Introduction of digital trunking technology for emergency communications

(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.