10.2 Professional Mobile Radio

Under the professional mobile radio communication understand the radio network that is available for a limited range of users (emergency, rescue, health services, law enforcement agencies, taxi stations, etc.). In this case, a common frequency resource is allocated for users grouped in working groups united by common interests. In such networks, both a group call and an individual call of a subscriber with the dialing of an additional number are usually provided.

The simplest network of professional mobile radio communications consists of a base station located approximately in the center of the serviced area (a fixed transceiver with an antenna raised to the height required to produce a stable signal in the service area) and mobile stations (radio stations on mobile objects). In the first mobile radio systems, each group was allocated a fixed working frequency with a group-wide or selective call to any of the subscribers of the group. The number of subscribers in the group was determined by the capabilities of the radio channel, and with an increase in the number of subscribers a new group was formed, which was allocated another radio channel (Fig.10.3, a).

Fig.10.3 Professional Mobile Radio System:
a) with radio channels assigned to the group; b) with access to any channel from the trunk

Currently, professional mobile radio systems use the principle of equal access of mobile subscribers to any of the dedicated radio network channels, called trunked (or trunked - from the English trunk - bundle, trunk). In trunk systems, when a call is received, a free channel is allocated to each pair of subscribers; after the end of the communication session, this channel is released and can be provided to another pair (Figure 10.3, b).

Since a mobile station can occupy any free channel for a communication session, the capacity is significantly higher in communication systems with an available channel beam than in communication systems with fixed channels.

Since in such systems the radio channel is not assigned to the mobile station, additional funds are required to search for a free channel, as well as time for its implementation. Selection of a free channel to a specific subscriber upon his request is usually carried out in one of two ways.

In the first method, the mobile station itself searches for a free channel for a communication session, and on each of the dedicated channels an attempt is made to establish communication with the base station. The search for a free channel delays the stage of establishing a connection; therefore, this principle of channel allocation is used with a small number of radio channels.

According to the second method, a special base station control channel is used to search for free channels. The control channels of each base station are spaced apart in frequency, and the mobile station scans all available control signals to search for the strongest signal (the nearest base station with a radio network zone structure). The control channel signal of the base station allows the mobile stations to synchronize with the base station, as well as to obtain information about the time slots that are available to transmit the call signal on this channel. In this time interval, the mobile station transmits to the base station the code of the called subscriber and its own code. The base station by the code of the called subscriber establishes the route of connection with another base station (if the called subscriber is in the service area of ​​another base station) and assigns subscribers free working channels.

An incoming call to the mobile station is received via the control channel of the base station, which is constantly scanned by all mobile stations operating in its zone.

In analogue professional mobile radio systems, the spacing between channels is typically 12.5 kHz. In this range, speech with amplitude modulation or frequency modulation with a slight frequency deviation can be transmitted, as well as data with a speed of up to 1200 bps can be transmitted. In most digital professional mobile radio systems, the frequency spacing between channels is 25 kHz.

In professional mobile radio systems, it is usually possible to quickly switch a communication channel to another carrier frequency, both when the received signal level decreases and when the interference level increases.

As an example of a mobile trunk system, consider the European TETRA standard. In this system, the frequency range of 380 ... 400 MHz is allocated for emergency services, and the frequency range of 400 ... 500 MHz is allocated to other users. Radio channels form a grid of frequencies with a spacing of 25 kHz and a separation frequency for receiving and transmitting - 10 MHz. A 25 kHz wide channel is alternately allocated to four user pairs (groups) (Fig. 10.4), that is, the principle of time-frequency channel separation is implemented.

Fig. 10.4 TETRA Transmission System Format

Each of the four users is alternately assigned a time interval of about 57 µs, during which two user information portions of 216 bits and three service information portions are transmitted (to synchronize the transmission, to optimize receiver parameter settings, and a guard interval to eliminate the influence of neighboring channels).

For speech encoding, a special speech coder (vocoder) is used, which forms a digital stream with a low speed (approximately 4.6 kbps). The digital form of the signal is a good basis for further speech encryption in order to increase the secrecy of transmission. In addition, the transmission of digital voice signals can be supplemented by the transfer of data in the format of a radio modem or a remote terminal.

The call setup time for the TETRA system does not exceed 0.3 s, which is acceptable for emergency services. It is possible to simultaneously send a message to multiple subscribers. There is also the mode of operation of two mobile stations directly with each other, bypassing the base station. This mode can be used in emergency situations, when the mobile station may be in a “radio shade” (in a cave, etc.)