# Design of Software-Defined Radio-Based Adaptable Packet Communication System for Small Satellites
## 1 Introduction
The second column of Table 1 shows the differences in distance between the edge points of the communication window, i.e., 5\({}^{\circ}\) and 90\({}^{\circ}\) elevation angles, for several low Earth orbits (LEO). The third column shows the power density loss between the same edge points excluding atmospheric attenuation. This value describes the variation in power in that specific orbit. Since we excluded the atmospheric loss, the power loss difference is equal to the difference in free space loss (FSL) between the edge points of the pass calculated by Equation (1) [8]. \[FSL=20\;log\;(4\pi d/\lambda) \tag{1}\]  
where  
* \(d\): slant range;
* \(\lambda\) : signal Wavelength. The main goal of the research is to dynamically change the data rate of the transmission so that it can exploit the change in the channel. This would increase the quantity of data that can be delivered within the short communication period available for satellites in low Earth orbit (LEO). This allows us to automatically tweak the transmitted signal characteristics and the transceivers' parameters, allowing the system to perform better in dynamic channels. Table 1 explains that satellites in lower orbit would benefit more from this adaptability than satellites in higher orbits. The distance the signal must travel to a nadir ground station is 10 times greater than the distance the signal must travel when that satellite is at the \begin{table}
\begin{tabular}{c c c} \hline \hline
**Orbit Altitude (km)** & **Distance Difference (km)** & **Power Loss Difference (dB)** \\ \hline
400 & 1404.533 & 13.086 \\
500 & 1577.976 & 12.373 \\
700 & 1863.173 & 11.274 \\
1000 & 2194.512 & 10.088 \\ \hline \hline \end{tabular}
\end{table} 
Table 1: The change in slant range and transmitted power loss for a satellite between 5\({}^{\circ}\) and 90\({}^{\circ}\) elevations, calculated for different orbits. The values do not consider the atmospheric loss and attenuation, which is also proportional to the slant range. Figure 2: Slant range vs. elevation of different orbits. acquisition of signal (AOS) point in a 1000 km altitude orbit. The distance is 20 times greater in a 400 km altitude orbit. Traditionally, SDRs are used to reduce the cost and the time required to develop communication systems [9]. This paper and similar recent studies focus on improving their performance. This paper's is significant because it is at the last step before implementing artificial intelligence (AI) in satellite communication systems. Adaptable systems allow users to adjust a satellite's amplifier along the pass to ensure compliance with regulations, such as power density limits, while delivering adequate power to the ground station. Adaptable systems allow satellites to maximally benefit from their potential by tuning the link budget factors in relation to the current transmission situation in order to deliver the maximum quantity of data to the user. As such, the novelty of the research paper is that the design optimizes the SDR system's performance for small satellites, and it uses tools accessible to most satellite developers. Most importantly, it intends to make the use of an adaptable system in CubeSats and small satellites a reality, given that such adaptable systems are not used in CubeSats yet. For a given received power, if we increase the rate of data transmission, the energy bit per noise density decreases. The designed adaptable system facilitates a higher rate of data delivery without increasing the transmission power when the received power increases due to smaller losses of signal. In this way, the required SNR at the receiver is maintained within the acceptable range. The relationship is shown by the following equations [10]. \[SNR=S/N=Pr/(B\times N0) \tag{2}\]  
\[Pr=~{}(R\times Eb) \tag{3}\]  
\[SNR=R/B\times Eb/N0 \tag{4}\]  
where  
* _SNR_: Signal-to-noise ratio;
* _Pr_: Received power;
* _B_: Bandwidth;
* \(N\)0: Noise density;
* _R_: Data rate;
* _Eb_: Energy per bit. Theoretically, whenever the SNR increases by 3 dB, we can transmit at double the data rate.