Direct-to-Cell: A First Look into Starlink's Direct Satellite-to-Device Radio Access Network through Crowdsourced Measurements

TL;DR

The paper addresses the lack of empirical understanding of Direct Satellite-to-Device (DS2D) connectivity by Starlink in collaboration with T-Mobile. It adopts a data-driven, crowdsourced measurement approach to quantify RAN performance, SCS usage, and projected DS2D capacity, using SINR-derived metrics and a LTE-based capacity model. Key findings show a strong link between DS2D satellite deployment and observed measurements, with measurements clustering in underserved, accessible areas like national parks; current per-beam throughput is around $3.1$ Mbps and could rise to $18.6$ Mbps under broader spectrum and NTN integration, subject to regulatory and device support factors. The work demonstrates the feasibility of DS2D in real-world conditions, informs spectrum policy, and highlights crowdsourced data as a valuable tool for monitoring, planning, and evaluating next-generation satellite-enabled mobile connectivity.

Abstract

Low Earth Orbit (LEO) satellite mega-constellations have emerged as a viable access solution for broadband connectivity in underserved areas. In 2024, Starlink, in partnership with T-Mobile, began beta testing an SMS-only Supplemental Coverage from Space (SCS) service. This marks the first large-scale deployment of Direct Satellite-to-Device (DS2D) communications, allowing unmodified smartphones to connect directly to spaceborne base stations. This paper presents the first measurement study of deployed DS2D technologies. Using crowdsourced mobile network data from the U.S. between October 2024 and July 2025, we provide evidence-based insights into the capabilities, limitations, and future evolution of DS2D technologies for extending mobile connectivity. We find a strong correlation between the number of satellites deployed, the number of unique cell identifiers measured, and the volume of measurements, concentrated in accessible areas with poor terrestrial network coverage, such as national parks and sparsely populated counties. Stable physical-layer measurements were observed throughout the period, with a 24-dB lower median RSRP and a 3-dB higher RSRQ compared to terrestrial networks, reflecting the SMS-only usage of the DS2D network during this period. Based on the SINR measurements collected, we estimate the expected performance of the announced DS2D mobile data service to be around 3 Mbps per beam in outdoor conditions. We also discuss strategies to expand this capacity up to 18 Mbps in the future, depending on key regulatory and business decisions, including allowable out-of-band emissions, permitted number of satellites, and availability of spectrum and orbital resources.

Figures (6)

• Figure 1: No. monthly unique ECI vs. No. operational satellites.
• Figure 2: Spatio-temporal evolution of observed Starlink's DS2D measurements from October 2024 to July 2025.
• Figure 3: Starlink's measurement counts vs. T-Mobile's coverage. 1. Death Valley National Park, 2. Yellowstone National Park.
• Figure 4: SCS share vs. population density by county.
• Figure 5: Biweekly letter-value plots of key RAN metrics.