Time-domain equalizer of cascaded filters for VDSL

Details Time-domain equalizer of cascaded filters for VDSL Time-domain equalizer of cascaded filters for VDSL

2000-02-04

2004-02-03

Tran, Khai (Department: 2631)

Pulse or digital communications

Equalizers

Reexamination Certificate

active

06687291

ABSTRACT:

TECHNICAL FIELD
The present invention relates to a VDSL transmission system of DMT(Discrete Multi-Tone) modulation/demodulation scheme, and more particularly, to a time-domain equalizer of cascaded dual filter in a VDSL transmission system which is adapted to maintain interoperability with an ADSL transmission system.
BACKGROUND OF THE INVENTION
FIG. 1
illustrates a functional block diagram showing a transmitter and a receiver of a DMT transmission system. Core technologies in the DMT transmission system is the RS coding, the TCM(Trellis Coded Modulation), the bit loading, the TEQ(Time-domain Equalizer), the FEQ(Frequency-domain Equalizer, and modulation and demodulation by using the IFFT(Inverse Fast Fourier Transform) and the FFT.
The operation of the modulation/demodulation device(a modem) using the Fourier Transform will be described.
A frequency domain DMT symbol X=[X
0
, X
1
, - - - , X
N−1
]
T
has N complex QAM symbols, wherein each of the QAM symbol forms a subchannel to form total N subchannels. An (n)th QAM symbol Xn[an (n)th subchannel symbol] is modulated with a digital carrier P
n
=[1, e
jwn
, e
j2wn
, - - - , e
j(N−1)wn
]
T
having a length N and a frequency &ohgr;
n
=2&pgr;n/N. Therefore, (m)th DMT symbol may be represented as X
m
=[X
m,0
, X
m,1
, - - - , X
m,N−1
]
T
, wherein, if an N number of QAM symbols are modulated with digital carrier and X
m,k
value at time k is calculated, X
m,k
=X
m,0*P0,k
+X
m,1*P1,k
+- - - +X
m,N−1*PN−1,k
is obtained, where
Pn,k
=e
jkwn
of a carrier P
n
at time k. That is, it can be known that X
m,k
is a (k)th signal obtained by subjecting X
m
to Fourier transform. Accordingly, a DMT modem can modulate N QAM symbols(N subchannel symbols) at a time by using N point inverse Fourier transform. Alikely, transmitted symbols can be demodulated by using the N point Fourier transform at a receiver terminal. In the DMT modulation/demodulation type transmission system, since the modulated signal is transmitted through a baseband, a resultant of a Fourier transform of the N sub-channel symbols at the transmitter should be a real value. For this, the N point IFFT at the transmitter is required to have a Hermitian symmetry characteristic of X
m,n
=X
*
m,N−n
, making only N/2 QAM symbols available actually for transmission after the modulation using the N point IFFT.
Once the DMT symbol X
m
=[X
m,0
, X
m,1
, - - - , X
m,N−1
]
T
modulated at the transmitter passes a digital equivalent channel of v+1 impulse response length, it induces an ISI(Inter-Symbol Interference) to the following DMT symbol X
m+1
, which can be eliminated as follows. That is, a CP(Cyclic Prefix) with a length v, i.e., X
m
=[X
m,0
, X
m,1
, - - - , X
m,N−1
]
T
is added in front of the DMT symbol x
m
to provide a symbol with a length N+v before transmission from the transmitter, and a guard time for giving up the first v signals is used at the receiver. In this instance, it is required at the receiver that an impulse response length of entire channel inclusive of a linear filter and a transmission line should made to be equal to, or below v+1, which is the CP length plus 1, by employing a fixed linear filter. The linear filter used in this instance is called as a TEQ(Time-domain Equalizer).
At the receiver, the CP is removed from the N+v numbers of signals passed through the TEQ, and the transmitted symbols are demodulated by using the Fourier transform, to obtain [X
m,N−1
, X
m,N−2
, - - - , X
m,N−1
]
T
, which is in a form of multiplication of transmission signals and transmission lines, at an (n)th subchannel in an (m)th DMT symbol if there is no noise on the transmission line. The Hn denotes a response at a channel frequency &ohgr;
n
