Nielson Outlines Results of U.S. TETRA Pilot
June 27, 2012
Photo courtesy Sepura
By Rick Nielson
About a year go, the Green Bay, Wis., TETRA pilot went fully operational
. This article summarizes why Nielson Communications Inc. (NCI) conducted the pilot, what was accomplished, what NCI’s future plans are concerning the pilot and technical details from the pilot. The purpose of the pilot was to confirm that the TETRA technology delivered all that it promised. NCI made mistakes in the past by implementing technology that had a lot of promise, but never delivered what was promised. NCI wanted to be sure its next technology move was the right one.
The TETRA pilot is composed of three Wisconsin sites in Green Bay, Appleton and Oshkosh. NCI operates multiple UHF analog trunking sites in northeast Wisconsin, and the pilot sites are three of those analog system locations. The existing analog antenna and combiner systems were used for the TETRA base stations, allowing NCI to compare the analog system performance to the TETRA system performance on an apples-to-apples basis. Other than the diversity receive antenna for the TETRA station, the same antenna system was used for both TETRA and analog trunking.
The base stations were coupled to the antenna system with transmit expansion cavities and additional receiver expansion ports using commonly available equipment developed in the United States. Two locations have basic DSL connections, and the Green Bay site is a wireless connection. All system functions reside on an IP network among sites. NCI staff can monitor the system health, traffic, data and other related parameters.
The analog trunking system used for comparison consists of 100-watt base stations, 30-watt mobiles and 4-watt portables. The TETRA system consists of 50-watt base stations, 10-watt mobiles and 1.8-watt portables. Sepura supplies the TETRA radio terminals, and Hytera Communications (formerly Rohde & Schwarz) manufactures the TETRA base stations used in the pilot.
TETRA is a four-slot TDMA system that allows four talk paths within a 25-kilohertz RF channel. TETRA is a digital system with a 36 kilobits per second (kbps) basic RF data rate where four time slots are allocated for various types of information, whether it is voice, data or control information. By allowing four talk paths in a single 25-kilohertz channel, TETRA qualifies as a 6.25-kilohertz channel equivalent, meaning that TETRA will be compliant for the next phase of FCC spectrum narrowbanding requirements.
There is also a special bit in the data stream called control frame 18, which is a dedicated management frame to configure a system for emergency traffic to assure emergency traffic always gets through. The TETRA system also controls mobile and portable transmit output power by measuring the uplink signal strength and automatically adjusting the power of the portable, saving battery life and reducing interference.
Considering TETRA requires a 25-kilohertz channel and it’s a trunked technology, FB8 channel designations are a must. New system licensing for TETRA is still possible, depending on the area of operation and availability of channel. Many 800 MHz systems are becoming obsolete, and with the 800 MHz trunked channels, TETRA is an ideal fit. The main issue with TETRA in the United States is the need for 25-kilohertz trunked channels. If an entity has these channels already or if it can get them, TETRA is a non-issue.
There has always been a question of TETRA performance compared with analog systems. On paper, TETRA RF specifications do not quite stack up to high-power, highly sensitive analog systems. With the pilot, NCI tested most of the TETRA features, along with the data functionality of TETRA. Following are five myths the NCI pilot addressed and debunked.
1. TETRA needs significantly more sites to provide similar coverage to current analog systems. Numerous presentations have stated that TETRA needs twice the number of sites as do other technologies. This is a false statement.
2. TETRA equipment costs more than analog systems. The cost of TETRA falls well below Project 25 (P25) equipment, and just slightly higher than Digital Mobile Radio (DMR) product offerings. TETRA’s feature set far outweighs the small difference in cost compared with other technologies.
3. TETRA mobiles and portables don’t perform as well as other technologies. In the pilot, NCI compared 1.8-watt TETRA portables to 4-watt analog portables along with 10-watt TETRA mobiles to 30-watt analog mobiles, and found little difference in performance because of TETRA’s digital receive diversity.
4. TETRA will interfere with other systems. This was one of the first tests NCI performed on the pilot, because the TETRA sites are located at busy tower locations. NCI set up an analog radio on the adjacent channel to the TETRA station. Receiver sensitivity was measured and recorded, with an external antenna on the radio, with the TETRA station not transmitting. The same receiver test was performed with the TETRA station in transmit condition. NCI found no degradation of receiver sensitivity with having the TETRA station transmitting or not.
During the test, NCI had to wait for a paging transmitter located on the opposite adjacent channel to the test receiver to come down before NCI could complete the test. The paging transmitter on the opposite side of the receiver channel was desensing the test receiver when in a transmit condition. Even though TETRA is a different communications standard, interoperability is achieved through trunked interfaces. Station manufacturers offer many options to interface to different technologies.
