To extend the flight duration of unmanned aerial vehicles (UAV), laser-powering systems have been proposed in recent times that can charge a flying UAV with laser beams from the ground. In this study, we analyze the potential of using a ground-based laser station to charge a fixed-wing UAV flying in a turbulent atmosphere. This presents a new design challenge in optimizing the UAV's trajectory by jointly considering atmospheric turbulence and UAV energy consumption. To overcome this, we derive theoretical models which capture the UAV velocity and position evolution along a circular trajectory in a turbulent atmosphere. Based on the derived models, we first compute the received energy loss at the UAV that depends on the UAV velocity (variance) as well as the trajectory radius. Then, based on the received energy loss and the energy consumption models of the UAV, the UAV trajectory is optimized with respect to velocity and trajectory radius to maximize the net laser power harvested by the UAV. Our results show that the optimal velocity and trajectory radius depend strongly on the levels of turbulence in the atmosphere.

For acquisition of narrow-beam free-space optical (FSO) terminals, a Global Positioning System (GPS) is typically required for coarse localization of the terminal.  However, the GPS signal may be shadowed, or may not be present at all,  especially in rough or unnameable terrains.   In this study, we propose a lidar-assisted acquisition of an unmanned aerial vehicle (UAV) for FSO communications in a poor GPS environment. Such an acquisition system consists of a lidar subsystem and an FSO acquisition subsystem: The lidar subsystem is used for coarse acquisition  of the UAV, whereas, the FSO subsystem is utilized for fine acquisition to obtain the UAV’s accurate position. This study investigates the optimal allocation of energy between the lidar and FSO subsystems to minimize the acquisition time. Here, we minimize the average acquisition time, and maximize the cumulative distribution function of acquisition time for a fixed threshold. We learn that an optimal value of the energy allocation factor exists that provides the best performance of the proposed system.

Due to their flexibility and low cost deployment, unmanned aerial vehicles (UAV) will most likely act as base stations and backhaul relays in the next generation of wireless communication systems. However, these UAVs—in the untethered mode—can only operate for a finite time due to limited energy they carry in their batteries. In free-space optical communications, one solution is to transport both the data and the energy from the source to the UAV through the laser beam—a concept known as simultaneous lightwave information and power transfer (SLIPT). In this study, we have analyzed the SLIPT scheme for laser-powered decode-and-forward UAV relays in an optical wireless backhaul. The major goal of this study is to optimally allocate the received beam energy between the decoding circuit, the transmitting circuit and the rotor block of the relay in order to maximize a quality-of-service metric such as maximum achievable rate, outage or error probabilities. As expected, we note that the optimal power allocation depends heavily on the source-relay and relay-destination channel conditions. In the final part of this study, we have maximized the operational time of the UAV relay given that the maximum achievable rate stays above a certain threshold in order to meet a minimum quality-of-service requirement.

The angular instability of unmanned-aerial-vehicle (UAV) relays as well as the turbulence-induced beam wander may lead to significant pointing error in free-space optical backhaul. The hop distance between each relay in a serial relay scheme is an important parameter that can be optimized in order to enhance the performance of the system. In this study, we have optimized the error and outage probabilities as a function of the hop distance vector for amplify-and-forward relays (AF) under a number of constraints such as the maximum and average relay power and the maximum relay gain. We have discovered that for the unconstrained optimization problem, the optimum hop distances is a strictly increasing sequence for both the error and outage probabilities. For maximum power constraint optimization, the optimum hop distances form a strictly increasing sequence, whereas for maximum gain constraint optimization, the optimum hop distances lead to a nondecreasing sequence.

Free-space optical (FSO) communications will play an important role in the backhaul of future generation of wireless networks in order to support high data rates. Because of narrow beamwidth inherent to an optical signal, acquisition and tracking form an important component of any FSO communication system. In this study, we have analyzed the optimization of received power allocation between tracking and data channels in an FSO receiver. The objective function that is optimized (minimized) are the probability of error and the probability of outage, and the optimization of power allocation is carried out as a function of parameters such as noise power, pointing error variance, pointing error correlation coefficient, and the threshold of outage. We have analyzed the optimization concerning the lognormal and exponentiated Weibull fading scenarios as well. We learn that the optimal power allocation is a function of the received signal-to-noise ratio: a lower signal-to-noise ratio dictates that a higher fraction of received power should be diverted to the tracking channel and vice versa.

Free-space optical (FSO) communications is an important technology that will be used for supporting high data-rates in the backhaul of next generation of wireless communication networks. In this paper, we have compared the probability of error performance of two types of receivers used in FSO today: a receiver based on a single detector, and a receiver based on an array of detectors. The performance of these two receivers is compared for a number of fusion algorithms for an array of detectors such as equal gain combiner (EGC), selection combiner (SC), switched combiner (SWC) and the maximal ratio combiner (MRC). From this study, we conclude that even though the array of detectors adds more noise in the sufficient statistic by virtue of large number of detectors, using more computationally expensive fusion algorithms (such as SC and MRC) can help us achieve a superior probability of error performance as opposed to a single-detector receiver for practical channel conditions.

Optical beam center position on an array of detectors is an important parameter that is essential for estimating the angle-of-arrival of the incoming signal beam. In this paper, we have examined the beam position estimation problem for photon-counting detector arrays, and to this end, we have derived and analyzed the Cramer-Rao lower bounds on the mean-square error of the unbiased estimators of the beam position. Furthermore, we have also derived the Cramer-Rao lower bounds of other beam parameters such as peak intensity, and the intensity of background radiation on the array. In this sense, we have considered a robust estimation of the beam position in which none of the parameters are assumed to be known beforehand. Additionally, we have derived the Cramer-Rao lower bounds of beam parameters for observations based on both pilot and data symbols of a pulse position modulation (PPM) scheme. Finally, we have considered a two-step estimation problem in which the peak intensity and background radiation are estimated using a method of moments estimator, and the beam center position is estimated with the help of a maximum likelihood estimator.

Pointing and acquisition are an important aspect of free-space optical communications because of the narrow beamwidth associated with the optical signal. In this paper, we have analyzed the pointing and acquisition problem in free-space optical communications for photon-counting detector arrays and Gaussian beams. In this regard, we have considered the maximum likelihood detection for detecting the location of the array, and analyzed the one-shot probabilities of missed detection and false alarm using the scaled Poisson approximation. Moreover, the upper/lower bounds on the probabilities of missed detection and false alarm for one complete scan are also derived, and these probabilities are compared with Monte Carlo approximations for a few cases. Additionally, the upper bounds on the acquisition time and the mean acquisition time are also derived. The upper bound on mean acquisition time is minimized numerically with respect to the beam radius for a constant signal-to-noise ratio scenario. Finally, the complementary distribution function of an upper bound on acquisition time is also calculated in a closed form. Our study concludes that an array of smaller detectors gives a better acquisition performance (in terms of acquisition time) as compared to one large detector of similar dimensions as the array.