energies

Article Concepts on Train-to-Ground Wireless Communication System for Hyperloop: Channel, Network Architecture, and Resource Management

Jiachi Zhang 1,2 , Liu Liu 1,*, Botao Han 1, Zheng Li 1, Tao Zhou 1, Kai Wang 1, Dong Wang 2 and Bo Ai 1,3 1 School of Electronic and Information Engineering, Beijing Jiaotong University, Beijing 100044, China; [email protected] (J.Z.); [email protected] (B.H.); [email protected] (Z.L.); [email protected] (T.Z.); [email protected] (K.W.); [email protected] (B.A.) 2 School of Rail Transportation, Shandong Jiaotong University, Jinan 250357, China; [email protected] 3 State Key Laboratory of Rail Traffic Control and Safety, Beijing Jiaotong University, Beijing 100044, China * Correspondence: [email protected]; Tel.: +86-010-51686315



Received: 8 July 2020; Accepted: 17 August 2020; Published: 20 August 2020


Abstract: Hyperloop is envisioned as a novel transportation way with merits of ultra-high velocity and great traveling comforts. In this paper, we present some concepts on the key technologies dedicated to the train-to-ground communication system based on some prevailing fifth-generation communication (5G) technologies from three aspects: wireless channel, network architecture, and resource management. First, we characterize the wireless channel of the distributed antenna system (DAS) using the propagation-graph channel modelling theory. Simulation reveals that a drastic Doppler shift variation appears when crossing the trackside antenna. Hence, the leaky waveguide system is a promising way to provide a stable receiving signal. In this regard, the radio coverage is briefly estimated. Second, a cloud architecture is utilized to integrate several successive trackside leaky waveguides into a logical cell to reduce the handover frequency. Moreover, based on a many-to-many mapping relationship between distributed units (DUs) and centralized units (CUs), a novel access network architecture is proposed to reduce the inevitable handover cost by using the graph theory. Simulation results show that this scheme can yield a low handover cost. Then, with regards to the ultra-reliable and low latency communication (uRLLC) traffic, a physical resource block (PRB) multiplexing scheme considering the latency requirements of each traffic type is exploited. Simulation presents that this scheme can maximize the throughput of non-critical mission communication services while guaranteeing the requirements of uRLLC traffic. Finally, in terms of the non-critical mission communication services, two cache-based resource management strategies are proposed to boost the throughput and reduce the midhaul link burden by pre-fetching and post-uploading schemes. Simulation demonstrates that the cache-based schemes can boost the throughput dramatically.

Keywords: hyperloop; train-to-ground communication; wireless channel; network architecture; resource management; multiplexing; cache-based strategies

1. Introduction From the development of China’s high-speed railway (HSR), it can be learned that high velocity brings about a number of benefits and merits such as shortening the travel time and helping establish socioeconomically balanced societies [1,2]. However, three inevitable factors primarily restrict the further acceleration, i.e., the mechanical resistance as well as the noise stemmed from the wheels and tracks, and the air resistance derived from air friction. According to reference [3], the actual measurement data demonstrates that the aerodynamic drag accounts for a majority of the resistance

Energies 2020, 13, 4309; doi:10.3390/en13174309 www.mdpi.com/journal/energies Energies 2020, 13, 4309 2 of 21

(over 80%) as the train proceeds at a velocity of 400 km/h, leading to a great waste of power energy obviously. Moreover, the generated mechanical noise will rise abruptly along with the acceleration (about seventh-order or eighth-order of speed) and become unbearable for passengers. Given above constraints, the maximum velocity of current HSR cannot exceed 600 km/h. Hyperloop is a novel designed transportation method, which refers to a maglev train (vactrain) moving forward at near or supersonic speed (1000–4000 km/h) inside a vacuum metal tube [4]. The closed transportation scenario together with maglev technology can yield a new way of transportation with the merits of low mechanical friction, low air resistance, and low noise mode regardless of all weather conditions, not to mention the great conveniences to customers and quality promotion of rail services due to the ultra-high-velocity. As such, maintaining a dependable and reliable communication link is a pivotal issue to guarantee the safe operation of the Hyperloop and provide real-time multimedia information. Rekindled by a white paper entitled “Hyperloop Alpha” published by Elon Musk in 2013 [5], the Hyperloop receives extensive attention both from academia and industry. In 2016, an American vactrain company, i.e., Hyperloop One, published the first study of the Hyperloop system, referring to a 28-min link connecting Helsinki and Stockholm [6]. Then, it first tested a full-scale Hyperloop with power propulsion system, braking system, etc., by using a test line in the northern desert of Las Vegas in 2017 [7]. In 2018, Hyperloop Transportation Technologies (HTT) company declared plans to establish Hyperloop lines in Toulouse, France, and Tongren, Guizhou, China [8]. In 2019, China underlined a series of key tasks for building national strength in transportation in a published official document, among which points out that efforts will also be made to develop the technologies of the low vacuum tube (tunnel) high-speed train [9]. Recently, TÜV SÜD published the first completed certification safety guidelines for Hyperloop applications which define the safety-related requirements relevant to this type of technology in 2020 [10]. The guidelines, which detail over 125 system requirements, are like a milestone on the road to maturity and commercialization of Hyperloop. As a new-born mode of transportation, the research on train-to-ground communication technology of the Hyperloop almost stays at its infancy stage currently. The primary communication challenge is the fast-fading channel along with the frequent handover effects caused by the ultra-high-speed. However, existing railway-dedicated communication including the widespread commercial used Global System for Mobile Communications-Railway (GSM-R) and Long Term Evolution-Railway (LTE-R) can only support a top speed of 500 km/h [11], which is far below the velocity of the Hyperloop. Currently, some military communication systems such as the common data link (CDL) can support a mobile station (MS) with a speed more than Mach 5 (over 6000 km/h) when providing a data rate of 200 kbps for uplink and 234 Mbps for downlink [12]. However, it is economically expensive and nation-security-endangered to deploy such a military communication system. Moreover, the deployment environment of CDL usually refers to a line-of-sight (LOS) scenario without any reflected radio waves, whereas the closed propagation environment will lead to a severe multipath effect. To our best knowledge, little research on the communication issue has been conducted, among which our team has performed a series of systematic works. In [13], Qiu summarized the challenges as well as communication quality-of-service (QoS) quantitatively and proposed a leaky waveguide solution for wireless coverage. In [14], we proposed two novel structures to reduce the Doppler effect using the distributed antenna system (DAS) method. In [15], Han performed a series of work on the wireless channel analysis, including Doppler shift, power delay spectrum, and multi-input and multi-output (MIMO) capacity, in the vacuum tube scenarios based on the propagation-graph channel modelling theory. However, [14] lacks a channel modelling-based simulation result, and [15] neglects the large-scale characterization, especially in terms of the path loss. As such, we adopt the two-slope path loss model to depict its attenuation trend in this paper. Moreover, the radio coverage analysis of both DAS and leaky waveguide system (LWS) is performed and the corresponding handover judgment conditions are presented herein. Energies 2020, 13, 4309 3 of 21

In terms of the network architecture, literature [16] and [17] proposed a novel baseband unit-to-remote radio unit (BBU-RRU) mapping relationship in the criss-cross railway lines for HSRs and presented an optimal baseband units (BBUs) pool selection algorithm. This scheme dedicated to the Hyperloop leads to a high equipment expenditure. Thereby, we apply this scheme in the 5G network architecture for the Hyperloop. Moreover, the trackside CUs can also provide services for public mobile cellular communication in the vicinity, indicating a full use of resources when no vactrain runs inside this cell. As for the physical resources management (mainly referring to the physical resource of time and frequency domain), the ultra-reliable and low latency communication (uRLLC) in 5G can satisfy the demands of those latency-sensitive communication services by using the flexible numerology [18]. Therefore, the multiplexing of uRLLC and enhanced mobile broadband (eMBB) traffic attracts extensive attention [19–21]. Different from the conventional multiplexing idea, i.e., puncturing the eMBB physical resource block (PRB) in the next mini-slot of the arriving time, we consider the unique required latency of each uRLLC traffic and propose a flexible multiplexing scheme. In our proposal, each uRLLC traffic is assigned to a PRB position within its maximum tolerable latency. A heuristic algorithm is employed to solve the optimization problem considering maximizing the eMBB throughput and minimizing the uRLLC power. With regards to the passengers Internet resource management, the cache-based algorithm can yield not only a high throughput but also a low transmission latency [22,23]. Herein, we employ the cache in trackside active antenna unit (AAU) and formulate two resource management schemes by using the predictable vactrain trajectory, which can result in a higher throughput performance than the conventional cache-based scheme. Although the cache-based prefetching idea for the HSRs has been explained in our previous work [24], we formulate another resource management scheme dedicated to the video surveillance data, i.e., post-uploading scheme. These works provide some insights into the design of the communication system, but also with some limitations, e.g., focusing only on parts of the communication procedure. As such, we aim to propose a wireless communication solution that can provide a reliable connection train-to-ground link from several important communication procedures. Moreover, it should be compatible with existing public mobile cellular communication systems not only to reduce the equipment expenditure but also to use mature and low-cost off-the-shelf technologies based on the previous work. In this paper, we propose a train-to-ground communication system solution dedicated to the Hyperloop, which mainly involves wireless channel, network architecture, multiplexing scheme of the physical resources, and the on-board passengers’ Internet resources management. Our contributions are listed as follows:

(1) In terms of the wireless access method, the challenges of the DAS method are analyzed from the aspect of Doppler effect. Simulation reveals that a severe Doppler shift appears when crossing the trackside antenna. Thereby, the LWS is a better option since it can yield a stable frequency shift compared to DAS. Then, we discuss the radio coverage based on the DAS and LWS. (2) To reduce the handover frequency stemmed from the ultra-high speed, a cloud architecture is used to integrate several successive roadside leaky waveguides into a logical cell. Then, a novel access network architecture dedicated to the Hyperloop is proposed based on a many-to-many mapping relationship between DUs and CUs. The graph theory is adopted to alleviate the inevitable handover cost when crossing different cells and the corresponding simulation result is provided. (3) As for the coexisting mission-critical and non-critical services, we propose a PRB multiplexing scheme dedicated to the Hyperloop by considering the tolerable latency margin of each type of traffic. Simulation shows this scheme can satisfy the low-latency demand and achieve a large network throughput in the physical layer. (4) To boost the on-board users’ quality of experience (QoE) as well as alleviate the midhaul link burden, two cache-based resource management strategies, i.e., the pre-fetching and post-uploading Energies 2020, 13, 4309 4 of 21

schemes are investigated to cope with such non-critical mission services. Simulation presents that this cache-based scheme can yield an enhancement in throughput.

