The 5G mobile communication systems are being de?ployed worldwide and China has been a global leader in the new technology.The number of base stations for 5G mobile communications is increasing by more than ten thousand every week in China.By the end of this year,about 500 000 base stations will be built up nationwide,cover?ing more than 300 cities.The beyond 5G (B5G)mobile systems with enhanced transmission technologies are also being devel?oped and attracting much attention.The emerging millimeter wave (mmW) and massive multiple-input multiple-output (MI?MO)technologies are expected to become key enabling technol?ogies for 5G/B5G mobile communications.In China,the mmW bands 24.75–27.5 GHz and 37–42.5 GHz have been allocat?ed for future 5G/B5G trials and it is anticipated that the 5G/B5G mobile communication systems based on mmW massive MIMO technologies will be ready for commercial use in 2022.As the key components in these communication systems,anten?nas and RF front ends play an important role in implementing ubiquitous connection among hundreds of different devices in 5G/B5G networks.

In this special issue,we have collected seven papers con?cerning antennas and RF technologies for 5G/B5G mobile communications.Beamforming and multibeam are crucial technologies for mmW massive MIMO based 5G/B5G appli?cations.Leaky-wave antennas (LWAs) are very suitable for beamforming in mmW bands.The paper“Leaky-Wave An?tennas for 5G/B5G Mobile Communication Systems:A Sur?vey”by HE et al.presents an overview of LWAs for 5G/B5G mobile communication systems.Classification and design methods of LWAs are introduced.The latest research prog?ress of LWAs for 5G/B5G mobile communication systems is demonstrated.Challenges and future research directions of LWAs are discussed.Low loss,simple structure and broad?band for beamforming LWAs are also essential for 5G/B5G mobile communication systems.Multibeam base station an?tennas are required to meet the increasing demands for high?er data rates in 5G/B5G networks.In the paper“Multibeam Antenna Based on Butler Matrix for 3G/LTE/5G/B5G Base Station Applications”,YE et al.propose a compact dualband dual-polarized two-beam antenna array and a wideband dual-polarized two-beam antenna array for base-station appli?cations.The dual-band dual-polarized two-beam antenna ar?ray is formed by two interleaved sub-arrays and two individu?al beamforming networks for different frequency operation.The wideband two-beam antenna array is composed of three 4 × 2 subarrays and two beam-forming networks.The pro?posed methods can easily be extended to the design of other multibeam base-station antenna arrays.Moreover,the paper“A Novel 28 GHz Phased Array Antenna for 5G Mobile Com?munications”by LI et al.presents a phased array antenna consisting of 16 × 16 patch elements.The antenna integrated with a wave control circuit can perform real-time beam scan?ning by reconfiguring the phase of an antenna unit.A proto?type of the antenna is fabricated and measured to demon?strate the feasibility of this approach.The phased array has the advantages of low power consumption,low cost,and con?formal geometry,suitable for base stations in B5G mmW mo?bile communication systems.

Antenna-in-package (AiP) technology has been widely ad?opted in mmW bands.It is believed that AiP technology may provide ultimate antenna solutions to 5G/B5G devices in the lower mmW bands.The paper“Design of Millimeter?Wave Antenna-in-Package (AiP) for 5G NR”by CHANG et al.dis?cusses the design of mmW AiP for 5G spectrum.The system architectures and design considerations for 5G phased arrays are presented.Beamforming IC and up/down converter chips,fabrication technologies and materials,antenna and transition design,and feeding networks and filters design are described.A 4×4 low temperature co-fired ceramic(LTCC)unit AiP mod?ule for the 5G band(27.5–28.35 GHz)is designed and manu?factured.A larger 8×8 array is also demonstrated by tiling upto four-unit modules on one single motherboard.The key de?sign considerations of the beamforming antenna array are low loss,compact system and small size.In the paper“Integrated 3D Fan-out Package of RF Microsystem and Antenna for 5G Communications”,XIA et al.investigate a three-dimensional(3D) fan-out packaging method for the integration of 5G com?munication RF microsystem and antennas.Through the dou?ble-sided wiring technology on the glass wafer,the fabrica?tion of 5G antenna arrays is realized.A slot coupling antenna for 5G communications is fabricated on a 12-inch glass wa?fer.The antenna can operate at 60 GHz with a maximum gain of 6 dBi,which implies a feasible solution to the 3D fan-out integration of RF microsystem and antennas for 5G communi?cations.Such a 3D fan-out integrated prototype is designed and manufactured.

Electronic design automation (EDA) software that can accu?rately simulate antennas,radio chips and microwave compo?nents is essential for 5G technologies.The paper“Electromag?netic Simulation with 3D FEM for Design Automation in 5G Era”by BALEWSKI et al.reviews challenges facing commer?cial tools for design of wireless devices where electromagnetic effects have to be taken into account.The focus is on simula?tion software based on finite-element method (FEM).Novel computational techniques based on FEM are introduced into state-of-the-art EDA software to accelerate numerical analy?sis,as well as to enable optimization,sensitivity analysis and interactive design tuning based on the rigorous electromagnet?ic model of a device.Several of these new techniques,helping to mitigate the most challenging issues related to FEM based simulation,are highlighted.It is demonstrated that these new computational techniques can significantly reduce the time needed for design closure with the acceleration rates reaching factors as large as tens or even over one hundred.

The paper“Robust Digital Predistortion for LTE/5G Power Amplifiers Utilizing Negative Feedback Iteration”by LIU et al.proposes a robust digital predistortion (DPD)technique uti?lizing negative feedback iteration for linearizing power amplifi?ers.Different from the conventional direct learning and indi?rect learning structure,the proposed DPD suggests a two-step method to identify the predistortion.A negative feedbackbased iteration is used to estimate the optimal DPD signal.The corresponding DPD parameters are extracted by forward modeling with the input signal and optimal DPD signal.The it?eration can be applied to both single-band and dual-band PAs,which will achieve superior linear performance than con?ventional direct learning DPD while having a relatively low computational complexity.

We would like to thank all the authors for their valuable contributions and all the reviewers for their timely and con?structive comments on the submitted manuscripts.We hope that this special issue is informative and useful for all readers.