  This paper studies the secrecy communication in an orthogonal frequency
division multiplexing (OFDM) system, where a source sends confidential
information to a destination in the presence of a potential eavesdropper. We
employ wireless powered cooperative jamming to improve the secrecy rate of this
system with the assistance of a cooperative jammer, which works in the
harvest-then-jam protocol over two time-slots. In the first slot, the source
sends dedicated energy signals to power the jammer; in the second slot, the
jammer uses the harvested energy to jam the eavesdropper, in order to protect
the simultaneous secrecy communication from the source to the destination. In
particular, we consider two types of receivers at the destination, namely
Type-I and Type-II receivers, which do not have and have the capability of
canceling the (a-priori known) jamming signals, respectively. For both types of
receivers, we maximize the secrecy rate at the destination by jointly
optimizing the transmit power allocation at the source and the jammer over
sub-carriers, as well as the time allocation between the two time-slots. First,
we present the globally optimal solution to this problem via the Lagrange dual
method, which, however, is of high implementation complexity. Next, to balance
tradeoff between the algorithm complexity and performance, we propose
alternative low-complexity solutions based on minorization maximization and
heuristic successive optimization, respectively. Simulation results show that
the proposed approaches significantly improve the secrecy rate, as compared to
benchmark schemes without joint power and time allocation.
