Study of BER of 64-QAM signal and OMI-window of feasible operationin analog/digital hybrid SCM transmission systems

In an analog/digital hybrid subcarrier multiplexed (SCM) transmission, carriers have generally been substituted for transmission signals such as AM vestigal sideband (AM-VSB) AM signals and M-QAM signals to evaluate transmission quality. In practical hybrid SCM, however, carriers are modulated by video signals or digital data, and the amplitude of a multiplexed signal composed of these modulation signals is more compressed than that of the carriers. This causes a decrease in the frequency of clipping of the multiplexed signal at the laser threshold. Consequently, the BER of the M-QAM signal in a practical hybrid SCM is lower than that of the experimental results for the same optical modulation index (OMI). However, it is difficult to prepare many practical modulation signals for experiments in a laboratory. Therefore, there is demand for a bit error rate (BER) analysis method for a multiplexed signal that includes the modulation signals needed to sufficiently evaluate the BER and determine the optimum OMI in a practical hybrid SCM. In this paper, we describe such a BER analysis method that can effectively estimate the BER in a practical hybrid SCM. In practical systems, the BER was greatly improved over the BER of a multiplexed signal of carriers. Furthermore, BER degradations due to clipping can be neglected for the AM signals in setting a practical OMI range. We used this analysis method to study the effective OMI range of AM and M-QAM signals. By assuming modulation signals, the OMI range is enlarged and, significantly, the OMI of an AM signal becomes suitable for setting practical values in AM-SCM transmission. This OMI range is more practical than those of former studies     

Amplitude density of infrequent clipping impulse noise andbit-error rate impairment in AM-VSB/M-QAM hybrid lightwave systems

Analogue video cavalcade services in cable access television (CATV) networks usually require a very high carrier-to-noise ratio (CNR) in order to provide a good quality picture. In consequence, practical lightwave hybrid amplitude modulation (AM) M-ary quadrature amplitude modulation (M-QAM) systems are found to suffer only infrequent clipping impulse noise, which can be provably modeled as a Poisson arriving pulse train. Based on this knowledge, a new expression for the probability density function (PDF) of the clipping noise at the output of a QAM demodulation matched filter is given which can be numerically evaluated with high accuracy. The bit-error rate (BER) performance prediction is then carried out for M-QAM signals within the hybrid system in the presence of an additive mixture of Gaussian and clipping impulse noise. The agreement between the analytical results and the experimental data is quite good     

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。     

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     

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.     

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。     

Performance Limitations of Subcarrier Multiplexed WDM Signal Transmissions Using QAM Modulation

We theoretically investigate the performance limitations of subcarrier multiplexed (SCM) wavelength-division-multiplexing (WDM) systems using optical double-sideband (DSB) modulated, 16-quadrature amplitude-modulated (QAM) signals. The performance limitations are investigated using crosstalk power and SCM channel spacing for various transmission conditions, including impairment factors such as dispersion and fiber nonlinearities for a single wavelength channel first. The effects of WDM channel spacing on SCM systems with multiwavelength channels are also evaluated via the calculated bit error rate (BER) performance, based on the performance limitations found in the single-wavelength simulation. This enables us to provide guidelines for the design of SCM/WDM systems for fiber-to-the-home (FTTH) network in WDM–passive optical network (PON) architecture, based on the performance limitations.      

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     

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     

M-ary amplitude shift keying OFDM system

Coherent M-ary amplitude-shift keying (MASK) is proposed for use in orthogonal frequency-division multiplexing (OFDM) systems. The frequency separation between subcarriers is only 1/2T instead of 1/T. With a slightly wider bandwidth, an /spl radic/M-ary ASK OFDM can achieve the same bit-error rate (BER) of M-ary quadrature amplitude modulation (QAM) OFDM and a better BER than that of M-ary phase-shift keying (MPSK) OFDM. The /spl radic/M-ary ASK OFDM has the same peak-to-average-power ratio as that of the M-ary QAM OFDM. The MASK OFDM can be implemented digitally and efficiently by fast cosine transform and demodulated by inverse fast cosine transform. Comparisons show that implementation complexity is reduced for additive white Gaussian noise channels with the use of the new scheme.     

