Low-Power wake up receiver based on Surface Acoustic Wave Correlator

Wireless Sensor Networks (WSN) consist of large number of distributed sensors nodes which are able to sense, read and transmit physical measurements such as temperature, humidity and pressure over wireless communication links. WSN nodes are often powered by batteries or can use energy harvesting methods from environmental energy sources. One of the major challenges in the design of WSN nodes is the high level of power dissipation for sensing, processing and communication. Operating at low-power levels reduces maintenance effort for periodic battery replacement or can even provide unlimited operation by means of energy harvesting. Since the communication process is the most power hungry process, ultra-low-power wireless communication is an enabler for network applications such as cyber-physical systems, Internet-of-Things and Industry 4.0 etc. Our research is based on Wake-up Receivers (WuR) architectures. Each of the WSN nodes contains a WuR which is always-on, listening for a wake-up signal from other nodes or the base station, and activating the node only when a wake-up signal is detected. By this scheme the communication with the base station becomes asynchronous, real-time and on-demand. Due to the centrally-coordinated, collision-free communication such WSNs can be scaled to very large node numbers. Designing always-on WuR at ultra-low-power dissipation levels makes the WSN nodes very energy efficient because they are only activated when a wake-up-signal is received. Additionally, the WuR must be robust to noise and co-channel interference in order to operate safely in parallel to other wireless systems. We investigate a novel radio architecture for the WuR using Linear Frequency Modulation (LFM) and passive analog signal processing by means of a Surface Acoustic Wave (SAW) correlator. The base station sends the required WSN node ID using LFM signal at 2.4 GHz. The node ID is encoded as chirp up or chirp down signal with chirping bandwidth of 80MHz. On the receiver side, the SAW chirp correlator demodulates the received LFM signal while suppressing other wireless signals. In order to achieve proper demodulation and high Signal-to-Noise Ratio (SNR), the SAW correlator is designed to behave like a Matched Filter (MF) which boosts up the SNR. After that the signal is amplified/detected by baseband amplifier stage, it is compared with the unique ID of the node, and the node's Wake up signal is asserted accordingly. Since the SAW correlator operates completely passive, the WuR can be implemented in a very energy-efficient way, without the need to use power hungry device such as Low Noise Amplifiers (LNA) or down conversion Local Oscillators (LO)