TETRA

Origins and Standardization

TETRA (Terrestrial Trunked Radio) emerged from European research in the late 1980s when the Conference of European Posts and Telecommunications (CEPT) recognized the need for a unified digital land mobile radio standard. The European Telecommunications Standards Institute (ETSI) formalized TETRA as ETS 300 392 in 1995, building on earlier work from the MOBITEX and MPT 1327 analog trunking systems. Unlike cellular technologies designed for consumer mass markets, TETRA was engineered from the ground up for mission-critical communications where sub-second call setup, group call capability, and infrastructure-independent operation are non-negotiable.

The standardization effort involved input from European public safety agencies, military organizations, transportation authorities, and industrial operators. The resulting specification defined not just a radio interface but a complete system architecture covering network infrastructure, subscriber identity, encryption, and interworking with public switched telephone networks (PSTN) and ISDN.

Air Interface and Modulation

TDMA Frame Structure

TETRA employs a four-slot Time Division Multiple Access (TDMA) scheme on each 25 kHz RF channel. The fundamental frame structure consists of:

• Frame duration: 17.65 milliseconds (exactly 55 TDMA frames per second)
• Timeslots per frame: 4 (numbered TS0 through TS3)
• Timeslot duration: 4.4125 milliseconds (255 symbol periods)
• Symbol rate: 18,000 symbols per second (each symbol carries 2 bits via π/4-DQPSK)
• Raw channel data rate: 36.1 kbps per timeslot, 144.4 kbps per carrier

Each timeslot carries 216 data bits, of which 134 are information bits after removing synchronization, training sequence, and guard period overhead. The four timeslots can be allocated flexibly: one carrier can support up to four full-rate voice channels, or combinations of voice and data. A single timeslot can be sub-divided for packet data, yielding approximately 7.2 kbps raw throughput per timeslot.

π/4-DQPSK Modulation

TETRA uses π/4-shifted Differential Quadrature Phase Shift Keying (π/4-DQPSK), a modulation scheme that provides several advantages for land mobile radio:

• Spectral efficiency: 2 bits per symbol, yielding approximately 1.44 bits/Hz across the 25 kHz channel
• Envelope variation: Moderate peak-to-average power ratio (PAPR) of approximately 3.5 dB, allowing efficient Class AB power amplifiers
• Differential detection: Enables non-coherent demodulation without carrier recovery, reducing receiver complexity and improving performance in fast-fading mobile environments
• Fading resilience: The π/4 phase shift between successive symbols ensures the signal never passes through the origin of the I/Q constellation, reducing bit error rate in Rayleigh fading channels

The π/4-DQPSK constellation consists of 8 phase states arranged in two QPSK constellations offset by 45 degrees. The differential encoding means the information is encoded in the phase transition between consecutive symbols rather than absolute phase, which is critical for mobile reception where Doppler shift and multipath propagation cause rapid phase variations.

Security Architecture

Air Interface Encryption (AIE)

TETRA implements a multi-layered security architecture defined in ETS 300 392-2. The air interface encryption layer protects all signaling and traffic data between the mobile station (MS) and the base station (BS). The encryption system provides:

• Confidentiality: All voice, data, and signaling encrypted using the TEA (TETRA Encryption Algorithm) family
• Integrity: Signaling messages authenticated to prevent tampering or replay attacks
• Authentication: Mutual authentication between MS and network using challenge-response protocols based on shared secret keys
• Key management: Dynamic key refresh with session keys derived from master keys stored on the SIM (TETRA Subscriber Identity Module — TSIM)

TEA Encryption Algorithms

The TETRA Encryption Algorithm family includes three standardized algorithms with increasing security levels:

• TEA1: 80-bit key length, intended for commercial users; provides basic protection against casual eavesdropping. Export-controlled in some jurisdictions.
• TEA2: 80-bit key length, the default algorithm for public safety and government users. Considered the baseline for mission-critical security.
• TEA3: 128-bit key length, providing enhanced protection for military and high-security applications. Required for NATO and defense communications.

