Archive for the ‘bandwidth’ Category

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.

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In the last post, I promised to discuss femtocells, picocells, etc. but that will need to wait a few days. To understand why different cell types are needed we must first discuss how cell phones are used. The bandwidth crisis is huge and multifaceted. Its roots stem from the changing nature of cell phone use. We must first look at and understand how cell phone use has changed and the emerging demands of the mobile market.

Along with the widespread adoption of the automobile, came the desire to have a telephone in the car. The first systems, circa 1946, were simply radios that connected to telephone offices and could be patched into the phone system by an operator. In the early 1960’s automatic dialing arrived. The TV series Burke’s Law, from the same era, had Amos Burke, played by Gene Barry, being chauffeured around in his Rolls-Royce Silver Cloud II which prominently featured a phone in the back seat. Everyone understood the message. Just like a Rolls Royce, radio phones were only for the very rich.

The problem with the early car phone systems was cost and capacity. The two were tightly related. Because the car phone was closely related to a standard radio set, only a few people could use the system at a time. A small user base meant high cost. Each car phone transmitted over a large distance. Today, CB radio is similar. With CB radio there is a limited number of channels and if one person is on a channel then that channel is tied up for a considerable distance.

In 1947  Douglas Ring and Rae Young of Bell Labs proposed hexagonal cells for car phones. However the technology didn’t exist and the system, as proposed, lacked a lot of necessary features. In 1970 Amos Joel, Jr., also of Bell labs,  invented an automatic call handoff system to transfer calls from cell to cell. In 1982 the FCC approved the Advanced Mobile Phone System and the cell phone as we know it today was being born. The concept dog cells together with calls being passed form cell to cell allowed many users on a given channel as long as they each used a different cell. This is a key concept. The idea is to generate additional cells as usage increases. With more and more usage comes the need for many more and much smaller cells.

The cell phone rapidly took off. Initially systems were bulky and often installed permanently in automobiles. This generalized the location specific nature people associated with telephones. Cell phones were associated with automobiles. A certain phone number might be for Bill’s car while another number was Bill’s house and a third his office. Phones had physical locations even if some of those locations were automobiles and able to move about.  The cell phone system itself only handled phones in cars.

Today the old concept of a cell phone being a car phone is all but gone. We use our cell phones in cars but we do so much more with them. The home phone is rapidly being replaced. This is an interesting generational change to observe. Older people generally hang on to the concept of a home phone and the associated home number. Younger people often dispensed with that old concept. For them, the phone number is tied not to a physical location or an object but to a person. In a few cases people have taken an intermediate step. There is still a home phone number, but it calls a cell phone rather than an old style phone line. Cordless phone systems, such as the Panasonic KX-TG6582T, allow linking a cell phone throughout the house in case you don’t want to carry your phone around. Even this is beginning to be passed by. Modern smartphones have become so multifaceted and embedded in our lives that we require their presence at all times. At work we hang on to the old office phone but more and more of our calls are over our cell phones. Why guess if a person is at his desk? Just call his cell.

The result of the move from car phone to personal phone has transformed how the bandwidth problem must be viewed. No longer is it adequate to think of adding capacity along major roadways. Cellular performance inside businesses is now important. Neighborhoods which used to have low call density except, perhaps, during rush hour now have high call density since home calls are being made over the cellular network. Solving the bandwidth issue will require attacking car, home and business. Furthermore, this will require more of a partnership arrangement between businesses, individuals and the cell companies.

The hardest problem to solve is the stadium problem. Imagine a stadium filled with 100,000 people all wanting to use their phones. Worse, imagine they all want to stream video to smartphones. Yikes! Solving the stadium problem is the most technically challenging hurdle the providers face. This particular problem will get a post all its own later in this series. For now the more tractable issues of home, car, and business are up for discussion.

Step one in solving the cellular bandwidth crisis is the most difficult one. It isn’t a technical issue and that is why it is so difficult. For things to improve the major players must have a change of heart. They must be willing to think out of the box and to take a broader view of the problem. Many times the first step to solving a problem is admitting there is one. That is only partially true in this case. The providers are well aware of the problem. They are in denial over the scope and the eventual backlash from consumers that will come but they do recognize that there is a shortage of bandwidth. Unfortunately, the providers are way too comfortable with an incremental and safe approach. What is needed are passionate people determined to bring pervasive connectivity with high bandwidth and constant availability. Along with this must be the courage to embark on a multi-pronged attack. As we’ll see in later posts, the technology is mostly here. There is an exception when we discuss the stadium problem but I’ll leave that as a teaser pointing to a later, more techie, post