# Multiport Single Element Mimo Antenna Systems: A Review
###### Abstract  
In response to the increasing demand for voice, data, and multimedia applications, the next generation of wireless communication systems is projected to provide faster data rates and better service quality to customers. Techniques such as Multiple-Input-Multiple-Output (MIMO) and diversity are being studied and implemented to meet the needs of next-generation wireless communication systems. Embedding multiple antennas into the same antenna system is seen as a promising solution, which can improve both the system's channel capacity and the communication link's quality. However, for small handheld and portable devices, embedding many antennas into a single device in a small area and at the same time providing good isolation becomes a challenge. Hence, designing a shared antenna system with multiple feed ports with equivalent or better performance characteristics as compared to the approach of multiple antennas with multiple feed ports is a promising advantage which can reduce the size and cost of manufacturing. This paper intends to provide an in-depth review of different MIMO antenna designs with common radiators covering various antenna design aspects such as isolation techniques, gain, efficiency, envelope correlation coefficient, and size, etc. There is also a discussion of the mathematical concepts of MIMO and different isolation techniques, as well as a comparative analysis of different shared radiator antenna designs. The literature review shows that only very few antennas' design with common radiator have been suggested in the available literature at present. Therefore, in this review paper, we have endeavored to study different antennas' designs with common radiator. A comparison is provided of their performance improvement techniques in a holistic way so that it can lead to further develop the common radiator multiport antenna systems and realize the promising advantages they offer. 5G communication; multi-element antenna; common radiator; single element; portable devices; SISO; wideband; MIMO; metamaterial; neutralization lines; parasitic material; mutual coupling; defected ground structure (DGS); planar inverted F antenna (PIFA); sub-6 GHz 2023/12/08/2023/  
to solve the data rate limitation problem experienced by SISO systems, due to its multiple antennas' capability which can also provide better system reliability and increased channel capacity [4; 5]. The MIMO antenna system can enhance the signal quality and the gain of an antenna but leads to complexity in design and an increase in size [6]. To attain a required level of signal independence and good isolation in MIMO operation, the multiple antennas should be placed with adequate separation from each other. But for portable devices, this approach would need to be given more space for multiple antennas, as well as additional feedline length [7]. The close placement of the antenna elements for MIMO operations can solve the space issue but results in higher mutual coupling among antenna elements. In this perspective, various isolation techniques and different types of isolation enhancement techniques are found in literature to improve the isolation in various antenna structures which are discussed in detail in Section 2. Through these techniques, the gain, bandwidth, envelop correlation coefficient (ECC), and efficiency can be enhanced. The antenna diversity technique used in MIMO systems enhances the performance of MIMO systems through multipath fading and co-channel interference reduction [8; 9]. Depending on the requirement, the diversity gain is achieved by employing different methods such as spatial, polarization, and pattern diversities. Several findings regarding the potential antenna designs suitable for portable devices and small handheld terminals for the MIMO applications have been described in the literature. Existing literatures discussed in Section 4 have presented works on numerous MIMO antenna designs.