Imagine talking to someone far away like in deep forest or in the middle of ocean or even in another country, using your mobile phone. Have you ever wondered how your voice or message travels such long distances? A big part of the answer lies up in the sky with satellites (assuming that there is no roaming service).

Satellites are like big mirrors in space that bounce signals from one point on Earth to another, including the signal from your mobile phone. This is how we can send messages, make calls, and use the internet on our phones, even when we're in places where there are no phone towers nearby.

In this note, we're going to talk about how satellites and mobile phones talk to each other. It's a bit like playing catch with a ball, but instead of throwing a ball back and forth, we're sending invisible signals up to the sky and back down to Earth. This technology has made it possible for us to stay connected with people no matter where they are, even in the middle of the ocean or on top of a mountain.

We'll also look at what makes this communication challenging, how smart people are working to make it better, and what exciting things we can expect in the future. So, join us as we explore this amazing technology that keeps us connected to the world.

• Challenges
• How to Workaround the challenges ?
• The Dream of Universal Access: Satellite Tech’s Brutal Trade-offs
• Technical Challenges and Mitigation - Time Delay
• Technical Challenges and Mitigation - Doppler Shift
• Really Deployed ?
• Starlink / Tmobile Direct to Cell
• Inmarsat
• AST/SpaceMobile

Challenges

Communicating between satellites and mobile phones, despite its widespread use and critical importance, faces several challenges. These challenges can affect how well and how reliably we can communicate, especially in remote areas or during critical situations.

• Latency Issues: Latency is the delay before a transfer of data begins following an instruction for its transfer. In satellite communication, signals have to travel thousands of kilometers to space and back, which can cause a noticeable delay. This is particularly problematic for real-time applications like video calls or online gaming.
• Implication on Implementation: The inherent delay in satellite communication due to the long distance signals must travel presents a challenge to meeting these latency requirements. To align with 3GPP standards, satellite communication systems need to implement techniques like onboard processing and optimized routing to reduce latency, ensuring compatibility with applications demanding low delay, such as VoLTE (Voice over LTE) and real-time gaming. ==> We may try hard to reduce the latency by data/signal processing on UE and Satellite side, but there wouldn't be much thing to do with the delay caused by propogation delay between UE and Satellite.
• Doppler Shift : One of the significant challenges is the Doppler shift. This phenomenon occurs because the satellite and the mobile phone are moving relative to each other. As the satellite orbits Earth, its speed causes the frequency of the signals it sends and receives to change slightly. This effect can make it difficult for the mobile phone to accurately pick up the signal, as the frequency might shift out of the phone's expected range. Overcoming the Doppler shift requires sophisticated technology in both satellites and mobile phones to adjust the frequencies of the signals they send and receive.
• Implication on Implementation: :The 3GPP standards include specific provisions for compensating frequency variations, including those caused by the Doppler effect, especially in high-speed scenarios like high-speed trains. For satellite communications, adapting these standards means developing more sophisticated Doppler shift compensation algorithms that can handle the relative movement between satellites and mobile phones. This ensures consistent signal quality and reliability across all mobile networks, including those extended through satellite links.
• Delay Spread : Another challenge is delay spread, which refers to variations in signal travel times due to different paths the signal might take. In satellite communication, while the open space minimizes obstacles, the immense distances the signal travels can cause parts of it to arrive at slightly different times. This variation can distort the received signal, affecting the quality and clarity of the communication.
• Implication on Implementation: : Delay spread, with its potential to distort signals, directly impacts the Orthogonal Frequency-Division Multiplexing (OFDM) technology used in LTE and 5G networks, as defined by 3GPP. OFDM is sensitive to timing discrepancies, which can cause inter-symbol interference. Addressing delay spread in satellite communications involves incorporating advanced equalization and timing correction techniques that are robust enough to handle the variations, thereby aligning with 3GPP standards that aim for high-quality, clear communication.
• Transmission Power : The vast distance between satellites and mobile phones also poses a challenge for transmission power. Transmitting a signal across thousands of miles requires significant power, but satellites are limited by their size, weight, and the solar energy they can collect. Balancing the need for a strong, clear signal with these power constraints requires careful engineering and power management.
• Implication on Implementation: : The 3GPP specifications detail power control mechanisms to optimize signal strength and quality while minimizing interference and power consumption. In the context of satellite communications, adhering to these specifications means developing satellite payloads and mobile devices that can operate effectively within the power constraints set by 3GPP. This involves leveraging highly efficient power amplifiers, smart power management systems, and adaptive transmission techniques to ensure that both uplink and downlink communications meet the power requirements and constraints defined by 3GPP.
• Receiver Sensitivity : On the receiving end, mobile phones and satellite must have the sensitivity to detect the satellite's signal, which can be weak and prone to interference. Improving a mobile phone's receiver sensitivity without significantly increasing its power consumption or cost is an ongoing challenge in the field.
• Implication on Implementation: : Receiver sensitivity is crucial for achieving the high-performance levels specified by 3GPP, particularly in challenging signal conditions. For satellite communications, enhancing receiver sensitivity to meet 3GPP standards involves deploying advanced noise reduction and signal processing technologies. This ensures that mobile phones and satellite receivers can detect and decode signals effectively, even at the low signal-to-noise ratios common in satellite links, thereby supporting the reliable, high-quality service expected from 3GPP-compliant networks.

