Oliver Klemp

Vehicular Channel Characterization and Its Implications for Wireless System Design and Performance

To make transportation safer, more efficient, and less harmful to the environment, traffic telematics services are currently being intensely investigated and developed. Such services require dependable wireless vehicle-to-infrastructure and vehicle-to-vehicle communications providing robust connectivity at moderate data rates. The development of such dependable vehicular communication systems and standards requires accurate models of the propagation channel in all relevant environments and scenarios. Key characteristics of vehicular channels are shadowing by other vehicles, high Doppler shifts, and inherent nonstationarity. All have major impact on the data packet transmission reliability and latency. This paper provides an overview of the existing vehicular channel measurements in a variety of important environments, and the observed channel characteristics (such as delay spreads and Doppler spreads) therein. We briefly discuss the available vehicular channel models and their respective merits and deficiencies. Finally, we discuss the implications for wireless system design with a strong focus on IEEE 802.11p. On the road towards a dependable vehicular network, room for improvements in coverage, reliability, scalability, and delay are highlighted, calling for evolutionary improvements in the IEEE 802.11p standard. Multiple antennas at the onboard units and roadside units are recommended to exploit spatial diversity for increased diversity and reliability. Evolutionary improvements in the physical (PHY) and medium access control (MAC) layers are required to yield dependable systems. Extensive references are provided.

Radio Channel Measurements at Street Intersections for Vehicle-to-Vehicle Safety Applications

This paper presents the results of an empirical study of wireless propagation channels for vehicle-to-vehicle communications in street intersections, a scenario especially important for collision avoidance applications. The results are derived from a channel measurement campaign performed at 5.6 GHz in four different types of urban intersections. We present results on typical power delay profiles, pathloss and delay spreads and discuss important propagation mechanisms. By comparing the results of the different intersections, we find that absence of line-of-sight is problematic for system coverage, especially when there are few other significant scattering objects in and around the intersection. Roadside buildings can create important propagation paths that account for a considerable part of the total received power.

5.9 GHz inter-vehicle communication at intersections: a validated non-line-of-sight path- loss and fading model

Inter-vehicle communication promises to prevent accidents by enabling applications such as cross-traffic assistance. This application requires information from vehicles in non-line-of-sight (NLOS) areas due to building at intersection corners. The periodic cooperative awareness messages are foreseen to be sent via 5.9 GHz IEEE 802.11p. While it is known that existing micro-cell models might not apply well, validated propagation models for vehicular 5.9 GHz NLOS conditions are still missing. In this article, we develop a 5.9 GHz NLOS path-loss and fading model based on real-world measurements at a representative selection of intersections in the city of Munich. We show that (a) the measurement data can very well be fitted to an analytical model, (b) the model incorporates specific geometric aspects in closed-form as well as normally distributed fading in NLOS conditions, and (c) the model is of low complexity, thus, could be used in large-scale packet-level simulations. A comparison to existing micro-cell models shows that our model significantly differs.

Real-world measurements of non-line-of-sight reception quality for 5.9GHz IEEE 802.11p at intersections

Vehicular Dedicated Short Range Communication (DSRC) promises to reduce accidents by enabling assistance systems such as cross-traffic assistance. This application requires movement information from vehicles that are in Non-Line-Of-Sight (NLOS) due to buildings at intersection corners. DSRC is foreseen to use IEEE 802.11p to deliver regular Cooperative Awareness Messages (CAM). Due to the high operational frequency of 5.9GHz, the ability to provide reliable NLOS reception is often put into question. We performed an extensive field test specifically targeted to measure DSRC NLOS reception quality. As a novelty, tested intersections were methodically selected to reflect typical urban intersections (in Munich) and the test setup was specifically designed to provide comparable and generalizable results. This allowed us to determine the influence of factors like building positions. The collected data shows that NLOS reception is possible. Reception rates stay mostly well above 50% for distances of 50 meters to intersection center with blocked LOS.

Overview of Vehicle-to-Vehicle Radio Channel Measurements for Collision Avoidance Applications

In this paper we present an overview of a vehicle-to-vehicle radio channel measurement campaign at 5.6GHz. The selected measurement scenarios are based on important safety-related applications. We explain why these scenarios are interesting from the aspect of radio propagation. Further we describe the power-delay profile and the Doppler spectral density of two situations especially suitable for collision avoidance applications: A traffic congestion situation where one car is overtaking another one, and a general line-of-sight obstruction between the transmitter and the receiver car. The evaluations show that in these situations the radio channel is highly influenced by the rich scattering environment. Most important scatterers are traffic signs, trucks, and bridges, whereas other cars do not significantly contribute to the multipath propagation.

