  In this paper, secure transmission of information over fading broadcast
channels is studied in the presence of statistical queueing constraints.
Effective capacity is employed as a performance metric to identify the secure
throughput of the system, i.e., effective secure throughput. It is assumed that
perfect channel side information (CSI) is available at both the transmitter and
the receivers. Initially, the scenario in which the transmitter sends common
messages to two receivers and confidential messages to one receiver is
considered. For this case, effective secure throughput region, which is the
region of constant arrival rates of common and confidential messages that can
be supported by the buffer-constrained transmitter and fading broadcast
channel, is defined. It is proven that this effective throughput region is
convex. Then, the optimal power control policies that achieve the boundary
points of the effective secure throughput region are investigated and an
algorithm for the numerical computation of the optimal power adaptation schemes
is provided. Subsequently, the special case in which the transmitter sends only
confidential messages to one receiver, is addressed in more detail. For this
case, effective secure throughput is formulated and two different power
adaptation policies are studied. In particular, it is noted that opportunistic
transmission is no longer optimal under buffer constraints and the transmitter
should not wait to send the data at a high rate until the main channel is much
better than the eavesdropper channel.
