Features

During network busy periods, that service allows access to network resources in order of user terminals call priority status.  As there are 16 levels of priority in TETRA, this service is very useful in providing different Grade of Service (GoS) levels (and tariff structures) during busy periods.

For example, front line officers would be provided with the highest priority levels in a Public Safety network to maintain the highest level of service access whilst routine users would be provided with lower priority levels.

This call service, of which the highest priority is the emergency call, provides the highest uplink priority and highest priority access to network resources. If a network is busy, the lowest priority communication is dropped to handle the emergency call.  Unlike 911, 112 or 999 initiated public network emergency calls (which can also be supported on TETRA) the TETRA emergency call can be initiated by using a dedicated switch located on the terminal.

Activating the emergency call automatically alerts the affiliated control room dispatcher and other terminal users in that persons talk group.

In TETRA a queue is provided in the trunking controller during network busy periods to store and handle calls on a First In First Out (FIFO) basis in order of user priority level.

The advantage is that a user only has to initiate a call request once, knowing that even in busy periods the call will be automatically established once a traffic channel becomes free, thus reducing user stress and frustration when contending with other users on a busy network.

This is probably the most basic voice service in TETRA but yet the most complex to support effectively and efficiently.  This is because group calls need to:

  • Use simple “Push To Talk” operation to provide fast call set-up group communications
  • Be operated and managed in particular ways to optimise network loading, some examples being:
  • Operate in simplex
  • Operate on a “preferred” site for optimum network loading
  • Have a defined area of operation (Area selection)
  • Have a very reliable call-set up signalling protocol to ensure all users in a group are connected together when a call is first initiated (call acknowledgment signalling is impractical for group calls)
  • Have priority mechanisms to ensure that specified users in a wide area group call (spanning multiple base station sites) are connected together when a network is busy

It is this complexity needed to support group calls that makes public cellular networks unsuitable, simply because they were originally designed to support “One to One” calls, unlike TETRA which was primarily designed to support group calls at the outset.

Direct Mode Operation (DMO) provides the ability for TETRA radio terminals to communicate directly with each independent on the TETRA network infrastructure.  DMO is not new and has been facility-mandated and used by many traditional PMR user organisations for several decades.

This service protects selected radio terminal users from being forced off the network as a result of pre-emptive calls (emergency calls) during busy periods.  When emergency calls are supported in a network, it is essential that only a small number of radio terminal users are provided with this facility as the objective of retaining important calls during busy periods could be lost.

Trunking

The main benefit of trunking is normally seen as spectrum efficiency, or more radio users per RF channel compared with a conventional radio channel for a given Grade of Service (GoS), brought about by the automatic and dynamic assignment of a small number of communication channels shared amongst a relatively large number of users.

Because trunking systems support more radio users than conventional systems, national administrations actively support the deployment of trunking systems as this helps reduce pressure on meeting PMR spectrum demands.

What users want is to solve all the operational problems associated with conventional PMR, yet still retain the simplicity of conventional open channel ‘all informed net’ operation.  The fundamental element of trunking that solves these conventional PMR problems is the use of a control channel.

Table 1 below lists some of the operational problems of conventional PMR and also lists how the use of trunking solves these problems.

Conventional PMR ProblemTrunking Solution
ContentionAll call requests are handled on the control channel for immediate call processing or in order of queue priority if the system is busy.
Manual Switching of ChannelsAutomatic cell handover takes away the need for manual channel selection
Inefficient Channel UtilisationThe automatic and dynamic assignment of a small number of communication channels shared amongst a relatively large number of users ensures an equal grade of service for all radio users on the system.
Lack of PrivacyThe dynamic and random allocation of channels makes it more difficult for a casual eavesdropper to monitor conversations.
Radio User AbuseAbuse is minimised as the identity of all radio users and the time and duration of messages are known and can therefore be easily traced to the abuser.

