I. Introduction
Recently, the evolution of wireless technology has been made dramatically. In the future, it is estimated that the need for Internet access will increase along with ever-increasing mobility [1]. The required service with a high data rate will be the solution that may solve future needs. Long-term evolution (LTE) technology was launched to overcome this issue. LTE represents a step toward advanced radio technology, which is designed to improve network capacity and speed for required bands of operation. Out of the LTE bands, LTE-R is specifically dedicated to railway systems, enabling high-speed wireless voice and data communication inside trains as well as inter-train communications. The wireless communication devices used for network communications need a wider frequency bandwidth because of escalating wireless service requirements. Recently, other frequency bands, such as the lower and upper 5G bands, were included in the service. It is of no doubt that the future 5G mobile networks that are focused on increasing the bandwidth, higher data rates, enhanced mobile broadband services, etc., will soon expand communication technology [2]. In the future, the LTE-R network may be combined with the future LTE and 5G cellular networks for better communication and public safety purposes. For this reason, the antenna element is an important part of these systems [2–4]. For the coverage of various bands (like LTE-R, LTE, and the lower and higher 5G bands), a multiband antenna or wideband antenna can be used for communication purposes.
Fig. 1 shows the railway communications scenario where the rooftop-mounted antenna needs to communicate with the base station antennas located nearby the tracks. Narrow and long path communication areas, such as tunnel environments, are scenarios distinct in railway communications. In these types of environments, end-fire directions are useful for proper communications along with higher antenna gain. Combining the two directional antennas, such as Yagi-Uda antennas and patch radiations, for the required bidirectional radiation pattern is the conventional method (e.g., [5, 6]). Meander line-folded dipoles, ring radiations, etc., are used in a cascaded form to obtain the bidirectional radiation pattern in the reported literature [7, 8]. At millimeter-frequency bands, a high gain is emphasized to overcome signal attenuation. The substrate-integrated waveguide (SIW) cavity antenna, grid antenna array (GAA), and Vivaldi antenna array are commonly available for 5G communications in the reported literature [9, 10].
Although not published in the literature, there are commercial products that have been developed for railway applications, as presented here. A rail antenna can be found in a product website [11], and the data sheet shows that the antenna can be used for multiband applications such as GSM 900, GSM 1800, GSM 1900, UMTS, Wi-Fi, and WiMAX bands. The given antenna can be mounted on the rooftop and has a size of 90 × 100 × 260 mm3 with a weight of 1.2 kg. Further antenna solutions for commercial and freight trains have been mentioned on the data sheet of PCTEL Inc. [12]. It has been shown that multiple antennas can be placed on the roof top of the train for various communication schemes such as land mobile radio (LMR), Wi-Fi, Bluetooth, global navigation satellite system (GNSS), and cellular communications. A multiband antenna for LTE/Cellular/Wi-Fi, GPS/GNSS applications has been made available [13], as the datasheet shows that the antenna was specifically designed for use on trains, trams, and busses operating on surface or underground systems. Rooftop antennas with GPS specially designed for use in trains, trams, and busses have also been made available [14]. This antenna covers all LTE, Wi-Fi, and WiMAX frequencies used worldwide, including global system for mobile communication for railway (GSM-R) and 700–6,000 MHz cellular frequencies. Antennas for railways have been developed for various cellular, and railway applications [15]. The size of the antenna is 200 mm in diameter and 60 mm in height. In general, the antennas used for railways are generally inverted F-antennas for rooftop applications, which are resistant against environmental problems such as snow and rain [16]. The communication-based control system (CBTC) also uses the Yagi-antennas in tunnel environments due to safety concerns [17]; these antennas are used to connect with the different directions properly due to their directional behaviors. The frequency bands reported for the railway applications are summarized in Table 1 [16, 18–22].
From the above review on the commercial antennas, however, with reference to the respective data sheets from the various product websites, it is observed that the details of the antenna structures are still missing in the research literature, and only the specification and antenna parameters are discussed. Also, antennas with higher 5G frequencies, such as 28 GHz band antennas, are missing from the literature for railway communications, although they could be the possible communications standard in the near future.
