Acoustic echo elimination in a digital mobile communications sys

Telecommunications – Transmitter and receiver at same station – Radiotelephone equipment detail

Patent

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Details

455561, 455296, 379407, 370289, H04B 700

Patent

active

060817321

DESCRIPTION:

BRIEF SUMMARY
The invention relates to a method and arrangement for eliminating acoustic echo generated in a mobile station in a digital mobile communications system.
On end-to-end connections of a data transmission system, such as a telephone network, long propagation delays often occur, as a result of which e.g. echo is detected in the case of normal speech when a signal is reflected from the far end of the connection back to the transmitting party.
Mainly two factors contribute to generating an echo: acoustic echo between the receiver and the microphone of a telephone, and electric echo, which is generated in the transmission systems of the transmission and reception directions of the connection.
Major sources of electric echo are hybrid circuits (2-wire to 4-wire transformers), which are located in terminal exchanges or at the remote subscriber stages in the fixed network. Subscriber lines of a fixed network are usually 2-wire lines for economical reasons. Connections between exchanges, in turn, are usually 4-wire lines.
As defined herein, the far end is that end of the transmission connection to which the speaker's own end returns as an echo, and the near end is that end of the transmission connection from which the echo is reflected back. Typically, the near end is a mobile station and the far end is another party, such as a PSTN subscriber.
Problems caused by returned echo are usually endeavoured to eliminate by means of an echo canceller or an echo suppressor. The echo canceller is a device processing a signal, such as a speech signal and used for reducing the echo by reducing the estimated echo from the echo (signal) occurring on the connection. The echo suppressor, in turn, disconnects the signal arriving from the near end when echo is present.
Prior art digital mobile communications systems are provided with echo cancellers, which prevent an echo returning from the public switched telephone network (PSTN) from being transmitted to the mobile subscriber. In mobile exchanges, echo cancellers of this kind are usually placed in the trunk circuits between the exchanges.
Echo returning from a mobile station is usually cancelled by means of an echo canceller placed in the actual mobile station. Such an echo canceller is usually based on an adaptive filter or comparing the levels of an output signal and an input signal. There are a large number of mobile stations in use nowadays in which the echo cancellation does not work sufficiently well, but a relatively low level, yet disturbing echo is transmitted to another party. In principle, the problem can be reduced by developing echo elimination methods for mobile stations, but it mainly improves the situation as far as new mobile station are concerned. Instead, it is difficult to update the software or equipment of the mobile stations that are already in use, because the mobile stations are already in possession of their users, and collecting them for service measures is time-demanding and costly. In the mobile communications system, there will thus always be such mobile stations whose echo elimination does not work sufficiently well, but causes disturbing echo to the other party. In digital mobile communications systems, speech transmission also takes place entirely digitally. From the point of view of the mobile network, the most limited resource is the radio path between the mobile stations and the base stations. In order to reduce the bandwidth required by one radio connection on the radio path, speech coding is employed in the transmission of speech, thus achieving a lower transfer rate, e.g. 16 or 8 kbit/s, compared with the transfer rate of 64 kbit/s typically used in the telephone networks. Both the mobile station and the mobile network must naturally comprise a speech encoder and a speech decoder for speech coding. On the side of the network, the speech coding functions may be placed in many alternative locations, such as at the base station or in association with the mobile exchange. Thus, in each mobile-terminating or -originating speech call, the speech conn

REFERENCES:
patent: 5222251 (1993-06-01), Roney, IV et al.
patent: 5995827 (1999-11-01), Gitlin et al.
GSM System for Mobile Communications; Michel Mouly, Marie Bernadette Pautet, 1992, pp. 216, 232-234, 249-259.
Recommendation GSM 05.04, Jan. 1991, pp. 1-3.
Recommendation GSM 05.05, Mar. 1991, pp. 1-19.
Recommendation GSM 05.10, Jan. 1991, pp. 1-6.
Patent Abstract of Japan, vol. 16, No. 550, E-1292, Abstract of JP,A, 4-207825 Nov. 19, 1992.

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