Radio Coverage in Tunnels
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The Trouble With Tunnels Nikki Swartz
Carriers have long considered tunnels and other subterranean environments dead spots that present coverage barriers, but you should think of them as opportunities. By providing coverage in tunnels, train and metro stations, you can reduce churn, become more competitive and even gain new subscribers. Consider that in New York, the Lincoln and Holland tunnels carry a combined 107,000 commuters between New Jersey and Manhattan daily. In Baltimore, 155, 000 vehicles travel the Fort McHenry and Harbor tunnels every day. And in Boston, more than 90,000 commuters will use the Ted Williams Tunnel each day when access limits are removed next year.
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If you lose customers when they drive into these tunnels, you may not get them back when they come out. Carriers are discovering that they can no longer afford not to provide coverage in subterranean environments. Despite the challenges -- wiring tunnels is expensive, difficult and time-consuming -providing coverage in tunnels and subway stations is important. If you don't do it, your competitor will. Cellular One and AT&T Wireless recently negotiated with the Massachusetts Bay Transportation Authority (MBTA) to wire Boston's subway for wireless service. MBTA officials and the carriers agreed
on 1-year pilot programs to equip seven subway stations. MBTA plans to open up bidding to cover the entire subway system soon. The Massachusetts Turnpike Authority also negotiated with Sprint PCS and AT&T Wireless so those carriers could wire the 1.6-mile Ted Williams Tunnel. AT&T Wireless also provides coverage in three other tunnels (the Sumner, Callahan and Prudential) in Boston as part of the pilot program with the MBTA, according to Marty Nee, AT&T Wireless public relations manager for New England. But many other carriers have not taken similar steps, mainly because it is not cheap. Tunnel coverage does not provide much return on investment. It takes a long time to see the payback, and port authority and transit fees can cost more than a typical landlord's fees. Regardless, carriers such as AT&T are realizing that having complete coverage justifies the cost. "With the volume of cars that go through, if you extrapolate that to the regular transit area, it would almost be like having a giant geographic hole in your system," Nee said. ENGINEERING SOLUTIONS There are several ways to design coverage for tunnels and subway stations, including the two most common: distributed antenna systems and leaky coaxial cable systems. First you should consider whether your priority is coverage, capacity or a combination. Another consideration is where you want the coverage. In subway stations, for example, you may want to cover only the stations or the tunnels as well. This determines where you will locate cell sites. Annelizabeth Rogers, Ortel wireless communication application engineer, said capacity usually is the biggest concern with underground applications, so you should use a dedicated base station to serve the area. But if coverage is your main concern, you can use a repeater so you don't have to buy a new base station for that area. According to Breck Lovinggood, Andrew business unit manager of RF/amps products, sourcing the signal is important when you're designing for underground coverage. To do that, you can tie in a microcell for additional capacity in that coverage area, or you can borrow that capacity from an existing cell site, either by fiber or by an off-air repeater. Off-air applications are quick and inexpensive to implement because you just put up an antenna outdoors and the distribution system indoors or underground.
