Coherent Optical Orthogonal Frequency Division Multiplexing Transmission Research Articles

Low complexity partition-recombined SLM scheme for reducing PAPR in coherent optical OFDM systems

High peak-to-average power ratio (PAPR) remains the main challenge of coherent optical orthogonal frequency division multiplexing (CO-OFDM) systems. Selective mapping (SLM) is a well-known effective PAPR reduction technique, but it suffers from high computational complexity. To reduce the PAPR of OFDM signals, a low-complexity, partition-recombined SLM (PRSLM) scheme is proposed. The PRSLM scheme can generate more candidate signals than that of the conventional-original SLM (COSLM) scheme. The real and imaginary components of time-domain symbols are first partitioned, and then recombined to generate new candidate signals, which reduces computational complexity significantly. A 40-GBaud single-polarization CO-OFDM transmission system is set up to evaluate the performance of the PRSLM scheme. Simulation results show that the proposed scheme achieves 61.9% reduction in complex multiplication complexity, and 60.3% reduction in addition complexity compared with the COSLM scheme when the FFT size is 128. Furthermore, the proposed scheme achieves a similar PAPR reduction performance to the COSLM scheme without bit error rate degradation. In a nutshell, the PRSLM scheme improves noise tolerance and implementation efficiency of high-speed optical communication as well as alleviates nonlinear impairments.

Flexible and fast multi‐level modulation format identification using optical orthogonal frequency division multiplexing redundancy subcarriers

AbstractCompared with the orthogonal frequency division multiplexing (OFDM) communication system, the subcarrier‐index modulation OFDM communication system can alleviate the problem of peak‐to‐average power ratio and the frequency offset sensitivity. Therefore, the problem of inter carrier interference (ICI) and low power of the radio frequency amplifier in OFDM communication system can be solved to some extent. A fast and flexible modulation format recognition (MFR) scheme utilizing OFDM redundancy subcarriers is proposed in order to solve the MFR problem in coherent optical OFDM transmission. The scheme uses the silent subcarrier position of OFDM to carry modulation format information (MFI) and the receiving end decodes the MFI index by comparing the modulus values of the redundant symbols. It can recognize the phase shift keying (PSK) and multiple quadrature amplitude modulation (M‐QAM) modulation formats block by block. The number of symbols required by the proposed MFR is equal to the number of OFDM subcarriers, and the average time to get the recognition result in a block is O(1). We numerically investigate the proposed MFR among 20 Gbaud 4/16/32/64 QAM and 8/16 PSK. The results show that the correct recognition rate of the proposed scheme is not affected by the modulation formats. In particular, the proposed MFR is a potential solution for future flexible optical networks.

Channel Equalization and Phase Noise Compensation Free DAPSK-OFDM Transmission for Coherent PON System

Channel equalization and phase noise compensation are required in coherent optical orthogonal frequency-division multiplexing (CO-OFDM) transmission systems for reliable performance. In general, redundant data such as training symbols and pilot tones are necessary for channel and phase noise estimation in quadrature amplitude modulation (QAM) based CO-OFDM systems, which reduces the spectral efficiency of the system. Especially, in the coherent passive optical network (PON), the redundancy increases as the number of subscribers increases. In this letter, we propose a differential amplitude shift keying (DAPSK)-based CO-OFDM transmission for PON system, which does not require channel equalization and phase noise compensation processes. The proposed DAPSK-based CO-OFDM transmission was experimentally demonstrated, and the performance according to receiver launched power and the number of subcarriers was compared with QAM-based CO-OFDM.

Joint timing/frequency offset estimation and correction based on FrFT encoded training symbols for PDM CO-OFDM systems.

A joint timing offset (TO) and frequency offset (FO) estimation algorithm is proposed for polarization division multiplexing (PDM) coherent optical orthogonal frequency-division multiplexing (CO-OFDM) systems. It is realized by taking the advantage of the time-frequency property of the fractional Fourier transformation (FrFT) encoded training symbols. Compared with the classical Schmidl & Cox method, the proposed algorithm exhibits robust estimation result of timing offset with poor optical signal-to-noise ratio (OSNR) and nonlinear interference. For the frequency offset estimation, a quite large FO estimation ranges of [-5GHz + 5GHz] can be achieved. The mean normalized estimation error can be kept under 0.002 and the max normalized estimation error is no more than 0.008. The feasibility and effectiveness of the proposed joint estimation algorithm has been verified by experiments. The transmission performances with [-5GHz + 5GHz] FO are compared under the OSNR range from 14 to 27dB in a 106.8Gbit/s 16-ary quadrature amplitude modulation (16-QAM) PDM CO-OFDM transmission system. The proposed TO/FO estimation algorithm performs robustly and accurately without any induced BER degradations.

