What is the Wireless Transmission?
Wireless transmission is a form of unguided media. Wireless communication involves no physical link established between two or more devices, communicating wirelessly. Wireless signals are spread over in the air and are received and interpreted by appropriate antennas.
Wireless transmission refers to a way of data transmission using wireless technology. Wireless transmission and wired transmission are corresponding.
With the increasing development of wireless technology, the application of wireless transmission technology is increasingly accepted by all walks of life. As a special-use method, wireless image transmission is gradually being favored by the majority of users. Its features such as easy installation, strong flexibility, and high-cost performance make monitoring systems in more industries adopt wireless transmission methods to establish connections between monitored points and monitoring centers.
Wireless monitoring technology has been widely used in modern transportation, transportation, water conservancy, shipping, railways, public security, fire protection, border inspection stations, forest fire protection, parks, scenic spots, factories, communities, and other fields.
Wireless transmission classification
Wireless transmission is divided into analog microwave transmission and digital microwave transmission.
Simulate microwave transmission
Schematic diagram of the analog microwave transmission system
Analog microwave transmission is to directly modulate the video signal on the microwave channel (microwave transmitter) and transmit it through the antenna. The monitoring center receives the microwave signal through the antenna and then demodulates the original video signal through the microwave receiver.
If you need to control the pan/tilt lens, add the corresponding command to control the transmitter in the monitoring center, and configure the corresponding command receiver in the monitoring front end. This monitoring method has been gradually replaced by digital microwaves, COFDM, 3G, CDMA, etc.
Digital microwave transmission
Schematic diagram of the digital microwave transmission system
Digital microwave transmission is to encode and compresses the video first, then modulate it through the digital microwave channel, and then transmits it through the antenna. The receiving end is the opposite. The antenna receives the signal, microwave despreads, video decompresses, and finally restores the analog video signal. It can also be the microwave.
After de-expansion, install the corresponding decoding software through the computer, and use the computer software to decompress the video, and the computer also supports functions such as recording, playback, management, cloud mirror control, alarm control, etc.; storage server, with disk array storage.
Wireless transmission frequency
From the application level, wireless image transmission systems are divided into two categories, one is fixed-point image monitoring and transmission systems, and the other is mobile video image transmission systems.
Fixed-point image monitoring and transmission system
Fixed-point wireless image monitoring and transmission systems are mainly used in situations where wired closed-circuit monitoring is inconvenient, such as port and dock monitoring systems, river water conservancy video and data monitoring, forest fire protection monitoring systems, urban security monitoring, construction sites, etc. The following describes different image transmission technologies from low to high-frequency bands.
Multiple image transmission technologies in the 1.1-2.4 GHz ISM frequency band
2.4 GHz image transmission equipment adopts spread spectrum technology, with two working modes, frequency hopping, and direct spread. The frequency hopping method has a low rate, with a throughput rate of about 2 Mbit/s, and strong anti-interference ability. Different frequency hopping sequences can also be used to achieve co-site multiplexing to increase capacity.
The direct spread method has a higher throughput rate, but the anti-interference performance is poor, and the use of multiple systems at the same site is restricted.
2.4 GHz image transmission can be based on the IEEE802.11b protocol, with a transmission rate of 11 Mbit/s, without the overhead in the transmission process, the actual effective rate is about 5.5-6 Mbit/s.
The IEEE 802.11g standard that was formulated later has a rate upper limit of 54 Mbit/s and can reach 108 Mbps in a special mode. This standard has high interoperability and can transmit several channels of MPEG-4 compressed images point-to-point.
Bluetooth technology, MESH, micro-cellular technology, etc. are also used in the 2.4 GHz frequency band.
1.2--3.5 GHz frequency band wireless access system
The 3.5 GHz wireless access system is a point-to-multipoint microwave communication technology, using FDD duplex mode, 16QAM, and 64QAM modulation mode, based on DOCSIS protocol.
Its working frequency band is relatively low, the loss of electric wave-free space is small, the performance of propagation rain attenuation is good, the access rate is high enough, and the equipment cost is relatively low.
