Global System for Mobile Communications (GSM) phones raise expectations among professional-mobile-radio users to levels that system makers are only now beginning to address. Not long ago, you'd need to be a member of the emergency services or a top executive to make calls from a mobile phone. Now, anyone with $100 to spend can roam most of Europe and enjoy direct access to international communications networks. Consumer models have a lot in common with professional user needs, but there are still compelling reasons why GSM can't deliver in the professional user market. With dedicated professional-mobile-radio backgrounds, alternative systems compete for private-mobile-radio (PMR) and public-access mobile-radio
(PAMR) deployments. Now, professional users can enjoy cellular like telephony and private radio dispatcher services with the same mobile handset.
The major competitors for European PMR/PAMR acceptance are terrestrial-trunked radio (Tetra) and Tetrapol, both of which have worldwide ambitions. Tetra is an open system from the start; the acronym plays on the Greek word for "four" and refers to the fact that there are four transmission channels per carrier. The similar-sounding Tetrapol started life as Matra's proprietary MatraCom MC9600 system. According to Tetra advocates, MatraCom became Tetrapol to take advantage of Tetra's high profile within the European Telecommunications Standards Institute's (ETSI's) standardization efforts . Yes, there are some politics here, but if the two systems must differ, users demand that they interoperate seamlessly.
Public safety demands PMR
Although today's GSM is ideal for personal users and most business users, PMR users demand far more from the system than current GSM networks provide. PMR users include members of ambulance, fire, police, and other public-safety organizations, plus military personnel. These users, regardless of their locations, demand unequivocal access to communications, fast call setup, and unquestionable security. For example, if an officer is out of a base station's range, he must be able to communicate directly with another officer's handset. This method is called "direct mode operation" (DMO). Other features that differentiate PMR from today's GSM include the ability to make pre-emptive priority (emergency) calls and group calls and efficient data-transmission modes—even from a fast-moving vehicle. PMR users also expect their systems to connect gracefully with all other network services, including GSM, integrated-services digital networks, LAN/wide-area networks, private circuits, and the public switched telephone network (PSTN).
Many of these features appeal to commercial users, too. While next-generation GSM stalls, professional-mobile-radio users see Tetra and Tetrapol as digital replacement technologies for the widely used MPT-1327 analogue system . From the user's perspective, both of these all-digital systems offer nearly identical capabilities. The differences appear buried in technical details that have considerable bearing on the systems' long-term prospect.
Architectures are nearly identical
At the highest level, Tetra and Tetrapol have functionally identical system architectures (Figure 1). Four main system interfaces support voice, circuit-switched, and packet-switched data services with various prioritization, protection, and privacy/security levels:
- The air interface ensures that manufacturers' transmission equipment communicates harmoniously within a Tetra or Tetrapol network.
- The terminal equipment interface supports mobile data applications.
- The DMO interface permits direct terminal-to-terminal and terminal-to-repeater-to-terminal communications that bypass the main network infrastructure.
- The intersystem interface (ISI) connects Tetra and Tetrapol networks from different manufacturers and provides a gateway to other network technologies.
The systems support shared user groups, maintaining mutual privacy and security using air interface encryption, which secures the transmission path, and end-to-end encryption, which provides a secure channel between user terminals for more confidential traffic. Access priority mechanisms ensure that essential calls can always complete without interruption with typical call setup times of around 300 msec. The radio controller, or dispatcher, can group users over the air and include them in a call that's in progress. The dispatcher can also remotely turn on a terminal and listen in to monitor a situation without the local user's knowledge, which suits incidents such as hijackings.
Similarly, an authorized user can listen to calls without the communicating parties identifying him. Other services include central call authorization and the ability to define user call areas.
Supplementary services include caller and called-party identification/identity suppression; call-waiting notification with or without caller identification; and call-forwarding, call-holding, and call-back facilities. Various call restriction schemes are available. For example, the dispatcher can prevent parties in a group call from hearing one another, meaning that each party communicates exclusively via the dispatcher. A mechanism can also pass control of a group call from the initiator to another user. Commercial users may appreciate the call charge facility that displays call-rate information and the accumulating call cost.