A detailed review of the antennas available in the required frequency bands is presented in this paper. The organization of the paper is as follows. Section II presents a review of single-band antennas, proposed dual-band antennas, and multiband antennas for generic communications. Section III presents the review of aerodynamic shark-fin radome cover design. Section IV presents the novel shark-fin antenna design studies with multiband behavior, including LTE-R, LTE, lower-5G, and 28 GHz frequency bands based on the reported research. Lastly, the conclusion is presented in Section V.
II. Antennas for Generic Communications
Various antenna design topologies for diverse frequency bands (LTE-R, LTE, lower and upper 5G) are reviewed in this section.
1. Single Band Antenna
A patch antenna on FR-4 substrate with partial ground plane has been studied for LTE700 MHz applications [23]. Since bandwidth (BW) improvement is also required, the rectangular slots on the main radiators are used to provide multiple current paths of about similar length at the resonance frequency. The reported antenna size is 135 mm (y-direction) × 32 mm (x-direction) × 1.6 mm (z-direction). The impedance BW of 87.3 MHz (|S11| dB <−10) is in the range of 697.6–784.9 MHz. A maximum antenna gain of 2.093 dB is observed along with the omnidirectional radiation in the xz-plane. Since the radiation performance of this antenna is omnidirectional, as can be seen in Fig. 2, it can be used for railway applications when installed on the train roof in the xz-plane and feeding from/through the train rooftop arrangements. However, a stability issue regarding train roof adjustments may occur, as the antenna is thin and lightweight. Some support is needed to secure it solid at the required place to prevent it from external environmental conditions, such as airflow, temperature, rainwater, lightning, etc.
Similarly, an antenna for the operating band of 4G LTE (single band) was reported in the literature with Yagi-biquad antenna configurations [24] and can be seen in Fig. 3. A bandwidth of about 100 MHz ranged from 2,364–2,464 MHz. The reflector, feeding, and director elements are considered as biquad-shaped structures with a small size variation. All the elements were fixed well on the boom structure, which was made of copper and ran along the length of the antenna elements. A gain of around 9 dB has been reported with a voltage standing wave ratio (VSWR) <1.5. This antenna has a directional radiation pattern, and if it is needed for railway communication, multiple antennas are required to provide omnidirectional radiation, and arrangements or adjustments (multiple ports/feeding network) may arise as issues. In addition, the metallic train roof effects may degrade antenna performance.
An antenna for bidirectional radiation has been presented with circular polarization, as seen in Fig. 4. The ground plane with an etched slot is sandwiched between the substrates at the front and back, and the CPW feed was used to feed the antenna. Two metasurfaces comprising of 2 × 4 corner-truncated patches were placed back-to-back on the top and bottom. This antenna has been studied for use in the 5.21–6.26 GHz frequency range [25]. This type of antenna can also be used in railway communications where front and back radiations are required. However, making the antenna omnidirectional for the multiple lower frequency bands and wide bandwidth is still promising.
Other single-band antennas for lower-5G millimeter-waves have also been reported in the literature [26–29]. Another antenna was reported with an array for 5G sub-6 GHz (5.57 GHz) applications [26]. The patch antenna configuration was used to design the antenna on the RT5880 substrate. A circular ring patch with a rectangular slot was used on one side of the substrate and a complete ground plane was used on the other side. Furthermore, a conventional feeding network was connected to the array to have a higher gain (12 dB) and directional radiation pattern in the required frequency band. Reduced mutual coupling was also considered in the designing of the antenna array. The total efficiency of the reported antenna array was observed to be more than 80% within the operating bandwidth. The antenna has been proposed for 5G mobile phones and handheld devices for wireless communications. Furthermore, antennas for millimeter-wave frequencies such as 28 GHz and 58.10 GHz have been reported and discussed adequately in the literature. A planar helix mobile phased antenna array has been reported for 5G communication systems with an operating frequency of 28 GHz [27], as shown in Fig. 5.
