Free-space line-of-sight (LOS) near-infrared (NIR) optics is a quite established field of research and application. Tropopsheric molecular and particle scattering at ultraviolet (UV) wavelengths can enable non-line-of-sight (NLOS) communication which brings robustness to blockage or shadowing. NLOS communication is particularly desirable to relax or eliminate pointing, acquisition and tracking requirements. NLOS-UV links can be used as an alternative to outdoor LOS-NIR links or in combination with existing optical or radiofrequency wireless links.
Analytical, experimental and numerical approaches have been used to determine the LOS-NIR and NLOS-UV channel impulse response and path loss. These studies demonstrate that UV channel is of multipath nature due to the volumetric scattering due to air molecules, aerosols and hydrometeors. Channel performances may be also degraded by high path losses and turbulence-induced fading as the link range increases. The proposed UVScat project aims at evaluating the potential of tropospheric NLOS-UV technology, focusing on channel modeling and system performance. The goal is to model the optical channel physical layer into account the complex interaction between transmitted UV radiation and tropospheric constituents such as gas molecules, aerosols and hydrometeors.
The tasks are the following: i) modeling atmospheric particulate scattering and turbulence at UV wavelengths; ii) development of an analytical wave-based model at UV wavelengths; iii) development of a Monte Carlo photon-based model at UV wavelengths; iv) simulation of UV communication channels and ozone scatterometer observations. The output of the UVScat project is supposed to be a feasibility analysis of the NLOS-UV technology (when compared to LOS-NIR links) and its potential for both telecommunciations and remote sensing as a basic step for a system and experimental future activity devoted to the verification of the proposed NLOS configurations.
The UVScat project aims at advancing the state of the art modeling of NLOS UV link within 3 major areas:
1) improvement of the hydrometeor scattering effects at UV wavelengths exploiting the work carried out for free-space-optics communication link within a Monte Carlo photon-based numerical model.
Water particles can have various shapes depending on their microphysics and thermodynamics in liquid, ice or mixed phases (Mori and Marzano, 2015). For fog droplets (few microns of diameter) and small raindrops (less than 1 mm of diameter) is reasonable to assume a spherical shape with pure water phase. For raindrops larger than 1 mm, the shape tends to oscillate around an oblate form up to about 8 mm diameter, beyond which the break-up phenomenon takes place. The snow particles may assume several shapes, starting from aligned vertical ice crystals up to horizontally oriented dendrites. Non-spherical particles can be effectively modeled as equi-volume spherical particles. Water particles can be distinguished and grouped according to their bulk characteristics and their statistical variability (Marzano and Ferrauto, 2005). These include minimum and maximum radius, density, size distribution, effective radius, mass concentration variability, shape parameter, fall velocity, and dielectric constant model. In this respect we can distinguish: i) advection fog (typical of maritime environment), radiation fog (usually continental), ii) rain; iii) graupel; iv) dry and wet snow. Current numerical UV NLOS models do not consider the microphysical and optical characterization of atmospheric cloud particles. The UVScat project will fill this gap by ingesting what accomplished by Mori and Marzano (2015).
2) introduction of the turbulence effects, basedon a meteorological parameterization, into the scattering bistatic UV channel model within an analytical wave-based model.
Tropospheric turbulence, including beam wandering and scintillation are phenomenon caused by local fluctuations of the complex refractive indices due variations of illumination, temperature and wind. These effects can be neglected on shortrange propagation (up to several hundred meters), but become significant on longer paths (several kilometers). This is particularly true for a NLOS links, where the intense turbulence scattering can increase the number of received photons. On the other hand, theoretical analysis (Rytov solution) indicates scintillation much greater in UV than in visible range. Turbulence can be parameterized in terms of measurable meteorological variables as shown in Carrozzo et al. (2014). The UVScat project will fill this gap by ingesting a physically-based structure constant model and insert the turbulence effects as an equivalent atmospheric scattering cross section.
3) exploitation of UV scattering channel for both estemporary NLOS communication and ozone remote sensing and feasibility of future experimental activity.
Traditional LOS communication systems include both radio frequency ones, char-acterized by licensed spectrum, low channel capacity and high power require-ments, and infrared ones with characteristic similar to UVC, but often affected by blockage. In this respect UVC technology represents an interesting alternative or complementary use to traditional systems in applications where low-power con-sumption and security are essential, whereas range and frequency bandwidth are of secondary importance. NLOS UVC feasibility and the potential of modula-tion/coding are, of course, further added values. NLOS UVC channel modeling has been significantly developed in the last decade, but a complete description of all propagation effects is still lacking. The UVproject will consider scenarios where UVC systems can be employed such as broad-ranging applications such as short-range data communication, mobile connections, surveil-lance sensor networks, homeland security, unattended ground sensor networks, and small communication systems in urban environments. A systematic assessment and exploitation of NLOS UVC availability in terms of geometrical configurations, atmospheric conditions and TX/RX system requirements is still needed and will be the goal of the UVScat feasibility study.
REFERENCES
- Carrozzo D, Mori S, and Marzano FS (2014) Modeling Scintillation Effects on Free Space Optical Links using Radiosounding Profile Data. 3rd Int Workshop on Optical Wireless Comm (IWOW), Funchal (Madeira Island, Portugal), 40-44. doi:10.1109/IWOW.2014.6950773.
- Marzano FS and Ferrauto G (2005) Generalized Eddington analytical model of azimuthal-ly-dependent radiance simulation in stratified media. Appl Optics 44: 6032-6048. doi: 10.1364/AO.44.006032
- Mori S and Marzano FS (2015) Microphysical Characterization of Free Space Optical Link due to Hydrometeor and Fog Effects, Applied optics, 54:6608-6840. doi: 10.1364/AO.54.006787.