Space-based cellular networks provide direct satellite-to-mobile connectivity

Rajesh Uppal September 10, 2022 Comm. & NW, Space Comments Off on Space-based cellular networks provide direct satellite-to-mobile connectivity 374 Views

Recently many companies have launched large constellations of satellites to deliver broadband Internet to serve nearly 2 billion people around the world that live in areas that lack mobile coverage, mostly far from major cities, which makes building a network of terrestrial cell towers to connect them prohibitively expensive. These companies such as SpaceX, OneWeb, Amazon, and Telesat, are planning LEO satellite constellations, but all of these services will require a user terminal like the DirecTV satellite dish or satellite phones to send and receive signals to low Earth orbit.

Satellite phones have been around for decades, but they are expensive and their brick-like form factor was inconvenient for everyday use. Now a handful of companies are working to fix this problem by building satellites that can connect to regular cell phones and provide high-bandwidth mobile data anywhere on earth.

AST SpaceMobile intends to build what it claims will be the first global cellular broadband network in space to operate directly with standard, unmodified mobile devices. The key advantage is it works with any existing mobile, and you just need to buy a subscription to connect to their network. And people like Vodafone and others are partnering with them.”

AST SpaceMobile’s mission is to eliminate the connectivity gaps faced by over five billion mobile subscribers worldwide and to bring cellular broadband to approximately half of the world’s population who remain unconnected. Their approach will mean that subscribers outside the reach of cellular coverage could have access to broadband speeds without having to invest in specialized hardware and be able to roam from land networks to space networks for the first time. Through its mobile network operator relationships, AST SpaceMobile has entered into agreements and understandings with mobile network operators which collectively service over 1.8 billion cellular customers.

Lynx

The Virginia-based company, Lynk says it has successfully demonstrated the ability to use ordinary, unmodified mobile telephones to connect to satellite Internet services. In Feb 2022, the company announced the successful completion of pre-commercial tests for its fifth “Shannon” satellite. The completion of this technical milestone advances toward Lynk’s planned deployment of its global ‘cell-towers-in-space’ service in 2022.

After some initial tests, the company said “hundreds” of mobile phones in the United States, United Kingdom, and the Bahamas were able to connect with the satellite as it passed overhead, as if it were a virtual cell phone tower in space. “Basically, our satellite looks to your cell phone like a standard cell tower,” said Charles Miller, the co-founder and chief executive of Lynk.

Lynk is building a network of small satellites to provide affordable cellular coverage to unmodified cellular devices, enabling messaging, data, IoT, and emergency communications everywhere on the planet. The genesis of Lynk stems from the 2014 Ebola outbreak in West Africa, where one of the company’s founders saw first-hand the need to connect Ebola frontline health workers in one of the most remote regions of the world. By inventing the technology that is solving major connectivity problems for the planet—we are helping to protect billions of people vulnerable to the diseases, disasters, and emergencies that can strike any of us … at any time.

These spacecraft have enabled Lynk to conduct the world’s first orbiting cell tower demonstrations for a space-based cellular network. In a very short period, the test program has already successfully demonstrated critical functionalities needed for satellite-to-phone messaging initially using GSM technology.

The difference with Lynk, Miller says, is that with its 1 m × 1 m satellite there is no terminal needed, nor even software to download. The service is intended to serve remote areas where a customer’s mobile provider, such as T-Mobile or Verizon in the United States, does not have coverage. So like when you travel to a foreign country and are asked whether you want to use a local service with international roaming charges, a similar message would pop up when exiting a mobile provider’s coverage area. Lynk, in this way, shares spectrum with the mobile network operator.

The current 4G/5G Mobile phones connected to terrestrial towers generally have a range limited to around 35 kilometers if the line of sight is not interrupted by hills, buildings, or foliage. The signal can in fact travel further, but the reception range is artificially limited by the highly accurate time frames of the mobile phone protocol and the curvature of the Earth.

