  How to design proper content placement strategies is one of the major areas
of interest in cache-enabled cellular networks. In this paper, we study the
probabilistic content placement optimization of base station (BS) caching with
cooperative transmission in the downlink of cellular networks. With placement
probability vector being the design parameter, non-coherent joint transmission
(NC-JT) and coherent joint transmission (C-JT) schemes are investigated
according to whether channel state information (CSI) is available. Using
stochastic geometry, we derive an integral expression for the successful
transmission probability (STP) in NC-JT scheme, and present an upper bound and
a tight approximation for the STP of the C-JT scheme. Next, we maximize the STP
in NC-JT and the approximation of STP in C-JT by optimizing the placement
probability vector, respectively. An algorithm is proposed and applied to both
optimization problems. By utilizing some properties of the STP, we obtain
globally optimal solutions in certain cases. Moreover, locally optimal
solutions in general cases are obtained by using the interior point method.
Finally, numerical results show the optimized placement strategy achieves
significant gains in STP over several comparative baselines both in NC-JT and
C-JT. The optimal STP in C-JT outperforms the one in NC-JT, indicating the
benefits of knowing CSI in cooperative transmission.
