Irregular Repetition Slotted ALOHA Scheme With Short Packets Under Rayleigh Fading Channels

Irregular repetition slotted ALOHA (IRSA) has attracted widespread attention as a promising modern access protocol for machine-type communications (MTC). Since IRSA is a typical short-packet transmission scheme, the decoding error probability caused by finite block-length is not negligible. In this paper, we establish an IRSA-based uplink access model considering the impact of finite block-length transmission. In particular, assuming Rayleigh fading channels, a detection-oriented power allocation strategy is employed to enable the AP to identify collision-free slots based on the power of received signal. We then perform detailed analysis of three performance metrics, i.e., system throughput, user invalid probability, and device energy efficiency, to investigate the optimal behavior of IRSA under the proposed model. It is validated that by carefully setting the two controllable parameters, i.e., fading threshold and packet length, a good compromise between throughput and user invalid probability can be achieved while maintaining appreciable energy efficiency. Furthermore, we derive a new upper bound on the maximal load that the system can support for IRSA, which is a generalization of the existing benchmark and much tighter in erasure scenarios.