Next generation wireless broadband access network requires a centralized radio access network (C-RAN) with high bandwidth and low latency of mobile fronthaul links. DWDM-PON is suitable for such fronthaul because of high bandwidth and low latency using dedicated wavelength channels. Auxiliary management and control channel (AMCC) superimposition scheme is advantageous for the management and control of DWDM-PON systems. In this paper, we review several types of superimposition and detection schemes that we have proposed and developed, and evaluate the impact of AMCC superimposition on the main signal as well as the quality of the superimposed signal.
Digital nonlinear compensation techniques have been thought to be keys to realize further spectrally efficient optical fiber communication systems. The most critical issue of the digital nonlinear compensation algorithms has been their computational complexity, or gate count of digital signal processing circuit. Among several approaches, digital nonlinear compensation algorithms based on perturbation analysis are attractive in terms of the hardware efficiency because the algorithms can compensate the accumulated nonlinear noise over all transmission spans with only one stage. In this paper, we discuss three approaches to sophisticate the perturbation nonlinear compensation. First, we illustrate a perturbation-based post-equalization method to improve the robustness to transceiver device imperfections. We next propose and numerically evaluate a symbol degeneration method to extend the perturbation nonlinear compensation methods to higher-order QAM without increasing the computational complexity. Finally, we discuss a sub-band processing of perturbation nonlinear compensation for further computational complexity reduction. By combining the perturbation method with Nyquist frequency division multiplexing, the computational complexity of perturbation calculation is reduced by a factor of more than 10 for 3000-km single-channel transmission of 128 Gbit/s dualpolarization QPSK with only 0.1 dB performance degradation.
Optical performance monitoring (OPM) is considered as an important tool in order to operate and manage dynamic, flexible, and thus complex photonic networks. In this paper, firstly we review recent studies on OPM and discuss its possible applications, such as failure diagnosis of transmitter, receiver, and other transport equipment, optimization of system reach design, and so on. We then present two different types of in-band OSNR monitor, consisting of an optical bandpass filter, a photo-detector, and a signal processer. Since the proposed monitor might be realized by the same hardware implementation as an optical channel monitor (OCM), this is potentially integrated with an OCM in a low-cost fashion. We also explain a BER monitor, which is realized by the same hardware configuration as the above in-band OSNR monitor. The BER in the method is estimated by monitoring OSNR including nonlinear noise as noise source and taking the imperfection of Tx, Rx, and other equipment into account. Finally we develop FPGA-based optical performance monitor prototype and experimentally demonstrate successful monitoring performance.
The high complexity of conventional intra-channel nonlinearity compensation algorithms, such as back-propagation,
is considered as the major obstacle for the implementation. To reduce the complexity, perturbation analysis is
applied because it considers multi-span transmission as one stage. In those perturbation based algorithms, such as
perturbation back-propagation (PBP) and perturbation pre-distortion, the number of required compensation stage is
much less than that of conventional back-propagation. To reduce the complexity further, the multi-tap finite impulse
response filter (FIR) in PBP is replaced with one-tap infinite impulse response (IIR) filter. The number of required
compensation stage of IIR PBP is only 15% of conventional back-propagation, whereas the complexity of each stage
is almost same. In perturbation pre-distortion, the proposed perturbation combination reduces the number of terms
from 19732 to 41, whereas no performance degradation is observed.
Nonlinear distortion is one of the major obstacles in DWDM systems with enhanced spectral efficiencies. In this paper
several approaches to address the issue of nonlinear impairments by means of digital signal processing are discussed.
Firstly, implementation-efficient and novel intra-channel nonlinear compensation schemes are proposed; one is based on
digital pre-distortion at the transmitter end and the other is based on digital back-propagation at the receiver end. The
virtues of the two approaches and implications to various applications are discussed; the pre-distortion technique is in
particular advantageous with QPSK modulation format; on the other hand, the improved version of digital back-propagation
is attractive in transceivers with adaptive or variable modulation/demodulation. Second, digital signal
processing algorithms to counteract inter-channel nonlinearities, namely cross-phase modulation, are discussed;
nonlinear polarization crosstalk canceller (NPCC) is proposed for mitigating the impact of nonlinear-induced fast
polarization crosstalk in dual-polarization systems (in the speed beyond MHz), which is too fast to be tracked by
ordinary polarization demultiplexing algorithms; improvement to the carrier phase recovery circuit and its combination
with NPCC are even more useful for further performance improvement. Numerical and experimental data are introduced
to support the above discussions.
Digital coherent receivers with data-rates of 100 Gbit/s based on dual-polarization quaternary phase shift keying (DPQPSK)
have become a reality. One research trend is now directing towards even higher bit-rates of 400 Gbit/s and 1
Tbit/s. However, it is also very desirable to improve the performance of the current basic 100 Gbit/s DP-QPSK.
Algorithms have a huge improvement potential and exemplary recent advances will be introduced in this paper.