TV White Space
TV White Space has been called beachfront property as it represents the first new frequencies available for unlicensed use in some time. The frequencies are becoming available because the guard bands necessary for analog television transmission are no longer needed with digital TV transmissions.
It’s not all good news for the users of the unlicensed bands though, because:
- Devices must work around existing broadcast TV signals.
- TV White Space is not a continuous amount of bandwidth, as current unlicensed bands are, but rather a set of discontiguous frequencies between 50 Mhz and 700 Mhz, with different frequencies available in different regions.
Regulations for utilizing the frequencies currently available for broadcast television now exist in the United States and the United Kingdom, with the rest of Europe and other parts of the world expected to follow. As technical and regulatory restrictions are introduced, it’s becoming apparent that Television White Space may not reach its full potential.
In the beginning, the transition from analog to digital television signals was assumed to free up massive amounts of bandwidth. Rather than auction off those frequencies, as has happened for cellular and other types of licensed transmissions, technology companies such as Google®, Microsoft®, Motorola®, and Dell® proposed to make the frequencies part of the unlicensed frequency bands similar to 900 MHz band in the U.S., 868-869 MHz in Europe, and the 2.4 GHz band worldwide. With the 2.4 GHz and 868/900 MHz bands getting crowded, additional bandwidth for unlicensed applications (which includes the vast majority of consumer products using Wi-Fi®, Bluetooth®, Zigbee®, cordless phones, etc.) is definitely needed. Utilizing the sub-Gigahertz frequencies available for television broadcast also has the benefit of better radio frequency (RF) propagation, because lower frequencies penetrate buildings and walls better than higher frequencies.
Immediately after Television White Space was proposed, there was pushback from television broadcasters and event coordinators, the latter of which use TV White Space frequencies for wireless microphones used in concerts, churches, sporting events and other large-scale events. These and other parties were concerned that opening up the available frequencies, which are mixed in with current television broadcasts, would start generating interference and transmission problems for their equipment. Broadcasters were particularly concerned about mobile/portable transceivers and very high power (up to 1 Watt) transceivers.
To address the concerns, government institutions such as the Federal Communication Commission (FCC) in the United States have adopted regulations that attempt to limit the possibility of interference by TV White Space equipment. That equipment must either refer to a sanctioned database of location-specific available frequencies or utilize a carrier sense to detect any available channels. The carrier sense equipment is limited to 50mW of output power and requires some stringent tests to ensure no interference. Fixed equipment that utilizes a TV White Space database is allowed to transmit up to 1 Watt, but portable devices are allowed to transmit only up to 40 mW or 100 mW depending on adjacent channel use. To access this TV database, the TV White Space equipment must know its position, typically through a GPS receiver, and must have access to the Internet or another IP network with access to the database.
Further complicating options for utilizing TV White Space is the local and regional availability of frequencies. The amount of bandwidth available depends greatly on the number of broadcast television providers in a given area. Large urban areas with a higher number of broadcast television stations will have less (or possibly even no) frequencies available for unlicensed use. The amount of White Space available is often inversely related to the number of users and applications which need the technology. Due to FCC rules regarding guard bands, cities including New York, Los Angeles, San Francisco and Miami have no availability for the higher power fixed stations at 1 Watt. Rather they are limited to only 40-50 mW for the output power on TV bands.
Technical and regulatory restrictions will reduce the use cases for TV White space. In the next post, we’ll look at what the current use cases are and what can be done by the FCC and other regulatory bodies to open up the spectrum.