Measurement Based Channel Characterization and Modeling for Vehicle-to-Vehicle Communications

Vehicle-to-Vehicle (V2V) communication is a challenging but fast growing technology that has potential to enhance traffic safety and efficiency. It can also provide environmental benefits in terms of reduced fuel consumption. The effectiveness and reliability of these applications highly depends on the quality of the V2V communication link, which rely upon the properties of the propagation channel. Therefore, understanding the properties of the propagation channel becomes extremely important. This thesis aims to fill some gaps of knowledge in V2V channel research by addressing four different topics. The first topic is channel characterization of some important safety critical scenarios (papers I and II). Second, is the accuracy or validation study of existing channel models for these safety critical scenarios (papers III and IV). Third, is about channel modeling (paper V) and, the fourth topic is the impact of antenna placement on vehicles and the possible diversity gains. This thesis consists of an introduction and six papers: Paper I presents a double directional analysis of vehicular channels based on channel measurement data. Using SAGE, a high-resolution algorithm for parameter estimation, we estimate channel parameters to identify underlying propagation mechanisms. It is found that, single-bounce reflections from static objects are dominating propagation mechanisms in the absence of line-of-sight (LOS). Directional spread is observed to be high, which encourages the use of diversity-based methods. Paper II presents results for V2V channel characterization based on channel measurements conducted for merging lanes on highway, and four-way street intersection scenarios. It is found that the merging lane scenario has the worst propagation condition due to lack of scatterers. Signal reception is possible only with the present LOS component given that the antenna has a good gain in the direction of LOS. Thus designing an antenna that has an omni-directional gain, or using multiple antennas that radiate towards different directions become more important for such safety critical scenarios. Paper III presents the results of an accuracy study of a deterministic ray tracing channel model for vehicle-to-vehicle (V2V) communication, that is compared against channel measurement data. It is found that the results from measurement and simulation show a good agreement especially in LOS situations where as in NLOS situations the simulations are accurate as far as existing physical phenomena of wave propagation are captured by the implemented algorithm. Paper IV presents the results of a validation study of a stochastic NLOS pathloss and fading model named VirtualSource11p for V2V communication in urban street intersections. The reference model is validated with the help of independent channel measurement data. It is found that the model is flexible and fits well to most of the measurements with a few exceptions, and we propose minor modifications to the model for increased accuracy. Paper V presents a shadow fading model targeting system simulations based on channel measurements. The model parameters are extracted from measurement data, which is separated into three categories; line-of-sight (LOS), LOS obstructed by vehicles (OLOS), and LOS blocked by buildings (NLOS), with the help of video information recorded during the measurements. It is found that vehicles obstructing the LOS induce an additional attenuation in the received signal power. The results from system level vehicular ad hoc network (VANET) simulations are also presented, showing that the LOS obstruction affects the packet reception probability and this can not be ignored. Paper VI investigates the impact of antenna placement based on channel measurements performed with four omni-directional antennas mounted on the roof, bumper, windscreen and left-side mirror of the transmitter and receiver cars. We use diversity combining methods to evaluate the performance differences for all possible single-input single-output (SIMO), multiple-input single-output (MISO) and multiple-input multiple-output (MIMO) link combinations. This investigation suggests that a pair of antennas with complementary properties, e.g., a roof mounted antenna together with a bumper antenna is a good solution for obtaining the best reception performance, in most of the propagation environments. In summary, this thesis describes the channel behavior for safety-critical scenarios by statistical means and models it so that the system performance can be assessed in a realistic manner. In addition to that the influence of different antenna arrangements has also been studied to exploit the spatial diversity and to mitigate the shadowing effects. The presented work can thus enable more efficient design of future V2V communication systems

