## 3 Related Work  
The literature contains multiple studies intended to address the experimental issues suffered during the development of satellite-based technologies. This section gathers relevant work and discusses their contributions.  
There are a few projects especially aimed at investigating transparent architectures in 5G. In particular, 5G-GOA [31] and 5G-LEO [32] are centered on the demonstration of 5G direct access in transparent satellite-based architectures, both adapting OpenAirInterface (OAI) to support the specific NTN conditions according to Release 17 [33]. The former project focuses on the GEO satellite systems. After preliminary in-lab validation with emulated satellite links [34; 35], the authors demonstrate the potential of their OAI implementation by deploying the solution with a real GEO satellite [35]. The latter project focuses on LEO satellite systems. Unlike 5G-GOA, 5G-LEO only includes in-lab demonstrations with a channel emulator, where the solution's performance is assessed [36]. Similar to the demonstration performed in 5G-GOA, authors in [37] also validate an OAI-based solution with a real transparent GEO satellite.  
The previous works contribute with the implementation of OAI features to support the direct access architecture in GEO and LEO. However, this objective does not encompass the exploration of alternative architectures or the integration of real LEO satellites into validations and tests.  
\begin{table}
\begin{tabular}{c c c c c c} \hline \hline
**Constellation** & **Operator** & **Altitude (Km)** & **Inclination (*)** & **Total Satellites** &
\begin{tabular}{c} **Launched** \\ **Satellites**[16; 17] \\ \end{tabular} \\ \hline Starlink [18; 19] & SpaceX & 328–1325 & 30–115.7 & 41,943 & 8737 \\ \hline OneWeb [20; 21] & Eutelsat & 1200 & 40–87.9 & 7088 & 660 \\ \hline Kuiper[22] & Amazon & 590–630 & 33–51.9 & 3236 & 29 \\ \hline Lightspeed [23] & Telesat & 1000–1325 & 37.4–99.5 & 1671 & 0 \\ \hline Gen4 [24] & Globalstar & 485–700 & 55–98 & 3080 & 0 \\ \hline Astra [25] & Astra Space & 380–700 & 0–98 & 13,620 & 0 \\ \hline GuoWang [26; 27] & China SatNet & 508–1145 & 30–85 & 12,992 & 0 \\ \hline Honghu-3 [28] & Hongqing & 340–550 & 48–60 & 10,000 & 0 \\ \hline \hline \end{tabular}
\end{table}
Table 1: Comparison of mega-constellation projects in mid-2025.  
Many studies also tackle the development of experimental testbeds. The authors in [38] present a testbed based on OAI which, employing Software-Defined Radio (SDR) modules, enables the deployment of direct access architectures for 5G NTN. It is capable of emulating transparent satellites in a LEO, a Medium Earth Orbit (MEO), a GEO, and even the L2 orbit. Likewise, the testbed introduced in [39] integrates a real LEO satellite to test 5G NTN technologies. However, the work only describes the future plans for this infrastructure, since the satellite has not been launched yet.  
Apart from testbed implementation, other studies also delve into indirect access architectures, i.e., satellites over the backhaul link. In [40], the authors describe their testbed and demonstrate the feasibility of implementing backhaul services with a real transparent GEO satellite. In a similar way, authors in [41] assess the performance of satellite backhaul connectivity based on Starlink by conducting a user-centric test campaign. This work is aligned with the present paper, but it is limited to the evaluation of the backhaul architecture.  
The scopes of the studies above have similarities with this paper. The current literature provides a comprehensive insight into the feasibility of 5G NTN, mainly for direct access architectures in a GEO. Nevertheless, the exploration of network architectures based on transparent LEO satellites has not been contemplated yet. This paper goes a step further by evaluating these architectures from the perspective of end users, as well as putting mega-constellations in the focus.