The remaining part of this paper proceeds as follows: Section2 presents the train-to-ground communication challenges and analyzes the communication service types in brief. Then, the possibility of two wireless access methods (the DAS and LWS) are assessed based on some simulation results in Section3. In Section4, a novel wireless access network architecture is illustrated in detail. As for the mission-critical traffic, the multiplexing approach is used in the physical layer and presented in Section5. Moreover, two cache-based resource management strategies mainly served for the non-critical services are illustrated based on our proposed network architecture in Section6. Finally, the conclusions are summarized in Section7. Some abbreviations and the corresponding definitions are listed in Table1 for convenience.

Table 1. Abbreviations and definitions.

Term Definition Term Definition HSR High-Speed Railway uRLLC ultra-Reliable Low Latency Communication Global System for Mobile GSM-R eMBB enhanced Mobile Broadband Communications-Railway LTE-R Long Term Evolution-Railway 5G The 5th generation Communication Based Train Control LOS Line-of-Sight CBTC System CU Centralized Unit SIL Safety Integrity Level DU Distributed Unit WDM Wavelength Division Multiplexing AAU Active Antenna Unit MIMO Multi-Input and Multi-Output DAS Distributed Antenna System CIR Channel Impulse Response LWS Leaky Waveguide System PL Path Loss LCX Leaky Coaxial Cable C-RAN Cloud Radio Access Network QoS Quality of Service NFV Network Functions Virtualization QoE Quality of Experience HARQ Hybrid Automatic Repeat reQuest KPI Key Performance Indicator MEC Mobile Edge Computing BER Bit Error Rate PSO Particle Swarm Optimization PRB Physical Resource Block QAM Quadrature Amplitude Modulation BBU Baseband Unit RRU Remote Radio Unit

2. Communication Challenges and Service Types

2.1. Communication Challenges The communication demands of the Hyperloop differ from the existing rail transportation dramatically, bringing about some new challenges on the train-to-ground communication system. (1) Special channel characterization. If adopting the public cellular network solution, the Doppler shift varies from the maximum to the minimum value rapidly as the Hyperloop travels across the trackside antenna at an ultra-high-speed, resulting in the drastic variation of the received signal [25]. Not to mention the severe penetration attenuation derived from the steel tube and the metal train body. (2) Except for the severe Doppler effect, another challenge caused by the high velocity is the frequent handover. Furthermore, all passengers insides the vactrain body move simultaneously, which leads to a phenomenon of signaling storm when dealing with a group handover. (3) Various types of communication services with strict demands. Different from rail transportation, the Hyperloop needs more types of mission-critical services with stringent requirements to guarantee the safe operation at such an ultra-high-speed. In addition, the passengers expect to enjoy in-journey communication services of good QoS, which requires a sufficient data rate. As such, the coexistence of both mission-critical and non-critical traffic calls for a novel resource allocation algorithm. Energies 2020, 13, 4309 5 of 21

Based on the above analysis, it can be learned that current mobile communication systems such as the GSM-R and LTE-R are incapable of dealing with train-to-ground communication challenges of the Hyperloop. Therefore, we put up with some schemes to investigate on this issue, which will be illustrated in the following sections.

2.2. Demands and Requirements of Communication Services The primary task of the communication system research and design is to identify every service type and quantify the corresponding requirements. As a new mode of transportation, the train-to-ground communication of Hyperloop requires a variety of communication services to assure the safe operation. In [13], we have already presented a summary of accurate communication key performance indicators (KPIs), including data rate, end-to-end transmission latency, and bit error rate (BER), which is presented in Table2.

Table 2. Key KPIs of train-to-ground communication services [13].

Data Data Rate/kbps Latency Requirement/ms BER Type Mission-critical services CBTC HSR Maglev Hyperloop CBTC HSR Maglev Hyperloop CBTC HSR Maglev Hyperloop TCS 5 1 10 6 10 6 10 6 10 6 200/train 1000/train 100 50 − − − − 6 6 6 6 OCS 40 40 10− 10− 10− 10− 2 3 5 5 OVCS 32/channel 100 100 40 40 10− 10− 10− 10− 3 6 3 3 TOSM 100 200 200 1000 300 150 300 300 10− 10− 10− 10− 3 3 3 3 VS 6000 4000 4000 18000 300 150 300 300 10− 10− 10− 10− UL:100 UL:100 UL:100 UL:100 PIS 300 300 300 300 10 6 10 3 10 6 10 6 DL:8000 DL:1000 DL:1000 DL:8000 − − − − Passenger multimedia services - 0.378—3.78 Gbps - -

Generally, the communication service types of the Hyperloop can be divided into two categories: mission-critical services and passenger communication service data from the aspects of the intended applications. The former one is used to ensure the security of train safe-operation and usually refers to the communication served for operation control system (OCS), traction control system (TCS), operational voice communication system (OVCS), video surveillance (VS), train operation status monitoring (TOSM), and passenger information service (PIS). Different from the HSR, the Hyperloop puts up with more stringent requirements on data rate, end-to-end latency, and BER, which can be regarded as the uRLLC requirements in the 5G communication system [26]. As for the passenger service, it mainly involves the service data such as on-vehicle video conference, online games, chatting, live broadcast, etc., generated by passengers, but having little effect on the safe-operation. We mainly consider the high-speed Internet access service with a large bandwidth requirement. Assume a per-passenger requires a data rate of 0.1–1 Gbps for 5G access, mobile user penetration rate of 90%, 5G terminal penetration rate of 80%, an activation rate of 70%, and a use rate of Internet access service to be 50%. Hyperloop has a total passenger capacity of 15. It follows that the total passenger throughput of the train is ((0.1, 1) 70% 50%) (15 90% 80%) = 0.378–3.78 Gbps, × × × × × which far exceeds the capability of the existing public mobile cellular system [27]. This type of service can be considered as the eMBB communication in 5G. Based on above analysis, we list some important train-to-ground wireless communication KPIs for communication based train control system (CBTC), HSR, Shanghai maglev, and Hyperloop in Table2[ 28,29]. Since detailed descriptions of each service along with the explanations about the KPI values are given in our previous work [13], we just present a brief overview herein. Energies 2020, 13, 4309 6 of 21 Energies 2020, 13, x FOR PEER REVIEW 6 of 22

VS, andThe PIS). mission-critical Similar to the communication HSR, all safety-related services accountcommunication for the safeservices operation of Hyperloop of the Hyperloop. should be Furthermore,categorized as they the SIL4, can be the divided highest into safety two level types that from means an aspect a catastrophic of the action impact objection: once it happens those related [27]. to theWith safety regards of the trainto the itself passenger (TCS and multimedia OCS data) serv andices public (PMS), safety the ones data (including rate, latency, OVCS, and TOSM, BER VS,requirements and PIS). Similarare not to theas HSR,stringent all safety-related as that of communicationthe mission-critical services services. of Hyperloop In addition, should the be categorizedcommunication as the demands SIL4, the vary highest among safety different level that pass meansengers a catastrophic even for the impact PMS. onceFor itexample, happens most [27]. InternetWith applications regards to theof the passenger business multimedia users usually services involve (PMS), web thebrowsing data rate, and latency,emailing, and which BER requirementsrequires a less are real-time not as stringent process asand that a small of the thro mission-criticalughput. The services. entertainment In addition, users the generally communication involve demandsInternet applications vary among requiring differentpassengers a large bandwidth even for theor PMS.real-time For example,process, e.g., most online Internet games, applications video, ofchatting, the business etc. users usually involve web browsing and emailing, which requires a less real-time process and a small throughput. The entertainment users generally involve Internet applications requiring3. Wireless a largeChannel bandwidth Analysis or real-time process, e.g., online games, video, chatting, etc. Hyperloop communication applications have strict requirements for QoS metrics, such as data 3. Wireless Channel Analysis rate, transmission delay, and BER. Due to these factors as well as a desire to use mature and low-cost technology,Hyperloop we communicationuse off-the-shelfapplications technologies have and strictadd requirementsapplications to for meet QoS specific metrics, services such as dataand rate,demands. transmission delay, and BER. Due to these factors as well as a desire to use mature and low-costExact technology, and detailed we usebroadband off-the-shelf wireless technologies channel and characterization add applications is toa meetprerequisite specific servicesfor the anddeployment demands. and performance analysis of the Hyperloop wireless communication system. Moreover, it providesExact and an detailed effective broadband evaluation wireless of further channel advanced characterization transmission is a prerequisite technologies for the and deployment resource andmanagement. performance analysis of the Hyperloop wireless communication system. Moreover, it provides an effective evaluation of further advanced transmission technologies and resource management. 3.1. Distributed Antenna System 3.1. Distributed Antenna System The DAS is an explicit way with the merits of easy to deploy and low cost of equipment. As shownThe in DAS Figure is an 1, explicitthe Hyperloop way with is theequipped merits ofwith easy MIMO to deploy antenna and low array cost for of wireless equipment. connection As shown to inthe Figure ground1, theAAUs. Hyperloop The on-board is equipped terminals with MIMO connect antenna with arraythe inside-train for wireless antennas connection via to WiFi the groundtechnology, AAUs. these The antennas on-board converge terminals all connect terminals with signal the inside-trainand forward antennas them to via the WiFi outside technology, MIMO theseantennas antennas via cable converge or vice all versa. terminals This signal two-hop and relay forward structure them toavoids the outside a radio MIMO wave antennas penetration via cableattenuation or vice of versa. the train This two-hopbody, guaranteeing relay structure a relatively avoids a radiostable wave received penetration amplitude. attenuation As such, of the trainwireless body, channel guaranteeing between a relatively ground stableantennas received and tr amplitude.ain antennas As such,is crucial the wireless to the channelcommunication between groundsystem performance. antennas and train antennas is crucial to the communication system performance.

