Next-generation Optical Access Networks Research Articles

Planning tools for next-generation DSP-based passive optical networks above 50G [Invited Tutorial

Next-generation optical access networks are evolving towards ultra-high bit rates (above 50 Gbps per wavelength) and extended fiber reach architectures. This trend will likely push the optoelectronics to their limits, thus requiring impairment compensation based on digital signal processing (DSP) techniques in the transceivers. In this paper, which is an invited follow-up of a tutorial given at ECOC 2023, we first present an overview of this evolving scenario and then propose a unified analytical model that is able to predict the performance of these new systems for both direct-detection and coherent transceiver types. We believe that this model can be useful for preliminary scalability studies of new access architectures (as it happens in international standardization bodies). Moreover, when they are deployed, it can be useful as a base for network planning tools, particularly if future transceivers will be, as expected, highly reconfigurable at the DSP level.

Alamouti-coded DSP algorithm with a simplified PTBC decoder for next-generation optical access networks

The use of Alamouti-coded polarization-time block code (A-PTBC) in combination with a simple single polarization coherent receiver enables phase-diverse coherent detection without any optical polarization tracking. However, applying this technique to high-speed single-carrier systems is not straightforward, as it requires specialized digital signal processing (DSP) algorithms for data recovery, which increases DSP complexity. In this paper, we propose a novel Alamouti-coded coherent algorithm designed to significantly reduce the complexity of the receiver DSP for data recovery. The proposed algorithm achieves the comparable performance to the conventional algorithm but requires only half the number of necessary equalizers for data recovery. We validate its performance through simulations and also experimentally demonstrate a 100 Gb/s 16-quadrature amplitude modulation (QAM) single-carrier coherent system employed the single-polarization coherent receiver over 20 km of standard single-mode fiber (SMF). Through the performance verification, the coherent system with the proposed algorithm exhibits performance comparable to that of the conventional Alamouti-coded coherent system and achieves a power budget of 34 dB when the transmit launch power is set to 7 dBm at a Bit Error Rate (BER) of 1 × 10−2 for 0-20 km fiber transmission.

Machine learning enhanced next-generation optical access networks—challenges and emerging solutions [Invited Tutorial

Optical access networks are envisioned to become increasingly complex as they support more and more diverse and immersive services, each with a different capacity, latency, and reliability need. While machine learning has been touted as a silver bullet that will intelligently manage network operations and resources to meet these demands, as it had been anticipated for core and metro networks, there exist various challenges that need to be addressed to progress machine learning models from research to production. In this tutorial, we first aim to motivate the continued push to advance optical access networks and rationalize the use of machine learning in these networks. We then highlight the challenges that are especially amplified due to the traffic dynamicity and heterogeneity, data scarcity, and computation-resource constraints of optical access networks. We discuss emerging machine learning approaches that are being explored to address these challenges. Finally, we consider a fast and self-adaptive machine learning enhanced dynamic bandwidth allocation scheme in an illustrative future use case of supporting immersive human-to-machine communications over the mobile fronthaul of next-generation mobile networks.

Flexible and adaptive coherent PON for next-generation optical access network [Invited

With the official adoption of the single-wavelength 50G PON standard at the International Telecommunication Union Study Group 15 (ITU-T SG15) meeting, both industry and academia have started extensive research on the next-generation optical access network beyond 50G. It is envisioned that 100G and 200G per wavelength and even beyond will be required in the next-generation optical access network to support the continuously growing bandwidth demands. Coherent passive optical network (C-PON), which is based on multi-access coherent optics and digital signal processing (DSP), appears to be a promising candidate for future 100/200G and even beyond single-wavelength PON, thanks to its superior sensitivity compared with current direct-detection based PONs. Actually, multi-access coherent optics brings many benefits to the optical access network beyond the improvement of sensitivity. In this invited paper, we review recent studies and progress on the flexible C-PON for optical access network in terms of access speed, multiple-access architecture, and deployment scenarios. Rate-adaptive coherent access is first introduced in this paper as a promising solution to achieve flexible C-PON. Adaptive modulation formats based on constellation shaping is highlighted to realize fine granularity of information rate and wide flexibility. We have also discussed the flexible multiplexing schemes beyond TDM, to take the full advantages of coherent access. Finally, we analyze the flexible deployment scenarios under the large power budgets thanks to coherent detection. All these flexibilities provided by coherent access would be interesting and beneficial in the next generation optical access network.

