  Cloud radio access network (C-RAN) has been recognized as a promising
architecture for next-generation wireless systems to \textcolor{black}{support}
the rapidly increasing demand for higher data rate. However, the performance of
C-RAN is limited by the backhaul capacities, especially for the wireless
deployment. While C-RAN with fixed BS caching has been demonstrated to reduce
backhaul consumption, it is more challenging to further optimize the cache
allocation at BSs with multi-cluster multicast backhaul, where the
inter-cluster interference induces additional non-convexity to the cache
optimization problem. Despite the challenges, we propose an accelerated
first-order algorithm, which achieves much higher content downloading sum-rate
than a second-order algorithm running for the same amount of time. Simulation
results demonstrate that, by simultaneously delivering the required contents to
different multicast clusters, the proposed algorithm achieves significantly
higher downloading sum-rate than those of time-division single-cluster
transmission schemes. Moreover, it is found that the proposed algorithm
allocates larger cache sizes to the farther BSs within the nearer clusters,
which provides insight to the superiority of the proposed cache allocation.
