| Information era has significantly changed people's communication methods. People prefer to develop broadband wireless communication devices that are integrated with video, voice and data and that support high-speed mobile transmission rather than being satisfied with low-speed digital services such as traditional voice service. However, there are limited resources and much interference in wireless channels, and heavy traffic and high real-time demand for video signals. Aiming at these problems, this thesis analyzes how COFDM technology is used to implement wireless video transmission under the condition of high moving speed.After briefly introducing COFDM technology theories, its major advantages and shortcomings, this thesis focuses on the whole design of COFDM-based clear video transmission system. Multi-path fading and doppler shift of wireless mobile channels would generate interference to signals, as a result, waveform of the received signal would be distorted, and channels would be interrupted if the interference is severe. Therefore, during system design, cascade coding combining Reed-Solomon codes and convolutional codes is utilized; in addition, time-direction and frequency-direction interleaves are added to alleviate channel fading. Channels would be estimated by inserting boosted pilots at the transmitting terminals. To adapt to different situations, many parameters are available and two working modes are designed, one is 2K mode and the other is 8K mode.When signals pass high power amplifier, they would be distorted in amplitude and phase, leading to spectrum spread and distortion of in-band signals, both of which would weaken performance of the whole system in turn. To reduce the nonlinear distortion of power amplifier, this thesis puts forward a improved technology named adaptive predistortion. It uses amplitude Predistorter to recompose amplifier AM/AM characteristic and then to inverse, and uses phase Predistorter to recompose AM/PM characteristic. The Matlab simulation result shows that if the bit error rate has the order of 10-4, and 16-QAM constellation modulation is applied, this technology is capable of reducing amplifier optimized output back-off by 7dB. |
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