RADIO GOES UNDERGROUND
Cabling is an essential part of building a wireless communications system that makes travel on the Metro safer for Buffalo, NY residents.
Although probably best known for football, excellent lake sailing and the world-famous Niagara Falls, Buffalo, NY, is also home to one of the few underground Metro systems in the United States. Known as the Niagara Frontier Transportation Authority (NFTA), the five-mile subterranean rail system provides the residents of this Western New York community an alternate means of transportation to the above-ground traffic congestion of buses, automobiles and trucks.
An underground wireless communications system connecting railroad workers, police officers and firefighters is the most important component of Metro safety. By providing voice communications to these essential services, the high standards for safety can be monitored and maintained.
In 1999, Radio Frequency Systems (RFS) participated as one of the product suppliers in modernizing the Metro’s 20-year-old, 450MHz backbone structure. The previous system used 7/8-inch single-slotted, corrugated radiating cable that was performing marginally and in need of repair. A newer 800MHz digital trunking radio system was to be installed in parallel with the older 450MHz system. Building and installing a wireless underground system that provides dual-band communications services using the combination of newer RF radiating backbone technology in long tunnels, and maintaining the existing 7/8-inch radiating cable infrastructure (where possible) in shorter tunnel sections offered a unique challenge.
With its background in providing wireless communications in subterranean environments in Hong Kong, Singapore, and many European cities, RFS proposed the use of optimized, smooth-wall radiating cable technology.
Unlike traditional, corrugated radiating cable that is fabricated by milling coaxial cable, smooth-wall radiating cable is punched to create specific antenna patterns that transverse the length of the cable. The radiating cable can therefore be optimized to provide improved RF performance.
A significant improvement in RF propagation was achieved in the longer tunnels of the Buffalo Metro by using a RAY 114B (11/4-foot) diameter cable that transmits and receives RF via radiating mode. This radiating cable provided better than 12dB of additional margin in the link budget and significantly increased the system’s overall RF performance.
To further demonstrate the wireless performance differences of radiating-mode vs. coupling-mode cable, the diagram on page 50 illustrates the RF power flow from a cable to a mobile antenna. In a typical vertical-slotted, coupling-mode cable (as used in the Metro’s older corrugated 7/8-inch radiating cable) the power flow is parallel to the cable axis. Electromagnetic energy is concentrated close to the cable and decreases rapidly with increased distance resulting in higher coupling loss. Coupling mode corresponds to leaks from the cable (i.e. leaky feeder) and is used in the system’s overall link budget analysis.
Radiating-mode cable incorporates horizontal slots that run longitudinally along the cable. RF power flows toward the receiving mobile antenna, lowering the overall coupling loss of the RF distribution system. The radiating cable proved to be more RF-efficient for the Metro’s wireless backbone than a coupling-mode cable of the same size. The increase in available RF receiver power permits the use of fewer bi-directional amplifiers, decreasing the cost and improving the overall system reliability.
The chart on page 48 graphically depicts the expected mean radiating cable coupling loss for different frequencies in the radiating mode of operation. In the specific case shown, the coupling loss of the RAY 114B cable was reduced as the frequency increased, resulting in an improved link budget for the 800MHz system.
When designing an RF system with optimized radiating cable, one must also be cognizant of avoiding the RF stop bands inherent with this type of product that increase longitudinal insertion loss within the band. For this particular product, RF stop bands are located at 320MHz, and its multiples are well outside the 450MHz and 800MHz radio systems used in this system.
With its new multiband RF system operational, the Niagara Metro became the first underground transport system in the United States to use the advanced smooth-wall, high-performance radiating cables. The 800MHz trunked system’s performance exceeded the RF technical specification requirements in all of the long Buffalo Metro tunnels where smooth-wall radiating cable was used.
In the shorter tunnel sections where the older 7/8-inch, corrugated radiating cable remains, damaged components were repaired, and the radiating cable backbone was enhanced.
High-performance cable taps were installed at predefined points along the length of the radiating cable. The cable taps provide low longitudinal insertion losses (0.7dB/tap) and coupling of 10dB.
Attached to each of these taps are 800MHz point-source antennas that improve the RF efficiency of this older cable. Although not as RF-efficient as the newer smooth-wall radiating cables, this hybrid approach of using radiating cables for the 400MHz system and antennas for the 800MHz system is a good alternative when the project budget does not permit the upgrade of the total backbone system.
Today the Buffalo Metro has a seamless dual-band trunking radio system that allows the use of the existing, older 450MHz mobile radios and the newer 800MHz replacement mobiles. The system provides for the smooth and economic transition from one radio technology to next-generation technology. The Metro ridership can take comfort in the fact that the carrier’s essential communications system has been significantly improved, providing them a safer environment when traveling across Buffalo in their subterranean world.
In the future, Buffalo’s new wireless mobile system will be able to accommodate new technologies for essential services (fire, police, train control) and for commercial use. Subject to the approval of the NFTA management, the radiating cable backbone can also be used to support commercial wireless paging, cellular and PCS services.
Because the system uses a broadband wireless design, unique new commercial services that are available today to riders in Hong Kong, Singapore, Munich, and other major cities could become a part of the Buffalo way of life.
