  This paper develops a new cooperative jamming protocol, termed
accumulate-and-jam (AnJ), to improve physical layer security in wireless
communications. Specifically, a full-duplex (FD) friendly jammer is deployed to
secure the direct communication between source and destination in the presence
of a passive eavesdropper. We consider the friendly jammer as an
energy-constrained node without embedded power supply but with an energy
harvesting unit and rechargeable energy storage; it can thus harvest energy
from the radio frequency (RF) signals transmitted by the source, accumulate the
energy in its battery, and then use this energy to perform cooperative jamming.
In the proposed AnJ protocol, based on the energy status of the jammer and the
channel state of source-destination link, the system operates in either
dedicated energy harvesting (DEH) or opportunistic energy harvesting (OEH)
mode. Thanks to the FD capability, the jammer also harvests energy from the
information-bearing signal that it overhears from the source. We study the
complex energy accumulation and consumption procedure at the jammer by
considering a practical finite-capacity energy storage, of which the long-term
stationary distribution is characterized through applying a discrete-state
Markov Chain. An alternative energy storage with infinite capacity is also
studied to serve as an upper bound. We further derive closed-form expressions
for two secrecy metrics, i.e., secrecy outage probability and probability of
positive secrecy capacity. In addition, the impact of imperfect channel state
information on the performance of our proposed protocol is also investigated.
Numerical results validate all theoretical analyses and reveal the merits of
the proposed AnJ protocol over its half-duplex counterpart.
