How SpaceX Direct-to-cell are going to overcome technical challenges using LTE standard for signals from Satellite to Smartphones?

As you probably know, engineers from Direct-to-cell project are claiming to put usual LTE eNBs on board of LEO satellites (with a speed ~7.7km/h) and make it real to receive/send information just by using unmodified usual smartphones.

Some facts :woman_astronaut: :

  • around 200 Direct-to-Cell Satellites already deployed.
  • this service in a test mode right now.
  • 1910-1915 MHz and 1990-1995 MHz bands (the “PCS G Block” of T-Mobile US) are used for testing aims.

As you know, LTE standard is designed as an “earth surface” network.

3GPP LTE terrestrial cellular network is designed/planned by taking into account the typical cell size (less then 15km), low user mobility (less then 500km/h), not moving infrastructure, all RRC/MAC times, subcarrier spacing and Doppler shifts, link budget, number and speed of HARQ processes and so on.

Yes, we know that for 5G NR there is 5G NTN (Rel 17 which describe all changes at radio to support that feature)

:man_student: Questions to you experts :man_teacher: :

  1. do you have an idea how they “improved” LTE to transmit signal from 400km range?
  2. how they receive insanely low signals from smartphones?
  3. what about mobility procedures (handovers/reselection)?
  4. what about S1/X2 or F1 interfaces (moving eNB on boards)?

So interesting topic!
Let’s share ideas and point of views :sunglasses:

  1. The signals from Satellite will be weak but above device sensitivity (typically between -116 to -121 dBm). This is initially meant to support only text messages and later to add voice and low speed data (< 2 or 4 Mbps).
  2. The satellite eNB will have large antenna array with much higher gain compared to normal eNB.
  3. The next generation satellites from Starlink will have point to point connectivity between them providing X2 connectivity and data forwarding.
  4. Each satellite will have a huge footprint (50 km or more) and provide coverage in no cellular coverage areas only, so not much traffic and device mobility. The handovers will be mostly due to Satellite mobility and that will be known in advance.

‎1/

  • Using techniques such as forward error correction & space diversity to send stronger signals.
  • Increasing the transmission power of satellites to deliver signals to Earth.
  • Use adaptive antennas

2/ Using technologies such as satellite based return link (STAR), improving the strength of the received signal and ensuring bi-directional communication.