Atlantic Cities

Why Your Cell Phone Drops Calls in Dense Cities

Why Your Cell Phone Drops Calls in Dense Cities
Flickr user ralphman, used under a Creative Commons license

Cell phones are small packages filled with big promises. They vow to let you cut two cords—phone and Internet—while compressing the necessary infrastructure into tall, tidy towers. No more dangling phone lines or unaesthetic cable boxes, just speedy connections to calls and emails. But the game is changing, and that same infrastructure that cell phones so handily pushed further from end users is steadily creeping back.

Cell towers are like other types of infrastructure in that they are inextricably linked with population density. More people means more towers. But where other industries can count on technological advances to help ease the problem, the rapid pace of development is actually compounding the problem for cell phone companies.

We’ve all experienced the expanding cell phone system’s shortcomings, from dropped calls to no service to a slow Internet connection. When one of those things happens, chances are it’s because too many people are crowded into one area. Poor service due to crowding is most apparent at concerts or other large events, but it’s becoming an everyday occurrence as more people use more connected devices. The solution is easy in the case of a concert—cell phone companies just truck in portable cell towers. But in the case of cities, the fix is a lot less temporary and looks a lot more like the wired infrastructure it superseded. Part of the problem stems from the way cellular networks were first conceived.

Early in the days of wireless communications, signals were often broadcast from one monolithic tower, much like radio stations are today. At the time, however, these cells didn’t communicate with each other. This made roaming tedious. Once you moved outside that area, you lost the signal. To continue communicating, you had to tune into a new tower. Still, these towers obviated the need for a massive single tower, and in doing so, carved up the map into different cells. Hence the biologically inspired epithet (though the term “cellular” wasn’t widely used until around 1970).

Early cells used two-way radio towers which broadcast signals in all directions. This was wasteful, though—to provide complete geographic coverage, each circular cell overlapped significantly. The coverage map looked like a series of Venn diagrams. This setup wasted valuable frequencies and required more towers than necessary. AT&T had kicked around the idea of a wireless phone system as early as the 1930s. It would be years before it came to fruition, though, in part because the government was hesitant to allocate the spectrum required to implement it at the time.

Part of the solution was a clever system of spectrum reuse enabled by a proposal from Bell Labs engineers Douglas H. Ring and W. Rae Young. The pair envisioned a series of hexagonal cells with towers located at the corners instead of the center. Three directional antennas at the top would cover 120 degrees each, meaning every tower would sit a the intersection of three cells instead of in the middle. In addition to eliminating coverage overlap among towers, Ring and Young’s design allowed frequencies to be more efficiently reused across a landscape, reducing overall demand. (The hexagonal cells also easily split into smaller units.) The pair wrote down their ideas in 1947, but it wasn’t until the late 1960s and early 1970s that solid state electronics were advanced enough to make them feasible.

Fast forward to today, and the spectrum problem is again rearing its head. Cellular networks are running out of spectrum, but this time it’s paradoxically because of advances in technology. People are buying cheaper, more capable wireless devices. Each of them nibbles a small chunk of frequency, with successive generations of cellular technology gobbling ever larger bites. Together, trends in population and device density and technological advancement have trimmed the amount of territory each cell tower can cover. For example, the transition from 2G to 3G cut tower coverage by 20 percent. The next generation of mobile phone technology—4G, also known as LTE—will cut even more.

Beyond building more of the classic 120-foot towers, wireless companies are looking to a new solution called femtocells. These cell “towers” are small and compact, looking more like a Wi-Fi base station than a piece of industrial infrastructure. Femtocells can more cost-effectively reduce the size of cells than typical towers, but they aren’t an easy solution. Each needs to be fed by a physical connection—backhaul, in industry parlance. But backhaul is expensive, and cell phone companies are loath to build it.

The need for more towers and more physical infrastructure seems inevitable, though. Indeed, it's a familiar pattern seen with maturing technologies. They often start with a monolithic scheme and progress toward a distributed approach. When the telephone was first introduced, an entire town may have had one phone. Party lines came next, and only a handful of houses had to share a phone. Soon every house had a telephone line, some even two or three. Water purification was once the purview of large municipal water facilities, but now we install taps throughout our homes and carry small water filters on backpacking trips. The same progression is happening in cellular networks today, with large towers that once supported an entire town or neighborhood being replaced by femtocells.

Barring a technological breakthrough, greater population density requires that the old technologies decentralize. People demand speedy access to abundant sources, and there are two ways to accomplish that—have people move closer together, or move the sources closer to the people. Chances are you’d rather not pick your next apartment based on proximity to a speedy cell phone tower, so the towers will need to come to you.

Tim De Chant is a writer based in Cambridge, Mass. His work has appeared in Ars Technica and Grist and he blogs about density at persquaremile.com. All posts »

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