Today I decided to go on a religious pilgrimage to my local cell phone base station.

After making the necessary offerings and sacrificing of a few chickens I turned on my RSSI test rig and began uploading live video to YouTube:Basically, I made about 1,000 measurements from an RSSI (Received Signal Strength Indicator), one every second, that I bought from ebay, based on the Analogue Devices AD8318. Datasheet HERE.

The hypothesis was that my cell phone would transmit at lower power when close to the base station.

With the antenna at the same 200mm from the phone, I drove out into the countryside and found a spot with really bad 4G coverage -120 Dbm compared to -71 Dbm in the town. I then repeated the RSSI test, storing all the data on a micro USB in the Arduino rig. And here's the result:There's a really obvious difference in behaviour between town and country. In the country my phone is transmitting at markedly higher power. I'm not sure if the graph is logarithmic or not - I'd have to look that up on the RSSI chip's datasheet.

Looking at the green countryside graph, there's two obvious 'plateaux' right at the bottom where the phone is doing nothing. We don't get that on the orange town graph because there's loads of other phones nearby transmitting on the same 4G band.

Why is this important? ........ Because it shows that either the base station can control the signal strength given out by each individual phone in it's catchment range by using a control signal ..... Or that the phone itself responds to low received signal strength by increasing it's transmit strength, guessing that it is a long way from the base station. In either case, the base station is probably going to get really annoyed if we boost our cell phone signal too much - no amount of sacrificed chickens is going to appease it's wrath! It's therefore really important to have individual gain controls on both the Rx and Tx in the cell phone booster.