Time-Domain Maximum-Likelihood Channel Estimation for PDM CO-OFDM Systems

The principle of orthogonal frequency-division multiplexing (OFDM) is to transmit the data through a large number of multiple orthogonal subcarriers. The coherent optical OFDM (CO-OFDM) is OFDM data that are being modulated to light frequency and being detected in coherent manner. CO-OFDM brings to optical communications the combination of two powerful techniques, coherent optical detection and OFDM. One of the primary challenges in the CO-OFDM system is to remove optical fiber nonlinear effects. This makes nonlinearity compensation a critical task of the CO-OFDM system. So a nonlinear equalizer (NLE) based on adaptive neuro-fuzzy inference system (ANFIS) is presented for CO-OFDM systems to mitigate nonlinearities on long-haul optical communications with high bit rate and bit error rate (BER)of the system. Various performance metrics were analyzed for the proposed ANFIS–NLE, and it is compared with existing techniques such as support vector machine and artificial neural network. From the experimental results, our proposed approach gives better performance in terms of BER and Q-factor on comparing with existing methods.

In this paper, a blind phase noise estimation method for a coherent optical orthogonal frequency-division multiplexing (CO-OFDM) system is considered. Based on the subspace approach, a simple and robust method is proposed to adaptively estimate and track phase noise in CO-OFDM systems. The idea behind this novel technique is to estimate the singular vectors that correspond to the smallest singular values of the noise subspace. A weighting parameter, which is derived based on the forward-backward linear prediction technique, is subsequently constructed using the obtained singular vectors of the noise subspace to adaptively estimate the phase noise in the system. Also, a variable step size is introduced to ensure an improved performance as well as stable convergence. Simulation results are shown to demonstrate the effectiveness as well as the efficiency of the proposed algorithm, which is implemented in a polarization multiplexed CO-OFDM system stressed by polarization mode dispersion, chromatic dispersion, and other polarization-dependent losses along the fiber link.

In this paper, it is proposed to enhance coherent optical orthogonal frequency-division multiplexing (OFDM) system by using index modulation in terms of bit error rate (BER) and spectral efficiency for different levels of the fiber nonlinearity. In the coherent optical OFDM with index modulation system (CO-OFDM-IM), the information is conveyed not only through activated subcarriers but also by indices of the active subcarrier that are dynamically updated at the transmitted symbol rates. Unlike the conventional coherent optical OFDM (CO-OFDM) system, adjusting the number of the active subcarriers in the CO-OFDM-IM system provides improved BER performance and reduction the signal peak-to-average-ratios, when the fiber nonlinearity becomes significant. The simulation results illustrate that the proposed CO-OFDM-IM system has better BER performance when the proposed CO-OFDM-IM system and the CO-OFDM system have similar spectral efficiency. Also, the CO-OFDM-IM system can provide better spectral efficiency than the CO-OFDM system at the low-to-medium order modulation such as BPSK, QPSK, and 16QAM.

An efficient channel estimation method called time-domain weighted average (TWA) algorithm is proposed for coherent optical orthogonal frequency division multiplexing (CO-OFDM) systems. On the premise of calculating the associated weight of channel transfer function, a more exact channel characteristic is obtained by calculating the weighted average of the pilot transfer function in this algorithm. Compared with time-domain average (TA) algorithm, the TWA algorithm can reach the same bit error rate (BER) with fewer pilots, and it improves the performance of CO-OFDM systems.

The statistical characterization of the nonlinear noise in Tbps Densely Spaced PDM CO-OFDM Systems under different modulation formats

In this research, the effect of laser phase noise fixing is investigated for different optical channel parameters such as dispersion, attenuation and link distance for Coherent Optical Orthogonal Frequency Division Multiplexing (CO-OFDM) systems. The effect of laser phase noise is examined at different data rates and link distances utilizing only one RF and optical carrier. In this research Phase Shift Keying (PSK) is used as a digital modulation format to maintain the maximum data rate with the acceptable BER value. By fixing the laser phase noise 1.7 Tb/s (111∗16 Gb/s) can be obtained from 191.4 to 185.9 THz at 50 GHz spacing over 1000 km link distance using only one optical cable in C and L bands according to the Telecommunication Standardization Sector (ITU-T). In that case high level modulation formats (N-PSK) should be performed with DWDM technique.

