The use of Machine Learning (ML) and Artificial Intelligence (AI) in smart transportation networks has increased significantly in the last few years. Among these ML and AI approaches, Reinforcement Learning (RL) has been shown to be a very promising approach by several authors. However, a problem with using Reinforcement Learning in Traffic Signal Control is the reliability of the trained RL agents due to the dynamically changing distribution of the input data with respect to the distribution of the data used for training. This presents a major challenge and a reliability problem for the trained network of AI agents and could have very undesirable and even detrimental consequences if a suitable solution is not found. Several researchers have tried to address this problem using different approaches. In particular, Meta Reinforcement Learning (Meta RL) promises to be an effective solution. In this paper, we evaluate and analyze a state-of-the-art Meta RL approach called MetaLight and show that, while under certain conditions MetaLight can indeed lead to reasonably good results, under some other conditions it might not perform well (with errors of up to 22%), suggesting that Meta RL schemes are often not robust enough and can even pose major reliability problems.

Intelligent Traffic Signal Control (ITSC) systems have attracted the attention of researchers and the general public alike as a means of alleviating traffic congestion. Recently, the vehicular wireless technologies have enabled a cost-efficient way to achieve ITSC by detecting vehicles using Vehicle to Infrastructure (V2I) wireless communications. Traditional ITSC algorithms, in most cases, assume that every vehicle is detected, such as by a camera or a loop detector, but a V2I implementation would detect only those vehicles equipped with wireless communications capability. We examine a family of transportation systems, which we will refer to as `Partially Detected Intelligent Transportation Systems'. An algorithm that can perform well under a small detection rate is highly desirable due to gradual increasing penetration rates of the underlying technologies such as Dedicated Short Range Communications (DSRC) technology. Reinforcement Learning (RL) approach in Artificial Intelligence (AI) could provide indispensable tools for such problems where only a small portion of vehicles are detected by the ITSC system. In this paper, we report a new RL algorithm for Partially Detected Intelligent Traffic Signal Control (PD-ITSC) systems. The performance of this system is studied under different car flows, detection rates, and types of the road network. Our system is able to efficiently reduce the average waiting time of vehicles at an intersection, even with a low detection rate, thus reducing the travel time of vehicles.

With the rapid advances in vehicle-to-vehicle (V2V) communications, interest for leveraging V2V communications for different applications has also grown exponentially. Leader selection is an important task required in a number of V2V communications related applications. In this paper, a distributed leader selection algorithm is introduced for vehicular ad-hoc networks (VANET) over unreliable V2V communications. The algorithm is simple, lightweight, and tightly integrated with SAE 2735 protocol for vehicular networks. The goal of the algorithm is to provide a robust leader selection method for a group of vehicles to determine a temporary leader in a short time, which could benefit several newly proposed applications based on V2V in traffic control and autonomous driving related areas. Simulation results have verified the excellent convergence time, robustness, and other aspects of the algorithm.

Traffic congestion in major cities around the globe is a daunting problem that is getting worse as the speed of urbanization keeps increasing. The role of traffic lights (TLs) at intersections in regulating flows cannot be underestimated. Even though there are a variety of actuated traffic signals based on cameras or loop detectors, the vast majority of existing TLs in the world employ a timer-based decision logic which is clearly not very effective. In this paper, we present a new scheme for controlling traffic at intersections, which is known as dedicated short-range communications (DSRCs)-actuated traffic control. The proposed approach leverages the presence of DSRC radios in vehicles and gives priority (by displaying green light) to approaches (roads) that include DSRC-equipped vehicles (such as 10% or 20% of vehicles having DSRC radios). Using this priority mechanism, it is shown that the average waiting time at each TL can be significantly reduced. This, in turn, can reduce the average commute time of urban workers during rush hours substantially. One of the great advantages of the presented approach is that it can function well with even a low percentage of DSRC-equipped vehicles. Given that many industry forecasts predict a gradual penetration rate for the DSRC technology, this is a very attractive feature. It is also shown that the proposed new approach leads to a cost-effective solution for urban traffic control since the hardware and software platforms needed for its implementation are low cost.

Autonomous vehicles have the potential to enhance road safety, reduce traffic pressure and improve the driving experience. With the on-board sensors, compute units, storage devices, and communication modules, autonomous vehicles are becoming integrated information systems. Compared to conventional centralized approaches and traditional clouds, the emerging vehicular clouds (v-clouds) technology is a more promising solution for utilizing such rich resources. In v-clouds, vehicles can communicate with one another, form self-organized vehicular ad-hoc networks (VANETs), collect real-time sensing data, conduct intensive computation, and disseminate information. However, the highly dynamic and heterogeneous nature of autonomous vehicles raises many issues when designing v-cloud systems. In this paper, we focus on the challenges in designing v-cloud computing architectures, providing effective routing protocols, securing v-cloud environments and enhancing the dependability of v-clouds. We review the state of the art and discuss open research issues.

Partially Detected Intelligent Traffic Signal Control(PD-ITSC) systems that can optimize traffic signals based on limited detected information could be a cost-efficient solution for mitigating traffic congestion in the future. In this paper, we focus on a particular problem in PD-ITSC - adaptation to changing environments. To this end, we investigate different reinforcement learning algorithms, including Q-Learning, Proximal Policy Optimization (PPO), Advantage Actor-Critic (A2C), and Actor-Criticwith Kronecker-Factored Trust-Region (ACKTR). Our findings suggest that RL algorithms can find optimal strategies under partial vehicle detection; however, policy-based algorithms can adapt to changing environments more efficiently than value-based algorithms. We use these findings to draw conclusions about the value of different models for PD-ITSC systems.

As traffic congestion becomes a huge problem for most developing and developed countries across the world, intelligent transportation systems (ITS) are becoming a hot topic that is attracting attention of researchers and the general public alike. In this paper, we demonstrate a specific implementation of an ITS system whereby traffic lights are actuated by DSRC radios installed in vehicles. More specifically, we report the design of prototype of a DSRC-Actuated Traffic Lights (DSRC-ATL) system. It is shown that this system can reduce the travel time and commute time significantly, especially during rush hours. Furthermore, the results reported in this paper do NOT assume or require all vehicles to be equipped with DSCR radios. Even with low penetration ratios, e.g., when only 20% of all vehicles in a city are equipped with DSRC radios, the overall performance of the designed system is superior to the current traffic control systems.

Traffic congestion is a daunting problem that is affecting the daily lives of billions of people across the world. Recently, a promising new traffic control scheme known as Virtual Traffic Lights (VTL) has been proposed for mitigating traffic congestion. VTL is an infrastructure free traffic control scheme that leverages the presence of Vehicle-to-Vehicle (V2V) communications. Such infrastructure free scheme has several benefits, such as alleviating traffic congestion; reducing the large cost of traffic lights and traffic control systems; reducing carbon emission, etc. This paper reports a DSRC-based prototype design effort on VTL using Dedicated Short Range Communications (DSRC) technology. The experiments performed show the feasibility of implementing VTL with DSRC technology. Preliminary results of the field tests conducted in Pittsburgh with vehicles using VTL equipment indicate that VTL is capable of coordinating traffic at intersections and reducing the commute time of people.

Distributed optimization methods have been widely studied in recent years as they could be instrumental in optimizing the operation of future power systems. Compared to the centralized approach, communications will have greater impact on distributed methods as their performance depends heavily on the information exchange among multiple control entities. However, it is not clear what communications infrastructures and technologies should be adopted for distributed methods in practice. To investigate this issue and bridge the gap between the power system communications research community and the distributed optimization research community, this paper models the communications infrastructures and schemes required by distributed methods using the OPNET modeler. The capabilities and limitations of centralized and distributed communications infrastructures are investigated in terms of communications delays incurred and their impact on the convergence of two distributed algorithms, namely, Alternating Direction Method of Multipliers (ADMM) and Optimality Condition Decomposition (OCD) for solving the AC Optimal Power Flow problem in the IEEE 118-bus system and in the large-scale Polish power system. Our simulation results show that the existing communications plane in power systems might not be able to support widely used distributed optimization techniques (such as ADMM and OCD), which is a major finding. In addition, our results indicate that the connectivity requirements of distributed optimization necessitate a mesh network architecture with additional communications links among substations.

Real-time monitoring of traffic density, road congestion, public transportation, and parking availability are key to realizing the vision of a smarter city and, with the advent of vehicular networking technologies, such as IEEE 802.11p and WAVE, this information can now be gathered directly from the vehicles in an urban area. To act as a backbone to the network of moving vehicles, collecting, aggregating, and disseminating their information, the use of parked cars has been proposed as an alternative to costly deployments of fixed roadside units. In this paper, we introduce novel mechanisms for parking vehicles to self-organize and form efficient vehicular support networks that provide widespread coverage to a city. These mechanisms are innovative in their ability to keep the network of parked cars under continuous optimization, in their multi-criteria decision process that can be focused on key network performance metrics, and in their ability to manage the battery usage of each car, rotating roadside unit roles between vehicles as required. We also present the first comprehensive study of the performance of such an approach, via realistic modeling of mobility, parking, and communication, through simulations, and an experimental verification of concepts that are key to self-organization. Our analysis brings strong evidence that parked cars can serve as an alternative to fixed roadside units, and organize to form networks that can support smarter transportation and mobility.

