Latency Constrained Non-Orthogonal Packets Scheduling With Finite Blocklength Codes

In this paper, we investigate the offline packets scheduling of a downlink system with two heterogeneous latency-constrained users. Specifically, user 1 is a latency-tolerant user which has one packet with long latency requirement, while user 2 is latency-sensitive user which has multiple packets with short latency requirements. To precisely characterize the transmission latency, the finite blocklength codes (FBC) capacity formula is used to measure the achievable date rates of the users. To cope with the heterogeneous latency requirements of users, a hybrid NOMA scheme is proposed. In the hybrid NOMA scheme, the packets of user 2 are non-orthogonally delivered with the packet of user 1 in a sequential way by taking the packets arrivals and deadlines into consideration. Due to complicate FBC capacity formula and the coupling effects of the variables, the formulated transmission energy minimization problem is challenging to solve. However, by properly utilizing the the fact that the transmission energy is monotonically decreasing with the blocklength, the optimal transmission blocklength and power of user 1 are derived in semiclosed-forms; while the remaining problem, for determining the transmission blocklength and power of user 2, is efficiently solved by using a successive upperbound minimization based algorithm. The simulation results demonstrate the efficacy of the proposed hybrid NOMA scheme.