Random Access and Uplink Shared Channel Resource Allocation With NOMA

In a cellular network, User Equipments (UEs) have to conduct the Random Access (RA) procedure with the Base Station (BS) before transmitting data. In the conventional four-step RA procedure, UEs perform RA competition by exchanging four messages with the BS by utilizing the limited resources. To support the massive Machine Type Communications (mMTC) in a 5G network, various solutions have been studied to reduce collision probability when many UEs perform RA competition simultaneously. By applying Non-Orthogonal Multiple Access (NOMA) to the RA procedure, the BS can decode multiple signals with different transmission power levels from multiple UEs by successive interference cancellation (SIC) technique and the RA delay can be reduced by decreasing collisions. To support latency-sensitive applications in a 5G network, the 3rd Generation Partnership Project (3GPP) adopts a two-step RA procedure which is a shortened version compared to a four-step RA procedure. However, it is difficult to apply NOMA to the two-step RA procedure since UEs perform the RA procedure before the BS allocates dedicated uplink resources to UEs. As researches have not been conducted on how many resources should be allocated by the BS in the NOMA-based two-step RA procedure, a new approach and resource allocation algorithm are required to utilize NOMA in the two-step RA procedure efficiently. In this paper, we propose a novel NOMA-based two-step RA procedure to increase the RA efficiency and reduce the RA delay. In the proposed two-step RA procedure, UEs determine the transmission power level based on the probability informed by the BS. The BS also allocates the optimal resource by estimating the number of UEs participating in RA. Through the proposed two-step RA with NOMA and optimal resource allocation algorithm, the RA efficiency can be reached up to 52%, compared to 37% of the conventional scheme, and the performance is evaluated by a Markov chain and simulations to show the improvement of the RA.