The system has relatively good coverage capabilities, usually reaching 5 km to 10 km, suitable for low-cost, large-area coverage applications of the prefecture, county, and city-level units; it can also complement each other with WLAN and LMDS to form a coverage area size match.
The organic complementary mode of multi-layer operation with dense user density. The problem is that the bandwidth is insufficient, only 30 MHz for the uplink and the downlink, which is difficult to use on a large scale.
1.3--5.8 GHz WLAN products
5.8 GHz WLAN products use OFDM orthogonal frequency division multiplexing technology. WLAN products in this frequency band are based on the IEEE802.11a protocol, and the transmission rate can reach 54 Mbit/s, and it can reach 108 Mbps in a special mode. According to the WLAN transmission protocol, in point-to-point applications, the effective rate is 20 Mbit/s;
In the case of point-to-six points, the effective transmission rate of each channel of the image is about 500 kbit/s, that is to say, the total transmission data volume is about 3 Mbit/s.
Wireless image transmission, basically solves the transmission problem of high-definition digital images in wireless networks, making it a reality to use the 5.8 GHz frequency band to transmit digital images on a large scale, which is especially suitable for urban security surveillance systems.
The working frequency of the ZWD-2422 wireless HD transmitter is 4.9GHz-5.9GHz. When it receives interference from other RF equipment or signals, it can automatically adjust to the appropriate frequency, so it is generally not 2.4G in the frequency band around 5G, and 3G will not interfere. ZWD-2422 wireless high-definition transmission.
WLAN transmits surveillance images, and the more mature one uses MPEG-4 image compression technology. When this compression technology is at a rate of 500 kbit/s, the compressed image resolution can reach 1CIF (352×288 pixels) ~ 2CIF.
In the case of a rate of 2 Mbit/s, this technology can transmit 4CIF (702×576 pixels, DVD definition) clear images.
The digitized image compressed by MPEG-4 is transmitted via the wireless channel and cooperated with the corresponding software to easily realize a networked and intelligent digital city security monitoring system.
2.4/5.8GHz products based on 802.11n, and 11n products are divided into AN and GN to work at 5.8GHz and 2.4GHz respectively, the transmission rate can reach 150, 300, 600 Mbps, and the effective transmission rate is 60, 160, and 300 Mbps. With HD With the development of cameras, this high-bandwidth 11N mode is very suitable for the transmission of high-definition cameras.
The cooperation of high-definition cameras and high-bandwidth wireless transmission equipment will gradually become the trend of wireless video surveillance.
Broadband fixed wireless access system in the 1.4-26 GHz frequency band
The LMDS system is a typical 26 GHz wireless access system, using three modulation methods: 64QAM, 16QAM, and QPSK. LMDS has greater bandwidth and two-way data transmission capabilities, can provide a variety of broadband interactive data and multimedia services, solve the bottleneck problem of the traditional local loop, and can meet the needs of high-speed broadband data, image communications, and broadband internet services.
The LMDS system covers a range of 3 kilometers to 5 kilometers and is suitable for metropolitan area networks. Because countries around the world have different working frequency band plans for LMDS, its compatibility is poor, rain attenuation performance is poor, and the cost is high.
Mobile video image transmission system
In addition to the demand for fixed-point image monitoring, the demand for mobile image transmission is also quite strong. Mobile video image transmission, widely used in public security command vehicles, traffic accident exploration vehicles, fire, and armed police field command vehicles and customs, oil fields, mining, water conservancy, electric power, finance, maritime, and other emergency and emergency command systems.
The main role is to The real-time images of the scene are transmitted back to the command center, so that the commanding and decision-making personnel of the command center are on the scene, improving the accuracy and timeliness of decision-making, and improving work efficiency.
Use CDMA, GPRS, 3G public mobile network to transmit images
The mobile transmission technology of the CDMA wireless network has many advantages, such as good confidentiality, strong anti-interference ability, anti-multipath fading, flexible configuration of system capacity, and low network construction cost.