Tetra and Tetrapol systems provide circuit data services that support permanent or switched circuits between terminals and between a terminal and a gateway. A connected packet data service provides X.25-level connectivity between terminals and other computers in the network. A connectionless packet data service transmits a single data packet from a base station to one or more terminals without establishing a logical connection. These services support applications such as database access, fax transmissions, file transfers, global-positioning-system access, Internet and Transmission Control Protocol/Internet Protocol access, radio paging, and video data transmission. New compression technologies will soon bring motion-picture transport to Tetra, with help from MPEG-4 and the International Telecommunications Union's SG15 work.
Air interfaces separate systems
Current Tetra and Tetrapol implementations occupy the 380- to 400-MHz bandwidth for PMR users and the 410- to 430-MHz bandwidth for PAMR users. Assisting member-state public-safety agencies to get on air, NATO relinquished the 20 MHz between 380 and 400 MHz. NATO's action emphasizes that the RF spectrum is congested, making frequency allocations rare. Witness the recent "frequency auctions" in the United States, in which assignments changed hands for billions of dollars at a time. In Europe, other frequencies that are reserved for commercial-trunked-radio use span 450 to 470 MHz and 870 to 933 MHz. No one is yet using these bands because the infrastructure isn't in place, but, as with GSM moving to 1800 MHz, acceptance will be rapid when market pressure dictates.
Clearly, the key system design parameter for any new radio system is frequency usage efficiency, an area in which Tetra and Tetrapol's design approaches fundamentally differ. Tetra's time-division multiple-access (TDMA) coding packs 16 channels into every 100 kHz of bandwidth, compared with four channels for GSM and eight channels for MPT-1327. Tetrapol uses frequency-division multiple-access (FDMA) coding, accommodating eight channels at 100 kHz using its normal 12.5-kHz channel spacing. Tetrapol specifications also permit a 10- kHz derivative. The 12.5-kHz value is significant because it is the same channel spacing value that analogue systems use, making it easier to fit new digital services into frequency allocations.
At first glance, Tetra appears to be today's most frequency-efficient medium. In practice, Tetrapol's FDMA system can prove more efficient in sparsely populated areas. FDMA demands a less sensitive receiver than does TDMA, and transceiver circuitry for FDMA is easier to implement. An ETSI working group summarized the respective advantages: For fewer than 10 channels per site and wide area coverage, FDMA wins out; for more than 15 channels per site and limited area coverage, TDMA is superior. Hoping to win ETSI approval for its system, the Tetrapol Forum is promoting the complementary roles of FDMA and TDMA.
Tetrapol relaxes barriers
Tetrapol's relative implementation ease partially accounts for why the system's roll-outs were the first to market. Two air-interface versions accommodate frequency bandwidths of 70 to 150 MHz and 150 to 933 MHz; the only differences between these two air interfaces are the interleaving and differential encoding formats that they use. Tetrapol terminals typically operate in half-duplex, press-to-talk mode, which saves the cost of a duplexer (the switch and filter circuits that allow simultaneous transmission and reception without the transmit signal swamping the receiver). Tetrapol also supports full-duplex exchanges for all radio equipment classes. The recommended uplink/downlink separation is 10 MHz in the 400-MHz band. A base station transmits on one to 24 radio channels. One channel is a control channel; the remaining channels carry user traffic. The FDMA approach requires base stations to dedicate one transmitter and one receiver to each radio channel.
Each Tetrapol user channel is a succession of 20-msec, 160-bit frames that support a gross 8-kbps data rate. The seven types of transmission frame are voice, data, high-data-rate, random-access, direct-mode-emergency, signalling, and training frames. A "super frame" comprises 200 consecutive frames, with each channel occupying one or more frames within this 4-sec time slot. The normal data-transmission rate is approximately 3.5 kbps; the high-data-rate option reaches 5 kbps. The Tetrapol codec digitizes speech at 6 kbps using regular code-excited linear-predictive coding, adding 2-kbps error protection, and requiring no speech echo cancellation processing.
