In the past, the wireless image transmission was mainly one-way analog TV broadcasting service, and a set of TV programs used a single frequency point. Single-frequency network can improve the utilization rate of frequency resources. However, when TV programs are broadcast at different places with the same frequency and the same frequency, they will interfere with each other. In addition, because the receiving or transmitting party is in a mobile state, both transmitting and receiving will encounter strong multipath interference, that is, echo interference. Therefore, The way to deal with echo interference may fundamentally affect the performance of a wireless high-definition video real-time transmission system.
OFDM can well solve the channel selective fading in multipath environment, but the channel flatness fading (that is, the fading that the amplitude of each carrier obeys Rayleigh distribution) has not been well overcome. OFDM which uses channel coding to solve this problem is called COFDM(Coded OFDM). Its basic principle is to combine frequency selective fading channel (frequency domain) with time-varying flat fading channel (time domain) to form time-frequency domain. In this domain, the signal to be modulated with high bit rate is divided according to certain rules, and then the time and frequency are distributed alternately. Then, they are connected by convolutional codes, so that the fading of coded data signals is statistically independent. If a signal suffers a negative echo loss at a certain carrier, statistically speaking, a positive echo will appear on another carrier, and the two compensate each other. Thereby improving the error resilience of the OFDM system.
The advantage of COFDM technology in wireless image transmission are as follows:
It is suitable for application in non-intervisibility and blocked environments such as urban areas, suburbs and buildings, and shows excellent “diffraction” and “penetration” capabilities. It is suitable for high-speed mobile transmission and can be applied to vehicles, ships, helicopters/unmanned aerial vehicles and other platforms.
COFDM equipment, which can be used in fixed-mobile and mobile-mobile rooms without any additional devices, is very suitable for installation on mobile platforms such as vehicles, ships, helicopters/unmanned aerial vehicles. Not only the transmission has high reliability, but also compared with the above schemes, it shows high cost performance because it does not need to configure additional “servo stabilization” devices. It is suitable for high-speed data transmission, and the speed is generally greater than 4M bps, which satisfies the transmission of high-quality video and audio.
COFDM has excellent anti-interference performance in complex electromagnetic environment. It has excellent performance against frequency selective fading or narrowband interference and interference between signal waveforms, and has strong anti-fading ability through the joint coding of each subcarrier.
Contact Person: Ms. Charlene Chen