Optically coherent direct modulated FM analog link with phase noisecanceling circuit

This paper demonstrates a new phase noise canceling (PNC) circuit for use with an optically coherent analog frequency modulation (FM) link employing directly modulated distributed feedback (DFB) lasers. Direct frequency modulation of a semiconductor laser is a highly efficient optical-to-electrical conversion process, which can lead to very low noise figure (NF) and high dynamic range (DR). However, the large laser phase noise found in semiconductor lasers significantly degrades the FM link performance. The PNC circuit is a simple means for canceling the laser phase noise while extracting the modulated signal and taking advantage of the high FM conversion efficiency of semiconductor lasers. The theoretical performance of the PNC PM link is discussed in terms of the signal-to-noise ratio (SNR), NF, and DR, and is compared to a Mach-Zehnder modulated link which uses high-power, solid-state lasers. Phase noise cancellation is demonstrated in an experimental PNC FM link. Comparison of a PNC FM link to an externally modulated AM (coherent) link shows a 31 dB improvement in the NF of the FM link, and a 10 dB improvement in DR. However at higher received optical powers phase-to-intensity noise limits the performance of both links to well below the theoretical calculations     

Envelope detection of a unit-index amplitude-modulated cartier accompanied by noise

The facts concerning the relative merits of linear and square-law envelope detection of a sinusoidally modulated carrier accompanied by Gaussian noise are clarified in this paper. The comparison when the carrier is weakly modulated and has mean power substantially greater than that of the noise in the input circuit has been fully covered in prior technical literature. In this special case the signal-to-noise ratio (SNR) in the detected output is the same for the two methods, and the distortion of the signal is worse with square-law operation because of inherent second-harmonic production. Since noise in the detected output is typically measured in the absence of signal modulation, the fact that the relative performance changes when the index of modulation approaches 100 percent tends to be overlooked. It is shown in the present paper that the noise performance of the square-law detector suffers a penalty approaching 1.8 dB when the carrier is strongly modulated. Also, the distortion in the square-law case is characterized by a second harmonic only 12 dB down from the fundamental, while the second harmonic in the output of the linear detector is practically negligible. These results are important in the evaluation of techniques in diversity reception.     

低信噪比条件下的调制方式识别

传统的调制方式识别方法通常假设接收信号是平稳随机过程,一般要在信噪比不低于10dB时才能取得较高的识别率。本文根据模拟调制信号的谱特性和谱相关特性,给出了低信噪比下的调制方式识别算法,理论分析和仿真结果表明,本文提出的算法,在低信噪比(3dB)条件下,仍然具有很高的识别率。     

OFDM link performance with companding for PAPR reduction in the presence of non-linear amplification

Use of companding for peak-to-average-power ratio (PAPR) control is explored for a link involving a nonlinear transmit power amplifier with orthogonal frequency division multiplexing (OFDM). Specifically, the objective of the study was to determine if companding using u-law compression/expansion at the transmitter/receiver, respectively, provides end-to-end performance gains relative to a system without companding. We consider the use of companding to ameliorate the impact of nonlinearities in the transmit amplifier. In the absence of companding, transmitter operation near saturation raises the signal level at the receiver but, because of the nonlinearities in the amplifier response, also results in distortion that impacts overall link performance. As the transmit power is backed-off from saturation, amplifier distortion is reduced, but error components due to lower SNR at the receiver become more significant. When companding is introduced in the system, the system is able to operate closer to saturation without substantial transmit distortion. However, requisite expansion of the compressed signal at the receiver yields noise amplification which can counteract any of the performance gains that would otherwise accrue from the increased SNR at the receiver. At issue is whether or not operating conditions exist (e.g., backoff, SNR, amplifier linearity model, etc) for which companding enhances the end-to-end performance relative to the link performance without companding. System simulation models were employed using Rapp's nonlinear power amplification models, where average symbol distance errors were used as performance metrics. We found that companding can provide very modest performance gains in comparison to systems that do not employ companding. Performance trends were corroborated in a hardware testbed with an amplifier chain, where average bit error rates were experimentally determined.     

