Pulse or digital communications – Transceivers – Modems
Reexamination Certificate
2000-04-03
2004-05-18
Tran, Khai (Department: 2631)
Pulse or digital communications
Transceivers
Modems
C375S219000, C370S290000
Reexamination Certificate
active
06738418
ABSTRACT:
RELATED APPLICATIONS
NONE
1. Technical Field
The present invention is generally related to the field of digital communications across a transmission line. It is particularly suited to xDSL implementations which use two-wire to four-wire conversion to allow full duplex operation. Thus, the present invention applies to ADSL Lite (ITU G.992.2), ADSL DMT, (ITU G.992.1) and ANSI T1.413 Issue 2, among others.
2. Background of the Invention
xDSL modems, such as ADSL, HDSL, SDSL and VDSL, among others, are well-known in the prior art. In such modems, a signal is sent over a twisted pair communication line which links a first transceiver to a second transceiver. A pair of xDSL modems arranged to communicate with each other use at least one established communication protocol.
One common communication protocol, found in ADSL and VDSL modems, is discrete multitone (DMT) modulation. In DMT modulation, an xDSL transmitter typically takes a complex-valued signal and places it for transmission in a DMT bin. The signal's coordinates in the complex plane signify the information that is to be sent to the receiver. Thus, in a typical protocol, one encodes two bits (00, 01, 10 or 11) per frequency bin using the complex plane. In DMT modulation, the signals typically comprise N discrete tones simultaneously carried over the twisted pair. The collection of discrete tones is commonly referred to as a symbol, and a sequence of such symbols containing a cyclic prefix, are transmitted in xDSL communications over a predetermined number of frequency bins, typically 128, 256 or 512, depending on the standard. The symbol can be sent downstream (from the ATU-C to the ATU-R) using a higher range of frequencies, or sent upstream(from the ATU-R to the ATU-C) using a lower range of frequencies. A more detailed description of xDSL communication, xDSL transceivers and equalizers can be found in U.S. Pat. No. 5,285,474 and U.S. Pat. No. 5,479,447, both to Chow et al., whose contents are incorporated by reference to the extent necessary to understand the present invention.
Much of a modem's communication functions are under the control of a signal processor. These communication functions may include such things as modulating and demodulating signals, echo cancellation, clipping mitigation, and filtering, among others. Thus, the signal processor is used to convert the transmitted and received digital signals from one form to another. The signal processor is typically implemented through a combination of hardware and executable software code. In the usual case, the signal processor includes a programmable computer, perhaps implemented as a reduced instruction set (RISC) computer, which handles only a handful of specific tasks. The processor is typically provided with at least one computer readable medium, such as a PROM, flash memory, CD-ROM, optical drive, hard drive, floppy drive or other non-volatile memory to store firmware and executable software code. The computer will usually also have a second computer readable medium, such as associated RAM or other volatile memory to provide workspace for data and additional software. The software code may all run on a single processor or controller, or may be distributed over two or more such processors or controllers.
In a given xDSL session between a pair of modems, the different frequency bins may experience different line noise levels. The noise level may depend on such factors as the crosstalk from other twisted pairs in a cable binder, and far end modem internal noise leakage, among other things, while the received signal level may depend on the length of the twisted pair transmission line. In the typical case, most frequency bins will have a signal-to-noise ratio that is sufficiently high to permit them to be used to transmit data. Other frequency bins, however, may have SNRs that are too low and so these frequency bins may not be used. These unused frequency bins represent bandwidth that is lost, and so it is generally recognized that the number of unused bins should be kept to a minimum, subject to maintaining good signal quality.
FIG. 1
shows a typical xDSL communication system
100
showing a central office modem
102
, commonly designated CO or ATU-C (ADSL transmission unit-Central), connected to a remote customer premises equipment modem
104
, commonly designated CPE or ATU-R (ADSL transmission unit-remote). The two modems
102
,
104
are connected by a twisted pair transmission line
106
, typically formed from copper or other conductor. The length of the twisted pair may vary, but is typically on the order of less than 20,000 feet, the length being dictated by the signal level transmitted from the far end transmitter, the cable attenuation of the transmitted signal, and the level of noise at the receiver Usually, in xDSL systems, one speaks of the “loop reach”, which expresses the allowable separation between the ATU-C and the ATU-R at various data transmission rates, e.g., 12,000 ft @ 1 Mbit/sec, 14,000 feet @ 900 Kbit/sec). Longer loop reach generally means that one can serve more customers, and so it is considered to be desirable to extend the loop reach as much as possible, while still maintaining good data rates.
Although modem
102
and modem
104
may have some differences due to the nature of their roles, one at ATU-C and the other at ATU-R, they have many characteristics and capabilities in common. Both modems have EMI and safety circuitry
110
a
,
110
b
, line transformer and associated filters
112
a
,
112
b
, and a hybrid circuit
114
a
,
114
b
which couples the two wires' differential mode signal from the twisted pair to the four wires (two each for the transmitter circuitry and the receiver circuitry). In addition, each has receiver circuitry
116
a
,
116
b
comprising one or more amplifiers and filters, and also transmitter circuitry
118
a
,
118
b
, also comprising one or more amplifiers and filters.
The transmitter circuitry typically includes a line driver and filter. The filtering is the combination of analog and digital frequency/time domain shaping. The filtering limits the energy contained in the regions above and below the transmitter pass band frequencies. In the case of FDM ADSL located at the remote site ATU-R, the low pass transmitter filtering limits the upstream generated signal energy which falls into the same frequency spectrum as the downstream receive spectrum. The low pass filtering does not limit the harmonic and inter-modulation distortion generated by the line driver which falls into the receive bandwidth. The upstream signal is transferred to the twisted pair interface via the hybrid. The hybrid is a 2-wire to 4-wire converter in which the 2-wire twisted pair transmission line interface is converted to a 2-wire receiver interface and a 2-wire transmitter interface.
In most modems, some transmitter energy couples from the transmitter into the receiver via the hybrid. This is called “echo”. In general, a designer of an xDSL modem does not know what the local echo power and the external noise energy in the same frequency band will be, since these parameters are a function of the operating environment. As such, it is common practice for a large “guard band” to be inserted between the upstream and the downstream transmit spectrums. The result is less stringent hardware implementation requirements for such items as A/D and D/A converters, analog filters, amplifiers, etc., but at the expense of a significant loss of data capacity for the modem user since no data can be carried within the guard band.
FIG. 2
shows a hypothetical channel for an ATU-R in which bins
6
-
29
are used for transmitting and bins
37
-
127
are used for receiving. The bins between
29
and
37
are not used because of the out-of-band energy
120
from the transmitter. Thus, these bins form a guard band, which represents unused bandwidth, and it is generally recognized that it is advantageous to reduce the width of this guard band to the extent possible. As also seen in FIG.,
2
, despite the presence of the guard b
Chen Raymond
Stiscia James J.
Day Jones
Tran Khai
Tran Khanh Cong
Virata Corporation
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