Cooperative Free-space Optical Systems Research Articles

Mitigating Jamming in FSO Cooperative Networks: A Buffer-Aided Relaying Approach

In this paper, we propose a buffer-aided relaying-based approach to counter the effects of jamming in free space optical (FSO) communication networks. An FSO cooperative system with a source node, a relay node, and a destination node is considered. The relay node consists of a finite-size buffer, and both relay and destination nodes are assumed to be affected by jamming. We use a Markov chain (MC) based approach to obtain the state transition matrix of the system, which is later used to evaluate the steady state probability of the proposed setup. A mixture of jamming and Gaussian noise is considered to evaluate the system’s performance. Novel expressions of the outage probability and average bit error rate of the considered FSO system are obtained. The system’s performance is then analyzed for different jamming powers and jamming probabilities. The proposed system is then compared with a non-buffer-aided (NBA) cooperative FSO system affected by a jammer; it is observed that the proposed buffer-aided FSO system significantly outperforms the NBA counterparts.

Asynchronous Transmission for Cooperative Free-Space Optical Communication System

Cooperative Free Space Optical (FSO) communication aims to combat atmospheric attenuation and turbulence by transmitting and receiving multiple copies of signal along different paths, and thus it relies on transmission redundancy. In this letter, we propose an asynchronous transmission scheme for cooperative FSO system. Different from the traditional cooperative FSO system with synchronous transmission, the source node in the proposed system can send different data to destination node. In order to recover the data of different links, a squared signal combination method is proposed and implemented at the receiver of destination. Therefore, compared to synchronous transmission, the asynchronous transmission not only brings diversity gain for cooperative FSO system, but also improves the channel utilization and thus the effective data rate. It is demonstrated that the Bit Error Rate (BER) of the asynchronous transmission is lower than that of the direct transmission and the synchronous transmission with equal data rate.

Switching-Based Cooperative Decode-and-Forward Relaying for Hybrid FSO/RF Networks

In this paper, we propose a switching scheme for a hybrid free-space optical (FSO)/radio frequency (RF) system with a selective decode-and-forward (DF) relay network. Specifically, the system transmits over FSO channels when the instantaneous signal-to-noise ratio (SNR) at the FSO receiver is greater than the threshold SNR. If the SNR drops below the threshold, the system switches and transmits over RF channels. The exact outage probability and average symbol error rate (SER) expressions are derived for a selective DF relay network with maximal ratio combining (MRC) assumed at the destination. In addition, the asymptotic outage and SER expressions with a lower computational complexity are derived and the diversity order is determined. The optimum value of the threshold SNR, which satisfies the target SER, has been calculated numerically for the proposed switching scheme. The theoretical results, which are validated by Monte Carlo simulations, show that the proposed switching scheme for a cooperative hybrid FSO/RF system drastically improves the performance compared to that of a single-hop (SH) switching-based hybrid FSO/RF and cooperative FSO systems.

High availability cooperative free space optical communication link with various diversity combining methods

The authors investigate the performance and reliability improvement of multiple relay cooperative free space optical (FSO) communication over direct link. Multiple gain combiner (MGC) method has been introduced as a diversity combining method which assures to provide a close to optimum performance as compared to maximal ratio combining (MRC). As compared to MRC, the proposed MGC technique does not require prior channel information and adaptive gain adjustment. Bit error rate (BER) performance improvement with different combining techniques, including MGC, in cooperative FSO system for different channel environments has been included in this study. BER performance for multiple relays for various combining techniques: selection combining, equal gain combining, MRC and MGC have been compared and it is seen that the performance of MGC is close to optimum MRC. High availability of the system is achieved by using multiple relays in the cooperative system. Markov model analysis for availability and mean time between failures of FSO systems in multiple relays cooperative communication has been done to verify the reliability improvement of a cooperative system with increasing number of relays.

Amplify-and-Forward Strategy Using MRC Reception Over FSO Channels with Pointing Errors

Free-space optical (FSO) systems offer ultrahigh data rates for a number of terrestrial applications. This paper presents what we believe is the first characterization of average bit error rate (BER) for a cooperative FSO system that uses channel side information-assisted amplify- and-forward (AF) relaying when line-of-sight (LOS) is available. This paper uses maximum ratio combining reception at the destination node to maximize the output signal-to-noise ratio under pointing error effects. An asymptotic closed-form expression for the BER is obtained over gamma–gamma fading channels with pointing errors. The developed asymptotic expression is used to analyze the diversity order gain in relation to the direct path link. The maximum diversity order is determined to be βSD + min(βSR, βRD), where βSD, βSR, and βRD are atmospheric turbulence parameters corresponding to the source–destination, source-relay, and relay-destination links, respectively. Moreover, optimal relay placement, which plays an important role in the performance, is derived by taking full advantage of asymptotic expressions without the need to use optimization methods. Obtained results corroborate that a higher diversity order gain and, hence, a better BER performance, is achieved compared to the direct path link and equivalent AF dual-hop transmissions.

