In the new era of railways, infrastructure, trains and travelers will be interconnected. In order to realize a seamless high-data rate wireless connectivity, up to dozens of GHz bandwidth is required. This motivates the exploration of the underutilized millimeter wave (mmWave) as well as the largely unexplored THz band. In this paper, we first identify relevant communication scenarios for railway applications. Then the specific challenges and estimates of the bandwidth requirements for high-data rate railway connectivity in these communication scenarios are described. Finally, we outline the major challenges on propagation channel modeling and provide a technical route for further studies.

The exponential growth of using wireless devices in recent years has motivated the exploration of the millimeter-wave (mmWave) frequency spectrum for multi-gigabit wireless communications. Recent advances in antenna technology, radio-frequency complementary metal-oxide semi-conductor (RF CMOS) process and high-speed baseband signal processing algorithms have promised the feasibility of millimeter-wave wireless communications. The multi-gigabit-per-second data rate of millimeter-wave wireless communication systems is leading to applications in many important scenarios, such as wireless personal area networks (WPAN), wireless personal area networks (WLAN), back-haul for cellular systems. The frequency bands include 28 GHz, 38 GHz, 45 GHz, 60 GHz, E-BAND, and even beyond 100 GHz.

The special issue aims to present some major achievements of the research and development in multi-gigabit millimeter-wave wireless communications. It includes four technical contributions from leading researchers in mmWave communications. The first paper entitled “Substrate-Integrated Waveguide-Based Monopulse Slot Antenna Arrays for 60 GHz Applications”, co-authored by ZHU, XUE and LIAO, presents the substrate-integrated waveguide (SIW)-based monopulse slot antenna arrays for the application of 60 GHz monopulse tracking systems. The second paper entitled “Millimeter Wave and THz Propagation Channel Modeling for High-Data Rate Railway Connectivity — Status and Open Challenges” is co-authored by Kürner, GUAN, Molisch, AI, HE, LI, TIAN, DOU and ZHONG. In this paper, the authors provide elementary discussions on bandwidth requirements of high-data rate railway connectivity, and highlight the open challenges in terms of wave propagation, static channel, and dynamic channel.

Co-authored by XU, MENG, MA and YEO, the third paper entitled “State of the Art in Passive Bandpass Filter Solutions for 60-GHz Communications”, reviews the state-of-the-art filter designs for 60-GHz applications. Design methodology, design technology, key performance parameters, similarities and differences, advantages and drawbacks, and future trends are explored and studied. The last (but not least) paper “Low-Power High-Efficiency Multi-Gigabit 60 GHz Transceiver Systems Routing in Vehicular Environments” is co-authored by Byeon and Park. It proposes low-power high-efficiency multi-gigabit 60 GHz transceiver systems for short-range communications. The antenna-in-package module with the transceiver demonstrates mobile-to-display 1080p Full-HD video transmission over a distance of 60 cm.

We would like to thank all the authors for choosing this special issue to publish their new research results and insights, all the reviewers for their valuable review comments which help to improve the technical quality and presentation of this special issue, and the editorial official of ZTE Communications for all the support and help during the editorial process of this special issue. We are sure this special issue will again be quite informative and a pleasure for you to browse through and read in depth.