基于多层非对称剪裁光正交频分复用的水下光无线通信系统

为了提高水下光无线通信(UOWC)的频谱效率和功率效率,提出了一种基于多层非对称剪裁光正交频分复用(Layered-ACO-OFDM)的UOWC系统.该系统通过对子载波分层调制的方法,充分利用了传统非对称剪裁光正交频分复用(ACO-OFDM)中损失的频谱资源,极大地提高了光无线通信系统的频谱效率.此外,Layered-ACO-OFDM系统不需要加入直流偏置,故功率效率也得到了大大提升.考虑了可见光在海水中传输过程中的吸收和散射效应,建立了基于蒙特卡罗方法的UOWC信道模型,并较为全面地研究了海水中不同叶绿素浓度和不同接收机参数下的信道响应.结果 表明,与传统ACO-OFDM相比,基于Layered-ACO-OFDM的UOWC系统具有频谱效率高的优势.同时,当信噪比为27 dB时,在清澈海水中其可以实现10m距离下1 Gb/s的高速率通信.     

限幅OFDM的一种信道估计优化方法

在OFDM 系统中,限幅常用于抑制信号的峰平比,而限幅会引入额外的限幅噪声,以致干扰数据和导引符号,恶化传输性能.本文针对限幅OFDM系统,提出一种基于导引符号的限幅噪声消除算法.该算法在插入导引符号之前,滤除位于导引位置的限幅噪声,从而改善信道估计性能.在此基础上,本文还提出了利用迭代算法以进一步提高信道估计精确度.本文对所提出的限幅噪声消除算法及迭代算法进行了仿真验证,结果表明,新算法能够明显改善信道估计,优化传输性能.     

基于信道估计的限幅OFDM信号检测性能分析

大峰均比是正交频分复用(OFDM)系统的主要缺陷之一，对时域OFDM信号限幅是通常采用的一种抑制发送信号峰值功率的方法，但是限幅产生的非线性干扰，使系统的信道估计和信号检测性能降低。本文分析了限幅干扰对信道估计以及基于估计信道信息的OFDM数据信号检测性能的影响，推导出了基于信道估计信号检测的信噪比与限幅参数之间的关系。利用信噪比和高斯信道的误符号率及误比特率公式，本文对频率选择性瑞利衰落信道环境限幅OFDM系统中信号的误符号率和误比特率进行估算。计算机仿真和估算的结果相吻合。     

Error probability of MPSK OFDM impaired by carrier frequency offset in AWGN channels

In this letter, we evaluate the symbol error probability of orthogonal frequency division multiplexing (OFDM) with M-ary (M>4) phase shift keying (MPSK) impaired by carrier frequency offset (CFO). A signal space decomposition (SSD) approach is proposed for additive white Gaussian noise (AWGN) channels, and then the symbol error rate (SER) performance is expressed as the sum of an infinite series with the Beaulieu series. The derived theoretical expression agrees well with the simulation results     

Better performance of OFDM using three special triangular constellations over AWGN and fading channels

Orthogonal frequency division multiplexing (OFDM) has been adopted as a major data transmission technique by many wireless communication standards. In this research, 3 new triangular constellations schemes, which are named as TRI1, TRI2, and TRI3, are introduced to replace for the well‐known rectangular quadrature amplitude modulation (QAM) constellation in OFDM modulation. In this study, it has been shown that these new schemes have 3 major advantages with respect to the QAM. The first advantage is their lower bit error rate, which results from the better usage of the constellation space with longer minimum distances. The 2 other advantages are a lower peak to average ratio and higher noise immunity. Both mathematical analysis and simulation results demonstrate that by applying high fading channels with additive white Gaussian noise and intersymbol interference impairment simultaneously, the proposed constellations exhibit a superior performance in criteria compared to the commonly used rectangular 16QAM and 64QAM constellations. As a result, they are good choice for high speed and real‐time multicarrier applications such as digital video broadcasting terrestrial at no extra cost.     

正交频分复用技术在光通信中的应用

光正交频分复用（O-OFDM）是最近几年被广泛研究和关注的新技术,表现出频谱效率高、色散补偿方便有效、减小系统非线性效果明显等诸多优点。文中首先给出了OFDM的基本原理,然后研究了O-OFDM的发展历程和相关研究成果,分析了它在具体应用中的优势和面临的主要问题,最后展望了未来O-OFDM技术的应用前景。     

OFDM for Optical Communications

Orthogonal frequency division multiplexing (OFDM) is a modulation technique which is now used in most new and emerging broadband wired and wireless communication systems because it is an effective solution to intersymbol interference caused by a dispersive channel. Very recently a number of researchers have shown that OFDM is also a promising technology for optical communications. This paper gives a tutorial overview of OFDM highlighting the aspects that are likely to be important in optical applications. To achieve good performance in optical systems OFDM must be adapted in various ways. The constraints imposed by single mode optical fiber, multimode optical fiber and optical wireless are discussed and the new forms of optical OFDM which have been developed are outlined. The main drawbacks of OFDM are its high peak to average power ratio and its sensitivity to phase noise and frequency offset. The impairments that these cause are described and their implications for optical systems discussed.      