Radial Basis Function Neural Network Nonlinear Equalizer for 16-QAM Coherent Optical OFDM

We propose a radial basis function neural network (RBFNN)-based nonlinear equalizer (NLE) for coherent optical orthogonal frequency division multiplexing (CO-OFDM) systems. The hidden layer neuron weights of the RBFNN-NLE are calculated using the $K$ -means clustering algorithm and the output layer weights are updated using the least mean square algorithm. With only 3% overhead in training, the proposed RBFNN-NLE was found to provide up to 4-dB performance improvement in terms of $Q$ -factor for 70-Gb/s 16-QAM CO-OFDM transmission over 1000 km ( $10\times 100$ km) fiber. Numerical results show that the operating data rate is 80 Gb/s at $Q = 6.25$ dB with the proposed RBFNN-NLE, compared with previously reported value of 70 Gb/s with artificial neural network-based NLE.

Demonstration of Phase-Conjugated Subcarrier Coding for Fiber Nonlinearity Compensation in CO-OFDM Transmission

In this paper, we demonstrate through computer simulation and experiment a novel subcarrier coding scheme combined with pre-electrical dispersion compensation (pre-EDC) for fiber nonlinearity mitigation in coherent optical orthogonal frequency division multiplexing (CO-OFDM) systems. As the frequency spacing in CO-OFDM systems is usually small (tens of MHz), neighbouring subcarriers tend to experience correlated nonlinear distortions after propagation over a fiber link. As a consequence, nonlinearity mitigation can be achieved by encoding and processing neighbouring OFDM subcarriers simultaneously. Herein, we propose to adopt the concept of dual phase conjugated twin wave for CO-OFDM transmission. Simulation and experimental results show that this simple technique combined with 50% pre-EDC can effectively offer up to 1.5 and 0.8 dB performance gains in CO-OFDM systems with BPSK and QPSK modulation formats, respectively.

Phase-Conjugated Pilots for Fibre Nonlinearity Compensation in CO-OFDM Transmission

In this paper, we demonstrate a novel fiber nonlinearity compensation technique for coherent optical orthogonal frequency-division multiplexing (CO-OFDM) systems based on the transmission of phase-conjugated pilots (PCPs). In this scheme, a portion of OFDM subcarriers (up to 50%) is transmitted with its phase conjugates, which is used at the receiver to estimate the nonlinear distortions in the respective subcarriers and other subcarriers, which are not accompanied by PCPs. Simulation and experimental results show that by varying the PCP overhead, a performance improvement up to 4 dB can be achieved. In addition, the proposed technique can be effectively applied in both single polarization and polarization-division multiplexed systems, in both single channel and wavelength-division multiplexing systems, thus, offering highest flexibility in implementations.

Pilot-based blind phase estimation for coherent optical OFDM system.

A pilot-based blind (PBB) phase estimation method for the coherent optical orthogonal frequency-division multiplexing (CO-OFDM) system is demonstrated in this paper. Instead of inserting the pilot-subcarriers that are loaded with the already known information in the OFDM signal, the unknown and specially designed signal is used to replace the signal on the pilot subcarriers to decrease the waste of spectrum and has demonstrated good performance in the phase noise compensation. Therefore, the spectral efficiency (SE) is further improved compared with the conventional pilot-aided (PA) phase noise estimation method. Both the proposed PBB and conventional PA estimation methods are compared in a CO-OFDM transmission experiment, which is modulated by 4 quadrature amplitude modulation (4-QAM) formats and transmitted over 1760-km standard single-mode fiber (SSMF) without optical dispersion compensation. The experimental results show that the proposed PBB method can achieve the similar performance as the conventional PA method.

Experimental Demonstration of CO-OFDM Systems Using Superimposed Pilot

We investigate the superimposed pilot (SP)-based channel estimation (CE) for long-haul coherent optical orthogonal frequency division multiplexing (CO-OFDM) systems. With specifically designed SP, we demonstrate the impacts that both SP and phase noise have on the performance of CE, from which we obtain the optimal signal-to-pilot power ratio. To enhance the estimation quality of the SP-aided scheme, we exploit an iterative decision feedback algorithm, and in turn, improve the system performance. Experimental results show that the SP-aided CE behaves robust to the phase noise when lasers with moderate linewidth (<;100 KHz), and can offer similar performance as the conventional preamble based methods without scarifying bandwidth efficiency. A CO-OFDM transmission over 5000 km of standard single mode fiber is successfully demonstrated using the SP-aided CE scheme, which shows a spectral efficiency gain of ~10%.

