Interference in IM/DD Optical Wireless Communication Networks

The emerging field of optical wireless communications (OWC) offers exceptional promise as a technology for next-generation wireless networks. High data rate capabilities and ultra-dense access point deployment will allow OWC to supplement traditional RF technologies and remove congestion from the crowded RF spectrum. When implementing OWC via intensity modulation with direct detection (IM/DD), peak optical power emission constrains the instantaneous optical power. Average optical power is also constrained by eye safety regulations and illumination requirements of infrared and visible light communication (VLC) systems, respectively. These constraints differ from the conventional electrical power constraint of RF and wireline systems. Accordingly, performance metrics such as signal-to-noise ratio (SNR) and signal-to-interferenceplus- noise ratio (SINR) have been redefined in relation to the optical channel constraints in order to provide fair comparison across OWC implementations. In densely deployed networks, interference has a significant effect on system performance. Two key properties simplify the analysis of RF networks: 1) the relationship between electrical power and interference variance is modulation agnostic and 2) many interferers are typically assumed. The former allows SINR to be defined in terms of the channel constraint and the latter allows the aggregate disturbance from interference-plus-noise to be modeled as an additive Gaussian random component. In OWC networks, the optical power constraints relate to interference variance in amodulation specific manner, and the highly directional optical channel creates instances where a small set of interferers dominate the aggregate disturbance. In this work, we first derive bounds on the variance of OWC interference under the constraints of an OWC channel. We then evaluate the accuracy of assuming that interference follows a Gaussian distribution. Finally, we show results of a study in a testbed environment with 15 VLC-enabled LED luminaires in order to empirically evaluate OWC interference characteristics.