Patent Publication Number: US-2016240928-A1

Title: Protection for wireless links at train carriage rooftops against jamming and interference

Description:
BACKGROUND 
     1. Technical Field 
     Embodiments of the subject matter disclosed herein relate to providing secure wireless intra-vehicular communications. 
     2. Discussion of Art 
     Intra-vehicular communication plays an important role in various public and cargo transportation systems (e.g., cruises, trams, metros, articulated buses, trains, and cargo ships) to ensure safety and stable operation of the vehicle. Intra-vehicular communication systems can be used for signaling and control purposes, provisioning of passenger information services, and management of systems on board or which interact with a vehicle (e.g., announcement systems, video surveillance, intercom, heating, ventilation, air-conditioning, broadband services and data-driven control systems). 
     Conventional wired intra-vehicular communication systems utilize wired lines laid along the vehicle body with interconnecting couplers at junction points or interfaces. In comparison with wireless transceivers, physical wires are cumbersome to install, maintain, troubleshoot, and replace. For example, couplers between carriages in articulated buses/trains/metros/trams are frequent points of failure, since the constant motion of the carriages causes the contacts of the couplers to wear out. 
     Furthermore, wired systems have fixed bandwidths, limited data rates, and a limited number of ports. Wired systems cannot be expanded without reinstalling wires across the vehicle. Thus, a wired communication system is expensive and not efficient to upgrade to accommodate future demands. Especially, wired systems are not scalable and practicable enough to provide individually customized user services (e.g., broadband access, multimedia services) for thousands of passengers. 
     Accordingly, it is expected that intra-vehicular communication systems will become increasingly reliant on wireless communication technology to support the efficient, reliable transmission and reception of data in and around the vehicle. The use of wireless technologies for intra-vehicular communication is an economical, expandable, reliable, and user-friendly alternative to wired communications. Moreover, it is easy to upgrade wireless systems to support emerging passenger related applications in the future. Hence, wireless communication is a natural fit for intra-vehicular communication. 
     However, existing architectures are not able to adequately address security challenges in wireless intra-vehicular communication systems. Specifically, open air transmission exposes the control and user traffic to third party attackers. In particular, an interruption of the control data may result in compromising the safety and the smooth operation of the vehicle. 
     BRIEF DESCRIPTION 
     In one embodiment, an intra-vehicular communication system is provided. The system includes, in one embodiment, a vehicle (e.g., rail vehicle) comprising a carriage top, an antenna mounted on an outer-vehicle side of the carriage top, and radio frequency absorbing material positioned proximate to the antenna and at least a portion of the carriage top. 
     In one embodiment, an intra-vehicular communication system is provided. The system includes a vehicle consist (e.g., rail vehicle consist) that comprises at least a first vehicle (e.g., a first rail vehicle) and a second vehicle (e.g., a second rail vehicle). The first vehicle has a first carriage top, a first antenna mounted on an outer-vehicle side of the first carriage top, and first radio frequency absorbing material positioned proximate to the first antenna and at least a portion of the first carriage top. The second vehicle of the plurality of vehicles has a second carriage top, a second antenna mounted on an outer-vehicle side of the second carriage top, and second radio frequency absorbing material positioned proximate to the second antenna and at least a portion of the second carriage top. 
     In one embodiment, an intra-vehicular communication method is provided. The method relates to hardening a wireless radio frequency communication system on a first vehicle of a consist of vehicles (e.g., rail vehicle consist) against a source of jamming or interference located within an interior region of any vehicle of the consist of vehicles. The method comprises installing radio frequency absorbing material proximate to an antenna of the wireless radio frequency communication system that is mounted on an outer-vehicle side of a carriage top of the first vehicle. The radio frequency absorbing material is configured to absorb electromagnetic energy over a responsive radio frequency bandwidth of the antenna. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       Reference is made to the accompanying drawings in which particular embodiments and further benefits of the invention are illustrated as described in more detail in the description below, in which: 
         FIG. 1  illustrates a rail vehicle consist (e.g., train) having a plurality of rail vehicles; 
         FIG. 2  illustrates a consist of rail vehicles; 
         FIG. 3  schematically illustrates a consist of rail vehicles; 
         FIG. 4  illustrates a system for wireless communication in the presence of a jamming source: 
         FIG. 5  illustrates a system for wireless communication in the presence of a jamming source with radio frequency absorbing materials between the jamming source and wireless communication components; 
         FIGS. 6A, 6B, 6C, 6D, 6E, and 6F  illustrate test results relating to  FIGS. 4 and 5 ; and 
         FIG. 7  illustrates a method for hardening a wireless communication system against jamming and interference. 
     
    
    
     DETAILED DESCRIPTION 
     Systems and methods for shielding wireless intra-vehicular communication from jamming are described herein. A system mitigating jamming of intra-vehicular wireless communication includes the use of radio frequency (RF) blocking or absorbing material between likely locations of jamming devices within unsecured areas of the vehicle and transceivers used for intra-vehicular communication. 