In this study, we proposed an improved Minimum Mean Square Error (MMSE) channel estimation algorithm for the coherent wireless underwater optical OFDM system aiming to improve parameters such as dispersion, attenuation and optical noise. The simulation results of the proposed algorithm was compared with the performance of Discrete Fourier Transform (DFT) based and time based Least Square (LS) and MMSE channel estimation techniques. The results demonstrated that the proposed approach improved the underwater wireless channel estimation performance and offers a promising alternative for the OFDM systems with less complexity and high efficiency.

One of the essential aspects of life is communication and in all communication systems, modulation and demodulation perform a crucial role in the transmission of transmitter-to-receiver data. Using various multiplexing techniques such as FDMA, CDMA and TDMA to work with the communication system, faced various problems such as low bit rate, multi-path fading, ISI and time dispersion. The incorporation of the OFDM approach gives an attractive solution to the aforementioned issues. OFDM transmitters and OFDM receivers with different modulation techniques are included in the OFDM-based different communication system. CO-OFDM was recently suggested and its extreme robustness against polarisation mode dispersion and chromatic dispersion was demonstrated in proof-of-concept transmission experiments. Optical fiber-based wired networks and also free space wireless networks require a premier technique for multiplexing i.e. CO-OFDM. In this paper, we investigated the performance of various modulation techniques which highlights several benefits such as channel robustness and greater efficiency of the available spectrum where multicarrier is used as high data rate bit streams from a single source are divided into the number of small subcarriers. OFDM is commonly regarded as discrete multitone (DMT) in this case. So in this review article, OFDM systems are also incorporated with their advantages and applications.

This paper mainly concerns on reducing the impact of chromatic dispersion (CD) and the nonlinear(NL) impairments in coherent optical orthogonal frequency division multiplexing (CO-OFDM) system. In effect, the system suffers from a severe problem due to a high peak to average power ratio (PAPR); and this consequently degrades the system performance. In this paper, a new method is proposed to mitigate the PAPR problem and thereby reduce the impact of nonlinearity on the performance of the CO-OFDM system with quadrature amplitude modulation QAM mapping. The recommended new technique is based on combining two effective ways of clipping method integrated with the L2-by3 method. The simulation results indicate that the proposed method was effectively able to improve system performance by enhancing system performance via bit error rate (BER), quality factor (QF), error vector magnitude (EVM) beside the Optical Signal-to-Noise Ratio (OSNR) without the need to add any digital processing at the receiver; and eventually decrease the receiver complexity. The proposed method reduced the peak to average power ratio (PAPR) about 3.8 dB and helped to increase transmission distance by approximately 200 km.

Orthogonal frequency-division multiplexing (OFDM) is resistant to frequency selective fading due to the longer symbol duration. However, mobile applications channel timing fluctuations in one OFDM signal cause intercarrier-interference (ICI), which reduces performance. This research presented the support vector regression (SVR) model-based channel estimation technique for coherent optical communication systems. Due to the coherent optical orthogonal frequency-division-multiplexed (COOFDM) system, a channel model is developed that includes linear fibre dispersion effects, noise from optical amplifiers, and inter-carrier interference generated by laser phase noise. As a result, for such a system, an accurate channel estimate is essential. Based on this concept, derivation of channel estimation and phase estimation for the system of CO-OFDM. The proposed method is tested and evaluated using MATLAB software. Computer simulation results for several standard methods such as extreme learning machines (ELM) and artificial neural networks (ANN) validate the feasibility of the suggested methodology. The CO-OFDM system’s transmission experiments and computer simulations prove that the support vector machine-based model following pilot-assisted phase estimation gives the optimal performance. Therefore, results depicted that the channel estimation utilizing the SVR model gives good performance than the other methods, thus the proposed model gives an accurate CE process, respectively.

