Comparative analysis of digital trunking radio communication standards.

Comparative analysis of digital trunking radio communication standards..

Ovchinnikov Andrey Mikhailovich

COMPARATIVE ANALYSIS OF DIGITAL TRUNKING RADIO COMMUNICATION STANDARDS

Previous issues of the journal (Nos. 3, 4 and 6 for 1999) examined the digital trunking radio communication standards TETRA, APCO 25 and Tetrapol, which are aimed at building mobile communication systems for law enforcement agencies and public safety services. In conclusion of the review of standards, this article attempts to conduct a comparative analysis of them based on various criteria.

Currently, the process of deployment of trunked radio networks around the world is characterized by the widespread introduction of digital systems. Almost all the world's leading equipment suppliers, system integrators and operators, as well as many large consumers of trunked radio services, have announced their transition to digital systems. The main competition in the market for standards aimed not only at ordinary corporate users, but also at representatives of law enforcement agencies and public safety services, is between TETRA, APCO 25 and Tetrapol.

In Russia, radio communication systems based on these standards have not yet been deployed. This is explained, first of all, by the fact that digital systems are significantly more expensive than analog ones, and the limited resources of departments and various associations do not allow them to actively engage in the process of digitalization of their communication networks. However, the transition to digital systems is inevitable, the prospects of trunking radio communication systems both in the world and in Russia are clearly associated with digital technologies. Digital radio communication systems provide users with a high level of services, various data transmission modes, increased communication security, the ability to integrate with fixed digital networks, etc.

Any publications on the subject of digital trunking radio communication systems in Russia are perceived in the light of the task of choosing either a federal or departmental digital standard. This article does not aim to recommend one or another standard to Russian users of trunking radio communication systems. The main task is to attempt to conduct a comparison according to a certain set of criteria so that the choice of a standard, which will be made by technical specialists, trunking system operators, potential consumers, would be conscious and justified.

If we set a goal to offer as many criteria as possible by which to compare digital standards, it turns out that there are an awful lot of such criteria. However, it is advisable to consider those criteria that are really important for consumers of trunking radio communication services. (For example, the user is unlikely to care about the signal modulation method or speech conversion algorithm used, but such indicators as communication range and speech signal reproduction quality are extremely important to him.)

All the indicators of digital standards that can be considered as criteria for comparison can be divided, with a certain degree of conventionality, into two groups: operational-technical and organizational-economic.

By operational and technical criteria we mean generalized technical indicators that are determined by the parameters of communication systems, such as the range and efficiency of communication, the degree of communication security, spectral efficiency, a set of communication services (both standard and special, oriented towards use by law enforcement agencies and public safety services). Each of these criteria is complex, i.e., in turn, consists of several indicators or depends on a certain set of parameters, which will be discussed below.

It is quite difficult to give a clear definition of organizational and economic criteria; it is much easier to simply list them. These include the cost indicators of communication systems, the possibility of allocating radio frequency spectrum resources, and the prospects for the development and distribution of each standard in the world. Compared to operational and technical indicators, these criteria have greater ambiguity and a much higher degree of subjectivity in their assessment. At the same time, organizational and economic indicators depend to a certain extent on technical ones, for example, the cost indicators are significantly affected by the communication range and spectral efficiency.

Operational and technical criteria

Generalized information on the TETRA, APCO 25 and Tetrapol standard systems and their main technical characteristics are presented in Table 1.

Table 1.

Considering the technical characteristics and functional capabilities of the presented trunking communication standards, it can be noted that all standards have high (relative to this class of mobile radio communication systems) technical indicators. The standards allow the use of duplex radio stations in their systems. The radio communication means of these standards use effective methods of speech conversion and noise-resistant coding of information. All standards ensure high communication efficiency and sufficient spectral efficiency.

