Multiplex communications – Duplex – Transmit/receive interaction control
Reexamination Certificate
1998-08-28
2001-10-02
Cangialosi, Salvatore (Department: 2661)
Multiplex communications
Duplex
Transmit/receive interaction control
C379S403000, C379S404000
Reexamination Certificate
active
06298046
ABSTRACT:
FIELD OF THE INVENTION
This invention relates generally to hybrid circuits, and more particularly to adaptive hybrid circuits for use with two-conductor transmission lines in full-duplex communications.
BACKGROUND OF THE INVENTION
In full-duplex communications over a two-conductor transmission line, it is desirable to remove from a composite receive signal that portion due to reflection of a local transmit (Tx) signal, to thereby yield a corrected receive (Rx) signal which closely approximates the true signal broadcasted from a remote transmitter. Such correction is generally accomplished using a signal separator, or “hybrid,” and a subtractor.
FIG. 1
shows an abbreviated electrical block diagram of a telecommunications system
100
, which includes a telecommunications apparatus
102
operative to communicate with a telecommunications apparatus
104
over a transmission line
106
. Telecommunications apparatus
102
, which may be a modem, typically includes a transceiver having a transmitter
108
and a receiver
110
, a conventional hybrid circuit
112
, a subtractor
114
, and a line interface
116
. Telecommunications apparatus
104
includes the same or similar components, at least a transceiver with a transmitter
118
and a receiver
120
coupled to a line interface
122
.
Conventional hybrid circuit
112
typically includes a conventional (2-port) balancing network
124
, a transmit source impedance (Z
S
)
126
, which is typically resistive, and a transmission line input impedance (Z
i
). Circuitry of
FIG. 1
is diagramed in an unbalanced (or ground-referenced) form for simplification, although balanced (or ungrounded) circuitry is frequently employed in practice.
Conventional balancing network
124
seeks to develop a voltage V
B
which is equal to, in magnitude and phase over the frequency range of interest, a reflected transmit signal component of voltage V
A
. Thus, a subtractor
114
may yield a subtractor output voltage V
C
=V
A
−V
B
which is equal to the incoming receive signal. For this to work, the voltage transfer function V
B
/V
Tx
of conventional balancing network
124
must be equal to that of the Z
S
-to-Z
i
voltage divider (Z
i
/Z
S
+Z
i
). This implies that conventional balancing network
124
must include elements that emulate the magnitude and phase variation behavior of Z
i
over the frequency range of interest.
Typically, transmission line
106
is a twisted pair of insulated conductors whose impedance magnitude as a function of frequency behaves as shown in a graph
500
of FIG.
5
. More particularly, graph
500
is representative of a twisted pair of insulated copper wires having AWG gauges
19
through
26
, typically used in cables for telephony, Integrated Services Digital Network (ISDN), Digital Subscriber Lines (xDSL), and related communication formats. A line impedance curve
502
(shown in solid) represents a long (e.g., 10,000 feet (3048 meters)), unimpaired, terminated transmission line. A line impedance curve
504
(shown in dotted) represents a transmission line impaired by a long bridged tap located near the line's input. These bridged taps are often installed on the main lines in anticipation of line sharing and are typically inaccessible and of unknown length. The phase behavior of such transmission lines is closely and predictably related to the impedance-magnitude behavior and, for brevity, is not separately shown. Conventional hybrids have been designed to accommodate these monotonically-decreasing impedance versus frequency behaviors at low frequencies, such as voiceband frequencies (generally about 300-3000 hertz (Hz) and low data rate communication frequencies (tens of kilohertz (kHz) and below, as in modems operative at 14.4 through 56 kilobits per second (kbps)).
FIGS. 2
,
3
, and
4
show various conventional balancing networks
200
,
300
, and
400
, respectively, for use in conventional hybrid circuit
112
of FIG.
1
. Each of conventional balancing networks
200
,
300
, and
400
includes an impedance structure
202
and an impedance structure
204
, with nodes
128
and
130
for coupling within conventional hybrid circuit
112
as indicated in FIG.
1
.
Conventional balancing network
300
of
FIG. 3
provides a fixed one-pole, one-zero transfer function, and has been proposed for 784 kbps 2B1Q High-data-rate Digital Subscriber Lines (HDSL) transceivers where the frequencies of primary interest extend to about 200 KHz. See “Generic Requirements for High-Bit-Rate Digital Subscriber Lines,”
Bellcore Technical Advisory
TA-NWT-001210, Issue 1, October 1991; and W. Y. Chen et al., “High Bit Rate Digital Subscriber Line Echo Cancellation,”
IEEE Journal on Selected Areas in Communications
, Vol. 9, No. 6, August 1991. Conventional balancing network
400
of
FIG. 4
implies adjustability of these pole and zero locations, which is described in detail in U.S. Pat. No. 4,096,362 (Crawford). Conventional balancing network
400
may include a magnitude-scaling component to accommodate the presence of bridged-tap line impairments, since the low frequency impact of such impairments simply uniformly scales the impedance magnitude over frequency. Multiple-pole, multiple-zero networks are also available to provide an arbitrarily close match to a given monotonically-decreasing impedance characteristic. Analysis and measurements show that such conventional balancing networks, whether fixedly or adaptively configured, are somewhat useful for unimpaired lines or for impaired lines with long bridged taps.
When one or more short bridged taps are present near a transmission line input, however, a conventional hybrid with one of these conventional balancing networks is found to be highly inadequate—or even counterproductive. In this context, a short bridged tap is one whose length is between roughly one-sixteenth wavelength at the maximum frequency of interest and several thousand feet. For example, short tap lengths would range from about two hundred to several thousand feet for 784-kbps HDSL.
FIG. 6
is a diagram which illustrates such a transmission line environment. A bridged tap
604
is tapped between transmission line sections
602
and
606
of a main line terminated by an impedance
608
. Here, bridged tap
604
has a length Y
2
located at distance Y
1
from the line's input. A distance Y
T
=Y
1
+Y
2
is the total distance from the line input to the end of the tap. While only a single local tap is shown, multiple local taps may also be present in such an environment.
FIG. 7
is a graph
700
showing some line impedance versus frequency behaviors of the environment shown in FIG.
6
. The numerical impedance values shown in graph
700
represent AWG #26 twisted-pair lines. A line impedance curve
702
(shown in solid) represents the behavior of a long unimpaired line; a line impedance curve
704
(shown in dotted) represents the behavior of a long line having a bridged tap of length 800 feet (244 meters) near the line's input; and a line impedance curve
706
(shown in dashed) represents a long line having a bridged tap of length 1600 feet (488 meters) near the line's input. These tap-impaired impedance behaviors are non-monotonic and oscillatory, and are not adequately emulated by conventional hybrids. The frequencies of impedance minima and maxima depend on tap length. Thus, a conventional hybrid designed for an 800-foot tap would badly mismatch a 1600-foot tap.
FIG. 8
is a graph
800
revealing a further complication in impedance matching. Line impedance curves
802
,
804
, and
806
of
FIG. 8
each correspond to a combined distance of 800 feet (244 meters) from the input to the end of the tap (distance Y
T
in the preceding discussion), but with different tap lengths and locations: line impedance curve
802
(shown in solid) represents an 800-foot (244-meter) tap at the input; line impedance curve
804
(shown in dotted) represents a 600-foot (183-meter) tap located 200 feet (61 meters) from the input; and line impedance curve
806
(shown in dashed) represents a
Cangialosi Salvatore
Cary Gray
RC Networks
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