Comment by onedognight | Hacker News Reader

Comment by onedognight | Hacker News Reader

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This is the basic idea from the white paper.

DIDO communication begins with the DIDO APs exchanging brief test signals with the DIDO user devices. By analyzing what happened to these test signals as they propagate through the wireless links, the DIDO Data Center determines precisely what will happen when it transmits data signals from the APs to users, and how the simultaneously transmitted signals will sum together when received by each user device. Then, the DIDO Data Center uses this analysis, along with the data each user is requesting (e.g. video from a website), to create precise waveforms for all of the APs that, when transmitted at once will sum together at each user device to create a clean, independent waveform carrying the data requested by that user. So, if there are 10 APs and 10 users all within range of each other, then 10 radio signals will sum together at each antenna of each user’s device to produce an independent waveform for each device with only that device’s data.

One question that I haven't seen answered yet is what happens if the receiving antenna is not stationary. It seems to me that if it is moving at the time that the test signal was sent out there is no guarantee that the signals will sum together properly for the receiver now that is in a different location. Most devices that are wireless are wireless because they spend most of the time moving around.

But perhaps this system of analyzing the test signals and figuring out how to create the waveforms that will sum up for the receiver happens so fast that it is not a problem to recalibrate constantly to take in account cell phones in moving cars, etc.

From the patent app: "As such, in one embodiment of the invention, the channel characterization matrix 616 at the Base Station is continually updated. In one embodiment, the Base Station 600 periodically (e.g., every 250 milliseconds) sends out a new training signal to each Client Device, and each Client Device continually transmits its channel characterization vector back to the Base Station 600 to ensure that the channel characterization remains accurate (e.g. if the environment changes so as to affect the channel or if a Client Device moves). In one embodiment, the training signal is interleaved within the actual data signal sent to each client device. Typically, the training signals are much lower throughput than the data signals, so this would have little impact on the overall throughput of the system. Accordingly, in this embodiment, the channel characterization matrix 616 may be updated continuously as the Base Station actively communicates with each Client Device, thereby maintaining an accurate channel characterization as the Client Devices move from one location to the next or if the environment changes so as to affect the channel. "

"Each client device continually transmits its channel characterization vector..."

Sounds like the scheme is going to wreak havoc on the battery life of a typical mobile device.

If this continual transmission is only done during periods of network usage, when you're already powering up the CPU and antenna chipset, how much of an additional burden would this be for the client device? I doubt it would be continuously transmitting when the device is in sleep mode, and I have no indication that this is a CPU-intensive task for the client device (the data center device is a different matter).

For the mobile phone use case, it seems like the transmitter would have to be emitting constantly in order to keep the base station appraised of its current position, for the purposes of computing the channel characterization. Otherwise, how would the base station know what transformations to perform on the outgoing signals to reach the mobile device in the event that a call is received? It's not like you could broadcast that information to everyone, because that would end up corrupting the spectrum for everyone else, which defeats the entire point of this system.

The whole scheme seems really dependent on knowing the position of all transmitters/receivers at all times.

The "paging channel" problem is already present in existing mobile radio systems. Solving it pretty much requires some form of TDMA — i.e. scheduled transmission — because only TDMA gives you the ability to actually power your radio off most of the time. Even Wi-Fi has features for this.

Why couldn't you reserve a tiny fraction of the bandwidth of the system (or some other but related spectra) for this sort of "calling transmitter #92" broadcast? Yeah, it'd impact everyone, but only finitely.

I thought of this, too. (I'm taking it as a given that this technology works as advertised at low speeds.)

At higher speeds, low frequency/long wavelengths ought to work better because the constructively interfering areas will be larger for any single receiver than they would be with a shorter wavelength. Also, when the receiver is traveling at constant velocity, it is very easy to predict where the receiver will be in the next frame. This covers a lot of cases (people on planes, cars, trains).

In fact, I can't think of a place where an average person (e.g., not someone flying a fighter aircraft) would be moving rapidly and unpredictably.

In terms of multi-path, I can well imagine that being inside a fast-moving car in a body of traffic isn't far off the worst-case scenario - not only is the receiver moving (unpredictably, if they're changing lanes), but there are a bunch of reflectors moving around them, also unpredictably.

I imagine there would be some overhead (e.g. each user transmitting a certain user-specific pattern every 1/2 second, and each AP transmitting a certain AP-specific pattern every 1/2 second), to calibrate the location of each user in AP-space. Fundamentally I think all the calculations involved would be fast linear algebra operations that could be done in hardware on the order of microseconds.

This brings to mind a very interesting idea as well. If the changing physical location of the receiver is causing changes in the composite wireless signal the device is receiving, and the base stations are recalculating and refreshing this all the time very quickly it could potentially be a very reliable and accurate GPS as well.

Actually, I don't think so. This isn't finding your physical location, then computing the correct profile for that location; it empirically probes the performance your location is experiencing, then directly uses that performance information. While you could analyze the results and at least take a stab at the physical location, it's quite possible it will work no better than cell tower triangulation.

Even trying to empirically map certain performance profiles to certain spaces may be impractical if the client radios and antennae performances differ enough to throw off the profiles, which if I understand this properly and given our proclivities for making things as cheap as possible, probably means this won't work either.

What happens if two clients are right next to each other? I wonder if you can still count on enough interference to give each one a totally different data stream.

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