Introduction of digital trunking technology for emergency communications

Introduction of digital trunking technology for emergency communications

The main application scope of the cluster communication system is departments and enterprises that have high requirements for command and dispatch functions, including government departments (such as the military, public security departments, national security departments, and emergency service departments), railways, water conservancy, power, civil aviation, and other units. As the economy develops, rental, logistics, property management, and factory manufacturing also increasingly require cluster communications. Trunked communication is gradually becoming a large-scale dedicated communication system other than public mobile communication. This article details the characteristics of emergency communications and cluster communications, the overview of domestic cluster systems, and the two domestic cluster communications systems.

1 Introduction

An emergency communication system is a system that provides communication functions in order to better coordinate different departments and efficiently handle these emergencies under some emergency tasks or disaster situations. The trunking communication system has unique dispatching function, group call function and fast call characteristics, so it plays an important role in the emergency communication system. The main application scope of the cluster communication system is departments and enterprises that have high requirements for command and dispatch functions, including government departments (such as the military, public security departments, national security departments, and emergency service departments), railways, water conservancy, power, civil aviation and other units. As the economy develops, rental, logistics, property management, and factory manufacturing also increasingly require cluster communications. Trunked communication is gradually becoming a large-scale dedicated communication system other than public mobile communication. Therefore, in addition to supporting basic cluster dispatch communication, the emergency communication system also needs to be able to quickly build a network, one-call, one-call, multi-service department linkage, good call quality, easy operation and many other uses in emergency situations Function.

2. Definition and characteristics of digital trunking communication

The International Radio Consultative Committee (CCIR) named the trunking communication system "Trunking CommunicaTIon System", and it is also called PMR (Private Mobile Radio) or SMS (Specialized Mobile System) abroad. China once translated Trunking CommunicaTIon System into trunk communication, and in 1987 it was translated into trunking mobile communication system, often called trunking communication system or trunking system.

A trunking communication system is a dedicated mobile communication system where multiple users (departments, groups) share a set of radio channels and use these channels dynamically. Compared with the public cellular mobile system, the trunking communication system has the following characteristics:

(1) The call is connected quickly;

(2) Users in the group share the forward channel;

(3) Half-duplex communication mode, PTT mode;

(4) Support private calls and group calls;

(5) When calling and speaking, you need to hold down the PTT button, and the called party does not need to go off-hook.

3. The current status of China's digital trunking technology development

Judging from the development status of digital trunking communication, the trunking industry is at an early stage of development. Compared with the development scale of public mobile communications, the scale of digital trunking communication is far behind. At the same time, the digital trunking communication system in use in China completely adopts foreign technology and equipment, and the entire industry is basically in a state monopolized by foreign manufacturers. Moreover, whether it is a TETRA system or an iDEN system, the openness of its standard is not high. The iDEN system is exclusively monopolized by Motorola. Although the TETRA system can be compatible at the air interface, the interconnection between various manufacturers ’systems cannot be achieved, which affects TETRA system development. The most direct problem caused by this situation is the high price of terminals and system equipment, or the high cost of system maintenance, upgrades and expansion, which to a large extent limits the development of digital trunking communications in China.

As China's trunking communication market has considerable market and development potential, many domestic telecommunications equipment manufacturers, such as ZTE, Huawei and Datang, are vigorously researching and developing digital trunking communication equipment and have developed to meet trunking communication requirements Digital trunking system equipment. Different from the current mature foreign cluster technologies, these systems have the independent intellectual property rights of many Chinese companies. The system architecture adopts the more advanced and open mobile communication technology, and on this basis, it introduces The independent patent technology has realized the key technology of the digital trunking system. On the other hand, compared with the monopoly and relative closure of foreign technology, the cluster system developed by domestic manufacturers can be fully disclosed and standardized.

In China, the current systems that can provide complete digital trunking equipment and terminals include the GSM-based digital trunking communication system (GT800 system) developed by Huawei and the CDMA-based digital trunking communication system (GoTa system) developed by ZTE Corporation.

4. GoTa system

The meaning of GoTa is an open cluster architecture (Global open Trunking Architechture), which was developed to meet the needs of users of digital trunking communication private networks and public networks.

