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In this paper, we propose a simple in-band optical signal to noise ratio (OSNR) monitoring method based on the beat noise RF power measurement. This method splits the incoming optical signal into two branches, and one of the branches is processed by an optical band pass filter. Then, the signals in the two branches are detected by balanced receiver and the beat noise can be calculated. The simulation results in VPItransmission 7.6 show that the power ratio between the beat noise and signal RF power could be used for OSNR estimation, which is insensitive to polarization mode dispersion impairment. Moreover, low speed photo detectors are used in this monitoring scheme providing immunity to chromatic dispersion, which also reduces the cost of the monitoring scheme.