## 1 Introduction  
Today, 6G networks have the potential to completely transform the Internet of Things (IoT) by facilitating applications that require ultra-low latency, broader spectrum resources, improved energy efficiency (EE), and widespread network coverage. These prerequisites are essential for the smooth operation of many Internet of Things scenarios, such as industrial automation, self-driving cars, smart cities, and remote healthcare [1, 2]. To accomplish these objectives, enhancements to the current communication technologies [3], which have already demonstrated their efficacy, have been suggested. These enhancements include ultra-massive multiple-input multiple-output capacity (UM-MIMO), ultra-dense networks (UDNs), and terahertz (THz) communication. Nevertheless, the continuous growth in the number of antennas/base stations (BSs) and the utilisation of extremely high carrier frequencies may result in higher energy consumption and hardware expenses. This is due to the necessity of installing more power-intensive and expensive radio frequency (RF) chains for signal conversion. However, deploying many active components that operate simultaneously at extremely high frequencies in wireless networks may only sometimes be advantageous, as doing so can introduce new challenges, including complex interference scenarios between users and cells, pilot contamination, and significant hardware impairments. Developing novel and economical methodologies for wireless communication systems is imperative to address the above-mentioned constraints. Driven by the rapid progress in metasurfaces and advanced fabrication technologies, reconfigurable intelligent surfaces (RISs) have emerged as highly promising solutions [4; 5; 6; 7]. A RIS is a two-dimensional human-made surface containing many low-cost passive elements. In utilising a smart controller connected to the RIS, the main advantage of the RIS is the ability to modify the direction of the wireless signals received by the RIS by manipulating the specific phase response of each element [5; 8]. Unlike conventional multi-antenna and relaying concepts that involve the active generation of wireless signals using expensive RF chains, RISs recycle existing signals in a network passively without requiring RF chains. Therefore, RISs are more cost-effective and ecologically sustainable than traditional active antenna systems. In addition, RISs can be easily incorporated into current wireless networks by installing them on various structures, including roadside billboards, building tops, building facades, windows, and even clothing worn by individuals [6; 9]. There are diverse approaches to integrating RISs in communication systems, which generally can be classified into different approaches, include reducing power consumption during transmission [10; 11; 12; 13; 14], improving spectra and EE [4; 15; 16; 17; 18; 19; 20; 21; 22; 23; 24; 25; 26], and creating an unobstructed LoS [27; 28; 29; 30; 31; 32; 33; 34]; a full analysis for the work addressed in previous researches is described in Section 3. Although the current STAR-RIS approaches elaborate on the improvement of communication systems, it still have some restrictions on the freedom of the communication systems. Currently, the transmitter and receiver must be positioned on the same side of the RIS, resulting in a radio coverage environment that covers only half of the surrounding space, either the reflection area or transmission area, as in Figure 1a,b. Nevertheless, the practical implementation of geographical limitations might only sometimes be adhered to, particularly when considering the more demanding requirements of the upcoming wireless generation of 6G. This limitation significantly restricts the adaptability and efficiency of RISs, as users are typically situated on both sides of an RIS. The innovative idea of transmitting and reflecting an RIS (T/R RIS) simultaneously was introduced to address this constraint in [35; 36; 37].