How to Workaround the challenges ?

Addressing the challenges of satellite communication involves innovative workarounds and technologies, particularly when considering the integration of satellite systems with conventional User Equipment (UE) and Non-Terrestrial Network (NTN) capable UE, as defined by 3GPP standards.

NOTE : I guess most of the demo(pre-deployment ?) shown in 2020 or earlier is based on the satellite and non NTN UE (regular mobile phone without supporting NTN) because it is before the finalization of 3GPP NTN specification or UE modem chipset availability and they claim that the communication is possible with existing mobile phone without any modification. But almost no technical details in terms of protocol issues. So my comments on non-NTN devices are from my speculation.

• Latency Issues
• Satellite: I can think of a few options.
• Choice of orbit (LEO or GEO). This would be the biggest factor.
• Choice of eNB/gNB configuration : RACH Preamble type, RAR window size, Contention Resolution Window, HARQ related parameters etc.
• Employing the advanced DSP to reduce the internal delay may do some help but not as much of orbit choice.
• Conventional UE: According to the claims from various satellite solution provider, there wouldn't be any special tricks required.
• NTN Capable UE: Compliance to NTN specification (e.g, handling large TA(Timing Advance), HARQ requirement)
• Doppler Shift
• Satellite: Design satellites and their payloads with advanced frequency compensation technologies that automatically adjust for Doppler shifts caused by their movement relative to the Earth. Doppler shift can be compensated both by Satellite and UE, but I think the compensation by Satellite would be the major factor.
• Conventional UE: According to the claims from various satellite solution provider, there wouldn't be any special tricks required.
• NTN Capable UE: How to handle Doppler shift is not directly specified by 3GPP, but advanced DSP(e.g CFO (Channel Frequency Offset) compensator, Advanced Channel Estimator / Equalizer) would be a great help.
• Delay Spread
• Satellite: Employ advanced modulation and coding schemes that are more resilient to the effects of delay spread, as well as smart beamforming technologies that can focus signals more directly to the intended receiver, reducing multipath interference.
• Conventional UE: According to the claims from various satellite solution provider, there wouldn't be any special tricks required.
• NTN Capable UE: NTN-capable UEs can use more advanced equalization and error correction protocols that are optimized for the unique challenges of satellite communication, including delay spread.
• Transmission Power
• Satellite: I think this should be handled mostly by Satellite using special techniques as below
• High Power / High Gain Antenna. Due to this, the satellites for cellular communication has antenna with huge size
• Advanced Beam Forming
• Conventional UE: There wouldn't be any breakthrough on UE side because the max power from UE is strictly limited by 3GPP.
• NTN Capable UE: There wouldn't be any breakthrough on UE side because the max power from UE is strictly limited by 3GPP
• Receiver Sensitivity
• Satellite: Like TX power, I think this should be handled mostly by Satellite using special techniques as below
• High Power / High Gain Antenna. Due to this, the satellites for cellular communication has antenna with huge size
• Advanced LNA.
• The environment around satellite (very low temperature comparing to terrestrial environment) can be a strong positive factor (i.e, very low thermal noise)
• Conventional UE: According to the claims from various satellite solution provider, there wouldn't be any special tricks required.
• NTN Capable UE: Application of high quality LNA, advanced Equalizer etc can help.

