The Degraded Poisson Wiretap Channel

TL;DR

This paper analyzes the secrecy capacity of the degraded Poisson wiretap channel in wireless optical communication, providing a closed-form expression and optimal coding strategies to enhance security.

Contribution

It offers the first closed-form secrecy capacity expression for the degraded Poisson wiretap channel and characterizes the rate-equivocation region with explicit code construction.

Findings

Closed-form secrecy capacity expression derived.

Complete rate-equivocation region characterized.

Optimal code construction using structured Wyner code.

Abstract

Providing security guarantees for wireless communication is critically important for today's applications. While previous work in this area has concentrated on radio frequency (RF) channels, providing security guarantees for RF channels is inherently difficult because they are prone to rapid variations due small scale fading. Wireless optical communication, on the other hand, is inherently more secure than RF communication due to the intrinsic aspects of the signal propagation in the optical and near-optical frequency range. In this paper, secure communication over wireless optical links is examined by studying the secrecy capacity of a direct detection system. For the degraded Poisson wiretap channel, a closed-form expression of the secrecy capacity is given. A complete characterization of the general rate-equivocation region is also presented. For achievability, an optimal code is explicitly constructed by using the structured code designed by Wyner for the Poisson channel. The converse is proved in two different ways: the first method relies only on simple properties of the conditional expectation and basic information theoretical inequalities, whereas the second method hinges on the recent link established between minimum mean square estimation and mutual information in Poisson channels.