A validated 5.9 GHz Non-Line-of-Sight path-loss and fading model for inter-vehicle communication

Inter-vehicle communication promises to prevent accidents by enabling applications such as cross-traffic assistance. This application requires information from vehicles in Non-Line-of-Sight (NLOS) areas due to building at intersection corners. The periodic Cooperative Awareness Messages (CAM) are foreseen to be sent via 5.9 GHz IEEE 802.11p. While it is known that existing micro-cell models might not apply well, validated propagation models for vehicular 5.9 GHz NLOS conditions are still missing. In this paper, we develop a 5.9 GHz NLOS path-loss and fading model based on real-world measurements at a representative selection of intersections in the city of Munich. We show that a) the measurement data can very well be fitted to an analytical model, b) the model incorporates specific geometric aspects in closed-form as well as normally distributed fading in NLOS, and c) the model is of low complexity, thus, could be used in large-scale packet-level simulations. A comparison to existing micro-cell models shows that our model significantly differs.

Safety on the Roads: LTE Alternatives for Sending ITS Messages

This article discusses different alternatives for sending intelligent transportation systems (ITS) messages using long-term evolution (LTE) networks. Specifically, it compares the unicast and evolved Multimedia Broadcast Multicast Services (eMBMS) transmission modes by means of system-level simulations and a cost modeling analysis. The optimum configuration of the eMBMS carrier is studied for the case of ITS services. This article also includes some recommendations on the configuration of the ITS server in charge of distributing safety messages as well as on its interaction with the mobile network operator (MNO). The results show that eMBMS is significantly more efficient in terms of resource consumption than the unicast mode, implying an important reduction of the delivery costs.

Automotive grade MIMO antenna setup and performance evaluation for LTE-communications

Multiple-input-multiple-output (MIMO) is a technique to achieve high data rates in mobile communication networks and is used by the Long-Term Evolution (LTE) standard. The integration of a two antenna MIMO system for usage in worldwide LTE networks into a conventional automotive roof top antenna is discussed. In-situ antenna measurement results of the final LTE MIMO configuration are shown. Furthermore initial test results of the integrated LTE antenna system in an 800 MHz live network are presented and a comparison to a reference system consisting of two quarter-wavelength monopole antennas with ideal spatial separation is provided.

Angle-Dependent Path Loss Measurements Impacted by Car Body Attenuation in 2.45 Ghz ISM Band

In the past few years more and more wireless applications (e.g. WLAN and Bluetooth) have migrated into the automotive domain. With the increasing number of devices, coexistence investigations in the 2.45 GHz ISM band are gaining importance to enable reliable communications with real time requirements like voice transmission and streaming applications. This paper presents initial results on angle dependent path loss measurements impacted by the influence of a car body on electro-magnetic wave propagation. To this effect, a measurement methodology was developed to gather the attenuation effects of the car according to different antenna positions and measurement angles. Various kinds of attenuation measurements were performed. Vehicle shell attenuation measurements were transformed into angular-dependent path loss metrics and related to vehicle-specific attenuation properties like window inserts, roof columns, trunk or engine hood. Focus of attention was drawn to free space attenuation and intra-vehicle attenuation measurements.

Analytical Approach for MIMO Performance and Electromagnetic Coupling in Linear Dipole Arrays

Multiple-input multiple-output (MIMO) antenna systems seem promising to establish wireless communication links with enhanced channel capacities and link budgets. One of the most challenging tasks in order to accurately predict the information theoretic channel capacities of next-generation wireless MIMO communication systems is the exact modeling of the interactions between real MIMO antenna configurations and suitable multipath channel scenarios. In order to derive analytical models that account for real antenna radiation patterns and means of pattern distortion by mutual coupling, a spherical eigenmode (SME) evaluation of antenna radiation patterns is presented. In conjunction with a statistical based channel model, the eigenmode expansion provides an efficient alternative to account for mutual coupling and to adopt an analytical approach in the calculation of antenna envelope correlation and MIMO channel capacity