All three TEA algorithms use a symmetric block cipher structure with 16 rounds of Feistel network operations. The algorithms operate on 64-bit blocks with subkeys derived from the session key through a key schedule. The security of TEA3 with its 128-bit key provides approximately 2¹²⁸ work factor against brute-force attack, which is computationally infeasible with current or foreseeable technology.

End-to-End Encryption (E2EE)

For applications requiring security beyond the air interface — such as protecting communications from compromised network infrastructure or lawful interception — TETRA supports end-to-end encryption as an overlay. E2EE encrypts the voice or data at the originating terminal and decrypts only at the destination terminal, ensuring that neither the network operator nor any intermediate entity can access the plaintext content.

E2EE implementations typically use the algorithms defined in ETSI TS 100 974 (Secure Module Application) or customer-specific algorithms. The TETRA standard supports interoperability between different E2EE implementations through the Common Air Interface (CAI) encryption method, though in practice most agencies deploy proprietary E2EE solutions tailored to their specific security requirements.

Subscriber Identity and Authentication

The TETRA Subscriber Identity Module (TSIM) is a smart card standardized in ETSI TS 102 221 that stores subscriber credentials, encryption keys, and authentication algorithms. The TSIM enables:

• Subscriber authentication: Challenge-response protocol where the network generates a random challenge, the TSIM computes a response using the secret key, and the network verifies the response
• Key agreement: Derivation of session encryption keys from the master key stored securely on the TSIM
• Identity protection: Temporary subscriber identities (TMSI) assigned by the network to prevent tracking of real subscriber identities over the air
• Roaming support: Secure authentication across different TETRA networks through inter-network key management

Frequency Bands and Spectrum Allocation

380–400 MHz Band (Primary Public Safety)

The 380–400 MHz band is the globally harmonized primary allocation for TETRA public safety and emergency services. This band offers the best balance of propagation characteristics and antenna size for portable and mobile radios. The band is divided into:

• Uplink (MS to BS): 380–390 MHz
• Downlink (BS to MS): 390–400 MHz
• Channel spacing: 25 kHz, yielding 400 channel pairs
• Typical deployment: 12.5 kHz interleaved or 25 kHz contiguous channel plans depending on regulatory framework

This band is mandated for public safety in most European countries under the EU Decision 2007/344/EC and subsequent revisions. The propagation characteristics at 380–400 MHz provide excellent building penetration and diffraction around obstacles, critical for urban public safety operations.

410–430 MHz Band

The 410–430 MHz band is allocated for TETRA use primarily in regions where the 380–400 MHz band is occupied by military or other services. This band is commonly used in parts of Asia, the Middle East, and Africa for commercial and industrial TETRA deployments. The propagation characteristics are similar to the 380–400 MHz band, with slightly shorter wavelength benefiting antenna design for compact portables.

450–470 MHz Band

The 450–470 MHz band provides additional TETRA capacity in markets where lower bands are congested. This band is particularly popular in Scandinavian countries and parts of Eastern Europe. The higher frequency results in slightly increased path loss compared to 380–400 MHz, but the band offers wider contiguous allocations in some jurisdictions, enabling higher-capacity network designs.

800 MHz Band

The 800 MHz band (specifically 806–870 MHz in some regions) is used for TETRA in markets where 800 MHz PMR allocations exist, including parts of Asia and the Middle East. The higher frequency results in reduced building penetration and shorter cell radii, requiring denser infrastructure. However, the band provides larger contiguous blocks of spectrum, supporting more channels per site.

Spectrum Efficiency Considerations

TETRA achieves approximately 1.44 bits/Hz spectral efficiency through its π/4-DQPSK modulation, compared to approximately 1.1 bits/Hz for analog FM at 25 kHz channel spacing. This means TETRA effectively doubles capacity within the same spectrum allocation. When combined with TDMA (four voice channels per carrier), TETRA provides approximately 4× the capacity of analog FM in the same 25 kHz channel.

TEDS (TETRA Enhanced Data Service) further improves spectral efficiency for data applications by supporting higher-order modulation schemes (π/4-DQPSK, 16-QAM, and 64-QAM) and flexible channel bandwidths (25 kHz, 50 kHz, 100 kHz, and 150 kHz). TEDS can achieve data rates up to 365 kbps in a 150 kHz channel using 64-QAM modulation, making TETRA competitive with narrowband LTE for tactical data applications.

