SAGE-Based Prediction of IQ Imbalance in OFDMA AF-FDR Uplink Relaying Systems

Abstract

In orthogonal frequency division multiple access (OFDMA) systems, the in- and quadrature-phase imbalance (IQI) remains a critical impairment. Previous research has predominantly focused on non-cooperative settings, assuming that the modulation type is known at the receiver. This work addresses the problem of IQI in uplink full-duplex relaying (FDR) systems that employ amplify-and-forward (AF) protocols with unknown modulation and fading conditions. Realistic conditions are introduced by modeling independent IQI at each node. Additionally, we exploited the data detection stage embedded in FDR systems as a valuable auxiliary tool to support the proposed estimator. Theoretical investigation demonstrated that the exact maximum-likelihood (ML) approach to the estimation problem is prohibitively complex for practical use. To address this issue, we implemented a simplified method using the space-alternating generalized expectation-maximization (SAGE) procedure, which facilitates the recursive computation of ML estimates. Comparative simulations demonstrate the effectiveness of the proposed method and its clear improvements over existing solutions.