光纤OFDM系统中的色散补偿技术研究

为了减小光纤的色度色散对光纤正交频分复用（OFDM）系统性能的影响,提出了将无线通信中的信道估计器引入光纤OFDM系统进行信道估计的色散补偿方法。在理论上分析了光纤的色度色散对于传输OFDM信号的影响,并在实验中采用梳状导频的形式,在每个OFDM码元的特定的子载波上插入导频,在接收端通过基于最小平方（LS）原则的LS估计器进行信道估计,得到了OFDM光纤传输的信道幅度响应和相位响应,通过使用LS估计器,直接调制的光OFDM信号在单模光纤中传输200km,误比特率低于10-6 ,功率代价小于2dB。结果表明,高频子载波较低频子载波更容易受到色散的影响,在光纤OFDM系统中引入信道估计器进行信道估计能够有效补偿由色度色散带来的相位偏移和幅度衰减。     

基于导频的室内可见光自适应OFDM通信系统信道估计

使用自适应OFDM技术改善可见光通信系统性能,需要以准确的信道估计为前提。研究可见光自适应OFDM通信系统的基于导频的信道估计方法,引入射频通信中常用的LMS方法,与可见光信道基于LS的估计方法进行了比较,并对其特性进行了分析,仿真实验结果表明,采用LMS估计方法在BER为10-3水平时,SNR优于LS估计方法2 dB,LMS信道估计方法信道跟踪能力较强,比LS法抗干扰。     

基于冲激响应加窗法的CO-OFDM系统信道估计研究

针对相干光正交频分复用(CO-OFDM)系统中光纤的色散和信号传输过程中噪声对系统可靠性的降低,提出了信道的冲激响应加窗(IRM,impulse response processed with window)算法,在最小二乘(LS)算法的基础上,通过时域加窗将信道冲激响应长度以外的噪声滤除,保证了信道冲激响应长度在OFDM保护间隔之内。在算法复杂度提高并不大的前提下,IRW算法系统误码率(BER)比LS算法降低了近1个数量级。仿真结果表明,在256个子载波的CO-OFDM系统中,BER为10-4时,IRW算法对系统信噪比(SNR)的要求比LS算法低了近2.5dB,而只比线性最小均方误差(LMMSE)算法高0.7dB;并且,IRW算法的复乘次数仅比LS算法高8倍,而比LMMSE算法的复乘次数低32倍。     

基于SF-MCMA均衡技术的直接检测光OFDM系统实验研究

提出了一种基于分频修正常模算法(SF-MCMA)的均衡技术补偿由光正交频分复用(O-OFDM)传输系统中色散引起的频率选择性衰减(FF)对信号的影响。理论分析了直接检测O-OFDM(DDO-OFDM)传输系统中O-OFDM信号因色散而产生的FF原因,并证明了对O-OFDM不同子载波信号进行SF-MCMA均衡可以纠正色散对OFDM信号的幅度失真和相位偏移的影响。通过实验表明,经100km标准单模光纤(SSMF)传输后,使用SF-MCMA均衡的信号比未使用该均衡的信号系统接收功率代价在(BER)为10e-4时降低了2dB。说明此均衡方法能够有效抵抗色散的影响。同时,此方法中O-OFDM信号不需要训练序列和导频,节省了带宽,可提高信号的频谱利用率。     

一种水下光通信中基于先验模型的预均衡方法

由于海水的散射特性,光在水下传输时会产生码间串扰(ISI)问题,传输环境较差时尤为严重。为解决该问题,文章将预均衡技术应用于基于直流偏置光正交频分复用(DCO-OFDM)调制的水下光通信系统中,并针对水下光的传输特性提出了一种基于先验信息的预均衡方法。在浑浊海港海水环境下与传统均衡算法进行了性能对比。仿真结果表明,在信噪比(SNR)较差的情况下,采用所提方法能够有效降低水下光通信系统的ISI,降低了误码可能性,提高了信道估计的可靠性。相比于最小二乘(LS)后均衡算法,误码率能下降约3.56 dB,相比于LS预均衡算法,误码率能下降约3.00 dB,相较于最小均方误差(MMSE)和最小均方(LMS)后均衡算法,误码率下降约1.85 dB。     

