# A New Hybrid Hierarchical Roadside Unit Deployment Scheme Combined with Parking Cars
## 4 Scenario Description and Basic Definition  
### The Concept of the t-RSU and Its Mechanism  
The U.S. Department of Transportation (DoT) was expected to deploy roadside infrastructure nationwide in 2008 but was impeded due to the lack of cooperation, funding, etc. Therefore, when it comes to the RSUs' deployment, it is of great necessity to take the RSUs' installation cost into consideration. Actually, we have proposed a connectivity-oriented maximum coverage RSU deployment scheme (CMCS) aiming to maximize the V2I communication performance in 2015 [21]. However, the notion of making the parked cars as temporary RSUs is not taken into consideration. In this research, we have systematically integrated an appropriate temporary RSU strategy with the CMCS. Hereinafter, we call the parked cars that are functioning the character of temporary RSU as t-RSU. The hybrid hierarchical roadside unit deployment scheme is divided into two parts: (1) For a specific parked car in a fixed area, whether it will serve the role as the t-RSU is determined by its coverage area, the existing coverage area (ECA) and whether it brings new coverage area or redundant coverage area (RCA). The areas of a single car are divided into many cells and the number of the cells is decided by the maximum transmission range of the parked cars, and the SCA represents the self-coverage area of the parked car. (2) We then optimize the deployment of the RSUs and the t-RSUs. The proposed RSU deployment scheme aims at maximizing the RSU connectivity while ensuring that the total installation cost does not exceed a given cost (referred to as \(C\) hereafter). Figure 3 gives a detailed description of the formation of t-RSU and SCA, ECA, and RCA. The green part of the graph indicates the coverage ECA of the existing t-RSU. The mixed color part represents the overlapping coverage area RCA of the current parked vehicle and the t-RSU. The white part represents the communication coverage SCA of the existing vehicle. For a specific parked vehicle in a fixed area, whether it can play the role of t-RSU depends on its coverage area, i.e., the existing coverage area and whether it brings a new coverage area or redundant coverage area. Figure 2: Typical connected environment of mixed traffic flow. We assume that the batteries of parked cars are energy-free for providing temporary RSU services. In fact, a typical IEEE 802.11p on-board unit (OBU) will not drain more than 1 W on average, which is a very generous upper limit. Considering that a small car's battery provides 480 Wh to 840 Wh [22], the temporary RSU service can last for about 20 days under the circumstances of fully draining the battery. All the VANET vehicles are assumed to have the same maximum transmission range. To make the t-RSU work in a relatively fixed place, which is of great necessity for the afterward RSU's installation deployment, it is necessary to give a detailed standard for choosing the candidate parking place. The candidate parking place may contain lots of parked cars. Before starting, the parking place is partitioned into many parts that here are defined by the vehicle's 2-hop maximum transmission range. For cooperative awareness applications (CAM), the European Telecommunications Standards Institute (ETSI) suggests that CAM messages at fixed broadcasting frequencies be in the range of 1 Hz to 20 Hz [23]. For energy-saving purposes, 1 Hz is used for the frequency of updating the information between the parks and the t-RSU in a single partition. A self-organizing method is here proposed by making the sleeping parked cars wake up regularly and listen to the IEEE 802.11p control channels (CCH) so that they can react to the changing situations in a parking area. While it is possible to obtain an optimized strategy result given complete geographic parking information, it is often impractical due to the huge computation cost. Another reason is that the states of the parking cars are dynamic and the constantly changing parking information.