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。     

Midinfrared Interband Cascade Laser for Free Space Optical Communication

A free space optical (FSO) link utilizing midinfrared (mid-IR) interband cascade lasers has been demonstrated in the 3- to 5- $mu{hbox {m}}$ atmospheric transmission window with data rates up to 70 Mb/s and bit-error rate (BER) less than $10^{-8}$ . The performance of the mid-IR FSO link has been compared with the performance of a near-IR link under various fog conditions using an indoor communication testbed. These experiments demonstrated the lower attenuation and scintillation advantages of a mid-IR FSO link.      

BER Performance of Free-Space Optical Transmission with Spatial Diversity

Free space optical (FSO) communications is a cost-effective and high bandwidth access technique, which has been receiving growing attention with recent commercialization successes. A major impairment in FSO links is the turbulence- induced fading which severely degrades the link performance. To mitigate turbulence-induced fading and, therefore, to improve the error rate performance, spatial diversity can be used over FSO links which involves the deployment of multiple laser transmitters/receivers. In this paper, we investigate the bit error rate (BER) performance of FSO links with spatial diversity over log- normal atmospheric turbulence fading channels, assuming both independent and correlated channels among transmitter/receiver apertures. Our analytical derivations build upon an approximation to the sum of correlated log-normal random variables. The derived BER expressions quantify the effect of spatial diversity and possible spatial correlations in a log-normal channel.     

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.     

A cross-layer analysis of TCP/link adaptation technologies over free-space optical links with Markov error model

This paper presents a comprehensive cross-layer framework on the performance of transmission control protocol (TCP) over a free-space optical (FSO) link, which employs automatic repeat request (ARQ) and adaptive modulation and coding (AMC) schemes. Not similar to conventional works in the literature of FSO, we conduct a Markov error model to accurately capture effects of burst errors caused by atmospheric turbulence on cross-layer operations. From the framework, we quantify the impacts of different parameters/settings of ARQ, AMC, and the FSO link on TCP throughput performance. We also discuss several optimization aspects for TCP performance.     

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.     

Machine Learning Algorithms for Predicting Electricity Consumption of Buildings

Inter-satellite optical wireless communication (IsOWC) is a developing free-space optical technology used to communicate among satellites in space. At the same time, SAC-OCDMA (spectral amplitude coding optical code division multiple access) is an encouraging research area in the domain of optical communication because of its high bandwidth, speed, huge capacity, and ability to carry bursty and asynchronous information transmission. The present paper is concerned with the hybrid IsOWC non-coherent SAC-OCDMA system based on PM-ZCC (Permutation Matrix Zero Cross-Correlation) code for long-range high data rate transmission. The advanced modulation format (CSRZ) and direct detection (DD) techniques have been used to design the proposed system. The system is designed for five stations (each carrying 10 Gb/s). The system's performance is investigated for pointing error (with and without) over a space distance up to 12000 km in terms of Q factor, eye diagrams, BER and SNR. Moreover, the performance of a single IsOWC link has also been compared with multiple IsOWC links for a distance of 6500 km at 10 Gb/s data rate. The results show that system performance improves by using CSRZ format and multiple ISL links.

     

Enhancing spectral efficiency of FSO system using adaptive SIM/M‐PSK and SIMO in the presence of atmospheric turbulence and pointing errors

This paper proposes an adaptive transmission modulation (ATM) technique for free‐space optical (FSO) links over gamma‐gamma turbulence channels.The ATM technique provides efficient utilization of the FSO channel capacity for improving spectral efficiency, by adapting the order of the phase‐shift keying modulation scheme, according to the channel conditions and the required bit error rate (BER). To overcome the channel degradation resulting from the turbulence effects as well as the pointing errors (PEs), single‐input multiple‐output (SIMO) system with maximal ratio combining (MRC) is proposed. Exact closed‐form expressions of BER and upper bound of the capacity are derived and verified by Monte Carlo simulations. The numerical results show that the proposed adaptive technique improves the spectral efficiency (SE) five times higher than the nonadaptive technique at the same BER threshold (10^?3).This improvement is achieved at signal‐to‐noise ratio (SNR) equals 27 and 42  dB in the case of atmospheric turbulence without and with PE, respectively. Furthermore, this SE could be obtained while the SNR = 30  dB by using ( 1 × 4 ) SIMO scheme with MRC and PE and having the same transmitting optical power.     

