Over-the-air wireless repeaters have been around since the early days of commercial cellular service. Back then, when base stations were few and far between, even in large cities, repeaters were used to good advantage for filling in coverage holes caused by terrain and urban clutter. But as networks grew, the additional base stations deployed to increase capacity and improve service quality reduced the need for such “outdoor” repeaters and their use declined, at least in North America.
 
In other parts of the world, particularly Asia, low-powered repeaters gained acceptance for solving indoor coverage problems. Such applications have yet to be widely embraced here. Indeed, U. S. carriers don’t appear to be particularly interested in repeaters at all, even though there are still plenty of coverage problems they could help resolve.

Two main reasons wireless carriers are ambivalent about repeaters are: One, they can be tricky to engineer; and two, if they aren’t carefully engineered, they can cause serious problems for the networks in which they reside. But now RF Window, a company exhibiting in the CTIA Wireless 2008 Korea Pavilion, is offering over-the-air repeater products it claims will eliminate these problems, making repeater engineering much simpler and less risky for network operators. This advance is enabled by RF Window’s patented “interference cancellation system” (ICS).

To appreciate what ICS can do, you must first consider how typical over-the-air repeaters work. Reduced to the basics, a repeater consists of two antennas, one pointing toward the “donor” base station and the other toward the area to be covered.

In between is a bi-directional RF amplifier that boosts the downlink and uplink signals so they can be retransmitted at a far higher level. To provide much coverage benefit, the gain of this amplifier generally has to be quite large, at least 65 dB or so. Therein lies the problem. In order to keep the repeater operating in a stable manner, the isolation between its two antennas has to be greater than its amplifier gain. In fact, with a bit of safety margin thrown in, the isolation between antennas typically has to be at least 80 dB, meaning that the signal transmitted by one antenna must be attenuated by at least 80 dB before it finds its way into the other antenna.

Figuring out how to obtain this much RF isolation with a practical installation is the main reason repeaters can be difficult to engineer. But the real problem is that if isolation is inadequate – that is, less than the repeater’s gain – the repeater can oscillate. If that happens, not only will the repeater fail to enhance coverage, it also may generate significant RF interference that can potentially disrupt operation of the entire network over a large area.

RF Window’s ICS technology addresses these repeater issues by greatly reducing the amount of RF isolation needed between the two antennas. By using sophisticated digital signal processing (DSP) techniques, ICS senses the signals “leaking” from one antenna to the other and injects equal, but 180° out of phase, signals to cancel them. The result: repeaters with high gain that can be installed with little regard for antenna isolation. In fact, the ability to operate at high amplifier gain means that repeaters can put out much higher transmit power. One RF Window repeater model has a total downlink power capacity of 20 W, rivaling that of regular base stations.

So far, RF Window repeaters have been deployed in Asia, South America, and Europe, but not in North America. If ICS provides the technology breakthrough that RF Window claims, that could soon change. 

Drucker is president of Drucker Associates. He may be contacted at edrucker@drucker-associates.com.