IEEE 802.11ac vs 802.11ad for V2X: How Many Frames Can We Aggregate?

Despite the high relative velocity, the coherence time in vehicle-to-everything (V2X) multipath fading channel does not linearly decrease with the increase in frequency or velocity, especially when directional propagation or beamforming is involved. Therefore, multiple data frames can be aggregated spanning a coherence time slot. Aggregation of data frames helps to avoid channel estimation after each frame transmission and contributes towards efficient utilization of bandwidth through reduction of protocol overhead. This ultimately leads to enhancement of throughput and data rate in vehicular environment. In this paper, we have addressed two types of frame aggregation, first, aggregated MAC service data unit (A-MSDU), where the medium access control (MAC) layer receives aggregated frames from the upper layers, and second, aggregated MAC protocol data unit (A-MPDU), where the MAC layer frames are aggregated and are send to physical (PHY) layer. Both these frame aggregation strategies are studied in the context of vehicle-to-infrastructure (V2I) and vehicle-to-vehicle (V2V) communications. Our ultimate goal is to compare network performance of IEEE 802.11ac and 802.11ad standards with different types of frame aggregation in V2I and V2V scenarios. IEEE 802.11ac and 802.11ad are the amendments to IEEE 802.11 and are possible candidates for realizing a multi-gigabit V2X wireless system, to satisfy emerging bandwidth hungry applications such as autonomous driving. We observed that frame aggregation is more effective in IEEE 802.11ac compared to 802.11ad. Also, in general, more aggregations are possible with A-MSDU than with A-MPDU. Further, the allowable number of frames to be aggregated for V2V is an order higher than V2I irrespective of the type of aggregation or standard, and higher aggregations are achieved in the lower range of beamwidth. velocity or frequency,