# Covert Communications in Active-RIS-Aided NOMA Systems: Element Grouping or Not?
## 1 Introduction  
Reconfigurable intelligent surfaces (RISs), which can be thought of as enablers to configure wireless propagation environments smartly, have been proposed and extensively developed for radar sensing and communications [1; 2; 3]. In particular, composed of a number of metasurface elements, RISs are capable of reflecting and refracting incident electromagnetic (EM) waves and imposing specified phases, which results in various EM functions, such as beamforming or interference nulling, by smartly configuring each element's amplitude and phase shift [4; 5; 6]. With the abundant spatial degree of freedom (DoF) provided by the RIS, the information security performance can be enhanced from the physical-layer security (PLS) perspective [7; 8], and the received signal-to-interference-plus-noise ratio (SINR) is also improved [9]. In comparison with traditional communication systems with radio frequency (RF) chains, RISs are expected to play a key role and to be one of the major technologies in future energy and spectrum-efficient wireless communications due to their straightforward structure. Also, the upcoming sixth-generation (6G) wireless networks regard RIS as a promising technology to facilitate massive connections [10]. In conventional passive RISs (PRISs), the metasurface elements are configured with low-power RF switches, allowing the noise to be ignored and resulting in relatively highpower efficiency. It has been shown that implementing PRISs with large apertures and high array gains enhanced the performance of wireless communication systems [11]. Unfortunately, a PRIS brings only minimal performance improvement due to the \"multiplicative fading\" effect [12]. Even if large-scale PRISs with high array gain are employed, they are still ineffective to enlarge coverage ranges of wireless communication systems. To overcome the limitations of PRISs and extend their applications, the idea of active RIS (ARIS) has been introduced recently to scatter the incident signals with both amplifications and adjusted phase shifts [13; 14; 15; 16]. Through amplifying incident waves, the strengths of the scattered signals are comparable to scenarios of direct link transmissions where the \"multiplicative fading\" effect is significant. Furthermore, an ARIS not only achieves an enhanced array gain and the required signal-to-noise ratio (SNR) at the receiver, but also becomes a PRIS when its amplifiers are turned off, making an ARIS more generalized than a PRIS. Different from amplify-and-forward (AF) relays that also receive and radiate the amplified signals, ARISs have distinct architectures using a phase-reconfigurable reflection-type amplifier for metasurface elements [17; 18]. Currently, ARISs can be implemented in several ways. For example, the incident waves can be reflected with orthogonal polarization by using an amplifying reflection array [19; 20; 21; 22; 23; 24]. Furthermore, by using a two-layer patch antenna and a phase-reconfigurable reflection amplifier to formulate an active element, the ARIS is capable of beam-scanning and amplification [25]. In addition, both ARISs and PRISs operate in the full-duplex transmission model with full-band response, which is more attractive than half-duplex AF relays equipped with costly RF chains. Recently, non-orthogonal multiple access (NOMA) technology has been incorporated as part of the upcoming 6G wireless networks [26]. In 6G wireless networks, NOMA plays a pivotal role in facilitating spectrally efficient massive wireless connections [27]. However, due to the open propagation of wireless signals, novel risks associated with information leakage and communication behavior exposure are introduced to the emerging NOMA systems. Ensuring the covertness of NOMA, especially preventing the exposure of NOMA communication behaviors, presents a significant and formidable challenge. In this context, covert communication emerges as a solution to address the security concerns in NOMA systems [10; 28]. In [29], covert communications in a PRIS-aided NOMA system were investigated.