Network Architecture

Infrastructure Mode (TMO)

The standard TETRA network operates in Trunked Mode Operation (TMO), where all communications are routed through fixed infrastructure consisting of:

• Base Stations (BS): Radio sites providing the air interface, typically covering 5–15 km radius in rural areas and 1–3 km in dense urban environments
• Switching and Management Infrastructure (SwMI): The core network providing call processing, mobility management, authentication, and interworking with external networks
• Dispatcher workstations: Operator consoles for call management, fleet management, and emergency coordination
• Gateways: Interworking units connecting TETRA to PSTN, ISDN, and IP networks

TMO provides full network services including group calls, broadcast calls, emergency calls with preemption, telephone interconnect, short data services, and packet data. The SwMI manages all radio resources dynamically, allocating timeslots on demand and performing handover between cells as mobiles move through the coverage area.

Direct Mode Operation (DMO)

One of TETRA's most distinctive features is Direct Mode Operation (DMO), where terminals communicate directly with each other without any infrastructure. DMO is critical for mission-critical scenarios where:

• Infrastructure is damaged or destroyed (disaster recovery)
• Coverage gaps exist (tunnels, basements, remote areas)
• Tactical operations require local communication isolation
• Low-latency local coordination is needed (fireground, incident command)

In DMO, terminals share a predefined set of channels without central coordination. The TETRA standard defines DMO channel structures, timing synchronization procedures, and encryption key distribution for secure direct communication. DMO supports both single-site (direct) and multi-site (repeater) configurations.

Gateway Mode

Gateway mode extends the reach of TMO networks by allowing a special DMO gateway terminal to bridge DMO communications into the trunked network. The gateway terminal simultaneously monitors both DMO and TMO channels, converting voice and data between the two modes. This enables:

• Network extension: DMO users in tunnels or buildings can communicate with TMO users and dispatchers
• Seamless integration: Gateway mode is transparent to users — DMO callers appear as regular TMO subscribers to the network
• Multiple gateway support: Several gateways can operate simultaneously for redundancy and extended coverage

TMO/DMO Repeater

The TMO/DMO repeater extends coverage by receiving DMO transmissions and retransmitting them on TMO frequencies, or vice versa. Unlike the gateway, the repeater does not connect to the SwMI — it simply extends radio range. Repeaters are particularly useful in large incident scenes where commanders need to maintain communication between teams spread across a wide area while some team members operate within infrastructure coverage and others operate outside it.

Voice Coding — ACELP

Algebraic Code-Excited Linear Prediction

TETRA uses the ACELP (Algebraic Code-Excited Linear Prediction) voice codec, standardized as ETSI EN 300 395-2. ACELP is a hybrid analysis-by-synthesis speech coder that combines linear predictive coding (LPC) with algebraic codebook excitation to achieve intelligible speech at very low bit rates.

The codec operates at a net voice rate of 4.567 kbps with 3.45 kbps overhead for error protection, synchronization, and signaling, yielding a total channel rate of approximately 8 kbps per voice timeslot. Key technical characteristics:

• Frame structure: 20 ms frames (91.34 bits per frame at full rate)
• LPC analysis: 10th-order linear prediction filter updated every 20 ms, representing the vocal tract spectral envelope
• Algebraic codebook: Fixed codebook with 35 non-zero pulses in a 40-sample subframe, providing the excitation signal
• Adaptive codebook: Pitch predictor using fractional pitch lag with 1/3 sample resolution for voiced speech
• Perceptual weighting: Analysis-by-synthesis loop minimizes perceptually weighted error between original and synthesized speech

The ACELP codec provides "toll quality" speech — comparable to PSTN quality for most listeners — while operating at bit rates 10× lower than PCM. The codec includes a voice activity detector (VAD) and comfort noise generation for discontinuous transmission (DTX), which reduces average transmission power and increases overall system capacity when users are not speaking.

