Impact of second-order PMD in a coherent optical orthogonal frequency division multiplexing system

The second-order, polarization-mode dispersion (PMD) effect for coherent optical orthogonal frequency division multiplexing (CO-OFDM) system in long-haul transmission with single-mode fiber was analyzed in detail. The closed-form expression is presented for the time impulse response of an OFDM signal affected by second-order PMD. Analysis and simulation results show that, compared with polarization-dependent chromatic dispersion (PCD), CO-OFDM system performance is mainly influenced by depolarization rate (DR) which can moderate phase noise induced by chromatic dispersion. For a fiber link of 800 km and DR of 30 ps, the system Q value is improved by about 1.3 dB over the case of no DR.     

Genetic algorithm quality of service design in resilient dense wavelength division multiplexing optical networks

The important role of quality of service (QoS) in deployment of a resilient dense wavelength division multiplexing (DWDM) backbone for global networks requires critical design-phase planning optimisation. The design issues of resilient DWDM networks for bandwidth and delay sensitive applications of dedicated path protection are addressed. A genetic algorithm (GA) model has been developed to solve the routing and wavelength assignment problem using binary variable-length chromosome encoding under two different schemes of bandwidth optimisation (BOS) and delay optimisation (DOS).The performance of the new GA-based resiliency model has been evaluated for four benchmark networks: PAN EUROPEAN, COST239, NSFNETand ARPA2. Simulation results show a superior capability and efficiency for the model to solve this complex, multi-constraint and nondeterministic polynomial-hard problem for BOS and DOS. The nonlinear nature of this process reveals a significant sensitivity for optical layer network topology on the optimum-design QoS. The results also demonstrate that the PAN EUROPEAN network shows the highest flexibility for primary path design, NSFNET for the secondary path and ARPA2 comes with the lowest design flexibility for both primary and secondary paths.     

Hybrid detectors for wireless orthogonal frequency division multiplexing with code division multiplexing systems based on reliability information

Orthogonal frequency division multiplexing with code division multiplexing (OFDM-CDM) is attractive for the next generation high-speed wireless systems due to the fact that the performance of OFDM-CDM systems can be considerably improved by employing a joint detection scheme such as the maximum likelihood (ML) detector. However, the complexity of the ML detector increases rapidly as the number of orthogonal spreading codes and/or the number of bits per modulation symbol increase. In this study, the authors introduce a unified detection model and propose two hybrid detectors, which combine zero forcing (ZF) with successive interference cancellation (SIC) and sphere detection (SD) algorithms, respectively. After obtaining the initial solution from the front-end ZF receiver, the proposed back-end algorithms are adopted to extend the potential solution list and search for the final result. The objective is to utilise the combination of a simplified linear equaliser and a comprehensive detection scheme to achieve enhanced performance and offer alternatives to the more complex and channel-estimation-sensitive minimum mean squared error (MMSE) scheme. The results show that the proposed hybrid detectors are able to achieve superior performance compared to the MMSE scheme and provides a significant performance improvement compared to the conventional OFDM counterpart.     

基于正交频分复用的水下转发器的设计

为解决长距离高速水声数据传输问题,采用接力式水声数据转发传输是一个很好的解决方案。结合正交频分复用技术的特点设计实现了一个数字式的水下转发器系统。系统功耗低、结构简单,可以很好地解决水声通信中存在的多途干扰和窄带宽问题,实现高速、可靠的水下数据转发的目的。湖试中,在6000m的距离内,3kHz带宽下,转发速率为4.32kb/s,误码率低于10-3。湖试结果表明,基于正交频分复用的水下转发器可以达到较高的转发速率和较低的误码率。     

Comparison between time shifted non-orthogonal frequency division multiplexing and OFDM

通过将时移参数引入多载波调制,形成了一种具有全新思想的时移非正交频分复用(TS-NFDM)调制技术,并通过理论分析和仿真实验将该技术的性能与当前在无线通信中扮演重要角色的正交频分复用(OFDM)技术进行了比较.比较结果表明,TS-NFDM技术具有明显的优势,其时移特性使其表现出许多优良的性能,其中最突出的是频带利用率高,约为OFDM的4.29倍,另外,信号功率的峰值均值比也明显低于OFDM.     

