Performance of orthogonal frequency division multiplexing based M-ary quadrature amplitude modulation in asymmetrical dual-hop mixed RF-FSO transmission system

This paper investigates the performance of a dual-hop mixed relay system with radio frequency (RF) and free-space optics (FSO) communication under the effect of pointing error (PE) and atmospheric turbulence (AT). This paper considers a system where RF and FSO links are cascaded. The RF link is modeled by Nakagami-m fading, and the FSO link is modeled as gamma–gamma (G-G) fading channel. Both the channel models use orthogonal frequency division multiplexing (OFDM) with M-ary quadrature amplitude modulation (QAM). The expressions for probability density function, cumulative distribution function, signal-to-noise ratio, and ergodic capacity are derived. The moment generating function (MGF) of fading and the bit error rate (BER) of the OFDM-based M-ary QAM scheme is derived in terms of Meijer's G-function. It has been observed that, in fixed gain relay systems, the modulation scheme's BER is dominated by the SNR of the RF link. While in a variable gain relay system, the turbulence conditions of the FSO system affect the SNR and the BER of the modulation method. The feasibility of heterodyne detection and intensity modulation direct detection (IM/DD) is analyzed in terms of outage probability and ergodic capacity. The results can be used to choose the optimal modulation order and relay system for QAM-OFDM-based optical wireless systems.     

Soft-switching hybrid FSO/RF links using short-length raptor codes: design and implementation

Free-space optical (FSO) links offer gigabit per second data rates and low system complexity, but suffer from atmospheric loss due to fog and scintillation. Radio-frequency (RF) links have lower data rates, but are relatively insensitive to weather. Hybrid FSO/RF links combine the advantages of both links. Currently, selection or ?hard-switching? is performed between FSO or RF links depending on feedback from the receiver. This technique is inefficient since only one medium is used at a time. In this paper, we develop a ?soft-switching? scheme for hybrid FSO/RF links using short-length Raptor codes. Raptor encoded packets are sent simultaneously on both links and the code adapts to the conditions on either link with very limited feedback. A set of short-length Raptor codes (k = 16 to 1024) are presented which are amenable to highspeed implementation. A practical Raptor encoder and decoder are implemented in an FPGA and shown to support a 714 Mbps data rate with a 97 mW power consumption and 26360 gate circuit scale. The performance of the switching algorithms is simulated in a realistic channel model based on climate data. For a 1 Gbps FSO link combined with a 96 Mbps WiMAX RF link, an average rate of over 472 Mbps is achieved using the implemented Raptor code while hard-switching techniques achieved 112 Mbps on average.     

Availability Evaluation of Ground-to-Air Hybrid FSO/RF Links

Recently, a hybrid architecture that utilizes the complementary nature of free-space optics (FSO) and radio frequency (RF) links with respect to their individual weather sensitivities was proposed to significantly increase availability for terrestrial broadband links. Based on this architecture, we developed a channel model integrating both the RF and FSO channels. Using the model and cloud distribution data obtained from the International Satellite Cloud Climatology Project, availability of an airborne hybrid FSO/RF link is evaluated. From the results, we conclude that if the FSO link alone is used, availability is greatly hampered by clouds due to attenuation and temporal dispersion. Contrarily, the RF signals are relatively immune to cloud influence, thus improving the hybrid link availability significantly. Furthermore, because of the significant temporal dispersion caused by multiple scattering of cloud particles, availability of FSO links can be improved by using frequency division schemes, though far from compensating for losses incurred by clouds.     

Performance analysis of a variable‐gain amplify‐and‐forward relayed mixed RF‐FSO system

In this work, an amplify‐and‐forward variable‐gain relayed mixed RF‐FSO system is studied. The considered dual‐hop system consists of a radio frequency (RF) link followed by a free space optical (FSO) channel. The RF link is affected by short‐term multipath fading and long‐term shadowing effects and is assumed to follow the generalized‐K fading distribution that approximates accurately several important distributions often used to model communication channels. The FSO channel experiences fading caused by atmospheric turbulence that is modeled by the gamma‐gamma distribution characterizing moderate and strong turbulence conditions. The FSO channel also suffers path loss and pointing error induced misalignment fading. The performance of the considered system is analyzed under the collective influence of distribution shaping parameters, pointing errors that result in misalignment fading, atmospheric turbulence, and path loss. The moment‐generating function of the Signal power to noise power ratio measured end‐to‐end for this system is derived. The cumulative distribution function for the Signal power to noise power ratio present between the source and destination receiver is also evaluated. Further, we investigate the error and outage performance and the average channel capacity for this system. The analytical expressions in closed form for the outage probability, symbol and bit error rate considering different modulation schemes and channel capacity are also derived. The mathematical expressions obtained are also demonstrated by numerical plots.     

