# Multiport Single Element Mimo Antenna Systems: A Review
## 4 Multiport Single Element System  
MIMO technology offers many benefits, including error-free and faster communication, as well as superior multipath propagation results. MIMO technology, which employs multiple trans-receive systems, also improves transmission and reception quality. The performance metrics required to characterize the behavior of multiport antenna systems is discussed in [34]. The author suggests that existing designs cannot fulfill the needs of future wireless handsets that can cover the 4G and 5G (upper and lower) frequency bands along with WLAN and WiMAX, while considering the wide bandwidth. Hence, novel designs should be implemented to satisfy the requirement. For 5G MIMO antenna base stations, the 3-D printing technology and metal plating technique are used together to fabricate metal-coated polymer antenna [43]. The fabricated antenna has an isolation of less than \(-14\) dB with antenna dimensions of 27.2 \(\times\) 27.2 \(\times\) 17 mm\({}^{3}\). Four identical box-folded PIFA MIMO antenna, designed for mobile handsets with a wide operating band of 1.84-2.69 GHz. However, the whole antenna system dimensions are large and not the most suitable for portable devices [43]. A dual-band MIMO antenna for LTE Terminal is presented in [44]. To reduce the mutual coupling, the coupling feed patch and capacitive load patch are used in this system, which reduces the current density on the ground plane and the slotted technology is used to achieve the dual band. In contrast, a two-element four-shaped MIMO printed antenna system designed with defected ground structure and micro-strip matching network is used to enhance the isolation [45]. An eight-port antenna array with each pair of antennas is placed symmetrically on the substrate at the four edges. Each antenna element is autonomously coupled with a feeding strip and a communal square loop is etched at each antenna pair. Good isolation is achieved through this configuration by orthogonal polarization, but the efficiency is compromised because of the non-availability of the external decoupling structure [46; 47]. A multiband antenna system for LTE application, discussed in [48], have 3D Inverted-F Antennas folded on a non-metalized section of PCB. A new resonance is created by inserting the parasitic radiating element to IFAs and high port-to-port isolation is achieved via a suspended neutralization line between the two ports. A novel MIMO antenna system with a nature-inspired flower shape is proposed for upcoming 5G communication systems [49], which covers the mmWave frequency band. In [50], a rectangular dielectric resonator antenna (RDRA) with circularly polarized response is designed which covers the frequency band from 4.4-4.8 GHz suitable for 5G NR (New Radio) Sub-6 GHz band applications. Similarly, an eight-element MIMO array structure is presented in [51], which covers the frequency range between 3.4 GHz to 3.6 GHz. This helps to surge the data throughput with intra-band contiguous carrier aggregation. This is a circular-monopole antenna for portable devices with each antenna element consisting of three microstrip transition printed on the top side of substrate and two L-shaped stubs at the ground plane to improve the isolation [52]. An eight-port planar antenna for 5G micro wireless access point applications is designed by integrating three monopole strips with different structures and resonant modes for a wider bandwidth. High isolation and low ECC are obtained through diverse protruded stubs and etched slots loaded on the defected ground plane [53]. In [54], a printed antenna is designed which is capable of multiband operation with coupled-fed monopoles providing good isolation of less than 15 dB over all the frequency bands. A hybrid antenna with 4G and 5G antenna modules combined operating at 3.5 GHz band capable of covering the C-band is designed [55]. Four-port MIMO antenna for sub-6 GHz 5G applications is designed on an FR4 substrate and the overall volume of the circuit board is 50 \(\times\) 50 \(\times\) 1.6 mm\({}^{3}\)[56].