The demand for a broadband wireless connection is nowadays no longer limited to stationary situations, but also required while traveling. Therefore, there exist combined efforts to also provide wireless access on high speed trains (HSTs), in order to add to the attractiveness of this means of transportation. Installing an additional relay on the train, to facilitate communication, is an approach that has already been extensively discussed in the literature. The possibility of a direct communication between the base station and the passenger has been neglected until now, despite it having numerous advantages. Therefore, a comparison between these two opposing approaches is presented in this article, accompanied by a detailed discussion of the related aspects. Additionally, we present simulation results for the two approaches when applying different schemes to supply a wireless connection. We also discuss the presented results from the perspectives of mobile operators and train operators. In the current market of train services, being able to provide mobile broadband access to costumers has become a main inducement for choosing this means of transportation. Due to the ubiquitous use of the Internet and the rapid adoption of novel devices such as smartphones and tablet computers, most passengers have become accustomed to experiencing high data rates and having the service following them no matter where they go. With the number of commuters expected to increase, high user mobility is also one of the most emphasized scenarios in the initiative for the fifth generation (5G) of wireless communications. LTE-Advanced (LTE-A), the contemporary standard for wireless communication, is not optimized for the challenges of high speed train (HST) scenarios. Hence, many train operators, mostly in collaboration with mobile operators, have increased efforts to satisfy the ever increasing requirements. There also exist international collaborations on a broad level to push for higher data rates and shorter latencies, for example the Shift2Rail initiative by the European Union. 1 Additionally, LTE for Railway (LTE-R) has been proposed in [1] as an evolution of Global System for Mobile Communications -Rail(way) (GSM-R), addressing required performance parameters and necessary adaptations on the architecture of the system. Nevertheless, LTE-R has not yet been standardized. Furthermore, the main concern of LTE-R is not passenger communication, which is the scope of this article. Therefore, we mostly focus on LTE-A. Wireless communications in HST scenarios is confronted with unique conditions that have a considerable impact on network planning. In particular, the scenarios are characterized by user equipments (UEs) being densely packed inside the train and moving at high speed, as well as the specific propagation effects in a diversity of different environments. Most publications on this topic assume a relay-based approach, assuming additional hardware installed on the train that communicates with the base station (BS) as well as with user equipments (UEs) without communicating directly ([2, 3]). However, the direct communication between UEs and BSs has its own advantages, but has not been given enough attention in the literature. This contribution provides an extensive comparison between the relay approach and the less studied direct link approach, and also discusses various other aspects that are specific to HST scenarios. The structure of the article is as follows. We explain general issues that arise in HST scenarios, and compare the relay and direct-link approaches. We then deal with further aspects of HST, showing dependencies between different parameters and discussing the system aspects from an operator's point of view. Conclusions are drawn in the final section. Wireless communication in HST scenarios exhibits several key differences compared with traditional considerations of a coverage oriented network. What is more, there is not the same amount of extensive experience with such scenarios as with classical networks. Thus, many effects have not been fully understood, and it is not clear which are the most significant in HST scenarios. The state of the art is discussed in the following, along with the aspects that are currently identified as most important. Following [4], newly built tracks for high speed trains should support at least 250 km/h, and many operational systems exceed this value. These speeds lead to a high Doppler shift, which causes several transceiver impairments such as channel estimation errors and inter carrier interference (ICI) in orthogonal frequency division multiplexing (OFDM) systems. Moreover, the channel characteristics along the tracks vary greatly. The surroundings show several distinct categories, which encompass certain different features that influence signal propagation. Among those are tunnels, trenches, cuttings, stations, viaduct-like structures, or bridges. An example of such a diverse environment is shown in Fig. 1 . An extensive description of such categories is described in [5] . In addition to the onerous propagation conditions, a large part of the signal may be shielded by the metal chassis of the carriage. If the signal