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.     

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 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.     

Hybrid optical/RF channels: characterization and performance study using low density parity check codes

A hybrid channel, consisting of a free space optical (FSO) link and a parallel radio frequency (RF) link, is considered. The FSO link carries most of the transmitted data while leaving only a small fraction of the data to be carried by the RF link. It is first shown that the capacity of the proposed hybrid structure is much higher than the capacity of a single FSO link. Next the performance of the hybrid channel is studied using a low density parity check code. A density evolution strategy for the hybrid channel is derived and a Gaussian approximation technique is presented. Using these techniques, the conditions for the convergence of the message passing algorithm in terms of minimum data carrying rate through the RF link are derived. Finally, numerical results showing the benefits of the hybrid channel under various channel conditions are presented.     

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.     

基于FSO系统的多进制LT码性能分析

为克服空间光通信(FSO)链路受大气湍流效应的影响,将LT码应用到FSO系统中.多进制LT码具有较高的传输效率.提出一种新型度分布函数来改善多进制LT码的译码性能.计算不同进制LT码的译码失败率,并模拟强湍流信道,对采用不同编码方案的FSO性能进行仿真.结果表明:采用新型度分布的LT码能够降低译码失败率,且能有效改善FSO系统在强湍流情况下的性能.     

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。     

800Mbps准循环LDPC码译码器的FPGA实现

本文提出了一种适用于准循环低密度校验码的低复杂度的高并行度译码器架构。通常准循环低密度校验码不适于设计有效的高并行度高吞吐量译码器。我们通过利用准循环低密度校验码的奇偶校验矩阵的结构特点,将其转化为块准循环结构,从而能够并行化处理译码算法的行与列操作。使用这个架构,我们在Xilinx Virtex-5 LX330 FPGA上实现了(8176,7154)有限几何LDPC码的译码器,在15次迭代的条件下其译码吞吐量达到800Mbps。      

Experimental Demonstration of a Hybrid Link for Mitigating Atmospheric Turbulence Effects in Free-Space Optical Communication

Atmospheric turbulence causes severe performance degradation in free-space optical communication links. However, by using a reliable low-bandwidth wireline or RF link in parallel, the performance of such links can be improved significantly. This is achieved by using error correction codes at the transmitter and decoding the received data jointly over the two links at the receiver. Our work describes the feasibility study of such a hybrid link. It is observed that a 10% use of the wireline link can result in more than 8-dB performance gain in the optical link.      

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.     

两种提高IEEE802.11b WLAN 物理层数据传输速率的编码技术

本文提出两种将IEEE802.11bWLAN物理层数据速率提高到16.5Mbps的编码和译码方法。这两个方案都继续采用原IEEE802.11b中对于5.5和11Mbps速率所采用的CCK编码方法。两个方案的编码过程变化不大，第一方案的译码还是采用对5.5和11Mbps数据率所使用的快速WALSH变换（FWT），第二个方案的译码被修改成适合16.5Mbps数据率的快速WALSH变换（FWT）。最后通过仿真比较了这两个方案在无多径干扰的AWGN信道上的性能，结果表明第二个方案优于第一个。     

基于网络丢包率的混合FSO/RF系统切换技术研究

利用C++ Builder编程软件提供的相关网络组件,提出了基于网络丢包率的混合FSO/RF系统切换判决机制,在网络中实现了对链路的监控功能,并采用无线路由器、光纤收发器等器件搭建了混合FSO/RF测试系统,对基于网络丢包率的混合系统切换机制进行了测试,结果表明采用网络切换判决方案,能够自动选择链路通信,保证数据不间断...     

A bit error rate analysis for TCP traffic over parallel free space photonics

Inter-satellite links (ISL) are a useful technology to transmit data to space stations and to communicate between satellites. However, there are serious limitations due to long delays and poor channel performance, resulting in high bit error rates (BER). In this paper, parallel transmission and the scaling of the Transport Control Protocol (TCP) window in free space optics (FSO) communications are analyzed in order to overcome these disadvantages in optical inter-satellite links. Latency and BER are the dominant effects that determine link performance. Thus, a physical, link, network and transport cross-layer analysis for FSO over ISL is presented in this paper. This analysis shows the advantages and disadvantages of using optical parallel transmission and TCP window scaling for free space optical links between stations and satellite constellations. The key contribution of this work is to simulate the effects of the BER and to link the results to packet error rate (PER) to determine the goodput for TCP transmissions by using a cross-layering approach. The results give evidence that wavelength division multiplexing (WDM) can mitigate the effects of long delay and high BER for a FSO communication using TCP.     

