Channel reciprocity improvement of secret key generation with loop-back transmissions

Secret key generation from wireless channel is an emerging technique to provide secure keys for message encryption. However, the measured channel state information (CSI) requires high reciprocity in order to obtain symmetric keys. In this paper, we propose a novel loop-back transmission scheme for time division duplex (TDD) systems, which is named as LB-TDD. The proposed LB-TDD scheme can effectively reduce the CSI non-reciprocity due to the hardware fingerprint interference and system synchronization offset. We initially discuss the benefits and optimizations of using LB-TDD for CSI reciprocity enhancement. After that, a secret key generation method based on orthogonal frequency division multiplexing (OFDM) probing signals is introduced. Finally, a channel model and a practical universal software radio peripheral (USRP) system is setup for both simulation and experimental verification. Evaluation results show that our proposed LB-TDD scheme can effectively eliminate CSI non-reciprocity for secret key generation.