Effectiveness of the Use of 3-dB Bandwidths of Multimode Fibres for Estimating the Transmission Performance of Adaptively Modulated Optical OFDM Signals Over IMDD Links

Detailed investigations of transmission capacities of adaptively modulated optical orthogonal frequency division multiplexed (AMOOFDM) signals against various multimode fibre (MMF) properties, are undertaken for the first time, based on 28000 statistically constructed impulse responses of optical amplification-free, MMF links using intensity modulation and direct detection. The utilization of 3-dB optical MMF bandwidths is identified to be a simple and accurate approach for estimating, without performing complicated simulations, AMOOFDM transmission capacities over any MMF links. For a link loss- (differential mode delay)- dominant performance region, the estimated signal capacities have very low relative standard deviations of $≪$ 6% ($≪$ 17%), which are independent of AMOOFDM modem parameters. Discussions are also made of practical implementation of the identified approach.      

The Influence of Directly Modulated DFB Lasers on the Transmission Performance of Carrier-Suppressed Single-Sideband Optical OFDM Signals Over IMDD SMF Systems

The transmission performance of carrier-suppressed, single-sideband (SSB), optical orthogonal frequency division multiplexing (OOFDM) signals is investigated numerically in intensity-modulation and direct-detection (IMDD) single-mode fiber (SMF) systems using directly modulated DFB lasers (DMLs), with special attention being given to the impact of DMLs on the system performance. The dependence of the optimum carrier suppression ratio is identified on DML operating condition and optical input power. It is shown that, for DML-based optical amplification-free metropolitan systems, 30 Gb/s over 80-km SMF transmission is achievable, which doubles the performance obtained by using double-sideband (DSB) OOFDM signals without carrier suppression, and that 10 Gb/s over 1200-km SMF transmission is also feasible for DML-based long-haul systems consisting of optical amplifiers. The results indicate that the SSB technique with an optimum carrier suppression ratio reduces considerably the DML-induced nonlinear effect, and leads to decreased susceptibility of the aforementioned transmission performance to variations in DML operating condition and optical input power.      

数字信道化技术中ADC的性能分析

在卫星通信系统中,数字信道化技术的应用越来越广泛,而模数转换器（ADC）作为数字信道化器前端不可缺少的一部分起着至关重要的作用。基于ADC基本原理和性能,仿真分析了在ADC量化位数不同的条件下,ADC对数字已调信号解调性能的影响。     

OFDM光网络中软件定义的信号传输性能优化

针对正交频分复用(OFDM)光网络,提出两项OFDM传输性能优化技术,分别可以抑制OFDM信号的旁瓣分量和降低峰均功率比(PAPR)。通过在每个OFDM符号前插入一段软件定义的旁瓣抑制码(SSC),OFDM信号旁瓣可以得到显著抑制,从而有效降低密集波分复用-正交频分复用(DWDM-OFDM)系统中的相邻载波干扰。此外,利用软件定义的方式更改保护频带(FGB),以降低OFDM信号的PAPR,改善信号传输性能。通过实验对两项OFDM传输性能优化技术进行了验证。     

ADC最佳方案的选择及其发展趋势

引言高速模数转换器(ADC)的性能特性对整个信号处理链路的设计影响巨大。系统设计师在考虑ADC对基带影响的同时,还必须考虑其对射频(RF)及数字电路系统的影响。充分了解高速ADC领域的最新进展对元器件选择至关重要。市场对高性能ADC有着强大的需求。虽然通信、成像、汽车和仪表市场用户群之间差异很大,但是低功耗已经成为用户共有的主要要求。此外,就是在实现低功率的同时还要具有高分辨率、高速度和高性能。本文通过介绍两个重要应用领域的情况来进一步阐述这一重要趋势。WCDMA基础设施要求高速ADC在第三代(3G)和WCDMA基站的接…     

相干光OFDM在高速光传输中的应用

相干光正交频分复用系统(CO-OFDM,Coherent Optical Orthogonal Frequency Division Multiplexing)可有效抑制光纤色度色散和偏振模色散,有望成为解决未来高速光传输的主流方案。这里结合光传输理论及正交频分复用系统(OFDM,Orthogonal Frequency Division Multiplex)技术原理,对CO-OFDM系统方案实现及信道等效模型进行了分析,通过数字仿真验证系统光调制器偏置点选择及信号均衡算法。理论分析和仿真结果表明,CO-OFDM系统采用单抽头频域均衡,可有效抑制光纤色散效应。光调制器偏置点选择在零点,可实现系统对OFDM信号的最佳线性调制,与传统强度调制/直接检测(IM/DD)系统比较,CO-OFDM系统品质因子有10 dB提高。     

Loss and Noise Characteristics of Microwave Direct Modulated Optical Links

Loss and noise figure of optical links transmitting microwave signals —e.g. for remote antennas in wireless communications — are dealt with.Conditions are rather general and include arbitrary matching conditions, shotnoise, and laser relative intensity noise. Specific and somewhat surprisingcharacteristics are derived. Among others, it is shown, that — likepassive, purely microwave systems — noise figure is not proportional toelectrical loss and it can be even lower than loss itself. Optical powershould be as low as possible — this limit is mainly determined by therequired dynamic range of the system. Loss and noise figure were measured inan experimental setup. Results of the measurements proved the theoreticalfindings rather well. Preliminary results on the transmission of 32TCM signalsover the link are briefly presented.     

