# Hybrid Multi-Antenna Techniques for V2X Communications--Prototyping and Experimentation
## 2 Distributed Multiple Antennas for V2X Communications  
### ESPAR Antennas  
There are plenty of multiple-input multiple-output (MIMO) techniques, that is, spatial multiplexing, diversity, and beam forming, that have been introduced in order to deal with the numerous V2X challenges and accomplish a satisfying V2X link performance. However, most of the antenna designs that have been used in the recent ITS systems can be considered as conventional antenna models with a single radiating element, that is, wire/printed monopole or PIFA, usually located in the side mirror or the vehicle's roof [17, 18]. These antenna designs do not endorse any digital techniques since they offer a fixed radiation pattern. Moreover, certain antenna array examples (or multiple antenna systems) have been developed for vehicles. Nevertheless, these antenna arrays demonstrate large dimensions, high complexity, high cost and various other drawbacks [19, 20, 21]. The same disadvantages are also encountered in a few proposed MIMO antenna systems that are integrated in a shark-fin structure [22, 23, 24].  
A reconfigurable antenna can be considered as an ideal candidate to substitute a multiple-antenna system and satisfy the cost and space limitations that are established by vehicle and truck manufacturers for V2X communication antennas. The ESPAR antennas constitute a special category of reconfigurable antennas that offer the feature of pattern reconfigurability by electronically adjusting the loads of the parasitic elements [25, 26]. Specifically, the ESPAR antennas are formed of a single active element and a specific number of parasitic elements that are distributed close to the active according to a certain geometrical arrangement (linear, circular etc.). The parasitic elements are located in a closer distance compared to the conventional antenna arrays and thus, they offer a smaller size, providing a relatively compact size to the final antenna system.  
Moreover, ESPAR antennas provide a reduced complexity and low fabrication expenses since they involve a single active element and a single RF chain. However, the main advantage of the ESPAR antennas is the ability to control their radiation pattern. In detail, the close proximity between the active and the parasitic elements generate significant mutual coupling effects and induce strong currents flowing on the parasitics that influence the total radiation pattern of the array. Therefore, by controlling the impedances of the parasitic elements either with RF switches (PIN diodes) or variable capacitances (varactors), it is fairly simple to adjust the mutual coupling and reconfigure the radiation pattern of the array. Pattern reconfigurability provides to the ESPAR antenna the capability to employ diversity, MIMO, and beam-forming techniques [27]. The superiority of the ESPAR antenna compared to a single monopole radiator for a V2X communication link is also demonstrated in Reference [28]. Simulation results using the IST-WINNER channel model for T2T links are presented, that verify the suitability of the 3-element ESPAR for the specific application.