Vehicular communications have been subject to a great development in recent years, with multiple applications, such as electronic payments, improving the convenience and comfort of drivers. Its communication network is supported by dedicated short range communications (DSRC), a system composed of onboard units (OBU) and roadside units (RSU).
A recently conceived different set-up for the tolling infrastructures consists of placing them in highway access roads, allowing a number of benefits over common gateway infrastructures, divided into several lanes and using complex systems. This paper presents an antenna array whose characteristics are according to the DSRC standards. Additionally, the array holds an innovative radiation pattern adjusted to the new approach requirements, with an almost uniform wide beamwidth along the road width, negligible side lobes, and operating in a significant bandwidth.
ANTENNA ARRAY DESIGN
The antenna element used consists of a circular microstrip patch (as shown in Figure 2), whose resonance frequency varies with the radius r. Additionally, two side slits (Δy × Δr) were created for LHCP generation. To adapt the antenna input impedance to the impedance of AFN transmission lines, a double quarter wavelength transformer was inserted (lt1 and lt2).
Figure 3 shows a comparison between the simulated and measured results, both of the S11 and axial radio of the manufactured antenna. It is possible to observe a good agreement between simulated and measured curves. In terms of S11—assuming the usual criterion for a good impedance matching an S11 < −10 dB—the patch has a measured bandwidth of 210 MHz (5.68–5.89 GHz).
The antenna was designed, simulated, and manufactured using the Rogers RO4725JXR substrate, whose main characteristics are: dielectric constant εr = 2.55, thickness h = 0.78 mm, and loss tangent tgδ = 0.0026. The prototype is shown in Figure 10, and presents global dimensions Wg × Lg = 286 × 218 mm2.
The Axial Ratio (AR) is a parameter that allows the quality of the circular polarization of an antenna to be characterized. Figure 12 illustrates the simulated and measured ARs of the designed antenna array. It is possible to observe an acceptable agreement between the two results, with only a small deviation.
DISCUSSION AND CONCLUSIONS
Vehicular communications is a topic with great attention and development, and it is expected that in the future vehicles will interact with each other, and with their surrounding environment through road infrastructures, serving as sensors on the road, collecting and sharing data. DSRC is a technology used to implement this vehicular network with multiple applications (such as electronic payments) that has increasing use due to its convenience to all stakeholders.
In Europe, there are two main types of tolling; either the vehicles pass through toll plazas divided into several separated lanes, some using electronic tolling and other with manual payments, or they pass through free flow structures in which the payment is only electronic and the various lanes are not separated, allowing passage without reducing the speed.
A reduced-speed passage is required in the first configuration, while the second requires a more complex system due to the presence of large interference between lanes. A new approach consists of bringing the tolling to the access roads due to the simplicity that it allows the whole system; however, due to the wider dimensions of these lanes, the RSU module must have an antenna having a wider radiation pattern to cover the entire lane.
Source: Campus Universitario de Santiago
Authors: Tiago Varum | Joao N. Matos | Pedro Pinho