Repermutation of MPEG Bitstream for Nonuniform 64 QAM Transmission

• Main Authors: Ching-Chih Kuo, 郭清渠
• Other Authors: Wen-Thong Chang
• Format: Others
• Language: en_US
• Published: 1994
• Online Access: http://ndltd.ncl.edu.tw/handle/53276455708408556985

碩士 === 國立交通大學 === 電信研究所 === 82 === The DCT coefficients in transform coding block are of different importance for the image quality. Therefore, during transform, they should be given different pro- tection to ensure that the most important coefficients are correctly received. In the standard multilevel uni- form QAM, all the bits in a symbol are treated as of equal importance, therefore, it is not optimal for the transmission of the transform coded video sequence. The idea of embedded modulation is an extention of the standard QAM. In this modulation,the constellation is partioned into groups such that the modulation distances of the most significant bits inter the groups are in- creased at the sacrifice of the modulation distances of the least significant bits within the group. With such constellation, transmited bits are given unequal pro- tection to fit the characteristics of the transform coe- fficients. For a two level grouping, the original Huffman code sequence is divided into two parts. The first parts consists of the lower order DCT coefficients and motion vectors and the second part consists of the higher order DCT coefficients. In 64 QAM modulation, the 6 bits pattern is formed with 2 bits from the first sequence and 4 bits from the second sequence. With such arrangement, under equal transmission power, most of the bit errors will occur in the higher order coefficients. The consequence will have significant impact on the subsequent error concealment. With standard uniform modulation,each coefficient is subject to equal error probability. While with embedded modulation, error is likely to happen only in the higher order coefficients such that the subsequent error correction will be much simplified. The objective of this study is to simulate the joint source- coding/modulation system and to evaluate its per- formance.