  As an alternative to downloading content from a cellular access network,
mobile devices could be used to store data files and distribute them through
device-to-device (D2D) communication. We consider a D2D-based storage community
that is comprised of mobile users. Assuming that transmitting data from a base
station to a mobile user consumes more energy than transmitting data between
two mobile users, we show that it can be beneficial to use redundant storage to
ensure that data files stay available to the community even if some of the
storing users leave the network. We derive a tractable closed-form equation
stating when redundancy should be used in order to minimize the expected energy
consumption of data retrieval. We find that replication is the preferred method
of adding redundancy as opposed to regenerating codes. Our findings are
verified by computer simulations.
