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Multi-Mode Massive MIMO Antenna Array

Multi-Mode Massive MIMO Antenna Array

In a massive MIMO system, the base station or access point is equipped with a large number of antenna elements. In conventional arrays, these elements have to be placed sufficiently apart in order to reduce correlation and mutual coupling, resulting in large array sizes. Employing multimode antennas as elements of the array offers the advantage of efficiently utilizing the available space. 

As a proof of concept, a complete demonstrator of an access point antenna array has been designed and realized (Fig. 1) [1]. It consists of 11 x 11 multi-mode antenna elements with dimensions  at 7.25 GHz. Each antenna element has four ports with integrated coupling elements and stripline distribution network.

Fig. 1: Multimode massive MIMO antenna array consisting of 11 x 11 four-port antenna elements (see insets) based on a square geometry with feed networks using power dividers and hybrid couplers realized in stripline technology. © 2020 IEEE [2]

The single antenna element is based on a square geometry. A modal analysis is conducted in order to find the dominant characteristic modes within the given frequency range. For the excitation of these modes, an iterative procedure is employed in which slots are cut into the plate, simulation ports are placed and the excited modes are checked. After that, a stripline distribution network consisting of couplers, power dividers and coaxial connectors is added in order to replace the simulation ports by physical coaxial ports and drive the ports with the proper phase relationships. In the end, an antenna element with four ports is created. 

All in all, the access point antenna array has  ports. By using multi-mode antenna elements, a considerable size reduction compared to conventional arrays can be achieved. For instance, the realized demonstrator with four ports per antenna element yields a size reduction of 54 % compared to a conventional array consisting of crossed dipoles.


References

[1] D. Manteuffel and R. Martens, "Compact Multimode Multielement Antenna for Indoor UWB Massive MIMO," in IEEE Transactions on Antennas and Propagation, vol. 64, no. 7, pp. 2689-2697, July 2016, doi: 10.1109/TAP.2016.2537388.

[2] N. L. Johannsen, N. Peitzmeier, P. A. Hoeher and D. Manteuffel, "On the Feasibility of Multi-Mode Antennas in UWB and IoT Applications below 10 GHz," in IEEE Communications Magazine, vol. 58, no. 3, pp. 69-75, March 2020, doi: 10.1109/MCOM.001.1900429.