Showing papers in "IEEE\/OSA Journal of Optical Communications and Networking in 2010"

Adaptive Modulation and Coding for Free-Space Optical Channels

Abstract: Adaptive modulation and coding can provide robust and spectrally efficient transmission over terrestrial free-space optical channels. Three adaptive modulation schemes are considered in this paper: (i) variable-rate variable-power adaptation, (ii) channel inversion, and (iii) truncated channel inversion schemes. It is shown that a simple channel inversion scheme performs comparable to a variable-rate variable-power adaptation scheme in the weak turbulence regime but faces significant performance degradation in the strong turbulence regime. We further study adaptive coding based on large-girth quasi-cyclic low-density parity-check- (LDPC-) coded modulation. It is shown by simulation that deep fades of the order of 30 dB and above in the regime of strong turbulence can be tolerated with the proposed scheme. It is demonstrated that communication in the saturation regime is possible with the proposed adaptive LDPC-coded modulation. We also determine the spectral efficiencies for the proposed adaptive modulation and adaptive coding schemes.

Study and Demonstration of Sleep and Adaptive Link Rate Control Mechanisms for Energy Efficient 10G-EPON

Abstract: This paper describes sleep and adaptive link rate (ALR) control functions to reduce the power consumption of optical network units (ONUs) in 10 Gb/s Ethernet passive optical network (10G-EPON) systems. A hybrid mechanism that includes both sleep and ALR control functions is proposed with a view to further improving the power-saving performance. The sleep control function switches the ONU mode, i.e., active or sleep, depending on the presence or absence of traffic. The ALR control function switches the downlink rates between an optical line terminal (OLT) and an ONU, i.e., 1 Gb/s or 10 Gb/s, depending on the quantity of traffic. The proposed hybrid mechanism offers effective power management of ONUs on the basis of the traffic conditions. The proposed hybrid mechanism is validated by a numerical simulation and an experiment.

Serial Free-Space Optical Relaying Communications Over Gamma-Gamma Atmospheric Turbulence Channels

Abstract: In this paper, a study on the end-to-end performance of multihop free-space optical wireless systems over turbulence-induced fading channels, modeled by the gamma-gamma distribution, is presented. Our analysis is carried out for systems employing amplify-and-forward channel-state-information-assisted or fixed-gain relays. To assess the statistical properties of the end-to-end signal-to-noise ratio for both considered systems, we derive novel closed-form expressions for the moment-generating function, the probability density function, and the cumulative distribution function of the product of rational powers of statistically independent squared gamma-gamma random variables. These statistical results are then applied to studying the outage probability and the average bit error probability of binary modulation schemes. Also, for the case of channel-state-information-assisted relays, an accurate asymptotic performance analysis at high SNR values is presented. Numerical examples compare analytical and simulation results, verifying the correctness of the proposed mathematical analysis.

Simplified Channel Model for Simulation of Free-Space Optical Communications

Abstract: Free-space optical (or optical wireless) communications represent an attractive technology for the realization of high-bandwidth wireless communications. However, for propagation through the atmosphere the characteristics of the optical signal are different from those of the signals from, e.g., fiber channels or radio-frequency wireless channels, and therefore the error characteristics on these links are also different. For evaluating fading mitigation techniques for optical wireless communications a channel model is needed that can be easily used by protocol designers. Existing channel models for optical wireless communications are based on atmospheric propagation theory and therefore require a deep physical understanding of the optical propagation through the atmosphere. The goal of this publication is to develop a simplified approach for modeling the received power dynamics of the atmospheric free-space optical channel. The proposed model consists of a random number generator and a low-pass filter and is therefore easy to implement and use. This approach is only valid for systems utilizing intensity modulation with direct detection, but this limitation is acceptable since most commercially available systems use this modulation format. The channel model is developed based on the statistics of received power measurements from a maritime-mobile link, a land-mobile link, and a satellite downlink.

Average Symbol Error Probability of General-Order Rectangular Quadrature Amplitude Modulation of Optical Wireless Communication Systems Over Atmospheric Turbulence Channels

Abstract: Using an accurate exponential bound for the Gaussian Q-function, we derive simple approximate closed-form expressions for the average symbol error probability (ASEP) of a free-space optical communication link using subcarrier intensity modulation (SIM) with general-order rectangular quadrature amplitude modulation (QAM) over atmospheric turbulence channels. To model the atmospheric turbulence conditions, the log-normal and the gamma-gamma distribution are used. Extensive numerical and computer simulation results are presented in order to verify the accuracy of the proposed mathematical analysis.

