Pulse or digital communications – Systems using alternating or pulsating current – Antinoise or distortion
Reexamination Certificate
1999-12-30
2002-06-25
Le, Amanda T. (Department: 2634)
Pulse or digital communications
Systems using alternating or pulsating current
Antinoise or distortion
C375S296000
Reexamination Certificate
active
06411657
ABSTRACT:
FIELD OF THE INVENTION
The present invention relates generally to high-speed data communications, and specifically to transmission of Very High Rate Digital Subscriber Line (VDSL) signals.
BACKGROUND OF THE INVENTION
Digital Subscriber Line (DSL) is a modem technology that enables broadband digital data to be transmitted over twisted-pair wire. DSL modems allow users to access digital networks at speeds tens to hundreds of times faster than current analog modems and basic ISDN service. A range of DSL standards has been defined, known generically as “xDSL,” wherein the various standards have different data rates and other associated features but share common principles of operation. VDSL (Very High Rate Digital Subscriber Line) is the next-generation technology in the DSL family, offering data rates up to 52 Mbit/s over short runs.
DSL modems transmit data that has been line coded (i.e., modulated) n accordance with either a single-carrier or a multi-carrier modulation scheme. Single-carrier schemes for VDSL include Quadrature Amplitude Modulation (QAM) and Carrierless Amplitude Modulation (CAP). These schemes are described, for example, by Gitlin et al., in
Data Communications Principles
(Plenum Press, New York, 1992), pp. 334-347, which is incorporated herein by reference. In QAM, input data values are mapped for transmission to a sequence of symbols, each having a certain amplitude and phase. Each symbol can be represented by a complex number, which is a point in a two-dimensional “constellation” of symbols. Data for VDSL transmission may be coded before modulation, using any of a variety of suitable coding schemes known in the art, or may alternatively be uncoded.
DSL transmission channels are often subject to severe inter-symbol interference, due to amplitude distortion in the frequency domain. The accepted solution to this problem is to use a decision feedback equalizer (DFE) in the receiver, in order to cancel interference from past signals. One of the problems caused by such a DFE is error propagation, since once an error has been introduced into one of the samples, the DFE will “remember” the error over many subsequent samples.
If the channel impulse response is known, a suitable Tomlinson-Harashima precoder can be used in the transmitter, and can eliminate the need for the DFE in the receiver. Precoders of this sort are described by Wei, in an article entitled, “Generalized Square and Hexagonal Constellations for Intersymbol-Interference Channels with Generalized Tomlinson-Harashima Precoders,” published in
IEEE Transactions on Communications
, 42:9 (September, 1994), pp. 2713-2721, which is incorporated herein by reference. The precoder in this context is intended to compensate for interference in a channel having an equivalent discrete-time response expressed as
1
+
∑
i
=
1
k
h
i
Z
-
i
.
The Tomlinson-Harashima precoder comprises a two-dimensional modulo device with a negative feedback loop. The modulo device takes each complex input symbol that it receives, r, into an output symbol s given by:
s
i
=r
i
−k
i
·2
L
(1)
wherein i=1,2, giving the real and imaginary parts of s and r; 2L is the modulo value; and k
i
is an integer such that −L<s<L. In the feedback loop, the symbols output by the modulo device are filtered by a digital filter having a discrete time response based on the equivalent discrete-time response of the channel, without the zero-order time-domain component. In other words, the filter response in the feedback loop is given by
∑
i
=
1
k
h
i
Z
-
i
.
The filtered feedback symbols are subtracted from the modulated symbols (whether coded or uncoded) that are input to the precoder for transmission.
In the receiver, the channel-distorted symbols are input to a modulo device, which is identical to that in the precoder. Assuming that the feedback filter response is well-matched to the actual response of the channel, the symbols output by the modulo device in the receiver will be identical, to within the white Gaussian noise added by the channel, to the modulated symbols that were input to the precoder for transmission. The output symbols can then be processed by a decision device or Viterbi decoder, as appropriate, to recover the input data.
U.S. Pat. No. 5,249,200, to Chen et al., whose disclosure is incorporated herein by reference, describes a device and method for combining precoding with symbol-rate spectral shaping. A data transmitter, which transmits signals to a receiver over a transmission channel, includes a Tomlinson precoding unit and a spectral shaping unit. The equivalent channel response is determined and conveyed to the preceding and shaping units, which adjust the spectral properties of the transmitted signals in accordance with the determined channel response. The precoding and shaping units may also be used independently of one another.
A further difficulty in transmitting data over twisted pair at DSL rates is that a substantial amount of radio-frequency (RF) radiation is inevitably emitted. It has been found that this emission can cause serious interference with amateur radio transmissions, particularly in the HF range. For this reason, emerging technical specifications for VDSL place strict upper limits on the radiation levels that VDSL systems are allowed to generate in HF bands that are set aside for amateur radio, such as 1.81-2.0 MHz, 3.5-4.0 MHz and other, higher-frequency bands. To meet these requirements, system designers typically add notch filters in the output circuits of their modems to attenuate signals in the forbidden frequency ranges. Such notch filters complicate the design not only of VDSL transmitters, but also of receivers. The VDSL receiver must compensate not only for distortion by the communication channel, but also for the distortion introduced in the transmitter output itself by the notch filters.
The conventional solution to this problem is to use an adaptive Decision Feedback Equalizer (DFE) in the receiver with a relatively large number of taps (together with a Forward Filter Equalizer—FFE). Decision feedback equalization is described, for example, in the above-mentioned book by Gitlin et al., incorporated herein by reference, pp. 500-513. The long DFE, with many taps, is undesirable for a number of reasons, including:
Error propagation—the longer the DFE, the longer will be the error bursts due to error propagation.
The equalizer might not converge to its optimal values, resulting in a performance loss, typically of ~1 dB. Advanced adaptation methods may decrease this performance loss, but at the cost of significant additional complexity.
Slower convergence of the adaptive equalizer.
SUMMARY OF THE INVENTION
It is an object of some aspects of the present invention to provide an improved high-speed data modem.
It is a further object of some aspects of the present invention to provide methods and apparatus that enable enhanced control of the spectral profile of high-speed data transmissions.
It is still a further object of some aspects of the present invention to provide improved methods and circuitry for notch filtering of digital data transmissions.
In preferred embodiments of the present invention, a high-speed data transmitter comprises a digital transmit (Tx) filter, which filters the symbols in the transmit oath of the modem in accordance with a specified spectral profile. The profile typically includes one or more notches, such as are required for eliminating radio-frequency interference (RFI) due to the transmitter in specified, forbidden frequency bands. The symbols to be filtered by the digital Tx filter are first precoded by a Tomlinson-Harashima precoder. The precoder comprises a modulo device, as described hereinabove, and a feedback filter having a response that is substantially equal to the response of the Tx notch filter, less a zero-order time-domain component of the Tx filter response. As a result of the preceding and digital filtering, the symbols output by the transmitter have an expan
Baum David
Reuven Ilan
Verbin Rami
Abelman ,Frayne & Schwab
Le Amanda T.
Tioga Technologies Inc.
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