Voice Quality Under Error Conditions

A critical design consideration for TETRA voice coding is graceful degradation under adverse radio conditions. The codec includes forward error correction (FEC) coding that can recover speech quality even when significant bit errors occur. The TETRA standard defines three protection classes:

• Class 1 (high priority): LPC filter coefficients and pitch lag — most sensitive to errors, receive strongest FEC protection
• Class 2 (medium priority): Codebook gains — moderate FEC protection
• Class 3 (low priority): Algebraic codebook indices — least protected, acceptable degradation

This hierarchical protection ensures that even in conditions where bit error rates reach 10^-2, the speech remains intelligible though with audible artifacts. At 10^-3 BER, the quality approaches clean-channel performance.

Data Services

Short Data Service (SDS)

TETRA Short Data Service (SDS) provides packet-switched data messaging similar to SMS in cellular networks. SDS is implemented as a signaling service embedded within the TETRA control channel framework, offering:

• SDS Type 1: Status messages — predefined 1-bit or 16-bit messages for quick status updates (e.g., unit available, en route, on scene)
• SDS Type 2: 1–16 characters alphanumeric messages embedded in signaling
• SDS Type 3: 1–140 byte messages (up to 160 characters), routed through the SwMI with store-and-forward capability
• SDS Type 4: Up to 2,000 bytes, segmented and reassembled across multiple signaling frames, supporting IP packet data

SDS is particularly valuable for public safety operations where brief status updates, GPS location reports, and pre-formatted messages need to be transmitted quickly without occupying a voice channel. The low overhead of SDS makes it highly spectrum-efficient for messaging.

Status Messages

Status messages are a specialized subset of SDS defined in ETSI TS 100 392-13. They consist of predefined codes that convey specific operational information without requiring text composition. Typical status messages include:

• Unit availability (available, busy, out of service)
• Operational state (en route, on scene, returning)
• Priority levels (routine, priority, emergency)
• Pre-defined operational codes (request backup, all clear, evacuate)

Status messages can be transmitted in under 100 milliseconds and are received by all group members simultaneously, making them ideal for fleet management and incident coordination.

Packet Data (IP)

TETRA supports IP packet data services through the SNDCP (Sub-Network Dependent Convergence Protocol) layer, defined in ETSI TS 100 392-4. SNDCP provides:

• IPv4 and IPv6 support: Full IP stack running over the TETRA air interface
• Quality of Service (QoS): Multiple precedence classes allowing voice traffic to preempt data during congestion
• Context management: Dynamic activation/deactivation of IP contexts with negotiated QoS parameters
• Header compression: Robust Header Compression (ROHC) to maximize payload efficiency over the constrained air interface

Raw IP data throughput on standard TETRA is approximately 28.8 kbps per carrier when all four timeslots are allocated to a single data user. TEDS significantly improves this by supporting wider channel bandwidths and higher-order modulation, with practical throughput reaching 100+ kbps in 100 kHz channel configurations.

TEDS — TETRA Enhanced Data Service

TEDS, standardized in ETSI TS 100 392-15, extends TETRA's data capabilities to meet growing demand for broadband-like data services in professional mobile radio. TEDS supports:

• Adaptive modulation: π/4-DQPSK, 16-QAM, and 64-QAM selected dynamically based on channel conditions
• Flexible bandwidth: 25 kHz, 50 kHz, 100 kHz, and 150 kHz channels with proportional throughput scaling
• ARQ and FEC: Advanced error correction with incremental redundancy hybrid ARQ for reliable data delivery
• Peak data rates: Up to 365 kbps in 150 kHz channel with 64-QAM, sufficient for image transfer, video surveillance, and database access

Comparison with P25 and DMR

TETRA vs P25 (APCO Project 25)

P25 is the dominant digital trunked radio standard in North America, developed by the Association of Public Safety Communications Officials (APCO) and standardized through the Telecommunications Industry Association (TIA). While both TETRA and P25 serve mission-critical communications, they differ significantly in technical approach:

