To go from half to full duplex requires solving a basic problem: A wireless device has trouble transmitting and receiving at the same time for the same reason you can’t hear a whisper while screaming at the top of your lungs: The interference is louder than the signal. Canceling that interference at the receiver involves subtracting the known transmitter signal. But that pithy description hardly does the task justice. Remember, the echo is a billion to a trillion times as loud as the signal you’re straining to hear, so you have to cancel reverberations extremely accurately. That means performing cancellation across several domains: radio frequency, analog, digital, and even within the antenna interface, as we will describe later. The cancellation in each domain must be coordinated with the cancellation in all the others.