Schedule Sequence Design for Broadcast in Multi-Channel Ad Hoc Networks

Ultra-reliable and low-latency communication (URLLC) is of capital importance for the 5G wireless networks and has received a lot of attention recently. Motivated by the hard delay requirement for URLLC, we aim at devising medium access control (MAC) schemes that can guarantee packet transmissions within a bounded delay. The model we considered is a multi-channel single-hop ad hoc network in which each node has packets to broadcast to other nodes. This broadcast scenario is common, and has been extensively studied in various scenarios such as safety message broadcast in vehicular ad hoc networks (VANETs). Most of the existing MAC schemes employ backoff algorithms to handle transmission failures caused by channel collisions, and thus cannot provide a hard guarantee on transmission delay. Moreover, these schemes rely on time synchronization and coordination among the nodes, which would cause heavy overhead for control message exchange. In this paper, we focus on devising MAC schemes to provide a hard guarantee on broadcast delay, without time synchronization and cooperation among the nodes. We employ the sequence scheme in which each node is pre-assigned a periodic sequence to schedule transmissions and receptions at each time slot. These sequences are referred to as schedule sequences. Since each node starts its transmission schedule independently, there exist relative time offsets among the schedule sequences they use. Previous sequence schemes for broadcast are only conducted in the single channel context. Our objective is to design schedule sequences such that in a multi-channel system, each node can transmit at least one packet to each of its neighbors successfully within a common period, no matter what the time offsets are. The sequence period should be designed as short as possible. In this paper, we analyze the lower bound on sequence period, and propose a sequence construction method by which the period can achieve the same order as the lower bound. We also consider the random scheme in which each node transmits or receives on a channel at each time slot with a pre-determined probability. The frame length and broadcast completion time under different schemes are compared by numerical studies.