The Dream of Universal Access: Satellite Tech’s Brutal Trade-offs

The idea of universal access provided by the communication between satellite and mobile phone would sound fancy. Imagine a world where your smartphone connects seamlessly to the internet, no matter where you stand—deep in a forest, adrift at sea, or atop a remote mountain. This is the tantalizing dream of universal access, fueled by satellite technology like Direct-to-Cell or NTN. However, making this happen isn’t easy. Companies face big challenges: huge antennas that make satellites expensive, lots of satellites that cost a fortune, and a tough choice between low orbits for fast signals or high orbits to cover more places. The dream of connecting everyone is amazing, but the real-world problems of cost and technology are just as huge.

In this section, I want to talk about a couple of critical trade-off (dilemmas) we need to overcome. I don't have any clear answer to this dilemmas but I wanted to clarify on what we should overcome (i.e, clear definition of problem itself).

Antenna Size and Satellite Design

To enable direct communication with standard user equipment (UE) like mobile phones—without requiring specialized hardware—the satellite needs a large antenna (e.g., AST SpaceMobile’s 64 m² or Starlink’s 25 m²). This increases the satellite's size, complexity, and cost.

Larger antennas are necessary to focus radio signals into a sufficiently strong beam to reach small, low-power devices like smartphones. This is because standard phones have limited transmission power and small antennas compared to dedicated satellite receivers. However, scaling up the antenna size poses challenges:

• Launch Constraints: Larger satellites require more powerful (and expensive) launch vehicles, and there are physical limits to how big a satellite can be while still fitting into a rocket fairing. Techniques like deployable antennas (e.g., phased arrays that unfold in space) help, but they add engineering complexity and risk of failure.
• Power Requirements: Bigger antennas and the associated electronics demand more power, necessitating larger solar panels and batteries, further driving up size, weight, and cost.
• Cost Implications: The manufacturing, testing, and deployment of such satellites become significantly more expensive, making it a high-stakes investment for companies aiming to scale this technology.

Orbit Altitude and Propagation Delay vs. Coverage

Lower orbits (e.g., Low Earth Orbit, LEO, at 300-2000 km) reduce propagation delay, which is critical for real-time applications like voice calls or gaming. However, LEO satellites have a smaller coverage footprint, requiring a large constellation for continuous service, which increases costs. Conversely, higher orbits (e.g., Geostationary Orbit, GEO, at ~36,000 km) allow fewer satellites to cover vast areas but introduce significant delays (~250-300 ms one-way), degrading performance.

• LEO Challenges: In LEO, satellites move rapidly relative to the Earth (e.g., orbiting every 90-120 minutes), so a single satellite can only serve a given area for a few minutes before handing off to another. This necessitates a dense constellation—hundreds or thousands of satellites (e.g., Starlink’s planned 42,000)—to ensure no coverage gaps. The sheer scale drives up costs for manufacturing, launch, and orbital management (e.g., collision avoidance, deorbiting).
• GEO Trade-offs: GEO satellites remain fixed over one spot, providing continuous coverage with just a few satellites (e.g., 3 GEO satellites can cover most of the planet). However, the long distance introduces latency that’s unacceptable for latency-sensitive applications. Additionally, the signal strength weakens over such distances, requiring either more powerful satellite transmitters or slight UE hardware modifications—contradicting the "no special hardware" goal.
• Middle Ground (MEO): Medium Earth Orbit (MEO, 2,000-20,000 km) offers a compromise, with moderate latency (~50-150 ms) and fewer satellites than LEO (e.g., O3b’s constellation uses ~20 satellites). However, MEO still requires more satellites than GEO and doesn’t fully solve the latency or coverage density issues.

