# Energy-Aware System Design for Autonomous Wireless Sensor Nodes: A Comprehensive Review
## 4 Energy Management
### Wake-Up Receiver for Sleeping/Idle Mode  
Using recent transceivers is usually associated to high power consumption during both transmit and receive phases. Some radio transceivers require an average of approximately 10 mA in receive mode and 20-30 mA in transmit mode for an ERP of 10-12 dBm. The use of those transceivers in energy autonomous WSNs is usually based on intermittent or duty cycling with certain transmitting, receiving and long sleep time intervals to save energy. During the sleep phase, the receiver is not active but also not completely switched off. Therefore, the system consumes power and latencies can massively increase. To satisfy the requirements regarding reaction time and low energy consumption, a separate so-called wake-up receiver (WuRx) (see Figure 8) with an ultra-low power consumption is required, allowing to switch the rest of the system completely off and have an always-on capability for on-demand communication with a power consumption in the \(\upmu W\) range. Figure 8: Block diagram of a typical wireless sensor node with a WuRx. Wake-up receivers are used in addition to the main receiver, so that the main receiver is only activated when requested. As soon as a request signal with a unique identifier is received, it wakes up the main processor and other peripheral devices. The wake-up packet detection can be realized with suitable circuits having an ultra-low power consumption. Improving the sensitivity and increasing the bit rate for instance would inevitably lead to an increase in power consumption. For decreasing the power consumption to an extreme low level, two main research approaches are followed. The first one focuses on developing new integrated circuits, allowing the optimization of power consumption in different frequency bands [99]. In the second one, off-the-shelf components are used to improve the time-to-market. For short range applications, different architectures based on passive components (see Figure 9) provide sensitivity improvements without a major increase in power consumption. In [100], a passive architecture consisting of an envelope detector based on Schottky diode followed with a 16-bit D-Flip-Flop serial detector is introduced. The achieved sensitivity is by \(-52\) dBm with a power consumption of \(45\) uW at a voltage of \(3\) V. The latency of the detection is in the range of \(80\) us with a bit rate of \(100\) kbit/s. In [101], a WuRx is introduced based on a passive front-end, with a digital baseband consumption of \(1.2\) uW when monitoring a wake-up packet (WuPt). The minimum input power needed for a successful detection is by \(-55\) dBm. A more sensitive WuRx is introduced in [102; 103], which consumes \(7.5\) uW at a voltage of \(3\) V with a sensitivity of \(-60\) dBm. The achieved latency is in the range of \(10\) ms with a bit rate of \(4.56\) kbit/s. The performance of the mentioned designs is generally limited by the Schottky diode noise figure for detecting OOK-modulated (On-Off-Keying) signals [103]. Other concerned works enhance the transmission power efficiency of the WuPt. In [104], power-optimized waves are used to carry WuPt data and to increase the radio frequency to direct current (RF-DC) conversion efficiency of the envelope detector. This increases the rectified peak voltage while holding the incident wave's average power constant. For long-range applications, a WuRx with higher sensitivity is required. For this reason, a low-noise amplifier (LNA) is generally added to boost the incoming signal to be properly detected by an envelope detector. Those architectures are known as tuned RF (TRF). Increasing the sensitivity based on a large gain would affect the noise figure of the following envelope detector. This leads to a significant amount of power. Therefore, smart duty cycling is generally used without increasing the intended short latency. In [105], a duty-cycle of \(0.6\%\) is applied on the TRF-based WuRx. It consumes an average power of \(8.5\) uW for a WuPt detection latency of \(8.1\) ms. The overall sensitivity is \(-73\) dBm.