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.     

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.     

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.     

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.     

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.     

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.     

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.     

Transmission surveillance and self-restoration against fibre fault for time division multiplexing using passive optical network

This study proposes a practical transmission surveillance and self-protection scheme for time division multiplexing using passive optical network (TDM-PON) with centralised monitoring and self-restorable apparatus. Troubleshooting a TDM-PON involves locating and identifying the source of an optical problem in what may be a complex optical network topology that includes several optical line terminals (OLTs), optical splitters, fibres and optical network units (ONUs). Since most components in the network are passive, a large part of the issues are due to dirty/damaged/misaligned connectors or breaks/macrobends in optical fibre cables. These will affect one, some or all subscribers in the network, depending on the location of the problems. The proposed scheme is able to prevent and detect the occurrence of fibre faults in a network system through centralised monitoring and remotely operate from a central office via Ethernet connection. Even with fibre fault prevention mechanisms, failures will still occur. Therefore fibre fault detection is required in order to detect potential faults and precisely localise the exact failure location.Whenever any failure occurs on the primary entity, the proposed system can protect and switch the failure line to the protection line to ensure that traffic flows continuously. Meanwhile, the failure information will be delivered to field engineers for taking appropriate recovery action to treat the fibre fault and failure link. One suggestion in point-to-multipoint (P2PM) applications has been proposed with the experimental results as the feasibility approach. This approach has bright prospects for improving the survivability and reliability as well as increasing the efficiency and monitoring capabilities in TDM-PON.     

Effect of number of sub-carriers, cyclic prefix and analogue to digital converter parameters on coherent optical orthogonal frequency division multiplexing modem's transmission performance

The number of sub-carriers, cyclic prefix (CP), and the characteristics of the analogue to digital converter (ADC), are significant parameters influencing the transmission capacity and distance of an optical orthogonal frequency division multiplexing (OOFDM) modem. Numerical simulations of coherent OOFDM modems are undertaken to investigate the effect of the number of sub-carriers, the CP length and ADC associated parameters such as the sampling speed, clipping ratio and quantisation bit on the system performance over single mode fibre (SMF) links for data rates up to 80 Gb/s. The use of a large number of sub-carriers is more effective in combating fibre dispersion when compared with employing a long CP. Moreover in the presence of fibre nonlinearities identifying the optimum number of sub-carriers is crucial factor for the modem performance. For various signal data rates up to 40 Gb/s, a set of data rate and transmission distance-dependent optimum ADC parameters are identified. These parameters give rise to negligible clipping and quantisation noise, moreover ADC sampling speed can increase dispersion tolerance while transmitting over SMF links.     

Superresolution by use of code division multiplexing

Traditional methods for superresolution have sacrificed field of view for resolution. These methods multiplexed different parts of signals' spectrum on different carriers, and thus managed to transfer a wider range of frequency, in a manner that is similar to frequency division multiplexing in classical communication. We propose code division multiplexing for such an application, which has been shown to have superior capabilities. To enable such mutiplexing we propose a unique setup that creates an incoherent cosine transform of the image. A theoretical analysis of the setup is obtained and later compared with the empirical results.     

Chaos-based multi-user time division multiplexing communication system

A chaos-based multi-user time division multiplexing (TDM) communication system has been proposed and its benchmark performance compared to that of the chaos-based direct sequence code division multiple access (DS-CDMA) system in the noisy and Rayleigh fading channels. The benchmark performance of the systems is investigated in terms of the bit error rate (BER) under the assumption of perfect synchronisation. The chaotic spreading signals, used to encrypt the binary messages, are generated using the logistic map. The degradation in performance of the systems in the Rayleigh fading channel when compared with the noisy channel is demonstrated. Furthermore, it is shown that in both noisy and Rayleigh fading channels, the chaos-based multi-user TDM system outperforms the chaos-based DS-CDMA system for a large number of users in the system, whereas the chaos-based DS-CDMA system yields better performance for low user numbers. The overall BER performance degradation for the chaos-based DS-CDMA multi-user system in noisy and Rayleigh fading channels is characterised by the flattening of the BER curves at low levels of noise due to the prevailing effects of the interuser interferences.     

Multichannel wavelength division multiplexing with photonic crystals

A multichannel wavelength-division-multiplexing system consisting of a two-dimensional photonic crystal is proposed. The system consists of two parts, a waveguiding element, realized by defects in a photonic crystal, and frequency-selective elements, realized by photonic crystal microcavities. Simulations, performed with a two-dimensional finite-difference time-domain technique with a perfectly matched layer absorbing boundary condition, showed the ability to filter an incident pulse into six spectral channels with a FWHM of 2 nm.     

Analytical performance of orthogonal frequency division multiplexing systems impaired by a non-linear high-power amplifier with memory

The performance of modern communication transmitters utilising multi-carrier modulation techniques is highly sensitive to non-linear distortions arising mainly from the high-power amplifier (HPA). In addition, the wideband characteristics of multi-carrier signals result in frequency-dependent distortions, typically known as memory effects. This study aims to analytically evaluate the impact of the distortion induced by non-linear HPA with memory on a multi-carrier signal exemplified by orthogonal frequency division multiplexing (OFDM). The behavioural model of HPA considered for theoretical analysis is a memory polynomial model, which is a truncated form of the Volterra series. In the theoretical framework developed, this study shows that memory in HPA considerably degrades the performance of OFDM in terms of symbol error rate (SER), and the distortion itself can be canonically characterised by a complex attenuation component and a non-linear noise component. Closed-form expressions for SER in additive white Gaussian noise channel are derived and the SER for fading channel is approximated by the adaptive Gauss-Kronrod quadrature method. Simulation results are shown for a realistic HPA, based on the Wiener-Hammerstein model, and compared with the analytical results to validate the proposed analysis.