Development and Assessment of Flexible Uniplanar SSPP-Based Endfire MIMO Antenna for B5G Radio Stripe Network Applications

An effective way to implement cell-free massive multiple-input-multiple-output (CF-mMIMO) network is the radio stripe network (RSN) concept, in which all access points (APs), that have antennas and other signal processing units needed to serve user equipments (UE), share a common stripe that can be easily deployed over existing infrastructures. This article proposes a MIMO antenna for the first time that satisfies all the requirements for the RSN applications, such as a flexible and an ultra-thin profile, a very low volume, and an endfire beam with a downward tilt to cover all UEs present below the deployed radio stripes. A spoof surface plasmon polariton (SSPP)-based structure is used over a flexible substrate to achieve a flexible and ultra-thin MIMO antenna. The SSPP transmission line (TL) uses U-shaped unit cells. An equivalent circuit model is proposed for the U-shaped SSPP unit cell to analyze the effect of the design parameters of the unit cell on the dispersion curve. A single metal-layered and simple hook-shaped design of an open-ended SSPP TL is used to achieve a 45° downward-tilted endfire radiation beam. Equivalent circuit models are proposed to understand the design evolutions of the antenna. A filtering mechanism is employed over the coplanar waveguide to the SSPP transition section to filter out unwanted higher frequencies. An SSPP-based director is shared between two orthogonally placed hook-shaped antenna elements to enhance the directivity. The fabricated antenna is placed on flat and curved surfaces made up of wood and brick wall for measurements and validation. The MIMO antenna has a maximum gain of 7.4 dBi with an impedance bandwidth of 42.2%, isolation up to 26.7 dB, and envelope correlation coefficient below 0.19. Besides its ultra-thin flexible profile and extremely low volume, the proposed MIMO antenna has a tilted beam and wideband operation, making it an excellent choice for beyond fifth-generation RSN applications.