Effect of electronic-ballast fluorescent lighting on wireless infrared links

Abstract

Fluorescent lamps driven by electronic ballasts emit an infrared (IR) signal that is periodically modulated at rates of tens of kilohertz, and which can severely impair the performance of IR wireless links. The impact of fluorescent interference can potentially be reduced by highpass electrical filtering, but such filtering induces intersymbol interference (ISI). The authors present expressions for the bit error rate (BER) of systems using on-off keying (OOK) and pulse-position modulation (PPM) in the presence of both a deterministic interfering signal and ISI. They have measured the interference waveforms from lamps driven by 22 and 45 kHz ballasts, and have used the measured waveforms to evaluate the performance of IR links using OOK and 2, 4, 8 and 16 PPM at bit rates of 1, 10, and 100 Mbit/s. When the fluorescent interference is normalised to the signal power required in the absence of this interference, the penalties incurred by OOK are found to be essentially independent of bit rate. At 1 Mbit/s, PPM suffers approximately the same penalties as OOK, but as the bit rate is increased, the degradation of PPM becomes progressively much smaller. In the absence of measures to prevent ISI, first-order highpass filtering is not effective in improving the performance of OOK, but can substantially improve PPM link performance at bit rates of 10 and 100 Mbit/s.