The city of Boston has a history of implementing technological breakthroughs for its public safety departments. In 1852, the city broke new ground in the use of technology when the world's first electronically transmitted alarm was sent to the fire department's dispatch center. More than 70 years later, the United States' first two-way public safety radio system between a fire department dispatch center and fire boats patrolling Boston Harbor was functional in 1923. Moving forward in time another 72 years, Boston became the site of a more advanced breakthrough in radio communications.
In late 1995, highway authorities opened a new tunnel under the Boston Inner Harbor. The new tunnel, christened the Ted Williams Tunnel in honor of the former Red Sox star and member of the Baseball Hall of Fame, connects the Greater Boston Area and Logan International Airport. Because the tunnel has yet to be opened to private-sector traffic, the vehicles traveling its 1.67-mile length are used by public safety or service agencies such as the local, county and state police departments, the Boston Fire Department, airport authorities, the Massachusetts Highway Department and the Massachusetts Turnpike Authority. According to highway department officials' projections, more than 18,000 vehicles will pass through the tunnel each day once the tunnel is open to the private sector.
For every public safety, transit department or airport authority vehicle that travels the tunnel, the need for constant contact with its dispatch center is crucial. Whether it's police squad cars or airport maintenance, each trip through the Ted Williams Tunnel could have an effect on the community's well being.
However, the walls of the tunnel pose a man-made barrier to radio frequency (RF) signals that carry radio communications. To work around this barrier, highway planners and officials needed a single radio antenna system that could support 31 channels simultaneously--a system the world never had seen before.
Boston city officials conducted a thorough site survey and presented specifications and requests for proposals to leading wireless communications equipment manufacturers and systems integrators, including our company. After the specs were reviewed, responding companies produced proposed block diagrams of their recommended antenna systems. Based on our technical response and willingness to custom-design necessary components for the unique assembly, Boston city and Massachusetts highway officials awarded the job to us.
As if that weren't enough The degree of difficulty didn't peak with creating an antenna system unique to the world of wireless communications. In addition to the challenges faced by the application engineering team in terms of delivering radio signals for 31 different channels along the complete expanse of the 1.67-mile tunnel, the Ted Williams Tunnel and the antenna system presented two other obstacles.
First, there was the physical impediment of the tunnel's walls and the depth at which the tunnel lies on the harbor floor. The antenna system would have to operate through the length of the tunnel with only two viable locations providing access to local donorto install the necessary equipment. Ventilation buildings on the surface at both ends of the tunnel were the installation sites for a variety of combiners, duplexers, multiplexers, phasing equipment and crossband coupler combiner systems.
The installation of the components in the ventilation buildings was only the first step in delivering RF propagation throughout the tunnel. A method of distributing RF signals along the curvature of the tunnels had to be devised. A distributed antenna system--small interiorstrategically placed throughout a structure--was ruled out because of the sheer number of antennas that would be required. The alternative turned out to be a system of leaky cable running along the roof of the tunnel. Leaky cable is a by-product of conventional coaxial cable with small slits cut though its layers that allow RF signals to seep out in amounts strong enough to cover small areas of square footage. Because the RF signals are distributed at regular intervals, the use of leaky cable precludes the need for additional antennas to be installed throughout the tunnel.
The second, and most difficult, challenge was to create a system that would carry all 31 radio channels necessary for providing wireless communications to the various agencies using the Ted Williams Tunnel. In the Boston area, public safety, highway department and airport agencies use frequencies ranging from 47MHz to 860MHz. Specific frequencies on the antenna system include UHF, VHF and 800MHz trunking systems. To prevent co-channel interference and to attenuate signals between channels, the application engineering team charged with integrating the system used a combination of filtering products and combiners, some of which were custom-designed because of the uniqueness of the project. The same components used to prevent co-channel interference inside the tunnel also eliminate the potential for simulcasts with above-ground systems.
Putting it in perspective The majority of public safety, airport and transit authority representatives driving through the Ted Williams Tunnel probably are unaware of the historic radio antenna system bringing them crucial wireless communications. However, those who participated in the design and installation of the antenna system--as well as department officials in charge of telecommunications for the agencies relying on the tunnel--are well aware of the breakthrough accomplished with the revolutionary system.
A comparison with radio antenna systems functioning in two of the world's more renowned tunnels puts the feat of the application engineers into a concise context. For example, the antenna system operating in the Washington Tunnel, part of the Washington, DC, subway system, supports only seven radio channels, or less than 25% of the channels carried though the Ted Williams Tunnel. In what is arguably the world's most famous tunnel, the Chunnel connecting England and France, an antenna system supports 15 radio channels.