Tetrapol transmissions use Gaussian minimum-shift keying (GMSK) modulation with a bandwidth/time product of 0.25. This heavy filtering maintains adjacent channel rejection, but needs accurate demodulation to avoid the intersymbol interference that compromises bit-error-rate performance. But for mobile use, GMSK simplifies RF- output-amplifier design because it's a constant envelope signal that can use efficient Class A/B designs. GMSK is relatively insensitive to the effects of inter-modulation between transmitters and multi path reflections. GMSK also limits wideband noise emissions that reduce spectral efficiency by requiring wide guard bands around individual frequency slots.
Tetra goes another route
Tetra uses a 25-kHz radio channel spacing that carries four user channels using TDMA coding. This approach is spectrally efficient and saves base station cost: One radio channel can serve four users. Transmit/receive spacing is 10 MHz in the 400-MHz band. A 14.167-msec TDMA frame combines four time slots of 510 bits each. Individual frames combine into an 18-frame multi frame that includes a control frame, providing Tetra's slow associated-control channel (SACCH). SACCH continuously maintains signaling information, guaranteeing that an emergency call can pre-empt other transmissions. Long or repetitive frame structures, such as encryption synchronization exchanges, combine into a 60-frame hyper frame that lasts 61.2 sec (Figure 2a).
Each user channel has a total bit rate of 9 kbps, with a best usable data rate of 7.2 kbps. By reserving or acquiring unused user channels on demand, the system combines as many as four streams to provide 28.8 kbps per radio channel. Two normal and heavy data protection levels reduce each user channel's capacity to 4.8 and 2.4 kbps, respectively. The speech codec uses adaptive code-excited linear predictive (ACELP) coding, compressing the incoming 14-bit, 8-kHz sampled data to a 4.8-kbps stream. This compression rate is equivalent to 60 msec of real-time speech per time slot, which receivers decode and expand back into the original format. TDMA coding allows terminals to time-shift incoming and outgoing speech data packets, providing full-duplex services without costly duplexer circuitry . For DMO, mobile terminals can transmit and receive on the same frequency by staggering time-slot transmit/receive operations (Figure 2b). 
Spectral efficiency costs
Tetra's efficiency and flexibility comes at a price. The modulation method is p/4 differential quadrature phase-shift keying (DQPSK), which codes modulation- bit pairs into an 18k-symbol/sec stream. One of four phase shifts relative to the carrier's current phase (45, 135, –45, and –135) yields eight phase points in a constellation diagram. The phase trajectory never passes through the origin, ensuring that signal amplitude never falls to zero during data transmissions (Figure 3a). The absolute phase is unimportant; the demodulator needs to detect only fast phase changes, easing frequency drift errors during message exchanges. These fast phase changes receive low pass filtering to constrain the transmission signal's bandwidth within a mask that specifies the same level within 12.5 kHz of centre frequency, yet demands –60 dBc rejection at 25 kHz. Root-raised-cosine (RRC) response filters meet transmission path filtering demands that, together with other coding and filtering tasks, require 60 MIPS DSP performance.
Transmit-path filtering spreads the constellation points into a "ball of wool" that the receiver's demodulator must resolve. The receiver must also resolve a weak signal at 3 dB in the presence of an unwanted 45-dB signal that's only 25 kHz away. Normal radio receivers place the heaviest filtering in their IF stages, but this approach doesn't work for Tetra because the intrinsically poor phase response—particularly where the filter rolls off—grossly distorts DQPSK symbols. Tetra receivers place their main filters at baseband, with RRC responses that match the transmitter path filters. The ball-of-wool shape becomes a tightly grouped constellation diagram that unambiguously delivers the original transmission symbols when you sample them at their centre points. Together, accurately matched transmit/receive-path RRC filters provide the overall raised-cosine channel response shape that provides the optimum bit-error-rate performance.
