Tackling Hidden Node Problem Utilizing Traffic Periodicity and Downlink Carrier Sense in LPWAN

Low-power wide-area networks (LPWANs), which achieve low-power consumption, enabling long-term battery operation and long-range communication capabilities, have emerged as a new standard for realizing massive wireless sensor networks (WSN). LPWANs are becoming increasingly popular due to low introduction costs, which stem from features such as using unlicensed bands and low-cost nodes. LPWANs are particularly useful for Internet-of-Things (IoT) applications that periodically collect information about specific observation targets. However, LPWAN generally adopts a simple medium access control (MAC), which significantly degrades communication quality due to packet collisions when the traffic load increases. Thus, MAC design is critical for realizing large-scale LPWANs. Carrier sense multiple access (CSMA) can autonomously avoid packet collisions. However, its performance is drastically deteriorated due to the hidden node problem in large-scale LPWANs. This paper proposes an autonomous distributed MAC strategy that can suppress the hidden node problem by utilizing traffic periodicity. The proposed method is designed carefully considering LPWAN-specific constraints, such as duty cycle limitations in unlicensed bands, low clock accuracy of nodes, and limited downlink communication opportunities. From numerical results, the proposed method improves the packet delivery rate (PDR) performance by up to approximately by 29%, 9% and 8% compared to ALOHA, CSMA-x, and the state-of-the-art LoRa MAC respectively.