AAU(active antenna unit) steel tube Rx(receiver) Tx(transmitter)

Figure 1. The sketch of the dist distributedributed antenna system.

In [15], we investigated the wireless channel characterization of this access scheme based on In [15], we investigated the wireless channel characterization of this access scheme based on the thepropagation-graph propagation-graph channel channel modeling modeling method method [30] [30 an] andd obtained obtained some some interesting interesting results. results. In In the simulation, the transmitter (Tx) is installed at the inne innerr side of the tube and the receiver (Rx) is set at the toptop ofof the the train train body. body. To To be compatiblebe compatible with with existing existing public public mobile mobile cellular cellular communication communication system, wesystem, consider we consider the 5G communication the 5G communication standard standa and setrd the and bandwidth set the bandwidth to 100 MHz to and100 MHz carrier and frequency carrier tofrequency 4.85 GHz. to The4.85 shapeGHz. The and shape size of and the size tube of is the set tube according is set according to [5]. The to Hyperloop [5]. The Hyperloop proceeds at proceeds a speed ofat a 1000 speed km of/h 1000 traveling km/h across traveling the groundacross the antenna ground (Tx) antenna located (Tx) at thelocated middle at the of themiddle tube. of the tube. Figure2 2aa plotsplots thethe emulatedemulated normalizednormalized channel impulse responses (CIRs) at different di fferent positions. positions. The strongest received power appears when the Rx arrivesarrives at the middle of the tube, i.e., the shortest distance between Tx and Rx. In In addition, addition, the the number number of of the the effective effective taps increases as the Hyperloop approaches the the ground ground antenna. antenna. The The Doppler Doppler effect effect reflects reflects the therelative relative motion motion between between Tx, Rx, Tx, and Rx, andsurroundings, surroundings, which which is extremely is extremely significant significant to tothe the channel channel characterization characterization for for the the Hyperloop. Hyperloop. Figure 2b shows the Doppler power spectrum considering the LOS, single bounce, and double

EnergiesEnergies 2020,2020 13, x, 13 FOR, 4309 PEER REVIEW 7 of 217 of 22 bounce components. Notably, the shape of the Doppler spectrum presents a feature of central Figure2b shows the Doppler power spectrum considering the LOS, single bounce, and double bounce symmetrycomponents. with the Notably, symmetry the shapepoint oflies the at Dopplerthe position spectrum of ground presents Tx. a The feature main of variation central symmetry tendency of the LOSwith component the symmetry is like point as lies a Z at shape the position with a ofturnin groundg angle Tx. The close main to 90°, variation whereas tendency the tendency of the LOS of the multipathcomponent component is like as is a Zlike shape a withS shape. a turning In additi angle closeon, their to 90◦ ,symmetry whereas the points tendency vary of thealong multipath with the vactrain’scomponent position. is like In a Ssummary, shape. In addition, the severe their symmetryDoppler pointseffect varyusing along the with DAS the can vactrain’s lead to position. a drastic amplitudeIn summary, variation the severeof the Dopplerreceived e ffsignal.ect using the DAS can lead to a drastic amplitude variation of the received signal.

(a) (b)

FigureFigure 2. Wireless 2. Wireless channel channel analysis analysis results results of of the the DAS: DAS: (a)) thethe normalizednormalized CIR; CIR; (b )(b the) the Doppler Doppler power power spectrumsspectrums at different at different positions. positions. 3.2. Leaky Waveguide System 3.2. Leaky Waveguide System Based on the analysis of the DAS results in Section 3.1, the severe Doppler effect will cause aBased dramatic on the fast analysis fading e ffofect the of DAS the received results signalsin Section especially 3.1, the in severe the vicinity Doppler of ground effect will antenna. cause a dramaticConsequently, fast fading it is noteffect suitable of the to bereceived implemented signal fors theespecially Hyperloop. in Thethe LWSvicinity and leakyof ground coaxial cableantenna. Consequently,(LCX) are widely it is not used suitable to provide to stablebe implem and suentedfficient for coverage the Hyperloop. in the confined The space LWS environment and leaky [31coaxial]. cableHowever, (LCX) are LCX widely cannot supportused to the provide communication stable atan ad high sufficient frequency coverage carrier [32 ].in Asthe such, confined we utilize space environmentthe LWS to[31]. provide However, effective LCX radio cannot coverage support for the the Hyperloop. communication Figure3 ata depictsa high frequency the access network carrier [32]. As such,architecture we utilize based the on LWS the to LWS. provide The LWS effective is connected radio coverage to the road for side the AAUs Hyperloop. via the cableFigure to 3a avoid depicts the accessthe penetration network architecture attenuation stemmed based on from the theLWS. steel The tube. LWS A roadsideis connected AAU to accounts the road for side the AAUs signal via processing of several leaky waveguides. It is worth noting that the conventional leaky waveguide emits the cable to avoid the penetration attenuation stemmed from the steel tube. A roadside AAU accounts a signal of which the equiphase surface direction is not perpendicular to train movement direction as for the signal processing of several leaky waveguides. It is worth noting that the conventional leaky shown in Figure3b. This phenomenon is caused by the fact that the currently used leaky rectangular waveguidewaveguide emits uses a fastsignal waves of aswhich the fundamental the equiphase mode surface to generate direction radiation. is not The perpendicular Doppler frequency to train movementshift of direction the LOS component as shown canin Figure be expressed 3b. This as phfd =enomenonv0 cos θ/λ is, where causedv0 byis the the velocity fact that and theλ currentlyis the usedwavelength. leaky rectangular Obviously, waveguide the fd keeps uses a constant fast waves as long as asthev0 fundamentaldoes not change. mode Then, to some generate off-the-shelf radiation. frequency compensation algorithms can be used to handle this situation directly.= Note thatθλ the AAU The Doppler frequency shift of the LOS component can be expressed as fvd 0 cos / , where connect to all leaky waveguides at the same side (left/right), otherwise, the Doppler shift will change v is the velocity and λ is the wavelength. Obviously, the f keeps a constant as long as v does 0 from f to f . d 0 d − d not change. Then, some off-the-shelf frequency compensation algorithms can be used to handle this situation directly. Note that the AAU connect to all leaky waveguides at the same side (left/right), − otherwise, the Doppler shift will change from fd to fd .

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Roadside AAU Cable steel tube Leaky waveguide system Rx

(a)

k0 k0 θ β 0

y x …… z β (β < k ) 0 0 0 (b)

FigureFigure 3. 3.The The leaky leaky waveguide waveguide system: system: (a )(a a) schema;a schema; (b ()b the) the radio radio propagation propagation direction. direction.

3.3.3.3. Radio Radio Covereage Covereage Analysis Analysis ToTo cope cope with with the the handover handover when when employing employing either either the DASthe DAS or the or LWS, the LWS, it is essential it is essential to analyze to analyze the efftheective effective radiocoverage. radio coverage. As for the As LWS, for the the radiatedLWS, the power radiated stemmed power from stemmed a slot can from be expresseda slot can as be

expressed as βz P = P0e− , (1) = −β z PPe0 , (1) where P0 is the transmit power, z means the distance from starting position of the leaky waveguide to thewhere slot position. P0 is theβ is transmit an attenuation power, constant z means with the a low distance value [ 33from]. In starting terms of position the DAS, of numerous the leaky references (e.g., [34]) state that the received power curve is divided into two regions, typically referred waveguide to the slot position. β is an attenuation constant with a low value [33]. In terms of the to as near and far region, i.e., the two-slope channel model. Two-slope channel models are typical DAS, numerous references (e.g., [34]) state that the received power curve is divided into two regions, representative of numerous empirical models based on measurements of the received signal strength, typically referred to as near and far region, i.e., the two-slope channel model. Two-slope channel which is expressed as models are typical representative of( numerousβz empirical models based on measurements of the P0e− , 0 z D received signal strength, whichP is= expressed as ≤ ≤ . (2) P e αND e αF(z D) , z > D 0 − · − − P ezD−β z ,0≤≤ In the near region, the PL slope αN=is steep,0 which is usually modeled as free space path loss. P  −α −−α . (2) N D ⋅>F ()zD In the far region, the waveguide effect appearsP0ee with few lower, order zD modes and the path loss slope is reduced significantly, denoted as αF. The break point, which is a point of transition from near to far 2 α region,In can the be near estimated region, by theD PL= aslope/λ, whereN isa steep,is the maximumwhich is usually transverse modeled dimension as free of space the tube path and loss. λ isIn the the signal far region, wavelength the waveguide in free space effect [34 appears]. with few lower order modes and the path loss slope Based on Equations (1) and (2), Figureα 4 plots the demonstration of the handover process of both is reduced significantly, denoted as F . The break point, which is a point of transition from near to LWS and DAS between two adjacent cells.2 De f f L and De f f D mean the effective coverage of the LWS far region, can be estimated by Da= / λ ,− where a is− the maximum transverse dimension of the and DAS, respectively. The handover will occur once it satisfies the A3 judgment condition, which is usuallytube and adopted λ is the in signal LTE-R wavelength [35]. in free space [34]. Based on Equations (1) and (2), Figure 4 plots the demonstration of the handover process of both Mn + O fn + Ocn HysD> Ms + O fDs + Ocs + O f f , (3) LWS and DAS between two adjacent cells.− eff− L and eff− D mean the effective coverage of the whereLWSM ands/M DAS,n is the respectively. signal quality The of handover source/target will occur AAU, onceO fs/ Oit fsatisfiesn is the specificthe A3 frequencyjudgment ocondition,ffset of sourcewhich/target is usually AAU, adoptedOcs/Oc ninis LTE-R the specific [35]. offset of source/target AAU. O f f is the offset parameter, ++− >+++ M nnOf Oc n Hys M ss Of Oc s Off , (3)

