Distributed Antenna System in Seattle's Transit Tunnels
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The following text was extracted from an RFP* written by Sound Transit. The RFP, published in October 2013, solicits bids to install additional radiax for cellular coverage.
Sound Transit currently owns and operates a Distributed Antenna System (DAS) in the DSTT and Beacon Hill tunnels. The system is tied to King County Radio System (KCRS), which is a mixed SmartNet and Trunked Digital P25 capable radio system (ASTRO25), manufactured by Motorola.
The largest system supported by RCS is the regional 800 MHz trunked radio system. The regional radio system is comprised of owners from the City of Seattle, Valley Communications Center, the East Side Public Safety Communications Agency and King County. RCS services and supports both the King County and Valley Communications Center subregional systems. Countywide this system consists of 27 transmitter sites, multiple interconnecting microwave and fiber systems, and multiple public safety dispatch centers. The system supports approximately 14,000 radio users in county and suburban agencies, including police, fire, Emergency Medical Services (EMS), general government functions, school districts, water and sewer districts, etc.
Currently, and only in the DSTT, there are 700 MHz radio channels supported by DAS that are in use by Metro Transit to contact the buses. Those channels will continue to be supported by the DAS as long as the DSTT has a mixed use (i.e. buses and trains).
Sound Transit is allowed to have transmitters distributed along underground structures, as long as no energy is radiated out of the tunnels. Any interference or intermodulation product to the 15th harmonic, higher than -120 dBm must be suppressed to avoid disturbing KCRS normal operation. The host may be instructed to stop operating their DAS if there is any suspected interaction between the DAS and KCRS. In such condition, the host may receive verbal instructions to stop operating the DAS from either Sound Transit officials and/or KCRS officials. The host then must turn off any of his equipment and proceed to demonstrate the interfering signals are not originated from the DAS equipment. The DAS will not be allowed to operate until any interfering signal has been suppressed to Sound Transit’s and/or KCRS satisfaction.
DSTT and Beacon Hill Tunnel Radiating Mode Cable
- The radiating cable portion of the hybrid distributive antenna system in the DSTT and Beacon Hill Tunnel segments shall use Andrew RTC6-C-1RN Radiax® 1-1/4” radiating mode radiating cable (or approved equivalent). The location and configuration of this radiating cable is shown in the Contract Drawings.
- Additional 7/8” inch radiating mode cable shall be used to provide coverage in the east and west shaft stairwells and the west emergency access ladders in the Beacon Hill Station and in the stairwell of the Pine Street Stub Tunnel vent shaft. These radiating mode cables shall be Andrew RTC5-C-1RN Radiax® radiating mode cable (or approved equivalent).
Beacon Hill Tunnel Coupled Mode Cable
Several runs of ½" coupled mode radiating cable shall be used to feed discrete distributed antennas in the Beacon Hill Station, as shown in the Contract Drawings. These coupled mode radiating cables shall be Andrew RXL4-1-RN Radiax® coupled mode cable (or approved equivalent). Plenum rated 1/2" coupled mode radiating cable shall be used in the east and west plenum spaces on the roof level of University Station in the DSTT, as shown in the Contract Drawings. These coupled mode radiating cables shall be Andrew RXP4-1 (or approved equivalent).
Discrete Distributed Antennas
- The discrete distributed antenna network used to provide coverage in each of the DSTT and Beacon Hill stations shall be fed via low insertion loss power dividers inserted in the tunnel side of each Distribution Hub, as shown in the block diagrams in the Contract Drawings.
- The locations of specific discrete antennas shown in the Contract Drawings is typical, and antenna locations may be selected to avoid conflicts with conduits, HVAC vents, lighting fixtures, and other obstructions.
RF Distribution Hub Locations
- RF distribution hubs that feed the hybrid radiating cable/distributed antenna systems in each tunnel are located as follows:
- University Station TC/C Room in the DSTT.
- Beacon Hill Station Communications Equipment Room, Transfer Level, near the center of the Beacon Hill Tunnel.
- The hubs provide the signal combining, amplification, and RF-to-fiber conversion functions required to distribute the signals from the tunnel distribution systems to the 800 MHz base station headend at the ST Operations and Maintenance Facility and vice versa. They also provide the interfaces between the ITAC backup repeaters and the distributive antenna systems in the DSTT and the Beacon Hill Tunnel and the interfaces between the 700 MHz KCRS/KCM shared talkgroup control stations and the distributive antenna system in the DSTT.
- The configurations of the University Station Distribution Hub and the Beacon Hill
- The Beacon Hill Distribution Hub is connected to the headend combining system at the following system headend location: Sound Transit Operations & Maintenance Facility (the Specific location and configuration of this equipment in the Central Communications Equipment Room is shown in the Contract Drawings ). The Tunnel Radio System interface to the 800 MHz base station equipment at this headend location is also shown in the Contract Drawings.
- The University Station Distribution Hub is connected to the headend combining system at the following system headend location: Sound Transit Operations & Maintenance Facility [800 MHz] (the Specific location and configuration of this equipment in the Central Communications Equipment Room is shown in Contract Drawings. The Tunnel Radio System interface to the 800 MHz base station equipment at this headend location is also shown in the Contract Drawings.
Various types of Andrew Radiax® cables are used; including: