Abstract-Emerging applications rely on wireless broadcast to disseminate time-critical information. For example, vehicular networks may exchange vehicle position and velocity information to enable safety applications. The number of nodes in one-hop communication range in such networks can be very large, leading to congestion and undesirable levels of packet collisions. Earlier work has examined such broadcasting protocols primarily from a MAC perspective and focused on selective aspects such as packet error rate. In this work, we propose a more comprehensive metric, the average system information age, which captures the requirement of such applications to maintain current state information from all other nearby nodes. We show that information age is minimized at an optimal operating point that lies between the extremes of maximum throughput and minimum delay. Also, via simulations we show that it cannot be achieved in 802.11 networks through pure MAC techniques such as contention window adaptation. This motivates our design of an applicationlayer broadcast rate adaptation algorithm. It uses local decisions at nodes in the network to adapt their messaging rate to keep the system age to a minimum. Our simulations and experiments with 300 ORBIT nodes show that the algorithm effectively adapts the messaging rates and minimizes the system age.

IEEE 802.11p is an emerging standard designed to provide wireless access in a vehicular environment. A major application of 802.11p-based Dedicated Short Range Communications (DSRC) is Vehicle to Vehicle (V2V) safety messaging. The US FCC has designated 75 MHz of bandwidth in the 5.9GHz band to support DSRC with a maximum permissible transmit power level of 33 dBm. In this paper we study the relationship between transmission power and packet reception in realistic propagation environments in an effort to understand the impact transmission power has on the performance of safety applications. The study is based on field tests with vehicles communicating using prototype DSRC radios.

In the US, the Dedicated Short Range Communications (DSRC) spectrum is organized into several channels. IEEE 1609.4 [2] defines an alternating-access channel switching scheme to enable a DSRC device to support applications concurrently on different channels. The default channel for Vehicle-to-Vehicle (V2V) safety messages is the Control Channel (CCH), but previous research has identified performance issues with this safety communication model. This paper proposes three alternative approaches for DSRC V2V safety communication in a multi-channel environment, all of which improve performance compared to the basic IEEE 1609.4 approach, while requiring only minor changes to the standards. The alternative approaches and the basic IEEE 1609.4 approach are evaluated via ns-2 simulations.

Abstract-Many vehicular safety applications rely on vehicles periodically broadcasting their position information and location trace. In very dense networks, such safety messaging can lead to offered traffic loads that saturate the shared wireless medium. One approach to address this problem is to reduce the frequency of location update messages when the movements of a vehicle can be predicted by nearby vehicles. In this paper, we study how predictable vehicular locations are, given a Global Positioning System trace of a vehicles recent path. We empirically evaluate the performance of linear and higher degree polynomial prediction algorithms using about 2500 vehicle traces collected under urban and highway driving conditions. We find that linear polynomial prediction using the two most recent known locations performs best. Also, traces with a time granularity of 0.2s are highly predictable in low speed urban environments, and a location update rate of 1Hz may suffice to represent urban vehicular movements. Lastly, the paper also evaluates compression of different time-granularity traces using line simplification and polynomial interpolation techniques to reduce message sizes.