By Roberto Marengon

 

People perceive the term digital as more advanced and better than analog, because digital presents a number of attractive advantages to users. Two digital technologies — TETRA and Digital Mobile Radio (DMR) — offer different benefits, but which is the right choice for your system? When a user decides to upgrade a mobile radio system, many factors should be considered. TETRA often presents high infrastructure costs. Are these costs appropriate for the given benefits compared with the emerging DMR solution? Understanding some of the key points that characterize each of the digital technologies will help you make the best decision.

 

 

TETRA standards development started in the European Telecommunications Standards Institute (ETSI) in the mid-1980s by a group of radio manufacturers. The initial requirement was a digital standard to replace the MPT 1327 analog trunked networks and to introduce a number of new features. The primary market segment intended for TETRA was the public access mobile radio (PAMR) market, where operators charge users service fees for trunked radio services. In the 1980s, many European countries had a strong interest in national PAMR systems for private dispatch communications and telephony. The standard specifications were written following the main needs of a PAMR system to maximize traffic capability.

 

ETSI chose a TDMA access and modulation schema to decrease the cell dimension and to increase traffic density. More traffic equals more business. ETSI spent several years specifying the standard because of the high complexity of the initial requirements — efficient linear modulation, vocoder selection, powerful protocols, field experimental tests and others.

 

During the same time, GSM networks expanded their coverage outside towns with reasonable and decreasing costs. As a result, an important part of the mobile radio market intended for TETRA was devoured by GSM technology. The huge effort needed to develop TETRA, together with the market contraction, produced economic disasters for many companies involved in TETRA’s development. The surviving companies identified a new market segment to help pay for the investments — public-safety services. The manufacturers, with the contribution of some European governments, oriented their efforts to the more profitable security market. TETRA is the most popular solution for public safety in Europe, although the costs and economic crisis have slowed implementation in some countries.

 

DMR’s history is shorter and more recent. The DMR standard was published by ETSI in 2006 and optimized at the end of 2007. DMR was developed to substitute analog two-way radio systems with a pin-to-pin digital solution, meaning the digital system should perform the same coverage and application schema — repeater, multisite network, simulcast and trunking — as the previous analog network. Motorola played an essential role in the specification activities and was the first actor in this new segment of the professional mobile radio (PMR) market. Less effort was needed to develop a DMR radio, because DMR is a less complex technology than TETRA. The standard offered a reasonable investment for several radio manufacturers. Motorola, Selex Communications, HYT, Funkwerk and Radio Activity offer commercial DMR products, and Tait, Team Simoco and Vertex Standard plan to offer DMR equipment.

 

 

DMR and TETRA are digital standards, and they provide some similar benefits to end users — encryption, which is more robust in TETRA; efficient data communications for GPS positioning, messaging and others; automatic handover and roaming; authentication; direct mode communications; and private and conference calls. The time division full-duplex operation of terminals is available only in TETRA. The DMR standard specifies it, but no terminals yet perform time division full-duplex operation.

 

Coverage versus Traffic. A customer should determine if it’s more important to fulfill high traffic needs or a large coverage area. While TETRA is designed to support high traffic volumes with small cells and a large number of channels, DMR works with large cells similar to analog systems, but DMR isn’t optimized to manage many channels. This point is important, because decreasing the cell radius may require more sites to cover the same area. DMR was developed to cover wide areas with low density.

 

The TETRA modulation schema is a linear four TDMA channels in a 25-kilohertz channel. The modulation implies that the maximum RF power available on TETRA portable terminals is lower than for DMR — up to 2 watts compared with 5 watts for DMR. Sensitivity is less in TETRA than DMR because of twice the bandwidth of the receiver. In addition, TETRA works only in the UHF bands, so the propagation effects also reduce the coverage capability. DMR uses a robust fixed envelope modulation schema that provides the same RF power and sensitivity of conventional analog systems. DMR offers similar coverage performance; no more sites are needed to change an existing system from analog to DMR technology. DMR performs with two TDMA channels in 12.5-kilohertz spacing per carrier, so it requires a more expensive branching system to increase the number of desired channels. Some DMR trunked systems are in development, but they are not available yet.

 

Network Backbone. There are several media, including copper lines, multiplexed audio lines, UHF narrowband channels, IP networks and public switched telephone network (PSTN), used to connect analog base stations. The TETRA interconnection backbone must ensure a bandwidth of several Megabits per second (Mbps) to serve the fast signaling needed for correct operations. It is hard to achieve such bandwidth in most analog connection supports. The DMR bandwidth requirement is low. A few tens of kilobits per second (kbps) are enough to carry both the two channels and all the related signaling. A narrowband UHF link also operating in nondirect visibility or a traditional phone modem can connect two or more DMR base stations. This is another example of DMR pin-to-pin substitution of an existing analog installation.

