Tamer A. ElBatt

Joint scheduling and power control for wireless ad hoc networks

In this paper, we introduce a cross-layer design framework to the multiple access problem in contention-based wireless ad hoc networks. The motivation for this study is twofold, limiting multiuser interference to increase single-hop throughput and reducing power consumption to prolong battery life. We focus on next neighbor transmissions where nodes are required to send information packets to their respective receivers subject to a constraint on the signal-to-interference-and-noise ratio. The multiple access problem is solved via two alternating phases, namely scheduling and power control. The scheduling algorithm is essential to coordinate the transmissions of independent users in order to eliminate strong levels of interference (e.g., self-interference) that cannot be overcome by power control. On the other hand, power control is executed in a distributed fashion to determine the admissible power vector, if one exists, that can be used by the scheduled users to satisfy their single-hop transmission requirements. This is done for two types of networks, namely time-division multiple-access (TDMA) and TDMA/code-division multiple-access wireless ad hoc networks.

Performance evaluation of safety applications over DSRC vehicular ad hoc networks

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

Joint scheduling and power control for wireless ad-hoc networks

In this paper, we introduce a cross-layer design framework to the multiple access problem in contention-based wireless ad hoc networks. The motivation for this study is twofold, limiting multiuser interference to increase single-hop throughput and reducing power consumption to prolong battery life. We focus on next neighbor transmissions where nodes are required to send information packets to their respective receivers subject to a constraint on the signal-to-interference-and-noise ratio. The multiple access problem is solved via two alternating phases, namely scheduling and power control. The scheduling algorithm is essential to coordinate the transmissions of independent users in order to eliminate strong levels of interference (e.g., self-interference) that cannot be overcome by power control. On the other hand, power control is executed in a distributed fashion to determine the admissible power vector, if one exists, that can be used by the scheduled users to satisfy their single-hop transmission requirements. This is done for two types of networks, namely time-division multiple-access (TDMA) and TDMA/code-division multiple-access wireless ad hoc networks.

Power management for throughput enhancement in wireless ad-hoc networks

We introduce the notion of power management within the context of wireless ad-hoc networks. More specifically, we investigate the effects of using different transmit powers on the average power consumption and end-to-end network throughput in a wireless ad-hoc environment. This power management approach would help in reducing the system power consumption and hence prolonging the battery life of mobile nodes. Furthermore, it improves the end-to-end network throughput as compared to other ad-hoc networks in which all mobile nodes use the same transmit power. The improvement is due to the achievement of a tradeoff between minimizing interference ranges, reduction in the average number of hops to reach a destination, reducing the probability of having isolated clusters, and reducing the average number of transmissions (including retransmissions due to collisions). The protocols would first dynamically determine an optimal connectivity range wherein they adapt their transmit powers so as to only reach a subset of the nodes in the network. The connectivity range would then be dynamically changed in a distributed manner so as to achieve the near optimal throughput. Minimal power routing is used to further enhance performance. Simulation studies are carried out in order to investigate these design approaches. It is seen a network with such a power managed scheme would achieve a better end-to-end throughput performance (about 10% improvement with a slotted aloha MAC protocol) and lower transmit power (about an 80% Improvement) than a network without such a scheme.

Cooperative collision warning using dedicated short range wireless communications

The emergence of the 802.11a-based Dedicated Short Range Communications (DSRC) standard and advances in mobile ad hoc networking create ample opportunity for supporting delay-critical vehicular safety applications in a secure, resource-efficient, and reliable manner. In this paper, we focus on the suitability of DSRC for a class of vehicular safety applications called Cooperative Collision Warning (CCW), where vehicles periodically broadcast short messages for the purposes of driver situational awareness and warning. First, we present latency and success probability results of Forward Collision Warning (FCW) applications over DSRC. Second, we explore two design issues that are highly relevant to CCW applications, namely performance trends with distance and potential avenues for broadcast enhancements. Simulation results reveal interesting insights and trade-offs related to application-perceived latency and packet success probability performance. For instance, we conjecture the existence of an optimal broadcast rate that minimizes our novel latency measure for safety applications, and we characterize it for plausible scenarios.