CDMA adopts the MPEG-4 compression method, and compresses the image with the MPEG-4 CIF format, which can reach a rate of about 2 frames per second; if the image is adjusted to the QCIF format, it can reach more than 10 frames per second.
GPRS is a wireless packet-switching technology based on the GSM system. It supports specific point-to-point and point-to-multipoint services and transmits data in packets. The GPRS peak rate exceeds 100 kbit/s, and the network capacity is allocated only when needed. This transmission method is called statistical multiplexing.
The main advantage of GPRS is that it is always online and billed according to the flow of traffic. It can access the Internet at any time without dialing, keep in touch with the network at any time, and has a high resource utilization rate.
3G technology has gradually replaced GPRS and CDMA and can achieve an effective rate of 384 kbit/s. In the urban area where the network is deployed, one CIF image can be transmitted in real-time, up to 20 frames per second.
Dedicated image transmission technology for emergency emergencies
For the image transmission system of some emergency command centers, it is often required to transmit the images of the emergency scene back to the command center. For example, the scene of major natural disasters, floods, fires, large gatherings of the masses, and important security missions.
This type of emergency image transmission system is not suitable for public network transmission, and it is better to use professional mobile image transmission equipment.
There are several technologies that can be used for mobile video image transmission.
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. It also has a higher transmission rate (up to 70 Mbit/ s～100 Mbit/s) and better QoS and security control.
The WiMAX802.16e coverage can reach 1 to 3 miles and is mainly positioned in the mobile wireless metropolitan area network environment. However, it is difficult for 802.16e to obtain enough global unified frequencies, and the construction cost and equipment price are relatively high.
Wireless Mesh (MESH) Technology
The wireless mesh (MESH) technology can realize high-speed data communication within a relatively close range. Using the 2.4 GHz frequency band, the effective bandwidth can reach 6 Mbit/s. This technology has a simple link design, flexible networking, and convenient maintenance.
Supports MeshController centralized management, terminal data does not need to be configured, and solutions are automatically generated. Support MeshController hot backup link, automatic roaming switching, and other functions.
Supports centralized management of MeshController user terminals and multiple verification methods to make the system more secure.
Support MeshController user flows control function can freely allocate flow according to user type, support speed limit, limit flow, limit online time, and other functions.
For fixed wireless image transmission, low-cost WLAN technology products can be used; for mobile video image transmission, public mobile networks or dedicated wireless image transmission technologies can be used. I hope that more colleagues can pay more attention to the problem of wireless image transmission in order to promote the development of the industry.
Wireless transfer method
Video baseband transmission
It is the most traditional way of television monitoring transmission. It does not do any processing on the 0～6MHz video baseband signal and directly transmits the analog signal through the coaxial cable (unbalanced).
Its advantage is that the short-distance transmission image signal loss is small, the cost is low, and the system is stable.
The disadvantage is that the transmission distance is short, and the high-frequency components above 300 meters attenuate greatly, and the image quality cannot be guaranteed; one video signal needs to be laid with a cable, and another cable is needed to transmit the control signal; its structure is a star structure, with a large amount of wiring and maintenance. Difficult, poor scalability, suitable for small systems.
Fiber optic transmission
The common ones are analog optical transceivers and digital optical transceivers, which are the best solution to solve tens or even hundreds of kilometers of TV surveillance transmission. The video and control signals are converted into laser signals for transmission in optical fibers.
Its advantages are long transmission distance, low attenuation, good anti-interference performance, and suitable for long-distance transmission.
The disadvantage is that it is not economical for the transmission of monitoring signals within a few kilometers; optical welding and maintenance require professional technical personnel and equipment operation and processing, high maintenance technical requirements, and it is not easy to upgrade and expand.
Network transmission
It is a monitoring transmission method that solves the long-distance and extremely scattered points between cities. It uses MPEG2/4 and H.264 audio and video compression formats to transmit monitoring signals.
Its advantage is that the network video server is used as the monitoring signal uploading equipment. As long as there is an Internet network, it can be monitored and controlled by installing remote monitoring software.