基于高度分集技术的低仰角测高影响因素分析

米波三坐标雷达有重要的军事价值,尽管目前有很多种测高技术包括高分辨技术、空间谱估计技术、复角技术、偏差补偿技术、频率捷变技术等,但其低仰角测高精度不高,因此有必要分析影响测高因素。本文利用高度分集测高技术,仿真分析了不同介质以及极化方式对反射系数的影响;信噪比,幅相特性不一致对测高精度的影响。     

基于短时分析的数字调制信号识别方法的研究

研究了一种新的数字调制信号识别方法。该方法用短时分析提取数字调制信号在幅度、频率和相位随时间变化的特征，并利用这些特征对各种数字调制信号进行识别。在加性高斯白噪声条件下给出了相应的最佳阈值，并通过仿真研究了该识别方法的性能。仿真结果表明该方法对噪声不敏感，在SNR为OdB时仍能获得90％以上的正确识别率。     

光纤时频传递系统的中继技术分析

为开展长距离高精度时间频率传输系统研究,对多级光纤时间频率传递级联系统的时延抖动进行理论分析,得出级联系统总时延抖动的一般公式。在实验上实现了两级级联50km光纤时频传递系统的闭环锁定,分别通过测试单级系统和级联系统的鉴相误差电压得到其时延抖动。通过实验结果和理论计算比较,分析了主要实验误差来源;通过分析掺铒光纤放大器(EDFA)放大自发辐射(ASE)产生的噪声对信噪比(SNR)的影响,系统频率稳定度由于信噪比的下降而产生劣化。     

Arc-sine modulation?a method of overcoming nonlinearities in transmission systems

A form of modulation is described which may be used in amplitude nonlinear systems. The signal is a form of phase modulation, but the use of synchronous detection means that it is threshold-free, and certain forms of distortion associated with angle modulation are not present. There is a gain of 6 dB over peak-power-limited a.m. For low-index p.m. systems which are distortion-limited, it may be possible to obtain substantial improvements in signal/noise ratio.     

Clipping Distortion Suppression of Directly Modulated Multi‐IF‐over‐Fiber Mobile Fronthaul Links Using Shunt Diode Predistorter

Herein, we demonstrate clipping distortion suppression of directly modulated multi‐IF‐over‐fiber links using a simple shunt diode predistorter. The dynamic range of a directly modulated analog fiber optic link is limited by nonlinear distortions caused by laser‐diode clipping. We investigate the link performance in the context of carrier‐to‐noise and distortion ratio (CNDR) and error vector magnitude (EVM) requirements when supporting LTE‐A services. We also design an analog predistorter with a shunt‐diode structure, and demonstrate experimentally that the predistorter has the ability to suppress clipping‐induced third‐order intermodulation distortions of the link by at most 14 dB. It also improves the CNDR and EVM of the 4‐IF‐multiplexed LTE‐A carriers by 7 dB and 2.9%, respectively.     

满足0.2级的光纤电流互感器信噪比分析

电流比值误差和相位误差是检验互感器的重要指标,噪声对其有较大影响.光纤电流互感器不同于传统电流互感器的重要特征之一即是存在较大噪声,在测量小电流时表现尤为显著,已经成为阻碍光纤电流互感器实用化进程的重要难题.分析了高斯白噪声背景下电流比值误差和电流相位误差的统计特征,研究了0.2S级下电流互感器数字输出信号信噪比所需满足的条件,探讨了电流互感器信噪比的计算方法以及影响信噪比的因素,并设计实验验证了理论分析的正确性.分析表明,为满足0.2S级光纤电流互感器信噪比需达到136,在额定电流以及数据更新率一定的情况下,仅有通过降低噪声以及增加敏感线圈缠绕圈数的方法可以提高系统信噪比.     