However, with Lynk’s patented, proven technology, the phone signal can reach an orbiting satellite 500 kilometers overhead without interrupting the mobile phone protocol. In short, the “cell tower in space” just looks like a standard cell tower to the phone in your pocket. Each satellite will use a modified version of terrestrial cell tower software that corrects for things like the Doppler frequency shift caused by the satellite rapidly passing overhead and the delay from sending a signal to space and back.

The satellites operate on a relatively low frequency compared to other communications satellites, which means they can tap into the part of the spectrum used by cell phones on earth. Miller says the company has developed an antenna that is both sensitive and powerful enough to communicate with cell phones on earth, but declined to get into specifics of the technology.

Lynk is starting small. With a single satellite, coverage is only available for a few minutes daily, across several degrees of latitude. With 10 satellites next year at an altitude of about 500 km, Miller said, the goal is to have coverage for much of the planet every several hours. By 2023, with about 100 satellites, there would be coverage every 5 to 20 minutes. To build a continuous, real-time network will require 1,500 satellites, he said.

“When people are presented with having nothing, or a satellite every 15 minutes, we are much better than nothing,” Miller said. “It’s a life-saving technology.” With limited bandwidth, initially the service will offer text messages only—focusing on the life-saving potential for users in the aftermath of a hurricane or lost on a mountaineering expedition or at sea. But eventually there will be enough satellites to serve broadband Internet, Miller said. Prices for the service will be set by the mobile network operator.

Lynx Technology

However, building an extraterrestrial mobile network has many challenges, first is the high mobility of around 17,000 mph, 300 miles above the ground. If you want to connect with cell phones from space, you need an antenna that is sensitive enough to collect their weak signals and powerful enough to return a signal that can be picked up by a cell receiver.

“The hard part is the uplink from the phone,” says Charles Miller, cofounder and CEO of Lynk, a satellite communications company based in Virginia. “You can’t change the phone to add more power. It needs to work out of pocket.”

Another challenge was compensating for the huge amount of doppler shift between the satellite and mobile phone on the ground. Existing phones and mobile networks are set up to accommodate bullet train speeds but not orbital velocities. Lynk engineers had to devise the technology for the satellite to perform this doppler compensation in space so that the phone “sees” what appears to be a fixed tower.

Lynk’s innovative approach to technology development has enabled an agile and rapid advance toward demonstrating the core technologies needed to deploy a space-based cellular network which is backward compatible for the cell phone in everyone’s pocket around the planet.

The company’s core technology is a software modification to a typical telecom software stack for LTE, GSM, and/or other 3GPP technologies. The initial step in the Lynk technology development roadmap was to develop, test, and demonstrate the ability of our proprietary telecom stack modifications to handle the Doppler shift and the propagation delay experienced between a standard cell phone and an orbiting satellite. Without these changes the phone would not work with a spaceborne “tower.”

Our objective was to prove that we could move telecommunications traffic between a base
station (or cell tower) in orbit and an unmodified standard cell phone on the ground. To achieve this goal, two critical objectives must be met: 1) write the software that can complete the connection, and 2) design and develop a spacecraft system that can fly it in orbit to conduct a successful field demonstration.

The technical and programmatic approach is to push the limits on space mission timelines and development cycles – squeezing design, build, test, flight iterations into time periods shorter than 6 months.

Lynk flies new space hardware every ~6 months to incrementally space qualify our LEO-to-phone space technologies. Each 6-month cycle includes the design, fabrication, assembly, and test of a new spacecraft iteration before handover for launch. During initial technology development, Lynk has leveraged the ISS and NASA’s cargo resupply (CRS) missions; these missions offer frequent and affordable access to orbit in support of Lynk’s rapid prototyping and agile development timelines.

