  To attain the targeted data rates of next generation cellular networks
requires dense deployment of small cells in addition to macro cells which
provide wide coverage. Dynamic radio resource management is crucial to the
success of such heterogeneous networks due to much more pronounced traffic and
interference variations in small cells. This work proposes a framework for
spectrum management organized according to two timescales, which include 1)
centralized optimization on a moderate timescale corresponding to typical
duration of user sessions (several seconds to minutes in today's networks), and
2) distributed spectrum allocation on a fast timescale corresponding to typical
latency requirements (a few milliseconds). An optimization problem is
formulated to allocate resources on the slower timescale with consideration of
(distributed) opportunistic scheduling on the faster timescale. Both fixed and
fully flexible user association schemes are considered. Iterative algorithms
are developed to solve these optimization problems efficiently for a cluster of
cells with guaranteed convergence. Simulation results demonstrate advantages of
the proposed framework and algorithms.
