Puttipong Mahasukhon

A Novel Adaptive Distributed Cooperative Relaying MAC Protocol for Vehicular Networks

Explosive growth in Information Technology has enabled many innovative application areas such as large-scale outdoor vehicular networks for vehicle-to-vehicle communications. By providing time-sensitive and location-aware information, vehicular networks can contribute to a safer and more efficient driving experience. However, the performance of vehicular networks requires robust and real-time data communications and is impacted by high mobility, intermittent connectivity, and unreliability of the wireless channel. In this paper, a novel adaptive distributed cooperative medium access control (ADC-MAC) protocol is proposed in order to address the inherent problems in the IEEE 802.11 standard when employed in vehicular networks. ADC-MAC exploits spatial diversities to maximize the system throughput as well as the service range of vehicular networks. This is accomplished through adaptively selecting the most suitable helper and transmission mode for transmit/receive pairs among direct transmission (DT), cooperative relay (CR) transmission and two-hop relay (TR) transmission, in accordance with the channel quality and the positioning of relay nodes. Both our Markov Chain modeling based theoretical analysis and ns-2 simulation experiments show that our ADC-MAC protocol outperforms existing schemes under the same network scenarios and maximizes the achieved system throughput and service distance.

A wireless test bed for mobile 802.11 and beyond

In this paper, we present our approach of establishing a wireless test bed as a part of our collaborations with the Federal Railroad Administration (FRA) for studying the performance of current and upcoming wireless technologies in a mobile railroad environment. The focus is on studying the impact of mobility on the wireless system throughput for moving trains with different velocities. We describe details of our test bed design including selection of the location, equipment as well as system topology and performance evaluations approach. We also present and discuss test results obtained from our test bed.

Evaluating saturation throughput performance of the IEEE 802.11 MAC under fading channels

Accurately modeling the IEEE 802.11 MAC is critical to design resource allocation, admission control and scheduling schemes for QoS provisioning in wireless LANs. However, it is difficult to model the saturation throughput performance of the IEEE 802.11 MAC due to the fact that the memory effect existing among different stages in the random backoff procedure. Therefore, Markov chain has been used to characterize the saturation throughput performance of the IEEE802.11 CSMA/CA protocol in many research efforts. However, one of the problems existing in the current research failed to consider all key practical issues altogether such as retry limit, backoff suspensions and the fading channel errors. Therefore, an accurate and realistic model of the IEEE 802.11 MAC protocol with fading channel errors is needed. In this work, we propose a new model based on Markov chain and validate its correctness by theoretical analyses and Ns-2 simulations. We also compared our results with other current major models. Our contributions in this work are: 1) taking the backoff suspension into considerations and derive a more accurate model than other current models; 2) introducing the fading channel errors into the modeling process and derive a close form equation of the saturation throughput performance of the 802.11 DCE simulation results show that the proposed model is same as accurate as other current models, and becomes much more accurate when considering fading channel errors.

Comparison of Throughput Performance for the IEEE 802.11a and 802.11g Networks

The 802. llg offers backward compatibility to 802.11b operating in 2.4 GHz with the same set of MAC parameters. However, this compatibility causes 802.1 lg to perform differently as 802.11a in terms of radio throughput performance, although both standards employ orthogonal frequency division multiplexing (OFDM) technology to achieve high data rate. In this paper, we compare and present an in- depth analysis of the throughput performance of both 802.11a and 802.1 lg standards. In order to compare the throughput performance of 802.11a and 802.1 lg, a Markov chain model of IEEE 802.11 MAC is adopted to evaluate the saturation throughput. Our analytical and experimental results show an inferior throughput performance for 802.1 lg even with the same OFDM technology as employed for 802.11a.

Distance and throughput measurements in mobile WiMAX test bed

In this paper, we present our approach of establishing a Mobile WiMAX test bed as a part of our collaborations with the Federal Railroad Administration (FRA) for studying the performance of current and upcoming broadband wireless technologies in a railroad environment. The focus is on studying the impact of mobility on the wireless system throughput for moving trains at high velocities. We describe details of our test bed and discuss test results obtained from our laboratory and test bed. Based on our measurements, we can achieve a throughput of approximately 2.9 Mbps with the most robust modulation scheme, a maximum distance of 11 kilometers, and a speed of 70 mph, making Mobile WiMAX a promising candidate for large-scale deployments in environments such as the railroad industry. Our goal is to provide a comprehensive measurement that will assist others in understanding Mobile WiMAX performance and guiding future network deployment.