Efficient direct position determination of orthogonal frequency division multiplexing signals

The authors present an efficient method for positioning of orthogonal frequency division multiplexing (OFDM) signals. The method, called direct position determination (DPD), enables one to determine the emitter position in a single search operation. In formerly published articles on DPD, the signal was assumed to be a narrowband Gaussian source, which was subjected to a rather simple flat-fading channel model. Here, the authors extend the DPD algorithm to handle the OFDM signals that propagate through a frequency-selective channel, and derive the least squares estimator (LSE) for the problem. In addition, the authors present a method for a fine 2D position estimation, which enables one to reduce the DPD computational complexity without compromising on its estimation accuracy.     

Wavelength division multiplexing addressed four-element fiber optical laser hydrophone array

A four-element hydrophone array system using a fiber Bragg grating laser system is described. The demodulation technique for the array combines a 3x3 coupler-based interferometer and three wavelength division multiplexers (WDMs). It is believed to be a new passive approach to use a WDM and a unique arithmetic method. The operational frequency limit depends only on the frequency of its electronic portion. The designed system has achieved a minimum of approximately 7.2x10(-7) pm/ radicalHz detectable wavelength perturbation for frequencies that are greater than 200 Hz.     

载波加权正交频分复用系统的同步性能分析

针对正交频分复用系统盲同步方法中载波加权的选择具有一定盲目性的问题,通过分析载波加权分别与系统误码率和载频定时估计误差之间的关系,推导出了载频定时估计的罗-克拉美界,然后通过权衡同步精确性和系统误码率之间的关系确定最优的载波加权.理论分析表明,在不同载波加权下载频定时估计误差和系统误码率存在一定程度的交换关系,由此通过权衡系统误码率和载频定时估计误差之间的关系可确定最优的载波加权.基于802.11a系统的仿真实验进一步表明,不同载波加权下的载频同步误差和系统误码率都能与各自的理论分析结果很好地吻合,最优的载波加权也使载频同步误差和系统误码率接近于理论分析的最优值.     

Dispersion-optimized optical fiber for high-speed long-haul dense wavelength division multiplexing transmission

Four non-zero-dispersion-shifted fibers with almost the same large effective area (A(eff)) and optimized dispersion properties are realized by novel index profile designing and modified vapor axial deposition and modified chemical vapor deposition processes. An A(eff) of greater than 71 μm(2) is obtained for the designed fibers. Three of the developed fibers with positive dispersion are improved by reducing the 1550 nm dispersion slope from 0.072 ps/nm(2)/km to 0.063 ps/nm(2)/km or 0.05 ps/nm(2)/km, increasing the 1550 nm dispersion from 4.972 ps/nm/km to 5.679 ps/nm/km or 7.776 ps/nm/km, and shifting the zero-dispersion wavelength from 1500 nm to 1450 nm. One of these fibers is in good agreement with G655D and G.656 fibers simultaneously, and another one with G655E and G.656 fibers; both fibers are beneficial to high-bit long-haul dense wavelength division multiplexing systems over S-, C-, and L-bands. The fourth developed fiber with negative dispersion is also improved by reducing the 1550 nm dispersion slope from 0.12 ps/nm(2)/km to 0.085 ps/nm(2)/km, increasing the 1550 nm dispersion from -4 ps/nm/km to -6.016 ps/nm/km, providing facilities for a submarine transmission system. Experimental measurements indicate that the developed fibers all have excellent optical transmission and good macrobending and splice performances.     

A transmission length limit for space division multiplexing in step-index silica optical fibres

ABSTRACT

By solving the power flow equation, we investigate the influence of mode coupling on space division multiplexing capability of three multimode step-index silica optical fibres with a different strengths of mode coupling. Results show that mode coupling significantly limits the length of these fibres at which the space division multiplexing can be realized with a minimal crosstalk between the neighbour optical channels. This is most pronounced in silica optical fibres with the strongest mode coupling. The two and three spatially multiplexed channels in the investigated step-index silica optical fibres can be employed with a minimal crosstalk up to the fibre lengths of few hundred of meters and few tens of meters, respectively. These lengths are much shorter than kilometer lengths at which these fibres are usually employed without space division multiplexing. Such characterization of optical fibres should be considered in designing an optical fibre transmission system for space division multiplexing.     

