## 2 Wireless Sensor Network  
Generally, a wireless sensor node consists of four basic components: power supply, sensing, processing and communication. Figure 1 visualizes the main blocks of a wireless sensor node schematically. The figure is taken from [33], but enhanced by the wake-up receiver.  
The wake-up receiver is an additional radio receiver that listens permanently to the wireless channel in the kilohertz range having a constant, but marginal power requirement in the area of \(\mu\)W. The low-frequency wake-up signal is modulated onto the high-frequency carrier signal and demodulated passively by the envelope detector. Figure 2 schematically shows a wake-up signal generated by means of on-off-keying modulation of the high-frequency carrier signal [34]. In the case of ultra-low power wake-up receivers, the envelope detector often merely consists of diodes and capacitors to ensure its energy efficiency [35; 36; 37]. As the signal is transmitted on the high-frequency carrier, the antenna receives the wake-up message with high efficiency. Signal losses happen mainly during the passive demodulation (envelope detection) of the wake-up signal.  
[PERSON] et al. present in [38] an overview of current low-power transceivers. With respect to energy consumption, the range goes from highly integrated concepts that require 0.1 \(\mu\)W to several solutions between 10 and 1000 \(\mu\)W.  
Figure 1: Schematic of a wireless sensor node including the basic components, power supply, sensing, processing and communication. The communication unit consists of the main radio and a wake-up receiver.  
Figure 2: Schematic of a low-frequency wake-up message on-off-keying modulated on the high-frequency carrier signal.  
The wireless nodes used in this work are based on the sensor nodes introduced in [34; 39; 40]. The microcontroller utilized on the boards is a 32-bit EFM32G222F128 manufactured by SiliconLabs running at 14 MHz. It provides several low power states to reduce energy consumption. In run mode, it draws around 2.5 mA and 0.9 \(\upmu\)A in deep sleep mode. Including all peripherals, the sensor node requires around 4.0 mA in run mode. The communication radio is a CC1101 from Texas Instruments. It has a current consumption of 34.2 mA when transmitting at +12 dBm output power at 868 MHz and around 16.4 mA when transmitting at 0 dBm. Its sensitivity is approximately in between \(-95\) dBm and \(-104\) dBm, depending on the data rate. The 125-kHz low-frequency receiver (AS3932) from AMS has a current consumption of around 3 \(\upmu\)A in listening mode. It correlates the incoming signal to a pre-configured address and creates an interrupt if send and stored addresses match. In combination with the matching network and envelope detector, the wake-up receiver has a sensitivity around \(-51\) dBm [34; 39]. Transmitting at 12 dBm, the wake-up range of this wake-up receiver could be determined by [39] as around 45 m.  
Using wake-up receivers can save energy as a wireless sensor node does not have to be active if no tasks need to be done. Furthermore, wake-up receivers provide a fully asynchronous communication, which means synchronization messages are obsolete. Due to this, wireless sensor nodes with wake-up receivers can communicate as soon as they have a message that needs to be delivered, but they can also accumulate sensor data to large data packets and stay in a very-low-power listening state during times when no tasks need to be done.