Hybrid NOMA Offloading for Delay-Sensitive Applications in MEC-Based NB-IoT Networks

Data traffic has grown exponentially with the rapid development of Narrowband Internet of Things (NB-IoT) technology. Nonorthogonal multiple access (NOMA) and mobile edge computing (MEC) are essential technologies to enhance the performance of NB-IoT networks. This article proposes a new hybrid NOMA offloading strategy, allowing an Internet of Things (IoT) device to execute NOMA with other devices at different periods until the task offloading is completed. An optimization problem is established to minimize the overall offloading delay. To find the solution to the problem, we first use the proposed offloading strategy to determine the pairing method and offloading order of the IoT devices. Then, we transform the delay optimization problem into the link rate maximization problem. Finally, the closed-form solution of the optimal power allocation scheme for each IoT device is derived according to the theoretical analysis and the Karush-Kuhn–Tucker (KKT) condition. The simulation results show that the proposed offloading strategy and power allocation scheme effectively reduce the overall offloading delay under different device numbers and data lengths, which exceeds the orthogonal multiple access (OMA) schemes, the pure NOMA scheme, and the iterative multiuser NOMA scheme in lowering offloading delay.