The disadvantage is that it is limited by network bandwidth and speed. ADSL can only transmit small images and low-quality images; only a few to ten frames per second can be transmitted. The animation effect is very obvious and there is a delay, so real-time monitoring cannot be achieved.
Microwave transmission
It is one of the solutions to monitor transmission in places where wiring is difficult for several kilometers or even dozens of kilometers. Using frequency modulation or amplitude modulation, the image is carried on a high-frequency carrier and converted into high-frequency electromagnetic waves for transmission in the air.
Its advantages are low overall cost, more stable performance, eliminating wiring and cable maintenance costs; it can dynamically transmit broadcast-level images in real-time, with good image transmission clarity, and completely real-time; flexible networking, good scalability, plug and play Use; low maintenance costs.
The disadvantage is that due to the use of microwave transmission, the frequency band is above 1GHz. Commonly used are the L band (1.0～2.0GHz), S-band (2.0～3.0GHz), Ku band (10～12GHz), and the transmission environment is an open space. Used in big cities, radio waves are more complicated and relatively easy to be affected by external electromagnetic interference;
Twisted pair transmission
(Balanced transmission) is also a type of video baseband transmission, which converts the 75Ω unbalanced mode to a balanced mode for the transmission.
It is a better solution to solve the problem of the transmission of surveillance images within 1 km, the electromagnetic environment is relatively complex and the occasion is better, and the surveillance image signal processing is transmitted in a balanced and symmetrical manner.
Its advantages are simple wiring, low cost, and strong resistance to common-mode interference.
Its disadvantage is that it can only solve the monitoring image transmission within 1Km, and a twisted pair can only transmit one image, which is not suitable for large and medium-sized monitoring;
The twisted-pair cable has a fragile texture and poor aging resistance, which is not suitable for field transmission;
The high-frequency component of the twisted pair transmission attenuates greatly, and the image color will suffer a great loss.
Broadband co-cable transmission
The video adopts technologies such as amplitude modulation, audio frequency modulation, and FSK data signal modulation, and dozens of monitoring images, audio, control, and alarm signals are integrated into a coaxial cable for bidirectional transmission.
Its advantage is that it makes full use of the resource space of the coaxial cable. Thirty channels of audio, video, and control signals are transmitted in two directions in the same cable;
Simple construction and convenient maintenance, saving a lot of material costs and construction costs;
Frequency division multiplexing technology solves the problem of scattered long-distance transmission points and difficult wiring to monitor transmission;
The radio frequency transmission mode only attenuates the carrier signal, the image signal attenuation is relatively small, and the brightness and chroma transmission are synchronously nested to ensure that the image quality reaches about 4;
The use of 75Ω coaxial unbalanced transmission makes it have a strong anti-interference ability, and the image quality can still be guaranteed in complex electromagnetic environments.
The disadvantage is the use of weak signal transmission, high system debugging technical requirements, and professional instruments must be used. If there is a problem with one device on the mainline, it may cause the entire system to have no image. In addition, the broadband modulation terminal needs to be supplied with an AC220V AC power supply (but most of the monitoring points have the condition of AC220V AC power supply).
Wireless SmartAir transmission
SmartAir technology is a unique single-antenna mode gigabit wireless high-speed transmission technology in the communications industry. It adopts multi-band OFDM air interface technology, TDMA low-latency scheduling technology, and advanced wireless communication technologies such as low-density parity-check code LDPC, adaptive modulation and coding AMC, and hybrid automatic retransmission HARQ to achieve a transmission rate of 1Gbps.
Advantage
- Low overall cost and more stable performance.
Only a one-time investment, with no need to dig trenches to bury pipes, it is especially suitable for occasions where the outdoor distance is long and has been decorated; the use of wireless monitoring can get rid of the shackles of cables, with a short installation period, convenient maintenance, strong capacity expansion, and rapid cost recovery The advantages.
- Flexible networking, good scalability, plug, and play.
Managers can quickly add new wireless monitoring points to the existing network, without having to lay the network or add equipment for new transmissions, and easily realize remote wireless monitoring.
