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 used 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.
To perform reliable sensing, in the basic operation mode on a single frequency band as described above (the “listenbefore-talk” mode) one has to allocate Quiet Times, in which no data transmission is permitted. Such periodic interruption of data transmission could impair the quality of service (QoS) of cognitive radio systems. This issue is addressed by an alternative operation mode proposed in IEEE 802.22 called Dynamic frequency hopping (DFH) where data transmission of the WRAN systems are performed in parallel with spectrum sensing without any interruption.
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 used 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.
To perform reliable sensing, in the basic operation mode on a single frequency band as described above (the “listenbefore-talk” mode) one has to allocate Quiet Times, in which no data transmission is permitted. Such periodic interruption of data transmission could impair the quality of service (QoS) of cognitive radio systems. This issue is addressed by an alternative operation mode proposed in IEEE 802.22 called Dynamic frequency hopping (DFH) where data transmission of the WRAN systems are performed in parallel with spectrum sensing without any interruption.
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