High-altitude airborne platforms inter-connected by free-space optical communications (FSOCs) have recently emerged as a promising solution for establishing wireless networks for rural and remote areas. The performance of FSOC system is severely degraded by the angle-of-arrival (AoA) fluctuation and pointing error. The precise alignment between the optical transmitter and receiver can be achieved by using the pointing, acquisition, and tracking (PAT), but it should work within the tight constraints of airborne platforms on size, weight, and power. It is also highly desirable that the PAT operates rapidly (e.g., without iteration for optimization) since the airborne platforms can be on the fast move. We propose a rapid and computation power-efficient adaptive beam control technique, where the beam sizes are adjusted without iterations at both the transmitter and receiver using non-mechanical variable-focus lenses, to mitigate the deleterious effects of AoA fluctuation and pointing error simultaneously. For this purpose, we provide the closed-form expressions about the optimum beam sizes at the transmitter and receiver for the outage probability. We carry out Monte Carlo simulations to validate the accuracy of our theoretical derivations. We show that the airborne FSOC system using the adaptive beam control technique outperforms the system having fixed beam sizes over wide ranges of AoA fluctuation and pointing error.

Free-space optical communications (FSOCs) are gaining popularity as a promising solution to the capacity crunch of aerial and satellite backhaul networks for next-generation mobile communication systems. A major technical challenge associated with such FSOC systems in motion is the pointing, acquisition, and tracking (PAT). To facilitate PAT, it is absolutely necessary to steer the optical beam over a wide angle and also adjust the beam divergence angle adaptively to the link condition and the amount of pointing error. The conventional methods to steer the optical beam and/or to adjust the beam divergence angle rely on mechanical mechanisms. Recently, the optical beam steering technique using variable focus lenses has been proposed and demonstrated. By using an on-axis variable focus lens followed by two decentered variable focus lenses, we can steer the optical beam over a wide range in a non-mechanical fashion. In this paper, we explore the possibility of using variable focus lenses for the realization of beam steering and adaptive beam control for FSOCs. For this purpose, we employ three variable focus lenses, one on-axis lens for adaptive beam divergence control and two decentered lenses for two-dimensional beam steering. We demonstrate the simultaneous beam steering and adaptive beam control capability of this variable focus lens-based system over a 104-m long free-space optical link. We evaluate the maximum beam steering angle and beam quality as we vary the offset distance of decentered variable focus lenses. Also evaluated experimentally is the impact of gravity on the beam quality. Finally, we show that the beam steering and adaptive beam control techniques implemented by variable focus lenses improve the performance of 10-Gb/s FSOC system considerably in the presence of pointing errors.

Pointing, acquisition, and tracking (PAT) is a major technical challenge of laser inter-satellite links (ISLs). For the fast establishment of laser link and the maximization of communication time, it is of importance to minimize the acquisition time. Satellite vibrations affect the PAT procedure adversely, and thus serve to increase the acquisition time. In this paper, we investigate through theoretical analysis the average acquisition time of laser ISLs in the presence of satellite vibrations. The analytic expression about the time taken from the beginning of spiral scan to the acquisition of the scan beam is provided in the presence of the pointing errors caused by vibrations. We also derive the optimum beam divergence angle for the acquisition time. The analyses are validated by Monte-Carlo computer simulations and a proof-of-concept experiment. The results show that the acquisition time can be minimized by adjusting the beam divergence angle adaptively to the link conditions.

Collecting light using multiple small apertures offers a lot of advantages over doing so using a single large aperture, including the effective mitigation of scintillation under strong turbulence, low implementation cost, and ease of scalability. In this work, we experimentally demonstrate a free-space optical communication system utilizing a 7-aperture direct-detection receiver based on the maximal ratio combining (MRC) scheme. For the implementation of MRC scheme blindly by using AC-coupled photo-receivers, we estimate the instantaneous signal-to-noise ratio (SNR) from the received signals time-averaged optimally for bit-error ratio performance. The experiment results show that the proposed blind scheme estimates the SNR accurately, and thus performs similarly to the pilot-symbol-aided scheme.

Free-space optical communications (FSOCs) have recently emerged as a promising solution for various communication scenarios. However, the pointing, acquisition, and tracking (PAT) is a technically challenging issue, especially in airborne FSOC systems. In this paper, we present an adaptive beam control technique combined with beaconless PAT using a variable focus lens. By using the lens whose focal length can be tuned electrically, we facilitate the coarse PAT by enlarging the beam size and also mitigate the adverse effects of pointing errors by adjusting the beam divergence angle adaptively to the link conditions. The major benefit of the proposed scheme is that the beam control can be implemented in a simple and compact manner, without relying on mechanical movements. We carry out a proof-of-concept experimental demonstration of the proposed scheme for a 10-Gb/s free-space optical link over 104 m. The demonstration shows that the probability of PAT success can be improved considerably by using the proposed scheme. It also shows that the proposed scheme could result in a system loss reduction of 6.4 dB and improve the tolerance to pointing errors up to 500 μrad for a target bit-error ratio of 10⁻³. The response time of the lens is measured to be <211 ms. We confirm experimentally that the receiver size and the amount of pointing error determine the optimum beam size at the receiver.

Satellite QKD/FSO systems, which facilitate quantum key distribution (QKD) over free-space optical (FSO) links between satellites and ground stations, present a promising pathway toward achieving global security in upcoming sixth-generation (6G) wireless communications. Our study focuses on a superior type of these systems, the satellite EB/CV-QKD/FSO, which utilizes the continuous-variable (CV) method for quantum state representation and the entanglement-based (EB) scheme for QKD implementation. We propose the use of optical phase-shift keying (QPSK) signaling and dual-threshold/heterodyne detection (DT/HD) receivers to bolster the reliability and feasibility of satellite EB/CV-QKD/FSO systems. Closed-form expressions for key system performance metrics are derived using improved channel modeling. Numerical results are presented to showcase the effects of channel impairments on the system performance. We also provide recommendations for optimal system setup parameters, aiming to enhance performance.