is first received by a relay, this can lead to a very strong multi-path component in addition to the line of sight (LOS) path. Another peculiarity of HST scenarios is the UE distribution and movement. In contrast to classical assumptions of random UE placement, all UEs are concentrated within the train. Their location and movement is approximately deterministic as it is predefined by the course of the tracks and the speed of the train. This implicates a certain a-priori knowledge that can be exploited to compensate for some of the above mentioned issues. The high UE concentration results in pulselike traffic in the cells along the tracks. This leads to BSs being either completely idle or confronted with a high load (in case the hardware is dedicated to serving the train) or to large performance fluctuations in a cell (in case the surrounding environment is also covered by the BS). From the high speed of the train it follows that small cells are traversed in a very short time, thus leading to a frequent necessity for handovers. When all UEs aboard a train simultaneously require a handover to the next cell, the control channel easily gets congested and suspends UEs from associating with the neighboring cell. As indicated above, the deterministic UE behavior may provide a considerable advantage for implementing elaborated handover schemes. It is commonly agreed that a satisfactory quality of service can only be achieved by employing dedicated hardware in an HST scenario, especially in the form of remote units (RUs). Due to their smaller size and cost compared to classical BSs, it is possible to install them close to the tracks and in great numbers. Several RUs can be connected to one BS via radio over fiber (RoF). This allows for very flexible routing options, as explained in the subsequent section. An example of an RU installation along the tracks is presented in Fig. 1 . In the remainder of this work we will assume such a scenario for our considerations. Due to the diversity of environments, the modeling of the fast fading channel becomes a complex task. Many considerations regarding wireless communication in HST scenarios are based on simulations applying the Winner channel model [6] . Even though the model is highly adjustable, it was not originally intended to represent the characteristics of a HST scenario. Furthermore, it does not reflect the dynamic changes in the channel characteristics from one category to another. Thus, such simulations only yield first-order statements that may considerably deviate from reality and may not appropriately reflect the specifics of the environment. 3GPP has recently introduced a 3D channel model in [7] . However, specific aspects for train communications are not yet included and need to be determined. In this section we consider the downlink direction exclusively. Similar considerations are valid for the uplink direction. Generally speaking, there exist two opposing approaches to provide wireless communications to passengers of an HST. In the first case, the UE directly associates with the BSs along the tracks, while in the second case this link is established via a relay, as shown in Fig. 2 . Subsequently, a comparison between these two approaches is drawn and the advantages and drawbacks of both approaches are discussed. In the relay scenario, one or several antennas are mounted on the outside of the train. These are connected to one or more relays which are then distributing the signal inside the train. This approach has the major advantage that the signal is not attenuated by the windows of the carriage. Moreover, with this setup the relay can be configured such that it appears as a single UE to the BS, thus significantly reducing the number of handovers. Therefore, all traffic is aggregated by the relays and then distributed to the UEs. While the quality of the experienced link might be considerably improved, the employment of relays does not come without cost. First, relays need to be licensed for the specific band they are operating on. This is of minor consequence when no borders are crossed. In smaller countries, which is the case in most of Europe, one train connection can easily span three or more countries. For each country, the relays need to be registered individually or else they need to be switched off, which leaves the system in a directlink state. Another consequence is that the employment of carriages becomes restricted to the countries/routes for which the relays on board are licensed and thus becomes less flexible. A second issues appears with the choice of the frequencies for the RU-to-relay and the relay-to-UE connection. If the same frequency is used on both link sections, thus only bypassing the penetration loss of the carriage, the UE might still receive a considerable amount of the desired signal by a direct link through the window. The relative receive power of these two links depends on the individual position of the UE and the actual penetration loss. Due to the latency caused by the relay, the signal might be perceived via several multi-path components that cannot be equalized. The aforementioned problem is completely avoided when two different frequencies are used on