Fluorescent lamps introduce a periodic interference signal in optical wireless receivers, which has the potential to severely degrade link performance. Usually, electrical high-pass filtering is employed to mitigate the effects of this interference, but this results in baseline wander or intersymbol interference. Digital pulse interval modulation (DPIM) is a modulation technique which has been shown to be suitable for deployment in optical wireless communication systems and this paper examines the effect of baseline wander on systems employing DPIM. A new expression is given for the slot autocorrelation function of DPIM and, from this, the power spectral density is calculated. The error performance of DPIM, as a function of high-pass filter (HPF) cut-on frequency, is compared with the more established techniques of on-off keying using non-return to zero signalling (OOK) and pulse position modulation (PPM), using both numerical analysis and computer simulation.
This paper investigates the performance of digital pulse interval modulation (DPIM) in the presence of multipath propagation and additive white Gaussian noise. To combat intersymbol interference (ISI) guard slots and a non-linear equaliser have been introduced. The average optical power requirements (AOPR) due to ISI for cases with/without guard slots and with equaliser are analysed using a ceiling-bounce model. Results obtained show that in the absence of equalisation, DPIM without guard slot offers a lower AOPR compared with on-off keying (OOK). Introducing guard slots gives a further reduction in AOPR by up to 4 dB due to the reduced duty cycle of the DPIM signal. The performance of DPIM without guard slot buy using an equaliser is found to be significantly better than DPIM with guard slots on a channel with severe ISI.
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