Evaluating the gain of directional antennas in linear VANETs using stochastic geometry

Maximizing the throughput of point-to-point communication has been the crux of wireless networks. In IEEE 802.11 networks, the first and prominent wireless technology, the model of point-to-point communication is still applicable today: the transmissions are between the wireless nodes and the access point, which usually serves as a gateway to the Internet. But this model is not well suited to more recent wireless systems such as Wireless Sensor Networks (WSNs) and Vehicular Ad Hoc NETworks (VANETs). In such networks, a very significant part of communication is between one node and its neighbors and simultaneous transmissions or, in other words spatial reuse, is required to insure good performance. When we consider communication from one node to its neighbor, an important metric is the density of successful simultaneous transmissions. Several studies such as [1], [2] have shown how this density of transmissions can be improved in Aloha or in CSMA networks. The aim of this paper is to show that the use of directional antennas can greatly improve the performance of the network in our neighbor-to-neighbor communication model because interference is greatly reduced. The model we build here allows a quantitative study of the performance and the improvement obtained with directional antennas to be be achieved. The study of Aloha (slotted and non-slotted) is very easy to accomplish and leads to closed formulas for the density of successful transmissions. The study of CSMA is more complex. We use a Matern selection process to mimic the behavior of CSMA in a random pattern of nodes distributed as a Poisson Point Process (PPP): each node receives a random mark and the nodes that have the smallest mark in their neighborhood are elected for transmission. Previous studies, such as [2], show that in CSMA networks, the density of successful transmissions is greatly influenced by the carrier sense detection threshold, which is one of the main parameters of CSMA. In this study we will assume that the carrier sense detection threshold is optimized to obtain the best performance of the CSMA network and our evaluations are performed under this condition. Our analytical models and our computation show that using directional antennas can lead to an improvement of up to more than 100% in the density of throughput compared to the normal use of unidirectional antennas.