The increased density of fixed and mobile wireless devices in indoor settings introduces novel challenges related to the deployment of multi-user access points (APs). Network “densification” includes the adoption of many replicated APs in order to meet aggregate demands divided up across the system. However, the close proximity of APs can create interference. Directional optical channels have been proposed to improve signal isolation between adjacent APs.In this paper we explore the use of controlled dynamic field of view (D-FOV) as a way to isolate resources. Specifically, we study (a) system resource reuse, (b) receiver scope, and (c) interference mitigation. Analysis and simulation show performance gains in overall system performance, and user SNR/SINR. We also show that D-FOV enables improved resource reuse, making full use of spatial diversity while mitigating interference effects on user devices. We validate results using data collected in a network and lighting testbed implementation. Results of the experimentation show good correspondence to the analysis and reveal the associated trade-offs that occur when moving between the span of narrow to wide FOVs.

Dense networks are characterized by the prevalence of wireless access points (APs) in close proximity to a population of user devices on a similar scale. This is in contrast with a legacy model of a single AP serving many devices. By increasing AP density, the aggregate data consumption of the system can be dramatically increased. In this paper we consider dense deployment of directional optical access points. In particular, we analyze how a user device can select and track a transmitter then adapt its field of view (FOV) to achieve the best signal quality within the array of APs. Using a geometric model for an indoor space and a reference optical channel model, we formulate an optimization problem with a goal of optimizing FOV for maximum signal to noise ratio (SNR). The results, applied as a proposed dynamic FOV technique with receiver tracking capability, show an average SNR increase of up to 40% when compared to a fixed receiver associated with a user device with dynamic translational/rotational motion. These results motivate the adoption of dynamic pointing and adaptive FOV at the receiver in order to realize improved performance for mobile devices in a dense optical wireless network.

The most common approach to light-based indoor positioning relies on multilateration of received signals to the mobile device. Any deficiencies in the fidelity of these light signals can significantly distort position estimates. In this paper, we propose a method to dynamically control the light distribution from the overhead luminaires to mitigate fading effects that would otherwise occur under static lighting. By manipulating the direction of the luminaire, effectively the dispersion pattern, we introduce signal diversity in the form of multiple pointing angles and light distributions. In addition to providing angle diversity, steering and then tracking sustains the maximal line-of-sight path between a source and receiver, which reduces angle-dependent attenuation and optimizes the signal-to-noise ratio for any coordinate without needing to change the physical properties of the source or receiver. This gain in signal strength combats the limited field-of-view of luminaires and photodiodes to provide better overall coverage, which translates directly to increase positioning accuracy, particularly in a 3D space. In the results, we show field-of-view gains of 43% and improvements in MSE of 20cm.

Orthogonal frequency division multiplexing (OFDM) is currently the most prominent modulation technique, mainly due to it's spectral efficiency. To improve the security performance of OFDM based systems, multiple security approaches are proposed in literature, including physical-layer (PHY) approaches. However, these techniques are technology specific, i.e. mostly designed for radio frequency (RF) transmission and cannot be directly deployed in other technologies such as optical wireless communications (OWC). In this paper, security aware OFDM (SA-OFDM) is presented as a technology independent PHY security approach designed to suppress eavesdropping. The novelty of SA-OFDM is not only its versatility in deployment, but also in its perception in combining PHY encryption and the receiver's architecture to improve security performance. SA-OFDM is tested under additive white Gaussian noise (AWGN) and Rayleigh channel models, indicating negligible impact on system performance for both RF and OWC links. Results show that the eavesdropper becomes oblivious to the transmitted information, i.e. has a bit-error-rate (BER) of 0.5, which is equivalent to random guessing. Even if the key is seized, the eavesdropper's BER does not exceed 10-2.

Providing optical wireless data access with the visible spectrum is a commercial reality. Products are available for point-to-point or point-to-multipoint solutions. However, there remain many important questions about how to successfully achieve “dual-use” systems in which both lighting and communications functions are realized. In particular, the necessary placement of lights for lighting and the characteristic of light signals to be out-of-view creates trade-offs especially when mobile devices can change their position, height, and orientation requiring cooperation with neighboring access points (APs). In this paper, we investigate outage probability for Visible Light Communication (VLC) links in a system in which these dimensions are varied. We propose a probabilistic model characterizing where to deploy indoor VLC access points considering the impact of handover delay and user velocity. Our model demonstrates the trade-offs that lie within this problem and we also provide a closed form upper bound on outage probability that guarantees outage-free regions for devices attempting handover. The model is used to demonstrate the impact of different VLC access point cell separations and to show how user device velocity impacts outage probability.

This study explores how the field‐of‐view (FOV) of a visible light communications (VLCs) receiver can be manipulated to realise the best signal‐to‐noise ratio (SNR) while supporting device mobility and optimal access point (AP) selection. The authors propose a dynamic FOV receiver that changes its aperture according to receiver velocity, location, and device orientation. The D‐FOV technique is evaluated through modelling, analysis, and experimentation in an indoor environment comprised of 15 VLC APs. The proposed approach is also realised as an algorithm that is studied through analysis and simulation. The results of the study indicate the efficacy of the approach including a 3X increase in predicted SNR over static FOV approaches based on measured received signal strength in the testbed. Additionally, the collected data reveal that D‐FOV increases effectiveness in the presence of noise. Finally, the study describes the tradeoffs among the number of VLC sources, FOV, user device velocity, and SNR as a performance metric.