One-dimensional to two-dimensional channel formatting with micro-optics for wavelength division multiplexing networks

We present a method of interfacing a conventional wavelength multiplexing and demultiplexing device with a two-dimensional (2-D) array of surface-active elements, such as micro-electromechanical system devices or photodetectors. We use diffractive optical elements to transform wavelength division multiplexed signals spread along a line into a 2-D array. We applied this technique to build a device that is compatible with an arrayed-waveguide grating. The theoretical insertion loss predicted by combining beam propagation and rigorous coupled-wave analysis is 2.75 dB. However, the experimental loss is 10 dB. The difference is due to the poor quality of the diffraction gratings. Nevertheless, the formatting operation was performed successfully.     

Precoded orthogonal frequency division multiplexing with hidden pilots for ultrawideband wireless communications

The authors propose a new hidden pilot scheme equipped with precoding and its application to orthogonal frequency division multiplexing-ultra-wideband (OFDM-UWB) systems. The proposed scheme can be thought of as an improvement over conventional hidden pilot schemes. By carefully designing precoder and hidden pilot from the view point of frequency diversity, channel estimation and the peak-to-average power ratio (PAPR), more frequency diversity gain and reduced PAPR can be achieved. In addition, the authors can support more pilots to estimate a channel providing mitigated self-interference between data symbol and hidden pilot with almost no loss of bandwidth efficiency in OFDM-based UWB communication systems. The authors show improved performance of the proposed scheme over the multiband OFDM scheme through simulations in a realistic UWB channel environment.     

Enhanced brain image retrieval using carrier frequency offset compensated orthogonal frequency division multiplexing for telemedicine applications

Wireless technology plays a vital role in the Electronic health (e‐health) applications such as telemedicine and remote patient monitoring. One of the major challenges of telemedicine applications is ensuring adequate quality of service and realizing precise diagnosis. Telemedicine employs image fusion to enhance the quality of medical image for better diagnosis. However, the transmission of medical image needs high data rate and bandwidth. Due to the robustness to multipath fading, high data rate transmission capability, high flexibility, high bandwidth efficiency, and easy implementation using fast Fourier transform, an orthogonal frequency division multiplexing (OFDM) is the suitable technique for medical image transmission in telemedicine applications This paper is focused on improving the quality of the fused brain image transmission over OFDM based telemedicine service using comb type pilot based carrier frequency offset compensation. The simulation results show that the proposed method offers better performance in terms of mean squared error, Euclidean distance, peak signal to noise ratio, bit error rate, carrier to interference ratio, and computational complexity.     

Low complexity blind estimation of residual carrier offset in orthogonal frequency division multiplexing based

The presence of residual carrier frequency offset (CFO) in an orthogonal frequency division multiplexing (OFDM) system leads to a loss of orthogonality between the subcarriers. This introduces inter-subcarrier interference and degrades the system performance significantly. In the literature, Liu and Tureli proposed a blind CFO estimation method based on the observation that in a typical OFDM system not all the subcarriers are used for data transmission. However, the computational complexity of such a method is very high. Based on practical considerations, we propose an approximate closed-form solution for the blind estimation of CFO that is easily implementable at a very low cost. We also propose a successive CFO estimation and compensation procedure, which reduces the performance degradation of the proposed algorithm as compared with the method of Liu and Tureli when relatively large CFO values are assumed. In addition, a decision-directed extension of the successive algorithm, which further improves the CFO estimation at a slightly higher complexity, is also given.     

An arbitrary mode converter with high precision for mode division multiplexing in optical fibers

In this paper, a high-precision arbitrary-mode converter is proposed for mode-division multiplexing optical fiber communication systems. As the kernel of this converter, a phase-only spatial light modulation based on simulated annealing algorithm is used as a spatial spectrum filter. The results demonstrate that our proposed mode converter is of high precision and is capable of converting different modes without any physical change.     

