  Caching at the base stations brings the contents closer to the users, reduces
the traffic through the backhaul links, and reduces the delay experienced by
the cellular users. The cellular network operator may charge the content
providers for caching their contents. Moreover, content providers may lose
their users if the users are not getting their desired quality of service, such
as maximum tolerable delay in Video on Demand services. In this paper, we study
the collaborative caching problem for a multicell-coordinated system from the
point of view of minimizing the total cost paid by the content providers. We
formulate the problem as an Integer Linear Program and prove its
NP-completeness. We also provide an online caching algorithm that does not
require any knowledge about the contents popularities. We prove that the online
algorithm achieves a competitive ratio of $\mathcal{O}(\log(n))$, and we show
that the best competitive ratio that any online algorithm can achieve is
$\Omega(\frac{\log(n)}{\log\log(n)})$. Therefore, our proposed caching
algorithm is provably efficient. Through simulations, we show that our online
algorithm performs very close to the optimal offline collaborative scheme, and
can outperform it when contents popularities are not properly estimated.
