Mobile Communications: Lessons from the MBTA

Oct. 6, 2016

The Boston subway system is one of the oldest in North America, with stations and tunnels that have been in continuous use for more than a century. In 2005, the Massachusetts Bay Transportation Authority (MBTA), decided to provide continuous, end-to-end cell phone coverage for Boston subway riders. Today the wireless network provides connectivity to 40 stations and more than 21 miles of underground and underwater tunnels. The network carries signal through curving tunnels, into century-old stations, and past flood-gates installed during the Big Dig.

The project began with analysis, moved to RF testing, distributed antenna system (DAS) network design and finally, implementation in five phases over 10 years.

Stage 1: Analysis. In the first stage, the design team walked the entire length of the tunnels at night when the trains were not running to understand the layout and identify any special considerations that might block signals, like large pipes or conduits crossing the tunnel or running along the wall. The design had to adapt to decades-old infrastructure in the tunnels and platforms. Unlike newer subways with more generous clearance, trains almost completely fill the old MBTA tunnels, leaving minimal space to allow radio signal to propagate or to install antennas and cable.

Stage 2: Testing and Gathering Signal Data. The RF signals from outdoor antennas penetrate into the tunnels and platforms to varying degrees. Parts of the track run above ground, where signals from service provider towers are strong enough to supply wireless service to riders. To assess signal strength everywhere in the system, the InSite Wireless team measured the pre-existing signal levels on trains and in stations to identify how strong these “background” signals were — since the system is designed to pick up where the outdoor signals leave off. The team also installed test antennas to transmit test signals and rode trains with specialized test equipment to capture actual, received signal levels. With a detailed empirical analysis of RF strengths throughout the system, the design team could identify exactly where to place the remote amplifiers and antennas.

Stage 3: Network Design. InSite worked closely with a strong project team of multiple partners who contributed to various aspects of the RF system design and fiber optic DAS deployment. To solve problem installation areas and along the longer curves, the designers specified radiating cable from radio frequency systems (RFS) instead of antennas. Radiating cable is regular antenna cable with slots cut into the outer conductor in such a way that the cable works like a sprinkler hose to “spray out” low levels of signal along its entire length. Jumper cable was threaded through cable sleeves around the flood-gates near South Station, to connect radiating cable sections and provide continuous wireless service while staying clear of the gates in case they are needed in the future.

When selecting DAS equipment, InSite chose various vendors at different times in the initial phases as equipment evolved. SOLiD joined the project team as Sprint’s preferred vendor adding 800 MHz SMR and a second 1900 MHz layer throughout the system in 2014. InSite also used SOLiD for the Back Bay Amtrak tunnel and Silver Line expansion to Logan Airport. SOLiD DAS solutions require only one fiber to support eight remote units, and the design is modular so it can be upgraded easily as carriers add new frequencies.

Stage 4: Network Buildout. The wireless network was expanded in five phases of construction between 2007 and 2015. By 2015, riders could use wireless services throughout the entire Boston subway system. The network includes 107 remote equipment locations, 454 DAS antennas, 642 remote amplifiers, 82,535 feet of radiating cable and over 21.1 miles of single mode fiber optic cable.

Five carriers have connected to the system, providing service to their subscribers: AT&T, Sprint, T-Mobile, Verizon, and Comcast (wifi on station platforms only). The MBTA network is one of the few neutral host systems in the U.S. that provides coverage throughout the underground platforms and tunnels, so that riders have uninterrupted service everywhere. Since passenger service begins at 5 am daily and ends at 1 am, we had a two-hour nightly window when tunnels were empty and wireless equipment installation could proceed safely.

Five Tips for a Successful Wireless Network

Over the course of this project, we learned what works for the success of a project of this size and scope. Here are five best practices:

1. Design for end-to-end wireless service. Nowadays people use their mobile phones everywhere and all the time. They expect to be continuously connected to their carrier’s wireless network. Dropped connections between stations and poor call quality lead to customer complaints. As you design your network, plan for end-to-end service on platforms, through tunnels, and above ground. This is a challenge in subways with 24-hour service, but in many cities, the network can be extended through the tunnels during off hours.

2. Plan for multiple carriers plus a strong referee. With multiple mobile carriers in most markets, the goal of the transit authority is wireless connectivity for most of the public – which means signing up multiple carriers like Verizon, Sprint, T-Mobile and AT&T. A neutral host can simplify the equation by working directly with carriers to add their services to the network and handle all the technical logistics for network connections.

3. Keep the head end above ground. With extremely limited space in the stations and tunnels as well as access and safety concerns, it makes sense to locate the head-end—where the carriers connect to the wireless network—at an above-ground location that offers both direct connection to the subway and unfettered access for carrier personnel.

4. Build for a hostile environment. Subways create a hostile environment for electronic equipment. The cabling, fiber enclosures and DAS equipment must be able to withstand extremes of heat and cold, vibrations from passing trains, moisture, and ambient brake dust – year-round, 24x7, without special maintenance. Choose your equipment carefully to meet these extreme conditions.

5. Future-proof the network. Don’t take a “good enough” approach to capacity. Instead, expect that your network will face ever-increasing traffic from riders as they tweet photos, watch videos, call friends and browse the Internet. From the outset, we thought ahead to support all carriers and all spectrum. The Boston subway network is designed for 4G now, but we are planning for 5G, with higher frequencies and greater demands from increased ridership.

We’re applying these best practices in other cities. The Los Angeles Metro subway’s DAS network is under construction, with the first five stations and tunnels operational, and InSite has been selected to design and host the wireless network in Atlanta’s MARTA citywide rail system. Both systems will offer full end-to-end coverage in stations and tunnels, and InSite will support all wireless carriers in a neutral host configuration.

Joe Mullin is chief technology officer-DAS at InSite Wireless Group LLC.