# A System-Level Methodology for the Design of Reliable Low-Power Wireless Sensor Networks
## 2 Sensor Node Energy Model
### Analytical Energy Model
#### 2.3.2 Wireless Transceiver and Sensor  
To evaluate the energy consumption per measurement of the wireless transceiver, both the transmitter and the receiver are considered. The current measurement setup is shown in Figure 5a. The studied transceiver uses a sub-1 GHz carrier, which has the capability to respond to the needs and concerns for long-range and low-power wireless connectivity [10; 54]. Computing the energy consumption per measurement during the active period is quite straightforward in this model. It can be estimated using  
\[E_{TRX}=V_{WMCU}\frac{l}{D_{R}}(I_{TX}+I_{RX}), \tag{3}\]  
where \(l\) is the packet length, \(D_{R}\) is the data rate, and \(I_{TX}\) and \(I_{RX}\) are the transmitter's and the receiver's currents during active mode, respectively, as shown in Figure 5b. In this study, the combined digital humidity, pressure, and temperature BME280 sensor from Bosch Sensortec was chosen. It is housed in a compact package, allowing the reduction of the overall sensor node's form factor. Both the measurement time and the current consumption depend on the oversampling mode of the three physical quantities. This means that, on the sensor's level, noise can be traded-off against latency and current consumption. Figure 5b clearly shows the different measurement phases (i.e., temperature, pressure, and humidity, respectively) and current consumption on the BME280 sensor where oversampling factors of 4, 2, and 1 were selected, respectively [50]. It also shows the current consumption profile on the transmitter and receiver. As expected, Figures 3 and 5b show that the transceiver is the most energy-consuming component of a SN. It should be noted that, in this case, the receiver is continuously listening, as depicted in Figure 5b. However, the estimations in this work consider it to be duty-cycled (i.e., only active during the reception period). This is because a simple point-to-point communication is assumed and implemented in this work. Yet, in WSN applications, communication protocols such as  
Figure 5: (**a**) Test setup for transceiver and sensor current measurement, and (**b**) current consumption breakdown of the sensor, transmitter, and receiver during one measurement. time division multiple access (TDMA) are used in order to allow the receiver the go into a power-saving mode when not communicating. Moreover, it can be seen from Figure 5b that the measurement starts right after the MCU's wake-up and takes a long time to finish. This means that the measurement done in the \(i^{th}\) period is actually transmitted in the \(\left(i+1\right)^{th}\) period. From an energy consumption point of view, the results remain the same. According to the datasheet [50], the sensor's measurement time can be calculated using  
\[t_{SENSOR}=2\;(T_{OVS}+P_{OVS}+H_{OVS}+1). \tag{4}\]  
The average current consumption during measurement can be calculated using  
\[I_{SENS}=\frac{I_{DDT}(1+2\;T_{OVS})+I_{DDP}(2\;P_{OVS}+0.5)+I_{DDH}(2\;H_{OVS }+0.5)}{t_{SENS}}. \tag{5}\]  
Therefore, the sensor's energy consumption is given by  
\[E_{SENS}=V_{SENS}\;I_{SENS}\;t_{SENS}, \tag{6}\]  
where \(V_{SENS}\) is the sensor's supply voltage. In order to see the impact of using different bit rates and output power levels and to evaluate the accuracy of the energy consumption models, measurements were performed at 50 kbps and 500 kbps, with an output power level going from -10 to 12 dBm. The results are depicted in Figure 6. The estimated energy was obtained using Equation (6) for the sensor, Equation (3) for the transceiver, and Equation (2) for the MCU. At this point, it can be assumed from Figure 6 that a better energy efficiency can be achieved with higher data rates, allowing the transceiver to go into a power-saving mode more quickly. However, this remains an assumption as it comes at the cost of a lower communication range. Therefore, the goal is to use the highest achievable data rate that can sustain the desired communication range.