     With reference to the drawings, like reference numerals designate identical or corresponding parts throughout the several views. However, the inclusion of like elements in different views does not mean a given embodiment necessarily includes such elements or that all embodiments of the invention include such elements. 
     “Software” or “computer program” as used herein includes, but is not limited to, one or more computer readable and/or executable instructions that cause a computer or other electronic device to perform functions, actions, and/or behave in a desired manner. The instructions may be embodied in various forms such as routines, algorithms, modules or programs including separate applications or code from dynamically linked libraries. Software may also be implemented in various forms such as a stand-alone program, a function call, a servlet, an applet, an application, instructions stored in a memory, part of an operating system or other type of executable instructions. It will be appreciated by one of ordinary skill in the art that the form of software is dependent on, for example, requirements of a desired application, the environment it runs on, and/or the desires of a designer/programmer or the like. 
     “Computer” or “processing device” or “computing device” or “processor” as used herein includes, but is not limited to, any programmed or programmable device that can store, retrieve, and process data. “Non-transitory computer-readable media” include, but are not limited to, a CD-ROM, a removable flash memory card, a hard disk drive, a magnetic tape, and a floppy disk. “Computer memory”, as used herein, refers to a storage device configured to store digital data or information which can be retrieved by a computer or processing element. The terms “controller” or “control system” or “control device” are used broadly herein and may be anything from a simple switching device, to one or more processors running computer-executable software instructions, to complex programmable and/or non-programmable logic circuitry. The terms “signal”, “data”, and “information” may be used interchangeably herein and may be in digital or analog form. 
     The term “functionality” as used herein may refer to the logical actions and supporting display screens of a system implemented in software and/or hardware. The term “electronically” as used herein may refer to performing a task using an electronic device or network, or any equivalent thereof (e.g., a fiber optic device or network, or some other form of digital device or network). The term “nodes” or “node devices” as used herein may refer to legacy equipment devices on a vehicle that are operatively connected to wired infrastructure within a secure zone such as, for example, electrical equipment associated with a rail vehicle. 
     The term “vehicle” refers to any transportation system of one or more coupled compartments. Specifically, as suggested throughout, vehicles need not be a single compartment, permanently-arranged transportation system (e.g., a sedan with fixed capacity), but can include multiple compartment variable arrangements (e.g., a multi-carriage train or bus which can add or remove individual carriages). A single vehicle herein may include multiple sources of power for mobility, such as a railway train having multiple locomotives. 
     In this regard, “intra-vehicular communication” refers to any communication within a single compartment, or between compartments of a vehicle. Intra-vehicular communication need not, and does not in aspects herein, occur within a single compartment, module, car, or carriage, et cetera, of a consist or other multi-part vehicle system. (“Consist” refers to plural vehicles mechanically or otherwise linked for coordinated travel along a route.) 
     As used herein, in regard to the relationship between radio frequency blocking materials and antennas. “proximate” means positioned in such a way as to block at least some radio frequency interference originating from a device below the antenna or another possible position of an interference source. In this regard, proximate can mean located on the same vehicle within a consist, or alternatively on another vehicle of a consist which moves with the vehicle on which the antenna is mounted. In specific instances, proximate may describe elements within a defined distance or angular arrangement, or intervening between two points (e.g., a compartment and an antenna). 
     Radio frequency absorbing material includes materials through which radio signals and/or other energy cannot pass, or materials which permit only a small fraction of such energy to cross between sides of the materials. In instances, radio frequency absorbing material does not reflect a substantial portion of radio signal at its incident strength, but rather permits it to pass partway through without transmitting a substantial portion of the signal through or in another direction (e.g., absorbs or scatters). An example of radio frequency absorbing material can include ferrite tile absorbing material, which can absorbs radio frequencies in a band between 30 MHz and 1 GHz. In another example, polyurethane foam loaded with carbon absorber material in a spectrum between 100 MHz and 40 GHz. These and other materials can be combined or used independently to absorb or block radio signals in these and other frequency bands. 
     While coupling of compartments to define the vehicle will generally involve some form of physical connection or contact, it will be appreciated upon review of the disclosures herein that compartments may function together at a distance (e.g., through magnetism or independent means of locomotion). In instances where at least one transportation compartment is utilized at a distance to another compartment, the vehicle may be defined as a plurality of compartments travelling a substantially matching path as a group in unison. This definition may also apply to other arrangements (e.g., mechanically linked cars or carriages). However, intra-vehicular communication need not be conducted between two cars, carriages, or compartments, and may be used to transfer data between components of a single car, bus, truck, et cetera, for control (e.g., braking, light actuation, locking of windows or doors) or convenience (e.g., stream non-control data). 
     Vehicles need not be confined to any particular type. While examples herein refer to rail vehicles, the disclosures herein are equally applicable to vehicles capable of travelling on improved or unimproved roads, over or through water, airborne, or in space. Further, aspects described herein are not defined to any particular class of vehicle. In a similar spirit, examples herein may be directed toward passenger rail or light rail, but are equally applicable to other classes of rail or transportation. 