With the rise of quantum computing and sophisticated cyber-attacks, traditional cryptographic methods face challenges in securing optical physical layer data transmission. This study proposes what we believe to be a novel approach by integrating biological genetic variation mechanisms (BGVM) with signal encryption in coherent optical orthogonal frequency division multiplexing (CO-OFDM) system. A mapping model between genetic structures and optical orthogonal frequency division multiplexing (OFDM) signals is constructed, and a dual encryption mechanism including mutation-guiding encryption mechanism (MGEN) and reorganization-guiding encryption mechanism (RGEN) is established to disrupt the data positions on subcarriers. In the system, the key sequences are generated by a four-dimensional hyperchaotic system, which exhibits good randomness and unpredictability. Experiment is successfully conducted to realize physical layer secure transmission of the CO-OFDM signal over 80 km standard single mode fiber (SSMF). The proposed scheme has low time complexity and the key space is extended to 101194 approximately, when the number of subcarriers N = 256.

In this letter, the deviation of the channel estimator based on intra-symbol frequency-domain averaging (ISFA) for coherent optical orthogonal frequency-division multiplexing (CO-OFDM) is investigated. The deviation-induced estimation error is derived analytically as a function of pulse broadening caused by chromatic dispersion and averaged noise power, and thereby the optimum averaging window size can be determined. To avoid the deviation, we propose an unbiased channel estimation algorithm based on the discrete Fresnel transform (DFnT) for CO-OFDM systems, utilizing the convolution-preservation property of DFnT for intra-symbol averaging. It is shown that the DFnT-based channel estimator converges to the actual channel response under estimation, and achieves better performance than the ISFA estimator, especially in highly dispersive channels. Finally, numerical results are provided to confirm the analysis and its advantages.

The mid-nonlinearity temporal inversion (MNTI) technology based on optical phase conjugate (OPC) is firstly proposed to compensate the chromatic dispersion and nonlinearity effect of the fiber link in coherent optical orthogonal frequency division multiplexing (CO-OFDM) system. System performance is analyzed under different transmission distance and different dispersion coefficient of fiber link. Simulation results show that, compared with mid-span spectral inversion (MSSI) method, at least 2-dB Q-factor improvement is achieved for the CO-OFDM system.

We propose a novel iteration-free blind phase noise estimation scheme for coherent optical orthogonal frequency division multiplexing (CO-OFDM) systems.In the new algorithm, the cost function is selected as the similar expression with real and imaginary parts as that in the modified constant modulus algorithm, and the new cost function is derived under some assumptions, where it is infinitely approximated by the sine and cosine functions.By means of the analytical formula of the cost function, the initial coarse common phase error can be obtained with only some samples, where the algorithm avoids computational complexity of conventional blind phase noise compensation scheme.In CO-OFDM systems with high-order modulation format (32 quadrature amplitude modulation) and narrow linewidth lasers, it is proved by the simulation results that the phase noise can be effectively compensated with the proposed blind estimation method.

For the first time, we propose and experimentally verify a novel low-complexity orthogonal least square (OLS) based radial basis function (RBF) neural network (NN) nonlinear equalizer for high-speed coherent optical orthogonal frequency division multiplexing (CO-OFDM) system. Its ability to compensate nonlinear impairments as well as linear impairments is comprehensively evaluated and compared with the linear equalizer. The impact of training overhead on system performance is also investigated. Results show that in a single-channel 40-Gb/s 16-quadrature amplitude modulation CO-OFDM system, with the training overhead of 4%, the maximum transmission distance is extended to 800 km at Q threshold of 8.7 dB, and RBF-NN outperforms the linear equalizer by 2.8 and 5.6 dB Q -factors after 800 and 600 km transmissions, respectively.