We are two decades into the new millennium, and the wireless communication pioneered in the previous century has become the foundation of every aspect of our lives. Wireless infrastructure is all but a utility, expected both indoors and out, from street level all the way to the top floor of the highest skyscraper. However, certain environments remain a challenge when it comes to wireless solutions. Tunnels are one such setting.
You do not have to be a miner deep underground to have a need for in-tunnel wireless connectivity. Millions of urban commuters pass through thousands of miles of rail and road tunnels every day, and for many, these tunnels represent wireless dead zones in which they are isolated from loved ones and locked out of their work.
However, tunnels do not have to be wireless wastelands. Although there are unique challenges in implementing in-tunnel wireless systems, there are emerging solutions that combat these challenges in an effective and affordable way. The following information examines these challenges and solutions to understand the best approach to in-tunnel wireless.
Tunnels, by their very nature, provide several obstacles for implementing wireless solutions. They are difficult to access and navigate, frequented as they are by vehicles and, in some cases, spanning just a few yards in diameter, and their shape can have different effects on different wireless signals depending on their frequency. Let’s consider these and other challenges of in-tunnel wireless solutions.
Logistics of Tunnel Work
The first challenge to implementing in-tunnel wireless solutions is simply accessing the tunnel itself. Most metro tunnels are in operation for most of the day, from the early morning to midnight or later. In some cases, tunnel operators limit maintenance windows to four hours or less. Compounding this problem is the fact that there are limited entry points to tunnels and limited space inside, requiring effective coordination among all workers. Depending on the work at hand, some projects cannot be left for the next available work window, but instead must be completed in one fell swoop.
Given these complicating factors, in-tunnel wireless solutions must be both simple to install and simple to maintain.
Despite the relatively simple geometry of tunnels, the specific architecture of a given tunnel has a large effect on wireless propagation within. Tunnels can range from narrow to wide, from circular to square, from straight to curved. Existing tunnel infrastructure such as tracks, pipes and cables affects wireless propagation and how signals should be distributed for the best results. As with all wireless installations, the choice of antenna in in-tunnel solutions is an important consideration that must reflect the local environment.
Optical Link Budget
Because tunnels can span several tens of miles, in-tunnel wireless systems must take careful account of optical link budgets. The fiber-optic cable linking head-end and remote radio units will be subject to loss — typically 0.5 dB/km for single-mode (SM) fiber-optic cable. Splicers and connectors along the way will typically cause 0.3 dB loss, and filters used for wavelength-division multiplexing (WDM) will account for a further loss of 0.8 dB to 1.2 dB (for coarse WDM, CWDM, this increases to 1.3 dB).
In a given 6-mile run of fiber optic cable, therefore, you must plan for 6 dB to 8 dB loss or even higher.
Above ground, the best practice for wireless system designers is to ensure minimal overlap in signals among sectors. In a tunnel deployment, zero overlap can create problems such as a high dropped call rate, high noise in the receiving path and uplink muting. To avoid this problem, in-tunnel wireless solutions may change cell radius, reposition antennas or force overlap by connecting sectors together. Carriers typically specify that one subway tunnel and one subway station constitute a sector, but other sectorizations are possible, based on the carrier’s preference.
As with any wireless deployment, in-tunnel wireless solutions must cope with the problem of interference. This includes passive intermodulation (PIM) and intermodulation (IM) interference from passive and active components, respectively. Interference can also include uplink noise from user devices operating at maximum power in an attempt to reach above ground macro cells. In busy subway stations, interference can also arise from many users trying to authenticate on the network simultaneously.
For in-tunnel wireless solutions, these sources of interference must be understood and measures must be taken to mitigate them.
Here is a closer look at the systems and components used to implement in-tunnel wireless solutions.
Distributed Antenna Systems
A properly designed distributed antenna system (DAS) ensures that wireless signals can propagate through the length of a tunnel. System operators commonly use DAS solutions in other environments where building materials can block signals or users require more capacity than generally would be available. A DAS either redirects external signals, such as those from a cell tower, or routes signals from a base transceiver station (BTS) provided by a wireless carrier. The system sends signals to antennas strategically distributed throughout a facility.
A DAS can be passive or active. A passive DAS uses a bidirectional amplifier (BDA) and coaxial cable combined with splitters and couplers to send signals directly to antennas. An active DAS consolidates signals at a head-end unit and sends them over fiber-optic cable to active remote radio units, which then feed into antennas. The typical length of most tunnels makes it necessary to use active systems to overcome signal loss. An active DAS can be analog or digital, referring to whether the head-end unit sends analog optic signals through the fiber-optic cables or whether it converts the RF signals to digital optical signals. For in-tunnel systems, digital DAS solutions generally provide a higher degree of control and flexibility, although analog systems can accommodate a larger bandwidth.
For in-tunnel wireless solutions, a popular approach for distributed antenna systems is an antenna that is literally distributed — in other words, a radiating cable.
