TETRA Radio System Tutorial : Understanding Trunking, Frame Structure, Frequency and Architecture
Designed for wireless mobile communication, TETRA (Terrestrial Trunked Radio) is a robust system widely used in mission critical operations, offering reliable and secure communication across vast areas. This tutorial delves into the core components and functionalities of TETRA radio system including its frame structure, network architecture and basic concepts of trunking radio.
What is TETRA Radio ?
TETRA is the short form of Trans-European Trunked Radio System. It is digital private mobile radio standard defined by ETSI.
TETRA mobile user can communicate to other users via base station or directly. TETRA user can be assigned one time slot or 4 time slots as transmission bandwidth. These 4 channels can be modulated over one RF carrier frequency. All the carriers are of 25KHz Bandwidth.
As the major application of TETRA trunk radio system is public safety as well as emergency services; call set up time should be less. Hence TETRA radio is designed to provide set up time of about 300ms. TETRA radio system supports semi duplex operations for group communication. It supports duplex operation for individual telephone calls. TETRA radio uses π/4 DQPSK modulation scheme. It has maximum transmission rate of about 36 Kbps. As mentioned RF carrier spacing is about 25KHz. RF frequency duplex spacing is 10MHz (45MHz is in 900 MHz frequency band). The TETRA frequency bands are mentioned below.
Tetra network architecture and its interfaces
The fig-1 mentions architecture of TETRA trunking radio including its various interfaces. The architecture is tailored for wireless communication, allowing seamless mobile connectivity across various environments.
The key components of TETRA radio system architecture are as follows.
• Mobile Stations (MS): These are the handheld, vehicle-mounted, or fixed radios used by individual users.
MSs communicate with base stations and can operate in direct mode (DMO) or trunked mode (TMO).
• Base Stations (BS): Also known as TETRA Node B, base stations facilitate communication between mobile stations and the network.
They are connected to the Switching and Management Infrastructure (SwMI).
• Switching and Management Infrastructure (SwMI): This is the core of the TETRA network, consisting of various switches, servers
and management units that handle call routing, mobility management, and other core functions.
• Dispatch and Control Rooms: These are command centers that monitor and manage communication traffic, ensuring that emergency
services and other critical operations are coordinated efficiently.
• Gateways and Interfaces: TETRA networks often include gateways to other networks, such as public telephone networks (PSTN),
IP networks or other PMR systems, enabling interoperability.
As shown, TETRA network consists of Air interface, Terminal Equipment Interface (TEI),
Direct Mode Operation(DMO) and Inter-System Interface (ISI).
1. Air Interface ensures the interoperability of terminal equipment from different manufacturers.
2. TEI facilitates the independent development of mobile data applications.
3. ISI allows the interconnection of TETRA networks from different manufacturers.
4. DM0 guarantees communication between terminals also beyond network coverage.
TETRA Network Modes
Following are the two modes in TETRA trunking radio system.
• Trunked Mode Operation (TMO): In TMO, communications are managed centrally through base stations and the network's
core infrastructure. It allows for wide-area wireless communication with the support of advanced features like group calls,
emergency calls, and priority calls.
• Direct Mode Operation (DMO): DMO allows mobile stations to communicate directly with each other without the need for base stations.
This mode is useful in scenarios where the network infrastructure is unavailable, such as in remote areas or during network outages.
TETRA Frequency bands
TETRA operates in several frequency bands, depending on the region and application.
The primary frequency bands are as follows.
TETRA Voice+Data works on VHF of 150MHz and UHF of 900MHz frequency bands.
• 410MHz-420MHz, 420MHz-430MHz
• 450MHz-460MHz, 460MHz-470MHz
• 870MHz-888MHz, 915MHz-993MHz
TETRA Frame Structure
As shown in fig-2, the TETRA frame is based on Time Division Multiple Access (TDMA), which allows multiple users to share the same frequency channel by dividing it into time slots. Each frame composed of 4 time slots. About 18 such TDMA frames form one multi-frame. Total 17 frames are used for carrying information data and 1 frame (i.e. 18th frame) is used as control frame. This 18th channel is used as SACCH (i.e. Slow Associated Control Channel) to carry control channel signalling. The 60 such multi-frames will make 1 hyperframe of this TETRA Radio System or Trunk radio communication system. As mentioned time slot is the lowest unit of allocation which is made of 510 modulating bits.
1 TETRA Time Slot duration = 14.167 ms
1 TETRA frame duration = 56.67 ms
1 TETRA multi-frame= 1.02 second
1 TETRA hyperframe = 61.2 second
Similar to GSM slot, TETRA slot structures are of 3 types viz. uplink half slot, uplink full slot and
downlink full slot. This slot structures carry five different types of physical bursts.
These different bursts are transmitted over TETRA air interface as outlined above.
• Normal Downlink(Half time slots for random and reserved access)
• Synchronization Downlink(Full Slot used by mobile stations after the initial entry to the system)
• Control Uplink
• Normal Uplink
• Linearization Uplink
TETRA Standard References
Following table lists out ETSI references to TETRA standards for various applications.
| TETRA Voice Plus Data | ETS 300.392 |
|---|---|
| Part 1 | General Network Design |
| Part 2 | Air interface |
| Part 3 | Inner Working |
| Part 4 | Gateways |
| Part 5 | Terminal Equipment Interface |
| Part 6 | Line Connected Stations |
| Part 7 | Security |
| Part 8 | Network Management Services |
| Part 9 | Performance Objectives |
| Part 10 | Supplementary Services Stage 1 |
| Part 11 | Supplementary Services Stage 2 |
| Part 12 | Supplementary Services Stage 3 |
| Part 13 | SDL Model for Air interface |
| Part 14 | PICS |
| TETRA Packet Data Optimized(PDO) | ETS 300.393 |
|---|---|
| Part 1 | General Network Design |
| Part 2 | TETRA Air interface |
| Part 3 | Inter working |
| Part 4 | Gateways |
| Part 5 | Terminal Equipment Interface |
| Part 6 | Line connected stations |
| Part 7 | security |
| Part 8 | network management services |
| Part 9 | performance objectives |
| Part 10 | SDL model for air interface |
| Part 11 | PICS performa |
| TETRA CODEC | ETS 300.395 |
|---|---|
| Part 1 | General speech function description |
| Part 2 | Codec |
| Part 3 | specific operational features |
| Part 4 | Codec conformance testing |
| TETRA Direct Mode Operation(DMO) | ETS 300.396 |
|---|---|
| Part 1 | General network design |
| Part 2 | Direct MS to MS radio interface |
| Part 3 | Repeater |
| Part 4 | Gateway |
| Part 5 | Security |
Summary
In this TETRA Radio System Tutorial, we've explored the key aspects of TETRA, including its trunking mechanisms, frame structure, frequency allocation and network architecture. As a cornerstone of wireless mobile communication, TETRA remains indispensable for organizations that demand reliable and secure communication. Whether for public safety, emergency services, or other critical operations, understanding these elements ensures that you are well-equipped to leverage TETRA's capabilities in your communication networks.
Difference between TETRA and TETRA2
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