In visible light communications (VLC) systems based on baseband intensity modulation (IM), the modulating signals are designed for a single-purpose to either support single-user access using single-carrier pulsed modulation such as On-Off keying (OOK) or multi-user access mainly using the multi-carrier orthogonal frequency division multiplexing (OFDM). In VLC based sensing, transmitted sinusoidal beacons are considered to identify the different light sources. A key challenge is that such systems are not capable of supporting simultaneous bit streaming and sensing to high-end devices already equipped with chip sets compatible with OFDM and resource limited low-end devices for Internet-of-Things (IoT). In this paper, we present a novel design of a modulating waveform to effectively tackle this issue. The proposed waveform allows high-end and low-end devices to receive the same baseband signal, extract the useful part and process without any interference. We provide a detailed presentation of the design, explanation of the demodulation schemes and performance evaluation. We validated the performance through simulation results showing bit-error rate (BER) curves using multi-carrier OFDM for high-end devices. The bit-error performance is maintained at different dimming levels while offering a parallel data stream using single-carrier modulation for low-end devices and a stream of sinusoidal beacons needed for the sensing mission.

As the technical feasibility of indoor Optical Wireless Communications becomes established, a variety of new system challenges emerge in the quest for higher and denser data rates when employing lighting, or luminaires, for VLC. Supporting device mobility is one such challenge wherein a mobile device equipped with an optical receiver will transit a field of optical access points (APs) and will require one or more handovers between APs. Of course, the nature of the handovers is dependent on the behavior of the user and the design constraints for the placement of the luminaires/APs within the indoor space. In this paper we focus on modeling a parameterized system for studying the handover performance for mobile users with different receiver characteristics including field of view (FOV) and orientation with respect to overhead APs. Results indicate the utility of our system models for characterizing handover performance for a reference indoor space and AP placement. We also describe several handover strategies providing performance gains in terms of reducing the number and frequency of handovers for a user in the system. Lastly, we show the linkage between a wide FOV and a narrow FOV in terms of optimizing system performance. In the future we expect to drive simulations based on representative mobility behavior derived from empirical models of user behavior in an indoor space.

A design framework is presented in this manuscript for a novel visible light communications (VLC)-based multi-tier waveform. Conventionally, VLC waveforms are designed to target specific services. In services that require high-speed access, multi-carrier modulation techniques, i.e., orthogonal frequency division multiplexing (OFDM), is considered. For lower-speed access services, single carrier modulation techniques are considered such as phase-shift keying (PSK) or pulse-position modulation (PPM). The proposed design offers a universal-and receiver-independent multi-tier waveform that is expected to serve the requirements of fifth-generation (SG) wireless networks and beyond, including high-speed connectivity, sensing and positioning services. This allows a wide variety of devices to extract a useful portion of the received waveform associated to the targeted service while ensuring inter-service-interference-free operation. In addition, the proposed design aims for cooperative transmission and dimming control to enhance the lighting environment for better user experience. The paper provides a detailed description of the design process and the experimental evaluation. The experimental results indicate that the designed waveform can offer dimming control over 60% of the light-emitting diode (LED) full dynamic range, while maintaining bit-error rate (BER) of 7×10-5for 64-quadrature amplitude modulation (64-QAM).

Visible light communication (VLC) has the potential to add significant capacity to short range wireless access technology by piggybacking data on light from overhead luminaires. However, an uplink is required to complete such a network, which introduces new issues. In this paper, we propose and implement a practical hybrid WiFi-VLC system that does not require a separate VLC uplink but rather aggregates WiFi and VLC downlinks and shares the WiFi uplink. Aggregated downlink bandwidth of the hybrid system is achieved by using a Linux bonding driver and media access control (MAC) address redirection. The throughput of the system is tested and compared with WiFi-only (one WiFi downlink) and asymmetric (one VLC downlink) systems under a congested WiFi environment. The evaluation results show that our system achieves aggregated downlink bandwidth that is approximately the summation of the downlink capacities of the WiFi-only and asymmetric systems. The study of the round-trip time (RTT) demonstrates the tradeoff between bandwidth utilization and latency that can be used in the design of load-balancing algorithms. Finally, the deployed system demonstrates feasibility in typical indoor space room dimensions.

We are at the dawn of an era in information and communication technology with unprecedented demand for connected and automated everything. Both Shannon theories and industrial advances have clearly evidenced that more densely packed networks and a much wider operating bandwidth are key drivers for meeting the escalating wireless network demands. Recently, there have been substantial research efforts on the exploitation of higher frequency bands, in particular the millimetre wave and optical wireless bands. After a decade of active research and development, and along with the maturity of device technology, Visible Light Communications (VLC) has emerged as a very promising technology to enable next generation digital innovations and support wide range of applications. This special issue on VLC focuses on three core thrusts of the discipline:Localisation, CommunicationandNetworking. The overall aim of the special issue is to inspire multi-disciplinary international communities to work together in order to achieve further research advances. Indeed, a total of 96 high quality papers were received from both academia and industry. After a careful peer-reviewing process, 17 papers were selected based on their combined novelty, rigour, and impact. Owing to the highly selective nature of JSAC, many other interesting papers were not selected for the special issue, but we hope that these papers might appear elsewhere.

