# Reviews Based on the Reconfigurable Intelligent Surface Technical Issues
## 2 Technical Considerations for RIS
### Reflective/Transmissive RIS and Hybrid RIS  
Most research on RISs has focused on reflective RISs. If each element of the RIS has a different hardware implementation (e.g., number of layers, size, thickness, number, and distribution of PIN diodes), the RIS can operate in the following three modes: transmissive [114], reflective, and hybrid transmissive and reflective [115], as shown in Figure 29, depending on the energy splitting for reflection and transmission [116; 117; 118]. A reflective RIS reflects the incident signal towards the user on the same side of the BS, while a transmissive RIS allows the signal to be transmitted towards the user on the opposite side. In the case of hybrid RIS, the RIS supports the dual functions of reflection and transmission [119]. A reflective RIS typically consists of three layers. The top layer is an array of 2D RIS unit cells that can directly interact with the incident signal, the middle layer is a conductor plate to reflect the incident propagating signal, and the bottom layer is a printed circuit connected to a controller, which can control the phase shift of the RIS unit cells. Two design schemes are widely used for transmissive RIS including the receiver-transmitter (Rx-Tx) structure and stacked FSS structure, as shown in Figure 30. In general, an Rx-Tx structure consists of three components: a receive (Rx) cell, a transmit (Tx) cell, and a phase shifter. A ground plane separates the Rx and Tx, allowing them to operate independently while avoiding mutual interference. The power transmission structure between them is usually implemented as a metal via or slot-coupling structure. When an incident wave arrives at the RIS, the Rx on one side of the RIS first receives the wave and converts it into an induced wave, which is then transmitted to the Tx placed on the other side of the RIS through a phase transition and re-radiated into free space. Therefore, the transmissive RIS structure can realize the MIMO function in practice because it can realize the beam steering characteristics for the incident wave. Stacked FSS structures are passive transmissive RIS  
Figure 28: RIS with relays. (**a**) RIS with relays [109]. (**b**) MISO system with relays. Adapted with permission from [113]. Copyright 2023 IEEE. Figure 29: Types of RISs. Adapted with permission from [114]. Copyright 2022 IEEE. structures that consist of multiple FSS layers on metal and dielectric substrates. As the radio waves propagate through each FSS layer, a constant value of phase change occurs, and cascading multiple layers can generally realize a larger phase change. Unlike the Rx-Tx type, the stacked FSS structure has no ground plane, and the spacing between each layer is also an important design parameter. The RIS ensures wireless connectivity over NLOS links in poor propagation conditions, such as when LOS links are blocked or shaded, and extends communication distance through controlled scattering and multipath components. However, reflective RISs only reflects signals within the front half space and blocks signals to users behind the RIS, resulting in poor performance for blocked users. This means that the service coverage is limited to 180\({}^{\circ}\) due to the location constraints of the nodes, which reduces the efficiency of RISs and can be a barrier to 6G small cell implementation. Therefore, a use case has been recently reported in [122] where users located on the sides of a surface not covered by transmitters can benefit from deploying RISs. Based on these considerations, an RIS design called hybrid RIS or Intelligent Omni Surface (IOS) has recently been introduced [123]. These RIS designs are also referred to as simultaneous transmit and reflect RIS (STAR-RIS) [124]. Unlike conventional RISs, hybrid RISs have dual functions of reflection and transmission, which can provide full spatial coverage to users located on either side of the surface to achieve omnidirectional wireless communication.