The challenge of extending your wireless reach

Overcoming the unpredictability of wireless

If it weren't for the laws of physics, the reliability of wireless might just match its convenience.

For both better and worse, though, the subtle art of radio involves sending information through the air without a physical connection. True, the technologies and techniques of radio aren't that different from those that we apply to wire, cable or fiber -- generate a carrier, modulate the carrier, send the information, then reverse the process. With wire, cable or fiber, the signal generally comes out pretty well at that other end, barring a physical disruption to the cable, which doesn't happen very often.

However, with signals that flow through the air, it can be quite miraculous that radio works at all. Sometimes, wireless signals arrive with sufficient strength and quality to be successfully detected and demodulated, and sometimes they don't. This leads to the greatest frustration experienced by wireless users -- not being able to communicate on the anytime, anywhere basis we have been led to expect.

There are many reasons for the temporary failure of a radio link, but the most common reason is fading. Radio waves naturally lose power with the square of the distance between transmitter and receiver. In other words, signals lose power exponentially. Throw in walls and other obstacles and the situation becomes worse. That's why in-building penetration of radio signals remains a hot topic among both engineers and end users.

Whose cell phone hasn't failed them indoors? Yet, there are good solutions to this problem that typically require adding more hardware. The simplest solutions are to either use more transmit power or higher-gain receivers and receiving antennas. However, these solutions have limitations; most notably, they need to be engineered into products. Also, transmit power is limited by government regulations. So we need to get even more clever.

One key way to be clever in this regard is to use distributed antenna systems (DAS) that are installed in buildings with large indoor areas, such as major office buildings, shopping malls, transportation hubs and similar venues. A DAS consists of a box that receives and transmits using wide-area services. This system then interfaces with antennas distributed throughout the structure or area, perhaps adding WLAN support in the process. DAS products are noted for their flexibility and adaptability. Two of the leading suppliers include LGC Wireless and MobileAccess.

There's another technique worth noting here, which can perhaps be thought of as the degenerate case of a DAS. This is the use of repeaters, which, as the name implies, take a signal and re-transmit it, thus boosting the original signal with just a tiny amount of latency added in the process.

Repeaters are an excellent way to solve smaller in-building coverage challenges. For an example, look at the Spotwave Z1900. This is a low-cost, residential class repeater that's easy to install and boosts signals in the cellular PCS (1.9 GHz) bands. Larger repeaters for commercial settings are also available.

Eventually, perhaps, mobile/mobile convergence will provide the ultimate solution to the in-building coverage challenge. This approach transparently merges wireless WANs with wireless LANs. Rather than extending the signal of a given system, this system hands off the connection to another type of radio. But while we're beginning to see the availability of both carrier-based and enterprise-provisioned solutions here, it will be some time before pure-convergence products are common. That's because cellular and WiMax carriers have a vested interest in expanding the footprint of their existing services, using their current handsets and adapters. As a result, repeaters and distributed antenna systems have a long and happy life ahead of them no matter which direction convergence goes in the near term.

We're only just scratching the surface of this problem with this brief column, but stayed tuned. There are other interesting solutions now appearing, including small base stations and particularly femtocells, which I'll cover in an upcoming column. It seems, in fact, that no matter what the challenge, wireless engineers are ready with a steady stream of innovations that continue to propel wireless towards ubiquity.

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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