Optical communications, both fiber-optics and optical wireless (OWC), have a proven potential for high speed data transmission due to the vast optical spectrum. Given the ever increasing wireless data demand, we look towards OWC as a supplemental medium in heterogeneous networks (HetNets). In such networks, fair comparison of available connections is required for optimal allocation of resources. This comparison is essential for provisioning and analysis as well as in the decision metrics for handover and aggregation. As the optical communications field has developed, Signal to Noise Ratio (SNR) has been defined in various ways. Instantaneous optical power constraints have led the fiber- optics community to define SNR relative to peak- to-peak optical signal power. Additional average optical power constraints have led the OWC community to observe SNR relative to average optical signal power rather than average electrical signal power. Accordingly, the relationship between SNR and link performance has also changed. While the various definitions offer fair comparison under similar constraints, relative analysis is necessary when comparing the quality of multiple links in a HetNet or performance of networks operating under different constraints. In this paper, we reconcile three common SNR definitions and use symbol error rate (SER) analysis for pulse amplitude modulation in order to exemplify how different definitions affect the SER versus SNR curves.

In positioning systems there is a fundamental tradeoff between accuracy and latency. A latency constrained optimization algorithm is proposed to be employed in conjunction with a prior published two-phase algorithm for estimating the location of a mobile node over the visible light channel. Instead of solely relying on triangulation, the mobile nodes estimate their locations through a two phase algorithm in which they firstly exploit the signal strength observables with unique IDs to establish a coarse estimate, and secondly use the azimuth and elevation observables to establish a fine estimate. In many cases, the fine estimate will improve upon the coarse estimate; however when triangulation fails, the algorithm yields the coarse estimate rather than a localization failure. Since the environment model relies on the primary requirement of adequate illumination, the number of LED anchors, and transmit power for communication functions are determined. The advantage of the two-phase algorithm is that it provides a framework to provide rapid RSS based positional estimates with accuracies bounded to within the illumination projection radius (48 cm median) of the distributed luminaries (anchor nodes) within less than 17 nanoseconds while requiring only a single luminaire in its field of view. However, when there are a sufficient number of luminaires in view the positioning algorithm can further improve the accuracy (14 cm median) through application of triangulation via angle of arrival measurements, at the expense of latency (upto tenths of seconds). The proposed framework allows a fundamental trade of accuracy and latency to be made depending on the observable parameters known to the positioning algorithm at run time.

Polar OFDM (P-OFDM) is an OFDM format that can double the spectral efficiency of existing real-valued and unipolar optical OFDM intensity-modulation with direct detection (IM/DD) optical communication systems. P-OFDM modulates the amplitude and phase of a complex OFDM signal to allow a real unipolar signal. In this paper, we investigate the effect of power allocation to the phase and the amplitude components of P-OFDM time-domain signal on (1) the effective signal-to-noise ratio (SNR) at the receiver (2) the peak-to-average power ratio (PAPR) under transmit power constraint and (3) the induced clipping noise under optical-source's dynamic-range and transmit power constraints. In addition to a 4 dB reduction in PAPR from ACO-OFDM, we demonstrate the improvement of throughput for P-OFDM over other optical OFDM methods when the signal power is limited.

Visible light communications (VLC) technology has a potential to complement the RF wireless technology for indoor coverage. It promises concurrent illumination control and wireless data transmission capabilities. The potential of advanced multi-carrier modulation techniques, i.e. orthogonal frequency-division multiplexing (OFDM), introduces new challenges in the development of driver circuits. The reverse polarity optical OFDM (RPO-OFDM) is a recent approach to realize compatibility between the analog OFDM signal and the industry well-known pulse-width modulation (PWM) for dimming control. In this paper, we present the implementation of a wireless access test-bed through visible light and dimming compatible RPO-OFDM. A driver topology capable of driving a string of light-emitting diodes (LEDs) using a RPO-OFDM signal is proposed. The driver alleviates the nonlinearity of LEDs by proper biasing and maintaining a quasi-linear range of operation, i.e. minimizes the OFDM signal clipping. Using a software defined radio (SDR) platform for online modulation and demodulation, and a range of un-coded quadrature amplitude modulation (QAM) constellations, the experimentally obtained measurements demonstrate linear wide-range dimming while independently maintaining a bit-error performance below the forward error correction (FEC). Error free bit-error rate (BER) performance without symbol level equalization is obtained for binary phase shift keying (BPSK) and 4-QAM.

Visible light communications (VLC) is emerging as a new alternative to the use of the existing and increasingly crowded radio frequency (RF) spectrum. VLC is unlicensed, has wide bandwidth, supports new levels of security due to the opacity of walls, and can be combined to provide both lighting and data communications for little net increase in energy cost. As part of a lighting system, VLC is ideal as a downlink technology in which data are delivered from overhead luminaries to receivers in the lighting field. However, realizing a symmetric optical channel is problematic because most receivers, such as mobile devices, are ill-suited for an optical uplink due to glare, device orientation, energy constraints. In this paper we propose and implement a hybrid solution in which the uplink challenge is resolved by the use of an asymmetric RF-VLC combination. VLC is used as a downlink, RF is used as an uplink, and the hybrid solution realizes full duplex communication without performance glare or throughput degradation expected in an all-VLC-based approach. Our proposed approach utilizes a software defined VLC platform (SDVLC) to implement the unidirectional optical wireless channel and a WiFi link as the back-channel. Experiments with the implemented prototype reveal that the integrated system outperforms conventional WiFi for crowded (congested) multiuser environments in term of throughput, and demonstrate functional access to full-duplex interactive applications such as web browsing with HTTP.