Energies 2020, 13, x FOR PEER REVIEW 9 of 22

where M s / M n is the signal quality of source/target AAU, Ofs / Ofn is the specific frequency offset

of source/target AAU, Ocs / Ocn is the specific offset of source/target AAU. Off is the offset

parameter, and Hys is the hysteresis parameter. Obviously, M s and Mn are determined by the Energies 2020, 13, 4309 9 of 21 β <<αα = 2 λ D D PL. Given that FN and Da/ , the eff− L and eff− D can be expressed as based on Equation (3), and Hys is the hysteresis parameter. Obviously, Ms and Mn are determined by the PL. Given that 2 −−β −−β D = a ()zzL D D ()zzL β < αF < αN and /λ, the0 ++−e f f L and e f f D can > be expressed1 +++ as based on Equation (3), P00e Of− nn Oc− Hys P e Of ss Oc Off , (4)

β(z zL ) β(zL z) P0e− − 0 + O fn + Ocn Hys > P0e− 1 − + O fs + Ocs + O f f , (4) −−β()zz − −−β()zz PeL0 ++− Of Oc Hys > P eL1 +++ Of Oc Off (5) β00(z zL ) nnβ(zL z) ss . P e− − 0 + O fn + Ocn Hys > P e− 1 − + O fs + Ocs + O f f . (5) 0 − 0

Distributed antenna system Leaky waveguide system Source cell Target cell handover β hysteresis

αN Deff-L

Power [dBm] Power αF DD Deff-D

z z z L0 zD0 breaking points D1 L1 distance z

Figure 4. The handover process based on the DAS and LWS. Figure 4. The handover process based on the DAS and LWS. Based on Equations (4) and (5), the radio coverage of the DAS and LWS between two adjacent cells Based on Equations (4) and (5), the radio coverage of the DAS and LWS between two adjacent when processing the handover can be calculated. Obviously, De f f L is usually larger than De f f D since − − thecells received when power processing slope the of the handover DAS is sharpercan be thancalculated. that of Obviously, the LWS. Considering Deff− L is usually that the larger effective than radioD coverage of a single AAUs is a constant, one trackside AAU connects to more antennas than thateff of− theD since leaky the waveguides received power consequently. slope of the In addition,DAS is sharper the handover than that failure of the probability LWS. Considering of DAS isthat higherthe effective than that radio of the coverage LWS. On of one a single side, itAAUs can be is explained a constant, by one the trackside low received AAU signal connects strength to more of theantennas DAS, on thethan other that hand, of the this leaky phenomenon waveguides derives consequently. from the drastic In addition, signal variation the handover caused by failure the time-varyingprobability Dopplerof DAS is shift. higher In conclusion,than that of thethe LWSLWS. is On a better one side, wireless it can access be explained method by option the low compared received tosignal the DAS, strength except of for the the DAS, high expenditureon the other cost. hand, this phenomenon derives from the drastic signal variation caused by the time-varying Doppler shift. In conclusion, the LWS is a better wireless access 4. Network Architecture method option compared to the DAS, except for the high expenditure cost. The other primary challenging issue stemmed from the ultra-high-speed is the extremely frequent handover,4. Network which Architecture is an integral part of every mobile communication system. For instance, the maximum cell rangeThe in other LTE-R primary is 12 km, challenging the residence issue time stemmed inside a cellfrom is aboutthe ultra-high-speed 43.2 s at a velocity is ofthe 1000 extremely km/h. Currently,frequent numeroushandover, in-depth which is research an integral has beenpart of conducted every mobile to alleviate communication the performance system. degradation For instance, causedthe maximum by the handover cell range from in LTE-R a variety is 12 of km, aspects the residence such as computingtime inside powera cell is or about handover 43.2 s at protocols. a velocity Forof example,1000 km/h. in [36 Currently,], Zhao proposed numerous a dual-link in-depth soft research handover has scheme been forconducted C/U plane to split alleviate network the inperformance HSR by deploying degradation a train caused relay by station the handover and two fr antennasom a variety on theof aspects train. Insuch [37 as], computing Song proposed power a handoveror handover trigger protocols. decision For schemeexample, by in using [36], Zhao the grey proposed system a dual-link theory to soft predict handover the received scheme signal for C/U quality.plane Thesesplit network works can in HSR cope by with depl theoying handover a train issue relay effi stationciently; and however, two antennas it needs on the the assistance train. In [37], of multi-linksSong proposed to achieve a handover a high-reliable trigger handover.decision scheme Hence, by it isusing hard the to implement grey system these theory schemes to predict for the the Hyperloopreceived signal sincethe quality. leaky These waveguide works system can cope emits with a signalthe handover that can issue hardly efficiently; affect the however, nearby cells. it needs theNetwork assistance architecture of multi-links refers to achieve to the way a high-relia networkble devices handover. and services Hence, areit is structuredhard to implement to serve thethese connectivityschemes for needs the Hyperloop of client devices. since the A good leaky network waveguide architecture system canemits provide a signal su ffithatcient can communication hardly affect the QoSnearby at a low cells. equipment cost and supports flexible configuration for diverse demands. Moreover, it can reduce the inevitable handover cost if possible. However, it is economically expensive to establish a new dedicated access network for the Hyperloop. Therefore, we aim to establish a network architecture based on the 5G architecture not only to provide the communication services for the Hyperloop, but also can be compatible with roadside public cellular networks. Energies 2020, 13, x FOR PEER REVIEW 11 of 22 Energies 2020, 13, 4309 10 of 21

4.1. Centralized Access Architecture 4.1. CentralizedCloud radio Access access Architecture network (C-RAN) is considered as a promising solution to provide high communicationCloud radio services access networkin the high-speed (C-RAN) mobile is considered scenario as as a promisingit is a clean, solution centralized to provide processing, high collaborativecommunication radio, services and real-time in the high-speed cloud computing mobile scenarioinfrastructure as it is wireless a clean, access centralized architecture processing, [38]. Ascollaborative such, we utilize radio, andthe real-timeC-RAN architecture cloud computing for the infrastructure Hyperloop communication wireless access based architecture on the [385G]. Asnetwork. such, we utilize the C-RAN architecture for the Hyperloop communication based on the 5G network. Generally, the C-RAN in 5G system consists of three functional entities, i.e., trackside ground AAUs, centralized centralized units units (CUs), (CUs), and and distributed distributed unit unitss (DUs) (DUs) [39]. [39 The]. TheCU mainly CU mainly processes processes the non- the real-timenon-real-time wireless wireless high-level high-level protocol, protocol, i.e., radio i.e., resource radio resource manageme managementnt and dual and connection. dual connection. CUs can beCUs deployed can be deployed on a general on a hardware general hardware platform platform together togetherwith mobile with edge mobile computing. edge computing. A DU mainly A DU processesmainly processes physical physical layer functions layer functions and the and real-tim the real-timee hybrid hybrid automatic automatic repeat repeat request request (HARQ) (HARQ) flow throughflow through a dedicated a dedicated equipment equipment platform platform or a genera or a generall + dedicated+ dedicated hybrid plat hybridform. platform. The AAU The contains AAU partscontains of physical parts of physicallayer function layer function and all andradio all freque radioncy frequency (RF) functionality, (RF) functionality, such as such the as transmit the transmit and receiveand receive functions, functions, filtering, filtering, and and amplification. amplification. With With the the merits merits of of easy-to-install, easy-to-install, the the compact compact AAUs are distributed along the line evenly to realize seamless wireless coverage. Considering the stringent uRLLC servicesservices demands, demands, the the roadside roadside AAUs AAUs and and DUs DUs are integrated are integrated into one into entity. one Theentity. CUs The together CUs withtogether mobile with edge mobile computing edge computing (MEC) platform (MEC) platfo and partsrm and of parts the core of the network core network functions functions are located are locatednear the near Hyperloop the Hyperloop lines, which lines, iswhich called is the called edge the cloud. edge Networkcloud. Network functions functions virtualization virtualization (NFV) (NFV)technology technology is used is for used the edgefor the cloud edge by cloud decoupling by decoupling the network the functionsnetwork functions from proprietary from proprietary hardware hardwareappliances appliances [40]. Hence, [40]. the Hence, edge cloudthe edge runs cloud in software runs in software on standardized on standardized computer computer servers. Usually,servers. Usually,a handover a handover involves the involves participation the participation of the core network. of the core By migrating network. parts By migrating of core network parts functionof core networkto the edge function cloud, to it the can edge reduce cloud, the latencyit can reduce evidently, the latency which caterseviden totly, the which demands caters of to uRLLC the demands traffic. Theof uRLLC AAUs traffic. connect The with AAUs the DUs connect via the with midhaul, the DUs whereas via the the midhaul, DUs are whereas connected the to DUs the CUsare connected via the F1 tointerface, the CUs i.e., via the the midhaul. F1 interface, i.e., the midhaul. Figure 55 presentspresents aa diagramdiagram ofof thethe C-RANC-RAN forfor thethe Hyperloop. Hyperloop. TheThe nearbynearby AAUsAAUs connectconnect withwith the LWS viavia thethe cablecable andand forwardforward datadata toto thethe edgeedge cloudcloud (or(or vicevice versa).versa). The on-board terminals connect with the antennas embedded inside the trai trainn via WiFi technology, these antennas converge all terminals signal and forward them to the outside multi-input and multi-output (MIMO) antennas via cable or vice versa. This tw two-hopo-hop relay structure avoids a radioradio wave penetration attenuation of the vactrain body, guaranteeingguaranteeing aa relativelyrelatively stablestable receivedreceived amplitude.amplitude. The edge cloud can obtain a timely and accurate information of the train position based on the monitoring and detecting devices distributed along the tube line. As such, it can en enableable the approaching cell and disenable the passing one as the train proceeds forward. This is the basic idea of the moving cell scheme, which integrates several adjacentadjacent AAUs AAUs connecting connecting to theto the same same edge edge cloud cloud into ainto logical a logical cell to achievecell to aachieve free-handover a free- handovereffect [41]. effect [41].

