High-speed Railway Communication Research Articles

3‐D Massive MIMO Channel Model for High‐Speed Railway Wireless Communication

AbstractIn the fifth generation (5G) wireless communication, high‐speed railway (HSR) communication is one of the most challenging scenarios. By adopting massive multi‐input multioutput (mMIMO) technology in HSR communication, the design of the underlying communication system becomes more challenging. Some new channel characteristics must be studied, such as nonstationarity in space, time, and frequency domains. In this paper, two models are established for the two states of HSR. When the HSR stops, a three‐dimensional (3‐D) stationary channel model based on the single‐ring distribution of the scatterers is established. When the HSR moves, a 3‐D nonstationary geometric random channel model (GRCM) based on the cylinder distribution of the scatterers by introducing the birth and death process of the propagation path and time‐varying characteristics of the channel parameters is established. Moreover, a time evolution algorithm for time‐varying channel parameters is proposed, and a modularization study of key parameter update algorithms is conducted. Finally, based on the proposed 3‐D nonstationary geometric random channel model, important time‐varying channel statistical characteristics, such as time autocorrelation function (ACF), spatial cross‐correlation function (CCF), Doppler power spectral density (DPSD), and stationary interval are studied. Simulation results indicate that the properties of the proposed channel models, verified by simulation results, can match well with those of the theoretical model.

5G Key Technologies for Smart Railways

Railway communications has attracted significant attention from both academia and industries due to the booming development of railways, especially high-speed railways (HSRs). To be in line with the vision of future smart rail communications, the rail transport industry needs to develop innovative communication network architectures and key technologies that ensure high-quality transmissions for both passengers and railway operations and control systems. Under high mobility and with safety, eco-friendliness, comfort, transparency, predictability, and reliability. Fifth-generation (5G) technologies could be a promising solution to dealing with the design challenges on high reliability and high throughput for HSR communications. Based on our in-depth analysis of smart rail traffic services and communication scenarios, we propose a network slicing architecture for a 5G-based HSR system. With a ray tracing-based analysis of radio wave propagation characteristics and channel models for millimeter wave (mmWave) bands in railway scenarios, we draw important conclusions with regard to appropriate operating frequency bands for HSRs. mymargin Specifically, we have identified significant 5G-based key technologies for HSRs, such as spatial modulation, fast channel estimation, cell-free massive multiple-input-multiple-output (MIMO), mmWave, efficient beamforming, wireless backhaul, ultrareliable low latency communications, and enhanced handover strategies. Based on these technologies, we have developed a complete framework of 5G technologies for smart railways and pointed out exciting future research directions.

Improvement and queuing analysis of the handover mechanism in the high-speed railway communication

This paper firstly analyzes the handover mechanism of vehicle-mounted base station technology in high speed railway system, and finds that the communication interruption of the physical layer will last for a period of time although the switch of MAC layer has been completed. In order to solve this problem, an improved switch mechanism is proposed in this paper in order to ensure the continuality of communication. Then the data transmission rule between RAUs and the train will be discussed through using queue theory. The original mechanism and the improved mechanism are modeled as the loss Geom/G/1 system with interruptions and vacation respectively. And the analytical equations of the loss rate and the waiting delay are deduced. Then, the numerical simulations of the two mechanisms are discussed and the relationship between the packet loss rate, the waiting delay, and the other parameters is obtained respectively. At the same time, the performance of the two mechanisms are compared by setting the same parameter values in order to verify the superiority of the proposed mechanism and its greatest advantage, that is, the improved mechanism can be compatible with all handover mechanisms.

