## 3 Circuit Level Perspectives of JCRS RF Hardware
### Antenna Implementation
#### 3.2.2 Antennas for Communications  
An effective MIMO system requires that the antennas are isolated from one another. The performances of antennas and propagation channels, including the radiation pattern, gain, side-lobe level, channel capacity, and impedance matching have been determined to be severely affected by the mutual coupling between MIMO antennas at the mmWave band. Typically, an antenna isolation strategy involves maintaining an inter-element distance over half of the free-space wavelength. However, an enormous board space is needed for this approach. For various frequency bands over the years, a number of techniques have been investigated to improve MIMO antenna isolation, but very few techniques have been investigated for the mmWave frequency band. Planar MIMO antennas operating in the mmWave spectrum can be classified based on their structural configuration, as slotted radiator, defected ground, and metasurface [47]. Waveguide represents another important classification.  
Slotted radiator: The majority of planar antennas are created on a single substrate, which lowers their fabrication complexity and minimizes their costs. Slots are incorporated into planar antennas to lower mutual coupling. In the radiating plane, slots typically have the following shapes: tapering [48], fractal [49], coplanar waveguide [50], etc. The main goal of the method is to avoid requirements for a decoupling framework by concentrating the highest current near the slits, which lowers the mutual coupling between MIMO antennas.  
Defected ground: The ground plane in planar antennas serves as a reflector, enabling EM waves to be normal to the radiating element. However, the ground plane may be adjusted from full to only a portion, known as defective ground structures (DGS), to accomplish better isolation between MIMO units [51]. In theory, defects on the ground plane interrupt the ground plane's current distribution. This perturbation modifies the properties of a radiating structure by adding some slot parameters to the transmission structure parameters, such as resistance, capacitance, and inductance. DGS may be etched into any geometric shape, such as a circle, rectangle, and ring, to improve the targeted parameter.  
Metasurface: The term \"metamaterial\" refers to a material that has been manufactured artificially with the intention of lowering the permeability, permittivity, and refractive index values [52]. When applied to the rear of the radiating device, a metamaterial construction with an EM band gap functions as a reflector to improve the radiation pattern on the broadside [53]. A \"metasurface\" signifies what is used to describe a metamaterial that has been etched on the same plane as an antenna element. Better isolation among radiating elements may be achieved by using a metasurface layer along the radiating element.  
Waveguide: Waveguides feature extremely low transmission losses in the mmWave spectrum in contrast with losses via printed transmission lines [54; 55]. The application of rectangular waveguides provides an extensive amount of flexibility in the choice and fabrication of antennas, including horn antennas [56], waveguide radiators [57], and wide-scan waveguide radiators [58]. Waveguides additionally exhibit remarkable isolation characteristics.