BER Analysis of 802.11b Networks Under Mobility

Most of the current 802.11b research activities are conducted under stationary environment. As the need for high-speed connectivity in mobile environments increases, it becomes necessary to understand the impact of mobility on performance characteristics of 802.11b. In this paper, we present an investigation of the mobility impact on performance of the 802.11b system with Rician/Rayleigh fading under different client velocities. This includes analysis of the Doppler shift caused by the velocity of transmitter and receiver, the multipath interference due to reflections and diffractions from terrains in the radio service coverage area, and other serious impairing factors. We have also studied the bit error rate performances for various velocities with different data rates. Our analysis includes our analytical and simulation data verified with the results from our 3.5 mile Federal Railroad Administration test bed on the BNSF railroad track in Nebraska. Our results show that the current implementation of 802.11b standard has promising potential to be used in a mobile environment if the line-of-sight path between transmitter and receiver exists.

A study on energy efficient multi-tier multi-hop wireless sensor networks for freight-train monitoring

The North American freight railroad industry is trying to leverage wireless sensor networks (WSN) onboard railcars for advanced monitoring and alerting. In railroad environments, freight train WSNs exhibit a linear chain-like topology of significant length. Thus, existing wireless technologies such as the IEEE 802.15.4 communication protocol, based on a star topology, are unable to provide reliable service. The end-to-end communication between nodes generally relies on individual nodes communicating with their respective neighbors to carry the information over multiple hops and deliver it to the preferred destination. The routing performance and reliability significantly degrades with increasing number of hops. We proposed a multitier multi-hop network which is designed to overcome these issues in large-scale multi-hop WSNs in railroad environments. This approach has significant advantages, such as more data bandwidth, higher reliability, and lower energy consumption. Our analytical results show that the proposed multi-tier communication approach spends energy more efficiently and utilizes less resource than the traditional chain topology onboard freight trains.

Type I HARQ performance modeling and evaluation of mobile WiMAX for network simulators

While the development of Mobile WiMAX devices is still an ongoing process, complete and accurate simulations become more important in order to study the performance of Mobile WiMAX based broadband wireless access networks. To further improve network simulation models for Mobile WiMAX, we have theoretically modeled the Hybrid Automatic Retransmission reQuest (HARQ) mechanism and evaluated its performance and accuracy. In this paper, we present the design and implementation methodology of the Mobile WiMAX HARQ simulation model in system-level network simulators in order to gain the benefit of link quality improved by enabling HARQ. Our results show that HARQ achieves higher throughput even at low signal strength which can be considered as non-line-of-sight (NLOS) environments. We believe the implementation of HARQ in the network simulation model and its evaluation studies will provide profound understanding to Mobile WiMAX field test studies.

IEEE 802.11b based ad hoc networking and its performance in mobile channels

It is widely believed that IEEE 802.11 standard is aimed mainly for fixed indoor wireless local area networks and is not suited for mobile applications, even though the IEEE 802.11b systems may work in either infrastructure mode or ad hoc mode. The impact of node mobility on ad hoc network performance has already been studied intensively, but these studies mostly do not consider temporal fluctuations of the mobile wireless channel due to the Doppler shift. An investigation of the mobility impact on the performance of IEEE 802.11b ad hoc systems with Rician/Rayleigh fading under different node velocities is presented. A comprehensive and in-depth analysis of the impacts of a multitude of different signal distortions on an IEEE 802.11b system performance is also presented. Specifically, the authors study the bit-error rate performances with respect to node velocities for different modulation schemes. The simulation results show that, owing to its extremely low implementation and deployment cost, the current IEEE 802.11b standard has its potential to be deployed in a mobile ad hoc environment if the line-of-sight path between transmitter and receiver exists.

Performance Analysis for Direction of Arrival Estimating Algorithms

Smart antennas have emerged as one of the most promising directions in supporting maximum communication link throughput. In this paper, we have investigated the impact of smart antennas on a complex mobile network such as a railroad wireless communications system. The objective is to analyze the selection of a Direction-Of-Arrival (DOA) estimation algorithm which provides the maximum efficiency when deployed in our railroad testbeds for wireless vehicular communication. Our findings are discussed to provide an in-depth understanding of how different algorithms should be selected to support efficient network operations.