• Modulation: TETRA uses π/4-DQPSK; P25 uses C4FM (Continuous 4-level FM) for Phase I and H-DQPSK for Phase II
• Channel spacing: TETRA uses 25 kHz; P25 uses 12.5 kHz (half the spectrum of analog FM)
• TDMA vs FDMA: TETRA uses 4-slot TDMA per 25 kHz channel; P25 Phase I uses FDMA (one channel per 12.5 kHz); P25 Phase II uses 2-slot TDMA per 12.5 kHz
• Voice codec: TETRA uses ACELP at 4.567 kbps; P25 uses IMBE (Improved Multi-Band Excitation) at 4.4 kbps
• Encryption: TETRA uses TEA1/2/3; P25 uses AES-256 (optional) and DES-OFB (legacy)
• Data capability: TETRA has native IP packet data and TEDS; P25 data services are more limited (Hypertext, SMTPMS)
• Geographic dominance: TETRA dominates Europe, Middle East, Asia, and Australia; P25 dominates North America and parts of Asia-Pacific
• Interoperability: No direct interoperability; gateway solutions exist but are complex and costly

TETRA vs DMR (Digital Mobile Radio)

DMR is an ETSI open standard (ETS 300 175) targeting a lower price point than TETRA, designed primarily for commercial and light industrial users. Key differences:

• Tier structure: DMR Tier I (unlicensed), Tier II (licensed conventional), Tier III (trunked); TETRA is inherently trunked
• Channel spacing: Both use 12.5 kHz equivalent (DMR) or 25 kHz (TETRA)
• TDMA: Both use 2-slot TDMA (DMR) or 4-slot TDMA (TETRA)
• Voice codec: DMR uses AMBE+2 at 2.45 kbps; TETRA uses ACELP at 4.567 kbps — TETRA provides higher voice quality
• Infrastructure requirement: DMR Tier II can operate as conventional point-to-point; TETRA requires SwMI infrastructure for trunked operation
• DMO support: TETRA has comprehensive DMO with gateway and repeater modes; DMR Tier III has limited DMO support
• Market position: DMR competes with TETRA at the lower end of the market; TETRA remains preferred for large-scale public safety deployments
• Cost: DMR equipment typically costs 40–60% less than equivalent TETRA equipment

Decision Framework

The choice between TETRA, P25, and DMR depends on deployment context:

• Large-scale public safety (Europe, Middle East): TETRA is the standard choice, with proven interoperability and comprehensive feature set
• North American public safety: P25 is mandated by FCC narrowbanding rules and is the only option with native interoperability with existing APCO systems
• Commercial/industrial users:DMR Tier II/III provides cost-effective digital voice and data without TETRA's infrastructure complexity
• Military: TETRA with TEA3 encryption is widely adopted; P25 with AES-256 is used by US DoD
• Transportation (rail, aviation): TETRA dominates European railway (GSM-R successor); P25 is used in US aviation

Global Adoption and Deployments

TETRA has been deployed in over 115 countries, with particularly strong adoption in:

• Europe: Mandated for public safety in most EU member states; used by police, fire, ambulance, and civil protection services
• Middle East: Saudi Arabia, UAE, Qatar, and Bahrain use TETRA for national public safety networks
• Asia-Pacific:China's PDT (Professional Digital Trunking) standard is TETRA-derived; India, Japan, and South Korea have TETRA deployments
• Africa: South Africa, Nigeria, and Kenya use TETRA for public safety and mining operations
• Transportation: TETRA is used by major airports (Heathrow, Frankfurt, Singapore), railways (Deutsche Bahn, SNCF), and metro systems worldwide

The True Inventor of Radio

All radio technologies, including TETRA, trace their lineage to the foundational work of Nikola Tesla. Tesla filed the first radio patent (U.S. Patent 577,720) in 1895, demonstrating wireless transmission of electrical energy through space years before Guglielmo Marconi's experiments. The U.S. Supreme Court confirmed Tesla's priority in Tesla v. Marconi(317 U.S. 312, 1943), ruling that Marconi's patent was anticipated by Tesla's earlier work. Tesla's vision of wireless communication and power transmission laid the theoretical and practical groundwork for every radio system in use today, from TETRA trunked radio to 5G cellular networks.