软切换链路下的混合编码与副载波调制方案

为了提高自由空间光/射频（FSO/RF）混合通信链路的性能，采用混合低密度奇偶校验（LDPC）编码与副载波相移键控/正交振幅调制（PSK/QAM）联合调制的方法，对不同传输比例下混合系统的误比特率性能进行了仿真分析，取得了不同信道条件下单链路和混合链路传输方案的误比特率数据。结果表明，在弱中强湍流条件下采用副载波二进制相移键控（BPSK）调制，相比开关键控（OOK）调制可获得大约4.4dB~5.2dB的增益。采用软切换的混合LDPC编码与副载波BPSK/16QAM调制方案，依据链路状态调整比例为1:1和3:1时，不同湍流强度下可获得大约0.3dB~7.4dB的性能增益。这一研究结果对于提高FSO/RF混合通信系统的全天候高效可靠传输性能具有重要的参考价值。     

Performance analysis of weather-dependent satellite–terrestrial network with rate adaptation hybrid free-space optical and radio frequency link

Due to the rapid development of satellite laser communication technology, free-space optical (FSO) links present a promising alternative to traditional radio frequency (RF) links. In this paper, taking the influence of weather factors into consideration, we investigate the performance of the hybrid FSO/RF links where the feeder link operates in the FSO band and the user link operates in the hybrid FSO/RF band. Specifically, the FSO feeder link is modeled by the gamma–gamma distribution in the presence of beam wander and pointing error, and the detection method adopts either the intensity modulation with direct intensity (IM/DD) or heterodyne detection. The RF user link is assumed to follow the shadowed Rician model. In addition, in order to improve the transmission rate of the link under the time-varying satellite–terrestrial channel, a rate adaptation scheme is proposed. The performance of the system under study is evaluated in terms of the outage probability, average bit error rate (BER), and average transmission rate. Our results provide some important insights, for example, (1) due to the constraints of the feeder link and weather factors, there is an upper limit on the outage performance and bit error rate of the hybrid link; (2) the adaptive transmission strategy can significantly improve the transmission rate of the link compared with traditional design.     

Weather effects on hybrid FSO/RF communication link

Free Space Optics (FSO) or optical wireless systems provide high data rate solution for bandwidth hungry communication applications. Carrier class availability is a necessity for wide scale acceptability which is extremely difficult to achieve in the case of optical wireless links. FSO links are highly weather-dependent and different weather effects reduce the link availability. Employing a hybrid network consisting of an FSO link and a back up link in the GHz frequency range renders high availability besides providing comparable data rates. In this paper effects of fog, rain and snow on FSO/GHz hybrid network are studied so that GHz frequencies with best complementary behaviour can be selected as a back up link. As a prime conclusion of the article, it is suggested that free space optical links can be supplemented with 40 GHz RF links to achieve near carrier class availability.     

Impact of transmission techniques in asymmetric RF/FSO system over Nakagami‐m and gamma‐gamma fading channels with pointing errors

In this paper, the performance of transmission techniques on the fixed‐gain amplify‐and‐forward–based asymmetric radio frequency/free space optical (RF/FSO) communication system is studied. The RF link and FSO link are, respectively, modeled by the Nakagami‐m and Gamma‐Gamma fading channels under the effect of zero boresight and non‐zero boresight pointing errors subject to heterodyne detection. Maximum ratio transmission (MRT) and orthogonal space‐time block coding (OSTBC) techniques are the transmission scenarios considered at the system source while selection combining is used for reception at the relay and destination for the signal detection. Moreover, a unified cumulative density distribution (CDF) of end‐to‐end signal‐to‐noise ratio is derived for the system. By utilizing this channel statistical CDF, the closed‐form expressions for the outage probability and average bit error rate for the M‐ary phase shift keying modulation are then obtained for the system. The analysis presented illustrates that both the atmospheric turbulence and pointing error significantly degrade the system performance. Based on this, the MRT transmission technique offers a better performance compared with the OSTBC techniques under the same system conditions. The accuracy of the analytical results is verified by Monte‐Carlo simulations.     

Adaptive hybrid free space optical/radio frequency communication system

Wireless communication has achieved lot of attention and the demand is continually increasing day by day. Radio frequency (RF) is highly attracted by various wireless communication applications. The RF spectrum is already very crowded and the rapid increase in the use of wireless services has led the problems of RF spectrum exhaustion and eventually RF spectrum deficit. Free space optical (FSO) communication is a viable technology with a plenty of bandwidth, license-free spectrum and interference free link. On the other hand, FSO channel is severely corrupted by atmospheric turbulence and non-predictive weather scenarios. We suggest a hybrid FSO/RF communication system in our previous research, which can mitigate the issues of the individual links. In this research, we investigate the performance of the proposed adaptive system for reliable data transmission. We develop modulation and power adaptive schemes for maximizing the mutual information. The proposed adaptive system is compared with non-adaptive system, which gives 2.75 dB gain for the joint power and 0.75 dB gain for the separate power constraint.     

