Wireless video surveillance system for armed police forces based on WiMAx technology

0 Introduction

Video surveillance systems have been widely used in the armed police force, which has greatly improved the armed police force's on-duty and combat readiness capabilities. However, the armed police force is stationed in a wide range, such as dams, important warehouses, mineral resource bases, bridges, tunnels, etc. The monitoring points are scattered and far away from the monitoring center. Video surveillance using traditional wired networks is often costly and difficult to implement. Secondly, the demand for real-time video surveillance is increasing, and higher requirements are also placed on the coverage and implementation distance of the same system. In these cases, mobile video surveillance based on a variety of wireless transmission methods has demonstrated irreplaceable advantages. Wireless LAN and wireless broadband access technology can connect multiple monitoring points and remote control centers, and can 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. It can ensure the transmission quality of video streams and make the deployment of monitoring systems at the access layer faster and easier. In this paper, combined with the characteristics of the armed police force, a monitoring system design based on WiMAX wireless broadband access technology is proposed. It is also suitable for the establishment of monitoring networks in other emergency situations, and the precautions in system construction are given.

1 WiMAX Technology Features and Advantages

WiMAX is a new wireless communication technology, the full name of which is Worldwide Interoperability for Microwave Access. It is a broadband wireless access metropolitan area network (BWAMAN) based on the IEEE 802.16 standard, which can provide high-speed connection to the Internet. WiMAX technology has a high data transmission rate, which can reach up to 75 Mb/s; it has a long transmission distance, with a maximum transmission radius of 50 km, and its network coverage area is 10 times that of 3G base stations.

1.1 WiMAX System Composition

The WiMAX network architecture is shown in Figure 1, which consists of a core network, base stations, user base stations, relay stations, user terminal equipment and network management. A typical WiMAX system includes one base station and multiple user base stations. Several relay stations can also be set up as needed to form a single-point to multi-point or multi-point to multi-point architecture.

Core network: The core network that WiMAX connects to is usually a traditional switching network or the Internet. WiMAX provides a connection interface between the core network and the base station.

Base station: Provides connection between user base stations and the core network. It usually uses sector antennas or omnidirectional antennas. It can provide flexible sub-channel deployment and configuration functions, and continuously upgrade and expand the network according to the status of the user group.

User base station: A type of base station that provides a relay connection between the base station and user terminal equipment. It usually uses a fixed antenna and is installed on the roof. A dynamic adaptive signal modulation mode is used between the base station and the user base station.

Relay station: usually used to improve the coverage capability of a base station, that is, to act as a relay station for information between a base station and several user base stations (or user terminal devices).

User terminal equipment: completes 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 WiMAX technology in the wireless video surveillance system of the armed police force

The video surveillance system used by the armed police force generally has a monitoring point within a few hundred meters, with the characteristics of wide coverage, multiple coverage points, and high quality requirements. However, the current wireless surveillance system mostly uses Wi-Fi-based IP wireless surveillance technology, which uses IEEE 802.11 as the wireless transmission standard, with a transmission distance of only a few hundred meters, a low data transmission rate, and an incomplete QoS mechanism, which limits its application in the armed police force.

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WiMAX's coverage fully meets the distribution characteristics of video surveillance points. There can be multiple monitoring points set up continuously within the coverage area 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), conveniently realizing network connection in remote areas.

WiMAX adopts a connection-oriented approach and defines a complete QoS mechanism. Its MAC layer supports four types of services: unsolicited broadband allocation service, real-time polling service, non-real-time polling service, and best-effort transmission service, providing users with data, video, and voice services with QoS performance, which can meet the QoS requirements required for video surveillance.

WiMAX, which uses the 802.16d standard and works in frequency division duplex mode (i.e., FDD mode), occupies a pair of 3.5 MHz frequencies. A sector can provide a total uplink and downlink bandwidth of more than 15 Mb/s. Each WiMAX client (i.e., WiMAX CPE - Client Produce Equipment) can provide a maximum throughput of 10 Mb/s, which can meet monitoring requirements with various bandwidth requirements. According to the 1 Mb/s bandwidth requirement for each monitoring point, a single WiMAX base station can support the video transmission requirements of about 10 monitoring points.

2 Armed Police Force Wireless Monitoring System Based on WiMAX Technology

This system consists of three subsystems: front-end video acquisition, network transmission, and video information management. The front-end video acquisition subsystem is mainly used for information monitoring. The network transmission subsystem completes the data exchange between the monitoring center and the front-end video acquisition in wireless and wired ways. The video information management subsystem mainly completes the storage, conversion, encryption, etc. of video information. The overall structure of the system is shown in Figure 2.

