  Caching is an efficient way to reduce network traffic congestion during peak
hours, by storing some content at the user's local cache memory, even without
knowledge of user's later demands. Maddah-Ali and Niesen proposed a two-phase
(placement phase and delivery phase) coded caching strategy for broadcast
channels with cache-aided users. This paper investigates the same model under
the constraint that content is placed uncoded within the caches, that is, when
bits of the files are simply copied within the caches. When the cache contents
are uncoded and the users' demands are revealed, the caching problem can be
connected to an index coding problem. This paper focuses on deriving
fundamental performance limits for the caching problem by using tools for the
index coding problem that were either known or are newly developed in this
work. First, a converse bound for the caching problem under the constraint of
uncoded cache placement is proposed based on the "acyclic index coding converse
bound". This converse bound is proved to be achievable by the Maddah-Ali and
Niesen's scheme when the number of files is not less than the number of users,
and by a newly derived index coding achievable scheme otherwise. The proposed
index coding achievable scheme is based on distributed source coding and
strictly improves on the widely used "composite (index) coding" achievable
bound and its improvements, and is of independent interest. An important
consequence of the findings of this paper is that advancements on the coded
caching problem posed by Maddah-Ali and Niesen are thus only possible by
considering strategies with coded placement phase. A recent work by Yu et al.
has however shown that coded cache placement can at most half the network load
compared to the results presented in this paper.
