IEEE 802.22 WRAN Standard
- an overview or tutorial about IEEE 802.22, the Wireless Regional Area Network, WRAN standard using Cognitive Radio technology.
The IEEE 802.22 standard defines a system for a Wireless Regional Area Network, WRAN that uses unused or white spaces within the television bands between 54 and 862 MHz, especially within rural areas where usage may be lower.
To achieve its aims, the 802.22 standard utilises cognitive radio technology to ensure that no undue interference is caused to television services using the television bands. In this way 802.22 is the first standard to fully incorporate the concept of cognitive radio.
The IEEE 802.22 WRAN standard is aimed at supporting license-exempt devices on a non-interfering basis in spectrum that is allocated to the TV Broadcast Service. With operating data rates comparable to those offered by many DSL / ADSL services it can provide broadband connectivity using spectrum that is nominally allocated to other services without causing any undue interference. In this way IEEE 802.22 makes effective use of the available spectrum without the need for new allocations.
IEEE 802.22 background
The IEEE 802.22 standard for a Wireless Regional Area Network or WRAN system has been borne out of a number of requirements, and also as a result of a development in many areas of technology.
In recent years there has been a significant proliferation in the number of wireless applications that have been deployed, and along with the more traditional services this has placed a significant amount of pressure on sharing the available spectrum. Coupled to this there is always a delay in re-allocating any spectrum that may come available.
In addition to this the occupancy levels of much of the spectrum that has already been allocated is relatively low. For example in the USA, not all the TV channels are used as it is necessary to allow guard bands between active high power transmitters to prevent mutual interference. Also not all stations are active all of the time. Therefore by organising other services around these constraints it is possible to gain greater spectrum utilisation without causing interference to other users. Despite the fact that the impetus for 802.22 is coming from the USA, the aim for the standard is that it can be used within any regulatory regime.
One particular technology that is key to the deployment of new services that may bring better spectrum utilisation is that of cognitive radios technology. By using this the radios can sense their environment and adapt accordingly. The use of cognitive radio technology is therefore key to the new IEEE 802.22 WRAN standard.
IEEE 802.22 standard history
The concept for 802.22 can trace its origins back to the first ideas for cognitive radio. With the development of technologies for the software defined radio, J Mitola in his doctoral thesis in 2000 coined the name "Cognitive Radio" for a form of radio that would change its performance by detecting its environment and changing accordingly.
In 2004 the FCC issued and NPRM (notice of proposed rulemaking) regarding the television spectrum. As a result in November 2004 the IEEE 802.22 working group was formed to develop a WRAN system that would deliver broadband connectivity particularly to rural areas by sharing the television spectrum.
By May 2006 draft v0.1 of the IEEE 802.22 standard was available, although much work was still required. Also discussions were required with broadcasters whose spectrum was being shared as they were fearful of interference and reduced revenues from advertising as a result.
The standard is expected to be completed by the first quarter of 2010 and with this some of the first networks could be deployed.
There are a number of elements that were set down for the basis of the 802.22 standard. These include items such as the system topology, system capacity and the projected coverage for the system. By setting these basic system parameters in place, the other areas fall into place.
- System topology: The system is intended to be a point to multipoint system, i.e. it has a base station with a number of users or Customer Premises Equipments, CPEs located within a cell. The base station obviously links back to the main network and transmits the data on the downlink to the various users and receivers data from the CPEs in the uplink. It also controls the medium access and addition to these traditional roles for a base station, it also manages the "cognitive radio" aspects of the system. It uses the CPEs to perform a distributed measurement of the signal levels of possible television (or other) signals on the various channels at their individual locations. These measurements are collected and collated and the base station decides whether any actions are to be taken. In this way the IEEE 802.22 standard is one of the first cognitive radio networks that has been defined.
- Coverage area: The coverage area for the IEEE 802.22 standard is much greater than many other IEEE 802 standards - 802.11, for example is limited to less than 50 metres in practice. However for 802.22, the specified range for a CPE is 33 km and in some instances base station coverage may extend to 100 km. To achieve the 33 km range, the power level of the CPE is 4 Watts EIRP (effective radiated power relative to an isotropic source).
- System capacity: The system has been defined to enable users to achieve a level of performance similar to that of DSL services available. This equates to a downlink or download speed of around 1.5 Mbps at the cell periphery and an uplink or upstream speed of 384 kbps. These figures assume 12 simultaneous users. To attain this the overall system capacity must be 18 Mpbs in the downlink direction.
In order to be able to meet these requirements using a 6 MHz television channel spectral efficiency of around 3 bits / sec / Hz are required to give the required physical layer raw data transfer rate.
IEEE 802.22 characteristics
The basic specification parameters of the IEEE 802.22 standard can be seen in the table below:
|Typical cell radius (km)||30 - 100 km|
|Methodology||Spectrum sensing to identify free channels|
|Channel bandwidth (MHz)||6, (7, 8)|
|Channel capacity||18 Mbps|
|User capacity||Downlink: 1.5Mbps
Uplink: 384 kbps
The IEEE 802.22 standard should provide additional usage of the huge amounts of broadcast spectrum that is available in many countries. As a result of the fact that 802.22 uses cognitive radio technology, it will be possible to ensure that no undue interference is caused to any existing services and users should not suffer any degradation in performance of their terrestrial television reception. Accordingly the use of IEEE 802.22 WRAN technology should enable more efficient use of the spectrum to be made as well as providing new services for users, especially in rural areas.
By Ian Poole
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