     As discussed, intra-vehicular communication herein is conducted wirelessly. Such wireless communication can be facilitated by, for example, a wireless local area network based on IEEE 802.11 standards. Such wireless communication techniques provide a variety of options in single or multi-band, over different channels or frequencies, all of which are embraced by the scope of this disclosure. Further, other wireless communications techniques such as Bluetooth, infrared, near-field communication, optical wireless communication, or according to various cellular network communication means (e.g., 3G, EDGE, 4G WiMAX, 4G LTE). 
     While aspects herein generally refer to wireless transceivers, such aspects are not intended to exclude independent transmitters or receivers, or other support modules (e.g., repeaters or relays), which can be incorporated alternatively or in combination with transceivers described herein. More generally. “wireless communication components” can embrace transceivers as well as all other components involved in wireless communication. 
     As discussed above, vehicles will become increasingly reliant on wireless communication not only for onboard conveniences but also for control of critical components related to vehicle operation and safety. For example, locomotive, environmental control, safety, fuel storage, and other systems are typically distributed across several carriages of a railway train, and end-to-end communication from the lead carriage to the caboose is necessary to the safe and dependable transit of the vehicle. In this regard, interference from the increasing number of unrelated wireless components, and, more dangerously, intentional jamming, have the potential to slow or stop railway progress, or even cause railway disasters. 
     Therefore, disclosures herein may provide systems and methods for improving the integrity and reliability of wireless intra-vehicular communication at least in part by shielding or hardening wireless intra-vehicular communications against jamming or interference signals within or in close proximity to the vehicle. Wireless intra-vehicular communications can be implemented using directional antennas associated with transceivers to provide improved performance, directivity, and signal strength. In this regard, antennas are oriented such that the main lobes of their transmission direction are oriented in a substantially aligned arrangement during most periods of vehicle travel (e.g., aligned unless antennas mounted on different carriages and train rounding a sharp bend). The antennas can maintain line of sight (e.g., mounted in open air on vehicle roof). In alternative or complementary arrangements, one or more antennas can be mounted without line of sight but such that transmission through the intervening materials remains effective for intra-vehicular communication. 
     To ensure the directional signals are not interrupted, material that absorbs or blocks radio frequency waves can be placed around transceivers, associated antennas, or other wireless communication components to mitigate or wholly block inadvertent or malicious signals capable of interfering with communication between two or more of the wireless communication components. In particular, material absorbing radio frequency waves can be placed between a wireless communication component and a potential source of jamming. The radio frequency absorbing material can be placed at the top of internal compartments of one or more carriages on an inner side, as one or more layers of intermediate material within a carriage top, or as a top layer of the carriage between the carriage and the wireless communication component. In this fashion, jamming signals from within a carriage will be denied a direct path to the wireless communication component, and jamming can be prevented. Similarly, this will also tend to block nearby ground-based interference sources. While radio frequency absorbing material are shown in the figures in contiguous configurations within or parallel to the roof, ceiling, or top of vehicles, it is understood that other configurations having multiple distinct sections and alternative sizes or geometries are embraced under the scope of this disclosure. 
     In arrangements where further security is required, partial walls or complete enclosures of radio frequency absorbing material can be arranged around wireless communication components such that the path between wireless communication components is unobstructed but interference from other directions is substantially blocked. Such further configurations can be employed to defeat threat of jamming or interference from airborne, elevated, or sources distant from the path of travel. 
     In configurations where radio frequency absorbing material is arranged in a configuration parallel to the top of the carriage, there may still be limited spaces where a jammer or other interference source within the carriage can be at least partially effective at interrupting traffic between wireless communication components. However, due to the geometric limitations imposed by the radio frequency absorbing material (e.g., a jammer must now be placed near the ceiling at the center of the carriage), the area of carriage to be searched by personnel is drastically reduced, and risk of discovery greatly increased. 
     Further anti-jamming or interference mitigation techniques can be employed in conjunction with radio frequency absorbing material and/or directional antennas. For example, anti-jamming modulation techniques, the use of frequency hopping signals, reduction in data rate, or spread spectrum techniques transmitting and receiving over a wide band can all be employed to further harden the wireless intra-vehicular communication systems and methods. 
     Various radio frequency absorbing or blocking materials can be utilized with aspects herein. Specific materials can be designed or configured to absorb electromagnetic energy of a responsive radio frequency bandwidth of antennas with which they are used in conjunction. For example, a radio frequency absorbing or blocking material can absorb or reflect transmissions with frequencies at or near 2.4 GHz, at or near 5.9 GHz, or between a range exactly or approximately bounded by the frequencies 2.4 GHz and 5.9 GHz. Particular materials used can include, but need not be limited to, dielectric loss type materials, magnetic loss type material, scattering type materials, and interference type materials. 
