Performance analysis preclipping AM/QAM hybrid lightwave systems

A model is presented for the analysis of the clipping noise in preclipping AM/QAM hybrid lightwave systems. It is shown that the clipping-induced degradation of BER performance can be reduced by preclipping the signal. It is also shown that the recurrence of the signal tips after passing through the low-pass filter presents a serious limit to the performance improvement achievable by the preclipping technique     

数字CATV系统中激光器削波对QAM信号影响的研究

目前的CATV网络不仅要传输一般的模拟信号,而且要能够传输QAM调制的数字信号。重点分析了数字CATV网络中数字频道和模拟频道的相互影响以及QAM调制的原理,讨论了在发射机端激光器削波对数字信号的影响,并给出了仿真结果。分析结果表明,QAM信号的误码率和QAM信号的光调制度、削波失真系数以及削波失真与高斯噪声之间的比例等都有密切的关系,激光器削波将严重恶化数字QAM信号的误码率。     

Asymptotic clipping noise distribution and its impact on M-ary QAMtransmission over optical fiber

Performance analysis for a hybrid subcarrier-multiplexed (SCM) AM/QAM transmission over an optical fiber is presented. It is shown that the bit-error-rate (BER) of M-ary QAM in such hybrid systems can be significantly affected due to occasional laser “clipping” of the SCM signal. Here we analytically determine the asymptotic distribution of the clipping noise by modeling it as a Poisson impulse train. The BER of M-ary QAM is then evaluated in terms of optical modulation indices of M-ary QAM and AM signals which in turn specify signal-to-noise ratio (SNR), impulsive index (clipping index) of the clipping noise, and power ratio of the Gaussian noise to the clipping noise. Numerical examples are given and compared with experimental data with reasonably good agreement for small SNR's. The results have application for estimating the BER's of digital signals for SCM analog/digital transmission over an optical fiber and for employing appropriate error correction codes and/or optimum receiver design for such environments     

Signal distortion and noise in AM-SCM transmission systemsemploying the feedforward linearized MQW-EA external modulator

In subcarrier multiplexing (SCM) optical video distribution systems, the nonlinear signal distortion generated by the combined action of laser chirp and fiber dispersion limits the transmission distance. This paper for the first time shows that low-chirp MQW (multiquantum well)-EA (electroabsorption) external modulators are applicable to AM-SCM transmission systems. The feedforward linearization technique is used to compensate the signal distortion due to the nonlinearity in the modulator's L-V (light power versus voltage) characteristic. The effectiveness of this type of modulator for suppressing dispersion-induced distortion is clarified experimentally. A feedforward linearized transmitter with composite second order (CSO) intermodulation distortion <-62 dBc, composite triple beat (CTB) intermodulation distortion <-59 dBc and a carrier-to-noise ratio (CNR) >50.5 dB is successfully constructed. It is shown that the transmitter can achieve a CSO <-59 dBc, a CTB <-57 dBc and a CNR >49.5 dB for a 32 channel AM signal and 10 km transmission. Furthermore, estimations of the signal distortion cancellation and the noise characteristic achieved with feedforward circuit modeling are shown. The results obtained here emphasize that MQW-EA external modulators are applicable to AM-SCM transmission systems     

AM/OFDM混合光传输系统误码特性

本文分析了在限幅噪声和高斯噪声环境下,在ＨＦＣ网中混合传输ＡＭ／ＯＦＤＭ信号时的误码特性,并与单载波ＡＭ／ＱＡＭ系统进行比较,结果表明ＯＦＤＭ系统由于多个子通道的稀释作用,其抗限幅噪声性能比单载波系统好。     

The impact of CSO/CTB distortion on BER characteristics by hybridmultichannel analog/QAM transmission systems

This article describes the measurements of composite second order (CSO), composite triple beats (CTB) frequency spectrum and time-varying distortion in the basic bandwidth (BB) (0-4 MHz) of the 64-QAM digital channels by multichannel AM/64-QAM optical and coaxial transmission systems. For any regular standard frequency plans (Europe, USA, NCTA, etc.) there is, for the I and Q components in the BB of the QAM channels, the harmonic series of the CSO/CTB with the first harmonic f ₁. It has been shown that for all the same laser-optics or coaxial transmission systems the CSO/CTB distortions spectrum form in the QAM BB 60-80 μs duration bursts of a series pulse with a pulse period T=1/f₁ and a pulse duration τ≈0.4/f_m, where f_m is the CSO/CTB harmonic with maximum amplitude in the QAM BB. The bursts result in the essential unevenness of error bits in time domain distribution. These bursts can deteriorate essential error correction by the interleaver-deinterleaver and Reed-Solomon (RS) forward error correction (FEC) code of the transmission systems also with a relative low bit error rate (BER) of about 10^-5 before FEC. There is, white Gaussian noise like, intermodulation distortion from interaction of digital-, digital-analog channels too, that have added white Gaussian noise     

