  In this letter, we investigate the security of a single-antenna rechargeable
source node in the presence of a multi-antenna rechargeable cooperative jammer
and a potential single-antenna eavesdropper. The batteries at the legitimate
transmitting nodes (i.e. the source node and the jamming node) are assumed to
be limited in capacity and are modeled as queueing systems. We investigate the
impact of the energy arrival rates at the batteries on the achievable secrecy
rates. In our energy-constrained network, we propose an efficient scheme to
enhance the system's security by optimizing the transmission times of the
source node. The jammer uses a subset of its antennas (and transmit
radio-frequency chains) to create a beamformer which maximizes the system's
secrecy rate while completely canceling the artificial noise at the legitimate
destination. Our numerical results demonstrate the significant average secrecy
rate gain of our proposed scheme.
