An Enhanced Covering Lemma for Multiterminal Source Coding
  An enhanced covering lemma for a Markov chain is proved in this paper, and then the distributed source coding problem of correlated general sources with one average distortion criterion under fixed-length coding is investigated. Based on the enhanced lemma, a sufficient and necessary condition for determining the achievability of rate-distortion triples is given. 
Shengtian Yang, Peiliang Qiu