Application of Cluster Communication Technology in GPS Vehicle Monitoring System

frozenviolet

Application of Cluster Communication Technology in GPS Vehicle Monitoring System [Copy link]

Abstract: This paper introduces the communication mode of GPS vehicle monitoring system. Based on the introduction of cluster system, this paper focuses on the application of cluster communication technology in vehicle monitoring system, including system composition, channel utilization mode, calling mode, synchronization mode, data transmission format and other contents.
Keywords: GPS, cluster communication, vehicle monitoring system
1. Overview
GPS vehicle monitoring system is a new vehicle management system that uses GPS receiver, wireless communication and geographic information technology to monitor, dispatch and command mobile users. According to the sub-function division, the vehicle monitoring system consists of three parts: positioning part, communication part and display part. Among them, the more commonly used communication methods are conventional communication, cluster communication and GSM short message service. The vehicle monitoring
system based on conventional communication is relatively simple in design, networking and use, but its scope of action is small; GSM short message service is currently more popular for positioning data transmission, with a large coverage range and can roam across the country, but its delay problem is the bottleneck restricting its development, and its calling function cannot be compared with the cluster system; the cluster system is a special dispatching and command system, which has incomparable advantages for some special units. This paper introduces the design and channel utilization of GPS vehicle monitoring system under cluster mode.
2. The trunking system
wireless trunking is actually a wireless communication system formed by automatically sharing several channels in a multi-channel relay (forwarding) system, and on this basis, small groups (groups) are combined into large groups (groups), plus many call, fault handling and other functions.
Compared with conventional communication methods, the trunking communication system has the advantages of high channel utilization, good service quality, low call blocking rate, privacy of calls, high system intelligence, switching function, and interconnection with PABX or PSTN.
There are many ways to classify the trunking communication system. According to the signaling type, there are analog signaling, digital signaling and hybrid signaling systems; according to the signaling method, there are common channel signaling and channel-associated signaling; according to the voice, there are digital trunking and analog trunking; according to the control method, there are centralized control, decentralized control, etc. At present, the most widely used is the analog trunking system with centralized control, and the signaling is based on the MPT1327 standard and the non-public standard. Based on the MPT1327 standard signaling system, the transmission of vehicle positioning data can be carried out in the signaling channel; and based on the non-public signaling standard, the vehicle positioning data can only be carried out in the voice channel. Accordingly, the digital information of the vehicle must be modulated before it can be transmitted in the voice channel.
3. GPS vehicle monitoring system based on cluster communication
3.1 System composition
The cluster-based GPS vehicle monitoring system is shown in Figure 1. Among them, the cluster network provides the system with a wireless communication link for voice and data transmission, and data transmission can be achieved using a variety of voice calling methods.
The monitoring center uses a vectorized electronic map as the background, and the monitoring software has the function of real-time display of all information of mobile users and information query. At the same time, the monitoring center also has the function of a differential base station, providing differential GPS correction information for mobile users within the entire monitoring range, so that the vehicle positioning accuracy is improved to within 10 meters. Each mobile user device consists of a cluster radio, a GPS receiver and a control logic circuit. The control circuit not only forms the logical timing required by the system, but also has data processing functions, including a modem.
3.2 Channel utilization method
In a cluster system with centralized control mode, the management of each channel is implemented on a control channel. Mobile users wait on the control channel, and both initiating and receiving calls are carried out on this channel. Once the channel is applied for, the calling party and the called party complete the voice contact on the applied channel, and then switch to the control channel to wait.
When using the voice channel to transmit vehicle positioning data, the radio station needs to apply for a channel every time the positioning information is transmitted. During the application time, no data is transmitted, and the time utilization rate is low; and when no idle channel is applied, the information of the vehicle will be lost. Therefore, the user who transmits data should set the priority to ensure the reliable transmission of data.
3.3 Calling mode
In the trunking communication system, the user's calling mode includes: single call, group call, group call, private call, etc. The vehicle monitoring system based on GPS adopts the differential mode to improve the positioning accuracy of the system. The differential information is sent by the monitoring center in the form of group call, and all users assigned to the same group can receive it.
In order to enable the data of each mobile user to be collected in real time, the time division multiple access (TDMA) method is adopted, that is, each vehicle initiates a call in sequence according to the timing shown in Figure 2. The time interval between two adjacent users includes the data sending time and the protection time (to prevent data from affecting each other). The number of users that can be accommodated in a sampling period is, where is the sampling period.
In order to prevent the monitoring center's voice scheduling of a user from affecting the data transmission of other users, voice calls and data calls are carried out in different groups. When the mobile user transmits data, it works in the data group and uses a dedicated channel. When the monitoring center needs to make a voice call to a user, it first sends a switching command to set the user's group number to the voice group, and then makes a voice call. The call is completed within the specified time limit, and the user automatically switches to the data group and then transmits data.
In the monitoring center, voice calls and data calls are completed using different wireless trunking radios.
3.4 Synchronization method
In the GPS vehicle monitoring system, asynchronous data transmission is usually used, and the receiving end and the sending end do not need to be synchronized. However, the data collection of each mobile user is time division multiple access mode, and the timing relationship between each other cannot be overlapped, that is, the mobile users must be synchronized.
There are two ways to synchronize each mobile user: ① Synchronize using the second pulse of the GPS receiver; ② Synchronize using the broadcast synchronization code sent by the monitoring center. The second pulse and the synchronization code are both provided to each mobile user with a unified time starting point, and the transmission time slots of different vehicles are generated by different delays. The monitoring center sends synchronization codes. One synchronization code can be sent in one sampling cycle, or multiple synchronization codes can be sent periodically.
3.5 Data transmission format
The data transmission in the monitoring system is divided into two parts: uplink and downlink. The uplink data mainly includes the real-time position, speed, time, and status (including alarms, faults, etc.) of the monitored vehicle; the downlink data mainly includes GPS differential correction information, using the general telegram in the RTCM-104 format. If it is synchronized by the monitoring center, it also includes a broadcast synchronization code.
1. Uplink data format
Data header User address Latitude Longitude Speed Time Vehicle status
2. Downlink data format
Data header Broadcast synchronization code RTCM telegram Dispatch command The user address
The uplink and downlink data headers are different, which are used to decode the uplink and downlink data. The user address refers to the identification number of each monitored vehicle. The dispatch command is the operation of the monitoring center on the vehicle at the corresponding address, such as emergency rescue, transportation to a certain place, shutting down the car engine, etc. The broadcast synchronization code is not used in the system using second pulse synchronization. The vehicle status is input by the driver according to the actual situation, such as robbery alarm, vehicle failure, etc.
4. Conclusion
The data transmission of GPS vehicle monitoring system can only be realized by voice channel at present. Because the bandwidth of voice channel is narrow and sub-audio signaling may be accompanied in the cluster system, special attention should be paid to the baud rate, modulation mode and data bandwidth when selecting modem. Generally, modems with FSK modulation mode, baud rate of 1200 or 2400 bps and data bandwidth limit within 3KHz can be selected.
The application of cluster communication technology in GPS vehicle monitoring system includes its system composition, cluster channel utilization method, calling method, synchronization method, data transmission format, etc. However, for the cluster system based on dedicated signaling, the channel application, maintenance, release and other processes and their occupancy time cannot be known exactly. Only rough data can be obtained through experimental means, which is a major obstacle to the expansion of the system. With the advent of the digital age, digital cluster systems have also emerged, and GPS vehicle monitoring systems based on digital cluster systems will also come out. Due to the high channel utilization rate of digital clusters, system expansion is relatively simple.