## 1 Introduction  
User mobility support is one of the main and the most attractive features of mobile wireless technologies. Furthermore, user devices typically support more than one wireless technology. Smartphones, for example, support access to mobile networks like 4G or 5G, but at the same time support WiFi access as well. Modern wireless technologies also represent the main driving force for emerging applications in areas such as V2X (Vehicle-to-everything), IoT (Internet of Things), eHealth, etc. [1]. The goal of modern communications is to utilize these available multiple network accesses to provide users better QoE (Quality of Experience), higher communication reliability and availability, and seamless communication sessions [2, 3]. Moreover, 5G technology has recognized this trend and has defined the ATSSS (Access Traffic Steering, Switching and Splitting) paradigm that is in line with the aforementioned goal [4]. Satellite technologies have always offered great coverage and access to communication in difficult-to-cover zones. However, the usual downsides are the price, limited upstream bandwidth, sensitivity to weather conditions and high latency. With this in mind, modern LEO (Low Earth Orbit) satellite communication systems have been emerging to provide much lower latencies and good bandwidth in both directions (downstream/upstream) at affordable prices that will probably be even lower in the near future. Starlink and OneWeb are typical and successful examples of such systems [5; 6]. Also, drone technology evolution and expansion will provide UAV (Unmanned Aerial Vehicle)-based extensions of mobile communication networks to support congested cell traffic offload and coverage expansion [7; 8]. Given the advancements in LEO satellite communication systems, these systems have been recognized as a good complement to 5G terrestrial systems to provide increased coverage, better reliability and availability, and bandwidth increase to mobile users [9; 10]. For these reasons, hybrid satellite-terrestrial network access is gaining attention as a good strategy approach in the near future that would enable many benefits to users [11; 12]. Vertical handover can be used to move the ongoing user session between different network technologies like between terrestrial and satellite network access [13]. However, the ATSSS paradigm expands this approach by enabling users to utilize both accesses at the same time. Steering and switching functions can be observed to utilize only one access at a given time, but splitting by definition assumes the usage of multiple network accesses at the same time. ATSSS implementation has two approaches--low level (the data link layer) and high level (the transport layer). Low level requires that the networks involved share the same 5G network core, while high level may even be implemented in a way that access networks are unaware of it. Also, it is worth noting that machine learning and artificial intelligence are becoming more and more popular approaches in optimizing multiple network access utilization according to user preferences such as price, latency, packet loss rate, etc. [14; 15]. To estimate the quality of multiple network accesses approaches and/or used techniques (optimization techniques, machine learning, etc. ), it would be very beneficial to have simulation frameworks and platforms in order to generate diverse and plentiful data [16]. Simulations can efficiently and economically provide some insight in performances of the tested approaches [17]. Based on these insights, the further tuning and elimination of detected issues can be performed. Software simulations are a more economical approach, but hardware-based simulations can provide a shorter simulation time and can use hardware accelerators to not only reduce the simulation time but also to enable the inspection of the features that are not feasible in software (for example, LDPC (Low-Density Parity Check) coding/decoding [18]).