1. A subject monitoring system for determining physiological information of a subject, comprising: a sensor configured to generate an intensity signal, wherein the sensor detects light attenuated by the subject; and processing equipment coupled to the sensor, wherein the processing equipment is configured to: calculate a first autocorrelation sequence of an optical intensity signal derived from the intensity signal; calculate a second autocorrelation sequence of the optical intensity signal; combine the first autocorrelation sequence and the second autocorrelation sequence; and determine a value indicative of physiological information of the subject based on the combined autocorrelation sequence.