Table 1: Conventional PMR problems solved by Trunking

It is important to note that the operational simplicity of conventional PMR ‘all informed net’ talk group communications is still retained by employing fast call set-up “Push To Talk” (PTT) operation on radio terminals.

trunking

Chart 1: Conventional and Trunked Radio User Loading Comparison

 

As the control channel acts as a signalling communications link between the Trunking Controller and all mobile radio terminals operating on the system, the Trunking Controller knows the status of the system at any moment in time as well as its historic usage, which is stored in its memory.  For example, the Trunking Controller knows:

  • The individual and group identity of all radio units registered on the system
  • The individual identity and time radio units registered on the system
  • The individual identity and time radio units de-registered from the system
  • The individual and group identity, time and duration of all messages

With additional intelligence in both the radio terminals and the trunking controller the advantages and benefits of trunking can be increased.  For example, the length of the control channel signalling messages can be increased by a set amount to accommodate a variety of new services and facilities.  Also, the trunking controller can be programmed to handle calls in a variety of ways as required by the operator of the system.

A four time slot TDMA technology was adopted in TETRA as it offered the optimum solution to balance the cost of equipment with that of supporting the services and facilities required by user organisations for a medium to high capacity network providing single site local RF coverage and/or multiple site wide area RF coverage.

RF Spectrum efficiency is a combination of three main factors being the occupied bandwidth per communication channel, the frequency re-use factor determined by the Carrier to Interference protection ratio C/I in dB’s and the trunking technology used.  As previously mentioned TETRA utilises the latest in trunking technology.

Also, the TDMA technology used in TETRA provides 4 independent communications channels in a 25 kHz RF bandwidth Channel, making it twice as efficient in occupied bandwidth terms as a traditional 12.5 kHz RF bandwidth FDMA channel.  Although FDMA technologies tend to have a better C/I performance than TDMA TETRA, the overall spectrum efficiency advantage lies with TETRA, especially for medium to high capacity networks.

tdma

 

Four time slot TDMA technology was adopted in TETRA as it offered the optimum solution to balance the cost of equipment with that of supporting the services and facilities required by user organisations for a medium to high capacity network providing single site local RF coverage and/or multiple site wide area RF coverage. A diagrammatic representation of the TDMA time slot structure used in TETERA is shown above.

RF Spectrum efficiency is a combination of three main factors being the occupied bandwidth per communication channel, the frequency re-use factor determined by the Carrier to Interference protection ratio C/I in dB’s and the trunking technology used. As previously mentioned TETRA utilises the latest in trunking technology.

tdma2

From the base station equipment configuration in above figure it can be seen that the FDMA solution requires 4 separate transceivers where as the TDMA solution only requires 1 transceiver. As a consequence, the FDMA solution requires a transmitter antenna combining and receiver splitting network to enable single transmit and receive antenna working.

Also, the RF power output of the FDMA transmitters will need to be higher in order to compensate for transmission losses in the transmit antenna combining network.

Because 4 slot TDMA already supports four independent communication paths, no antenna combining equipment is required, thereby saving space as well as cost.

Because of using TDMA technology, the cost and equipment space at base station sites can be significantly reduced compared with traditional FDMA technology trunking solutions. Another advantage of TDMA technology is that it enables new services and facilities to be supported with minimum cost.  Some examples are:

 

The ‘laws of physics’ limits the maximum data rate in a given RF channel bandwidth.  Assuming the same modulation scheme, the wider the channel bandwidth the higher the data rate.  Because TDMA uses wider channels than FDMA, the combined data rate on a single RF carrier is greater.

The net data rate in TDMA is better than FDMA in poor RF propagation conditions.  This is because Automatic Repeat Requests (ARQ’s) are required when received data is corrupted as a result of RF fading.  As TDMA terminal devices effectively operate in full duplex ARQ’s can be sent efficiently after each time slot transmission instead of waiting until the end of each voice transmission, as is usually the case with FDMA.

In TDMA any number of time slots up to the maximum limit of the technology being employed can be combined to increase data throughput as required for specific applications.

Because of the TDMA time slot structure it is possible to assign one time slot to support voice and the next time slot to support data in a two slot transmission from radio terminals.  This capability effectively allows a single radio terminal to concurrently transmit or receive voice and data at the same time.

TDMA technology inherently supports full duplex communications.  Although full duplex voice communications can be supported on FDMA systems the need for duplex operation requires RF screening between the transmitter and receiver and also a duplexer to allow single antenna working.  Because of this, duplex FDMA radio terminals are usually bulkier and more costly to produce than TDMA terminals, which do not need RF screening or antenna duplexers.