A survey on fiber nonlinearity compensation for 400 Gbps and beyond optical communication systems

Optical communication systems represent the backbone of modern communication networks. Since their deployment, different fiber technologies have been used to deal with optical fiber impairments such as dispersion-shifted fibers and dispersion-compensation fibers. In recent years, thanks to the introduction of coherent detection based systems, fiber impairments can be mitigated using digital signal processing (DSP) algorithms. Coherent systems are used in the current 100 Gbps wavelength-division multiplexing (WDM) standard technology. They allow the increase of spectral efficiency by using multi-level modulation formats, and are combined with DSP techniques to combat the linear fiber distortions. In addition to linear impairments, the next generation 400 Gbps/1 Tbps WDM systems are also more affected by the fiber nonlinearity due to the Kerr effect. At high input power, the fiber nonlinear effects become more important and their compensation is required to improve the transmission performance. Several approaches have been proposed to deal with the fiber nonlinearity. In this paper, after a brief description of the Kerr-induced nonlinear effects, a survey on the fiber nonlinearity compensation (NLC) techniques is provided. We focus on the well-known NLC techniques and discuss their performance, as well as their implementation and complexity. An extension of the inter-subcarrier nonlinear interference canceler approach is also proposed. A performance evaluation of the well-known NLC techniques and the proposed approach is provided in the context of Nyquist and super-Nyquist superchannel systems.Comment: Accepted in the IEEE Communications Surveys and Tutorial

Waveforms for the Massive MIMO Downlink: Amplifier Efficiency, Distortion and Performance

In massive MIMO, most precoders result in downlink signals that suffer from high PAR, independently of modulation order and whether single-carrier or OFDM transmission is used. The high PAR lowers the power efficiency of the base station amplifiers. To increase power efficiency, low-PAR precoders have been proposed. In this article, we compare different transmission schemes for massive MIMO in terms of the power consumed by the amplifiers. It is found that (i) OFDM and single-carrier transmission have the same performance over a hardened massive MIMO channel and (ii) when the higher amplifier power efficiency of low-PAR precoding is taken into account, conventional and low-PAR precoders lead to approximately the same power consumption. Since downlink signals with low PAR allow for simpler and cheaper hardware, than signals with high PAR, therefore, the results suggest that low-PAR precoding with either single-carrier or OFDM transmission should be used in a massive MIMO base station

A 300-800MHz Tunable Filter and Linearized LNA applied in a Low-Noise Harmonic-Rejection RF-Sampling Receiver

A multiband flexible RF-sampling receiver aimed at software-defined radio is presented. The wideband RF sampling function is enabled by a recently proposed discrete-time mixing downconverter. This work exploits a voltage-sensing LNA preceded by a tunable LC pre-filter with one external coil to demonstrate an RF-sampling receiver with low noise figure (NF) and high harmonic rejection (HR). The second-order LC filter provides voltage pre-gain and attenuates the source noise aliasing, and it also improves the HR ratio of the sampling downconverter. The LNA consists of a simple amplifier topology built from inverters and resistors to improve the third-order nonlinearity via an enhanced voltage mirror technique. The RF-sampling receiver employs 8 times oversampling covering 300 to 800 MHz in two RF sub-bands. The chip is realized in 65 nm CMOS and the measured gain across the band is between 22 and 28 dB, while achieving a NF between 0.8 to 4.3 dB. The IIP2 varies between +38 and +49 dBm and the IIP3 between -14 dBm and -9 dBm, and the third and fifth order HR ratios are more than 60 dB. The LNA and downconverter consumes 6 mW, and the clock generator takes 12 mW at 800 MHz RF.\ud \u

Nonlinearity mitigation in phase-sensitively amplified optical transmission links

The fundamental limitations in fiber-optic communication are caused by optical amplifier noise and the nonlinear response of the optical fibers. The quantum-limited noise figure of erbium-doped fiber amplifier (EDFA) or any phase-insensitive amplifier is 3 dB. However, the noise added bythe amplification can be reduced using phase-sensitive amplifier (PSA) whose quantum-limited noise figure is 0 dB. PSAs can also compensatefor the nonlinear distortions from the optical fiber with copier-PSA implementation. At the transmitter, a copier which is nothing but aphase-insensitive amplifier is used to create a conjugated copy of the signal. The signal and idler are co-propagated in the span, experiencingcorrelated nonlinear distortions. The nonlinear distortions are reduced by the all-optical coherent superposition of the signal and idler in thePSA.In this work, an analytical investigation is performed for the nonlinearity mitigation using the PSAs, by calculating the residual nonlineardistortion after the coherent superposition in PSAs. The optical bandwidth and the dispersion map dependence on the nonlinearity mitigationin the PSAs are analytically and experimentally studied. A modified Volterra nonlinear equalizer (VNLE) is used to reduce the residual nonlineardistortions after PSAs. Experiments were performed to show that PSAs can mitigate cross-phase modulation (XPM), which was evidentby observing the constellation diagrams. The maximum allowed launch power increase was also measured to quantify the XPM mitigation. Tothe best of our knowledge, this is the first experiment that showed the mitigation of XPM in a phase-sensitively amplified transmission link.Also, the effectiveness in mitigating self-phase modulation (SPM) and XPM using a PSA is studied

Approaching the non-linear Shannon limit

We review the recent progress of information theory in optical communications, and describe the current experimental results and associated advances in various individual technologies which increase the information capacity. We confirm the widely held belief that the reported capacities are approaching the fundamental limits imposed by signal-to-noise ratio and the distributed non-linearity of conventional optical fibres, resulting in the reduction in the growth rate of communication capacity. We also discuss the techniques which are promising to increase and/or approach the information capacity limit

A survey on RF and microwave doherty power amplifier for mobile handset applications

This survey addresses the cutting-edge load modulation microwave and radio frequency power amplifiers for next-generation wireless communication standards. The basic operational principle of the Doherty amplifier and its defective behavior that has been originated by transistor characteristics will be presented. Moreover, advance design architectures for enhancing the Doherty power amplifier’s performance in terms of higher efficiency and wider bandwidth characteristics, as well as the compact design techniques of Doherty amplifier that meets the requirements of legacy 5G handset applications, will be discussed.Agencia Estatal de Investigación | Ref. TEC2017-88242-C3-2-RFundação para a Ciência e a Tecnologia | Ref. UIDP/50008/201

Experimental demonstration of digital predistortion for orthogonal frequency-division multiplexing-radio over fibre links near laser resonance

Radio over fibre (RoF), an enabling technology for distribution of wireless broadband service signals through analogue optical links, suffers from non-linear distortion. Digital predistortion has been demonstrated as an effective approach to overcome the RoF non-linearity. However, questions remain as to how the approach performs close to laser resonance, a region of significant dynamic non-linearity, and how resilient the approach is to changes in input signal and link operating conditions. In this work, the performance of a digital predistortion approach is studied for directly modulated orthogonal frequency-division multiplexing RoF links operating from 2.47 to 3.7 GHz. It extends previous works to higher frequencies, and to higher quadrature amplitude modulation (QAM) levels. In addition, the resilience of the predistortion approach to changes in modulation level of QAM schemes, and average power levels are investigated, and a novel predistortion training approach is proposed and demonstrated. Both memoryless and memory polynomial predistorter models, and a simple off-line least-squares-based identification method, are used, with excellent performance improvements demonstrated up to 3.0 GHz