Joint relay selection and link scheduling in cooperative free-space optical system

Free-space optical (FSO) communication system is vulnerable to link disruption due to the propagation loss and atmospheric turbulence. Cooperative FSO system has been proposed to mitigate the adverse effects of link disruption by the cooperative transmission in assistance of relay. However, most of the related works focus on the outage probability analysis and put less attention on the resource utilization. In fact, there exists a waste of bandwidth resource due to time slot waiting and more FSO transceivers are occupied. We mainly focus on the relay selection and link scheduling mechanisms of cooperative FSO system to improve not only network reliability but also resource utilization. We first propose a relay selection algorithm as a solution to combat channel fading. In the relay selection algorithm, two cooperative modes including intrarelay-set and inter-relay-set cooperative modes are proposed to enhance network capacity. Then, a link scheduling algorithm is further proposed to improve the bandwidth utilization and reduce the occupancy rate of transceivers by utilizing more sufficiently the idle FSO links. Simulation results show that the proposed algorithms can provide significant performance gain for cooperative FSO system in reliability and resource utilization and are highly suitable for the growing traffic demands.

Ergodic Capacity Analysis of Decode-and-Forward Relay-Assisted FSO Systems Over Alpha–Mu Fading Channels Considering Pointing Errors

The ergodic capacity of decode-and-forward (DF) relay-assisted free-space optical (FSO) communication systems when line of sight is available is analyzed over gamma–gamma fading channels with pointing errors. A novel closed-form approximate ergodic capacity expression is obtained in terms of the H-Fox function for a three-way FSO communications system when the $\alpha{-} \mu$ distribution to efficiently approximate the probability density function (PDF) of the sum of gamma–gamma with pointing-error variates is considered. Moreover, we present novel asymptotic expressions at high signal-to-noise ratio (SNR) , as well as low SNR for the ergodic capacity of DF relay-assisted FSO systems. The main contribution in this paper lies in an in-depth analysis about the impact of pointing errors on the ergodic capacity for cooperative FSO systems. In order to maintain the same performance in terms of capacity, it is corroborated that the presence of pointing errors requires an increase in SNR, which is related to the fraction of the collected power at the receive aperture, i.e., $A_{0} $ . Simulation results are further demonstrated to confirm the accuracy and usefulness of the derived results.

Ergodic capacity analysis for DF strategies in cooperative FSO systems.

This paper focuses on the ergodic capacity analysis in the context of cooperative free-space optical (FSO) systems when the line of sight is available. Novel asymptotic closed-form expressions for the ergodic capacity corresponding to two different decode-and-forward (DF) strategies are obtained for a cooperative FSO communication system. Here, the atmospheric turbulence is modeled by a gamma-gamma distribution of parameters α and β which allows to study a wide range of turbulence conditions (moderate-to-strong) as well as the effect of the misalignment with zero boresight. It is demonstrated that cooperative communications are able to achieve not only a better performance in terms of the error rate performance as well as outage probability than direct transmission, but also in terms of the channel capacity in the context of FSO systems without much increase in hardware. In this way, a 3-way FSO communication setup is considered, in which the cooperative protocol can be applied to achieve a greater ergodic capacity compared to a direct transmission. It can be concluded that a greater and robust capacity strongly dependent on the relay location is achieved compared to a direct transmission without cooperative communication when line of sight is available. Here, the line of sight is taken into account in order to achieve a significant robustness under different turbulence conditions and more severe pointing errors regardless of the relay location. Simulation results are further demonstrated to confirm the accuracy and usefulness of the derived results.

Adaptive selective relaying in cooperative free-space optical systems over atmospheric turbulence and misalignment fading channels

In this paper, a novel adaptive cooperative protocol with multiple relays using detect-and-forward (DF) over atmospheric turbulence channels with pointing errors is proposed. The adaptive DF cooperative protocol here analyzed is based on the selection of the optical path, source-destination or different source-relay links, with a greater value of fading gain or irradiance, maintaining a high diversity order. Closed-form asymptotic bit error-rate (BER) expressions are obtained for a cooperative free-space optical (FSO) communication system with Nr relays, when the irradiance of the transmitted optical beam is susceptible to either a wide range of turbulence conditions, following a gamma-gamma distribution of parameters α and β, or pointing errors, following a misalignment fading model where the effect of beam width, detector size and jitter variance is considered. A greater robustness for different link distances and pointing errors is corroborated by the obtained results if compared with similar cooperative schemes or equivalent multiple-input multiple-output (MIMO) systems. Simulation results are further demonstrated to confirm the accuracy and usefulness of the derived results.

Performance Analysis of Selective Relaying in Cooperative Free-Space Optical Systems

In this paper, we evaluate the performance of a selective-relaying (SR) protocol that is suitable for cooperative free-space optical (FSO) communications with any number of relays. The SR strategy is based on transmitting all information symbols either along the direct link between the source and the destination or along one of the indirect links via the relays. We derive closed-form expressions of upper-bounds on the average error probability that can be achieved over Rayleigh and lognormal fading channels. We also provide an asymptotic analysis for quantifying the diversity gain and reduction in fading variance that can be realized by the cooperative scheme. The performance analysis shows the superiority of the SR scheme over the strategy that corresponds to activating all available links.

Cooperative FSO Systems: Performance Analysis and Optimal Power Allocation

In this paper, we investigate the cooperative diversity technique as a candidate solution for combating turbulence-induced fading over free-space optical (FSO) links. In particular, we propose a novel cooperation strategy that is suitable for quantum-limited FSO systems with any number of relays and we derive closed-form expressions for the error performance of this strategy. In scenarios where the channel-state-information (CSI) is available at the different nodes, we propose an optimal power allocation strategy that satisfies the Karush-Kuhn-Tucker (KKT) conditions and that further boosts the performance of FSO networks. It turned out that this closed-form optimal solution corresponds to transmitting the entire optical power along the “strongest link” between the source and the destination nodes. A simple procedure is proposed for selecting this link and for distributing the power among its different hops.