Core Network

CU MEC Edge cloud

F1 interface

AAU+DU Cable AAU+DU AAU+DU steel tube Leaky waveguide system

Current cell Next cell Figure 5. TheThe diagram of the C-RAN architecture for Hyperloop.

Despite the fact that this this architecture architecture can can cope cope wi withth the the frequent frequent handover handover efficiently, efficiently, this benefit benefit will disappear as thethe HyperloopHyperloop travelstravels acrossacross twotwo edgeedge clouds.clouds. This challenging issue will be addressed in the following section.

EnergiesEnergies 20202020,, 1133,, x 4309 FOR PEER REVIEW 1111 of of 21 21

4.2. A Novel Network Architecture 4.2. A Novel Network Architecture To handle the inevitable handover during a long Hyperloop line, some improvements are proposedTo handle to boost the inevitablethe communication handover during performance a long Hyperloop therein. As line, traveling some improvements across two adjacent are proposed edge cloudsto boost during the communication a long-distance performance Hyperloop therein.line, the Asinevi travelingtable cloud across-to- twocloud adjacent handover edge yields clouds a certain during resourcea long-distance migration Hyperloop cost. Hence, line, we the aim inevitable to reduce cloud-to-cloud the cost as much handover as possible yields by extending a certain resourcethe CU– DUmigration mapping cost. relationship Hence, we to aim a more to reduce flexible the one. cost Generally, as much as one possible edge cloud by extending connects the to CU–DU several roadsidemapping AAU relationships + DUs tothr aough more the flexible F1 interface one. Generally, [39], whereas one edgea single cloud AA connectsU + DU can to several only connect roadside to oneAAUs edge+ DUs cloud, through i.e., one the-to F1-many interface mapping [39], whereas relationship a single [16] AAU. Given+ DU the can fact only that connect it is economically to one edge expensivecloud, i.e., to one-to-many establish a dedicated mapping relationshipnetwork to the [16 ].Hyperloop, Given the the fact communication that it is economically system expensivedeployed alongto establish the Hyperloop a dedicated line network should also to the provide Hyperloop, communication the communication services to system the nearby deployed public along cellular the systemHyperloop or the line nearby should other also railway provide lines communication to make full servicesuse of the to equipment the nearby resources. public cellular In this system regard, or wethe upgrade nearby other the star railway architecture lines to maketo a flexi fullble use mesh of the architecture. equipment resources.In other words, In this one regard, AAU we + upgradeDU can connectthe star to architecture more than toone a flexibleedge cloud mesh in architecture.its neighbor, i.e., In other the many words,-to- onemany AAU mapping+ DU relationship. can connect to moreFigure thanone 6 shows edge cloud the diagram in its neighbor, of the proposed i.e., the many-to-many network architecture, mapping relationship.where subfigure (a) is a schemaFigure of the6 shows conventional the diagram one-to of-many the proposed mapping network relationship architecture, and subfigure where (b) Figure presents6a is the a schema many- toof-many the conventional mapping relationship. one-to-many Compared mapping to relationship subfigure (a), and the Figure many6-tob- presentsmany architecture the many-to-many enables a moremapping flexible relationship. handover Compared scheme. In to addition Figure6,a, this the flexible many-to-many meshed architecturearchitecture enablescan yield a morea high flexible safety robustness.handover scheme. For instance, In addition, if the link this connecting flexible meshed cloud architecture 1 to 3 suffers can an yield interruption a high safety accidently, robustness. the HyperloopFor instance, can if still the linkmaintain connecting an uninterrupted cloud 1 to 3 connection suffers an interruptionvia the link from accidently, cloud 1 the to 2 Hyperloop and link from can cloudstill maintain 2 to 3. an uninterrupted connection via the link from cloud 1 to 2 and link from cloud 2 to 3.

cloud 1 cloud 3

cell 4 cell 2 cell 3

cloud 4 cell 1

cloud 2

Movement direction

(a)

Midhaul link New added midhaul link cloud 1 cloud 3 Link between cloud centers Hyperloop line cell 4 cell 2 cell 3 AAU+DU cell 1 cloud 4 Resources migration direction cloud 2 Movement direction

(b) FigureFigure 6 6.. TheThe diagram diagram of of the the network network architecture: architecture: ( (aa)) one one-to-many;-to-many; ( (bb)) many many-to-many.-to-many.

AsAs the the Hyperloop Hyperloop proceeds proceeds inside inside a a cell, cell, it it will will choose choose an an edge edge cloud cloud to to communicate communicate according according toto each cloud’s cloud’s real-time real-time computational computational burden burden status status as well as well as the as topography the topography structure. structure. The burden The burdenstatus of status the cloud of the center cloud varies center at varies different at timedifferent even time without even the without Hyperloop. the Hyperloop. However, However, compared comparedto the ultra-high-speed to the ultra-high of- thespeed Hyperloop, of the Hyperloop, the burden the status burden can status be regarded can be regarded as static as within static within a short ainterval. short interval. During During this interval, this interval, assume assume the Hyperloop the Hyperloop travels acrosstravels several across cellsseveral involving cells involvingNT edge clouds. Consider the cloud-to-cloud handover, a weighted undirected graph can be established by NT edge clouds. Consider the cloud-to-cloud handover, a weighted undirected graph can be abstracting the cells and edge clouds into vertex sets and abstracting the cloud-to-cloud links into established by abstracting the cells and edge clouds into vertex sets and abstracting the cloud-to- edge sets. cloud links into edge sets. As shown in Figure 6, the Hyperloop proceeds across from cell 1 to 4 successively, involving 4 nearby edge clouds, i.e., cloud 1 to 4. Figure 7 presents an abstract graph of this topographical

Energies 2020, 13, x FOR PEER REVIEW 13 of 22 relationship. The vertical direction means the successive passing cells along the Hyperloop line, whereas the horizontal direction denotes cloud centers. Those vertexes with deep color in one row refer to those edge clouds that are connected to the same cell. The numbers marked inside each vertex circle denote the currently available computational capability of the edge cloud. As the Hyperloop Energies 2020, 13, 4309 12 of 21 travels across two cells belonging to two different edge clouds, a group handover occurs with a resource migration cost (e.g., the drop rate, BER, throughput, etc.). The cost gcc' between edge cloud c and cAs′ can shown be calculated in Figure as6, the Hyperloop proceeds across from cell 1 to 4 successively, involving 4 nearby edge clouds, i.e., cloud 1 to 4. Figure7 presents an abstract graph of this topographical relationship. The vertical direction means 0, the successivecc passing= ' cells along the Hyperloop line, whereas the horizontal direction denotes=→ cloud centers. Those vertexes with deep color in one row refer gcc''' ND a'/ cc F ', cc c c '. (6) to those edge clouds that are connected to the+∞ same,' cell. Thecc numbers → marked inside each vertex circle denote the currently available computational capability of the edge cloud. As the Hyperloop travels acrossIn Equation two cells (6), belonging the first tocase two indicates different that edge no handover clouds, a groupoccurs handoverwithin the occurssame edge with cloud, a resource i.e., migration= cost (e.g., the drop rate, BER, throughput, etc.). The cost gcc between edge cloud c and c0 cc' . The second case means that the cost is proportional to the active0 passenger number N a as can be calculated as  well as the normalized distance D'cc' between these0, two cloudsc = c0 and inversely proportional to the  =  gcc0 FN' aD0cc0 /F0cc0 , c c0 . (6) normalized capacity of the clouds’ link  cc' . The third case→ infers that two clouds that are not  + , c c physically linked cannot perform a handover. ∞ → 0

cloud1 cloud2 cloud3 cloud4

cell1

cell2

cell3

cell4

FigureFigure 7. 7. TheThe schematic schematic diagram diagram of of graph graph theory. theory.

AnotherIn Equation consideration (6), the first when case selecting indicates the that cloud no handover is the corresponding occurs within available the same computational edge cloud, i.e., capacityc = c0. Thesince second a heavy-burdened case means that edge the center cost is may proportional reduce the to theprocessing active passenger efficiency number of the NHyperloopa as well as communicationthe normalized tasks. distance As Dsuch,0cc0 between the total thesehandover two cloudscost should and inversely be a sum proportionalof these two tocosts, the which normalized can capacity of theη clouds’++ link F0cc . The thirdη case∈ infers that two clouds that are not physically linked be marked as gggcc''() c c 0 , where (0,1) is a constant used to tune the primary of these cannot perform a handover. two costs. Another consideration when selecting the cloud is the corresponding available computational Finally, our goal is to minimize the total group handover cost during the whole Hyperloop line capacity since a heavy-burdened edge center may reduce the processing efficiency of the Hyperloop by selecting a certain edge center at different logical cells, which can be expressed as communication tasks. As such, the total handover cost should be a sum of these two costs, which can be marked as ηgcc + (gc + gc ), where ηηϕ(0, ϕ 1) is a constant++ ϕ used to ϕ tune the primary of these two costs. 0 0 min {lc,(1),'' l++ cgg cc ( lc , c (1),'' l c g c )} ϕϕ,  ∈ (7) Finally, our goal is to minimizelc,(1),' l+ c l the total group handover cost during the whole Hyperloop line by selecting a certain edge center at different logical cells, which can be expressed as where ϕ ∈{0,1} denotes the selection variable that the logical cell l selects the cloud c to lc, X min ηϕ ϕ g +ϕ (ϕ g + ϕ g ) (7) communicate as the Hyperloop travels linside,c (l+1 )it,c 0 andcc0 (1),'lc+l,c c represents(l+1),c0 cthe0 selection of the next ϕl,c,ϕ(l+1),c { } 0 l logical cell (l + 1). The selection variables are constraint by following formulas where ϕl,c 0, 1 denotes the selection variable thatϕ the= logical cell l selects the cloud c to communicate ∈ { } lc, 1, as the Hyperloop travels inside it and ϕ(l+1),c represents the selection of the next logical cell (l +(8)1). 0c The selection variables are constraint by following formulas X ϕl,c = 1, (8) c

