Secrecy extraction from correlated fading channels: An upper bound

The reciprocal nature of wireless radio channels can be well employed to extract a secret key between two terminals. The maximum size of a key that can be shared is characterized by the mutual information between the observations of two radios. We consider a correlated fading channel and a closed form for the mutual information is derived. It is found that the secrecy capacity is largely limited by the maximum Doppler frequency.