What are the challenges in full-duplex wireless and how is self-interference cancelled?

Full-duplex transmits and receives simultaneously on same frequency, potentially doubling spectral efficiency. Main challenge: Self-interference (SI) from transmitter to receiver can be 100+ dB stronger than desired signal. Cancellation stages: Antenna isolation (separation, directional antennas, circulators, 15-40dB), RF cancellation (inject inverted TX signal, 20-40dB), and Digital cancellation (estimate and subtract residual SI, 30-50dB). Challenges: Hardware nonlinearities and noise limit achievable cancellation, and receiver chain must handle large SI without saturation. Applications: Relaying, wireless backhaul, and collision detection. Practical implementations achieve total 110dB+ cancellation.