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.     

Nonsingular Constant Modulus Equalizer for PDM-QPSK Coherent Optical Receivers

Adaptive electronic equalizers using the constant modulus algorithm (CMA) algorithm often converge to a singular coefficient matrix that produces the same signal at multiple outputs. We address this issue in the context of optical communications systems with polarization-division multiplexing and coherent receivers. We study, by computer simulation, the performance of multiuser CMA equalizer, an enhanced CMA equalizer initially proposed for use in wireless multiuser and later multiple-input/multiple-output communications systems. We show that the proposed adaptive electronic equalizer does not exhibit singularities and, therefore, is superior to the commonly used CMA equalizer.     

Multirate signal processing techniques for high-speed communication over power lines

With the widespread use of Internet and multimedia applications, there is renewed interest in utilizing the electric power lines for transmission of high data rate communication signals. Power lines offer a convenient and pervasive communication medium that does not require new cable installation. However, power lines are classified as a noisy and unpredictable communication channel. Advanced signal processing techniques, such as wavelet filter banks and multirate systems, can be utilized to improve the quality of communication over power lines. In this article an overview of the diverse applications of wavelet filter banks in power line communications is presented with particular reference to wavelet-based multicarrier modulation techniques. Moreover, some suggestions are given regarding the future applications of multirate signal processing techniques in PLC.     

新型的光纤无源器件——光纤法布里-珀罗调谐滤波器

本文介绍了国外可调谐光纤法布里-珀罗(FFP)滤波器不同类型的结构、制作、主要参数及其用途。每一种滤波器都可覆盖一个独立的由谐振腔长度所决定的自由光谱区。滤波器采用压电元件调谐,在信号处理、传感器、波分复用(WDM)系统、相干光通信系统、特别是近几年才发展起来的光频复用(OFDM)系统中将得到广泛的应用。     

光纤通信网络在高速数据采集系统中的应用

基于光纤通信网络对一种高速数据采集系统进行了设计。通过多路数据采集方式,并与光纤通信网络相结合,使得高速数据采集能力得到大幅提高。该系统主要由处理系统、采集模块及光纤网络共同构成,光纤通信网络前端采集模块在模拟滤波、采样完成后,通过处理器实现信号的传输。此外,通过光数据采集系统实验,证明了在高速数据采集系统中应用光纤通信网络能实现速率在10 Gbit·s-1以下的光数据接入采集。     

Optical wavelength converters

Devices and techniques for shifting the frequency/wavelength of an optical carrier wave are reviewed, with particular reference to applications in wavelength/frequency division multiplexed optical communication systems. Considered first are devices which make use of semiconductor laser amplifiers or oscillators. Some of these techniques come close to satisfying the requirements for a suitable wavelength shifter, though none appear to offer the potential for a transparent network independent of data modulation format. A second main category, using nonlinear optical materials, promises to satisfy the requirements for frequency translation in a network which is fully transparent to the modulation format. However, the conversion efficiencies currently available appear too low for cost-effective communication systems engineering so that further advances in nonlinear optical materials/techniques are still required. It seems likely that guided wave techniques will provide the best means of improving the efficiency by increasing the interaction length and the optical intensity for a given power. Schematic examples of such a converter are discussed. All of the existing techniques have some deficiencies compared to an ideal system so that wavelength conversion for optical communication systems is still a problem in search of a good solution     

雷达通信一体化的六种主要技术体制

雷达通信等信息系统在信号产生、传输与处理等方面呈现同构化特性,其一体化已成为发展趋势。文中梳理了大量雷达通信一体化领域的国内外文献,分析了雷达通信一体化的必要性与可行性,总结归纳了实现雷达通信一体化的六种主要的技术体制。提出了新一代集分布式、网络化、一体化三大特性于一体的网络雷达对抗系统的雷达通信一体化的性能要求,陈述了对新一代电子信息系统的雷达通信一体化研究的迫切性。     

Handoff with DSP Support: Enabling Seamless Voice Communications across Heterogeneous Telephony Systems on Dual-Mode Mobile Devices

In this paper we investigate the problem of voice communications across heterogeneous telephony systems on dual-mode (WiFi and GSM) mobile devices. Since GSM is a circuit-switched telephony system, existing solutions that are based on packet-switched network protocols cannot be used. We show in this paper that an enabling technology for seamless voice communications across circuit-switched and packet-switched telephony systems is the support of digital signal processing (DSP) techniques during handoffs. To substantiate our argument, we start with a framework based on the Session Initiation Protocol (SIP) for vertical handoffs on dual-mode mobile devices. We then identify the key obstacle in achieving seamless handoffs across circuit-switched and packet-switched systems, and explain why DSP support is necessary in this context. We propose a solution that incorporates time alignment and time scaling algorithms during handoffs for supporting seamless voice communications across heterogeneous telephony systems. We conduct testbed experiments using a GSM-WiFi dual-mode notebook and evaluate the quality of speech when the call is migrated from WiFi to GSM networks. Evaluation results show that such a cross-disciplinary solution involving signal processing and networking can effectively support seamless voice communications across heterogeneous telephony systems.     

