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September 29, 2021

How Lynk Proved Direct Two-way Satellite-to-Mobile-Phone Connectivity

By Tyghe Speidel

On 14 September 2021, Lynk became the first company in history to create a successful two-way connection between a standard mobile cellular device on Earth and a satellite cell tower. Lynk’s 5th satellite (Shannon) has now completed two-way registration and signaling for hundreds of mobile phones in 3 different countries — the United States, the United Kingdom, and the Bahamas. 

It has been five years since we invented the method of two-way direct communication between a satellite and existing, standard mobile devices. And now we have proven it from space.

Lynk has solved the ‘Uplink Challenge’. Our 5th satellite has successfully completed its commissioning tests which includes our antenna, radio, and core mobile processing tests demonstrating the ability to receive signals from terrestrial subscriber phones on Earth, that are out of terrestrial coverage, despite the noise from millions of other phones communicating terrestrially in the same mobile operator spectrum at the same time in our spot beam. We have repeatedly demonstrated the intricate two-way signaling required for a phone to connect to our cell tower in space, involving multiple instances of uplink and downlink messages, including a device request for channel access, and then the corresponding authentication and location update procedures. To date, we’ve done this with hundreds of phones in the UK, the Bahamas, and the US. This has never been proven before with a satellite cell tower and Lynk has done it.  As part of the testing, we have proven our signaling link budget (the signal power analysis) to close the link with existing mobile phones, as well as proven our ability to not interfere with existing terrestrial mobile operations while operating in the same bands.

Below is a step-by-step description and flow diagram of the two-way registration process that has been successfully demonstrated in multiple locations around the world with all kinds of standard mobile phones and mobile devices.  This message flow represents the typical interactions that occur between a standard terrestrial cell tower and a cellular device. The only critical difference is that this signaling flow is occurring between Lynk’s “cell-tower-in-space” and a mobile device on the ground. This critical distinction is where Lynk’s proprietary intellectual property is so vital.  Current mobile standards developed by 3GPP are not designed to operate at over 500 km distances between the phone and tower, nor when the tower is traveling at 28,000 km/h (17,000 mph)!  To talk to existing cell phones on Earth requires sophisticated radio and baseband processing techniques that allow the satellite to appear to terrestrial phones as though it’s a regular terrestrial tower.  The satellite also needs special technology to pick out phones trying to communicate with it, given that millions of phones may exist within the satellite antenna spot beam.  To work with existing phones, they can’t know the difference, and our testing verifies that this technology works.

The registration flow described below is a universal procedure; any existing mobile device on the planet can conduct this procedure to allow itself a mobile connection to Lynk’s satellite network. This is an important milestone for Lynk and for the future of communications around the world.

We are now commencing tests that will demonstrate the initial mobile services we plan to roll out in the middle of 2022, and can’t wait to share those results as soon as possible.

Note:

  • Downlink refers to a transmission from the satellite tower to the mobile device on the Earth.
  • Uplink refers to a transmission from the mobile device to the satellite tower. 

Registration Procedure 

  1. Lynk’s satellite points toward the Earth and transmits information on the downlink broadcast control channel to phones on the ground within the beam. 
  2. UPLINK #1: Mobile devices within the beam who are out of terrestrial coverage, receive Lynk’s broadcast control channel  signal, and synchronize to the downlink frame. After this, they respond with an uplink signal to request a channel assignment using what is called the “Random Access Channel”.
  3. Lynk’s satellite then repeatedly collects the requests of dozens of mobile phones, all in parallel, and responds with a downlink signal to each of these phones and individually assigns them each a “channel”. This downlink signal is called an “Immediate Assignment”.
  4. UPLINK #2:  Dozens of phones, in parallel, receive the immediate channel assignments, and send a “location update request”, which includes the devices’ mobile identity (aka IMSI). 
  5. At this point, the satellite cell tower processes the IMSI identities in the location update requests using an onboard database (the HLR) and takes one of two actions:
    • If the identity of the phone is that of a Lynk subscriber, the satellite sends a Lynk “Authentication Request” to ask for additional device identity information to complete the authentication process. Currently only Lynk test devices are authorized Lynk subscribers. 
    • If the identity of the phone is NOT that of a Lynk subscriber, the satellite sends a “location update reject” signal. Since there are always handsets operating outside of existing cellular coverage, Lynk has rejected thousands of phones in our test areas that were not Lynk subscribers. Until now, this rejection always occured in a terrestrial setting when a subscriber’s device attempts to connect to another terrestrial network that it is not subscribed to use, and is invisible to the user.
  6. UPLINK #3:  The Lynk mobile devices on Earth that receive an “Authentication Request” send an “Authentication Response” with the additional information needed to confirm it is an authorized subscriber and officially complete the authentication process. 
  7. After the authentication process is complete, Lynk’s satellite completes the location update procedure by sending the device a “location update accept” signal, and then a subsequent “Channel release” signal to make room for another call flow with another handset. 
  8. UPLINK #4:  The Lynk mobile device which has now completed the authentication and location update procedure sends an uplink to confirm the “Channel release”. At this point, the device camps on the Lynk satellite network until it needs to send or receive data from the satellite. 

On a given overpass test lasting just 2 minutes, we have multiple independent proof points, including 1) completing all 4 uplinks to the satellite from Lynk devices, and 2) “rejecting” many phones that are not currently subscribers on the Lynk satellite network. The rejection of a huge number of non-Lynk devices provides additional proof that Lynk’s signal processing and cellular base station technology on the spacecraft is working with real world handsets on the ground, consistent with existing mobile standards, as we can rapidly discriminate between unauthorized commercial handsets and Lynk’s authorized test devices.