# Hybrid Multi-Antenna Techniques for V2X Communications--Prototyping and Experimentation
## 3 Detailed Description of the Diversity Engine
The rationale of the diversity algorithm is the following:  
* The pattern reconfiguration for the RF chains depends on the MRC output value; if it falls below a predefined threshold (thrs\({}_{1}\)), the reconfiguration procedure will initiate. Therefore, the pattern combination will not change if the SNR of the current selection does not fall below thrs\({}_{1}\), despite the fact that a different combination with better performance may exist. In this context, continuous unnecessary changes are avoided, which results to a complexity reduction and  
Figure 6: Diversity engine with two RF modules (one per side mirror) with two RF chains and reconfigurable antennas per module. Implemented diversity engine—algorithmic representation. avoidance of synchronization problems. At the same time, important performance degradation is avoided, since the use of thrs\({}_{1}\) ensures that the MRC-SNR remains relatively high. * When on transmit-mode, the transceiver will select either the omni pattern for single message broadcast, or the pattern combination that was decided during reception in order to optimize the link between B and A. * On the other hand, if reconfigurations have not been performed for more than thrs\({}_{2}\) received packets (parameter timer of Figure 6, the diversity engine will attempt to reconfigure and search for a better pattern combination, despite the fact that the SNR threshold is not violated. The reconfiguration trigger through the timer is used in order to periodically force the system to search for better pattern combinations and improve performance, even if the SNR remains relatively high. The reconfiguration procedure is presented in Figure 7. It is performed in two stages, since there is no explicit channel state feedback mechanism defined in the standards. During the first stage (variable check = false in Figures 6 and 7), for each pair of antennas, that is, each side mirror in our implementation (defined by counter j), the engine estimates the received SNR. The main reconfiguration rule is that for each j, the RF chain with the minimum SNR is set to omni and the one with the maximum SNR is selected for reconfiguration and is denoted as \"active-directive\". This policy will allow us to exploit maximally the directive gain of the \"active-directive\" antenna, while we maintain the use of an omni antenna for the exchange of broadcast messages through all directions. For the case of the \"active-directive\" antenna, the first reconfiguration step is defined as follows: If the current pattern is directive (pattern 1 or 2), then the omni pattern, 0, is selected. Otherwise, directional patterns are selected in a random manner. The status of the previous pattern combination is saved and the same procedure is performed for the other pair of antennas (or all j's existing in the system). At the second stage (check = true), the selected pattern for the active RF chain of each antenna pair is evaluated and the system validates the reconfiguration procedure as successful or failed. If the measured SNR is increased compared to the previous formation, then the reconfiguration is assessed  
Figure 7: Reconfiguration function of the diversity engine. as successful. On the other hand, if the estimated SNR is decreased, the reconfiguration attempt is considered failed and two scenarios exist depending on the formation of the current pattern: if it is omni, then the previous selected pattern is restored, since it is assessed as the optimum for the current radio channel state and despite the fact that reconfiguration was triggered, no SNR improvement can be achieved; if it is directive, the opposite directive pattern is selected, since the current selection does not favor propagation towards the desired direction. In any case, at least two antennas are set to omni. This is necessary for the rapidly varying vehicular network, since the vehicle should be able to receive/transmit beacon and traffic messages from/to all directions as part of the safety-related, single message services of the ITS framework.