Design of voiceband echo cancellers

Abstract

High-speed full-duplex data transmission over the general switched telephone network has been achieved with the use of echo cancellers. Echo cancellers are adaptive filters used to cancel the echo which is generated in the telephone network due to impedance mismatches. Frequency offset is considered to be a major problem affecting the performance of echo cancellers. The existing solution of this problem is to use a data-driven echo canceller having a cross-coupled structure which is followed by a rotator and a phase-locked loop (PLL). A joint adaptation for the canceller taps and the PLL is performed. In this thesis, a vioceband data-driven echo canceller is designed. It is tested through simulation under various impairements with the emphasis on the frequency offset. The existing solution of frequency offset effect is also investigated through the simulation. Results of simulation show that the computation complexity and the degradation due to the phase hit are two main drawbacks of this solution. A new technique for frequency offset compensation called 'sequential adaptation' is proposed and tested here through simulation. The proposed technique is seen to be effective in terms of computation complexity and overcoming the phase hit effect. Moreover, a new approach to phase estimation during data mode is proposed. This approach utilizes the idea of the sign algorithm and contributes in the direction of reducing the complexity of the system.