This paper investigates optimal power allocation
strategies over a bank of independent parallel Gaussian wiretap
channels where a legitimate transmitter and a legitimate receiver
communicate in the presence of an eavesdropper and a friendly
jammer. We characterize the optimal power allocation strategy
for the jammer in several scenarios, providing algorithm to
obtain the optimal solutions. In our scenario, which is applicable
to current OFDM communications systems, we demonstrate
that the optimal jammer power allocation strategy can lead to
considerable secrecy gains and we provide a set of results that
cast further insight into the problem.