Experimental Study of Channel Frequency-Correlation in an Indoor Multipath Environment for Wireless Key Generation

Channel randomness in wireless communication systems can be employed for generating two copies of a key bits sequence in a given pair of nodes. Among others, the differential phase method that uses phase samples of two coherent frequency tones for generating high-entropy key bits sequence seems to be a promising technique. Unfortunately, implementation of the differential phase method relies on correct specification of the frequency separation of the probing tones, which requires additional study of the frequency-correlation properties of the propagation medium. This paper is devoted to experimental study of the channel frequency-correlation properties in a typical indoor environment. Frequency autocorrelation functions are experimentally determined for the signal envelope and carrier phase on four different indoor radio links. The estimates of the channel coherence bandwidth and multipath delay spread are performed, and the influence of the line-of-sight component (LOS) is considered. Our experiments showed that, depending on the LOS intensity, the coherence bandwidth on typical indoor radio links of 20-meters length varies from 8 MHz to 25 MHz (for the correlation level of 0.2), whereas for samples of the carrier phase, the coherence bandwidth is somewhat narrower and varies from 6 MHz to 19 MHz.