Overview of TETRA Private Mobile Radio (PMR)
- an overview, summary or tutorial about the basics of TETRA radio used for
Private Mobile radio (PMR) applications, and in particular by the emergency
services
TETRA is a modern standard for digital Private Mobile Radio (PMR)
and Public Access Mobile Radio (PAMR). It offers many advantages including
flexibility, security, ease of use and offers fast call set-up times. This makes
it an ideal choice for many business communications requirements.
The name TETRA stands for TErrestrial Trunked RAdio. Aimed at
a variety of users including the police, ambulance and fire services, it is
equally applicable for utilities, public access, fleet management, transport
services, and many other users. It offers the advantages of digital radio whilst
still maintaining the advantages of a PMR system.
Tetra radio beginnings
Work started on the development of the TETRA standards in 1990 and has relied on
the support of the European Commission and the ETSI members. Experience gained
in the development of the highly successful GSM cellular radio standard, as well
as experience from the development and use of trunked radio systems has also
been used to fashion the TETRA standard. In addition to this the process has
gained from the co-operation of manufacturers, users, operators and industry
experts. With this combined expertise the first standards were ready in 1995 to
enable manufacturers to design their equipment to interoperate successfully.
Tetra radio features
TETRA radio offers many new and valuable features. These include a fast call
set-up time, which is a particularly important requirement for the emergency
services. It also has excellent group communication support, direct mode
operation between individual radios, packet data and circuit data transfer
services, better economy of frequency spectrum use than the previous PMR radio
systems and in addition to this it provides advanced security features. The
system also supports a number of other features including call hold, call
barring, call diversion, and ambience listening.
The TETRA radio system uses Time Division Multiple Access (TDMA)
technology with 4 user channels on one radio carrier and 25 kHz spacing between
carriers. This makes it inherently more efficient than its predecessors in the
way that it uses the frequency spectrum. Data can be transmitted at 7.2 kbits
per second for a single channel. This can be increased four fold to 28.8 kbits
per second when multi-slot operation is employed.
For emergency services in Europe the frequency bands 380-383
MHz and 390-393 MHz have been allocated. These bands can be expanded to cover
all or part of the spectrum from 383-395 MHz and 393-395 MHz should this be
needed. For civil systems in Europe the frequency bands 410-430 MHz, 870-876 MHz
/ 915-921 MHz, 450-470 MHz, 385-390 MHz / 395-399,9 MHz, have been allocated.
TETRA radio trunking facility provides a pooling of all radio
channels that are then allocated on demand to individual users, in both voice
and data modes. By the provision of national and multi-national networks,
national and international roaming can be supported, the user being in constant
communication. TETRA supports point-to-point, and point-to-multipoint
communications both by the use of the TETRA infrastructure and by the use of
Direct Mode without infrastructure.
In addition to this it is possible for TETRA radio to operate
in a secure format. The digital data can be encrypted before transmissions,
making the system inherently secure. This may be required for some covert
operations or for the police services.
TETRA radio operation
There are three different modes in which TETRA can be run:
- Voice plus Data (V+D)
- Direct Mode Operation (DMO)
- Packet Data Optimised (PDO)
The most commonly used mode is V+D. This mode allows
switching between speech and data transmissions, and can even carry both by
using different slots in the same channel. Full duplex is supported with base
station and mobile radio units frequencies normally being offset by about 10 MHz
to enable interference levels between the transmitter and receiver in the
station to be reduced to an acceptable level.
DMO is used for direct communication between two mobile units
and supports both voice and data, however full duplex is not supported in this
mode. Only simplex is used. This is particularly useful as it allows the mobile
stations to communicate with each other even when they are outside the range of
the base station.
The third mode, PDO is optimised for data only transmissions.
It has been devised with the idea that much higher volumes of data will be
needed in the future and it is anticipated that further developments will be
built upon this standard.
Data structures
TETRA radio uses TDMA techniques. This enables much greater spectrum efficiency
than was possible with previous PMR systems because it allows several users to
share a single frequency. As the speech is digitised, both voice and data are
transmitted digitally and multiplexed into the four slots on each channel.
Digitisation of the speech is accomplished using a system that enables the data
to be transmitted at a rate of only 4.567 kbits/second. This low data rate can
be achieved because the process that is used takes into account the fact that
the waveform is human speech rather than any varying waveform. The digitisation
process also has the advantage that it renders the transmission secure from
casual listeners. For greater levels of security that might be required by the
police or other similar organisations it is possible to encrypt the data. This
would be achieved by using an additional security or encryption module.
The data transmitted by the base station has to allow room
for the control data. This is achieved by splitting what is termed a multiframe
lasting 1.02 seconds into 18 frames and allowing the control data to be
transmitted every 18th frame. Each frame is then split into four time slots. A
frame lasts 56.667 mS. Each time slot then takes up 14.167 mS. Of the 14.167mS
only 14 milliseconds is used. The remaining time is required for the transmitter
to ramp up and down. The data structure has a length of 255 symbols or 510
modulation bits. It consists of a start sequence that is followed by 216 bits of
scrambled data, a sequence of 52 bits of what is termed a training sequence. A
further 216 bits of scrambled data follows and then the stream is completed by a
stop sequence. The training sequence in the middle of the data is required to
allow the receiver to adjust its equaliser for optimum reception of the whole
message.
The data is modulated onto the carrier using differential
quaternary phase shift keying. This modulation method shifts the phase of the RF
carrier in steps of � pi /4 or �3 pi /4 depending upon the data to be
transmitted. Once generated the RF signal is filtered to remove any sidebands
that extend out beyond the allotted bandwidth. These are generated by the sharp
transitions in the digital data. A form of filter with a root raised cosine
response and a roll off factor of 0.35 is used. Similarly the incoming signal is
filtered in the same way to aid recovery of the data.
Additionally, TETRA radio uses error tolerant modulation and
encoding formats. The data is prepared with redundant information that can be
used to provide error detection and correction. The transmitter of each mobile
station is only active during the time slot that the system assigns it to use.
As a result the data is transmitted in bursts. The fact that the transmitter is
only active for part of the time has the advantage that the drain on the battery
of the mobile station is not as great as if the transmitter was radiating a
signal continuously. The base station however normally radiates continuously as
it has many mobile stations to service.
One important feature of TETRA is that the call set up time
is short. It occurs in less than 300 mS and can be as little as 150 mS when
operating in DMO. This is much shorter than the time it takes for a standard
cellular telecommunications system to connect. This is very important for the
emergency services where time delays can be very critical.
Further TETRA radio developments
While TETRA radio is a major improvement over the previous PMR systems in
operation, additional data capacity is always needed. In view of the higher data
capabilities now being offered by the cellular services, the TETRA radio
standard is being updated to enable it to keep pace with other comparable
technologies. In this way, TETRA will be able to offer commercial users the
advantages of a PMR service alongside the data capabilities of a cellular
network.
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