Signaling systems for simplex radio stations.

Simplex radio station signaling systems..

Vorobyov Sergey Viktorovich

SIMPLEX RADIO STATION SIGNALING SYSTEMS

The article examines the most well-known selective calling systems used in modern simplex radio stations. Their main characteristics are given. The features of use in radio communication networks are shown.

In the work [1] (Vorobyov S.V., Ovchinnikov A.M. “Some options for organizing operational radio communications of security and protection services”, Special Equipment. No. 1-2, 1999.) describes options for constructing the simplest simplex radio networks with fixed channel assignment. Such networks are capable of solving a wide range of problems related to information exchange between mobile subscribers. With low traffic, the deployment and operation of such networks is in many cases simpler and cheaper than other types of radio communication.

One of the ways to increase the efficiency of using radio frequency resources in networks with fixed channel assignment is to allocate one radio channel to several independent groups of subscribers. To ensure their independent operation, each radio station or group of radio stations is assigned one or more identification numbers (IN). Several IDs assigned to a radio station allow it to operate in different groups. This case is illustrated by Fig. 1. Subscriber 1’s radio station is assigned identification numbers ID1, ID2, and ID3, subscriber 2 — ID 1, groups 1 and 2 — ID3, ID2, and ID4. Subscriber 1 participates in negotiations with all network subscribers, subscriber 2 can exchange messages only with subscriber 1, and groups 1 and 2 — with each other without the participation of subscriber 1. Thus, Fig. 1 shows 4 communication directions. The number of subscriber communication directions coincides with the number of IDs assigned to the subscriber.

Fig. 1.

To transmit a message to a specific subscriber or group of subscribers, it is necessary to transmit the corresponding ID. The transmission of IDs in mobile radio networks with fixed channel assignment is carried out using selective calling systems (SCS).

This article examines the most well-known SIVs used in modern mobile radio networks.

SIV are sets of additional signals transmitted during a communication session. Each SI is assigned a signal or combination of SIV signals. These signals are generated by a special block (encoder) in the transmitter and are decoded on the receiving side. When waiting for a call, radio stations receive all signals on a given radio channel, but do not output them to the loudspeaker; when receiving SIV signals corresponding to the specified SIV, the receiver opens the radio station's audio path, i.e. voices the message.

SIV signals must meet a number of specific requirements, in particular, they must be easily detected and distinguished in conditions of high noise and interference, not degrade the quality of voice message reception, etc. One of the main characteristics of SIV is the maximum number of SIV generated by the system signals.

Experts distinguish two classes of SIV: tonal and subtonal (subtonal) depending on the frequency range they occupy (see Fig. 2). Each class distinguishes between analog and digital systems.

Fig. 2

Tone system signals occupy a frequency band that coincides with the speech signal band (usually 300…3400 Hz), so they are transmitted for a short time at the beginning of the message. The advantage of such systems is the ability to form a large address space. At the same time, with an unstable communication channel, the moment of transmission of a tone selective call may coincide with the fading of the signal in the receiver, which will lead to the omission of the entire message. Thus, the disadvantage of this type of signaling is the relatively low reliability of establishing a communication channel, especially in urban network conditions.

Subtone selective calling system signals occupy a frequency band of 50…250 Hz, i.e. they are below the band of the transmitted speech signal, and are effectively filtered by a filter in the low-frequency path of the receiver. Therefore, they are continuously transmitted throughout the entire communication session, which allows for more reliable channel establishment and control, especially with unstable communication. The disadvantage of subtone selective calling systems is a decrease in the share of carrier oscillation modulation per information message, as well as a limited address space due to a narrow frequency band.

Let's consider the most well-known SICs.

DTMF

DTMF (Dual-tone multifrequency) refers to analog tone SIV.

This signal was developed for use in wired telephone networks and is intended for tone dialing when working with electronic and quasi-electronic automatic telephone exchanges. A distinctive feature of tone dialing is the ability to dial additional numbers after connecting the automatic telephone exchange line, which allows for secondary connections at the subscriber device level. The parameters of this signal are standardized by GOST 25554-82. The DTMF alphabet is limited to decimal digits and additional symbols A, B, C, D, *, #. Additional symbols allow for an extended set of codes to be entered. Each element of the alphabet is assigned two of the eight frequencies. In this case, the frequencies are divided into two groups of four frequencies:

• lower – 697 Hz, 770 Hz, 852 Hz, 941 Hz;
• upper – 1209 Hz, 1336 Hz, 1477 Hz, 1633 Hz.

Each DTMF symbol is coded by the sum of two harmonic oscillations. The duration of a dual-frequency message must be no less than 40 ms, and a pause no less than 25 ms. Frequency stability is no worse than ± 1.5%. Table 1 shows the correspondence between the symbol and the frequency combination.

