# Hybrid Multi-Antenna Techniques for V2X Communications--Prototyping and Experimentation
## 3 Detailed Description of the Diversity Engine  
Let's consider two vehicles (A and B), which are equipped with the diversity engine depicted in Figure 6. At the base of the engine, an MRC is employed, which combines the received signal-to-noise ratio (SNR) from the four independent RF chains. In the course of the research activities, other diversity schemes were also tested, that is, equal gain combining (EGC), antenna selection and minimum mean square error (MMSE). The MRC was selected as the one with the best performance, despite the fact that MMSE is theoretically optimal in terms of SNR. However, the MMSE requires knowledge of the noise and interference variance, thus, for its real-world implementation a noise power estimator was also used. Due to the estimation error and the non stationary nature of the noise+interference level in dynamic vehicular environments, the MMSE diversity performance diverged from its expected optimal behavior. Uncorrelated signal reception is desired in order to maximize the diversity gain and this is ensured due to the increased distance among the ESPARs. In order to evaluate the received SNRs, the engine uses measurements of the error vector magnitude (EVM) at the received quadrature phase-shift keying (QPSK) constellation of pilot symbols, in conjunction with the automatic gain controller (AGC) values for each RF chain. It is also assumed that the system hosts two sets of applications. The first set contains services and applications in broadcast mode of operation (i.e., reception of cooperative awareness messages), while the second set involves the direct communication of A and B, implementing, for example, a platoon. Thus, in the investigated scenario, the two transceivers host two services, a single message service and an A-to-B unicast. In order to describe the simple but yet efficient operation of the diversity engine, an exemplary use case is presented where A and B are communicating directly with the use of a V2X standard (ITS-G5 or C-V2X/PC5). Initially, it is assumed that B receives an ITS packet. The received packet is processed and if originated by A, it is forwarded to the diversity engine, which uses the selected metrics (EVM and AGC values) in order to decide whether to change the currently selected combination of patterns of the four ESPAR antennas. All engine decisions are taken during reception. During the transmission phase, B employs the combination of patterns selected during the receiving phase. Due to the reciprocity principle, it is expected that the pattern that optimizes reception from A, also offers increased power towards A. The same operation is also followed by vehicle A. Since many unicast,  
Figure 5: Diversity engine with two radio frequency (RF) modules (one per side mirror) with two RF chains and reconfigurable antennas per module. Simplified block diagram. multicast, and broadcast applications are required to operate in parallel, functionalities that ensure coexistence of omni and directive patterns were also developed, as demonstrated below. During the field trials of the ROADART project, the implemented diversity engine was able to support various modes of operation, including random selection of pattern combinations for each RF chain or user-defined manual pattern selection. Here, we focus on a simple, automated, standard-agnostic, and yet remarkably efficient approach presented in the flow graphs of Figures 6 and 7. During the field trials, the 3-element ESPAR with PIN diodes were used--providing three active patters per antenna (the notation used is 0: omni, 1: front, 2: back). As shown in Figure 6, the basic operation of the diversity scheme depends on two thresholds (thrs\({}_{1}\) and thrs\({}_{2}\)) related with the two control parameters, that is, MRC SNR and a timer, respectively. Initially, all patterns are set to omni.