GoTa's air interface has been optimized and transformed on the basis of cdma2000 technology, so that it can meet the technical requirements of modern trunking communication. The call method adopted by GoTa is the unique PTT voice call in trunking communication. In order to increase the call connection speed, GoTa defines a set of corresponding system structure and protocol stack to meet the fast connection of trunking communication system; in order to support the group Calling, GoTa, optimizes the air interface, so as to achieve the purpose that users in the same group under the same cell / carrier frequency can share the same air channel when calling. GoTa also uses a sharing method when processing communication connections, which will reduce the delay when the network processes calls. For users, the process of channel selection and allocation is transparent. Therefore, GoTa has the technical advantages of fast access, high channel efficiency and spectrum usage, high user privacy, easy scalability, and support for many types of services.

4.1 The network structure of GoTa system

The network structure of the GoTa system is shown in Figure 1. The system includes the following logic function modules and subsystems.

Figure 1 GoTa system network structure

(1) Cluster terminal

The trunk terminal provides users with various cluster services of the CDMA trunking system, and also provides users with traditional ordinary telephone services, supplementary services, short message services, and data services. The terminal is connected to the cluster system through the air interface to provide cluster scheduling functions.

(2) Base station subsystem BSS

The base station subsystem BSS is composed of a base station transceiver BTS and a base station controller BSC, which mainly completes the access functions of cluster services, data services and ordinary telephone services. The base station subsystem BSS is connected to the dispatching subsystem and the CDMA core network (including the switching subsystem and the packet subsystem) through standard interfaces to meet various service requirements of the cluster terminal, including cluster services, telecommunications services, and data services.

(3) Dispatching subsystem DSS

It is composed of DCC and DHR. DSS mainly completes the cluster scheduling business. DCC is the general control point for cluster calls, and it completes the processing of cluster dispatch calls, including identifying cluster users, establishing various cluster calls, such as private calls and group calls, and judging cluster PTT requests. As a dispatch control center, DCC also receives the cluster voice data from the reverse link and distributes it to the corresponding forward link according to the nature of the call.

DHR completes database management and configuration functions, provides group registration and group member registration for cluster users, and provides local information of cluster members and business rights records of cluster members to complete call statistics and billing functions.

(4) Dispatching desk

The dispatching station is a terminal with special authority in the user group, and can be divided into a mobile dispatching station and a fixed dispatching station.

● The fixed dispatching station is connected to the dispatching subsystem and the operation and maintenance center at the same time. It can initiate and participate in dispatching services such as group call and broadcast, and can realize the management function of group members.

● The mobile dispatcher can initiate dispatching services such as group calls and broadcast calls. It has a higher priority in the control of the voice of group calls and has a simple group user management function.

(5) CDMA core network

The CDMA core network includes a switching subsystem and a packet subsystem. The switching subsystem mainly completes the telephone interconnection service, ordinary telephone call service and some value-added services, and provides storage and management functions for user information of related business functions. The main function of the packet subsystem is to provide users with data service channels, thereby providing users with high-speed packet data service services.

4.2 Key technologies

Compared with GSM and CDMA public cellular mobile communication systems, there are two unique key technologies in the digital trunking system: business channel sharing and rapid call establishment.

(1) Sharing of forward wireless service channels

Because the trunking system provides group call services, the CDMA system is for one-to-one calls. Therefore, when implementing a PTT group call, the CDMA system must be modified to achieve forward resource sharing.

In order to achieve forward traffic channel sharing, the same modulation parameters must be used for the forward traffic channels used by the group of users, that is, the forward traffic channels assigned by each user in the same group have the same long code mask and WalshCode, so that the same group under the same carrier / sector will use the same channel, so as to achieve the purpose of forward channel sharing.

In order to achieve the purpose of group users sharing on the forward channel, the corresponding parameters (long code mask and WalshCode) must be added to the signaling process. The forward shared service channel is supported by F-FCH / F-SCH, without changing the structure of the forward channel. Unlike the original cdma 1x forward code channel, the private long code mask originally set when the code channel was established Change to a special public long code mask required or specified by the dispatch group; different groups use different long code masks and WalshCodes. Compared with the cdma 1x forward service channel, Walsh code resources are the same as voice users, and the channel structure is basically unchanged; the long code mask is calculated by an encryption algorithm.

