Mobile Hotspot Network System for High-Speed Railway Communications Using Millimeter Waves

Abstract

We propose a millimeter wave (MMW)-based mobile hotspot network (MHN) system for application in high-speed railways that is capable of supporting a peak backhaul link throughput of 1 Gbps per train at 400 km/h. The MHN system can be implemented in subways and high-speed trains to support passengers with smart devices and provide access to the Internet. The proposed system can overcome the inherent high path loss in MMW through system designs and high antenna gains. We present a simulation of the system performance that shows that a fixed beamforming strategy can provide high signal-tointerference-plus-noise-ratio similar to those of an adaptive beamforming strategy, with the exception of 15% of the train path in which the network can use link adaptation with low-order modulation formats or trigger a handover to maintain the connection. We also demonstrate the feasibility of the MHN system using a test bed deployed in Seoul subway line 8. The backhaul link throughput varies instantaneously between 200 Mbps and 500 Mbps depending on the SNR variations while the train is running. During the field trial, the smartphones used could make connections through offloading. Keywords: Millimeter waves, MHN, Railway communication, Doppler shift, High-speed scenarios, 5G.Manuscript received Apr. 15, 2016; revised July 11, 2016; accepted July 20, 2016 I. IntroductionMobile data traffic has been increasing along with the increased usage of smartphones and the evolution of various networks. Cisco has published a white paper that reported that mobile data traffic has grown 4,000-fold over the past ten years, reaching 3.7 exabytes per month as of 2015 [1]. Due to the ubiquity of handheld devices, there is demand for mobile data not only at home and work, but also on buses and trains. Smartphone users are able to connect to the Internet through cellular networks or utilize offloading, if available. Offloading occurs at the user or device level when users are switched from cellular connections to Wi-Fi or small-cell access connections. The white paper [1] disclosed that mobile offloading exceeded cellular traffic in 2015 for the first time. To be specific, 51% of the total amount of mobile data traffic was offloaded onto fixed networks through Wi-Fi or femtocells in 2015. When we consider these statistics, we envision that more and more of the smart devices connected to mobile hotspots, for example, on buses and subway/high-speed trains, will be connected to networks, and a good portion of such traffic will be offloaded through Wi-Fi.Dual-layer, two-tier, or hierarchical two-hop networks are all terms used to describe offloading-related architectures [2], [3]. Offloading can be applied in vehicles using mobile relays that are deployed in the topology. Vehicles, such as subways and high-speed trains, can be viewed as mobile hotspots that have a large number of users at the same location. The smartphones and tablets of users can connect to a network through Wi-Fi access points (APs) inside a train car. To connect to public land mobil...

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