## 1 Introduction  
In the dynamic landscape of contemporary broadband communications, the pursuit of higher data rates aligns seamlessly with the quest for widespread connectivity. This becomes particularly crucial as more tourists explore remote natural areas, where traditional communication infrastructure is either absent or severely limited. The challenge of providing mobile services in such areas persists, and current satellite solutions, though capable, often demand costly and cumbersome user equipment, discouraging a substantial user base and relegating these systems to niche applications. A promising solution to overcome this challenge involves delivering broadband satellite services directly to regular handsets, specifically unmodified smartphones. This approach has proven successful in transforming niche applications into mainstream phenomena, as exemplified by the integration of GPS functionality into widely adopted consumer gadgets like smartphones and tablets. Supporting LTE/NR satellite services with regular handsets could potentially provide a viable solution to access broadband services in remote areas. However, implementing direct-to-device (D2D) satellite systems in terrestrial spectrum bands poses significant challenges, especially when dealing with unmodified smartphones. These challenges encompass interference issues, Doppler shifts, which affect signal frequency and phase, causing distortion and reception problems, and delays complicating the realization of Hybrid Automatic Repeat Request (HARQ) protocols, which are crucial for reducing the Bit Error Rate (BER) in channels, and the inherent limitations of smartphones' relatively low power, leading to constraints in link budget and throughput. Regulatory challenges also arise due to the operation of these systems in terrestrial spectrum bands, falling outside traditional satellite service allocations. This study delves into the multifaceted issue of interference in D2D satellite systems, exploring potential problems that may arise when implementing such systems. A notable challenge is the necessity to utilize terrestrial spectrum bands for satellite services, which were not originally intended for mobile satellite services. Some companies, like AST SpaceMobile and Lynk, have ventured into this domain, launching test satellites capable of providing broadband services in UHF terrestrial mobile bands. Although utilizing UHF bands offers advantages due to their excellent propagation characteristics and favorable link budgets, it comes with the constraint of requiring larger antennas, a challenge imposed by payload mass limitations [1]. To address the antenna size limitation, one potential solution is to operate in higher terrestrial spectrum bands. SpaceX and T-Mobile's collaboration plans to launch second-generation Starlink satellites serving regular handsets within the n25 band, specifically 1910-1915 MHz (Earth-to-space) and 1990-1995 MHz (space-to-Earth). However, this frequency band presents a challenge, as the space-to-Earth link overlaps with the mobile satellite service (MSS) S-band, using the 1980-2010 MHz (Earth-to-space) and 2170-2200 MHz (space-to-Earth) bands. This overlap introduces the potential for reverse space-to-space link interference, a complex issue due to radio waves' propagation in a vacuum with minimal attenuation. Additionally, the S-band, defined as the n256 band in 3 GPP for NTN networks, is supported by future smartphones, creating an overlap with other potential D2D systems built on future NTN networks. This paper presents an interference analysis conducted between second-generation Starlink satellites and MSS and NTN systems, focusing on the frequency band 1990-1995 MHz. It specifically considers typical MSSs, which are based on ITU filings from various MSS networks and designed to offer voice calls, IoT services, and data transfer services in the S-band. The study also explores the potential interference with NTN systems in the n256 band based on 3 GPP specifications.