Critically, Tetra's DQPSK modulation requires a linear (Class A) RF power amplifier to preserve coding integrity. This requirement is a big problem, because Class A amplifiers are notoriously power-hungry and present battery life difficulties. Tetra radio equipment comes in 25, 10, 3, and 1W power classes and, like Tetrapol radios, dynamically adjust output power to provide adequate field strength by interpreting received signal strength and bit-error-rate measurements. This capability is important, but the amplifier design is crucial. Cartesian loop designs promise to laniaries relatively efficient, nonlinear Class A/B amplifiers to match Class A performance. The basic idea is to feed the output RF signal back into a quadrature demodulator whose uncorrected output contains the nonlinearities that the power amplifier introduces. Applying feedback corrections to the transmission quadrature modulator laniaries the output signal . Linear Modulation Technology (www.linearmod.com) specializes in overcoming many practical difficulties to make this approach work. The company is developing a Cartesian loop ASIC that targets Tetra mobiles but has not yet assigned the part a number.
Little application-specific silicon is available for either Tetra or Tetrapol. Atmel has just released its Tetrapol AT75C801 baseband IC, which integrates the audio interface, inphase/quadrature (I/Q) modem, a radio interface, a processor, and peripherals. The 32-bit RISC ARM7 processor core design features internal register banks. These banks provide near-instantaneous context switching for real-time use. A JTAG port accesses the processor's in-circuit-emulation features and supports boundary-scan testing. The memory controller accesses the on-chip 512 X32-bit RAM, the 256X32-bit ROM that contains the system boot program, and as much as 16 Mbytes of off-chip memory via an 8/16-bit interface. Peripherals include an interrupt controller, multiple serial ports, and assorted I/O lines that can service a keypad and an LCD.
The AT75C801's voice codec has direct microphone and speaker interfaces, and a digital serial link to an off-chip DSP that performs voice and channel coding/decoding. The I/Q codec handles Gaussian-filtered minimum-shift-keying modulation. A calibration mode reduces the differential offset between I and Q channels to 1 mV, minimising transmission noise. The ARM core controls the radio programmer, which provides the serial interface between internal circuitry and as many as eight off-chip radio circuits. The RF controller generates an analogue signal that is proportional to channel frequency, and a gain-control circuit controls transmitter power and the on/off ramps that smooth the beginning and end of a radio burst.
Other radio features include receive- signal-strength indication and digital-reception-offset measurement and compensation. On-chip power management complements the 3V-dc device, which has a maximum static power consumption of just 25 µA. The device is now available for around $30 (50,000). The custom-designed micro BGA-256 package is optimized for analogue I/O signal integrity. Development tools include an evaluation module and protocol stack software. Atmel is also working on a Tetra IC with the code-name TETSI-A, which the company plans to release later this year.
Consumer Microcircuits has a baseband processor (CMX980A) that provides the interface between a terminal or base station's analogue and RF blocks and performs most of the DSP-intensive operations, including critical RRC filtering. Although primarily targeting Tetra, the chip's architecture lends itself to Tetrapol deployments, as well as dual-standard use (notably Tetra/Tetrapol or Tetra/MPT-1327). In Tetra use, the chip handles the DQPSK modulation, filtering, digital-to-analogue conversion, and transmission on/off amplitude ramping. Analogue I/Q signals pass to the RF carrier modulator for up conversion, amplification, and transmission. You can program output signal amplitude and correct output offsets. For multi system use, you can directly program the digital filters, which have default coefficient values that are set for Tetra.
The CMX980A's receive path also digitizes and filters baseband analogue I/Q signals with offset correction and filter-coefficient programming capabilities. Accurately filtered digital I/Q streams pass to an off-chip DQPSK demodulator for symbol decoding and de-multiplexing before voice decoding and digital-to-analogue conversion. The CMX980A includes auxiliary ADCs and DACs that you can use for functions such as receive-signal-strength indication, automatic frequency and gain control, and voltage-standing-wave monitoring for RF power amplifier protection.
Direct links to internal nodes let you monitor the chip's performance in real time, easing development concerns. Development support includes the EV9800 pc board, which has a Windows interface for easy control and sample software routines. Power requirements for the CMX980A are 3.3 to 5V dc, demanding 50 µA in standby and around 15 mA when active. Available now in 44-pin PLCC or PQFP packages, the CMX980A's costs around $30 (1000).
What's next?
