  Information leakage rate is an intuitive metric that reflects the level of
security in a wireless communication system, however, there are few studies
taking it into consideration. Existing work on information leakage rate has two
major limitations due to the complicated expression for the leakage rate: 1)
the analytical and numerical results give few insights into the trade-off
between system throughput and information leakage rate; 2) and the
corresponding optimal designs of transmission rates are not analytically
tractable. To overcome such limitations and obtain an in-depth understanding of
information leakage rate in secure wireless communications, we propose an
approximation for the average information leakage rate in the fixed-rate
transmission scheme. Different from the complicated expression for information
leakage rate in the literature, our proposed approximation has a low-complexity
expression, and hence, it is easy for further analysis. Based on our
approximation, the corresponding approximate optimal transmission rates are
obtained for two transmission schemes with different design objectives. Through
analytical and numerical results, we find that for the system maximizing
throughput subject to information leakage rate constraint, the throughput is an
upward convex non-decreasing function of the security constraint and much too
loose security constraint does not contribute to higher throughput; while for
the system minimizing information leakage rate subject to throughput
constraint, the average information leakage rate is a lower convex increasing
function of the throughput constraint.