短长度分布式喷泉码的性能分析

LT码是喷泉码的一种,由于深空通信不能实现反馈,距离远,变时延的传输特性,LT码适合用于深空通信中,由于传统的单信源通信已经不能满足当前的通信需求,多信源多中继通信系统成为了研究的重点。介绍了短长度分布式二、四信源的通信模型,以及各自的度分布,并选取了码长较短、删除概率较小这几种情形通过仿真验证了各自的性能。仿真结果表明,二、四信源分布式LT码比LT码有更好的性能。     

新型度分布的LT码在自由空间光通信中应用

由于自由空间光通信(FSO)受到大气湍流效应的影响,运用信道编码技术来保证FSO系统的稳定性。研究LT码原理,并将其引入FSO系统。分析LT码度分布需要满足的条件,引用泊松鲁棒孤子度分布来减小LT码译码开销、降低编译码复杂度,并应用于FSO系统来提高FSO系统性能。采用MATLAB进行仿真,比较泊松鲁棒孤子度分布与鲁棒孤子度分布的性能,并模拟在不同湍流信道中的FSO系统误码率。结果表明,泊松鲁棒孤子度分布具有较小的平均度值,能够减小LT的译码开销、降低编译码复杂度;且应用泊松鲁棒孤子度分布的LT码能够降低FSO系统中的误码率,保证FSO系统稳定。     

适用于大气弱湍流信道的自适应码率极化码

为了提高光通信链路在大气弱湍流信道下的解码性能和传输效率,基于极化码的信息位嵌套特性,设计了一种自适应码率极化码。该码字在弱湍流信道中能充分地极化,纠错效果较好。为了调节码率,引入CRC校验码作为发送端的停止标志,逐次发送更低码率的码字直到译码结果通过校验,此时的码字码率即是保证可靠传输的最大码率。不同湍流强度下的仿真结果表明,在误帧率为10-8时,相比传统极化码,自适应码率极化码可以获得1.7~2.3 dB的性能增益。对自适应码率极化码的时延进行了仿真分析,并结合误帧率得到了自适应码率极化码的信息吞吐率,结果表明,在弱湍流信道中,自适应码率极化码的信息吞吐率能满足FSO的传输需求。     

基于BM算法的BCH码的译码硬件实现

BCH码是一种理论上比较成熟的代数码型,在电力通信系统,GSM标准的语音和数据业务,以及卫星通信和数字广播通信（DVB-S2）等多个领域均有着广泛的应用。基于幂次运算,在线性反馈移位寄存器（LFSR）下实现了基于Berlekamp-Massey（BM）时域迭代译码算法的整个译码器构架,以及BM简化算法的硬件设计。通过计算机模拟仿真表明,两种算法的译码速率分别可达到32Mbps,37Mbps。     

Energy Efficient Coded Communication for IEEE 802.15.4 Compliant Wireless Sensor Networks

The multipath nature of the wireless environment does not provide reliable links for robust communication in wireless senor networks (WSNs). These unreliable links increase the error level to a greater extent and therefore, reduces battery life. Hence, there arises a need for developing energy efficient forward error correction code that avoids more energy consuming Automatic Repeat request (ARQ) scheme used in WSNs to improve link reliability. In this paper, we consider a simple block error correction codes such as cyclic and Bose Chaudhuri Hocquenghem (BCH) codes to be used in IEEE 802.15.4 RF transceiver based sensor nodes. The simulations are performed to measure network parameters such as bit error rate, and energy spent per bit under Rayleigh fading channel environment. It is found that BCH code with code rate of 0.8 provides coding gain of 1.6 dB when compared with cyclic and ARQ schemes and proves to be an energy efficient code among the codes considered.     

A Reconfigurable TDMP Decoder for Raptor Codes

A Raptor code is a concatenation of a fixed rate precode and a Luby-Transform (LT) code that can be used as a rateless error-correcting code over communication channels. By definition, Raptor codes are characterized by irregularity features such as dynamic rate, check-degree variability, and joint coding, which make the design of hardware-efficient decoders a challenging task. In this paper, serial turbo decoding of architecture-aware Raptor codes is mapped into sequential row processing of a regular matrix by using a combination of code enhancements and architectural optimizations. The proposed mapping approach is based on three basic steps: (1) applying systematic permutations on the source matrix of the Raptor code, (2) confining LT random encoding to pseudo-random permutation of messages and periodic selection of row-splitting scenarios, and (3) developing a reconfigurable parallel check-node processor that attains a constant throughput while processing LT- and LDPC-nodes of varying degrees and count. The decoder scheduling is, thus, made simple and uniform across both LDPC and LT decoding. A serial decoder implementing the proposed approach was synthesized in 65 nm, 1.2 V CMOS technology. Hardware simulations show that the decoder, decoding a rate-0.4 code instance, achieves a throughput of 36 Mb/s at SNR of 1.5 dB, dissipates an average power of 27 mW and occupies an area of 0.55 mm².     

Channel Coding for Future Space Missions: New Requirements and Trends

Future space missions will put severe constraints on communication links in terms of data rates, bandwidth occupancy, complexity, and performance. The requirements imposed by the new missions and their consequences on channel code design are presented in the first part of the paper. All relevant issues, including code rates, frame lengths, modulation formats, performance metrics, complexity, and others, are discussed. In the second part of the paper, long erasure correcting codes are presented and their properties explained. These codes operate at the upper layers of the space link protocol and constitute an attractive new frontier for zero packet loss in future space communications.