The switching to another channel by the secondary user (CR CPE) occurs when an incumbent comes on. However, the switching will not be done to an adjacent channel.

For example if the frequency is set from 54 to 72 MHz with 6 MHz bandwith per channel then 

Channel 1 -> 54 to 60 MHz

Channel 2 -> 60 to 66 MHz

Channel 3 -> 66 to 72 MHz

If the incumbent comes on in channel 1, then the secondary user will switch to channel 3. 

It will not switch to channel 2 since channel 2 is an adjacent channel

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