  Coded caching schemes with low subpacketization and small transmission rate
are desirable in practice due to the requirement of low implementation
complexity and efficiency of the transmission. Placement delivery arrays (PDA
in short) can be used to generate coded caching schemes. However, many known
coded caching schemes have large memory ratios. In this paper, we realize that
some schemes with low subpacketization generated by PDAs do not fully use the
users' caching content to create multicasting opportunities and thus propose to
overcome this drawback. As an application, we obtain two new schemes with low
subpacketizations, which have significantly advantages on the memory ratio and
transmission rate compared with the original scheme.