2.1 Front-end video acquisition subsystem

The front-end video acquisition subsystem is located at the monitoring point and is mainly used to collect monitoring data, including wireless cameras, PTZ, decoders and other equipment. The transmission access of the front-end system must adopt digital access mode, 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 used in this system is a digital camera device that integrates multiple advanced technologies such as video compression technology, network technology, and embedded technology. It integrates lenses, optical filters, image sensors, video compression cards, wireless network cards and other devices, and can realize many functions such as video acquisition, video compression and wireless network transmission, so that the work of the monitoring point can be completed independently without the assistance of a computer. The wireless camera has its own independent IP address, so that users in the monitoring center and the metropolitan area network can use a standard browser to access the network camera according to the IP address, view real-time images and monitoring data; authorized users can remotely control the actions of the camera and the PTZ lens or configure the system through the network management system, so that the target can be monitored in an all-round way.

The PTZ is a device for installing and supporting the camera. It is divided into two types: fixed PTZ and electric PTZ. Fixed PTZ is suitable for situations where the monitoring range is not large. After the camera is installed on the fixed PTZ, the horizontal and pitch angles of the camera can be adjusted. After reaching the best working posture, just lock the adjustment state. Electric PTZ is suitable for scanning and monitoring a large area. It can expand the monitoring range of the camera and accurately operate and position by receiving signals from the controller. Since the protocols used by PTZs and control devices of different manufacturers are different, the decoder is mainly used to transfer digital signals sent by the control end.

2.2 Network transmission subsystem

The network transmission subsystem consists of WiMAX CPE, wireless base station and IP bearer network. As the wireless client terminal of the WiMAX network, 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 completes data exchange with the IP bearer network at the same time. A single WiMAX sector station can support multiple CPEs. Each CPE is responsible for accessing one or more video surveillance front ends. The CPE's RF module converts the signal to a suitable frequency and sends it to the WiMAX base station via the antenna to achieve the convergence of video signals from multiple monitoring points.

2.3 Video Information Management Subsystem

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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. It completes tasks such as video information storage, management, query, system configuration, and real-time monitoring. The monitoring management server provides business logic control for the entire video monitoring system, completes video image switching or distribution in the monitoring center, configures parameters for WiMAX CPE, and views monitoring information of designated points in real time. It is the core business implementation platform for video monitoring. The database server completes the storage, management, and backup of video monitoring information, and provides historical data browsing, retrieval, and other services for other servers and authorized users in the LAN.

When 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. If it is, it will be forwarded to the designated user, such as the TV wall or other users in the network, and the video information will be transmitted to the video storage server for archiving management. At the same time, the monitoring management server receives the management instructions of the administrator and sends the instructions to the designated CPE device to perform monitoring point parameter configuration and other tasks, as shown in Figure 3.

3. Precautions for system construction

The following four points also need to be paid attention to during the construction and use of the system to ensure the smooth application of the system.

The site selection of the base station should be based on the actual business distribution situation. The commanding heights in the center of the business-intensive area (on a tower or a mountain top) should be selected as the site of the central base station to maximize the reliability of the wireless link, avoid adverse factors such as terrain and obstructions, and ensure the quality of video information transmission.

Anti-interference ability of wireless system Ground wireless access system is often subject to adjacent frequency or co-frequency interference from other sectors. In order to effectively overcome the influence of interference, first of all, equipment with anti-interference technology design should be used as much as possible during construction. Secondly, reasonable network planning must be carried out to select the location of base stations and the location and direction of user-end antennas to ensure that the mutual interference between user terminals in the system is reduced to a minimum. In the project, a system with adaptive switching technology of multiple modulation modes such as BIT/SK, QPSK, 16QAM, 64QAM, etc. is selected to fully guarantee the system's large-capacity data transmission support capability and the reliability of wireless links.

Frequency planning and load balancing At the beginning of system planning, the flexibility of antenna configuration and the characteristics of hotspot distribution should be fully considered on the basis of existing frequencies, and the frequencies should be used efficiently under the premise of meeting coverage and capacity. After the system is opened, it is still necessary to track and observe the traffic of each sector of each base station for a long time, and adjust and correct the sectors that are not compatible with the actual capacity in real time. For example, if a sector is designed according to the capacity of 10 Mb/s, and it is found through observation that the demand of this sector is significantly less than 10 Mb/s, then the excess carriers from this sector will be moved to other sectors with large traffic demand to improve the frequency utilization of the entire system and maintain load balance.

Video image compression technology The amount of video image information is huge, while the wireless channel bandwidth is relatively limited. Correctly selecting the compression standard can ensure smooth transmission of video information. It is recommended to use H. 264 technology, which saves about 50% of the outgoing code stream compared to H. 263, can greatly reduce the transmitted data, and well solve the outstanding 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, easy installation, and strong flexibility. It is suitable for the armed police force to conduct large-scale temporary places, where the monitoring system needs to be quickly built or in marginal areas where wired access is not easy to obtain. It has practical significance for improving the combat effectiveness of the armed police force. As WiMAX technology becomes more mature, its application will play an important role in real-time monitoring systems.