I. Introduction
Although conventional cryptographic security mechanisms are essential to the overall problem of security, the openness of wireless medium poses both threats and opportunities for securing transmission. The openness of the transmission medium makes eavesdropping extremely easy-anyone within communication range can listen to the traffic in the air, and possibly extract information. However, the unique properties of wireless medium might provide ways of combating such security threats. For example, due to random fading, the intended receiver will have a channel different from an eavesdroppers's when she is at a reasonable distance away. This difference can be utilized to secure the communication between the transmitter and the intended receiver. In this paper, we consider the secret wireless communication problem for multiple antenna system from an information-theoretic view.