Several techniques for reducing the rare words effects have been introduced in the literature, including spelling expansion (Habash, 2008), dictionary term expansion (Habash, 2008), proper name transliteration (Daumé and Jagarlamudi, 2011), treating words as a sequence of characters (Luong and Manning, 2016), subword units (Sennrich et al., 2015), and reading text as bytes (Gillick et al., 2015).
most of these techniques still have no mechanism for handling low frequency characters, which are the target of this work.