White Spaces: Competition and Survival in the RF Jungle
By Ken Hamberg
On February 17, 2009, we'll see the end of virtually all analog television broadcasting throughout the United States. For several years television stations have been simultaneously broadcasting on two separate channels, in both analog and digital. The fact is that DTV stations can co-exist on adjacent channels without interference, which combined with the opening of so many channels formerly occupied by analog transmissions, creates a lot of prime real estate in the electro-magnetic spectrum for reallocation and purchase by other wireless users and services besides broadcasting.
This is an ongoing process that wireless microphone customers throughout the country need to seriously address, because we use that space, too. The good news is that the bad news isn't all that bad. Here at B&H we'll continue to monitor the situation on Capitol Hill, and we'll keep you updated on any new developments that affect our customers concerning this issue.
White Spaces and Tight Spaces
White spaces are the unoccupied channels in the broadband RF (Radio Frequency) spectrum. Wireless microphone systems, both fixed and frequency-agile, operate in white spaces all the time in both the VHF and UHF bands. The availability of white spaces in the U.S. varies from city to city, reflecting the number of channels in use by TV stations.
Wireless mics are an essential audio component in the music, broadcast and entertainment industries, and users are very aware of the problems associated with interference and dropouts in heavily congested RF areas, as is anyone owning a cell phone. Both Congress and the FCC are very interested in opening up white spaces used by wireless microphones for supervised use by unlicensed wireless broadband devices as well.
Unlicensed devices include cordless telephones, WLAN devices, cell phones, PDA's, and Bluetooth devices. Most wireless microphone systems, intercoms, 2-way radios, and other communications systems and devices are licensed.
The FCC, in preparation for the February 2009 deadline, has reallocated blocks of channels, many of which were available to wireless mic users, and either restricted their use or auctioned them off to the private sector for the development of wireless services. Licenses have been purchased by AT&T, Qualcomm, and a number of local wireless service providers. Let's make this clear: White Spaces aren't for sale, frequency blocks and TV channels are.
The FCC's agenda is to open up the spectrum to and broaden the infrastructure of unlicensed broadband technology, creating competition between wireless broadband, cable and DSL that can only benefit the consumer in the long run.
The U.S. is currently ranked No.13 worldwide in broadband penetration, lagging behind Korea and Norway among other countries. FCC commissioner and engineer Michael Copps has a particular ax to grind with regard to the unavailability of affordable, reliable broadband in underserved municipalities and rural areas, as do many congressmen.
The area on the electro-magnetic road map vacated by analog television happens to be where RF travels best in terms of distance and indoor reception. Think of WiFi with a longer range and more reliable reception and you'll see what the FCC has in mind for America's broadband future.
Representatives from Shure, Audio-Technica and Sennheiser have been testifying on Capitol Hill, along with the NAB, NFL, Disney and others from the broadcast and entertainment fields, lobbying against white space allocation for unlicensed broadband devices based on the inevitable congestion and resulting interference.
They're up against groups such as the White Space Coalition, a consortium that includes the technology giants Google and Microsoft, Dell and Hewlett-Packard, Samsung and Philips, and Intel and Earthlink. Both Philips and Microsoft have submitted prototype broadband devices (known as white space devices or WSD's) for testing by the FCC. These devices must pass a series of rigorous tests to prove that they will not interfere with television broadcasts in any way. Approved WSD's may appear in retail in February, 2009, though as yet none has gotten the thumbs-up.
Your Slice of the Pie
Let's take a look at the new spectrum roadmap as it's been drawn by the FCC, and see how it affects wireless microphone use.
• Channels 2-51 (54-698MHz) will be re-occupied by digital television, and channels 14-51 will be assigned to DTV stations in the UHF band. Historically channels 14-20 (470-512MHz) have been underutilized and white spaces in this range will probably not be available to unlicensed wireless consumer devices. This is good news for wireless mic users.
• Channels 52-69 (700-806MHz) have been reallocated and are off-limits to television. Channels 63, 64, 68 and 69 are reserved for Public Safety radio communications, and the remaining blocks have been or will be auctioned off to the private sector for wireless service and infrastructure development. This is bad news for those of you with wireless mic systems in this range. Frequency-agile systems may be re-tuned at the factory or by a repair service, and should be. Fixed-frequency systems in this range will no longer be usable in the U.S. after February, 2009, although they'll work as well as they always have outside of the U.S.
• The 944-952MHz range is a usable frequency range that may be exploited for future wireless microphone development. It's been used primarily for wireless links between radio station studios and their remote station transmitter sites. This part of the spectrum is not in use in many areas of the U.S, nor is it used by other wireless systems or devices.
• The 2.4-2.483GHz range, while teeming with consumer-grade wireless activity, is limited to low-power devices and is subject to international restrictions on broadcast use. This means that lower-output wireless mic transmitters (10-25mW) should function nicely. Sabine already has a professional 2.4GHz series of wireless systems in the market place, and systems from other manufacturers will undoubtedly follow.
The ways of the FCC may be a bit secretive and mysterious, but there are some brilliant and experienced engineering minds among the 5 commissioners on the panel. The controversial internet broadband device in question is under intense scrutiny, and since the maximum output power allowed wireless mic transmitters is 250mW (though typical systems output at 10-50mW), one has to doubt that the FCC would allow someone's download to interrupt, say, a Super Bowl broadcast (or for that matter, your gig at a local club) because broadband devices were permitted to exceed the 250mW limit. That being said, interference has always been and will continue to be a problem in wireless communications.
Wireless microphone technology is quite sophisticated, and is subject to pretty stringent requirements. The frequency-scanning technology now in place in many wireless mic receivers has proven to be a great solution for many interference problems by detecting local TV channels, or other wireless mic frequencies already in use, and selecting the optimum channels. One example of similar technologies already developed and under accelerated refinement for unlicensed broadband device applications may be found in the form of spectrum sensing capability.
Spectrum sensing is basically a scanning function built into WSD's enabling them to listen for (called a listen-before-talk strategy) and detect the signals of DTV channels, wireless microphones and other wireless activity in a local area. The idea is that the devices will accurately "report" the presence of other wireless activity and correctly identify the channels and types of signals in use, enabling the WSD user to avoid interference. The FCC has requested that the prototypes submitted for testing exhibit spectrum sensing capability at a signal threshold as low as -116dBm/6MHz, while showing that the devices will not interfere with DTV and wireless mic transmissions.
According to Edgar Reihl, the Director of Advanced Development at Shure: "The basic requirement is that the interference range of the wireless broadband transmitter, which is the distance within which the broadband device will interfere with a wireless microphone, must not exceed the range from which the device can reliably detect the presence of a wireless microphone signal in order to avoid it". Conversely, as to whether or not a wireless mic receiver can detect the low-level signal of a wireless broadband device on a given channel remains to be seen.
The point is that the FCC will have to hold wireless broadband developers to the same standards of quality and accountability to which it has held the audio and broadcast industries, if the redrawn spectrum roadmap is going to lead us to a bright new wireless future.
Special thanks to Mark Brunner from Shure for permission to use the charts shown in this article. Special thanks to Shure's Edgar Reihl for providing valuable technical information related to its content.
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