both sections of the link. For example, a dedicated frequency of a mobile operator is used from RU to the relay and a second frequency, e.g. in the ISM band, is used to provide WiFi inside. This setup is frequently referred to in the literature [2] . Nonetheless, it only provides a data connection for the passengers. Since only the relay is visible as a single UE for the BS, the passengers are not accessible for mobility management. The performance of this setup may also considerably depend on the number of antennas and relays per train/carriage. It must be scrutinized whether the relays should work individually (e.g. one per carriage) or if the received signals should be combined. This is also affected by the possibility of connecting all relays to all carriages. Since a cable connection will not be feasible in practice, near-field communication standards at higher frequencies than the traditional 6 GHz band (e.g. in the upper mmWave band) may be considered for this task. The direct-link approach assumes a direct connection between RU and UE. In comparison to the above scenario, the signal does experience a severe penetration loss into the carriage in this case. As the chassis of the carriages is usually made of metal, the signal enters the train mainly through the windows. However, the penetration loss may greatly vary among window types, as they are mostly metal coated themselves. Attenuation values range from 20 dB to 40 dB for metal coated windows of a German ICE-train [8] , but other types of train exhibit different values, e.g. as observed in [3] with a combined range of 10 dB to 40 dB. Note that these values reflect the situation for current carriages in use. Since the interest among train operators is increasingly to provide the best quality of experience to their customers, the design of future trains is likely to be adapted to the demands of the wireless link. Among various options is the possibility of introducing windows with small penetration loss (omitting the metal coating). Another option is to include apertures in the chassis or the window itself, by incorporating materials that are more permeable for electromagnetic waves, e.g. carbon fiber materials. Examples of such apertures are indicated in Fig. 2 . In order to compare the relay and direct-link setups, LTE-A system level simulations were carried out. The system model comprises a scenario with four equidistantly spaced sites that are placed directly next to the train tracks (thus exploiting available railway infrastructure). Each site employs two RUs pointing in opposite directions along the tracks. RUs facing away from the train are not considered interferers. Two RU collaboration schemes are compared: • Each site is connected to an individual BS (baseline scheme). • The two dominant RUs are connected to the same BS (cooperation scheme). A fully occupied German ICE-train with 460 passengers is regarded, with 10 percent of the passengers having an active wireless connection. For the direct-link setup a penetration loss of 30 dB is applied referring to the mean value of the aforementioned penetration losses. No penetration loss is considered for the relay setup. The simulation parameters are summarized in Table 1 . Simulations were performed with the Vienna LTE-A Downlink System Level Simulator. 2 Simulation results are shown in the UEs in the direct setup experience a penetration loss of 30 dBs, and the relays are operating under ideal conditions (e.g. no processing delays and no overhead). Taking into account further aspects as discussed earlier makes the direct-link approach a notable alternative. From the curves it is further found that the performance can also be improved by advanced collaboration schemes. The throughput peaks in the vicinity of the base stations can be exploited by a sophisticated scheduler scheme. Regarding the traffic type, UEs with delay sensitive data could be assigned resources such that their requirements are fulfilled. UEs with best-effort traffic models could mostly be served when the train is closer to an RU and a higher total data rate is available. While it is possible in the relay approach to let the whole train appear as the equivalent of one or two UEs (given the proper frequency setup), this option is not available for the directlink approach. Each active passenger appears as an individual UE. Therefore, many handovers must be executed when the train moves from one cell to another. Considering the high speeds and that even "inactive" passengers, not actively transmitting data, must be handed over to the next cell, this can lead to a considerable amount of traffic on the control channel and might in extreme cases lead to blocking of UEs. As mentioned above, the deterministic location of the train and the semi-deterministic location of the UEs (confined inside the train, but not known in particular) enables new concepts of handover and cell extension, including moving cell, smart handover schemes, and sliding handover. The moving cell concept is shown in Fig. 4 . For both approaches, relay and direct-link, it increases the cell length from around 1 km to a maximum of up to 50 km. Commercial systems with the necessary capabilities for that task are already available. 