All-Optical Network Coding

Abstract: We investigate the application of network coding to all-optical networks from both the algorithmic and infrastructural perspectives. We study the effectiveness of using network coding for optical-layer dedicated protection of multicast traffic that provides robustness against link failures in the network. We present a heuristic for solving this problem and compare it with both inefficient optimal methods and non-network-coding approaches. Our experiments show that our heuristic provides near-optimal performance while significantly outperforming existing approaches for dedicated multicast protection. We also propose architectures for specialized all-optical circuits capable of performing the processing required for network coding and show how these devices can be effectively deployed in an all-optical multicast network.

WDM-RoF-PON Architecture for Flexible Wireless and Wire-Line Layout

Abstract: We experimentally demonstrate what we believe to be a novel wavelength division multiplexing-passive optical networking (WDM-PON) architecture compatible with 60 GHz radio-over-fiber systems to simultaneously provide wireless or wired services. We place each wireless/wire-line channel in one WDM slot to carry a 2.5 Gb/s orthogonal frequency division multiplexing (OFDM) signal and reuse downstream wavelengths of the wireless and wire-line channels for uplink by an on-off-keying modulation format signal. For the independent design for wireless and wire-line channels, the layouts of wireless or wire-line channels are flexible for different applications.

Diversity–Multiplexing Trade-Off in Coherent Free-Space Optical Systems With Multiple Receivers

Abstract: In this paper, from an information theory point of view, we investigate the performance of a coherent free-space optical (FSO) communication system with multiple receive apertures over atmospheric turbulence channels. Our study builds on a recently introduced statistical model that characterizes the combined effects of turbulence-induced wavefront distortion and amplitude fluctuation in coherent receivers with phase compensation. We investigate the link reliability as quantified by “diversity gain” and the relationship between the link reliability and the spectral efficiency as quantified by “diversity-multiplexing trade-off (DMT).” Our results provide insight into the performance mechanisms of coherent FSO systems and demonstrate significant performance gains that can be obtained through the deployment of multiple receive apertures and phase compensation techniques.

Generating Realistic Optical Transport Network Topologies

Abstract: We address the problem of generating physical realistic optical transport network topologies. This type of network has characteristics that differ from scale-free networks, such as the Internet. Based on the analysis of a set of real transport topologies, we identify and assess relevant characteristics. A method to generate realistic topologies is proposed. The proposed method is validated by comparing the characteristics of computer-generated and real-world optical transport networks.

Subcarrier Intensity Modulated Free-Space Optical Communications in K-Distributed Turbulence Channels

Abstract: In this paper, the bit error rate (BER) performance of a subcarrier intensity-modulated (SIM) free-space optical (FSO) communications system using binary phase shift keying (BPSK) is investigated over a K-distributed turbulence channel. First, the performance is analyzed employing a negative exponential turbulence model, and an exact closed-form expression is derived for the BER. Then, it is shown that the probability density function (PDF) of the K distribution can be approximated accurately by a finite sum of weighted negative exponential PDFs. Based on this interesting result and by using the closed-form expression, which is derived for the case of a negative exponential model, an approximate, closed-form expression for the BER of the BPSK-based SIM FSO over a K channel is derived. Moreover, to improve the BER performance, spatial diversity using selection combining (SC) is considered. It is shown that the PDF of the resulting channel irradiance corresponding to the SC diversity scheme over a K channel can be approximated accurately by a finite linear combination of negative exponential functions. The derived approximate PDF accurately estimates the PDF of the channel irradiance for arbitrary values of diversity order and is valid for a wide range of channel parameters. Then, an approximate, closed-form expression is derived for the average BER of the BPSK-based SIM FSO system employing the SC diversity technique over a K channel. Numerical results presented in this paper show that the derived approximate expressions are very accurate and can be used as efficient tools for performance analysis of the system.

