  Recently, physical layer (PHY) caching has been proposed to exploit the
dynamic side information induced by caches at base stations (BSs) to support
Coordinated Multi-Point (CoMP) and achieve huge degrees of freedom (DoF) gains.
Due to the limited cache storage capacity, the performance of PHY caching
depends heavily on the cache content placement algorithm. In existing
algorithms, the cache content placement is adaptive to the long-term popularity
distribution in an offline manner. We propose an online PHY caching framework
which adapts the cache content placement to microscopic spatial and temporary
popularity variations to fully exploit the benefits of PHY caching.
Specifically, the joint optimization of online cache content placement and
content delivery is formulated as a mixed-timescale drift minimization problem
to increase the CoMP opportunity and reduce the cache content placement cost.
We propose a low-complexity algorithm to obtain a throughput-optimal solution.
Moreover, we provide a closed-form characterization of the maximum sum DoF in
the stability region and study the impact of key system parameters on the
stability region. Simulations show that the proposed online PHY caching
framework achieves large gain over existing solutions.
