  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.
