Date Added: Oct 2009
In this paper, the authors consider a scenario where a source node wishes to broadcast two confidential messages for two respective receivers via a Gaussian MIMO broadcast channel. A wire-tapper also receives the transmitted signal via another MIMO channel. First they assumed that the channels are degraded and the wire-tapper has the worst channel. They establish the capacity region of this scenario. The achievability scheme is a combination of the superposition of Gaussian codes and randomization within the layers which they will refer to as Secret Superposition Coding. For the outerbound, they use the notion of enhanced channel to show that the secret superposition of Gaussian codes is optimal.