基于DFT的信道估计算法在CO-OFDM系统中研究

信道估计作为相干光OFDM(CO-OFDM)系统的一种 关键技术对系统的性能有着十分重要的影响, 光纤通信系统通过信道估计,可以将信号经过光纤传输后的影响,如损耗、色散、非线性等 ,通过在发送 端发送已知数据,在接收端利用均衡等恢复出原始信号。本文首先介绍了相干光OFDM系统, 并使用 optisystem和matlab软件搭建了相干光OFDM系统仿真平台,提出了适用于相干光OFDM系统 的基于DFT 算法及其进一步的改进算法,并在此平台上进行了仿真试验。结果表明,基于DFT算法及其 改进算法能够 很好地提高相干光OFDM系统的传输性能,改进的DFT算法与DFT算法在BER的值 为10－4的条件下,有1dB的光信噪比增益;与LS算法比较, 有3dB的光信噪比增益。     

基于训练序列进行系统优化的60GHz正交频分复用-光载无线通信系统

分析了60 GHz正交频分复用-光载无线通信(OFDM-RoF)系统中OFDM信号经光纤传输后在光电检测时存在的子载波间互拍干扰(SSBI)现象。提出使用交叉型训练序列结构的OFDM信号来降低SSBI的影响。实验证明,在无色散补偿的情况下,2.5 Gb/s交叉型训练序列结构的基带OFDM信号在58 GHz毫米波系统中经光纤传输20 km后,在误码率为10-3时,其功率代价仅为0.5 dB,而连续型训练序列结构OFDM的功率代价为2 dB。说明基于交叉型训练序列结构的OFDM信号的传输性能在抵抗SSBI方面要明显优于连续型训练序列结构OFDM信号。     

一种改进的载波抑制调制光毫米波信号的产生方案

为了克服光纤无线(ROF)系统中色散对光载波抑制(OCS)调制光毫米波信号传输的影响,提出一种改进的OCS调制方案。使用双驱动马赫-曾德尔调制器(MZM),通过调整两路输入射频信号相位、基带信号增益和直流偏置电压将2.5Gbit/s数据信号仅调制到(OCS)信号的一个边带上传输。理论分析表明,与传统OCS调制光毫米波信号产生方案相比,本文方案解决了色度色散引起的码元走离问题,大大增加了传输距离。仿真实验结果表明,经过110km光纤传输后信号的眼图仍然十分清晰,在BER=10-10条件下,信号经过20、40和60km光纤传输后的功率代价分别为0.78、1.7和1.9dB。     

双模式80波分通道1000km少模光纤传输

为了解决急剧提升的通信系统容量需求与长距离传输等问题，通过实验验证了超大容量的少模光纤传输。在超大容量需求的背景下，同时使用波分复用、模分复用、偏振复用三种复用技术进行信号传输，凭借自研的低损耗六模渐变型光纤（各模式衰减约为0.2 dB/km），实现了覆盖C波段共80个通道，每个通道双模双偏振信号的1000 km传输。考虑到超长距离传输带来的色散和双模双偏振带来的串扰，在进行接收端离线数字信号处理（DSP）时首先使用频域色散补偿算法进行色散补偿，并在下采样和时钟恢复后联合利用多输入多输出-频域最小均方算法（MIMO-FDLMS）和多输入多输出-时域最小均方算法（MIMO-TDLMS）进行信道均衡和色散补偿。在28%冗余的低密度奇偶校验（LDPC）信道编码软判决前向纠错（SD-FEC）阈值5.2×10-2条件下，实现了总的线传输速率40.96 Tbit/s，净速率高达32 Tbit/s。     

