A physiological monitoring system may determine respiration information from a PPG signal. The system may analyze the PPG signal with respect to itself by associating values of the PPG signal with values of a time-delayed version of the PPG signal to create pairs of associated values. A subset of associated values may be identified. Respiration metric values may be determined based on the subset of pairs. The respiration metric values may be amplitude values and/or time values corresponding to the subset of pairs. The respiration metric values may be analyzed using autocorrelation, cross-correlation, or other signal processing techniques to determine respiration information such as respiration rate.
