# Energy-Efficient UAVs Deployment for QoS-Guaranteed VoWiFi Service
## 3 QoS and UAVs Endurance in VoWiFi Networks
### Modeling QoS in VoWiFi  
Each aerial AP and its associated stations can be studied as an independent IEEE 802.11 system, as illustrated in Figure 2. Although all stations have a common Medium Access Control protocol (i.e., MAC sub-layer), the Modulation and Coding Scheme (MCS) set on each station is automatically self-configured seeking, the greatest possible data bit-rate achievable with the present signal strength. Table 2 shows different MCSs defined for OFDM modulations (e.g., 802.11 a/g) and the minimum signal strength required for each one.  
Figure 1: UAV-based VoWiFi network.  
VoIP is a mature technology that counts with several methods for the assessment of voice quality [44]. At the planning stage, the method predominantly used in the literature for QoS assessment is the E-Model [45] since it allows one to estimate the speech quality coming down to the effect that the VoIP traffic load has on network performance in terms of packet loss and delay. The E-model provides a quality score termed \(R\) factor from 0 (poor) to 100 (excellent) which can be readily obtained using the following expression [12; 46; 47]:  
\[R =R_{0}-\underbrace{I_{e}+(95-I_{e})\frac{P_{pl}}{\frac{P_{pl}}{ BustrR}+B_{pl}}}_{l_{e,eff}}\] \[-\underbrace{(0.024d+0.11\cdot(d-177.3)\cdot H(d-177.3))}_{l_{d}} \tag{1}\]  
where:  
* \(I_{e,eff}\) represents a combination between impairment equipment parameter at zero packet loss (\(I_{e}\)), and a function that depends on \(I_{e}\), the packet loss rate, and packet loss behaviour. \(I_{e}\) is a codec-dependent constant associated with codec compression degradation (a list of values from ITU-T codecs were presented in ITU-T Rec. G.113 Appendix I), \(B_{pl}\) represents the codec  
\begin{table}
\begin{tabular}{c c c c} \hline \hline
**Modulation** & **Coding Rate** & \
\begin{tabular}{c} **Data Rate** \\ **(Mb/s)** \\ \end{tabular} &
\begin{tabular}{c} **Sensitivity** \\ **(dBm)** \\ \end{tabular} \\ \hline BPSK & \(1/2\) & \(6\) & \(-82\) \\ BPSK & \(3/4\) & \(9\) & \(-81\) \\ QPSK & \(1/2\) & \(12\) & \(-79\) \\ QPSK & \(3/4\) & \(18\) & \(-77\) \\
16-QAM & \(1/2\) & \(24\) & \(-74\) \\
16-QAM & \(3/4\) & \(36\) & \(-70\) \\
64-QAM & \(2/3\) & \(48\) & \(-66\) \\
64-QAM & \(3/4\) & \(54\) & \(-65\) \\ \hline \hline \end{tabular}
\end{table}
Table 2: 802.11 sensitivity relations for OFDM modulations with 20 MHz channels.  
Figure 2: Communication system for each drone.  
packet loss robustness, which also has a specific value for each codec (listed in ITU-T Rec. G117 Appendix I), \(P_{pl}\) represents the packet loss rate (in %) in the WiFi channel and finally \(BurstR\) characterizes the burst ratio (i.e., equals 1 if packet loss if random and greater otherwise). In this paper, we use the G.711 codec (\(I_{e}=0\) and \(B_{pl}=25.1\)) and assume random losses (i.e., \(BurstR=1\)).
* \(I_{d}\) accounts for impairments associated with the delay in the communication chain. A widely accepted approximation for \(I_{d}\) can be obtained from one-way delay in the communication path \(d\), where \(H\) is the Heaviside function (\(H(x)=0\) for \(x<0\) and \(H(x)=1\) for \(x>0\). In our case \(x=d-177.3\)). The one-way delay \(d\) (in milliseconds) includes the delay in the WiFi network plus 20 ms for the packetization interval of the codec.  
All terms in Equation (1) are known but the network delay \(d\) and packet loss rate \(P_{pl}\). The values for these two factors will be obtained from our WiFi mac-sublayer analytical model for each UAV deployed (we assume that backhaul networks do not introduce any negative impacts into QoS (i.e., infinite bandwidth), or it can be considered as a constant in \(R\)) (and its associated users). \(R\) is one of our objective function constraints, and its computation will be addressed in Section 5.2.2.