1550-nm volume holography for optical communication devices

The two-lambda method can provide a strategic approach to implement all-optical devices for communication wavelength division multiplexing (WDM) signal processing based on volume holography. By writing holograms at 488 nm in LiNbO/sub 3/:Fe and reading them in the third window of optical communication systems (1550 nm), the feasibility of WDM demultiplexers and holographic memories for digital bytes is here demonstrated.     

A nonlinear electrical equalizer with decision feedback for OOK optical communication systems

In this paper we introduce a nonlinear equalizer using the Radial Basis Function (RBF) network with decision feedback equalizer (DFE) for electronic dispersion compensation in optical communication systems with on-off-keying and a direct detection receiver. The RBF method introduces a non-linear equalization technique suitable for optical communication direct detection systems that include nonlinear transformation at the photodetector. A bit error rate performance comparison shows that the RBF with DFE out performs the RBF without DFE and achieves similar results provided by maximum likelihood sequence estimator.     

Performance of smart lightwave receivers with linear equalization

Signal processing techniques can be used to reduce linear and nonlinear distortion in high-speed lightwave systems caused by fiber dispersion and nonideal responses of optoelectronic and electronic components. The improvement in the performance of 2.5 and 10 Gb/s intensity modulation, direct detection systems is assessed for receivers which utilize an analog taped delay line equalizer to compensate for signal distortion. Synchronous and fractionally spaced equalizers are evaluated. Smart receivers that jointly optimize the decision time, decision threshold, and equalizer tap weights under a minimum bit error ration criterion are considered. This yields the optimum system performance and allows consideration of both reduced distortion and enhanced noise arising from the signal processing. The effectiveness of the equalization is determined as a function of several important system parameters. Three-tap and five-tap synchronous equalizers yield virtually the same improvement in receiver sensitivity. Depending on the system, a five-tap fractionally spaced equalizer with half-bit-period tap spacing may or may not be significantly more effective than a three-tap synchronous equalizer     

Solid state optical space switches for network cross-connect andprotection applications

Network protection and reconfiguration is becoming increasingly important in fiber optic communications systems. This is driven by the intense traffic and high cost of lost high-data-rate optical connections. Optical cross-connects at the nodes in transmission systems are developing rapidly in response. A key optical component required for these applications is an optical space switch. Since the required timescale of network reconfiguration at the optical level is on the order of 50 ms to prevent electrical intervention. The optical space switching speed must be approximately 5 ms or faster. The demand created by these applications has motivated the development of a solid state optical space switch based on a novel planar-waveguide technology. This planar integrated optics technology relies on the thermo-optic effect in specialized optical polymer materials and results in reliable optical space switches without moving parts to wear out. This article reviews the state of the art in solid state optical space switches based on thermo-optic polymers and applications of these switches in network communication systems. The same polymer-based planar waveguide technology used to make the solid state optical space switches of today provides the basis for WDM devices. Electro-optic modulators, and devices integrating several functions (space switching, wavelength multiplexing, light generation and detection) in one component in the years to come     

Optical networks research at UCL

New dynamic optical switching technologies and adaptive transmission techniques will allow increased capacity and flexibility in future core and metro networks. The Optical Networks Group at UCL has carried out research focused on these areas, in collaboration with industry and other university research groups, for over a decade. In this paper, the technical challenges to be overcome in implementing dynamic, high-capacity networks are discussed, and recent results of the group’s work are presented. New network architectures based on wavelength-routing and optical burst switching have been proposed and analysed, and the experimental demonstration of burst switching using fast wavelength-tunable lasers is described. High-capacity dense wavelength division multiplexed transmission systems, using advanced regeneration and equalisation techniques, have been studied, both theoretically, and experimentally using a dynamically reconfigurable recirculating fibre loop. Research results on advanced optical signal formats, all-optical 3R regeneration at 40 Gbit/s channel rates, adaptive analogue and digital electronic processing techniques and optical finite impulse response equalisation are presented. Finally, the development of optical performance monitoring technology, a key component of future all-optical networks, is described. The paper concludes with a prediction of future trends in optical communications research.     

Send a hologram [photorefractive materials]

As computers and the Internet become faster and faster, more and more information is transmitted, received, processed, and stored everyday. The demand for high-speed, large-capacity information systems is pushing scientists and engineers to explore all possible approaches, including electrical and optical means. Photorefractive materials and devices are becoming viable alternatives for information systems. Photorefractive materials, including traditional electrooptic photorefractive crystals as well as photopolymers and photosensitive glasses, have demonstrated their potential in information systems. In this article we describe several applications of various photorefractive materials in information storage, processing, and communication systems. Specifically, we briefly discuss the applications of the traditional electrooptic photorefractive crystals and photopolymers in volume holographic data storage (VHDS) and information processing. Then, we discuss our recent works on the applications of photopolymers, holographic polymer dispersed liquid crystals (H-PDLC), and photosensitive glasses in photonic devices for optical fiber communications.     