A comprehensive implementation of the envisioned traffic safety and efficiency applications of the IEEE 802.11p and Wireless Access for Vehicular Environments (WAVE) standards assume the premise of the use of Dedicated Short-Range Communications (DSRC) technology both as on-board units and as roadside units (RSUs). The high cost associated with RSUs, however, has so far prevented massive deployment of RSUs. Finding alternative solutions to this longstanding problem is therefore very important. In this paper, we propose a self-organizing network approach to using parked cars in urban areas as RSUs. This self-organizing network approach enables parked cars to create coverage maps based on received signal strength and make important decisions, such as if and when a parked car should serve as an RSU. Our results show the feasibility and cost-effectiveness of the proposed approach, which is able to provide excellent coverage using only a small fraction of the cars parked in a city.

Vehicular Ad-hoc Networks (VANETs) are an emerging field, whereby vehicle-to-vehicle communications can enable many new applications such as safety and entertainment services. Most VANET applications are enabled by different routing protocols. The design of such routing protocols, however, is quite challenging due to the dynamic nature of nodes (vehicles) in VANETs. To exploit the unique characteristics of VANET nodes, we design a moving-zone based architecture in which vehicles collaborate with one another to form dynamic moving zones so as to facilitate information dissemination. We propose a novel approach that introduces moving object modeling and indexing techniques from the theory of large moving object databases into the design of VANET routing protocols. The results of extensive simulation studies carried out on real road maps demonstrate the superiority of our approach compared with both clustering and non-clustering based routing protocols.

Due to the increasing number of sensors deployed in modern vehicles, intra-vehicular wireless sensor networks (IVWSNs) have recently received a lot of attention in the automotive industry, as they can reduce the amount of wiring harness inside a vehicle. By removing the wires, car manufacturers can reduce the weight of a vehicle and improve engine performance, fuel economy, and reliability. In addition to these direct benefits, an IVWSN is a versatile platform that can support other vehicular applications as well. An example application, known as a side blind zone alert (SBZA) system, which monitors the blind zone of the vehicle and alerts the driver in a timely manner to prevent collisions, is discussed in this paper. The performance of the IVWSN-based SBZA system is evaluated via real experiments conducted on two test vehicles. Our results show that the proposed system can achieve approximately 95%-99% detection rate with less than 15% false alarm rate. Compared with commercial systems using radars or cameras, the main benefit of the IVWSN-based SBZA is substantially lower cost.

We study the effect of radio signal shadowing dynamics, caused by vehicles and by buildings, on the performance of beaconing protocols in Inter-Vehicular Communication (IVC). Recent research indicates that beaconing, i.e., one hop message broadcast, shows excellent characteristics and can outperform other communication approaches for both safety and efficiency applications, which require low latency and wide area information dissemination, respectively. To mitigate the broadcast storm problem, adaptive beaconing solutions have been proposed and designed. We show how shadowing dynamics of moving obstacles hurt IVC, reducing the performance of beaconing protocols. To the best of our knowledge, this is one of the first studies on identifying the problem and the underlying challenges and proposing the opportunities presented by such challenges. Shadowing also limits the risk of overloading the wireless channel. We demonstrate how these challenges and opportunities can be taken into account and outline a novel approach to dynamic beaconing. It provides low-latency communication (i.e., very short beaconing intervals), while ensuring not to overload the wireless channel. The presented simulation results substantiate our theoretical considerations.

In sparse highway vehicular networks, the high probability for network disconnection at the initial stages of introducing the DSRC technology can be mitigated by the deployment of fixed infrastructure points known as Road Side Units (RSU). However, due to the cost associated with the deployment and maintenance of significant numbers of RSUs, it is highly unlikely that the majority of highways will be seeing RSU support in the near future. In this paper we study the impact of specific vehicular network parameters in the communication delays in infrastructure-less highway scenarios: first, the deceleration of vehicles, and consequently, a decrease in their separation from succeeding vehicles; and second, the transmission power of the IEEE 802.11p radio, which can be increased to achieve faster connectivity with the succeeding vehicle. Our results show that the connectivity of sparse vehicular networks can be improved substantially by varying these parameters.

Vehicle-to-Vehicle (V2V) communication is a core technology for enabling safety and non-safety applications in next generation intelligent transportation systems. Due to relatively low heights of the antennas, V2V communication is often influenced by topographic features, man-made structures, and other vehicles located between the communicating vehicles. On highways, it was shown experimentally that vehicles can obstruct the line of sight (LOS) communication up to 50 percent of the time; furthermore, a single obstructing vehicle can reduce the power at the receiver by more than 20 dB. Based on both experimental measurements and simulations performed using a validated channel model, we show that the elevated position of antennas on tall vehicles improves communication performance. Tall vehicles can significantly increase the effective communication range, with an improvement of up to 50 percent in certain scenarios. Using these findings, we propose a new V2V relaying scheme called tall vehicle relaying (TVR) that takes advantage of better channel characteristics provided by tall vehicles. TVR distinguishes between tall and short vehicles and, where appropriate, chooses tall vehicles as next hop relays. We investigate TVR's system-level performance through a combination of link-level experiments and system-level simulations and show that it outperforms existing techniques.

Intra-Vehicular Wireless Sensor Network (IVWSN) is a new automotive architecture that applies wireless technologies to the communications between Electrical Control Units (ECUs) and sensors. It can potentially help achieve better fuel economy, reduce wiring complexity, and support additional new applications. In the existing works, most of the popular wireless technologies applied on IVWSNs occupy the same 2.4 GHz ISM frequency bands as WiFi and Bluetooth do. It is therefore essential to evaluate the performance of IVWSNs under interference from WiFi and Bluetooth devices, especially when these devices are inside the vehicle. In this paper, we report the results of a comprehensive experimental study of IVWSNs based on ZigBee and Bluetooth Low Energy under WiFi and Bluetooth interference. The impact of the interference from Bluetooth and WiFi devices can be clearly observed from the experiments. The results of the experiments conducted suggest that Bluetooth Low Energy technology outperforms ZigBee technology in the context of IVWSNs when WiFi interference exists in the car.

We study the effect of radio signal shadowing dynamics, caused by vehicles and by buildings, on the performance of beaconing protocols in Inter-Vehicular Communication (IVC). Recent research indicates that beaconing, i.e., one hop message broadcast, shows excellent characteristics and can outperform other communication approaches for both safety and efficiency applications, which require low latency and wide area information dissemination, respectively. We show how shadowing dynamics of moving obstacles hurt IVC, reducing the performance of beaconing protocols. At the same time, shadowing also limits the risk of overloading the wireless channel. To the best of our knowledge, this is the first study identifying the problems and resulting possibilities of such dynamic radio shadowing. We demonstrate how these challenges and opportunities can be taken into account and outline a novel approach to dynamic beaconing. It provides low-latency communication (i.e., very short beaconing intervals), while ensuring not to overload the wireless channel. The presented simulation results substantiate our theoretical considerations.

Virtual Traffic Lights (VTL) is a recently proposed self-organizing traffic control scheme that has the potential to mitigate traffic congestion in urban areas. This paper reports a prototype design effort on Virtual Traffic Lights using Android-based smartphones. The experiments performed show the feasibility of implementing VTL using smartphones' WiFi devices.

In this paper, we propose a novel scheme for the design of grouping of radio-frequency identification (RFID) tags, based on the Chinese remainder theorem (CRT). Grouping allows verifying the integrity of a collection of objects without the requirement for accessing external systems, and can be extended to identify missing objects. Motivated by the redundancy property of the Chinese remainder representation, we propose grouping of RFID tags via the CRT. The proposed scheme not only provides designated decoding guarantees, but also offers flexibility in constructing group generation matrices. We also characterize the key objects needed to study decoding guarantees of grouping and its extended counterpart, called rank-deficient and dead-end sets, respectively, which enable theoretical analyses of error rates. The two key objects are related to the minimum and stopping distances of a linear code, respectively. As such, the characterization offers direct connection with coding theory that helps in the understanding of the verification/identification problems being studied. Theoretical and simulation results are presented, demonstrating that the proposed scheme is an efficient approach to the design of grouping of RFID tags.

The traffic control systems currently used in rail transportation are heavily infrastructure-based and therefore very costly. This work proposes a biologically inspired solution for the coordination of trains in dense railway networks, avoiding collisions with considerably higher cost efficiency than the state of the art solutions. This solution does not require track mounted infrastructure, instead it relies on the continuous cooperation between trains. Inspired by self-organizing biological systems, two simple algorithms are proposed for managing points of conflict or switches where the right of way or priority of trains has to be carefully handled. The first-level algorithm avoids local collisions by the exchange of status information through beacons. The second-level algorithm uses one-hop or multi-hop communication to share the network status among trains and coordinates the trains in possible conflict zones around switches or in shunting yards. The proposed solution can decrease the signalling cost of railway transport substantially. This efficient approach can significantly reduce the probability of accidents caused by human and hardware errors. In addition, the proposed approach can lead to significant time and energy savings in railway transport. The biologically inspired solution proposed in this paper might lead to a paradigm shift in the signalling system used in rail transportation.