     Turning now to the drawings,  FIG. 1  illustrates a consist  100  having a plurality of rail vehicles, e.g., carriages. (Rail vehicles are an example of one possible type of vehicle.) As illustrated, the plurality of rail vehicles specifically includes a first rail vehicle  110 , a second rail vehicle  130 , and a third rail vehicle  150 . The first rail vehicle (e.g., carriage)  110  includes a carriage top  112 , an outer vehicle side  120 , an inner vehicle side  118 , and an inner vehicle region  124 . The carriage top  112  includes the roof, ceiling, or outer surface above the compartment(s) defined by the inner vehicle region  124 . Similarly, the second rail vehicle  130  includes a carriage top  132 , an outer vehicle side  140 , an inner vehicle side  138 , and an inner vehicle region  144 , and the third rail vehicle  150  includes a carriage top  152 , an outer vehicle side  160 , an inner vehicle side  158 , and an inner vehicle region  164 . One or more of the plurality of rail vehicles of the consist  100  can include a communication system having wireless communication components. As illustrated, the first rail vehicle  110  includes a first communication system  122 , and the second rail vehicle  130  includes a second communication system  142 . 
     As illustrated, the first rail vehicle  110  and the second rail vehicle  130  are configured for communication between the first communication system  122  and the second communication system  142  using a first antenna  114  and a second antenna  134 . The first antenna  114  and the second antenna  134  can be highly directive antennas, and generally have their main lobes approximating beam solid angle substantially aligned. As illustrated, the first antenna  114  is mounted atop the first rail vehicle  110  and the second antenna  134  is mounted atop the second rail vehicle  130 , providing an uninterrupted line of sight between the first antenna  114  and the second antenna  124 . By providing an uninterrupted line of sight, standard antennas will suffer only minimal loss in communicating wirelessly, and directional antenna use can be optimized by allowing alignment of the primary directions of transmission. 
     The first rail vehicle  110  includes a first radio frequency absorbing material  116  positioned proximate to the first antenna  114 . The first radio frequency absorbing material  116  can be at least a portion of the first carriage top  112 , positioned between the antenna and carriage top  112 , within carriage top  112 , and/or on an inner-vehicle side  118  of carriage top  112 . Similarly, the second rail vehicle  130  includes a second radio frequency absorbing material  136  positioned proximate to second antenna  134 . The second radio frequency absorbing material  136  can be at least a portion of the first carriage top  132 , positioned between the antenna and carriage top  132 , within carriage top  132 , and/or on an inner-vehicle side  138  of carriage top  132 . While the first radio frequency absorbing material  116  and the second radio frequency absorbing material  136  (and other elements) are shown in a generally symmetrical fashion in  FIG. 1  and other illustrations herein, it is appreciated that such symmetrical configurations are not required, and asymmetrical or unequal arrangements are embraced by the scope of this disclosure in configurations alternative to that of, e.g.,  FIG. 1 . 
     Arranged as such, the first radio frequency absorbing material  116  substantially blocks the first antenna  114  from jamming or interference signals positioned below the antenna  114 , and the second radio frequency absorbing material  136  substantially blocks the second antenna  134  from jamming or interference signals positioned below the antenna  114 . Because carriage top  112  and carriage top  132  are in an elevated position, and due to the mobile nature of the consist  100 , possible jamming or interference signals would be highly effective from within or alongside the consist  100  but for the employment of first radio frequency absorbing material and second radio frequency absorbing material. 
     By way of example, a jammer  190  is illustrated in the inner vehicle region  164  of the third rail vehicle  150 . In some configurations, the third inner vehicle region  164  can be, for example, a freight or passenger compartment. Jammer  190  broadcasts a jamming signal, which can be broadcast in all directions from jammer  190  or directionally. At least a portion of the jamming signal can be represented by one or both of jamming vector(s)  191 . As can be appreciated, a “dead space” in which jamming signals are prevented from introducing interference occurs in the space blocked by first radio frequency absorbing material  116  and/or second radio frequency absorbing material  136 . 
     To allow flexibility in discontinuities between the first rail vehicle  110 , the second rail vehicle  130 , and the third rail vehicle  150 , gaps may occur in the radio frequency absorbing material, and it may be possible to orient a jammer or other interference source in a position thwarting the geometries of the first radio frequency absorbing material  116  and/or second radio frequency absorbing material  136 . However, because of the limited areas where jamming may be even partially effective, the spaces to be searched to locate the source of jamming (or other interference) is limited to a very small fraction of the consist  100 . Further, radio frequency absorbing materials may be employed to cover all overhead areas, including flexible or jointed materials covering or overhanging discontinuities between first rail vehicle  110 , second rail vehicle  130 , and/or third rail vehicle  150 . In this fashion, jamming and interference from below the first antenna  114  and/or the second antenna  134  can be prevented. 
     Further, positioning of the first antenna  114 , second antenna  134 , or other components can be rearranged to optimize transmission characteristics or shielding from jamming or interference. For example, to better protect against jamming or interference threats aboard the consist  100 , the first antenna  114  may be positioned toward a front or rear end of the first rail vehicle  110 , and/or raised or lowered based on the arrangement of the first radio frequency absorbing material  116 . 
