When LoRaWAN Meets CSMA: Trends, Challenges, and Opportunities

Long-Range Wide-Area Network (LoRaWAN) has been increasingly deployed to serve as the wireless networking platforms for various Internet-of-Things applications. However, an inherent drawback of LoRaWAN is that the signal transmissions of LoRaWAN end devices frequently collide with each other, which leads to data decoding failure. This is because LoRaWAN enables wireless channel access mainly based on the naive ALOHA protocol where each end device is allowed to transmit regardless of others' ongoing transmissions. In this context, the past few years have witnessed the prosperity of enhanced channel access protocol design for LoRaWAN. Particularly, carrier-sense multiple access (CSMA) has gained more attention recently since it enables channel access in a distributed manner without additional signaling overhead between LoRaWAN end devices and gateways. More importantly, implementing CSMA in LoRaWAN is a challenging and interesting problem since signals in LoRaWAN can traverse below the noise floor, making traditional carrier-sensing techniques fail to detect ongoing transmissions. By considering the necessity and importance of this topic, this article aims to provide a comprehensive and timely review of CSMA-based LoRaWAN for the first time. We start by analyzing the trends of existing LoRaWAN-oriented CSMA studies. As the key possible technique to enable CSMA in practical LoRaWAN, Channel Activity Detection (CAD) and its associated challenges are then deeply investigated with a real-world measurement study. Based on the limitations of CAD, we finally provide several research insights for CAD-based CSMA in LoRaWAN.