NMT testing – description of the standard, measuring instruments
Standard NMT (abbreviation for Nordic Mobile Telephone) is an analog mobile communication standard in the frequency range from 453 to 468 MHz. Developed jointly by the communications administrations of Denmark, Finland, Norway and Sweden for the organization of joint public radiotelephone communications in the Scandinavian countries. It was first launched in 1981 in Sweden, Norway, Finland and Denmark, and later in some European countries, the CIS, the Middle East and Asia.
In the NMT standard, mobile stations are fully compatible with all base stations in the system, regardless of country. All mobile subscribers have the opportunity to work in any of the countries included in the system. The system is intended primarily to serve land mobile subscribers, but in some cases it can also serve subscribers of maritime mobile services at short distances from the coast.
Principle of operation
The principle of operation of a mobile radio communication system is based on interaction with a fixed telephone network. Mobile communication networks include:
MSC – mobile communication switching center;
BTS – base stations;
MS – mobile stations.
The Mobile Switching Center (MSC) controls the mobile radio system and is the interface between the mobile station and the fixed telephone network. Each MSC serves a group of base stations. The set of BTSs served by one MSC forms a service area (TA - traffic area).
The system is designed in such a way that, depending on the significance of subscribers, it can provide them with some advantages, such as speed dialing, priority, etc.
At each base station, one channel is used as a call channel, it is marked with a special identification signal. One or more other channels, when free, are marked with another signal to indicate that the channel is free. Mobile stations within the coverage area of the base station are constantly receiving on the call channel. However, under certain circumstances, the MSC may allow the call channel to be used for conversation. This feature can only be used when there are no free communication channels at the base station.
In addition to signals distinguishing between call channels and communication channels, there are signals that identify the service area and country in which the mobile station is located, as well as signals that indicate the channel number. All service signals are digital and are transmitted at a rate of 1200/1800 bps FFSK modulation (Fast Frequency Shift Keying – zero corresponds to a frequency of 1800 Hz, and one to 1200 Hz). The conversation itself is an analog signal so that it can be eavesdropped on by a "normal" receiver.
Operating frequency range
Operating frequencies are in two bands: 453-457.5 MHz and 463-467.5 MHz, which are used for radio communication between mobile and base stations and between base and mobile stations, respectively. Duplex spacing of receiving and transmitting channels in the standard NMT-450 equals 10 MHz. The frequency spacing of adjacent channels is 25 (20) kHz.
Since the total number of radio frequencies available in the system is limited, in order to increase the capacity of the communication system, the formation of small communication zones ("small cells") is envisaged. However, as a consequence, the probability of reaching the border of the service area of the base station to another, controlled by the same radiotelephone switch, increases. Moreover, the output power of the transmitters of all mobile stations is automatically reduced by the command of the radiotelephone switch when the station enters the "small cell" area. The same power reduction procedure is used to reduce interference when mobile stations are close to base stations with normal coverage areas.
Comparison of NMT-450 and NMT-900 standards
At present, about 40 countries have adopted the standards NMT-450 and NMT-900. The NMT-450 standard has been adopted in the CIS as the main analog mobile communication standard. The principles of building cellular radio communication systems of the NMT-450 and NMT-900 standards practically coincide. Both cellular systems are based on the NMT-450 specification. The main differences of the more advanced NMT-900 standard were mainly related to the introduction of a small-sized manual station into the subscriber equipment, the improvement of management and the development of communication services. Small handheld stations are currently being developed for the NMT-450 standard as well. In addition, the NMT-900 adds a new frame structure, officially defined by the specification document. The frame includes additional information, prefixes and line signals. The part of the specification that relates to MSC BTS interactions has been changed (for example, self-testing,
Currently, the NMT-450 standard has been finalized and its characteristics have been brought to the level of the NMT-900 standard. The new version received the designation NMT-450J. Key enhancements include increased productivity, improved performance of handheld telephones, and protection of access to the communications network using Subscriber Identificati Security/SIS.
Identification of the subscriber (subscriber station) is carried out by a special key (SAK) recorded in the mobile station. The same key is contained in the identification register installed in the switching center. The identification procedure is carried out with each new call from the mobile station. Cellular mobile communication systems of the NMT-450J and NMT-900 standards provide subscribers with a wide range of services. In addition to the transmission of voice messages at the local, long distance and international levels, NMT networks allow you to send faxes and have access to various databases, while the data transfer rate should not exceed 4.8 kbps.
Subscribers are provided with the following services: call forwarding to another number, call restriction, that is, the duration of conversations, conference calls for three subscribers, organization of user groups with reduced dialing and other services. Based on the NMT standards, wireless communication systems for fixed subscribers (WLL) have been developed. All subscriber devices, including telefaxes and data modems, can be included in the system of this radiotelephone communication as a user interface. From the point of view of the subscriber, this communication option does not differ from wired telephone communication. Also, the numbering of a wireless subscriber telephone may not differ from the numbering of a wired network. In the wireless subscriber access system WILL, speech and signaling can be encrypted at the radio interface.
Prospects for NMT technology
In the process of modernization, new versions of the standard are being developed. To increase the capacity of cellular networks of the NMT-450 standard, it is planned to use a frequency grid step of 12.5 kHz instead of the standard step of 25 kHz, which will increase the number of working channels from 180 to 359. But only owners of new phone models will be able to use this privilege. It is supposed to use time division of channels, as in digital systems (work of several subscribers on one frequency).
In European countries, this standard has one prospect – a complete rejection of NMT-450 and the transition to IMT-MC-450 technology (CDMA2000).