Effects of CSI Knowledge on Secrecy of Threshold-Selection Decode-and-Forward Relaying

TL;DR

This paper analyzes how channel state information (CSI) impacts the secrecy performance of a threshold-selection decode-and-forward relay system, deriving closed-form expressions for secrecy outage probability and ergodic secrecy rate.

Contribution

It provides new analytical expressions for secrecy metrics considering CSI availability in threshold-selection DF relaying systems.

Findings

CSI improves secrecy performance, especially at low rates and SNRs.

Both hops can limit secure communication, but affect secrecy differently.

Source-eavesdropper link SNR is critical for security.

Abstract

This paper considers secrecy of a three node cooperative wireless system in the presence of a passive eavesdropper. The threshold-selection decode-and-forward (DF) relay is considered, which can decode the source message correctly only if a predefined signal-to-noise ratio (SNR) is achieved. The effects of channel state information (CSI) availability on secrecy outage probability (SOP) and ergodic secrecy rate (ESR) are investigated, and closed-form expressions are derived. Diversity is achieved from the direct and relaying paths both at the destination and at the eavesdropper by combinations of maximal-ratio combining (MRC) and selection combining (SC) schemes. An asymptotic analysis is provided when each hop SNR is the same in the balanced case and when it is different in the unbalanced case. The analysis shows that both hops can be a bottleneck for secure communication; however, they do not affect the secrecy identically. While it is observed that CSI knowledge can improve secrecy, the amount of improvement for SOP is more when the required rate is low and for ESR when the operating SNR is also low. It is also shown that the source to eavesdropper link SNR is more crucial for secure communication.