Error statistics of 64-QAM signal in AM/64-QAM hybrid optical transmission

This paper describes the performance of an error correction system based on the DAVIC specifications. It also presents an error statistics property and its analytical model for the impulse noise induced by clipping in amplitude modulation (AM)/64 quadrature amplitude modulation (QAM) hybrid optical transmission (hybrid transmission). We found in hybrid transmission that the errors cannot be completely corrected by the error correction of DAVIC after hit error rate (BER) measurement with error correction, and then the error statistics of the 64-QAM channel were evaluated. It was confirmed that errors occasionally occur in long bursts that exceed an interleaver block length of the error correction when the impulse noise degrades the 64-QAM signal's BER. We believe that the error burst is caused by the slowed fluctuation of the envelope of a frequency division multiplex (FDM) signal in hybrid transmission. Once a portion of the envelope amplitude reaches the clipping level, the duration of maintaining such a condition is much longer than the 64-QAM symbol time. Accordingly, the errors occasionally run for more than 100 symbols. We apply Rice's fading duration distribution model to explain this error burst occurrence mechanism and its statistical manner and discuss this model's validity by comparison between the theoretical and measurement data of error statistics property. Consequently, we found that the theoretical value by the proposed model is in excellent agreement with the measurement data and confirmed that the proposed model is valid for analyzing the error manner for the clipping induced error.     

Design of Broadband Highly Linear IQ Modulator Using a 0.5$mu$m E/D-PHEMT Process for Millimeter-Wave Applications

A broadband highly linear IQ modulator using a 0.5-mum enhancement/depletion-pseudomorphic high-electron mobility transistor process is presented in this letter. An innovative broadside/edge coupler is proposed to apply to the IQ modulator. The chip size is only 1times1 mm², including radio frequency and baseband PADs. The sideband and local oscillation suppressions of the modulator are better than -33 and -15 dBc, respectively. At a carrier frequency of 60 GHz with a 64 quadrature amplitude modulation (QAM) modulation, the modulator demonstrates an error vector magnitude of within 3%, and an adjacent channel power ratio of better than -40 dBc. To the best of the authors' knowledge, this work demonstrates the best modulation quality with a 64 QAM modulation up to 60 GHz among all the reported reflection-type IQ modulators.     

Impact of noise and nonlinear distortion due to clipping on the BERperformance of a 64-QAM signal in hybrid AM-VSB/QAM optical fibertransmission system

Experimental BER measurements of a 64-QAM signal in a hybrid AM-VSB/QAM optical fiber transmission system are shown to be in good agreement with a corrected and reinterpreted theory of impulsive and Gaussian noise based on Middleton's class B impulsive noise model. Fiber dispersion induced distortion of the chirped laser output is also shown to adversely affect the QAM signal     

降低可见光OFDM系统限幅失真的部分线性压扩变换研究

针对可见光正交频分复用（OFDM）系统发光二极管（LED）限幅失真严重的问题,提出一种部分线性压扩变换方法。输出光功率约束较大或较小时,在发射端对直流光正交频分复用（DCO-OFDM）双极性信号中的正信号或负信号进行线性压缩变换,在接收端对压扩信号进行扩展变换可以有效地减小限幅噪声。输入信号功率一定时,在有效信噪比最大原则下对部分线性压扩变换系数和直流偏置进行优化,系统能够获得最佳性能。仿真实验表明,随着约束光功率增大,系统性能越差,且压缩系数的最优值也越小。随着QAM调制阶数的增加,部分线性压扩变换获得的性能增益越小。      

Theoretical and experimental analysis of clipping-induced impulsivenoise in AM-VSB subcarrier multiplexed lightwave systems

