# Design of Software-Defined Radio-Based Adaptable Packet Communication System for Small Satellites
## 1 Introduction  
Software-defined radio (SDR) is a flexible technology that enables the design of an adaptive communications system. Accordingly, a generic hardware design can be used to address various communication needs, with varying frequencies, modulation schemes and data rates [1]. The radio implementation process includes setting the filtering parameters, such as the pass and stop band frequencies, as well as digital quadrature transformations and data rate adjustments using up and down sampling processes. Recent improvements in analogue to digital converter technology have led to the development of software-defined radios with digital receivers [2]. Many satellites have adopted the SDR architecture as an experimental or secondary subsystem, but this year--according to the ESA--the first fully SDR commercial satellite was launched: the Eutelast satellite \"Quantum\". Eutelast Quantum will be the first genera- to universal satellites able to serve any region of the world and adjust to new business without the need to procure and launch an entirely new satellite. The first Quantum satellite will have a launch mass of 3500 kg, a power of 5 kW, and an all-Ku-band communications payload mass of 450 kg [3]. Amateur operators have built many SDR-based ground stations;however, it is not yet common for amateur or educational small satellites to use SDR as the main communication subsystem. However, this will change in the near future, given the amount of research being carried out in this field and the benefits of SDRs over traditional radios, such as reconfigurability and adaptability. Adaptation is one of the smartest use of the SDR. The European Space Agency (ESA) is currently funding the Satellite Adaptive Communication Channel project (SACC), a pioneer system intended to increase the data sent from satellites. Similarly, this paper develops a design to be implemented in Raspberry Pi devices, which are used as satellite subsystem processors. The design takes advantage of the SDR to reduce the cost and simplify the complexity of the electronics design; moreover, it boosts the communication system's performance by implementing feedback and adaptable techniques. In satellite communication, the Earth-satellite link is established only when the Earth station enters the beam width of the satellite's antenna [4]. As shown in Figure 1, the slant range is the distance between the satellite and its ground station. Satellites arise to each specific ground station at the furthest point, then move closer until reaching a minimum distance at 90\({}^{\circ}\) elevation. Signal attenuation is highly dependent on the length of the line-of-sight path. As such, the maximum slant range value should be considered when designing the link budget. The designing of any communication system starts with the link budget analysis [5]. The link budgets of satellites are designed to maintain connectivity in the worst case of communication. However, to utilize the communication windows, the signal's characteristics may be altered in order to deliver more data to the ground station within the same period of time [6]. When the satellite approaches the ground station's horizon, the signal must travel a significantly greater distance than when the satellite is at the zenith of its path over the ground station. The graph in Figure 2 shows the typical communication window line of sight range in kilometers for different altitude orbits. Figure 1: Slant range variation according to the satellite’s position. Small spacecraft (SmallSats or small satellites) include spacecraft with a mass less than 180 kg [7]. CubeSats fall under the definition of small satellites. The standards dictate that they weigh 1.33 kg maximum and are 1 cm \(\times\) 1 cm \(\times\) 1 cm. The distance between a satellite in ISS orbit and its ground station can reach more than three times the distance when the satellite is at zenith. This directly affects the received signal strength at the ground station.