In this paper, the position of erbium-doped fiber amplifier (EDFA) in an intensity-modulation and direct-detection (IM/DD) optical fiber communication system is optimized to suppress a part of chromatic dispersion (CD) caused distortions and alleviate the burden of digital signal processing. The results demonstrate that, for a system with a certain signal rate and launch optical power, the transmission fiber length is longer with optimizing the position of the EDFA. If a 20% overhead soft-decision forward error correction threshold of 2.7×10−2 is considered, the CD-uncompensated transmission fiber length of a 50 Gb/s four-level pulse amplitude modulation (PAM4) system can be increased by 34.78% compared with the non-optimized system. Moreover, the launch optical power of the optimized system can be decreased by 6 dB.
A fiber nonlinearity compensation scheme based on an unsupervised-learning neural network is proposed. In the proposed scheme, labels in the training data and weights of the neural network are iteratively updated until converging. To validate the proposed scheme, a 3200 km dual-polarization 16-QAM simulation link and an 1800 km single-polarization experimental link were carried out. Simulation and experiment results validate that the proposed method can achieve the same equalization performance as the supervised-learning-neural-network-based scheme, without any pre-defined training data.
We present a method for complex spectrum analysis of periodic optical signal with multi-level modulation format. The information of the complex spectrum can be extracted with ultra-low sampling rate in the proposed method, and the temporal information reconstructed by inverse Fourier transform can be used to monitor the performance of the optical signal. Besides, the proposed method is transparent to symbol rate and modulation format. The spectral amplitudes and phases are measured accurately in the simulation based on the proposed method for a 28 GBaud Quadrature-Phase-ShiftKeying (QPSK) signal with a 400-bit pattern and a 28 GBaud 16-Quadrature-Amplitude-Modulation(16QAM) signal with a 560-bit pattern. Moreover, we analyze the performance of the reconstructed signal under different linewidth and optical signal-to-noise ratio (OSNR) by simulation. The Q-factor of the reconstructed 28-GBaud QPSK signal reduces when the linewidth of laser increases and the OSNR of signal decreases.
In this paper, we proposed a promising nonlinearity (NL) estimation technique based on error vector correlation (EVC) function, which is robust against ASE noise. The method is demonstrated to estimate NL power accurately in 112 Gb/s polarization-multiplexed quadrature phase-shift keying (PM-QPSK) simulation system, 224 Gb/s PM-16-quadrature amplitude modulation (PM-16-QAM) system and 672 Gb/s wavelength division multiplexed (WDM) PM-16-QAM system. For all of these three systems, the maximum estimation error of NL power is lower than 1 dB for 2400 km transmission link with launch power varying from 0 to 8 dBm per channel when the OSNR range used in simulation of QPSK and 16-QAM system are 12 ~ 30 dB and 18 ~ 36 dB respectively.
Indoor positioning systems have been gaining traction in recent years, and visible light positioning (VLP) may be the most promising technology. We propose an indoor VLP method based on three light-emitting diodes and an image sensor. We regard the lamp plane composed of three lamps as a rigid body and the process of lamp plane imaging as the process of rotating and scaling the rigid body. By solving the matrix transformation, the two-dimensional (2-D) coordinates and azimuth angle of the image sensor are obtained with the focal length of the image sensor unknown. Azimuth angle plays an important role in other positioning methods, such as pedestrian navigation methods. The experimental results show that the 2-D positioning maximum error is only 3.21 cm in the horizontal condition, and the azimuth angle error is within 5 deg when the tilt angle is <7 deg.
Time-domain CD equalization using finite impulse response (FIR) filter is now a common approach for coherent optical fiber communication systems. The complex weights of FIR taps are calculated from a truncated impulse response of the CD transfer function, and the modulus of the complex weights is constant. In our work, we take the limited bandwidth of a single channel signal into account and propose weighted FIRs to improve the performance of CD equalization. The key in weighted FIR filters is the selection and optimization of weighted functions. In order to present the performance of different types of weighted FIR filters, a square-root raised cosine FIR (SRRC-FIR) and a Gaussian FIR (GS-FIR) are investigated. The optimization of square-root raised cosine FIR and Gaussian FIR are made in term of the bit rate error (BER) of QPSK and 16QAM coherent detection signal. The results demonstrate that the optimized parameters of the weighted filters are independent of the modulation format, symbol rate and the length of transmission fiber. With the optimized weighted FIRs, the BER of CD equalization signal is decreased significantly. Although this paper has investigated two types of weighted FIR filters, i.e. SRRC-FIR filter and GS-FIR filter, the principle of weighted FIR can also be extended to other symmetric functions super Gaussian function, hyperbolic secant function and etc.
The optimal order in fractional Fourier transform (FrFT) can be used to estimate chromatic dispersion (CD) and nonlinearity in an optical fiber transmission system. In this paper, we propose a novel method to estimate CD with lower computation complexity in fractional domain. The computation complexity can be reduced by 103 times with the same measurement accuracy compared with one step method when the number of samples is 8192 and search step is 0.0001. The correctness of the novel method for optimal order searching is proved by chirp parameter estimation for linear frequency modulation (LFM) signals. The measurement relative error is only 0.02%. For CD estimation, the maximum estimation error ratio is 0.338% and 0.564% for 28Gbit/s quadrature phase-shift keying (QPSK) and 16 quadrature amplitude modulation (16QAM) optical fiber transmission systems over 100 km~2000 km SSMF.
The characteristics of white LED transmission through a smoke screen is critical for visible light communication through a smoke screen. Based on the Mie scattering theory, the Monte Carlo transmission model is established. Based on the probability density function, the white LED sampling model is established according to the measured spectrum of a white LED and the distribution angle of the lambert model. The sampling model of smoke screen particle diameter is also established according to its distribution. We simulate numerically the influence the smoke thickness, the smoke concentration and the angle of irradiance of white LED on transmittance of the white LED. We construct a white LED smoke transmission experiment system. The measured result on the light transmittance and the smoke concentration agreed with the simulated result, and demonstrated the validity of simulation model for visible light transmission channel through a smoke screen.
In this paper, we introduce circle 16 quadrature amplitude modulation (C-16QAM) modulation format in a high spectral efficiency zero-guard-interval (ZGI) coherent optical (CO) orthogonal frequency-division multiplexing (OFDM) system. At transmitter, the C-16QAM has advantages over the conventional square 16QAM in terms of transmission distance and tolerance to laser linewidth and fiber nonlinearities. ZGI CO-OFDM enables to take away the cyclic prefix (CP), so it has the benefit of higher spectral efficiency compared with the conventional CO-OFDM system. At receiver, in order to compensate chromatic dispersion (CD) and phase noise in a single channel ZGI CO-OFDM system, we studied the overlapped frequency domain equalizer (OFDE) and two carrier phase recovery (CPR) algorithms. We simulate the above systems and the results demonstrate that with the C-16QAM, a 28GBaud ZGI CO-OFDM system can have the longer transmission distance, the higher tolerance to laser linewidth and fiber nonlinearities with contrast to the conventional square 16QAM.
We propose an accurate and nondata-aided chromatic dispersion (CD) estimation method involving the use of the cross-correlation function of two heterodyne detection signals for coherent optical communication systems. Simulations are implemented to verify the feasibility of the proposed method for 28-GBaud coherent systems with different modulation formats. The results show that the proposed method has high accuracy for measuring CD and has good robustness against laser phase noise, amplified spontaneous emission noise, and nonlinear impairments.
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