  It is well-known that wireless channel reciprocity together with fading can
be exploited to generate a common secret key between two legitimate
communication partners. This can be achieved by exchanging known deterministic
pilot signals between both partners from which the random fading gains can be
estimated and processed. However, the entropy and thus quality of the generated
key depends on the channel coherence time. This can result in poor key
generation rates in a low mobility environment, where the fading gains are
nearly constant. Therefore, wide-spread deployment of wireless channel-based
secret key generation is limited. To overcome these issues, we follow up on a
recent idea which uses unknown random pilots and enables "on-the-fly" key
generation. In addition, the scheme is able to incorporate local sources of
randomness but performance bounds are hard to obtain with standard methods. In
this paper, we analyse such a scheme analytically and derive achievable key
rates in the Alice-Bob-Eve setting. For this purpose, we develop a novel
approximation model which is inspired by the linear deterministic and the lower
triangular deterministic model. Using this model, we can derive key rates for
specific scenarios. We claim that our novel approach provides an intuitive and
clear framework to analyse similar key generation problems.
