Feature Issue on
Optical Access Networks (OAN) The passive optical network (PON) is
an optical fiber based network architecture, which can provide much higher bandwidth
in the access network compared to traditional copper-based networks. Incorporating
wavelength-division multiplexing (WDM) in a PON allows one to support much higher
bandwidth compared to the standard PON, which operates in the "single-wavelength
mode" where one wavelength is used for upstream transmission and a separate one is
used for downstream transmission. We present a comprehensive review of various
aspects of WDM-PONs proposed in the literature. This includes enabling device
technologies for WDM-PONs and network architectures, as well as the corresponding
protocols and services that may be deployed on a WDM-PON. The WDM-PON will become a
revolutionary and scalable broadband access technology that will provide high
bandwidth to end users.

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Wavelength-division-multiplexed passive optical network (WDM-PON) technologies for broadband access: a review (Invited)

The main bandwidth bottleneck in today's networks is in the access segment. To address that bottleneck, broadband fiber access technologies such as passive optical networks (PONs) are an indispensable solution. The industry has selected time-division multiplexing (TDM) for current PON deployments. To satisfy future bandwidth demands, however, next-generation PON systems are being investigated to provide even higher performance. In this paper, we first review current TDM-PONs; we designate them as generation C. Next, we review next-generation PON systems, which we categorize into C+1 and C+2 generations. We expect C+1 systems to provide economic near-term bandwidth upgrade by overlaying new services on current TDM-PONs. For the long term, C+2 systems will provide more dramatic system improvement using wavelength division multiplexing technologies. Some C+2 architectures require new infrastructures and/or equipment, whereas others employ a more evolutionary approach. We also review key enabling components and technologies for C+1 and C+2 generations and point out important topics for future research.

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Next-Generation Optical Access Networks

The advent of digital signal processing (DSP) to optical coherent detection means that more phase estimation options are available, compared to the earlier generation where phase-locked loops (PLLs) were invariably deployed in synchronous coherent receivers. Several phase estimation methods are numerically modeled: the maximum a posteriori (MAP) phase estimate, decision directed estimate, power law-Wiener filter estimate and power law-PLL estimate. An asynchronous coherent detection case is also modeled. The phase estimates are evaluated with respect to their tolerance of finite laser linewidth and their suitability for implementation in a parallel digital processor. Laser phase noise causes transmission system performance to be degraded by excess bit errors and cycle slips. The optimal phase estimate is the MAP estimate, and it is also included as a baseline. The power law-Wiener filter phase estimate is found to perform only marginally worse than the MAP estimate. It must be recast using a look-ahead computation to be implemented in a parallel digital processor, and the impact is investigated of the increase in the number of computations required. Differential logical detection is often used to reduce the impact of cycle slip events, and the implications of this operation on the bit error rate are studied. It is found that by choosing the correct FEC scheme differential logical detection does not increase the Q-factor penalty.

Phase Estimation Methods for Optical Coherent Detection Using Digital Signal Processing

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]

Passive optical networks (PONs) are currently evolving into next-generation PONs (NG-PONs) which aim at achieving higher data rates, wavelength channel counts, number of optical network units (ONUs), and extended coverage compared to their conventional counterparts. Due to the increased number of stages and ONUs, NG-PONs face significant challenges to provide the same level of survivability like conventional PONs without exceeding the budget constraints of cost-sensitive access networks. Toward this end, partial optical protection, in combination with interconnecting a subset of ONUs through a wireless mesh network (WMN) front-end, are promising solutions to render NG-PONs survivable in a cost-effective manner. In this paper, we present a probabilistic analysis of the survivability of NG-PONs and hybrid fiber-wireless (FiWi) access networks, taking both optical and wireless protection into account. In addition, we propose different selection schemes to wirelessly upgrade a subset of ONUs, and investigate their performance for a wide range of fiber link failure scenarios and different NG-PON topologies.

Survivability Analysis of Next-Generation Passive Optical Networks and Fiber-Wireless Access Networks

We propose a hybrid optical-wireless access network that consists of reconfigurable optical backhaul and wireless mesh networks (WMNs). The complementary characteristics of wireless and optical networks are combined to provide a broadband and ubiquitous last-mile connection. Wireless mesh routers are deployed to penetrate the vicinity of end users for a flexible and ubiquitous connection. It eliminates massive and geographically scattered deployment of physical infrastructure to reach the end users. The broadband optical backhaul consists of optical ring and multiple tree networks, connecting the central hub and WMNs. The ends of the optical tree networks connect to the wireless gateway routers of WMNs. A hybrid time-division-multiplexing (TDM)/wavelength-division-multiplexing (WDM) optical backhaul is realized by wavelength-multiplexing multiple TDM-passive-optical-network streams. This hybrid architecture provides graceful scalability, cost effectiveness, and bandwidth efficiency. To adapt to a change of the overall demand in different districts, reconfigurability is implemented in the optical backhaul utilizing tunable optical transceivers. An experimental test bed is implemented to evaluate the reconfigurable scheme. Given the synergy of the optical backhaul and WMNs, we propose an integrated-routing algorithm to achieve load balancing on this hybrid architecture. The simulation using NS2 shows an approximately 25% throughput improvement with load balancing.

Hybrid Architecture and Integrated Routing in a Scalable Optical–Wireless Access Network

Homodyne phase-shift-keying systems can achieve the best receiver sensitivity and the longest transmission distance among all optical communication systems. This paper reviews recent research efforts in the field and examines future possibilities that might lead toward potential practical use of these systems. Additionally, phase estimation techniques based on feed-forward phase recovery and digital delay-lock loop approaches are examined, simulated, and compared

Homodyne Phase-Shift-Keying Systems: Past Challenges and Future Opportunities

Passive optical networks have been identified as promising access solutions that can open the first-mile bottleneck, bringing gigabits-per-second data rates to end users. Current TDM PONs enjoy low cost by sharing resources in time, but suffer from limited capacity. In the future, WDM technology may be employed to achieve high performance. In this article we introduce a novel PON employing dynamic wavelength allocation to provide bandwidth sharing across multiple physical PONs. Tunable lasers, arrayed waveguide gratings, and coarse/fine filtering combine to create a flexible new optical access solution. The network's excellent scalability can bridge the gap between conventional TDM PONs and WDM PONs. The powerful architecture is a promising candidate for next-generation optical access networks.

SUCCESS-DWA: a highly scalable and cost-effective optical access network

The deployment of passive optical networks for broadband access has been largely recognized as a promising solution to open the first-/last-mile bottleneck. We propose and demonstrate a new hybrid TDM/WDM PON architecture that jointly serves multiple physical PONs to enjoy statistical multiplexing gain as well as cost sharing. This article describes design and implementation issues of the key building blocks, including fast tunable lasers, burst-mode receivers, and scheduling algorithms with quality of service support. A prototype testbed is constructed, and transmission experiments are performed to show the effectiveness of the integrated system. Coexistence with current PONs and future evolutional paths are described as well. Experimental explorations of the new network architecture involve advanced photonic and electronic subsystems, which inspires developments of the next-generation optical access networks.

Wei-Tao Shaw

Papers

Next-Generation Optical Access Networks

Hybrid Architecture and Integrated Routing in a Scalable Optical–Wireless Access Network

Homodyne Phase-Shift-Keying Systems: Past Challenges and Future Opportunities

SUCCESS-DWA: a highly scalable and cost-effective optical access network

Success PON demonstrator: experimental exploration of next-generation optical access networks