Thursday, December 14, 2006

The Interference that doesn't interfere

Interference is relentlessly advanced as the primary reason to be skeptical about sharing or reallocating low-frequency wireless capacity. Interference claims are often made with little or no technical support. The value and pervasiveness of television are simply invoked, with a challenge that users will not welcome anything which degrades that service.

To a na├»ve observer, it seems logical that adding additional transmitters in the same or adjacent frequencies to those in use causes interference. This is not necessarily the case. “Interference” is not, as commonly assumed, a physical property of wireless signals. One wireless transmission will not deflect another, nor do two coexisting transmissions tied to the same frequency and location necessarily prevent either from being received.

Cellular telephony systems demonstrate this principle: they use small cells around towers to achieve “space-based frequency re-use.” If there are more users than can effectively share one tower, the network operator can replace it with several shorter-range towers. All the users are still operating nearby, in the “same” spectrum.

Interference is a property of devices. If a receiver is unable to distinguish its intended signal from extraneous background noise, we say the transmission is subject to interference. Changing the receivers and transmitters can make that interference go away. Even with devices as simple as televisions, we can all appreciate that a better set or antenna produces a more robust picture. In other words, it experiences less interference.

Modern wireless devices like mobile phone and WiFi transceivers are sophisticated at modulating their signals and adapting to their surroundings, reducing the experience of interference. In the broadcast context, however, the existing equipment does not have those properties. There are hundreds of millions of televisions in the field, which use the most straightforward and spectrum-inefficient transmission technique: high-power, narrowband broadcasting on a single channel.

Nonetheless, it is quite possible to unlock more of the capacity in low-frequency spectrum without causing significant harm to existing broadcast services.

First, making available wireless capacity in frequencies currently allocated to television does not necessarily require sharing of occupied frequencies. A great deal of the so-called “broadcast” spectrum is simply un-used.

Kevin Werbach

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