Study On Echo Cancellation For Telecommunication Systems

Echo cancellation has been a bottleneck in modern telecommunication systems. This paper firstly introduced the cause of the echoes and basic principles of two kinds of echo cancellations technologies, line echo cancellation (LEC) and acoustic echo cancellation (AEC), as well as the disadvantages of these two technologies. Subsequently, it researched on one of the most important basic adaptive algorithm for echo cancellation, LMS Algorithm, which is the abbreviation name for Least Mean Square Algorithm. However, LMS's convergence behavior depends on the power spectrum of input signals-the quicker input signals fluctuate, the slower the LMS converges.Normalized LMS Algorithm could resolve this problem. For white noise signal, where all frequencies have the same energy, the NLMS performs well. But the human speech has more energy in low frequencies then in high frequencies. Therefore, a NLMS provides good echo cancellation for low frequencies and poor echo cancellation for high frequencies. A pre-whitening filter in front of the echo cancellation filter transforms human speech into something more "white noise" alike - the energy of high frequency signals is similar to the energy of low frequency signals. The processing accuracy of NLMS can then be improved greatly. At the end of the paper, the simulation was given with C language and Matlab, which well proved the views above.