Earlier this year, Vodafone and AST SpaceMobile made headlines by successfully making the world’s first satellite video call on a smartphone. This first-of-its-kind achievement has opened the door to a wider conversation in the satellite and telecom industries about the viability of space-based cellular networks, the technological advancements that are needed for more robust non-terrestrial cellular networks and the challenges and limiting factors to making cellular NTN capabilities a reality.

According to Luke Ibbetson, Head of Research & Development at Vodafone, space-based cellular connectivity is still in its infancy. “I think it’s very early days in terms of the penetration of the service and the level of capability that the networks have at this present time,” he said.

Ibbetson explained that Vodafone has been partnering with AST SpaceMobile to build out a constellation that will be capable of operationalizing cellular services from space. “We’re in the early stages of building out that constellation,” said Ibbetson. “We had a fantastic achievement a few weeks ago when our chief executive made the first 4G video call over that network. We’re very pleased to see that it’s starting to mature from a technology perspective. We only have five satellites in orbit so far, and those are performing very well. But it is a very intermittent service that we’re using just for testing at this point. It will require further satellite launches in the coming year before we’re able to offer a persistent service.”

It is not just the technological aspects that are still in the early stages. Some industry analysts point to a need for a clearer business model for space-based cellular connectivity and services. “There is considerable excitement about the potential for new LEO direct-to-device constellations…however, the business model remains uncertain,” said Tim Farrar, Founder of TMF Associates, a mobile satellite services consulting and research firm. “To date, the only proven model is that employed by Apple and Globalstar, where Apple has been paying Globalstar for capacity and providing free messaging services to iPhone users in order to stimulate increased sales of their phones. It is particularly unclear if cellphone users will be prepared to pay for satellite connectivity if they rarely use the satellite service.”

The Tech and Spectrum Challenges Facing Space-Based Cellular Networks

To get space-based cellular connectivity out of its infancy and into a more mature state, a combination of technological and regulary challenges need to be addressed. “If you are trying to provide a meaningful broadband service, and you do not have the right technology, in terms of being able to provide beams with enough focus to guarantee that you can’t interfere with terrestrial users and adjacent frequency bands, then it means you are left with power restrictions from those space-based cells that restrict the service then to very low-rate data or text messaging,” said Ibbetson. “If, on the other hand, you have technology that allows you to form very tightly focused beams with very good radiation patterns, then you can navigate the road to regulations and provide a genuine full service from orbit - albeit one that’s always going to be capacity constrained.”

According to Jaydee Griffith, Managing Director of ATIS’ Next G Alliance, spectrum sharing between terrestrial and non-terrestrial networks will be a challenge that must be solved to fully support space-based cellular networks. “How do we enable that spectrum sharing between terrestrial and non-terrestrial, space-based networks to ensure there’s no interference and so we can maximize the efficacy of the spectrum?” Griffith asked. “I think some companies say they may have some proprietary solutions to making that happen, but it’s something that I think a lot of the community is going to have to start agreeing on. It is good to have those solutions for a competitive advantage, but for the widespread coverage that both the mobile industry and consumers want, it’s going to probably have to be standardized.”

Farrar agreed that spectrum is a limiting factor to providing cellular services from space. “Spectrum availability is a major challenge, because terrestrial allocations are fragmented across - and sometimes within - countries,” he explained. “Terrestrial spectrum is also very valuable and most mobile operators will be unwilling to dedicate their spectrum resources to providing satellite coverage, except in the most remote areas. Using existing satellite spectrum is the obvious alternative, as seen in Globalstar’s partnership with Apple, because this is licensed globally and does not interfere with terrestrial networks.”

The Satellite Network Requirements

As for the satellite network requirements to fully enable space-based cellular connectivity, Ibbetson points to the need for large antennas. “One requirement is that you need physically large aperture antennas,” he said. “You are inherently dealing with cellular devices that have very low emitted power, and you need to be able to pick that up in orbit. Similarly, you need to have large capture areas for the solar power to form beams with the right energy and to close the downlink again, because you have limited received sensitivity on these devices.”

Another satellite requirement that must be considered is the compensation for delay in Doppler, which can either be done on the satellite itself or on cellular devices. “What we are predominantly seeing for most of the constellations that are going up is putting all of the compensation for delay in Doppler on the satellite itself,” explained Griffith. “There is some discussion about simplifying the satellites by putting some of that compensation on the device, but doing so is going to increase costs and increase device power requirements.”

The question still remains: will space-based cellular networks ever replace terrestrial networks? According to Ibbetson, the answer is no. “What we aspire to do is to provide a full extension of our network so that we can provide full coverage for all of the services that people would normally expect their smartphones to be able to handle,” said Ibbetson. “It’s not something that will be able to replace the terrestrial network. It is very much a complement.”

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