  This paper investigates multi-mode and orbital angular momentum (OAM) mode
data transmission techniques by a using a circular antenna array, operating at
28 GHz. The classical mode excitation of the latter is modified such that in
the horizontal plane the antenna array operates as multimode transmitter (i.e.,
it provides broad- , uni- and/or multi-cast transmission), while in the
vertical direction OAM transmission occurs: this dual-functionality by a single
antenna-array at 28 GHz is presented for the first time. Specifically, it can
transmit/receive in either broad-, uni-, multi-cast mode in the horizontal
plane and also, it is capable of generating up to 15 spatially orthogonal OAM
modes in the vertical direction. The proposed circular array is designed using
twelve, low-complexity, multi-layer microstrip patch antennas with high
radiation efficiency. It was tested through full electromagnetic analysis in
terms of impedance matching, mutual coupling and radiation pattern. It was,
also, fabricated and measured: good agreement between simulated and measurement
results was observed. The proposed antenna array is perfect candidate for high
spectral efficiency data transmission for 5G and beyond wireless applications.

Signal Processing