Aggregate results from the first 4 missions (~18 months) of rapid technology development and on-orbit testing

Spacecraft flight hardware and software qualified, verified, and improved through 4
design, build, integration, test, and fly iterations. The Lynk 02 payload was designed, prototyped, assembled, tested, licensed, and put through the NASA safety review process in less than 5 months.
Verification and validation of the payload design and link budget to within 1 dB of models. Ground teams were focused on validating payload RF front end performance with signal
measurements on the ground.
Demonstrated movement of SMS text message from a satellite to a standard smartphone
using the cellular broadcast protocol
Demonstrated ability to control and reprogram satellite in orbit using both ground and
inter-satellite links. Lynk 02 mission demonstrated our ability to handle commanding, collect telemetry, and reprogram flight and payload computer software through and inter-satellite link between the payload and a commercial communications constellation
36 testing partners (including 27 MNOs representing 1.5 billion subscribers, or 25% of the
global telecommunications market)
Test licenses were acquired in 11 countries around the planet
No evidence of interference with cellular networks on the ground during testing

AST SpaceMobile

Lynk is not the only company working to directly connect terrestrial phones with satellites. A Texas-based company, Midland, Texas, headquartered AST SpaceMobile, launched a small test satellite called BlueWalker 1 in 2019, validating its satellite-to-cellular architecture. The spacecraft successfully managed communications delays from low Earth orbit and the effects of doppler in a satellite-to-ground cellular environment using the 4G-LTE protocol, the company said.

AST’s next prototype spacecraft, BlueWalker 3, is expected to launch aboard a SpaceX mission from Cape Canaveral, Florida, in 2022. The BlueWalker 3, which is a 693-square-foot phased array for planned direct-to-cell phone connectivity at 4G/5G speeds will validate the company’s space-to-cell network for testing with mobile network operators. The spacecraft has an aperture of 64 square meters and is designed to communicate directly with cell phones via 3GPP standard frequencies.

AST’s system will consist of dozens of small, pizza box-sized satellites flying in formation as they receive cell signals. Some experts estimated that AST is building a new, unproven type of satellite constellation that’s a riff on so-called “fractionated satellites,” which divide the capability of one large satellite among several smaller ones.

According to AST’s founder and CEO Abel Avellan, the system isn’t truly fractionated because each of the small satellites will have the same capabilities, rather than splitting the functionality of one larger satellite. But the formation will be managed by a large control satellite, which will direct network traffic and satellite movement like a conductor leading an orchestra. Although the later versions of the satellites in AST’s system will communicate with one another over Wi-Fi or a similar wireless protocol, Avellan says the first satellites to go up will be physically connected.

The advantage of AST’s approach is that the satellites can be spread out over hundreds of feet. Since each satellite is itself a receiver and is working in tandem with the others, this has the effect of creating a massive antenna. “In essence we are building a very, very large satellite with a lot of power that can connect directly to a handset,” says Avellan. “Our system is a replica of the terrestrial network in space.

The company has entered into agreements and understandings with mobile network operators that collectively cover approximately 1.5 billion mobile subscribers. Partners in this effort are leading global wireless infrastructure companies that include Vodafone, Rakuten, and American Tower.

However, AST has yet to obtain permission from the Federal Communications Commission for access to the US market. Previously, NASA has raised concerns about the large size of the proposed satellites. In order to provide service, AST plans to build spacecraft with large phased array antennas—900 square meters. According to NASA, in planning for potential conjunctions with other satellites and debris in this orbit, this would require proscribing a “hard-body radius” of 30 meters, or as much as 10 times larger than other satellites.

Maneuvering around the proposed SpaceMobile constellation would be extraordinarily taxing, NASA said. “For the completed constellation of 243 satellites, one can expect 1,500 mitigation actions per year and perhaps 15,000 planning activities,” the space agency stated. “This would equate to four maneuvers and 40 active planning activities on any given day.”

Finally, the space agency is concerned because AST has never built a satellite remotely close in size to the 1-ton or larger vehicles that will populate its constellation. Given this lack of experience, it is expected that 10 percent or more of the satellites may fail, making them unable to maneuver to avoid collisions. NASA found the risk of a catastrophic collision to be “unacceptably high.”