Table 1

In radio networks with fixed channel assignment, DTMF is used to separate independent subscriber groups, selective calling, remote initialization of radio devices, such as radio repeaters, dialing a telephone exchange number when communicating with subscribers of a telephone network, etc. Usually, the DTMF code is dialed manually by the user from the radio keyboard in the signal transmission mode. This creates certain inconveniences, in particular, additional occupation of the radio channel during dialing, the need to equip the radio with a keyboard, etc. Currently, some radio stations allow you to pre-program several numbers into memory and automatically call them by pressing one key.

It should be noted that since this signal was originally intended for use in wired telephone networks, its parameters do not fully meet the requirements of selective calling for radio networks. In particular, modulation of the carrier by two harmonic oscillations simultaneously places increased demands on the linearity of the modulation-demodulation path of the radio station. At the same time, its radiation power per oscillation also decreases, which leads to a decrease in the noise immunity of the signal.

The advantage of the DTMF signal is its compatibility with the tone signaling system of modern automatic telephone exchanges and wired subscriber equipment, which makes it possible to organize the interaction of telephone networks and radio networks with fixed channel assignment quite simply.

Select 5

Select 5 – refers to analog tone SIV.

This system was developed specifically for use in radio networks.

The system alphabet consists of decimal digits from 0 to 9, and symbols G, B, C, D, F, R. Each element of the alphabet is assigned one frequency.

The Select 5 selective call signal consists of a sequence of alphabet elements of a certain length (usually from 5 to 7 elements). Each sequence corresponds to a certain ID. For example, some Motorola radios allow one channel to be assigned up to 5 sequences of up to 7 selective call elements.

There are six known modifications of this system, differing in individual frequency values ​​and message durations. Table 2 shows the alphabet elements and the corresponding frequencies, as well as the message durations for various modifications of the system.

Alphabet elements 0 and G are often used for selective calling of groups, element R is used to repeat the previous element.

Table 2

An example of the use of the Select 5 system is the selective calling procedure described in CCIR Recommendation 256.

CTCSS, PL

CTCSS (Continuous Tone Controlled Squelch System) analogue subtone SIV.

The CTCSS selective calling system contains a set of forty subtone frequencies lying in the range of 67.0 … 250.3 Hz. The frequencies are divided into 3 groups. Group A includes 17 frequencies, B — 16, and C 7. The distribution of frequency ratings by groups is shown in Table 3. To separate subscribers working on the same radio frequency, it is recommended to use frequencies within the same group. A subscriber's selective call is provided by transmitting one of the frequencies together with a message during the entire communication session. Separation of the information signal and the selective call on the receiving side is carried out by bandpass filters.

Motorola registered this signal system under the Private Line® (PL) trademark, introducing a correspondence between the frequency and the symbolic code. This correspondence is shown in Table 3.

Table 3

The disadvantages of this signal system include the limited address space. It should be noted that the signal frequencies are very densely located in the subtonal band, therefore, in order to ensure reliable separation of subscribers, increased requirements are imposed on their stability on the transmitting side and the selectivity of the CTCSS signal decoder on the receiving side.

The advantages of the CTCSS system include the absence of the need for preliminary dialing and transmission of the subscriber code, the possibility of reliable control of the channel quality during the communication session.

DCS (DPL) code

DCS (Digital Continuous Squelch) – belongs to the class of digital subtonal SIV.

This system is intended for use in radio communication networks.

The system contains 80 binary sequences. The list of sequences in octal format is presented in Table 4. When forming a signal, the sequence is encoded with a noise-resistant code and is cyclically added to the speech signal during the entire communication session. One cycle contains 23 bits. Fig. 3 shows an example of a sequence with number 114.

Fig. 3

The transmission rate of the sequence is selected in such a way that its spectrum is below the spectrum of the speech message. Therefore, their separation in the receiver is possible with the help of filters. The digital signal arriving at the modulator has the form “without returning to zero (see Fig. 3).

Table 4

Motorola has registered this signaling system under the trademark Digital Private Line® (DPL).

This selective calling system contains more numbers than CTCSS, but it requires more complex hardware for its implementation. It should be noted that the use of a digital signal with a “no return to zero” form creates certain problems in most modern radio stations, in which modulation is carried out in the VCO of the frequency synthesizer. This is caused by the inclusion of a part of the spectrum of the selective call signal in the frequency band of the VCO control signal, which leads to distortion of the DCS signal form (see Fig. 4). For this reason, this system includes sequences with a balanced structure, i.e. the number of zeros in the sequence is approximately equal to the number of ones, which helps minimize signal distortion.

Fig. 4

The main characteristics of known SIVs considered give an idea of ​​their capabilities and allow us to assess the feasibility of using one or another type in real conditions of communication network operation.