With a distributed antenna system, you distribute point-source antennas to provide continuous coverage. The antennas transmit the same frequency at different locations. You get a relatively continuous signal-level coverage, and it allows you to keep that signal low to avoid interference. Alternately, you can use a leaky coaxial cable system, which includes regular coax cable but with slits that provide openings for the RF to leak out. Leaky coax allows you to operate the radio at lower power because copper or fiber carries the signal. This method provides continuous RF coverage along the entire length of the cable. According to Rogers, you should use fiber optics to cover a subterranean environment because of the distances that you're covering. Fiber optic has low attenuation compared to coaxial cable. With a fiberoptic based system, you can change your antenna's location without changing the output. But with an active coax system, when you move your antenna, you must change the whole design because of noise contributions, amplifiers and output power. A CASE FOR LEAKY COAX Currently, AT&T Wireless provides coverage to Grand Central Station, one railroad tunnel, and the Lincoln and Holland tunnels, which connect commuters from New Jersey to Manhattan. The carrier has taken different design approaches to each. According to Steve Chen, AT&T Wireless RF engineering manager for Manhattan, AT&T's cell-site equipment for the Lincoln Tunnel is located in two vent buildings, one on each side of the river. Cable in the tubes provides coverage in both directions from the vents. "If you picture one of the vent buildings in Manhattan, it would cover the tunnel from the vent building toward Manhattan, which is the portal or the exit of the tunnel and halfway into the tunnel," Chen said. "On the New Jersey side, the vent would cover halfway into the tunnel and toward the outside." The Lincoln Tunnel is three miles long and has three tubes and three cell sites, and the Holland Tunnel is two-and-a-half miles long, with two tubes and two cell sites. One of the biggest challenges, Chen said, was deciding whether to use a distributed antenna system or a leaky coax design for these tunnels. Because of the length and curves of the tunnels, Chen said an antenna system would not have worked. RF waves do not bend with curves, and it would have required many antennas. "When you do a distributed antenna system, you have to have splitters and more connectors in the tunnel, and maintenance will be more of a problem than if you have one piece of leaky coax that runs the length of the tube," he explained.
In addition, Chen said the Port Authority of New York and New Jersey cleans the tunnels every evening with machines that scrub the side walls and ceilings to remove dirt and grime. Constantly putting water on connectors and antennas would cause problems that don't affect cables. Finding a location for the cable in the tunnels is critical because the cable has to be high enough so that cars, buses and trucks don't block the RF signal. According to Chen, AT&T did not use repeaters because there were only two access points -- one at the end of the tunnel and one at the beginning of the tunnel. Putting any kind of amplifier inside would have created another maintenance problem. "That's a key because if we can't get to the tunnels when the site's down or the repeater's out, it's a big problem," he said. Chen said AT&T Wireless also avoids using repeaters in tunnels because they usually don't have alarming features, and you don't know when they're down until a customer calls and says he can't get on the system. There is one disadvantage to the leaky coax option: Usually it is more expensive than antennas, Chen said. SUPPORTING DISTRIBUTED ANTENNAS According to Chen, AT&T Wireless uses a distributed antenna system instead of leaky coax in Grand Central Station because it's easier to control the RF and cover a wider area with antenna technology. "With all of the platforms, you'd have to run a lot of leaky coax because the RF doesn't travel far from the radius," he said. The Fort McHenry and Harbor tunnels in Baltimore required a similar coverage approach. Khoa Khuu, AT&T Wireless spectrum planning engineer, decided that a variation of the distributed antenna system would best fit those tunnels. Khuu said there is an outside cell site at the entrance to each tunnel. His team members drove past the cell site to see how far the signal strength went into the tunnel. When the signal dropped off below the acceptable coverage level, they placed an antenna inside the tunnel to hand off coverage. The carrier had to work closely with Baltimore authorities, who gave specific instructions on where to place the equipment.
