Artificial Noise Aided Secure Transmission in Active RIS-Assisted CF System Without Eavesdroppers' CSI

Reconfigurable Intelligent Surface (RIS) has emerged as a key technology capable of economically and efficiently reconfiguring wireless communication environments, and has been widely utilized to assist secure transmission in cell-free (CF) system in recent years. However, due to the presence of multiplicative fading effects, traditional passive RIS typically performs poorly when deployed far from the access point (AP) and users. Additionally, eavesdroppers (Eves) are malicious users hidden within the system, making it difficult for the system to acquire their channel state information (CSI). To address above issues, this paper investigates secure transmission of CF system aided by multiple active RISs. By considering no Eves' CSI, we propose an artificial noise (AN) aided transmssion strategy to enhance the secure transmission of system. In this strategy, the system total power is divided into two parts at the APs, one for the signal transmission and one for the AN signaling. To allocate the first part of power, we propose an efficient alternating optimization (AO) algorithm to optimize the active and passive beamformers at APs and RISs by ensuring that the users' quality of service (QoS) for communication is satisfied. To allocate the second part of residual power, we propose equal and unequal power allocation strategies for transmitting artificial noise (AN) so as to degrade Eves' signal reception performance. The simulation results show that compared with the benchmark schemes, our proposed method with the aide of active RISs in CF system can more effectively suppresse the performance of Eves, which in turn significantly enhances the secrecy rate for users in the system.