In mobile phone repeater system, if outdoor antenna too close to the indoor antenna, oscillation (commonly named "feedback") can occur. It's a positive loop between the system's output and input, like putting a microphone up close to a speaker.
Oscillation occurred when broadcasted signals from indoor antenna get picked back up by the outdoor antenna and passed through the mobile phone repeater again. The result of oscillation in mobile repeater system is mobile phone can receive a high signal, but it's a spurious signal with background noises. If mobile repeater system is suffering from oscillation for few weeks, the repeater device will stop work.
How to Prevent Oscillation
The immediate solution of prevents oscillation is set both outdoor and indoor antennas for enough apart and not point to each other to ensure that the outdoor antenna does not receive any signal from the indoor antenna. While install repeater system for home/office/building, you can install the outdoor antenna at a higher horizontal level to avoid the rebroadcasting range of the indoor antenna. In a vehicle, the metal roof usually provides enough of a barrier between the outdoor and indoor antennas to prevent oscillation interference.
If separation is not enough to prevent oscillation, you can add some shielding such as solid wall between outdoor and indoor antennas.