High-speed railway (HSR) brings convenience to peoples' lives and is generally considered as one of the most sustainable developments for ground transportation. One of the important parts of HSR construction is the signaling system, which is also called the “operation control system,” where wireless communications play a key role in the transmission of train control data. We discuss in detail the main differences in scientific research for wireless communications between the HSR operation scenarios and the conventional public land mobile scenarios. The latest research progress in wireless channel modeling in viaducts, cuttings, and tunnels scenarios are discussed. The characteristics of nonstationary channel and the line-of-sight (LOS) sparse and LOS multiple-input-multiple-output channels, which are the typical channels in HSR scenarios, are analyzed. Some novel concepts such as composite transportation and key challenging techniques such as train-to-train communication, vacuum maglev train techniques, the security for HSR, and the fifth-generation wireless communications related techniques for future HSR development for safer, more comfortable, and more secure HSR operation are also discussed.

Challenges Toward Wireless Communications for High-Speed Railway

Future 5th generation networks are expected to enable three key services—enhanced mobile broadband, massive machine type communications and ultra-reliable and low latency communications (URLLC). As per the 3rd generation partnership project URLLC requirements, it is expected that the reliability of one transmission of a 32 byte packet will be at least 99.999% and the latency will be at most 1 ms. This unprecedented level of reliability and latency will yield various new applications, such as smart grids, industrial automation and intelligent transport systems. In this survey we present potential future URLLC applications, and summarize the corresponding reliability and latency requirements. We provide a comprehensive discussion on physical (PHY) and medium access control (MAC) layer techniques that enable URLLC, addressing both licensed and unlicensed bands. This paper evaluates the relevant PHY and MAC techniques for their ability to improve the reliability and reduce the latency. We identify that enabling long-term evolution to coexist in the unlicensed spectrum is also a potential enabler of URLLC in the unlicensed band, and provide numerical evaluations. Lastly, this paper discusses the potential future research directions and challenges in achieving the URLLC requirements.

/pdf/enabling-technologies-for-ultra-reliable-and-low-latency-44un8h2u5m.pdf

Enabling Technologies for Ultra-Reliable and Low Latency Communications: From PHY and MAC Layer Perspectives

The recent development of high-speed trains (HSTs) as an emerging high mobility transportation system, and the growing demands of broadband services for HST users, introduce new challenges to wireless communication systems for HSTs. Accurate and efficient channel models considering both large-scale and non-stationary small-scale fading characteristics are crucial for the design, performance evaluation, and parameter optimization of HST wireless communication systems. However, the characteristics of the underlying HST channels have not yet been sufficiently investigated. This paper first provides a comprehensive review of the measurement campaigns conducted in different HST scenarios and then addresses the recent advances in HST channel models. Finally, key challenges of HST channel measurements and models are discussed and several research directions in this area are outlined.

Channel Measurements and Models for High-Speed Train Communication Systems: A Survey

Orthogonal frequency division multiplexing (OFDM) is a superior technology for the high-speed data rate of wire-line and wireless communication systems. The OFDM has many advantages over other techniques such as its high capacity and immunity against multipath fading channels. However, one of the main drawbacks of the OFDM system is the high-peak-to-average power ratio (PAPR) that leads the system to produce in-band distortion and out-of-band radiation because of the non-linearity of the high-power amplifiers. Therefore, numerous techniques have been proposed to overcome the PAPR problem such as selective mapping, partial transmit sequence (PTS), clipping, and nonlinear companding. In this paper, the PTS technique was analytically reviewed as one of the important methods to reduce the high PAPR problem. The PAPR performance and the computational complexity level are discussed in terms of modifying the PTS technique in the frequency domain, time domain and modulation stage (inverse fast Fourier transform block). Moreover, the numerical statistic comparison of the current modified-PTS methods is introduced, and the criteria for selecting the suitable modified-PTS method in the OFDM system are also given. The simulation and the numerical calculations results show that the rows exchange-interleaving PTS scheme is the best method for reducing the PAPR value with low complexly in the frequency domain, and the cooperative PTS method is the best among the modulation stage methods, while the cyclic shift sequence PTS method achieves the superior performance in PAPR reduction and computational complexity for the time domain methods.

/pdf/a-review-of-partial-transmit-sequence-for-papr-reduction-in-59x9kyvr2j.pdf

A Review of Partial Transmit Sequence for PAPR Reduction in the OFDM Systems

Tactile Internet of Things (IoT) requires ultraresponsive and ultrareliable connections for massive IoT devices. As a promising enabler of tactile IoT, grant-free nonorthogonal multiple access (NOMA) exploits the joint benefit of grant-free access and nonorthogonal transmissions to achieve low latency massive access. However, it suffers from the reduced reliability caused by random interference. Hence, we formulate a variational optimization problem to improve the reliability of grant-free NOMA. Due to the intractability of this problem, we resort to deep learning by parameterizing the intractable variational function with a specially designed deep neural network, which incorporates random user activation and symbol spreading. The network is trained according to a novel multiloss function where a confidence penalty based on the user activation probability is considered. The spreading signatures are automatically generated while training, which matches the highly automatic applications in tactile IoT. The significant reliability gain of our scheme is validated by simulations.

