Pulse or digital communications – Equalizers – Automatic
Reexamination Certificate
1998-12-18
2001-07-10
Bocure, Tesfaldet (Department: 2631)
Pulse or digital communications
Equalizers
Automatic
Reexamination Certificate
active
06259729
ABSTRACT:
BACKGROUND OF THE INVENTION
The present invention relates to a method of and an apparatus for training tap coefficients of an adaptive equalizer, and particularly to the method of and the apparatus for optimizing tap coefficients of a FIR (Finite Impulse Response) filter used as an adaptive equalizer to equalize a multicarrier data signal that has been transmitted through a distorting channel.
In a multicarrier data transmission system, input digital data are grouped into blocks, called symbols, of a certain number of parallel bits. The parallel bits of a symbol are further divided into a plurality of bit sets, and each of the bit sets is used to modulate each one of the same number of carrier signals of different frequencies. A preferred method of modulation/demodulation is a modulation to use an IFFT (Inverse Fast Fourier Transformation) and a demodulation to use a FFT (Fast Fourier Transformation).
FIG. 12
is a block diagram illustrating a system configuration of the multicarrier data transmission system, having a transmitter
300
comprising an encoder
120
, an IFFT circuit
130
and a D/A (Digital to Analog) converter
140
, and a receiver
400
for receiving a multicarrier data signal transmitted from the transmitter
300
through a transmission channel
200
, comprising an A/D (Analog to Digital) converter
410
, a FFT circuit
430
, a FEQ (Frequency-domain EQualization) circuit
440
and a decoder
450
. As to an adaptive equalizer
420
and a training circuit
500
, they will be described afterwards.
When a symbol consists of binary data of 512 bits for modulating 256 carrier signals, for example, the encoder
120
divides the binary data into 256 sets of 2 bits, and encodes each n-th (n=1 to 256) component of a 256- dimensional frequency-domain vector representing the 256 carrier signals by n-th of the 256 sets of 2 bits, as follows. When the logic of n-th 2-bit set is {0, 0}, {0, 1}, {1, 0} or {1, 1}, the n-th component of the frequency-domain vector is encoded as 1+j, 1−j, −1+j or −1−j, for example, j being an imaginary.
The frequency-domain vector thus encoded is transformed into a time-domain digital signal by the IFFT circuit
130
and converted by the D/A converter
140
into an analog signal to be transmitted through the transmission channel
200
as the multicarrier data signal.
The multicarrier data signal received by the receiver
400
is sampled and converted into a time-domain digital signal by the A/D converter
410
and further transformed into a frequency-domain vector by the FFT circuit
430
. The FEQ circuit
440
performs frequency-domain equalization of the frequency-domain vector for compensating distortion of the frequency-domain vector due to attenuation and delay caused through the transmission channel
200
, and the decoder
450
reproduces the symbol data by decoding each component of the frequency-domain vector.
However, when duration of the impulse response of the transmission channel
200
is not negligible compared to symbol length, inter-symbol interference, that is, interference of a symbol with a preceding or a following symbol, or inter-channel interference, that is, interference of a signal of a carrier frequency with signals of neighboring carrier frequencies due to transmission distortion becomes dominant and impossible to be compensated by the above frequency-domain equalization.
A method developed for dealing with this problem is to shorten duration of the impulse response by compensating and equalizing the time-domain digital signal samples by the A/D converter
410
, by performing convolution of the time-domain digital signal through a FIR filter
420
provided between the A/D converter
410
and the FFT circuit
430
, and the training circuit
500
for optimizing tap coefficients of the FIR filter
420
so as to correctly equalize the transmission characteristic of the transmission channel
200
.
A usual method of optimizing the tap coefficients of the FIR filter
420
is to repeatedly generate and transmit a PRBS (Pseudo-Random Binary Sequence) from the transmitter
300
, and to make each of the tap coefficients converge into an optimum value at the receiver
400
by comparing the signal received from the transmitter
300
with a corresponding signal obtained from the same PRBS generated at the receiver side. The FIR filter
420
which has variable tap coefficients to be optimized for equalizing duration of the impulse response is hereinafter called the adaptive equalizer, and a process of and a means for optimizing the tap coefficients are called the training and the training circuit.
The present invention pertains to the training method and the training circuit for stably and rapidly optimizing tap coefficients of the adaptive equalizer.
As a prior art of the training method, there is a technique disclosed in a U.S. Pat. No. 5,285,474.
FIG. 13
is a block diagram illustrating a training circuit according to the prior art. The training circuit of
FIG. 13
consists of a transmitter
100
and a receiver
1000
and performs training of tap coefficients of an adaptive equalizer (not depicted in
FIG. 13
) provided in the receiver
1000
for equalizing signal distortion due to transmission characteristic of a transmission channel
200
connecting the transmitter
200
and the receiver
1000
.
The transmitter
100
comprises a first PRBS generator
110
for generating a PRBS signal, a first encoder
120
for encoding the PRBS signal into a frequency-domain transmission signal vector X, an IFFT circuit
130
for transforming the frequency-domain transmission signal vector X into a time-domain transmission signal x(D). (Hereinafter, a frequency-domain vector is denoted by a capital letter and a time-domain signal obtained by processing the frequency-domain vector with IFFT is expressed as a function of discrete delay variable D denoted by a corresponding small letter.)
The time-domain transmission signal x(D) is converted into analog signal, transmitted through the transmission channel
200
, received by the receiver
1000
and converted again into a time-domain reception signal y(D). (Ordinary elements such as D/A and A/D converters are omitted to depict in
FIG. 13.
)
Here, following equation stands;
y
(
D
)=
x
(
D
)*
h
(
D
)+
n
(
D
)
wherein h(D) and n(D) represent the impulse response and the noise signal of the transmission channel
200
, and the operator ‘*’ denotes convolution operation.
The above equation is expressed as Y=XH+N in the frequency domain.
The receiver
1000
comprises a second PRBS generator
1200
for generating a replica of the PRBS signal generated by the first PRBS generator
110
, a second encoder
1250
for generating a frequency-domain training vector X′ by encoding a frequency domain vector with the replica of the PRBS signal in the same way and in synchronization with the first encoder
120
, a target-impulse-response update means
1300
, a target-impulse-response windowing means
1400
, a tap-coefficient update means
1500
and a tap-coefficient windowing means
1600
.
The target-impulse-response update means
1300
, the target-impulse-response windowing means
1400
, and tap-coefficient update means
1500
and the tap-coefficient windowing means
1600
operate so as to make tap coefficients of the adaptive equalizer having L taps (L being a fixed integer) converge into optimum values which enable the adaptive equalizer to equalize and shorten the duration of the impulse response H, or h(D), of the transmission channel within v taps (v being another fixed integer), that is, within target duration of the equalized impulse response, by updating transitional values of the target impulse response and the tap coefficients alternately and repeatedly, referring to the reception signal y(D) and the training vector X′.
In the following paragraphs, outlines of operation of the target-impulse-response update means
1300
, the target-impulse-response windowing means
1400
, the tap-coefficient
Bocure Tesfaldet
NEC Corporation
Scully Scott Murphy & Presser
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