Node Density Estimation in VANETs Using Received Signal Power

Accurately estimating node density in Vehicular Ad hoc Networks, VANETs, is a challenging and crucial task. Various approaches exist, yet none takes advantage of physical layer parameters in a distributed fashion. This paper describes a framework that allows individual nodes to estimate the node density of their surrounding network independent of beacon messages and other infrastructure-based information. The proposal relies on three factors: 1) a discrete event simulator to estimate the average number of nodes transmitting simultaneously; 2) a realistic channel model for VANETs environment; and 3) a node density estimation technique. This work provides every vehicle on the road with two equations indicating the relation between 1) received signal strength versus simultaneously transmitting nodes, and 2) simultaneously transmitting nodes versus node density. Access to these equations enables individual nodes to estimate their real-time surrounding node density. The system is designed to work for the most complicated scenarios where nodes have no information about the topology of the network and, accordingly, the results indicate that the system is reasonably reliable and accurate. The outcome of this work has various applications and can be used for any protocol that is affected by node density

Vehicular Communication in Obstructed and Non Line-of-Sight Scenarios

Since the invention of the first car available to masses, the 1908 Ford Model T, technology has advanced towards making car travel safer for occupants and bystanders. In recent years, wireless communication has been introduced in the vehicular industry as a means to avoid accidents and save lives.Wireless communication may sometimes be challenging due to obstacles in the physical world that interact with wireless signals. Such obstacles may be dynamic, e.g. other vehicles in the traffic flow, or static, e.g. nearby buildings. Two scenarios are defined to describe those cases. The obstructed line-of-sight (OLOS) scenario is described as the case where a smaller obstacle, usually a vehicle, is placed in-between a transmitter and a receiver. This obstacle usually partially blocks communication and the receiver often moves in an out of the line-of-sight. The non line-of-sight (NLOS) scenario is described as the case where a larger obstacle completely blocks communication between a transmitter and a receiver. An example would be a building at an intersection which shadows the communication between two vehicles. In this thesis the OLOS and NLOS scenarios are investigated from different points of view.In chapter 2, a road side unit (RSU) that has been constructed and evaluated for integrating older vehicles without wireless communication with newer vehicles using wireless communication is described. Older vehicles are being detected using a universal medium-range radar and their position and speed vectors are broadcasted wirelessly to newer vehicles. Tests have been performed by using the system in parallel with wireless enabled vehicles; by comparing the content in the messages obtained from both systems, the RSU has been found to perform adequately. Accuracy tests have been performed on the system and Kalman filtering has been applied to improve the accuracy even further.Chapter 3 focuses on the OLOS scenario. A truck as an obstacle for wireless vehicular communication is being investigated. Real life measurements have been performed to characterize and model the wireless channel around the truck. The distance dependent path loss and additional shadowing loss due to the truck is analyzed through dynamic measurements. The large scale fading, delay and Doppler spreads are characterized as a measure of the channel dispersion in the time and frequency domains. It has been found that a truck as an obstacle reduces the received power by 12 and 13 dB on average in rural and highway scenarios, respectively. Also, the dispersion in time and frequency domains is highly increased when the line-of-sight is obstructed by the truck. A model for power contributions due to diffraction around the truck has also been proposed and evaluated using the previously mentioned real life measurements. It has been found that communication may actually be possible using solely diffraction around a truck as a propagation mechanism.Finally, in chapter 4 a wireless channel emulator that has been constructed and evaluated is described. Modem manufacturers face a challenge when designing and implementing equipment for highly dynamic environments found in vehicular communication. For testing and evaluation real-life measurements with vehicles are required, which is often an expensive and slow process. The channel emulator proposed is designed and implemented using a software defined radio (SDR). The emulator together with the proposed test methodology enables quick on-bench evaluation of wireless modems. It may also be used to evaluate modem performance in different NLOS and OLOS scenarios

Measurement of TOA Using Frequency Domain Techniques for Indoor Geolocation

Frequency domain techniques have been widely used in indoor radio propagation measurements and modeling for telecommunication applications. This work addresses measurement of the time of arrival (TOA) of the first path for geolocation applications using results of frequency domain channel measurements. First, we analyze the effect upon TOA measurement accuracy due to: sampling period of the radio channel in the frequency domain, sampling period in the time domain used for detection of the TOA and the windowing filter used before transformation to the time domain. Then, we provide some results of measurement made in line of sight (LOS) and Obstructed LOS (OLOS) indoor environments in order to compare the characteristics of the measured TOA in these two important scenarios for indoor geolocation applications. Finally, we compare the measurement results with the ray tracing based model that had been developed previously for indoor geolocation applications

