Joint Source and Channel Coding of Embedded Wavelet-based Coded Images using LDPC Codes

摘要

The transmission of images over heterogeneous networks to a large variety of terminals induces the need for efficient scalable image coding, and adapted error control mechanisms to protect the compressed bit stream against degradations caused by packet losses. In this context, we propose an optimal joint source channel coding (JSCC) approach combined with unequal error protection (UEP) of the transmitted data packets enabling an error-resilient transmission of embedded wavelet-based coded images over binary erasure channels. We theoretically show that the average expected rate-distortion function of the separately encoded subbands is convex with monotonically decreasing slopes and prove that a similar conclusion with bounds on the allowable code rates can be drawn when transmitting an increasing number of fixed-length packets. Based on these proofs we conclude that the developed JSCC-allocation process can rely on a Lagrangian-multiplier method. In order to reduce the computational complexity of the allocation mechanism, we propose a novel Viterbi-based search-algorithm, which determines for every subband the near-optimum number of packets and their corresponding code-rates. We show that our proposed solution results in significant complexity reductions while providing very near-to-optimal performance. The experimental results show that when the channel estimation matches the real channel that the use of UEP can deliver significantly better results than the use of EEP.