Acoustic echo is caused by acoustic coupling between microphone and loudspeaker/receiver,such as 
1 Direct air patch between microphone and speaker/receiver
2 Sound reflected by surroundings
3 Handset inside path between microphone and speaker or vibration coupling


The acoustic echo signal is picked up by microphone in the uplink side and then transmitted to the far-end 
side . Therefore , if the acoustic echo exists, the far end user will hear his/her own voice. An approach
to prevent the niticeable echo is necessary.