0 Introduction The video surveillance system has been widely used in the armed police force, which has greatly enhanced the ability of the armed police force to prepare for the battle. However, the armed police force has a wide range of stations, such as dams, important warehouses, mineral resource bases, bridges, tunnels, etc. The monitoring points are scattered and far from the monitoring center. Video surveillance using traditional wired networks is often costly and difficult to achieve, followed by real-time video. There is an increasing demand for monitoring, and higher requirements are placed on the coverage and implementation distance of the same system. In these cases, mobile video surveillance based on multiple wireless transmission methods presents an irreplaceable advantage. Wireless LAN and wireless broadband access technology can connect multiple monitoring points and remote control centers, and quickly establish a wireless monitoring network in the shortest time.
Currently commonly used wireless access technologies include Wi-Fi, microwave, and WiMAX. Compared with other technologies, WiMAX has the advantages of long transmission distance, high access rate and high bandwidth, which can ensure the transmission quality of video streams and make the deployment of the monitoring system at the access layer faster and easier. Based on the characteristics of the armed police force, this paper proposes a monitoring system design scheme based on WiMAX wireless broadband access technology, which is also applicable to the establishment of monitoring network under other emergency conditions, and gives the precautions in system construction.
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1 WiMAX technology features and advantages WiMAX is a new type of wireless communication technology, the full name of Worldwide Interoperability for Micro-wave Access, is based on the IEEE 802.16 standard broadband wireless access metropolitan area network ( Broadband Wireless Access Metropolitan Area Network, BWAMAN) provides high-speed connectivity to the Internet. WiMAX technology has a high data transmission rate of up to 75 Mb/s; a long transmission distance, a maximum transmission radius of 50 km, and a network coverage area 10 times that of a 3G base station.
1.1 WiMAX system composition The WiMAX network architecture is shown in Figure 1. It consists of the core network, base station, user base station, relay station, user terminal equipment and network management. A typical WiMAX system includes a base station and a plurality of user base stations, and a plurality of relay stations can be set as needed to form a single point to multi-point or multi-point to multi-point architecture.
Core network: The core network to which WiMAx is connected is usually a traditional switched network or the Internet. WiMAX provides a connection interface between the core network and the base station.
Base station: Provides the connection between the user base station and the core network, usually using a sector antenna or an omnidirectional antenna, which provides flexible sub-channel deployment and configuration functions, and continuously upgrades the extended network according to the user community.
User base station: A type of base station that provides a relay connection between a base station and a user terminal device, usually using a fixed antenna and being mounted on a roof. A dynamic adaptive signal modulation mode is adopted between the base station and the user base station.
Relay station: It is usually used to improve the coverage capability of a base station, that is, a relay station that serves as information between a base station and several user base stations (or user terminal equipment).
User terminal equipment: Complete specific application functions, such as video surveillance.
Network management system: used to monitor and control all base stations and user base stations in the network, providing functions such as query, status monitoring, software download, and system parameter configuration.
1.2 Advantages of the wireless video surveillance system of the armed police force used by the WiMAX technology The video surveillance system used by the armed police force generally has a monitoring point within a few hundred meters, which has the characteristics of wide coverage, many coverage points and high quality requirements. At present, the wireless monitoring system mostly uses Wi-Fi-based IP wireless monitoring technology, which adopts IEEE 802.11 as a wireless transmission standard, and the transmission distance is only a few hundred meters, the data transmission rate is low, and the QoS mechanism is not complete, which limits Its application in the Armed Police Force.
WiMAX's coverage capability fully satisfies the distribution characteristics of video surveillance points. Multiple consecutive monitoring points can exist within the coverage of a WiMAX base station. At the same time, WiMAX has the characteristics of non-line-of-sight transmission, which can meet the transmission requirements in complex environments. WiMAX can also be used as a wireless extension of wired network access (Cable, DSL) to easily realize network connections in remote areas.
WiMAX uses a connection-oriented approach that defines a complete QoS mechanism. Its MAC layer supports four kinds of services: unsolicited broadband distribution service, real-time polling service, non-real-time polling service, best-effort transmission service, providing users with QoS performance data, video and voice services to meet video surveillance needs. QoS requirements.
WiMAX operating in the frequency division duplex mode (FDD mode) using the 802.16d standard occupies a pair of 3.5 MHz frequencies, and the total uplink and downlink bandwidth provided by one sector exceeds 15 Mb/s; WiMAX clients (ie WiMAX CPE - Client Produce Equipment, customer premises equipment products) can provide up to 10 Mb / s of throughput, to meet the monitoring requirements of various bandwidth requirements; 1 Mb / s bandwidth requirements per monitoring point A single WiMAX base station can support about 10 monitoring point video transmission requirements.
2 Wireless Monitoring System of Armed Police Force Based on WiMAX Technology This system consists of three subsystems: front-end video acquisition, network transmission and video information management. The front-end video capture subsystem is mainly used for information monitoring. The network transmission subsystem performs data exchange between the monitoring center and the front-end video acquisition in a wireless and wired manner. The video information management subsystem mainly completes the storage, conversion, and encryption of video information. The overall structure of the system is shown in Figure 2.
2.1 Front-end video capture subsystem The front-end video capture subsystem is located at the monitoring point and mainly collects monitoring data, including wireless cameras, pan/tilt, and decoders. The transmission access of the front-end system must adopt the digital access method, and the image is converted into a digital signal at the front end and then transmitted to the monitoring center to reduce the transmission bandwidth.