A new theoretical and experimental analysis of clipping-induced impulsive noise in directly modulated AM-VSB/digital hybrid lightwave systems is presented. The theoretical model is based on asymptotic clipping and shot noise theories, and the probability density of combined Gaussian and impulsive noise is obtained by performing a numerical inverse Fourier transform on the closed-form characteristic function. Similar analyzes in previously published work have resulted in closed-form expressions for the BER in hybrid AM-VSB/QAM systems, but these models, which do not use the numerical inverse Fourier transform, have either required the use of an experimentally measured clipping rate using a spectrum analyzer or have had limited BER prediction accuracy. Application of our theory to experimental results for the BER in a hybrid AM-VSB/64 QAM system demonstrates that the performance of the model if better than the models of previously published works. Theoretical analysis of a similar hybrid system employing the N-VSB HDTV digital modulation format rather than QAM indicates that an improvement in BER is achieved if 8-VSB at double the symbol rate is used instead of 64 QAM. We also present the first experimental characterization of the amplitude and time distribution of (1) low-frequency bandpass and (2) down-converted impulsive noise and we show that the time interval between clipping events has a high probability of being 167 μs, 1.5 μs, or 4 ms in an NTSC system with unmodulated carriers. These results have potential impact on forward error correcting codes used to ameliorate the degrading effects of clipping     

Performance characteristics and applications of hybrid multichannelANI-VSB/M-QAM video lightwave transmission systems

The performance characteristics and applications of hybrid multichannel amplitude modulation vestigal sideband (AM-VSB)/M-quadrature amplitude modulation (QAM) video lightwave transmission systems operating at either 1310 or 1550 nm are reviewed. These systems can transport up to 80 AM-VSB video channels and more than 30 64/256-QAM digital video channels over a standard single-mode fiber (SMF) using a single laser transmitter. Five main transmission impairment mechanisms for these systems are reviewed as follows: (a) clipping-induced impulse noise, (b) bursty nonlinear distortions, (c) multiple optical reflections, (d) stimulated Brillouin scattering, and (e) self-phase modulation. For AM-QAM video lightwave trunking applications, the in-line erbium-doped fiber amplifier (EDFA) selection is discussed using a frequency-domain simulation model. Such lightwave trunking systems can provide an AM carrier-to-noise ratio (CNR) greater than 50 dB with composite second order (CSO) and composite-triple-beat (CTB) distortions less than -65 dBc, and nearly error-free transmission (BER⩽10^-9) for the 64-QAM channels with signal-to-noise ratio (SNR) of 30-dB or better. Comparison between 64-QAM and 256-QAM video channel transmission and the effect of the QAM channels on the AM-VSB channels are also presented. The implications of these results and others in hybrid multichannel AM-QAM video lightwave trunking systems are discussed     

针对线性群时延畸变的加权多模盲均衡算法

群时延失真是影响卫星通信系统误码性能的重要因素之一。本文分析了高速卫星信道的线性群时延模型,依此模型提出了改进的加权多模盲均衡算法(MWMMA)。该算法引入Sigmoid函数的变形形式,构造了加权幂指数随迭代误差自适应变化的函数关系,在不需要通过大量仿真实验预先确定模式转变门限值的前提下,利用迭代误差的递减实现算法从MCMA模式通过多模自适应切换为DD模式。通过对多种算法的仿真比较,得到该算法不仅能使通过群时延信道的16QAM信号收敛集中,而且具有更小的稳态剩余码间干扰;同时,基于该算法对64QAM和256QAM传输信号的均衡仿真结果,验证了该算法对高阶QAM信号的均衡具有一定的适用性。      

Load modulation power amplifier with lumped-element combiner for IEEE 802.11b/g WLAN applications

A new monolithic microwave integrated circuit power amplifier for 802.11b/g wireless local area network (WLAN) has been implemented using the load modulation concept of a Doherty amplifier. The /spl lambda//4 transmission line for the load modulation circuit of the carrier amplifier is replaced by a lumped element based /spl pi/-network, which dual functions as an output matching network, simultaneously. This amplifier shows that error vector magnitude is about 4.6% and power added efficiency (PAE) about 31.8% at P/sub out/ of 19 dBm for a 802.11g 64QAM signal. PAE of the power amplifier is about 49.6%, and adjacent channel leakage ratio below 37.2dBc at 11 MHz offset at P/sub out/ of 23 dBm for the 802.11b complementary code keying signal     

Pseudobinary and Pseudoquaternary Detection Processes for Linearly Distorted Multilevel QAM Signals

The paper describes two new detection processes suitable for QAM signals that have 16 or more levels (possible data symbol values) and that have been subjected to linear distortion, causing severe intersymbol interference in the received signal. The detectors are developments of the pseudobinary systems previously described, and are designed to operate in the presence of severe intersymbol interference. Results of computer simulation tests are presented, comparing the tolerances to additive white Gaussian noise of various arrangements of each detector with those of more conventional detectors. The tests have been carried out on a model of a 19 200 bit/s synchronous serial data transmission system, operating with a 64-level QAM signal that is transmitted, in turn, over two different telephone circuits.     

