Direct-to-cell (D2C) satellite internet—like what SpaceX (Starlink Direct-to-Cell) and AST SpaceMobile are building—is possible mainly because of advances in RF electronics, antennas, and signal processing. It wasn’t possible 10 years ago!

Uplink is harder because the transmitter is tiny, weak, and power-limited (the phone), while the receiver is far away (the satellite).

The core problem (electronics perspective)

A normal smartphone:

• Transmits ~0.2–1 W (23–30 dBm)
• Uses a tiny, inefficient antenna
• Was designed to talk to a cell tower a few km away

A satellite:

• Is 500–700 km away
• Sees huge free-space path loss (~160–170 dB)
• Must receive very weak signals

👉 The electronics must detect micro-volt level RF signals buried in noise and transmit back without draining the phone battery.

 

Key electronic technologies that make it possible

1. Massive phased-array antennas (on the satellite)

This is the biggest enabler.

What changed?

• Thousands of antenna elements
• Each element has:
  • RF front-end
  • Phase shifter
  • Gain control
• All controlled digitally

Why it matters

• Signals from many elements add coherently
• Effective gain becomes 30–40 dBi
• This compensates for the phone’s weak transmitter

📌 AST SpaceMobile satellites literally unfold into large RF “apertures” (~60–70 m²).

 

Ultra-low-noise RF front ends

The satellite receiver chain is extremely sensitive.

Key components:

• Low-Noise Amplifiers (LNAs) with noise figures ≈ 0.5–1 dB
• High-linearity mixers
• High-resolution ADCs

Electronics trick:

First amplifier dominates the entire noise budget (Friis equation)

Without modern GaAs / GaN / CMOS RF LNAs, D2C would not work.

 

Digital beamforming & DSP (very important)

Older satellites used analog beams.
D2C satellites use digital beamforming.

Electronics involved:

• High-speed ADCs
• FPGAs / radiation-hardened SoCs
• Massive DSP pipelines

What DSP does:

• Separates thousands of phones simultaneously
• Tracks Doppler shifts (~±40 kHz at LEO speeds)
• Cancels interference
• Dynamically steers beams

📌 This is only possible because space-qualified digital electronics have become powerful enough.

 

Using standard cellular frequencies

This is subtle but critical.

D2C uses:

• LTE / 5G NR waveforms
• Licensed cellular bands (e.g. 700 MHz, 850 MHz)

Why electronics matter here:

• Lower frequency = less path loss
• Smartphone RF chips already support these bands
• Satellite electronics must implement full cellular base-station functionality

So the satellite is electronically a:

Flying cell tower with a giant phased array

 

Advanced power electronics on the satellite

Big antennas + DSP = big power draw.

Needed:

• High-efficiency DC-DC converters
• Gallium Nitride (GaN) power amplifiers
• Smart power management ICs

GaN is key because:

• Higher efficiency
• Higher power density
• Better thermal handling in vacuum

 

Compare the two transmitters (electronics reality)

Smartphone (uplink transmitter)

• Output power: ~0.2–1 W (23–30 dBm)
• Antenna gain: −3 to 0 dBi
• Efficiency: poor (hand, body absorption)
• Strict SAR & battery limits

Satellite (downlink transmitter)

• Output power: 10–100+ W RF (40–50 dBm)
• Antenna gain: 30–40 dBi
• No human exposure limits
• Large solar arrays + batteries

📌 The satellite can transmit ~1,000,000× more effective power than the phone.

 

Effective Isotropic Radiated Power (EIRP)

EIRP = Transmit Power + Antenna Gain

Uplink (phone → satellite)

• Tx power: 23 dBm
• Antenna gain: −2 dBi
• EIRP ≈ 21 dBm (~0.13 W)

Downlink (satellite → phone)

• Tx power: 45 dBm
• Antenna gain: 35 dBi
• EIRP ≈ 80 dBm (100,000 W effective)

That’s a ~60 dB difference
→ 1 million times stronger downlink

 

Receiver sensitivity: who listens better?

Phone receiver (downlink)

• Noise figure: ~1–2 dB
• Bandwidth optimized
• Very short cable losses
• Designed to hear weak base stations

Satellite receiver (uplink)

• Noise figure: ~0.5–1 dB (excellent)
• But:
  • Huge bandwidth
  • Space radiation limits
  • Long RF distribution networks

Even with a better receiver, the satellite is still trying to hear a whisper from 600 km away.

 

Free-space path loss (the killer)

Path loss increases with distance:

FSPL(dB) (For LEO satellite (~600 km, 800 MHz)): ~161–165 dB loss

This loss hits uplink and downlink equally — but only one side has power to spare.

 

5. Antenna physics makes uplink brutal

Phone antenna:

• Tiny compared to wavelength
• Detuned by the hand
• Random orientation
• Polarization mismatch

Satellite antenna:

• Actively steered phased array
• Perfect polarization
• Huge effective aperture

👉 Antenna inefficiency hurts uplink much more, because there’s no transmit power margin.

 

Thermal noise sets a hard limit

Thermal noise floor:

N=−174 dBm/HzN = -174

If uplink bandwidth is wide:

• Noise power increases
• Required SNR increases
• Phone cannot increase power

Result:

Uplink data rate must be low

This is why early D2C supports:

• SMS
• IoT-like data
• Very low bitrate voice

 Doppler + timing hurt uplink more

Satellite speed ≈ 7.5 km/s

Effects:

• Doppler shift ±40 kHz
• Fast timing changes
• Random access bursts from phones

The satellite must:

• Detect unsynchronized, weak uplink bursts
• Correct Doppler per user
• Separate thousands of phones

Downlink is synchronized and controlled — much easier.

 

Regulatory & safety limits (electronics constraint)

Phones are limited by:

• SAR (Specific Absorption Rate)
• Battery heating
• PA linearity
• Efficiency at low supply voltage

You cannot “just increase power” in uplink.

 

Practical consequence in system design

This is why:

• Uplink uses very robust modulation (QPSK, BPSK)
• Low coding rates
• Narrow bandwidth
• Long symbol durations

Downlink can:

• Use higher-order modulation
• Use wider bandwidth
• Serve many users at once

 

One-line engineering summary

Downlink is a shout from space; uplink is a whisper from Earth.

The entire electronics challenge of D2C is:

Designing satellite receivers, antennas, and DSP that can hear that whisper.