Novel transmitter IQ imbalance compensation algorithm using training symbols in PDM CO-OFDM system

We propose a novel training symbol structure for polarization division multiplexed (PDM) coherent optical orthogonal frequency division multiplexing (CO-OFDM) system. We deduce the mathematical model for the scheme. Based on the given model, a novel compensation method is introduced to compensate the IQ imbalance. The theoretical analysis is validated by numerical simulation, in which we apply the Quadrature Phase Shift Keying (QPSK) modulation at a data rate of 75.211Gb/s over 50 km and data rate of 100Gb/s over 320 km transmission. Compared with the commonly used approach, the proposed scheme demonstrates better system BER performance against IQ imbalance.

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.

Transmission and reception of PDM dual-subcarrier coherent 16QAM-OFDM signals

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.

In this letter, we propose and experimentally investigate a training-aided iterative frequency offset estimation (IFOE) method, which can track dynamic frequency offset (FO) drift in coherent optical orthogonal frequency division multiplexing (CO-OFDM) systems. In contrast to existing FO estimation algorithms that compute specific FO values, in IFOE, only the sign value of FO is judged and recorded without any phase calculation. The estimated FO is updated once for every data block based on FO signs obtained from multiple previous blocks, which helps combat noise and allows the shortening of the employed training symbol. We experimentally investigate the performance of IFOE in a 160-km CO-OFDM system. The results show that IFOE achieves superior bit error rate performance to a one-time FO estimator (without FO tracking) and approaches the performance of multiple separate FO estimators that use longer training symbols.

In this paper, we propose a modified constellation reshaping method for peak-to-average power ratio (PAPR) reduction in polarization division multiplexing (PDM) coherent optical orthogonal frequency division multiplexing (CO-OFDM) systems based on the selective mapping (SLM) method. A modified quadrature amplitude modulation (M-QAM) constellation is generated that has the same minimum distance as that of the conventional QAM (C-QAM) constellation. The M-SLM method is denoted as the M-QAM constellations combining with the traditional SLM method. At the receiver side, the mean square errors between the constellations of the received data and each M-QAM rotated by different phase rotation factors are calculated. Then, the phase rotation factor with the minimum mean square error is selected to recover the original data. The advantage of the proposed method is that it does not need to send additional side information, while it possesses nearly the same PAPR reduction and bit error rate performance as the traditional SLM method.

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 study, the authors presented discrete Fourier transform (DFT)-based channel estimation algorithms for a coherent optical orthogonal frequency division multiplexing (CO-OFDM) transmission system. Influence of the proposed algorithms to the performance and computational complexity of the CO-OFDM system was analysed and compared with other published algorithms. Their simulation results demonstrated that the improved DFT-based channel estimation algorithms are significantly efficient in our experimental conditions. Those algorithms can be considered as efficiently compromised systems in balance out the outcome of performance and the requirement on computational complexity. Such relatively economic and practical approaches have provided an optimistic prospect in studies of transmission systems for future optical communications.

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.

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.

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.

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.

In this paper, we propose a linear polarization coding scheme (LPC) combined with the phase conjugated twin signals (PCTS) technique, referred to as LPC-PCTS, for fiber nonlinearity mitigation in coherent optical orthogonal frequency division multiplexing (CO-OFDM) systems. The LPC linearly combines the data symbols on the adjacent subcarriers of the OFDM symbol, one at full amplitude and the other at half amplitude. The linearly coded data is then transmitted as phase conjugate pairs on the same subcarriers of the two OFDM symbols on the two orthogonal polarizations. The nonlinear distortions added to these subcarriers are essentially anti-correlated, since they carry phase conjugate pairs of data. At the receiver, the coherent superposition of the information symbols received on these pairs of subcarriers eventually leads to the cancellation of the nonlinear distortions. We conducted numerical simulation of a single channel 200 Gb/s CO-OFDM system employing the LPC-PCTS technique. The results show that a Q-factor improvement of 2.3 dB and 1.7 dB with and without the dispersion symmetry, respectively, when compared to the recently proposed phase conjugated subcarrier coding (PCSC) technique, at an average launch power of 3 dBm. In addition, our proposed LPCPCTS technique shows a significant performance improvement when compared to the 16-quadrature amplitude modulation (QAM) with phase conjugated twin waves (PCTW) scheme, at the same spectral efficiency, for an uncompensated transmission distance of 2800 km.

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.

We proposed and demonstrated an optimized in-band optical signal-to-noise ratio (OSNR) monitoring method for polarization division multiplexed coherent optical coherent optical orthogonal frequency division multiplexing (PDM-CO-OFDM) system with CAZAC equalization and Raman amplification, and it shows satisfactory accuracy in a wide range.