Direct Mode Operation (DMO)

Direct Mode Operation (DMO) provides the ability for TETRA radio terminals to communicate directly with each independent of the TETRA network infrastructure. DMO is not new and has been a facility mandated and used by many traditional PMR user organizations for several decades. The primary requirement for DMO has been brought about by the need to balance the RF Coverage, Grade of Service (GoS) and Reliability of a network with that of the network’s overall cost. The requirement for DMO makes the use of public cellular networks unsuitable.

As well as allowing direct communications in situations where network coverage is not available, DMO also includes the possibility of using a sequence of one or more TETRA terminals as relays. This functionality is called DMO gateway (from DMO to TMO) or DMO repeater (from DMO to DMO). In emergency situations this feature allows direct communications underground or in areas of bad coverage.

Impact of TETRA signals

..Safety limits for exposure to radio waves..

TETRA is subject to the same safety guidelines as other mobile communications products and systems. These are set by independent expert organizations, like the International Commission on Non-Ionizing Radiation Protection (ICNIRP). The guidelines, which are endorsed by the WHO and other authorities around the world, set limits for exposure based on extensive reviews of the scientific evidence. The guidelines incorporate a substantial margin of safety, designed to protect both users and the general public. Member

companies of the TETRA Industry Group rigorously design and test their products and systems to make sure they comply fully with the safety guidelines.

The transmitting power of a TETRA portable radio is comparable to that of a mobile phone, typically 1 Watt at peak power, with average power of 0.25 Watt. TETRA terminals also use a feature called Adaptive Power Control, which continually adjusts the power output to the lowest level needed to maintain reliable communication with the base station.

A study carried out by the UK Defense Science and Technology Laboratory (DSTL) looked at the impact of TETRA signals on calcium concentrations in neuronal cells. The results were published in the

International Journal of Radiation Biology in December 2005. As well as simulating the maximum exposure that can arise from TETRA handsets, several lower power levels were also used to see if there could be "power windows" where TETRA signals cause a reaction. No effects of TETRA on calcium responses were found at any of the power levels used in the measurement.

Advantages of TETRA network

The main advantages of TETRA over other technologies (such as GSM) are:

•The much lower frequency used gives longer range, which in turn permits very high levels of geographic coverage with a smaller number of transmitters, thus cutting infrastructure costs.

•During a voice call, the communications are not interrupted when moving to another network site! This is a unique feature which dPMR and DMR do not offer.

•High spectral efficiency - 4 channels in 25 kHz and no guard bands, compared to GSM with 8 channels in 200 kHz and guard bands.

•Very fast call set-up - a one to many group call is generally set-up within 0.5 seconds (typical less than 250 msec for a single node call) compared with the many seconds (typically 7 to 10s) that are required for a GSM network.

•Works at high relative speeds >400 km/h.

•The system contains several mechanisms, designed into the protocols and radio parameters, to ensure communication success even during overload situations (e.g., during major public events or disaster situations), thus calls will always get through unlike in cellular systems. The system also supports a range of emergency calling modes.

•TETRA infrastructure is usually separate from (but connected to) that of the public (mobile) phone networks, resulting in (normally) no call charges for the system owners, substantially more diverse and resilient communications and it is easy to customize and integrate with data applications (vehicle location, GIS databases, dispatch systems, etc.).

•Unlike most cellular technologies, TETRA networks typically provide a number of fall-back modes such as the ability for a base station to process local calls. So called 'mission critical' networks can be built with TETRA where all aspects are fail-safe/multiple-redundant.

•In the absence of a network mobiles/portables can use 'direct mode' whereby they share channels directly (walkie-talkie mode).

•Gateway mode - where a single mobile with connection to the network can act as a relay for other nearby mobiles that are out of range of the infrastructure.

•Unlike cellular technologies, which connect one subscriber to one other subscriber (one-to-one), TETRA is built to do one-to-one, one-to-many and many-to-many. These operational modes are directly relevant to the public safety and professional users.

Introduction to TETRA network

TErrestrial Trunked RAdio (TETRA) is an open digital trunked radio standard defined by the European Telecommunications Standardization institute (ETSI) to meet the needs of the most demanding Private Mobile Radio users such as Police, Emergency Services, Energy Distributors, Manufacturing, Petro-Chemical, fire department etc.

TETRA is a professional mobile radio and two-way transceiver (colloquially known as a walkie talkie) specification.

ETSI has developed this standard in co-operation with leading manufacturers, system operators and users. The main purpose of this standard was to define a series of open interfaces, services and facilities, in sufficient detail to enable independent manufacturers to develop infrastructure and terminal products that would fully interoperate with each other.

TETRA uses Time Division Multiple Access (TDMA) with four user channels on one radio carrier and 25 kHz spacing between carriers. Both point-to-point and point-to-multipoint transfer can be used.

This means excellent efficiency of frequency spectrum.

In addition to voice and dispatch services, the TETRA system supports several types of data communication. Sending text messages, Status messages and short data services (SDS) are provided over the system's main control channel.

For protection against eavesdropping; air interface encryption and end-to-end encryption is available.

The common mode of operation is in a group calling mode in which a single button push will connect the user to the users in a selected call group and/or a dispatcher.

TETRA terminals can act as mobile phones(cell phones), with a full-duplex direct connection to other TETRA Users or the PSTN.

Emergency buttons, provided on the terminals, enable the users to transmit emergency signals, to the dispatcher, overriding any other activity taking place at the same time.

The frequency range for public safety systems are from 380-400 MHz and for the commercial applications are from 410-430 MHz &  and from 870-876 MHz.

..Facilities of TETRA..

•Group call

•Pre-emptive priority call (Emergency Call)

•Call Retention

•Priority Call

•Busy Queuing

•Direct Mode Operation (DMO)

•Dynamic Group Number Assignment (DGNA)

•Ambience listening

•Area Selection

•Late Entry

•Voice Encryption