Multi-transceiver optical wireless spherical structures for MANETs

Due to its high bandwidth spectrum, Free-Space- Optical (FSO) communication has the potential to bridge the capacity gap between backbone fiber links and mobile ad-hoc links, especially in the last-mile. Though FSO can solve the wireless capacity problem, it brings new challenges such as frequent disruption of wireless communication links (intermittent connectivity) and the line-of-sight (LOS) requirements. In this paper, we study a multi-transceiver spherical FSO structure as a basic building block for enabling optical spectrum in mobile ad-hoc networking. We outline optimal designs of such multi-transceiver subsystems such that coverage is maximized and crosstalk among neighboring transceivers is minimized. We propose a low-level packaging architecture capable of handling hundreds of transceivers on a single structure. We also present MANET transport performance over such multi-element mobile FSO structures in comparison to legacy RF-based MANETs.     

Free-space-optical mobile ad hoc networks: Auto-configurable building blocks

Existence of line of sight (LOS) and alignment between the communicating antennas is one of the key requirements for free-space-optical (FSO) communication. To ensure uninterrupted data flow, auto-aligning transmitter and receiver modules are necessary. We propose a new FSO node design that uses spherical surfaces covered with transmitter and receiver modules for maintaining optical links even when nodes are in relative motion. The spherical FSO node provides angular diversity in 3-dimensions, and hence provides an LOS at any orientation as long as there are no obstacles in between the communicating nodes. For proof-of-concept, we designed and tested an auto-configurable circuit, integrated with light sources and detectors placed on spherical surfaces. We demonstrated communication between a stationary and a mobile node using these initial prototypes of such FSO structures. We also performed the necessary theoretical analysis to demonstrate scalability of our FSO node designs to longer distances as well as feasibility of denser packaging of transceivers on such nodes.

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.     

Sum rate–based relay selection strategy in FSO systems

We present a relay selection strategy for bidirectional free‐space optical (FSO) communication systems. A parallel relayed FSO network consisting of multiple decode‐and‐forward protocol based two‐way relays (TWRs) is considered. Using the proposed relay selection strategy, one TWR is selected to establish an FSO communication link between two non‐line‐of‐sight FSO transceivers. As the two‐way communication process is characterized by achievable sum rate of the system, the relay selection strategy is designed such that the achievable sum rate is maximum. Considering a Gamma‐Gamma distributed optical channel with path loss and pointing errors, we present a mathematical framework for the proposed relay selection strategy.     

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...     

基于NS2的非视距紫外通信模型的建立与仿真研究

紫外光通信由于可以实现非视距通信方式,全天候全方位工作、抗干扰能力强等特点而逐步变为当前无线光网络的一个研究热点。分析了紫外光非视距通信方式的节点覆盖范围、链路模型以及传输容量;构建了紫外光非视距通信模型,包括传输模型和多收发器节点模型,添加了支持紫外非视距通信的定向静态路由协议;利用建立的模型对紫外光非视距通信的各种...     

Performance Characterization of Spatial Diversity Based Optical Wireless Communication over Atmospheric Turbulence Channels

Optical wireless communication is regarded as the next-generation high-speed technology. It has demonstrated its capability to deliver data faster than any other state-of-the-art wireless communication technique. This technology has drawn attention as a means of implementing reliable high capacity outdoor systems that cannot be implemented by conventional fiber optics. It has emerged recently as an efficient solution to match the larger bandwidth and high data rates requirement of the upcoming wireless communication systems. However, although FSO (free space optics) system has many appealing features, it has rather disappointing performance for long links due to the degrading effects of atmospheric turbulence-induced fading. In the presence of such type of performance impairments, the received signal exhibits random intensity fluctuations, which increase the BER (bit error-rate), where the severe weather conditions can have a detrimental impact on the performance, which may result in an inadequate availability. The MIMO wireless optical procedure, in which the spatial dimensions are used to improve the reliability and spectral efficiency of point-to-point links, provides a promising approach to mitigate turbulence effects due to its powerful performance enhancing capabilities.In this paper, we investigate a terrestrial atmospheric FSO communication system operating under the influence of strong atmospheric turbulences. Additionally, the MIMO technique with equal gain combining (EGC) is used in this work to enhance the data rate of the proposed system. Atmospheric turbulence impacts are modeled as a lognormal channel with due regard for geometric losses. With the use of NRZ line coding, an FSO highly sensitive receiver using either avalanche photodetector (APD) or PIN is designed and simulated for best system performance. The preference is achieved by using Bessel and Gaussian filters. It has been found that APD receiver using Gaussian filter is suitable for long-range links with APD gain value of 3. Also, the selection of APD gain is critical to the system performance. In addition, the optimal value of APD gain required for best system performance decreases as the size of the MIMO technique increases. The achievable performance improvements including received power levels, BER and Q-factor are also discussed. The results show that the system with optical amplifier at the transmitter gives an optimum performance. In addition the system performance is enhanced in most weather conditions by using an amplified 2×2 MIMO-FSO system with booster amplifier.     

