## 1 Introduction  
MIMO (multiple-input, multiple-output) antennas have emerged as an effective solution to enhance the wireless communication system's capacity and reliability [1]. They are increasingly being used in GHz frequency applications [2], which demand high data rates and reliable communication links [3]. MIMO antenna systems provide higher throughput by transmitting multiple signals simultaneously using multiple antennas [4]. The number of antennas used for transmitting and receiving in a wireless communication system determined the total number of antennas utilized in a multiple-input multiple-output (MIMO) system. [5, 6, 7]. The GHz frequency range includes frequencies from 1 GHz to 30 GHz [8, 9, 10, 11]. These frequencies are widely used in various applications, such as Wi-Fi [12], cellular communication [13], satellite communication [14], and radar systems [15]. MIMO antennas can be designed and optimized for GHz frequency applications to meet specific system requirements. The design of a MIMO antenna for GHz frequency applications involves considering various parameters such as operating frequency, bandwidth, polarization, gain, radiation pattern, and size. One of the most critical aspects of designing a MIMO antenna for GHz frequency applications is the operating frequency. The operating frequency should be selected based on the specific application requirements. For example, the most common Wi-Fi frequency bands are 2.5 GHz and 5 GHz, while cellular communication systemsoperate in the 700 MHz to 2.7 GHz frequency range. The operating frequency of the MIMO antenna is determined by the size and shape of the antenna elements [16; 17]. The operating frequency and bandwidth of a MIMO antenna are heavily influenced by parameters such as the shape and size of the antenna, and on the basis of these parameters, antennas have been categorized into two distinct groups: (1) microstrip antennas [18; 19] and (2) slot antennas [20; 21]. Microstrip antennas are widely used in GHz frequency applications due to their compact size, small height and width, and compatibility with a wide range of electrical devices. A microstrip antenna has a ground plane and a conducting patch on opposite sides of a dielectric substrate. In addition, the conducting patch is usually fed using a coaxial cable [22] or a microstrip line [23]. In the microstrip antenna, the dimension of the patch decides the frequency of operation [24]. A smaller patch size results in a higher operating frequency, while a larger patch size results in a lower operating frequency. Slot antennas are another type of MIMO antenna used in GHz frequency applications. Slot antennas consist of a metal plate with a slot or a narrow opening in the center. The slot is usually excited using a microstrip line or a coaxial cable. Slot antennas are easy to fabricate and offer a wide bandwidth [25]. The frequency of operation of a slot antenna is directly dependent on the dimensions of the slot. A smaller slot size results in a higher operating frequency, while a larger slot size results in a lower operating frequency. The performance of MIMO antennas for GHz frequency applications is evaluated based on various parameters such as radiation pattern, gain, efficiency, and isolation. MIMO antennas can be designed to operate at different frequencies, depending on the specific application. In GHz frequency applications, the design of MIMO antennas requires careful consideration of the antenna's physical dimensions and radiation properties to ensure optimal performance. The operating frequency of MIMO antennas in GHz applications can range from a few GHz up to several tens of GHz. One important consideration in designing MIMO antennas for GHz applications is the physical size of the antennas. At higher frequencies, the wavelength of the electromagnetic waves becomes shorter, and the size of the antennas must be proportionally reduced to achieve the desired performance. This can be challenging, as smaller antennas typically have lower gain and efficiency than larger antennas.