Si Photonic-Integrated Chip Assembly With Waveguide Ge Avalanche Photodiode for 10 Gbps L-Band Optical Access Networks

The receiving characteristics of the silicon photonic-integrated chip assembly consisting of transimpedance amplifiers and butt-joint waveguide germanium avalanche photodiodes with lateral separated absorption and multiplication (SAM) structures were demonstrated based on conventional complementary metal-oxide-semiconductor processes. We experimentally verified a clear open eye diagram at 10 Gbps with a reverse bias of 17 V. Receiving sensitivities of <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"><tex-math notation="LaTeX">$-$</tex-math></inline-formula> 22.8 dBm and <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"><tex-math notation="LaTeX">$-$</tex-math></inline-formula> 22.0 dBm were obtained for the best and worst polarizations, respectively, with optical signals that had wavelengths longer than those of the absorption edges of Ge at a bit rate of 10 Gbps and a wavelength of 1600 nm. The proposed waveguide butt-joint germanium avalanche photodiodes with lateral SAM structures can be used to receive long wavelengths of downstream signals with low polarization dependence in next-generation optical access network systems.

Dynamic Subcarrier Assignment in OFDMA-PONs Based on Deep Reinforcement Learning

Orthogonal Frequency Division Multiplexing Access Passive Optical Networks (OFDMA-PONs), a solution for the next-generation optical access network, allows multiple optical network units (ONUs) to dynamically share subcarriers (SCs) to support efficient bandwidth allocation. In uplink transmission, multiple ONUs can share orthogonal low bit rate SCs to transmit data at different time slots (TSs) during the transmission cycle. In this paper, the dynamic subcarrier allocation (DSA) scheme based on deep reinforcement learning (DRL) is proposed for various ONU bandwidth requests. The novel scheme jointly allocates time slots, subcarriers, and modulation formats in a dynamic and flexible manner. The ONU can save transmit power by using a lower order modulation format while meeting the delay requirement. The simulation part demonstrates how the proposed DRL-based DSA scheme can be adapted to various situations, including 1) variation in the size of ONU bandwidth requests, and 2) variation in the weight of different indicators. The extensive simulation results show that, for the first time, the proposed DRL-based DSA algorithm achieves optimal traffic latency with substantial power saving, compared with the traditional two-dimensional DSA algorithms.

Guest Editorial: Next-Generation Optical Access Networks to Support Super-Broadband Services and 5G/6G Mobile Networks

We are pleased to introduce to our readers the following Special Issue (SI) on Next-Generation Optical Access networks to support Super-Broadband Services and 5G/6G Mobile Networks in IEEE Network. In recent years, optical access networks have been playing a growing role in mobile networks and will become even more prominent with 5G/6G due to the demand for extreme bandwidth, coverage, synchronization, and low latency. The success of future mobile networks will not just be about new radio access technologies. The deployment of future wireless technologies will be dependent on the ability of optical access networks to seamlessly connect to radio access networks (RANs) and edge/cloud computing resources. Many technologies are currently being developed for optical networks for convergence and access to increase throughput, improve energy efficiency, and simplify network design, deployment, and operation. This SI is intended to provide recent advances in optical networking technologies, concentrating on optical access networks and the convergence of integrated fiber and wireless access technologies.

