Performance Analysis and Optimization of Downlink Transmission in LoRaWAN Class B Mode

Low-power wide-area (LPWA) networks have been proposed to satisfy the features of massive machine-type communication (mMTC) of Internet-of-Things (IoT) networks with the large number of end devices, such as low data rates, high network scalability, wide area coverage, and delay tolerance with very low energy cost. The LoRa wide-area network (LoRaWAN) is one of the leading technologies among LPWA networks (LPWANs) and supports three types of medium access control (MAC) options: Class A, Class B, and Class C, each of which are used to address different needs of various applications. Specifically, Class B is designed to reduce downlink frame transmission delay while the end device maintains a relatively low energy consumption. In this article, we propose an analytical model of LoRaWAN Class B mode, focusing on the delay, the data throughput, and the energy consumption for downlink frame transmission using the M/G/1 queueing model. Based on the analytical model, a cost function considering both the average waiting time of the frame in the gateway and the average energy consumption of an end device is proposed. Using the cost function, we derive the optimal number of ping slots which maximizes the value of the cost function. Results show the tradeoff relation between the waiting time of the frame in the gateway and the energy consumption of an end device, and it is verified that the optimal number of ping slots increases as the traffic density increases.