ECHO CANCELLATION in Wireless communication

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.   
Based on Application Specific Integrated circuit (ASIC) technology this new echo canceller utilizes high speed digital signal processing techniques to model and subtract the echo from the echo return path. More sophisticated digital interface multi-channel echo canceller systems were also developed to address new echo problems associated with long distance telephony system.

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.
In the late 1970s, COMSAT Tele Systems developed and sold first commercial   analog echo canceller, which were mainly digital devices with an analog Interface to the network. The result was a new digital echo cancellation technique that outperformed existing suppression based techniques, creating the improve network performance. The 1990s have witnessed explosive growth in the wireless telecommunications industry. With wireless telephony being widely implemented and competition increasing as new wireless carriers enter the market, superior voice transmission  quality and customers service have now become key determining factors for subscribers evaluating a carrier’s network.

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. The result of this reflection is creation of multipath echo and multiple harmonics of echo, which, unless eliminated ,are transmitted back to the distant end and are heard by the talker as echo.
There are four audio source in a vehicle:

1. Desired local speech.
2. Undesired direct coupling of remote speech from speaker to microphone
3. Undesired remote speech reflected from roof, windscreen, side windows etc.
4. Undesired local speech reflected from roof, windscreen, side windows etc.

b) Hybrid Echo
Hybrid echo is the primary source of echo generated from the public-switched telephone network (PSTN). This electrically generated echo is created as voice signals are transmitted across the network via the hybrid connection at the two-wire/four-wire PSTN conversion points, reflecting electrical energy back to the speaker from the four-wire circuit. The signal path between to telephones, involving a call other than a Local one requires amplification using a four-wire circuit. Although not a factor in itself on digital cellular networks, Hybrid echo becomes a problem in PSTN-originated calls. The cost and cabling required rules out the idea of running a four-wire circuit out to the subscriber’s premise from the local exchange. For this reason, an alternative solution had to be found. Hence, the four-wire trunk circuits were converted to two-wire local. Cabling, using a device called a ‘hybrid’
Unfortunately, the hybrid is by a nature a leaky device. As voice signals pass from the four-wire to two-wire portion of the network, the energy in the four-wire section is reflected back on itself ,creating the echo speech. Provided that the total round –trip delay occurs within just a few milliseconds (i.e. within 28ms ),it generate a sense that the is live by adding sidetone, which make a positive contribution to the quality of the call.  In cases where the total network delay exceeds 36ms however, the positive benefits disappears ,and intrusive echo results. The actual amount of signals reflected back depends on how well the balanced circuit of the hybrid matches the two-wire line. In the vast majority of cases, the match is poor, resulting in a considerable  level of signal reflecting back. This is measured as echo return loss(ERL). The higher ERL,the lower the reflected signals back to the talker and vice versa.

CAUSES OF ECHO

Acoustic echo apart, background noise  is generated through the network when analog and digital phones are operated in hands-free mode. As   additional sounds are directed and indirectly picked up by the microphone, multipath audio is created and transmitted back to the talker. The surrounding noise, whether in an automobile or in a crowded, public  environment, passes through the digital cellular vocoder, causing distorted speech for the wireline caller. Digital processing delays and speech-compression techniques further contribute to echo generation and degraded voice quality in wireless networks .Delay are encountered as signals are processed through various routes within the networks, including copper wire ,fiber optic lines, microwave connections, international gateways, and satellite transmission. This is especially true with mixed technology digital networks, where calls are processed across numerous network infrastructures. In today’s digital wireless networks, voice paths are processed at two points in the network within the mobile handset and at the radio  frequency (RF) Interface of the network. As calls are processed through  vocoders in the network, speech processing delays ranging from  80ms to 100ms are introduced, resulting in an unacceptable total end-to-end dealy of 160ms to200ms. As a result echo cancellation devices are required within the wireless network to eliminate the hybrid and acoustic echoes in a digital wireless call

THE COMBINED PROBLEM ON DIGITAL CELLULAR  NETWORKS

To deal  with  hybrid echo created by vocoder processing delays, it is mandatory for digital cellular mobile calls to have a group echo canceller installed-even for local calls. As a result , all calls on to the PSTN must pass . Through an echo canceller to remove what would otherwise be a noticeable and annoying  echo

THE PROCESS OF ECHO CANCELLATION

In modern telephone networks, echo cancellers are typically positioned in the digital circuit, The process of canceling echo involves two steps. First, as the call is set up, the echo canceller employs a digital adaptive filter to set up a model or characterization of the voice signal and echo passing through the echo canceller. As a voice path  passes back through the cancellation system, the echo canceller compares the signal and the model to cancel existing echo dynamically. This process removes more than 80 to90 percent of the echo across the network. The second process utilizes a non-linear processor (NLP) to eliminate the remaining residual echo by attenuating the signal below the noise floor.