Towards characterising and classifying communication-based automotive applications from a wireless networking perspective

Together, the Dedicated Short Range Communication (DSRC) and Vehicular Ad Hoc Network (VANET) technologies provide a unique opportunity to develop and introduce various types of communication- based automotive technologies to the marketplace. To date, many applications have been identified by the automotive community. Given the large number and diverse nature of these applications, it is advantageous to develop a systematic classification methodology to facilitate future DSRC and VANET research. Toward this objective, in this paper, we present a study that goes through two major steps: characterisation and classification. First, we focus on a set of representative applications and characterise them with respect to plausible application- and networking-related attributes. The characterisation process not only strengthens our understanding of the applications but also sets the stage for the classification step since it reveals numerous application commonalities. Thus, we have categorised the given applications into seven generic classes, with the consideration of balancing the trade-off between exploiting as many application similarities as possible while preserving their salient differences. This is of paramount importance to facilitate performance analysis of newly designed protocols. Finally, we have identified key performance metrics for each class of applications, which, we hope, could bridge the gap between the automotive and wireless networking communities. Accordingly, the proposed classes are envisioned to play a dual-role: facilitate application simulation and performance evaluation and guide DSRC and VANET protocol research and development.

Performance evaluation of multiple access protocols for ad hoc networks using directional antennas

In this paper, we introduce a novel reservation based multiple access protocol for ad hoc networks using directional antennas. First, we investigate the limitations of the extreme reservation schemes, namely omni-directional and directional reservations. We highlight the trade-off between spatial reuse (favors directional reservation) and control/data packet collisions (favors omni-directional reservation). Next, we show that the so-called hybrid reservation schemes fail to balance the trade-off as well. Therefore, we introduce a novel algorithm that balances the aforementioned trade-off via sending reservation messages that carry information about the required direction of transmission, in all unblocked directions. In addition, we introduce candidate techniques for handling new types of collisions inherent to directional antennas. Finally, we conduct a simulation study that shows considerable performance gains of the proposed scheme over the omni-directional, directional, and hybrid reservation paradigms.

High-availability free space optical and RF hybrid wireless networks

We introduce hybrid free-space optical and RF wireless links as potential technology for designing next-generation broadband wireless networks. We present various design challenges and potential solutions for real-time link performance characterization and adaptation for enhanced performance during adverse weather conditions. First, we introduce the hybrid wireless architecture and emphasize its significant role in achieving ubiquitous carrier-grade wireless connectivity. Second, we propose a link monitoring scheme that accurately reflects the performance of optical wireless links under various weather conditions. In addition, we examine the role of known link performance restoration schemes - power and data rate control. Third, we propose two novel link restoration schemes that efficiently utilize the hybrid architecture: dynamic load switching and multihop routing. Finally, the article describes an elaborate field testbed based on the hybrid architecture and various link restoration techniques. The dynamic load switching scheme is shown to have a profound impact on the overall hybrid link availability. The results, recorded from the experiments during extreme weather conditions, validate the impact of the hybrid architecture concept and conclusively prove the availability and reliability of the architecture in achieving sustained highspeed wireless connectivity.

Potential for Intra-Vehicle Wireless Automotive Sensor Networks

We propose using a wireless network to facilitate communications between sensors/switches and control units located within a vehicle. In a typical modern vehicle, the most demanding sensor will require a latency of approximately less than 1 msec with throughput of 12 kbps. Further, the network will need to support about 15 sensors with this requirement. The least demanding sensor will require a latency of approximately 50 msec with data throughput rate of 5 bps and will need to support about 20 of these types of devices. Initial part of this paper gives an overview of the issues spanning several layers of the protocol stack. Then, we focus on the Medium access control (MAC) layer and derive necessary design parameters based on given network requirements. We evaluate the IEEE 802.15.4 standard with respect to its suitability for use in a prospective intra-vehicle wireless sensor network.

DSRC Channel Fading Analysis from Empirical Measurement

Vehicle-to-vehicle communication research has gained tremendous interest in recent years due to its importance in facilitating future active safety and telematics applications. However, with limited literature available, there is lack of understanding for modeling mobile-to-mobile channels in vehicular environments. In this paper, we conduct preliminary channel fading statistical analysis of empirical measurement data from devices compatible with the emerging dedicated short range communications (DSRC) standard. In particular, we use the Nakagami distribution to analyze the received signal strength (i.e., RSSI) from empirical measurement, and characterize the fading statistics as a function of distance for vehicular operating environment. Our analysis reveals that fading can be approximated by Rician distribution within 100 m while it seems to follow Rayleigh distribution beyond 100 m. The outcome of the study could be used for vehicular network simulation.