High efficiency wireless optical links in high transmission speed wireless optical communication networks

Power link budgets are prepared for wireless optical communication systems to illustrate the optical losses that happen during transmission. This paper has presented optical wireless links, which offer ultra multi gigabit per second data rates and low system complexity. For ground space and/or terrestrial communication systems, these links suffer from atmospheric loss mainly due to fog, and scintillation. Optical wireless links provide high bandwidth solution to the last mile access bottleneck. However, an appreciable availability of the link is always a concern. Wireless optical links are highly weather dependent, and fog is the major attenuating factor reducing the link availability. Link margin, received signal power, transmission bit rate, bit rate distance product, signal‐to‐noise ratio, and BER are the major interesting design parameters in the current study. Copyright © 2013 John Wiley & Sons, Ltd.     

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.     

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.

     

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.     

Fiber-Distributed Indoor High Bitrate Optical Wireless System

In this paper, we describe a wideband Indoor Optical Wireless distribution system based on an infrared communication channel using a dedicated distribution architecture. The idea is to provide narrow line-of-sight indoor free-space optical cells at very high rates through an optical fiber network. The wavelength used is 1,550 nm for eye safety and optical power budget reasons. To validate the system performance using standard On-Off Keying modulation, we calculate the power budget and simulate the overall link, showing that an implementation with commercially available components can lead to 2.5 Gbps operational optical wireless links.     

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.      

Optical Digital High-Speed Transmission: General Considerations and Experimental Results

Laboratory experiments on digital optical transmission systems at bit rates of 1 and 2 Gbits/s are described. Systems with graded-index and single-mode fibers in the optical short and long wavelength region were investigated. All systems include complete circuits for clock and signal regeneration. Special emphasis was laid on the development of electronic circuits for gigabit signal processing and on the investigations of the noise sources of the optical channel, which appear especially pronounced in broad-band systems. The experimental results confirm the possibility to set up reliable high-speed optical transmission systems under laboratory conditions with available components. The remaining problems are of optical and not of electronic nature, despite the fact that monolithic integrated circuits for gigabit applications are hardly commercially available today.     

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.     

Data Parallelization by Optical MIMO Transmission Over Multimode Fiber With Intermodal Coupling

Data parallelization by means of optical multiple-input multiple-output (MIMO) transmission over dispersive multimode fiber (MMF), with a high degree of modal coupling but not accounting for intermodal dispersion, is investigated by developing an analytical model for direct detection of MMF MIMO frequency-flat transmission with mutually incoherent sources. The MIMO channel performance is derived in terms of a new formulation of a channel matrix for modal group powers accounting, for the first time, for modal coupling. For fixed aggregate signaling rate and power budget, for uncoded bit streams, increasing the number of output detectors improves the bit error ratio (BER)-similarly to wireless MIMO. However, contrary to wireless MIMO, increasing the number of input ports actually yields a BER penalty, which is traceable to the quadratic nature of photodetection. We finally establish the feasibility of enhancing the aggregate bit rate using multiple inputs in the case that the individual single-input-single-output channels are band limited, e.g., given optical data sources each at 2 Gb/s, it is possible to attain a 12-Gb/s signaling rate over several hundreds of meters of MMF at 10^-10 BER, by utilizing six such inputs into the MIMO system, while incurring just a small average power penalty of approximately 2 dB/channel. The current model assumes strong intermodal coupling and neglects ISI influence over distances of up to hundreds of meters at gigabit rates, providing a first step in the optical MIMO analysis. On the other hand, similar scenario is practically met for shorter distances (up to 100 m) for the novel types of plastic optical fibers.     

Transmission Characteristics of 120-GHz-Band Wireless Link Using Radio-on-Fiber Technologies

The transmission characteristics of a 120-GHz-band millimeter-wave wireless link are described. The wireless link uses photonic technologies for generation, modulation, and transmission of millimeter-wave signals. This configuration enables set up of the photonic millimeter-wave generator and transmitter core separately; therefore, the wireless link can be used as a kind of radio-over-fiber system. The effects of transmitting 120-GHz-band optical subcarrier signals through single-mode fibers were theoretically and experimentally investigated. It was confirmed that the time shift of the code edges, because of chromatic dispersion, limits the transmission distance. A data stream at 10-Gbit/s was successfully transmitted over the 120-GHz-band millimeter-wave wireless link, with a bit error rate (BER) below $10^{-12}$ over a distance of 250 m. The results also demonstrated the stability of the wireless link, which satisfied the 10-Gb Ethernet standard under clear weather conditions.      

Aggregation defragmentation algorithm based on channel density and minimum gap in elastic optical networks

Free-space optics (FSO) is a data relaying technology, which requires a direct line of sight between the transmitter and the receiver units for reliable transmission. FSO communication links have many merits such as high modulation bandwidth, high data transmission rates, low cost, and easy installation process. The performance of FSO link is affected by certain external parameters such as absorption, scintillation, and atmospheric attenuation due to different weather conditions. This paper reports the designing and simulative comparison of two wavelength division multiplexing-based FSO links under rain and snow weather conditions. The proposed system reports successful transmission of \(32\times 10\) Gbps of data along a link distance of 16.5 and 1.07 km under rain and snow weather conditions, respectively, with acceptable performance levels (\(Q\sim \) 6 dB and \(\hbox {BER} \le 10^{-9}\)).     