Biologically inspired approaches have the potential to solve many of the difficult networking problems awaiting practical solutions. The recently proposed Virtual Traffic Lights (VTL) is one example (or instance) of this powerful approach for solving some fundamental transportation problems[1], [2]. The successful operation of VTL scheme ultimately depends on the local rules used by vehicles approaching an intersection for electing a leader that manages the traffic at that intersection by serving as a virtual traffic light. In this paper, it is shown that by using a different set of local rules at intersections, one can support priority management of emergency vehicles in a self-organized manner. The proposed VTL-Priority Intersection Control (VTL-PIC) protocol can detect the presence of an emergency vehicle and assign priority to the emergency vehicle at an intersection. Large-scale simulation results show that while the travel time of emergency vehicles can be reduced significantly, the impact of the proposed algorithm on the travel time of other vehicles is negligible.

The gradual penetration of Dedicated Short Range Communications Radio (DSRC) technology in the years to come is a formidable problem that could adversely affect the implementation of safety and non-safety applications. In this paper, we propose a solution that might mitigate the negative impact of the partial and gradual penetration problem of DSRC technology. The proposed solution is based on a self-organizing network paradigm that draws its inspiration from biological systems, such as social insect colonies [1]. By designing the local rules and a distributed algorithm needed to perform this function, it is shown that DSRC equipped cars can indeed serve as RSUs. Results show that the message reachability and connectivity in urban vehicular networks can be increased substantially in a cost-effective manner as the proposed approach does not entail deployment of infrastructure based RSUs.

Nowadays, there are more than 150 sensors inside a car which are connected via wires to the Electronic Control Units (ECUs). The increase in the number of sensors put into a car adds significant weight to the car and makes the wiring system more complex. Intra-car Wireless Sensor Network (WSN) is therefore a promising solution for automobile industry in the near future as it eliminates the amount of wiring harness and simplifies the wiring structure. The reduction in the weight of a car leads in a more efficient engine performance, better fuel economy, and reliability. In addition to the direct benefits, the Intra-car WSNs can be used for other applications. An example shown in this paper is a BLind spot Information System (BLIS). The Intra-car WSN-based BLIS is evaluated via real experiments on vehicles. The results show a comparable or even higher detection rate with other BLISs (i.e., based on cameras and radars). The main benefit of Intra- car WSNs-based BLIS is cost; the proposed system does not require additional hardware components (e.g., cameras or radars).

The reliability of communication in vehicular networks depends mostly on the density of DSRC-enabled vehicles that form the network. In highway scenarios, and depending on the time of day, the probability of having a disconnected vehicular network can be very high, which hinders communication reliability. To improve communication in these scenarios, infrastructure points known as Road Side Units (RSU) may be used. RSUs, however, have an associated cost, and therefore the number of RSUs needs to be minimized while still providing a significant improvement on communications. In this paper we study the effect of including RSUs as relay nodes to improve communication in highway scenarios. We model the average time taken to propagate a packet to disconnected nodes (denoted as re-healing time) when considering both scenarios of connected and disconnected RSUs. We then compare the results of both these models and of a model with no RSUs. Results show significant improvements with RSU deployments, both connected and disconnected, particularly in multi-cluster communication scenarios.

A thorough understanding of the communications channel between vehicles is essential for realistic modeling of Vehicular Ad Hoc Networks (VANETs) and the development of related technology and applications. The impact of vehicles as obstacles on vehicle-to-vehicle (V2V) communication has been largely neglected in VANET research, especially in simulations. Useful models accounting for vehicles as obstacles must satisfy a number of requirements, most notably accurate positioning, realistic mobility patterns, realistic propagation characteristics, and manageable complexity. We present a model that satisfies all of these requirements. Vehicles are modeled as physical obstacles affecting the V2V communication. The proposed model accounts for vehicles as three-dimensional obstacles and takes into account their impact on the LOS obstruction, received signal power, and the packet reception rate. We utilize two real world highway datasets collected via stereoscopic aerial photography to test our proposed model, and we confirm the importance of modeling the effects of obstructing vehicles through experimental measurements. Our results show considerable obstruction of LOS due to vehicles. By obstructing the LOS, vehicles induce significant attenuation and packet loss. The algorithm behind the proposed model allows for computationally efficient implementation in VANET simulators. It is also shown that by modeling the vehicles as obstacles, significant realism can be added to existing simulators with clear implications on the design of upper layer protocols.

Vehicular ad hoc networks (VANETs) have emerged as a serious and promising candidate for providing ubiquitous communications both in urban and highway scenarios. Consequently, nowadays it is widely believed that VANETs will be able to support both safety and non-safety applications. For both classes of applications, since a zero-infrastructure is the typical premise assumed, it is crucial to understand the dynamics of network connectivity when one operates without relying on any telecommunications infrastructure. Using the key metrics of interest (such as link duration, connection duration, and re-healing time) we provide a comprehensive framework for network connectivity of urban VANETs. Our study, in addition to extensive simulations based on a new Cellular Automata Model for mobility, also provides a comprehensive analytical framework. This analytical framework leads to closed form results which facilitate physical insight into the impact of key system parameters on network connectivity. The predictions of our analytical framework also shed light on which type of safety and non-safety applications can be supported by urban VANETs.

In this paper, we investigate the protective relays used in electric power systems and their role in large-scale blackouts. After reviewing the state of the art, to mitigate future blackouts, we propose a new machine learning approach for protective relays based on binary hypothesis testing, support vector machines (SVMs), and communications between the protective relays and the supervisory control and data acquisition (SCADA), which we call smart protective relays. The goal of smart relays is to classify and discriminate the normal conditions from fault conditions via local measurements. It is shown that the proposed SVM-based smart relays can detect the location of an initial fault using local current, voltage, real power, and reactive power measurements, and by monitoring these metrics, they can make a correct decision even when the state of the system changes after some equipment failure. We show that by making an intelligent decision on whether and when to trip, and communicating the changes observed to SCADA for fast and intelligent decision making, SVM-based smart relays have the potential to mitigate large-scale blackouts and confine them to much smaller areas. By deploying SVM-based smart relays only at relatively few locations where they have the highest probability to be tripped incorrectly, the probability of cascade of failures and a blackout can be greatly reduced.

Modeling complicated vehicular traffic behavior and the associated network connectivity in urban areas has been a challenging task for the past several years. In this paper, we study how intersections and two-dimensional road topology affect the connectivity behavior of traffic in urban areas. To better understand this phenomenon, we investigate both static and dynamic aspects of network connectivity in urban areas and analyze several characteristics such as network connectivity, path redundancy, re-healing time, etc. Based on a comprehensive study, we make several key observations and provide insights into the behavior of network connectivity whereby critical routing problems are identified and suggestions are made for the key components of an appropriate routing protocol for different vehicular ad hoc network (VANET) applications in urban areas.

Connectivity analysis of Vehicular Ad Hoc Networks (VANETs) involves the study of static and dynamic characteristics of the network. Realistic mobility models are essential for both aspects, in order to produce valid configurations of the position of vehicles in one static instant, and to dynamically move such vehicles in an interval of time. Focusing on static connectivity evaluation, we extend the state-of-the-art of the realism of urban mobility models for VANETs, by proposing and implementing an analysis based on a stereoscopic aerial survey over a large European city. Our static connectivity metrics show important differences compared to the model and simulation based results described in the VANET literature.

Due to the increasing oil prices, reducing the weight of future cars will be important. One way to do this is by replacing wired sensors and their cables with wireless sensors. This could reduce the weight of cars, thus improving fuel efficiency. The replacement is anticipated to reduce the weight of an average passenger vehicle by up to 50 kg. However, such a replacement raises security concerns due to the intrinsic vulnerability of wireless technologies. In addressing security problems for intra-car wireless sensor networks, existing methods, focusing on energy efficiency and providing a low security level, are unfortunately not applicable. Our research focuses on providing a real-time and a high security solution. In this paper, we make four main contributions. First, we propose a systematic methodology to select security algorithms for real-time wireless sensor networks. Second, we determine the most effective combination with respect to the execution time and the security level by using our methodology. Third, the reported results are shown to be directly applicable to off-the-shelf sensor products and real-time wireless sensor networks, since we evaluated the performance on a real sensor platform and the real-time requirements for a car are much more stringent than other real-time systems. Finally, we suggest a mathematical methodology to increase the security level which imposes a burden only to adversaries, not to sensors.

