# A Multimode Fusion-Based Aviation Communication System
## 2 System Design
### Link Calculation  
Communication using electromagnetic waves as a medium in wireless communication, electromagnetic wave propagation can be influenced by propagation distance, propagation medium, and its characteristics. It is critical to research electromagnetic wave propagation in aviation wireless communication. Figure 1: Architecture diagram of an aviation multimode fusion communication system. The radio can penetrate the ionosphere with a frequency above 30 MHz, and the ability to bypass the obstacles is weak. The basic mode is direct wave propagation between transmission and reception, and free space propagation is an ideal propagation model. Assuming an omnidirectional radiation source, for the transmission power in all directions, the unit is W. According to the principle of the conservation of energy, the energy radiated by the source per unit of time is always distributed on the sphere (or any necessary sphere) surrounding the source. Therefore, as the radius of the omnidirectional radiation sphere increases, the power or power flux density per unit area is  
\[\Phi_{d}=\frac{P_{t}}{4\pi d^{2}} \tag{1}\]  
where \(4\pi d^{2}\) means the superficial area of the ball, and the typical unit of power flux density is W/m\({}^{2}\). The power \(P_{r}\) received by the receiving antenna is determined by the size and orientation of the antenna relative to the receiver. In the effective area of the antenna \(A_{eff}\), the power received by the receiving antenna absorption cross-section is  
\[P_{r}=\Phi_{d}A_{eff}=\frac{P_{t}}{4\pi d^{2}}A_{eff} \tag{2}\]  
An omnidirectional transmitting antenna can be regarded as a point source, and according to electromagnetic theory, its effective area in any direction \(A_{isotropic}\) is  
\[A_{isotropic}=\frac{\lambda^{2}}{4\pi} \tag{3}\]  
where \(\lambda\) is the mean wavelength of the radiation waves. Bring Equation (3) into Equation (2), and the relationship between the transmission power and reception power of omnidirectional antennas is  
\[P_{r}=\frac{P_{t}}{\left(\frac{4\pi d}{\lambda}\right)^{2}} \tag{4}\]  
Based on Equation (4), the free space path loss can be summarized as  
\[L=\frac{P_{t}}{P_{r}}=\left(\frac{4\pi d}{\lambda}\right)^{2}. \tag{5}\]  
where \(L\) represents the free space path loss between two omnidirectional antennas, also known as diffusion loss. During signal transmission, the signal intensity difference between the transmitted signal and the received signal can reach more than ten orders of magnitude, and the power of a certain transmitter can reach the kilowatt (kW) level or even the megawatt (MW) level; in contrast, the power of the receiver can reach the picowatt (pW) level. In general, the intensity, gain, and loss values of the signal are usually represented in decibels (dB):  
\[L=10 lg\frac{P_{t}}{P_{r}}=C+20 lgf+20 lgd(\mathrm{dB}). \tag{6}\]  
where \(f\) means the transmission signal frequency, the unit is MHz, and \(d\) means propagation distance, the unit is km. As shown in Equation (6), the free space path loss \(L\) is only related to the transmission signal frequency \(f\) and the propagation distance \(d\). \(C\) = 32.45 is the unit conversion factor, and it changes with the units of \(f\) and \(d\). For instance, when \(f\) is in MHz and \(d\) is in miles, the value of \(C\) is 36.52. Nevertheless, when \(f\) is Hz and \(d\) is m, the value should be \(-147.5\)When using anisotropic antennas, the free space loss related to antenna reception and transmission power is  
\[P_{r}=\frac{P_{\mathrm{f}}G_{\mathrm{f}}G_{r}}{\left(\frac{4\pi d}{\lambda} \right)^{2}}=\frac{P_{\mathrm{f}}G_{\mathrm{f}}G_{r}}{L}. \tag{7}\]  
where \(G_{\mathrm{f}}\) and \(G_{r}\) means transmitter antenna gain and receiving antenna gain, respectively. Equation (7) is called the Friis equation.