Some embodiments herein relate to a method in a base station for informing a user equipment of a power offset to be used at the user equipment, which base station is controlled by a radio network controller. It relates to the generation of a focused ultrasound acoustic signal and the receiving of echo signals from the wall of a body lumen with an array of acoustic transducers wrapped around the circumference of the capsule. The switching frequency adjuster generates an adjusted error signal in response to an error signal generated by the error amplifier.