  Coded caching is a technique that leverages locally cached contents at the
end users to reduce the network's peak-time communication load. Coded caching
has been shown to achieve significant performance gains compared to uncoded
schemes and is thus considered a promising technique to boost performance in
future networks by effectively trading off bandwidth for storage. The original
coded caching model introduced by Maddah-Ali and Niesen does not consider the
case where some users involved in the placement phase, may be offline during
the delivery phase. If so, the delivery may not start or it may be wasteful to
perform the delivery with fictitious demands for the offline users. In
addition, the active users may require their demand to be kept private. This
paper formally defines a coded caching system where some users are offline, and
investigates the optimal performance with and without demand privacy against
colluding users. For this novel coded caching model with offline users,
achievable and converse bounds are proposed. These bounds are shown to meet
under certain conditions, and otherwise to be to within a constant
multiplicative gap of one another. In addition, the proposed achievable schemes
have lower subpacketization and lower load compared to baseline schemes (that
trivially extend known schemes so as to accommodate for privacy) in some memory
regimes.
