Multiple descriptions with feed-forward: A single-letter achievable rate region

The two-channel multiple descriptions problem for an i.i.d source, with feed-forward to one or both side-decoders is considered. A single-letter achievable rate-region is derived; it enlarges the best known rate-region for multiple descriptions without feed-forward. The proof of the result uses a block-Markov superposition source coding strategy. In point-to-point source coding, feed-forward does not decrease the rate-distortion function of an i.i.d source. In contrast, an example is provided to show that the derived region can be larger than the optimal multiple description rate region without feed-forward.