Joint IQ Imbalance and Carrier Frequency Offset Compensation Using TFI-OFDM in Cell-Free Networks

Cell-free network architectures are a promising candidate for sixth-generation (6G) communications, as densely distributed access points (APs) flexibly accommodate traffic demands and mitigate inter-cell interference. In practical cell-free systems employing direct-conversion receivers, however, performance is severely degraded by analog front-end impairments such as in-phase/quadrature (IQ) imbalance and carrier frequency offset (CFO). Conventional orthogonal frequency division multiplexing (OFDM)-based algorithms address these impairments separately, but their joint impact is insufficiently mitigated because IQ imbalance and CFO mutually interfere, leaving residual errors when either is estimated first. To overcome this, we extend our previously proposed adaptive compensation scheme based on time-frequency interferometry-OFDM (TFI-OFDM) by introducing a decision-feedback mechanism. Preliminary CFO estimation and compensation are first performed to suppress inter-symbol interference (ISI), followed by joint estimation and compensation of IQ imbalance and CFO via decision feedback, achieving accurate channel estimation with low pilot overhead. Simulation results demonstrate that the proposed scheme effectively mitigates the mutual interference of both impairments, achieving bit-error-rate (BER) performance close to an ideal impairment-free system. These results confirm that TFI-OFDM-based joint compensation with decision feedback is a promising approach for practical 6G cell-free deployments.