Adaptive wireless channel probing for shared key generation

Generating a shared key between two parties from the wireless channel is of increasing interest. The procedure for obtaining information from wireless channel is called channel probing. Previous works used a constant channel probing rate to generate a key, but they neither consider the tradeoff between the bit generation rate (BGR) and channel resource consumption, nor adjust the probing rate according to different scenarios. In order to satisfy users' requirement for BGR and to use the wireless channel efficiently, we first build a mathematical model of channel probing and derive the relationship between BGR and probing rate. Second, we introduce an adaptive channel probing system based on Lempel-Ziv complexity (LZ76) and Proportional-Integral-Derivative (PID) controller. Our scheme uses LZ76 to estimate the entropy rate of the channel statistics, e.g. the Received Signal Strength (RSS), and uses the PID controller to control the channel probing rate. Our experiments show that this system is able to dynamically adjust its probing rate to achieve a desired BGR under different moving speeds, different mobile types, and different sites. Our results also show that the standard deviation of the LZ76 calculator is less than 0.15 bits/s. The PID controller is able to stabilize the bit generation rate at a desired value with mean error of less than 0.9 bits/s.