Wavelength-division multiplexing free-space optical interconnect networks for massively parallel processing systems

Wavelength-division multiplexing (WDM) techniques provide many advantages for building optical interconnect networks for massively parallel processing (MPP) systems. A design for a 1024-channel network for MPP systems based on the interconnection-cached network with vertical-cavity surface-emitting laser (VCSEL) arrays with one wavelength is described. We then show how a WDM version with four different wavelengths can increase the channel density. We also show how a WDM system can reduce the fan-in loss by a factor of 4. All the VCSEL's in each array are of the same wavelength, while different arrays use different wavelengths. We describe our experimental WDM subsystem containing four VCSEL arrays, operating at wavelengths of 843, 950, 970, and 980 nm, and three different WDM filters for multiplexing-demultiplexing. We present the operational results of the subsystem at 1 Gbit/s per channel.     

Monochromatic heterodyne fiber-optic profile sensor for spatially resolved velocity measurements with frequency division multiplexing

Investigating shear flows is important in technical applications as well as in fundamental research. Velocity measurements with high spatial resolution are necessary. Laser Doppler anemometry allows nonintrusive precise measurements, but the spatial resolution is limited by the size of the measurement volume to approximately 50 microm. A new laser Doppler profile sensor is proposed, enabling determination of the velocity profile inside the measurement volume. Two fringe systems with contrary fringe spacing gradients are generated to determine the position as well as the velocity of passing tracer particles. Physically discriminating between the two measuring channels is done by a frequency-division-multiplexing technique with acousto-optic modulators. A frequency-doubled Nd:YAG laser and a fiber-optic measuring head were employed, resulting in a portable and flexible sensor. In the center of the measurement volume of approximately 1-mm length, a spatial resolution of approximately 5 microm was obtained. Spatially resolved measurements of the Blasius velocity profile are presented. Small velocities as low as 3 cm/s are measured. The sensor is applied in a wind tunnel to determine the wall shear stress of a boundary layer flow. All measurement results show good agreement with the theoretical prediction.     

Effect of stress on performance of dense wavelength division multiplexing filters: optical properties

Thin-film filters used for dense wavelength division multiplexing (DWDM) applications are processed by a variety of deposition techniques, including ion-beam sputtering. Ion-beam sputtering produces high-quality coatings and provides flexibility of coating materials. However, DWDM filters consisting of oxide films that are reactively deposited by ion-beam sputtering, as in most sputter techniques, typically exhibit high levels of compressive stress. This affects the optical characteristics of the filters. Details of the filter passband characteristics and wave-front distortion illustrate the influence of the stress. Spatial variation of the stress on the filter surface causes the filter center wavelength to have spatial variation, and it causes the filter to have an asymmetric passband characteristic.     

Geometric superresolution by code division multiplexing

In many highly resolved optical systems the resolution is limited not by the optics but by the CCD's nonzero pixel size. As a result, overall resolution is decreased. Here we propose a novel approach to enhancing resolution beyond the limit set by the CCD's pixels. This method does not involve additional mechanical elements, such as those used for microscans. In this scheme neither the CCD nor additional elements are moved. The geometric superresolving procedure is based on code-division multiplexing, with all its inherent benefits, such as relative noise immunity to single-tone interference. A setup is proposed for coherent and incoherent illumination, with slight modifications for the latter. A theoretical analysis of the setup is presented and compared with empirical results. This scheme is shown to enhance one-dimensional image resolution with the use of only a simple mask that doubles image resolution. This method can easily be expanded to two-dimensional images and to resolution-enhancement factors greater than 2.     

基于短时傅里叶变换的水声OFDM时间同步方法

正交频分复用(Orthogonal Frequency Division Multiplexing,OFDM)技术具有抗多径、频带利用充分、传输数据率高的优点,在水声通信中具有重要的发展前景。由于多普勒频移和多径时延的存在,需对OFDM信号进行时间同步。同步不准会引起OFDM水声通信中符号间干扰和子载波间干扰,提出了一种基于叠加单频序列的短时傅里叶变换(STFT)时间同步方法,该方案首先提取叠加在OFDM符号上的单频信号,对该信号进行短时傅里叶变换,随后对其时频幅度谱进行分析,利用平坦区域确定同步时刻。仿真和试验结果表明,该方案有效可行。