Editorial Special Article Collection on Antennas and Propagation for Emerging 5G/6G Communications

Wireless communication systems have become key players in enabling highly interactive and context-aware scenarios, with a broad range of applications and sectors, spanning from industry, logistics, healthcare, transportation, or massive telemetry, just to name a few. The need for connectivity has been further spanned by new ways of handling and organizing information, within the framework of the Internet of Things and the evolution towards proactive and predictive environments, such as those foreseen in Cognitive cities. This evolution has led to the need of ever increasingly demanding key performance indicators of wireless communication devices and systems of current 5G and future 6G networks, with transmission rates in the Tbps range, latencies of <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$100~\mu $ </tex-math></inline-formula> s, reliability values of 99.99999%, or device densities of <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$10^{7}$ </tex-math></inline-formula> devices/km2, among others. These requirements pose relevant challenges in different domains, which in turn has catalyzed research efforts to meet these stringent demands related with devices, channel and system design and development. In this way, new antenna concepts are proposed, propagation analysis and measurement techniques are explored, and new application scenarios and domains are envisaged. With the emergence of high frequency ranges, large antenna arrays, and metasurfaces for communication systems, the near field boundary definition is becoming increasingly important. As a result of high path losses at mmWave and THz frequencies, the 6G communication systems are anticipated to operate mostly in the near field, where a plane wave approximation can result in a significant phase error during beam steering. This simple conclusion has resulted in re-evaluation of many antenna design principles that were taken for granted during the era of small antenna apertures and sub-6 GHz propagation. This is particularly visible through the analysis of reconfigurable intelligent surfaces (RIS), which are considered in <xref ref-type="bibr" rid="ref1" xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">[1]</xref> . RIS apertures are large w.r.t. operational frequency and they are expected to operate predominantly in near field, which necessitates the generation of near field codebooks and imaging patterns. The performance of RIS metasurfaces in near field is considered in <xref ref-type="bibr" rid="ref2" xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">[2]</xref> , that involves generation of near field codebooks and imaging patterns.