- Low maintenance cost.
The wireless monitoring and maintenance are maintained by the network provider, and the front-end equipment is a plug-and-play, maintenance-free system.
- The wireless monitoring system is a combination of monitoring and wireless transmission technology
It can transmit live information from different locations to the wireless monitoring center through wireless communication in real-time, and automatically form a video database for future retrieval.
- In the wireless monitoring system, the wireless monitoring center obtains the video information of the monitored point in real time, and the video information is continuous and clear.
At wireless monitoring points, cameras are usually used for real-time collection of on-site conditions. The cameras are connected through wireless video transmission equipment and send data signals to the monitoring center through radio waves.
Application of wireless transmission
Wireless monitoring has a wide range of applications, including scattered security monitoring, traffic monitoring, industrial monitoring, home monitoring, and many other fields. Such as:
- Wireless monitoring of cash machines, bank tellers, supermarkets, factories, etc.
- Nursing centers, kindergartens, and schools provide remote wireless monitoring services
- Unattended systems for power stations and telecommunication base stations
- Wireless monitoring system for petroleum, drilling, exploration, etc.
- Intelligent building, intelligent community wireless monitoring system
- Assembly line wireless monitoring system, warehouse wireless monitoring system
- Remote wireless monitoring of forests, water sources, and river resources
- Outdoor equipment wireless supervision
- Wireless monitoring system for traffic conditions of bridges, tunnels, and intersections
- Wireless video transmission system for tourist attractions, large factories, and construction sites
- Forest fire prevention wireless video transmission system
- Wireless video transmission system for ports, docks, and border checkpoints
Long-distance wireless transmission technology
The wireless communication technology widely used in remote areas mainly includes GPRS/CDMA, digital radio, spread spectrum microwave, wireless network bridge and satellite communication, shortwave communication technology, etc. It is mainly used in more remote or unsuitable areas, such as coal mines, offshore, polluted or harsh environments, etc.
Wireless transmission technology classification
GPRS/CDMA wireless communication technology
GPRS (General Packet Radio Service) is a wireless packet switching technology based on the GSM communication system developed and operated by China Mobile. It is a technology between the second and third generations.
GPRS network supports both circuit-type data and packet-switched data at the same time so that the GPRS network can be easily connected to the Internet. Compared with the original circuit-switched data transmission method of the GSM network, GRRS’s packet-switching technology has real-time online and pay-per-quantity. High-speed transmission and other advantages.
CDMA (the English abbreviation for Code Division Multiple Access) is a new wireless communication system based on code division technology and multiple access technologies operated by China Telecom. Its principle is based on spread spectrum technology.
Digital radio communication
Digital radio is the abbreviation of digital wireless data transmission radio. It is a wireless data transmission station that adopts digital signal processing, digital modulation and demodulation, forward error correction, equalization soft decision, and other functions.
Most of the working frequencies of digital radio stations use the 220-240MHz or 400-470MHz frequency bands, which are compatible with digital phones, have good real-time data transmission, dedicated data transmission channels, are a one-time investment, no operating fees, are suitable for harsh environments, and stability Good to wait for the advantages.
The effective coverage radius of digital radio stations is about tens of kilometers, which can cover a city or a certain area. Digital radio stations usually provide a standard RS-232 data interface, which can be directly connected with computers, data collectors, RTUs, PLCs, data terminals, GPS receivers, digital cameras, etc.
It has been widely used in various industries and has been applied in various industries such as aerospace, railway, electric power, petroleum, meteorology, earthquake, etc. It has also made great progress and development in the SCADA field of remote control, telemetry, signal shaking, and remote sensing.
Spread spectrum microwave communication
Spread spectrum communication, that is, spread spectrum communication technology refers to a communication technology in which the bandwidth of the signal used to transmit information is much larger than the bandwidth of the information itself. It was first used in military communications.
The basic principle of its transmission is to modulate the transmitted information with a pseudo-random code sequence (spreading code). The rate of the pseudo-random code is much greater than the rate of transmitting the information. At this time, the bandwidth occupied by the transmitted signal is much greater than that required by the information itself.
