MIMO antenna systems happen to be the vital cog of wireless communication system. The implementation of which currently has generated a plethora of issues to be sorted out by the researchers worldwide. One of the major issues in the development of multiple element antenna arrays for MIMO applications is mitigation of mutual coupling between the antenna elements that leads to severe degradation of system performance. This deleterious effect impacts antenna system parameters like bandwidth, gain, efficiency and impedance matching over the desired frequency range besides the radiation pattern and signal-to-interference-noise ratio (SINR). Since its vicious tentacles are spread over these crucial parameters, mutual coupling is the root cause of MIMO system performance degradation.
In multielement antennas, isolation enhancement is a major research problem that set off an extensive exploration of a variety of decoupling techniques. In the unsophisticated primitive techniques, miniaturization was the main casualty and the antenna systems became bulky and were not cost effective. Moreover, design complexity was an issue. MTM based antenna array systems are most suited for compact massive MIMO applications, since the compromise needed in the low-profile aspect of their design is almost negligible. While these exotic factitious materials revolutionized the realm of antenna design, isolation enhancement in MTM inspired multielement antennas useful in long term evolution (LTE) systems and array antennas for massive MIMO systems is a challenge currently confronting academicians and industrial scientists alike.
Mutual coupling in microstrip based and/or printed MTM antenna systems originates from surface wave propagation. Several novel decoupling techniques have been employed in MTM inspired antenna systems in the later years of current decade. Many innovative designs of multielement MTM antenna systems incorporating metasurface walls, electromagnetic bandgap (EBG) structures etc for decoupling the antenna elements have appeared in literature2,3. Since mutual coupling suppression down to the levels of -30dB is desirable which is a thumb rule in MIMO antenna system designs, scientists worldwide started their journey in quest of directions for optimizing MIMO communication systems through enhanced isolation of array elements which constitute the array antenna. Measurements on prototypes of designs achieving mutual coupling suppression even down to less than − 46.5dB levels have been reported in the literature4. The impact of MTM on antenna system designs is phenomenal5–8. Likewise, it offers many avenues to address a specific issue while designing an efficient multielement antenna system.
A class of MTM structures known as Electromagnetic bandgap structures or metasurface corrugations exhibit stop band and pass band characteristics for surface wave propagation. This property of EBG structures has been successfully utilized for attenuation of surface waves. Isolation and bandwidth enhancement have been achieved through two layer tunable EBG structures in unison with slit patch arrays placed between two monopole antennas3,9. Two closely spaced meander line antennas are effectively decoupled through an MTM substrate and this antenna system is reported to achieve an isolation of 12-19dB over 5.1-5.9GHz frequency band2. This simple design offers improved impedance matching and gain. Unwanted frequency bands are notched for reducing multipath fading effects in four element UWB MIMO antenna system17 and envelope correlation coefficient (ECC) of less than 0.02 is maintained across the operating frequency band 18.
An MTM mushroom wall has been used to improve isolation in a four element MIMO antenna system10. In this design, a crossed double layer mushroom wall structure integrated with substrate- integrated cavity-backed slot antenna elements and this system was shown to achieve an isolation of 42dB with an envelope correlation coefficient well below 0.02 within the operational band of 2.39-2.45GHz. In this design the height of the antenna system is rather high, limiting its use in MIMO applications. An array antenna decoupling surface (ADS) consisting of a thin substrate with flowery patterns of metal patches was tried out to suppress mutual coupling in a 4-element antenna and was found to bring down the mutual coupling to -30dB level11. Here the unwanted coupled waves are cancelled by controlling the partially diffracted waves from the ADS through carefully designing the metallic patch patterns. Though this method is promising, its usefulness is limited and it can be applied only for 2 x 2 arrays. As the number of elements in an array antenna increases, the patch patterns on the ADS become very complicated1.
A Jerusalem Cross (JC) MTM unit cells based thin planar lens MIMO antenna system with seven elements was constructed and demonstrated to achieve mutual coupling levels lower than − 30dB which satisfies the threshold level desired by MIMO system requirement. However, compactness of the system suffers due to the large distance between the metalens and the element feeds and it is an issue that needs to be optimized for its suitability in MIMO communication systems12. Mutual coupling reduction in Dielectric Resonator antennas useful for 60GHz MIMO system has been attained through a metasurface shield4. The constructed prototype achieved mutual coupling levels of -30 to -46.5dB in the 59.3–64.8GHz frequency band.
A MIMO antenna system employing a frequency selective surface (FSS) wall has been reported to achieve − 30dB isolation levels13. The FSS walls of this system were optimized for the operating band of 57-63GHz to achieve this level which is a thumb-rule requirement in a MIMO system1. Envelope correlation coefficient (ECC) is an important parameter while analysing the diversity performance characteristics of a MIMO antenna system. This system has reportedly achieved an ultra-low ECC of 5x10− 6 making this system appropriate to MIMO applications in this frequency band. FSS is an attractive candidate for mitigation of mutual coupling because of the flexibility it provides as far as the operating band is concerned. Yet another system operating in the frequency band of 30GHz employing crossed-dipole structures on the FSS has been reported14 and it is very clear from the results of the investigations, varying degrees of isolation enhancement are achievable through optimization of the FSS for operation in a frequency band of interest. The air gap technique19 is implemented in aperture coupled antenna design to improve the gain.
Recently surface wave attenuation in a patch array antenna was accomplished using a capacitively loaded loop metamaterial (CLL-MTM) superstrate and more than 55dB isolation was achieved at a centre frequency of 3.3GHz. This CLL-MTM will be very much useful in LTE communication systems as it can be used to improve the isolation of array antennas already in use, eliminating the necessity of replacing them with new ones15. A self-diplexing MIMO antenna improving the isolation has been reported20,21. In this paper a dual element antenna system is proposed for MIMO applications in the frequency band of 7.525-9.1GHz. To reduce the mutual coupling an MTM array is inserted in between two CSRR radiating elements. The radiating elements are stimulated through aperture coupling technique to increase the gain.