Optical wireless communication (OWC) refers to transmission in unguided propagation media through the use of optical carriers, i.e., visible, infrared (IR), and ultraviolet (UV) bands. In this survey, we focus on outdoor terrestrial OWC links which operate in near IR band. These are widely referred to as free space optical (FSO) communication in the literature. FSO systems are used for high rate communication between two fixed points over distances up to several kilometers. In comparison to radio-frequency (RF) counterparts, FSO links have a very high optical bandwidth available, allowing much higher data rates. They are appealing for a wide range of applications such as metropolitan area network (MAN) extension, local area network (LAN)-to-LAN connectivity, fiber back-up, backhaul for wireless cellular networks, disaster recovery, high definition TV and medical image/video transmission, wireless video surveillance/monitoring, and quantum key distribution among others. Despite the major advantages of FSO technology and variety of its application areas, its widespread use has been hampered by its rather disappointing link reliability particularly in long ranges due to atmospheric turbulence-induced fading and sensitivity to weather conditions. In the last five years or so, there has been a surge of interest in FSO research to address these major technical challenges. Several innovative physical layer concepts, originally introduced in the context of RF systems, such as multiple-input multiple-output communication, cooperative diversity, and adaptive transmission have been recently explored for the design of next generation FSO systems. In this paper, we present an up-to-date survey on FSO communication systems. The first part describes FSO channel models and transmitter/receiver structures. In the second part, we provide details on information theoretical limits of FSO channels and algorithmic-level system design research activities to approach these limits. Specific topics include advances in modulation, channel coding, spatial/cooperative diversity techniques, adaptive transmission, and hybrid RF/FSO systems.

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Survey on Free Space Optical Communication: A Communication Theory Perspective

Power Aware Routing in Mobile Ad Hoc Networks

Cognitive radio (CR) technology is envisaged to solve the problems in wireless networks resulting from the limited available spectrum and the inefficiency in the spectrum usage by exploiting the existing wireless spectrum opportunistically. CR networks, equipped with the intrinsic capabilities of the cognitive radio, will provide an ultimate spectrum-aware communication paradigm in wireless communications. CR networks, however, impose unique challenges due to the high fluctuation in the available spectrum as well as diverse quality-of-service (QoS) requirements. Specifically, in cognitive radio ad hoc networks (CRAHNs), the distributed multi-hop architecture, the dynamic network topology, and the time and location varying spectrum availability are some of the key distinguishing factors. In this paper, intrinsic properties and current research challenges of the CRAHNs are presented. First, novel spectrum management functionalities such as spectrum sensing, spectrum sharing, and spectrum decision, and spectrum mobility are introduced from the viewpoint of a network requiring distributed coordination. A particular emphasis is given to distributed coordination between CR users through the establishment of a common control channel. Moreover, the influence of these functions on the performance of the upper layer protocols, such as the network layer, and transport layer protocols are investigated and open research issues in these areas are also outlined. Finally, a new direction called the commons model is explained, where CRAHN users may independently regulate their own operation based on pre-decided spectrum etiquette.

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CRAHNs: Cognitive radio ad hoc networks

Recent advances in hardware, software, and communication technologies are enabling the design and implementation of a whole range of different types of networks that are being deployed in various environments. One such network that has received a lot of interest in the last couple of years is the Vehicular Ad-Hoc Network (VANET). VANET has become an active area of research, standardization, and development because it has tremendous potential to improve vehicle and road safety, traffic efficiency, and convenience as well as comfort to both drivers and passengers. Recent research efforts have placed a strong emphasis on novel VANET design architectures and implementations. A lot of VANET research work have focused on specific areas including routing, broadcasting, Quality of Service (QoS), and security. We survey some of the recent research results in these areas. We present a review of wireless access standards for VANETs, and describe some of the recent VANET trials and deployments in the US, Japan, and the European Union. In addition, we also briefly present some of the simulators currently available to VANET researchers for VANET simulations and we assess their benefits and limitations. Finally, we outline some of the VANET research challenges that still need to be addressed to enable the ubiquitous deployment and widespead adoption of scalable, reliable, robust, and secure VANET architectures, protocols, technologies, and services.

Vehicular ad hoc networks (VANETS): status, results, and challenges

Cognitive radio (CR) is the enabling technology for supporting dynamic spectrum access: the policy that addresses the spectrum scarcity problem that is encountered in many countries. Thus, CR is widely regarded as one of the most promising technologies for future wireless communications. To make radios and wireless networks truly cognitive, however, is by no means a simple task, and it requires collaborative effort from various research communities, including communications theory, networking engineering, signal processing, game theory, software-hardware joint design, and reconfigurable antenna and radio-frequency design. In this paper, we provide a systematic overview on CR networking and communications by looking at the key functions of the physical (PHY), medium access control (MAC), and network layers involved in a CR design and how these layers are crossly related. In particular, for the PHY layer, we will address signal processing techniques for spectrum sensing, cooperative spectrum sensing, and transceiver design for cognitive spectrum access. For the MAC layer, we review sensing scheduling schemes, sensing-access tradeoff design, spectrum-aware access MAC, and CR MAC protocols. In the network layer, cognitive radio network (CRN) tomography, spectrum-aware routing, and quality-of-service (QoS) control will be addressed. Emerging CRNs that are actively developed by various standardization committees and spectrum-sharing economics will also be reviewed. Finally, we point out several open questions and challenges that are related to the CRN design.