Today’s digital cellular network technologies, namely TDMA, CDMA, and GSM, require significantly more processing power to transmit signal paths through the channels . As these technologies become even more sophisticated, echo control will be more complex. Echo cancellers designed with standard digital signal processors (DSPs), which share processing time in a circuit within a channel or across channels, provide a maximum of only 128ms of cancellation and are unable to cancel acoustic echo. With network delays occurring in excess of 160ms in today’s mixed-signal network infrastructures, a more powerful, application-specific echo-cancellation technology is required to control echo across wireless networks effectively.

CONTROLLING OF ECHO

a) Controlling Acoustic Echo
In echo cancellation, complex algorithmic procedures are used to compute speech models. This involves generating the sum from reflected echoes of the original speech, and then  subtracting this from any signal the microphone picks up. The result is the purified speech of the person talking. The format of this echo prediction must be learned by the echo canceller in a process known as adaptation. It might be said that the parameters learned from the adaptation process generate the prediction of the echo signal, which  then forms an audio picture of the room in which the microphone is located. Figure 5 shows the basic operation of an echo canceller in a conference room type of situation.
During the conversation period, this   audio picture constantly alters, and, in turn, the canceller must adapt continually. The time required for the echo canceller to fully learn the acoustic picture of the room is called the convergence time. The best convergence time recorded is 50ms, and any increase in this number results in syllables of echo being detected. Other important performance criteria involve the acoustic echo canceller’s ability to handle acoustic tail circuit delay. This is the time span of the acoustic picture and roughly represents the delay in time for the last significant echo to arrive at the microphone. The optimum requirement for this is currently set at 270ms-any time below this could result in echoes being received by the microphone outside the ability of the  echo canceller to remove them, and hence in participants hearing the echoes.
b) Controlling Complex Echo in a Wireless Digital Network
Although acoustic echo is present in every  hands-free mobile call, the amount of echo depends on the particular handset design and model that the mobile user has. On the market are a few excellent handsets that limit the echo present,  but, due to strong price   pressures, most handsets do not control  the echo very well at all-in fact, some phones on the market have been determined to have a terminal compiling loss of 24db. Echo becomes a  problem when the processing inherent to the digital wireless network adds an additional delay (typically in excess of 180ms round-trip). This combination makes for totally unacceptable call quality for the fixed network customer
This back-to-back configuration ensures a high audio quality for both PSTN and mobile customers. In addition, the echo canceller’s software configuration is designed to provide a detailed analysis of background noises, including acoustic echo from the mobile user’s end. Some echo cancellers incorporate a user-settable network delay, which enables network operators to fine-tune the echo control to suit their parameters via a menu option on the canceller’s hand-held terminal or on the network management system (NMS).

THE FUTURE OF ECHO-CANCELLATIUON TECHNOLOGY

New digital cellular networks and network scenarios reflect a significant change-taking place in the operation of echo cancellers. Instead of  being a means of simple echo control, echo cancellers have now become highly sophisticated transmission equipment at the center of highly complex networks. Network operators and telcos implementing echo cancellation across their networks will hold the key to improved call quality, directly impacting their ability to provide enhanced network performance, maintain customer loyalty, increase talk time revenues, and reduce subscriber churn.

CONCLUSION

In this paper, the nature of echo cancellation, their different types, causes, the process of echo cancellation and controlling the echo has been proposed. The operation of the proposed echo cancellation structure in such system has been described and also the controlling of echo signal are taken into account. It has been discussed that the presence of echo in the transmission signal reduces the ability to achieve the natural conversation. Applying effective echo control via the echo-cancellation platform is one way of improving the overall call clearly on digital cellular network. The two distinct areas of echo cancellation are –‘acoustic’ and ‘hybrid’ echo cancellation. The explosive growth in the wireless telecommunication industry in this decade has been a result of new digital handset in the market also the new digital network infrastructure such as TDMA. CDMA and GSM and the numerous network carriers. Improvement in network technology and echo cancellation techniques is expected to bring digital wireless telephony much closer matching wireline quality.          

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