Secrecy of Massive MIMO Systems Under Antenna Failure

Abstract

As the fifth generation of mobile communication systems (5G) is being deployed, massive multiple-input multiple-output (MIMO) serves as one of its enabling technologies where reliable and ultra low latency communications are achieved while being power and spectrum efficient. This thesis studies the security aspect of massive MIMO system from the physical layer point of view. In particular, we analyze and evaluate the secrecy performance of massive MIMO system while undergoing antenna failure. This involves a threat model that can be characterized by a multiple-input single-output single-eavesdropper (MISOSE) wiretap channel in which the eavesdropper (Eve) performs pilot contamination attack (PCA) against a target receiver (Bob) in order to increase the decodablility of the confidential signal intended for Bob. For a block fading MISOSE wiretap channel, a lower bound on the achievable ergodic secrecy rate is derived with a closed form provided when maximum ratio (MR) precoder is selected at the transmitting BSs. We also show that a tight lower bound can be obtained by treating the received signal at Bob as being received over an interference channel and by considering complete knowledge of channel state information (CSI) that is being shared between the legitimate transmission parties. The analysis carried here considers two types of propagation channels, namely, spatially correlated and uncorrelated channels. Both analytical and numerical results show that a massive MIMO system provides secure transmission even under significantly high level of antenna failure. The results also show that for the same precoding scheme the spatially correlated channels yield higher ergodic secrecy rate than their uncorrelated counterpart albeit being impacted more from the antenna failure. However, the analysis indicates that antenna failure penalty for secure communication over spatially correlated channels can be significantly severe if the failed antenna elements coincided with the dominant eigendirections of Bob’s channel. The asymptotic results show that increasing BS antennas would not increase the achievable ergodic secrecy rate over spatially uncorrelated channels as long as the precoding scheme is selected based on imperfect CSI since PC allows the coherent interference at Bob to have a non-zero finite limit which eventually drives the achievable secrecy rate to be bounded. In contrast, no such limit on achievable ergodic secrecy rate exists for transmission over spatially correlated channels given that the channels of interfering users have distinct spatial characteristics.