How do I get the radio frequency in ble sample #ble


I'm trying the sample of bluetooth 
I use central and peripheral(use gatt) code load in two boards
I want to Implement frequency hopping to send the data
but I don't know how to start 
 what api should I call , so that I can know the frequency channel which is using and change the frequency channel  ?
I also found " bt_le_set_chan_map " can set the channel map ,but how to use this api?


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