     While  FIG. 1  and other drawings herein depict a three carriage arrangement for the vehicle consist, it is understood this depiction only provides one example, and that other configurations having more or fewer carriages or component vehicles are embraced under the scope herein. Further, any number of other components (e.g., antennas, radio frequency absorbing materials) can be employed on one or more carriages in varying identical or dissimilar combinations without departing from the scope or spirit herein. For example, one carriage may have two antennas and one radio frequency absorbing material, while another carriage may have one antenna and two radio frequency absorbing materials. Directional and conventional antennas, multiple types of radio frequency absorbing materials, carriages of different design, and other variants can be utilized in single embodiments, in addition to the substantially symmetrical configurations depicted. 
       FIG. 2  illustrates an embodiment of a consist  200  having a plurality of rail vehicles, e.g., carriages. In the embodiment illustrated, the plurality of rail vehicles specifically includes a first rail vehicle  210 , a second rail vehicle  230 , and a third rail vehicle  250 . The first rail vehicle (e.g., carriage)  210  includes a carriage top  212 , an outer vehicle side  220 , an inner vehicle side  218 , and an inner vehicle region  224 . The carriage top  212  includes the roof, ceiling, or outer surface above the compartment(s) defined by the inner vehicle region  224 . Similarly, the second rail vehicle  230  includes a carriage top  232 , an outer vehicle side  240 , an inner vehicle side  238 , and an inner vehicle region  244 , and the third rail vehicle  250  includes a carriage top  252 , an outer vehicle side  260 , an inner vehicle side  258 , and an inner vehicle region  264 . One or more of the plurality of rail vehicles of the consist  200  can include a communication system having wireless communication components. As illustrated, the first rail vehicle  210  includes a first communication system  222 , and the second rail vehicle  230  includes a second communication system  242 . 
     The first communication system  222  is configured to communicate, at least in part, using a first antenna  214 , which is shielded from interference and jamming by a first radio frequency absorbing material  216 . The second communication system  242  is configured to communicate, at least in part, using a second antenna  234 , which is shielded from interference and jamming by a second radio frequency absorbing material  236 . 
     To better facilitate communication between the first communication system  222  and the second communication system  242 , the third rail vehicle  250  can have a repeater  268  that is configured to relay signals broadcast from the first antenna  214  to the second antenna  234 , and/or from the second antenna  234  to the first antenna  214 . The repeater  268  can be hardened in a fashion similar to the first antenna  214  and the second antenna  234 . This is done by integration of a third radio frequency absorbing material  256  below, within, and/or above third carriage top  252 . The third radio frequency absorbing material  256  prevents exploitation of or interference with the repeater  268 , ensuring the integrity of the distributed communication system rather than simply insulating endpoint antennas. Such additional hardening is not limited to the repeater  268 , and radio frequency absorbing materials or other techniques can be used to protect various other components susceptible to harm or interruption from wireless jamming or interference signals. 
       FIG. 3  illustrates another embodiment employing alternative geometries of radio frequency absorbing materials. A consist  300  has a plurality of rail vehicles (e.g., carriages). In the embodiment illustrated, the plurality of rail vehicles specifically includes a first rail vehicle  310 , a second rail vehicle  330 , and a third rail vehicle  350 . The first rail vehicle (e.g., carriage)  310  includes a carriage top  312 , an outer vehicle side  320 , an inner vehicle side  318 , and an inner vehicle region  324 . The carriage top  312  includes the roof, ceiling, or outer surface above the compartment(s) defined by the inner vehicle region  324 . Similarly, the second rail vehicle  330  includes a carriage top  332 , an outer vehicle side  340 , an inner vehicle side  338 , and an inner vehicle region  344 , and the third rail vehicle  350  includes a carriage top  352 , an outer vehicle side  360 , an inner vehicle side  358 , and an inner vehicle region  364 . One or more of the plurality of rail vehicles of the consist  300  can include a communication system having wireless communication components. As illustrated, the first rail vehicle  310  includes a first communication system  322 , and the second rail vehicle  330  includes a second communication system  342 . 
     The first communication system  322  is configured to communicate, at least in part, using a first antenna  314 , which is shielded from interference and jamming by a first radio frequency absorbing material  316 . The second communication system  342  is configured to communicate, at least in part, using a second antenna  334 , which is shielded from interference and jamming by a second radio frequency absorbing material  336 . 
     The first radio frequency absorbing material  316  has a vertical extension  317  creating a vertical barrier at an angle to the planar section  315  of the first radio frequency absorbing material  316  substantially parallel to the carriage top  312 . The vertical extension  317  can be incident to the planar section  315  at any angle, but is shown at a substantially perpendicular angle in  FIG. 3 . The vertical extension  317  can be sized and oriented to avoid blocking or interrupting signals transmitted from or received by the first antenna  314 . For example, the vertical extension  317  can be limited to a height below the height of the main lobe of transmission where the first antenna  314  is a directional antenna. 
     The second radio frequency absorbing material  336  includes an enclosure  337  surrounding the second antenna  334 . The enclosure  337  is shown substantially continuous with a planar section  335  of the second radio frequency absorbing material  336  (e.g., second radio frequency absorbing material is substantially monolithic with the enclosure  337  and the planar section  335  built as a single structure), but may be a distinct component in some configurations. As illustrated, one side of the enclosure can be left open to permit clear line of sight or minimize signal disruption sending and/or receiving to, e.g., the first antenna  314  or another wireless communication component. 
