Scalable Source Coding With Causal Side Information and a Causal Helper

We consider a successive refinement source coding model in which each receiver observes its own side-information causally and is required to form its reconstruction in a causal manner. Furthermore, a one way conference link of given capacity allows Decoder 1 - the helper - to send causal descriptions of what it has received so far to Decoder 2. A complete characterization of the rate distortion region is provided. The optimal helper is a scalar quantizer of its side-information which depends on the corresponding first-stage message symbol sent by the encoder. Thus, the imposition of the causality constraint on the side informations as well as on the helper's conference both renders the problem tractable and simplifies the optimal code.