You should install cell sites and equipment as close to the tunnel as possible, and preferably midspan because this allows for the shortest tunnel distances. Once AT&T determined where to place equipment, it measured the cable length from the equipment location to the antenna point. "When we got the cable length, we calculated the link budget to determine the power output we needed to balance the uplink and the downlink path -- that's one of the critical designs in our system," Khuu said. One of the challenges was the tremendous cable run -- sometimes as long as 700 feet -- from the equipment location to the antenna. When AT&T engineers determined the link budget, they found that the coverage at the middle of the tunnel did not meet their requirements. "One way to solve that problem was to improve the uplink signal by using diversity, putting more than one receive antenna at thetunnel, and also by the use of the low-noise amplifier at the antenna to improve our uplink signal," explained Rich Davis, AT&T Wireless RF engineer and team leader. According to Khuu, the Harbor Tunnel was an easy design because it is a mile-and-a-quarter long and fairly straight. It has two tubes, so AT&T could design it with one cell site at the end of the tunnel. The McHenry Tunnel was more complicated because it has four tubes, it's a mile-and-a-quarter long and has a curve in the middle, and several other carriers provide service in the tunnel. Khuu, however, designed a customized system that enabled AT&T to provide coverage with just one cell site. Davis said Khuu used a series of hybrid splitters and combiners on one of the sectors that was able to serve two tubes with what would traditionally have required another set of sites. "Basically, he saved a full cell site at either side of the tunnel with that design," Davis said. "It's not a leaky coax system; it's antennas at either end of the tunnel pointed toward the middle of the tunnel. In a way, it's a distributed antenna system, but really it's more a regular cell site just plopping the antennas in either end of the tunnel, putting them underground and shooting the signal toward the center of the tunnel." The design allows AT&T to provide coverage inside and outside the tunnel. Khuu carefully selected antennas so that as subscribers drive past antennas in the tunnel, there's enough signal on the backside to extend coverage to the outside site. NECESSARY NEGOTIATIONS In addition to design issues, carriers have found that working with competitors can be challenging, but successful. For the Lincoln and Holland tunnels, AT&T Wireless has an exclusive contract with the Port Authority for several months, and when that time period ends, it will allow Bell Atlantic on the system and share cables with that carrier.
"It can be hard to negotiate with competitors -- if you put a lot of money into the construction of the cabling and the infrastructure, you obviously want to recoup the money from your competitor, and the competitor doesn't want to pay," Chen said. In the McHenry and Harbor tunnels in Baltimore, several carriers have their own antenna systems, but each carrier's design is a little different. According to Khuu, it took some coordination because AT&T shared some brackets in the tunnels with another carrier's mounting spaces. The main concern was keeping the systems far enough apart to avoid interference issues, he said. For example, when AT&T installed antennas 10 months ago, another carrier was putting up equipment in the same tunnel at the same time. "We both wanted the same mounting location, so we worked with each other to come up with sharing the bracket to mount the antennas," Khuu said. "The tunnel authority noticed the conflict and said we'd have to work it out amongst ourselves. We worked with the other carrier's engineers to share the space." Another challenge, according to Chen, is negotiating with the Port Authority. For example, with the Lincoln and Holland tunnels, AT&T had to install not only its own RF, but also Bell Atlantic's RF and the Port Authority's 2-way radio system. "They also wanted to put in AM/FM radio over the airways and rebroadcast in the tunnels, so we had to provide for all four of those," Chen said. "Any time you have four different RF sources, the chances of having co-channel interference is higher." To avert problems, the carrier created specially made cross-band couplers to eliminate interference. But the challenges did not end there. Chen said AT&T also had to adhere to power limitations. If it put more power into the system than Bell Atlantic or the Port Authority did, it would create interference. Carriers also have to get approval from the tunnel authority before installing equipment. There are stringent requirements on where antennas can be mounted within the tunnel because of the authority's cleaning equipment, Davis said. Accessing equipment for maintenance also can be challenging. For example, if thereis a problem with equipment located at the vent, Davis said AT&T has to coordinate access through a government agency. SUBTERRANEAN PROGRESS According to Paul Sergeant, Nortel senior manager of CDMA product marketing, unlike AT&T Wireless, most carriers are just starting to think about wiring subterranean environments. Many providers still are concentrating on adding capacity above ground. However, as
more carriers enter the market, they all will want to put facilities in tunnels and underground locations. But not everyone may have the opportunity. Usually, the public transportation system takes bids from wireless operators in the respective markets. "That could be a significant challenge because if you only have one bid, you won't be able to have competition, and you may not be able to get your service into that tunnel," said Alan Post, Nortel TDMA product line manager. If you are given the opportunity, you will want to be prepared to face the design issues ahead. Because of access challenges, it is important that your equipment is small, unobtrusive and reliable so it needs little maintenance. And although it may take awhile to negotiate, the end result will benefit all carriers in the marketplace that are fighting for coverage in the same spots.