## 1 Introduction
Wideband MIMO antennas with tuning possibilities across upper mid-frequency bands have been considered as one of the most suitable solutions [11]. The literature also discusses on several optimized MIMO antenna designs for 5G smartphones such as compact antennas using low proximity [18], compact size [19], and multi-band MIMO antenna using monopole [20; 21], dipole [22], and patch antennas [23; 24]. Nevertheless, these designs are subject to certain drawbacks, for instance, additional coaxial local conversion losses [17; 23], the radiation characteristics of wideband MIMO antennas being compromised [17; 25], and higher design gradients. For instance, Ref. [26] presented a ten-element dual-band MIMO antenna with a neutralizing line decoupling structure, but it has a low efficiency of only 45% and an isolation of only 12 dB. It is further known that the number of antenna ports must be raised in order to provide support to more users, increase channel capacity, and yield higher data rates [27]. Thus, extensive studies have been dedicated to design eight-element antennas for different 5G applications [1; 28; 29; 30; 31; 32; 33; 34; 35]. However, low antenna flexibility is a common issue with most current research works; therefore, there is a need for highly flexible eight-element antennas that are optimized for 5G mid-band frequencies. The novelty and contributions of our paper are as follows:  
* We proposed a Pi-shaped ten-element MIMO antenna that consists of a decoupling structure that offers isolation of greater than \(-10\) dB across the upper mid-band (LTE 46/47, n102), with a fractional bandwidth of 32.72%. * **SAR analysis**: The maximum and minimum SARs observed in hand simulation for 1 g of tissue were 0.118 W/kg and 0.0606 W/kg, and the maximum and minimum SARs observed in the presence of head simulation for 10 g of tissue were 18.7 W/kg and 1.72 W/kg. * The mutual coupling performance of the proposed ten-element antenna was examined, and the results show good values for the envelope correlation coefficient (ECC) \(<\) 0.03, which proves the appropriateness of the antenna for real-time diversity environments. Therefore, using the method of independent frequency control, an extensive parametric study was carried out on different design parameters. Moreover, a Pi-shaped ten-element MIMO antenna array was designed and measured for the verification of the design concept presented in this work. The obtained experimental results show a high isolation of more than 12 dB, presenting the best level of high-data-rate broadband system (HDBS) performance. The antenna array was optimized for total efficiency at a resonance frequency of 5.8 GHz in simulation (76.12-91.57%), in the presence of a hand (33.2-83.43%) and head (32.4-75.3%), and in measurements (68.7-90.2%). The article is structured as follows: Section 2 deals with the methodological aspect of the antenna design and offers a definition accompanied by an explanation of its geometry. Section 3 displays and discusses the S-parameters, radiation patterns, and MIMO parameters of the achieved antenna array, with an equal focus on the simulated and experimentally measured outcomes. Last, Section 4 presents the discussions and conclusions for the study.