Repeated preamble based carrier frequency offset estimation in the presence of I/Q imbalance

The paper proposes a novel estimation of carrier frequency offset (CFO) and I/Q imbalance, based on the popular repeated preamble. By investigating the nonlinear least squares cost function of CFO estimation, we discover a general relation among three arbitrary pilot symbols and derive several useful linear equations. Then, we propose a corresponding linear least squares estimation of unsigned CFO with explicit closed-form solution. Using the tentative CFO estimate, the I/Q imbalance can be estimated with sign ambiguity. Notice that the taps of the FIR filter representing frequency-dependent imbalance in practical frond-ends are constrained, we also propose a novel computation-free detection method to solve the sign ambiguity.