In the next major phase of mobile telecommunications standards “5G,” Visible Light Communication (VLC) technology or light fidelity (Li-Fi) has great potential to be a breakthrough technology in the future of wireless Internet access. We propose a novel real-valued unipolar version of orthogonal frequency division multiplexing (OFDM) that is suitable for direct intensity modulation with direct detection (IM/DD) optical wireless systems including VLC. Without additional forms of interference estimation and cancelation to recover the symbols, the Spectral and Energy Efficient OFDM (SEE-OFDM) almost doubles the spectral efficiency of unipolar optical OFDM formats. In our scheme, multiple signals are generated and added/transmitted together, where both odd and even indexed subcarriers of the inverse fast Fourier transform (IFFT) operation carry data and are not affected by any kind of interference, (e.g., clipping). Monte Carlo simulations under additive white Gaussian noise (AWGN) show gains of up to 6dB in signal-to-noise ratio (SNR) compared to the conventional energy-efficient asymmetrically clipped optical OFDM (ACO-OFDM). Moreover, a peak-to-average power ratio (PAPR) reduction of 2.5dB is obtained as a bonus. Therefore, advantages such as increased data rate and reduced PAPR make the proposed SEE-OFDM very attractive for optical wireless systems.

Spatial modulation (SM) and spatial multiplexing (SMP) are two multiple-input multiple-output (MIMO) techniques for transmitting data over an indoor optical wireless channel. Receivers for SM and SMP can be of the non-imaging type, in which case the channel matrix coefficients can be highly correlated, or of the imaging type, which can reduce the degree of correlation and improve overall system performance. In this work, we propose a new framework to analyze the performance of imaging MIMO systems. This framework is applied to characterize the performance of SM and SMP under both imaging and non-imaging receivers. Results of our analysis indicate that imaging receivers can provide significant signal-to-noise ratio (SNR) improvements up to 45dB under SM and SMP as compared to the use of non-imaging receivers. Finally, the application of the proposed analysis framework indicates specific design principles to optimize imaging receiver parameters.

The monitoring of coastal environments is of great interest to biologists and environmental protection organizations with video cameras being the dominant sensing modality. However, it is recognized that video analysis of maritime scenes is very challenging on account of background animation (water reflections, waves) and very large field of view. We propose a practical approach to the detection of three salient events, namely boats, motor vehicles and people appearing close to the shoreline, and their subsequent summarization. Our approach consists of three fundamental steps: region-of-interest (ROI) localization by means of behavior subtraction, ROI validation by means of feature-covariance-based object recognition, and event summarization by means of video condensation. The goal is to distill hours of video data down to a few short segments containing only salient events, thus allowing human operators to expeditiously study a coastal scene. We demonstrate the effectiveness of our approach on long videos taken at Great Point, Nantucket, Massachusetts.

A two phase hybrid algorithm for estimating the location of a mobile node, which has the capability of measuring signal strength, azimuth, and elevation, in a smart space environment over the visible light channel is proposed. In contrast to conventional triangulation approaches which are performed in a simplified plane, the smart room architecture requires a non-planar solution due to the illumination requirement. Furthermore, conventional triangulation approaches can at times produce numerically ill-defined solutions, thereby prohibiting a notion of target location. Instead of solely relying on triangulation, the mobile node estimate their locations through a two phase approach in which they firstly exploit the signal strength observables with unique IDs to establish a coarse estimate, and secondly use the azimuth and elevation observables to establish a fine estimate. In many cases, the fine estimate will improve upon the coarse estimate; however when triangulation fails, the algorithm yield the coarse estimate rather than a localization failure. Since the environment model relies on the primary requirement of adequate illumination, the number of LED anchors and transmit power for communication functions are determined. Simulation results confirm the effectiveness of the hybrid two phase localization approach in a smart space indoor environment by having a median coarse phase accuracy of 34.88 cm and a fine phase median accuracy of 13.95 cm.

Recent developments in solid-state light-emitting diode (LED) materials and devices are driving a resurgence in the use of free-space optics (FSO) for wireless broadband communication. This technology uses the visible spectrum provided by ‘white’ LEDs, which are becoming ubiquitous in lighting and has some desirable properties competitive with existing radio-frequency communications. By leveraging the low-cost nature of LEDs and lighting units there are many opportunities to exploit this medium for widespread optical communication deployment. The optical medium, however, has distinct characteristics that must be managed, such as directionality and susceptibility to noise sources in the visible spectrum. In this study, the authors present a new indoor FSO communication system, also known as a visible-light communication system that achieves satisfactory data rates while supporting multiple access under line-of-sight constraints. A hexagonal device design is proposed and investigated in the context to two communication protocols designed to manage point-to-point and point-to-host cases. Theoretical analysis and simulation of the two protocols using this hexagonal transceiver design indicate suitability for addressing high data-rate communications between peer devices; or between multiple devices using the peer-to-host model. A new medium access control scheme will also discussed.