A Reduced Delay Method for Improving the Performance of Sigma-Delta Analog-to-Digital Converters (ADC)

Excess loop delay in a continuous-time switched-current modulator causes a stability problem and degrades the modulator's dynamic range. This paper presents a simple and effective way to reduce the loop delay and improve the modulator's performance. The loop delay of the ADC is reduced by feeding the predicted next state to the comparator. With reduced loop delay, a larger loop gain is allowed without a stability problem, and hence, the dynamic range of the ADC is improved. A new circuit architecture to realize a second-order modulator with this method is also presented. From the simulation result, the new architecture shows a 6–10 dB improvement in dynamic range for a second-order modulator.     

Transmission of Wavelength-Division-Multiplexed Channels With Coherent Optical OFDM

Transmission performance for wavelength-division-multiplexed (WDM) systems with coherent optical orthogonal frequency-division multiplexing is simulated including the fiber nonlinearity effect. The simulation shows that the system Q of the WDM channels at 10 Gb/s is over 13.0 dB for a transmission up to 4800 km of standard single-mode fiber without dispersion compensation     

Application of the Kalman Filter in Linearity Testing of Analog-to-Digital Converters

This work introduces application of the Kalman Filter in linearity test of Analog-to-Digital Converters (ADCs). The Kalman Filter can be used to suppress errors in the histogram counts, based on characteristics of the test environment and the device under test, and achieve high accuracy within short test time. In this paper, appropriate system and noise models, and a corresponding form of the Kalman Filter are developed, while all necessary parameters are obtained from experimental measurements. Simulation and experiment results show that the INL _k test errors are reduced by more than 50% by applying the proposed method. Consequently, this method can achieve desired accuracy with significantly fewer samples as compared to the conventional algorithm, and shorten the linearity test time by a factor of four or higher. This method is valuable for effectively reducing the production test time and cost for ADCs.     

Optimizing the End-to-End Performance of Reliable Flows over Wireless Links

Pure end-to-end error recovery fails as a general solution to optimize throughput when wireless links form parts of the end-to-end path. It can lead to decreased end-to-end throughput, an unfair load on best-effort networks, and a waste of valuable radio resources. Link layer error recovery over wireless links is essential for reliable flows to avoid these problems. We demonstrate this through an analysis of a large set of block erasure traces measured in different real-world radio environments, with both stationary and mobile hosts. Our analysis is based on a case study of the circuit-switched data service implemented in GSM. We show that the throughput on this wireless channel can be increased by using a larger (fixed) frame size for the reliable link layer protocol. This yields an improvement of up to 25% when the channel quality is good and 18% even under poor radio conditions. Our results suggest that adaptive frame length control could further increase the channel throughput. Finally, we discuss link and transport layer error control mechanisms and their interactions with end-to-end congestion control schemes. For reliable flows, we argue in favor of highly persistent error recovery and lossless handover schemes implemented at the link layer.     

短波正交频分复用(OFDM)传输性能研究

短波通信作为一种重要的通信方式一直受到信道带宽的限制,加之传输信道的不稳定性致使其传输效果不佳,如何有效的提高频带利用率、改善传输性能是短波通信的重要研究内容。正交频分复用(OFDM)是一种多载波数字调制技术,在提高频谱利用率、克服多径方面发挥了重要作用。文章以Simulink为平台构建短波OFDM通信系统仿真模型,并在频偏和多径条件下进行仿真,分析仿真结果可以看到,频偏和多径对系统的传输效果都有影响,其中频偏对通信系统的影响较为严重。     

Transmission Characteristics Evaluation Under Bad Weather Conditions in Optical Wireless Links with Different Optical Transmission Windows

Optical wireless links offer 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, scintillation and precipitation. Optical Wireless link provides high bandwidth solution to the last mile access bottleneck. However, an appreciable availability of the link is always a concern. Wireless Optics links are highly weather dependent and fog is the major attenuating factor reducing the link availability. Optical wireless links offer gigabit per second data rates and low system complexity. For ground space and or terrestrial communication scenarios, these links suffer from atmospheric loss mainly due to fog, scintillation and precipitation signals and then to upgrade the transmission bit rate distance product for ultra long transmission links. This paper has presented the bad weather effects such as rain, fog, snow, and scattering losses on the transmission performance of wireless optical communication systems. It is taken into account the study of bit error rate, maximum signal to noise ratio, maximum transmission optical path lengths and maximum transmission bit rates under these bad operating conditions.     

