Anyone in the eastern portion of the U.S. this week who was forced to evacuate an office, home or school following Tuesday's magnitude 5.8 earthquake soon noticed that cell phone service was spotty or, in many cases, nonexistent. For New Yorkers herded outside of their skyscrapers and into the streets, it was a communication blackout reminiscent of (although of course not the same as) the 9/11 terrorist attacks. In both situations, mobile phone users were unable to connect to the cell network to communicate with loved ones.

Whereas the 9/11 cell phone outage was the result of many factors—including the downing of cell phone towers—this week's problems (though brief) were caused purely by volume. Countless cell phone users were fighting for limited access, leaving most without service. With Hurricane Irene bearing down on the East Coast, one is left to question the reliability of mobile phones in the face of serious emergencies.

Scientific American spoke with Andrea Goldsmith, a Stanford University electrical engineering professor and researcher at the school's Wireless Systems Lab, about why mobile phone users are often unable to connect during emergencies, as well as options for improving cell network performance when it matters most.

[An edited transcript of the interview follows.]


What is happening when, during an emergency, you try to use your mobile phone and can't get a signal?
There's a finite capacity that the cell phone system can support at any given base station. Say it's 10 users or 100 users talking on their phones. When the 101st user calls in, there's no channel available for that user to occupy. That's a little bit of a simplistic description, but that's effectively what happens. When there's an event like an earthquake that causes people to want to make more phone calls or want to access the Internet more, the systems just become overloaded.

How have cell-phone services changed since 9/11?
Cell phones are coming up now on their fourth generation of technology, so we've had two evolutions of cell phone technology since 2001. Most of the work in between evolutions has been aimed at getting higher data rates because people now also use their phones for accessing the Internet and for exchanging videos and pictures. There have been improvements in capacity, but not enough to keep up with just a tsunami of traffic across the wireless networks.

With the introduction of smartphones, many mobile phone users have the ability to connect over the cell network or via Wi-Fi. Shouldn't this provide additional capacity during emergencies that wasn't there years ago?
Cell networks and Wi-Fi both have limitations. Cell phone networks have a lot less bandwidth than Wi-Fi, but at least the cell phone service providers own the part of the spectrum over which their customers communicate. This enables the cell providers to guarantee a certain quality of service. Still, it's not easy for them to add capacity because there isn't a lot of spectrum available for them to purchase.

Wi-Fi has access to a large amount of bandwidth for connecting to the Internet but doesn't have exclusive rights to that space. Smartphones have to share the spectrum with a large number of different wireless devices, including baby monitors, cordless phones and Bluetooth gadgets. With Wi-Fi, when there are a lot of people trying to get on the network, nobody can get on.

So your ability to connect via the cellular network or Wi-Fi really depends on the people around you?
Yes. If you went into a Starbucks in Times Square, say, probably you wouldn't be able to get on the Internet via Wi-Fi. And if you were in the part of the city where the cell phone towers were not overloaded, you probably got a connection just fine.

What can be done to improve cell networks so they are less likely to be overloaded during emergencies?
Typically you design any system for average use. The problem is, if it's designed for average use and there's an event that's statistically unusual, typically you're going to overload the capacity of the system. One option is to just build in extra capacity, and one way of doing that is to make the cells smaller. Right now the coverage of the base station that your phone talks to is fairly large. These cell towers put out about 10 watts of power, so they can, for example, cover a fairly large region of New York City. If a large cell can support something like 1,000 users in Midtown and you make that cell smaller, so that it covers a couple of blocks, you can now support 1,000 users within a much smaller region.

In terms of making these cells smaller, would these be public cells, or would people start carrying around their own mini cell towers with them?
Small cell towers known as picocells and microcells are public. They're part of the architecture laid out by the cellular operators. Femtocells are desktop cell phone towers that a company or an individual would buy this for better cell phone coverage in their office or home [they can enhance a signal within a radius of 10 to 200 meters around the device].

In the case of an emergency like the earthquake or hurricane, would you have been more likely to get a signal if you had your own femtocell?
There are a number of challenges with femtocells. Earlier femtocells didn't manage their interference with one another well and weren't able to hand off to the larger cell towers outdoors. So a femtocell might have helped if you made a call indoors and stayed indoors. The question is how to do dynamic optimization of all the picocells, microcells and femtocells so that you're adapting not just to traffic changes under normal conditions but in particular under emergency situations. If an earthquake or the 9/11 attacks take out a number of towers the way the cell phone towers are configured now, service providers have to manually reroute cellular traffic; they can't adapt to dynamic changes in traffic.

The ability to do this dynamically is referred to as self-organized networks, or SONs. Combining that capability with small cells has tremendous potential to increase both the capacity and the ability to adapt to changes that are unpredictable.

How would self-organized networks, or SONs, be implemented into a cellular network?
It's a software network management tool for base stations. It can sit in the cloud, or it can sit on the mobile gateway that's managing all of the cell phone base stations.

What are some other options for improving wireless network performance?
Obviously, if capacity is a problem, throwing more spectrum at it is a way to increase capacity. Given that there just isn't a lot of spectrum available, another option is cognitive radio, which can take spectrum that's already occupied and use it more intelligently. In this way users sharing the spectrum, as they do with Wi-Fi, don't interfere with the licensed users of the spectrum, such as broadcasters. You would have to have a cell phone equipped with cognitive radio software that can find spectrum holes. For example, if you're trying to make a mobile phone call and can't get a signal, your phone would switch into cognitive radio mode and scan not just the cell phone bands but other bands for some piece of spectrum it could use.

That's a promising technology, but I'm skeptical that in the near term it will solve the capacity problem or even be viable politically. Spectrum is so precious to broadcasters and other spectrum bandwidth owners that they don't trust that secondary users can share their space without interfering.

Certainly, this distrust has been a big part of the controversy over the use of so-called white spaces between licensed bandwidth by wireless gadgets.  What impact will the expansion of 4G networks have on these emergency bottlenecks that we currently experience?
Newer, 4G networks are optimized for data in the sense that they use the same network that the Internet is using. Telephone operators might just leave voice traffic on their 3G networks, which are still fairly new and not going away anytime soon. Voice requires a lot less bandwidth than data, which would then be pushed to 4G networks. In that case, things will be better in an earthquake because when you walk outside and try to make a phone call you won't be competing with people using their smartphones to send data across the same network.