Channel Estimation for Full-Duplex OFDM-Based Ambient Backscatter Communication Systems With I/Q Imbalance

Ambient backscatter communication (AmBC) has attracted significant attention as a viable paradigm for energy efficient communication in the next generation of Internet-of-Things (IoT). Channel estimation, a vital task in AmBC receiver design, becomes more challenging when accounting for more practical scenarios such as frequency selective fading and the presence of hardware impairments such as I/Q imbalance. Full-duplex communication further complicates channel estimation, due to the presence of self-interference. In this work, we consider OFDM-based full-duplex AmBC systems affected by I/Q imbalance, and propose two methods for the joint estimation of the channel and I/Q imbalance. The first algorithm is a pilot-based estimator, while the second is a semi-blind estimator based on the concept of decision-directed (DD) estimation. In addition, we propose a novel design of appropriate pilot sequences to optimize the performance of the proposed methods. As benchmarks on estimation performance, we obtain analytical expressions for the pilot-based and semi-blind Cramer-Rao bounds (CRBs). Our simulations demonstrate that both proposed estimators converge to their respective bounds. The proposed methods offer different tradeoffs between accuracy and computational complexity, which are suitable for different use cases.