Throughput and delay analysis for hybrid radio-frequency and free-space-optical (RF/FSO) networks

In this paper the per-node throughput and end-to-end delay of randomly deployed (i.e. ad-hoc) hybrid radio frequency - free space optics (RF/FSO) networks are studied. The hybrid RF/FSO network consists of an RF ad hoc network of n nodes, f(n) of them, termed ‘super nodes’, are equipped with an additional FSO transceiver with transmission range s(n). Every RF and FSO transceiver is able to transmit at a maximum data rate of W ₁ and W ₂ bits/sec, respectively. An upper bound on the per node throughput capacity is derived. In order to prove that this upper bound is achievable, a hybrid routing scheme is designed whereby the data traffic is divided into two classes and assigned different forwarding strategies. The capacity improvement with the support of FSO nodes is evaluated and compared against the corresponding results for pure RF wireless networks. Under optimal throughput scaling, the scaling of average end-to-end delay is derived. A significant gain in throughput capacity and a notable reduction in delay will be achieved if \(f(n) = \Upomega\left(\frac{1}{s(n)}\sqrt{\frac{n}{\log n}}\cdot \frac{W_1}{W_2} \right)\). Furthermore, it is found that for fixed W ₁, f(n) and n where f(n) < n, there is no capacity incentive to increase the FSO data rate beyond a critical value. In addition, both throughput and delay can achieve linear scaling by properly adjusting the FSO transmission range and the number of FSO nodes.     

Physical layer security for dual‐hop FSO/RF system using generalized ΓΓ/η−μ fading channels

We investigate the physical layer security of decode‐and‐forward–relayed free space optics (FSO)/radio frequency (RF) communication system. In this network, the eavesdropper exists after relay node and overhears RF transmission. Further, FSO being a line‐of‐sight transmission is assumed to be secure from eavesdroppers. Here, we have the Gamma‐Gamma (ΓΓ) distribution for FSO link and generalized η?μ distribution for RF link. The security for information transmission to the legitimate user in the presence of an eavesdropper is measured in terms of secrecy capacity and secrecy outage probability. Deriving the probability density function and cumulative distribution function of end‐to‐end signal‐to‐noise ratio, the closed‐form expressions for security parameters are achieved. The numerical analysis of the proposed system is done under the influence of atmospheric turbulence effects and various fading conditions. The results have been verified through simulation.     

Outage analysis of the hybrid free-space optical and radio-frequency channel

We study the hybrid free-space optical (FSO) and radio-frequency (RF) channel from an information theoretic perspective. Since both links operate at vastly different carrier frequencies, we model the hybrid channel as a pair of parallel channels. Moreover, since the FSO channel signals at a higher rate than the RF channel, we incorporate this key feature in the parallel channel model. Both channels experience fading due to scintillation, which is slow compared to typical signalling rates. Under this framework, we study the fundamental limits of the hybrid channel. In particular, we analyse the outage probability in the large signal-to-noise ratio (SNR) regime, and obtain the outage diversity or SNR exponent of the hybrid system. First we consider the case when only the receiver has perfect channel state information (CSIR case), and obtain the exponents for general scintillation distributions. These exponents relate key system design parameters to the asymptotic outage performance and illustrate the benefits of using hybrid systems with respect to independent FSO or RF links. We next consider the case when perfect CSI is known at both the receiver and transmitter, and derive the optimal power allocation strategy that minimises the outage probability subject to peak and average power constraints. The optimal solution involves non-convex optimisation, which is intractable in practical systems. We therefore propose a suboptimal algorithm that achieves significant power savings (on the order of tens of dBs) over uniform power allocation. We show that the suboptimal algorithm has the same diversity as the optimal power allocation strategy.     

具有能量收集的双向RF/FSO中继系统性能分析

设计了一种在大气湍流和指向误差影响下的带有能量收集的混合双向RF/FSO中继传输系统。RF信号部分采用能量分割的方式进行能量收集,FSO信号部分则将其信号中的直流电分量所携带的能量收集起来存储在中继器中。其中射频RF链路采用Nakagamim信道衰落模型,自由空间光FSO链路采用Gamma Gamma信道衰落模型。并且计算出了两条链路各自的累积分布函数CDF。采用DF的中继协作方式,并利用Meijei G函数推导出系统中断概率的闭合表达式,最终通过仿真来验证结果的正确性。     