Timing Jitter Analysis and Mitigation in Hybrid OFDM-DFMA PONs

Hybrid orthogonal frequency division multiplexing-digital filter multiple access passive optical networks (OFDM-DFMA PONs) offer a cost-effective solution to the challenging requirements of next-generation optical access networks and 5G and beyond radio access networks. It is crucial to consider the impact of timing jitter in any ADC/DAC-based system, therefore this paper presents an in-depth investigation into the impacts of DAC/ADC timing jitter on the hybrid OFDM-DFMA PON's performance. We introduce improved accuracy white and coloured, DAC and ADC timing jitter models, applicable to any DSP-based transmission system. We prove that DAC and ADC timing jitter effects are virtually identical and investigate the effects of white/coloured timing jitter on upstream performance in hybrid OFDM-DFMA PONs and determine the associated jitter-induced optical power penalties. To mitigate against the timing jitter-induced performance degradations, a simple, but highly effective DSP-based technique is implemented which increases robustness against the timing jitter effects and significantly reduces timing jitter-induced optical power penalties. This consequently relaxes DAC/ADC sampling clock jitter requirements and so reduces implementation costs. White (coloured) timing jitter effects are shown to be independent of (dependent on) ONU operating frequency band and a trade-off between DAC and ADC jitter levels can be exploited to reduce ONU costs.

A Survey on Cost Effective Optical Network Unit Configurations

The technology of Wavelength division multiplexing-Passive optical networks WDM-PONs is basically developed based on the approach of broadcast-and-select in which an identical aggregation of wavelengths is directed to different destinations using a power splitter. Later, it was developed based on the approach of wavelength routing in which an aggregation of wavelengths is separated and directed in such a way that each individual wavelength in the aggregation can reach a specific destination using an arrayed waveguide grating (AWGs). It is one of the highly nominated technologies for next-generation optical access network NG-OAN. This is due to its ability to overcome the limitation imposed in the currently standardized TDM-PONs (E-PON, G-PON, 10G-EPON, and XG-PON), such as their shared-traffic nature and allowable power budget. However, the inventory of wavelength-specific transmitters needed in this technology especially at subscriber premises (i.e. optical network unit ONU) where cost is most sensitive, and their associated operation, administration, and management costs remains a major issue that needs to be settled before widespread commercial success can be achieved. This attracts many researchers to concentrate their efforts to develop a cost-effective ONU. Many proposals were found in the literature that aims to realize this desirable goal with most of them agreed up on that it can be achieved by enabling ONU mass production. They also agreed up on that enabling the ONU mass production can only be achieved by unifying its design. In this paper, the author provides a review of various state-of-the-art configurations, proposed to unify the ONU design.

Highly Reliable Transmission System for Next-Generation Optical Access Network Based on Silicon Modulator With Maximum-Ratio Combined Receiver

A novel maximum-ratio combined receiver (MRC-RX) is proposed and demonstrated through a proof-of-concept experiment, conducted to mitigate the system performance degradation caused by the low linearity of the silicon modulator when deployed in passive optical network (PON)-based access. By employing the proposed MRC-RX, an optimized receiver sensitivity is achieved, which enables the mitigation of the performance degradation induced by the silicon modulator, and a more reliable transmission in both cases of low and high received power. In the proof-of-concept experiment, at a bit error rate of 1.0 × 10 <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">-4</sup> , the receiver sensitivity of the MRC-RX was improved by 3.5 dB and 6.7 dB compared with the lite coherent receiver (lite CO-RX) and direct detection receiver (DD-RX), respectively. Compared with a system employing a commercial lithium niobate (LiNbO3) modulator, a maximum improvement of ~5.6 dB in the receiver sensitivity was obtained for the MRC-RX in the silicon modulator system. Thus, the system performance degradation caused by the low linearity of the silicon modulator can be effectively reduced by employing the MRC-RX. The proposed method makes sense to the building of next generation high reliable photonics integrated access by employing silicon modulators.