QAM and QPRS digital broadband cable systems

The performance of high-level modulation schemes in non-linear broadband digital communication systems is investigated. The tolerance of 16-, 64-, 256-QAM and 49-, 225-QPRS schemes to these non-linearities is given in terms of their phase margins. Our research results show that high level QAM schemes are less sensitive to non-linearities when operated in offset mode. Comparison of non-linearly amplified broadband QAM and QPRS schemes reveals that QPRS schemes need less carrier-to-noise ratio (CNR) than corresponding QAM schemes for identical operating conditions. Also, when the non-linearities are not severe, the degradation in performance is controlled by AM/PM characteristic of the broadband amplifier.     

基于分数阶Fourier变换减小OFDM系统PAPR的研究

介绍了基于分数阶Fourier变换的OFDM系统,给出了这种系统中经过变换后的信号的表达形式,探讨了该系统在不同的调制方式下（QAM调制和QPSK调制）使用Clipping算法降低系统PAPR的情况,并使用Matlab对不同阶数的CCDF进行了仿真。仿真结果表明,QAM调制下基于分数阶Fourier变换的Clipping算法性能不如基于傅立叶变换的Clipping算法好,而在QPSK调制方式下基于分数阶Fourier变换的OFDM系统PAPR性能比基于傅立叶变换的OFDM系统性能有了显著的提高。     

An accurate digital baseband predistorter design for linearization of RF power amplifiers by a genetic algorithm based Hammerstein structure

In this paper, a novel digital predistorter design based on the Hammerstein structure is proposed in order to linearize radio frequency power amplifiers. A genetic algorithm optimization method has been proposed to accurately identify the coefficients of a Wiener model for the power amplifier. Digital predistorter design based on the proposed Hammerstein model has been carried out according to the accurate Wiener model. The validation of the suggested model is carried out using the simulation of the power amplifier and the digital predistortion excited by 64QAM signals in the advanced design system software. According to the simulation results, the criterion of an adjacent channel power ratio decreased by about 16 dB. The simulation results show the adjacent channel power ratio of almost ??46 dBc. In order to assess the feasibility of the proposed predistorter, it is completely implemented in the Kintex FPGA using Vivado HLS. This proposed model enables a more accurate modeling of nonlinear distortion and memory effects compared to the previous linearization methods. This paper presents the new linearization method using the genetic algorithm based Hammerstein structure.     

Significant performance advantage of electroabsorption modulatorintegrated distributed feedback laser (EML) transmitter in transportingmulticarrier QAM signals

Data are presented that show that, for transporting quadrature amplitude modulated (QAM) radiofrequency (RF) subcarriers in suboctave frequency range, electroabsorption modulator integrated distributed feedback lasers (EMLs) can he modulated with significantly higher (2.5 times) modulation index without any in-band signal distortion as compared to the directly or externally modulated (using LiNbO₃ MZ modulator) distributed feedback (DFB) laser transmitters in the 1.55-μm band. This occurs when the selection of frequency range and biasing the modulator section of an EML is such that the third-order intermodulation distortion is suppressed and the second-order distortion is outside the in-band frequency range. We have used an equivalent of 64- and 256-QAM 40 carriers in 550-800 MHz with the test carrier modulated with 5.063 M symbols per second. For a given bit error rate (BER), the receiver sensitivity was as much as 4 dB (optical) higher with an EML-based transmitter compared to all other transmitters. The results are presented for QAM signals but they are equally applicable for other formats of digital modulation of RF carriers, such as quadrature phase shift keying (QPSK) in suboctave frequency range     

Performance of Reduced-Bandwidth 16 QAM with Decision-Feedback Equalization

This paper presents and investigates the performance of a reduced-bandwidth 16 QAM (RB-16 QAM) signaling technique which employs severe narrow-band filtering and decision-feedback equalization in the receiver to compensate for the resulting intersymbol interference. The overall filtering is designed so as to provide the spectral efficiency of 64 QAM. RB-16 QAM is compared to 64 QAM in terms of its performance on additive white Gaussian noise channels, in multipath fading environment, as well as in terms of its sensitivity to modem imperfections including carrier and timing phase errors, filter imperfections, nonlinear distortion and sinusoidal interference. The results show that depending on the spectral shaping filters and the equalizer used, RB16 QAM can be significantly more advantageous than 64 QAM. A most interesting finding is that while RB-16 QAM is comparable to 64 QAM in terms of its spectral efficiency, its robustness against system imperfections is very much like that of conventional 16 QAM.