Delay-tolerant network (DTN) architectures have recently been proposed as a means to enable efficient routing of messages in vehicular area networks (VANETs), which are characterized by alternating periods of connectivity and disconnection. Under such architectures, when multihop connectivity is available, messages propagate at the speed of radio over connected vehicles. On the other hand, when vehicles are disconnected, messages are carried by vehicles and propagate at vehicle speed. Our goal in this paper is to analytically determine what gains are achieved by DTN architectures and under which conditions, using the average message propagation speed as the primary metric of interest. We develop an analytical model for a bidirectional linear network of vehicles, as found on highways. We derive both upper and lower bounds on the average message propagation speed by exploiting a connection with the classical pattern-matching problem in probability theory. The bounds reveal an interesting phase transition behavior. Specifically, we find out that, below a certain critical threshold, which is a function of the traffic density in each direction, the average message speed is the same as the average vehicle speed, i.e., DTN architectures provide no gain. On the other hand, we determine another threshold above which the average message speed quickly increases as a function of traffic density and approaches radio speed. Based on the bounds, we also develop an approximation model for the average message propagation speed that we validate through numerical simulations.

Enabling safety in vehicles is an ongoing challenge for the automotive sector. One approach towards enhancing safety is to increase knowledge within a vehicle of the actions of vehicles in the vicinity. Increased awareness is essential for activating the safety systems to take evasive or precautionary actions in the event of an incident. Wireless radio communication has emerged as a key enabler for exchanging safety information. Several initiatives across the world have considered various radio communication technologies to implement safety communication. However, there are significant constraints to utilizing wireless radio communication. In this article, we discuss briefly the challenges in enabling safety communication with wireless radio in the context of vehicular networks. We introduce the on-going work in utilizing free space optical communications as an enabler for inter-vehicle safety communication. As a first step, we compare with the current 802.11 standard implementation for achievable performance. Given that the two technologies are inherently different, directional versus omni-directional, we seek to identify the scenarios where each technology is best suited. Particularly, we compare packet delivery ratio (PDR), throughput and average packet delay of the two enabling technologies, under assumptions, in the context of increasing vehicle traffic density. Our results demonstrate that a directional technology such as free-space optics is less susceptible to contention scenarios. As a result, the performance in high density scenarios is better than that can be achieved from using omni-directional long-range technologies such as 802.11.

Recent advances in wireless communications technology and low-power, low-cost CMOS imaging sensors enable a new sensing modality employing ubiquitous distributed video sensing. In many video sensing applications multimedia streams are often required by the base station to provide detailed information of the target spot in the sensor field. However, limited bandwidth, unstable network environment, and data transmission interference prevent the large-scale deployment of such applications without new strategies for energy and data capacity management. In this paper we propose a dynamic path formation algorithm based on our data path throughput estimation model. Coupled with a distributed TDMA packet scheduling scheme, this path formation algorithm can establish throughput-aware video delivery path between source and destination. OPNET simulation results indicate that the throughput estimation is accurate and our proposed TDMA scheme is preferred for streaming applications. We also explore the use of distributed mobile base stations as an option to improve the egress of video data streams from the system.

Deploying large-scale sensor networks involves the programming of many devices based on a desired mission. Techniques for reprogramming devices in situ have been investigated to mitigate the effort required when program updates are required or when the mission of the system changes. We consider a technique that is intended to support multiple concurrent missions by the system by exploiting available resources of the sensor network. In essence, our model is based on a tasking scheme, a common framework for the interchange and instantiation of tasks on multiple devices, and the use of attributes defining the resources in the system. A prototype system has been implemented to demonstrate and validate the concepts using the Imote2, a 32-bit mote architecture that has been configured with embedded Linux enabled with Java. Several applications have been rendered as tasks that are injected into a multi-node sensor network. Results demonstrate the support of concurrent overlaid applications in the system permitting task injection, maintenance, and termination. Performance evaluation of the scheme indicates benefits over an epidemic model of code dissemination.

With the decrease in the cost and in the size of computing devices, wireless sensor networks (WSNET) have the potential of being composed by an extremely large number of nodes offering multiple services. Such networks have the capability of executing multiple tasks concurrently by allocating simply a fraction of their resources. Alternatively, many smaller wireless networks may collaborate to execute a larger, unforeseen application. In both cases a routing scheme other than the prevailing one may improve the efficiency of the task or the application being executed, reducing the energy consumption in the network. We posit that to fully tap into the potential of such networks a new routing infrastructure is needed, one that allows switching between different routing schemes dynamically as required by the applications being deployed, the conditions of the network as a whole and the existing locality information. We show in this paper how dynamic routing scheme selection can be achieved when sensor networks are overlaid with a virtual attribute based cluster hierarchy. We present analytical results for our scheme and show the expected improvement that can be achieved.

In this paper, we present an analytical model for delay tolerant message propagation in a dynamic vehicular network. The analysis provides upper and lower bounds for message propagation as function of traffic density, vehicle speed and radio range. The model is an extension of previous work which considered a particular network setting. The results from the analytical model are compared with simulation results for various vehicular traffic densities. The work demonstrates that increased mobility of vehicles actually aids in messaging contrary to the expectation that it would be a hindrance due to frequent topology changes. An increase in vehicle speed from 0 m/s to 20 m/s results in a corresponding increase in message propagation rate of 200 m/s for vehicular density of 25 vehicles/km.