Extended L-band monolithically tunable InAs/InP quantum-dash multimode laser with integrated amplifier

A two-sectioned InAs/InP quantum dash laser structure is proposed and investigated as a monolithic broadband multimodal tunable laser with an integrated semiconductor optical amplifier. The optical power-injection current and spectral characteristics of the device at different operating conditions demonstrated a total wavelength tunability of ∼15.8 nm in the extended-L-band (∼1615 to ∼1630.8 nm) window with ∼2.0 times −3 dB bandwidth enhancement. Furthermore, due to the unique tunability mechanism of forward biasing the amplifier section, the device exhibits simultaneous wavelength tuning as well as optical amplification features, with an estimated gain of ∼8.5 dB affirmed by an increase in the wall-plug efficiency up to 6.8% from 3.9%, shown by its single-section counterpart. This demonstration paves a potential platform for the deployment of broadband quantum-dash laser-amplifiers as unified light sources in next-generation optical access networks.

Centralized vs. Decentralized Bandwidth Allocation for Supporting Green Fog Integration in Next-Generation Optical Access Networks

The surging numbers of wireless devices and the accompanying exponential growth of user traffic have led edge and fog computing to become essential components in future access networks. By bringing more processing toward the edge of the network, they enable the realization of various demanding applications while supporting numerous resource-constrained devices. Their integration with passive optical networks and their huge offered bandwidth has therefore attracted much research interest. In this article, we review these computing concepts and examine key considerations for their integration with long-reach optical access networks when the employed upstream bandwidth allocation is either centralized or decentralized.

Transmission Performance and Cost Analysis of Multi-Carrier-based Wavelength Division Multiplexed Passive Optical Access Network

AbstractMulti-tone-based wavelength division multiplexed passive optical networks are considered to provide cost-effective access network solution. We analysed and revealed the transmission performance, power budget concern and the cost investigation of the multi-tone-based WDM PON. A differential phase shift keying and re-modulated return to zero on/off keying signals for downlink and uplink, respectively, are transmitted via a single-mode fibre. The transmission performance of the proposed architecture is analysed using key parameters of bit error rate and optical signal to noise ratio using different data rates and channel spacing. The simulation results are in agreement with the theoretical analysis. The power budget is calculated for both upstream and downstream data signals showing excellent system margins for any unseen losses. The proposed setup is also analysed in terms of cost to prove it the cost-effective solution for the next-generation optical access networks.

Optical Communications and Networks

As the second issue in 2020 of the Optical Communications and Networks Series (OCNS), this series selects articles that cover interesting advances on devices, systems and networks regarding optical communications and networking. We are pleased to witness active innovations in this area, which will make the data exchange and processing in the future Internet more adaptive, application- aware, and cost-eff ective. For this issue, we have selected four contributions that address (i) interleaved data converter architectures for future high-speed fiber optical communications; (ii) improved modulation designs for visible light communications; (iii) new bandwidth allocation schemes for supporting green fog integration in next-generation optical access networks; and (iv) a techno-economic study on optical network disaggregation employing open-source software business models.

Sparse-fast-Fourier-transform-based quick synchronization for optical direct detection orthogonal frequency division multiplexing systems.

Orthogonal frequency division multiplexing (OFDM) has always been a promising candidate for optical access networks. However, it is very sensitive to synchronization errors and requires complex digital signal processing to eliminate this influence, thus increasing the computation complexity, delay, and system cost, which hampers its applications in cost-sensitive and low-latency scenarios of future optical access networks. To deal with this issue, a sparse-fast-Fourier-transform (FFT)-based quick synchronization algorithm for optical direct detection OFDM systems is proposed and demonstrated with greatly reduced computation complexity. Detailed simulations and experimental verifications along 50km standard single-mode-fiber transmission prove the efficiency, accuracy, and feasibility of the sparse-FFT-based synchronization technique in cost- and delay-sensitive applications for next-generation optical access networks.

Review on Wavelength Division Multiplexing Free Space Optics

Abstract Wavelength division multiplexing-based free space optics (WDM FSO) has emerged as a potential communication network candidate for last-mile access among other applications. FSO has received much attention in the last few years as a complement as well as an alternative to radio frequency-based communication due to spectrum crisis among other reasons. On the other hand, WDM has been considered as one of the next-generation optical access network candidates for bandwidth efficiency and increased data rates. A hybrid network of these two technologies thus has emerged as another research direction. In this article, we present the background, progress and the current state of WDM FSO.