光通信系统Q因子测量和原理

10Gbit/s及其以上速率光传输系统非线性效应很强,对系统最终的BER性能有着举足轻重的影响。人们倾向采用Q因子来衡量系统性能。Q因子被定义为在接收机判决电路信噪比,这种检测到的电SNR最终决定物理层系统BER。可以适用于各种信号格式和速率的数字客户信号,而且不需要解开帧结构,比较简单易行。本具体讨论Q因子的定义及与OSNR之间的关系。同时介绍ITU关于Q因子的测试方法和对测试设备的要求。     

Effects of intermodulation distortions due to the joint action ofdynamic chirping and “dispersive” transmission of SCMoptical signals

The effect of the joint action of the AM-FM conversion due to chirping in a semiconductor laser and the chromatic dispersion of the optical fiber has been so far analyzed in the case of static chirping in hybrid multichannel AM/QAM optical fiber transmission systems, based on subcarrier-multiplexing (SCM) technique. The static approach is not exhaustive in the case of channeling covering the whole bandwidth, that is ≈1 GHz, for which the dynamic chirping must be considered, as well. In this letter, analytical expressions of the power spectral density of the intermodulation distortions due to “dispersive” transmission of chirped intensity-modulated signals are calculated, considering both the static and the dynamic effects. As a consequence of this analysis, an accurate method for the determination of the small-signal photon decay rate Γ_P of the semiconductor laser is proposed, which is based on electrical measurements of the second-order intermodulation products     

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     

基于Nyquist脉冲副载波调制的短距离光纤传输技术研究

提出了一种适用于强度调制直接检测(IM/DD) 光系统的信号调制格式。这种基于Nyquist脉冲half cycle副载波调制(SCM)信号能有效提高频谱效率,为了减少传递函数引起的系统的性能 恶化,采 用了数字预均衡和后均衡实现对光纤损伤的补偿;建立了偏振复用(PDM)IM/DD系统 平台, 分析了Nyquist脉冲SCM信号在系统中的PAPR性能,实现了72Gbit/s 64-QAM N yquist脉冲SCM信号经过20km标准单模光纤(SMF)传输后其误码性能 小于前向纠错(FEC)阀值3．8×10－3。     

Fiber-optic transmissions of microwave 8-phase-PSK and 16-aryquadrature-amplitude-modulated signals at the 1.3-μm wavelengthregion

Transmissions of a 6-GHz 8-phase phase-shift-keyed (8φ-PSK) signal over a 12.5-km single-mode fiber with a 5-dB power margin and bit-error-rate (BER) of 10^-10 and a 6-GHz 16-ary quadrature-amplitude-modulated (QAM) signal over the same distance with a 2-dB power margin and BER of 10^-12 were demonstrated. The 8φ-PSK digital modem operated at 78Mb/s and the 16-QAM digital modem operated at 90 Mb/s. A high-speed multimode InGaAsP laser diode and a high-speed p-i-n diode were used in both fiber-optic transmission systems. Floor characteristics and power penalties observed in the BER performances of both systems were found to be caused by the intensity noise of the laser diode, particularly the reflection-induced intensity noise     

A Communication System Based on Walsh–Hadamard Codes and Constellations for Fading Environments

In this paper, we investigate a new digital communication system based on Walsh-Hadamard codes. The modulator maps a uniformly distributed binary source to a set of symbols with a Gaussian-like distribution. These symbols are then mapped to a 2-D constellation in such a way that performance is improved when transmission occurs over Rayleigh fading channels. We assess the performance of these modulation schemes when the channel is of the nonfrequency-selective Rayleigh fading plus white Gaussian noise type and compare the results to conventional quadrature-amplitude modulation (QAM) schemes with comparable spectral efficiencies. The proposed system provides excellent robustness to fading without the use of common diversity schemes employed in M-ary signaling systems to improve bit-error-rate (BER) performance in such environments.     

