The paper proposes a new set of normalization techniques for precoding/beamforming matrices applicable to broadband multiuser multiple-input multiple-output (MIMO) satellite systems. The proposed techniques adapt known normalization methods to account for the signal attenuation experienced by users due to the degradation of antenna gain and free space losses towards the edge of the coverage. We use, as an example, an array-fed reflector (AFR) antenna onboard a satellite in geosynchronous orbit (GEO), which provides a favorable trade-off between high-directivity, reconfigurability, and the requirement for digital processing, but suffers from high scan losses away from broadside due to optical aberrations when considered for global coverage applications. Three different precoding/beamforming techniques are employed, namely zero forcing (ZF), minimum mean squared error (MMSE), and matched filtering (MF). Low-complexity power normalization techniques digitally applied after the beamformer are introduced that, in the absence of any atmospheric effects, lead to iso-flux-like characteristics whilst satisfying the power constraint per feed. In comparison with other methods reported in the literature, mainly based on iterative algorithms, the proposed techniques consist in closed-form expressions to provide uniform signal-to-noise ratio (SNR) and signal-to-noise plus interference ratio (SNIR) across the users without significant impact on the payload sum rate. Numerical results are presented to comparatively demonstrate the achieved performance in terms of total capacity and distribution of SNR and SNIR at various noise and interference scenarios.

This paper presents the methodology followed for the design of a quad-ridged flared horn (QRFH) feed for the VLBI Global Observing System (VGOS) antennas. The study demonstrates the potential undesired effects caused by the presence of non-fundamental modes at the throat of the horn antenna, and how they can be avoided following some design considerations. The approach consists of a separate design of the feeding transition and the flared section of the horn, and it can be extrapolated to other implementations. Firstly, the geometry of the coaxial to quad-ridged waveguide (QRWG) transition is defined. The proposed design technique ensures negligible propagation of higher order modes at the waveguide interface. Secondly, the ridges and sidewall profiles of the flared part are optimized to achieve the desired performance in terms of input reflection and aperture efficiency on the VGOS 13.2-meter displaced-axis two-reflector radio telescope. Following the optimized design, five QRFH antenna units have been manufactured and tested, all showing reflection coefficients better than -10 dB and predicted aperture efficiencies better than 55 % in the band from 2.2 to 14 GHz. The excellent agreement between simulations and experimental results demonstrates the validity of the proposed methodology.

HERAS is a tool developed in Matlab for the analysis of reflector antennas using physical optics (PO) theory. Its graphical user interface (GUI) and source code are freely available for educational and research purposes. It has the necessity of being a flexible tool to provide adaptability to system engineering requirements and can also be of interest to antenna engineers working on the design of reflector antennas. Due to the increasing demand of broadband services, satellite communications systems are becoming highly complex in order to meet connectivity requirements. To fully exploit the benefits of these systems, multidimensional optimisations are crucial, which call for an efficient estimation of the coverage characteristics. Reflector-based solutions are one of the preferred architectures for very high-throughput satellite (VHTS) systems. A detailed description of HERAS is presented in this paper which has been validated with available commercial software packages. In addition, some examples are described in which the tool has been used for the efficient estimation of VHTS systems performance.