Inter-Symbol Interferences Deteriorated Ultraviolet Communications Using Photon-Counting Receivers

The strong scattering effects of ultraviolet (UV) signals in the atmosphere enables the non-line-of-sight (NLOS) ability of UV communications, but introduces large time dispersion of the channel impulse response (CIR), which inevitably results in serious inter-symbol interferences (ISIs). Because a larger transmitting rate can introduce a larger ISI, it is meaningful to study the impact of CIR on the ISI and further quantify the achievable information rate (AIR) of NLOS UV communications. In this paper, we analyzed the influence of both coplanar and non-coplanar geometries on the CIR for multiple scattering effects and proposed a modified Gamma function (MGF) to precisely characterize the CIR. Based on the MGF model, we quantified the ISIs and derived the bit-error rate (BER) and the AIR under thermal noise for both on-off keying (OOK) modulation and 2-pulse-position modulation (2-PPM) using photon-counting receivers. The simulation results showed that the proposed MGF model can achieve higher fitting precision of both CIRs and BERs compared with the existing Gamma function model. Besides, we demonstrated that when the thermal noise photons are less than 10, the OOK and 2-PPM can achieve an AIR of 15 Mbit/s and 13 Mbit/s, respectively, under typical system geometries.