OFDM can well solve channel selective fading in multipath environment, but channel flat fading (that is, the fading in which the amplitude of each carrier obeys the Rayleigh distribution) has not been well overcome. OFDM that uses channel coding to solve this problem is called COFDM (Coded OFDM). The basic principle is to combine the frequency selective fading channel (frequency domain) and the time-varying flat fading channel (time domain) together to form the time-frequency domain. In this domain, the signal to be modulated with a high bit rate is divided according to certain rules and then interleaved in time and frequency. Then they are connected with a convolutional code, so that the fading suffered by the coded data signal is statistically independent. If the signal suffers a negative echo loss at a certain carrier, statistically speaking, a positive echo will appear on another carrier, and the two compensate and cancel each other. Therefore, the anti-error performance of the OFDM system is improved.

COFDM (coded orthogonal frequency division multiplexing), the abbreviation of Coded Orthogonal Frequency Division Multiplexing, is currently the most advanced and most promising modulation technology in the world. Its basic principle is to convert the high-speed data stream into several sub-channels with lower transmission rate through serial-to-parallel conversion for transmission.

Coding (C) means that the channel coding adopts a convolutional coding method with a variable coding rate to meet the protection requirements of data of different importance; Orthogonal Frequency Division (OFD) refers to the use of a large number of carriers (subcarriers), which have equal The frequency interval is an integer multiple of a basic oscillation frequency; multiplexing (M) means that multiple data sources are interleaved and distributed on the above-mentioned large number of carriers to form a channel.

1. In the middle of the last century, people proposed a multi-carrier communication scheme with frequency band aliasing, choosing mutually orthogonal carrier frequencies as subcarriers, which is what we call COFDM. This "orthogonality" refers to the exact mathematical relationship between carrier frequencies. According to this assumption, COFDM can not only make full use of channel bandwidth, but also avoid using high-speed equalization and anti-burst noise errors. COFDM is a special multi-carrier communication scheme. The information flow of a single user is serially/parallel converted into multiple low-rate code streams, and each code stream is sent with a subcarrier. Instead of using bandpass filters to separate subcarriers, COFDM uses Fast Fourier Transform (FFT) to select waveforms that remain orthogonal despite aliasing.
2. COFDM technology belongs to multi-carrier modulation (Multi-Carrier Modulation, MCM) technology. Some documents mix OFDM and MCM, which is actually not rigorous enough. MCM and COFDM are commonly used in wireless channels. The difference between them is that COFDM technology specifically refers to dividing a channel into orthogonal sub-channels, which has a high channel utilization rate; while MCM can be more channel division methods.
3. The introduction of COFDM technology is actually to improve the spectrum utilization of the carrier, or to improve the modulation of multi-carriers. Its characteristic is that each sub-carrier is orthogonal to each other, so that the spectrum after spread spectrum modulation can overlap each other, thereby reducing the Mutual interference between subcarriers. The modulation method used by each carrier of COFDM can be different. Each carrier can choose different modulation methods according to different channel conditions, such as BPSK, QPSK, 8PSK, 16QAM, 64QAM, etc., based on the principle of the best balance between spectrum utilization and bit error rate. The COFDM technology uses adaptive modulation, and selects different modulation methods according to the quality of the channel. COFDM also adopts the coordinated working mode of power control and adaptive modulation. When the channel is good, the transmission power remains unchanged, and the modulation mode (such as 64QAM) can be enhanced, or the transmission power can be reduced when the modulation mode is low (such as QPSK).
4. COFDM technology is the basis of the HPA Alliance (HomePlug Powerline Alliance) industrial specification. It uses a discontinuous multi-tone technology to combine a large number of signals in different frequencies called carriers into a single signal to complete signal transmission. Because this technology has the ability to transmit signals under clutter interference, it is often used in transmission media that are susceptible to external interference or have poor ability to resist external interference.
5. COFDM is the abbreviation of Coded Orthogonal Frequency Division Multiplexing, which is the most advanced and most promising modulation technology in the world. Its practical value lies in supporting applications that break through the limit of line-of-sight. It is a technology that makes full use of radio spectrum resources and has good immunity to noise and interference. Diffraction and penetration of obstructions are COFDM technologies. core. Its basic principle is to convert the high-speed data stream into several sub-channels with lower transmission rate through serial-to-parallel conversion for transmission.
6. COFDM technology can separate multiple digital signals simultaneously and can operate safely around interfering signals. It is precisely because of this special signal "penetrating ability" that COFDM technology is deeply loved and welcomed by communication equipment manufacturers. COFDM technology can continuously monitor the sudden change of communication characteristics on the transmission medium. The ability of the communication path to transmit data will change with time. COFDM can dynamically adapt to it, and switch on and off the corresponding carrier to ensure continuous progress. successful newsletter. COFDM technology is especially suitable for use in high-rise buildings, densely populated and geographically prominent places, areas where signals are spread, and places where high-speed data is transmitted.

