Employing inverse return-to-zero and Manchester formats for uplink wavelength reuse in RoF systems

The inverse return-zero (IRZ) modulation format and Manchester format were investigated as wavelength reusing schemes in the RoF downlink systems. Since both of the modulation formats featured power remaining in the bit slot regardless of the bit value, the downlink optical carrier can be reused as the uplink light source. The performances of two formats were analyzed in the symmetrical and asymmetrical duplex RoF system. It was found that IRZ is more suitable for symmetrical system, and the duty cycle of IRZ pulse is the critical factor on the asymmetrical RoF system performance. However the Manchester format demonstrated the large system tolerance on asymmetrical duplex RoF system with the receiver sensitivity degradation within 1 dB.     

Digital signal processing for coherent multi-level modulation formats

This paper presents a review on the recent progress of digital signal processing （DSP） in the high-speed optical transmission system using single-carrier based multi-level modulation formats. The principle of several digital phase and polarization tracking algorithms recently proposed and demonstrated for future spectrally-efficient optical transmission system are discussed in detail. A novel DSP-based interference mitigation algorithm for the single-ended coherent receiver has been included. Recent technology advance on transmitter-side DSP such as digital pulse-shaping, pre-equalization and digital nonlinear compensation has also been discussed.     

室内可见光无载波幅度相位调制系统性能分析

针对室内可见光通信调制技术问题,提出翻转光无载波幅度相位调制和单极性光无载波幅度相位调制两种功率有效的调制方案,二者分别采用"正、负模块极性分组"以及"零值位置极性编码"方法实现信号单极性处理,以满足可见光通信"强度调制/直接检测"的要求.基于朗伯辐射模型,考虑到高斯背景光噪声的可见光直射传输信道,推导了包括直流偏置光无载波幅度相位调制在内的三种调制方案的误比特率闭式表达式,仿真验证了其准确性.在此基础上,分析比较了三者频带利用率,讨论了信道参量对光无载波幅度相位调制系统误码性能的影响,结果表明,在5m×5m×3m的室内场景下,与发射机辐射角为30°和45°相比,0°时的系统误码性能分别优于6.9dB和29.9dB;收发机距离为1m时,误码性能比2m时改善近12dB.     

Performance of directly modulated OFDM data over multimode fiber link

The performance of orthogonal-frequency-division-multiplexing (OFDM) technology in the multimode fiber (MMF) optical communication system is investigated numerically. The mode dispersion in MMF is analyzed by utilizing the optical angular spectrum analysis method. Simulation results in terms of bit-error-rate (BER) and signal-to-noise ratio (SNR) show that the transmission distance can reach up to 2.5-km at 10^− 3 BER for 40-Gb/s data. Results of different modulation formats further indicate that better performance can be achieved by adopting multi-level modulation format.     

Dual-ring 16-Star QAM direct and coherent detection in 100 Gb/s optically amplified fiber transmission: simulation

This paper presents the transmission performances, by simulation, of optical communication systems over 10 spans of dispersion compensated and optically amplified fiber distance with a bit rate of 100 Gb/s, by employing modulation formats of two amplitude levels and 8 phase states per amplitude level, the 2R-16-Star QAM constellation under direct and coherent detection with and without phase estimation. Different ring ratios of the amplitude levels are examined and associate transmission performances are reported. Optical signal to noise ratio is achieved with 18 dB and 23 dB for back to back and long haul transmission cases with a dispersion tolerance of  ±67 ps/nm at 2 dB power penalty of the eye opening at 100 Gb/s. Monte–Carlo simulation is also performed and a receiver sensitivity of  − 15 dBm is achieved for a BER of 10⁻⁵under direct detection after 1100 km of dispersion-compensated and optically amplified transmission. Transmission performances, bit error rate versus receiver sensitivity, are also confirmed with the use of the eye diagram and associate multiple-peaks statistical spectral density distribution. For 100 Gb/s 2R-16-Star QAM coherent transmission, an improvement of the receiver sensitivity of 2.5 dB and 3.5 dB is obtained for coherent detection without phase estimation and respectively. Under coherent detection with phase estimation, the chromatic dispersion tolerance reaches  ±100 ps/nm for a 2 dB eye opening penalty at 100 Gb/s bit rate. Comparative studies of the transmission performances of the Star and Square QAM modulation formats are also conducted under the fiber linear and nonlinear effects and detection with and without phase estimation.     