相位调制微波光传输链路特性分析（特邀）

常规的微波光子系统采用强度调制方式实现微波信号的电光转换,由于调制器采用马赫-曾德尔干涉结构(MZI),系统性能不仅受到自身正弦响应特性的制约,而且需要进行偏置点控制,因此存在动态范围受限、系统控制复杂以及3 dB固有损耗带来的效率不足的问题,而采用相位调制可避免该问题。围绕相位调制光传输链路,为了完成相位调制信号的光电解调,文章提出采用薄膜滤波器通过边带抑制与边带选通两种方式实现相位调制到强度调制的转换,并分析了链路射频性能与器件参数之间的映射关系。实测对比了相位调制与常规强度调制链路之间的传输特性,通过分析可知,在相同链路配置条件下,相位调制链路具有更高的传输效率,而且光滤波带来的均衡作用,使得相位调制链路的3 dB带宽比强度调制链路大两倍。     

相干光OFDM系统中直接判决均衡器的性能研究

理论上分析了相干光正交频分复用(CO-OFDM)系统中相位噪声(PN)和色度色散(CD)对星座图的影响,并详细探讨了CO-OFDM系统中基于导频的直接判决均衡器(DDE)的原理,还对其性能进行了仿真分析。仿真结果表明:在不同的传输距离下,DDE的误码率都能呈瀑布状下降至10-3这一公认的前向纠错(FEC)门限,使得后续的FEC模块可以让误码率迅速降低。此外,在激光器线宽小于100kHz时,DDE的误码性能并没有大幅度的提升。因此,在选择DDE作为均衡器的CO-OFDM系统中,应折中考虑激光器线宽和误码性能的关系,基于成本考虑建议采用100kHz激光器。     

Performance of Optical OFDM in Ultralong-Haul WDM Lightwave Systems

We show, using simulations, that a combination of orthogonal frequency division multiplexing (OFDM) and optical single sideband modulation can be used to compensate for chromatic dispersion in ultralong-haul wavelength-division multiplexed (WDM) systems. OFDM provides a high spectral efficiency, does not require a reverse feedback path for compensation, and has a better sensitivity than nonreturn to zero. This paper provides design rules for 800-4000-km optical-OFDM systems. The effects of WDM channel number and spacing, fiber dispersion, and input power per channel on the received Q are studied using extensive numerical simulations. These effects are summarized as a set of design rules     

基于Wigner-Hough变换的线性调频信号检测性能

根据Wigner-Hough变换思想和检测理论,分析了线性调频信号的检测性能,给出了虚警概率和检测概率的数学表达式,该表达式是Hough变换的积累单元数和检测门限的函数,然后对概率密度函数进行了合理近似,并给出了Hough变换的积累单元数计算公式,最后仿真分析了基于Wigner-Hough变换对线性调频信号相干积累相对于傅立叶变换的优越性.     

基于Wigner-Hough变换的LFM信号检测性能分析

根据Wigner-Hough变换思想和广义似然比检验理论,分析了双门限情况下线性调频信号的检测性能,给出了虚警概率和检测概率的数学表达式,该表达式是Hough变换的积累单元数和检测门限的函数,然后对概率密度函数进行了合理近似,并给出了Hough变换的积累单元数计算公式,最后的计算机仿真比较了傅立叶变换和Wigner-Hough变换的LFM(Linear Frequency Modulated)信号检测性能和基于Wigner-Hough变换的不同目标起伏类型的LFM信号检测性能.     

Precompensation for Narrow Optical Filtering of 10-Gb/s Intensity Modulated Signals

Signals in spectrally efficient optical networks can be distorted by the narrowing of the overall bandwidth that occurs when optical add-drop multiplexers are concatenated. In this letter, it is demonstrated that the performance implications of this bandwidth narrowing can be mitigated by precompensation using arbitrary optical waveform generation. The synthesis of the required voltage waveforms for a dual-drive Mach-Zehnder modulator reduces the penalty in receiver sensitivity due to narrow optical filtering from 6.5 to 1.9 dB. Simultaneous precompensation for fiber dispersion is also demonstrated.     

OFDM在ROF系统中的传输性能研究

对正交频分复用(OFDM)信号在光纤/无线接入系统传输中的噪声特性进行了分析.采用激光器的静态模型,对由于激光器非线性所产生的OFDM信号的带内噪声和邻道噪声干扰在理论上进行了计算,并进一步分析了光调制指数对系统载噪比的影响,此分析方法为OFDM-ROF系统非线性分析,以及系统中半导体激光器光调制指数的选择提供了理论分析途径.