On respawn, the previously in use radio radio should be selected and turned on. If the player had a previously set frequency, it should be restored. Otherwise, the radio should be set to the current group frequency. Changing groups should set the new group frequency as default until the player manually sets a new frequency.

Note that the current default behaviour, with randomly changing channels on respawn, is so frustrating that the following mod has been created to rectify the situation: https://forums.bohemia.net/forums/topic/239013-respawnradio/

This should be baseline behaviour. The mod should not be necessary.