What is the self-interference cancellation challenge in full duplex wireless and what RF techniques help?

The self-interference cancellation challenge in full-duplex wireless arises because the device transmits and receives on the same frequency at the same time, meaning the transmitter's powerful signal (typically +20 to +30 dBm) appears at the receiver input along with the desired received signal (which may be as weak as -90 to -100 dBm). The self-interference must be canceled by 100-130 dB to reduce it below the receiver noise floor. RF techniques that help: antenna cancellation (use separate TX and RX antennas with: physical separation (provides 20-40 dB of isolation depending on distance and frequency), polarization isolation (cross-polarized antennas provide 15-25 dB additional isolation), and directional isolation (using directional antennas pointing in different directions); total antenna isolation: 30-60 dB), analog RF cancellation (create an inverted copy of the TX signal and add it to the RX signal path before the LNA; the cancellation circuit includes: a delay line (matching the propagation delay from TX antenna to RX antenna), a variable attenuator (matching the self-interference amplitude), and a variable phase shifter (matching the self-interference phase for 180-degree cancellation); analog cancellation provides: 20-40 dB of additional suppression; the cancellation must track the TX signal in real-time (adaptive cancellation) because the self-interference channel changes with the environment), and digital cancellation (performed in the DSP after the ADC; the digital baseband processor subtracts a digital estimate of the self-interference from the received signal; provides: 20-40 dB of additional suppression; limited by: ADC dynamic range (the ADC must digitize both the large self-interference and the small desired signal without clipping or losing resolution)). Total cancellation: 80-120 dB with all three stages combined.