Simulcast Project Dallas/Ft. Worth

Transcription

1 Simulcast Project Dallas/Ft. Worth Simulcast Forum 21 Las Vegas, Nevada Gary Gilbert Engineering Services/Sr. Project Manager 3002 Century Drive Rowlett, TX March 30, 2017

2 North Texas Tollway Authority (NTTA) NTTA operates more than 950 toll miles in the Dallas/Ft. Worth region. Texas DPS is contracted to patrol the tollways. The NTTA provides a radio system and a dispatch center for the troopers. This radio system is VHF and currently consists of 6 transmit/receive sites and 4 receive only sites. A system upgrade is underway to increase the system to 13 transmit/receive and 2 receive only sites. As this is a VHF simulcast system, the engineering, equipment selection and deployment is crucial. Equipment must be the same throughout the full system.

3 Legacy System The original system was covered by a single transmitter, centrally located, on a tower with a DB224 mounted on top at 140. There were 6 receive only sites spread out. This system had holes in the coverage, especially in the boundaries, in the concrete canyons and newly built toll roads.

4 Simulcast Requirements As the metroplex grew, more toll roads were added. As NTTA provides radio coverage for the troopers, a simulcast system was required. Link America was chosen to design and provide the simulcast coverage.

5 Issues were found: System was supposed to already be simulcast, but our investigation showed one transmit site was providing the coverage. With the system already being simulcast, no work was to be performed on the legacy system, except to upgrade the Master Site to accommodate the new sites. Phase 1

6 Budgetary issues did not allow a full system buildout but a change order did allow us to build the legacy system into a simulcast system. Existing sites and antennas had to be used. These antennas were either DB222 or DB224 antennas, all mounted on the top of buildings and poles at heights of 50 to 140. Phase 1

7 Phase 1 Link America built out 2 other legacy sites as transmitter/receivers and added 3 transmit/receive sites and 1 receive only site to cover the new tollroads. Within the limitations and the existing omnidirectional antennas, overlapping coverage had to be minimized.

8 Coverage Issues With antennas and site locations fixed, overlap coverage caused issues. Transmit power was tweaked to maximize quality coverage. After working with NTTA and performing our drive testing, numerous areas were noted to have issues. Every problem area was investigated to find the causes: 1. Buildings/terrain shadowing transmit signal, or 2. Due to heights, terrain features and types of antennas, the overlap coverage caused interference and loss of radio signal.

9 Another phase of the project was added to clear these problem areas: A total of 13 transmit/receive sites will be utilized with only 2 receive only sites. Antennas are being changed out to either a 6 degree downtilt omnidirectional or corner reflectors to isolate the VHF transmit signal better. Phase 2

10 Coverage / Timing

11 Primary Equipment Spectracom SecureSync GPS Located at all Transmitter sites Synchronizes all sites to Master site Intraplex Provides PL/CTCSS tone to the Master III Located at all Transmitter sites and Master site Transports Audio Between Master site and Slave sites Synchronizes transmitted audio between all sites Two types: NX is the Master and LX are Slave

12 Primary Equipment Convex 2240A Audio/PTT distributor Located at Master Site Distributes PTT and Voted Audio to Voice Optimizers NBL4 Voice Optimizer Located at Master Site Adjusts audio levels going to each Transmitter

13 Primary Equipment Voter Receiver Receives audio from all sites and selects single receive audio to pass on to transmitters. Currently in process of changing out to new digital JPS voter receiver system. Harris Mastr III VHF transmitter/receiver Transmit power adjustable from 12W to 100W

14 Simulcast Timing The SNC-101S and SNC-101T modules accept the reference timing input from the GPS receivers at each location. The SNC- 101S module at the origination point provides the timing reference signals. The SNC-101T module at each transmitter site compares the locally received GPS signal to the timing reference information arriving over the network connection from the origination point. GPS receivers at all end points in the network provide extremely precise frequency and time references. The SynchroCast3 system uses the GPS frequency reference to calibrate the transmitter frequency directly, while using the time reference to adjust a variable time delay mechanism that automatically compensates for delay changes in the audio distribution network. The timing comparator receives input from a local timing reference of the transmitter site GPS receiver plus the master timing reference that arrives over the link, with exactly the same network delay as its accompanying audio signal. The comparator measures the time offset between these two inputs and sends commands to the local system controller to create the overall delay required to compensate for any variations in the network delay. The system operates automatically once the initial installation and alignment are complete, keeping the total delay to each transmitter constant even if the actual path delay changes, as might occur if the network gets routed to an alternate path.

15 Master Site Block Diagram

16 Transmitter/Receiver Sites

17 Lessons Learned Ensure all equipment is identical in model and version numbers. Original Mastr III transmitters had different internal versions of boards (pre-p25) that did not have the same internal timing as the newer versions of Mastr III transmitters or upgraded features. This required more test driving and setup adjustments. Original Intraplex units did not have the up-to-date features that assists in timing as current models have. All transmitters and Intraplex units are now the same throughout the system.

18 Future Upgrade Due to a recent outage due to a construction crew cutting the fiber to the Master Site, a Redundant Master Site is planned. A mirror image of the Master Site will be installed at a different location. The fiber will be redirected so that a new Master network switch (with redundant switch) will route traffic to each Master Site. If the Master Site drops, the output of the Master network switch will reroute traffic to the redundant master site.

19 Questions? Link America. LLC (214) Century Dr. Rowlett, Texas 75088