Why the future of wireless cells is small

Smaller cells offer greater capacity

"Cellular" has become part of the vernacular of almost every culture on Earth, and with good reason. Cellular phones have become ubiquitous, and we're rapidly approaching 3 billion cellular users worldwide -- half the population of the planet -- a milestone that we may meet this year.

The "cell" has become the basic unit for provisioning wireless communications, from wireless personal area networks to local area, metro-area and wide-area networks. However, we are now in the middle of an important evolutionary process that will move the cell to a new level of capability, which will have a far-reaching effect for users everywhere.

To explain, let's go back to the precellular days -- the 1950s. Back then, mobile phones were implemented using great big boxes with vacuum tubes and similar ancient technology, and were the end of a system known as MPS -- the Mobile Phone System. If you look carefully in TV shows from the '50s, you can sometimes see these on the transmission humps of police cars and similar vehicles.

The problems with MPS were many. For one, you had to place a call through a mobile operator, and you could not always connect to a desired number. But most important, the capacity of the system was very limited. There was only one stationary, high-power base station covering a very large area. When all of the available frequencies were occupied, the next caller was out of luck. This architecture is still in use today, and is known as point-to-multipoint (P2P or PTMP). Today, however, it's used primarily in fixed -- not mobile -- wireless applications such as fixed WiMax.

As it turns out, researchers at Bell Labs had already seen the shortcomings of MPS in the 1940s and had begun work on AMPS -- the Advanced Mobile Phone System. AMPS still used fixed base stations, but allowed a call to be handed off from one base station to another as a user roamed out of range of one so-called cell and into range of another.

It's easy to see how this model can be extended to cover an arbitrarily large area -- just add more cells. Frequencies can be reused over distance, meaning this system makes efficient use of the scarce resource that is the radio spectrum. This system also enables more users to be on the air in a given area, which means cellular operators can make more money. Since the advent of AMPS, we've gone digital (2G) and added broadband (3G). But while there are relatively new capabilities, the overall model and philosophy behind cellular has stayed the same.

The biggest problem in cellular today is where to put the cells. This is largely dictated by the laws of physics, but other laws, such as local zoning and aesthetic regulations, often come into play. With the need to support ever-more calls, it's often necessary for carriers to split cells into smaller cells to increase capacity. But cellular telephone equipment is physically large and expensive, with big antenna towers and the need for real estate.

This has led to a renewed interest in reducing the size of cells, creating what are called femtocells. Making cells smaller is a very good idea because frequencies can be reused even more, the cost of equipment and the required real estate goes down, and overall capacity goes up, with users spread among more cells. But the disadvantage is obvious -- more cells must be installed.

The concept of femtocells is similar to the microcells used in wireless LANs, which typically have a coverage radius of no more than 100 meters. Bluetooth has the concept of picocells, with even less coverage -- typically no more than 30 meters. Femtocells (so named not because they're smaller than picocells but, rather, simply for market differentiation) have been noted as an alternative for the metro-scale deployment of WiMax and could, in fact, be used for any cellular deployment. I've been a strong advocate of this concept for as long as I can remember.

But a big issue is that all these femtocells must be interconnected, and that's where it gets tricky. Doing so with wire could get very expensive if new wiring is run to each cell. This brings us to meshes, which implement backhaul over the air by relaying between mesh nodes. Many use "mesh" synonymously with "Wi-Fi mesh" but, in reality, mesh techniques can be used with any radio technology. And we can also mix multiple radio technologies in any given situation.

It's clear to me that the future of urban wireless deployments is in very small cells (no matter what we call them) using mesh techniques for backhaul. This is the next step in the road to wireless ubiquity and one that will have a very positive effect on capacity, reliability and cost. Small cells are indeed the future of wireless.

Craig J. Mathias is a principal at Farpoint Group, an advisory firm specializing in wireless networking and mobile computing. He can be reached at craig@farpointgroup.com.

Copyright © 2007 IDG Communications, Inc.

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