Reducing the effects of intersymbol interference in diffuse DPIM optical wireless communications

Abstract

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.

Recent developments in indoor optical wireless systems

“…In DPIM, each block of log 2 L data bits is mapped onto one of L possible symbols, each differs in length [27]. Information is encoded by varying the number of empty slots between adjacent pulses, and hence the redundancy of empty slots following a pulse in PPM symbol is removed.…”

“…During detection, the slots contained within the guard slot following the correctly detected pulse are automatically assigned zero values, regardless of the level of the sampled output. The error performance with/without equalisation for DPIM is presented in [27].…”

A synopsis of modulation techniques for wireless infrared communication

“…The slot error rate and spectral properties of DPPM are also valid for the DPIM. To reduce the channel induced intersymbol interference, the use of one or more guard band(s)/slot(s) immediately after the pulse has been introduced by [19]. But for a highly dispersive channel, to achieve an acceptable error performance one would still require an equalizer at the receiver [19].…”

Bit error performance of diffuse indoor optical wireless channel pulse position modulation system employing artificial neural networks for channel equalisation

“…Mobility could be improved by utilizing a wide beam angle optical transmitter. In non-LOS links with no requirement for alignment, the existence of multipath links between the transmitter and the receiver eliminates the shadowing effect and blocking probability, but at the cost of increased path loss, reduced bandwidth and increased ISI compared with the LOS links [6,9,11,12].…”

“…Although in OW systems the combination of intensity modulation and direct detection (IM/DD) prevents multipath fading, ISI still constitutes a major system impairment especially at high bit rates in an indoor environment [12][13][14]. In dispersive environment, the maximum likelihood sequence detection (MLSD) technique has been shown to give the optimum result; however, its complexity and delay prohibit its use in many applications [15].…”