Signal-Dependent Shot and Relative Intensity Noise in Channel Estimation of Laser Diode-based Indoor VLC Systems

Laser diode (LD) powered luminaires are already gaining traction in automotive applications and are expected to be increasingly deployed in homes and offices in the near future. This gives a unique opportunity to build visible light communication (VLC) systems with ultra-high-bandwidths offered by the LDs. However, LDs are subject to some inherent noise sources, notably signal-dependent shot noise (SDSN) and relative intensity noise (RIN). RIN basically describes the instability in the power level of a laser and quantifies the output power fluctuations. SDSN arises from variations in the photodetector’s response to optical signals. Our research explores the interplay between SDSN and RIN, revealing their combined adverse effect on channel estimation accuracy in a VLC system. Towards this direction, we first derive the Cramér-Rao lower bound (CLRB) in the presence of the SDSN and the RIN, which gives a lower estimate for the variance of an unbiased estimator. Then, we present the derivation of least square (LS) and maximum likelihood (ML) channel estimators. Furthermore, we present the optimal receiver in ML sense and compare it with a simple threshold detector as a sub-optimal solution, quantifying the impact of channel estimation accuracy on both receivers. Our study results reveal the adverse impact of the joint presence of SDSN and RIN on the channel estimation error; consequently, it degrades the BER of the two proposed receivers. Moreover, RIN emerges as the dominant noise source, especially at higher levels of transmitted power.