Routing in ad hoc wireless networks does not simply consist in finding a route with shortest length (as in wired networks with virtually error-free communication links), but it requires the creation of a stable and good quality communication route to avoid any unnecessary packet loss. In this paper,we discuss physical layer-oriented routing in ad hoc wireless networks, and we analyse the potential advantages of combining the use of power control (PC) with the chosen routing strategy. More precisely, we propose a modified ad hoc on-demand distance vector (MAODV) routing protocol, with and without PC, derived from the AODV-routing protocol by considering the bit error rate at the end of a multi-hop path as the metric to be minimised for route selection. In other words, we consider routing with a physical layer-oriented quality of service criterion, and we analyse the system performance in scenarios with either strong line-of-sight (LOS) or shadowed communications. Although in a scenario with strong LOS communications there are a few cases where the MAODV-PC protocol offers the best performance, in the presence of shadowed communications the proposed physical layer-oriented strategy is not attractive.

While many interesting dynamic load balancing schemes have been proposed for efficient use of limited bandwidth and to increase the capacity of congested or hot spots (or cells) in wireless networks, to date, a comprehensive mathematical framework which encompasses all of these schemes does not exist. In this paper, we provide a unified mathematical framework for dynamic load balancing, which leads to closed-form performance expressions for evaluating the performance of some of the most important dynamic load balancing strategies proposed in the literature. To the best of our knowledge, this is the first generic theoretical framework that can be used to evaluate the performance of many different dynamic load balancing schemes with simple closed-form results. The accuracy of the results predicted by these analytical expressions derived from the theoretical framework is checked by comparing these results with simulation results provided in the literature for well-known schemes.

A new statistical channel model known as the Multiple Scatterer Channel (MSC) is developed to capture the time variations of both line-of-sight (LOS) and non-line-of-sight (NLOS) fixed indoor communication (FIC) channels. It is shown that, when the statistics of the signal scatterers (e.g., speed distributions and locations) are known, the MSC model can capture the time variations of any FIC channel because its parameters can be estimated via ray tracing.

Due to an increasing number of sensors deployed in cars, recently there is a growing interest in implementing a wireless sensor network within a car. In this paper, we report the results of packet transmission experiments using ZigBee sensor nodes within a car under various scenarios. The results of the experiments suggest that both Received Signal Strength Indicator (RSSI) and Link Quality Indicator (LQI) can only be used as a threshold-based indicator to evaluate the link quality - indicating poor link quality when dropping below a certain threshold. Preliminary experimental results show that a detection algorithm developed by the authors based on RSSI/LQI/error patterns and an adaptive strategy might increase the goodput performance of the link while improving power consumption of the radio.

While transparent WDM optical networks become more and more popular as the basis of the Next Generation Internet (NGI) infrastructure, such networks raise many unique security issues. The existing protection schemes which only consider unintended failures and only rely on postmortem detection and reaction are not sufficient to provide security assurance for such infrastructures which require timely protection from malicious sabotage as well as inadvertent faults. Moreover, as may have been observed from past practices, providing a particular solution specialized to each incessantly discovered new problems would not dramatically improve the situation. In order to increase the security of future networks we will need to imbed intelligence into the network such that they can continuously learn from the experience in faults and self-organize to protect themselves from potential failures caused by malicious new attacks or ordinary reliability problems. In this paper, we present results with simple vulnerability and attack scenarios in order to demonstrate how the self-organization helps to adapts against new vulnerabilities and avoid attacks.

Vehicular ad hoc wireless network (VANET) exhibits a bipolar behavior in terms of network topology: fully connected topology with high traffic volume or sparsely connected topology when traffic volume is low. In this work, we develop a statistical traffic model based on the data collected on 1-80 freeway in California in order to study key performance metrics of interest in disconnected VANETs, such as average re-healing time (or the network restoration time). Our results show that, depending on the sparsity of vehicles, the network re-healing time can vary from a few seconds to several minutes. This suggests that, a new ad hoc routing protocol will be needed as the conventional ad hoc routing protocols such as dynamic source routing (DSR) and ad hoc on-demand distance vector routing (AODV) will not work with such long re-healing times.

A vehicular ad hoc network (VANET) may exhibit a bipolar behavior, i.e., the network can either be fully connected or sparsely connected depending on the time of day or on the market penetration rate of the wireless communication devices. In this paper, we use empirical vehicle traffic data measured on 1-80 freeway in California to develop a comprehensive analytical framework to study the disconnected network phenomenon and its network characteristics. These characteristics shed light on the key routing performance metrics of interest in disconnected VANETs, such as the average time taken to propagate a packet to disconnected nodes (i.e., the re-healing time). Our results show that, depending on the sparsity of vehicles or the market penetration rate of cars using Dedicated Short Range Communication (DSRC) technology, the network re-healing time can vary from a few seconds to several minutes. This suggests that, for vehicular safety applications, a new ad hoc routing protocol will be needed as the conventional ad hoc routing protocols such as Dynamic Source Routing (DSR) and Ad Hoc On-Demand Distance Vector Routing (AODV) will not work with such long re-healing times. In addition, the developed analytical framework and its predictions provide valuable insights into the VANET routing performance in the disconnected network regime.

This paper investigates whether and when route reservation-based (RB) communication can yield better delay performance than non-reservation-based (NRB) communication in ad hoc wireless networks. In addition to posing this fundamental question, the requirements (in terms of route discovery, medium access control (MAC) protocol, and pipelining, etc.) for making RB switching superior to NRB switching are also identified. A novel analytical framework is developed and the network performance under both RB and NRB schemes is quantified. It is shown that if the aforementioned requirements are met, then RB schemes can indeed yield better delay performance than NRB schemes. This advantage, however, comes at the expense of lower throughput and goodput compared to NRB schemes

Routing protocols developed for ad hoc wireless networks use broadcast transmission to either discover a route or disseminate information. More specifically, reactive routing protocols has to flood the network with a route request (RREQ) message in order to find an optimal route to the destination. Several applications developed for vehicular ad hoc wireless networks (VANET), which is a subset of MANET, rely on broadcast to propagate useful traffic information to other vehicles located within a certain geographical area. However, the conventional broadcast mechanism may lead to the so-called broadcast storm problem. In this paper, we explore how serious the broadcast storm problem is in both MANET and VANET by examining how broadcast packets propagate in a 2-dimensional open area and on a straight road or highway scenarios. In addition, we propose three novel distributed broadcast suppression techniques; i.e., weighted p-persistence, slotted 1-persistence, and slotted p- persistence schemes. Our simulation results show that the proposed schemes can achieve up to 90% reduction in packet loss rate while keeping the end-to-end delay at acceptable levels for most VANET applications. They can also be used together with the route discovery process to guide the routing protocols to select routes with fewer hop counts.

Current cellular networks are not self-organizing in nature; e.g., they are vulnerable to base station (BS) failures or malfunctions due to congestion (a large amount of traffic) or due to emergencies. To address this problem, use of fixed relay nodes (FRNs) in a geographical area (which may or may not be covered by base stations or access points) was recently proposed to introduce self-organizing features to cellular wireless networks, such as enabling traffic forwarding in case of congestion and handling base station failures. In this paper, we provide an algorithm for choosing system parameters in a joint cellular and fixed relay network such that quality of service (QoS) requirements in terms of delay are satisfied. In particular, we show how to determine the optimum number of neighbors to achieve small average path lengths, high coverage, and low vulnerability to failures.

The remarkable surge in research on ad hoc wireless networks is largely due to their potential in offering insfrastructureless communications. While initially studied adopting a "conventional" networking approach, based upon years of research experience on wired computer networks (with virtually error-free communication links), the presence of unreliable wireless communication links necessitates a communication-theoretic foundation for the design and analysis of wireless ad hoc networks. Such a novel comprehensive "bottom-up" perspective was, for the first time, presented in OK Tonguz et al. (2006). In this paper, we summarize the communication-theoretic framework, where the impact of physical layer on the network performance and its interaction with higher layers is taken into account. The main goal of this paper is to provide the reader with the intuition behind the comprehensive approach, rather than the mathematical details of the approach

In this paper we investigate the potential application of RFID technology in future cars as a cheaper, more reliable, and secure wireless alternative to the current wired operation in the engine compartment and other parts of a car where several sensors are connected to adapters which, in turn, are connected to a serial data bus for carrying the measured sensor data to various embedded microprocessors. The preliminary measurement results show that the coherence bandwidths of various channels between a reader and several RFID tags placed into different locations in the car are sufficiently large for the purpose of in-car sensor network communications while the power losses of certain channels are quite excessive and a special distributed antenna system may be required to deal with the problem.

