Energy-Efficient Wake-Up Radio MAC Protocols Without Duty Cycling inWireless Sensor Networks (WSNs): A Classification and ComparativeReview

Wake-up Radio Medium Access Control (WuR-MAC) protocols represent a vital advancement in Wireless Sensor Networks (WSNs), which aim to reconcile energy efficiency with network responsiveness. This study presents a detailed classification and analysis of WuR-MAC protocols that function without duty cycling. These are systematically categorised into Always Active WuR-MAC and Radio Triggered WuR-MAC Protocols. The Always Active WuR-MAC protocols, among the earliest developed, ensure high responsiveness by maintaining a continuously active, low-power radio, separately for transmission and reception of wake-up frames, albeit at the cost of increased energy consumption and a limited wake-up range. In contrast, Radio Triggered WuR-MACs are further divided into Passive WuRx and Ultra-low Power (ULP) Active WuRxbased protocols. Passive WuRx achieves exceptional energy savings by harvesting energy from incoming signals; however, it suffers from a constrained activation range and wake-up time, limiting the amount of power available to switch on the wake-up circuit. Conversely, ULP-Active WuRx protocols strike a more favourable balance between energy consumption and wake-up range, making them suitable for applications demanding extended coverage and energy efficiency. The classification highlights how evolving trade-offs in power consumption and wake-up capabilities have shaped protocol development, corroborating findings from earlier studies. This review aims to guide future protocol design by underscoring the technological nuances and performance implications of WuR-MAC strategies without duty cycling.