EM-Based Modulation Identification for OFDM Transmissions in FD-AF Relaying Systems

Abstract

This work provides a novel modulation identification (MI) framework for cooperative wireless networks operating with a full-duplex (FD) amplify-and-forward (AF) relay. The proposed framework is designed for an orthogonal frequency division multiplexing (OFDM) setting, enabling it to successfully tackle the issues posed by wideband frequency-selective fading channels. Unlike conventional approaches, the framework utilizes the delay produced by the relay to provide intrinsic diversity that improves the distinguishability of various modulation forms. Furthermore, channel impulse response estimation and timing recovery are performed jointly as auxiliary tasks to reinforce the reliability of the proposed MI process. An iterative expectation–maximization (EM) procedure is employed to obtain the maximum-likelihood (ML) estimates of the unknown parameters while maintaining practical computational complexity. The simulation findings show that the proposed algorithm delivers marked improvements over the existing techniques, particularly in scenarios affected by timing misalignment and channel uncertainty.