TERRESTRIAL TRUNKED RADIO
Joe Nielson
Abstract
TETRA is a communications standard developed by the European Telecommunications
Standards Institute (ETSI). It stands for TErrestrial Trunked Radio, and previously stood for
Trans-European Trunked Radio. The standard was originally started in 1989 and was created to
better serve the needs of Private Mobile Radio (PMR) systems. Due to a growing amount of
users, and a non-growing amount of radio spectrum, there is a high need for spectrum efficiency
in order to allow the maximum amount of users to have access to communications. TETRA
provides a new way to solve this problem, along with others such as: secure speech and data
transfer, fast call setup, and even custom applications can be implemented. The list of features
that TETRA supports is quite long, I will be discussing more of them in this paper. I will also
include an overview of radio trunking methods.
Introduction
TETRA technology is in use throughout the world and is considered an “open” standard,
meaning anyone can use it. Its main purpose was to develop specific interface standards with
enough detail to allow different radio manufacturers to use them in order to fulfill the needs of
the commercial and public safety PMR markets. It is the first truly open digital private mobile
radio standard and therefore results in a more competitive market and lower prices for the user.
The openness also allows different equipment from different manufacturers to fully
communicate with each other.
Basics of Radio communication
Portable radios (the walkie-talkie kind) and mobile radios (the ones in a vehicle) typically do not
communicate directly with one another. When a user makes a call on a radio, the signal goes to
a tower usually located on a hill or someplace that would get a good signal. The tower has
antennas and when a signal is received by a portable or mobile radio, it repeats that signal to all
of the other radios or emergency centers within the system. In a small town where there is not a
large need for radio usage, a conventional system can sometimes be used. Conventional systems
only use one channel and all radios and repeaters are programmed for one frequency only. If
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someone wants to use it, they first have to check to make sure that no one else is using it
currently. [1]
The next step would be to use a pair of frequencies in the same way as mentioned above which
would support a slightly larger group of users. An example would be like having one frequency
for northern operations, and another frequency for southern operations. In order to receive all
traffic, you would need to program all radios with both frequencies.
Trunking is the next step up which instead of using designated frequencies for certain operations,
it can pool together any number of frequencies and allow a controller to choose an available one.
If a radio user makes a call on the system a trunking controller uses any available channel in the
pool of channels available to repeat the message across the system. In order to keep the
firefighters messages off of the police radios, a unique identifier is used. This identifier is used
to keep track of a talkgroup. Each radio needs to have talkgroups programmed into them in
order to receive messages they want, and to not receive messages they do not want. A talkgroup
in a trunked system would be the equivalent to the dedicated frequency in a conventional system.
[2]
Bandwidth
Messages are delivered across radio waves through the use of modulation. This can be done by
either amplitude modulation or frequency modulation with frequency modulation being the more
prominent one. This modulation causes the signals to be produced around the radio frequency
carrier. In order to make sure that one frequency does not get interweaved into another
frequency there are certain formulas used. For example, if a channel is operating at a carrier
signal of 155.150 MHz with a bandwidth of 50 kHz, then the actual data that modulates the
155.150 MHz frequency cannot go above 155.175 kHz, or below 155.125 kHz. If the signal
goes above or below those frequencies then it will cause interference for another user assuming
there is another user adjacent to them in the spectrum. Figure 1 below shows part of a radio
wave and how the bandwidth is calculated. f0 is the carrier frequency and (f2-f1) is the
bandwidth.
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Figure 1: Shows part of a radio wave and how a theoretical bandwidth is calculated.
Analog vs. Digital Radio
In today’s world it seems like almost every piece of technology is coming out with a digital
version that is almost always better. In the PMR market the analog version will probably be an
option for some people for several more years, but digital radio will soon take over fully.
Typically five things come into play when trying to decide on the type of radio system needed;
voice quality, radio frequency coverage, non-voice services, security and cost.
Voice quality over radio is typically determined by multiple things such as clarity, distortion,
noise, and transmission delay. A digital radio system needs to encode and decode the voice
signals at the transmitting and receiving ends through the use of a codec. A codec is something
that can both encode and decode a data stream, in this case a voice signal. A voice signal in a
digital system has to be translated in a way so it best represents the voice sample in the codecs
reference table. Since something like background noise cannot be translated into something on
the codecs table, it makes digital radio very good at eliminating background noise. This does
have a downfall in that if a digital signal gets corrupted, the quality drops very rapidly.