Wireless networking is currently dominated by radio frequency (RF) techniques. However, the soon-to-be ubiquity of LED-based lighting motivated by significant energy savings provides an opportunistic deployment of widespread free-space optical (FSO) communications. LED-based network transceivers have a variety of competitive advantages over RF including high bandwidth density, security, energy consumption, and aesthetics. They also use a highly reusable unregulated part of the spectrum (visible light). In this paper we describe results from a pilot project to demonstrate the viability of an optical free-space visible light transceiver as a basis for indoor wireless networking. Inexpensive, commercial, off-the shelf LEDs and photodiodes were used to construct two prototypes; a simplex channel as expected as a component of an asymmetric/hybrid RF-FSO system, and a full-duplex channel demonstrating the ability to isolate multiple channels. On-off keying (OOK) was applied without observable flicker in the target modulation ranges. Results indicate the viability of creating inexpensive FSO transceivers that might be embedded in commercial lighting products to support ceiling-to-floor distances of approximately 3 m.

Abstract – Data routing in large wireless sensor networks is challenged by requirements for scalability, robustness, and energy use, as well as the possibility that globally unique identifiers for each node are nonexistent. Moreover, if the sensor network serves multiple functions or users, its routing function must be energy efficient when responding to information requests (inquiries) that can arrive at high rates, require diverse data types, and target subsets of the sensor network. We propose virtual “containment ” hierarchies based on relevant attributes as a more efficient mechanism to disseminate inquiries rather than the use of flooding schemes. We show algorithms that support clustering sensors according to these hierarchies and implement clusterhead failure recovery and load balancing among cluster members. We show that our framework has significant bandwidth gains over a flooding-based scheme under the scenario considered using analytical techniques.

Deployed wireless sensor networks are usually tightly coupled with specific tasks and therefore present unique communication needs. With technological advances that are making easy deployment of wireless sensor networks a reality, mechanisms that allow data exchange between sensor systems deployed at different times and initially for different purposes are necessary. Otherwise, sensors which are co-located and can collaborate with one another to increase capacity and/or performance may be prevented from doing so due to the inability to exchange data with one another. To this end we propose a semantic-based routing architecture that enables internetworking of sensor systems. In this architecture each sensor system can define its own address names that reflect application-level needs, but which follows a specific naming structure. Semantic routers are elected in the network and process packets based on both the address name and the naming structure. Processing is based on rules that can be modified according to changes in application-level requirements or changes in the naming structure. We describe this architecture and show that it enables easy internetworking of sensor systems deployed at different times and for different applications, enables data forwarding to only relevant regions in the network (as opposed to network-wide flooding), supports application-level communication needs, and supports hierarchical routing schemes.

We propose a task graph based framework for modeling and execution of distributed applications in mobile ad hoc networks. Our framework represents a distributed application by a graph composed of nodes and edges in which the nodes logically represent application sub-tasks that need to be completed and the edges represent associations, with certain attributes, between nodes. During application run-time, suitable devices that can complete the sub-tasks and can satisfy the attributes of the associations between them are selected on-the-fly to execute the application. New devices are selected to continue application execution if old devices become unavailable due to mobility. Thus, we decouple the application from a specific set of devices and allow its execution if there is at least one suitable device in the network for carrying out each of the required sub-tasks. We propose an application execution protocol to realize this vision and show simulation results which indicate that our approach is practical for environments with low user/device mobility.

A novel distributed approach for executing distributed tasks on mobile ad hoc networks (MANETs) is presented. A distributed application is represented as a complex task comprised of simpler sub-tasks that need to be performed on different categories of computing devices with specialized roles. The dependencies induced by logical patterns of data flow between these specialized devices which are responsible for performing the aforementioned sub-tasks, yield a task graph representation for a given application. We present a simple and efficient distributed algorithm for dynamic discovery and selection of suitable devices in a MANET from among a number of them providing the same functionality. We refer to this process as instantiation which is carried out with respect to the proposed task graph representation of the application. We also present a distributed algorithm for detecting disruptions in application execution that can occur due to device mobility. The algorithm then recovers quickly from the situation by re-instantiating affected parts of the task graph, if possible. Finally, we propose metrics for evaluating the performance of these algorithms and report simulation results for a variety of application scenarios.

We propose a novel four-step hybrid approach for retrieval and composition of video newscasts based on information contained in different metadata sets. In the first step, we use conventional retrieval techniques to isolate video segments from the data universe using segment metadata. In the second step, retrieved segments are clustered into potential news items using a dynamic technique sensitive to the information contained in the segments. In the third step, we apply a transitive search technique to increase the recall of the retrieval system. In the final step, we increase recall performance by identifying segments possessing creation-time relationships. A quantitative analysis of the performance of the process on a newscast composition shows an increase in recall by 59 percent over the conventional keyword-based search technique used in the first step.

Video-on-demand (VoD) has been an active area of research for the past few years in the multimedia research community. However, there have not been many significant commercial deployments of VoD owing to the inadequacy of per user bandwidth and the lack of a good business model.1 Significant research efforts have been directed towards reduction of network bandwidth requirements, improvement of server utilization, and minimization of start-up latency. In this paper, we investigate another aspect of VoD systems which has been largely neglected by the research community, namely, pricing models for VoD systems. We believe that the price charged to a user for an on-demand video stream should influence the rate of user arrivals into the VoD system and in turn should depend upon quality-of-service (QoS) factors such as initial start-up latency. We briefly describe some simple pricing models and analyze the tradeoffs involved in such scenarios from a profit maximization point of view. We further explore secondary content insertion (ad-insertion) which was proposed elsewhere [1] not only as a technique for reducing the resource requirements at the server and the network, but also as a means of subsidizing VoD content to the end user. We treat the rate of ad insertion as another QoS factor and demonstrate how it can influence the price of movie delivery.