A Novel Energy-Efficient Transmission Scheme in CO-OFDM Elastic Optical Networks

The use of orthogonal frequency division multiplexing (OFDM) technology enables an optical transmission system to break the limitation of wavelength grids due to legacy of wavelength division multiplexing. This constructs a flexible and elastic transmission paradigm so as to achieve high spectrum efficiency and flexibility of fiber resource usage. This paper introduces a novel adaptive transmission strategy in elastic coherent optical OFDM transmission systems, aiming to optimize the system operation in terms of energy and spectrum consumptions for a transmission demand with a required data rate. By jointly considering the nonlinear effects of Mach-Zehnder modulator and amplified spontaneous emission noise from optical amplifiers, as well as the performance impairment due to high peak-to-average-power ratio (PAPR) in the electronic domain, we first provide an analytical model on the bit error rate performance for a single-elastic optical transmission line. To achieve an efficient PAPR reduction, we introduce a new method called simplified null switching, which is considered very suitable in the elastic optical transmission systems due to lower computation complexity and little dependence on the channel side information. Based on the analytical model, an optimization problem is formulated based on the proposed analytical model and solved via mathematical programming. Case studies via extensive numerical experiments are conducted to verify the proposed analytical model and gain better understanding on the solutions of formulated optimization problem.

Bit Error Rate Estimation Methods for QPSK CO-OFDM Transmission

Coherent optical orthogonal frequency division multiplexing (CO-OFDM) is an attractive transmission technique to virtually eliminate intersymbol interference caused by chromatic dispersion and polarization-mode dispersion. Design, development, and operation of CO-OFDM systems require simple, efficient, and reliable methods of their performance evaluation. In this paper, we demonstrate an accurate bit error rate estimation method for QPSK CO-OFDM transmission based on the probability density function of the received QPSK symbols. By comparing with other known approaches, including data-aided and nondata-aided error vector magnitude, we show that the proposed method offers the most accurate estimate of the system performance for both single channel and wavelength division multiplexing QPSK CO-OFDM transmission systems.

Blind equalization for dual-polarization two-subcarrier coherent QPSK-OFDM signals

Dual-polarization two-subcarrier coherent optical orthogonal frequency division multiplexing (CO-OFDM) transmission and reception is successfully demonstrated with blind equalization. A two-subcarrier quadrature phase shift keyed OFDM (QPSK-OFDM) signal can be equalized as a 9-ary quadrature amplitude modulation signal in the time domain with the cascaded multimodulus algorithm equalization method. The nonlinear effect resistance and transmission distance can be enhanced compared with the traditional CO-OFDM transmission system based on frequency equalization with training sequence.

Weighted Inter-frame Averaging-Based Channel Estimation for CO-OFDM System

We propose an efficient channel estimation method for coherent optical orthogonal frequency-division multiplexing (CO-OFDM) based on weighted inter-frame averaging (WIFA), in which the channel estimation results of the adjacent frames are averaged to increase the estimation accuracy. The principle of the proposed WIFA method is theoretically analyzed, and the benefit is demonstrated by a polarization-division-multiplexed CO-OFDM transmission experiment. The channel magnitude and phase differences among adjacent frames are investigated. The effectiveness of WIFA method, combined with conventional least square method, time-domain averaging method, and intra-symbol frequency-domain averaging method, respectively, is demonstrated through 240-km standard single-mode fiber transmission with different launched power values. The experimental results show that the WIFA method can significantly enhance transmission performance.

A novel two-stage phase noise estimation algorithm for coherent optical OFDM systems

In coherent optical orthogonal frequency-division-multiplexing (CO-OFDM) receivers, the phase noise caused by the instabilities of the local oscillator is a potentially serious problem. In order to deal with the common phase error (CPE) caused by the phase noise in actual CO-OFDM receiver, a novel two-stage Least Mean Square (LMS) phase noise adaptive estimation algorithm is proposed. In the proposed estimation algorithm, an initial coarse estimation of the optical carrier phase to exploit the knowledge of pilot symbols is followed by a fine LMS adaptive estimation. The main advantage of the proposed method is that the phase-ambiguity problem is avoided after a cycle slip and the Bit Error Rate (BER) performance is improved. Furthermore, the phase noise effect induced by the transmitter laser and receiver Local Oscillator (LO) is analyzed. A simple CO-OFDM transmission system with QPSK to apply the proposed adaptive estimation algorithm is simulated, the simulation analysis shows that the proposed adaptive estimation algorithm has a more excellent performance.

Analysis of Partial Pilot Filling Phase Noise Compensation for CO-OFDM Systems

In coherent optical orthogonal frequency division multiplexing (CO-OFDM) systems, phase noise of transmitter and local oscillator lasers has an important impact on system performance. In this letter, a partial pilot filling compensation scheme is analyzed and compared to conventional common phase error compensation and RF-pilot-based phase noise compensation. The effects of phase noise for various laser linewidths and of received optical power on system performance are evaluated. A CO-OFDM transmission system with 4-QAM is simulated with VPI transmissionMaker V8.7.

