Isolation improvement and user-effect modeling for antenna arrays in 5G and beyond

The evolution of mobile phone systems has been fast in the past decades with 5G systems closely being deployed. This development of mobile networks requires new techniques to solve the ever-increasing demand for the number of users, even higher data rates, and the problem of spectrum scarcity. This dissertation describes several novel electromagnetic designs and modelling methods that enable 5G and beyond communication systems. In-band full-duplex (IBFD) communication utilizes the same frequency band for receiving and transmitting simultaneously. Thus, the spectral efficiency is doubled in the best case compared to current half-duplex systems. However, the major problem of IBFD is the strong self-interference (SI) between the transmitter and the receiver on the same device. To make IBFD communication feasible, in the first part of this thesis, electromagnetic wavetraps are proposed as a method to improve isolation between the receiving and transmitting antennas of a device. Wavetraps are resonant structures usually placed between antennas to decouple them. They suppress surface currents and scatter the transmitted electromagnetic fields suitably to cancel out the naturally coupled fields at the location of the receiving antenna, thus increasing the antenna isolation. Planar wavetraps are designed for a multiple-input-multiple-output (MIMO) IBFD relay where planar antennas are used. The methodology for designing and optimising wavetraps is established utilizing the theory of characteristic modes. The clear positive impact of the wavetraps on antenna isolation is experimentally verified both in the anechoic chamber and in realistic multipath environments. The effects of multipath environment on the obtainable isolation are characterized. The thesis furthermore introduces an antenna decoupling structure for bi-directional IBFD communications, where the isolation between omnidirectional collinear dipoles is significantly enhanced. Communication at millimetre-waves (mm-waves) is essential in future mobile communications due to wide available bandwidth that enables very high data rates. The second part of the thesis considers the effect of a human body to mm-wave handset antennas operating at 28 GHz and 60 GHz. The effect of the proximity of fingers to handset antennas is studied giving novel approaches how to mitigate the effect by utilizing beamsteering and by a thin reflector decreasing the shadowing over 20 dB. Furthermore, characterization of shadowing and scattering due to an entire human body is made possible at 60 GHz with a novel simulation model which reduces the computational complexity while keeping the modelling accuracy. Mm-wave base stations in urban areas need to be deployed densely in future communication systems to cope with expected severe link blockage. The third part of the thesis introduces a PCB-based beamsteerable end-fire antenna array at 28 GHz for low-cost base stations. The antenna array is integrated with a switch network and high gain antenna elements, obtaining high measured realized gain even with the high losses of PCB technology at 28 GHz.