ASIC enables broad access to space-based mobile network

U.K. custom design house Ensilica has announced it is developing the cellular ASIC that will enable AST SpaceMobile’s planned space-based cellular broadband network. The ASIC will be a key communications component of the spacecraft electronics payload for AST SpaceMobile.

Ensilica’s business development director, Paul Morris said, “The big challenge of making this work is the limited power supply from the solar cells. If you imagine a satellite, as it goes around the earth, with a low earth orbit satellite you’re in the sun for a certain proportion of the orbit, and then you’re in total darkness. So, in the small time you’re in the sun, you can charge and then the rest of the time you’re running on batteries. That limits the power budget. The challenge they brought to us was to build a modem design that met all their communications specifications, that met the power budget.

“And the only way of doing that was to come up with a novel architecture and to use a very advanced semiconductor process node,” Morris continued. “We can’t say which one it is, but we can say it’s an advanced node and these weren’t available even a small number of years ago. It wasn’t possible to do this before. You could never have got the signal processing down low enough to actually run it off the solar cells. And that’s really the breakthrough, I think.”

Inherent to the challenge is meeting the performance requirements within the power budget. This means managing the tradeoffs between bandwidth, transmit power, receive sensitivity, and power consumption, all of which are at odds with each other.

Nokia radio technology to enable AST SpaceMobile’s direct-to-cell phone connectivity from space

Nokia wins five-year global 4G and 5G deal from AST SpaceMobile in July 2022.
AST SpaceMobile plans to bring cellular connectivity directly to 4G and 5G devices via low Earth-orbiting satellites in collaboration with mobile network operators.

Nokia’s AirScale Single RAN equipment aims to enable AST SpaceMobile in providing mobile services to new and existing subscribers in regions currently not served by terrestrial communication networks. This includes connecting devices globally on land, at sea, or in flight. Nokia will provide equipment from its comprehensive, energy-efficient AirScale portfolio including its AirScale base stations powered by its latest generation of Nokia’s ReefShark System-on-Chip (SoC) chipsets. AST SpaceMobile will benefit from Nokia’s modular baseband plug-in cards which add capacity where it is needed offering flexibility and efficiency. Nokia will also provide its NetAct solution for network management and seamless daily network operations as well as optimization and technical support services.

“Connectivity should be considered an essential service like water, electricity, or gas. Everyone should be able to have access to universal broadband services that will ensure that no one is left behind,” said Tommi Uitto, President of Mobile Networks at Nokia. “Nokia has a long history of delivering connectivity solutions that have had a major and positive impact on society. We have worked closely with AST SpaceMobile on this important initiative for two years which seeks to provide crucial connectivity from space to underserved communities around the world. We are of course proud our technology is playing an important role in underpinning the networks.”

AST SpaceMobile plans to launch its BlueWalker 3 satellite for testing in early to mid-September from Cape Canaveral, Florida. BlueWalker 3 is a low Earth orbiting satellite and has an aperture of approximately 64 square meters (693 square feet), which is designed to communicate directly with cellular devices via 3GPP standard frequencies. Ultimately, AST SpaceMobile is aiming to deploy approximately 100 satellites to achieve substantial global mobile coverage.

“With the integration of Nokia’s AirScale system, AST SpaceMobile and Nokia are taking an important step toward closing connectivity gaps all over the world,” said Scott Wisniewski, Chief Strategy Officer at AST SpaceMobile. “Nokia is supporting us with dozens of engineers and development professionals, including leading architecture research experts at Bell Labs, the world-renowned industrial research arm of Nokia. In the coming months, we are scheduled to launch our BlueWalker 3 test satellite into low Earth orbit, which has a 64-square meter phased array antenna designed for direct-to-cell connectivity. With this satellite, we plan to conduct testing all over the world with leading mobile network operators, leveraging Nokia’s technology solutions on the ground.”