In the past, wireless image transmission was mainly based on one-way analog TV broadcast services. A single TV program uses a single frequency point. A single-frequency network can improve the utilization of frequency resources. When the TV programs are broadcast frequently, they will interfere with each other.
In addition, because the receiving or transmitting party is in a moving state, both transmitting and receiving will encounter strong multipath interference, that is, echo interference. Therefore, the processing method of echo interference may fundamentally affect a wireless high-definition video. For the performance of real-time transmission systems, OFDM can well solve the channel selective fading in a multipath environment, but the flatness fading of the channel (that is, the fading of the amplitude of each carrier following 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 a frequency-selective fading channel (frequency domain) with a time-varying flat fading channel (time domain) to form a time-frequency domain.
In this domain, high-bit-rate signals to be modulated are divided according to certain rules, and then time and frequency interleaved distribution is performed. Then use convolutional codes to connect them, so that the fading suffered by the encoded data signal is statistically independent. If the signal suffers a negative echo loss at one carrier, statistically a positive echo will appear at another carrier, and the two compensate and cancel each other. So as to improve the anti-error performance of the OFDM system.
The advantages of COFDM technology applied to wireless image transmission are as follows:
It is suitable for applications in non-viewing and blocking environments such as urban areas, suburbs, and buildings, and exhibits excellent "diffraction" and "penetration" capabilities. Suitable for high-speed transmission, it can be applied to platforms such as vehicles, ships, helicopters and drones.
COFDM equipment, which does not require any additional equipment, can achieve fixed-mobile, mobile-mobile use, is very suitable for installation on vehicles, ships, helicopters / drones and other mobile platforms. Not only does the transmission have high reliability, but compared with the above scheme, it does not need to be equipped with an additional "servo stable" device, so it shows a high cost performance. Suitable for high-speed data transmission, the rate is generally greater than 4M bps, which meets the transmission of high-quality video and audio.