# Energy Aware Cluster-Based Routing in Flying Ad-Hoc Networks
## 3 EALC Methodology
### SNR and PLR  
Signal to noise ratio (SNR) is the ratio between the signal level and noise level. It indicates the strength of the received signal minus the noise power. Packet loss ratio (PLR) is a ratio of the total number of packets sent to the number of packets lost. In general, PLR depends on the path loss and noise interference, where path loss is determined by the transmission medium. Non-conductive materials such as buildings, hills human bodies, etc. produces absorptions and increase the path loss. The relationship between SNR, PLR and path loss can be derived as follows:  
\[PLR=1-(1-0.5e^{-0.78125+SNR})^{8f} \tag{2}\]  
where:  
\[SNR=P_{T}-PL(d)-P_{n}\]  
\[PL(d)=20\log\left(\frac{4\pi d}{\lambda}\right)\]  
\(P_{T}\) is transmission power, \"\(f\)\" is frame size, \(PL(d)\) is path loss, \(P_{n}\) is noise power and \"\(d\)\" is the distance between transmitting and receiving nodes. Figure 2 visualizes the relationship between SNR and PLR. An increase in SNR will lower the PLR, but at points SNR \(>P2\) and SNR \(<P1\), there is no change in PLR. If we increase \(P_{T}\) such that SNR \(>P2\), PLR remains at a minimum. Increasing \(P_{T}\) beyond this point does not improve PLR, but it only leads to high energy consumption. If we are able to estimate the maximum \"\(d\)\" (max transmission range) according to the application requirements, we can adjust \(P_{T}\) such that SNR \(=P2\) at maximum \"\(d\)\". At this point, \(P_{T}\) is low enough to achieve minimum PLR. Hence, node energy can be optimized by adjusting \(P_{T}\) according to the operational requirements.  
In order to find the transmission range requirement of the application, we divide the operation area or grid into four sub-grids. Nodes receive 3D positional information (latitude, longitude, and height) from each other. Based on this information, they calculate the average distance (using Equation (3)) from all nodes within the sub-grid:  
\[Avg_{Dis}=\frac{\sum_{i=1}^{n}d}{n} \tag{3}\]  
\"\(n\)\" is the number of nodes in sub-grid and \"\(d\)\" is the distance of each node from all other nodes in the same sub-grid. This average distance is then used to select the transmission power level of all nodes in the sub-grid. After defining the receiver sensitivity and computing the path loss at the average distance, transmission power must be set high enough such that SNR at the outer edge of transmission range must provide minimum PLR. After computing the \(P_{T}\) adopt the predefined power level that is the most appropriate to this \(P_{T}\). In this way, we select the transmission power according to an operational requirement. The selected power level is enough to maintain the contact with other nodes in the same sub-grid and low enough to save the node energy.  
Nearby flying nodes are likely to maintain their neighborhood with nodes in same sub-grid for some period of time. During flying, a node may change its sub-grid and can adapt the transmission power according to the current settings of the sub-grid. The main reason for dividing the operational area into four sub-grids is that it is found to be an optimal value. If we increase the sub-grid area (i.e., decreasing the number of sub-grids), it results in long average distances which cause the setting of high transmission power. Because the nodes only communicate with or through their nearby located nodes, setting high power will waste the node energy. On the other hand, division into a greater number of sub-grids results in very small sub-grid areas, which leads to frequent movement of nodes into sub-grids and they have to adopt transmission power accordingly. Frequent switching of power levels produces fluctuation in the electric current of the UAVs' circuitry.