Code Family for Modified Spectral-Amplitude-Coding OCDMA Systems and Performance Analysis

Abstract: A novel family of codes for modified spectral-amplitude-coding optical code division multiple access (SAC-OCDMA) is introduced. The proposed codes exist for more processing gains than the previously reported codes do. In the network using these codes, the number of users can be extended without any essential changes in the previous transmitters. In this study, we propose a construction method for these codes and compare their performance with previously reported codes.

Routing and Wavelength Assignment of Static Manycast Demands Over All-Optical Wavelength-Routed WDM Networks

Abstract: In this paper we present the static manycast routing and wavelength assignment (MA-RWA) problem along with heuristics and an integer linear program (ILP) to solve it. Manycast is a point-to-multipoint communication paradigm with applications in e-Science, Grid, and cloud computing. A manycast request specifies a candidate set of destinations, of which a subset must be reached. To solve MA-RWA, a light-tree must be assigned to each manycast request in a static set such that the number of wavelengths required is minimized. We present two heuristics, the shortest path heuristic (SPT) and the lambda path heuristic (LPH), a tabu search meta-heuristic (TS), and an ILP formulation. We show that TS provides results close to the optimal solution (from the ILP) for small networks. We then show that TS provides a 10% improvement over LPH and a 30%-40% improvement over SPT for various realistic networks.

Lightpath Establishment Assisted by Offline QoT Estimation in Transparent Optical Networks

Abstract: We propose lightpath establishment techniques for transparent optical networks, valid for both centralized path computation (i.e., performed by a path computation element-PCE) and distributed path computation (i.e., performed by each ingress node). The proposed techniques rely on correlating the end-to-end quality of transmissions (QoT) (e.g., optical-to-signal-noise ratio-OSNR) of lightpaths already set up to evaluate the QoT of lightpaths to be established. The correlation is leveraged by two end-to-end estimation frameworks called “network kriging” and “norm l2 minimization.” Each framework is used in turn to estimate end-to-end parameters that describe the QoT for a lightpath to be established, based on measured parameters from other lightpaths that were previously established or probed. Simulations are carried out for both PCE and distributed networking scenarios on a sample optical transparent network. The proposed lightpath establishment techniques reduce the probability that a setup attempt is unsuccessful due to unacceptable QoT and reduce the number of setup attempts to successfully establish lightpaths. Simulation results are also provided to show that our techniques permit a fast convergence of QoT information at PCE or ingress nodes in the context of dynamic networks.

Clustering Methods for Hierarchical Traffic Grooming in Large-Scale Mesh WDM Networks

Abstract: We consider a hierarchical approach for traffic grooming in large multiwavelength networks of a general topology. Inspired by similar concepts in the airline industry, we decompose the network into clusters, and select a hub node in each cluster to groom traffic originating and terminating locally. At the second level of the hierarchy, the hub nodes form a virtual cluster for the purpose of grooming intra-cluster traffic. Clustering and hierarchical grooming enables us to cope with large network sizes and facilitates the control and management of traffic and network resources. Yet, determining the size and composition of clusters so as to yield good grooming solutions is a challenging task. We identify the grooming-specific factors affecting the selection of clusters, and we develop a parameterized clustering algorithm that can achieve a desired trade-off among various goals. We also obtain lower bounds on two important objectives in traffic grooming: the number of lightpaths and wavelengths needed to carry the subwavelength traffic. We demonstrate the effectiveness of clustering and hierarchical grooming by presenting the results of experiments on two network topologies that are substantially larger than those considered in previous traffic grooming studies.

Analysis of Four-Wave Mixing Suppression in Fiber-Optic OFDM Transmission Systems With an Optical Phase Conjugation Module

Abstract: Coherent optical orthogonal frequency-division multiplexed (OFDM) systems must be carefully designed to minimize the detrimental impact of fiber nonlinearity manifested through four-wave mixing (FWM). Because of the small subcarrier spacing associated with OFDM, a significant fraction of FWM processes is well matched, resulting in a rapid buildup of FWM light with propagation distance. In this paper, we consider optical phase conjugation (OPC) as an approach to suppress such well-matched FWM processes. An analytical formula accurately predicting the degree of suppression is derived and discussed. It is shown that when combined with the methods previously proposed in the literature, the application of OPC can dramatically reduce the overall FWM power accumulated within the link for a wide range of crucial design parameters.

