Lecture
In modern conditions, the demand for the exchange of various information, including between moving objects, is increasing. Radio communication systems in which the equipment of one or several communication participants is located on mobile objects (automobiles, ships ***, airplanes, portable and portable structures) are called mobile (mobile) radio communication systems. Depending on the nature of the services provided by such systems at the present stage, the following main types of mobile communication can be distinguished:
Personal radio call systems (pager radio communication systems) are designed to be operatively transmitted to a mobile subscriber who is in the coverage area of a communication system with a limited message. Since a two-way communication is not provided for in the paging system, less organization costs are required, by increasing the power of the central transmitting station, the requirements for the mobile receiver can be reduced: the receiver sensitivity, power consumption, weight, size, etc. can be reduced.
Personal radio call systems are built primarily for serving large areas. At the same time, there are many systems of limited use, for example, for intra-production communications within a single building or within the boundaries of a group of buildings.
In its most general form, a personal radio call system is a set of portable receivers of mobile subscribers and a common radio transmitting center that provides preparation and emission of address information. The system of personal radio call works as follows.
To collect and generate the transmitted messages, the radio transmitting center contains a system for collecting information and a pager terminal. The information collection system may receive messages in various formats: via the public telephone network, via computer networks, etc. These messages are processed by operators or automatic processing services (for messages arriving via digital channels) and are sent to a paging terminal. The paging terminal performs the main processing of incoming messages in accordance with the protocol used. The codes for each message are buffered and queued for previously received messages. Each message is assigned a specific address. Then these messages are emitted in batches (groups) with a certain periodicity within the service area of the network. To do this, using transmitters, the signals of the pager terminal modulate the carrier frequency (usually in the meter or decimeter wave bands) radiated into space for a short time.
The compact receiver (pager) of the mobile network subscriber is constantly in standby mode in the coverage area of the communication system. Each receiver has its own individual number (address) for receiving the information addressed to it. To do this, subscriber pagers constantly analyze the addresses of all incoming messages. If the address of the received message matches the subscriber number, this message is processed and stored in the receiver's memory. The subscriber is notified of the received message with a sound or light signal or with the vibration of the case.
Data transmission in different communication systems is carried out using various protocols. A protocol is a set of rules that establish the order of interaction between communication partners. The protocol defines the basic characteristics of information exchange: the format of the transmitted messages, the transmission rate, the ability to counteract interference, the type of modulation, etc.
One of the first common mobile communication systems used the POCSAG protocol, which allows to transmit digital, alphabetic, and tonal messages in the form of a bit stream with a speed of 512 bit / s and frequency shift keying. At the same time, the transfer of a logical unit corresponds to a decrease in the frequency of the emitted signal by 4.5 kHz, the transfer of a logical zero corresponds to an increase in frequency by 4.5 kHz.
Bit combinations are code words. The format of code words in the POCSAG is presented in Figure 10.1. In the POCSAG standard, the code word is 32 bits. There are two types of code words: the information code word and the synchronization code word. In turn, the information code words are divided into an address code word and a message code word that are distinguished by the first bits, called flags. The bit corresponding to the address codeword flag is 0; the bit corresponding to the message flag is 1.
| Bits | one | 2 ... 21 | 22 ... 32 |
| Field | Flag | Information bits | Check bits |
| Address word | 0 | Address Code | Control code |
| Word message | one | Message Code | Control code |
Fig.10.1 Format of code words of the POCSAG standard In the information word, bits 2 through 21 make up the field of information bits. In the address word, the combination of the bits of this field determines the number of the called subscriber, and in the message word they define the transmitted user information. Thus, this system can contain 221 addresses (of which each pager can have 4 addresses).The remaining bits of the information word (22 through 32) make up the check bits field and are used for error detection and correction. In this system, the check bits form a code that allows you to correct up to two erroneously received bits in each code word. Two informational words are combined in one frame. In turn, the synchronization code word and 8 information frames are combined into a packet. The format of the transmission cycle of the personal radio call system is shown in Figure 10.2. All pager addresses are divided into 8 groups, and each group is assigned one frame time for reception. The receivers of each group analyze the addresses of incoming messages while receiving only “their” frame. While receiving other frames, the receiver is in standby mode, which increases the battery life and increases the system capacity (the number of possible subscribers). The synchronization word has a specific format and is transmitted in each packet for periodic synchronization of the components of a personal radio call system.
A further development of the POCSAG standard was an increase in the transmission rate to 1200 bps, and then to 2400 bps. More high-speed data transfer protocols (up to 6400 bps) are provided by the ERMES and FLEX protocols. Work is underway to expand personal radio call networks by adding new service areas. In this case, both synchronous and sequential work of all transmitting centers can be organized. Duplex communication systems are also known when a receipt arrives at the radio transmitting center (reply about receiving a message) and, possibly, some additional information. | |||
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Devices for the reception and processing of radio signals, Transmission, reception and processing of signals
Terms: Devices for the reception and processing of radio signals, Transmission, reception and processing of signals