3 The working principle is that many remote units (RUs) are associated with a central control entity via RoF. When the train leaves the range of one RU, the central unit will reroute the data-stream toward the next RU along the tracks. In that manner, the cell "travels" with the train (in an abstract sense, this can be interpreted as a semi-static beamforming scheme), enabling a transparent and smooth transition, without the need for handovers over longer distances. Considering a speed of 250 km/h, the transition of a cell of 1 km length takes approximately 15 seconds and a handover becomes necessary. With the moving cell concept, the period of reoccurring handovers is enhanced to 12 minutes. The concept is further detailed in [9] . Smart handover schemes have been reported, e.g. in [10] . Such schemes improve handover performance, but assume cells simply touching each other on the outer cell borders. Nonetheless, they do not omit the issue that many handovers must be handled simultaneously when transitioning from one cell to the next. To further alleviate this issue, a possible solution would be to spread out the handover region wider than just between the two RUs at the cell borders. We call this a sliding handover. It is conceptually illustrated in Fig. 4 . In the figure, it is observed that the cell overlap spans three RUs. When the train enters the handover zone, the control entity signals to a fraction of the UEs to perform a handover, while the others remain in the current cell. To find the optimal point for initializing the handover, the knowledge of the exact train position and the predefined trajectory (given by the tracks) can be exploited. In order to generate such an overlap region, the RU in the middle of the handover region is shared by both cell control centers. It can also transmit on different frequency bands and exploit coordinated multipoint (CoMP) schemes in order to avoid excessive interference in the handover zone. Fraction-wise handover avoids overloading the control channel. To further smooth the handover situation, the handover zone can be extended arbitrarily. This effectively leads to a 3 http://www.kathrein. de/indoor reduction of the total cell length (because of the longer overlap), which is, however, only a small fraction compared to the total length. The aspects of wireless communications in HST scenarios that have been discussed in the previous sections considerably affect many design choices for an actual system. The choice between the relay and direct-link approaches is aggravated by the fact that many of the presented aspects are interdependent, thus influencing each other. In this section these interdependencies are discussed from the perspectives of train operators and mobile-operators. The interior of a train is characterized by many objects such as seats and baggage racks as well as passengers that reflect or absorb electro-magnetic waves. For the direct-link and the relay approaches, this has to be taken into account. The actual position of the UE might have an even greater impact than the characteristics of the outdoor link. In the relay approach, only the characteristics of the interior of the carriage need to be considered for the second part of the link. For the direct-link approach, a possible loss due to penetration through compartment walls needs to be considered. Such a path can even be dominant over one passing directly through the window closest to the UE, since penetration through windows might show a strong dependence on the angle of incidence. This has to be considered in the modeling of the direct-link channel. Possible shadowing of a train passing by in the opposite direction constitutes a further issue that might be included as a possible environment scenario, which occurs with low probability. The selection of the carrier frequency also considerably impacts the overall system performance. The most relevant options are: • Licensed bands (i.e. LTE-Advanced (LTE-A) frequencies). • Unlicensed bands (industrial, scientific and medical (ISM) bands). • Free bands in mmWaves. On the one hand, licensed bands allow access to mobility management. On the other, distinct bands might experience a considerably different interference environment. In rural scenarios the interference from neighboring BSs can become a major issue, while for ISM bands, due to the limited equivalent isotropically radiated power (EIRP), and due to the lack of close interferers, the interference will be smaller. The limitation of the radiated power could be compensated with the deployment of very cost efficient RUs along the tracks, being spaced at very short distances. The application of mmWave is currently a hot topic for dense static deployment scenarios; however, their applicability for high mobility scenarios has yet to be scrutinized. 