Review of saw devices and their signal processing applications in space communications

An increasing number of advanced signal processing techniques are being investigated in satellite communications systems as the ground and space segments require more and more sophisticated, light-weight, low-power-consumption and cost-effective equipment. RF signal processing techniques based on surface acoustic wave (SAW) devices can be suitably exploited in these systems, constituting a valid alternative to digital baseband processing. This paper aims at presenting the most recent applications of SAW devices to RF signal processing in satellite communication systems. They span the frequency range from about 10 MHz to 10 GHz. Interaction of SAWs with laser beams is also illustrated.     

Chip-scale optical interconnects and optical data processing using silicon photonic devices

Recent advances in the density and complexity of photonic integrated circuits have facilitated possible implementation of chip-scale optical communication systems. Chip-scale optical interconnects and optical data processing are two important functions to transmit and process signal in the optical domain. Silicon photonics offers a promising platform to enable chip-scale optical interconnects and optical data processing using silicon photonic devices. In this paper, we review our recent progress in the design, modeling, and fabrication of silicon photonic devices and their applications in chip-scale optical interconnects and optical data processing with advanced modulation formats. For chip-scale optical interconnects, we experimentally demonstrate digital signal transmissions in silicon microring and silicon vertical slot waveguide. Terabit chip-scale optical interconnect is demonstrated in the experiment. Also, we experimentally demonstrate analog signal transmissions in silicon microring and silicon photonic crystal nanocavity. For chip-scale optical data processing, we experimentally demonstrate all-optical wavelength conversion using a silicon waveguide, simultaneous polarization and wavelength demultiplexing using 2D grating coupler connected with microrings, two-mode (de)multiplexing using a tapered asymmetrical grating-assisted contra-directional coupler, and two-/three-mode (de)multiplexing using asymmetrical directional converter. In addition, we propose and simulate chip-scale optical data exchange, chip-scale high-base optical computing, and chip-scale optical coding/decoding by using nonlinear interactions in a silicon-organic hybrid slot waveguide. The obtained theoretical and experimental results of chip-scale optical interconnects and optical data processing indicate possible integration of optical communication functions on a monolithic chip.     

Design of a transversal equalizer for electronic dispersion compensation in optical communication links

A programmable transversal equalizer for electronic dispersion compensation(EDC) in optical fiber communication systems is developed.Based on the SiGe technology with a cut-off frequency of 80 GHz,the equalizer consists of 6 seriesparallel amplifiers as delay units and 7 Gilbert variable gain amplifiers as taps,which ensure that the equalizer can work at the bit rate of 10 Gb/s.With different tap gains,the forward voltage gain of the transversal equalizer varies,which demonstrates that the equalizer has various filtering characteristics such as low pass filtering,band pass filtering,band reject filtering,and notch filtering,so it can effectively simulate the inverse transfer function of dispersive channels in optical communications,and can be used for compensating the inter-symbol interference and other nonlinear problems caused by dispersion.The equalizer(including pads) occupies an area of 0.40 mm × 1.08 mm,and its total power dissipation is 400 mW with 3.3 V power supply.     

Digital Equalization of Chromatic Dispersion and Polarization Mode Dispersion

In this paper, we consider a fractionally spaced equalizer (FSE) for electronic compensation of chromatic dispersion (CD) and polarization-mode dispersion (PMD) in a dually polarized (polarization-multiplexed) coherent optical communications system. Our results show that the FSE can compensate any arbitrary amount of CD and first-order PMD distortion, provided that the oversampling rate is at least 3/2 and that a sufficient number of equalizer taps are used. In contrast, the amount of CD and PMD that can be corrected by a symbol-rate equalizer only approaches an asymptotic limit, and increasing the number of taps has no effect on performance due to aliasing that causes signal cancellation and noise enhancement.     

The Birth of Lightwave Technology and Its Implications to Microwaves

Ever since the invention of lasers in the late 1950's, the use of coherent optical radiation for communication and signal processing with multigigahertz bandwidth has been a major research goal in electron devices, microwaves, quantum electronics, and optics. In order to realize that goal, the key issues that need to be addressed include a) how to transmit optical radiation with low propagation loss and signal distortion, b) how to effectively interface optical devices with electronic devices, c) how to modulate, multiplex, switch, and detect optical radiation at such high data rates, and d) how to solve the materials and fabrication technology problems. In the early 1960's, research on optical communications was concerned primarily with transmission of laser radiation through the atmosphere and the pipes.