基于超结构光纤光栅的全光编解码性能分析

分析了光码分多址(OCDMA)系统中超结构光纤光栅(SSFBG)编解码器的相关特性, 考虑了输入脉冲宽度、SSFBG编解码光栅之间的波长偏移以及光栅的折射率调制振幅对全光编解码性能的影响。结果表明, 随着输入脉冲宽度和编解码光栅之间的波长偏移量的增加, 自相关峰值旁瓣比和自互相关峰值比下降, 即编解码性能出现下降; 编解码器的插入损耗和相关性能间存在矛盾, 需要折中考虑SSFBG折射率调制振幅的选取。建立了基于SSFBG编解码器的时域相位编码OCDMA系统的数学模型, 考虑了差拍噪声、多址干扰、接收机噪声以及接收机的带宽限制对系统性能的影响, 采用全光阈值技术和turbo编码来提高相干扩时OCDMA的系统性能。     

Simulation of high capacity 40 Gb/s long haul DWDM system using different modulation formats and dispersion compensation schemes in the presence of Kerr's effect

In this paper, simulative analysis of 40 Gb/s long haul (500-2000 km) DWDM system with ultra high capacity upto 1.28 Tb/s has been carried out for carrier-suppressed return-to-zero (CSRZ), duobinary return-to-zero (DRZ) and modified duobinary return-to-zero (MDRZ) modulation formats. The DWDM system has been analyzed for the pre, post and symmetrical dispersion compensation schemes for 16 Channels with 25 GHz channel spacing in order to find the optimum modulation format for a high bit rate optical transmission system. The effect of variation in input power and transmission distances is observed in terms of Q value and eye opening for various formats. It is found that symmetrical compensation is superior to pre and post dispersion compensation schemes. It has also been observed that the performance of DWDM system is severely limited by the four-wave mixing (FWM) effect and is determined that MDRZ format seems to be the best choice for the transmission distance beyond 1550 km despite slightly more complex transmitter and receiver configuration. Further, symmetrical compensation scheme has been investigated for 32×40 Gb/s MDRZ format for faithful transmission over 1450 km.     

Analysis of inter-satellite free-space optical link performance considering different system parameters

The welcome and adaptation of optical wireless technology by the modern era has brought forward the concept of an inter-satellite free-space optical communication system. In the present work, I study the combined effect of selection of different operating wavelengths and detector types along with the pointing errors at the transmitter and receiver side on the performance of an inter-satellite free-space optical link. The link performance has been optimized by measuring and analyzing the bit error rate and quality-factor of received signal under different scenarios. Performance of the inter-satellite link has also been investigated considering different modulation formats and data rates for LEO and MEO distances.     

Performance analysis of FSO links under strong atmospheric turbulence conditions using various modulation schemes

Strong atmospheric turbulence is a major hindrance in wireless optical communication systems. In this paper, the performance of a wireless optical communication system is analyzed using different modulation formats such as, binary phase shift keying-subcarrier intensity modulation (BPSK-SIM), differential phase shift keying (DPSK), differential phase shift keying-subcarrier intensity modulation (DPSK-SIM), M-ary pulse position modulation (M-PPM) and polarization shift keying (PolSK). The atmospheric channel is modeled for strong atmospheric turbulences with combined effect of turbulence and pointing errors. Novel closed-form analytical expressions for average bit error rate (BER), channel capacity and outage probability for the various modulation techniques, viz. BPSK-SIM, DPSK, DPSK-SIM, PolSK and M-PPM are derived. The simulated results for BER, channel capacity and outage probability of various modulation techniques are plotted and analyzed.     

Toward an optimum use of the optical channel capacity

Abstract

In recent decades, in addition to long-haul, high-capacity links, research activity in optical fiber communication systems has addressed possible advantageous applications in distribution networks, local and metropolitan area networks, CATV backbones and distribution, and high-speed computer networks. As a consequence, new possibilities in optical transmission systems have been explored to face the new requirements in terms of capacity, spectral efficiency, and receiver sensitivity. Among the possible solutions capable to exploit effectively the potentials offered by the optical channel, novel multilevel modulation formats have been proposed for future communication environment. This paper reviews multilevel modulation techniques for optical transmission: a comparative analysis is performed between conventional modulations, such as N-PSK and N-QAM, and new modulation/demodulation techniques, such as N-4QSK, N-SPSK, and PM-DD, taking into account both system performance and technological constraints.     

