FDD isolation is a simultaneous leakage budget; TDD isolation is a state-and-time budget. In FDD, duplexer rejection, transmitter noise, antenna isolation and receiver selectivity must protect reception continuously while the transmitter is active. In TDD, the switch or circulator, PA turn-off, leakage during the opposite state, timing guard, transient blanking and receiver recovery must all be acceptable before the receive slot is used.
Derive the minimum net isolation from transmitter power and the maximum leakage the receiver can tolerate at each relevant frequency. If +40 dBm is present in the transmit path and the receiver can tolerate no more than -30 dBm at that leakage frequency, the arithmetic minimum is 70 dB before design, temperature, production and measurement margin. Allocate it across duplexer or switch, antenna separation, filtering, shielding and routing; do not assign the entire value to one catalog component.
Reject an isolation claim measured only at center frequency, low power or a static switch state. Verify band edges, transmitter noise outside the wanted band, harmonics, multiple carriers, temperature, mismatch, switching trajectories, control skew, PA decay, LNA protection, AGC recovery and the installed antenna configuration. The RFQ should state duplex mode, bands, guard interval, simultaneous carriers, transmit power, receiver blocker and damage limits, permitted desensitization, switching time and the conducted or radiated reference planes.