The proposed array displays circular polarization in the end-fire direction. Over 65° of axial ratio beamwidth and 7 GHz of axial ratio bandwidth were achieved in the proposed design and discussed in detail. An efficiency of 50% at 5 dBi gain is achieved by the proposed phased mobile antenna array with eight elements.
A double F-slot patch antenna has also been discussed in the literature for 58.10 GHz applications [28]. The directivity and gain of the double F-slot patch antenna were 7.55 dBi and 5.99 dBi, respectively. Matching the microstrip antenna and the 75-Ω microstrip line is achieved by the inset feed technique. The reported approach in this work offers major advantages in millimeter-wave applications, such as radio, television, satellite, and radar. Various microstrip antennas, low-profile and compact antennas, wideband antennas and antenna arrays, antennas with beam-steering capability, and frequency tunability have been reported and discussed in the literature for 5G communications [29].
Another work on the 5G antennas for 28 GHz applications has been reported in the literature [30]. The defected ground structure was incorporated into the ground plane to reduce the size of the antenna. The antenna can be seen in the inset of Fig. 6 along with the S-parameters. The size of the antenna is compact in nature and can be applied to devices in which limited space is required. However, to make it more directional, stacked configurations can be made; due to the alignment issue of the stacking layers, the performance can be degraded due to the size limitations of the port connection used.
2. Dual-Band Antennas
Antennas for dual-band applications for LTE-R and LTE applications are discussed here. The proposed antenna with its top view, side view, and current distribution for the dual-band frequencies can be seen in Fig. 7. The antenna consists of a substrate (FR4) with a dielectric constant of 4.4 (tanδ = 0.025) and a thickness of 1.6 mm. The elliptical ring patch (conducting layer) is the main radiator designed to operate for 700 MHz applications. The inner dimensions of the elliptical ring are W3 (minor axis, 22 mm) and Wy1 (major axis, 64 mm). The width of the ring is denoted as t. CPW feeding was used to feed the proposed antenna. The ground plane dimensions (in mm) are Lg × Wsub (47 × 60). The feed line width is W1 (5 mm). The gap between the ground plane and the feed line is k4 (0.5 mm). The F-shaped slots with slot width k2 are integrated into the ground plane of the antenna for observing another band to make the antenna for dual-band operations.
The slot in the ground plane creates the new current path, resulting in one more frequency along with the main frequency. The geometrical parameters of the slot are k1, k3, and t1 (4.5 mm). The proposed antenna has an optimized size of Lsub × Wsub (180 mm × 60 mm) for dual-band (700 MHz and 2,100 MHz) applications. The antenna parameters, such as the elliptical ring width (t), ground plane (Lg), F-Shaped slots (k1, k2, k3), and feed line with (W1), are the key parameters to observe the antenna for dual-band operation for the required frequency bands. The optimized parameter values for the dual-band operation with −10 dB |S11| dB for t, Lg, k1, k2, k3, and W1 are 3, 46, 35.5, 0.5, 22.5, and 5.0 (all dimensions are in mm), respectively.
The current distribution is also discussed here, and it can be observed that for the 700 MHz band, the current path is longer, and for the 2,100 MHz band, the current path is smaller. These paths are mainly formed around the F-shaped slot in the ground plane of the antenna with a higher intensity of current distribution. The red color shows the higher value of intensity, while the blue shows the lower. The green color shows the current intensity in the range between the values of the red and blue colors. The fabricated antenna can be seen in the inset of Fig. 8, along with the S-parameters. The measurement results follow the same trend as in the simulations. However, a frequency shift can also be observed for both frequencies; this may be caused by fabrication errors. The bandwidths observed for the 700 MHz and 2,100 MHz bands are 157 MHz and 160 MHz, respectively. Furthermore, radiation patterns are also studied to observe the radiation behavior for the required frequencies. The radiation pattern for both frequencies exhibit the omnidirectional radiation pattern observed in a previous work [25]. The maximum gain for the 700 MHz band is 2.36 dBi. For the 2,100 MHz frequency band, it is 5.38 dBi. In addition, the radiation patterns are omnidirectional for both frequencies (i.e., 700 MHz and 2,100 MHz). The above antenna works on dual-band performance with the LTE-R band and LTE band frequencies of operation.
