Hybrid fiber-wireless networks incorporating WDM technology for fixed wireless access operating in the sub-millimeter-wave and millimeter-wave (mm-wave) frequency regions are being actively pursued to provide untethered connectivity for ultrahigh bandwidth communications. The architecture of such radio networks requires a large number of antenna base-stations with high throughput to be deployed to maximize the geographical coverage with the main switching and routing functionalities located in a centralized location. The transportation of mm-wave wireless signals within the hybrid network is subject to several impairments including low opto-electronic conversion efficiency, fiber chromatic dispersion and also degradation due to nonlinearities along the link. One of the major technical challenges in implementing such networks lies in the mitigation of these various optical impairments that the wireless signals experience within the hybrid network. In this paper, we present an overview of different techniques to optically transport mm-wave wireless signals and to overcome impairments associated with the transport of the wireless signals. We also review the different designs of subsystems for integrating fiber-wireless technology onto existing optical infrastructure.

Fiber-Wireless Networks and Subsystem Technologies

Driven primarily by cloud service and data-center applications, short-reach optical communication has become a key market segment and growing research area in recent years. Short-reach systems are characterized by direct detection-based receiver configurations and other low-cost and small form factor components that induce transmission impairments unforeseen in their coherent counterparts. Innovative signaling and digital signal processing (DSP) play a pivotal role in enabling these components to realize their ultimate potentials and meet data rate requirements in cost-effective manners. This paper presents an overview of recent DSP developments for short-reach communications systems and discusses future trends.

Digital Signal Processing for Short-Reach Optical Communications: A Review of Current Technologies and Future Trends

Erbium-doped fiber devices have been extraordinarily successful due to their broad optical gain around 1.5–1.6 μm. Er-doped fiber amplifiers enable efficient, stable amplification of high-speed, wavelength-division-multiplexed signals, thus continue to dominate as part of the backbone of longhaul telecommunications networks. At the same time, Er-doped fiber lasers see many applications in telecommunications as well as in biomedical and sensing environments. Over the last 20 years significant efforts have been made to bring these advantages to the chip level. Device integration decreases the overall size and cost and potentially allows for the combination of many functions on a single tiny chip. Besides technological issues connected to the shorter device lengths and correspondingly higher Er concentrations required for high gain, the choice of appropriate host material as well as many design issues come into play in such devices. In this contribution the important developments in the field of Er-doped integrated waveguide amplifiers and lasers are reviewed and current and future potential applications are explored. The vision of integrating such Er-doped gain devices with other, passive materials platforms, such as silicon photonics, is discussed.

Erbium-doped integrated waveguide amplifiers and lasers

Polarization management is very important for photonic integrated circuits (PICs) and their applications. Due to geometrical anisotropy and fabrication inaccuracies, the characteristics of the guided transverse-electrical (TE) and transverse-magnetic (TM) modes are generally different. Polarization-dependent dispersion and polarization-dependent loss are such manifestations in PICs. These issues become more severe in high index contrast structures such as nanophotonic waveguides made of silicon-on-insulator (SOI), which has been regarded as a good platform for optical interconnects because of the compatibility with CMOS processing. Recently, polarization division multiplexing (PDM) with coherent detection using silicon photonics has also attracted much attention. This trend further highlights the importance of polarization management in silicon PICs. The authors review their work on polarization management for silicon PICs using the polarization independence and polarization diversity methods. Polarization issues and solutions in PICs made of SOI nanowires and ridge waveguides are discussed.

Polarization management for silicon photonic integrated circuits

The wavelength-division-multiplexed passive optical network (WDM-PON) is considered to be the next evolutionary solution for a simplified and future-proofed access system that can accommodate exponential traffic growth and bandwidth-hungry new applications. WDM-PON mitigates the complicated time-sharing and power budget issues in time-division-multiplexed PON (TDM-PON) by providing virtual point-to-point optical connectivity to multiple end users through a dedicated pair of wavelengths. There are a few hurdles to overcome before WDM-PON sees widespread deployment. Several key enabling technologies for converged WDM-PON systems are demonstrated, including the techniques for longer reach, higher data rate, and higher spectral efficiency. The cost-efficient architectures are designed for single-source systems and resilient protection for traffic restoration. We also develop the integrated schemes with radio-over-fiber (RoF)-based optical-wireless access systems to serve both fixed and mobile users in the converged optical platform.

