- IEEE 802.22
IEEE 802.22 is a new
working groupof IEEE 802 LAN/MAN standards committee which aims at constructing Wireless Regional Area Network ( WRAN) utilizing white spaces (channels that are not already used) in the allocated TV frequency spectrum. The use of the spectrum will be used in an opportunistic way in order not interfere with any TVchannel that is transmitting.
The IEEE 802.22 working group on Wireless Regional Area Networks is the youngest group of the IEEE 802 LAN/MAN standards committee. Its project, formally called as "Standard for Wireless Regional Area Networks (WRAN) - Specific requirements - Part 22: Cognitive Wireless RAN Medium Access Control (MAC) and Physical Layer (
PHY) Specifications: Policies and procedures for operation in the TV Bands" focuses on constructing a consistent, national fixed point-to-multipointWRAN that will utilize UHF/VHF TV bands between 54 and 862 MHz. Specific TV channels as well as the guard bands of these channels are planned to be used for communication in IEEE 802.22.
Because the IEEE 802.22 working group was formed in 2004, nothing has yet been specified regarding the particular functionalities of the PHY/MAC layers. However the
IEEE, together with the FCC, is pursuing a centralized approach for available spectrum discovery. Specifically each Access Point(AP) would be armed with a GPSreceiver which would allow its position to be reported. This information would be sent back to centralized servers (in the USA these would be managed by the FCC), which would respond with the information about available free TV channels and guard bands in the area of the AP. Other proposals would allow local spectrum sensing only, where the AP would decide by itself which channels are available for communication. A combination of these two approaches is also envisioned.
Overview of the WRAN Topology
802.22 specifies that the network should operate in a point to multipoint basis (
P2MP). The system will be formed by base stations (BS, above mentioned as Access Points, AP's) and customer-premises equipments (CPE). The CPE's will be attached to a BS via a wireless link in the frequencies mentioned above. The BS's will control the medium access for all the CPE's attached to it. One key feature of the WRAN Base Stations is that they will be capable of performing a "distributed sensing". This is that the CPE's will be sensing the spectrum and will be sending periodic reports to the BS informing it about what they sense. The BS, with the information gathered, will evaluate whether a change is necessary in the channel utilized, or on the contrary, if it should stay transmitting and receiving in the same one.
An approach to the PHY layer
The goal of this layer is to provide excellent, yet simple, performance. The PHY layer must be able to adapt to different conditions and also needs to be flexible for jumping from channel to channel without errors in transmission or losing clients (CPE's). This flexibility is also required for being able to dynamically adjust the bandwidth, modulation and coding schemes. OFDMA will be the modulation scheme for transmission in up and downlinks. With
OFDMAit will be possible to achieve this fast adaptation needed for the BS's and CPE's.By using just one TV channel (a TV channel has a bandwidth of 6 MHz; in some countries they can be of 7 or 8 MHz) the approximate maximum bit rate is 19 Mbit/s at a 30 km distance. The speed and distance achieved is not enough to fulfill the requirements of the standard. The feature "Channel Bonding" deals with this problem. Channel Bonding consists in utilizing more than one channel for Tx / Rx. This allows the system to have a bigger bandwidth which will be reflected in a better system performance.
An approach to the MAC layer
This layer will be based in Cognitive Radio Technology. It needs to be also able to adapt dynamically to changes in the environment by sensing the spectrum. The MAC layer will consist of two structures: Frame and Superframe. A superframe will be formed by many frames. The superframe will have an SCH (Superframe Control Header) and a preamble. These will be sent by the BS in every channel that it's possible to transmit and not cause interference. When a CPE is turned on, it will sense the spectrum, find out which channels are available and will receive all the needed information to attach to the BS. Two different types of spectrum measurement will be done by the CPE: "in-band" and "out-of-band". The in-band measurement consists in sensing the actual channel that is being used by the BS and CPE. The out-of-band measurement will consist in sensing the rest of the channels. The MAC layer will perform two different types of sensing in either in-band or out-of-band measurements: "fast sensing" and "fine sensing". Fast sensing will consist in sensing at speeds of under 1ms per channel. This sensing is performed by the CPE and the BS and the BS's will gather all the information and will decide if there is something new to be done. The fine sensing takes more time (approximately 25 ms per channel or more) and it is utilized based on the outcome of the previous fast sensing mechanism.These sensing mechanisms are primarily used to identify if there is an incumbent transmitting, and if there is a need to avoid interfering with it.
White Spaces Coalition
* [http://www.ieee802.org/22/ IEEE 802.22 working group]
Cordeiro, Challapali, Birru & Shankar; "IEEE 802.22: An Introduction to the First Wireless Standard based on Cognitive Radios"; Journal of Communications, Vol. 1, No. 1, April 2006.
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