NPDCCH Period Adaptation and Downlink Scheduling for NB-IoT Networks

The third-generation partnership project (3GPP) has defined a new radio access network protocol, called the narrowband Internet of Things (NB-IoT). To accommodate devices with diverse signal qualities, the base station can flexibly adjust the lengths of the two important radio resources, the narrowband physical downlink control channel (NPDCCH) and narrowband physical downlink-shared channel (NPDSCH), to serve the devices under one coverage enhancement level. The interval between two consecutive NPDCCH is called an NPDCCH period. Given the NPDCCH period, the base station should allocate the two radio resources for the devices to receive downlink data. The NPDCCH period length and resource allocation significantly affect the radio resource utilization of NB-IoT networks. In this article, we investigate the NPDCCH period adaptation and the NB-IoT scheduling problem over NB-IoT networks. The objective is to minimize the consumed radio resource. We prove that the scheduling problem is $\mathcal {NP}$ -hard and cannot be approximated with a ratio better than 3/2. Then, we propose two algorithms based on our observations to solve the target problems and show that the proposed scheduling algorithm is a 2-approximation algorithm. The simulation results evaluate the efficacy of the proposed algorithms and provide useful insights into the NPDCCH period adaptation and scheduling design for NB-IoT networks.