Number of Satellites and Investment Scale

A LEO constellation requires a massive number of satellites for continuous coverage, demanding enormous upfront investment. Higher orbits reduce the satellite count but compromise performance due to delay.

• LEO Economics: Building, launching, and maintaining thousands of satellites (e.g., Starlink’s $10+ billion investment) is a capital-intensive endeavor. Companies must also account for satellite lifespan (5-7 years in LEO due to atmospheric drag) and replacement costs. However, LEO’s lower latency and higher throughput potential (e.g., 20-150 Mbps for Starlink) make it attractive for consumer markets if costs can be amortized over millions of users.
• GEO Economics: GEO satellites are expensive individually (hundreds of millions each), but fewer are needed, reducing total constellation costs. However, their high latency limits them to applications like broadcasting or low-bandwidth IoT, not broadband or real-time mobile services.
• Spectrum and Interference: Both approaches require access to radio spectrum, which is a finite and regulated resource. LEO constellations may face interference challenges due to their density, while GEO satellites must compete with existing users in higher bands.

Technical Challenges and Mitigation - Time Delay

Delivering LTE and 5G services directly from low‑Earth‑orbit satellites forces protocols built for terrestrial cell sites to cope with a propagation delay an order of magnitude larger than they were originally designed for. Random‑access handshakes, HARQ acknowledgements, timing‑advance corrections, and other millisecond‑level control loops must still complete on time even though each signal now travels hundreds of kilometres into space and back. This section maps these tight air‑interface budgets against the true radio round‑trip of a LEO link, then shows how careful orbit geometry, on‑board base‑station processing, existing flexibility in LTE‑TDD and NR timing tables, and—when necessary—upper‑layer retransmission techniques keep every critical timer inside spec without modifying today’s handsets

NOTE : Most of the mitigation techniques mentioned here is not specified in 3GPP and mostly proprietary which are not explicitely open to public. Each of the satellite service provider would use different cocktails of the tricks and some of them would use other tricks which is not mentioned here. So take these tricks in this secion as a kind of potential/feasible tricks for brain storming.

Cellular Timing Budgets vs. LEO Satellite Round‑Trip Delay

Conventional cellular protocols have always relied on sub‑millisecond reflexes between a handset and a tower a few kilometres away, yet a low‑Earth‑orbit link stretches that path to hundreds of kilometres without breaking the rhythm—if every micro‑process in the stack is re‑examined. The table below isolates each timing‑critical loop in LTE and NR, lays its terrestrial budget beside the raw round‑trip that space imposes, then reveals whether the margin survives untouched or must be rescued by clever engineering

Generic mitigation toolbox

The tricks listed in this section is an extension to the list in previous section. The table below lists a set of practical tricks that let a normal LTE or 5G phone work even when its “cell tower” is a fast‑moving satellite in low orbit. Each trick tackles one of the big space problems—extra delay, fast Doppler shifts, or gaps in coverage—while leaving the phone itself unchanged. Some ideas use room that already exists in the standards, like a longer ACK time in LTE‑TDD. Others add a small, fixed delay on purpose so the up‑link and down‑link keep the same rhythm. When tight timing still cannot be met, the network simply repeats messages or lets a higher layer retry. Finally, operators turn on only the easiest services first, such as text messages and alerts, and wait until more satellites are in the sky before enabling voice and full data. Together, these methods hide the extra space delay and make the satellite cell feel almost the same as an ordinary ground cell to the user.

NOTE: On‑orbit processing, geometry‑aware scheduling, existing LTE‑TDD / NR k₁ flexibility, the option to drop L1 HARQ, and longer TTIs let a LEO satellite satisfy every critical timing window—even without any handset NTN extensions. 3GPP Rel‑17 merely formalises what these work‑arounds already achieve.

Technical Challenges and Mitigation - Doppler Shift

When a satellite rushes overhead at nearly 8 km /s, every radio signal between the phone and its “space cell” slides tens of kilohertz up or down in frequency, and that shift is changing hundreds of hertz every second. This section will describe on how this fast, large Doppler shift collides with the frequency‑error limits that LTE and 5 G were written for, and then list the common engineering tricks that can mitigate those problems.

