Secure Degrees of Freedom for Active RIS Aided Interference Channel with Confidential Messages

In this paper, we investigate the secure degrees of freedom (SDoF) of an active reconfigurable intelligent surface (RIS)-aided two-user multiple-input multiple-output (MIMO) interference channel with confidential messages. In this model, each transmitter sends confidential messages to its intended receiver while ensuring that the unintended receiver cannot decode them. To model a typical multi-cell interference scenario, we assume all transmitters have the same antenna configuration, and all receivers have the same antenna configuration. Compared to prior works on RIS-aided interference channels, we consider not only the interference encountered during decoding but also the constraints required for secure transmission. Our results show that an active RIS can enhance the SDoF, even when secure transmission is not feasible in the original interference channel. Based on the different antenna configurations, we propose two distinct RIS beamforming strategies to efficiently utilize RIS resources. Several regimes are defined based on different strategies, the number of RIS elements, and the number of antennas. We present the SDoF outer bounds along with corresponding proofs for each regime using information-theoretic methods and RIS beamforming design. Among these regimes, achievable schemes for each possible SDoF value are then identified, utilizing zero-forcing decoding, symbol extension and interference alignment. Finally, the SDoF of the active RIS aided interference channel with confidential messages is determined.