Not sure whether this is worth to share with all the bimmers.
My ride is 535i GT with idrive. I had the same problem with radio frequency as the car was imported from Japan. As you all know, there is no more frequency available after 90 hz. One day when I played with the radio tuning, I accidentally discovered that my 77.2hz was playing 97.2hz radio station. Same went to my 83.3hz, it was playing 93.3hz. With this discovery, I am no longer require to add any radio booster/receiver.