NOTE : Most of the mitigation techniques mentioned here is not specified in 3GPP and mostly proprietary which are not explicitely open to public. Each of the satellite service provider would use different cocktails of the tricks and some of them would use other tricks which is not mentioned here. So take these tricks in this secion as a kind of potential/feasible tricks for brain storming.

Cellular Doppler Budgets vs. LEO Satellite Doppler

A signal sent from a phone to a low‑Earth‑orbit satellite is hit by a Doppler shift that can move its frequency tens of kilohertz and change thousands of hertz every second, far more than a ground network ever sees. The table compares those fast, large shifts with the much smaller frequency‑error limits built into LTE and 5G procedures such as cell search, sub‑carrier alignment, and PRACH timing. It then shows the standard tricks engineers rely on to keep each procedure inside its original tolerance. Together these measures let an ordinary handset lock onto and stay connected to a cell that is racing across the sky.

Generic mitigation toolbox — Doppler shift

The list below gathers the practical fixes engineers apply so the handset never notices that its serving tower is racing through space. Some solutions are simple—wait until the satellite is high overhead or shift the satellite’s carrier by the expected amount—while others work in the background, such as tiny frequency tweaks every OFDM symbol or extra‑wide sub‑carrier spacing. A few tactics give the phone more room to find the signal, repeating the sync blocks or asking the handset to scan a wider band. Together these measures chase the Doppler error down to a level that fits inside the normal LTE and 5 G limits, allowing everyday devices to connect as if the cell were standing still.

Really Deployed ?

Satellite to cellular communication is not a wish-to-have technology any more. There are several solutions that are already deployed (or at least demonstrated / proved working). Followings are a list of these solution :

Starlink / Tmobile Direct to Cell

Source : SPACEX SENDS FIRST TEXT MESSAGES VIA ITS NEWLY LAUNCHED DIRECT TO CELL SATELLITES

• Satellite Orbit : LEO
• Radio Access Technology : 4G
• Frequency/Band : 1910-1915 and 1990-1995MHz radio bands (the PCS “G block”)
• Channel Bandwidth : 1.4Mhz, 5 Mhz
• Max Data Rate : 2 Mbps
• Antenna Dimension : roughly 25 square meters in size.
• Partner Terestrial Operator : T-Mobile

Inmarsat

Source : Satellites - inmarsat

• Satellite Orbit : GEO

• Radio Access Technology : 4G

• Frequency/Band : L-band (?)

AST/SpaceMobile

Source : AST/SpaceMobile

• Satellite Orbit : LEO

• Radio Access Technology : 4G / 5G

• Frequency Band : 850 MHz cellular A and B block as well as certain lower 700 MHz B and C block

• Antenna Dimension : approximately 64 square meters (693 square feet) , approximately 223 square meters with block 2/3

• Test UE : Samsung Galaxy S22
• Partner Terestrial Operator : AT&T
• Further Details : This Note