Low-Complexity Beam Selection Scheme for High Speed Railway Communications

Large-scale multiple input multiple output (MIMO) assisted beamforming based on millimeter waves is an effective way to increase system capacity and data transmission rate for high speed railway (HSR) communication systems. To reduce the number of handovers and improve system throughput, multi-stream beamforming with different beamwidths is used as the transmit beam at the base station (BS). However, when the train moves toward the cell edge, the inter-beam ambiguity (IBA) between multiple beams becomes more and more serious, and the performance of the system will decrease significantly if multiple beams are still activated simultaneously. In order to overcome such difficulty, a low-complexity beam selection scheme based on multi-stream beamforming is proposed in this paper. With this methodology, when the train is in the vicinity of the BS, all beams are activated to achieve the performance of high throughput. Neverthless, when the train moves at the cell edge, only one beam is activated due to the insupportable IBA. The optimal beam switching position could be calculated according to the requirements of the angular resolution. Observing from the theoretical analysis and the simulation results, the proposed scheme can achieve low complementation complexity with high throughput performance.

Impact of Doppler shift error on least‐squares MIMO channel estimation for high‐speed railway

Owing to the availability of Doppler shift measurement and the existence of dominant line-of-sight (LOS) path in high-speed railway (HSR) communications, Doppler shift is often pre-compensated on the received signal before performing further processing. In this study, the authors address the impact of erroneous Doppler shift measurement on the tracking performance of least-squares multiple-input multiple-output (MIMO) channel estimation for HSR. The closed-form tracking mean-square error (MSE) is derived over time-varying Rician fading channel assuming Gaussian distributed Doppler shift measurement. By transforming the Gaussian distributed measurement into its characteristic function and performing approximations with practical considerations, the tracking MSE of the approach with Doppler shift pre-compensation is written as a function of that with no pre-compensation, contributed from the previous research. Accordingly, they are able to develop a simple relation between MSE gain and MSE loss for channels with dominant LOS path. It is revealed that erroneous Doppler shift measurement impacts stronger LOS or faster time-varying channel more severely. However, if the measurement is perfect, the presence of a dominant LOS path provides tremendous gain approximately equal to the Rician factor in high signal-to-noise ratio (SNR) regime.

High-Speed Railway Communication System Using Linear-Cell-Based Radio-Over-Fiber Network and Its Field Trial in 90-GHz Bands

A linear-cell-based radio-over-fiber (LC-RoF) system is proposed and demonstrated for efficient mobile communication in high-speed trains without hard handover processes. The configuration of the LC-RoF network and possible router architectures are discussed for the field trial test of the proposed system and for advanced systems in the future. Via the LC-RoF network, 90-GHz, millimeter-wave radio access between the ground and a train car is operated in a centralized manner. The resulting throughput of 1.5 Gbit/s is achieved in the field trial test, which is comprised of 250-Mbaud four-subcarrier differential quadrature phase-shift keying with a forward-error correction code implemented in field-programmable gate arrays to the Shinkansen train traveling at 240 km/h without any interruptions in the connections at the change of radio access units.

A Fast Hybrid Beamforming Scheme for High-Speed Railway Communications

With the increasingly demand for reliable mobile communication service in the high-speed railway (HSR) system, the service stability of HSR communication is of great concern in recent research. Focusing on ensuring the quality of services (QoS) and system throughput, massive multi-input multi-output (massive MIMO) and beamforming technologies have been widely applied. In this paper, aiming to minimize the communication outage probability (OP) and not to decrease spectrum efficiency (SE) too much, we propose a fast HBF (F-HBF) scheme for HSR communications. The proposed scheme uses a low-complexity beam-searching algorithm to trace high-speed trains in real time. The simulation results verify that the proposed scheme can significantly reduce OP without too much SE degradation.

Recent Breakthroughs on Angle-of-Arrival Estimation for Millimeter-Wave High-Speed Railway Communication

With significantly improved efficiency, largescale hybrid antenna arrays with tens to hundreds of antennas have great potential to support millimeter-wave (mmWave) communication for high-speed railway (HSR) applications. The significant beamforming gains rely on fast and accurate estimation of the angle-of-arrival (AoA), but this can be impeded by the high train speed, the cost/energy oriented design of arrays, and the severe attenuation of mmWave signals. This article reviews these challenges, and discusses the limitations of existing AoA estimation techniques under hybrid antenna array settings. The article further reveals a few recent theoretical breakthroughs that can potentially enable fast and reliable estimation, even based on severely attenuated signals. Under a speed setting of 500 km/h, a performance study is carried out to confirm the significant improvements of estimation accuracy and subsequent beamforming gains as the results of the breakthroughs.