X ϕ = 1, (9) (l+1),c0 c0 Energies 2020, 13, x FOR PEER REVIEW 13 of 21

 1, Energies 2020, 13, 4309  (lc 1), ' 13 of(9) 21 c'

which means that one logical cell can only select one edge cloud to communicate as the Hyperloop whichtravels means inside thatit. Optimization one logical cell problem can only (7) together select one with edge Equations cloud to (8) communicate and (9) can asbe theregarde Hyperloopd as an travelsNP-hard inside problem it. Optimization and it can be problem solved (7) by together the Floyd with–Warshall Equations algorithm. (8) and Details (9) can solution be regarded can as be anfound NP-hard in our problem previous and work it can[16,17] be solved. Based byon thethe Floyd–Warshallproposal, the group algorithm. handover Details cost can solution be alleviated can be foundevidently in our to ensure previous a high work on [16-board,17]. Based communication on the proposal, QoS together the group with handover reliable costand cansafe be operation alleviated of evidentlythe Hyperloop. to ensure a high on-board communication QoS together with reliable and safe operation of the Hyperloop. 5. Wireless Physical Resources Management 5. Wireless Physical Resources Management Based on the aforementioned analysis in Section 2, the two main types of communication Based on the aforementioned analysis in Section2, the two main types of communication services, services, i.e., the mission-critical service and passenger Internet service can be categorized as the i.e., the mission-critical service and passenger Internet service can be categorized as the eMBB and eMBB and uRLLC services. Since these two application cases are supported simultaneously in 5G, uRLLC services. Since these two application cases are supported simultaneously in 5G, the joint the joint physical resource scheduling of eMBB and uRLLC traffic is another challenging issue to the physical resource scheduling of eMBB and uRLLC traffic is another challenging issue to the Hyperloop. Hyperloop. Thanks to the flexible frame structure of the 5G, a time slot can be divided into several Thanks to the flexible frame structure of the 5G, a time slot can be divided into several mini-time mini-time slots to cope with the uRLLC traffic. The concept of multiplexing is to deal with these two slots to cope with the uRLLC traffic. The concept of multiplexing is to deal with these two coexisting coexisting services by puncturing/superposing parts of PRBs assigned to eMBB traffic for the services by puncturing/superposing parts of PRBs assigned to eMBB traffic for the sporadically arrived sporadically arrived uRLLC packets at the next mini-slot boundaries [19]. uRLLC packets at the next mini-slot boundaries [19]. Currently, much work about multiplexing has been investigated, such as [19,20,21], which Currently, much work about multiplexing has been investigated, such as [19–21], which mainly mainly aim to boost the throughput of the eMBB services subject to the instant puncturing of the aim to boost the throughput of the eMBB services subject to the instant puncturing of the uRLLC traffic uRLLC traffic on the eMBB packet transmission upon arrival. However, the primary issue of these on the eMBB packet transmission upon arrival. However, the primary issue of these research is that research is that all sporadically arrived uRLLC traffic are treated equally without any discrimination. all sporadically arrived uRLLC traffic are treated equally without any discrimination. Consequently, Consequently, these methods cannot process the multiple types of uRLLC traffic with different KPIs these methods cannot process the multiple types of uRLLC traffic with different KPIs for Hyperloop for Hyperloop and that is the issue we try to solve. and that is the issue we try to solve. Figure 8 presents a diagram of the proposed wireless PRB multiplexing of the eMBB and uRLLC Figure8 presents a diagram of the proposed wireless PRB multiplexing of the eMBB and uRLLC traffic. As shown in this figure, each time slot with one millisecond time duration is divided into M traffic. As shown in this figure, each time slot with one millisecond time duration is divided into M mini-slots equally so as to achieve the stringent latency requirement of uRLLC. Four different colors mini-slots equally so as to achieve the stringent latency requirement of uRLLC. Four different colors represent 4 eMBB services users, which are scheduled at the slot boundary periodically. In addition, represent 4 eMBB services users, which are scheduled at the slot boundary periodically. In addition, the proportional fairness algorithm is adopted to guarantee the data rate fairness among different the proportional fairness algorithm is adopted to guarantee the data rate fairness among different users. Three types of the uRLLC traffic arrive sporadically during the time slot whose PRBs are users. Three types of the uRLLC traffic arrive sporadically during the time slot whose PRBs are already already allocated to different eMBB users and should be processed before the next time slot due to allocated to different eMBB users and should be processed before the next time slot due to the strict the strict latency requirements. latency requirements.

OVCS TCS OCS Frequency eMBB user 1 eMBB user 2 eMBB user 3 eMBB user 4 uRLLC TCS Subcarrier spacing uRLLC OCS uRLLC OVCS Latency margin PRB Time One eMBB time slot minislot Figure 8. The diagram of the physical resource multiplexing of the eMBB and uRLLC. Figure 8. The diagram of the physical resource multiplexing of the eMBB and uRLLC. Given the fact that the 5G supports a flexible frame structure, each type of uRLLC traffic can be allocated to a PRB with specific bandwidth and time duration [18]. Specifically, a traffic with a Given the fact that the 5G supports a flexible frame structure, each type of uRLLC traffic can be stringent latency requirement is allocated to a PRB with small time duration but large bandwidth. allocated to a PRB with specific bandwidth and time duration [18]. Specifically, a traffic with a To cope with the multi-types of uRLLC traffic for Hyperloop, we propose a novel latency-margin-based multiplexing scheme herein. In contrast to the conventional multiplexing scheme, multi-types of uRLLC traffic will be processed not at the next mini-slot directly. Instead, it is permitted to be allocated Energies 2020, 13, 4309 14 of 21 within its corresponding latency margin. For instance, two different types of uRLLC traffic arrive at the eighth mini-slot in Figure8, the system ought to deal with OVCS tra ffic in no longer than next three mini-slots (latency margin: three mini-slots), whereas it would cope with the TCS and OCS traffic within next mini-slot (latency margin: one mini-slot). Obviously, the latency-margin-based idea enables great puncturing flexibility of PRB when multiplexing. The goal of the proposed latency-margin-based multiplexing scheme is to maximize the throughput of all eMBB traffic and minimize the allocated power of the uRLLC traffic when guaranteeing the latency and BER demands of the sporadically arrived uRLLC traffic. Let n0 denote the channel noise power, ht, f means the channel gain of the PRB at the t-th mini-slot and f -th subcarrier. Several M-quadrature amplitude modulation (QAM) modes can be chosen based on the BER and data rate requirements as well as the channel gain ht, f . Then, this optimization problem can be expressed as

min αTuRLLC + βPuRLLC , (10) s tq, fq,p { } tq+i, fq+j

f Nt N   Q q q p ht +i f +j X X X  tq+i, fq+j q , q  uRLLC   s.t. T = ∆ f ∆t log21 +  (11)  n0∆ f  q=1 i=1 j

f N Nt XQ Xq Xq PuRLLC = p (12) tq+i, fq+j q=1 j=0 i=0

  require Ps M, γ P (13) q q ≤ q 2 pq htq+i, fq+j p = k k (14) tq+i, fq+j f t Nq Nq P P 2 htq+i, fq+j j=0 i=0 k k