Radio Frequency (RF) coverage depends on many factors out of the scope of this paper such as
receiver sensitivity and propagation characteristics of the frequency being used. The following
diagram shows a representation of digital and analog radio voice qualities and how they are
affected as distance is increased. It is assumed that the differences in the chart are not
representing any particular technology and is more of a general description. [11]
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Figure 2: A representation of digital and analog radio voice qualities and how they are affected
as distance is increased.
Analog voice quality will fade more gradually as the signal strength decreases or distance
increases, while digital voice quality will drop off more suddenly.
In terms of non-voice services, a digital system offers a higher data throughput for the same
channel bandwidth. This is due to digital systems being designed to transmit digital information
which is voice and/or data with no difference between the two. An analog system would have to
have a separate method to transmit data for a non-voice service.
Security in a digital system is easier to implement simply because the algorithms used to encrypt
messages are digital. Authentication of users can also be implemented in most digital systems
making it even more secure.[11]
TETRA
TDMA, FDMA, CDMA
TETRA uses TDMA which stands for Time Division Multiple Access. The process of TDMA
involves digitally modulating a single frequency in order to increase the number of independent
communication channels. Specifically it uses 4 channels interleaved into one 25 kHz channel.
Instead of just one user being able to use the single 25 kHz channel, it can now be used by up to
4 different users. This creates both a cost savings in frequency needed and base stations or
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repeaters needed. It can support a gross bit rate of 36kbits/s, with 7.2kbits/s per TDMA channel.
The difference in the 28.8kbits/s (4*7.2) is from overhead of the TDMA structure.
Each TDMA frame of four slots is grouped further as 18 frames, which, combined, form a
multiframe. In circuit mode (as opposed to packet mode) voice and data, is compressed into 17
TDMA frames allowing for a control signaling frame to be used without stopping the flow of
data.
Frequency Division Multiple Access or FDMA is the most common and oldest method of radio
communication multiple access. In the 1950’s, the hardware technology was not as good as it is
today and that meant that only a certain area of radio frequency spectrum was able to support
private mobile radio communications. Each channel was subdivided into 50 kHz spacing to
prevent the channels that were next to each other from interfering. As the hardware and
technology became more reliable the spectrum was then divided further into 25 kHz channels.
Code Division Multiple Access or CDMA uses both frequency and time distribution on a given
amount of radio frequency spectrum. Due to this frequency and time distribution scheme, it
makes it harder to avoid problems during actual implementation and is rarely used. A CDMA
system requires 1 MHz of frequency allocation per base station or repeater site, which becomes
almost impractical for the private mobile radio market as most PMR users do not have that much
frequency available. [4]
Basic Services
TETRA offers the basic group call service as do most other radio systems. It also implements an
individual calling service or a one-to-one call. There is also an acknowledged group call and
broadcast call. All of which can be in a clear or encrypted mode.
Short Data Service (SDS) up to 256 bytes
A SDS message can be up to 256 bytes and be either a point to point or point to multipoint
message. The relatively short message size allows this service to be supported on the control
time slot. Four types of data transport are supported: 16 bits, 32 bits, 64 bits, and 2,039 bits (256
bytes). Some examples of applications that this service can serve are: status messaging, text
messaging, automatic vehicle location, automated dispatch and telemetry. A status message
could be used to update a supervisor and let them know a job is in progress or an officer is on
duty or off duty. This status message could be sent with the push of only one or two buttons
making it very quick and easy for an employee. Text messaging can be used to send a location
of an incident to nearby responders. The text form makes it an easy way for a quick reference
and can be viewed again in case the location is forgotten. Many taxi companies also have their
own private data networks for status messaging and text messaging. Automated dispatch could
be used to automatically send out a work request to the nearest service vehicle in the area by
using Automatic Vehicle Location. Telemetry could be used for alarm applications such as
power failure or equipment failure.
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Circuit mode data
There are three levels of quality or protection in the TETRA circuit mode data service and are
used on a single time slot communication channel. A high quality rate of 2400 bit/s, a “normal”
rate of 4800 bit/s, and low quality rate of 7200 bits/s are all defined. The quality is determined
by noise and fading which can corrupt data.