Attack-Aware Wavelength Assignment for Localization of In-band Crosstalk Attack Propagation

Abstract: The high data rates employed by wavelength division multiplexing transparent optical networks make them most suitable for today's growing network traffic demands. However, their transparency imposes several vulnerabilities in network security, enabling malicious signals to propagate from the source to other parts of the network without losing their attacking capabilities. Furthermore, detecting, locating the source, and localizing the spreading of such physical-layer attacks is more difficult since monitoring must be performed in the optical domain. While most failure and attack management approaches focus on network recovery after a fault or an attack has already occurred, we suggest a novel safety strategy, proposing a prevention-oriented method to aid attack localization and source identification in the planning phase. In this paper, we propose attack-aware wavelength assignment that minimizes the worst-case potential propagation of in-band crosstalk jamming attacks. We define a new objective criterion for the wavelength assignment (WA) problem, called the propagating crosstalk attack radius (P-CAR), and develop heuristic algorithms aimed at minimizing both the P-CAR and the number of wavelengths used. Our aim is to achieve better protection, but without the need for extra resources. We compare our algorithms with existing WA approaches, illustrating their benefits with respect to transparent optical networks' security, as well as the associated wavelength utilization.

Techno-economic Evaluation of FTTC/VDSL and FTTH Roll-Out Scenarios: Discounted Cash Flows and Real Option Valuation

Abstract: Due to the increasing demand in data rates telecom operators are faced with the question of when to upgrade their access network infrastructure toward fiber-to-the-home (FTTH). In this paper a techno-economic study of fiber-to-the-cabinet/very high bit rate digital subscriber line (FTTC/VDSL) and FTTH deployments is performed. The economics and risks associated with the incumbent's decision to invest in dense urban and urban areas are analyzed using both discount cash flow (DCF) analysis and real options analysis (ROA). The results revealed that in the case of investment subsidization, the option value to expand in a later phase can significantly improve the financial metrics of the business cases. The analysis made is based on the ECOSYS techno-economic methodology and tool.

Design and Simulation of Filterless Optical Networks: Problem Definition and Performance Evaluation

Abstract: Filterless optical networks based on advanced transmission technologies and passive optical interconnections between nodes offer a lower-cost alternative to optical networks based on active photonic switching. A design and simulation platform is proposed for studying these novel network architectures and looking at their performance characteristics. Simulation results are presented for three reference network topologies, along with a comparative cost and performance study of active photonic and passive filterless optical network solutions.

SLA-Aware Dynamic Bandwidth Allocation for QoS in EPONs

Abstract: The Ethernet passive optical network (EPON) is one of the most promising broadband access networks. We propose a new dynamic scheduling algorithm, referred to as cyclic-polling-based dynamic bandwidth allocation with service level agreements (CPBA-SLA), for service differentiation that meets the service level agreements between the OLT and ONUs. The proposed dynamic bandwidth allocation (DBA) scheme provides a constant and predictable average packet delay and an improved delay jitter of the expedited forwarding traffic without the influence of load variations. A performance evaluation shows the effectiveness of the proposed DBA scheme.

Performance Analysis of a Laser Ground-Station-to-Satellite Link With Modulated Gamma-Distributed Irradiance Fluctuations

Abstract: The performance of a ground-station-to-space laser uplink with a Gaussian-beam wave model subject to turbulence and beam wander effects is the topic of the present study. The modulated gamma distribution is used to describe the combined effect of the above two deteriorating factors. At first, a versatile expression of the above probability density function is deduced. We then derive novel closed-form expressions for its cumulative distribution function and the moment-generating function. The scintillation index and the probability of fade are hence readily evaluated. The analysis is completed with the evaluation of the bit error rate assuming heterodyne detection with differential phase-shift keying. In order to attain an adequate error rate target, we incorporate diversity at the satellite receiver. A proper simulation scenario is adopted, and numerical results are provided to verify the accuracy of the derived expressions.

Lightpath Reconfiguration in WDM Networks

Abstract: Lightpath reconfiguration is a networking task that can be performed in order to improve resource utilization. The lightpath reconfiguration problem becomes nontrivial when a new set of lightpaths requires the release of resources previously seized by the (working) lightpaths currently in place, but, in order to ensure continuity of the traffic flow, the working lightpaths cannot be torn down before the new ones are set up. Under this condition the reconfiguration can fall into a deadlock state, and deadlocks can only be solved by temporary disruption of some connections. At this point, traffic disruptions are necessary, and network operators must compensate customers with penalty fees for the service disruption. In this paper we focus on minimizing the number of simultaneously disrupted connections at any time during the reconfiguration process. In this paper, we propose a mixed-integer program (MIP) model, an exact algorithm, and a heuristic for solving the problem considering our objective.

