Throughput Optimization of Receiver-Limited Optical Wireless Communication System

To allow a good signal-to-noise ratio (SNR) over a wide bandwidth for signals arriving from an unknown angle, many optical wireless communication (OWC) systems need a wide-aperture high-bandwidth detector. However, the physical properties of a photodiode (PD) challenge the design for high bandwidth and a large detection area, simultaneously. This problem is also known as the bandwidth versus area trade-off of an amplified PD receiver. In this paper, we study the limitations of a single-PD OWC receiver on throughput, particularly on bandwidth, photon capturing area, and electrical noise, to formulate a model that evaluates and optimizes the throughput of a communication link for different signal modulation schemes. We optimize the signal power spectral density (PSD) of pulse-amplitude modulation (PAM) and of DC-biased optical orthogonal frequency-division multiplexing (DCO-OFDM) modulation for throughput, considering that large modulation bandwidths suffer disproportionally from noise enhancement in typical receiver amplifiers. This worsens if large PDs are used, but we show that to a large extent, the corresponding stronger signal can compensate for the increased noise. We study the optimum detector size, in combination with adaptations of the spectrum and, in particular, the maximum frequency of the modulation. While the limitations of photonic emitters have been extensively studied before, this is one of the first papers that models how the receiver limits the throughput of PAM and OFDM.