=2&pgr;n/N, a frequency characteristic of (n)th subchannel. The subchannel frequency characteristic is by using an FEQ(Frequency-domain Equalizer), which has one complex tap that is adaptively renewed by using the LMS(Least Mean Square) algorithm, to form an inversion of a transmission function of the transmission line on the whole. By doing so, an identical determination circuit can be made available at the receiver for entire subchannels, but without any enhancement of the performance.
FIG. 2
illustrates a functional block diagram showing a system of the time-domain equalizer in FIG.
1
.
As described before, in the transmission system employing the TEQ, an impulse response length of entire channel is limited to v+1 by using the TEQ, and, in order to obtain a linear filter w(D), which is the TEQ, a system as shown in
FIG. 2
is used. And, once an optimal value of the linear filter is obtained by using the system, only w(D) is used in actual data transmission.
Referring to
FIG. 2
, it can be known that, in the DMT transmission system, a transmitter output signal at an initial stage x(D) passes through a transmission filter p(t)
21
, a receiver match filter p*(−t)
23
, and a TEQ w(D)
25
, and on the same time, through an object filter b(D)
27
. A subtracter
26
subtracts a signal from the TEQ
25
from the signal from the object filter
27
, to provide a difference that is an error signal “e(D)=x(D)b(D)−y(D)w(D)”. The object filter b(D) is an adaptive linear filter having a (v+1) number of taps.
The error signal e(D) of the TEQ obtained thus should be minimized, by means of algorithms, such as MMSE-DFE(Minimum Mean Square Error-Decision Feedback Equalizer) method, LS(Least Square) method, Eigen Value method, and the like, and modified versions of the foregoing methods for easy implementation of the methods. When the error is minimized by using the above algorithm, an entire transmission function of the transmission filter p(t)
21
, the receiver match filter p*(−t)
23
, and the TEQ w(D)
25
has a (v+1) number of impulse responses, thereby the DMT modulated symbol X
m
giving no ISI to a following symbol X
m+1
. Thus, by obtaining an optimal TEQ by using a given algorithm and fixing a tap on the obtained TEQ, initialization of the TEQ is completed.
In this instance, though impulse responses of the object filter b(D) are limited to v+1, impulse responses of the TEQ w(D)
25
are not limited to v+1. Particularly, provided a number of subchannels used in the DMT transmission system increases, and a sampling frequency of a digital to analog converter and an analog to digital converter increases along with an increase of entire signal band, a length of digital equivalent impulse response of a channel having a length of time fixed physically increases. In this instance, a number of taps of the TEQ should be increased along with an increase of a size of the subchannel, for limiting impulse responses of entire channels to v+1, effectively.
The TEQ in an ADSL(Asymmetric Digital Subscriber Line) transmission system employs an FIR filter of approx. 25 taps. However, since a transmission system having maximum 4096 subchannels and a maximum sampling rate of 35.328 MHz, such as VDSL, is involved in an increase of hardware complexity as the number of subchannels increases, and has an overall operation speed increase, that impedes implementation of the TEQ by using the aforementioned method, it is required to reduce a number of taps of the TEQ, effectively.
As described, besides the problem of the hardware complexity in implementation of the TEQ in a VDSL transmission system, the VDSL transmission system has a problem of interchangeability with the ADSL transmission system. That is, the VDSL transmission system should be capable of accepting even a case when the transmitter or the receiver has the ADSL transmission system, for which the VDSL transmission system is required to maintain the TEQ system that the ADSL transmission system uses for maintaining interchangeability with the ADSL transmission system without addition of separate hard

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