Grant-Free NOMA: A Low Complexity Power Control Through User Clustering

This paper proposes synchronous grant-free non-orthogonal multiple access (GF-NOMA) frameworks that effectively integrate UE clustering and low-complexity power control to facilitate power reception disparity required by the power domain NOMA. While single-level GF-NOMA (SGF-NOMA) designates an identical transmit power for all user equipments (UEs),  multi-level GF-NOMA (MGF-NOMA) groups UEs into partitions based on the sounding reference signals strength and assign partitions with different identical power levels. Based on the objective of interest (e.g., max-sum or max-min rate), the proposed UE clustering scheme iteratively admits UEs to form clusters, whose size is dynamically determined based on the number of UEs and available resource blocks (RBs). Once the UEs are acknowledged with power levels and allocated RBs through random access response (RAR) messages, UEs can transmit anytime without grant acquisition. Numerical results show that the proposed GF-NOMA frameworks can compute clusters in the order of milliseconds for hundreds of UEs. The MGF-NOMA can reach up to 96-99% of the optimal benchmark max-sum rate, the SGF-NOMA reaches 87% of the optimal benchmark max-sum rate at the same power consumption. Since the MGF-NOMA and optimal benchmark enforces the strongest and weakest channel UEs to transmit at maximum and minimum transmit powers, respectively, the SGF-NOMA also offers a significantly higher energy consumption fairness and network lifetime as all UEs consume equal transmit powers. While the MGF-NOMA delivers an inferior max-min rate performance, the SGF-NOMA is shown to reach 3x10⁶ MbpJ energy efficiency compared to 1x10⁷ MbpJ