The IEEE 802.22 Working Group started its activities in 2004 with the aim of achieving a regional radio network. The frequency bands used are primarily the digital dividend, that is to say the frequency bands that will be freed when television will fully and finally digital. In France, it is planned for 2010. In the rest of the world, this should be spread between now and 2015.
The studies focus on television “white channels” that is to say frequencies or television bands that are not used or that are very bad. The idea is to introduce an opportunistic transmission that uses the unused strips at certain times on television, taking care not to interfere with the TV channels.
The frequencies used lie below 1 GHz. They have the television tape properties: penetration through the walls, very good range and speed.
Customers will be equipped with GPS or Galileo systems to locate the transmitter. After querying a central database, they will receive the frequency on which to transmit with the power and appropriate directivity.
These “cognitive” radio principles have been studied for a few years so much better to use the spectrum. Network conditions and user behavior may imply a change in the frequency, power, etc.
In summary, cognitive radio defines the ability to use a tape with license without the operator who uses or embarrassed. Like many bands are not used, it is possible to recover a considerable bandwidth.
We can define two types of cognitive radio:
• Full Cognitive Radio, where all the parameters are used.
• Spectrum Sensing Cognitive Radio, where only the radio frequency is taken into account.
Two subassemblies are also discernible:
• use of cognitive radio in a licensed environment, such as proposed by the IEEE 802.15.2;
• use of an unlicensed frequency band as that proposed by IEEE 802.19.
Cognitive radio has been especially developed as part of SDR (Software Defined Radio) and was especially interested in Spectrum Sensing Cognitive Radio solution in TV bands. The main difficulty is obviously to detect the use of the band by its legal owner, then stop secondary emissions and detect that the tape is not used again. The detection of the energy is not really enough to be sure that we will not disrupt redundant signals. The use is carried by an OFDM technology, in which only the sub-bands corresponding to the free bands are used.
IEEE 802.22 working in point-to-multipoint mode, a signal can be broadcast over a large area and simultaneously address a set of points. The system consists of base stations, or BS (Base Station), and terminal equipment. The base station receives from the terminal equipment regular reports of listening carrier. From this information, the base station may decide to change or not to set the frequency and transmission power of the terminal equipment.
The physical support works and determines the OFDM sub-bands to be used, and the encoding modulation. Trials show that the program on the basis of a television station channel used to route twenty megabits per second for 30 km.