Adapting Rate and Power for Maximizing Secrecy Energy Efficiency

We present the optimum rate and power adaptation rule that maximizes the average secrecy energy efficiency (SEE) subject to an average transmission power constraint. The SEE is defined as the outage secrecy capacity, the largest secrecy rate, such that the outage probability is less than a certain value, divided by the total power consumption (bits per joule). We characterize the SEE gain provided by varying the rate and/or the power, and discuss the impact of the number of antennas on the optimum adaptation rule.