  We consider a system where an agent (Alice) aims at transmitting a message to
a second agent (Bob) over a set of parallel channels, while keeping it secret
from a third agent (Eve) by using physical layer security techniques. We assume
that Alice perfectly knows the set of channels with respect to Bob, but she has
only a statistical knowledge of the channels with respect to Eve. We derive
bounds on the achievable outage secrecy rates, by considering coding either
within each channel or across all parallel channels. Transmit power is adapted
to the channel conditions, with a constraint on the average power over the
whole transmission. We also focus on the maximum cumulative outage secrecy rate
that can be achieved. Moreover, in order to assess the performance in a real
life scenario, we consider the use of practical error correcting codes. We
extend the definitions of security gap and equivocation rate, previously
applied to the single additive white Gaussian noise channel, to Rayleigh
distributed parallel channels, on the basis of the error rate targets and the
outage probability. Bounds on these metrics are also derived, taking into
account the statistics of the parallel channels. Numerical results are
provided, that confirm the feasibility of the considered physical layer
security techniques.