Power conservation is one of the most important issues in wireless ad hoc and sensor networks, where nodes are likely to rely on limited battery power. Transmitting at unnecessarily high power not only reduces the lifetime of the nodes and the network, but also introduces excessive interference. It is in the network designer's best interest to have each node transmit at the lowest possible power while preserving network connectivity. In this paper, we investigate the optimal common transmit power, defined as the minimum transmit power used by all nodes necessary to guarantee network connectivity. This is desirable in sensor networks where nodes are relatively simple and it is difficult to modify the transmit power after deployment. The optimal transmit power derived in this paper is subject to the specific routing and medium access control (MAC) protocols considered; however, the approach can be extended to other routing and MAC protocols as well. In deriving the optimal transmit power, we distinguish ourselves from a conventional graph-theoretic approach by taking realistic physical layer characteristics into consideration. In fact, connectivity in this paper is defined in terms of a quality of service (QoS) constraint given by the maximum tolerable bit error rate (BER) at the end of a multihop route with an average number of hops

The performance of most dynamic load balancing schemes, proposed to solve the hot spot problem in cellular systems, depends on the user distribution within a cell. In this paper, we first study the impact of user distribution on the call blocking probability performance that can be achieved by two key dynamic load balancing schemes (i.e., channel borrowing without locking (CBWL) and integrated cellular and ad hoc relay (iCAR) system (H. Jiang and S.S Rappaport, 1994)-(H. Wu et al., 2001)). We show that the achievable call blocking probability performance is location-dependent, and iCAR and CBWL schemes might not be able to achieve 2% call blocking probability for all user distributions, unless major modifications are made to CBWL or iCAR to overcome local congestion problems. Therefore, in this paper, we propose to use a joint dynamic load balancing scheme that combines the features of CBWL and iCAR schemes. Our objective is to guarantee a low call blocking probability in congested cells irrespective of user location.

Several dynamic load balancing schemes have been proposed in the literature to solve the hot spot problem in cellular networks. In O. K. Tonguz and E. Yanmaz (2003), a unified theoretical framework for the performance analysis of dynamic load balancing schemes in cellular networks have been developed, and closed-form expressions for the call blocking probability of several existing dynamic load balancing schemes have been derived. However, to the best of our knowledge, an analysis showing which one of these dynamic load balancing schemes is optimum, in terms of the achievable call blocking probability, has not been reported before. In this paper, we explore, for the first time, if a global optimum dynamic load balancing scheme exists, and we examine if and under which conditions the existing dynamic load balancing schemes can achieve optimality.

In this paper, we compare the performance of two possible switching techniques for ad hoc wireless networks: namely, reservation-based (RB) and non-reservation-based (NRB) switching. Fundamental network performance metrics, such as throughput and delay, are studied to understand the tradeoffs between the two switching schemes. In particular, we identify the circumstances where each of these two schemes is better than the other.

Power conservation is one of the most important issues for ad hoc wireless networks where nodes are likely to rely on limited battery power. Transmitting at unnecessarily high power not only reduces the lifetime of the nodes and the network, but also introduces excessive interference. It is in the network designer's best interest to have each node transmit at the lowest possible power while preserving network connectivity. In this paper, we investigate the optimal common transmit power, defined as the minimum transmit power used by all nodes necessary to guarantee network connectivity. In particular, we show that for a given route BER and node spatial density, there exists a global optimal data rate at which the transmit power can be globally minimized. Moreover, we also show that there exists a critical node spatial density at which the optimal transmit power is the minimum possible for a given data rate and a given route BER.

In this paper, we propose to use a dynamic load balancing scheme that utilizes fixed relay stations placed in a cellular geographical coverage area to improve the handover performance in cellular networks. To this end, we develop closed-form performance expressions and derive the handover dropping probability in terms of the main system parameters, such as the new call and handover arrival rates, the number of fixed and load-balancing channels per cell, the load balancing probability for new and handover calls, etc. Our results show that by employing a dynamic load balancing scheme, not only the new call blocking probability, but also the handover dropping probability can be improved substantially, without reserving any channels or assigning any priority to handover calls. In fact, employing dynamic load balancing without any guard channels outperforms a conventional system that employs the well-known guard channel method (GCM).

In this paper, we investigate the impact of the channel utilization ratio (CUR) on the performance of ad hoc wireless networks. Given that a node can hold a multi-hop route for a time interval defined as the reserved channel utilization interval (RCUI), we assume that the node effectively utilizes the reserved route for an interval defined as the effective channel utilization interval (ECUI), the duration of which corresponds to the duration of the message to be transmitted. This models a realistic scenario, where a node may use the shared radio medium for only a portion of the reservation interval. Defining the CUR as the ratio between the RCUI duration and the ECUI duration, we develop an analytical framework which leads to the evaluation of the "optimal" CUR for the maximization of the effective transport capacity, a concept recently introduced by the authors and representing the actual bandwidth-distance product carried by the network.

In this paper, the performance of two possible switching schemes for ad hoc wireless networks, namely reservation-based (RB) and non-reservation-based (NRB) is investigated. It is shown that route reservation would be the scheme of choice in a scenario where delay is a constraint, the network is dense, and/or nodes move with moderate speed. However, a non-reservation scheme is preferable in a scenario where the network is sparse, there is heavy traffic load, nodes are static, and/or delay is not a major concern.

While many studies of admission control schemes assume exponentially distributed cell residual time, field measurements have shown that cell residual time, in reality, follows a heavy-tailed distribution; in particular, a Pareto distribution. These measurements were made in Wireless Andrew, an enterprise-wide broadband microcellular wireless network at Carnegie Mellon University. In this paper, we investigate the impact of Pareto distributed cell residual time on the admission threshold of the distributed admission control scheme (DACS), which is one of the well-known admission control schemes. Our results show that under Pareto-distributed cell residual time, a system that implements DACS can accommodate or "sign on" more simultaneous connections than the exponentially distributed case.

Fault and attack management has become a very important issue for network operators that are interested to offer a secure and resilient network capable to prevent and localize, as accurately as possible, any failure (fault or attack) that may occur. Hence, an efficient failure location method is needed. To locate failures in opaque optical networks, existing methods which allow monitoring of the optical signal at every regeneration site can be used. However, to the best of our knowledge, no method exists today that performs failure location for transparent optical networks. Such networks are more vulnerable to failures than opaque networks since failures propagate without being isolated due to optoelectronic conversions. In this paper, we present a failure location algorithm that aims to locate single and multiple failures in transparent optical networks. The failure location algorithm developed in this paper can cope with ideal scenarios (i.e., no false and/or lost alarms), as well as with nonideal scenarios having false and/or lost alarms.

In this paper, we report theoretical and experimental channel-capacity estimates of heating, ventilation, and air conditioning (HVAC) ducts based on multicarrier transmission that uses M-ary quadrature amplitude modulation and measured channel responses at the 2.4-GHz industrial, scientific, and medical band. It is shown theoretically that data rates in excess of 1 Gb/s are possible over distances up to 500 m in straight ducts in which reflections have been suppressed. Our experimental results also show that even in the case of more complex HVAC duct networks (i.e., HVAC duct networks that include bends, tees, etc.) data rates over 2 Gb/s are possible. Our estimations in this case are valid for distances of up to 22 m, which was the maximum distance of our experimental setup. These experimental results, measured with a large-scale testbed set up at Carnegie Mellon University, Pittsburgh, PA, albeit limited in terms of transmitter-receiver separation distance, provide further evidence on the potential of HVAC systems as an attractive solution for providing communications in indoor wireless networks.

In this paper, we propose a simple semi-analytical approach for the evaluation of the impact of mobility on the bit error rate (BER) performance of multi-hop ad hoc wireless networks. Analytical expressions, relating the BER at the end of a multi-hop route with the mobility characteristics of the nodes and the routing strategy, are derived. Two node mobility models are considered: direction-persistent (DP) and direction-nonpersistent (DNP). In particular, two network switching scenarios are analyzed: (i) opportunistic non-reservation-based switching (ONRBS), where a message flows from source to destination by opportunistically choosing the available shortest consecutive links; and (ii) reservation-based switching (RBS), where, after the creation of a multi-hop route from source to destination, the message is "forced" to flow over the reserved links, regardless of their actual lengths. The network performance is evaluated in ideal (without inter-node interference, INI) and realistic (with INI) cases. The improved robustness against mobility offered by ONRBS, with respect to RBS, is analyzed and quantified.

In this paper, we present a performance analysis for different implementations of handover in indoor wireless networks (IWN) that use heating, ventilation, and air conditioning (HVAC) ducts as communication channels. If FDMA/TDMA technology is used, our results show that the handover performance, i.e., new call blocking and handover dropping probabilities, of an IWN that uses HVAC ducts are up to 6.6 times better for practical scenarios compared to those of traditional IWN (i.e., IWN that do not use HVAC ducts). Our results also indicate that for scenarios under investigation, using a single access point to serve one floor and its staircase region achieves the best handover performance for IWN that use HVAC ducts. The solution to the handover problem in HVAC-IWN is tightly coupled with the coverage, capacity, and load balancing issues in indoor wireless networks. We show, for the first time, that the proposed solution to the handover problem in HVAC-IWN results in an increase in capacity per coverage area, an increase in the radius of coverage for an access point, and can be used to achieve load balancing in WLAN that use IEEE 802.11 technology.

Multi-element antenna arrays at both transmitter and receiver systems have emerged as a new technique for wireless communication systems in rich multipath environments. HVAC ducts, viewed as over-moded waveguide network systems at RF frequencies, have the multipath richness through various propagating modes and reflections within the system. This paper studies multiple-input multiple-output (MIMO) systems for HVAC ducts. Computations based on measured channel responses of a 6/spl times/6 MIMO system in 0.3 m diameter HVAC ducts in the 2.4 GHz ISM band demonstrate a five-fold increase in mean capacity over single antenna systems for an SNR of 30 dB.