1. A large amount of data can also be sent under narrow bandwidth: COFDM technology can separate at least 1000 digital signals at the same time, and the ability to operate safely around interfering signals will directly threaten the CDMA technology that has become popular in the market today. The trend of further development and growth is precisely because of this special signal "penetrating ability" that makes COFDM technology popular and popular with European communication operators and mobile phone manufacturers, such as California Cisco Systems, New York Flarion Institute of Technology and Lucent Institute of Technology and others have begun to use this technology, and the Canadian Wi-LAN Institute of Technology has also begun to use this technology.
2. COFDM technology can continuously monitor the sudden change of communication characteristics on the transmission medium: Since the ability of the communication path to transmit data will change over time, COFDM can dynamically adapt to it and switch on and off the corresponding carrier To ensure continued successful communications;
3. This technology can automatically detect which specific carrier has high signal attenuation or interference pulse under the transmission medium, and then take appropriate modulation measures to enable the carrier at the specified frequency to communicate successfully;
4. COFDM technology is especially suitable for use in high-rise buildings, densely populated and geographically prominent places, and areas where signals are spread. Both high-speed data transmission and digital voice broadcasting hope to reduce the impact of multipath effects on signals.
5. It can effectively counter the interference between signal waveforms, and is suitable for high-speed data transmission in multipath environments and fading channels. When frequency selective fading occurs due to multipath transmission in the channel, only the subcarriers falling in the frequency band depression and the information carried by them are affected, and other subcarriers are not damaged, so the overall bit error rate performance of the system is much better many.
6. Through the joint coding of each subcarrier, it has strong anti-fading ability. The COFDM technology itself has already utilized the frequency diversity of the channel, if the fading is not particularly serious, there is no need to add a time domain equalizer. By jointly encoding each channel, the system performance can be improved.
7. COFDM technology is highly resistant to narrowband interference, because these interferences only affect a small part of the sub-channels.
8. The implementation method of OFDM based on IFFT/FFT can be selected;
9. The channel utilization rate is very high, which is especially important in the wireless environment with limited spectrum resources; when the number of sub-carriers is large, the spectrum utilization rate of the system tends to be 2Baud/Hz.
10. The application of COFDM technology in wireless image transmission has the following unique advantages:
11. Applied in non-visible and obstructed environments, the excellent "diffraction" and "penetration" capabilities make it suitable for real-time wireless image transmission in urban areas, suburbs, and buildings. Traditional microwave equipment must be visible under visible conditions. (There must be no obstruction between the two sending and receiving points) to establish a wireless link channel, so the use is greatly restricted by the environment. It is necessary to inspect the application environment in advance, select and test the sending and receiving points, adjust the direction of the antenna, and calculate the height of the antenna, etc. , the workload is very heavy and cumbersome, which not only directly restricts the transmission and reception of audio and video, but also greatly reduces the reliability and work efficiency of the system.
    COFDM wireless image equipment has completely changed this situation. Due to its technical characteristics such as multi-carrier, COFDM equipment has the advantages of "non-line-of-sight" and "diffraction" transmission. Realize the stable transmission of images, not affected by the environment or affected by the environment. The system adopts an omnidirectional antenna, which can set up a wireless transmission link in the shortest time. The acquisition end and the receiving end can also move freely without being restricted by the direction. The system is simple, reliable, and flexible in application.
12. It is suitable for wireless transmission of real-time images in high-speed movement, and can use microwave (digital microwave, spread spectrum microwave) and wireless LAN equipment on platforms such as vehicles, ships, and helicopters. Due to technical system reasons, the acquisition terminal cannot be independently realized And the receiving end transmits images in real time during high-speed movement. When using microwave and wireless LAN equipment on vehicles and ships for wireless image transmission, the usual solution is to configure an additional "servo stabilization" device to solve problems such as electromagnetic wave orientation, tracking, and stabilization, but it can only be used under certain conditions. Realize the transmission of mobile point to fixed point under the environment, and the image is often interrupted, which seriously affects the effect of transmission and reception. The engineering is complex, the reliability is reduced, and the cost is extremely high.
    But for COFDM equipment, it does not need any additional devices, and it can be used between fixed-mobile, mobile-mobile, and is very suitable for installation on mobile platforms such as vehicles, ships, and helicopters. Not only the transmission has high reliability, but also shows high cost performance.
13. The transmission bandwidth is high, which is suitable for high bit rate and high image quality audio and video transmission. The image bit rate can generally be greater than 4M bps. In general digital microwave and spread spectrum microwave transmission links, although MPEG-2 encoding is used, the channel mostly adopts 2M rate, such as E1, so that the decoded image resolution can reach 720×576, but the image compressed code stream is only 1M Left and right, unable to meet the specific requirements of the receiving end for post-audio and video analysis, storage, and editing.
    Each subcarrier of COFDM technology can choose high-speed modulation such as QPSK, 16QAM, 64QAM, etc., and the combined channel rate is generally greater than 4M bps. Therefore, high-quality codec images such as 4:2:0 and 4:2:2 in MPEG-2 can be transmitted, the image resolution of the receiving end can reach 720×576 or 720×480, and the code stream can be around 6M. The image quality is close to that of DVD, which can fully meet the specific requirements of the receiving end for post-audio and video analysis, storage, and editing.
14. In a complex electromagnetic environment, COFDM has excellent anti-interference performance against frequency selective fading or narrow-band interference and interference between signal waveforms. Through the joint coding of each sub-carrier, it has a strong anti-fading ability. In single-carrier systems (such as digital microwave, spread-spectrum microwave, etc.), a single fading or interference can cause the entire communication link to fail, but in a multi-carrier COFDM system, only a small part of the sub-carriers will be interfered, and these sub-carriers The channel can also use error-correcting codes for error correction to ensure a low bit error rate for transmission.

