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Improved PPM modulations for high spectral efficiency IR-WLAN

Abstract:

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 PPM standard receiver. In order to reduce the bandwidth a pulse-conformation system is added to the basic PPM emitter. It is based on a differential subcodification 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. In this study we have worked with a receiver that uses a maximum likelihood criterion with block decision. Measures 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 performances of both PPM and IPPM modulations versus white gaussian noise are similar (less than 1 dB of difference). Both models are also equally sensitive to low frequency interference, results show that their behavior in a highly lit environment is similar. Their performances 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 4-IPPM scheme improves the performances against jitter of the basic 4-PPM 2Mb/s around 3dB for an error probability of 10-6.