Pulse or digital communications – Systems using alternating or pulsating current – Plural channels for transmission of a single pulse train
Reexamination Certificate
1998-07-02
2001-07-10
Chin, Stephen (Department: 2734)
Pulse or digital communications
Systems using alternating or pulsating current
Plural channels for transmission of a single pulse train
C375S222000, C375S342000, C709S241000
Reexamination Certificate
active
06259743
ABSTRACT:
FIELD OF THE INVENTION
The present invention relates to communications equipment, and, more particularly, to the use of blind equalization in a receiver.
BACKGROUND OF THE INVENTION
Carrierless amplitude modulation/phase modulation (CAP) is a bandwidth-efficient two-dimensional passband line code. (Additional information on a CAP communications system can be found in J. J. Werner, “Tutorial on Carrierless AM/PM—Part I—Fundamentals and Digital CAP Transmitter,”
Contribution to ANSI X
3
T
9.5
TP/PMD Working Group
, Minneapolis, Jun. 23, 1992.) CAP is closely related to the more familiar quadrature amplitude modulation (QAM) transmission scheme. In voiceband modems, QAM has been used for over 25 years, while CAP has been used for over 15 years. However, CAP is simpler to implement digitally. Illustrative prior art transceiver structures for the QAM and CAP transmission schemes are shown in
FIGS. 1 and 2
, respectively. Both
FIGS. 1 and 2
illustrate two-dimensional encoding where a complex symbol, A
n
, is applied to the transmitter portion (where A
n
=a
n
+jb
n
), and a recovered complex symbol, Â
n
, is provided by the receiver portion, where Â
n
=â
n
+j{circumflex over (b)}
n
. With respect to other notation used in these FIGS., g(t) (e.g., see
FIG. 1
) is the impulse response of a baseband shaping filter, e
in
(t) and e
qu
(t) are equalizers for the in-phase and quadrature components, respectively, and p(t) and {tilde over (p)}(t) are impulse responses of a passband shaping filter which form a Hilbert pair (e.g., see FIG.
2
).
To observe the difference between QAM and CAP transceivers, notice in
FIG. 1
that the conventional QAM transmitter consists of a baseband pulse shaping filter followed by a modulator. The idealized QAM receiver for the case of no intersymbol interference (ISI) and additive Gaussian noise inverts these operations first using a demodulator and then a matched filter. An equivalent representation of the QAM transceiver is shown in
FIG. 3
, where the filtering and modulation operations have been reversed. This is known as the passband representation because the filtering is done at passband. To compensate for ISI, the matched filter has been replaced by an equalizer. Finally, removing the modulator and demodulator in
FIG. 3
yields the CAP transceiver of
FIG. 2
, which attains the same theoretical performance as QAM but is simpler to implement digitally.
Currently, some broadband access applications, such as VDSL (Very high rate Digital Subscriber Line), may require either a CAP receiver or a QAM receiver. Some in the art have proposed simply putting both the CAP receiver and the QAM receiver into one receiver—in effect having a dual structure receiver with a CAP section (having its own equalizer) and a separate QAM section (with its own equalizer). To further complicate matters, this dual structure receiver may require the use of blind equalization techniques in both the QAM section and the CAP section. In this case, there is no training signal for the dual structure receiver to use to identify the type of modulation. As such, the dual structure receiver must first independently converge both the equalizer in the QAM section and the equalizer in the CAP section, and then make a decision as to the type of modulation before switching to the correct steady state operating mode.
An alternative approach is described in the above-referenced U.S. patent application of Werner et al., entitled “Blind Start-Up Of A Dual Mode CAP-QAM Receiver.” In this approach, a receiver utilizes a single equalizer for supporting both a CAP mode of operation and a QAM mode of operation.
SUMMARY OF THE INVENTION
The receiver structure described in the above-referenced U.S. Patent application of Werner et al., entitled “Blind Start-Up Of A Dual Mode CAP-QAM Receiver” may require the use of an additional constellation phase compensator for correct decoding of a QAM signal. However, I have realized that this receiver structure can be further improved to eliminate the additional phase compensator. In particular, and in accordance with the invention, a dual-mode receiver uses a hybrid cost function that provides for automatic constellation phase recovery regardless of whether a CAP signal or a QAM signal has been transmitted.
In an embodiment of the invention, a receiver comprises an adaptive filter that utilizes a combined cost function during blind start-up. This combined cost function is the superposition of a cost function based on one type of received signal, e.g., QAM, with a cost function based on another type of received signal, e.g., CAP.
In another embodiment of the invention, a receiver comprises an adaptive filter that alternates between a cost function based on one type of received signal, e.g., QAM, and a cost function based on another type of received signal, e.g., CAP.
In accordance with a feature of the invention, a method is described that uses information about (a) the expected constellations, and (b) the values before and after a rotator of the receiver for deciding what type of signal is being received.
REFERENCES:
patent: 4227152 (1980-10-01), Godard et al.
patent: 5809074 (1998-09-01), Werner et al.
patent: 5835731 (1998-11-01), Werner et al.
patent: 6088389 (2000-07-01), Larson
patent: 6101217 (2000-08-01), Gut
Neil K. Jablon, C.W. Farrow, and Shao-Ning Chou, Timing recovery for blind Equalization, 1988, Maple press,p.p. 112-118.*
Neil k. Jablon, Joint Blind Equalization, Carrier Recovery, and Timing Recovery for High-Order QAM Signal Constellations, Jun. 1992, IEEE vol. 40 No. 6, p.p. 1383-1398.
Al-Beshrawi Tony
Chin Stephen
Lucent Technologies - Inc.
Opalach Joseph J.
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