echo cancellation in wireless communicetion

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ABSRACT
In this paper, the echo cancellation network, there. Different types causes, the process of echo cancellation and controlling the echo are Present. This paper also discuss the nature of echo and how echo cancellation technique is helpful in improving the quality standards of mobile call. The echo cancellation technique reduces the background noise and remove hybrid and acoustic echo before any transcoder process.
INTRODUCTION
Now a days the use of wireless phone are increases. This wireless Phones becomes the essential part of the communication network. Day to day the Network get expand and become more complicated because of the competion between the wireless carrier. The echo is generated during the transmission in the network. In addition they eliminate double talk capabilities and greatly reducing the ability to achieve the natural conversation. An echo cancellation in such system has to deal with the cancellation of acoustic (multi-path)echoes generated with each syllables of speech.



HISTORY OF ECHO CANCELLATION
The late 1950s marked the birth of echo control in the telecommunication. Industry with the development of the first echo suppression devices. Although echo suppressers reduced echo caused by transmission problems in the network, they also reduced in choppy first syllables and artificial volume adjustment. Echo cancellation theory was developed in early 1960s by AT &T Bell Labs, by COMSAT Tele Systems. COMSAT design the first analog echo canceller system to demonstrate feasibility and performance of satellite communication networks. These systems were not commercially viable, however, because of their size and manufacturing costs.



TYPES OF ECHO
a) Acoustic echo
Acoustic echo is generated with analog and digital handsets, With the degree of echo related to the type and quality of equipment used. This form of echo is produced by poor voice coupling between erapiece and microphone in handsets and hands-free devices. Further voice degradation is caused as a voice compressing encoding /decoding devices (vocoders) process the voice paths within the handsets and in wireless networks. This results in returned echo signals with highly variable properties. Acoustic echo was first encountered with the early video/audio conferencing studios and –as figure 1 shows-now also occurring typical mobile situations, such as peoples are driving theirs cars. In this situations sound from a loudspeakers is heard by a listener, as intented. However, this same sound also is picked up by the microphones, both directly and indirectly, after bouncing off the roofs, windows, and seats of the car.