Novel Monitoring Technique for Passive Optical Networks Based on Optical Frequency Domain Reflectometry and Fiber Bragg Gratings

Abstract: An efficient monitoring method having a very short measurement time (a few seconds) is proposed. The equal-length branches can be effectively monitored, but also the information of temperature at any place in the network can be obtained. The feasibility of this technique is experimentally demonstrated.

Efficient Video-on-Demand Streaming for Broadband Access Networks

Abstract: We introduce three important factors for efficient video-on-demand (VoD) streaming strategy in broadband access networks such as a passive optical network (PON): an efficient streaming scheme for bandwidth savings, optimal use of deployed network bandwidth, and proactive use of user storage facilities. To achieve these goals, we propose efficient algorithms such as video adaptive streaming (VAST), video greedy adaptive streaming (VGAST), and video greedy adaptive streaming with proactive buffering (VGAST-PB). The VAST and VGAST algorithms use a patching scheme to save on network bandwidth for video streaming and increase the efficiency by adaptively changing the speed of patch streams and the patching window based on the time of the day and video popularity. VGAST-PB further improves the efficiency of VGAST by proactively buffering the popular videos. The proposed algorithms can be applied in fiber-to-the-x broadband access networks. To analyze the algorithms, we consider a typical 2.488 Gbps gigabit PON network providing VoD service to 512 households. We use a daily video request model and a video popularity model to determine the adaptive streaming speed and dynamic patching window. Numerical results show that, when the available network bandwidth is reduced below the required level due to background traffic, the efficient algorithms can con-siderably reduce the average user waiting time and the number of waiting requests.

Noncoherent Spectral/Spatial OCDMA System Using Two-Dimensional Hybrid Codes

Abstract: We propose newly constructed two-dimensional hybrid codes (2-D hybrid codes) and the corresponding system architecture for a spectral/spatial optical code division multiple access (OCDMA) system. We employ unidimensional (1-D) integer lattice codes and 1-D perfect difference codes to construct the 2-D hybrid codes. This proposed system can suppress the phase-induced intensity noise (PIIN) and has the multiuser interference (MUI) cancellation property. In comparison with the other systems using the 2-D maximal-area matrices codes (2-D MM codes), the 2-D perfect difference codes (2-D PD codes), and the 2-D spatial division multiplexing balanced incomplete block design codes (2-D S-BIBD codes), the numerical results show that the proposed system has the superior performance.

Thresholding-Based Optimal Detection of Wireless Optical Signals

Abstract: The receiver design for a high-speed free-space (wireless) optics (FSO) signal is necessarily highly complex when channel state information (CSI) is not available. Currently, although most approaches provide high detection performance in terms of bit error, receiver design is difficult to implement. This paper proposes two practical thresholding-based detection schemes, which offer significant improvement to receiver throughput on a computational load basis when CSI is not available. The first is based on a simple maximum likelihood (ML) function where the bit error rate (BER) is the same as conventional symbol-by-symbol detection. This method, however, causes a loss of BER performance. The second uses the aid of pilot-symbol-assisted modulation (PSAM) to modify the ML function when channel coefficients are temporally correlated. While numerical analysis based on this method shows that the BER performance in a log-normally distributed fading channel is very close to detection achieved with perfect CSI, the receiver suffers from increased complexity. If random processes for fading and noise are assumed as stationary and given that the detection threshold is quickly calculated and applied during a given period, such complexity of PSAM-based and symbol-by-symbol detection methods can be reduced.

