listening to a Bluetooth LE device in the background

I am developing an application that requires Bluetooth LE support. In any case, I need to trigger an activity in my app after the device has sent a string. The problem is that this has to be in every case - even if the app is minimised or the phone is asleep. There are several solutions on the web for background activity, but I want to ask how can this be solved so that it works reliably and I can be sure that the action will occur? Thank you for your suggestions for solutions to the problem.