Determining the minimum number of neighboring nodes required to guarantee full connectivity, i.e., to ensure that a node can reach, through multiple hops, any other node in the network, is an important problem in ad hoc wireless networks. In this paper, we consider reservation-based wireless networks with stationary and uniform (on average) node spatial distribution. Assuming that any communication route is a sequence of minimum length hops, we show that, in an ideal case without inter-node interference (INI) and on the basis of a suitable definition of transmission range, the minimum number of neighbors required for full connectivity is, on average, /spl pi/. Full connectivity is guaranteed if the transmitted power (in the case of fixed node spatial density) or, equivalently, the node spatial density (in the case of fixed transmitted power) are larger than critical minimum values. In a realistic case with INI, we prove that there are situations where full connectivity cannot be guaranteed, regardless of the number of neighbors or the transmitted power.

We consider a novel discrete-time model to analyze the performance of circuit-switched sensor networks. In particular, we assume that a node, after reserving a multi-hop communication route to the desired destination, holds it for a time interval defined as reserved channel utilization interval (RCUI) and utilizes it for an effective channel utilization interval (ECUI). A realistic network communication scenario with inter-node interference (INI) and a reservation-based medium access control (MAC) protocol with finite numbers of (active) routes (FNR) in the network is first considered, and the average interference power is evaluated through a novel combinatorial analysis. Results are presented in terms of effective transport capacity and channel utilization ratio (CUR). In particular, we show that for very low values of the packet generation rate at each node, activation of the maximum possible number of routes guarantees no loss, in terms of effective transport capacity, with respect to an ideal (no INI) scenario. However, this comes at the expense of a very low utilization: in other words, once a multi-hop route has been reserved, its effective utilization time must be a few orders of magnitude lower than the duration of the reservation interval.

We have introduced a novel concept called "managed mobility", and addressed the mobility of the relaying devices called mobile ad hoc relaying stations (MARSs) in the integrated cellular and ad hoc relaying (iCAR) system. We anticipate that the idea of managed mobility proposed in this paper for the iCAR system as well as the mobility management strategies may also be applied in other ad hoc networks, such as the self-reconfigurable sensor network, to reduce additional node deployment cost and increase fault tolerance.

In this paper, we present an analytical framework for dynamic priority queueing of handover calls in wireless networks. The framework employs a queueing discipline with two classes of priority for handover calls. Two queues, first priority and second priority, are employed for the two priority classes of handover calls. The priority of queued handover calls is not based only on the received signal strength, but also on the remaining time in the overlap region between two cells. We also incorporate a priority transition between handover calls in the queue; specifically, a second-priority handover call in the second-priority queue, based on certain criteria, can become a first-priority handover call and join the first-priority handover queue. In addition, the event that a handover call could finish its call while waiting in the queue is taken into account in the analysis. This event was not taken into consideration in previous related studies and, as a result, these previous studies overestimate handover failure probability. Our results also show that the predictions of the analytical framework developed in this paper are in very good agreement with simulation results. The developed analytical framework is comprehensive and can also cope with several priority schemes proposed by other researchers in the literature. For example, it is shown that, under certain conditions, the proposed framework converges to first-in-first-out queueing of handover calls. One can easily modify the proposed framework to incorporate priority schemes that use guard channels for handover calls. It is also shown that one could potentially use the framework developed in this paper in integrated voice/data networks, as well as for handover between different network types. The proposed analytical framework is anticipated to be a very useful tool in evaluating performance of present and future wireless networks employing dynamic priority queueing for handovers and in designing more efficient handover algorithms.

The capacity of wireless networks can be increased via dynamic load balancing/sharing by employing overlay networks on top of the existing cellular networks. One such recently proposed system is the integrated cellular and ad hoc relay (iCAR) system, where an overlay ad hoc network is employed to use the resources efficiently by dynamically balancing the load of the hot spots in the cellular network, and to provide quality-of-service to subscribers, no matter where they are located and when the request is made. It is assumed that this overlay network operates in the 2.4-GHz Industrial, Scientific, and Medical (ISM) band and, hence, the number of available ISM-band relay channels used for load balancing will be limited due to other users' interference at a given point in time. In this paper, the impact of ISM-band interference on the performance of iCAR systems, which is a representative hybrid wireless network, is studied, and it is shown that dynamic load balancing and sharing capabilities of iCAR systems are strictly dependent on the availability of the ISM-band relay channels. In addition to quantifying the impact of the number of available relay channels on the performance of iCAR systems, a simple channel assignment scheme to reduce the performance degradation due to other users' interference is also provided. Results show that this interference avoidance technique can improve the realistic performance of iCAR-like hybrid wireless networks by 12%-23% when the interferers are uniformly distributed in the ISM-band.

In this paper, bit error rate (BER) performance and connectivity characteristics of multi-hop ad hoc wireless networks are analyzed under a circuit-switched network communication scheme characterized by the creation of a multi-hop communication route, through intermediate relay nodes, for each source-destination pair. The proposed transmission scheme is packetized yet it does not employ retransmissions: in this sense, it can be considered as a hybrid scheme between circuit switching and packet switching. The ideal limiting performance under the assumption of no inter-node interference (INI) is evaluated. In particular, the concept of minimum spatial energy density is introduced, and quantified with a precise expression in the case of uncoded binary phase shift keying (BPSK) transmission. A realistic scenario with INI is then considered, and two different medium access control (MAC) protocols are proposed: Aloha and "per-route" carrier sense multiple access (PR-CSMA). In both cases, the BER performance is analyzed. Results show that MAC and physical layers are strictly interrelated, and designing one without considering the other may lead to wrong choices in ad hoc wireless network design.

Many interesting dynamic load balancing schemes have been proposed for efficient use of the available frequency spectrum, and, in particular, to increase the capacity of congested or hot cells in a cellular network. We provide a simple theoretical framework which leads to closed-form expressions for evaluating the performance of some of the most important dynamic load balancing strategies proposed in the literature. To the best of our knowledge, this is the first generalized theoretical framework that can be used to evaluate the performance of many different dynamic load balancing schemes with simple closed-form results. The accuracy of the developed expressions is checked and verified by comparing their results with simulation results provided in the literature. There is an excellent match between the predictions of our theoretical framework and the simulation results.

In this paper, we present an innovative solution to the handover problem in multi-story buildings using HVAC ducts for wireless communications. The proposed solution is based on a new system architecture design of the indoor wireless networks that use the heating, ventilation, and air conditioning (HVAC) ducts as a communication channel. Exploiting the system architecture design and the time diversity at the mobile terminal, we show that intra-floor handover can be eliminated, while inter-floor handover can be substantially reduced due to the fact that multiple floors can be served via the same access point. Our results indicate that the mean number of handovers per call achieved in the indoor wireless networks that use HVAC ducts reduces; e.g., by up to a factor of nine for the illustrative example considered in this paper compared to that of indoor wireless networks that do not use HVAC ducts. The improvement; i.e., reduction in the mean number of handovers in multi-story building, depends on the number of access points used.

While most analytical studies on handover performance assume exponential channel holding times, recent field measurements have shown that channel holding time follows a mixed lognormal distribution. In this paper, the impact of mixed lognormal channel holding time on the handover performance of guard channel scheme with first-in first-out (GCM-FIFO) queuing examined. Our results show that the handover performance of GCM-FIFO is almost the same as the one when the channel holding time follows an exponential distribution. Hence, to evaluate the handover performance of PCS networks where the channel holding time follows a mixed lognormal distribution, one could use the analytical framework developed for the case of exponential channel holding time distributions.

The integrated cellular ad hoc relaying (iCAR) system is a representative heterogeneous wireless system, proposed to address the congestion problem in the wireless networks. In this paper, we present an analytic model based on Markov chains for the queuing delay performance of iCAR. Our results show that the new call requests in iCAR have a significantly lower queuing delay than that of the conventional cellular system. The analytic model developed in this paper may serve as the guideline for the delay performance evaluation of the next generation heterogeneous wireless systems.

In this paper, a simplified analysis technique for the integrated cellular and Ad hoc relay (iCAR) systems is presented. First, a simple two-cell system is analyzed using a multi-dimensional Markov-chain. The performance metric employed is the call blocking probability of each cell in the system. To this end, first a closed-form expression for the call blocking probability in the two-cell system is provided. Then, it is shown that these closed-form expressions could be used to analyze more practical systems. The accuracy of the developed simple analytical expressions is checked and verified by comparing the results predicted by these analytical expressions with simulation results. It is shown that there is an excellent match between analytical and simulation results.

