  In this paper, we investigate the transmission delay of cache-aided broadcast
networks with user cooperation. Novel coded caching schemes are proposed for
both centralized and decentralized caching settings, by efficiently exploiting
time and cache resources and creating parallel data delivery at the server and
users. We derive a lower bound on the transmission delay and show that the
proposed centralized coded caching scheme is \emph{order-optimal} in the sense
that it achieves a constant multiplicative gap within the lower bound. Our
decentralized coded caching scheme is also order-optimal when each user's cache
size is larger than the threshold $N(1-\sqrt[{K-1}]{ {1}/{(K+1)}})$
(approaching 0 as $K\to \infty$), where $K$ is the total number of users and
$N$ is the size of file library. Moreover, for both the centralized and
decentralized caching settings, our schemes obtain an additional
\emph{cooperation gain} offered by user cooperation and an additional
\emph{parallel gain} offered by the parallel transmission among the server and
users. It is shown that in order to reduce the transmission delay, the number
of users parallelly sending signals should be appropriately chosen according to
user's cache size, and alway letting more users parallelly send information
could cause high transmission delay.
