently power efficiency is an important criterion when evaluating modulation techniques. From the error probability analysis carried out on the non-distorting additive white Gaussian noise channel, it is shown that DPIM is able to trade off power efficiency against bandwidth efficiency by increasing the number of bits per symbol. Furthermore, by encoding an additional bit per symbol, DPIM can outperform pulse position modulation (PPM) both in terms of power efficiency and bandwidth efficiency when simple threshold detection is employed. Indoor optical wireless systems generally operate in the presence of intense ambient light, emanating from both natural and artificial sources. Along with contributing to the generation of shot noise, artificial ambient light sources also introduce a periodic interference signal which can have a detrimental effect on link performance. Original analysis is presented which examines the error performance of DPIM in the presence of interference from a fluorescent lamp driven by a high-frequency electronic ballast, which is potentially the most degrading source of ambient light. It is found that such interference results in an average optical power requirement that is almost independent of the bit rate. The analysis then goes on to consider the effectiveness of electrical high-pass filtering as a simple means of mitigating the effect of the interference, taking into account the baseline wander introduced by the high-pass filter. DPIM was found to be more susceptible to the effects of baseline wander compared with PPM, a finding which is supported by the original analysis carried out on the power spectral density of the scheme. Consequently, whilst electrical high-pass filtering was found to be very effective at high bit rates, significant power penalties are still incurred at low to medium bit rates. In non-directed line of sight and diffuse link configurations, multipath propagation gives rise to intersymbol interference (ISI), which must be taken into account for data rates above 10 Mbit/s. Original analysis is presented which examines the unequalized performance of DPIM in the presence of ISI. From this analysis, it is found that on any given channel, the improved bandwidth efficiency of DPIM results in lower average optical power penalties, compared with PPM. One novel technique which can be used to make DPIM more resistant to the effects of ISI is to add a guard band to each symbol, immediately following the pulse. Original contributions are presented which evaluate the effectiveness of this technique. To quantify the results obtained, analysis is also carried out on DPIM using a zero-forcing decision feedback equalizer (ZF-DFE), which represents a more traditional approach to mitigating the effects of ISI. It is shown that the guard band technique offers a similar level of performance to the ZF-DFE on all but the most severe channels, and has the advantage of reduced cost and complexity compared with implementing a ZF-DFE. To support the theoretical and simulated performance of DPIM carried out in this thesis, details are given of a prototype 2.5 Mbit/s diffuse infrared link employing 16-DPIM which has been designed and constructed. The error performance of the link is measured under a variety of ambient light conditions, and the effectiveness of electrical high-pass filtering in mitigating the resulting interference is assessed. It is shown that whilst a fluorescent lamp driven by a high frequency electronic ballast has the potential to significantly degrade link performance, the power penalty introduced by this source can be made manageable by careful selection of the high-pass filter cut-on frequency.