I. Introduction
Multilevel diversity coding system (MDCS) is a special model of multi-source networks [1], which is developed from diversity coding systems (DCS) [2]. The sources in MDCS are hierarchical, with the higher-level sources being decrypted after the lower-level ones. One subclass of MDCS is symmetric MDCS (SMDCS) [3] [4], where all decoders that access the same number of encoders have the same decoding level. Another subclass of MDCS is asymmetric MDCS (AMDCS). In [5], the most general case of AMDCS was proposed, where all the 2L − 1 access structures for L encoders correspond to different decryption levels. More recent results on general MDCS with more than three encoders and further general multi-source networks can be found in [6].