Heating, ventilation, and air conditioning (HVAC) ducts in buildings behave as multimode waveguides when excited at radio frequencies and thus, can be used to distribute radio signals. The channel properties of the ducts are different from the properties of a usual indoor propagation channel. In this paper, we describe physical mechanisms which affect the HVAC channel impulse response and analyze their influence on the delay spread. Those mechanisms include antenna coupling, attenuation, and three types of dispersion: intramodal, intermodal, and multipath. We analyze each type separately and explore the behavior of the delay spread as a function of distance in straight ducts. Experimental channel measurements taken on real ducts confirm the validity of our model.

We present a simple path loss prediction model for link budget analysis in indoor wireless local area networks (LANs) that use heating, ventilation, and air conditioning (HVAC) ducts. The model we propose predicts the average power at each location in an HVAC duct network. This prediction model greatly simplifies the analysis for a complex duct network, making it a convenient and simple tool for system design. Our prediction model Is verified by experimental measurements.

Coping with congestion or hot spots is one of the common problems faced by the wireless service providers. To handle this hot spot or congestion problem in a cellular network, several dynamic load balancing schemes based on channel borrowing have previously been proposed. A recent approach to dynamic load balancing is the Integrated Cellular and Ad hoc Relay (iCAR) system. iCAR employs ad hoc relay stations (ARS's) in the cellular network to balance traffic loads efficiently and to share channels between cells via primary and secondary relaying. In this paper, it is shown that the realistic performance of ICAR systems in terms of dynamic load balancing and load sharing is heavily dependent upon the number of available ISM-band ARS channels. Results also indicate that the number of ARS channels required for dynamic load balancing is much more than the number of ARS channels required for load sharing (i.e., for bringing the call blocking probability of a hot spot to 2%).

We report theoretical channel capacity estimates of heating, ventilation, and air-conditioning (HVAC) ducts based on multi-carrier transmission that uses M-QAM modulation and measured channel responses at 2.4 GHz Industrial, Scientific, and Medical (ISM) band. It is shown that, data rates in excess of 1 Gbps are possible over distances up to 500 m in "matched" ducts (one can think of "matched" ducts as user-friendly ducts, since "matching" can, in principle, eliminate all the multipath reflections in HVAC ducts). Our work also shows that data rates in excess of 300 Mbps are possible over distances up to 500 m even in the presence of significant multipath reflections.

One of the common problems faced by the wireless service providers worldwide is coping with congestion or hot spots. To handle this hot spot problem, methods that combine the existing cellular networks with ad hoc networks have been proposed. Integrated Cellular and Ad Hoc Relay (iCAR) system employs ad hoc relay stations (ARSs) within the cellular network to balance traffic loads efficiently and to share channels between cells via primary and secondary relaying. These ARSs operate in the ISM band, and therefore, do not cause interference to the cellular band. When analyzing the performance of WAR systems, there are several factors that should be taken into account These factors include the coverage area of the ARSs, the number of ARS channels, the placement of ARSs, etc. In this paper, the impact of the number of ARS channels on the performance of WAR systems is studied. To this end, a multi-dimensional Markov-chain analysis is performed for a simplified two-cell system model. Results show that, with a proper amount of ARS coverage within each cell the call blocking probabilities can be decreased significantly with a small number of channels. Results also suggest that by increasing the number of ARS channels perfect load balancing can be achieved.

An approximate analysis is presented which can be used to predict the performance of power and signal-to-noise ratio (SNR) equalization schemes when applied to dense wavelength-division multiplexing (DWDM) lightwave systems employing erbium-doped fiber amplifier (EDFA) cascades. Expressions are provided which relate the maximum number of amplifiers, EDFA gain imbalance, bit rate (R/sub b/), transmitter power, receiver dynamic range and number of channels. The relative advantages of these two equalization strategies are quantified by comparing the maximum number of amplifiers allowed by each scheme. It is shown that, while SNR equalization represents, on balance, the more desirable equalization strategy for future EDFA-based DWDM lightwave transmission systems, under certain conditions power equalization may be a better choice. When employing an APD receiver, for instance, power equalization can support 1.9 times more amplifiers than SNR equalization. However, when employing the more conventional preamplified PIN/FET receiver, SNR equalization can support 1.7 times more amplifiers than power equalization.

We propose a dynamic multipath routing (DMPR) scheme to improve resource utilization of a network for a given quality of service (QoS) in carrying delay sensitive traffic. The proposed scheme takes advantage of available alternate routes which can be used beneficially in communication networks or switches, without increasing the network latency. Through a simulation study of circuit-switched, virtual circuit-switched (ATM) and packet-switched (Internet) networks we demonstrate the benefit of using DMPR over a single-path routing (SPR) scheme. We also discuss the implementation aspects of the DMPR scheme in data networks and the overhead associated with it.

iCAR is a new wireless architecture based on the integration of cellular and modern ad-hoc relaying technologies. We analyze its performance and compare it with conventional cellular system. In particular, we prove that due to the ability of ad-hoc relay stations (ARS) to relay traffic from one cell to another cell dynamically. iCAR has a lower system-wide call blocking probability than any corresponding cellular system without ARS, even if traffic can be evenly distributed among cells. We also study two typical scenarios and present some numeric results.

In this paper, an analytical framework which employs a delay-dependent queue discipline with two classes of priority for handoff calls is considered. Results obtained with the analytical framework are compared with the simulation results of Ebersman and Tonguz (1999) and it is shown that the accuracy of the analytical approximation is better than 95% for traffic loads of practical interest. The benefit of delay-dependent queueing compared to FIFO queueing increases as the handoff traffic increases.

An analytical framework which employs a multiple queuing scheme (MQS) for handoff calls in wireless networks is considered. Results show that, MQS and delay-dependent priority queuing (DDPQ) reported in Xhafa and Tonguz (2001) reduce handoff dropping probability by 40% when compared to the first-in-first-out (FIFO) scheme. It is also shown that, if one compares the MQS and DDPQ schemes, MQS achieves lower handoff dropping probability when the percentage of high-mobility handoff call users is greater than 38%.

Integrated cellular and ad hoc relaying systems (iCAR) is a new wireless system architecture based on the integration of cellular and modern ad hoc relaying technologies. It addresses the congestion problem due to unbalanced traffic in a cellular system and provides interoperability for heterogeneous networks. The iCAR system can efficiently balance traffic loads between cells by using ad hoc relaying stations (ARS) to relay traffic from one cell to another dynamically. This not only increases the system's capacity cost effectively, but also reduces the transmission power for mobile hosts and extends system coverage. We compare the performance of the iCAR system with conventional cellular systems in terms of the call blocking/dropping probability, throughput, and signaling overhead via analysis and simulation. Our results show that with a limited number of ARSs and some increase in the signaling overhead (as well as hardware complexity), the call blocking/dropping probability in a congested cell and the overall system can be reduced.

A fundamental problem in current cellular systems is limited capacity. Adding to this problem is unbalanced traffic among the cells. Given the explosion of the wireless traffic, especially wireless data traffic for Internet/Web access, and limited spectrum available for licensing, congestion will occur in some cells, resulting in blocked new calls and dropped handoffs due to the lack of available data channels (or DCHs). Since the locations of the congested cells vary from time to time (e.g. downtown on Monday morning, or amusement parks on Sunday afternoon), it's difficult to guarantee a sufficient amount of resources in each cell in a cost-effective way. In this paper, we propose to integrate the cellular infrastructure with modern wireless/mobile relaying technologies to achieve dynamic load balancing among different cells. Our basic idea is to place a number of mobile relay stations (or MRSs) within each cell to divert traffic in one (possibly congested) cell to another (non-congested) cell.

The impact of random user motion on intelligent paging schemes is examined and quantified. To this end, a straight-line motion pattern and two Markov chains are presented to model the random movement of mobile users (MUs) within the location area (LA). The first Markov chain describes a two-dimensional random walk. The second gives a bias to a particular section of the LA. Results show that the cost of intelligent paging in the two cases of Markovian motion is more expensive than that of a simple straight-line motion. In this work, we also consider all-step intelligent paging and show that if intelligence is applied in all steps, a substantial reduction of at least 60% in paging cost can be achieved over sequential and one-step intelligent paging schemes.

Wavelength conversion has been shown to reduce the probability of blocking in both circuit-switching and packet-switching wavelength routed optical networks (WRONs). The effectiveness of the blocking reduction depends on the topology, and is known to be best for meshed topologies, where the average number of hops per path is large. This paper shows that by exploiting wavelength conversion, routing without buffers, known as hot-potato, becomes an interesting option for packet switching WRONs with meshed topologies, such as Manhattan Street (MS) Network and ShuffleNet (SN). The results show that, by using more than 4 wavelengths, a 64 node MS or SN network can work at full load with a hop delay within one hop from its lowest achievable value. We also show that using delay-line routing buffers at the node is a much more effective way of reducing blocking than using wavelength conversion.

