Recent Advances in Equalization Technologies for Short-Reach Optical Links Based on PAM4 Modulation: A Review

Abstract

In recent years, short-reach optical links have attracted much more attention and have come to constitute a key market segment due to the rapid development of data-center applications and cloud services. Four-level pulse amplitude modulation (PAM4) is a promising modulation format to provide both a high data rate and relatively low cost for short-reach optical links. However, the direct detector and low-cost components also pose immense challenges, which are unforeseen in coherent transmission. To compensate for the impairments and to truly meet data rate requirements in a cost-effective manner, various digital signal processing (DSP) technologies have been proposed and investigated for short-reach PAM4 optical links. In this paper, an overview of the latest progress on DSP equalization technologies is provided for short-reach optical links based on PAM4 modulation. We not only introduce the configuration and challenges of the transmission system, but also cover the principles and performance of different equalizers and some improved methods. Moreover, machine learning algorithms are discussed as well to mitigate the nonlinear distortion for next-generation short-reach PAM4 links. Finally, a summary of various equalization technologies is illustrated and our perspective for the future trend is given. Appl. Sci. 2019, 9, 2342 2 of 22 multi-tone (DMT), and quadrature amplitude modulation (QAM) based on Kramers-Kronig (KK) receiver [18]. Considering the power consumption and implementation complexity, the most attractive format in short-reach optical links is PAM4, which is believed as a highly promising candidate for the next-generation passive optical network (NG-PON) and has been ratified by IEEE 802.3 bs for 400 Gbps Ethernet transmission [19][20][21][22][23][24][25].However, the low-pass filtering effects induced by the limited bandwidth of transmitter and receiver can cause severely inter-symbol interference (ISI). Low-cost devices such as lasers, modulators, photodiodes (PD) and trans-impedance amplifiers (TIA) also produce nonlinear distortions like level-dependent skew and level-dependent noise. Furthermore, the interaction between chromatic dispersion (CD) and direct detection will lead to a power-fading effect, where the detected signal may contain frequency notches after several kilometers transmission at a high symbol rate. Therefore, various equalization technologies based on digital signal processing (DSP) have been investigated for short-reach links over the years . Conventional equalizers, such as the feed-forward equalizer (FFE) or decision feedback equalizer (DFE), are employed to compensate for the linear impairments induced by limited bandwidth and CD [26][27][28], while the equalizer based on the Volterra series is utilized to mitigate nonlinear distortion [29][30][31][32]. Some new equalization techniques, such as direct detection-faster than Nyquist (DD-FTN) algorithm [33][34][35], intensity directed FFE/DFE (ID-FFE/ID-DFE) algorithm [36,37] and joint clock recovery and FFE (CR-FFE) algorithm...

Special Issue on Advanced DSP Techniques for High-Capacity and Energy-Efficient Optical Fiber Communications

“…• Recent Advances in Equalization Technologies for Short-Reach Optical Links Based on PAM4 Modulation: A Review [4] The authors review the latest progress on DSP equalization technologies for short-reach optical links based on four-level pulse amplitude modulation (PAM4) modulation. They introduce the configuration and challenges of the transmission system, and cover the principles and performance of different equalizers and some improved methods.…”

Simplified Soft-Output Direct Detection FTN Algorithm for 56-Gb/S Optical PAM-4 System Using 10G-Class Optics

“…Recently, the high-speed transmission systems with bandwidth-limited devices have been investigated using the advanced DSP algorithms [21]. Feed-forward equalization (FFE) is a commonly used DSP algorithm to eliminate ISI, whereas FFE enhances the in-band noise resulting in the bit error rate (BER) performance degradation [22].…”

Tunable Optoelectronic Chromatic Dispersion Compensation Based on Machine Learning for Short-Reach Transmission

“…Nonetheless, transmission in the O-band has high attenuation, which reduces the length of the transmission link [2]. Alternatively, solutions in the C-band, which address the intersymbol interference induced by dispersion, can also be considered [3][4][5].…”

“…DD-receivers are the current alternative, but CD cannot be fully compensated in the digital domain as only the power is detected in the receiver. Different options for inter-and intra-data center interconnection, as well as for mobile fronthaul application for operation on the C-band have already been discussed in recent years [4,5]. A few of them operate on PAM-4 modulation and different options for DSP-based receiver equalization, such as Volterra and maximum likelihood sequence estimation (MLSE) [7][8][9] and DSP-based pre-distortion, such as Tomlinson Harashima precoding and the kramers-kronig receiver [10,11].…”