Application of COFDM Wireless Transmission Technology in Video Surveillance
The wireless image transmission scheme using COFDM technology has good non-line-of-sight transmission and high-speed mobile transmission performance, and can provide real-time images and sounds of DVD quality. The on-site live audio and video can be transmitted flexibly and rapidly directly through vehicle-mounted or portable equipment or transmitted back to the command center through a relay station, optical fiber network, etc. The equipment can establish long-distance links with other microwave, satellite, and optical fiber communication equipment to build a practical and effective image transmission system. The main application environment of wireless image transmission equipment of COFDM technology is: urban building blocking environment, between buildings, inside and outside of buildings, between underground and ground of buildings; used in mobile; sea image, aerial image transmission, etc. It is a high-performance wireless image that is urgently needed by domestic public security, army, armed police, fire protection, civil air defense (civil defense), water conservancy, maritime affairs, customs, radio and television and other industries in security, field command, mission reconnaissance, disaster rescue, live broadcast and other tasks transmission device.

hanhsx wireless image real-time transmission monitoring system adopts COFDM technology, which can ensure stable transmission and real-time monitoring under high-speed movement, anti-fading and multi-path interference (moving speed can reach 150 km/h), and provide high-definition broadcast-level DVD quality Image, strong non-line-of-sight transmission capability, ideal for urban blocking environment applications known as extremely difficult "canyon communications", without antenna orientation. The product has the following characteristics: audio and video digital transmission, real-time image transmission, small volume of acquisition and transmitter, strong mobility, flexible and convenient, can be used by hand, encrypted transmission, good confidentiality, and the link transmission distance can reach 10KM-50KM.

The product is widely used in public security, fire protection, traffic police, civil air defense emergency, urban management law enforcement, environmental protection monitoring, fire emergency, water conservancy and flood control, electric power emergency, railway emergency, maritime law enforcement, sea surveillance inspection, customs border defense, dock monitoring, forest fire prevention, oil field anti-theft , military reconnaissance and other fields, suitable for real-time mobile transmission and monitoring of high-quality images in various complex environments such as urban areas, seas, and mountains.