Similarly, dual-band performance with LTE-R and lower 5G has also been reported by using the parasitic elements on the second layer in the stacked antenna configuration [30]. The LTE-R (700 MHz) and lower 5G (3.5 GHz) bands that can be used in railway communications were considered for the study. Their antenna geometry consists of an elliptical ring patch antenna along with stacked parasitic elements (circular patches with varying radius) for the higher band operation. The antenna was designed on the FR4 substrate. Two layers of FR4 substrates were used, one for the elliptical ring antenna and another for the stacked circular patches above the feeding line. The antenna was fabricated, and the measurement results have shown good agreement with the concept for the dual-band operation of antennas.
3. Multiband Antennas
A multiband antenna has been reported as a top-mounted train antenna to work for GSM, LTE, and Wi-Fi communication systems [31]. The proposed antenna has a double-sided planar dipole-type configuration with a single feeding port. Roof mounting and coupling effects have been considered in this work, and thereafter the structure was optimized and the measurement or the installation scenario can be seen in Fig. 9. The antenna operates in the bands of 825–960 MHz, 1.7–2.7 GHz, and 5.7–5.9 GHz with |S11| < −10 dB. The geometrical dimensions of the antenna are 116 mm × 40 mm × 1.6 mm. An antenna gain of 4.27–9.82 dBi with omnidirectional (horizontal) coverage was realized to ensure reliable mobile communication. Good agreements were observed between measured and simulated results, and it is reported that this antenna can be used for railway communications.
So far, the antennas for LTE-R, LTE, lower 5G, and 5G bands (single band, dual-band, and multiband) reported in the literature have been reviewed. Some antennas are applicable for railway communications. However, a smaller height for better fixing on the rooftop and covering of the antenna for external protection is required for railway application. Furthermore, the omnidirectional radiation pattern is preferred to improve communication in all directions (horizontally) to outside antennas at low frequencies (< 5 GHz). Along with the antennas, antenna covers are needed to prevent damage due to external conditions, such as water, heat, airflow, etc. The cover must be designed with lower air resistance/better aerodynamic performance in mind so that it can be fitted well on the rooftop of a train or vehicle without minimizing the attenuation on the antenna’s transmitting and receiving performance. In the following section, a study on the aerodynamic performance [16] of the radome cover is presented.
III. Radome Cover Design
An antenna radome is an enclosure/covering structure for antenna, which is used to protect antennas in harsh environments. It is made of material that has minimum attenuation on the transmission and reception of electromagnetic signals by the antenna. A circular-shaped radome cover has been discussed in the literature for RFID antenna elements. Two different feeding mechanisms are used to control horizontal and vertical polarizations [32]. This kind of radome is expected to have good performance when installed in a fixed location to communicate with the other antennas. Furthermore, an automobile antenna unit for roadside communication has been discussed in the literature [33]. The objective shows the communication between the vehicle and the roadside infrastructure. The antenna elements for various applications such as dedicated short range communication (DSRC), GPS, WLAN, GSM were arranged one after the other in a low mutual coupling configuration, and a radome cover was used according to cover all the antennas. Various studies on the different types of radome cover have been reported in the literature [34–36].
From the literature, the size of the cover is modified according to the antenna elements and does not have the specific well-known general shape of the radome, such as circular or rectangular. Various antennas for rooftop configurations have been reported in the literature [37–39], but the use and development of radome structures for railway communications is absent. A shark-fin antenna for the vehicular rooftop application has also been reported for the multiband antenna to operate at GSM/LTE/cellular/DSRC systems [40], but the already-existing commercial shark-fin rooftop quad-band antenna module was used for radome purposes. Recently, a detailed study on cover design along with aerodynamic simulations has been reported [16] and discussed in detail.