Fast simulation of high-speed railway fading channel using finite-state Markov models

The design and performance evaluation of the high-speed railway (HSR) communications are inseparable from the accurate and efficient high-speed channel simulator. The variation on path loss, shadowing and fast fading are the key characteristics of HSR channels. Although finite-state Markov chain (FSMC) channel models have been extensively investigated to describe fading channels, most of the previous channel simulators cannot reflect all the above key characteristics. In this paper, we propose a novel FSMC model based on channel simulator with the path loss, shadowing and fast fading in HSR. The accurate closed-form expression of state transition probabilities between channel states is derived to break the simplified assumption of first-order Markov chain. To validate the accuracy of the new FSMC channel simulator, the comparison analysis between the proposed channel simulator and the actual measurements are provided.

Location-Aided mMIMO Channel Tracking and Hybrid Beamforming for High-Speed Railway Communications: An Angle-Domain Approach

Wireless communication for high-speed railways (HSRs) has received considerable attention in recent years. The fast time-varying and doubly selective fading properties of channels pose a substantial threat to the link performance. Meanwhile, the nonnegligible Doppler frequency offset (DFO) resulting from the high-speed motion induces intercarrier interference. In this paper, we propose an angle-domain channel tracking and hybrid beamforming scheme for HSR by investigating the time and spatial properties of channels. A large-scale uniform linear antenna array is applied at the base station to provide high spatial resolution. The DFO is compensated through beam alignment and capturing the precise angular information from signals. Then, the spatial beam gain is tracked during the angle-of-arrival time, which can be used to reconstruct the full-dimensional channel state information. Finally, the hybrid beamforming is used to realize data transmission. The key idea of tracking is, applying a linear Kalman filter to modify the time-varying beam gain, while the main idea of hybrid beamforming is utilizing nonorthogonal beams to compress the channel dimension and decrease the computational complexity. The superiority of the proposed scheme is evaluated through simulations.

Geometry-Based Stochastic Channel Model for High-Speed Railway Communications

Reliable wireless communication links are a critical but challenging aspect for high-speed railway (HSR). To develop and analyze such systems, accurate models for the propagation channel are required. The radio channel properties in HSR scenarios are different from those in cellular scenarios not only due to the high velocity, but also due to the special construction elements and the type of surroundings along the train lines. We performed a measurement campaign at a carrier frequency of 2.4 GHz on the Guangzhou–Shenzhen dedicated high-speed train line in China. Motivated by the results from these measurements, we propose the concept of active scatterer region as an improvement for geometry-based stochastic channel models for HSR, to better describe the non-stationary properties of such channels. Further contributions to the total channel impulse response are also studied, including the line-of-sight component and the scattering component from overhead line poles. Each type of component is modeled and fully parameterized based on the measurement results.

Measurements and Analysis of Short-Term Fading Behavior in High-Speed Railway Communication Networks

This paper presents the analysis of short-term fading behavior in high-speed railway (HSR) communication networks, based on passive channel measurements. A long-term evolution (LTE)-based channel sounding system is used to measure the propagation characteristics in a dedicated HSR LTE network with multiple scenarios, such as rural, station, and suburban, and with the echo channel effect (ECE) caused by the same signaling from different remote radio units. The measurement data are partitioned into four cases, involving rural, rural with the ECE, station, and suburban, aiming to investigate the impact of two factors, e.g., the scenario and the ECE, on the propagation characteristics. Based on the processed data, the short-term fading behavior is characterized in terms of the fading severity and the time-frequency-space dispersion in the four cases. The distance-dependent models and statistical models of Ricean K-factor, delay spread, Doppler spread, and angular spread are proposed and compared with previous paper. The presented results will be useful in time-frequency-space channel modeling for future HSR mobile communication systems.

