Space-air-ground integrated networks (SAGIN) within UAV-assisted free-space optical (FSO) communication systems can efficiently accommodate massive connections and provide highly reliable and seamless communications. In this work, we investigate the performance of a UAV-assisted, asymmetric, dual-hop radio frequency (RF)/FSO system with the amplified-and-forward relay protocol for the SAGIN. Specifically, the shadowed Rician fading is utilized in this study to characterize the shadowing effect on the RF signal for the satellite-to-UAV link. Meanwhile, the atmospheric turbulence effect on the optical signal for the UAV-to-terrestrial user link is modeled by the Málaga distribution fading, in view of the pointing error impairments. For comparison, the heterodyne detection technique is employed, as well as the intensity modulation with direct detection technique, in improving performance of the relay system. Thus, we derive the closed-form expressions for the cumulative distribution function, probability density function, the moment generating function, and particularly, the moments of the end-to-end RF/FSO system, in terms of Meijer’s G-function. Utilizing these derived formulae, the precise closed-form expressions for the outage probability, the average bit error rate (BER) with various modulation schemes, and the ergodic capacity are given. In specific, the tight asymptotic results for the outage probability and the average BER at the high SNR regions are derived using the asymptotic expansion of Meijer’s G-function. Furthermore, closed-form expressions are presented for the case that the FSO link experiences Gamma-Gamma distribution by changing some specific parameters. Finally, extensive numerical results validate the theoretical results with the Monte-Carlo simulation.

Unmanned aerial vehicles (UAVs) can be harnessed as relay nodes in free-space optical (FSO) communication systems to realize a flexible and cost-effective approach for delivering on-demand communication in next-generation wireless communications. However, fading channels can adversely affect the performance of UAV-assisted dual-hop FSO systems owing to various factors. This paper presents a unified system performance analysis of UAV-assisted dual-hop FSO/FSO systems with the amplify-and-forward relaying protocol and intensity modulation/direct detection technique. For general applicability, the influence of the attenuation loss, atmospheric turbulence, pointing error impairments, and angle-of-arrival (AOA) fluctuations on the FSO link between the source and the UAV is considered. Meanwhile, the FSO link between the UAV and the destination is modeled using Málaga distribution channels, which are affected by atmospheric turbulence. The tight closed-form expressions for the outage probability and the average bit error rate of the dual-hop system are derived, followed with the diversity order of the system in the high signal-to-noise ratio range. Furthermore, the impact of various system parameters and channel parameters correlated with the attenuation loss, atmospheric turbulence, pointing error impairments, and AOA fluctuations on the system performance is studied, and a detailed comparative investigation of the various modulation schemes is conducted. Finally, simulation results are presented to validate the analytical results.

A hybrid satellite-terrestrial relay network (HSTRN) has been envisioned as a promising communication architecture for future wireless communications, which can prominently reduce the impact of shadow fading and expand service coverage. In this paper, we conduct theoretical performance analysis for a dual-hop communication system, consisting of multiple relays and multiple users in the HSTRN with the amplify-and-forward protocol. For the shadowing effect and the multipath effect, we introduce the shadowed-Rician distribution and Nakagami- $m$ fading to characterize the channel models for the satellite-relay and the relay-user links, respectively. Additionally, the opportunistic scheduling scheme is employed, where the optimal relay and the optimal user are selected from the alternative nodes with the maximum instantaneous signal-to-noise ratio. Thereafter, the analytical expressions including ergodic capacity and average symbol error rate (SER) are derived. More specifically, we also present the asymptotic behavior of the average SER at the high signal-to-noise ratio (SRN) regime. The theoretical analysis is validated by the numerical results. Moreover, the results also reveal that the performance of the system can be improved by increasing the number of relays and users under different channel parameters and various modulation schemes.

The integration of free-space optical (FSO) communications and relay technology has been envisioned as a promising and effective solution to achieving substantial performance gains for future deep space communications systems. In this paper, the system performance of a dual-hop FSO/FSO relay framework for deep space communications is studied, to the best of our knowledge, for the first time, in the presence of plasma absorption, coronal turbulence, and pointing error impairments. More specifically, the Málaga fading distribution model is employed to characterize the coronal channel, since it has a large scope for the scintillation index ranging from weak to strong turbulence. In addition, we present closed-form expressions for the probability density function, the cumulative distribution function, the moment function, and the generalized moments function of the signal-to-noise ratio (SNR) for the whole system by employing an amplify-and-forward relaying protocol. We capitalize on these unified formulations to develop analytical expressions for the critical performance metrics, such as the outage probability, average bit error rate, and ergodic capacity of the whole relay system, considering different combination of the heterodyne detection and intensity modulation with direct-detection techniques in the two hops. To provide more insights into deep space communications system, we also carry out an asymptotic analysis of the metrics in the high SNR regime via an asymptotic expansion of Meijer's G function. The results of Monte Carlo simulations verify all the expressions derived here and prove the improvement of the relay-assisted system than via a single FSO link. Finally, analytical results are presented to reveal the impact of the parameters on the system performance.

The propagation performance of a free-space optical (FSO) communication system in an atmospheric environment is restricted and degraded due to the influence of atmospheric turbulence. In this paper, both the lognormal and Gamma–Gamma channel models are employed to characterize this turbulence under weak-to-strong conditions. In addition, the average bit error rate and average channel capacity of an FSO communication system under the influence of background noise, thermal noise and quantum noise (resulting from the environment, the device, manual operation, etc.) are considered. Moreover, the comparison of system performance under different turbulence conditions and various noises are conducted. Simulation results reveal that thermal noise has a dominant effect on the FSO system. In addition, both the channel parameters and the system parameters have a significant influence on the performance of an FSO communication system.

With the maturity of 5G and Intelligent Transportation Systems (ITS) technologies and the prospect of Beyond 5G (B5G) and 6G technologies, the limited lifetime and computing of mobile devices pose significant challenges to Quality of Service (QoS). In addition, the problem of inefficient use of computing, storage, communication, and other resources still exists in communication systems. In response to the above issues, Multi-tier Computing Networks (MTCNs) migrate computationally intensive tasks to the cloud, fog, or edge with sufficient resources, thereby realizing energy-efficient collaborative computing and multi-dimensional resource sharing. However, in the MTCN environment with complex heterogeneity, and high-intensity dynamics, how to provide sustainable solutions for resource scheduling strategies is a meaningful issue. Inspired by Virtual Network Embedding (VNE) to decouple physical network configuration, we propose a multi-target-aware dynamic resource scheduling algorithm for MTCN to improve resource flexibility, which is the first attempt in this direction. Specifically, we consider differentiated QoS requirements like computing, storage, bandwidth, delay, etc., and establish multi-target-aware embedded constraints. Additionally, we present a Deep Reinforcement Learning (DRL)-based scheduling network that can interact scientifically and efficiently with the MTCN environment. It extracts environmental information as state input to better focus on dynamic characteristics as well as calculates candidate nodes and links using a three-layer network architecture and related constraints. Furthermore, the learning process is optimized through the combination of the reward mechanism and the gradient descent mechanism. Finally, comparison experiments on three widely used evaluation indicators (long-term average revenue, long-term average revenue-cost ratio, and VNR acceptance rate) verify that the proposed algorithm has made an average improvement of $19.042\%$ , $2.563\%$ , and $3.932\%$ respectively compared with all baselines.