Improved PPM modulations for high spectral efficiency IR-WLAN

This article presents a modified PPM modulation technique for infrared wireless LAN that improves the spectral efficiency of the system significantly. The resulting signal (improved PPM or IPPM), which is based on FQPSK IJF filters, is jitter- and ISI-free and can be recovered with any standard PPM receiver. In order to reduce the bandwidth a pulse-conformation system is added to the basic PPM emitter. It is based on a differential sub-codification of the original PPM signal. IPPM presents a waveform similar to the frequency response of a raised-cosine filter with α=0.25. This signal can be recovered with any standard PPM receiver. We have worked with a receiver that uses a maximum likelihood criterion with block decision. Measurements show that the bandwidth required for a 4-IPPM signal, with a rate of 2 Mb/s, is reduced to 3.6 MHz, for the same quality criterion of -21 dBr. The spectral efficiency for -21 dBr rises from 0.2 to 0.55 b/s/Hz. The performance of both PPM and IPPM modulation versus white Gaussian noise are similar (less than 1 dB difference). Both models are also equally sensitive to low frequency interference, results show that their behavior in a highly lit environment is similar. Their performance can be improved by the use of BDFE (block decision feedback equalizer) presenting also similar results. IPPM is more robust against jitter than PPM. Extensive simulation results show that the 4-IPPM scheme improves the performance against jitter of the basic 4-PPM 2 Mb/s around 3 dB for an error probability of 10^-6