LLR-based coded performance analysis of cognitive dual-hop BICM systems

In this work, we analyze the performance of a cooperative two-hop cognitive relaying system using the “detect-and-forward” strategy. The cooperation between secondary users, in the underlay spectrum sharing mode, is under an interference constraint imposed by the primary network. The proposed analytical method is based on the probabilistic characterization of the bits reliability metrics given under the form of logarithmic likelihood ratios (LLRs). First, we use closed-form expressions for the probability density function of the LLRs to calculate the theoretically achievable rates and compare them with the practical throughput of a simulated turbo-coded transmission. Then, we derive tight approximations for the end-to-end coded bit error rate (BER) of the secondary transmissions, compare them with simulation results using convolutional coding schemes, and discuss their application using turbo-codes. The derivations accuracy and the system behavior are evaluated for different cooperation configurations and under different spectrum sharing conditions.