Random Access Protocol for Massive Internet of Things Connectivity in Space–Air–Ground-Integrated Networks

Space–air–ground-integrated networks (SAGINs) are receiving a lot of attention as a candidate for an extension to nonterrestrial networks beyond the limit of terrestrial networks. SAGIN can be a means to satisfy various Quality of Service (QoS) by providing several communication links in different characteristics. To fully utilize the advantage of SAGIN, a protocol for informing user equipment (UE) of an appropriate base station layer of SAGIN is needed according to QoS requirements. In addition, the development of the protocol should consider the characteristics of the UE. In particular, when the UE is a machine-type communication device (MTCD) constituting a massive Internet of Things network, random access congestion issue should be considered. To solve the problems, we propose a random access protocol. We propose an indicator named the virtual deadline indicator (VDI) to inform MTCD which layer to try for random access. MTCD attempts random access to one layer of SAGIN by comparing the remaining deadlines of traffic and the point of the VDI. To minimize the deadline expiration rate, we propose an algorithm to shift the VDI position. The base station estimates the number of MTCDs and remaining deadline distribution by using the number of idle preambles to find the VDI location. Finally, we compare the performance of the protocol through simulation with three benchmark schemes. We confirm that the proposed protocol reduces the deadline expiration rate without compromising the Age of Information and energy efficiency.