Design and Development of Hybrid FSO/RF Communication System with Auto-Tracking Mechanism

Abstract

Optical wireless communication (OWC) has an enormous potential to support massive data transmission requirements. In an outdoor environment, OWC with long-range communication is referred to as free space optics (FSO) in the literature. FSO is one of the technologies which supports high bandwidth, unlicensed spectrum, high security, immune to interference and ease of installation. These attractive features of FSO provide a viable solution to the last mile problem in broadband wireless transmission. FSO communication remains sensitive to environmental conditions, scintillation and pointing errors despite many advantages. The design of FSO communication must consider all the above channel impairments. This research work focuses on designing reliable and available FSO communication under different channel conditions. The design of a hybrid FSO/RF communication with an auto-tracking system defines the scope of this work. Approaches adopted for the design of a hybrid FSO/RF system are enumerated below: • Initially, the performance of FSO communication in various atmospheric conditions and pointing losses are measured. In this regard, different weather conditions (i.e., rain, fog, and snow), atmospheric turbulence, and geometrical losses are taken into account to evaluate the FSO system performance. Also, an experimental testbed for FSO communication is designed and implemented with an indoor atmospheric chamber to replicate the weather conditions. An image under different foggy and atmospheric turbulence conditions is transmitted to evaluate the performance. • In FSO communication, fog, atmospheric turbulence, and pointing errors are the major bottlenecks that degrade the performance significantly. On the other hand, RF communication is less prone to the above issues. A hybrid FSO/RF is designed and implemented with an auto-tracking system. An efficient machine learning (ML) aided switching mechanism is proposed for selecting the appropriate communication link based on the current weather conditions. The ML model is trained with different weather conditions to estimate the link margin (LM) of FSO communication. A switching decision is taken based on the LM estimation. • Even under clear weather conditions, the performance of FSO communication is primarily dependent on atmospheric fading and a strict line-of-sight (LoS) condition,i.e., pointing errors. Therefore, a statistical model for intensity fluctuation due to the atmospheric turbulence and pointing error is derived. A closed-form probability density function (PDF) for pointing errors comprised of boresight and jitter error is derived. Two auto-tracking mechanisms are designed and developed at the transmitter to combat the issues of boresight and jitter error. First, a coarse tracking system is designed with a magnetometer sensor on-board to minimize the boresight error. Any leftover boresight error, known as jitter, is compensated by a fine-tuning and closed-loop feedback system. The proposed auto-tracking mechanism has been experimented with a wide range of non-zero boresight angles, from ±10◦ to ±180◦ with a fine-tuning of jitter up to 8 cm. The performance of the proposed system has been analyzed in terms of outage probability. The analytical results are compared with simulation, measurement, and existing methods. • FSO is considered primarily for establishing point-to-point (PtP) communication. A novel point-to-multipoint (PtM) tracking mechanism for FSO communication is designed and implemented to meet the increasing demand for mobile platforms and mechanisms to establish PtM connections. The alignment mechanism consists of two tracking systems, i.e., coarse tracking and fine-tuning. The coarse tracking is responsible for locating FSO transceivers placed at different positions using magnetometer sensors connected to the cloud. On the other hand, the fine-tuning mechanism overcomes high-frequency amplitude oscillation or jitter with a displacement range of 8 cm. The link is continuously monitored, and if the link is in inoperable condition or not established due to extreme weather conditions, the reference line-of-sight (LoS) angle is updated in the cloud to establish an alternate connection. The stability of the proposed system is analyzed in terms of root locus and step response. The proposed PtM system is validated experimentally with FSO terminals located at different distances over indoor and outdoor environments. The proposed method achieved an alignment time of less than 5 seconds for 10 degrees of misalignment.