The toughest bandwidth problem to solve is the stadium conundrum. Imagine one hundred thousand people in a football stadium all wanting to watch concurrent games on their mobile devices. Even if you put a cell tower at the stadium you run into Shannon’s Law. There is a limit to how much data can be carried by a cell tower. The amount is directly proportional to the bandwidth, i.e. the spectrum space, allocated. You have probably run into Shannon’s Law and its consequences while staying at a hotel when a supposedly high bandwidth WiFi connection yielded slow data speeds. That’s because the bandwidth was being shared with many others and you were only getting a small percentage of the available data speed. Some attempts have been made at mitigating this. Today we see WiFi systems with beam forming and MIMO. These help but not enough. However, understanding the basic concept behind these techniques points the way to better solutions.
To understand how this can be attacked, start with the fact that a laptop connecting to a wireless network access point in San Francisco doesn’t interfere with one connecting to an access point in New York. Both laptops get the maximum speed. While Shannon’s Law applies to each individual connection, one doesn’t impact the other. The same is true in the cellular system. A cell phone in San Francisco doesn’t impact the data link speed of one in New York. Now imagine if each laptop in a hotel had a very directional antenna pointed at a wireless access point with that access point also having a very directional antenna pointed back at the laptop. Each laptop would be linked to its own access point and because of directional antennas wouldn’t see, i.e. interfere with, the other laptops in the hotel. The result would be full data speed for each laptop. As just described this is impractical. Some attempts are made at this by having access points generate a more directional signal or by using multiple antennas in an access point and the laptop to generate phased arrays and thereby a directional signal. However, this is woefully inadequate.
I need to digress and talk about some of the problems with other approaches I mentioned in earlier posts. Just putting in a bunch of femtocells can cause problems. The cells can interfere with each other. To properly utilize dense populations of cells, they need to be intelligently aggregated. One method is to have adjacent cells use different channels i.e. different parts of the spectrum. That way interference is reduced. If you think of spectrum spaces A and B then a string of towers in a row can be assigned A-B-A-B-A-B and so on. In a two dimensional system, the four color mapping theorem tells us we need only segment the spectrum into four channels to make sure no two adjacent towers are on the same channel. None of this is new to either the cellular providers or the equipment manufacturers and there additional techniques for handling cell to cell interference. Companies like AirWalk and Ubiquisys have attacked the issue. Tied into this is the problem of cell to cell handoff. This can be a particularly difficult problem for small, dense arrays of cells. The handoff issue is most easily solved when cell to cell movement is slow such as in an office space. A stadium is another place where movement is slow.
An interesting alternative approach to the above problem has been generated by Rearden Wireless. Rearden calls it DIDO for Distributed-Input-Distributed-Output Wireless Technology. The concept is to use massive numbers of access points but to use them in concert. By timing the transmissions of each one, the radio waves from each antenna will sum but only at the desired location i.e. the desired device. Simultaneously, similar phased signals are overlaid to communicate with other devices. The available bandwidth is directly proportional to the number of access devices. Think of it as phased arrays taken to the next level. A more detailed description can be found here. There has been some hype surrounding DIDO and some of it may be self promotion by the inventors. The hype usually revolves around a claim that Rearden has violated Shannon’s Law. However, that is far from the case. Just like multiple cells circumvent Shannon’s Law to some extent, DIDO uses multiple access points to do the same. The method is very creative. It remains to be seen how well DIDO competes with other methods of using large numbers of access points in a relatively small area.
There is no silver bullet for the stadium problem. Even innovative approaches like DIDO will require massive numbers of access points and even then functionality will depend on only a small fraction of the people in a stadium streaming data at the same time. As smartphones combine with data in the cloud to drive cellular data usage, we can’t afford to wait for the perfect solution.