Multiplex communications – Duplex – Transmit/receive interaction control
Reexamination Certificate
1998-06-10
2001-11-13
Cangialosi, Salvatore (Department: 2661)
Multiplex communications
Duplex
Transmit/receive interaction control
C370S286000
Reexamination Certificate
active
06317419
ABSTRACT:
FIELD OF THE INVENTION
The present invention relates generally to echo cancellation techniques in a digital communication system. More particularly, the present invention relates to an echo canceler training scheme that contemplates the presence of digital impairments or other memoryless nonlinearities within the echo channel.
BACKGROUND OF THE INVENTION
56 kbps modems are very soon to be standardized in accordance with the V.90 modem recommendation. V.90 modems employ pulse code modulation (PCM) techniques and take advantage of the digital nature of the public switched telephone network (PSTN) to obtain a higher data rate in the downstream direction, i.e., from a server modem to a client modem.
FIG. 1
depicts a conceptual diagram of a typical 56 kbps communication path using current PCM modem technology. A central site, such as an internet service provider (ISP)
100
, is digitally connected to a telephone network
130
through a transmitter
110
and a receiver
120
resident at an ISP modem
105
. The network
130
is connected to a local loop
150
through a central office line card
140
. The line card typically has a PCM codec implemented therein. The local loop
150
is connected to the user's personal computer (PC)
170
at the user's site through the user's modem
160
. As can be appreciated by those skilled in the art, the connection between the ISP modem transmitter
110
to the telephone network
130
is a digital connection that supports a typical data rate of about 64 kbps. Since the parameters of the telephone network
130
and line card
140
are dictated and set by the operating specifications of the network (and particularly the use of the &mgr;-law or A-law signal point constellations), the central site transmitter
110
is configured to transmit the digital data in a particular way to fully exploit its digital connection to the network.
FIG. 2
illustrates a portion of a digital server modem
200
, which includes a transmitter
202
and a receiver
204
. As with many practical data communication systems, near-end echo (represented by an echo path
206
) associated with a transmit signal may be present in a signal received by server modem
200
. The characteristics of the near-end echo signal may be dictated by functional components in the upstream and downstream channels and/or processing performed within the telephone network. The echo signal combines with the intended receive signal and the “corrupted” receive signal is then processed by server modem
200
. An echo canceler
208
is employed by server modem
200
to compensate for the near end echo. In an ideal modem system, a duplicate echo signal generated by echo canceler
208
is subtracted from the signal received by server modem
200
.
In V.90 modems, the presence of digital impairments in echo path
206
complicates the echo cancellation process. These digital impairments, such as robbed bit signaling (RBS) and digital pads, are nonlinear in nature and usually memoryless; conventional echo cancellation techniques for modeling the overall echo path using a linear time invariant (LTI) filter may not provide optimal results. Instead, the echo path
206
can be effectively modeled as a nonlinear memoryless mapping, an LTI filter, and another nonlinear memoryless mapping. Furthermore, current echo cancellation techniques for V.90 server modems have several shortcomings. One proposed solution utilizes a probing signal that is transmitted by digital server modem
200
and received by the analog client modem. The client modem detects and analyzes the downstream digital impairments and sends information back to server modem
200
. The information is utilized by echo canceler
208
to establish and model the characteristics of the downstream digital impairments in addition to analog aspects of echo path
206
. This technique is undesirable because server modem
200
must rely upon the detection and analytical capabilities of the client modem. Performance errors caused by the client modem can adversely affect the server modem echo cancellation procedure and can adversely affect the ultimate resolution of the receive signal.
Furthermore, the current scheme for sending back the mapping information from the analog client modem to the digital server modem
200
assumes that the digital impairment source always maps one PCM level to another PCM level. This assumption may not be valid for some situations. In addition, the above proposed scheme only analyzes the downstream characteristics of echo path
206
. The client modem has no way to directly determine those aspects of echo path
206
that are associated with any upstream digital impairment features of the modem system. Although server modem
200
may be configured to estimate such upstream digital impairment features in echo path
206
, an adaptive and direct measurement of echo path
206
may be more robust and reliable.
Finally, the accuracy of this proposed echo cancellation technique may not be optimal. Due to elements in the downstream path over which the probing, signal must travel, the amount of noise in the probing signal may exceed desirable levels. Excessive noise levels or nonlinearities present in the downstream channel (which may not necessarily exist in echo path
206
) can corrupt the determination of the downstream digital impairment characteristics and can lead to inaccurate echo cancellation.
SUMMARY OF THE INVENTION
Accordingly, it is an advantage of the present invention that an improved echo cancellation technique suitable for V.90 server modems is provided.
Another advantage of the present invention is that it provides an echo canceler training technique that is dedicated to the server modem such that information from the client modem is unnecessary.
Another advantage is that the present invention can employ an echo canceler training procedure that utilizes a line probing signal which also serves as a downstream digital impairment learning signal.
A further advantage is that the present invention may be configured to compensate for digital impairments and other nonlinear distortions that may be present in the near-end echo path.
Another advantage is that the preferred echo canceler training process does not assume that the digital impairment sources always map one PCM level to another PCM level.
Another advantage is that a server modem echo canceler training technique according to the present invention is more robust and reliable than prior art techniques.
Another advantage is that the present invention provides a server modem echo canceler training scheme having less exposure to system noise than previous solutions.
The above and other advantages of the present invention may be carried out in one form by a method for training and adaptive updating of a server modem echo canceler in a data communication system having a transmitter configured to transmit signals over a downstream communication channel and a receiver configured to receive signals over an upstream communication channel. where echo signals follow an analog echo path from the transmitter to the receiver. The preferred method involves transmitting a training signal from the transmitter, determining, in response to the training signal, digital impairments present within the echo path, and obtaining, in response to the training signal, the transfer characteristics of the echo path.
REFERENCES:
patent: 4571720 (1986-02-01), Chevreau et al.
patent: 4780883 (1988-10-01), O'Connor et al.
patent: 4970715 (1990-11-01), McMahan
patent: 4977591 (1990-12-01), Chen et al.
patent: 4987569 (1991-01-01), Ling et al.
patent: 5329586 (1994-07-01), Agazzi
patent: 5761247 (1998-06-01), Betts et al.
patent: 5764699 (1998-06-01), Needham et al.
patent: 5889823 (1999-05-01), Agazzi et al.
patent: 5909426 (1999-06-01), Liau et al.
patent: 5982768 (1999-11-01), Bellenger et al.
patent: 6058110 (2000-05-01), Bellenger et al.
patent: 0 708 537 A1 (1995-02-01), None
patent: WO 98/08310 (1997-05-01), None
patent: WO 98/17044 (1997-10-01), None
patent: WO 99/37051 (1998-01-01), Non
Olafsson Sverrir
Zhou Zhenyu
Cangialosi Salvatore
Conexant Systems Inc.
Farjami & Farjami LLP
LandOfFree
Method and apparatus for training an echo canceler in a PCM... does not yet have a rating. At this time, there are no reviews or comments for this patent.
If you have personal experience with Method and apparatus for training an echo canceler in a PCM..., we encourage you to share that experience with our LandOfFree.com community. Your opinion is very important and Method and apparatus for training an echo canceler in a PCM... will most certainly appreciate the feedback.
Profile ID: LFUS-PAI-O-2590332