Modulating Retroreflector Based Free Space Optical Link for UAV-to-Ground Communications

Abstract

Weight reduction and low power consumption are key requirements in the next generation of unmanned aerial vehicle (UAV) networks. Employing modulating retro-reflector (MRR)-based free space optical (FSO) technology is an innovative technique for UAV-to-ground communication in order to reduce the payload weight and power consumption of UAVs which leads to increased maneuverability and flight time of UAV. In this paper, we consider an MRR-based FSO system for UAVto-ground communication. We will show that the performance of the considered system is very sensitive to tracking errors. Therefore, to assess the benefits of MRR-based UAV deployment for FSO communications, the MRR-based UAV FSO channel is characterized by taking into account tracking system errors along with UAV's orientation fluctuations, link length, UAV's height, optical beam divergence angle, effective area of MRR, atmospheric turbulence and optical channel loss in the doublepass channels. To enable effective performance analysis, tractable and closed-form expressions are derived for probability density function of end-to-end signal to noise ratio, outage probability and bit error rate of the considered system under both weak-tomoderate and moderate-to-strong atmospheric turbulence conditions. The accuracy of the analytical expressions is verified by extensive simulations. Analytical results are then used to study the relationship between the optimal system design and tracking system errors.Index Terms-Angle of arrival (AoA) fluctuations, FSO communications, UAV, modulating retro-reflector (MRR). I. INTRODUCTIONU NMANNED aerial vehicles (UAVs) have been introduced to overcome many of the shortcomings of the current terrestrial infrastructure by operating as aerial communication nodes and providing robust line-of-sight (LoS) connectivity to ground devices [1], [2]. UAVs deployed as flying communication nodes using radio frequency (RF) will interfere with ground devices, hence degrading the performance of the ground network. Employing UAVs equipped with free space optical (FSO) technology is a promising method for future ultra dense wireless networks [3]. FSO communication systems use very small beam divergence which is physically inaccessible to RF technologies, thus, making FSO links extremely secure. Moreover, FSO system is much faster, easier to deploy, more compact, and cheaper than RF [4]- [6]. However, vulnerability to signal blockage is one of the fundamental limitations of FSO links that essentially confines the receiver to be placed within the line-of-sight (LoS) of the transmitter. A potential application of UAV-assisted FSO systems is in dense cities with tall buildings where a UAV can act as an aerial relay to connect source and destination nodes where the LoS between ground optical nodes is interrupted by tall buildings [7], [8].