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The wireless communication system is steered towards the millimeter wave spectrum to achieve low latency and high-speed data rate. The MIMO antennas aid in attaining a higher data rate. The prominent spectrum at millimeter wave is Ka-band, suitable for short-range communication. The <inline-formula> <tex-math notation="LaTeX">$\vert \text{S}$ </tex-math></inline-formula>-parameter<inline-formula> <tex-math notation="LaTeX">$\vert $ </tex-math></inline-formula> response and radiation pattern of the existing MIMO antenna at this band are relatively unstable. Hence it encouraged to design and develop a four-element MIMO antenna operating at Ka-band. The antenna is a circular ring shape with two concentric rings with a plus-shape stub overlayed on circular rings. The structure is developed in four-stage with the comprehension of characteristic mode theory (CMA). The proposed structure generated Mode 2 as an efficient mode, with minor Modes 3 and 5 contributing for resonance out of five modes. The overall antenna profile is <inline-formula> <tex-math notation="LaTeX">$3.27\lambda _{0} \times 3.74\lambda _{0}$ </tex-math></inline-formula> (where <inline-formula> <tex-math notation="LaTeX">$\lambda _{0}$ </tex-math></inline-formula> is the wavelength at a resonance frequency of 28 GHz). The novel decoupling structure has improved the isolation to 30 dB and increased the bandwidth. The antenna has an operating bandwidth of 24.1-30.9 GHz, with a maximum gain of 6.5 dBi. The <inline-formula> <tex-math notation="LaTeX">$\vert \text{S}$ </tex-math></inline-formula>-parameter<inline-formula> <tex-math notation="LaTeX">$\vert $ </tex-math></inline-formula> from all the ports has an exact and stable response. The proposed antenna has resulted in bidirectional radiation tilted at an angle of <inline-formula> <tex-math notation="LaTeX">$334^{0}$ </tex-math></inline-formula> and <inline-formula> <tex-math notation="LaTeX">$210^{0}$ </tex-math></inline-formula> in the XZ plane. In the YZ plane, it has a triple beam. The radiation pattern is also stable throughout the bandwidth. The proposed MIMO antenna has a symmetrical design, demonstrating the possibility of expansion to n-element MIMO through a six-element MIMO antenna design. The article also presents the channel capacity, path loss, and link margin calculation for designed antenna line-of-sight (LOS) communication. The antenna has been evaluated with diversity parameters such as ECC, DC, CCL, TARC, and MEG.