Frequently Asked Questions - Spectrum scarcity or spectrum hoarding?
Although all available spectrum is currently allocated in developed countries, many independent studies have found that the total amount of spectrum in use at any one time in any one place is a tiny fraction of the total. This is due to the way spectrum was originally allocated and to the fact that spectrum is often used intermittently; for instance some TV broadcasting stations do not transmit 24 hours a day.
As a consequence, a radically new way to use spectrum has been suggested. Instead of leasing spectrum to a given organization on an exclusive basis, a new dynamic spectrum management paradigm proposes to use whatever spectrum is available in a certain place at a certain time and switch to another frequency whenever interference is detected in a given band.
Of course to implement dynamic spectrum access requires new technologies and new legislation and many vested interests are fighting this, alleging possible interference. The key issue is how to determine when a particular spectrum band is really being used in a particular geographic region and how to move quickly to a new frequency band when an existing user with higher priority is detected. Thus in the VHF and UHF bands, television broadcasters transmitting at high power in specific frequencies and regions would have first priority. They are the primary license-holders in the spectrum. TVWS broadband devices would have a secondary priority and would be obliged to ensure that they do not interfere with the primary license holder.
The technology to accomplish this feat has been demonstrated and implemented in the new IEEE802.22 standard recently approved, as well as in other standards currently being considered.
Stimulated by the impressive success of WiFi (due mostly to the use of unlicensed – or open spectrum), the IEEE created a working group to address the requirements of a Wireless Regional Area Network. The challenge was to develop a technology suitable for long distance transmission that could be deployed in different countries (each with quite different spectrum allocations). The IEEE focused on spectrum currently allocated to TV broadcasting which spans approximately 50 to 800 MHz This range of spectrum is not currently used in its entirety all the time, so there are “white spaces”, fallow regions that can be re-used for bidirectional communications. In rural areas all over the world, but specially in developing countries, large portions of spectrum are currently under-utilized.
The IEEE802.22 standard is likely to enable dynamic spectrum access in a similar manner that the IEEE802.11 (WiFi) standard did to open spectrum. Of course not all spectrum can be liberated at once; a gradual process is required as the many technical, legal, economic and political hurdles are solved. There is no doubt, however, that IEEE802.22 paves the way to the future of spectrum allocation.
To assess the availability of a given frequency channel at a given time, two methods are being considered: channel sensing and a database of primary users in a given geographic location at a given time.
Channel sensing means that before using a channel, the base stations will listen to the channel first to determine whether it is already in use. If in use, the base station will try another channel and repeat this procedure until a free channel is found. The device will continue to sense at regular intervals to account for the possibility of stations coming alive at any time.
Although this method should be sufficient to detect and avoid spectrum interference, current spectrum holders have successfully lobbied the regulators to force implementation of the second method, which is much more complicated and imposes additional costs in consumer equipment.
The second method establishes an “off limit” zone in a given channel by building a database of the existing transmission stations, including their position and respective coverage area. A new station wishing to transmit must first determine its exact position (so it must have a GPS receiver or other means to find out the geographic location) and then interrogate the database to ascertain that its present location is not in the forbidden zone of the channel it is attempting to use. To interrogate the database, it must have Internet access by some other means (ADSL – Asymmetrical Digital Subscriber Loop -, Cable, Satellite, or Cellular), besides the 802.22 radio (which cannot be used until the channel is confirmed as available). This adds an additional burden to the station hardware and translates into additional cost.
The FCC has been promoting the building of a database of registered users of TVWS spectrum and have authorized 10 different private enterprises to build, operate and maintain such repositories. Field trials of TVWS technology are now being conducted in the U.S. and elsewhere.
In the U.K., the telecommunications regulator, OFCOM, is conducting TVWS trials. OFCOM is currently using the database method.
Although IEEE802.22 has received the most publicity, several competing standards to leverage TV white spaces for two-way communication services are currently being explored. These include:
- IEEE802.11af – this amendment builds on the enormous success of IEEE802.11 by adapting the same technology to the frequency bands allocated to TV transmission. This adaptation relieves spectrum crowding in the 2.4 GHz band and offers greater range due to use of lower transmission frequencies. A IEEE802.11 working group is discussing details.
- IEEE802.16h – This amendment of the 802.16 standard was ratified in 2010 and describes the mechanism for implementing the protocol in uncoordinated operation, licensed or license-exempt applications. Although most deployments have been in the 5 GHz band, it can also be applied to the TV bands frequencies and can profit from the significant deployments of WiMAX (Wireless Microwave Access) systems in many countries.