Junhyeong Kim

Distributed antenna system-based millimeter-wave mobile broadband communication system for high speed trains

This paper proposes a distributed antenna system (DAS)-based millimeter-wave mobile broadband communication system for HSTs in order to meet the growing demand of high-speed communication services in HSTs. Based on the proposed network structure taking advantages of DAS and millimeter-wave, we present a proper design of physical layer (PHY) such as frame structure. Throughout the paper, we show the feasibility of the system with moderate implementation complexity, and the simulation results also demonstrate that it is possible to provide gigabit-per-second (Gbps) data services for HSTs.

On Millimeter Wave and THz Mobile Radio Channel for Smart Rail Mobility

As a widely acknowledged efficient and green transportation model, rail traffic is expected to evolve into a new era of “smart rail mobility” whereby infrastructure, trains, and travelers will be interconnected to achieve optimized mobility, higher safety, and lower costs. Thus, a seamless high-data rate wireless connectivity with up to dozens of gigahertz bandwidth is required. Such a huge bandwidth requirement motivates the exploration of the underutilized millimeter (mm) wave and terahertz (THz) bands. In this paper, the motivations for developing millimeter wave and THz communications for railway are clarified by first defining the applications and scenarios required for smart rail mobility. Ray-tracing simulations at 100 GHz imply that to form high-gain directional antenna beams, dynamic beamforming strategies and advanced handover design are critical for the feasibility of THz communications to enable smart rail mobility.

Development of Millimeter-Wave Communication Modem for Mobile Wireless Backhaul in Mobile Hotspot Network

The current cellular communications are optimized for low mobility users, meaning that their performance is degraded at high speed. Therefore, passengers in a high-speed train experience very poor radio link quality due to the significantly large number of simultaneous handovers. In addition, wireless data traffic is expanding exponentially in trains, subways and buses due to the widespread use of smartphones and mobile devices. To solve the inherent problem of cellular communication networks and meet the growing traffic demand, this paper proposes the mobile hotspot network of a millimeter-wave communication system as a mobile wireless backhaul. This paper describes the physical layer design of uplink and downlink in the proposed system, and the performances of uplink and downlink are evaluated under Rician fading channel conditions. The implemented baseband prototype of the proposed millimeter-wave communication modem is presented. This system can provide a Gbps data rate service in high-speed trains carrying hundreds of wireless Internet users.

A Periodic Frequency Band Rotation Scheme for Multi-Cell Coordination Clustering

Lots of attention to achieve cell edge performance has been made in future wireless networks. Through multi-cell coordination clustering, we can reduce co-channel interference, resulting in cell edge performance improvement. This letter proposes a periodic frequency band rotation clustering scheme for joint transmissions in multi-cell configuration. Through mathematical interference analysis, we show that the inter-cluster interference can be reduced and cell edge data rate is improved over existing schemes.

Joint processing in multi-cell coordinated shared relay network

This paper investigates multi-cell coordinated shared relay with joint processing (JP) scheme in interference limited network. In general, multi-cell coordinated shared relay network increases the Signal to Interference-plus-Noise Ratio (SINR) in cell edge users, but resource usage efficiency is decreased by two-phase transmission of relay protocol. We propose JP of cooperating BSs and shared relay station (RS) in the relay transmission phase (second phase) to increase resource usage efficiency, and introduce the SVD precoding with phase correction based on the multi-cell coordinated shared RS network to mitigate other cell JP interference in the cluster. Simulation results show that the proposed scheme leads to network capacity enhancement.

A study on millimeter-wave beamforming for high-speed train communication

This paper investigates three millimeter-wave beamforming (BF) scenarios for high-speed train (HST) communication that can potentially help to meet the explosively growing demand for Internet services in HSTs. Throughout the paper, we introduce the BF schemes based on the beam pattern that we designed and show the performance of three BF scenarios via the simulation, which demonstrates that in the case of adaptive BF applied to both transmitter (TX) and receiver (RX), the highest SINR can be obtained. It is also shown that the different BF schemes are appropriate for different HST location and the fixed BF can achieve very similar performance to that of adaptive BF in most of time. Consequently, we have gained a valuable insight into designing a mmWave BF based HST communication system from feasibility and implementation perspective.

Channel Characterization for Mobile Hotspot Network in Subway Tunnels at 30 GHz Band

Mobile Hotspot Network (MHN) is designed as a new communication system for high speed mobile group vehicles and aiming at providing multi-Giga bps data rate by using millimeter wave (mmWave). According to the MHN setup, the channel at 30 GHz band is simulated by a ray-tracing tool in a typical straight subway tunnel. In order to assess the system physical layer performance, system simulations based on entire channel characteristics provide an profound significance. the channel parameters such as path loss, power delay profile, decorrelation distance, Rician K-factor, Doppler characteristics, etc., are extracted and analyzed for verifying system feasibility and can serve as a guide for physical layer design.

Utility-Aware Network Coding in Wireless Butterfly Networks

This paper investigates the effect of combined network coding and power control based on network utility under wireless butterfly networks. Although it is well known that network coding improves network performances, it might not be always advantageous for a hostile wireless environment due to error propagation problem through relaying. Therefore, in this paper, the crossover point for profit-and-loss of network coding and non-network coding according to wireless link quality is derived, and then we show that network utility in terms of spectral efficiency per power is improved, and average power consumption is reduced by adaptively applying relay mode control and power control, which is of bang-bang type with a threshold in wireless butterfly networks.

On the high-speed railway communication at 30 GHz band: Feasibility and channel characteristics

Simulations for studying novel high-speed railway millimeter-wave radio propagation channels in urban city are presented in this paper by using verified ray-tracing tool. Based on setups of three antenna beam steering strategies at transceiver, the comparisons among these strategies are given by system feasibility and radio channel characteristics. Numerical results of the simulations are shown to illustrate that adaptive antenna beam steering could greatly improve system performances. However, jointly using fixed and adaptive beam steering is recommended as a trade off between technology implementation and channel performance. Moreover, the system effects from different distances between base station and railroad track are investigated. By selecting a reasonable distance, the detailed radio channel characteristics are extracted including delay spread and Doppler spread, which could serve as a guide for future physical layer design.

Efficient Doppler mitigation for high-speed rail communications

Since the high-speed rail (HSR) communications are suffering from the huge Doppler shift, an efficient Doppler mitigation method is proposed in this paper. The proposed method mainly concerns about the mitigation of Doppler shift in OFDM based systems, especially for the case of employing mmWave with very high mobility. The proposed method can be used for both downlink and uplink of the system and it is quite robust to the time-varying mobility. Furthermore, the pilot-aided channel estimation for Doppler compensation is not necessary in most of the cases, which indicates the pilot density in time-domain can be reduced. Simulation results show that by using the proposed method, the residual Doppler shift can be compressed to 0 Hz with the probability of 90%.