(2) Quick call establishment

Another important feature of PTT calls in the trunking system is fast connection. If the CDMA system is not modified, the call setup time will not meet the needs of the PTT call application.

In order to solve the fast connection, we must start from two aspects. One is to transform the signaling process, reduce unnecessary processes and negotiations, and try to use parallel processing to save users' access time and meet the PTT calling method. On the other hand, it is possible to increase the number of base station access channels by using enhanced access channels and by allowing cluster mobile stations to increase the initial transmit power and power increase step size, as well as improve the system's receiving sensitivity and optimize search strategies. The access trial control is completed in a very short time. In addition, in order to improve the access success rate of the mobile station and the stability of the call, the system can consider the use of Access Probe Handoff, Access Handoff, Channel-Assignament into Soft / Softer Handoff and other technologies.

5. GT800 system

GT800 is a digital trunking system based on GSM-R for independent technological innovation, compatible with GSM and TD-SCDMA wireless access technologies, and meeting the command and dispatch needs of industry users. GT800 is implemented in two stages according to the maturity period of basic technology development:

● Phase Ⅰ: Positioning cluster for common network operation, using GSM wireless access technology, supporting the existing mainstream cluster system business functions, and the performance index meets or exceeds the existing cluster system (connection speed, etc.).

● Phase Ⅱ: positioning private network and common network cluster, based on Phase I core network enhancement, supporting GSM and TD-SCDMA access technology at the same time, through terminal dual-mode evolution to TD-SCDMA.

5.1 The network structure of GT800 system

The network structure of the GT800 system is shown in Figure 2. The system includes the following logic function modules and subsystems.

Figure 2 GT800 digital trunking system networking structure

(1) Network subsystem

The network subsystem mainly completes the exchange function and the database function required for customer data and mobility management and security management. NSS is composed of a series of functional entities. A network subsystem NSS may include several MSC, VLR and HLR. All functional entities communicate with each other through the No.7 signaling protocol. The group call register (GCR, Group Call Register) contains the group ID and the group call area. The group call area and the group ID are called a group. Call for reference. Among them, the GCR connected to the anchor MSC contains a list of dispatch stations related to the group call reference, a list of relay MSCs, and a list of cells directly controlled by the anchor MSC. The GCR connected to the relay MSC contains the anchor MSC address related to the group call reference and the list of cells controlled by the relay MSC.

(2) Base station subsystem

The base station subsystem BSS is the most direct basic component of the cluster system in relation to wireless transmission. It directly connects to the mobile station through the wireless interface and is responsible for wireless signal transmission and reception and wireless resource management. On the other hand, the base station subsystem is connected to the mobile service switching center (MSC) in the core network to implement communication connections between mobile users or between mobile users and fixed network users, and to transmit system signals and user information.

(3) Cluster terminal

The cluster terminal consists of two parts, a mobile terminal (MT) and a subscriber identification module (SIM). Supports point-to-point private calls, broadcast calls, and group calls.

5.2 Key technologies

(1) Channel sharing

GT800 adopts GSM physical channel structure logical channel, adding group call channel and broadcast notification channel. In the GSM system, each mobile phone is allocated a separate channel for talking, while in the GT800 group call, all mobile phones use the same group call channel for monitoring. The number of monitored mobile phones is not limited, saving wireless channel resources.

(2) Quick call establishment

The call setup time is mainly consumed in the interactive messages between the air interface and the mobile phone. GT800 adopts the method of reducing merged messages and system synchronization processing technology, which saves the time of call establishment. When a group call is established, the group members immediately join the group call according to the channel description in the broadcast notification message, without any message interaction with the network, which speeds up the response.

6. Conclusion

Because cluster communication has the characteristics of rapid call establishment, and supports group calls, broadcast calls, and various supplementary services, it is very suitable for dispatch communication between various departments in emergency communication. At the same time, the cluster communication also has the functions of weakening faults, single-station operation and direct connection between terminals, which is suitable for applications in bursts, harsh and without basic network. With the continuous development of emergency communications in China, trunking communications will play an important role in it.

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