Key Technologies of WDM-PON for Future Converged Optical Broadband Access Networks [Invited]

With a record 200Gbps 50-piece-FPGA implementation, we investigate performance of a concatenated-staircase-Hamming code proposed for OIF-400GZR. FPGA emulations reveal an error flare at 10−10 BER lowering the predicted BER threshold from 1.25e-2 to 1.21e-2.

FPGA Investigation on Error-Floor Performance of a Concatenated Staircase and Hamming Code for 400G-ZR Forward Error Correction

Digital signal processing is the core functional block in beyond-100G transmission systems with a high symbol rate and/or high spectral efficiency We review recent progress on high symbol rate signal generation and detection, and corresponding advanced algorithms for signal equalization and compensation at the transmitters (Tx) and the receivers (Rx), respectively Thanks to the joint Tx-side optical pre-equalization (pre-EQ) and Rx-side partial response maximum likelihood sequence estimation, significant performance improvement can be achieved by reducing the penalty caused by bandwidth limitations of the optical and electronic components A joint pre-EQ and look-up table predistortion technique is also reviewed in this paper, which is demonstrated to be an effective method for mitigating devices' nonlinearity impairment We exemplify how these advanced algorithms enable high symbol rate beyond-100G transmission systems with signal generation based on either electronic time-division multiplexing or digital-to-analog converters

High Symbol Rate Signal Generation and Detection With Linear and Nonlinear Signal Processing

This research work proposed and experimentally demonstrated beyond-100GBd probabilistically shaped coherent optics that support the transmission of a ~1.2 Tbps coded line rate and a ~0.9 Tbps net bit rate per optical carrier over metro-regional distances. Pursuing next-generation transmission systems having both high capacity approaching terabits per wavelength and high spectral efficiency require engineering efforts from different aspects, which may include the enhancement of analog-to-digital or digital-to-analog interfaces, the improvement of channel characteristics, and also the realization of better receiver sensitivities, preferably through DSP techniques such as probabilistic shaping (PS). By doing so, this work achieved a single-channel and a 112.5-GHz WDM transmission of 900G wavelengths over 800 and 400 km, respectively, which employed a PS DP-64QAM modulation format at 105 GBd, yielding a net spectral efficiency of 8.05 bits/s/Hz. Such a high symbol rate was demonstrated using a commercially available DAC unit without band-interleaving. The enhanced optical channel consists of ITU-T G.654 compliant Terawave SLA+ fiber spans with the backward-pumped Raman amplifiers. In addition, with the same PS DP-64QAM modulation format and the same enhanced channel, this research work also demonstrated that 400G wavelengths can be delivered over 2800 km supporting long-haul applications.

Approaching Terabits Per Carrier Metro-Regional Transmission Using Beyond-100GBd Coherent Optics With Probabilistically Shaped DP-64QAM Modulation

We demonstrate three-channel WDM transmission of 12.84-Tb/s/ch polarization division multiplexed (PDM) 64-QAM over 320-km SMF-28 with hybrid amplification spans. Each 10-Tb/s superchannel comprises one hundred Nyquist DWDM subchannels in 1275-GHz spectrum occupancy, yielding a spectral efficiency of 7.9 b/s/Hz. The BER of all channels after transmission over 320 km is measured smaller than 2×10-2, the third generation FEC limit.

30-Tb/s (3×12.84-Tb/s) signal transmission over 320km using PDM 64-QAM modulation

With a novel adaptive pre-equalization scheme based on DD-LMS, four 100-GHz channels, each carrying a 480-Gb/s dual-carrier PM-8QAM signal, are successfully transmitted over 2550-km SSMF with EDFA-only amplification.

Hung-Chang Chien

Papers

FPGA Investigation on Error-Floor Performance of a Concatenated Staircase and Hamming Code for 400G-ZR Forward Error Correction

High Symbol Rate Signal Generation and Detection With Linear and Nonlinear Signal Processing

Approaching Terabits Per Carrier Metro-Regional Transmission Using Beyond-100GBd Coherent Optics With Probabilistically Shaped DP-64QAM Modulation

30-Tb/s (3×12.84-Tb/s) signal transmission over 320km using PDM 64-QAM modulation

Transmission of 480-Gb/s Dual-carrier PM-8QAM over 2550km SMF-28 Using Adaptive Pre-equalization