From a technical point of view, the main differences between the TETRA standard, on the one hand, and APCO 25 and Tetrapol, on the other, are determined by the method of dividing the communication channels. For the TETRA standard, this is time-division multiple access (TDMA), and for APCO 25 and Tetrapol, it is frequency-division multiple access (FDMA). Let's consider how this main difference, as well as other technical parameters, affect the main operational and technical indicators.

Communication range

The communication range is usually understood as the maximum distance between two radio stations, at which stable communication with the required quality is ensured. In this case, the communication is considered stable when the ratio of the communication sessions conducted to the total number of communication attempts exceeds the specified value. The required quality is understood as the quality of speech signal reception, at which the specified intelligibility is maintained.

The communication range depends on a large number of factors, which can be divided into 3 main groups:

• factors determined by the conditions of use of communication equipment (antenna installation height, terrain, interference environment, etc.);
• factors determined by the technical parameters implemented in communication equipment (transmitter power, receiving path sensitivity, antenna gain, etc.);
• factors determined directly by the principles of communication channel construction laid down in the standard (communication channel bandwidth, information rate in the channel, signal modulation method, speech coding algorithm, noise-immune coding methods).

Naturally, it is only possible to correctly compare digital trunking radio communication standards by the last group of factors, since other groups depend either on operating conditions or are determined by the quality of production of radio equipment.

It should be understood that, in principle, FDMA systems provide a greater communication range (all other parameters being equal) compared to TDMA systems. This is explained by the lower signal energy per bit of information. It is known that the signal energy Ec is defined as

Ec = Pc Tc, where

Pc is the power and Tc is the signal duration. It is clear that with a decrease in the signal duration (the transmission time of one information bit for a digital system), the energy decreases proportionally. For example, for TETRA systems, with four information channels on one physical frequency, the equivalent power per information bit is 4 times less than in FDMA systems, which is equivalent to a decrease in the communication range by approximately 40%.

Another factor that affects the reduction of the communication range in TDMA systems compared to FDMA systems is the stability of the communication channel during multipath signal propagation, which occurs in dense urban areas or hilly terrain due to signal reflection from buildings and other obstacles and leads to the appearance of a radio echo. The reflected signal has a greater effect, the greater its ratio to the signal duration. Therefore, reducing the duration of the information bit in TDMA systems worsens the quality of reception in multipath conditions. (In principle, it is possible to achieve compensation for signal delay, but this requires the use of different types of receivers for different signal propagation conditions.)

Many sources provide data on approximately twofold reduction of communication range in TDMA systems compared to systems with frequency division multiplexing. For example, according to official data of the International Telecommunication Union (see “Project 25/TETRA Comparison”, RadioResource International, 1/2000), the range of a TETRA base station for a portable radio station is 3.8 km in suburban conditions and 17.5 km for a mobile station in rural conditions. The coverage area in these conditions of APCO 25 base stations with frequency division multiplexing is 2 times greater (7.6 and 35 km, respectively).

Communication efficiency

The main parameter characterizing the efficiency of communication is the time of establishing a connection (communication channel) between subscribers. If we consider the time of establishing a communication channel within the coverage area of ​​one base station, then all standards have close indicators, within the range from 0.2 to 0.5 s. However, as some experts rightly note (see, for example, V.V. Aleshin, S.I. Sergeev, Digital Trunking for Law Enforcement Agencies, Technologies and Communications, No. 6, 1999), the advantage of standards using FDMA (Tetrapol, APCO 25) is that the minimum duration of establishing a connection is maintained over a larger territory, since the communication range for these standards is greater. For subscribers of TETRA standard networks, on average, the probability of being in different service areas is higher. In this case, the call will go through the switch, which will inevitably increase the time of establishing a connection. In addition, there is a risk that all repeater channels are occupied in the called subscriber's zone, and even in the case of a preemptive call, time will be required to break one of the current connections. Thus, in general, it can be said that statistically the connection establishment time for transmitting voice messages in Tetrapol and APCO 25 networks is shorter than in the TETRA standard.