     The benefit of such alternative embodiments can be appreciated in view of a situation where a jammer  390  is within the third rail vehicle  350 . The jammer  390  broadcasts a jamming signal, represented at least in part by a jamming vector  391 . As shown, by including the vertical extension  317 , the standoff between the first antenna  314  and spaces influenced by a jamming signal from jammer  390  is increased without obstructing communication related to the first antenna  314  or dramatically increasing the size of the radio frequency absorbing material  316 . 
     The jammer  390 ′ illustrates the benefit of using the enclosure  337  to counter an external jamming threat. The jammer  390 ′ broadcasts a jamming signal represented at least in part by a jamming vector  391 ′. As shown, the planar section  335  would not protect the second antenna  334  from aerial or other external (e.g., outside the consist  300 ) jamming threats arranged at an elevated height or sufficient distance. The enclosure  337  defeats such threats by shielding the second antenna  334  in directions on which the second antenna  334  does not transmit or receive. In this fashion, a jammer must be placed within or very close to a transmission path to interrupt signals between the second antenna  334  and the first antenna  314 , making jamming extremely difficult for static and even mobile external threats. Further, while the enclosure  337  is shown approximately fitted to the second antenna  334 , protection can be further enhanced by using a lengthened enclosure with the second antenna  334  set back under a further length of the enclosure  337  (e.g., portion of the enclosure  337  closer to the first antenna  314  empty, portion of the enclosure  337  farther from the first antenna  314  houses the second antenna  334 ). In an example, the enclosure  337  can be approximately the length of the second rail vehicle  330 , thereby shielding a portion of the space through which the second antenna  334  transmits and receives, and making it nearly geometrically impossible to jam the second antenna  334  without arranging the jammer  390 ′ directly in the line of sight of the second antenna  334 . 
       FIGS. 4 and 5  are provided to illustrate the benefit of using radio frequency absorbing materials in conjunction with antennas to combat jamming and interference.  FIG. 4  illustrates a system  400  in which a first antenna  410  and a second antenna  420  exchange information wirelessly in conjunction with a first wireless communication system  412  and a second wireless communication system  422 , respectively. Endpoints  414  and  424  are employed to provide data for transmission and reception, and assess the effects of jamming by a jamming source  430 , arranged between the endpoints  414  and  424 .  FIG. 5  illustrates a system  500  in which a first antenna  510  and a second antenna  520  exchange information wirelessly in conjunction with a first wireless communication system  512  and a second wireless communication system  522 , respectively. Endpoints  514  and  524  are employed to provide data for transmission and reception, and assess the effects of jamming by a jamming source  530 , arranged between the endpoints  514  and  524 . The distinction between  FIGS. 4 and 5  is the integration of a first radio frequency absorbing material  516  and a second radio frequency absorbing material  526  blocking the jamming source  530  from the first antenna  510  and the second antenna  520 , which is not present in system  400 . In contrast, therefore, the jamming source  430  has line of sight to first antenna  410  and second antenna  420 . 
     The spatial arrangement of tests conducted in  FIGS. 4 and 5  can be arranged to mimic those existing in vehicles where radio frequency absorbing material is to be integrated. For example, as implemented in some arrangements of intra-vehicular communication on trains or other consists, first antennas  410 / 510  and second antennas  420 / 520  can be separated by approximately 4.5 meters, and elevated approximately 2 meters above jammers  430 / 530  (e.g., mimicking the approximate relative location of a jammer in a passenger or freight compartment). Other configurations placing first antennas  410 / 510  and second antennas  420 / 520  at different distances, heights, angles, or intervening media can be utilized to replicate conditions for other vehicles. 
     Results of tests conducted in accordance with  FIGS. 4 and 5  are illustrated in  FIGS. 6A-6F . The test results were produced by arranging first antennas  410 / 510  and second antennas  420 / 520  approximately 4.5 meters apart and 2 meters above jamming sources  430 / 530 , with an air transmission media. In the arrangement used to produce the results of  FIGS. 6A-6F , first antennas  410 / 510  and second antennas  420 / 520  are single input, single output directional antennas broadcasting over dual band 802.11 wireless with a center frequency of 2452 MHz (2.4 GHz). Jammers  430 / 530  have a jamming signal bandwidth of 20 MHz centered at 2452 MHz. 
       FIGS. 6A and 6B  show transmission throughput in megabytes per second over a period of time with a jamming signal power of 0 dBm for system  400  and system  500 , respectively. As can be appreciated based on the dramatic improvement in throughput in  FIG. 6B , throughput between the first antenna  510  and the second antenna  520  is nearly triple, on average, the throughput between first antenna  410  and second antenna  420 .  FIGS. 6C and 6D  show the impact of increasing the jamming signal power to 10 dBm, where inclusion of the first radio frequency absorbing material  516  and the second radio frequency absorbing material  526  provides a much higher maximum transmission throughput between the first antenna  510  and the second antenna  520  and approximately triple the average transmission throughput in comparison to the first antenna  410  and the second antenna  420 . This improvement is further emphasized in  FIGS. 6E and 6F , which show the impact of increasing the jamming signal power to 20 dBm. Inclusion of the first radio frequency absorbing material  516  and the second radio frequency absorbing material  526  provides a maximum transmission throughput and average throughput between the first antenna  510  and the second antenna  520  over four times that between the first antenna  410  and the second antenna  420 . 
