Pulse or digital communications – Equalizers – Automatic
Reexamination Certificate
1998-08-28
2001-10-16
Chin, Stephen (Department: 2634)
Pulse or digital communications
Equalizers
Automatic
C375S350000, C708S322000, C708S323000
Reexamination Certificate
active
06304598
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
This invention relates to systems for, and methods of, providing for the transmission and reception of signals through unshielded twisted pairs of wires within a communications system. The invention particularly relates to systems for, and methods of, reducing the power dissipation in devices within a communications system and to systems for, and methods of, reducing such power dissipation in communications systems having high throughputs. A “high throughput” as used within the context of this disclosure may include, but is not limited to, one gigabit (GB) per second.
2. Description of Related Art
A basic communications system is illustrated in FIG.
1
. The system includes a hub and a plurality of computers serviced by the hub in a local area network (LAN). Four computers are shown by way of illustration but a different number of computers may be contained within the system. Each of the computers is usually displaced from the hub by a distance which may be as great as approximately one hundred meters (100 m.). The computers are also displaced from each other. The hub is connected to each of the computers by a communications line. Each communication line includes unshielded twisted pairs of wires or cables. Generally, the wires or cables are formed from copper. Four unshielded twisted pairs of wires are provided in each communication line between each computer and the hub. The system shown in
FIG. 1
is operative with several categories of unshielded twisted pairs of cables designated as categories
3
,
4
and
5
in the telecommunications industry. Category
3
cables are the poorest quality (and lowest cost) and category
6
and
7
cables are the best quality (and highest cost).
Associated with each communications system is a “throughput”. The throughput of a system is the rate at which the system processes data and is usually expressed in bits/second. Most communications systems have throughputs of 10 megabits (Mb)/second or 100 Mb/second. A rapidly evolving area of communications system technology enables 1 Gb/second full-duplex communication over existing category-
5
unshielded twisted pair cables. Such a system is commonly referred to as “Gigabit Ethernet.”
A portion of a typical Gigabit Ethernet is shown in FIG.
2
. The Gigabit Ethernet provides for transmission of digital signals between one of the computers and the hub and the reception of such signals at the other of the computer and the hub. A similar system can be provided for each of the computers. The system includes a gigabit medium independent interface (GMII) block which receives data in byte-wide format at a specified rate, for example 125 MHz, and passes the data onto the physical coding sublayer (PCS) which performs scrambling, coding, and a variety of control functions. The PCS encodes bits from the GMII into 5-level pulse amplitude modulation (PAM) signals. The five symbol levels are −2, −1, 0, +1, and +2. Communication between the computer and hub is achieved using four unshielded twisted pairs of wires or cables, each operating at 250 Mb/second, and eight transceivers, one positioned at each end of a unshielded twisted pair. The full-duplex bidirectional operation provides for the use of hybrid circuits at the two ends of each unshielded twisted pair. The hybrid controls access to the communication line, thereby allowing for full-duplex bidirectional operation between the transceivers at each end of the communications line.
A common problem associated with communications systems employing multiple unshielded twisted pairs and multiple transceivers is the introduction of crosstalk and echo noise or impairment signals into the transmission signals. Noise is inherent in all such communications systems regardless of the system throughput. However, the effects of these impairment signals are magnified in Gigabit Ethernet. Impairment signals include echo, near-end crosstalk (NEXT), and far-end crosstalk (FEXT) signals. As a result of these impairment signals the performance of the transceivers, particularly the receiver portion, is degraded.
NEXT is an impairment signal that results from capacitive and inductive coupling of the signals from the near-end transmitters to the input of the receivers. The NEXT impairment signals encountered by the receiver in transceiver A are shown in FIG.
3
. The crosstalk signals from transmitters B, C, and D appear as noise to receiver A, which is attempting to detect the direct signal from transmitter E. Each of the receivers in the system encounters the same effect and accordingly the signals passing through the receivers experience signal degradation due to NEXT impairment signals. For clarity of
FIG. 3
, only the NEXT impairment experienced by receiver A is illustrated.
Similarly, because of the bidirectional nature of the communications systems, an echo impairment signal is produced by each transmitter on the receiver contained within the same transceiver as the transmitter. The echo impairment signal encountered by the receiver in each transceiver is shown in FIG.
4
. The crosstalk signals from transmitters appear as noise to the receivers, which are attempting to detect the signal from the transmitter at the opposite end of the communications line. Each of the receivers in the system encounters the same effect and accordingly the signals passing through the receivers experience signal distortion due to the echo impairment signal.
Far-end crosstalk (FEXT) is an impairment that results from capacitive coupling of the signal from the far-end transmitters to the input of the receivers. The FEXT impairment signals encountered by the receiver in transceiver A are shown in FIG.
5
. The crosstalk signals from transmitters F, G, and H appears as noise to receiver A, which is attempting to detect the direct signal from transmitter E. Each of the receivers in the system encounters the same effect and accordingly the signals passing through the receivers experience signal distortion due to the FEXT impairment signal. For clarity of
FIG. 5
only the FEXT impairment experienced by receiver A is illustrated.
Four transceivers at one end of a communications line are illustrated in detail in FIG.
6
. The components of the transceivers are shown as overlapping blocks, with each layer corresponding to one of the transceivers. The GMII, PCS, and hybrid of
FIG. 6
correspond to the GMII, PCS, and hybrid of FIG.
2
and are considered to be separate from the transceiver. The combination of the transceiver and hybrid forms one “channel” of the communications system. Accordingly,
FIG. 6
illustrates four channels, each of which operates in a similar manner. The transmitter portion of each transceiver includes a pulse-shaping filter and a digital-to-analog (D/A) converter. The receiver portion of each transceiver includes an analog-to-digital (A/D) converter, a first-in first-out (FIFO) buffer, a digital adaptive equalizer system including a feed-forward equalizer (FFE) and a detector. The receiver portion also includes a timing recovery system and a near-end noise reduction system including a NEXT cancellation system and an echo canceller. The NEXT cancellation system and the echo canceller typically include numerous adaptive filters.
Characteristics of the communication line, e. g., length, may impact the ability of the NEXT cancellation system and echo cancellers to effectively cancel NEXT and echo noise. Measurements of typical cable responses, as well as simulation, show that in order to provide an adequate level of cancellation of these sources of interference, “long” echo and NEXT cancellers are required. The term “long” is used to describe a canceller having a large number of taps as necessitated by the characteristics of the cable. For example
FIG. 7
shows the echo impulse response for a 100 m cable with a characteristic impedance of 85 ohm and 100 ohm terminations. Although the nominal characteristic impedance is 100 ohm, manufacturing standards allow for a 15% tolerance. The mismatch in impeda
Agazzi Oscar E.
Creigh John L.
Hatamian Mehdi
Broadcom Corporation
Chin Stephen
Christie, Parke & Hale, LLP
Ha Dac V.
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