A method of decoding an encoded audio bitstream and generating frequency bandwidth extension is disclosed. The method includes decoding the audio bitstream to produce a decoded low band audio signal and generate a low band excitation spectrum corresponding to a low frequency band. A sub-band area is identified within the low frequency band using a parameter which indicates energy information of a low band spectral envelope. A high band excitation spectrum is generated for a high frequency band by copying a sub-band excitation spectrum from the identified sub-band area to a high sub-band area corresponding to the high frequency band. Using the generated high band excitation spectrum, an extended high band audio signal is generated by applying a high band spectral envelope. The extended high band audio signal is added to the decoded low band audio signal to generate an audio output signal having an extended frequency bandwidth.