A radiating cable, also called a leaky feeder, is a coaxial cable that acts as a distributed antenna. Manufacturers design radiating cables with periodic gaps (or apertures) in the outer conductor that allow the interior RF signals to radiate — analogous to the signal leaking from the cable. The apertures also serve to receive wireless signals broadcast from other sources, such as two-way radios, allowing the signals to propagate through the radiating cable and, therefore, travel much farther than they would otherwise. The design of the outer apertures can vary greatly among different radiating cables to influence the frequency range most suited to the leaky feeder.
Radiating cables are a natural fit for in-tunnel wireless solutions and serve as an effective alternative to traditional antennas, such as Yagi antennas. Radiating cables can cover the long distances typical of tunnels while providing consistent coverage along their length. Because of their leakiness, radiating cables must be used with signal amplifiers at regular intervals, depending on their rated longitudinal loss (e.g., 3 dBm per 100 meters of cable) which increases with signal frequency. In general, an amplifier is needed for roughly every three-tenths of a mile of cable, depending on frequency bands and cable size. Designers of in-tunnel wireless solutions also must consider the coupling loss of radiating cables, which refers to the loss between the cable and end user device. For best results, the radiating cable should be in line-of-sight of end devices, not hidden behind false ceilings or placed within cable ducts.
To understand the variety and parameters of radiating cables better, we take as an example the Radiaflex series of radiating cables from Radio Frequency Systems (RFS). Radiaflex cables comprise seven series tailored to different in-tunnel (or other confined coverage) wireless solutions, encompassing all major services from 75 MHz to 6 GHz. Some Radiaflex cables provide wideband coverage for multiband and multi-operator applications, while others are optimized for narrower high-frequency applications. All Radiaflex cables boast low longitudinal and coupling losses, which are critical characteristics for system design and total cost. Radiaflex cables conform to major international flame- and fire-retardancy standards
The choice of antennas in a wireless solution can have a significant effect on the cost, look and performance of a given system. For in-tunnel solutions, there are pros and cons to balance in the choice between enclosed Yagi antennas and radiating cables. With their controlled pattern of radiation perpendicular to the cable, radiating cables provide uniform coverage throughout the tunnel, whereas enclosed Yagi antennas provide less consistent coverage. Furthermore, radiating cables require little clearance from tunnel walls, while Yagi antennas require more space. Clearance can be a problem in tunnels, where space is at a premium. The advantage of Yagi antennas for in-tunnel solutions is that they are easier to install and cost less than radiating cables, which require full tunnel access and installation that is more complex.
Effective in-tunnel wireless solutions are critical to ensure the safety of tunnel crews and commuters. Tunnels are particularly challenging environments in the event of disasters such as fire, because they are confined spaces with few exit points. It is therefore imperative to support safety workers with reliable wireless communications in any scenario.
Furthermore, as transportation technology continues to improve and vehicles become increasingly networked, in-tunnel wireless communications will be necessary for the reliability of connected systems. Self-driving vehicles may rely on edge or cloud compute capabilities to function safely in all situations, and tunnels should not disrupt any required mission-critical connectivity.
To ensure safety, in-tunnel wireless solutions must accommodate a wide range of wireless technologies, from two-way emergency bands — such as Family Radio Service (FRS) and General Mobile Radio Service (GMRS) in North America and Terrestrial Trunked Radio (TETRA) and Private Mobile Radio (PMR) in Europe — to modern cellular bands up to 5G and beyond. Note that although sub-6 GHz 5G is attainable in tunnels with the right equipment, it is unlikely that millimeter-wave (mmWave) 5G will be practical for such environments. In road tunnels, FM and digital audio broadcasting (DAB) services must also be supported, both for safety and to ensure commuter satisfaction.
Although the complexity of wireless systems generally increases with the number of technologies supported, some in-tunnel solutions are specially designed to accommodate a wide range of wireless technologies. The RFS Radiaflex radiating cables, for example, support multiple operators with continuous coverage in bands in all standardized 4G and 5G frequency bands simultaneously to mission-critical spectrum.
The equipment used for in-tunnel wireless solutions should be designed to withstand hazardous conditions, such as fires, to ensure it continues to function during an emergency. Some wireless equipment providers offer fire-resistant components to ensure uninterrupted service during emergencies, such as the RFS DragonSkin coaxial cable, which is fire-resistant up to 1000°C and is UL 2196-certified for low-smoke, zero-halogen (LSZH) emissions. Cable installation must also account for the potential of fire. For example, if radiating cables are installed with clamps or cable ties, it is important to incorporate metal versions of these fasteners alongside the cheaper plastic clamps or ties. These prevent the cable from detaching in the event of a fire that causes plastic fasteners to fail, ensuring the cable remains functional and keeping it clear of potential escape routes.
Although in-tunnel wireless systems face challenges not found in other environments, the solutions discussed here are designed to surmount these obstacles and ensure that tunnels are both safe and enjoyable. To design wireless systems that meet all the needs of a given tunnel while minimizing total cost of ownership (TCO), it is important to collaborate with the right wireless suppliers. Look for providers with a proven background of successful in-tunnel deployments, like RFS, who can draw upon their ample experience with in-tunnel wireless solutions to offer fit-for-purpose equipment and informed advice.