Numerical simulation of clipping induced distortion in externallymodulated lightwave AM-SCM systems

For the first time to our knowledge the results of numerical simulation, and experimental verification, of clipping induced distortion in externally modulated lightwave AM-SCM CATV systems are presented. Comparison of these results with clipping induced distortion in directly modulated systems show a remarkable outcome: under the same modulation conditions the composite triple beat (CTB) distortion component in externally modulated systems is 6 dB higher than in directly modulated systems     

Impact of finite frequency deviation on the performance ofdual-filter heterodyne FSK lightwave systems

A detailed theoretical analysis is given of the impact of finite frequency deviation on the sensitivity of dual-filter heterodyne frequency-shift-keying (FSK) lightwave systems. The analysis provides closed-form signal-to-noise ratio (SNR) results for estimating the bit-error-ratio (BER) performance of the system. These closed-form results provide an insight into the impact of finite frequency deviation 2Δf_d, laser linewidth Δν, bit rate R_b, and IF filter bandwidths on the system performance. Simulation results indicate that the accuracy of the approximate theory presented is within 1 dB for linewidths up to 22% when BER=10^-9. It is shown that there is a well-defined relationship between the choice of frequency deviation and the tolerable amount of laser phase noise. The sensitivity degradation can be very severe for a fixed linewidth as the frequency deviation gets smaller     

Doppler-rate estimation for burst digital transmission

We propose a new algorithm for Doppler rate estimation in burst-mode phase shift keying (PSK) transmissions. This issue may arise in mobile radio links when the received signal experiences significant time-varying Doppler distortion, as in low-earth-orbit satellite systems. The algorithm is based on the transmission of a training sequence and has a feedforward structure that is easy to implement in digital form. Its estimation accuracy is close to the Cramer-Rao bound even at SNR values as low as 0 dB. Comparisons with earlier methods are discussed     

基于多级维纳滤波的盲信噪比估计

提出了一种基于多级维纳滤波(MSWF: Multistage Wiener Filter)的盲信噪比估计算法。结合信号子空间分解理论,该方法利用多级维纳滤波器的相关相减结构(CSA: Correlation Subtraction Algorithm)前向递推实现含噪信号空间分解,避免了传统方法对信号自相关矩阵进行复杂的特征值分解运算,并以此估计信号功率和噪声功率来完成盲信噪比估计。在加性高斯白噪声(AWGN)信道条件下进行信噪比估计仿真,仿真表明,当实际信噪比在-7~25dB范围内时,估计器的估计标准偏差小于0.5dB,且性能优于常规方法。设定实际信噪比为10dB,当接收码元数目为100时,对所有仿真的调制方式信噪比估计标准偏差小于0.35dB,证明了估计器在小样本支撑环境下实现信噪比快速盲估计的能力。      

Linewidth-insensitive coherent AM optical links: design,performance, and potential applications

The use of coherent detection in analog optical links offers several advantages over direct detection: improved receiver sensitivity, inherent frequency translation, and the ability to utilize angle modulation and separate wavelength division multiplexed (WDM) signals. In this paper, we investigate an externally modulated coherent AM optical link. We study the dynamic range of the coherent AM link, considering receiver noise, laser phase noise, laser relative intensity noise (RIN), and system nonlinearities. With proper selection of the receiver's IF bandwidth, the coherent AM link can be made insensitive to the laser linewidth. For optical powers less than 5 mW, RIN of less than -160 dB/Hz reduces the spurious-free dynamic range (SFDR) by less than 3 db with the use of a balanced receiver. The external modulator nonlinearity is the dominant nonideal effect; it reduces the SFDR by 5-19 dB from the theoretical limit for 100% modulation index. We compare the performance of the coherent AM link with that of a conventional direct detection link for two applications: point-to-point links and distribution networks. When the received optical power is less than 1 mW, the coherent link can provide higher SFDR than the direct detection link. Thus, coherent links are well-suited for long distance point-to-point links and FM video distribution systems     

