  A heterogeneous system, where small networks (e.g., small cell or WiFi) boost
the system throughput under the umbrella of a large network (e.g., large cell),
is a promising architecture for the 5G wireless communication networks, where
green and sustainable communication is also a key aspect. Renewable energy
based communication via energy harvesting (EH) devices is one of such green
technology candidates. In this paper, we study an uplink transmission scenario
under a heterogeneous network hierarchy, where each mobile user (MU) is powered
by a sustainable energy supply, capable of both deterministic access to the
large network via one private channel, and dynamic access to a small network
with certain probability via one common channel shared by multiple MUs.
Considering a general EH model, i.e., energy arrivals are time-correlated, we
study an opportunistic transmission scheme and aim to maximize the average
throughput for each MU, which jointly exploits the statistics and current
states of the private channel, common channel, battery level, and EH rate.
Applying a simple yet efficient "save-then-transmit" scheme, the throughput
maximization problem is cast as a "rate-of-return" optimal stopping problem.
The optimal stopping rule is proved to has a time-dependent threshold-based
structure for the case with general Markovian system dynamics, and degrades to
a pure threshold policy for the case with independent and identically
distributed system dynamics. As performance benchmarks, the optimal power
allocation scheme with conventional power supplies is also examined. Finally,
numerical results are presented, and a new concept of "EH diversity" is
discussed.
