Fixed point image monitoring transmission system
Wireless image transmission system is divided into two categories from the application level, one is a fixed-point image monitoring transmission system, the second is a mobile video image transmission system.
The fixed-point wireless image monitoring transmission system is mainly used in occasions where wired closed-circuit monitoring is inconvenient, such as monitoring systems for harbor terminals, video and data monitoring for river water conservancy, forest fire prevention monitoring systems, and urban security monitoring. The following describes the different image transmission technologies from low to high frequency bands.
1.1--2.4GHz ISM band of a variety of image transmission technology 2.4GHz image transmission equipment using spread spectrum technology, there are two kinds of frequency hopping and direct-scaling work. The frequency hopping mode is low, the throughput rate is about 2 Mbit/s, and the anti-jamming capability is strong. Different frequency hopping sequences can also be used to realize the same-site multiplexing to increase the capacity. The direct-spread mode has a high throughput rate, but its anti-interference performance is poor, and multiple sets of systems are restricted from the same site.
2.4GHz image transmission can be based on the IEEE802.11b protocol, the transmission rate is 11Mbit/s, and the overhead in the transmission process is removed. The actual effective rate is about 3.8Mbit/s. Later developed IEEE802.11g standard, the upper limit of the rate reached 54Mbit/s. The standard has high interoperability. Point-to-point can transmit several MPEG-4 compressed images.
Also used in the 2.4GHz band are Bluetooth technology, HomeRF technology, MESH, and microcellular technology. With the gradual expansion of the scope of application, the 2.4 GHz band is in full-load operation, and its rate issues, safety issues, and mutual compatibility issues merit further study.
1.2-3.5GHz Wireless Access System The 3.5GHz wireless access system is a point-to-multipoint microwave communication technology using FDD duplex mode, using 16QAM and 64QAM modulation methods and based on the DOCSOS protocol. Its operating frequency band is relatively low, the radio wave free space loss is small, the propagation rain attenuation performance is good, the access rate is high enough, and the equipment cost is relatively low. The system has a relatively good coverage, usually 5km ~ 10km, suitable for the application of low-cost, large-area coverage at the prefecture and city level units; it can also be complemented with WLAN and LMDS to form a coverage area size and user Density densely matched multi-layered organic complementary mode of operation. The existing problem is that the bandwidth is insufficient. Only 30 MHz in the uplink and downlink, and it is difficult to use on a large scale.
1.3--5.8GHz WLAN Products The 5.8GHz WLAN products use orthogonal frequency division multiplexing technology. In this band, the WLAN products are based on the IEEE802.11a protocol and the transmission rate can reach 54 Mbit/s. According to the WLAN transmission protocol, when the peer-to-peer application is used, the effective rate is 20 Mbit/s; in the case of point-to-six, the effective transmission rate of each image is about 500 kbit/s, which means that the total transmission data volume is 3 Mbit. /s or so. For the transmission of wireless images, the problem of "high-definition digital image transmission in wireless networks" is basically solved, making it possible to transmit digital images over a wide range of frequencies in the 5.8 GHz band, and is particularly suitable for urban security monitoring systems.
WLAN transmission monitoring images, now more mature is the use of MPEG-4 image compression technology. This compression technology at 500kbit/s rate, the compressed image clarity can reach 1CIF (352 × 288 pixels) ~ 2CIF. At a rate of 2 Mbit/s, this technology can transmit 4CIF (702 × 576 pixels, DVD clarity) sharp images. The digitized image after compression by MPEG-4 is transmitted through a wireless channel, and with the corresponding software, it is easy to realize a networked and intelligent digital urban security monitoring system.
The 5.8GHz-band WLAN products have poor air relay, point-to-point connections are not economical, they are not suitable for small devices, and the technical cost is too high. At the same time, the 5.8GHz band faces spectrum control in some areas.
1.4--5.8GHz wireless transmission network iMAX point-to-multipoint wireless access system is a typical wireless transmission private network, with its strong non-line-of-sight capabilities, high capacity, long transmission distance, advanced encryption and QoS mechanisms and other outstanding The feature provides users with an enhanced broadband wireless access solution. Excellent OFDM (Orthogonal Frequency Division Multiplexing) technology and high receiving sensitivity of -92dbm are used to solve long-distance video transmission.