     In view of the exemplary devices and elements described herein, methodologies that may be implemented in accordance with the disclosed subject matter will be better appreciated with reference to the flow charts. While for purposes of simplicity of explanation, the methodologies are shown and described as a series of block steps, the claimed subject matter is not limited by the order of the block steps, as some block steps may occur in different orders and/or concurrently with other block steps from what is depicted and described herein. Moreover, not all illustrated block steps may be required to implement the methods described herein, and aspects described as alternative or complementary steps, or as functionality of systems or apparatuses herein, may comprise a portion of methods even if not so illustrated. 
       FIG. 7  illustrates a method  700  of hardening a wireless radio frequency communication system, on a first vehicle (e.g., first rail vehicle) of a consist of vehicles (e.g., consist of rail vehicles), against a source of jamming or interference located within an interior region of any vehicle (e.g., any rail vehicle) of the consist of vehicles. Method  700  begins at  702  and proceeds to  704 , which comprises installing radio frequency absorbing material proximate to an antenna of the wireless radio frequency communication system mounted on an outer-vehicle side of a carriage top of the first vehicle. In some configurations, the radio frequency absorbing material is configured to absorb electromagnetic energy over a responsive radio frequency bandwidth of the antenna. After installation is complete, method  700  proceeds to end at  706 . The method may further include supplemental steps of: providing the radio frequency absorbing material; positioning the radio frequency absorbing material at the location proximate to the antenna; after installation, testing the radio frequency absorbing material as installed, in terms of its effectiveness in absorbing electromagnetic energy over the responsive radio frequency bandwidth of the antenna; and/or based on the testing, re-positioning the radio frequency absorbing material, adding additional radio frequency absorbing material to increase the size of an area covered by the radio frequency absorbing material, and/or adding additional radio frequency absorbing material to increase a thickness of the radio frequency absorbing material. 
     The aforementioned systems, components, architectures, environments, and the like have been described with respect to interaction between several components and/or elements. Such devices and elements can include those elements or sub-elements specified therein, some of the specified elements or sub-elements, and/or additional elements. Further yet, one or more elements and/or sub-elements may be combined into a single component to provide aggregate functionality. The elements may also interact with one or more other elements not specifically described herein for the sake of brevity, but known by one of ordinary skill in the art. 
     Discussing specific embodiments, in one embodiment a rail vehicle comprises a carriage top, an antenna mounted on an outer-vehicle side of the carriage top, and radio frequency absorbing material positioned proximate to the antenna and at least a portion of the carriage top (e.g., near the antenna). In an optional embodiment, the radio frequency absorbing material is positioned between the antenna and the carriage top. Another option provides the radio frequency absorbing material positioned within the carriage top. Still another option provides the radio frequency absorbing material positioned on an inner-vehicle side of the carriage top. The radio frequency absorbing material can be configured to absorb electromagnetic energy over a responsive radio frequency bandwidth of the antenna, and may be configured to operate at or around one or more of the frequencies of 2.4 GHz and 5.9 GHz. The radio frequency absorbing material can comprise one or more of a dielectric loss material, a magnetic loss material, a scattering material, and an interference material. 
     In another embodiment, a consist having a plurality of rail vehicles is provided. The consist comprises at least a first rail vehicle and a second rail vehicle of the plurality of rail vehicles. The first rail vehicle has a first carriage top, a first antenna mounted on an outer-vehicle side of the first carriage top, and first radio frequency absorbing material positioned proximate to the first antenna and at least a portion of the first carriage top (e.g., near the first antenna). The second rail vehicle of the plurality of rail vehicles has a second carriage top, a second antenna mounted on an outer-vehicle side of the second carriage top, and second radio frequency absorbing material positioned proximate to the second antenna and at least a portion of the second carriage top near the second antenna. The consist can further include a first communication system located within an interior region of the first rail vehicle and operably connected to the first antenna, and a second communication system located within an interior region of the second rail vehicle operably connected to the second antenna. In such arrangements, the first communication system and the second communication system can be configured to wirelessly communicate with each other by direct line-of-sight between the first antenna and the second antenna. Further configurations can include a third rail vehicle of the plurality of rail vehicles having a third carriage top, a repeater apparatus mounted on an outer-vehicle side of the third carriage top, and a third radio frequency absorbing material positioned proximate to the repeater apparatus and at least a portion of the third carriage top near the repeater apparatus. In such configurations, a first communication system located within an interior region of the first rail vehicle and operably connected to the first antenna can be configured to wirelessly communicate through the repeater apparatus with a second communication system located within an interior region of the second rail vehicle and operably connected to the second antenna. In a further embodiment, the first radio frequency absorbing material is configured to block a direct line-of-sight between the first antenna and a radio frequency jamming device located anywhere within an interior region of any of the plurality of rail vehicles, and/or the second radio frequency absorbing material is configured to block a direct line-of-sight between the second antenna and a radio frequency jamming device located anywhere within an interior region of any of the plurality of rail vehicles. In addition, in arrangements having a third rail vehicle having a repeater, the third radio frequency absorbing material is configured in at least one embodiment to block a direct line-of-sight between the repeater apparatus and a radio frequency jamming device located anywhere within an interior region of any of the plurality of rail vehicles. 
