Cognitive Detect-and-Forward relay channels: Soft metrics and performance analysis

In this paper, we analyze the performance of a cooperative two-hop cognitive relay system using “Detect-and-Forward” strategy where the relay demodulates the overheard signal and forwards the detected binary words. The cooperation between secondary users is based on the underlay approach, while adhering to the interference constraint on the primary user, i.e., the limited amount of interference which the primary user can tolerate. The proposed method is based on the probabilistic characterization of the reliability metrics given under the form of L-values. First, we derive analytical expressions of the cumulative distribution function and the probability density function of the L-values. Then, we use the obtained expressions to calculate the theoretically achievable rates. Simulation results demonstrate the accuracy of our derivations for different cooperation configurations and conditions.