## 5 Frequency Manipulation and Communication Feasibility
### ISL Communication Feasibility  
In order to check the feasibility of the ISL communication network, a link budget has been computed. Several communication parameters of the ISL network including the SDR output power, SDR transceiver sensitivity, antenna gain, and all loses associated with the communication links were measured. Frequencies of 400.15 MHz and 460.00 MHz were considered in this study. The output transmission power of the SDR transceiver and cable  
Figure 16: Demonstration of frequency manipulation: (**a**) 460.00 MHz to 400.15 MHz; (**b**) 400.15 MHz to 460.00 MHz. losses were measured as 0.1 W and 0.2 dB, respectively. Sensitivity (dBm) is the minimum power or signal strength that the SDR receiver can detect the transmitted information as having been. The minimum power was obtained by continuously attenuating the transmitted SS signal from Sat \(A\) with variable attenuators until the receiver at Sat \(B\) could not decode the SS signals. In order to ensure the integrity and accuracy of the SDR sensitivity, an RF shield box was utilized as shown in Figure 17.  
The RF shield provides a typical measurement environment which eliminates the influence of external unwanted noise, interferences, and internal signal reflections that may distort the signal. Moreover, a cable test environment was utilized to determine the optimum and typical sensitivity of the transceiver. The SDR receiver sensitivity value of \(-116\) dBm was obtained based on the following equation:  
\[R_{x}=T_{x}-L_{VA}-L_{c} \tag{10}\]  
where \(R_{x}\) (dBm) is the received signal power at Sat \(B\), \(T_{x}\) (dBm) is the transmitter output power at Sat \(A\), \(L_{VA}\) (dB) is the variable attenuation value, and \(L_{c}\) (dB) are RF cable losses. Table 3 summarises the sensitivity test results. The decoding success rate at Sat \(B\) was determined based on the SDR internal receiving gain which was tuned from 40 dB to its maximum value of 76 dB. When the variable attenuation was increased and the signal could not be decoded, the SDR gain values were increased until successful decoding occurred. This process was repeated until the SDR  
Figure 17: Sensitivity test setup. \begin{table}
\begin{tabular}{c c c c c c c c c} \hline \hline \multirow{2}{*}{\(T_{x}\) (dBm)} & \multirow{2}{*}{\(L_{VA}\) (dB)} & \multirow{2}{*}{\(L_{VA}\)+\(L_{c}\) (dB)} & \multirow{2}{*}{\(R_{x}\) (dBm)} & \multicolumn{8}{c}{**Success Rate Based on**} \\ \cline{5-8}  & & & & & \multicolumn{4}{c}{**SDR Internal \(R_{x}\) Gain (dB)**} \\ \cline{5-8}  & & & & **40** & **50** & **60** & **70** & **75** & **76** \\ \hline
20 & \(-100\) & \(-100.2\) & \(-80.2\) & o & & & & \\ \hline
20 & \(-120\) & \(-120.2\) & \(-100.2\) & x & \(\Delta\) & o & & & \\ \hline
20 & \(-130\) & \(-130.2\) & \(-110.2\) & x & \(\Delta\) & o & & & \\ \hline
20 & \(-135\) & \(-135.2\) & \(-115.2\) & & x & \(\Delta\) & o & & \\ \hline
20 & \(-136\) & \(-136.2\) & \(-116.2\) & & & & \(\Delta\) & \(\Delta\) & o \\ \hline
20 & \(-137\) & \(-137.2\) & \(-117.2\) & & & & x & x & \(\Delta\) \\ \hline
20 & \(-137\) & \(-138.2\) & \(-118.2\) & & & & & & x \\ \hline \hline \end{tabular}
\end{table} 
Table 3: Sensitivity test results (o: full packets decoded, \(\Delta\) : partial decoding, x: failed to decode). threshold sensitivity of \(-116.2\) dBm was reached at the maximum SDR receiver gain of \(76\) dBi and the decoding processing was no longer possible. The internal SDR transmitting gain at Sat \(A\) was maintained at its maximum value of \(89\) dB. A maximum separation distance of \(4600\) km large enough to probe water vapor profiles near Earth was assumed. Since this distance is large, use of a power amplifier was considered. A power amplifier of \(30\) W that can be used for small satellite sizes was considered [60]. A dipole antenna with theoretical gain of \(2.1\) dBi was used in the link budget estimations [61]. Other losses including antenna pointing, polarization, and transmission line losses were measured.