空间光通信中平台振动对误码率影响的抑制

在自由空间光通信(FSO)中,通信端机所处的平台在各种随机干扰力下发生振动,使正常的通信光路发生偏移,对误码率(BER)产生影响。为了减小振动引起的误码率,提出通过控制接收端光斑形状和大小来抑制误码率,并给出利用像散来获得指定光斑的设计方法和实例。结果表明,在实际通信环境中,通过选取合适的光斑参量可以使振动引起的误码率达到最小,通过调节发射端光学天线参量可以获得指定的光斑参量。对于振动不对称的情况,接收端光斑应该选择为椭圆形。     

自由空间光通信中LT码的编码技术研究

为了克服大气湍流对自由空间光通信(FSO)链路的影响,将LT码应用到FSO系统中,并结合基于部分信息LT码的修正转移鲁棒孤子分布(ISRSD),模拟了不同大气湍流强度信道,采用不同的编码方案进行模拟比较,结果表明:与RS码相比,误码率为10-4时,LT码获得大约2 dB的编码增益;相比于RSD,ISRSD能使译码开销降低1％～2％,且LT码采用ISRSD后,误码率为10-7时,系统获得0.2～0.5 dB的编码增益.     

RoFSO系统中4FSK仿真及其误比特率性能分析

将射频技术应用到自由空间光通信具有独到的优势。为了研究自由空间光载射频技术,在四进制频移键控调制解调理论基础上,以自由空间光通信为应用背景,采用仿真,从时域角度对四进制频移键控系统进行了编码及解码研究,并推导了经过高斯噪声和瑞利信道后系统误比特率及信道容量。两种解调法的系统误比特率仿真结果表明,当信噪比为12dB时,过零检测法的系统误比特率达到10-15,明显优于相干解调法。     

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.     

基于激光雷达观测数据的无线光通信系统性能研究

在分析大气衰减效应对无线光(FSO)通信性能影响 时,获取及时准确的能见度数据是关键。首先,分析了 目前获取能见度方法的不足之处,提出由激光雷达(LADAR)反演得到的数据可以更好地满足 FSO通信性能分析的 需求;其次,在介绍了利用LADAR观测数据反演水平能见度方法的基础上,以烟台1月18日典型天气为 例,获取了全天的能见度数据,并结合80%的相对湿度阈值对雾霾的转化过程进行分析 ;最后,结合 实测数据,通过设置适当的通信系统参数,对该日天气条件下的误码率(BE R)进行了实时分析,并得出雾比霾更 容易引起通信系统性能的降低。本文所得结论,将为建立实际的FSO通信链路提供 指导。     

无线光通信系统误码性能分析及方法研究

误码率(BER)是衡量无线光通信系统设计优劣的重要指标,如何正确统计误码性能显得尤为重要。利用Matlab软件,研究基于蒙特卡罗仿真实现无线光通信系统误码性能分析的方法。介绍了无线光通信系统以及利用蒙特卡罗仿真进行性能估计的原理;研究了误码性能仿真方法,详细地给出了仿真中的信源产生方法、信道模型、信噪比参数计算方法、误码率统计方法等,并给出了部分核心Matlab程序;给出了基于低密度奇偶校验(LDPC)码、脉冲位置调制(PPM)的无线光通信系统仿真图,详细介绍了各仿真参数的设置,并在不同天气和不同信噪比条件下统计了系统性能。统计结果表明,此性能分析方法准确可行。     

Development and Performance Investigation of a Single-Channel 160 Gbps Free Space Optics Transmission Link Using Higher Order Modulation Scheme

This article reports the development of a high bit rate terrestrial free space optics (FSO) transmission link. Spectral-efficient polarisation division multiplexed-quadrature phase-shift keyed signals are used to transmit high bit rate data. Coherent receiver is proposed to ameliorate the demodulator performance under environmental conditions. Also, digital signal processing techniques at the receiver are used to mitigate the adverse channel effects on the information signal. The bit error rate analysis for different environmental conditions is carried out using numerical simulations and the results demonstrate reliable 160 Gbps transmission. The impact of atmospheric scintillation due to turbulent channel conditions on the link performance is also investigated. Further, the transmission performance is compared with previous reported studies which shows that the system demonstrates better achievable range and information bit rate performance. The reported work provides a suitable reference to realize bandwidth-efficient high-capacity FSO links under dynamic weather conditions.

     

1.28 terabit/s (32x40 Gbit/s) wdm transmission system for free space optical communications

We review a novel Free Space Optical (FSO) system that represents a significant breakthrough in the area of FSO communications. The system encompasses a pair of novel terminals: these allow direct and transparent optical connection to common single mode fibers and include a dedicated electronic control unit that effectively tracks the signal beam wandering due to atmospheric turbulence and mechanical vibrations. Further improvement in the signal power stabilization is achieved by means of saturated EDFAs. These solutions allow to realize a new FSO system, which is tested in a double-pass FSO link between two buildings in Pisa, Italy. When the terminals are fed by common WDM signals they allow enough power budget and margins to support a record high capacity transmission (32×40 Gbit/s), with a enormous improvement of stability (six hours with no error burst). During day-long transmission, the system behavior has been deeply characterized to correlate any increase of bit error ratio (BER) to the FSO control parameters.     

