Loop-Back based Physical-Layer Key Generation in FDD System Under Hardware Impairments

In the physical-layer key generation, the channel reciprocity is vulnerable to hardware impairments (HIs) that can adversely impact the performance of key generation, and the security of loop-back mechanism based schemes in frequency division duplex (FDD) systems remains to be improved. To enhance both channel reciprocity and system security, this paper proposes a secure and highly reciprocal key generation scheme in FDD system by increasing the number of loop backs. By transmitting signals via different frequency bands in the loop-back mechanism, the proposed scheme can eliminate the adverse effects of the variations in HIs across different frequency bands and can improve system security. The normalized mean square error performance of the proposed scheme is theoretically analyzed and the security of the proposed method is illustrated. The simulation and experimental results demonstrate that by considering HIs differences during the design process, the proposed loop-back mechanism based scheme can enhance the channel reciprocity and achieve higher key generation ratio, and lower key disagreement rate compared with the state-of-the-art method.