It seems that one of the hottest topics in the industry is Direct-to-Device (D2D) cellular service that can bypass terrestrial cell towers and connect cellphones directly to satellites. I’ve already seen claims being made that D2D technology will solve the rural cellular issue since everybody will have cell coverage. Today’s blog looks at some of the reality behind these claims.

There are a lot of players suddenly in this space. Not surprisingly, SpaceX / Starlink already has hundreds of satellites in orbit and plans for 15,000. AST SpaceMobile is partnering with AT&T and Verizon. Globalstar originally pioneered Apple’s SOS emergency test service and was recently acquired by Amazon, which recently applied for licenses for 5,105 satellites for D2D. Lynk Global has partnered with more than 45 cellular companies worldwide. EchoStar and Viasat have also announced plans to enter the market.

Basic Technology. The satellites will circle at heights between 200 and 550 miles above Earth and will be traveling at 17,000 miles per hour. At least for now, nobody has figured out how to hand off calls between a satellite and terrestrial cellular network. There are some limitations for D2D that are largely defined by physics.

• Can’t Work Indoors. Your cellphone transmits at under 0.5 watts of power—the maximum power level set by the FCC. Cell signals lose from 50% to 90% of the signal strength when passing through a wall. While that usually is enough power to reach a cell tower located within 2-3 miles of your house, it’s not nearly enough power to reach a satellite.
• Receiver Limits. Starlink has been achieving data speeds up to 300 Mbps download for home broadband. Expect speeds to be far less for a D2D due to the much smaller antenna surface area in a cellphone compared to a Starlink receiver dish. In general, the larger the size of the antenna, the more bits that can be gathered. Receivers for Starlink range from 11.75 X 15.7 inches for the Starlink Mini to 22.7 X 20.1 inches for the higher-performance receiver. The typical cellphone has a receiver of around 6.5 X 7 millimeters, meaning that even the Starlink Mini has 2,600 times more surface area for receiving a signal from the satellite than a typical cellphone.
• Area Speed Coverage Limits. The planned coverage area for the next generation of D2D Starlink satellite is around 45 square miles. The newer satellites will be using massive adjustable antenna beams along with 40 to 65 MHz channels. This means that a single data stream from a satellite will have an overall data capacity between 40 and 150 Mbps download, depending on the spectrum being used. Starlink is planning on 16 simultaneous full-power beams from the next generation D2D satellites. The challenge is that this spectrum must be shared among all users inside a beam footprint. That means a maximum of 2.4 Gbps of bandwidth to share in a 45 square mile area—a minuscule amount of bandwidth compared to what can be delivered by cell towers serving the same area. This is compounded by a limitation on the simultaneous number of users that can be connected at the same time.

What does this all mean from a practical perspective? The D2D satellites will be able to connect to a large number of people who are outdoors, within the overall limitation of the number of simultaneous people that can be connected at one time. We won’t know until somebody shows us what this will mean for connecting a lot of people in a small area, like at a rural rock concert.

Early trials have shown that D2D has some capacity for making a connection to a moving vehicle. This is never going to be as guaranteed as when standing outdoors, and it will likely depend on the type of vehicle and the speed being driven.

D2D is not going to work directly indoors. It’s possible that somebody will develop an outdoor receiver that can bounce a signal to go in through a window. It would make sense to do this in conjunction with something like a Starlink receiver.

It’s hard to envision D2D as a standalone cellular service, except possibly for the rare person who spends most of their time outdoors.

To be fair to the industry, at least for now, the big D2D providers aren’t touting it as a standalone cellular service. They are talking about this being an add-on to normal cellular plans. This doesn’t make much sense in urban areas where normal cell coverage is already ubiquitous and strong. But this could be an interesting additive to a cell plan for folks who either live or travel a lot in rural areas. It’s a cool technology, but a lot of the excitement could be empty hype, which seems to be the norm for most new technology products.

There is only one statistic that is really going to matter for D2D, which is the number of people willing to pony up a monthly fee to use it. The one thing going for the new technology is that there is a lot more serious demand for any kind of cellular coverage in the third world than there is in the rural U.S. Maybe that’s the real market they are shooting for.