Flexible real-time gigabit free-space optic demonstrator

In this paper, we demonstrate a real-time high-resolution analogue-to-digital-based free-space optics system. We utilise a field-programmable gate array platform complete with system-on-chip to flexibly implement the orthogonal frequency division multiplexing (OFDM) with up to 2048 sub-carriers and carrier-less amplitude and phase (m-CAP) modulation with up to 20 sub-bands in a real free-space optical link for the first time. We also consider high order modulation formats up to 64-QAM (6 bits/symbol) to achieve a raw data rate up to 1.8 Gb/s. Previous real-time free-space optics transceivers were either analog-based or restricted to on-off keying (1 bit/symbol). We emulate the fog environment utilising neutral density filters and evaluate the performance of the link under increasingly poor visibility conditions. We show that OFDM and m-CAP offer similar performances in the link environment, however, we finally show that m-CAP offers increased robustness and receiver sensitivity in low visibility settings.