4 A possible application could be to radiate mmWaves from the tracks (i.e. from surface level) to the train base (assuming antennas are installed there). The choice of a relay approach or a direct-link approach also strongly impacts the optimal placement and orientation of the RUs. While transmitter directions being aligned along the tracks work well in combination with relays, a positioning off the tracks with an antenna orientation perpendicular to the track-orientation can be beneficial for the direct-link approach, as it may overcome issues with a shallow angle of incidence, causing high penetration loss through the windows. Besides all the technical aspects, mobile operators will also consider economic factors such as CAPEX and OPEX, political issues, necessary agreements, and complexity of implementation. The two key players in a HST scenario are the train operators and the mobile operators, whose perspectives might considerably differ from each other. For a mobile operator, the direct-link approach comes with the benefit of not having to rely on additional hardware being installed on the train. Thus, their system for supplying wireless access becomes more independent. The type of train only determines the expected penetration characteristics. Additionally, the mobile operator has all options for mobility management as opposed to a relay scenario with WiFi on the second part of the link. A train operator might also favor the direct approach, as they are not obligated to install and maintain additional hardware, but more importantly for not having to deal with any legal issues when trains are crossing borders. For both sides this approach comes with the benefit of reduced necessity to synchronize with each other. Along with this first decision comes the question of the average performance that should be supplied to the passengers. Most importantly this affects the optimal placement and distance of the RUs. The total amount of RUs and connected hardware along the tracks (e.g. control entities, acquired sites) determines the CAPEX and OPEX of the whole system. Thus, RU spacing needs to be optimized to provide the required performance while minimizing the cost. Modifying the carriages might change the cost picture completely, thereby offloading some of the cost to the train operator. For example, installing windows with a low penetration loss or carriages with electro-magnetic apertures and employing the direct-link approach will make it possible to significantly increase the spacing of the RUs, thus substantially reducing CAPEX and OPEX. An interesting question for mobile operators is whether the deployed hardware should also be used to supply the vicinity of the train tracks. From a cost perspective, this can be profitable but, simultaneously, it increases the complexity of network planning. For train operators, it is beneficial to keep control over mobile communications when the task for supplying wireless access for their trains is not completely handed off to a mobile operator. On the one hand, the goal of mobile operators is mostly to achieve good coverage with minimal effort. Thus, they will focus on lucrative portions of highly frequented routes. On the other hand, train operators have the goal of providing coverage for the whole rail network, with a minimum performance guaranteed everywhere. Thus, the competitiveness of the train operator is consistently increased, as the additional service is provided comprehensively. Currently, there is growing interest among train operators to employ the hardware for mobile communications as well as for conveying control data for the trains. Since this aspect will be critical for the security of the system, it will be the operators' desire to ensure the dependability of the communication system, including aspects such as stringent latency constraints. From this perspective, many aforementioned arguments need to be reevaluated. For example, train operators typically do not buy their own licensed frequency bands. On the other hand, freely accessible bands such as the ISM bands come with the disadvantage of unpredictable interference, which is not feasible for security-relevant systems. We discussed a variety of different aspects of HST scenarios in this article, with the main focus on the comparison of the direct-link and relay approaches. It was observed that the directlink approach suffers from high penetration loss and a high number of simultaneous handovers, which can be overcome by utilizing materials with lower penetration losses (for windows and/ or carriage walls), and by using sophisticated mobility management schemes such as a moving cell or a sliding handover. Moreover, with sophisticated scheduling schemes, delay-sensitive and best-effort traffic can be differentiated, depending on the train's position. If the technical issues with the direct-link approach can be overcome, it makes possible a less complex system setup and also avoids legal issues with additional hardware installed on the trains. Further considerations of train and mobile operators increase the complexity of the decision for specific system aspects, with cost issues and other factors coming into play, thus making such scenarios subject to multi-objective optimization.  ACKNOWLEDGEMENTS
Mueller et al. REF perform a simulation study to compare the performance of using a system based on train access terminals to direct communication with the UEs on the train.
Providing current and future cellular services to high speed trains
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