KEYWORDS: Polarization, Digital signal processing, Optical communications, Telecommunications, Oscillators, Logic, Receivers, Data processing, Field programmable gate arrays
Compared with the fiber channel, the changes of polarization state and the channel characteristics of the two polarization states are relatively slowly in space optical communication channel. At the same time, there have no serious PMD effect in the atmosphere. Because of the relative stability of polarization states in space optical communication systems, we propose and experimentally validate a method that performs low-speed polarization de- multiplexing processing first, and then performs low-speed linear channel equalization for each polarization channel in the DSP process. While the polarization de-multiplexing is realized by ICC algorithm, and the channel equalization is realized by CMA. What’s more, the tap coefficients of ICC algorithm and CMA are able to update by processing the received information sequence in low speed with low-speed logic cells in this method. Therefore, the advantage of this method is that the logic resource consumption in hardware implementation will be reduced by reuse of the low-speed logic cells during FPGA processing while the ICC algorithm and CMA are running. In order to verify the effectiveness of the scheme, we build a communication system that 20-GBd PM-QPSK signals are sent at the transmit-side, and a coherent receiver with local oscillator at the receive-side. By analyzing the constellation diagram and comparing the BER after the ICC algorithm and CMA processing with that after CMA processing, we know that this method simplifies the DSP effectively without significant impact on communication quality.
KEYWORDS: Orthogonal frequency division multiplexing, Receivers, Signal to noise ratio, Detection and tracking algorithms, Passive optical networks, Numerical simulations, Signal processing, Analog electronics, Computing systems, Transmitters
An extensive comparison between iteration interference cancellation and symbol predistortion in an analog domain generated ratio frequency tone based virtual single sideband direct detection orthogonal frequency division multiplexing system is presented on the basis of equalization principles, computational complexity, algorithm performance, and robustness. The analysis results show that the two algorithms have the same computational complexity. Symbol predistortion is more suitable for the downstream scheme in the passive optical network, while iteration interference cancellation is suitable for the upstream scheme since, for these two scenarios, the computational complexities are both located at the optical line terminal. The numerical simulation and 40 Gbps experimental results both indicate that the iteration interference cancellation algorithm exhibits a better performance and is more robust.
Intensity modulation and direct detection signal are sensitive to power fading and nonlinear intersymbol interference (ISI) induced by modulator chirp, fiber dispersion, and square-law photo-detection. We propose and experimentally demonstrate a Nyquist 4-ary pulse amplitude modulation and direct detection scheme relying on pulse-shaping with an electrical filter and optical equalization with a vestigial-sideband (VSB) filter in the transmitter. The power fading could be eliminated by using the VSB filter. Compared with conventional 4-ary pulse amplitude modulation, the Nyquist signal has a stronger resistance to nonlinear ISI.
The intersymbol interference caused by dispersion, chirp, and a vestigial sideband filter in intensity modulation and a direct detection single carrier system is analyzed theoretically and numerically. An iterative nonlinear intersymbol interference cancellation technique is proposed and experimentally demonstrated in a 40-Gbps 16-QAM Mach-Zehnder modulator-based vestigial sideband intensity modulation and direct detection half-cycle Nyquist–subcarrier modulation system over a 100-km uncompensated standard single-mode fiber transmission for the first time. The experimental results show that 2.2-dB receiver sensitivity improvement is obtained at the forward error correction limit by using the iterative technique.
We experimentally demonstrate the superior performance of a 40-Gbps 16-QAM half-cycle Nyquist subcarrier modulation (SCM) transmission over a 100-km uncompensated standard single-mode fiber using dual-drive Mach-Zehnder modulator-based vestigial sideband intensity modulation and direct detection. The impact of modulator chirp on the system performance is experimentally evaluated. This Nyquist-SCM technique is compared with optical orthogonal frequency division multiplexing in both back-to-back and 100-km transmission experiments, and the results show that the Nyquist system has a better performance.
KEYWORDS: Signal to noise ratio, Orthogonal frequency division multiplexing, Singular optics, Frequency division multiplexing, Modulators, Receivers, Telecommunications, Signal generators, Transmitters, Amplitude modulation
We propose and experimentally demonstrate the superior performance of a 40-Gbps 16-quadrature amplitude modulation virtual single sideband (VSSB) direct detection optical frequency division multiplexing system with symbol predistortion to mitigate subcarriers-to-subcarriers beating interference. The VSSB signal is generated in the electronic domain by combining the baseband orthogonal frequency division multiplexing signals and sinusoidal waves without a frequency gap to maximize the electrical and optical spectral efficiencies. The results show that 5-dB optical signal to noise ratio sensitivity improvement is obtained by using symbol predistortion.
The presence of vestigial-sideband optical filter, dispersion and chirp of modulator increases subcarrier to subcarrier intermixing interference (SSII), which significantly restricts transmission performance. For the first time, the iterative interference cancellation method is introduced to calculate and eliminate SSII. Furthermore, we successfully demonstrate a 40-Gbps, 16-QAM, dual drive Mach-Zehnder modulator (MZM)-based system transmission through 100-km uncompensated standard single-mode fiber. The impact of chirp on iterative algorithm is also experimentally evaluated by setting different optical phase modulation amplitudes on the two arms of the dual drive MZM.
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