Study On Propagation Models Of Ultraviolet Communication Systems

The solar-blind ultraviolet communication (UVC) is a newly developing technology in optical wireless communication with potential advantages of non-line-of-sight (NLOS), stable and reliable link, anti-interference, safety, high security, and full-weather operation. There are uncertainties in the transmission of optical signals because of the randomness of UVC channels caused by atmospheric scattering and optical turbulence effects. Hence, the research on the characteristics of the atmospheric channel is of great significance to UVC.This dissertation makes innovative theoretical researches on the atmospheric scattering channel and especially on the channel characteristics of NLOS UVC. The main achievements are the following:1. A universal NLOS ultraviolet single-scatter model in noncoplanar geometry is first proposed by using the filamentary ray method to accurately calculate the received energy. We discuss in detail the different intersection cases of the receiver (Rx) field-of-view (FOV) and a filamentary ray emitted with any pointing angles within the transmitter (Tx) beam. Our model removes the restriction in previous single-scatter models that the Tx beam and the Rx FOV must have coplanar axes.2. We propose an approximate single-scatter model for noncoplanar geometry. Simulation results show that the approximate model can estimate rapidly the UVC link performance while the arbitrarily pointing Tx beam or Rx FOV is narrow. Compared with previous approximate models, our model can provide a more accurate estimation in more general cases, especially in the ultraviolet Ad Hoc network cases which need to calculate rapidly the scattering power. 3. Based on our single-scatter model, we derive the delay spread and impulse response function for the UVC and verify them by Monte-Carlo (MC) simulations. Further, through the MC multiple-scatter model for noncoplanar geometry, we first analyze the difference in the received energy and pulse broadening between the single and multiple scatter models for different Tx and Rx pointing and different communication ranges, and summarize the UVC geometric parameters that the single-scatter model has to satisfy to be applied.4. A NLOS UVC turbulence model considering turbulence-induced scintillation attenuation (SA) is proposed in the random turbid medium. Then, a single-scatter power model considering the scattering effects of turbulent eddies is proposed in the random turbulent medium. We analyze the power spectrum density models for UVC and the relationship between the single-scatter irradiance and various parameters, such as turbulence characteristics, wavelength and communication range. We also study the correlation between the scattering effect in the random turbulent medium and Rayleigh and Mie scatterings.5. We derive the joint spatial distribution of turbulence-induced fading for the NLOS link by considering the correlation of fading at multiple points in space. Then, we analyze the performance of the UVC with various spatial diversity reception schemes, such as maximum-likelihood (ML), selection combining (SC), maximal ratio combining (MRC), and equal gain combining (EGC). The simulation results show that when the atmospheric turbulence is weak, the ML achieves a slightly better performance than MRC and EGC; as the turbulence becomes stronger, MRC and EGC outperform ML in the high signal-to-noise ratio cases.