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.     

机载激光通信链路的传输信道及性能分析

对于机载激光通信的传输信道,分析了大气信道中的衰减、光强闪烁、光束漂移及气动光学效应等复合效应,并推导了接收光束尺寸、链路中断概率和误码率等传输性能。仿真分析表明：一方面,接收光束尺寸受光束漂移及气动光学效应的影响而增大,且在低空条件下的光束尺寸主要受到大气湍流的影响而导致其大于高空条件的;另一方面,长距离的机载激光通信链路需考虑光束漂移的影响,并可通过优化发射光束大小进行抑制。     

Performance Evaluation of Short Range Underwater Optical Wireless Communications for Different Ocean Water Types

This paper has presented our interesting in wireless underwater communications for different ocean water types. Recent interest in ocean exploration has brought about a desire for developing wireless communication techniques in this challenging environment. Due to its high attenuation in water, a radio frequency (RF) carrier is not the optimum choice. Acoustic techniques have made tremendous progress in establishing wireless underwater links, but they are ultimately limited in bandwidth. In traditional communication systems, constructing a link budget is often relatively straight forward. In the case of underwater optical systems the variations in the optical properties of ocean water lead to interesting problems when considering the feasibility and reliability of underwater optical links. The main focus of this paper is to construct an underwater link budget which includes the effects of scattering and absorption of realistic ocean water. As well as we have developed the underwater optical wireless communication systems to have shorter ranges, that can provide higher bandwidth (up to several hundred Mbit/s) communications by the assistant of exciting high brightness blue LED sources, and laser diodes suggest that high speed optical links can be viable for short range application. The received signal power, signal to noise ratio, bit error rate, transmitted signal bandwidth, and transmission bit rates are the major interesting parameters for different ocean water mediums as a criteria of the best signal transmission characteristics of short wireless optical communications over wide range of the affecting parameters.     

大气对近地面无线激光通信链路的影响

由于无线光通信有通信容量大,保密性好,抗电磁干扰,结构简单等优点,近年来随着适合无线光通信应用的先进通信电子设备等技术的成熟,又掀起了对其研究的热潮,无线光通信被视为下一代光通信的发展方向之一。首先指出了无线光通信的技术优势并回顾了其发展史,阐述了大气对近地面无线激光通信系统链路的影响,重点研究了大气的衰减和大气湍流效应中的光强(大气)闪烁、光束弯曲和漂移及光束展宽,并对大气闪烁进行了仿真。最后对近地面无线光通信进行了展望。     

Performance Evaluation of a Novel Converged Architecture for Digital-Video Transmission Over Optical Wireless Channels

A robust channel coding architecture for multigigabit-per-second digital-video transmission over the optical wireless channel is introduced and evaluated. The proposed scheme combines low-density parity-check coding with channel interleaving to improve the transmission over turbulent temporally correlated optical wireless channels while satisfying real-time video delay constraints. Frame error rates of the presented code design are evaluated via simulation for intensity-modulation/direct-detection optical wireless links in both lognormal- and Rayleigh-fading channels. Results indicate that the performance of the proposed system is effective across a large range of atmospheric turbulence strengths and achieves significant temporal diversity in moderately long (10 ms) temporal correlation times while satisfying a 0.3-s real-time delay constraint. Moreover, the proposed design is shown to outperform the Reed-Solomon codes prevalent in the legacy fiber and wireless digital-television distribution systems.     

Performance of Hybrid Error Control Schemes of Satellite Channels

The effectiveness of hybrid error control schemes involving forward error correction (FEC) and automatic repeat request (ARQ) is examined for satellite channels. The principal features of the channel are: large round-trip transmission delay due to the satellite link, and burst errors introduced by the terrestrial links that connect the users to the satellite link. The performance is estimated for two channels described by Fritchman's simple partitioned finite-state Markov model, and is compared to that obtainable if the channel is considered as a binary symmetric channel of the same bit error probability. Results show that the hybrid schemes offer substantial improvement over ARQ and FEC, and that an optimum exists for the number of errors corrected to obtain maximum throughput efficiency.     

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.     

Maximizing the Transmission Performance of Adaptively Modulated Optical OFDM Signals in Multimode-Fiber Links by Optimizing Analog-to-Digital Converters

Based on a comprehensive theoretical model of a recently proposed novel technique known as adaptively modulated optical orthogonal frequency-division multiplexing (AMOOFDM), investigations are undertaken into the impact of an analog-to-digital converter involved in the AMOOFDM modem on the transmission performance of AMOOFDM signals in unamplified intensity-modulation and direct-detection (IMDD) multimode-flber (MMF)-based links. It is found that signal quantization and clipping effects are significant in determining the maximum achievable transmission performance of the AMOOFDM modem. A minimum quantization bit value of ten and optimum clipping ratio of 13 dB are identified, based on which, the transmission performance is maximized. It is shown that 40-Gb/s- over-220-m and 32-Gb/s-over-300-m IMDD-AMOOFDM signal transmission at 1550 nm with loss margins of about 15 dB is feasible in the installed worst case 62.5-mum MMF links having 3-dB effective bandwidths as small as 150 MHz ldr km. Meanwhile, excellent performance, robustness to fiber types, and variation in launch conditions and signal bit rates is observed. In addition, discussions are presented of the potential of 100-Gb/s AMOOFDM signal transmission over installed MMF links.     

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.