Scheduling and Decoding of Downlink Control Channel in 3GPP Narrowband-IoT

Abstract

Narrowband Internet of Things (NB-IoT) is a low power wide area network technology introduced by the $3^{rd}$ Generation Partnership Project (3GPP). It is a derivative of the existing 3GPP Long Term Evolution (LTE) that will enable cellular service to a massive number of IoT devices. In comparison with LTE and 5G New Radio, the NB-IoT devices will be of low cost, low throughput, and delay-tolerant. The reduction in available bandwidth and introduction of repetitions for achieving wider coverage requires modified Narrowband Physical Downlink Control Channel (NPDCCH) search space design and decoding as compared to the LTE. Hence, in this paper, we first explain the NPDCCH physical layer procedures, along with the search space decoding. Unlike LTE, there is no channel feedback mechanism in NB-IoT. Therefore, we propose a novel resource mapping scheme for NPDCCH based on the uplink reference signals. We perform system-level simulations and analyze the impact of the proposed mapping for varying operating frequencies and channel conditions. Further, the NB-IoT devices have limitations on the battery power, and hence, the existing control channel schedulers cannot be reused for the NB-IoT scenario. Thus, we propose a novel scheduler for NPDCCH. We have also modified the current state-of-the-art algorithms to meet the NPDCCH constraints and compared them against the proposed scheduler. We derive bounds for such scheduling algorithms and show that the proposed scheduler additionally conserves up to 25% of the IoT device battery power. Through Monte Carlo simulations, we show that the proposed scheduler better achieves the various trade-offs between power consumption, search space utilization, and fairness as compared to the existing schedulers.