277.6-Gb/s LDPC-coded CO-OFDM transmission system with low PAPR based on subcarrier pre-filtering technology

This paper proposes a novel subcarrier pre-filtering method to reduce the peak-to-average power ratio (PAPR) in the coherent optical orthogonal frequency division multiplexing (CO-OFDM) system. It can mitigate the fiber nonlinearity without damnifying the signal waveform or inducing any redundant information. The experiment successfully demonstrates a 277.6-Gb/s low-density-parity-check (LDPC) coded CO-OFDM system over 200-km fiber via a subcarrier pre-filtering technology. More than 4-dB improvement in PAPR is achieved compared with that of the conventional CO-OFDM, and the nonlinear threshold can be improved by 2.1dB at the bit error rate (BER) of 5×10−2.

Fiber nonlinearity compensation for CO-OFDM transmission with 10.7-Gb/s NRZ-OOK neighbors

Fiber nonlinearity limits the use of coherent optical orthogonal frequency division multiplexing (CO-OFDM) to upgrade wavelength-division multiplexing (WDM) systems using legacy non-return-to-zero-on-off-keying (NRZ-OOK) channels. This letter proposes to compensate for the fiber nonlinearity of CO-OFDM with NRZ-OOK neighbors by combining digital signal processing (DSP)-based self-phase modulation (SPM) post-compensation with pilot-tone-based cross-phase modulation (XPM) compensation. The simulation results demonstrate that the optimum low-pass filter bandwidth for pilot-tone-based XPM compensation depends on the pilot-to-signal ratio value and launch optical power. Our method allows a 4-dB increase in the launch power for a 40-Gb/s single polarization CO-OFDM channel placed in the middle of six 10.7-Gb/s NRZ channels in a 50-GHz space and 1 200-km WDM system.

Nonlinear phase noise in coherent optical OFDM transmission systems

We derive an analytical formula to estimate the variance of nonlinear phase noise caused by the interaction of amplified spontaneous emission (ASE) noise with fiber nonlinearity such as self-phase modulation (SPM), cross-phase modulation (XPM), and four-wave mixing (FWM) in coherent orthogonal frequency division multiplexing (OFDM) systems. The analytical results agree very well with numerical simulations, enabling the study of the nonlinear penalties in long-haul coherent OFDM systems without extensive numerical simulation. Our results show that the nonlinear phase noise induced by FWM is significantly larger than that induced by SPM and XPM, which is in contrast to traditional WDM systems where ASE-FWM interaction is negligible in quasi-linear systems. We also found that fiber chromatic dispersion can reduce the nonlinear phase noise. The variance of the total phase noise increases linearly with the bit rate, and does not depend significantly on the number of subcarriers for systems with moderate fiber chromatic dispersion.

Data-aided adaptive weighted channel equalizer for coherent optical OFDM

We report an adaptive weighted channel equalizer (AWCE) for orthogonal frequency division multiplexing (OFDM) and study its performance for long-haul coherent optical OFDM (CO-OFDM) transmission systems. This equalizer updates the equalization parameters on a symbol-by-symbol basis thus can track slight drifts of the optical channel. This is suitable to combat polarization mode dispersion (PMD) degradation while increasing the periodicity of pilot symbols which can be translated into a significant overhead reduction. Furthermore, AWCE can increase the precision of RF-pilot enabled phase noise estimation in the presence of noise, using data-aided phase noise estimation. Simulation results corroborate the capability of AWCE in both overhead reduction and improving the quality of the phase noise compensation (PNC).

No-Guard-Interval Coherent Optical OFDM for 100-Gb/s Long-Haul WDM Transmission

This paper describes coherent optical orthogonal frequency division multiplexing (CO-OFDM) techniques for the long-haul transmission of 100-Gb/s-class channels. First, we discuss the configurations of the transmitter and receiver that implement the optical multiplexing/demultiplexing techniques for high-speed CO-OFDM transmission. Next, we review the no-guard-interval (No-GI) CO-OFDM transmission scheme which utilizes optical multiplexing for OFDM signal generation and the intradyne receiver configuration with digital signal processing (DSP). We examine the transmission characteristics of the proposed scheme, and show that No-GI CO-OFDM offers compact signal spectra and superior performance with regard to tolerance against optical amplifier noise and polarization-mode dispersion (PMD). We then introduce long-haul high-capacity transmission experiments employing No-GI CO-OFDM; 13.4 Tb/s (134 times 111 Gb/s) transmission is successfully demonstrated over 3600 km of ITU-T G.652 single-mode fiber without using optical dispersion compensation.