光纤SCM通信系统容量的极限

本文以光纤CATV传输系统为例.研究分析了SCM传输技术中容量的限制。导出了各种副载波调制型式(数字信号的PSK、FSK、ASK,模拟信号的FM、AM和AM-VSB)的容量极限值与系统中主要器件技术参数间的关系。     

色散平坦光纤中的高速率PM-16QAM信号传输研究

提出了一种高速率、偏振复用、正交幅度调制信号的色散平坦光纤传输系统,传输速率分别为160 Gbps和256 Gbps,调制格式为PM-16QAM。实验研究了色散平坦光纤链路系统的传输特性,并分别与非零色散位移光纤和标准单模光纤链路传输特性做了比较。实验结果表明,较低输入光功率情况下,PM-16QAM信号在160 Gbps传输50 km时,经色散平坦光纤传输后的误差矢量幅度EVM优于经非零色散位移光纤传输情况0.5%,比特误码率BER优于非零色散位移光纤传输情况两个数量级;色散平坦光纤链路能更好地衰减旁瓣噪声;256 Gbps传输50 km和75 km时,仅在色散平坦光纤链路传输后可以较好地解调出信号;传输距离越长,保持较好特性时输入光功率范围越小。对比160 Gbps和256 Gbps情况,高速率PM-16QAM信号在色散平坦光纤链路的传输特性优于非零色散位移光纤和标准单模光纤链路的传输特性,传输速率越高、传输距离越长效果越明显。     

Superposition Coded Modulation With Peak-Power Limitation

We apply clipping to superposition coded modulation (SCM) systems to reduce the peak-to-average power ratio (PAPR) of the transmitted signal. The impact on performance is investigated by evaluating the mutual information driven by the induced peak-power-limited input signals. It is shown that the rate loss is marginal for moderate clipping thresholds if optimal encoding/decoding is used. This fact is confirmed in examples where capacity-approaching component codes are used together with the maximum a posteriori probability (MAP) detection. In order to reduce the detection complexity of SCM with a large number of layers, we develop a suboptimal soft compensation (SC) method that is combined with soft-input soft-output (SISO) decoding algorithms in an iterative manner. A variety of simulation results for additive white Gaussian noise (AWGN) and fading channels are presented. It is shown that with the proposed method, the effect of clipping can be efficiently compensated and a good tradeoff between PAPR and bit-error rate (BER) can be achieved. Comparisons with other coded modulation schemes demonstrate that SCM offers significant advantages for high-rate transmissions over fading channels.      

Corona Based Optimal Node Deployment Distribution in Wireless Sensor Networks

This paper shows the trade off between different modulation techniques such as multi level quadrature amplitude modulation, multi level phase shift keying, and multi level differential phase shift keying for upgrading direct detection optical orthogonal frequency division multiplexing systems with possible transmission distance up to 15,000 km and total bit rate of 2.56 Tb/s. The 2.56 Tb/s signal is generated by multiplexing 64 OFDM signals with 40 Gb/s for each OFDM. Variations of optical signal to noise ratio (OSNR), signal to noise ratio (SNR), and bit error rate (BER) are studied with the variations of transmission distance. Maximum radio frequency power spectrum, and output electrical power after decoder are measured for different multi level modulation techniques with carrier frequency. It is observed that multi level QAM has presented better performance than multi level PSK and finally multi level DPSK in optical OFDM systems. Maximum output power after decoder is enhanced with both 32-PSK, and 64-QAM. Quadrature signal amplitude level at encoder is upgraded with 64-QAM. It is noticed that OSNR, SNR, and BER are improved using 4-QAM OFDM system than either QPSK or 4-DPSK.