In my last post, I described how wireless networks are starting to bump up against “Shannon’s Law”, essentially running out of ways to cram more data into existing radio spectrum.
Privately-held Artemis has proposed a novel solution to this problem. They call their technology “pCell”, short for Personal Cell. They make use of a technique called “Distributed Input/Distributed Output” or DIDO, and sometimes it appears that way in press reports. (Side Note: Notice the subtle marketing shift there. Dido was the founder of Carthage and a Greek hero, but a villain in Roman mythology. Artemis is a Greek goddesses, and a highly revered one at that, sister to Apollo.)
In current mobile systems, we take a piece of radio spectrum and divide it up in many clever ways so that each phone has a unique signal to and from the base station. With pCell, each cell in the system sends out a unique signal to all the devices it is talking to, but each device receives it in such a way that it can interpret its own message. A unique signal to each device (current approach) versus a common signal with each device taking its own unique interpretation (pCell).
In the press, Artemis describes this as ‘sidestepping’ Shannon’s Law. Shannon’s Law says you can only divide up a signal into a finite number of pieces. Instead, Artemis does not divide up the signal, it transmits a signal in such a way that each device can take what it needs from the signal.
This is phenomenally complicated math, so a key piece of Artemis is that the computation that determines each signal is not handled by the pCell in the field. Instead, all that data from the pCell is sent back to Artemis servers and the math is performed ‘in the cloud’. This company claims this allows the system to scale massively. It also has the benefit of making the pCell’s themselves relatively cheap to build, they do not need a lot of processors or memory.
Artemis claims that this approach frees up immense amounts of capacity. And in that video demo they stream a different HD video to ten iPhones simultaneously from a single pCell.
In theory, this system should scale nicely. The pCells have a range somewhere between Wi-Fi access points and cellular base stations (but have the power to reach much further). They are small, and so should be relatively inexpensive to deploy in large numbers. And since the hard work is done in the cloud, adding additional users is just a matter of spinning up more servers. (A topic for a different day, but the price of that function is dropping rapidly as well.) This seems to be one more example of the rapidly plummeting cost of computing is having an impact in the physical world.
All of this is pretty exciting. If it works.
I still have a lot of questions. First, how do transmissions from the phone to the cell work (i.e. the uplink)? Second, how does that math work? Third, how mobile is the system, meaning how fast can you travel between cells? And what happens when lots of devices are moving about? In today’s cellular systems, this problem sucks up considerable amounts of capacity to handle.
It sounds like Artemis has worked out these problems, but in my next post, I will look at how they have been parsimonious in their disclosure about the technical side of their system.