The Broadband Fixed Wireless Access System (LMDS) system in the 1.5- to 26-GHz band is a typical 26-GHz wireless access system that uses 64QAM, 16QAM, and QPSK. LMDS has greater bandwidth and bidirectional data transmission capabilities, can provide a variety of broadband interactive data and multimedia services, to solve the bottleneck of the traditional local loop, to meet the needs of high-speed broadband data, image communications and broadband internet services. The LMDS system covers a range of 3 km to 5 km and is suitable for metropolitan area networks. Because countries in the world have different plans for the working frequency band of LMDS, their compatibility is poor, their rain attenuation performance is poor, and their cost is high.
To sum up, for urban digital surveillance systems, the use of WLAN technology over 2.4 GHz as a fixed point of the image transmission is entirely feasible, but also the development trend.
Mobile video image transmission system
In addition to the need for image monitoring at a fixed point, the need for mobile image transmission is also very strong. Mobile video image transmission is widely used in *command vehicles, traffic accident exploration vehicles, fire control *field command vehicles and customs, oil fields, mining, water conservancy, electric power, finance, maritime affairs, and other emergency and emergency command systems. The main role is to The real-time image of the scene is transmitted back to the command center so that the commanding and decision-making personnel of the command center are immersed in the environment, the accuracy and timeliness of the decision-making is improved, and the work efficiency is improved. Fujitsu has introduced the use of public network and special technology for mobile video transmission.
2.1 Using CDMA, GPRS public mobile network to transmit images The mobile transmission technology of CDMA wireless networks has many advantages: good confidentiality, strong anti-interference ability, resistance to multi-path fading, flexible configuration of system capacity, and low network construction cost. CDMA adopts MPEG-4 compression method, compresses the picture with MPEG-4 CIF format, can reach the speed of about 2 frames per second; If adjust the picture to QCIF form, can reach more than 10 frames per second. However, for the security system, the low transmission frame rate is generally used to ensure the clarity of the transmission, because only the image clarity above the CIF can meet the needs of investigation and forensics. If you want to further improve the real-time transmission rate of live images, a simple solution is to use multiple CDMA network cards bundled to increase the transmission rate of wireless channels. Currently, there are 2 to 3 routers bundled on the market. The cost of adding a network adapter is to increase equipment costs and usage costs. With the continuous development of video compression technology, the transmission rate of 3 to 4 frames per second on a single network card is achievable. If every 3 to 4 frames of CIF format images can be transmitted, it can meet the safety of general mobile public transport facilities. Monitoring requirements.
GPRS is a wireless packet switching technology based on the GSM system that supports specific point-to-point and point-to-multipoint services and transmits data in the form of "packet". The GPRS peak rate exceeds 100 kbit/s, and the network capacity is only allocated when needed. This transmission method is called statistical multiplexing. The main advantage of GPRS is that it is always online and traffic-based. It can connect to the Internet at any time without dial-up, maintain contact with the network at any time, and have high resource utilization.
Another option that can be expected is the 3G system. Now that the world has entered the deployment stage, the effective rate that can be achieved so far is 384 kbit/s. It will bring about a revolutionary progress in the mobile video transmission system. However, it should be noted that even if the speed is increased a lot, do not think that all mobile transportation facilities can transmit images back to the monitoring center at the same time because the concept is almost impossible for public network image transmission.
2.2 Dedicated image transmission technology for emergency emergencies For some emergency command center image transmission systems, it is often required to transmit the image of the emergency scene back to the command center. For example, there are major natural disasters, floods, fire scenes, large-scale mass gatherings, and important safety and security tasks. This kind of emergency image transmission system should not use public network transmission. It is better to use professional mobile image transmission equipment. However, at present, China has not yet specifically planned frequencies for this. There are several technologies that can be used for mobile video transmission.
2.2.1 WiMAX
WiMAX is a point-to-multipoint broadband wireless access technology. WiMAX adopts a series of new technologies such as dynamic adaptive modulation, flexible system resource parameters and multi-carrier modulation, and has a high-speed transmission capability (up to 70 Mbit/s). ~100Mbit/s) and better QoS and security controls. WiMAX 802.16e covers a range of 1 to 3 miles and is mainly located in a mobile wireless metropolitan area network environment. However, it is difficult for 802.16e to obtain enough global unified frequencies, and the construction costs and equipment prices are relatively high.
2.2.2 Wireless Mesh (MESH) Technology Wireless "MESH" technology can realize high-speed data communication in a close range. With the 2.4 GHz frequency band, the effective bandwidth can reach 6 Mbit/s. This kind of technical link is simple in design, flexible in networking, and easy to maintain.
For fixed wireless image transmission, lower cost WLAN technology products can be used; for mobile video image transmission, public mobile network or dedicated wireless image transmission technology can be adopted. Fujitec hopes that more peers can further pay attention to the issue of wireless image transmission to promote the development of the industry.
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