The bandwidth realizes the spectrum expansion, and at the same time, the radio power spectral density transmitted to space is also greatly reduced. At the receiving end, the same spreading code is used for relative demodulation and information data recovery!
Its main characteristics are strong anti-noise ability; strong anti-interference ability; strong anti-fading ability; strong anti-multipath interference ability; easy multimedia communication networking; good safe communication ability; no interference with other systems of the same kind, etc.,
At the same time, it has the characteristics of long transmission distance and wide coverage, which is especially suitable for field networking applications.
Wireless network bridge
The wireless network bridge is the product of the combination of wireless radio frequency technology and traditional wired network bridge technology. The wireless bridge is designed for point-to-point network interconnection using wireless (microwave) for long-distance data transmission.
A wireless bridge is a storage and forwarding device that realizes LAN interconnection at the link layer. It can be used for long-distance (up to 50Km) and high-speed (up to 100Mbps) wireless networking between fixed digital devices and other fixed digital devices.
Spread-spectrum microwave and wireless bridge technologies can both be used to transmit large data signal transmission services such as video surveillance, which requires relatively high bandwidth.
Satellite communications
Satellite communication refers to a technology that uses artificial earth satellites as relay stations to forward radio signals, thereby realizing communication between multiple ground stations. It is the inheritance and development of terrestrial microwave communication.
The satellite communication system usually consists of two parts, namely the satellite end and the ground end. The satellite end is in the air and is mainly used to amplify the signal sent by the ground station and then forward it to other ground stations. The ground station is mainly used to control and track satellites, and to realize the connection of the ground communication system to the satellite communication system.
Satellites can be divided into synchronous satellites and non-synchronous satellites. The operating direction and period of a synchronous satellite in the air are the same as the rotation direction and period of the earth.
From any position on the ground, the satellite is stationary; the operation of a non-synchronous satellite period is greater than or less than the earth’s operating period, and its orbital height and inclination shape can be adjusted according to needs.
The characteristics of satellite communication are wide coverage, wide working frequency, good communication quality, and not being restricted by geographical conditions, and the cost having nothing to do with the communication distance.
It is mainly used in the fields of international communication, domestic communication, military communication, mobile communication, and radio and television. The main disadvantage of satellite communication is that the communication has a certain delay.
For example, when you make a satellite phone call, you cannot hear the other party’s answer immediately. The main reason is satellites. The transmission distance of communication is long, and there is a certain delay in the transmission of radio waves in the air.
Shortwave communication
According to the division of the International Radio Advisory Committee, shortwave refers to electromagnetic waves with a wavelength of 100m-10m and a frequency of 3MHZ-30MHZ. Shortwave communication refers to radio communication using shortwave, also known as high frequency (HF) communication.
Shortwave communication can be divided into ground wave propagation and skywave propagation. The attenuation of ground wave propagation increases with the increase of the operating frequency. Under the same ground conditions, the higher the frequency, the greater the attenuation.
The use of ground waves is only suitable for short-distance communication, and its operating frequency is generally selected below 5MHZ. The ground wave propagation is less affected by the weather and is relatively stable, and the channel parameters basically do not change with time, so the channel can be regarded as a constant parameter channel.
Skywave propagation is a way that radio waves are reflected by the ionosphere for long-distance communication. After the electromagnetic waves projected obliquely are reflected by the ionosphere, they can reach the ground thousands of kilometers away.
The propagation loss of sky waves is much smaller than that of ground waves. After multiple reflections between the ground and the ionosphere, they can reach extremely far places. Therefore, sky waves can be used for global communications.
Skywave propagation is extremely unstable due to the severe impact of ionospheric changes and multipath propagation, and its channel parameters change sharply with time, so it is called variable parameter channel.
The characteristics of shortwave communication are low construction and maintenance costs, short period, simple equipment, easy circuit scheduling, strong anti-destroy ability, narrow frequency band, small communication capacity, and poor stability of skywave channel signal transmission.
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