Po-Lung Tien

Next-generation OFDMA-based passive optical network architecture supporting radio-over-fiber

In this paper, we propose a novel architecture for next-generation orthogonal frequency division multiple access (OFDMA)-based passive optical networks (PONs), referred to as ROFPON. Besides carrying local broadband OFDMA data, ROFPON seamlessly supports radio-over-fiber (RoF) transports between the central office and multiple remote antennas at end users without using costly WDM lasers. We analytically and experimentally study the receiver sensitivity to OFDMA signals and the radio frequency (RF) signals performance. By corroborating simulation results with experimental results, we discuss the determination of crucial system parameters, such as the optimal broadband-to-radio power ratio, and the exploitation of a notch filter for removing RF interference. Experimental results show that the integrated 10 Gb/s OFDMA and three 20 MHz RF signals are successfully transported both downstream and upstream over a 20 km single-mode-fiber PON. Finally, experimental results demonstrate that QPSK-encoded WiMAX format RF signals are transmitted/relayed upstream with E-O-E conversion at each optical network unit (ONU), and received error-free at the optical line terminal after cascading 32 ONUs.

A Lagrangean relaxation approach to routing and wavelength assignment for multi-granularity optical WDM networks

In this paper, we propose an efficient approximation approach, called Lagrangean relaxation with heuristics (LRH), aimed to resolve routing and wavelength assignment (RWA) for multi-granularity WDM networks facilitating fiber, waveband, and lambda switching capabilities. The task is first formulated as a combinatorial optimization problem in which the bottleneck link utilization is to be minimized. The LRH approach performs constraint relaxation and derives a lower-bound solution index according to a set of Lagrangean multipliers generated through subgradient-based iterations. In parallel, using the generated Lagrangean multipliers, the LRH approach employs a new heuristic algorithm to arrive at a near-optimal upper-bound solution. With lower and upper bounds, we delineate the performance of LRH with respect to accuracy and convergence speed under different parameter settings. We further draw comparisons between LRH and a typical linear programming (LP) approach via experiments over the widely-used NSFNET and three randomly generated networks. Numerical results demonstrate that LRH outperforms the LP approach in both accuracy and computational time complexity particularly for larger sized networks.

HOPSMAN: An Experimental Testbed System for a 10-Gb/s Optical Packet-Switched WDM Metro Ring Network

For future WDM MANs, optical packet-switching has been considered to be a promising paradigm that efficiently supports a wide range of Internet-based applications having time-varying and high bandwidth demands and stringent delay requirements. This article presents the design of an experimental testbed system for a high-performance optical packet-switched WDM metro ring network, HOPSMAN. HOPSMAN boasts three crucial features. First, it has a scalable architecture in which the number of nodes is unconstrained by the number of wavelengths. Second, HOPSMAN nodes are equipped with high-speed photonic hardware components, including fast tunable receivers and optical slot erasers, capable of performing speedy optical packet-switching operations. Third, HOPSMAN incorporates a MAC scheme that embodies efficient and dynamic bandwidth allocation, resulting in exceptional delay-throughput performance. The article presents the key hardware components by highlighting the challenging issues we faced and the solutions we proposed for the testbed implementation. Finally, to demonstrate the feasibility of HOPSMAN, the article describes the experimental setup and presents the results obtained from running a commercially available remote media player application on the system.

A High-Performance OFDMA PON System Architecture and Medium Access Control

Orthogonal frequency-division multiplexing (OFDM) passive optical network (PON) has been considered to be a promising next-generation broadband wired access solution. However, based on the current tree-based architecture, existing OFDM PON systems face severe challenges when increasing the scalability and data-rate performance. In this paper, we propose a high-performance virtual-tree orthogonal frequency-division multiple access PON system (VTOPS). With the virtual-tree architecture and coupled with the use of inexpensive direct modulation, VTOPS features high reliability, scalability, spectrum efficiency, and cost effectiveness all at once. For governing the flexible/fair access and dynamic allocation of bandwidth, VTOPS incorporates a rate-based medium access control (MAC) scheme. The MAC scheme performs dynamic rate adjustment using a neural-fuzzy system. By adjusting the system parameters, the MAC scheme can achieve a wide range of delay and fairness performance under a variety of traffic patterns. Finally, we show both theoretical and experimental results to demonstrate that, by applying the power pre-emphasis algorithm and adaptive subchannel modulation, VTOPS achieves 40 Gb/s downlink and 40 Gb/s uplink transmissions, using low-cost 10 GHz directly modulated lasers.

