Performance of Di-code Pulse Position Modulation Technique in Diffuse Indoor Wireless Optical Communication Systems

The bandwidth of the indoor optical wireless Pulse Position Modulation (PPM) system can be
exploited to improve the receiver sensitivity by using Di-code Pulse Position Modulation (Di-PPM)
technique. This paper analysis the receiver sensitivity of indoor optical wireless Di-PPM systems, over a slightly dispersive channel, uses a bandwidth-limited PIN-bipolar (PINBJT) optical receiver, zero guard interval and pre-detection filter based on a simple third-order Butterworth filter, that authorizes the receiver to operate over channels with bandwidth as low as 1.2 times the bit rate. The received pulse shape is a convolution between the impulse response of a diffuse optical wireless link in a ceiling bounce model and rectangular waveform, the operating bit-rate chosen is 100Mbit/s. In this paper the performance analysis is extended in order to include the effects of intersymbol interference
(ISI) on error probability and some important errors: wrong slot, erasure and false alarm. A
mathematical model is then presented showing how the transfer function of the indoor optical wireless Di-PPM receiver can be calculated. Results are obtained and the pulse shape at input of receiver decision device is illustrated by using MathCAD software.