  Physical layer security has recently been regarded as an emerging technique
to complement and improve the communication security in future wireless
networks. The current research and development in physical layer security is
often based on the ideal assumption of perfect channel knowledge or the
capability of variable-rate transmissions. In this work, we study the secure
transmission design in more practical scenarios by considering channel
estimation errors at the receiver and investigating both fixed-rate and
variable-rate transmissions. Assuming quasi-static fading channels, we design
secure on-off transmission schemes to maximize the throughput subject to a
constraint on secrecy outage probability. For systems with given and fixed
encoding rates, we show how the optimal on-off transmission thresholds and the
achievable throughput vary with the amount of knowledge on the eavesdropper's
channel. In particular, our design covers the interesting case where the
eavesdropper also uses the pilots sent from the transmitter to obtain imperfect
channel estimation. An interesting observation is that using too much pilot
power can harm the throughput of secure transmission if both the legitimate
receiver and the eavesdropper have channel estimation errors, while the secure
transmission always benefits from increasing pilot power when only the
legitimate receiver has channel estimation errors but not the eavesdropper.
When the encoding rates are controllable parameters to design, we further
derive both a non-adaptive and an adaptive rate transmission schemes by jointly
optimizing the encoding rates and the on-off transmission thresholds to
maximize the throughput of secure transmissions.
