Echo cancelers enable full-duplex operation for hands-free mobile telephone

A Hands-Free unit has been desired since the early development of mobile telephone. The availability of a Hands-Free unit to supplement the standard telephone handset has an obvious advantage. By using Hands-Free, the mobile subscriber has both hands available to drive the automobile while carrying on a mobile telephone conversation. The addition of a microphone and loudspeaker to existing mobile telephone equipment, however, is not trivially straightforward. Because electronic echo returning from the land telephone network is already present, the acoustic echo introduced by the coupling between loudspeaker and microphone completes a signal loop. In order to insure stability and thereby prevent oscillations, the gain around this signal loop must be less than unity. The recent development of adaptive echo canceler technology can be used to overcome these difficulties and makes Hands-Free mobile telephone with full-duplex operation feasible. This paper documents work which was undertaken to investigate the aforementioned claim. The Hands-Free problem is defined, the limitations of echo suppression are illustrated, a solution utilizing adaptive echo cancellation is presented, and experimental results are reported.