In this paper we revisit hybrid analog-digital precoding systems with emphasis on their modelling and radio-frequency (RF) losses, to realistically evaluate their benefits in 5G system implementations.For this, we decompose the analog beamforming networks (ABFN) as a bank of commonly used RF components and formulate realistic model constraints based on their S-parameters. Specifically, we concentrate on fully-connected ABFN (FC-ABFN) and Butler networks for implementing the discrete Fourier transform (DFT) in the RF domain. The results presented in this paper reveal that the performance and energy efficiency of hybrid precoding systems are severely affected, once practical factors are considered in the overall design. In this context, we also show that Butler RF networks are capable of providing better performances than FC-ABFN for systems with a large number of RF chains. Index TermsAnalog beamforming networks, hybrid precoding, millimeter wave, massive MIMO, Butler matrix.

In this paper, we explore reduced-connectivity radio frequency (RF) switching networks for reducing the analog hardware complexity and switching power losses in antenna selection (AS) systems. In particular, we analyze different hardware architectures for implementing the RF switching matrices required in AS designs with a reduced number of RF chains. We explicitly show that fully-flexible switching matrices, which facilitate the selection of any possible subset of antennas and attain the maximum theoretical sum rates of AS, present numerous drawbacks such as the introduction of significant insertion losses, particularly pronounced in massive multiple-input multiple-output (MIMO) systems. Since these disadvantages make fully-flexible switching suboptimal in the energy efficiency sense, we further consider partially-connected switching networks as an alternative switching architecture with reduced hardware complexity, which we characterize in this work. In this context, we also analyze the impact of reduced switching connectivity on the analog hardware and digital signal processing of AS schemes that rely on received signal power information. Overall, the analytical and simulation results shown in this paper demonstrate that partially-connected switching maximizes the energy efficiency of massive MIMO systems for a reduced number of RF chains, while fully-flexible switching offers sub-optimal energy efficiency benefits due to its significant switching power losses.

The work presented in this letter demonstrates that nonuniform transmission lines (NUTLs) can be used for pulse shaping applications in ultra wideband pulsed systems and presents an approach to their design for this application. The pulsed waveforms generated are modified Hermite polynomials and encoded pulse trains thereof and are characterised by a broad bandwidth ranging from near DC to over 10 GHz and relatively long temporal duration in the region of nanoseconds. These pulses are more mathematically complex and at least three times higher bandwidth than previously has been reported in the literature. It is also believed to be the first time that modified Hermite polynomial pulses have been generated in practice. The NUTL design approach is verified by test structures fabricated on FR4 circuit board. The results demonstrate that NUTLs, which are completely passive structures, allow generation of complex pulse shapes in applications requiring fast, transient waveforms.

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