# Hybrid Active-Passive Reconfigurable Intelligent Surface for Cooperative Transmission Systems
## 1 Introduction  
With the approach of the beyond fifth generation mobile network (B5G)/6G era, the massive increase in wireless devices and smart terminals has made the traditional network environment increasingly complex, bringing serious communication problems [1]. Gratefully, the emergence of reconfigurable intelligent surfaces (RISs) presents an effective solution to these problems [2]. In recent years, RISs, which are considered one of the sixth generation wireless systems' main technologies, have attracted wide attention in academia and industry because of their ability to change communication performance by using the implied randomness in the propagation environment [3]. RIS is an electromagnetic artificial surface made up of many near-passive reflecting elements (REs), which independently and efficiently adjusts the amplitude and phase of the incident signal through a controller. The controller can be programmed to reconfigure the electromagnetic signal, thus eliminating the need for elaborate decoding, encoding and radio frequency (RF) processing operations [4; 5; 6]. Moreover, passive RIS is reliant on tunable electronic circuits (such as varactors or PIN diodes) with low power consumption to change the phase, so the power consumption is almost zero [7]. As RIS is lightweight, it can be installed on a wide range of scattering surfaces, like buildings, roadside signage, windows, walls, etc. [8; 9]. From the introduction of the RIS concept to the present day, a large number of studies and ideas have emerged regarding the performance analysis of RIS-assisted communication systems. In [10], the basic performance of RIS-assisted single-input single-output (SISO) communication systems under mixed Rayleigh and Rice fading channels were investigated, and expressions for the ergodic capacity (EC) and the outage probability (OP) were obtained using the central limit theorem (CLT) by assuming that the REs were sufficientlylarge. In parallel, the authors discovered that using RIS with N REs enabled the system achieve an effective signal-to-noise ratio (SNR) gain of \(N^{2}\). In [11], the authors proposed a wireless single-cell system with RIS assistance that demonstrated that passive RIS systems were less costly than conventional massive multiple-input multiple-output (MIMO) systems or multiple-antenna amplify-and-forward (AF) relaying with significantly improved energy efficiency, coverage, and achievable rates for wireless communication systems. The statistical channel state information (CSI)-based RIS-assisted MIMO system for secure transmission was studied in [12], confirming that there was an increase in the ergodic secrecy rate as the number of RIS REs increased. In addition, RIS can be integrated with various wireless technologies, such as millimeter wave (mmWave), non-orthogonal multiple access (NOMA), free space optics (FSO), visible light communication (VLC), unmanned aerial vehicle (UAV), and two-way communication, through which different communication objectives can be achieved [13; 14; 15; 16; 17; 18]. While passive RIS has been proven to have many advantages that are unmatched by other technologies, passive RIS also has some shortcomings. For example, passive RIS-assisted systems may be limited by \"multiplicative fading\" effects. To be specific, the reflected signal needs to travel over a channel that is cascaded and made up of transmitter-RIS and RIS-receiver channels [19], and the path loss of this cascaded channel is a relationship in which the two channel losses are multiplied, rather than added. The \"multiplicative fading\" effect has the dire consequence that, in many cases with strong direct links, the expected capacity gain cannot be achieved using RIS [20]. If we want to increase the gain in the RIS-assisted system to a feasible level, the passive RIS necessitates a high number of REs, which results in an oversized RIS surface, which is not a viable solution in practical situations.