On the degradedness of fast fading Gaussian multiple-antenna wiretap channels with statistical channel state information at the transmitter

The characterization of secrecy capacities of Gaussian wiretap channels depends on the knowledge of channel state information at the transmitter (CSIT). When there is only statistical CSIT, the secrecy capacity is unknown in general. In this paper we investigate the relation between the usual multivariate stochastic order and the degradedness among the legitimate receiver's and eavesdropper's channels under fast fading with only statistical CSIT. The considered transmitter has multiple antennas and the legitimate receiver and the eavesdropper both have multiple antennas. Based on the technique of coupling we derive criteria to check the degradedness of the fast fading Gaussian wiretap channel including some commonly used channel models even there is only statistical CSIT. We illustrate one example of a Rician 2× 2×2 channel where the ergodic secrecy capacity can be derived under different K-factors.