Packet mode data
Packet data is also supported in TETRA in either ‘Connection Oriented’ or ‘Connection-less’
configurations. In a connection oriented configuration a data message is not sent until the actual
link is set up between the sender and receiver. With a connection-less configuration, the data
message is sent out before a connection to the destination has been reached and just relies on the
network to forward the message when the sufficient resources become available. Some
applications of the packet data service include database lookup where an employee could query a
database to see if a part is in stock.
Multi-Slot Data
More than one time slot can be used to transfer data in order to increase the data transfer rate. If
two slots are used a data rate of up to 14400 bits/s is achievable. With three time slots a data rate
of up to 21600 bits/s and with four up to 28800 bits/s can be implemented.
There are also defined “Essential” services in release 1:
Access Priority prevents loss of control channel integrity. This can happen during prolonged
call request activity above the system’s physical limits. During times of congestion, uplink
access priority is dynamically controlled and lower priority users are denied from making call
requests.
Ambience listening allows a dispatcher to place a radio into Ambience Listening mode without
any indication that it is happening. It allows the dispatcher to pick up background noise within
the range of the radios microphone. This could be important for someone transporting valuable
materials or for a trucking company that has employees with health risks. It also brings up issues
about invasion of privacy as well.
Area Selection allows a dispatcher to select a different base station to make a call and helps to
improve network loading.
Call Authorized by Dispatcher allows a dispatcher to verify a call request before the call
actually goes through the system.
Discreet Listening allows any authorized radio user to listen to any communication without
being identified, even on different channels.
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Dynamic Group Number Assignment allows the creation of unique groups of users. This
would be useful for Police, Fire and EMS during a major disaster when close coordination is
needed between the three groups.
Late Entry will automatically switch a user’s radio over to the correct channel if they turn it on
and a conversation is already ongoing.
Pre-emptive Priority Call gives the caller the highest uplink priority and highest access priority
to the network. This can be enabled be either pressing a designated button on the radio, or
dialing a series of buttons. If the network is busy at the time of this call, then the lowest priority
call on the network will be dropped to make room.
Priority Call enables a 16-tier call priority service during peak busy periods.
Optional Supplementary Services:
Calling Line Identification Presentation – Displays the unit ID of the calling party.
Connected Line Identification Present – Displays the unit ID of the called party
Calling/Connected Line – blocks the caller ID
Talking Party Identification – the ID of a transmitting radio unit is displayed in all other radio
units on the call
Call Forward Unconditional - This will forward a call to another radio regardless of the called
radios current status.
Call Forward on Subscriber Busy - If a radio is currently engaged in a call and is called by
someone else, the call can be forwarded to another radio.
Call Forward on Subscriber Not Reachable – If a radio terminal is not reachable and someone
tries calling it the message can be forwarded automatically to another radio.
Call Forward on No Reply – A user can have their radio programmed to forward calls to
another radio if they do not answer.
List Search Call – If multiple radio terminals are not reachable, the call can be forwarded to a
predefined list of radio terminals to ensure the call is answered.
Short Number Addressing – “Speed Dialing”
Call Waiting – Ability to recognize when another person is calling during an engaged call
Call Hold – Ability to pick up another incoming phone call, and then continue the previous call
after the second one has ended
Call Completion Busy – If a called user is busy, then the second callers request is put in a queue
until the called user if free and then the call will be initiated.
Call Completion No Reply – If a call is made to a user that does not answer, then the next time
that called user becomes active on the network, a call is automatically made to the caller from
before.
Include Call - A radio user can include another radio user in an already existing call.
Call Barring – allows a network to block both incoming and outgoing calls from a defined list
of users and even on a per network basis.
ISI – Inter-System Interface
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The TETRA Inter-System Interface allows different TETRA systems to communicate with each
other. At first thought this would seem unnecessary to a typical PMR user. The question being
is why would a public safety employee located in Green Bay, on Green Bay’s private radio
network want to communicate with a public safety employee in Platteville? To answer this I will
first go into why the ISI was originally thought of. The Schengen Agreements in Europe
involved a harmonizing of the countries in the European Union (EU) and their Police
Departments. They wanted to have better cooperation with cross-border Police Departments and
customs. In the EU this can be easier to understand since the countries are close together and
having a criminal moving from country to country would make it harder on the Police
Departments. Other than public safety, there are many PMR markets that are nationwide or
international and would have lots of benefits from implementing a TETRA system.
