A couple years ago, Nielson Networks President, Rick Nielson, attended a TETRA Association presentation by Phil Kidner at an IWCE convention. Rick was impressed with what TETRA could do, especially with how smoothly data and voice were integrated onto a single RF carrier. Rick developed a relationship with Rohde and Schwarz (developer of infrastructure equipment) along with Sepura plc (developer of radio hardware), and began discussing the possibility of piloting this techology in the United States.
In December, 2010, technicians from Nielson Communications met in Germany, and received training from Rohde and Schwartz and Sepura experts on how to build and maintain a small TETRA network in the United States. We started with basic TETRA theory, and left able to program some base stations and terminals. On January 11, 2011, radio hardware arrived in Green Bay, along with filtering and antennas from Comprod. Our goal was to compare analog and TETRA radio perfomance, and the best way to do that was to bolt TETRA into our existing LTR antenna system on the tower.
Construction of the TETRA system began in mid-January, with the preparation of three tower sites - DePere, Appleton, and Oshkosh. Each site required the diversity antenna and filter installation. At our Green Bay office, on January 19, we made our first TETRA transmissions. On January 25 and 26, we installed the final pieces of networking components, allowing the installed base stations to fully communicate with each other. Enhancements to the system, such as the telephone interfaces, began later that week.
Testing of the radio system began in early February, where we compared driving observations against radio propogation software predictions. We began comparing the TETRA's digital signals with the analog LTR system, and found that the cover patterns were quite comparable. Similairly, we observed that when the TETRA range collapsed, the analog LTR system was very distorted and unusable as well.. The amazing thing is that TETRA is using 1/2 of the power of the LTR system on transmitting from the repeater (TETRA=50W, LTR=100W), and 1/3 of the power transmitting from the mobile radio (TETRA=10W, LTR=30W).
Further range tests continued during the summer months, during greatest leaf growth, to document how well the signals would propogate during the wost-case scenerio.
Below are links to larger images of our coverage study. The YELLOW map is a plot of the TETRA base stations receiving signals from a Sepura TETRA mobile transmitting 10W of RF power. The BLUE map is a Kenwood LTR radio transmitting 30W to the same antenna that the TETRA radio is using. The BLUE-GREEN map is a mixture of the other two maps... if you see a greenish color, then both systems are operating. If you see a yellow dot or a blue dot, then that particular system "wins" over the other. Looking closely at the maps, you will see that the analog signals do travel a touch further, however, nowhere near the myth of 1/2 signal coverage. Our pilot demonstrated that "twice the infrastructure" is not required for TETRA.
We also had the fortune of performing a qualitative drive test, as displayed on the MAP WITH TRACES. During the summer of 2011, we used a CRIBS test unit, generously provided by MACLtd of Great Britian, to perform a real-time collection of signal strength data, and plot this information onto a map. We were able to verify our model's projections, and observe signal strengths along several major highways within the coverage area. These samples were collected during the summer, where tree leaves and other foliage would have the maximum ability to disrupt the signal. The rings are at 5 mile increments, and we used a 1/4 wave atop of a common car to represent our case study system.
|TETRA RX PLOT - 10W||LTR RX PLOT - 30W||Combined RX PLOT.
Dark Blue = Analog "win"
|TETRA Drive Verification|
Since construction of the pilot, we have tested out a variety of components to learn more about TETRA.
Fiplex sent us a bi-directional amplifier (BDA) which is often used to replicate signals inside of a building, such as a hospital. We used their BDA for almost a year, without issue. Fiplex makes a variety of BDA solutions, covering multiple frequencies, providing solutions for a variety of projects. For example, a hospital could install TETRA, and Fiplex can setup a BDA that would re-transmit both TETRA and cellular telephone signals.
AVL capability, using Bitea's radio solution, allows us to track via GPS, location information on where the units are on a Google Map. We can also record the path taken by a unit, for later analysis / documentation.
Telephony was setup uising an Asterisk flavor called Trixbox. Using this telephone solution, we were able to make full-duplex telephony calls from the radios to an office extension, or to landline telephones. The system can also handle incoming calls, in the form of an auto-attendant, or ringing multiple devices at the same time. For instance, if you call into the TETRA system, you can ring my desk phone, and my TETRA radio at the same time. As a user, I may answer the device most convienient to me.
Dispatching was setup after the pilot, using Sieman's software to create dynamic groups. We also expiremented with discrete and ambient listening. We were able to record and playback calls from the computer station, and using a microphone attached to the computer, communicate with radios in the field.
Job Applications were setup to demonstrate the data power of TETRA. With Sepura's assistance, we setup a job ticket application that a radio user to receive, via texting, an address and a problem description. The user may then accept or decline the assignment, again via texting, with the system making a time stamp on the user's decision.