What does the future hold for PMR users? Tetrapol and Tetra are highly complex systems, so be sure to check the vendor's Web sites for more details. Further developments are in the pipeline. In an attempt to ease Tetra's market penetration, there's a "thin Tetra," or "Tetra-6/Tetra-12," proposal. This proposal adapts a subset of the four-channel specification to suit low-density applications. At the high end, digital advanced wireless services seeks to bring asynchronous transfer mode, 155-Mbps data services to Tetra. Neither proposal is yet advanced.
Atmel's director for corporate alliances, Ben Altieri, believes that Tetrapol will continue to win big business in the short term because it's easier to implement:
"Tetra may win the majority share when designers fully overcome issues such as battery life," says Altieri. "But the two systems will complement each another and dominate the two-way radio business for the next decade. After this time, GSM's UMTS (universal mobile telecommunications service) and its efficient W-CDMA (wideband code-division multiple access) technology threatens to steal real market share from professional mobile radio."
Consumer Microcircuits' marketing manager, Matthew Phillips observes that the Tetra specification demands that manufacturers develop a more sophisticated technology than GSM but with smaller economies of scale.
"The equipment must still be affordable, and this year will finally see many new product releases," says Phillips. Consumer Microcircuits believes that Tetra will be a commercial success and will set the global standard for digital two-way radio. Phillips contends that chip manufacturers will need to respond with more highly integrated devices that offer lower power consumption and cost.
"As communication technologies evolve," he says, "Tetra will face challenges from GSM-R and, eventually, UMTS. One thing is certain: Companies involved in the two-way radio business will face ever-increasing customer demands for ways to develop new products as quickly as possible."
| Tetra and Tetrapol share origin Tetra and Tetrapol are similar because both systems have roughly parallel backgrounds. Matra conceived Tetrapol around 1987. Matra first sold the system, which targeted private-mobile-radio (PMR) use, to the French Gendarmerie as "Rubis." To accelerate market acceptance and help create a de facto standard, the company released the technology into the public domain in 1994. Matra has repeatedly tried for formal acceptance from the European Telecommunications Standards Institute (ETSI) but so far has failed. However, the technology has received certification from the International Telecommunication Union and recognition from the Schengen group. Now, Matra Nortel, the company continues with its publicly available standard (PAS) effort. (For more information, see www.tetrapol.com.) By the end of 1998, 30 organizations in 15 countries had specified Tetrapol networks with a total user base approaching 500,000. European user countries include the Czech Republic, France, Germany, Romania, Slovakia, Spain, and Switzerland. Tetrapol equipment suppliers include Matra Nortel and Siemens.
Tetra dates from the early 1990s, but enjoyed a slower roll-out than its main competitor—principally because of difficulties with international standardization efforts and technical implementation. This position is now changing fast, with 32 contracts awarded to infrastructure and terminal equipment suppliers. In the first quarter of 1999, these contracts accounted for 2200 sites and 200,000 terminals. ETSI approved Tetra with its ETS 300 392 through 396 standards. (You can download these standards at www.etsi.com.) Tetra proponents suggest that Tetrapol is unlikely to achieve ETSI status, because ETSI doesn't mandate standards that compete for common ground. The Tetra Memorandum of Understanding (www.tetramou.com) represents Tetra manufacturers and users. Tetra equipment suppliers include Cleartone, GEC-Marconi Communications, Motorola, Nokia, and Simoco.
European countries with Tetra projects include Austria, Belgium, Croatia, Estonia, Finland, Germany, Greece, Hungary, Italy, the Netherlands, Norway, Portugal, Romania, Spain, Switzerland, and the United Kingdom. Many of these countries' political decisions to use Tetra originate from 1985's Schengen Treaty, which sought to ease cross-border traffic within the European Union. The Schengen Treaty mandates that member nation states have interoperable communications systems for their public-safety agencies, and that public-safety authorities are legally required to consider Tetra. Recognising that another technology exists, the Schengen telecommunications group has also requested that Tetra and Tetrapol inter-work. Tetrapol's latest intersystem-interface specification attempts to address this request.
