Study Of Adaptive Video Coding Method Based On The Quality Of Wireless Channel Transmission

Rail transit is a national key development derection because of its large transportation capacity and punctuality. Train-ground wireless communication technology is one of key technologies to ensure its operation reliability and safety. Because of the narrow bandwidth of the traditional GSM-R (Global System for Mobile Communications -Railway) train-ground communication system, it can not meet the needs of real-time video surveillance and other large amounts of data transmission. Applying some new high-bandwidth technologies such as 4G/LTE (Long Term Evolution) in the field of wireless train-ground communication has much broader application prospects. Using 4G network to accomplish the train-ground communication can greatly reduce operating costs, but a single transmission channel of 4G has some problems such as instability, poor reliability and signal coverage. This paper mainly studies the multimode wireless train-ground communication technology and video adaptive coding technology to meet the needs of real-time train video surveillance. The main work of this paper is:(1) A multi-channel wireless communication system with channel selection strategy is designed. By detecting some performance parameters of the current available networks and applying the analytic hierarchy process method and entropy method to determine the weight of each parameter, a comprehensive assessment parameter is set to periodically evaluate the status of the current available wireless networks, and then select an appropriate channel to transmit video stream data.(2) The average bit rate algorithm of the X264 encoder is improved based on the bitrate control principle of H.264. Target bitrate can dynamically adjust to the current network status. A compensation algorithm is added to modify the frame layer quantitative parameter (QP) which is calculated by the original algorithm based on the difference between the target bitrate and actual output bitrate.And the coding buffer size setting is improved according to the difference.(3) A simple train-ground video surveillance system has been built. On the vehicle side which is built on the embedded platform, two 3G/4G modules compose a dual-channel multimode wireless communication network, a USB camera is used for the video input. A program is designed to achieve the wireless network status detection, video capturing, coding and sending process. On the server side, VLC player is applied to decode and display. The feasibility of the multi-channel wireless communication channel selection strategy and the improved average bitrate (ABR) algorithm are verified on this platform.The experimental results shows that the improved average bit rate (ABR) algorithm has good adaptability to the current wireless network and can easily meet the requirements of real time and quality performance in video monitoring.