## 1 Introduction  
In recent years, due to the rapid development of modern wireless communication systems [1, 2] and radar technology [3], multi-antenna systems and multiple-input multiple-output (MIMO) antenna arrays have attracted much attention. At the same time, with the requirements of emerging technologies such as 5G (mobile terminals and base stations), the Internet of Things (IoT), and unmanned driving, the trend of miniaturization and integration of microwave systems has also made the physical distance between antennas smaller and smaller. Therefore, for a MIMO antenna system, decoupling between antennas to improve isolation is essential to promote the overall system performance [4]. The coupling of the MIMO antenna system can be expressed by the transmission coefficient between antenna elements, which is essentially the mutual interference of the spatial electric field or surface waves between the antennas. The coupling between antennas is usually reduced by the main polarization configuration and path cancellation. The main polarization configuration method uses the spatial electric field distribution of the antenna, that is, the adjacent antenna adopts different polarization methods to achieve high isolation, such as directional antennas [5, 6] and dipole antennas [7]. Path cancellation technology can add a coupling structure to introduce a new coupling path to cancel the original coupling [8, 9]. The frequency selective surface (FSS) is a commonly used technique in the path cancellation method. FSS is a periodic structure with the same two-dimensional array elements arranged on a dielectric substrate [10]. As a spatial filter, FSS can pass or block electromagnetic waves in a specific frequency range in free space. Therefore, it is possible to design a compact multi-antenna system (such as a multiple-input multiple-output (MIMO) system) that works in different frequency bands at the same time by using the frequency selection characteristics of FSS. Moreover, FSS is also widely used in the decoupling of multi-antenna systems to obtain better system performance in a compact antenna configuration [11; 12; 13; 14]. In the context of the explosive growth of 5G electronic devices, more properties such as light weight, high thermal conduction, low cost, and easy e-waste disposal are required. Although traditional metals that are applied in current electronic devices can bring good electronic properties, their characteristics of high specific gravity, strong rigidity, and easy oxidation, as well as the significant chemical pollution caused by the preparation process and the process of fabricating electronic devices, are not desirable [15; 16; 17]. Thus, the development of metal alternative materials has attracted much research attention. Graphene, as an outstanding carbon-based material, has excellent electrical, mechanical, thermal, and optical properties [18; 19]. Compared with traditional metal materials, graphene has high chemical stability and environmental affinity [20]. In the potential applications of future wireless communication systems, graphene is very suitable to being an alternative material rather than metal, such as antennas, attenuator [21; 22; 23; 24; 25]. However, single-layer or few-layer graphene sheets need to be deposited on substrate and thus challenging to be uniformly prepared in large sizes. In addition, the post-processing of micro-graphene to obtain a patterned shape requires sophisticated equipment. More importantly, the surface resistance of micro-graphene in the microwave frequency band is as high as hundreds of ohms, which will seriously damage the efficiency of electronic devices [26; 27]. The macroscopic graphene assembly film (GAF) formed by stacking graphene nanosheets is another approach of applying graphene material. GAF has inherited the excellent properties of carbon-based materials and can be prepared industrially, which has received extensive research attention [28; 29; 30].