We employ grant-based access with retransmissions for multiple users with small payloads, particularly at low spectral efficiency (SE). The radio resources are allocated via non-orthogonal multiple access (NOMA) in the time into T slots and frequency dimensions, with a measure of non-orthogonality η. Retransmissions are stored in a receiver buffer with a finite size Cbuf and combined via Hybrid Automatic Repeat reQuest (HARQ), using Chase Combining (CC) and Incremental Redundancy (IR). We determine the best scaling for the SE (bits/rdof) and for the user density J/n, for a given number of users J and a blocklength n, versus signal-to- noise ratio (SNR, ρ) per bit, i.e., the ratio Eb/N0, for the sum-rate optimal regime and when the interference is treated as noise (TIN), using a finite blocklength analysis. Contrasting the classical scheme (no retransmissions) with CC-NOMA, CC-OMA, and IR-OMA strategies in TIN and sum-rate optimal cases, the numerical results on the SE demonstrate that CC-NOMA outperforms, almost in all regimes, the other approaches. For high Cbuf and small η, IR-OMA could surpass CC-NOMA. At low Eb/N0, the SE of CC-OMA with TIN, as it exploits CC and offers lower interference, can approach the trend of CC-NOMA and outperform the other TIN-based methods. In the sum-rate optimal regime, the scalings of J/n versus Eb/N0 deteriorate with T , yet from the most degraded to the least, the ordering of the schemes is as (i) classical, (ii) CC-OMA, (iii) IR-OMA, and
(iv) CC-NOMA, demonstrating the robustness of CC-NOMA. Contrasting TIN models at low ρ, the scalings of J/n for CC-based models improve the best, whereas, at high ρ, the scaling of CC-NOMA is poor due to higher interference, and CC-OMA becomes prominent due to combining retransmissions and its reduced interference. The scaling results are applicable over a range of η, T , Cbuf , and J , at low received SNR. The proposed analytical framework provides insights into resource allocation in grant-based access and specific 5G
use cases for massive ultra-reliable low-latency communications (URLLC) uplink access.