5. TETRA is a new, untested technology. This is by far the greatest myth of all. TETRA has been around for many years and is a mature technology that is continually improving. TETRA is used by public safety and militaries throughout the world, including the U.S. military.
TETRA is a standards-based communications protocol with multiple applications available, and equipment vendors offer a variety of terminal units. Most TETRA manufacturers have type accepted their equipment for use in the U.S. NCI tested the following basic features with the pilot.
Direct Mode Operation (DMO) — The TETRA radios will talk directly to each other outside of the TETRA network coverage, similar to a simplex radio conversation.
Trunk Mode Operation (TMO) — The TETRA radios operate on the network with all network functionality.
Gateway Mode — When at the edge of the TETRA network range, a mobile in a good network signal area may be put in gateway mode, allowing other radios out of network range to talk through the gateway radio back to the network and similar to a vehicular repeater system.
Repeater Mode — Allows a mobile or portable to be a repeater system for other radios outside of the trunked network coverage.
Half-Duplex or Full-Duplex Operation — The radios will perform group calls using half-duplex push-to-talk (PTT) type communications, as well as full-duplex communications. Full duplex requires TMO registration.
Full-Duplex Telephone Interconnection — The system will allow full-duplex calls via session initiation protocol (SIP) connection. Cellular type calls are a standard feature. SIP interfacing allows easy telephone integration. Features such as caller ID, call forwarding and multiline calling are possible with the TETRA SIP interface and a properly equipped SIP telephony server.
Emergency Calling — The system is designed to allow emergency calls to go through even on a fully busy system.
Short Data Messages (SDS) — The system allows SDS messaging without detracting from voice traffic capacity. The SDS messages are sent on the control channel.
Vehicle Tracking — The system allows vehicle tracking, again without any impact on system loading as the datagrams are sent on the control channel. A variety of software packages make vehicle tracking easy to implement. GPS data is not heard on the talk paths.
Software Dispatch Consoles — A variety of IP-based software dispatch console products are available.
Packet Data — TETRA offers IP packet data transmission, allowing IP connectivity anywhere on the radio network.
Circuit-Switched Data — TETRA offers circuit-switched data capability for circuit-switched applications.
High Ambient Noise Operation — The TETRA voice vocoder allows superior voice quality. The human voice will be transmitted through most any type of ambient noise condition. This includes helicopters, sirens and diesel engines. NCI stood by a chop saw and an air compressor in operation, and the voice audio came through clear, without any hint that the saw and compressor were in operation.
Telemetry Data Operation — NCI used TETRA modems from Funk-Electronic Piciorgros. It was easy to set up a demonstration fire alarm system that used the control channel data capability. The TMO-100 offers complete flexibility by communicating on all communications protocols, serial or IP based. The modem is standard with analog and digital remote terminal unit (RTU) inputs.
Dynamic Group Allocation — The TETRA system will allow users to be dynamically grouped as needed. One use for this was allowing utility companies to automatically change the radios to the same group when they are within a certain radius of an outage, which is done by programming the radius around a specified GPS coordinate.
To test the performance of the system, NCI completed and recorded locations and signal strengths. TETRA radios display signal strength and signal quality. NCI then used the field data to calibrate the software model. When in the field, the operator made voice contact with both the analog and TETRA units, and the voice quality was recorded at each location. NCI compared the systems, and they were close in coverage. When the TETRA audio started to get errors, indicated by the warbling or digital type audio, the analog system was scratchy and difficult to hear. Talk-back performance between the analog and TETRA systems was almost identical. The 10-watt mobile and 1.8-watt portables were performing as well as the 30-watt and 4-watt analog equipment. The reason for the improvement in talk-back coverage is because of the receive diversity antenna system. By using the diversity system, the talk-back gain achieved is a minimum of 5 dB.
The results confirmed that the coverage of the analog system was slightly better than the TETRA system, but most users would not tolerate the audio quality of the analog at the extended ranges.
NCI plans to bring a node to trade shows to show the complete TETRA feature set live. NCI also plans to investigate more TETRA data system implementations, as well as host live demonstrations in Green Bay and webinars.
Rick Nielson, PE, is president of Nielson Communications Inc. (NCI), established in 1974. NCI is a wireless communications products and services provider located in northern Wisconsin with offices in Green Bay, Neenah and Sturgeon Bay. Contact Nielson at firstname.lastname@example.org.