Dev.SN ♥ developers
Sections
Dev.SN
from the chewing-through-my-data-plan-that-much-faster dept.
SpallsHurgenson writes "Steve Perlman is ready to give you a personal cell phone signal that follows you from place to place, a signal that's about 1,000 times faster than what you have today because you needn't share it with anyone else.
"It's a complete rewrite of the wireless rulebook," says Perlman. The technology is now called pCell - short for "personal cell" - and it allows streaming video and other data to phones with a speed and a smoothness you're unlikely to achieve over current cell networks.
Perlman's invention - formerly known as DIDO - discards the current arrangement of cells shared by many users, giving each phone its own tiny cell, a bubble of signal that goes wherever the phone goes. This "personal cell" provides just as much network bandwidth as today's cells, Perlman says, but you needn't share the bandwidth with anyone else. The result is a significantly faster signal."
(Score: 2, Informative) by Foobar Bazbot on Monday February 24 2014, @07:19PM
Yes, one tower per independent transmitter is an oversimplification. No, two towers with phased arrays is not the same thing. Note that DIDO is a particular, extreme subset of MIMO -- while using multiple towers with phased arrays is definitely MIMO, it's not the same , and doesn't have the same characteristics as DIDO.
Picture two towers with at least two DoF each, so you can transmit desired signals on two beams each. (In this example, steering nulls and/or more beams won't help.) We have two arbitrarily placed clients.s1 and s2 are the desired signals to be received by clients 1 and 2. A1, A2, B1, B2 are the transmitted signals in each beam from tower A or B to client 1 or 2. Ignoring propagation delay and loss, we get:
which is trivially solved (e.g. A1=s1, B2=s2, B1=A2=0)) with 2 DoF to spare.
Adding client 3 at the intersection of beams A2 and B1 yields:
which also trivially solvable. Letting A1=s1, B1=0, then A2=s3 and B2=s2-A2=s2-s3.
But adding a fourth client at the intersection of beams A1 and B2 yields:
Which is of course, singular, and has no solution. Suddenly our four DOF aren't enough to serve four clients, and increasing the DoF on each array doesn't help as long as they're localized in two towers, and the 4 clients are in this geometric arrangement.
The radial range of coherence of an array or cluster of transmitters increases with range; at long range (relative to the array size), you get purely radial beams and nulls extending to infinity, which are usually desired, thus a compact (relative to target distance, still large compared to wavelength) array is commonly chosen. But for DIDO, those beams can interfere in multiple places, yielding singularities beyond this as shown in the example above. Since the idea of DIDO is not to form beams for their own sake, but to form local "hotspots" of coherent signal, it's far better to have the array quite large compared to the target distance, or in other words to have the transmitters distributed (hence the name) throughout the volume in which clients operate.
In practice, lumping a few transmitters on each tower, but still having several towers in view at any time, gets you enough distribution -- one per tower (or even three per tower, with sector antennas) isn't needed.
Sure, you've just described TDMA. But that's what we're (supposedly) trying to get away from -- DIDO gives each client all the bandwidth, all the time. If you argue that each client doesn't need that (at least WRT mobile phones), because we're each actually using full bandwidth only a small fraction of the time, (and not all the same fraction), you're absolutely right, and that's one reason why building out more smaller cells makes more sense than DIDO -- you can stop when 99% of clients have enough bandwidth 99% of the time.
I'm not sure DIDO makes any practical sense for anything, but if it does, it's for applications where very limited bandwidth is possible, and going with smaller cells containing fewer users is for one reason or another impractical. The NVIS example shown in the DIDO white paper is the sort of thing that could make sense.
userscript: Expand/Collapse buttons on every comment [userscripts.org]