针对带限时变信道上的渐进图像传输，提出了一种信源信道联合编码码率分配快速算法，首先参考一组速率兼容信道码字的误码性能划分信道分区，在各分区内缩小可用信道码率集后，通过前后向码率搜索求解最佳码率分配。该算法运算复杂度低，计算次数比启发式码率搜索算法降低了一个数量级，缩短了运算时间。因此，将其应用于自适应传输系统，可根据信道状况快速调整码率分配。仿真结果表明，接收端重建图像的PSNR值始终在29.5 dB以上，同时波动范围小于4 dB，具有优良稳定的传输质量。

A fast rate allocation algorithm of joint source and channel coding is proposed for progressive image transmission on band-limited and time varying channels. Firstly the channel is segmented into several operating regions according to the error-probability performance of a set of rate-compatible channel codes. In each operating region, the number of available channel codes in the rate set is reduced, and the optimal error protection scheme is obtained by forward and backward rate searching. The proposed algorithm has a low complexity, and its computation number is only ten percent of that of the heuristic search algorithm, thus the operation time is reduced. When the fast algorithm is applied in adaptive transmission systems, the rate allocation can be quickly adjusted according to channel conditions. Simulation results show that the PSNR value of reconstructed image is always above 29.5 dB and the fluctuation range is less than 4 dB, which shows a good and stable transmission quality.