Experimental 25-Mb/s wireless infrared link using 4-PPM with scalar decision-feedback equalization

We report a non-line-of-sight optical link for short-range, indoor data transmission at 25 Mb/s. The optical link uses a ceiling-directed beam and a wide-field-of-view receiver. The system uses 4-pulse-position modulation (4-PPM) with scalar decision-feedback equalization (SDFE), achieving low bit-error rates with a range of two meters in the presence of background light and multipath-induced intersymbol interference. We observe that the gain in electrical SNR efficiency from equalization is about 0.5 dB larger than theory, which we attribute to timing-offset errors in the unequalized system. We verify that the near-DC noise caused by electronic ballast-driven fluorescent lamps has a negligible impact on the system performance