In this paper, the downlink file transmission within a finite lifetime is
optimized with the assistance of wireless cache nodes. Specifically, the number
of requests within the lifetime of one file is modeled as a Poisson point
process. The base station multicasts files to downlink users and the selected
the cache nodes, so that the cache nodes can help to forward the files in the
next file request. Thus we formulate the downlink transmission as a Markov
decision process with random number of stages, where transmission power and
time on each transmission are the control policy. Due to random number of file
transmissions, we first proposed a revised Bellman's equation, where the
optimal control policy can be derived. In order to address the prohibitively
huge state space, we also introduce a low-complexity sub-optimal solution based
on an linear approximation of the value function. The approximated value
function can be calculated analytically, so that conventional numerical value
iteration can be eliminated. Moreover, the gap between the approximated value
function and the real value function is bounded analytically. It is shown by
simulation that, with the approximated MDP approach, the proposed algorithm can
significantly reduce the resource consumption at the base station.Comment: Submitted for IEEE ICC 201