# Energy-Aware System Design for Autonomous Wireless Sensor Nodes: A Comprehensive Review
## 5 Energy Saving on Network Level
### Radio Optimization  
The radio module is the most energy consuming element for a sensor node. To decrease the overpower consumption during wireless communication, it is important to optimize the radio parameters by applying cognitive radio and cooperative communications techniques, optimizing antenna direction, modulation, and power transmission [108]. Cognitive radio (CR) techniques are used to reduce the power consumption for packet retransmission, resultant from packet losses, by alternating the access to multiple channels opportunistically. Sensor nodes can choose a communication channel in the wireless spectrum automatically and adjust the transmission and reception parameters according to channel conditions [109]. The authors of [110] showed that using CR improve the network lifetime by 13% when compared to the one without CR.  
Cooperative communication techniques aim to significantly enhance the quality of the received signal using several single-antenna systems working together to form a virtual multi-antenna transmitter [111]. The importance of these techniques is the use of relay nodes as intermediate nodes, whereby the energy consumption is proportional to the  
Figure 10: Energy saving mechanisms in wireless sensor networks.  
square of the distance; therefore, decreasing the distance between the source and the destination reduces the energy considerably.  
Directional antennas contribute to energy conservation by reducing the interference and data retransmission created by omnidirectional antennas [112]. While a node transmits data in a specific direction, a nearby node can also transmit at the same time without interference, thus saving a significant amount of energy. In the case a node intends to send a data packet to a neighbor node, it only sends in the specified direction, thus enhancing energy efficiency. However, using directional antennas requires finding the appropriate direction and parameters.  
Modulation optimization techniques are interesting for determining the optimal modulation parameters of the radio with minimal power consumption [113]. Within short ranges, the circuit power consumption is higher than the consumed power for the transmission, which is the opposite for long distances. For this reason, it is important to set the appropriate modulation parameters that establish a trade-off between the transmission signal power and distance between nodes.  
Transmission power control (TPC) is used to improve the energy by adjusting the radio transmission power according to some link quality parameters such as the link quality indicator (LQI) and the received signal strength indicator (RSSI) [114]. By reducing the transmission power, the interference is decreasing. In addition, higher transmission power allows the physical layer to incorporate the modulation at a higher bit/baud ratio. This can maximize the bandwidth in the case of heavy traffic. However, the delay in data transmission is significantly increased as more hops are involved to send a data packet. TPC protocols dissipate high energy during the initialization process.