f N Nt Pq Pq 2 p h t +i, f +j tq+i, fq+j j=0 i=0 q q γq = (15) t f NqNq ∆ f

(tq+i, fq+j) (tq +i , fq +j ) = ∅; q , q0; q, q0 1, 2, ... , Q (16) ∩ 0 0 0 0 ∈ { } 0 t tarr dq, t N∗ (17) ≤ q − ≤ q ∈ f 1 f N f N + 1, f N∗ (18) ≤ q ≤ − q q ∈ f where Nt and N mean the time-frequency length of the allocated PRB to uRLLC traffic q(q Q). q q ≤ N f is the number of all subcarriers, (tq, fq) is the starting position in time-frequency domain of traffic require q, and all the power allocated to it is denoted as pq. Pq and dq are the corresponding required BER and latency margin, respectively. α and β are two parameters used to tune the primary of these two objections. γq is the BER of traffic q. When the power pq stays a constant, the maximum γq can be obtained from Equation (15) according to the Lagrange Multiplier Method. Equation (11) means the throughput of the eMBB traffic after subtracting that of the uRLLC traffic. Equation (12) infers to total power allocated to the uRLLC traffic. Equation (13) considers the BER demands when multiplexing. Equation (16) means that the PRBs of two different uRLLC traffic get no overlapping parts. Equations (17) and (18) imply that the allocated position of the PRBs are located within the time-frequency limits. This optimization issue is a NP-hard problem, which is complex to find a certain solution. The heuristic algorithms are often used to solve such questions. Herein, the particle swarm optimization Energies 2020, 13, x FOR PEER REVIEW 16 of 22 corresponding required BER and latency margin, respectively. α and β are two parameters used γ to tune the primary of these two objections. q is the BER of traffic q . When the power pq stays γ a constant, the maximum q can be obtained from Equation (15) according to the Lagrange Multiplier Method. Equation (11) means the throughput of the eMBB traffic after subtracting that of the uRLLC traffic. Equation (12) infers to total power allocated to the uRLLC traffic. Equation (13) considers the BER demands when multiplexing. Equation (16) means that the PRBs of two different uRLLC traffic get no overlapping parts. Equations (17) and (18) imply that the allocated position of the PRBs are located within the time-frequency limits. This optimization issue is a NP-hard problem, which is complex to find a certain solution. The Energiesheuristic2020 algorithms, 13, 4309 are often used to solve such questions. Herein, the particle swarm optimization15 of 21 (PSO) algorithm is employed to deal with it [42]. For each particle, it contains three unsolved variables, i.e., (,,)pt f . During the initialization, the initial position and velocity of each particle are (PSO) algorithmqqq is employed to deal with it [42]. For each particle, it contains three unsolved variables, randomlyi.e., (pq, tq, generated.fq). During If the a initialization,particle’s position the initial satisfies position Equations and velocity (13), (17), of each and particle (18), it are is randomlya feasible positiongenerated. that If can a particle’s be used for position further satisfies process; Equations otherwise (13), an infeasible (17), and (18),position it is will a feasible be discarded. position After that updatingcan be used the for position, further Equation process; (16) otherwise is used an to infeasiblecheck all the position position. will If be a particle’s discarded. position After updatingoverlaps withthe position, any other’s Equation position, (16) then is used change to check its position all the position.randomly. If a particle’s position overlaps with any other’sFigure position, 9 presents then change the simulation its position results randomly. of two multiplexing scheme. Subfigure (a) shows the PRB Figurepositions9 presents of different the simulationuRLLC traffic results (1 for of VS, two 2 multiplexing for PIS, 3 for scheme.OCS, and Figure 4 for9 TCS)a shows based the on PRB the positionsconventional of discheme,fferent uRLLCi.e., processing traffic (1 the for arrived VS, 2 foruR PIS,LLC 3traffic for OCS, in the and next 4 formini-slot. TCS) based In contrast, on the subfigureconventional (b) scheme,demonstrates i.e., processing our proposed the arrived scheme, uRLLC where tra ffithec in background the next mini-slot. color implies In contrast, the correspondingFigure9b demonstrates channel ourgain proposed (white color scheme, refers where to a high the value). background α and color β implies are set the at correspondingdifferent value tochannel investigate gain (whitethe primary color refersbetween to athe high throughput value). α and βuRLLCare set power. at different The first value case to investigatefocuses on the primaryminimization between of thethe throughputuRLLC power and uRLLCand the power. corresponding The first case PRB focuses is allocated on the minimizationto time-frequency of the positionsuRLLC power with a and high the value. corresponding The second PRB case is puts allocated an emphasis to time-frequency on the minimization positions of with the athroughput. high value. TheAs such, second the case PRB puts is allocated an emphasis to time-frequency on the minimization positions of the with throughput. a low channel As such, gain the value. PRB is The allocated third caseto time-frequency takes both objections positions into with account a low an channeld yields gain a relatively value. The compromise third case takesresult. both objections into account and yields a relatively compromise result.

(a) (b)

Figure 9. AA diagram diagram of of the PRB positions of uRLLC tratraffic:ffic: ( a)) the conventional scheme; ( b) the proposed scheme.

Based on the above scheme, the coexisting of eMBB and uRLLC uRLLC traffic traffic can be handled handled efficiently efficiently to maximize the throughput of all eMBB traffic traffic andand guarantee the latency as well as BER demands of the uRLLC tratraffic.ffic.

6. Passengers Passengers Internet Resources Management The ultra-high-speed not only exerts a heavy impact on the physical wireless channel characterization, but also influences the in-journey passenger Internet access experience. On one hand, the short residence time inside a cell yields a low network throughput. On the other hand, the low-quality wireless channel also decreases the cell throughput to some degree. Generally, the communication system fetches the requested contents from the Internet network through edge cloud and the core network, which consequently generates a large transmission delay, not to mention the transmission latency in the fronthaul, midhaul, and backhaul links. Hence, it is of great importance to investigate resource strategies to improve the QoS at ultra-high-speed. A promising scheme to boost the communication performance is to add a cache device to the nearby base station (BS) [24,39]. Numerous prevailing pre-fetching methods have been investigated by leveraging the cache equipment. Likewise, we add a cache device to the roadside AAUs to enable the following two cache-based resource management schemes. Energies 2020, 13, 4309 16 of 21

6.1. Cache-Based Pre-Fetching Scheme In the conventional pre-fetching scheme, the load balancer will pre-store some popular Internet resources into the cache device preliminarily. As the passengers request for an Internet content, the load balancer will search it in the pre-stored cache first. If found, it will be transmitted to the user instead of being fetched from the Internet via the core network [22]. The conventional cache scheme seems attractive since it can reduce the transmission latency greatly. However, if a passenger requests an un-stored content, the load balancer can do nothing but fetch it from the Internet. Hence, we aim to propose a novel cache-based pre-fetching scheme to deal with such situation. Generally, the requested contents of small file size can be transferred to users within a short interval. As for those contents of large file size such as the high-definition video (HDV), they usually need a long transmission time, which reduces the communication QoE dramatically. Thanks to the breakpoint transmission technology, a video content of large file size can be chunked into a group of small segments to be independently downloaded and consumed by the Hyperloop on-board passengers. The basic idea of our proposal is to pre-store parts of the requested contents (un-stored in current cache) to the cache of next load balancer as shown in Figure 10. Then the passenger can fetch it directly from the cache as the Hyperloop enters next logical cell. The detailed scheme is formulated as follows:

(1) As a passenger u requests for an Internet content, it will arrive at the roadside load balancer first via the wireless link. (2) The load balancer searches the requested content in its cache database. If found, then transfer the content to the passengers via the wireless link directly. (3) If not found, fetch it from the Internet via the edge cloud. Simultaneously, estimate the transmission completion time ∆tcomplete based on the average allocated data rate mentioned in Equation (15), which can be expressed as ∆tcomplete = S f ile/Ru(t), (19)

where S f ile is the file size of the requested content. Calculate the remaining residence time ∆tresidence based on Equation (19) inside current logical cell according to Hyperloop position. (4) If ∆tcomplete > ∆tresidence, i.e., the requested content will not be transferred to the passenger within the current logical cell completely, broadcast this content request to the nearby edge clouds. (5) Assess the burden status of the nearby edge clouds which connects to the load balancer that covers the next logical cell. Select an edge cloud with little communication burden and download the rest part of the requested content from the Internet and store them in the cache of the next Energiesload 2020 balancer., 13, x FOR PEER REVIEW 18 of 22 (6) As the Hyperloop enters the next logical cell, the passenger can fetch the remaining (6) As the Hyperloop enters the next logical cell, the passenger can fetch the remaining transmission-unfinished content from the cache via the wireless link directly. transmission-unfinished content from the cache via the wireless link directly.

4. Broadcast the request that the un-stored Cloud 3 Cloud 1 content cannot be transferred within current cell 5. Download it from the Internet and pre-store in the cache of next cell

2. Search it in the cache Cache 3. Fetch unstored contents via the cloud AAU+DU

1. Requesting content

Current logical cell Next logical cell 6. Transfer it to the Hyperloop as it enters the coverage of next cell Cloud 2

Figure 10. The diagram of the pre-fetching scheme. Figure 10. The diagram of the pre-fetching scheme.

In conclusion, we propose a novel pre-fetching scheme by coordinating multiple edge clouds based on the proposed network architecture mentioned in Section 4. The merits of this proposal lie not only in reducing the content transmission delay but also making more margin for the transmission of mission-critical services data. Figure 11 presents the simulation results of the pre-downloading scheme. The red line denotes the maximum transmission rate of the midhaul, whereas three schemes will be compared, i.e., Scheme 1: without cache, Scheme 2: conventional cache-based strategy (store some popular Internet content in the cache in advance), and Scheme 2: our proposal. The vactrain starts from AAU1 and proceeds across AAU2. The actual throughput of three schemes stays the same in AAU1. Notably, Scheme 3 enables the pre-downloading procedure as the midhaul turns congested. Then, in AAU2, an abrupt throughput ascent of Scheme 3 appears since it pre-downloads some requested contents to the cache. This cache-based scheme presents an inspiring throughput performance especially in terms of the congested wired link. Transmission rate/Mbps Transmission

Figure 11. The network throughput at different positions.

6.2. Cache-Based Post-Uploading Scheme As described in Section 2, multi-types of communication services will be transferred via the midhaul link. Usually, the optical-fiber-made midhaul can provide a sufficient bandwidth (in Gb/s) by the wavelength division multiplexing (WDM) technology. However, the processing latency mainly lies in the photoelectric signal conversion, which may endanger the transmission of the mission-critical services. Moreover, the coexistence of multi-types of services implies that several types of traffic attempt to use a common midhaul simultaneously and encounter a data collision [43,44]. As such, high-priority-traffic should be guaranteed first.

Energies 2020, 13, x FOR PEER REVIEW 18 of 22

(6) As the Hyperloop enters the next logical cell, the passenger can fetch the remaining transmission-unfinished content from the cache via the wireless link directly.

4. Broadcast the request that the un-stored Cloud 3 Cloud 1 content cannot be transferred within current cell 5. Download it from the Internet and pre-store in the cache of next cell

2. Search it in the cache Cache 3. Fetch unstored contents via the cloud AAU+DU

1. Requesting content

Current logical cell Next logical cell 6. Transfer it to the Hyperloop as it enters the coverage of next cell Cloud 2

Figure 10. The diagram of the pre-fetching scheme.

