The reliable, low-latency exchange of sensor and control information between vehicles enables cooperative driving which allows to significantly increase road safety and traffic efficiency. Vehicular short-range communication based on IEEE 802.11p was specifically designed for safety-critical Vehicle-to-Vehicle (V2V) communication. Its successor 802.11bd promises reliability enhancements by employing packet repetitions, which allow to exploit the wireless channel diversity through packet combining. However, as the added channel load increases the interference is the root cause of access loss, repetitions require a careful medium allocation. This work provides a novel comprehensive study on the trade-off between combining gain and access loss based on extensive performance modeling of both time- and frequency-domain repetitions. The results show significant combining gains which increase with the channel selectivity, and a benefit of frequency diversity at low vehicle mobility that motivates the use of multi-channel repetitions. However, as the related load increase leads to more collisions, the observed gains quickly diminish in multi-user scenarios, resulting in rising packet loss and access delays. To maximize the gains the trade-off is investigated based on system-level simulations with variable traffic load. A methodology to determine the optimal number of repetitions for a given channel state expressed by the Channel Busy Ratio (CBR) is presented. Based on the CBR thresholds, the distributed allocation under time-varying load conditions is evaluated. The results show a trade-off between allocation stability, achieved by short and steady-state performance, achieved by long observations. To deal with this trade-off, the dynamic adaptation of the CBR observation based on channel behavior is proposed. Further, the multi-channel repetitions are investigated, showing worse oscillations due to the bandwidth increase. Strategies to suppress oscillations are discussed, among which an iterative allocation which estimates the impact of the allocation decision, shows the best results. As with the introduction of the Cellular V2X (C-V2X) technology mixed deployments are inevitable, the coordination of the heterogeneous technologies has become crucial to fulfill the requirements of the diverse vehicular applications. Finally, the redundant operation of 802.11p/bd and C- V2X in ad-hoc mode is investigated as a further option to improve the communication reliability. Overall this work contributes to the understanding of how to enhance the reliability of V2V communications by distributed repetition allocation. It can be concluded that a threshold-based repetition allocation allows to increase the reliability at the cost of a tolerable delay increase.