Coverage prediction and optimization algorithms for indoor environments

A heuristic algorithm is developed for the prediction of indoor coverage. Measurements on one floor of an office building are performed to investigate propagation characteristics and validations with very limited additional tuning are performed on another floor of the same building and in three other buildings. The prediction method relies on the free-space loss model for every environment, this way intending to reduce the dependency of the model on the environment upon which the model is based, as is the case with many other models. The applicability of the algorithm to a wireless testbed network with fixed WiFi 802.11b/g nodes is discussed based on a site survey. The prediction algorithm can easily be implemented in network planning algorithms, as will be illustrated with a network reduction and a network optimization algorithm. We aim to provide an physically intuitive, yet accurate prediction of the path loss for different building types

Performance of TOA Estimation Algorithms in Different Indoor Multipath Conditions

Using Time of Arrival (TOA) as ranging metric is the most popular technique for accurate indoor positioning. Accuracy of measuring the distance using TOA is sensitive to the bandwidth of the system and the multipath condition between the wireless terminal and the access point. In a telecommunication-specific application, the channel is divided into Line of Sight (LOS) and Obstructed Line of Sight (OLOS) based on the existence of physical obstruction between the transmitter and receiver. In indoor geolocation application, with extensive multipath conditions, the emphasis is placed on the behavior of the first path and the channel conditions are classified as Dominant Direct Path (DDP), Nondominant Direct Path (NDDP) and Undetected Direct Path (UDP). In general, as the bandwidth increases the distance measurement error decreases. However, for the so called UDP conditions the system exhibits substantially high distance measurement errors that can not be eliminated with the increase in the bandwidth of the system. Based on existing measurements performed in CWINS, WPI a measurement database that contains adequate number of measurement samples of all the different classification is created. Comparative analysis of TOA estimation in different multipath conditions is carried out using the measurement database. The performance of super-resolution and traditional TOA estimation algorithms are then compared in LOS, OLOS DDP, NDDP and UDP conditions. Finally, the analysis of the effect of system bandwidth on the behavior of the TOA of the first path is presented

Distance Measurement Error Modeling for Time-of-Arrival Based Indoor Geolocation

In spite of major research initiatives by DARPA and other research organizations, precise indoor geolocation still remains as a challenge facing the research community. The core of this challenge is to understand the cause of large ranging errors in estimating the time of arrival (TOA) of the direct path between the transmitter and the receiver. Results of wideband measurement in variety of indoor areas reveal that large ranging errors are caused by severe multipath conditions and frequent occurrence of undetected direct path (UDP) situations. Empirical models for the behavior of the ranging error, which we refer to as the distance measurement error (DME), its relation to the distance between the transmitter and the receiver and the bandwidth of the system is needed for development of localization algorithms for precise indoor geolocation. The main objective of this dissertation is to design a direct empirical model for the behavior of the DME. In order to achieve this objective we provide a framework for modeling of DME, which relates the error to the distance between the transmitter and the receiver and bandwidth of the system. Using this framework we first designed a set of preliminary models for the behavior of the DME based on the CWINS proprietary measurement calibrated ray-tracing simulation tool. Then, we collected a database of 2934 UWB channel impulse response measurements at 3-8GHz in four different buildings to incorporate a variety of building materials and architectures. This database was used for the design of more in depth and realistic models for the behavior of the DME. The DME is divided into two components, Multipath-DME (MDME) and UDP-DME (UDME). Based on the empirical data, models for the behavior of each of these components are developed. These models reflect the sensitivity to bandwidth and show that by increasing the bandwidth MDME decreases. However in UDME the behavior is complicated. At first it reduces as we increase the bandwidth but after a certain bandwidth it starts to increase. In addition to these models through an analysis on direct path power versus the total power the average probability of having a UDP was calculated