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Transmitter in-phase and quadrature (Tx IQ) mismatch is one of the major obstacles preventing coherent optical orthogonal frequency division multiplexing systems from achieving high data rates. Several Tx IQ mismatch compensation methods at the receiver side have been proposed based on the mismatch estimation by the use of pilot signals. In this letter, we propose a novel pilot system for the Tx IQ mismatch estimation to improve the channel-estimation performance. The pilot scheme exploits the conjugate redundancy of the subcarrier mirror images due to the Tx IQ mismatch and allows the estimation of the Tx IQ mismatch factor and channel parameter independently. The advantages of the proposed algorithm are proved via mean square error analysis, and the bit error rate improvement is shown in simulations.