Low-Complexity Energy-Efficient Power Allocation With Buffer Constraint in HSR Communications

In this paper, we investigate the energy-efficient power allocation for downlink distributed antenna system (DAS) with buffer size constraint in high-speed railway (HSR) scenarios. We design the power allocation at different times for HSR communications, utilizing the characteristics of fixed moving path of trains. A key issue we discuss is the matching problem between the data arrival process and instantaneous wireless transmission process under the buffer size constraint. We formulate the energy-efficient power allocation problem with the requirement of buffer size as a non-convex optimization problem. By a parameterized transformation and an iterative algorithm, we derive the optimal solution at different time phases. Then, to reduce complexity of the multiple iterations, we present a low-complexity algorithm by analyzing the feasible region of the optimal solution. Simulation results demonstrate that the proposed power allocation schemes can achieve the optimal energy efficiency (EE) and avoid data overflow. Moreover, with a smaller number of iterations, the low-complexity algorithm can achieve the same EE performance as the optimal algorithm based on the bisection method.

Group mobility by cooperative communication for high speed railway

The high speed railway (HSR) provides more convenience to people, so the main attention is given to provide the reliable communication inside the train. It is the challenging task due to the high mobility of train and frequent need of handoff to each communicating users. In this paper, the individual handoff problem is solved by group handoff by using moving relays (MR) through cooperative communication. The system model for non-cooperative and cooperative communication in multi-tier heterogeneous network is proposed. Two different scenario for HSR communication i.e. without MR or direct communication and with MR, are discussed. Moreover, effect of interference, noise signal and power control factor are also considered in this paper. The performance of system model is analyzed by handoff probability, outage probability, coverage probability and transmission capacity. The results show that the outage probability and handoff probability decreases and coverage probability and transmission capacity increases as the number of MR increases due to cooperative communication. The analytical results are also verified by the Monte-Carlo simulation.

Channel Modeling for Future High-Speed Railway Communication Systems: A Survey

The widespread popularity of high-speed railways (HSRs) urges a critical demand on high-data-rate railway communication services for both train operation and passenger experience. To satisfy the ever-increasing requirements, future HSR communication systems, such as long-term evolution for railway (LTE-R), fifth generation (5G) on HSR, and 5G for railway (5G-R), and corresponding transmission technologies, e.g., mobile relay, coordinated multipoint, massive multiple-input multiple-output (MIMO), and millimeter-wave (mmWave), have recently attracted much attention. Radio channel modeling is the foundation of design and evaluation of wireless systems and transmission technologies. This paper focuses on a survey of channel modeling for the future HSR communication systems. The significant requirements of future HSR channel models are highlighted, and recent advances in the HSR channel modeling are reviewed. Finally, potential research directions for future HSR channel modeling are outlined.

Toward Traffic Patterns in High-Speed Railway Communication Systems: Power Allocation and Access Selection

In high-speed railway communication systems, the distributed antenna systems are usually employed to mitigate frequent handover and enhance the signal-to-noise ratio to receivers. In this case, jointly optimizing downlink power allocation with antenna selection (PAWAS) can enhance system energy efficiency, while the channel state and traffic density are taken into account. Besides, two typical kinds of terrains with sparse and rich scatterings and three traffic patterns including delay-sensitive, -insensitive, and hybrid traffics are investigated in this paper. We show that severe small-scale fading decreases the ergodic capacity, which is quantitatively analyzed, and proved to be proportional to the number of selected transmit antennas. In addition, in case of delay-sensitive traffic, we show that the PAWAS can be viewed as generalized channel-inversion associated with transmit antenna selection. In case of delay-insensitive traffic, we show that when multiple antennas are selected, the power allocation can be viewed as channel-inversion, whereas when single antenna is selected, it is traditional waterfilling. In case of hybrid traffic, we prove that the optimal PAWAS method can be given by separately solving the PAWAS of its delay-sensitive and -insensitive parts. Simulation results validate our theoretical results and demonstrate that proposed PAWAS can minimize the average transmit power in cases of arbitrary traffic density and channel states.

