Research On Key Techniques Of Wireless Video Coding And Transmission

With the rapid development of wireless communication technology and the increase of wireless networks bandwidth, it is possible to transmit multimedia content in mobile circumstance. Video communication is the kernel of multimedia operation because vision is the most important information media of human being. On the other hand, the transmission of video content over wireless networks poses big technical challenge due to the specific characteristics of wireless channel such as high bit error rate. Sponsored by National 863 project"Digital Video, Audio Encoding, Transmission, Testing and Applying Model"(No. 2002AA119010), and National Natural Science Foundation"Research on Universal Rate-Distortion Model over Wireless Channel"(No. 60372067), this dissertation is focusing on the key technologies of video coding and transmission framework aiming to decrease the computation complexity and enhance the error resilience ability.The wireless video coding and transmission system studied in this dissertation is introduced firstly. Then the theory background and characteristics in each technical component are analyzed. Afterwards, this dissertation is focusing on the key technologies of source codec and channel codec.In source coding part, two fast inter prediction modes decision algorithms are proposed to reduce the computation complexity of H.264/AVC inter prediction scheme. Based on the research of the relationship between the optimal coding mode of a macro block (MB) and its residual measure after motion estimation and compensation, the candidate inter modes used in rate distortion optimization framework can be limited in a small mode subgroup. Due to the overlapped mode groups and dynamic adjusted thresholds adopted in the algorithms, the optimal coding mode lie within the chosen mode subgroup with great possibility which leads to extensive computation reduction with acceptable loss in quality. Experiment results demonstrate the validity of the proposed algorithms.Complexity scalable video coding techniques are desirable in low-end and high-end wireless video communication applications. The complexity scalability of H.264/AVC motion estimation is explored firstly, then a complexity scalable inter prediction algorithm based on spatial correlation is proposed. This algorithm uses a probability table to relate the prediction modes of adjacent MBs to that of the current MB. For each combination of adjacent MBs, the probability table includes a list of prediction modes in order of expected occurrence. Based on this table, an ordering of the most probable, next most probable prediction mode etc. is determined. Hence the number of modes used in inter prediction can be adjusted under different complexity constraints while the optimal mode is guaranteed to lie in the chosen modes with great possibility. Experiment results show that this algorithm has 4 computation scales with gracefully degraded quality.In channel codec part, firstly the performance of Turbo codes under the influence of channel codec parameters and channel statistic is investigated. Based on the simulation results, a bit error rate (BER) model over AWGN channel is deduced using mathematical tools such as curve fitting etc. Given certain parameter set such as code rate, interleaver length and wireless channel signal to noise ratio (SNR), the BER in receiver end can be estimated using this model in sender end thus can be used in joint source-channel coding and end-to-end rate distortion optimization. Experiment results prove that this model bears high accuracy and more importantly, this model indicates that empirical model can be used in the research on Turbo codes performance while the theoretical analysis is very difficult.In source decoding part, error concealment techniques are investigated to reduce the influence of high BER in wireless channel on the reconstructed image quality in decoder. And a novel temporal-spatio error concealment scheme is proposed. Firstly a directional interpolation based spatial error concealment algorithm is used which introduces a method similar with the intra prediction in the encoder. This algorithm uses the intra prediction modes of adjacent blocks to predict the optimal intra prediction mode of current impaired block, and then the interpolation is processed in the predicted direction. Secondly an enhanced temporal error concealment algorithm is proposed. This algorithm divides the impaired MB into four subblocks. Each subblock is substituted by the values of block in the reference frame pointed to by the optimal motion vector (MV) in the candidate MV group generated by the MVs of neighboring blocks. And the principle to measure the best match is modified so that the match results could be more precise. Finally, a switch strategy between spatial error concealment algorithm and temporal error concealment algorithm is addressed. It chooses an appropriate algorithm based on the adjacent MBs coding modes in order to adapt to the impact of the scene change. Experimental results demonstrated that the algorithms proposed are prior to the algorithms in H.264/AVC both in objective and subjective quality.