Secure Degrees of Freedom of RIS-Assisted MIMO Broadcast Channel With Confidential Messages

This letter investigates the secure degrees of freedom (SDoF) of reconfigurable intelligent surface (RIS) aided 2-user multiple-input multiple-output (MIMO) broadcast channel with confidential messages. In this model, transmitter sends corresponding secure messages to each receiver and ensure the other receiver cannot decode them. Compared to existing studies on broadcast channel with confidential messages, our model employs an active RIS that reduces the rank of channel matrices, enlarging the nullspace for secure transmission to increase the SDoF. In particular, our model can realize secure communication even when the SDoF of original model is zero. According to the different configurations of antennas, the problem is divided into several regimes. We present outer bound and corresponding proof for each regime by utilizing theoretical methods and RIS beamforming. Then we propose universal achievable schemes for different regimes by using zero-forcing decoding. Finally, we determine the exact SDoF of RIS-assisted MIMO broadcast channel with confidential messages and its SDoF gain compared to original model.