Cognitive radio networking and communications: an overview

In this paper we conduct a feasibility study of delay-critical safety applications over vehicular ad hoc networks based on the emerging dedicated short range communications (DSRC) standard. In particular, we quantify the bit error rate, throughput and latency associated with vehicle collision avoidance applications running on top of mobile ad hoc networks employing the physical and MAC layers of DSRC. Towards this objective, the study goes through two phases. First, we conduct a detailed simulation study of the DSRC physical layer in order to judge the link bit error rate performance under a wide variety of vehicles speeds and multi-path delay spreads. We observe that the physical layer is highly immune to large delay spreads that might arise in the highway environment whereas performance degrades considerably at high speeds in a multi-path environment. Second, we develop a simulation testbed for a DSRC vehicular ad hoc network executing vehicle collision avoidance applications in an attempt to gauge the level of support the DSRC standard provides for this type of applications. Initial results reveal that DSRC achieves promising latency performance, yet, the throughput performance needs further improvement.

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Performance evaluation of safety applications over DSRC vehicular ad hoc networks

In this paper, we propose the virtual dynamic backbone protocol (VDBP) for ad hoc networks. VDBP constructs a backbone from a subset of the nodes in the network satisfying these two properties: (i) Any node in the network is either a backbone node or is a neighbor of a backbone node (dominating set property), (ii) any pair of backbone nodes is connected via other backbone nodes (connectivity property). VDBP provides explicit mechanisms to handle the mobility of the nodes and depends on local computations to a large extent. Thus it is highly self-organizing even under highly dynamic topologies. Such a backbone can be potentially used by a routing protocol to disseminate broadcast or multicast messages as well as to handle QoS traffic in mobile ad hoc networks. We present the performance of VDBP in terms of backbone size and connectivity using simulations.

Virtual dynamic backbone for mobile ad hoc networks

We propose four fractal point processes (FPPs) as novel approaches to modeling and analyzing various types of self-similar traffic: the fractal renewal process (FRP), the superposition of several fractal renewal processes (Sup-FRP), the fractal-shot-noise-driven Poisson process (FSNDP), and the fractal-binomial-noise-driven Poisson process (FBNDP). These models fall into two classes depending on their construction. A study of these models provides a thorough understanding of how self-similarity arises in computer network traffic. We find that (i) all these models are (second-order) self-similar in nature; (ii) the Hurst parameter alone does not fully capture the burstiness of a typical self-similar process; (iii) the heavy-tailed property is not a necessary condition to yield self-similarity; and (iv) these models permit parsimonious modeling (using only 2-5 parameters) and fast simulation. Simulation verifies that these models exhibit a fractal behavior over a wide range of time scales.

Point process approaches to the modeling and analysis of self-similar traffic .I. Model construction

This paper presents the single-radio adaptive channel (SRAC) algorithm which enables dynamic spectrum access in multi-hop wireless ad hoc networks where each node has only one half-duplex radio (transceiver). Designed as a relatively independent module, SRAC can upgrade various existing single-radio legacy medium access control (MAC) protocols to be dynamic spectrum access capable, achieving efficient use of the spectrum, relaxing their operating conditions, and naturally supporting multicast applications. The SRAC algorithm is characterized by three features: (a) dynamic channelization in response to jamming, primary spectrum users and channel load, (b) "cross channel communications", and (c) as-needed use of spectrum. We evaluate the performance of SRAC through analysis and QualNet simulations.

Single-Radio Adaptive Channel Algorithm for Spectrum Agile Wireless Ad Hoc Networks

We construct five new fractal traffic models based on the unified framework of fractal point processes (FPPs), and analyze their asymptotic statistical properties. Two of them are on-off types, suitable for characterizing an aggregate fractal traffic source. The other three provide a structure suitable for flow modeling, allowing one to gain a quantitative understanding of how application- or flow-level fractal dynamics (such as user activity, session/flow arrivals, duration, and volume distributions) affect packet-level fractal dynamics. Such a structure, together with analytic results obtained in this work, opens an entirely new horizon that enables us to explain the causes and origins of complex multiple-time-scale behavior of packet-level dynamics in a quantitative manner, i.e., multi-fractal patterns over small time scales and mono-fractal behavior over large time scales. Consequently, they provide network traffic engineering researchers and practitioners with practical and flexible tools for analyzing, devising and performing various traffic engineering studies. All of the FPPs are implemented in OPNET as IP-routable traffic generators.

Bo Ryu

Papers

Performance evaluation of safety applications over DSRC vehicular ad hoc networks

Virtual dynamic backbone for mobile ad hoc networks

Point process approaches to the modeling and analysis of self-similar traffic .I. Model construction

Single-Radio Adaptive Channel Algorithm for Spectrum Agile Wireless Ad Hoc Networks

Fractal traffic models for Internet simulation