Hybrid beamforming is a combination of digital and analogue beamforming techniques to achieve a trade-off between performance and cost, and it plays an important role for millimetre wave and massive MIMO based communication systems.
In this thesis, two novel designs based on hybrid beamforming structures are first proposed, which together with the corresponding inter-subarray coding schemes, can achieve multi-beam multiplexing for arbitrary directions to serve corresponding users. In the first design, based on the relationship of directions between the two required beams, the adjacent antenna spacing is regarded as a variable to be determined, while in the second design, the adjacent antenna spacing is fixed and an iterative optimisation procedure is proposed to solve the problem based on a least-square formulation.
As extended versions of the above design with a fixed antenna spacing, three novel designs considering practical application constraints are proposed to achieve multi-beam multiplexing for corresponding users. In the first scheme, to reduce the implementation complexity, one nearly equal magnitude constraint is imposed on the analogue coefficients so that beamforming can be achieved by pure phase shifters after the normalisation of magnitudes. In the second scheme, a robust design against steering vector errors is considered to guarantee limited variation of resultant beam responses due to various steering vector errors. In the third scheme, by considering both the equal magnitude constraint and steering vector errors simultaneously, a robust design with equal magnitude constraint is proposed.
Furthermore, when a large number of antennas are available, for a required level of beamforming performance, we may not need to employ all of them for each scenario. As a result, further complexity reduction can be achieved by only employing a subset of
the available antennas based on the sub-aperture and overlapped subarray architectures. Moreover, by considering the polarisation states of the signals in the crossed-dipole based array, the best set of crossed-dipole antennas and dipoles can be selected to achieve the same multi-beam multiplexing effect.
