A method of processing breathing signals of a subject includes obtaining breathing signals of a subject, obtaining a signal-phase histogram using the breathing signals, wherein the signal-phase histogram comprises a plurality of data points, each of the data points having at least a phase value and a signal value, determining a reference value using at least some of the plurality of data points from the signal-phase histogram, determining whether a difference between the reference value and a signal value that is associated with a current respiratory cycle exceeds a threshold, and generating an output when the difference exceeds the threshold. A method of predicting breathing signal is also provided.
