Exact Cramer-Rao Bounds for Semi-blind Channel Estimation in Amplify-and-Forward Two-Way Relay Networks

In this paper, we derive the exact Cramer-Rao bounds (CRBs) for semi-blind channel estimation in amplifyand-forward two-way relay networks employing square QAM modulation. The derived bounds are used to show that the semiblind approach, which exploits both the transmitted pilots and transmitted data symbols, can provide substantial improvements in estimation accuracy over the training-based approach which only uses pilot symbols to estimate the channel parameters. We also derive the more tractable modified CRB which accurately approximates the exact CRB at high SNR for low modulation orders.