A nonsignatory to the Schengen Treaty, the United Kingdom has an active Tetra role that's stimulated by the UK Home Office's public-safety radio communications project. In a deal worth £1.5 billion, Tetra networks will replace all police mobile systems by 2003. Quadrant, a consortium that comprises British Telecom (www.bt.com), Motorola, Nokia, and TRW is leading the project. The project's main objective is designing a national network that caters to current and future police needs, together with the needs of other public-safety agencies. British Telecom will provide the network infrastructure and service, and the other members will compete for equipment supply. A pilot service will begin in June 1999 with full roll-out commencing next year. Unwilling to wait, the West Midlands Ambulance Service already has a six-site Tetra system, which Simoco supplies. Meanwhile, Dolphin is spearheading commercial efforts. The company is developing a trans-European Tetra network and expects to complete its UK national coverage next year. |
| For more information... |
| For more information on products such as those discussed in this article, circle the appropriate numbers on the Information Retrieval Service card or use EDN Europe's Express Request service. When you contact any of the following manufacturers directly, please let them know you read about their products in EDN Europe. |
| ATM Forum
Brussels, Belgium
+32-2-761-6677
www.atmforum.com
Circle No. 490 |
ETSI (European Telecommunications Standards In stitute)
Sophia Antipolis, France
+33 4 92 94 43 95
www.etsi.com
Circle No. 491 |
ITU (International Telecommunication Union)
Geneva, Switzerland
www.itu.ch
Circle No. 492 |
| MPEG (Moving Picture Expert Group)
www.mpeg.org
Circle No. 493 |
Tetra Memorandum of Understanding
Taastrup, Denmark
+45-43-34-5508
www.tetramou.com
Circle No. 494 |
Tetrapol Forum
Bois d'Arcy, France
fax +33-1-30-45-28-35
www.tetrapol.com
Circle No. 495 |
| Atmel
Camberley, UK
+44-1276-686677
www.atmel.com
Circle No. 496 |
Cleartone
Pontypool, UK
+44-1495-752255
www.cleartone.co.uk
Circle No. 497 |
Consumer Microcircuits
Witham, UK
+44-1376-513833
www.cmlmicro.com
Circle No. 498 |
| Dolphin Telecommunications
Basingstoke, UK
+44-1256-811822
www.dolphin-telecom.co.uk
Circle No. 499 |
Ericsson
Stockholm, Sweden
+46-8-757-0889
www.ericsson.com
Circle No. 500 |
GEC-Marconi Communications
Chelmsford, UK
+44-1245-353221
www.marconi-mobile.it
Circle No. 501 |
| Linear Modulation Technology
Bath, UK
+44-1761-413174
www.linearmod.com
Circle No. 502 |
Matra Nortel
Bois d'Arcy, France
+33-1-34-60-70-00
www.matracom.com
Circle No. 503 |
Motorola
Basingstoke, UK
+44-1256-358211
www.motorola.com
Circle No. 504 |
| Nokia
Helsinki, Finland
+358-9-5112-1
www.nokia.com
Circle No. 505 |
Quadrant
London, UK
+44-0-171-932-7489
www.quadrant.org.uk
Circle No. 506 |
Siemens
Zurich, Switzerland
+41-1-495-48-49
www.siemens.ch
Circle No. 507 |
| Simoco
Cambridge, UK
+44-1223-876000
www.simoco.co.uk
Circle No. 508 |
TRW
Cleveland, OH, USA
+1-216-291-7000
www.trw.com
Circle No. 509 |
Tags: ACELP, adaptive code-excited linear predictive, ADC, ASIC, AT75C801, CMX980A, DAC, differential quadrature phase-shift keying, DMO, DQPSK, ETSI, European Telecommunications Standards Institute, EV9800, FDMA, frequency-division multiple-access, full-duplex, Gaussian minimum-shift keying, Global System for Mobile Communication, GMSK, GSM, GSM-R, integrated-services digital network, intersystem interface, ISDN, ISI, JTAG, LAN/wide-area network, Linear Modulation, MPEG-4, PAMR, PLCC, PMR, PQFP, private-mobile-radio, PSTN, public switched telephone network, public-access mobile-radio, RISC ARM7, Root-raised-cosine, RRC, SACCH, SG15, slow associated-control channel, TDMA, TETSI-A, time-division multiple access, UMTS, W-CDMA, wideband code-division multiple access
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