Progressive optimal error protection of embedded codes

The embedded wavelet image coders such as SPIHT [1] or JPEG 2000 [2] allow an efficient progressive transmission of digital images. However, the bitstreams generated by these coders are very sensitive to channel noise, and error protection is necessary to ensure acceptable reconstruction fidelity. One of the most successful protection systems for embedded wavelet coders was recently proposed by Sherwood and Zeger [3] who partitioned the bitstream of information bits into packets of fixed length and protected these packets by using a concatenation of a cyclic redundancycheck (CRC) coder for error detection and a rate-compatible punctured convolutional (RCPC) coder for error correction. A challenging task is to find an optimal error protection for such systems [4]; that is, an assignment of the available channel rates to the packets that minimizes the expected reconstruction error (measured, for example, by the peaksignal-to-noise ratio (PSNR)) subject to a total bit rate constraint. An alternative proposed in [4] is to maximize the expected number of error-free received source bits for this target total bit rate. Though suboptimal in the PSNR sense, this approach has two advantages. First, an optimal protection can be determined with a fast algorithm [4, 5]. Second, the solution is independent of both the source coder and the test image. Thus, the algorithm can also be implemented by the receiver, avoiding the need for sending side information, which would have to be strongly protected