DMO – Direct Mode Operation
Direct Mode Operation (DMO) is a technology that enables TETRA radios to communicate
directly with each other much like a walkie-talkie. This direct communication is not over the
TETRA network and is exclusively between the sending and receiving parties. A DMO
‘Gateway’ or a higher RF power mobile radio can be put into ‘Gateway’ mode. This allows a
lower power portable radio to communicate in DMO to the gateway enabled mobile radio. The
gateway would then serve as a repeater and relay the message from the DMO enabled portable
radio, (which would be out of range of the Trunked Mode Operation TMO network), to the
desired TETRA network. A higher powered mobile radio can also be used as a ‘local’ repeater
running solely in DMO. This would repeat all messages received to all nearby radio users in
DMO. When a user is in DMO there are two possibilities for channel assignment. DMO can use
either one single frequency anywhere in the 20 MHz spectrum, or a two frequency channel with
one each in a 10 MHz sub-band.
There is also a ‘Dual Watch’ mode that can be enabled which allows a radio to listen for TMO
calls while in DMO and vice versa. This can be useful when a radio user is not sure when they
will be back in a TMO service area again. There are also many more different subconfigurations possible.
TEDS – Tetra Enhanced Data Service
TETRA also has a release 2 where an enhanced data service is available. This new enhanced
packet-data service can allow for up to 10 times the amount of data transfer speeds. The TEDS
also enables a TETRA network to better interact with other technologies such as 3G or cellular
technologies. It also is fully backwards compatible and can fully interact with previous TETRA
standards.
TETRA in the United States
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TETRA currently is not allowed in the United States for a couple of reasons. The first problem
is that the TETRA standard is not fully compliant with certain Federal Communications
Commission (FCC) rules and therefore would be illegal to use in the United States. The TETRA
Association has already requested that the FCC waive the rules that TETRA is not compliant
with and the Commission has already granted some of the waivers. Another reason for non-US
operation is that Motorola has many Intellectual Property Rights on the standards due to their
help in defining them. There have been statements made by Motorola about them cooperating
fully with allowing the standard in the US. However, the fact that allowing the TETRA standard
to be used in the US would make it a much more competitive market for Motorola, thus making
it much less desirable for the company to want it allowed.
Conclusion
As with all current technologies TETRA is constantly evolving and getting better and as a
consequence older technologies do not stay current for very long. TETRA will probably
continue to have more releases and increase its functionality. Currently it is popular in other
countries outside of the United States and will probably be coming to the United States soon.
However the success of TETRA will always be dependent on competing technologies and it will
either have to constantly be evolving or it will fall behind.
References
[1] Signal Harbor. (2005). Understanding trunking. Retrieved from
http://www.signalharbor.com/sr/05apr/index.html
[2] Hayes, W. (2003, May 29). What is trunking. Retrieved from
http://www.thebriarpatch.org/trunking/
[3] TETRA Association. (n.d.). Tetra. Retrieved from
http://www.tetramou.com/about/page/12027
[4] TETRA Association. (2006, January). Tetra technology advantages & benefits. Retrieved
from http://www.tetramou.com/Library/Documents/Why_TETRA/Technology Benefits.pdf
[5] Krishna, S. (2012, April 12). Non coherent demodulation of pi/4 dqpsk (tetra). Retrieved
from http://www.dsplog.com/2010/04/12/non-coherent-demodulation-of-pi4-dqpsk-tetra
[6 ]Barrus, J. (2012, March 5). Fresh ideas in two-way communications. Retrieved from
http://communities.motorola.com/community/two-way_communications/blog
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[7 ]Charan, L. (2006). Complex technology made real. Retrieved from
http://www.complextoreal.com
[8] ETSI. (2007, October). Etsi TR 102 580 v1.1.1. Retrieved from
http://pda.etsi.org/exchangefolder/tr_102580v010101p.pdf
[9] University of California, Berkeley. (Producer). (2000). The radio spectrum. [Print Photo].
Retrieved from http://ptolemy.eecs.berkeley.edu/eecs20/sidebars/radio/spectrum.gif
[10] Ascom. (n.d.). Tetra - terrestrial trunked radio. Retrieved from
http://www.ascom.com/en/tetra-article.pdf
[11] Gray, D. (2003). TETRA the advocate's handbook: from paper to promise. Looe, Cornwall:
TETRA Advocate.