On the Performance of Millimeter Wave-based RF-FSO Links with HARQ Feedback

This paper studies the performance of hybrid radio-frequency (RF) and free-space optical (FSO) links in the cases with and without hybrid automatic repeat request (HARQ). Considering millimeter wave (mmwave) characteristics in the RF link and pointing errors in the FSO link, we derive closed-form expressions for the message decoding probabilities as well as the throughput and the outage probability of the RF-FSO setups. We also evaluate the effect of various parameters such as power amplifiers efficiency, different transmission techniques in the FSO link, pointing errors in the FSO link as well as different coherence times/symbol rates of the RF and the FSO links on the throughput and outage probability. The results show the efficiency of the RF-FSO links in different conditions. Moreover, the HARQ can effectively improve the outage probability/energy efficiency, and compensate the effect of hardware impairments in RF-FSO links.Comment: Under review in PIMRC'201

Performance Analysis of ARQ-based RF-FSO Links

In this letter, we study the performance of hybrid radio-frequency (RF) and free-space optical (FSO) links using automatic repeat request (ARQ). We derive closed-form expressions for the message decoding probabilities, throughput, and outage probability with different relative coherence times of the RF and FSO links. We also evaluate the effect of adaptive power allocation between the ARQ retransmissions on the system performance. The results show that joint implementation of the RF and FSO links leads to substantial performance improvement, compared to the cases with only the RF or the FSO link

Diversity Techniques in Free-Space Optical Communications

Free-space optical (FSO) communication offers significant technical and operational advantages such as higher bandwidth capacity, robustness to electromagnetic interference, a high degree of spatial confinement (bringing virtually unlimited reuse and inherent security), low power requirements, and unregulated spectrum. FSO communication can be deployed as an efficient solution for a wide range of applications such as last-mile access, fiber backup, back-haul for wireless cellular networks, and disaster recovery among others. Although FSO system have many appealing features, they have rather disappointing performance for long links due to the degrading effects of atmospheric turbulence-induced fading. In this dissertation, we investigate different diversity techniques to boost the performance of FSO systems in the presence of the atmospheric turbulence-induced fading. In Chapter 3, we investigate receive diversity in coherent FSO systems considering both turbulence-induced amplitude and phase fluctuations under weak turbulence regime. To mitigate the wavefront phase distortion effect, modal compensation is deployed at the receiver. Under the assumption of Rician channel that models the combined effects of the atmospheric fading and modal compensation, we derive outage probability and diversity- multiplexing tradeoff of such systems. Our results show that, at high signal to noise ratio (SNR) regime, the diversity gain as great as the number of receiving apertures is achieved. Moreover, it is found that the modal compensation provides finite-SNR diversity advantages in coherent receivers. In Chapter 4, we investigate multi-hop transmission (serial relaying) as a form of diversity technique to combat atmospheric fading in coherent FSO systems. Utilizing the same channel model as in Chapter 3 and considering decode-and-forward relaying strategy, we quantify the outage probability and the finite-SNR diversity-multiplexing tradeoff of this relaying scheme. Exploiting the fact that fading variance is distance-dependent in the atmospheric channel, our results demonstrate that the multi-hop transmission takes advantage of the resulting shorter hops and yields significant performance improvements in the presence of fading. In Chapter 5, we study hybrid-ARQ protocols in coherent FSO communications over Gamma-Gamma atmospheric fading channels. We investigate and compare the performance of three hybrid-ARQ protocols in terms of the outage probability and throughput. Furthermore, we characterize the outage performance at high-SNR regime by diversity and coding gains. Our results provide insight into the performance mechanisms of different hybrid-ARQ protocols in coherent FSO systems and demonstrate that hybrid-ARQ significantly improves the outage performance of a coherent FSO system particularly in strong turbulence regime. In Chapter 6, we investigate parallel relaying in an intensity modulation/direct detection (IM/DD) FSO system. Assuming Gamma-Gamma fading model, we analyze both decode-and-forward and amplify-and-forward modes of cooperation. Focusing on high SNR regime, we investigate the outage probability and characterize it by the diversity and coding gains. The diversity-multiplexing tradeoff expression of each cooperation mode is also derived. Our performance analysis reveals that large energy savings can be achieved through the use of parallel relaying in FSO systems