YouTube

• Inmarsat Overview - OCENSSupport (2021)
• AST SpaceMobile: Transforming how the world connects - AST SpaceMobile(Sep 2021)
• How Vodafone and AST SpaceMobile plan to connect for a better future - AST SpaceMobile(Feb 2022)
• Coverage Above & Beyond | T-Mobile - T-Mobile (Sep 2022)
• Nokia explains how AST SpaceMobile is designed to provide 4G/5G cellular broadband from orbit - AST SpaceMobile (Oct 2022)
• How does Starlink Satellite Internet Work? - Branch Education (2022)
• Watch Elon Musk's SpaceX and T-Mobile Reveal Mobile Service For Areas with No Cell Coverage - CNET Hightlights (Aug 2022)
• SpaceX + T-Mobile Update - SpaceX (Aug 2022)
• How AT&T and AST SpaceMobile plan to extend coverage for consumers, businesses & first responders - AST SpaceMobile (Dec 2022)
• Testing 5G NTN in the Lab - VIAVI Soultions (Jan 2023)
• I-6 F2 technology - Inmarsat (Feb 2023)
• First-Ever 5G Connectivity from Space to Everyday Smartphones Achieved by AST SpaceMobile - AST SpaceMobile (Sep 2023)
• SpaceX Starlink Direct To Cell Is Here - JCristina (Oct 2023)
• Starlink’s Direct to Cell Satellite Phone Service Set to Launch in 2024 - OCWorkbenchTechTV (Oct 2023)
• How AST SpaceMobile Plans to Beam Broadband From Space to Your Phone - AST SpaceMobile (Dec 2023)
• Direct to Cell Technology in 2024: A Quick Summary - Build a CubeSat (Apr 2024)
• Starlink Direct to Cell: How it works, common misconceptions, and answers to your questions - DISHYtech(Dec 2024)
• Starlink Direct to Cell Beta Now Open - DISHYtech(Dec 2024)
• T-Mobile Starlink Direct to Cell Review | The Worst It Will Ever Be- DISHYtech(Feb 2025)
• T-Mobile Starlink Direct-to-Cellular Service Pricing & Now Available to 'Everyone' - Mobile Internet Resource Center (Feb 2025)
• The future of Satellite & NTN | Expert insights from MWC25 - GSMA (Mar 2025)
• 6G@UT Forum 2025: AST SpaceMobile System and Supplemental Coverage from Space for D2C - Lindmark Melo (Apr 2025)

Reference

• NTN - Sharetechnote
• NTN Part 1 – 3GPP Rel. 17 – Non-Terrestrial Networks - WirelessMove(Feb 2023)
• NTN Part 2 – NB-IoT over Satellite - WirelessMoves (Mar 2023)
• NTN – Part 3 – 3GPP Rel. 17 – System Architecture for Satellite Services- WirelessMoves (Mar 2023)
• NTN – Part 4 – Satellite and Mobile Operator Mix and Match - WirelessMoves (Mar 2023)
• NTN – Part 5 – Why Does the UE Need a GPS Fix? - WirelessMoves (Mar 2023)
• NTN – Part 6 – Satellite Mobility - WirelessMoves (Apr 2023)
• Tradespace Exploration of the Next Generation Communication Satellites - MIT Open Access Articles (2019)
• T-Mobile Takes Coverage Above and Beyond With SpaceX - T-Mobile (Aug 2022)
• SpaceX and T-Mobile partner for direct-to-cellphone satellite service - Space News (Aug 2022)
• Visual Magnitude of the BlueWalker 3 Satellite - arXiv (Nov 2022)
• MediaTek and Inmarsat join forces on two-way satellite communications direct to smartphones, IOT devices and cars - inmarsat (Apr 2023)
• AT&T leases spectrum to AST SpaceMobile - FIERCE Wireless (May 2023)
• AST SpaceMobile Confirms 4G Capabilities to Everyday Smartphones Directly From Space - AST/SpaceMobile(Jun 2023)
• AST SpaceMobile Achieves Space-Based 5G Cellular Broadband Connectivity From Everyday Smartphones, Another Historic World First - AST/SpaceMobile (Sep 2023)
• Satellite to cellular: Everything you need to know about satellite communication on smartphones, including Galaxy S24 - Pocket-lint (Oct 2023)
• Emergency SOS via satellite: Everything you need to know about Apple's iPhone safety feature - Pocket-lint(Sep 2023)
• AST SpaceMobile adjusts launch plan for potential direct-to-device customer- Space News (Nov 2023)
• Android and iPhone satellite connectivity: Here's what you need to know - Android Authority (Nov 2023)
• SPACEX SENDS FIRST TEXT MESSAGES VIA ITS NEWLY LAUNCHED DIRECT TO CELL SATELLITES - STARLINK
• SpaceX deploys direct-to-smartphone satellites in first launch of 2024 - SPACENEWS (Jan 2024)
• SpaceX Inches Toward Cellular Starlink Service With First 'Direct to Cell' Launch - PC Magazine (Jan 2024)
• Starlink Successfully Tests Space Direct to Cell Mobile Service - ISPreview (Jan 2024)
• Satellites - inmarsat
• Mobile Environment Coverage Issues and Delay Spread Calculations in Modern Communications Systems - Science Alert