Four-wave mixing (FWM) is the most serious fiber nonlinearity associated with low-input optical power levels in long-haul multichannel optical systems employing dispersion-shifted fiber. To reduce the crosstalk due to FWM, a generalized suboptimum unequally spaced channel allocation (S-USCA) technique is proposed and investigated. Even though the developed technique is useful in combating FWM crosstalk in wavelength division multiplexing (WDM) lightwave systems with up to 12 channels, its main virtue is in designing multichannel WDM lightwave systems with more than 12 channels. Comparisons of power penalty due to FWM between equal channel spacing (ECS) systems and the S-USCA systems are presented. It is shown that for an intensity modulation/direct detection (IM/DD) transmission system operating in an optical bandwidth of 16 nm with 0 dBm (1 mW) peak optical input power per channel, while a conventional ECS WDM system with 0.84-nm channel spacing cannot even achieve a bit-error rate (BER)=10/sup -9/, the suboptimum technique developed in this paper, for the same minimum channel spacing, can achieve a BER=10/sup -9/ with an FWM crosstalk power of less than 1 dB at the worst channel in a 20-channel WDM system.

For pt.I see ibid., vol.46, no.8, p.1027-37 (1998). Four-wave mixing (FWM) in dispersion-shifted optical fiber is a major problem associated with low optical input power levels in optical wavelength-division multiplexed (WDM) systems. To reduce the crosstalk caused by FWM, a generalized suboptimum unequally spaced channel allocation (S-BISCA) technique has been proposed. While the S-USCA technique reduces the PWM power substantially, it also reduces the minimum channel spacing compared to conventional equal channel spacing (ECS) systems when the same number of carrier channels are accommodated in a fixed optical bandwidth. This results in more interchannel interference (ICI) when employing the S-USCA scheme. The power penalty of the ECS and the S-USCA systems caused by crosstalk and frequency drift are investigated and compared in this paper. The superior system performance region, where S-USCA systems out perform ECS systems, is also quantified. For 20-channel systems using an amplitude-shift keying (ASK) heterodyne detection scheme, for instance, results show that the S-USCA technique pays less power penalty up to bit rates of 5.5, 7.5, and 9.5 Gb/s, when all channels have identical states of polarization and the launched input power per channel P/sub in/, equals to -6, -3, and 0 dBm, respectively.

The teletraffic/transmission performance of regular two-connected multi-hop cell-switching optical networks in uniform traffic under hybrid store-and-forward is presented. The Manhattan street (MS) network and Shufflenet (SN) are compared in terms of average queueing delay, queue size, transit delay, throughput, and bit error rate (BER) for intensity modulation/direct-detection (IM/DD) both analytically and by simulation. A hybrid semi-transparent store-and-forward node architecture is presented. Cells are electronically stored just in the case of conflict to avoid deflection, otherwise the cells will traverse the node without opto-electronic conversion. This architecture performs well, in terms of throughput, propagation delay and BER. It is also shown that by combining deflection routing with the store-and-forward scheme the network can accommodate two different bit-rates. This suggests that the proposed hybrid scheme may have good potential for future multimedia networks.

The transmission performance of regular two-connected multi-hop transparent optical networks in uniform traffic under hot-potato, single-buffer deflection routing schemes is presented. The Manhattan Street (MS) network and ShuffleNet (SN) are compared in terms of bit error rate (BER) and packet error rate (PER) both analytically and by simulation. We implement a novel strategy of analysis, in which the transmission performance evaluation is linked to the traffic randomness of the networks. Amplifier spontaneous emission (ASE) noise, and device-induced crosstalk severely limit the characteristics of the network, such as propagation distance, sustainable traffic, and bit-rate. Our results indicate that under the same load the BER performance of single-buffer deflection routing is worse than hot-potato. However, at BER=10/sup -9/ single-buffer has a higher throughput than hot-potato. It is shown that the feasibility of deflection routing in transparent networks with MS and SN topologies heavily depends on the power coupling coefficient of the routing space switch used in each node.

The impact of finite intermediate frequency (IF) on the performance of heterodyne ASK lightwave systems is examined and quantified in the presence of laser phase noise and shot noise. For negligible linewidths, it is shown that certain finite choices of IF (R/sub b/,3R/sub b//2,2R/sub b/,5R/sub b//2, etc.) lead to the same ideal bit-error-rate (BER) performance as infinite choices of IF. Results indicate that for negligible linewidths the worst case sensitivity penalty is 0.9 dB for proper heterodyne detection and occurs when f/sub IF/=1.25 R/sub b/. For nonnegligible linewidths (e.g., when /spl Delta//spl nu/T/spl ges/0.04) the sensitivity penalty is always less than 0.9 dB for finite choices of IF. The analysis presented does lead to a closed-form signal-to-noise ratio (SNR) expression at the decision gate of the receiver which can readily be used for BER and sensitivity penalty computations. The SNR expression provided includes all the key system parameters of interest such as system bit rate (R/sub b/), the peak IF SNR (/spl xi/), laser linewidth (/spl Delta//spl nu/), and the IF filter expansion factor (/spl alpha/). The findings of this work suggest that the number of channels in a multichannel heterodyne ASK lightwave system can be increased substantially by properly choosing a small value for the IF at the expense of a small penalty <1 dB. On the negative side, IF frequency stabilization becomes a more critical requirement in multichannel systems employing small values of IF.

A hybrid network architecture using subcarrier multiplexed optical fiber (SCMOL) and coaxial cable is proposed for the access link of PCS between tiny passive radio base stations in microcells and remote central base stations. Results indicate that the required number of laser diodes, photodetectors and amount of fiber could be reduced significantly with the proposed hybrid access link compared to the point-to-point SCMOL link and the network cost would decrease proportionally.

The amount of fiber required, propagation delay, and length of the longest link are significant design constraints in spatially large networks. This paper examines these characteristics from the viewpoint of growth and compares basic networks with hierarchical ones in terms of these characteristics. Results show that, when considering growth from three nodes, a star network randomly placed with a uniform distribution uses less fiber than a dual ring until there are 57 nodes. As the networks become large, the star has the smallest propagation delay and the dual ring uses the least amount of fiber. A two-level network having a star on the upper level and dual rings on the lower network level performs well in both categories by using 1.38 times as much fiber as the dual ring and having 1.65 times the propagation delay of a star as the number of nodes becomes large.

Handoffs in a mobile cellular communications environment will become an increasingly important issue as cell sizes shrink to accommodate an increasingly large demand for services. A new handoff ordering method is proposed which can be used to provide rapid handovers with a smaller percentage of dropped calls than other methods. Signal prediction priority queuing (SPPQ) is a generic queuing method which can be adapted to almost any handover technique for the benefit of decreased dropped calls. In a typical personal communications system (PCS) environment, it is shown that for realistic call blocking probabilities (2-6%), SPPQ leads to about 15% fewer dropped calls compared to first in first out (FIFO) queuing. This benefit comes at the expense of a slight increase (<1%) in blocked call percentage. The proposed SPPQ scheme is also compared to a method called the measurement based priority scheme (MBPS) and, based on worst case scenarios, it is shown to outperform MBPS as well.

Efficient mobility management for mobile (MS) or portable stations (PS-hand-off, channel assignment, and location management-will play an important role in future personal communication systems (PCS). Among these tasks, location management plays a critical role for wide area roaming. The key elements of location management are location registration/updating, and paging. Efficient paging algorithms may generate relatively less paging traffic such that larger location areas may become plausible to reduce location updating and signalling traffic. Deciding optimum location area size is also affected by the call arrival rate to the cell, cell boundary crossing rate of portable stations, cell size, and the allocated number of voice channels per cell. In this paper, different paging techniques are compared to find the optimum location management method for future PCS environment. To this end, a new technique called paging area updating method (PAUM) is introduced and its performance is analysed. It is shown that, for a low cell boundary crossing rate of portable stations, the proposed PAUM technique outperforms other paging techniques, especially when the call arrival rate is high.

Rapidly increasing demand for wireless services makes the backbone network design a crucial design issue in metropolitan area microcellular networks. The optimum backbone network in conjunction with the subcarrier multiplexed optical link to microcell is examined. First, the number of microcells is computed for the metropolitan area of New York city taking into consideration the typical traffic characteristics of various wireless services in such urban areas. Then, feasible and cost-effective network topologies and configurations are studied. The required number of microcells increases from 600 to 3,500 and the coverable cell radius decreases from 800 to 320 meters when the subscription ratio increases from 10% to 60% in a city, such as New York, of 14 million population and 300 square miles area. In terms of the overall network cost, the optimum number of mobile switching centers may range from 5 to 1,000 depending on the subscription ratio and the cost of mobile switching centers. In order to make the backbone network robust to possible fluctuations in subscription ratio and costs of components, an exact forecast of them is required.

The feasibility of fiber-in-the-loop (FITL) is analyzed, dividing the local loop into four segments based on density: the urban business area, the urban residential area, suburban new growth areas, and a rural allocation area. A cost model is constructed around key network elements using realistic prices found in the marketplace. This model takes into account the impact of the synchronous optical network (SONET), remote powering issues, and network upgradability to broadband services. As a means to determine the feasibility of FITL in the future, the cost models of the network elements are subject to projected industry cost trends, and the analysis is repeated for three different points in time (1992, 1995, and 1998) to account for price reductions due to experience. Regulatory issues and their impact on the economic feasibility of FITL are also analyzed.>