At the same time, the data transfer rate, which is also an indicator of communication efficiency, is becoming increasingly important in modern mobile radio networks. For the TETRA standard, it can reach 28.8 kbps (when using all four time intervals to transmit the data array). For FDMA standards, it is several times less: for Tetrapol — 8000 bps, for APCO 25 — 9600 bps.

Communication Security

The concept of communication security includes requirements for ensuring the secrecy of negotiations (excluding the possibility of extracting information from communication channels by anyone other than the authorized recipient) and protection against unauthorized access to the system (excluding the possibility of taking over control of the system and attempts to disable it, protection against «double» etc.).

If we compare the standards themselves, and not the systems and complexes of technical means based on them, we can say that all standards have a comparable degree of both information protection and protection from unauthorized access. They provide the ability to use standard information protection algorithms, as well as the ability to use original algorithms developed by users of radio communication networks.

Spectral efficiency

The main indicator of the spectral efficiency of a communication system is the effective frequency band per voice channel, which determines how many communication channels can be accommodated in the fixed frequency band allocated for the deployment of the communication network. Table 1 shows that TETRA has an advantage in this indicator compared to standards with frequency division of channels. The APCO 25 standard also declares an effective frequency band equal to 6.25 kHz, but this will only be achieved in the second phase of the project.

Set of communication services

The functional capabilities provided by the digital trunking radio communication standards systems are given in Table 2.

Table 2.

Note: (n/s — no information)

Considering the functional capabilities of the presented trunking communication standards, it can be said that they provide a comparable level of communication services. All standards allow building various configurations of communication networks, provide various modes of voice and data transmission, communication with public switched telephone networks (PSTN) and fixed networks. The standards allow using duplex radio stations in their systems. The more developed Tetrapol and TETRA standards have some advantage, which implement the “dual surveillance” and open channel modes, which are extremely useful for public safety services. However, given the rapid development of standards and the constant expansion of the functions of communication systems, it is quite possible that APCO 25 will soon provide the same capabilities.

Fulfillment of special requirements for public safety radio communication systems

Information on the availability of some specific communication services aimed at public safety representatives is presented in Table 3. It can also be noted that the TETRA, APCO 25, and Tetrapol standards provide a comparable level of special services.

An interesting additional service, the availability of which is not known in standards other than Tetrapol, is provided by the auxiliary service of simulating radio subscriber activity. In this mode, constant traffic is maintained in the selected zone. During a break in negotiations, the base station periodically sends signals over communication channels that are difficult to distinguish from information signals. Such a service significantly complicates the capabilities of intruders monitoring the traffic of a specific subscriber or group of subscribers, who, in particular, may be law enforcement officers.

Table 3.

Organizational and economic criteria

Radio frequency spectrum resources

Availability of radio frequency spectrum (RFS) resources for deployment of a radio communication system is the most important criterion for choosing a particular system. In this case, the most promising standards are those that provide the ability to build communication networks in the widest range.

TETRA systems theoretically provide the ability to work in a very wide range (150-900 MHz). At the same time, manufacturers currently offer mainly equipment that operates only in the range allocated in Europe for building TETRA networks — 380-385/390-395 and 410-430/450-470 MHz, although there is already information about projects for systems in the 800 MHz range.

APCO 25 systems, in accordance with functional and technical requirements, provide the ability to operate in any of the ranges allocated for mobile radio communications.

The Tetrapol standard limits the upper frequency of its systems to 520 MHz. In reality, most operating systems use the 380-400 MHz range.

An important criterion for comparing standards is the frequency resource required to deploy a communication network with the same number of subscribers and the same radio coverage area. There can be no single answer here. On the one hand, the TETRA standard has better spectral efficiency, on the other — Tetrapol and APCO 25 provide a larger radius of the base station service area. Therefore, for TETRA systems, smaller radio frequency spectrum resources will be required for radio communication networks with very intensive traffic, and the advantages of Tetrapol and APCO 25 will be evident for communication networks with low traffic and a wide coverage area.

