Pulse or digital communications – Receivers – Automatic gain control
Reexamination Certificate
1998-11-25
2002-09-17
Chin, Stephen (Department: 2734)
Pulse or digital communications
Receivers
Automatic gain control
C375S229000
Reexamination Certificate
active
06452987
ABSTRACT:
FIELD OF THE INVENTION
This invention relates generally to communications and, more particularly, to high-speed data communications systems.
BACKGROUND OF THE INVENTION
Plain Old Telephone Service (POTS) is typically deployed to individual subscribers over a twisted pair of wires. Today, in addition to voice services, more and more subscribers want high-speed data access to, e.g., the Internet, over this twisted pair. A technology that increases the transmission capacity over a twisted pair is Asymmetric Digital Subscriber Line (ADSL), which provides transmission capabilities of millions of bits per second (Mbps),
One standard for ADSL transmission is ANSI T1.413 (e.g., see “Standards Project for Interfaces Relating to Carrier to Custom Connection of Asymmetrical Digital Subscriber Line (ADSL) Equipment”,
T
1
E
1
Specification,
1997). This standard specifies the use of Discrete Multi-Tone (DMT) modulation. DMT modulation is a form of orthogonal frequency division multiplexing (OFDM) and utilizes multiple carriers (also sometimes referred to as subcarriers, channels, or bins; hereafter the term bin is used) for conveying information. In DMT modulation, the allocated frequency range is divided into K bins, K>1, each bin separated by approximately 4 kHz. In such an approach, a DMT-based ADSL system transmits what is referred to as “multi-tone symbols” or “DMT symbols.”
In discrete multitone (DMT) receivers, a frequency domain equalizer (FEQ) is used to converge the symbol constellation for each bin. The FEQ is normally a one-tap complex finite impulse response (FIR) filter adapted by a Least Mean-Square (LMS) based algorithm (e.g., see, Chow, J. K., Tu, J. C. and Cioffi, J. M., “A Discrete Multitone Transceiver System for HDSL Applications,”
IEEE J. on Selected Areas in Commun.,
vol. 9,
no.
Aug., 6, 1991; and Lee, I. Chow J. S. and Cioffi, J. M., “Performance Evaluation of a Fast Computation Algorithm for the DMT in High-Speed Subscriber Line,”
IEEE J on Selected Area in Commun.
vol. 13,
no.
9, 1995). Unfortunately, the convergence rate is slow (e.g., on the order of 20 secs.) if the FEQ is updated by the LMS algorithm or most other standard equalization algorithms. This slow convergence is a result of the dependency of convergence rate on received signal power. For example, for an 8 kft (kilo-feet) loop, received signals (bins) at high frequencies have about 45 dB more power attenuation than the received bins at low frequencies—resulting in different convergence rates for different parts of the received DMT signal.
SUMMARY OF THE INVENTION
We have realized a method and apparatus that improves convergence speed and enhances tracking capability in a DMT receiver. In particular, and in accordance with the principles of the invention, a DMT receiver uses an automatic gain control (AGC) element and a rotator element for use in equalizing the received DMT signal in each bin. As a result, the dependency of convergence rate on input power is reduced.
In an embodiment of the invention, during start-up a DMT receiver uses an automatic gain control (AGC) element and a rotator element in place of a one-tap complex finite impulse response (FIR) filter (as noted above updated using the LMS algorithm). This technique is applicable to receivers using non-blind equalization and blind equalization. In the latter case there is an additional benefit—it eliminates the need for a training sequence.
REFERENCES:
patent: 5809074 (1996-11-01), Werner et al.
patent: 5793807 (1998-08-01), Werner et al.
patent: 6137809 (2000-10-01), Martinez et al.
patent: 6219378 (2001-04-01), Wu
patent: 6272108 (2001-08-01), Chapman
patent: 6307889 (2001-10-01), Chun
G. A. Dumont et al., “The Multimodulus Blind Equalization Algorithm”, (IEEE, Proc. Thirteenth Int'l. Conference on Digital Signal Processing, Santorini, Greece, 1997) pp. 127-130.
U.S. patent application Ser. No. 09/109,364, entitled “Blind Start-Up Of A Dual Mode CAP-QAM Receiver”, filed Jul. 2, 1998.
Larsson Patrik
Werner Jean-Jacques
Yang Jian
Chin Stephen
Kim Kevin
Lucent Technologies - Inc.
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