Because the entire cellular network is depending on it. Our telephone network is a really marvelous thing that we take totally for granted. For the better part of 125 years, telephone lines, microwave links, and finally cell towers have shared the landscape with the more natural wonders of this world, and we humble humans can barely imagine harming it short of driving a large SUV into a telephone pole (an activity which, I can assure you, is even less fun for you than it is for the telephone pole.)
Yet, that simple cell booster in your home, the one you depend on to eliminate dead spots, the one you can’t live without… could potentially destroy the whole thing. That is, it could destroy the whole thing if it didn’t have complicated auto gain control circuitry.
Auto gain control is used to make sure that the output from your cell booster’s indoor antenna doesn’t reach its outdoor antenna. If it did, it could set up a feedback loop. You’re familiar with feedback loops when someone talks into a microphone that’s too loud or too close to a speaker. They’re no fun and someone usually runs to turn the mike down. Now, imagine that feedback loop happening in your cell booster. You wouldn’t hear it, not unless you were on the phone. It would disrupt your phone’s internet service, but that’s just the beginning of it. The feedback could be broadcast back to the cell tower in an ever growing, ever-loudening cascade of chaos. The feedback could spread to other towers, and before you know it, the entire system is down and it could possibly spread to the landline system as well, since cell towers rely on landlines for communications.
That would be a nightmare, and it was a big concern for the FCC when early cell boosters came out. Cell carriers warned of the dangers of those boosters. 2014’s new rules meant that every booster carried new advanced auto gain control circuitry, even those ones with manual settings for gain. The system listens constantly for the slightest hint of feedback, at far lower levels than you could detect, and if there is any, the power is reduced. It happens so quickly you can’t detect it.
If the system uses full auto gain, as all modern boosters do, it’s constantly increasing power until it is on the verge of feeding back, and then reducing. power to avoid feedback. By doing that, the booster will never threaten the whole system that we have come to rely on.
It’s all part of the service your cell booster provides, and you never even realized it.