PL approximation of DBPSK in DF-based cooperative FSO network with pointing error

In this paper, we utilize piecewise linear (PL) approximation to analyze the performance of cooperative free space optical (FSO) network employing differentially modulated binary phase shift keying (DBPSK) data with multiple decode-and-forward (DF) relays. The maximum-likelihood (ML) decoding rule at the destination is approximated by PL approximation which considers the possibility of erroneous relaying and performs very similar to the ML decoder with reduced decoding complexity. The atmospheric fading optical links are modeled by Gamma–Gamma distribution subject to both types of detection techniques, i.e., heterodyne detection and intensity modulation/direct detection (IM/DD) with pointing error. We analytically formulate the probability of error for the multiple-DF relay-based FSO network. However, the novel unified expression of average bit error rate (BER) of PL decoder with single relay and single source to destination pair is derived. Further, we also derive the asymptotic approximate BER of DF-FSO network with multiple relays at high signal-to-noise ratio (SNR) of source to relay links considering heterodyne detection with negligible pointing error. In addition, the unified closed-form expressions of outage probability with single and multiple DF relays are derived in terms of Meijer G function. The expression of outage probability is examined at high SNR in order to obtain analytical diversity order. The impact of different power distribution techniques on outage probability is determined by utilizing power distribution parameters. The derived analytical results are validated through simulation.     

1 Gbps实时传输的自由空间光通信链路损耗

远距离自由空间光学（FSO）通信系统面临的最大挑战是在大气湍流影响下信号传输会造成光强度衰减/波动,导致通信链路中断。文中提出了一种基于通信系统接收功率的对数正态统计来计算由湍流引起的通信链路损耗的方法,可评估指导FSO通信系统中的系统参数。文中模拟了不同强度湍流影响,接收终端口径为2 cm、20 cm条件下,850 nm、1 550 nm波长的光通信链路损耗与传输距离的关系。然后利用模拟分析结果设计了一个接收口径为20 cm的FSO通信系统,在强湍流条件下完成~2 km距离传输高清图像和视频。FSO通信系统的传输速率为1 Gpbs,与4 G网络相比,可以满足大量无压缩数据流传输的清晰度和实时性。     

Performance Analysis of Free Space Optical Networks Using the Beta-Average Recursive Estimator

The present paper aims to investigate and analyze the performance of a point-to-point optical network under a free-space optical (FSO) communication system. Free-space optical communication is a sophisticated technique that has been employed as a channel model of optical wireless communication to transfer large amounts of data at high speeds. However, two important issues, that could affect the source of the FSO link, have been added. The first one is related to chromatic dispersion which is attributed to the chirping phenomenon of the optical signal; the second issue concerns the secondary jamming power that can be generated by some Radio Frequencies from base stations. It is important to note that the effects of atmospheric turbulence, misalignment fading, and atmospheric attenuation as well as geometric losses were also taken into account. The purpose of this research work is to overcome these effects, in order to increase the data transmission rate from 1.25 to 50 Gbps in the proposed system. Consequently, a very efficient solution is suggested to correct these problems and to make the FSO link more reliable using the Beta-Average Recursive Estimator. In addition, the numerical results obtained are presented for the purpose of validating our proposal through the evaluation of the performance of a transmission link in terms of the bit error rate and Q-factor. In the end, the resulting conclusions are listed, explained and discussed.

     

Optical ISL tracking considerations in meeting ISDN requirements

Burst errors in an optical intersatellite link (ISL) are short term (of the order of a millisecond) degradations of the link due to antenna mistracking. These errors degrade the average bit error rate (BER), decrease coding gain, complicate calculations of tracking accuracy, and give rise to concern about the quality of the link for carrying digital signals. After reviewing previous work on these topics, the digital performance of the link is examined in terms of how these considerations affect the ability of an optical ISL to meet ISDN (Integrated Services Digital Network) criteria for an ISL. A link with on-board regeneration is assumed. Since no allocation has yet been made for an ISL, an allocation of 25 per cent of the degradation permitted for the satellite link by CCITT Rec. G.821 and CCIR Rec. 614 is assumed here. It is found (1) that if the requirement derived from Rec. 614 is met, the requirements derived from Rec. G.821 are also met and (2) that an optical ISL should have little difficulty in meeting these requirements.