高速线性光采样用被动锁模光纤激光器重复频率优化

线性光采样是一种测量基于先进调制码型的高速光信号的有效手段, 而被动锁模光纤激光器是其实施所需的关键组件. 本文在介绍线性光采样工作原理的基础上, 首次分析得到被动锁模光纤激光器重复频率与待测信号光线宽的约束关系, 对于正交相移键控(QPSK)信号, 当信号光线宽与采样光脉冲重复频率的比值小于1.5×10^-3 时, 高速信号的相位噪声对线性光采样带来的损伤可以忽略不计. 利用95.984 MHz重复频率的被动锁模光纤激光器对线宽为100 kHz速率为28 Gbaud的QPSK信号开展相关实验, 通过标准数字相干接收算法可以得到与传统高速示波器相同的星座图, 理论分析与实验结果完全符合. 这一研究结果有助于线性光采样用被动锁模光纤激光器的优化设计.     

Investigation on wavelength re-modulated bi-directional passive optical network for different modulation formats

In this paper, the cost effective bi-directional passive optical network architecture with wavelength remodulated scheme is investigated. To realize the cost-effective PON, remodulation scheme is used, in which the downstream optical signal is reused as a carrier for the upstream transmission as it eliminates the need for an extra laser source at optical network units. The performance of proposed passive optical network is analyzed and compared for various modulation formats such as Non Return to Zero (NRZ), Return to Zero (RZ) and On–Off Keying (OOK) with 64 optical networks units (ONUs) at different traffic speed for downlink and uplink, respectively. It has been observed that the most suitable data format for proposed PON network is NRZ. Further the proposed system performance is compared with the current state-of-the-art PON architectures.     

Improvement of transmission performance of adaptively modulated optical OFDM signals

An adaptively modulated optical orthogonal frequency division multiplexing (OOFDM) based system has been simulated in present research. Performance analysis in terms of bit error rate (BER) and signal to noise ratio (SNR) has been carried over the system using fixed modulation for different M-array quadrature amplitude modulation (M-QAM). An adaptive modulation over OOFDM has been analyzed over three different algorithmic schemes. Impact of adaptive modulation on spectral efficiency has also been investigated for OOFDM. Simulation results show the enhancement in terms of spectral efficiency and capacity for adaptively modulated OOFDM system.     

An optical continuous phase FSK modulation scheme with an arbitrary modulation index over long-haul transmission fiber link

This paper presents an optical continuous phase frequency shift keying (CPFSK) modulation scheme with an arbitrary modulation index. The detailed principle on the optical CPFSK generation is derived and analyzed, which includes the special case of the minimum-shift keying (MSK) with a modulation index h = 1/2. The differential detection and the coherent detection of CPFSK are also depicted. The performances of the four kinds of the optical CPFSK modulated system with a 40 Gb/s modulation rate whose modulation index are h = 1/2, h = 2/3, h = 3/4 and h = 1 are simulated via the spectral efficiency and the receiver sensitivity over fiber link respectively. In addition, comparison with the differential phase shift keying (DPSK) is taken. Through the calculation of the spectral efficiency of each modulation formats, CPFSK has higher spectral efficiency than DPSK with the same optical devices. The transmission performances of our CPFSK over the fiber link change better as the modulation index increases under the condition of the first order dispersion of the fiber link is completely compensated. Through simulations, a 1200 km transmission distance can be achieved with a modulation index h = 1.     

Performance of phase compensated coherent free space optical communications through non-Kolmogorov turbulence

The bit-error-rate (BER) performance of coherent free-space optical (FSO) links employing phase compensation techniques is investigated in weak non-Kolmogorov turbulence. Assuming that the amplitude fading and phase fluctuation follow lognormal model and Gaussian distribution respectively and using the expression of non-Kolmogorov turbulence in terms of Zernike polynomials, the signal-to-noise ratio (SNR) at the coherent receiver is analyzed and as a special case, a new closed-form expression using chi-square distribution is obtained. Thus, the influence of different compensation modes and normalized receiver diameter on BER performance is evaluated and an optimum normalized receiver diameter is suggested to achieve the minimum BER. Moreover, the impact of outer scale L₀ and the exponent value α in non-Kolmogorov spectrum is studied with the optimum diameter, which reveals that the BER has an obvious decrease with larger values of L₀ and α.     

