# Energy-Aware System Design for Autonomous Wireless Sensor Nodes: A Comprehensive Review
## 4 Energy Management
### Control of Operation Modes  
In contrast to classical or battery powered systems, the operation of wireless sensor systems using energy harvesting is usually limited by the amount of harvested energy. In most cases, the limitation of the available energy prohibits continuous and full system function all the time. Instead, power and energy consumption must be distributed well over time by defining several system operation modes and controlling them depending on the energy and power balance. This means, for example:  
* Store energy if the input is high, e.g., in solar supplied systems to bridge the night  
\begin{table}
\begin{tabular}{c c c c} \hline \hline
**Wireless Node Operation Mode** & **Pansamp NRG 2.0 [72; 73]** & **Decawave DWM1000 [74]** & **9 XTend [75]** \\ \hline
**Off** & 5 \(\mu\)A & - & 5 \(\mu\)A \\
**Idle** & - & 13.4 mA & 147 \(\mu\)A \\
**Sleep** & 1–2 \(\mu\)A & 200–550 nA & 1.6–10 mA \\
**Transmission** & 36 mA (868 MHz) & 111 mA (3.5 GHz) & - \\
**Reception** & 18 mA (868 MHz) & 154 mA (3.5 GHz) & - \\ \hline \hline \end{tabular}
\end{table}
Table 2: Drained current of different wireless sensor node for different activity modes.  
* Use energy only if really needed, e.g., do not send data if the value has not changed
* Reduce the standby current as much as possible
* Switch off unnecessary parts  
Figure 6 gives an overview of the general classification of operation modes. Every sensor system has its specification, e.g., one measurement and one data transmission per time unit. If this functionality can be fulfilled, it can be described as a \"regular operation\". If the available energy is lower than required for the \"regular operation\", the functionality must be reduced. This means that less than one measurement/transmission per time unit is possible, i.e., the system will work, but the performance is less than expected. This would be a disadvantage and might even cause problems if the gap between two measurements is too long for the application.  
If the available energy is not sufficient to acquire and send any data, the system will stay in standby. Usually, in this state of operation, it can still convert and store energy, so that eventually at least an emergency signal can be sent. In some cases, for example for a system supplied by solar energy, there are peaks of incoming power. In these cases, situations might occur where the energy storage elements are already filled and the available income still surpasses the \"regular operation\" and even the \"increased functionality\" by far. This state as well as the standby state are unpreferred and can be handled by the dimensioning of the energy storage part of the system or using more storage units. The usual goal of the system design is to reach the \"regular operation\" state most of the time and usually only deviate with reduced or increased (preferred) operation. The change from one operating mode to another is triggered by conditions and thresholds related to the specification of the sensor node, its typical duty cycle and the expected energy consumption for every subtask as well as the actual available energy in the system.