Channel Measurement, Simulation, and Analysis for High-Speed Railway Communications in 5G Millimeter-Wave Band

More people prefer to using rail traffic for travel or for commuting due to its convenience and flexibility. As the record of the maximum speed of rail has been continuously broken and new applications are foreseen, the high-speed railway (HSR) communication system requires higher data rate with seamless connectivity, and therefore, the system design faces new challenges to support high mobility. Millimeter-wave (mmWave) technologies are considered as candidates to provide wideband communication. However, mmWave is rarely explored in HSR scenarios. In this paper, channel characteristics are studied in the 5G mmWave band for typical HSR scenarios, including urban, rural, and tunnel, with straight and curved route shapes. Based on the wideband measurements conducted in the tunnel scenario by using the “mobile hotspot network” system, a 3-D ray tracer (RT) is calibrated and validated to explore more channel characteristics in different HSR scenarios. Through extensive RT simulations with 500-MHz bandwidth centered at 25.25 GHz, the power contributions of the multipath components are studied, and the dominant reflection orders are determined for each scenario. Path loss is analyzed, and the breakpoint is observed. Other key parameters, such as Doppler shifts, coherence time, polarization ratios, and so on, are studied. Suggestions on symbol rate, sub-frame bandwidth, and polarization configuration are provided to guide the 5G mmWave communication system design in typical HSR scenarios.

Investigation of cross-correlation characteristics for multi-link channels in high-speed railway scenarios

This paper investigates the cross-correlation characteristics of large-scale parameters (LSPs) and small-scale fading (SSF) for high-speed railway (HSR) multilink propagation scenarios, based on realistic measurements conducted on Beijing to Tianjin HSR line in China. A long-term evolution-based channel sounding system is utilized in the measurements to obtain the channel data. By applying a proposed time-delay based dynamic partition method, multi-link channel impulse responses are extracted from the raw channel data. Then, the statistical results of LSPs, including shadow fading, K-factor, and root-mean-square delay spread are derived and the cross-correlation coefficients of these LPSs are calculated. Moreover, the SSF spatial correlation and cross-correlation of SSF are analyzed. These results can be used to exploit multi-link channel model and to optimize the next-generation HSR communication system.

Dynamic Channel Model With Overhead Line Poles for High-Speed Railway Communications

In order to develop reliable communication links for high-speed railway (HSR) systems, accurate models for the propagation channel are required. The radio channel properties in HSR scenarios are different from those in cellular scenarios not only due to the high velocity, but also due to the special construction elements and the type of surroundings along the train lines. This letter focuses on the influence of overhead line poles along the railway on the HSR communication channel. The overhead line poles affect the line-of-sight communication path periodically, and also act as static and periodic scatterers along railway lines. We performed a measurement campaign at a carrier frequency of 2.4 GHz on the Guangzhou-Shenzhen dedicated high-speed train line in China. Based on the measurement result we propose a dynamic channel model considering periodic poles for HSR communication.

Location-Aware ICI Reduction in MIMO-OFDM Downlinks for High-Speed Railway Communication Systems

High mobility may destroy the orthogonality of subcarriers in OFDM systems, resulting in inter- carrier interference (ICI), which may greatly reduce the service quantity of high-speed railway (HSR) wireless communications. This paper focuses on ICI mitigation in the HSR downlinks with distributed transmit antennas.For such a system, its key feature is that the ICIs are caused by multiple carrier frequency offsets corresponding to multiple transmit antennas. Meanwhile, the channel of HSR is fast time varying, which is another big challenge in the system design. In order to get a good performance, low complexity real-time ICI reduction is necessary. To this end, we first analyzed the property of the ICI matrix in AWGN and Rician scenarios, respectively. Then, we propose corresponding low complexity ICI reduction methods based on location information. For evaluating the effectiveness of the proposed method, the expectation and variance of remaining interference after ICI reduction is analyzed with respect to Rician K-factor. In addition, the service quantity and the bandwidth and computation cost are also discussed. Numerical results are presented to verify our theoretical analysis and the effectiveness of proposed ICI reduction methods. One important observation is that our proposed ICI mitigation method can achieve almost the same service quantity with that obtained on the case without ICI at 300km/h,that is, ICI has been completely eliminated. Numerical results also show that the scenarios with Rician K-factors over 30dB can be considered as AWGN scenarios, which may provide valuable insights on future system designs.