On Performance Improvement Of The Physical Layer In Vehicular Ad Hoc Networks

With the increasing emergence of wireless communications technologies and their rapid expansion into most other technology sectors, the automotive industry is actively searching ways to use wireless communications to create a competitive advantage in the marketplace. In this context, vehicular ad-hoc networks (VANETs) have been used for many applications, such as safety, rescue, exploration, military, and communication redundancy systems in non-populated areas, as well as its ordinary applications in urban and highways environments as an essential part of intelligent transport system (ITS). Unfortunately, the penetration of this technology is still weak and much research remains to be done, to bring alive the vision of scalable, reliable, efficient VANETs. The quality of communication (QoC) primarily depends on the reliability of the physical layer (PHY) which encountered many issues and challenges in VANET. The high mobility of the vehicles causes substantial fragmentation and topology variability in VANET. The performance of the PHY is one of the important factors that play a key role toward improving the communication quality. The PHY must address the limited bandwidth and the applications requirements as well as the unstable and fragmented network topology.The major research and objective of this dissertation is to bring to attention the physical layer issues, challenges, and research opportunities related to the PHY in various wireless communication applications of VANETs. The main contributions and innovation are as follows.1. Add a considerable improvement to the performance of the communication system in VANETs by exploiting theoretical methods to develop new schemes which improve the performance of the OFDM system in the PHY without increasing the bandwidth or the cost and complexity of the system. This work presents a general model to describe the transmission process of the PHY in vehicular communication scenarios. The proposed model tackles some challenging characteristics of vehicular communication and theoretically investigates the communication behavior in different transmission scenarios. The PHY transmission process, which composed of many intricate steps is theoretically described in this model. The structure of this model corresponds to the PHY of the IEEE802.11p. The proposed PHY model also describes the characteristics of the vehicular radio channel. Simplified channel models which determine the most important characteristics of the channels in parametric form are developed. The proposed model is later used to evaluate the performance and study the challenges of the PHY, and consequently to improve the overall communication efficiency in vehicular networks.2. Utilize algebraic lattice theory and other mathematical theories to achieve performance improvement for the OFDM system that can directly lead to significant improvement in the performance of the PHY in VANET. To improve the communication reliability, a multi-order OFDM frequency diversity approach appropriate for vehicular networks is proposed using properties of order theory and Hamming distance. The frequency diversity is obtained by specifying proper correlations among the transmitted symbols. As sub-channels experience independent fading, at least one of the symbols may have robust signal, which can be used by the receiver to detect other symbols.3. Proposes a novel symbol timing synchronization technique for vehicular networks, by exploiting the power difference between OFDM subcarriers and the phase shift due to timing errors. The proposed scheme minimizes both the ICI and ISI interferences which results due to symbol time offset (STO) using three deferent approaches. In the first approach, the phase shift in the subcarrier that caused by the timing error is monitored through the first received OFDM symbol to estimate the timing offset. In this sense, this estimator exploits the phase shift due to timing error to estimate the timing error. Hence, we referred to it throughout this dissertation as the phase shift monitoring estimator (PSME). The second approach minimizes the power difference between adjacent subcarriers within the same OFDM symbol based on the assumption that the channel response on those subcarriers is approximately equal. This estimator utilizes the subcarriers only in the frequency domain, hence, it is referred to as the frequency domain accumulative power difference estimator (FD-APDE). The third estimator is designed by measuring the power difference between subcarriers with equal index values in two consecutive OFDM symbols. This estimator is derived based on the assumption that the channel is changing slowly in the time domain with respect to two consecutive OFDM symbols. Therefore, it is denoted as the time domain accumulative power difference estimator (TD-APDE). The proposed estimators are totally blind because the pre-knowledge of the transmitted data or the channel state information (CSI) are not required. However, the presence of these information greatly enhances the performance of these estimators. The proposed estimators are combined to complement each other for providing timing synchronization in different VANET’ scenarios.4. The channel estimation in VANET using least square (LS) and MMSE channel estimator is comprehensively investigated, and a new approach is developed to enhance the PHY performance, which consequently, increase the QoC in VANET. The modified technique combined both LS and MMSE with DFT and SVD for better performance improvements and complexity reduction.5. This work also exploits the randomness inherent of the PHY to provide a security solution for vehicular network. It investigates the capabilities of exploiting the noise and interference, which are usually seen as handicaps, as a source for the randomness in the security system of the vehicular networks. A secret key establishment technique for vehicular networks using the special properties and randomness inherent of the wireless channel is presented. The variation over time of the Received Signal Strength (RSS), which is caused by the vehicles’ movement, noise and multipath fading, is quantized and exploited for generating a secret key. Our comprehensive simulations show that the proposed key extraction technique suits VANETs rather than other communication systems, owing to the multi environments operation of VANET that causes more randomness. The proposed approach can extract high or low rate secret key with high entropy rate and less information exchange between legitimate vehicles. The extracted secret key can be employed in many security systems to support providing of security services in VANET.