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Implementation of an Indoor Test Bed for the Evaluation of Car-To-Car Communication Services
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A promising approach for improving the capacity of Wireless Mesh Networks is by making use of multiple non-overlapping RF channels. Multi-channel protocols have the advantage that several devices can transmit in parallel within a collision domain on distinct channels. When using IEEE 802.11b/g/a most protocol designers assume 3 and 12 non-overlapping channels, respectively. However, this simplified assumption does not hold. We present results from measurements that show that the number of available non-interfering channels depends on the antenna separation, PHY modulation, RF band, traffic pattern and whether single- or multi-radio systems are used. The problem is caused by adjacent channel interference (ACI) where nearby transmitters "bleed over" to other frequencies and either cause spurious carrier sensing or frame corruption. For nearby transceivers, as in the factory defaults of multi-radio devices, this results in at most two non- interfering channels, one within 2.4 GHz and the other within the 5 GHz band. Only if the distance between the antennas is increased, non-interfering channels within the bands themselves become available. Moreover, our comparison of single- and multi- radio systems allows us to isolate ACI from board crosstalk and radiation leakage of which only the multi-radio systems seem to suffer. Finally, we show how a packet-level simulator can be improved to realistically incorporate ACI. With the help of this simulator more confident statements about the performance of various multi-channel protocols can be made.
VANETS extend the driver's horizon to decrease traffic accidents by utilizing IEEE 802.11p wireless networks. In high density traffic situations an overloaded communication channel leads to significant packet loss. To overcome this problem multiple channels can be used in IEEE 802.11p. This itself leads to adjacent channel interference and impacts the performance of nearby channels. In this paper we define performance require- ments with respect to packet loss and communication range. We show the impact of ACI on simultaneous channel usage by using our multi channel propagation extension for JiST/SWANS. We evaluate the performance on adjacent channels with respect to maximum channel load and variable transmit power. Our results show that the simultaneous usage of nearby channels is an serious issue when no channel access synchronization is applied. I. INTRODUCTION The enhancement of traffic safety and traffic efficiency has become an emerging topic in the automotive industry as well as in the research community. Driver assistance systems based on dedicated short range communication (DSRC) are considered as the key technology to reduce the number of traffic accidents. Cooperative systems like Vehicular Ad-Hoc Networks (VANETS) are designed to enhance drivers' hori- zons by exchanging information between vehicles and thus enhance the awareness of critical situations. In this context VANETS are a new form of spontaneous wireless networks with the characteristics of high node velocity and a highly dynamic network topology. VANETS are based on the IEEE 802.11p wireless access in vehicular environment (WAVE) protocol which provides the possibility of per-packet transmit power control and bit rate adjustment. Basically two types of messages can be distinguished by exchanging broadcast data in VANETS. First, Decentralized Environment Notifications (DENs) which provide information about already existing situ- ations that have been detected by the vehicle's sensors. Second, Cooperative Awareness Messages (CAMs) that transmit the position and status information of a vehicle e.g. speed and acceleration. CAMs are received by neighboring vehicles to calculate if potential collisions exist e.g. in intersection or lane change situations. Due to the high mobility CAMs have to be sent with an increased transmit rate to ensure up-to-date information on the receiver side. The communication range for high critical information is defined by a minimum of 300m according to the Crash Avoidance Metrics Partnership (2) and the evaluations in (3).
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