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]

We proposed and experimentally demonstrated a novel hybrid subcarrier modulation (H-SCM) technique to generate a spectral-efficient 60-GHz optical millimeter-wave (mm-wave) that carries independent 2.5-Gb/s wireless and 10-Gb/s wireline signals using intensity and phase modulation, respectively. The frequency beating components of the 60-GHz channel due to interleaved and imbalanced optical path in H-SCM are numerically analyzed and experimentally measured in terms of timing jitter and amplitude fluctuation. The generated 60-GHz optical mm-wave signal using H-SCM with phase noise variance of about 0.5 is demonstrated in a radio-over-fiber testbed propagating through a combined distance of 25-km fiber and 4-m free space. The power penalties for the received wireless and wireline signals are 1 and 0.2 dB at 10-9 bit-error rate, respectively.

Simultaneous Transmission of Wireless and Wireline Services Using a Single 60-GHz Radio-Over-Fiber Channel by Coherent Subcarrier Modulation

The performance of dual-carrier 400G solutions based on three high-order quadrature amplitude modulation (QAM) formats (8/16/32-QAM) is investigated on the same platform. We first study the benefit and penalty differences of gray and differential coding, and then experimentally compare OSNR sensitivities and transmission performance using flexible transceiver configuration and the same erbium-doped fiber amplifiers-only standard single-mode fiber link. Experimental results show the implementation penalties are 1.85 dB for 42-GBd polarization multiplexed (PM)-8QAM, which is 1 and 1.5 dB better than 32-GBd PM-16QAM and 25-GBd PM-32QAM at 5.92-dB Q
 2
-factor (corresponding to a bit error rate of 2.4E-2), respectively. It is also found that maximum reaches of 2460 km for 42.6-GBd PM-8QAM, 1640 km for 32-GBd 16QAM, and 820 km for 25-GBd PM-32QAM are achieved. Furthermore, the performances of cascaded multimodulus algorithm and decision-directed least mean square (DD-LMS) algorithms are also compared. The improvement by addition of one sample/symbol-based DD-LMS digital filter is investigated as well.

Performance Comparison of Dual-Carrier 400G With 8/16/32-QAM Modulation Formats

A centrally-controlled, bi-directional WDM-PON architecture has been demonstrated. The self- survivable scheme can protect all types of failures in distribution and feeder fibers, remote nodes, and transmitters in CO and ONUs.

A Self-Survivable WDM-PON Architecture with Centralized Wavelength Monitoring, Protection and Restoration for both Upstream and Downstream Links

We experimentally demonstrate the coherent transmission system with the highest ETDM-based symbol rate of 128.8-GBaud over record breaking distances. We successfully transmitted single-carrier 515.2-Gb/s PDM-QPSK/9-QAM signals over 10,130km/6,078-km, respectively, over 100km spans of TeraWave SLA + fiber. To the best of our knowledge, it is the highest ETDM-based symbol rate reported so far, and the longest WDM transmission distance with single-carrier 400G signals. For the first time, the 515.2-Gb/s single-carrier PDM-QPSK signals in 200-GHz-grid are successfully transmitted over distance above 10,000km in terrestrial transmission environment. We have also demonstrated the transmission of single carrier 128.8-GBaud filtered QPSK signals in 100-GHz-grid over 6,078-km, which has the line spectral efficiency (SE) of 5.152 (b/s/Hz).

Transmission of single-carrier 400G signals (515.2-Gb/s) based on 128.8-GBaud PDM QPSK over 10,130- and 6,078 km terrestrial fiber links.

With joint LUT-based pre-distortion and DBP-based post-compensation to mitigate the opto-electronic components and fiber nonlinearity impairments, we demonstrate the unrepeatered transmission of 1.6Tb/s based on 4-lane 400G single-carrier PDM-16QAM over 205-km SSMF without distributed amplifier.

1.6Tb/s (4×400G) unrepeatered transmission over 205-km SSMF using 65-GBaud PDM-16QAM with joint LUT pre-distortion and post DBP nonlinearity compensation

Hung-Chang Chien

Papers

Simultaneous Transmission of Wireless and Wireline Services Using a Single 60-GHz Radio-Over-Fiber Channel by Coherent Subcarrier Modulation

Performance Comparison of Dual-Carrier 400G With 8/16/32-QAM Modulation Formats

A Self-Survivable WDM-PON Architecture with Centralized Wavelength Monitoring, Protection and Restoration for both Upstream and Downstream Links

Transmission of single-carrier 400G signals (515.2-Gb/s) based on 128.8-GBaud PDM QPSK over 10,130- and 6,078 km terrestrial fiber links.

1.6Tb/s (4×400G) unrepeatered transmission over 205-km SSMF using 65-GBaud PDM-16QAM with joint LUT pre-distortion and post DBP nonlinearity compensation