Recent advances in optical modulators and folded-path channel simulation bring an extra dimension to ongoing hybrid Free-Space Optics (FSO) and wireless RF networks. Typical FSO transceiver system design and network integration differ from emerging interrogators targeting Modulating Retro Reflector (MRR). At long ranges and in hostile environments, MRR edge nodes provide vital information such as relative GPS coordinates, unit identification, and integrated sensors data. To optimize network scalability, dynamic, instantaneous critical data flow, and real-time response, FSO transceivers and interrogators should overlap in physical layer performance and network layers interface. In this paper, we review system specification, requirement and channel impact on one-pass FSO and RF links as well as double-pass MRR links. Then, we state the underlying challenges on their network integration and ways to optimize performance by re-designing the system physical layer and identifying the corresponding network requirements.
FSO communication by retro-modulation is an innovative way to convey information between an optical transceiver and a semi-passive unit that reflects signals denoted by Modulating Retro Reflector (MRR). This concept is completely different from typical FSO communication systems consisting of two transceivers operating in full-duplex mode. In this paper, we review this new concept, analyze atmospheric effects due to folded paths propagation, evaluate different optical modulator technologies suitable for these applications and their impacts on system performance, and compare analysis results to the commonly implemented active intelligent RFID tags.
This paper is written in response to many inquiries about 10 j. Free-Space Optics (FSO) performance compared to shorter wavelengths. The increasing interest in better understanding FSO weather effects is the result of carrier requests as well as recent progress in analyzing fog effects on FSO signal propagation. Extensive studies in modeling fog and simulating FSO attenuations revealed the complexity behind estimating FSO link availability in a given geographical location. There are many different types of fog that are inhomogeneous along the propagation path. Each type is characterized by the water droplet sizes and their concentrations, which are used in Mie scattering theory to compute FSO signal attenuation. As a result, some vendors are augmenting their FSO links with a microwave backup link or simply investigating other wavelengths claimed to be more resistant to fog such as 10 jt. In this paper, we analyze ways to improve FSO link availability, 10 t improvement compared to shorter wavelengths, and challenges behind successful microwave backup installation.
One of the biggest challenges facing Free-Space Optics deployment is proper understanding of optical signal propagation in different atmospheric conditions. In an earlier study by the author (30), attenuation by rain was analyzed and successfully modeled for infrared signal transmission. In this paper, we focus on attenuation due to scattering by haze, fog and low clouds droplets using the original Mie Scattering theory. Relying on published experimental results on infrared propagation, electromagnetic waves scattering by spherical droplet, atmospheric physics and thermodynamics, UlmTech developed a computer-based platform, Simulight, which simulates infrared signal (750 nm-12 μm) propagation in haze, fog, low clouds, rain and clear weather. Optical signals are scattered by fog droplets during transmission in the forward direction preventing the receiver from detecting the minimum required power. Weather databases describe foggy conditions by measuring the visibility parameter, which is, in general, defined as the maximum distance that the visible 550 nm signal can travel while distinguishing between the target object and its background at 2% contrast. Extrapolating optical signal attenuations beyond 550 nm using only visibility is not as straightforward as stated by the Kruse equation which is unfortunately widely used. We conclude that it is essential to understand atmospheric droplet sizes and their distributions based on measured attenuations to effectively estimate infrared attenuation. We focus on three types of popular fogs: Evolving, Stable and Selective.
With recent advances and interest in Free-Space Optics (FSO) for commercial deployments, more attention has been placed on FSO weather effects and the availability of global weather databases. The Meteorological Visual Range (Visibility) is considered one of the main weather parameters necessary to estimate FSO attenuation due to haze, fog and low clouds. Proper understanding of visibility measurements conducted throughout the years is essential. Unfortunately, such information is missing from most of the databases, leaving FSO players no choice but to use the standard visibility equation based on 2% contrast and other assumptions on the source luminance and its background. Another challenge is that visibility is measured using the visual wavelength of 550 nm. Extrapolating the measured attenuations to longer infrared wavelengths is not trivial and involves extensive experimentations. Scattering of electromagnetic waves by spherical droplets of different sizes is considered to simulate FSO scattering effects. This paper serves as an introduction to a series of publications regarding simulation of FSO atmospheric propagation. This first part focuses on attenuation due to rainfall. Additional weather parameters, such as rainfall rate, temperature and relative humidity are considered to effectively build the rain model. Comparison with already published experimental measurement is performed to validate the model. The scattering cross section due to rain is derived from the density of different raindrop sizes and the raindrops fall velocity is derived from the overall rainfall rate. Absorption due the presence of water vapor is computed using the temperature and relative humidity measurements.
12 Recently, free space optical communication systems have been discussed as solutions to provide high-speed local loop connectivity in metropolitan area network environments. Free space optical technology is capable of providing much higher bandwidth than any other competing wireless connectivity options such as classical RF or microwave systems. A widespread concern regarding free space optical systems is related to its capability to provide the high availability figures commonly envisioned by network service providers. Wireless microwave systems tend to have high availability figures, but at the expense of the ability to operate at higher data rates. LightPointe suggests an integrated network solution that combines the benefits of both technologies.
12 A free-space laser link closes an otherwise all-fiber SONET ring, demonstrating for the first time the feasibility of using wireless optics as a back-up to fiber in an application demanding the highest levels of statistical availability and sub-50-ms protection-restoral times. This experiment demonstrates that protocol-transparent wireless optical links can be readily internetworked with industry- standard fiber-based protection protocols to achieve SONET restoral times in the event of a fiber cut. By using the wireless optics as a back-up to fiber rather than as the primary link, end-users are normally protected from the unavoidable burst errors and outages that can arise on a wireless optical link in the event of anomalously poor atmospheric visibility or unanticipated line-of-sight obstructions. While an all-fiber SONET ring operating over physically diverse paths is generally preferred, hybrid fiber/air rings operating over physically-diverse paths (fiber as one path and air as the other) will easily meet or exceed existing Bellcore availability standards for SONET rings. The hybrid part-fiber, part-air ring advantageously protects customers from fiber cuts (a.k.a. `backhoe fade') and may be preferable to over service via either an unprotected fiber spur or over a `collapsed' fiber ring made up of fiber segments sharing a common conduit. The experiment is performed at an OC-12 (622 Mbps) data rate in a point-to-consecutive point configuration which demonstrates the use of a relay site to work-around a line- of-sight obstruction.