## 3 System Model
### Energy Consumption Model  
In this subsection, we discuss the radio energy consumption of an SN's communication unit in the proposed system.  
In EEUCH, the communication link is between the SN and UAV, while in the existing benchmark protocols this link is between the SNs or between the SNs and BS. However, the proximity of the UAV to the SNs enables that the link between the UAV and the SNs can also use the IEEE 802.15.4 (ZigBee) and IEEE802.11 ah (WiFi) [35] protocols. These protocols are medium range that could achieve transmissions several hundred meters long. These are the same technologies which are used in the existing benchmark routing protocols [29], so the radio model does not alter much on changing the channel from SN or BS to UAV. The hardware energy consumption model of an SN is shown in Figure 3.  
The radio energy consumption of the SNs is due to their two major hardware components, i.e., (i) transmit electronics and (ii) transmit amplifier. The energy consumed by  
Figure 3: Radio energy consumption model.  
the transmit electronics is due to digital coding, modulation, filtering, and spreading of digital signal at transmitter, to transmit \(m\) number of bits, which is given as \(m\times E_{elec}\). This energy consumption is the same for the receiver as well. Moreover, the energy consumed by transmit amplifier depends on the distance \(d\) between transmitter and receiver antennas, amplification factor \(\varepsilon_{amp}\), and path loss exponent \(\lambda\), to transmit \(m\) number of bits. It is given as \(m\times\varepsilon_{fs}\times d^{\lambda}\). The minimum transmit energy consumed by the SN required to achieve acceptable received \(E_{b}/N_{0}\) at the receiver (UAV) is given in (2) below, where \(E_{b}\) is received energy per bit and \(N_{0}\) is the noise power spectral density [13; 19; 20; 21; 22; 23; 24]. It is noteworthy that two separate channel models, free space (\(d^{2}\)) and multipath (\(d^{4}\)) as mentioned in Equation (2), are considered in this study. These simplified models are based on certain assumptions, as described in [36]. It is also important to mention that the benchmark and other routing protocols follow the same energy consumption model.  
\[E_{TX}=\left\{\begin{aligned} & mE_{elec}+m\varepsilon_{fs}d^{2}, \text{ if }d\leq d_{0}\\ & mE_{elec}+m\varepsilon_{mp}d^{4},\text{ if }d>d_{0}\end{aligned}\right.. \tag{2}\]  
In (2), \(E_{TX}\) is the amount of energy consumed to transmit \(m\) number of bits, \(E_{elec}\) is the amount of energy required for transmit electronics to process a single bit, parameter \(\varepsilon_{fs}\) and \(\varepsilon_{mp}\) are the amplification factors in case of free-space and multipath, respectively, and \(d_{0}=\sqrt{\varepsilon_{fs}/\varepsilon_{mp}}\).  
The energy consumed by the radio amplifier is significantly higher than the energy consumed by the transmit electronics. The difference in energy consumption between the two is directly related to the distance between the transmitter and the receiver antennas, given a specific channel model [18; 19; 23; 24]. The efficient delivery of data is critical to prolonging the lifetime of a network. Therefore, this work proposes an energy-efficient routing protocol to achieve this goal.