Gamma-gamma大气湍流下自由空间光通信的性能

自由空间光通信在各种应用中得到了广泛关注,但大气湍流效应导致系统链路性能恶化.假设大气湍流信道是无记忆平稳遍历并且为加性高斯白噪声(AWGN),自由空间光通信系统采用开关键控(OOK)强度调制直接检测(ID/DD),研究了gamma-gamma大气湍流信道下自由空间光通信的性能.利用MeijerG函数推导出了自由空间光...     

Effect of clear atmospheric turbulence on quality of free space optical communications in Yemen

Free space optical (FSO) communication is one of the most recently developed modes of wireless communication.FSO is a technique used to convey data carried by a laser beam through the atmosphere.While FSO offers a broadband service,it requires a line of sight communication between the transmitter and receiver.The atmosphere has effects on the laser beam passing through it.For instance,the quality of data received is affected by the scattering and atmospheric turbulence.The atmospheric turbulence is caused by both temporary and special random fluctuations of the refractive index along the optical propagation path.Clear air turbulence impairs the performance of the FSO due to the fluctuation in the intensity of the laser beam.By referring to the two criteria,namely bit error rate (BER) and signal to noise ratio (SNR),this work includes analysis of the effect of atmospheric turbulence on FSO systems in Yemen by using an appropriate model.     

Impact of pointing errors and turbulence effects on POLSK and coherent OWC-based FSO system over generalized turbulence channel model

The emerging technology in wireless communication, free space optics (FSO) offers a myriad of merits over current radio frequency links due to its wide license-free bandwidth, ease of installation, high security features and viable cost for short-distance communication. Its unparallel high-speed data rate and immunity against electromagnetic interference makes FSO the emerging technology of today. Atmospheric conditions like absorption, scattering, turbulence and pointing error prevail during wireless transmission. Through this paper we aim at elucidating the effect of the error introduced by misalignment between the transmitter and receiver that are ideally required to be in the line of sight rendering pointing error. Pointing error and turbulence effects are the main limitation parameters for our analysis. For this purpose, we have taken into consideration three different modulation techniques polarization shift keying, coherent OWC and on–off keying FSO communication system. We derived the novel expressions for the average spectral efficiency (ASE) over the generalized turbulence model for these modulation techniques. The ASE is analyzed against average transmitted optical power with the maximum ASE of 50 bits/s/Hz at the transmitted power of 10 dBm for coherent OWC technique.     

Free-Space Optical Communications with Channel Shortening Filter and Viterbi Equalizer

This paper is about signal processing techniques for airborne free-space optical (FSO) wireless communications through clouds. FSO channel is known to be accompanied by multi-scattering, which causes severe intersymbol interference. Unlike traditional FSO systems relying on optical signal processing devices, this research paper investigates the feasibility of digital signal processing schemes applied to FSO. By having a channel shortening equalizer (CSE or TEQ for time equalization) at the transmitter and a Viterbi equalizer at the receiver, reliable communication at low-to-mid level optical thickness values is shown to be feasible. We also propose how existing TEQ algorithms can be modified to fit into TEQ-in-transmitter configurations. Performance is evaluated in BER, and compared with conventional equalizations.     

APD阵列探测器在自由空间光通信上的应用研究

自由空间光通信(free space optical communication,FSO)技术是通信前沿技术之一,雪崩光电二极管(avalanche photodiode,APD)阵列探测器以其高灵敏度、高带宽、低噪声的优势应用于FSO系统,成为国内外研究热点。本文分析了FSO对APD阵列探测器的性能需求,回顾了国内外近年来用于无线光通信的APD阵列的研究现状,其中重点介绍了美国林肯实验室以及海军实验室的研究成果,最后对APD阵列探测器在FSO应用的发展方向进行了预测与总结。     

