Michael Hempel

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 Survey of Energy-Efficient Compression and Communication Techniques for Multimedia in Resource Constrained Systems

Advances in wireless multimedia communication technologies enable new types of pervasive and ubiquitous applications such as mobile health care, environmental monitoring, facility monitoring and traffic surveillance. Among different factors concerned, energy efficiency is one of the most challenging issues in multimedia communication due to the resource constraints, and the requirements for high bandwidth and low transmission delay. In this survey, we provide a broad picture of the state-of-the-art energy efficient techniques that have been proposed in wireless multimedia communication for resource-constrained systems such as wireless sensor networks and mobile devices. Following the essential stages required for multimedia communication, we categorize these techniques into two groups: multimedia compression techniques and multimedia transmission techniques. In the first group, we introduce the state-of-the-art compression algorithms and perform analyses and evaluations on energy efficiency in applying these compression algorithms to resource-constrained multimedia transmission systems. In the second group, we will further categorize the energy efficient transmission techniques into two sub-categories according to their different communication architectures. We review both cross-layer communication, including Unequal Error Protection (UEP), and independent-layer communication, focusing on Routing, MAC, and Physical layer protocols. We present the basic problem statement and objectives of these techniques, and survey multiple potential approaches that have been reported in the literature. Our focus in this survey is to provide insight into different research directions to improve energy efficiency in wireless multimedia communication protocols for future developments.

Index-Based Selective Audio Encryption for Wireless Multimedia Sensor Networks

Wireless multimedia sensor networks (WMSNs) support many acoustic applications for audio surveillance, animal tracking/vocalization, human health monitoring, etc. However, resource constraints in sensor networks (such as limited battery power, bandwidth/computation capability, etc.) pose challenges for the quality and security of audio data transmission and processing. The security is a critical issue since audio information can be accessed or even manipulated in WMSNs. In order to ensure security, audio quality and energy efficiency, we propose an index-based selective audio encryption scheme for WMSNs. The scheme protects data transmissions by incorporating both resource allocation and selective encryption based on modified discrete cosine transform (MDCT). In this proposed scheme, the audio data importance is leveraged using the MDCT audio index, and wireless audio data transmission proceeds with energy efficient selective encryption. The simulation results show that the proposed approach offers a significant gain in terms of energy efficiency, encryption performance and audio transmission quality.

On Energy Efficient Encryption for Video Streaming in Wireless Sensor Networks

Selective encryption for video streaming was proposed for efficient multimedia content protection. However, the issues on joint optimization of video quality, content protection, and communication energy efficiency in a wireless sensor network (WSN) have not been fully addressed in the literature. In this paper, we propose a scheme to optimize the energy, distortion, and encryption performance of video streaming in WSNs. The contribution of this work is twofold. First, a channel-aware selective encryption approach is proposed to minimize the extra encryption dependency overhead at the application layer. Second, an unequal error protection (UEP)-based network resource allocation scheme is proposed to improve the communication efficiency at the lower layers. Simulation experiments demonstrate that the proposed joint selective encryption and resource allocation scheme can improve the video transmission quality significantly with guaranteed content protection and energy efficiency.

Secure Stochastic ECG Signals Based on Gaussian Mixture Model for

The blood circulation system in a human body provides a unique and natural trust zone for secure data communications in wireless healthcare systems such as body area networks. Unfortunately, biometric signal authentication using physiological attributes in wireless healthcare has not been extensively studied. In this paper, we propose a data authentication approach utilizing electrocardiography (ECG) signal patterns for reducing key exchange overhead. The major contribution of this research is to apply stochastic pattern recognition techniques in wireless healthcare. In the proposed approach, the inter-pulse interval (IPI) signal pattern at transmitter side is summarized as a biometric authentication key using Gaussian mixture model (GMM). At the receiver side, a light-weight signature verification scheme is adopted that uses IPI signals gathered locally at the receiver. The proposed authentication scheme has the advantage of high sample misalignment tolerance. In our earlier work, we had demonstrated the concept of stochastic authentication for ECG signal, but the signature verification process and GMM authentication performance under time synchronization and various sample points were not discussed. Here, we present a new set of analytical and experimental results to demonstrate that the proposed stochastic authentication approach achieves a low half total error rate in ECG signals verification.

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With the technological advancement in body area sensor networks (BASNs), low cost high quality electrocardiographic (ECG) diagnosis systems have become important equipment for healthcare service providers. However, energy consumption and data security with ECG systems in BASNs are still two major challenges to tackle. In this study, we investigate the properties of compressed ECG data for energy saving as an effort to devise a selective encryption mechanism and a two-rate unequal error protection (UEP) scheme. The proposed selective encryption mechanism provides a simple and yet effective security solution for an ECG sensor-based communication platform, where only one percent of data is encrypted without compromising ECG data security. This part of the encrypted data is essential to ECG data quality due to its unequally important contribution to distortion reduction. The two-rate UEP scheme achieves a significant additional energy saving due to its unequal investment of communication energy to the outcomes of the selective encryption, and thus, it maintains a high ECG data transmission quality. Our results show the improvements in communication energy saving of about 40%, and demonstrate a higher transmission quality and security measured in terms of wavelet-based weighted percent root-mean-squared difference.

-Healthcare Systems

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.

Assurance of Energy Efficiency and Data Security for ECG Transmission in BASNs

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.

A wireless test bed for mobile 802.11 and beyond

Long Term Evolution (LTE) has been introduced by 3GPP (3rd Generation Partnership Project) and is poised to dominate the 4th generation (4G) of mobile telecommunication networks. In this article, we present an in-depth analysis of the LTE physical layers characteristics and its performance. Our work is unique in providing a detailed performance study based on Release 8 of the 3GPP standard. Several works have discussed the LTE performance; however, most have been restricted to limited scenarios. Our work shows a more comprehensive investigation of the maximum data throughput under different conditions and scenarios. Our performance study includes TDD and FDD operational modes for uplink and downlink transmissions in different antenna diversity schemes, data modulation, and code rates. Our results show that LTE (3GPP-Release 8) supports downlink throughputs of up to 300Mbps and for the uplink a throughput of up to 75 Mbps.

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

Numerous MAC protocols have been proposed for stationary wireless sensor networks (WSNs). However, there have been very few approaches proposed to make the MAC layer in WSNs suitable for mobile scenarios. We propose a new MAC layer protocol called MOBMAC to support mobility in WSNs. MOBMAC uses an adaptive frame size approach to overcome the effect of frame losses caused by the Doppler shifts in mobile scenarios. An extended Kalman filter (EKF) is used to predict the frame size for each transmission, which enhances the energy efficiency of the system and minimizes latencies. Our study shows that under mobile scenarios, the MOBMAC reduces energy consumption by 60% and shows decrease of 25% in latency in comparison with the well known base protocol