Down the Block and Around the Corner The Impact of Radio Propagation on Inter-vehicle Wireless Communication John S. Otto, Fabi´ an E. Bustamante, and Randall A. Berry Department of Electrical Engineering and Computer Science Northwestern University Evanston, United States {jotto,fabianb,rberry}@eecs.northwestern.edu Abstract Vehicular networks are emerging as a new distributed system environment with myriad possible applications. Most studies on vehicular networks are carried out via simulation, given the logistical and economical problems with large- scale deployments. This paper investigates the impact of realistic radio propagation settings on the evaluation of VANET-based systems. Using a set of instrumented cars, we collected IEEE 802.11b signal propagation measure- ments between vehicles in a variety of urban and suburban environments. We found that signal propagation between vehicles varies in different settings, especially between line- of-sight (“down the block”) and non line-of-sight (“around the corner”) communication in the same setting. Using a probabilistic shadowing model, we evaluate the impact of different parameter settings on the performance of an epidemic data dissemination protocol and discuss the im- plications of our findings. We also suggest a variation of a basic signal propagation model that incorporates additional realism without sacrificing scalability by taking advantage of environmental information, including node locations and street information. 1. Introduction Vehicular networks are emerging as a new distributed system environment with myriad possible applications that range from traffic information systems and road safety [1]– [5] to urban sensing and entertainment [6]. Vehicular ad- hoc networks (VANETs) provide infrastructureless, rapidly deployable, self-configurable network connectivity. The net- work is made of vehicles interconnected by wireless links and willing to store and forward data for their peers. As vehicles move freely and organize themselves arbitrarily, message routing is done dynamically based on network con- nectivity. Compared with other ad-hoc networks, VANETs are particularly challenging due in part to the vehicles’ high rate of mobility and the numerous signal-weakening obstructions, such as buildings, in their environments. Because of logistic and economic limitations, most studies on mobile ad-hoc networks are carried out via simulation, typically using models of wireless networking components layered above a discrete event simulator. Previous work has pointed out that many of these models may be based on “too simplistic” assumptions [7], [8] and some have tried to quan- tify the potential risks of such assumptions (e.g. about radio propagation models [9]–[11] or node mobility patterns [12], [13]). While more complex and realistic models are available (e.g. McKown and Hamilton’s ray-tracing [14]), their use is limited to relatively small networks and not practical for evaluating typically large-scale VANETs. This paper investigates the impact of more realistic con- figurations for radio propagation models on the evaluation of VANET-based systems. Using a set of instrumented vehicles, we experimentally characterize IEEE 802.11b signal propa- gation between vehicles in a variety of urban and suburban areas in the Chicago area. We found that signal propaga- tion varies depending on the environment, and especially between line-of-sight (LOS) and non-LOS (“Around-the- Corner”–ATC) communication in the same environment. For instance, we found that the effective communication window for two vehicles in an urban setting can be nearly 35% shorter than that in an open field (45 sec. to 70 sec.). Total throughput can vary between settings by as much as 42% (1.9 MB in an urban setting and 3.3 MB in an open field). To understand the implications of these variations on the performance of common VANET applications, we evaluate a simple epidemic protocol using parameter settings derived from our measurements. Epidemic-based data dissemination protocols are fairly well understood and commonly adopted by many applications in this domain [15]. For our study, we employ the JiST/SWANS wireless simulator [16] and rely on STRAW [13] to model vehicular mobility. We model radio propagation using the JiST/SWANS implementation of the probabilistic shadowing model with log-distance path loss [17]. We show that, while a single parameter setting may be sufficient to model simple, open-field like environments, it is not enough to accurately characterize more complex settings. In an open field environment, the ATC and LOS path loss