Extending Fibre Nonlinear Interference Power Modelling to Account for General Dual-Polarisation 4D Modulation Formats

In optical communications, four-dimensional (4D) modulation formats encode information onto the quadrature components of two arbitrary orthogonal states of polarisation of the optical field. Many analytical models available in the optical communication literature allow, within a first-order perturbation framework, the computation of the average power of the nonlinear interference (NLI) accumulated in coherent fibre-optic transmission systems. However, all such models only operate under the assumption of transmitted polarisation-multiplexed two-dimensional (PM-2D) modulation formats, which only represent a limited subset of the possible dual-polarisation 4D (DP-4D) formats. Namely, only those where data transmitted on each polarisation channel are mutually independent and identically distributed. This paper presents a step-by-step mathematical derivation of the extension of existing NLI models to the class of arbitrary DP-4D modulation formats. In particular, the methodology adopted follows the one of the popular enhanced Gaussian noise model, albeit dropping most assumptions on the geometry and statistic of the transmitted 4D modulation format. The resulting expressions show that, whilst in the PM-2D case the NLI power depends only on different statistical high-order moments of each polarisation component, for a general DP-4D constellation, several other cross-polarisation correlations also need to be taken into account.     

Toward an optimum use of the optical channel capacity

In recent decades, in addition to long-haul, high-capacity links, research activity in optical fiber communication systems has addressed possible advantageous applications in distribution networks, local and metropolitan area networks, CATV backbones and distribution, and high-speed computer networks. As a consequence, new possibilities in optical transmission systems have been explored to face the new requirements in terms of capacity, spectral efficiency, and receiver sensitivity. Among the possible solutions capable to exploit effectively the potentials offered by the optical channel, novel multilevel modulation formats have been proposed for future communication environment. This paper reviews multilevel modulation techniques for optical transmission: a comparative analysis is performed between conventional modulations, such as N-PSK and N-QAM, and new modulation/demodulation techniques, such as N-4QSK, N-SPSK, and PM-DD, taking into account both system performance and technological constraints.     

Ground-to-Satellite Optical Communication Link Performance with Spatial Diversity in Weak Atmospheric Turbulence

Abstract

In this article, performance improvement of a free-space optical communication link with spatial diversity is evaluated for coherent (sub-carrier BPSK and sub-carrier QPSK) and non-coherent (OOK and Q-PPM) modulation schemes in an atmospheric turbulence fading channel. Based on Kolmogorov's theory, the link performance characterized by the signal-to-noise ratio or bit error rate is evaluated for both modulation schemes. It is observed that there is performance improvement with the increase in the order of transmit diversity for all the modulation schemes. Sub-carrier BPSK modulation outperforms the other modulation schemes in terms of the minimum signal-to-noise ratio requirement for a given bit error rate. Also, the effect of correlation among different transmitting antenna beams on the link performance is analyzed.     

Double Security Level in Optical Communication Systems Using Combined Modulation Formats

A new optical communication system exhibiting a double security level is presented. Information coding is based on combined modulation formats of two data streams over the same optical carrier. Incoherent and coherent optical encryption algorithms are presented and are theoretically analyzed. The optical encryption method is transparent to data bit rates. Furthermore, it requires "on-line" data decoding with no possibility of recording and further processing. The incoherent scheme is very practical and can be implemented in existing optical communications systems, using currently available technology. Computer experiments of the incoherent method indicate less than 25 dB of signal to noise ratio required for 10-9 bit error rate and ineligible information decoding time of more than 10 8 years.     

Performance analysis of different modulation formats in 4-channel CATV transmission system using OADM

In this paper we have analyzed the performance of different modulation formats for a four-channel WDM CATV system using optical ad drop multiplexers and the impact of frequency and wavelength on Q-value, and eye opening is observed for added and dropped channels at different lengths.