(A) study on downlink cell-free massive MIMO with channel hardening

Abstract

This dissertation considers a downlink cell-free massive multiple-input multiple-output (CF-mMIMO) network with channel hardening. At first, we reveal that the pilot contamination further impairs the channel hardening and show that performance evaluations and system designs based on the assumption of strong channel hardening are inappropriate. Motivated by the technical problem, we introduce a modified bound on the achievable rate of downlink CF-mMIMO networks. Based on the modified bound, we propose a power control which adapts itself to the level of channel hardening. The performance comparison demonstrates that the proposed power control allows one to design a better system. Secondly, we investigate secure communications in a downlink CF-mMIMO network which is attacked by multiple passive eavesdroppers. It will be revealed that the distributed access points in the network allow not only legitimate users but also eavesdroppers to reap the diversity gain, which seriously degrades the secrecy performance. Motivated by this, we propose an artificial noise-aided secure power control under passive eavesdropping aiming to achieve a higher secrecy rate and/or guarantee security. In addition, we devise a secure power control without the assumption of strong channel hardening and propose a secure power control over spatially correlated Rician channels. The performance of the proposed scheme is evaluated and compared with various power control schemes, which clearly shows that the proposed scheme outperforms all the competing schemes and the power control design based on the assumption of channel hardening underestimates the secrecy performance.