Intelligent p-Cycle Protection for Dynamic Multicast Sessions in WDM Networks

Abstract: In WDM networks, it is important to maintain the survivability of communication sessions when link failure occurs due to the high bandwidth provided by a fiber link. Link failures have more serious impact on multicast sessions than on unicast sessions since a link used by a multicast session may carry traffic to multiple destinations. Thus, it is more critical to protect multicast sessions against single link failures. Researchers have proposed various protection schemes for multicast sessions, including tree-based, path-based, and segment-based schemes. Tree-based schemes suffer excessive use of network capacity, whereas path-based and segment-based schemes require long restoration times. In this paper, we propose a new p-cycle-based dynamic multicast protection scheme named IpC, which achieves both fast restoration and high capacity efficiency. The main feature of IpC is that it computes high efficiency p-cycles on demand to protect dynamic multicast sessions as they arrive. Extensive simulations have been conducted to evaluate the proposed IpC scheme, and the results show that it outperforms an existing p-cycle-based scheme.

Routing and Wavelength Assignment Problem in PCE-Based Wavelength-Switched Optical Networks

Abstract: As a key problem in wavelength-switched optical networks (WSONs), in which wavelengths are the most important resources, the routing and wavelength assignment (RWA) problem has been researched extensively. Due to the advantages of path computation in multilayer and multidomain networks, especially in multiconstraint scenarios, the path computation element (PCE) has been introduced into WSONs, which provides a better approach to the solution of the RWA problem in WSONs. This paper focuses on the RWA problem in PCE-based WSONs. A PCE-based routing framework and two PCE-based routing models are first designed, and then two distributed resource reservation schemes based on forward reservation protocol (FRP) and backward reservation protocol (BRP) are proposed and simulated in a PCE-based WSON. Finally, eight PCE-based RWA schemes are proposed in WSONs, the performances of which have been investigated and validated using the discrete event simulation tool OMNeT++. Numeric results show that the RWA (first-fit) scheme has the best performance in terms of blocking probability and average delay time.

Quality of Service Provisioning in Optical CDMA Packet Networks

Abstract: An enhanced media access control (MAC) layer protocol that uses the signaling method benefits of the physical layer in order to provide different levels of quality to different users in optical code-division multiple-access (OCDMA) packet networks is presented. In the proposed network architecture the users are categorized into two classes of service, one having a higher quality level and the other having a lower quality level. Users of each class transmit at the same power level and different from the other classes’ users. Also, the MAC of each user estimates the amount of interference on the channel and adjusts the packet transmission’s time to improve network performance. Through simulation it is shown that the combination of appropriate power assignment to users and proper MAC algorithm parameters can provide various quality of service (QoS) metric levels on metrics such as normalized throughput and packet error rate. This is achieved by dividing the available resources of the OCDMA network between the users of each class. To make the QoS provider method more practical in data communication networks, we have studied the fairness issue by defining two parameters related to the normalized throughput of each class.

Wavelength Dimensioning of Optical Transport Networks Over Nongeosychronous Satellite Constellations

Abstract: Massive data traffic demand leads to the proposal of satellite constellation networks providing broadband communications for users. The key technology to satisfy the high capacity requirement of future communications between satellites is the optical intersatellite link (ISL). Wavelength-routed optical transport networks over nongeosychronous satellite constellations (WROTNS) show the most promising future, since the networking by means of wavelength division multiplexing (WDM) ISLs with wavelength routing can not only efficiently utilize the optical amplifier bandwidth on board the satellite but also simplify routing decisions and minimize processing delays on board. In the paper we consider that the physical ISL topologies of the WROTNS are the arbitrarily connected networks. Based on the first satellite constellation with the proposal to include WDM ISLs and wavelength routing, its time-variant optical ISL topology is analyzed, and then a large number of randomly generated connected networks with distinct topologies are investigated to estimate the bounds of wavelength requirements. The results show that wavelength requirements of the WROTNS are determined by percentage of active terminals, maximum admissible ISLs hops, and the number of optical ISL terminals on board, and they are also time dependent. The topologically regular WROTNS are studied as well. These works are helpful to evaluate the design of constellation networking.

Intelligent Dynamic Bandwidth Allocation Algorithm in Upstream EPONs

Abstract: An upstream access scheme for Ethernet passive optical networks, called the intelligent fuzzy-logic-based dynamic bandwidth allocation algorithm (IFLDBA) has been proposed. The algorithm provides the allotment of bandwidth between optical network units (ONUs) and within ONUs. Fuzzy logic is used to improve the bandwidth allocation within each ONU. We compare the IFLDBA algorithm with the existing broadcast polling algorithm. The results show that IFLDBA has bandwidth utilization of up to 21% better and lower delay than the broadcast polling algorithm.