## 2 Literature Review  
Recently, several researchers have carried out intensive work on MIMO antennas for mobile phones [9; 10; 11; 12; 13; 14; 15; 16; 17; 18; 19; 20; 21; 22; 23; 24; 25]. However, the space limitation in mobile phones brings challenges in integrating a greater number of antennas with a good value of isolation and lower ECC. Single-port [9], dual-port [10; 11; 12; 13; 14; 15; 16], and quad-port [17; 18; 19; 20; 21; 22; 23; 24; 25] MIMO antennas are proposed for smartphone terminals by researchers. The single-port antenna proposed in [9] resonates at a dual-frequency band; however, the efficiency of the antenna is much lower, and it does not cover the 5G band. Dual-port smartphone antennas resonating at dual [10; 11; 12], quad [13], wideband [14; 15], and ultrawideband [16] frequency bands are also proposed. The antenna proposed in [10; 11; 12] has acceptable gain, efficiency, and MIMO performance; however, it resonates at mm-wave 5G band, thus not covering the required sub-6-GHz 5G band. The antenna in [11; 12; 13] has a thickness of 5 and 6 mm, respectively, occupying a larger space within the smartphone and thus hindering practical usage. Antennas in [14; 15] cover applications such as LTE/Wireless Local Area Network (WLAN) bands; however, the gain in [14] is negative and no efficiency is calculated, whereas the antenna in [15] operates only in the LTE band. The antenna in [16] achieves wide bandwidth, very high isolation, and the lowest thickness (0.508 mm) as compared to other two-port antennas; however, to increase the processing capability while attaining much higher download/upload speeds for applications such as mobile Internet of Things (IoT) and improved data rates, a greater number of antenna elements are needed. To achieve the same, various four-port antennas [17; 18; 19; 20; 21; 22; 23; 24; 25] are proposed, out of which the antenna in [18; 20; 21; 22; 23; 25] operates at the sub-6 GHz 5G band while other antennas are resonating at LTE [19] and WLAN [24] bands. The antenna in [18] has a decent frequency response and isolation without using any decoupling techniques; however, the efficiency and SAR analysis of the antenna are not evaluated while the gain achieved is also lower than the proposed antenna. The four-element antenna [20] is printed on the front and back of the substrate, where lack of gain and SAR evaluation with lower efficiency make the antenna less usable in practical smartphone applications. Even though SAR analysis is carried out in [21], the lower bandwidth in the 5G regime with a lack of planar antenna geometry leads to the occupation of more space and thus makes the antenna less practical for slim mobile phones. Even after achieving decent frequency response and MIMO performance, the lack of SAR analysis and lower flexibility make the antennas in [22; 23; 25] a weak contender as compared to the proposed antenna for WLAN and sub-6 GHz 5G smartphone applications. Moreover, no bending analysis is carried out in any of the dual or four-port antennas discussed. The novel contributions of the proposed work are as follows:  
1. The antenna is a four-port structure with all the four elements connected to a common ground plane. 2. The antenna operates in the entire sub-6 GHz 5G band (3.3 GHz-5 GHz) along with the WLAN band (2.4 GHz and 5 GHz). 3. SAR analysis is carried out at two resonant peaks (3.5 and 5.1 GHz) in order to ensure the safe usage of the antenna in mobile terminals. Bending analysis is performed, which shows decent performance in terms of MIMO diversity, transmission, and scattering parameters that guarantee the practical usage of the antenna for slim and foldable smartphone devices. * The antenna has demonstrated high gain (>4 dBi), high inter-elemental isolation (>17.5 dB), and reasonable efficiency (85%) with all the diversity parameters such as ECC, DG, TARC, MEG, and channel capacity meeting the requirements of MIMO antennas. Scattering, transmission, and MIMO parameters matched well under normal as well as bending conditions.