基于上行非正交多址接入技术的星空地融合网络性能分析

针对光电混合的星空地融合网络上行链路,该文研究了多天线波束成形技术和上行非正交多址接入(NOMA)技术相结合的系统遍历和速率性能。首先,在无人机采用多天线和上行NOMA技术条件下,为实现系统和速率最大化,提出了一种基于统计信道状态信息的波束成形方案。接着,假设卫星-无人机链路采用自由空间光链路且服从伽马-伽马衰落,无人机-地面用户链路采用射频链路且服从相关瑞利衰落,推导了系统和速率的闭合表达式。最后,通过数值仿真验证了理论分析的正确性。仿真结果表明,与正交多址接入(OMA)方案相比,所提方案提高了系统性能,并且与基准波束成形(BF)方案相比,所提方案具有更好的性能优势。     

美军机载射频与FSO混合通信发展述评

机载光与射频混合通信系统概念是美国防先期研究计划局( DARPA)提出的创新性概念,用以为机载平台提供定向高速宽带数据链,支持空基骨干网等宽带通信应用。目前该概念已经进行了概念验证以及多次演示和测试。概述了美军机载光与射频混合通信研究进展,分析了相关关键技术,指出了发展趋势,并给出了相关的启示和建议。     

Mixed RF/FSO bidirectional system achieving spectral efficiency

The performance of two-way relay (TWR)-assisted mixed radio-frequency/free-space optical (RF/FSO) system is evaluated in this letter. The proposed system employs decode-and-forward relaying phenomena where the relay is basically an interfacing node between two source nodes \(S_1\) and \(S_2\), where \(S_1\) supports RF signal, while \(S_2\) supports FSO signal. The TWR-assisted system helps in achieving spectral efficiency by managing bidirectional communication in three time slots, thus maximizing the achievable rate of the network. The RF link is subjected to generalized \(\eta -\mu \) distribution, and the optical channel is affected by path loss, pointing errors and gamma–gamma (gg) distributed atmospheric turbulence. The novel expressions for the probability density function and cumulative distribution function of the equivalent end-to-end signal-to-noise ratio (SNR) are derived. Capitalizing on these derived statistics of end-to-end SNR, the expressions of outage probability and the bit-error rate for different binary modulations and M-ary modulations are provided.     

一种SUTN网络中用户接入信道的调度算法

针对多用户的卫星-无人机-地面通信网络(SUTN)的用户接入信道问题,提出基于遍历容量的调度算法(ECSA)。卫星通过空间光通信(FSO)向无人机(UAV)传输数据;UAV采用无线通信射频(RF)链路向用户传输数据。ECSA算法通过推导无人机端和用户端的信噪比,再推导出信噪比的概率密度函数,然后依据概率密度函数计算用户的遍历容量;最后,依据公平原则,并结合用户的遍历容量,计算用户的目标函数,将具有最大的目标函数值的用户优先接入信道。仿真结果表明,相比于轮流调度算法,提出的ECSA算法有效提高了系统容量。     

Analysis of effective capacity for free-space optical communication systems over gamma-gamma turbulence channels with pointing errors

To facilitate the efficient support of quality-of-service (QoS) for promising free-space optical (FSO) communication systems, it is essential to model and analyze FSO channels in terms of delay QoS. However, most existing works focus on the average capacity and outage capacity for FSO, which are not enough to characterize the effective transmission data rate when delay-sensitive service is applied. In this paper, the effective capacity of FSO communication systems under statistical QoS provisioning constraints is investigated to meet heterogeneous traffic demands. A novel closed-form expression for effective capacity is derived under the combined effects of atmospheric turbulence conditions, pointing errors, beam widths, detector sizes and QoS exponents. The obtained results reveal the effects of some significant parameters on effective capacity, which can be used for the design of FSO systems carrying a wide range of services with diverse QoS requirements.     

瞄准误差和Gamma-Gamma大气湍流下信号束散角的优化分析

在自由空间激光通信(FSO)系统中,瞄准误差和大气湍流严重影响系统通信链路,是链路性能恶化的两大主要因素.通过对联合信道的理论分析,建立了信道模型,并利用Meijer G函数推导出了直接探测系统和相干探测系统平均信道容量的表达式.通过在Matlab中的仿真结果可知,在给定瞄准误差和大气湍流的条件下,信号光的光束束散角并非越小越好,而是存在最优束散角,该束散角不仅可以保证系统性能良好,而且使得发射系统功率取得最小值,并且采用相干探测系统可以抑制大气湍流对系统的影响,有效提高系统的性能.     

Fiber optics without fiber

Low-power infrared beams, which do not harm the eyes, are the means by which free-space optics (FSO) technology transmits data through the air between transceivers, or link heads, mounted on roof-tops or behind windows. It works over distances of several hundred meters to a few kilometers, depending upon atmospheric conditions. The authors believe that FSO could be the ultimate solution for high-speed residential access. Instead of hybrid fiber-coax systems, hybrid fiber-laser systems may turn out to be the best way to deliver high data rates directly to the home