Millimeter-wave network architectures for future high-speed railway communications: Challenges and solutions

The shortage and congestion of lower spectra motivate the exploration of the broadband and underutilized mmWave to be used in future public mobile communications, and subsequently future HSR mobile communications, which has the potential to provide multi-gigabit rate radio access for train passengers. However, due to the inferior propagation characteristics of mmWave and particularity of HSR scenarios, there are many design challenges ahead. In this article, we tackle those challenges by developing technical solutions for mmWave broadband HSR systems. We first propose feasible multiple access techniques and frame structures based on OFDM and SC communications, respectively. We then present promising train-trackside network architectures based on different MIMO techniques, including BF and SM, which have been viewed as the key enabling technologies to realize mmWave communications in outdoor environments. Moreover, we discuss the inherent defects of each architecture and offer the corresponding solutions or recommendations. Finally, we conduct performance evaluation, and discuss the advantages and disadvantages of the proposed architectures. We hope this article will stimulate further research on the innovative use of mmWave in high-speed rail systems.