The wireless camera selected by the system is a digital camera device that integrates various advanced technologies such as video compression technology, network technology and embedded. Integrated with lens, optical filter, image sensor, video compression card, wireless network card and other devices, it can realize many functions such as video capture, video compression and wireless network transmission, so that the monitoring point can be completed independently without the assistance of the computer. The wireless camera has its own independent IP address, so users in the monitoring center and metropolitan area network can use the standard browser to access the network camera according to the IP address, view real-time images and monitor data; authorized users can remotely control the camera through the network management system. And the movement of the PTZ lens or the configuration of the system, so that the target can be fully monitored.
The gimbal is a device for mounting and supporting the camera. It is divided into two fixed pan/tilt and electric pan/tilt. The fixed gimbal is suitable for the case where the monitoring range is not large. After installing the camera on the fixed gimbal, the horizontal and vertical angles of the camera can be adjusted. After the best working posture is reached, it is only necessary to lock the adjustment state. The electric pan/tilt is suitable for scanning and monitoring a wide range. It can expand the surveillance range of the camera and accurately locate the signal by receiving signals from the controller. Since the protocols used by different manufacturers' pan/tilt and control devices are different, the decoder is mainly used for digital signals sent by the relay control terminal.
2.2 Network Transmission Subsystem The network transmission subsystem consists of WiMAx CPE, radio base station and IP bearer network. As a wireless client terminal of the WiMAX network, the WiMAX CPE is deployed at the monitoring point and uses a small beam angle directional antenna to complete data exchange with the base station. The wireless base station mainly accesses multiple WiMAX CPE devices, and simultaneously performs data exchange with the IP bearer network. A single WiMAX sector station can support multiple CPEs. Each CPE is responsible for accessing one or more video surveillance front ends. The CPE RF module converts the signal to an appropriate frequency and transmits it to the WiMAx base station via an antenna to implement video for multiple monitoring points. The convergence of signals.
2.3 Video Information Management Subsystem The video information management subsystem is located in the monitoring center and is mainly composed of a video storage server and a monitoring management server, which complete tasks such as storage, management, query, system configuration and real-time monitoring of video information. The monitoring management server provides business logic control for the entire video surveillance system, completes the video image switching or distribution of the monitoring center, configures parameters of the WiMAX CPE, and displays the monitoring information of the specified point in real time, which is the core service implementation platform of the video monitoring. The database server completes the storage, management, and backup of video surveillance information, and provides historical data browsing and retrieval services for other servers and authorized users of the local area network.
After the video information is transmitted to the monitoring center, the monitoring management server first analyzes the video information to determine whether it is the monitoring information that the authorized user needs to browse in real time, and if so, forwards it to the designated user, such as a video wall or other users in the network. At the same time, the video information is transmitted to the video storage server for archival management. At the same time, the monitoring management server receives the management instruction of the management personnel, and sends the instruction to the designated CPE device to perform monitoring point parameter configuration and the like, as shown in FIG. 3 .
3 System construction precautions The following four points should be paid attention to during the construction and use of the system to ensure the smooth application of the system.
Base station location selection should be based on the actual situation of service distribution, select the commanding height of the center of the business-intensive area (on a tower or the top of the mountain) as the site of the central base station, try to improve the reliability of the wireless link, and avoid adverse factors such as terrain and obstructions. Guarantee the quality of video information transmission.
Wireless system anti-interference ability Terrestrial wireless access systems are often subject to adjacent or co-channel interference from other sectors. In order to effectively overcome the influence of interference, firstly, equipment with anti-interference technology design should be selected as much as possible in the construction. Secondly, reasonable network planning must be adopted to select the location of the base station and the position and direction of the antenna at the customer end to ensure the connection between the user terminals in the system. Mutual interference is reduced to a minimum. In the project, a system with adaptive switching technologies such as BIT/SK, QPSK, 16QAM, 64QAM, etc., can fully guarantee the system's large-capacity data transmission support capability and the reliability of the wireless link.
At the beginning of the frequency planning and load balancing system planning, the flexibility of the antenna configuration and the hotspot distribution characteristics should be fully considered on the basis of the existing frequency, and the frequency of use should be used efficiently under the premise of covering coverage and capacity. After the system is turned on, it is still necessary to perform long-term tracking observation on the traffic of each sector of each base station, and adjust the sector that is incompatible with the actual capacity in real time. If a sector is designed according to the capacity of 10 Mb/s, it is observed that the demand of the sector is significantly less than 10 Mb/s, and the excess carrier is moved from the sector to other sectors with large traffic demand. In order to improve the frequency utilization of the entire system and maintain load balancing.
The video image compression technology has a huge amount of video image information, and the bandwidth of the wireless channel is relatively limited. The correct selection of the compression standard can ensure the smooth transmission of video information. It is recommended to use H. 264 technology, than H. 263 saves about 50% of the outgoing code stream, which can greatly reduce the transmitted data, and solve the prominent contradiction between image information and bandwidth.
4 Conclusion This paper proposes a video surveillance system based on WiMAX wireless broadband access technology, which has the advantages of wide monitoring range, convenient installation and strong flexibility. It is suitable for large-scale temporary places for armed police forces, and it is difficult to quickly build monitoring systems or marginal areas. The local application of wired access has practical significance for improving the combat effectiveness of the armed police force. As WiMAX technology matures, its applications will play an important role in real-time monitoring systems.
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