In general, the purpose of a radome cover is to lower the air resistance on the antenna element. Air resistance describes the forces that oppose the relative motion of an object as it passes through the air. It is dependent on several factors, including the density of the air, the area of the object, its velocity, and other properties of the object. Aerodynamic simulations to account for the drag coefficient are required to design the proper cover for the antennas. In this work [16], two covers (rectangular box and shark fin) are analyzed as a candidate of the radome structure for the antenna elements. The size limitation for the rooftop antenna is not taken into consideration as opposed to the mobile phone antenna, where small, compact antennas are required. The first radome cover is a rectangular box cover with an air-incident area of 90 mm (height) × 64 mm (width), and the other one is a shark-fin-shaped cover. This airflow mechanism has been reported for various values of wind speed and direction, as shown in Fig. 10. It was observed that, in the rectangular box cover, the airflow was not uniform (higher intensity was seen at the back side of the object), which resulted in the larger drag force on the object, thereby increasing the air resistance. It was also observed that the airflow was smooth (uniform flow of air through the surface) in the case of shark-fin cover compared to the rectangular box cover. The respective drag coefficient to study air resistance was also discussed and observed—due to smooth airflow and lower drag coefficient, the shark-fin cover radome is expected to have more stability than that of the rectangularly shaped cover.
The acrylonitrile butadiene styrene (ABS) material, which is a low-cost, easy-to-machine material for structural applications, with a dielectric permittivity of 2.1 was fabricated and used to analyze the radome structure for the antenna. The thickness effect of the cover on the antenna [16] has also been investigated and reported. It was observed that the impact of the cover on the antenna increased with the thickness of the cover. Thicker material may increase the size of the cover for both the inner and outer sides; the reference point was taken/considered as the center point of the cover thickness. When the thickness of the cover exceeded 4 mm, an intersection with the antenna elements was observed, which changed the antenna characteristics dramatically. Therefore, the thickness was 2 mm for the proposed antenna. The same radome has been used for millimeter-wave antennas at 28 GHz for railway communications [41]. The antenna structure with the shark-fin radome (white color) and rooftop measurement on the train can be seen in the literature [16, 41].
IV. Multiband Antenna Development for Railway Communications
This section discusses a multiband antenna for the LTE-R, LTE, lower, and upper 5G operations. Fig. 11 shows the proposed shark-fin antenna. A thick metallic element (gray color) can be seen above the ground (in different views).
Four separate elements (two for horizontal polarization and two for vertical polarization) can also be seen inside the sharkfin cover with four different feed points. The design of the proposed antenna was divided into two parts.
1. Triple Band Antenna Covering LTE-R, LTE, and Lower 5G Applications
The requirement of the multiband antennas is desired such that, if needed, multiple bands can be operated by a single antenna element. A triple-band antenna design and reported results [16] were discussed, and a detailed study has been provided for railway communications. The S-parameter measurements have shown good agreement with the simulations. Also, the small ground plane and large ground plane (for the real scenario) were considered. For the measurement on the large ground plane, the antenna was kept on the rooftop of the train (the research facility provided at the Korean Railroad Research Institute [KRRI], South Korea) in consideration of the real environment, and the vector network analyzer (VNA) was connected to the same place. The S-parameters have indicated the desired bands of operation with |S11| < −10 dB. Since the antenna ground plane is large enough compared to the antenna element, negligible effects were observed in the rooftop measurement. Furthermore, the radiation performance was measured in an anechoic chamber. As a result, omnidirectional radiation was observed, which is useful for the desired communication in railways.
2. 28 GHz Antenna for Railway Communications
Antenna design studies for railway communications for the millimeter-wave band (28 GHz) have been reported recently [41]. It is considered that the rooftop antenna must be connected to the base station antennas, which are located on either side of the track. In this case, the antenna on the rooftop can effectively communicate with the front and back sides of the train for proper railway communications. The shark-fin cover, dualpolarization behavior, and multiport antenna were also considered in the reported work. The reported antenna element design study was carried out in various steps. The complete antenna element was designed on a single substrate. The initial antenna element was designed with four elements with a feed line coupled in a gap-coupled mechanism. The feeding point was fixed at the center point. Later, the parasitic elements are added to the initial elements in three steps. The improved bandwidth along with the reduced side lobe level was obtained in the final antenna design, in which the radiation directions (main lobe) reached the 84º directions. The complete antenna was fabricated, and the S-parameters were measured. An approximately 2 GHz bandwidth was observed similar to the simulation results.