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。     

光OFDM系统中几种同步算法的对比分析

对光OFDM系统中的三种同步算法进行了理论分析,基于仿真实验,针对定时度量、定时偏差、丢帧率三个性能参数进行了对比分析。仿真分析表明T.Schmidl＆D.Cox算法由于循环前缀（Cyclic Prefix,CP）缘故使得其定位精确性欠佳,偏差较大,无法进行精确定位,但算法稳定性及丢帧率方面具有很好的性能。而H.Minn和Li-xun Huang＆Li-jun Sun两种同步算法定位精确性相对较好,偏差较小,但在稳定性及丢帧率方面,后者优于前者。因而在这三种同步算法中,Li-xun Huang＆Li-jun Sun算法性能较好,适应性较强,更具实际应用价值。     

PMD-Supported Coherent Optical OFDM Systems

Although polarization-mode dispersion (PMD) greatly impairs conventional high-speed single-carrier systems, it is shown that for multicarrier systems such as coherent optical orthogonal frequency-division-multiplexed systems (CO-OFDM), not only does PMD not cause any impairment, but it also provides a benefit of polarization diversity against polarization-dependent-loss-induced fading and consequently improves the system margin. The PMD benefit to fiber nonlinearity reduction in CO-OFDM systems is also predicted     

光OFDM在接入网中的应用及仿真分析

正交频分复用(OFDM)频谱利用率高、抗干扰性强、能够有效对抗单模光纤偏振模色散,在高速光通信系统中有很好的应用前景.利用Matlab和Optisystem,对强度调制(IM)/直接检测(DD)的单模光纤系统进行了性能仿真,结果显示在不进行色散补偿和光放大的前提下能够传输40km,满足接入网的要求.     

Phase Estimation for Coherent Optical OFDM

Phase estimation is one of the enabling functionalities in coherent optical orthogonal frequency-division-multiplexing (CO-OFDM) receivers. In this letter, we compare pilot-aided and data-aided phase estimation methods for a CO-OFDM transmission experiment at 8 Gb/s over 1000-km standard single-mode fiber without optical dispersion compensation. We also show that as few as five subcarriers are sufficient for pilot-aided phase estimation     

Bit and Power Loading for Coherent Optical OFDM

We show the first experiment of bit and power loading for coherent optical orthogonal frequency-division-multiplexing (CO-OFDM) systems. The data rate of CO-OFDM systems can be dynamically adjusted according to the channel condition. The system performance can be further improved through optimal power loading into each modulation band.     

超100Gb/秒的光OFDM在光长度系统的传输

100 G Ethernet is considered to become the next generation Ethernet standard for IP networks. Typical 100 Gb/s transmission systems and their performance are presented. Comparision and analysis for 100 Gb/s transmission systems have been discussed. It is demonstrated that optical OFDM can be used in future 100 Gb/s/ch and long-haul system.     

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     

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。     

Maximum-Likelihood Phase and Channel Estimation for Coherent Optical OFDM

We present a channel model for a coherent optical orthogonal frequency-division-multiplexed (CO-OFDM) system including linear fiber dispersion effects and noises from optical amplifiers and intercarrier interference induced by laser phase noise. Based upon this model, we derive maximum-likelihood (ML) phase estimation and channel estimation for the CO-OFDM system. Both computer simulation and transmission experiment of the CO-OFDM system show that the ML decision-feedback following pilot-assisted phase estimation gives the optimal performance.     

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     

Theoretical and Experimental Investigations of Direct-Detected RF-Tone-Assisted Optical OFDM Systems

In this paper, we propose and experimentally demonstrate a radio frequency (RF)-tone-assisted optical orthogonal frequency-division multiplexing (OFDM) transmission. By inserting an RF tone at the edge of the signal band and biasing the Mach–Zehnder modulator (MZM) at the null point, the proposed system has a better sensitivity and chromatic dispersion (CD) tolerance compared to the previous intensity-modulated single-sideband OFDM (SSB-OFDM). We show analytically that the majority of the linear channel impairments, such as the transmitter, CD, optical filtering, and receiver, can be compensated for by a simple zero-forcing equalizer. Besides, the optimum value of the important parameter, carrier-to-signal-power ratio (CSPR), is analytically obtained and supported via the experimental results. We also observe that the relatively worse sensitivity of the previous SSB-OFDM can be attributed to the limited CSPR. We experimentally demonstrate a 10-Gb/s, 8 quadrature-amplitude modulation (QAM) RF-tone-assisted OFDM transmission, and show that our system has a ${sim}$5-dB better sensitivity compared to the previous intensity-modulated SSB-OFDM and exhibits a negligible transmission penalty after 260-km uncompensated standard single-mode fiber (SSMF).      

Equivalent Lumped Elements of DC-Biased Thin Film Microstrip Line in MMICs

This letter investigates the equivalent lumped elements of the thin film microstrip line (TFML) with different DC-bias voltages for the first time. The wide-band frequency-dependent characteristic impedance Z_o(f) and equivalent lumped elements such as R(f), L(f), G(f), and C(f) are accurately characterized for DC-biased TFML on a 20-m thick low dielectric constant (K) polyimide up to 50 GHz. The experimental results shows the DC-biased TFML can be extensively applied to the voltage-controlled passive and active integrated circuits.