We present and evaluate an optimal scheduling algorithm for inserting secondary content for improving resource utilization in VoD systems. The algorithm runs in polynomial time, and is optimal with respect to the total bandwidth usage over the merging interval. We present constraints on content insertion which make the overall QoS of the delivered stream acceptable, and show how our algorithm can satisfy these constraints. We discuss dynamic scenarios with user arrivals and interactions, and show by simulations that content insertion reduces the channel bandwidth requirement to almost half.

Video production involves the process of capturing, editing and composing video segments for delivery to a consumer. A composition must yield a coherent presentation of an event or narrative. This process can be automated if appropriate domain-specific metadata are associated with video segments and composition techniques are established. Automation leads to the support of dynamic composition and customization for applications such as news on demand. In this paper, we present techniques to achieve dynamic, real-time and cohesive video composition and customization. We also identify metrics for evaluating our techniques with respect to existing manually produced video-based news. The results of such an evaluation show that the quality of automatic composition is comparable to-and in some cases, better than-broadcast news video composition. The results also validate the assertions on which the automatic composition techniques are based.

Large scale clustered continuous media (CM) servers deployed in applications like video-on-demand have high availability requirements.In the event of server failure, streams from the failed servers must be reassigned to healthy servers with minimum service disruption.Such servers may also suffer from periods of transient overload resulting from a high degree of customer interactivity.For example, in a video-on-demand system if a large number of users are viewing a favorite game, many of them can simultaneously request a replay of an interesting part of the game.This requires a large number of '5nteractive" channels within a short period of time and can result in a transient server overload.In this paper we propose solutions for graceful recovery from overload scenarios arising out of server failure or customer interactions.Rapid resource reclamation is key to overload tolerance, and our proposed solution is based on rate adaptive stream merging and content insertion techniques, We also utilize conventional time-sharing techniques to handle transient overload.We show that while merging is necessary for achieving overload tolerance, it is not sufficient, and for a complete solution, content insertion is required.Specifically, we consider a general clustered CM server architecture model where multiple servers can fail simultaneously.We develop a model for resource shortfalls that occur as a result of overload on failure.We also describe optimal polynomial time algorithms for recovering resources to the maximum extent possible, by clustering streams in real time.

Multimedia data have specific temporal presentation requirements. For example in video conferencing applications the voice and images of the participants must be delivered and presented synchronously. These requirements can be achieved by scheduling or managing system resources. We present a technique called limited a priori scheduling (LAP) to manage the delivery channel from source to destination for digital multimedia data. By using delay estimation a LAP scheduler can retrieve stored digital media spanning arbitrary networks with unspecified delays. The use of delay estimation also facilitates selective degradation of service in bandwidth and buffer limited situations. Such degradation enables the continuous real-time playout and synchronization of various media arriving from different sources. The performance of the LAP scheduler is described based on implementation and experimentation using Ethernet.

The synchronization problem for audio/visual reproduction has consumed engineers time since the advent of recorded audio and images and the first multimedia productions. As computers have evolved to support programmed reproduction of multimedia information, the complexity but not the character or interest have changed. In the digital domain, synchronization problems exist due to data distribution and communications, random events caused by human-computer interaction, and general computer and communications performance limitations. This rich research domain has led to numerous approaches to the modeling and execution of multimedia synchronization scenarios. Unfortunately, these approaches are difficult to compare and evaluate due to their varied theoretical bases and modeling techniques. We develop a uniform, theoretical foundation for discussing multimedia synchronization and temporal specification. We propose a temporal reference framework and use it to compare existing temporal specification schemes and their relationships to multimedia synchronization. The ensuing comparison of existing specification and synchronization techniques demonstrates the utility of the framework.

In the future we.envision systems that will provide video information delivery services to customers on a very large scale. These systems must provide customers with mechanisms to select programs of their choice from live broadcasts. Customers should also be provided with easy means of browsing and accessing prerecorded digital data (e.g., distributed digital multimedia libraries), and downloading data from other information sources. To be viable for such large information sets, these systems must understand customer preferences and tailor the available information to the customer's needs. To support this vision, a number of issues must be addressed and obstacles overcome. Intuitive interfaces, powerful query formulation and evaluation techniques, comprehensive data models, and flexible presentation functionalities must be developed. To realize these components, an effective query evaluation engine with the capabilities of query resolution in different content-specific formats (e.g., by graphics, by image, by sound) and in different domain-specific models (e.g., database of movies, database of newsclips) should be present. Additionally, the digital video database will require an efficient indexing system for easy access to the stored information. In this paper we discuss existing research trends in this area and requirements for future data delivery systems. An overview of video indexing is presented followed by a discussion on current indexing techniques.

Multimedia computing requires support for heterogeneous data types with differing storage, communication and delivery requirements. Continuous media data types, such as audio and video, impose delivery requirements that are not satisfied by conventional physical storage organizations. In this paper, we describe a physical organization for multimedia data based on the need to support the delivery of multiple playout sessions from a single rotating-disk storage device. Our model relates disk characteristics to the different media recording and playback rates and derives their storage pattern. This storage organization guarantees that as long as a multimedia delivery process is running, starvation will never occur. Furthermore, we derive bandwidth and buffer constraints for disk access and present an approach to minimize latencies for non-continuous media stored on the same device. The analysis and numerical results indicate the feasibility of using conventional rotating magnetic disk storage devices to support multiple sessions for video-on-demand applications.

