In this paper, a novel microstrip line-fed meander-line-based four-elements quad band Multiple Input and Multiple Output (MIMO) antenna is proposed with a gain enhancement technique. The proposed structure resonates at four bands simultaneously, that is, 1.23, 2.45, 3.5 and 4.9 GHz, which resemble GPS L2, Wi-Fi, Wi-MAX and WLAN wireless application bands, respectively. The unit element is extended to four elements MIMO antenna structure exhibiting isolation of more than 22 dB between the adjacent elements without disturbing the resonant frequencies. In order to enhance the gain, two orthogonal microstrip lines are incorporated between the antenna elements which result in significant gain improvement over all the four resonances. Furthermore, the diversity performance of the MIMO structure is analyzed. The Envelope Co-Relation Coefficient (ECC), Diversity Gain (DG), Channel Capacity Loss (CCL), Mean Effective Gain (MEG) and Multiplexing Efficiency are obtained as 0.003, 10 dB, 0.0025 bps/Hz, −3 dB (almost) and 0.64 (min.), respectively, which are competent and compatible with practical wireless applications. The Total Active Reflection Coefficient (TARC) resembles the characteristic of the individual antenna elements. The layout area of the overall MIMO antenna is 0.33 λ × 0.29 λ, where λ is the free-space wavelength corresponding to the lowest resonance. The advantage of the proposed structure has been assessed by comparing it with previously reported MIMO structures based on number of antenna elements, isolation, gain, CCL and compactness. A prototype of the proposed MIMO structure is fabricated, and the measured results are found to be aligned with the simulated results.

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A ten-elements MIMO antenna with improved isolation and reduction of X-polar radiation (XPR) in millimeter (mm) wave region is presented. The individual antenna element consists of stub loaded coplanar waveguide (CPW) structure with elliptical shaped radiator. Defected ground structure (DGS) has been incorporated by engraving partial elliptical shaped slots at the coplanar ground to reduce the mutual coupling effect between the adjacent antenna pairs of the MIMO. The backside of the substrate has a metallic plane with defects which is not connected to the coplanar ground of the antenna. This defected metal structure (DMS) acts as a reflector to improve the radiation characteristics by lowering of XPR as well as improving the co-pol to X-pol isolation. The proposed MIMO structure has a fractional bandwidth of 23.52% (22.5GHz-28.5 GHz) which entails the mm-wave 5G frequency spectrum n258 (24.25-27.5 GHz). The isolation| characteristics of the proposed MIMO antenna are below -35 dB over the operating bandwidth for any pair of adjacent antenna elements which imply significantly improved isolation between the adjacent antenna elements with highly compact layout area of 3.4λ × 8.5λ, where λ is wavelength corresponding to the mid-point of the bandwidth. The inter-element spacing of the MIMO structure is 0.44 λ. The main attractive feature offered by this structure is reduction of XPR in the operating frequency range thus improving the co-pol to X-pol isolation up to 20 dB and 28 dB in the boresight direction in both the XZ and YZ planes respectively. Furthermore, the co-pol to X-pol isolation is maintained above 15 dB almost throughout the elevation angular region -180 o to 180 o thereby negating the hindrance imposed by surface waves in mm-wave region. The diversity performances in terms of Envelope Correlation Coefficient (ECC), Diversity Gain (DG) and Channel Capacity Loss (CCL) are obtained as 0.1 × 10 -6 , 10 dB and 0.005 bps/Hz respectively which are within the acceptable limits. Mean effective gain (MEG) values with respect to the excited port 2 and port 3 is identical and approximately -3dB thus indicating the mean value of these two ports to be almost 1 dB. The multiplexing efficiency of the proposed MIMO antenna is satisfactory over the bandwidth. The group delay is investigated and found to be almost constant with a variation of 1.2 ns thus indicating linear phase characteristic of the antenna. The proposed structure is compared with other mm-wave MIMO antennas and found to be advantageous with respect to the number of antenna elements, compactness, low mutual coupling, co-pol to X-pol isolation, and ECC. A prototype of the proposed 10-elements MIMO is fabricated, and the measured results resemble with the simulated results.INDEX TERMS Mm-wave MIMO, DGS, mutual coupling reduction, co-pol to X-pol isolation, DMS. I. INTRODUCTIONMm-wave MIMO antenna is an essential component for MIMO systems for achieving a complete package of high data stream, multifunctional applications and future generation communication sy...