一种基于奇异值分解的改进信噪比盲估计算法

针对空间分解类信噪比(SNR)估计算法中子空间维数估计复杂度较高,低信噪比下估计偏差较大的问题,提出了一种改进的子空间维数估计算法。该算法首先利用样本自相关矩阵的奇异值序列进行后向差分得到梯度序列,对梯度序列每一项与后5项之和的比值进行搜索,最大比值所对应的奇异值序号作为信号子空间维数,最后计算信噪比。合适数据长度下的仿真结果表明：在信噪比-5 dB~20 dB范围内,常规通信信号的信噪比估计平均偏差小于0.5 dB,标准差小于1 dB;该算法提升了低信噪比下的估计性能,运算量较小,无需知道调制方式、载波频率、符号率等先验信息,在低信噪比时对信噪比时变的跟踪估计更为准确,且对复杂高阶调制信号同样适用。     

Performance improvement of RoF transmission link by using 120° hybrid coupler in OSSB generation

This paper presents an analysis, simulation and comparison of the performance of Optical single sideband radio over fiber system based on a dual drive Mach Zehender modulator using 90° and 120° hybrid coupler including the effects of phase noise from RF signal oscillator and laser source, fiber dispersion. Signal to noise ratio (SNR) is significantly influenced by phase noise from RF signal oscillator. On comparison with conventional 90° hybrid coupler system, the performance of the considered system improves by 0.78 dB in terms of SNR, when RF and laser phase noises are increased.     

Digital correction of PWM switching amplifiers

Pulsewidth modulated (PWM) signals for driving a switching audio amplifier can be synthesized in the digital domain with extremely high linearity and precision. However, nonidealities associated with the power stage degrade output performance. A method to digitally correct for these nonidealities, resulting in very low total harmonic distortion (THD) and high signal-to-noise ratio (SNR) performance, is presented. This method also provides excellent rejection of power supply noise which is otherwise absent in digital PWM amplifiers. To meet noise requirements for hi-fi audio, the feedback structure is a fourth-order structure which shapes the noise beyond the audio band. The method has been implemented on a bread board, and state-of-the-art performance was achieved. Total harmonic distortion of 85 dB and dynamic range of 100 dB was measured using Audio Precision test equipment.     

IF signal transmission at 60 GHz-band using direct modulation of atwo-mode locked Fabry-Perot slave laser

We demonstrate the fiber transmission of an IF signal at 60 GHz-band using a two-mode injection-locked Fabry-Perot slave laser for fiber radio systems. The IF signal can be generated by the CW modulation of the slave laser without using an optical intensity-modulator. We confirm the fast response of the modulated slave laser and a 77 dB dynamic range for signal modulation against third-order distortion. The IF signals power deviation caused by fiber dispersion is also investigated     

A Low-Cost Digital Matched Filter for Arbitrary Constant-Envelope Spread-Spectrum Waveforms

In this paper a noncoherent digital matched filter (DMF) with binary quantization is presented which shows good performance for arbitrarily angle modulated constant-envelope waveforms having large time-bandwidth products. The considered DMF represents an extension of the noncoherent DMF with one-bit digitization suggested previously by Turin for binary PSK signals. Analysis of the filter performance in the presence of white Gaussian noise results in a general expression which allows the output SNR of both DMF's to be calculated for any spread-spectrum waveform. Evaluating this expression reveals that the extended DMF can be matched to arbitrary waveforms having constant envelopes in such a way that the SNR loss with reference to the corresponding analog matched filter is less than 2.87 dB. The degradation is as low as 1.96 dB for waveforms with biphase or quadriphase signal samples such as binary PN-PSK, PNQPSK, and offset-PN-QPSK, but also PN-MSK. If both DMF's are applied to waveforms having complex valued envelopes, the extended DMF may outperform Turin's DMF by as much as 3 dB.