Energy Efficiency of Repetition Coding and Parallel Coding Relaying Under Partial Secrecy Regime

This paper evaluates the secure energy efficiency (SEE) of a cooperative network subject to partial secrecy requirements, implemented through a fractional equivocation parameter θ ∈ (0, 1] that allows partial secrecy when θ <; 1. We assume that only the channel state information (CSI) of the legitimate channel is available, while the CSI with respect to the eavesdropper is unknown. Then, we propose a CSI-aided decode-and-forward (DF) scheme, in which the transmitter uses the available CSI in order to choose between direct and cooperative paths. Moreover, the relay employs either repetition coding (CSI-RC), i.e., source and relay use the same codebook, or parallel coding (CSI-PC), when different codebooks are used. By resorting to the Dinkelbach algorithm, we propose a joint power allocation scheme, which also optimizes θ to maximize the SEE. Our schemes are compared with the traditional DF, amplify- and-forward, and cooperative jamming (CJ). In most scenarios, CSI-RC performs best in terms of SEE. Nevertheless, we observe that CSI-PC achieves the highest SEE when θ → 1 and if the relay is close to either the transmitter or the receiver. Moreover, CJ also stands out to maximize the SEE if the relay is placed closer to the eavesdropper. In addition, the influence of θ in the system performance is evaluated, showing that a joint θ and power optimization considerably improves the SEE.