Economic efficiency

Today, digital radio communication system equipment is significantly more expensive than analog systems. As a rule, the cost of contracts concluded is a commercial secret, but it should be understood that when deploying a system of any of the presented digital radio communication standards, servicing several hundred subscribers, we are talking not about thousands, but about millions of dollars. Judging by the advertising information of foreign companies, the cost of subscriber radio stations operating in digital standards can fluctuate between 800 and 4 thousand dollars, and a significant share of the cost can be determined by the presence of modules or software for information protection.

Comparison of the economic efficiency of systems of different standards cannot be considered in isolation from the category of mobile radio communication systems. For the creation of communication networks with a small load, wide territorial coverage and a number of channels within 10, a more optimal option (including in terms of cost) is the use of FDMA systems, which include APCO 25 (Phase I) and Tetrapol. This is explained by the larger radius of the service areas of FDMA systems compared to TDMA systems. According to estimates given in the technical report of the Tetrapol PAS standard, the cost of basic equipment for a multi-zone radio network implemented on the basis of TDMA, in relation to a system with frequency division multiplexing (with the same cost of a unit of equipment) will be 30-50% higher.

However, for communication networks with intensive traffic and a number of channels in one zone more than 15, it is preferable to use systems with time division of channels, which include TETRA.

It should be noted that the APCO 25 standard (Phase II) will have universality, providing the ability to build systems with both frequency and time division of channels.

Prospects for the development of systems of these standards in the world

If we compare digital trunking radio communication standards by the number of networks in operation, the number of users, and the total coverage area, then the undoubted leadership here belongs to the Tetrapol standard. Currently, more than 35 large radio communication networks have been deployed in 21 countries around the world, serving about 0.5 million subscribers. The coverage area of ​​the operating communication networks is 600,000 km2. It should be noted that the Tetrapol standard is popular in all regions: in addition to Europe, radio communication networks have been deployed in Southeast Asia, the Near and Middle East, and Latin America. Since the first communication network was put into operation in 1994, we can say that the standard has been sufficiently developed, and users are much less likely to encounter errors in the software of stationary equipment. Despite the fact that the standard is corporate, the Tetrapol standard is supported by a large number of large equipment manufacturers.

In terms of the number of existing communication network projects, the TETRA standard is not inferior to Tetrapol, but most projects are at the initial stage: trial operation of pilot networks or deployment of communication systems. So far, almost all communication networks are concentrated in Europe. Perhaps, the TETRA standard is supported by the largest number of leading manufacturers, and not only European ones. Such leading companies as Motorola (Dimetra system), Nokia (Nokia TETRA), OTE Marconi (ELETTRA) have released their systems based on the TETRA standard.

The APCO 25 standard is just beginning its transition to the deployment stage of communication networks. For now, the equipment of the ASTRO system by Motorola is actually being manufactured. There are projects for several networks in the USA, the first contract for the supply of equipment to Europe has been concluded (the communication system of the British customs).

When choosing a radio communication standard, it is necessary to take into account the information about whether the standard is open or corporate (closed).

The Tetrapol corporate standard is the property of its developer, Matra. Equipment can only be purchased from a limited number of manufacturers.

Open standards, which include TETRA and APCO 25, ensure the creation of a competitive environment, attracting a large number of manufacturers of basic equipment, subscriber radio stations, and test equipment for the production of compatible radio equipment, which helps reduce their cost. Access to standard specifications is provided to any organizations and firms that have joined the relevant association. Users who choose an open radio communication standard do not become dependent on a single manufacturer and can change equipment suppliers. Open standards are supported by government and law enforcement agencies, large companies in many countries around the world, and are also supported by the world's leading manufacturers of element and node base.

All this allows us to say that open standards are more likely to conquer the trunking radio communication systems market in the future.