Reciprocity for Physical Layer Security with Wireless FDD and in Wireline Communications

The reciprocity of the channel transfer functions between legitimate users changing randomly delivers the common randomness used in physical-layer key generation. Time-Division Duplexing (TDD) provides this symmetry of the bidirectional channel measurements, despite of uncorrelated noise on both sides and possible hardware imperfections. We now show that also Frequency-Division Duplexing (FDD) yields a usable symmetry as far as the direction of arrival is concerned. Even in wireline communications, be it over power lines or twisted pairs, transfer and far-end crosstalk functions provide useful symmetries, where especially key generation for power line communication may become important for IoT applications.