基于LDPC-TCM OFDM的60GHz光载毫米波传输性能实验研究

提出采用低密度奇偶校验码-网络编码调制(LDPC-TCM)提高正交频分复用(OFDM)信号在60GHz光载无线通信(ROF)系统中的传输性能。研制了60GHz LDPC-TCM OFDM-RoF系统,实验比较LDPC-OFDM信号和LDPC-TCM OFDM信号在60GHz光载毫米波的性能,当误码率(BER)为10-3时,经过光纤传输20km后,LDPC-OFDM信号的接收机灵敏度比原始OFDM信息提高了15dB,而LDPC-TCM OFDM信号比LDPC OFDM信号提高1dB。结果说明,LDPC-TCMOFDM信号比LDPC OFDM信号在光链路传输系统中具有更佳的纠错能力和更能降低光链路中色散的影响。     

利用自适应均衡滤波器改善大气湍流信道OWC性能

光无线通信是近年来无线通信领域的研究热点之一。大气湍流是影响光无线通信的重要因素,特别是在高速数字通信中会产生严重的码间干扰。本文分析了大气湍流信道的特性,提出采用自适应LMS均衡技术改善大气湍流信道的性能,对采用OOK调制方式的OWC系统均衡前后的误码率性能进行了分析比较。仿真结果表明自适应LMS均衡技术可以将系统的性能提高约10dB。     

Adaptive frequency-domain equalization and diversity combining forbroadband wireless communications

We introduce a new kind of adaptive equalizer that operates in the spatial-frequency domain and uses either least mean square (LMS) or recursive least squares (RLS) adaptive processing. We simulate the equalizer's performance in an 8-Mb/s quaternary phase-shift keying (QPSK) link over a frequency-selective Rayleigh fading multipath channel with ~3 μs RMS delay spread, corresponding to 60 symbols of dispersion. With the RLS algorithm and two diversity branches, our results show rapid convergence and channel tracking for a range of mobile speeds (up to ~100 mi/h). With a mobile speed of 40 mi/h, for example, the equalizer achieves an average bit error rate (BER) of 10 ^-4 at a signal-to-noise ratio (SNR) of 15 dB, falling short of optimum linear receiver performance by about 4 dB. Moreover, it requires only ~50 complex operations per detected bit, i.e., ~400 M operations per second, which is close to achievable with state-of-the-art digital signal processing technology. An equivalent time-domain equalizer, if it converged at all, would require orders-of-magnitude more processing     

A nonlinear equalization for a PAM4 IM/DD system using MZM

The nonlinear distortion caused by the Mach-Zehnder modulator (MZM) is one of the main limiting factors for the improvement of the transmission performance of the intensity modulation and direct detection (IM/DD) optical com-communication proposed. Compared with the traditional VNLE that uses the least mean square (LMS) to calculate the tap coefficients,the improved VNLE uses the least square (LS) method to obtain more stable convergence. The simulation results show that the VNLE based on LS has better performance when solving complex nonlinear damage. For 25 Gbaud 4-level pulse amplitude modulation (PAM4) signals, the improved VNLE can reduce the bit error rate (BER) to below 10-4 in a 7-km-long single-mode optical fiber transmission system. In addition, in order to make the BER below 10-3, the transmission distance that the improved VNLE can withstand is about 1.5 km longer than that of the traditional VNLE.     

Experimental evaluation of the BER performance in optical OFDM system based on discrete Hartley transform precoding

We experimentally assess the bit error rate （BER） performance of an intensity modulation/direct detection （IM/DD） optical orthogonal frequency division multiplexing （OFDM） system based on discrete Hartley transform （DHT） precoding in single-mode fiber （SMF） link for 2.5 Gbit/s quadrature phase shift keying （QPSK） OFDM symbol rate. The experimental results show that for the optical OFDM system based on DHT-precoding, the receiver sensitivity at the BER of 10-4 after 100 km SMF transmission is about 1.5 dBm better than that of the original QPSK OFDM signal, and the DHT-precoded OFDM QPSK signal can achieve approximately 1.3 dB of peak-to-average power ratio （PAPR） reduction.     