Optimization of urban optical wireless communication systems

Urban optical wireless communication systems are considered a "last mile" technology. An optical wireless communication system uses the atmosphere as a propagation medium. In order to provide line-of-sight (LOS), the transceivers are placed on high-rise buildings. However, dynamic wind loads, thermal expansion, and weak earthquakes cause buildings to sway. These sways require the designer to increase the transmitter beam divergence angle so as to maintain LOS between the transmitter and the receiver. It is clear that an overly wide divergence angle increases the required laser power, and, as a result, terminal cost and complexity increase. On the other hand, an overly narrow beam divergence angle may result in cutoff in communication when there is building sway. In this paper, we derive a mathematical model to minimize transmitter power and optimize transmitter gain (divergence angle) as a function of the building-sway statistics, the communication system parameters, and the required bit-error probability (BEP). Reduction in laser power could improve overall system performances and cost. For example, for BEP of 10/sup -9/, we can attain at least a 4-dB reduction of the required transmitter power in comparison to a system with both half and twice the optimum beam divergence angle.     

Measurement‐based link scheduling for maritime mesh networks with directional antennas

The proliferation of wireless transceivers and the availability of the unlicensed band has given a boost to the deployment of wireless networks, with IEEE802.11/WiFi being the major driver in this arena. In this research, we consider a wireless mesh network designed for long‐distance communication with a typical deployment scenario of a maritime mesh network. This network uses an antenna system made up of multiple fixed‐beamwidth antennas. Compared to most other directional antenna schemes which use directional antenna for transmission and omni‐directional antenna for reception, our system uses directional antennas for both transmission and reception where a pair of transmitter–receiver antennas needs to be aligned and have an acceptable channel quality before transmission can take place. Through efficient use of directional antennas for both transmission and reception, and spatial reuse in transmission, we are able to realize a high‐capacity mesh network. In this paper, we present a practical approach to achieve contention‐free medium access, namely, a measurement‐based link‐scheduling algorithm. We evaluate the performance of the link‐scheduling algorithm using simulations and show that it is able to exploit the spatial diversity provided by the directional antennas to outperform comparable schemes for wireless mesh networks. We also briefly discuss implementation issues to demonstrate the viability of the approach. Copyright © 2010 John Wiley & Sons, Ltd.     

双向四节点自由空间光通信中继协作策略

为了扩大自由空间光（FSD,free space optical）通信网络的覆盖范围,研究了感知协作FSO通信策略。策略的物理模型为直线型放置的4个节点要通过中间2个节点的协作完成两对节点间光波信号的交换。首先讨论了联合无线网络编码（JN）和叠加编码感知接收（SCCR）的概念及其相关分析,然后对FSO通信网络模型提出了...     

Analysis of Outage Probability in Wavelength Diversity Based FSO Link Under Gamma–Gamma Fading with Varying Atmospheric Attenuation

Wireless Personal Communications - Free space optical (FSO) communication links offers an emerging alternative to RF communication links because of high speed, secure and cost-effective options to...     

Performance prediction and optimization of a coherent phasemodulated low noise analog optical link operating at microwavefrequencies

Analog optical communication links operating at microwave frequencies are useful for applications like antenna remoting, transceivers, optical signal distribution (CATV), etc. In recent years great progress has been achieved on AM optical links, however, very little has been published on the use of coherent optical links for analog applications. In this paper, we present the analysis, calculated performance, and design guidelines for a coherent phase modulated analog optical link. The performance of this link is compared to that of AM links, and a substantially improved performance is predicted     

Error rate performance of coded free-space optical links over strong turbulence channels

Error control coding can be used over free-space optical (FSO) links to mitigate turbulence-induced fading. We present error rate performance bounds for coded FSO communication systems operating over atmospheric turbulence channels, which are modeled as a correlated K distribution under strong turbulence conditions. We derive an upper bound on the pairwise error probability (PEP) and then apply the union-bound technique in conjunction with the derived PEP to obtain upper bounds on the bit error rate. Simulation results are further demonstrated to verify the analytical results.     

A survey of free space optical networks

Free Space Optical (FSO) networks, also known as optical wireless networks, have emerged as viable candidates for broadband wireless communications in the near future. The range of the potential application of FSO networks is extensive, from home to satellite. However, FSO networks have not been popularized because of insufficient availability and reliability. Researchers have focused on the problems in the physical layer in order to exploit the properties of wireless optical channels. However, recent technological developments with successful results make it practical to explore the advantages of the high bandwidth. Some researchers have begun to focus on the problems of network and upper layers in FSO networks. In this survey, we classify prospective global FSO networks into three subnetworks and give an account of them. We also present state-of-the-art research and discuss what kinds of challenges exist.