The radiation pattern was also measured in an anechoic chamber at the Electromagnetic Technology Institute (EMTI) center in Seoul. The main beam directions were observed in the end-fire directions at around 84°, which were similar to the simulation results and can be used in railway communications to communicate well in the front and back directions. To achieve the Circular polarization, two linearly polarized antennas are perpendicularly arranged. Two pairs of horizontal and vertical antennas were arranged in a fitting structure made through 3D printing to fix all the antennas well in the shark-fin cover. The antenna’s S-parameters, along with the mutual coupling between the antennas, were adequately studied and reported. The link budget calculations were also considered with the designed antenna. The same antenna was considered for both receiving and transmitting antennas. For a given power level, the received power was calculated, and a link of around 2 km could be achieved. The proposed study has shown that the antenna base station locations can be 1 km from each other and still retain proper communication.
Both antennas (Antenna 1 and Antenna 2) [16, 41] were combined inside a shark-fin cover discussed in Section III, and measurements were carried out to determine the impedance bandwidth for each operating band. Fig. 12(a) shows the sharkfin antenna with a white-colored radome structure. The sharkfin cover can be seen on the ground plane. Fig. 12(b) shows the triple-band antenna along with the four elements’ arrangement structure for additional 28 GHz operation [41]. The slotted ground was made for feeding to the two horizontal antennas located about the ground plane. Two horizontal antennas (H1 and H2) and two vertical positioned antennas can be seen on the ground plane (fixed on the arrangement structure) along with the lower frequency thick and large antenna element. The multiband antenna has a total of five port antennas, one port for lower frequency operation and four ports for 28 GHz operations along with two different combinations for dual polarizations. It is expected that due to the reflections from any nearby obstacles, the polarization may change from linear to vertical or vice-versa. For this reason, the dual polarization configuration has been used. Also, in a practical or real scenario, the base station antenna for the high frequency operations could be installed on both sides of the track, a little far distance from the center axis along the track and thereby a tilt in the main beam is required. Therefore, the pair of horizontal/vertical element on the either side of the low frequency element has been considered for the communications. The all-configuration fits well inside the designed radome cover for the low-frequency antenna.
The S-parameters were measured with the VNA and can be seen in Fig. 13. It is observed that the complete antenna shows sufficient and required bandwidths for all bands, such as LTE-R, LTE, lower, and upper 5G bands. It is expected that the radiation performance of the antenna for the various frequency bands will have a lower effect on the configuration discussed above. One reason for this is that the thicker antenna (low-frequency antenna) is much larger than the high-frequency antenna, thereby providing sufficient free space for the radiation without any shielding. Another reason is that the high-frequency antenna elements have the end-fire radiation along the longest dimension of the antenna element that becomes parallel to the low-frequency antenna element. This configuration also does not affect the impedance bandwidth of the antenna at operating bands, as discussed in Fig. 11. Each antenna radiation performance, S-parameters, and mutual coupling between the horizontal and vertical elements for 28 GHz elements are adequately discussed and reported. From the above analysis, it is expected that the antenna will be a suitable candidate for future advances in railway communications.
V. Conclusion
In this paper, a review of the antenna with various bands, such as LTE-R, LTE, lower, and upper 5G bands, is presented for railway communications. For railway communications, antennas can be mounted on the rooftop such that the desired omnidirectional pattern can be obtained for communication. In addition, to protect the antenna from heat, water, and other external elements, the dielectric structure (such as a shark-fin cover) is required to cover the antenna so that it may be used in future advanced railway communications. Further, a multiband antenna with an aerodynamic radome cover is discussed as the multiport antenna configuration. The high-frequency antenna has been shown to communicate with the front and back side located antenna. The base station antennas are meant to be positioned on either side of the tracks. Therefore, some angular tilt/bend is needed to have efficient communication with them. The front and back radiation may be considered as power losses in the same direction. The beamforming to overcome the above scenario will be considered as a future work for railway communications.
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