Secrecy Outage Capacity of Fading Channels

TL;DR

This paper investigates the limits of secure communication over fading channels under outage constraints, proposing strategies for different levels of channel information and analyzing key buffer effects.

Contribution

It extends outage capacity to secrecy constraints, providing new capacity characterizations and optimal power control policies for scenarios with varying channel knowledge.

Findings

Optimal power control combines channel inversion and ergodic strategies.

Secrecy outage capacity is characterized under different CSI assumptions.

Key buffer overflow impacts overall outage probability.

Abstract

This paper considers point to point secure communication over flat fading channels under an outage constraint. More specifically, we extend the definition of outage capacity to account for the secrecy constraint and obtain sharp characterizations of the corresponding fundamental limits under two different assumptions on the transmitter CSI (Channel state information). First, we find the outage secrecy capacity assuming that the transmitter has perfect knowledge of the legitimate and eavesdropper channel gains. In this scenario, the capacity achieving scheme relies on opportunistically exchanging private keys between the legitimate nodes. These keys are stored in a key buffer and later used to secure delay sensitive data using the Vernam's one time pad technique. We then extend our results to the more practical scenario where the transmitter is assumed to know only the legitimate channel gain. Here, our achievability arguments rely on privacy amplification techniques to generate secret key bits. In the two cases, we also characterize the optimal power control policies which, interestingly, turn out to be a judicious combination of channel inversion and the optimal ergodic strategy. Finally, we analyze the effect of key buffer overflow on the overall outage probability.