In conclusion,Energies 2020, 13 ,we 4309 propose a novel pre-fetching scheme by coordinating multiple17 edge of 21 clouds based on the proposed network architecture mentioned in Section 4. The merits of this proposal lie not only inIn reducing conclusion, the we proposecontent a noveltransmission pre-fetching delay scheme but by coordinatingalso making multiple more edge margin clouds for the transmissionbased of on mission-critical the proposed network services architecture data. mentioned in Section4. The merits of this proposal lie not Figureonly 11 in reducingpresents the the content simulation transmission results delay of but the also pre-downloading making more margin scheme. for the transmissionThe red line of denotes the maximummission-critical transmission services data.rate of the midhaul, whereas three schemes will be compared, i.e., Figure 11 presents the simulation results of the pre-downloading scheme. The red line denotes the Scheme 1: without cache, Scheme 2: conventional cache-based strategy (store some popular Internet maximum transmission rate of the midhaul, whereas three schemes will be compared, i.e., Scheme 1: content withoutin the cache cache, Schemein advance), 2: conventional and Scheme cache-based 2: our strategy proposal. (store The some vactrain popular Internet starts contentfrom AAU1 in and proceedsthe across cache AAU2. in advance), The and actual Scheme throughput 2: our proposal. of three The schemes vactrain starts stays from the AAU1same andin AAU1. proceeds Notably, Scheme across3 enables AAU2. the The pre-downloading actual throughput ofprocedure three schemes as the stays midhaul the same turns in AAU1. congested. Notably, SchemeThen, in 3 AAU2, an abruptenables throughput the pre-downloading ascent of Scheme procedure 3 appears as the midhaul since turnsit pre-downloads congested. Then, some in AAU2, requested an abrupt contents to throughput ascent of Scheme 3 appears since it pre-downloads some requested contents to the cache. the cache. This cache-based scheme presents an inspiring throughput performance especially in terms This cache-based scheme presents an inspiring throughput performance especially in terms of the of the congestedcongested wired link. link. Transmission rate/Mbps Transmission

FigureFigure 11. The 11. The network network throughput throughput at at di differentfferent positions. positions. 6.2. Cache-Based Post-Uploading Scheme 6.2. Cache-Based Post-Uploading Scheme As described in Section2, multi-types of communication services will be transferred via the As midhauldescribed link. in Usually, Section the 2, optical-fiber-made multi-types of midhaul communication can provide a services sufficient bandwidthwill be transferred (in Gb/s) by via the midhaulthe link. wavelength Usually, division the optical-fiber-made multiplexing (WDM) technology.midhaul can However, provide the processinga sufficient latency bandwidth mainly lies (in Gb/s) in the photoelectric signal conversion, which may endanger the transmission of the mission-critical by the wavelength division multiplexing (WDM) technology. However, the processing latency services. Moreover, the coexistence of multi-types of services implies that several types of traffic mainly liesattempt in the to use photoelectric a common midhaul signal simultaneously conversion, and which encounter may aendanger data collision the [43 transmission,44]. As such, of the mission-criticalhigh-priority-tra services.ffic shouldMoreover, be guaranteed the coexistence first. of multi-types of services implies that several types of trafficThe attempt video surveillance to use a datacommon plays amidhaul key role insimultaneously monitoring the Hyperloopand encounter real-time a status,data collision [43,44]. Asfault such, pre-discovery high-priority-traffic and diagnosis. should Data of be this guaranteed type is usually first. not as latency-sensitive as the other mission-critical data but accounts for a large file size. Therefore, we stagger these data to make more bandwidth for those traffic with higher priorities as shown in Figure 12[ 41]. The proposed scheme can be summarized as follows:

(1) Collect the video surveillance data and transfer it to the nearby load balancer via the wireless link. Then store the data to the cache instead of being forwarded to the cloud center directly. (2) Detect the burden status of nearby edge centers and get prepared to forward it to an available edge center with less communication burden. Then transmit the data to an available cloud via the midhaul. This process usually occurs when the Hyperloop has left the current logical cell. Energies 2020, 13, x FOR PEER REVIEW 19 of 22

The video surveillance data plays a key role in monitoring the Hyperloop real-time status, fault pre-discovery and diagnosis. Data of this type is usually not as latency-sensitive as the other mission- critical data but accounts for a large file size. Therefore, we stagger these data to make more bandwidth for those traffic with higher priorities as shown in Figure 12 [41]. The proposed scheme can be summarized as follows: (1) Collect the video surveillance data and transfer it to the nearby load balancer via the wireless link. Then store the data to the cache instead of being forwarded to the cloud center directly. (2) Detect the burden status of nearby edge centers and get prepared to forward it to an available edge center with less communication burden. Then transmit the data to an available cloud via Energies 2020, 13, 4309 18 of 21 the midhaul. This process usually occurs when the Hyperloop has left the current logical cell.

Cloud 1 Cloud 3 1. Store the video surveillance data into the load balancer cache

Cache AAU+DU

Leaky waveguide system

Current logical cell Cloud 2 2. Post-upload the video surveillance data to the clouds when avaliable

FigureFigure 12.12. TheThe diagramdiagram ofof thethe post-uploadingpost-uploading scheme.scheme.

AnotherAnother issueissue aboutabout thesethese twotwo cache-basedcache-based schemesschemes isis thethe exactexact storagestorage sizesize ofof thethe cache.cache. ItIt cancan bebe roughlyroughly estimatedestimated thatthat thethe cachecache storagestorage sizesize isis nono largerlargerthan than

S ==+ (2F + F )T , (20) cacheSFFTcache(2f routhal frouthalwireless wirelesscell ) cell , (20) wherewhere thethe firstfirst partpart meansmeans thethe sumsum ofof thethe pre-fetchedpre-fetched filefile sizesize generatedgenerated inin thethe previousprevious logicallogical cellcell andand thethe downloadingdownloading partpart inin thethe currentcurrent cellcell viavia thethe samesame midhaul.midhaul. TheThe latterlatter meansmeans thethe maximummaximum uploadeduploaded file file size size via via a wireless a wireless link link when when traveling trave insideling inside the current the current cell. Based cell. on Based these on two these schemes, two theschemes, network the throughput network throughput can be greatly can improved, be greatly and improved, a more reliable and linka more can bereliable established link forcan thebe transmissionestablished for of the mission-critical transmission services. of mission-critical services.

7.7. ConclusionsConclusions TheThe HyperloopHyperloop adoptingadopting thethe magneticmagnetic levitationlevitation andand vacuumvacuum tubetube technologiestechnologies cancan achieveachieve anan ultra-high-velocityultra-high-velocity ofof overover 10001000 kmkm/h,/h, whichwhich bringsbrings greatgreat challengeschallenges toto thethe existingexisting communicationcommunication systems.systems. InIn thisthis paper,paper, wewe proposepropose aa train-to-groundtrain-to-ground wirelesswireless communicationcommunication systemsystem solutionsolution basedbased onon thethe prevailing prevailing 5G system,5G system, involving involving wireless wireless channel, networkchannel, architecture, network architecture, resource management. resource Ourmanagement. contributions Our togethercontributions with futuretogether works with can future be summarized works can be as summarized follows: as follows: (1)(1) The feasibilitiesfeasibilities of of two two wireless wireless access access methods, methods, i.e., the i.e., DAS the and DAS LWS, and are analyzed.LWS, are Specifically, analyzed. theSpecifically, Doppler powerthe Doppler spectrums power for thespectrums DAS at diforfferent the DAS positions at different are characterized. positions are Then, characterized. we analyze theThen, radio we coverageanalyze the from radio an aspectcoverage of thefrom handover. an aspect In of future, the handover. the accurate In future, radio propagationthe accurate characterizationradio propagation of thecharacterization LWS in the near of the field LWS will in be the investigated, near field will especially, be investigated, the Doppler especially, effect of the LWSDoppler should effect be of analyzed the LWS in should detail. be analyzed in detail. (2)(2) As for the network structure, C-RANC-RAN isis utilizedutilized toto integrateintegrate severalseveral nearbynearby AAUsAAUs intointo aa logicallogical cell, achieving a free-handover eeffectffect insideinside thisthis cell.cell. To dealdeal withwith thethe inevitable groupgroup handoverhandover when traveling across didifferentfferent macromacro cells,cells, aa novelnovel accessaccess networknetwork structurestructure isis investigatedinvestigated toto reduce thethe resourceresource migration migration cost. cost. However, However, such such proposal proposal can can alleviate alleviate the the cost cost evidently evidently in the in mesh Hyperloop lines, but exerts little impacts on the single sparse Hyperloop line, which will be solved in future. (3) In terms of the coexistence of eMBB and uRLLC traffic, we propose a novel PRB multiplexing scheme considering the latency margin of mission-critical services, which aims to maximize the network throughput subject to the stringent requirements of different types of uRLLC traffic. Though we proposed a solution based on the PSO algorithm, an optimization solution with a closed-form expression will be much helpful especially in terms of the low-latency traffic. (4) To enhance the QoE of passengers’ Internet access, a cache-based mechanism of “staggering the peak” of data transmission (including pre-fetching and post-uploading schemes) is proposed to boost the transmission performance. In the simulation, we only consider the coordination of two Energies 2020, 13, 4309 19 of 21

adjacent AAU. However, it can be inferred that the joint of more AAUs will definitely yield a better throughput performance, which will be investigated in the future.

In summary, we make our endeavors to investigate on the train-to-ground communication system for Hyperloop from the aspect of the whole system architecture. We wish our works could enlighten the future research of the Hyperloop.

Author Contributions: Conceptualization, J.Z., L.L., T.Z., and B.A.; investigation, J.Z., Z.L., and D.W.; methodology, J.Z., B.H., Z.L., K.W., and B.A.; project administration, L.L.; supervision, L.L.; writing—original draft, J.Z. and L.L.; and writing—review and editing, J.Z. and L.L. All authors have read and agreed to the published version of the manuscript. Funding: This research was supported by the Fundamental Research Funds for the Central Universities under grant 2018JBZ102, and the Beijing Nova Programme Interdisciplinary Cooperation Project (Z191100001119016). Acknowledgments: The authors thank the anonymous reviewers for their constructive comments which are much helpful for enhancing the quality and presentation of this paper. Conflicts of Interest: The authors declare no conflict of interest.

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