SPARM: Spatially Pipelined ACK Aggregation for Reliable Multicast in Directional MAC

In this paper, we propose an ACK-based reliable multicast protocol for directional antennas referred to as spatially pipelined ACK aggregation for reliable multicast (SPARM). To resolve the problems of ACK implosion and ACK collection latency, the SPARM exploits (1) spatial reuse and (2) pipelining of ACK aggregation. In the SPARM, receivers sequentially aggregate their ACKs while the sender multicasts a data frame to the next beam, both removing the ACK implosion problem and reducing ACK collection latency. Also, we prove that the aggregation process does not interfere with the sender multicasting to the next beam. Performance evaluation shows that the proposed SPARM has full reliability and outperforms the existing schemes with respect to the throughput by about 200%.