Intelligent voice smoother for silence-suppressed voice over Internet

When transporting voice data with silence suppression over the Internet, the problem of jitter introduced from the network often renders the speech unintelligible. It is thus indispensable to offer intramedia synchronization to remove jitter while retaining minimal playout delay (PD). We propose a neural network (NN)-based intravoice synchronization mechanism, called the intelligent voice smoother (IVoS). The IVoS is composed of three components: (1) the smoother buffer; (2) the NN traffic predictor; and (3) the constant bit rate (CBR) enforcer. Newly arriving frames, assumed to follow a generic Markov modulated Bernoulli process (MMBP), are queued in the smoother buffer. The NN traffic predictor employs an online-trained back propagation NN (BPNN) to predict three traffic characteristics of every newly encountered talkspurt period. Based on the predicted characteristics, the CBR enforcer derives an adaptive buffering delay (ABD) by means of a near-optimal simple closed-form formula. It then imposes the delay on the playout of the first frame in the talkspurt period. The CBR enforcer in turn regulates CBR-based departures for the remaining frames of the talkspurt, aiming at assuring minimal mean and variance of distortion of talkspurts (DOT) and mean PD. Simulation results reveal that, compared to three other playout approaches, the IVoS achieves superior playout, yielding negligible DOT and PD, irrespective of traffic variation.

QoS burstification for optical burst switched WDM networks

In this paper, we propose a QoS burstification (QBT) mechanism on the basis of a new notion window. The QBT mechanism efficiently emulates the burst behavior by exerting simple FIFO service within the window and weight-proportional service at the window boundary. As will be shown, QBT achieves fairness and mean-burstification-delay guarantees among various traffic classes.

A 40-Gb/s OFDM PON System Based on 10-GHz EAM and 10-GHz Direct-Detection PIN

This letter demonstrates a 40-Gb/s optical double-sideband (ODSB) orthogonal frequency-division multiplexing passive optical network (OFDM-PON) system using a cost-effective 10-GHz-bandwidth electroabsorption modulator (EAM). By employing a subcarrier-adaptive modulation format and pre-emphasis, we successfully achieve 20-km EAM-based single-mode-fiber (SMF) transmission.

A Lagrangean relaxation-based approach for routing and wavelength assignment in multigranularity optical WDM networks

Optical wavelength-division multiplexed (WDM) networks often include optical cross-connects with multigranularity switching capability, such as switching on a single lambda, a waveband, or an entire fiber basis. In addition, it has been shown that routing and wavelength assignment (RWA) in an arbitrary mesh WDM network is an NP-complete problem. In this paper, we propose an efficient approximation approach, called Lagrangean relaxation with heuristics (LRH), aimed to resolve RWA in multigranularity WDM networks particularly with lambda and fiber switches. The task is first formulated as a combinatorial optimization problem in which the bottleneck link utilization is to be minimized. The LRH approach performs constraint relaxation and derives a lower-bound solution index according to a set of Lagrangean multipliers generated through subgradient-based iterations. In parallel, using the generated Lagrangean multipliers, the LRH approach employs a new heuristic algorithm to arrive at a near-optimal upper-bound solution. With lower and upper bounds, we conduct a performance study on LRH with respect to accuracy and convergence speed under different parameter settings. We further draw comparisons between LRH and an existing practical approach via experiments over randomly generated and several well-known large sized networks. Numerical results demonstrate that LRH outperforms the existing approach in both accuracy and computational time complexity, particularly for larger sized networks.

A novel OFDMA-PON architecture toward seamless broadband and wireless integration

We propose a novel OFDMA-PON architecture enabling seamless integration of broadband data and wireless signals from multiple remote antennas. We demonstrate that three 20MHz wireless channels at 2.1GHz-band are trimly overlaid with 10Gbps OFDMA-PON signal.

An Energy and Cost Efficient WDM/OFDMA PON System: Design and Demonstration

In this paper, we propose an energy and cost efficient WDM/OFDMA PON system, referred to as NEWOPS. Based on a new two-tiered semi-tree architecture, NEWOPS allows a large number of ONUs to be connected to OLT using only a handful of colorless transceivers. Due to the architecture, NEWOPS not only facilitates high spectral efficiency and flexible sub-channel bandwidth allocation, but also reduces the Rayleigh backscattering (RB) problem and achieves high energy and cost efficiency. Experimental results demonstrate that, taking only 5-GHz bandwidth for each wavelength, NEWOPS successfully achieves 20-Gb/s downstream and 10-Gb/s upstream OFDM-xQAM transmissions via the same fiber trunk. We depict that, to support 40 ONUs for example, NEWOPS achieves energy efficiency that is three to six times higher than that of a typical 10-Gb/s WDM/OFDM PON.