Projection-Based Precoding for Secure Cell-Free Massive MIMO Networks

This paper proposes novel projection-based precoding schemes to enhance physical layer security in cell-free massive multiple-input multiple-output (MIMO) systems. Assuming availability of channel state information (CSI) for both legitimate users (Bob) and passive eavesdroppers (Eve), we develop three secure precoding designs based on classical linear strategies: maximum ratio transmission (MRT), zero-forcing (ZF), and minimum mean square error (MMSE). The first approach projects these linear precoders onto the orthogonal complement of Eve's channel space. The second applies this projection directly to the legitimate users' channel, before performing linear precoding on the resulting “secure” channel matrix. At last, we introduce a novel constrained MMSE-based precoder that incorporates explicit secrecy constraints to reduce signal leakage toward Eve. Reformulations of these precoders yield simpler expressions and reveal equivalences among various projected schemes. Analytical derivations, supported by simulations in realistic environments, show that the proposed methods can fully suppress Eve's received power even in the presence of channel estimation errors, thereby significantly enhancing secrecy performance over other state-of-the-art secure schemes.