Throughput and Energy Tradeoffs for Retransmission-based Random Access Protocols

The fifth-generation of wireless communication networks is required to support a range of use cases such as enhanced mobile broadband (eMBB), ultra-reliable, low-latency communications (URLLC), massive machine-type communications (mMTCs), with heterogeneous data rate, delay, and power requirements. The 4G LTE air interface is designed to support fewer devices with large payloads, and uses extra overhead to enable scheduled access, which is not justified for small payload sizes. In this paper, we employ a random access communication model with retransmissions for multiple users with small payloads at the low spectral efficiency regime. The radio resources are split non-orthogonally in the time and frequency dimensions. Retransmissions are combined via different Hybrid Automatic Repeat reQuest (HARQ) methods, namely Chase Combining and Incremental Redundancy with a finite buffer size constraint Cbuf, via a conventional matched filter receiver. We determine the best scaling of the spectral efficiency (SE) versus signal-to-noise ratio (SNR) ρ per bit and the scaling for the user density (number of users per real degrees of freedom, rdof) versus SNR per bit, for the sum-optimal regime and when the interference is treated as noise, using a Shannon capacity approximation. Numerical results show that the scaling results are applicable over a range of η, T, Cbuf, J, at low received SNR values. The proposed analytical framework can provide insights for resource allocation strategies in general random access systems and in specific 5G use cases for massive URLLC uplink access.