Source: Gap Wireless
To obtain a PDF copy of the “In-Tunnel Wireless Report,” click here.
According to RFS CEO Monika Maurer, few companies have a history of world firsts and inventions that date back to 1900. “We are very proud of our legacy and our groundbreaking innovation,” she said in a YouTube video.
A news release from RFS marking the occasion says that the company has been responsible for industry developments ranging from wire in 1900 to radiating cable for connectivity in tunnels and is currently working to ease the evolution to 5G wireless communications.
The company traces its beginning to German engineer Louis Hackethal. A commemorative video RFS produced says Hackethal invented the first insulated wire for telecommunications.
The company Hackethal-Draht-Gesellschaft in Hannover was created to make cables.
Throughout its history, RFS has been responsible for a number of key inventions in cable and antenna technology. These range from creating that first insulated wire for telecommunications in 1900, to designing in 1972, under the name KabelMetal, the first radiating cables to deliver connectivity in tunnels, which are used in 41 percent of metro rapid-transit train stations worldwide.
In 1983, the video explains, KabelMetal was part of mergers that led to the creation of Radio Frequency Systems.
This year, the company’s research-and-development (R&D) teams developed Dragonskin, the first standalone cable to meet the most stringent fire safety standards, while integrating 5G-ready capabilities across its entire portfolio.
The news release says the company has manufacturing sites, R&D centers and offices in 20 countries and deployments at some of the world’s most recognizable sites. In 2017, it delivered the broadcast solution that sits on top of the One World Trade Center overlooking New York City.
RFS has contributed articles to AGL Magazine and other magazines I edited going back to 1983. Just one example: RFS contributed articles that helped readers to install coaxial cable on towers and install connectors properly. Maybe one of their articles helped you in your job. The articles RFS contributed certainly helped me in my job, and my thanks to the company for that.
“At RFS, innovation is in our DNA,” Maurer said. “We are inventors, and we are focused on your future.”
Here’s to a wonderful future for RFS. 120 years, so far. Imagine that!
Radio Frequency Systems (RFS) has introduced the APXVBLL20X-C and APXVBLL20X-C-I20 models of its RF X-TREME Triple-Band Antenna, which facilitate triple-band site upgrades for reduced cell interference in high traffic areas. BLL RF X-TREME antennas can be used for multiple bands such as LTE 700, LTE 800, Digital Dividend 2, CDMA, GSM, DCS, UMTS and LTE 2.6. With the RF X-TREME portfolio, RFS provides the capacity of three full-band antennas by orienting them side by side to achieve high gain and optimal performance in a single package. Using the entire antenna length for every band instead of the traditional method of stacking antennas on top of one another allows operators to move from a dual-band antenna to a triple-band antenna of the same length and maintain similar gain levels. Providing full-band coverage on every port enables operators to now implement 4xRx and 4xMIMO on any of the higher frequency bands, enhancing cell-edge performance with fewer base stations – which is particularly effective for LTE advanced. www.rfsworld.com
Radio Frequency Systems (RFS) has introduced the APXVBLL20X-C and APXVBLL20X-C-I20 models of its RF X-TREME Triple-Band Antenna. The latest ultra-broadband antennas facilitate triple-band site upgrades for reduced cell interference in high traffic areas. BLL RF X-TREME antennas can be used for multiple bands such as LTE 700, LTE 800, Digital Dividend 2, CDMA, GSM, DCS, UMTS and LTE 2.6.
With RF X-TREME, RFS provides the capacity of three full-band antennas by orienting them side-by-side to achieve high gain and optimal performance in a single package. Providing full-band coverage on every port enables operators to implement 4xRx and 4xMIMO on higher frequency bands for cell-edge performance with fewer base stations.
The aerodynamic BLL models support 694-2690MHz frequency bands. They are triple-band cross-polarized with three arrays (6 ports), 1×694-960 / 2×1695-2690. The radome design reduces wind load and minimizes tower loading. RFS triple-band antennas offer broad variable tilt range from 0-10 degrees, beneficial in dense areas, while maintaining 28dB isolation premium performance.
Variable electrical downtilt enhances precision for controlling intercell interference; tilt is remotely adjustable according to AISG/3GPP standards. The antennas offer integrated RET, manual overdrive and tilt indicator to streamline installation and ease-of-use. Integrated RET serial numbers on the antenna radome ease on-site commissioning and secure remote mapping from OMC. www.rfsworld.com
Radio Frequency Systems (RFS) offers products to help carriers support the new AWS-3 frequency bands. All RFS 65-degree core antenna models are already compatible with AWS-3, and AWS-3 support will be extended to every other RFS antenna model. RFS’ product portfolio for AWS-3 includes new models of filters, diplexers and tower-mounted amplifiers to include AWS-3 paired spectrum, 1755 MHa to 1780 MHz and 2155 MHz to 2180 MHz, all with the same RF performance. The products feature lightweight, low insertion loss, IP67 Class protection and a new triplexer with DC sensing. www.rfsworld.com