Adaptive bandwidth mechanism using dual rate OLT for energy efficient WDM–TDM passive optical network

This paper reviews various energy efficient approaches in existence and proposes a Hybrid WDM–TDM PON architecture that allows the adaptive bandwidth allocation mechanism to reduce central office power consumption with acceptable performance. Our proposed architecture allows sending two signals, one broadband and other narrowband to each optical networking unit so an appropriate signal can be utilized according to the traffic demand. In case of very low traffic, only narrowband signal is used and a significant amount of energy consumption and OPEX is reduced. By using $$2\times \hbox {N}$$ power splitter and interleaver, proposed architecture provides broadcasting at both broadband and narrowband signal depending on the required link rate. This further reduces energy consumption and OPEX by avoiding the transmission of same signal from multiple sources. Offered data rates to the optical distribution networks (ODNs) may also be varied by doubling the wavelength spacing of remote node AWG so that two contiguous wavelengths can be transmitted at each port or ODN. This provides the geographical dynamic bandwidth allocation. Proposed architecture also support simultaneous transmission of both broadband and narrowband signals to the ODN to provide bandwidth scalability and network extensibility for supporting future access network in terms of new users and data rates. As two signals are reaching to any ODN, resiliency against OLT TRx and line card failure is also achieved. The performance of the proposed design is verified by simulation results in terms of bit error rate and receiver sensitivity to demonstrate its feasibility for the next-generation optical access network.

Techno-Economic Analysis of Active Optical Network Migration Toward Next-Generation Optical Access

Active optical networks (AONs) have been one of the most deployed fiber access solutions in Europe. However, with the increasing traffic demand, the capacity of the existing AONs is becoming insufficient. For the legacy AONs, there are two major variants of architectures, namely, point-to-point and active star. Considering the different characteristics of these two AON architectures, this paper proposes and analyzes several migration paths toward next-generation optical access (NGOA) networks offering a minimum 300 Mbit/s sustainable bit rate and 1 Gbit/s peak bit rate to every end customer. Furthermore, this paper provides detailed descriptions of the network cost modeling and the processes for AON migration. The total cost of ownership (TCO) is evaluated for the proposed migration paths, taking into account different migration starting times, customer penetration, node consolidation, and business roles in the fiber access networks. The migration from AON to NGOA can be economically feasible. The results indicate that a network provider plays a key business role and is responsible for the major part of the TCO for AON migration. Moreover, performing node consolidation during AON migration can be beneficial from a cost point of view, especially in rural areas.

Introduction to the Special Issue on Optical Network Design and Modeling

The ONDM 2016 Special Issue is a collection of outstanding optical networking papers, chosen among the ONDM 2016 most highly ranked papers. The papers in this special issue cover a broad range of topics including dynamic elastic optical networks, network virtualization, next-generation optical access networks, spatial division multiplexing networking, and multi-layer design.

A Frequency Reuse-Based Design for Flexible and Scalable Passive Optical Networks (PONs)

The accelerated growth in the bandwidth demand emphasizes the necessity to evolve from the currently deployed gigabit-class passive optical networks (PONs) to the next-generation optical access networks (NG-OANs). Different architectures were proposed in the literature in order to create a NG-OAN that is able to fulfilling the aforementioned goal. In this paper, a time-division multiplexing (TDM)/dense wavelength-division multiplexing (DWDM) scheme was proposed. The proposed scheme is sought to satisfy the current and future anticipated bandwidth demands. The architecture we proposed was able to allow different bit rate optical line terminals OLTs to use the same frequency band, and transmit their services over a 24 km shared feeder to 16 passive remote terminals (PRTs) with 16 ONU group for each. Each group can accommodate up to 16 ONU, total of 256 ONU/PRT, resulting in overall system capacity 4096 ONU. The architecture also allows the independent-upgradeability for each optical network terminal ONU.