 

Network Management. For an emergency organization, it is essential to efficiently manage all the communications resources during disaster events. Many emergency services agencies have internal technical services to guarantee fast and secure operations in extreme cases. In these occurrences, complexity should be avoided, and technical staff must be able to easily modify, repair or reconfigure the system. A crash in a node of the system should not cause a communications blackout. The complexity of TETRA infrastructure elements — base stations and switching nodes — are not easy to understand, manage and reconfigure. TETRA networks outsource maintenance activities. DMR systems are similar to well-known analog networks, and the main operations don’t differ significantly. Staff skilled in analog radio can be trained to efficiently operate DMR infrastructure, so mission-critical organizations can have full control of their mobile communications resources.

 

Analog-to-Digital Migration. Because of its analog/digital dual-mode feature, DMR offers a natural migration path from analog to digital radio systems. The DMR standard covers all the most used analog solutions such as single site, multisite, simulcast and trunking. A customer can start to implement DMR radio infrastructure over an existing analog network, reusing sites, antennas, power supplies, branching, connection links and frequency licenses. If problems are discovered, they can be solved more easily in analog mode than in digital. The customer can gradually implement the new DMR equipment working in dual mode and interoperating with the analog network.

 

TETRA migration presents a different scenario: TETRA is a revolution in the radio approach for infrastructure aspects and for end-user operations. TETRA offers few possibilities of interoperability between digital and analog terminals. TETRA is a trunked-only standard and operates in UHF spectrum in Europe. There are also TETRA systems in the Middle East, Africa, Asia, and South and Latin America. Tuning a new digital system could require some time to solve unknown problems in an unfamiliar environment. The migration from an analog system to TETRA produces an abrupt discontinuity and requires a switch to the new radio system at the same time.

 

Spectrum Efficiency. The spectral efficiency of TETRA must be considered accurately. TETRA gives three (plus one for control) channels in 25-kilohertz channel spacing compared with the 6.25-kilohertz channel of DMR when no control channel is requested. The lower carrier to interference (C/I) protection ratio of the TETRA modulation and the poor adjacent channel selectivity reduce the spectral efficiency. It takes more distance to reuse the same frequency and causes more spectral pollution. TETRA requires a continuous control channel on air, so the power supply requirements of a base station may be several times greater than for DMR base stations. Unlike DMR, it is difficult to use solar cells to supply TETRA infrastructure.

 

The major difference is the modulation scheme. For pi/4-DQPSK modulation, TETRA requires super linear power amplifiers that result in high DC power consumption and poor efficiency because of the combined amplitude and phase modulation. The constant envelope modulation of DMR, 4FSK frequency modulation, has an efficiency advantage over TETRA of a factor of about two to three.

 

Vendor Environment. TETRA takes advantage of a mature multivendor environment. The major manufacturers have standardized and made interoperable radio platforms, so it is easy for a customer to select the best supplier. Hundreds of successful installations provide confidence about the stability of the TETRA solution. DMR is too young for such an environment. There are some important ETSI unspecified points for DMR such as the vocoder (AMBE II+ will likely be the de facto standard), interoperability tests, encryption coding and most services including positioning. The manufacturers said they need interoperability to take advantage of a true open market, but DMR presents some proprietary features. Some manufacturers are developing trunked solutions for DMR, but based on available information, each vendor protocol looks different and incompatible.

 

Costs. TETRA was designed to cover the PAMR market with every imaginable feature, so complexity and development costs contribute to the final product cost. The trunking approach requires fast and expensive dedicated switch nodes to run the full protocol. The connections between base stations and the nodes must support several Mbps of bandwidth to avoid excessive signaling delays. The number of sites required to perform the same coverage of an existing analog system may be twice or more. More sites mean not only a higher initial investment, but also fixed costs because of maintenance, site rent, frequency licenses and backbone links. Moreover, a linear modulation requires more expensive hardware and implies higher energy consumption and dissipation.

 

TETRA offers advantages and disadvantages compared with DMR, and it is often the best choice for medium- to high-capacity trunked networks with high traffic volume and low coverage areas, such as large industrial sites, large campuses, airports and similar locations. DMR offers a true replacement of analog systems with all the benefits of a digital solution. Some care has to be taken when a trunked system is needed due to the proprietary and not-yet-available DMR trunked solutions. DMR is especially suited to large areas with relatively low traffic applications where simulcast gives the best performance. 

 

 

Roberto Marengon is the managing director of Radio Activity, a radio communications engineering firm. He started his career at Alcatel, developing the first Italian 900 MHz cordless telephone. He was the research and development manager of Prod el SpA (formerly Marconi Group and now Selex Communications) for more than 10 years, developing a new generation of simulcast networks. In 1998, he started engineering company SIEL TRE Srl, focused on maritime radio applications, and launched Radio Activity in 2003. E-mail comments to r.marengon@radioactivity-tlc.it. 

 

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