Fuzzy stochastic gradient decision feedback equalizer for VSBterrestrial HDTV broadcasting

We present a fuzzy stochastic gradient (FSG) decision feedback equalizer (DFE) for VSB terrestrial HDTV broadcasting. This equalizer employs a well-designed fuzzy Takagi-Sugeno (1985) model to automatically regulate the step size of the descent gradient vector, combining a fast convergence rate and a low excess mean square error (MSE). The only penalty paid is a slight increase in the computational complexity compared with the LMS algorithm. Simulation results show that this equalizer provides 3.5 dB signal-to-noise ratio (SNR) improvement at a BER of 3.0×10^-6 with respect to the conventional LMS DFE recommended by the Grand Alliance     

Compensating for Nonlinear Effects in Coherent Detection Optical Transmission Systems

Fiber nonlinearity is one of the most important limiters of capacity in coherent optical communications. In this paper, we review two nonlinear compensation methods: digital backward propagation (BP) and nonlinear electrical equalizer (NLEE) based on the time-domain Volterra series. These compensation algorithms are implemented in a single-channel 50 Gb/s coherent optical single-carrier frequency-division multiplexed (CO-SCFDM) system transmitting over 10 × 80 km of standard single-mode fiber (SSMF).     

少模光纤通信系统中的自适应频域均衡算法

少模光纤模式复用存在模式耦合和差分模式时延,必须通过自适应均衡算法补偿。为了降低长距离少模光纤通信系统中自适应均衡算法的复杂度,采用基于变步长-频域块最小均方算法的多输入多输出均衡器对2×2模分复用系统解复用。利用频域块最小均方自适应算法修正均衡器权系数,并通过变步长函数调整步长因子,兼顾算法收敛速度和收敛性能。算法可通过快速傅里叶变换降低计算复杂度。在112Gbit/s的1000km少模光纤高速通信仿真系统中,保证相同收敛速度情况下,提高信号Q2因子3.7dB,并在可编程现场门阵列上验证了100km少模光纤通信系统时的算法性能。结果表明,该算法能够实现模分复用系统的信号解复用,达到快速收敛、低稳态失调的目的。     

基于压扩变换的直接检测O-OFDM系统的实验研究

仿真研究并实验验证了直接检测光正交频分复用（O-OFDM）系统中引入压扩变换（CT）技术可降低OFDM信号峰值平均功率比（PAPR）和提高接收灵敏度。仿真结果表明,压扩系数μ越大,信号PAPR越低;随着μ的增大,系统误码率（BER）呈现先下降后上升的趋势。权衡PAPR的降低度和系统BER,选择μ=2时为最佳。实验研究表...     

MLSE Equalizers for Frequency Discrimination Receiver of MSK Optical Transmission System

We propose maximum-likelihood sequence estimator (MLSE) equalizers based on either Viterbi algorithm or template matching temple matching (TM) for the equalization of impairments imposed on the minimum shift keying (MSK) modulation formats in long haul transmission without optical dispersion compensation. The TM-MLSE equalizer is proposed as a simplified alternative for the Viterbi-MLSE equalizer. It is verified that the Viterbi-MLSE equalizer can operate optimally when noise approaches a Gaussian distribution. Simulation results of the performances of the two MLSE equalizers for optical frequency discrimination receiver-based optical MSK systems are described. The transmission performance is evaluated in terms of: (1) the chromatic dispersion (CD) tolerance for both Viterbi-MLSE and TM-MLSE equalizers; (2) transmission distance limits of Viterbi-MLSE equalizers with various number of states; (3)the robustness to fiber polarization mode dispersion (PMD) of Viterbi-MLSE equalizers; and (4) performance improvements for Viterbi-MLSE equalizers when utilizing sampling schemes with two and four samples per bit over the conventional single sample per bit. With a small number of states (64 states), the non-compensating optical link can equivalently reach up to approximately 928 km SSMF for 10 Gb/s transmission or 58 km SSMF for 40 Gb/s. The performance of 16-state Viterbi-MLSE equalizers for optical frequency discrimination receiver (OFDR)-based optical MSK transmission systems for PMD mitigation is also numerically investigated. The performance of Viterbi-MLSE equalizers can be further improved by using the sampling schemes with multiple samples per bit compared to the conventional single sample bit. The equalizer also offers high robustness to fiber PMD impairment.