     Another embodiment relates to a method of hardening a wireless radio frequency communication system on a first rail vehicle of a consist of rail vehicles against a source of jamming or interference located within an interior region of any rail vehicle of the consist of rail vehicles. The method comprises installing radio frequency absorbing material proximate to an antenna of the wireless radio frequency communication system mounted on an outer-vehicle side of a carriage top of the first rail vehicle. The radio frequency absorbing material can be configured to absorb electromagnetic energy over a responsive radio frequency bandwidth of the antenna. In further arrangements, the radio frequency absorbing material is installed in one or more positions including between the antenna and the carriage top, within the carriage top, or on an inner-vehicle side of the carriage top. The radio frequency absorbing material can be configured to block a direct line-of-sight between the antenna and a source of jamming or interference located anywhere within an interior region of any rail vehicle of the consist of rail vehicles. The radio frequency absorbing material can include one or more of a dielectric loss material, a magnetic loss material, a scattering material, and an interference material. 
     In addition to the communication systems described, others can be integrated aboard various vehicles and used for other forms of communication within or beyond a vehicle. For example, an internet connection can be enabled with access points aboard a train or other consist. However, the access points would not be shielded from jamming, as this would interfere with passenger ability to connect to the access points. In this manner, there can be more than one wireless communication means used, with wireless communications dedicated to vehicle function hardened against jamming or interference and less critical systems arranged outside hardened areas to facilitate connection stability and bandwidth within compartments of the vehicle. 
     A vehicle (e.g., rail vehicle or otherwise) can comprise a carriage top, an antenna mounted on an outer-vehicle side of the carriage top, and radio frequency absorbing material positioned proximate to the antenna and configured to block at least some radio frequency interference originating from a device below the antenna (e.g., inside an interior of the vehicle). For example, the radio frequency absorbing material may be configured to absorb electromagnetic energy over a responsive radio frequency bandwidth of the antenna. According to an aspect, the radio frequency absorbing material is different from some or all of the materials of the carriage top, e.g., the carriage top may be made of metal sheeting, and the radio frequency absorbing material is a different material than the metal sheeting. According to another aspect, additionally or alternatively, the radio frequency absorbing material is not a structural component of the vehicle, that is, it is not required for maintaining structural stability of the vehicle. 
     Although a “carriage” is indicated in certain examples herein as being a type of rail vehicle, this term is not limited to rail vehicles unless otherwise explicitly specified as such. Thus, unless otherwise specified, uses such as “carriage top” refer to vehicle rooftops generally. 
     In the specification and claims, reference will be made to a number of terms that have the following meanings. The singular forms “a”, “an” and “the” include plural referents unless the context clearly dictates otherwise. Approximating language, as used herein throughout the specification and claims, may be applied to modify any quantitative representation that could permissibly vary without resulting in a change in the basic function to which it is related. Accordingly, a value modified by a term such as “about” is not to be limited to the precise value specified. In some instances, the approximating language may correspond to the precision of an instrument for measuring the value. Similarly, “free” may be used in combination with a term, and may include an insubstantial number, or trace amounts, while still being considered free of the modified term. Moreover, unless specifically stated otherwise, any use of the terms “first,” “second,” etc., do not denote any order or importance, but rather the terms “first,” “second,” etc., are used to distinguish one element from another. 
     As used herein, the terms “may” and “may be” indicate a possibility of an occurrence within a set of circumstances; a possession of a specified property, characteristic or function; and/or qualify another verb by expressing one or more of an ability, capability, or possibility associated with the qualified verb. Accordingly, usage of “may” and “may be” indicates that a modified term is apparently appropriate, capable, or suitable for an indicated capacity, function, or usage, while taking into account that in some circumstances the modified term may sometimes not be appropriate, capable, or suitable. For example, in some circumstances an event or capacity can be expected, while in other circumstances the event or capacity cannot occur—this distinction is captured by the terms “may” and “may be.” 
     This written description uses examples to disclose the invention, including the best mode, and also to enable one of ordinary skill in the art to practice the invention, including making and using any devices or systems and performing any incorporated methods. The patentable scope of the invention is defined by the claims, and may include other examples that occur to one of ordinary skill in the art. Such other examples are intended to be within the scope of the claims if they have structural elements that do not different from the literal language of the claims, or if they include equivalent structural elements with insubstantial differences from the literal language of the claims.