# Design of a Compact Multiband Monopole Antenna with MIMO Mutual Coupling Reduction
## 1 Introduction  
In today's digital age, the Internet has become an integral part of our daily lives. From smartphones and laptops to smart homes and IoT devices, people's demand for faster, more reliable wireless connections continues to grow. These demands bring significant challenges to wireless communication technology, including the need for larger bandwidth and more antennas. Therefore, the Wi-Fi Alliance proposed the latest Wi-Fi 7 standard to address these demands. The Wi-Fi 7 standard features tri-band operation (supporting 2.4 GHz, 5 GHz, and 6 GHz bands), ultra-wide 320 MHz channels, 4096 QAM modulation, up to 16 \(\times\) 16 MIMO, and multi-link operation (MLO) [1]. 3 GPP also launched the fifth generation of mobile communications (5G). The performance goals of 5G include high data rates, reduced latency, energy savings, cost reduction, increased system capacity, and large-scale device connectivity. 5G NR (New Radio) includes the FR1 (sub-6 GHz) and FR2 (mmWave) frequency ranges [2,3]. FR1 covers frequencies from 410 MHz to 7.125 GHz, with 3.5 GHz being the most popular. FR2 covers frequencies from 24.25 GHz to 52.6 GHz.  
Therefore, many researchers have proposed UWB antennas [4,5] to satisfy the required frequency bands. However, these antennas often have larger dimensions and do not cover the Wi-Fi 2.45 GHz band. Many multiband antennas [6, 7, 8, 9, 10, 11, 12, 13] have been proposed, but they also do not cover the required frequency bands. A method using loading metamaterials to achieve multifrequency operation in monopole antennas is proposed in [14]. Another more complex method, using two-dimensional (2-D) resonant-type composite right/left-handed transmission lines to achieve multiband and high gain in patch antennas, is proposedin [15]. This method can cascade to three or five cells as a series patch antenna with beam steering functionality. In multiple antenna MIMO systems, a critical issue is the reduction of mutual coupling between antennas, especially at lower frequencies. In [6], a miniaturized antenna with engraved inductor and capacitor is presented. This design utilizes opposite conduction currents on the two radiators to eliminate coupling. In [7], mutual coupling reduction is achieved by using different polarization directions and a cross-shaped ground resonator. In [8; 12], a similar ground resonator is employed to achieve mutual coupling reduction. In [13], neutralization lines are connected between two antennas to achieve mutual coupling reduction and improve the reflection coefficient. In [16; 17; 18], four different polarization directions are used to suppress mutual coupling. However, this method is more challenging to implement in real applications. In [19], a metamaterial PCB with split-ring resonators is added between two dielectric resonator antennas to suppress mutual coupling. The split-ring resonators can be designed for the required suppression frequency. However, adding the metamaterial PCB will increase the overall size of the antenna. In [20; 21], similar single-negative electric metamaterials (MTMs) were placed in the middle between two closely spaced (\(\lambda_{o}/8\)) patch antennas to achieve excellent mutual coupling reduction.  
In this paper, the authors propose a method to suppress mutual coupling between antennas, which can be designed for the required suppression frequencies without the need for additional components or an increase in the overall size of the antenna.