Analysis of non-reciprocity factors in extracting secret key from wireless channels for practical indoor scenarios

Recently, extracting shared symmetric key from wireless channel is a promising approach in various application areas. Channel reciprocity and randomness are the foundations for the physical layer key generation approach. However, various practical factors cause the non-reciprocity of channel characteristics. This paper examined the influence of hardware devices deviation and switch time delay on the non-reciprocity of channel state information (CSI) in an Orthogonal Frequency-Division Multiplexing (OFDM) system. The experiments were performed in various indoor scenarios on the platform of Universal Radio Peripheral (USRP). The CSI estimates of two communicating entities are collected from GNU Radio to investigate the reciprocity and redundancy. Our experiment results indicate that hardware devices deviation causes uneven non-reciprocity over bandwidth and the reciprocity drops dramatically when the switching time delays exceed the coherence time. In addition, the time redundancy of CSI estimates is mainly affected by moving speed while the frequency redundancy is affected by center frequency, propagation environment and bandwidth.