Mass Transit Systems Employ Advanced Tunnel Radio Communications Systems

Welcome to the June issue of GL's Consulting Newsletter

Continuing our series on mass transit communications, we focus on the need to maintain vital radio communications links within subway tunnels, stations and pubic access areas. Modern technologies enable extending the aboveground mobile radio coverage to the underground seamlessly and cost-effectively. Regardless of whether the communications system is above ground or below ground, many of the requirements remain the same:

  • Tunnel radio communications for first responders as well as system maintenance and operation personnel
  • Inter-agency communication and interoperability in tunnels via centralized intelligent command and control center
  • Growing homeland security concerns and need to access voice and data quickly in tunnels
  • Emergency alerts and public safety concerns in tunnels during emergencies
  • Automatic Vehicle Location and Advanced Mobile Data Terminals within tunnels
  • Next Train Arrivals information over the radio links

To meet the above requirements tunnel radio systems can encompass many system technologies - selecting the right one is sometimes not as easy as it looks.

Some of the requirements of tunnel radio systems are:

  • Continuous coverage over the entire length of the tunnel i.e. no dead spots
  • Clear audio throughout with little or no interference
  • Reliable system operation under harsh tunnel environmental conditions
  • Trunked radio channels across many bands
  • Ease of system operation and maintenance

Some of the more interesting points about tunnel radio systems are given below. These are taken from our experience, measurements, and research.

  • Signals propagate better at higher frequencies than lower frequencies, i.e. 800 MHz is better than UHF which is better than VHF. For radio coverage - i.e. reflection is better than absorption.
  • From a propagation point of view - straight tunnels are better than curved tunnels; metal tunnels are better than concrete.
  • BDA - or Bidirectional Amplifiers are a common inexpensive solution for extending coverage, but supporting accessories such as cabling, antennas, filters, and power supplies can increase cost. Unless adjusted properly, interference could be an issue.
  • Leaky coax provides uniform coverage throughout a tunnel, but electromagnetic interference (EMI) is an issue.
  • If EMI is an issue, fiber optic cable can be used to transport RF.
  • A vehicular repeater can be used for temporary and portable situations.
  • Audio switches can be used to connect one radio system to another, but capacity can be an issue.

If you would like some valuable links and a list of vendors of tunnel radio systems - give us a call for a list of vendors. Here are some tunnel projects that GL has worked on:

Tunnel Radio Communications for Metro Train System in Baltimore, MD (MTA)


GL Communications provided system design and technical specifications for the Baltimore Metro Subway to extend system-wide UHF radio communications inside the tunnel and underground stations. We also provided construction management that included inspection and quality assurance services of the newly installed redundant off-air radio system interface, fiber-fed bi-directional amplifiers and RF distribution infrastructure inside the tunnels and underground station platforms.

Tunnel Radio Communications for Metro Train System in Washington DC (WMATA)


Using RF measurement systems, GL's engineers analyzed the signal strength within WMATA's tunnels and identified locations of poor RF coverage. Our analysis and engineering support helped the customer implement remedial actions to improve the RF coverage within the metro tunnels.

Tunnel Radio Communications for Metro Train System in South Eastern Pennsylvania (SEPTA)


GL provided system design and technical specifications for radio communications of aboveground, garage repeaters and underground systems inside tunnels for SEPTA's transit network. Our services included surveys, studies, description, definition and documentation of existing and proposed systems.

Tunnel Radio Communications for Massachusetts Bay Transportation Authority (MBTA)


GL conducted field measurements and propagation studies for a system-wide radio project for MBTA that included detailed link budget analysis for tunnel and aboveground segments. Our studies helped MBTA assess the potential for success of radio communications within the service area.

We hope you will continue to enjoy reading our newsletters. Future issues will highlight GL's support for projects such as:

  • WiMAX and WiFi technologies
  • Simulcast Trunked Radio Systems
  • HVAC and Environmental Control of Outdoor Equipment Cabinets
  • Metro Fire Security Management System
  • Agency-wide Customer Information System for a Transit Network

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