Deep Learning Aided Grant-Free NOMA Toward Reliable Low-Latency Access in Tactile Internet of Things

The next-generation cellular network is to provide a large variety of services for different kinds of terminals, from traditional voice and data services over mobile phones to small packet transmission over massive machine-type terminals. Although orthogonal-subcarrier-based waveforms are widely used nowadays in many practical systems, they can hardly meet the requirements in the coming 5G networks. Therefore, more flexible waveforms have been proposed to address the unprecedented challenges. In this paper, we will provide comprehensive analysis and comparison for typical waveforms. To offer an insightful analysis, we will not only introduce the basic principles of the waveforms, but also reveal the underlying characteristics. Moreover, a comprehensive comparison in terms of different performance metrics will also be presented in thispaper, which provides an overall understanding of the new waveforms.

Waveform Design for 5G Networks: Analysis and Comparison

Future IoT applications provoke the connectivity and reliability requirements of machine-type communications, which yield mMTC and URLLC, respectively. In mMTC, grant-free NOMA is adopted to serve massive machine-type devices with low signaling overhead. The challenge of grantfree NOMA for mMTC is the collision induced by non-orthogonal resource allocation. Various grantfree NOMA techniques have been proposed to address this challenge, aiming at high connectivity. In this article, we introduce a novel grant-free NOMA approach, namely coded tandem spreading multiple access (CTSMA). The basic idea is that the data packet of each user is divided into segments, and redundancy segments are generated by coding through the data segments, and multiple orthogonal spreading codes are utilized to tandemly spread the data symbols of different segments from one user. Thereby, collision only occurs on the segments using identical spreading codes, which can be resolved by the redundancy segments to achieve reliable access. In addition to the transmitter and receiver designs, a closedform expression of the collision resolution capability is derived in this article to reveal the trade-off among the connectivity, the reliability, and the user data rate when using CTSMA. Theoretical analysis shows that CTSMA achieves both high connectivity and ultra-reliability at the expense of user data rate.

Coded Tandem Spreading Multiple Access for Massive Machine-Type Communications

Internet of things (IoT) and machine-type-communication (MTC) will play an important role in 5G communication systems. Combining the simplicity of Orthogonal Frequency Division Multiplexing (OFDM) with the advantages of Filtered Bank Based Multi-Carrier (FBMC), Universal Filtered Multi-Carrier (UFMC) is robust to frequency offset and timing offset, and supports segmented spectrum communication by filtering a series of successive subcarriers, but its Instantaneous-to-Average Power Ratio (IAPR) is high. The nonlinearity distortion may cause large out-of-band radiation and Block Error Rate (BLER) performance degradation. In this paper, to further reduce IAPR and enable more reliable communication, an IAPR reduction method Discrete Fourier Transform-Spread UFMC (DFT-S-UFMC) is proposed. Simulation results show that UFMC has link level performance gain over DFT-S-UFMC, but IAPR of DFT-S-UFMC is lower than UFMC.

DFT-Spread Universal Filtered Multi-Carrier for 5G

As the next generation cellular system, 5G network is required to provide a large variety of services for different kinds of terminals, from traditional voice and data services over mobile phones to small packet transmission over massive machine-type terminals. Although orthogonal-subcarrier based waveform has been widely used nowadays in many practical systems, it can hardly meet the future requirements in the coming 5G networks. Therefore, more flexible waveforms have been proposed to address the unprecedented challenges. In this article, we will provide comprehensive analysis and comparison for the typical waveform candidates. To obtain insightful analysis, we will not only introduce the basic principles of the waveforms but also reveal the underlying characteristics of each waveform. Moreover, a comprehensive comparison in terms of different performance metrics will be also presented in this article, which provide an overall understanding of the new waveforms.

pdf/waveform-candidates-for-5g-networks-analysis-and-comparison-4fusnyjotq.pdf

Waveform Candidates for 5G Networks: Analysis and Comparison

With the rapid development of high-speed railway (HSR) system, there is an increasing demand on providing high quality multimedia services for HSR passengers. In this paper, we investigate the downlink resource allocation problem for multimedia services delivery in HSR orthogonal frequency division multiple access (OFDMA) system. Taking the statistical delay-QoS requirements into account, we formulate the problem as a mixed integer non-linear programming (MINLP), which aims at maximizing the system throughput over a trip of the train. With the help of integer constraint relaxation, this problem can be simplified into a convex optimization problem. To solve it, a stochastic approximation based dynamic resource allocation algorithm is developed. Finally, the numerical results are presented to show the effectiveness of the proposed resource allocation algorithm.

Delay-Aware Dynamic Resource Allocation in High-Speed Railway Networks

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