Concurrent retrieval of continuous media from a physical storage device can be achieved by interleaving data and providing a suitable scheduling algorithm. Scheduling approaches that exploit gains from statistical multiplexing are susceptible to a nonzero probability of frame loss due to the variable-bit-rate characteristic of compressed video. With interframe encoding schemes (such as specified by the MPEG standard), the losses propagate, resulting in a net loss of frames that exceeds the fraction of lost data. In this paper, we describe a mechanism for the storage and retrieval of MPEG-encoded video from a single disk storage system. The scheme balances the need for the reliable delivery of MPEG frames with the desire to support the largest number of sessions. Our approach reorganizes the MPEG-encoded video stream based on the relative importance of the frames and maps them to the storage device geometry. The reorganization reduces the impact of frames lost due to missed deadlines and distributes the frame losses over time and among sessions. Simulation results show that the new approach improves performance when compared to conventional storage and scheduling schemes.

A coherent computer-based multimedia presentation requires the use of real-time scheduling theory applied to system resources including storage devices, the bus, and network. In this paper we present a scheduling technique which is essentially static in nature during periods of network quiescence, but becomes dynamic in response to detection of network load changes of users interaction. This mechanism, called the limited a priori (LAP) scheduler, is an extension of earlier work on scheduling which assumed guaranteed system loading characteristics. The LAP schedules determines network load changes and estimates packet delay using a dynamic filtering algorithm. We show the bounds of accuracy for this technique and describe its suitability with respect to multi-media traffic across an FDDI network.

Video-on-demand represents a key demonstrative application for enabling multimedia technology in communication, database, and interface research.This application requires solving a number of diverse technical problems including the data synchronization problem for time-dependent data delivery.In this paper we describe the general requirements of video-ondemand and introduce a system supporting content-based retrieval and playback for the structure and content of digital motion pictures.In our model we capture domain-specific information for motion pictures and provide access to individual scenes of movies through queries on a temporal database.We describe our implementation of this service using existing workstation and storage technology.

Multimedia data often have time dependencies that must be satisfied at presentation time. To support a general-purpose multimedia information system, these timing relationships must be managed to provide utility to both the data presentation system and the multimedia author. New conceptual models for capturing these timing relationships, and managing them as part of a database are proposed. Specifically, n-ary and reverse temporal relations are introduced and defined along with their temporal constraints. These new relations are a generalization of earlier temporal models and establish the basis for conceptual database structures and temporal access control algorithms to facilitate forward, reverse, and partial-interval evaluation during multimedia object playout. The proposed relations are defined to ensure a property of monotonically increasing playout deadlines to facilitate both real-time deadline-driven playout scheduling or optimistic interval-based process playout. A translation of the conceptual models to a structure suitable for a relational database is presented.>

The authors provide an overall assessment of the current state-of-the art in multimedia information technology, the future directions and the engineering challenges faced by the designers and researchers working in this area. They describe how the newly emerging networking and high performance storage technologies can provide unique opportunities for building interesting multimedia applications, which were unconceivable previously.>

Multimedia applications describe unique requirements that must be met by computer network and operating system components. In particular, the time-dependencies of multimedia data require mechanisms to ensure timely and predictable delivery of data from their sources to destinations. For single medium applications which have relatively constant bandwidth utilization, connections from source to destination can be tailored to moderate ranges of data rates. On the other hand, due to the large variation in multimedia object sizes and concurrency in object presentation, multimedia applications can require a correspondingly large variation in required bandwidth over the life of a connection. Data of these types may not arrive in time to meet the intended playout schedule when the capacity of the channel is exceeded. In this paper we present an approach to remedying this situation by effectively smoothing the bandwidth requirement over time via a scheduling mechanism.

Protocols to provide synchronization of data elements with arbitrary temporal relationships of both stream and non-stream broadband traffic types are proposed. It is specified that the provision of a synchronization function be performed within a packet switched network, and, accordingly, a two-level communication architecture is presented. The lower level, called the network synchronization protocol (NSP), provides the ability to establish and maintain individual connections with specified synchronization characteristics. The upper level, the application synchronization protocol (ASP), supports an integrated synchronization service for multimedia applications. The ASP identifies the temporal relationships among an application's data objects and manages the synchronization of arriving data for playout. The proposed NSP and ASP are mapped to the session and application layers of the open-systems-interconnection (OSI) reference model, respectively.>

The authors propose a technique for formally specifying and modeling the temporal composition of multimedia data. The proposed model is based on timed Petri nets and the logic of temporal intervals. A strategy based on the inter media timing relationships established by the proposed modeling tool is presented for constructing a database schema to facilitate data storage and retrieval of data elements. An algorithm which allows the retrieval of media elements from the database in a manner which preserves the temporal requirements of the initial specification is proposed. The proposed model accomplishes the specification of synchronization requirements for complex structures of temporally related objects.>

A technique is presented for the formal specification and modeling of multimedia composition with respect to intermedia timing. The proposed model is based on the logic of temporal intervals and timed Petri nets. A strategy is evinced for constructing a database schema to facilitate data storage and retrieval of media elements based on the temporal relationship established by the proposed modeling tool. An algorithm which allows the retrieval of media elements from the constructed database in a manner which preserves the temporal requirements of the initial specification is presented. Using the proposed model, the synchronization requirements of complex structures of temporally related objects can be easily specified.>