A circuit for detecting a predetermined symbol of a digital data stream includes a frequency shifter, a correlator, a filter and a decision unit. The frequency shifter performs inverse-frequency shifting upon a first data to generate a first frequency-shifted data. The correlator calculates correlation upon the first frequency-shifted data and a second data to generate a correlated data. The filter, coupled to the correlator, filters and the correlated data to generate a filtered correlated data according to a time-domain windowing length. The decision unit, coupled to the filter, determines the predetermined symbol from the digital data stream according to the filtered correlated data.