Pulse or digital communications – Equalizers – Automatic
Reexamination Certificate
1999-02-24
2004-01-13
Liu, Shuwang (Department: 2734)
Pulse or digital communications
Equalizers
Automatic
C375S229000, C375S231000, C375S233000, C375S235000
Reexamination Certificate
active
06678317
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to an adaptive equalizer device and a method for controlling an adaptive equalizer which are applied to technical fields such as digital communication for digital cable television (CATV), for example.
2. Description of the Background Art
Digital communications such as digital cable television are coming into practice as high-speed data communication systems replacing conventional analog communications. In a digital communication like the digital cable television, a received signal IN transmitted from a transmitting side to a receiving side has the features shown in FIG.
12
. That is to say, in a QAM (Quadrature Amplitude Modulation) transmission/reception method, the received signal IN is transmitted for each predetermined period (symbol period SP), as n, n+1, n+2, . . . . The received signal IN includes transmitted data called symbols and a carrier at a high frequency for carrying the symbols.
The digital cable television is susceptible to multipath transmission in which radio waves propagate through a plurality of paths because of reflection at an end of a cable, for example. The presence of multiple delay waves has been confirmed, which occur when a plurality of radio waves caused by multipath transmission overlap. The multiple delay waves cause frequency-selective fading, and the frequency-selective fading causes distortion in the received signal IN.
Next, a conventional receiver will be described.
FIG. 13
is a block diagram showing part of a structure of a conventional receiving processing circuit of QAM transmission/reception system. First, an I/Q demodulator
1
removes carrier from the received signal IN and separates the received signal IN into I (In-phase) component xi and Q (Quadrature) component xq. The input signal x(n) indicates the component xi or the component xq. The character (n) indicates that it is a signal in the nth symbol period SP.
The carrier is not completely removed by the I/Q demodulator
1
, but may remain in the input signal x(n). The carrier remaining unremoved in the input signal x(n) is referred to as a carrier frequency error.
Next, the carrier frequency error, distortion due to frequency-selective fading, and other components in unwanted frequency bands and noise are removed from the input signal x(n), to obtain an output signal y(n). That is to say, the matched filter
2
removes components in unwanted frequency bands and noise from the input signal x(n). The carrier frequency error removing circuit
3
removes the carrier frequency error from the input signal x(n). The adaptive equalizer
4
removes the distortion due to frequency-selective fading from the input signal x(n), and outputs the distortion-removed input signal x(n) as the output signal y(n). The carrier frequency error detecting circuit
5
detects the carrier frequency error from the output signal y(n) and feedback controls the carrier frequency error removing circuit
3
with the detected result R
1
. Thus the carrier frequency error removing circuit
3
more completely removes the carrier frequency error from the input signal x(n).
FIG. 14
is a conceptual diagram showing an example of the output signal y(n). The output signal y(n) is composed of 64 pieces of data, for example. The 64 pieces of data each include I component yi and Q component yq. In
FIG. 14
, the components yi and yq are plotted as dots on the I-Q coordinates, which is called a symbol point arrangement diagram. The character S in
FIG. 14
indicates a symbol, which is a group of 64 dots. In
FIG. 14
, 64 dots form a symbol S, which is called 64-value QAM. When an carrier frequency error occurs, the symbol S rotates as shown in
FIG. 15
, for example. When the symbol S rotates, processing to the received signal IN becomes difficult.
The carrier frequency error causes the symbol S to rotate for the following reason. In the QAM transmission/reception method, as shown in equation (1) below, the I axis component zi is multiplied by a cosine signal cos(wt) and the Q-axis component zq is multiplied by a sine signal sin(wt), and a transmitted signal IN
0
obtained by adding them is sent from the transmitting side to the receiving side.
IN
0
=zi
·cos(wt)+
zq
·sin(wt) (1)
In equation (1), w indicates the carrier frequency of the transmitted signal IN
0
, and t indicates time. The received signal IN is expressed by equation (2) below.
IN=zi
·cos(wat)+
zq
·sin(wat) (2)
In equation (2), wa indicates the carrier frequency of the received signal IN.
On the receiving side, the input signal N is multiplied by a cosine signal cos(wt), as shown in equation (3).
IN
·cos(wt)=
zi
·cos(wt)·cos(wat)+
zq
·sin(wt)·cos(wat)=
zi
·[cos{(
w−wa
)
t
}+cos{(
w+wa
)
t
}]/2
+zq
·[sin{(
w+wa
)
t
}+sin{(
w−wa
)
t
}]/2 (3)
Only the low-frequency component is taken out from equation (3), as xi. That is to say,
xi=zi
/2·cos{(
w−wa
)
t}+zq
/2·sin{(
w−wa
)
t}
(4)
Further, on the receiving side, the input signal IN is multiplied by a sine signal sin(wt), as shown in equation (5).
IN
·sin(wt)=
zi
·cos(wt)·sin(wat)+
zq
·sin(wt)·sin(wat)=
zi
·[sin{(
w+wa
)
t
}−sin{(
w−wa
)
t
}]/2
+zq
·[cos{(
w−wa
)
t
}−cos{(
w+wa
)
t
}]/2 (5)
Only the low-frequency component is taken out from equation (5), as xq. That it to say,
xq=−zi
/2·sin{(
w−wa
)
t}+zq
/2·cos{(
w−wa
)
t }
(6)
The I/Q demodulator
1
on the receiving side takes out the components xi and xq from the received signal IN in this way.
In equations (4) and (6), when wa is equal to w, then
xi=zi
/2
xq=zq
/2
Where no frequency component remains. That it to say, no carrier frequency error occurs.
However, when wa is not equal to w, frequency component remains. That is to say, a carrier frequency error occurs. Equations (4) and (6) can be expressed in a matrix as
(
xi
xq
)
=
(
cos
{
(
w
-
wa
)
t
}
sin
{
(
w
-
wa
)
t
}
-
sin
{
(
w
-
wa
)
t
}
cos
{
(
w
-
wa
)
t
}
)
·
(
zi
/
2
zq
/
2
)
(
7
)
Equation (7) shows rotation. It is seen from equation (7) that the presence of carrier frequency error causes the symbol S to rotate.
FIG. 16
shows basic structure of the conventional adaptive equalizer
4
. The conventional adaptive equalizer
4
includes a discrete filter
6
, which serves as the main element, an error detecting circuit
7
, and a coefficient updating circuit
8
. The discrete filter
6
includes shift registers SR
0
to SR
L−1
, multipliers M
0
to M
L−1
, and adders A
1
to A
L−1
.
Next, operation made by the discrete filter
6
will be described. The discrete filter
6
removes distortion caused by frequency-selective fading from the input signal x(n) and outputs the distortion-removed input signal x(n) as the output signal y(n). The shift registers SR
0
to SR
L−1
delay the input signal x(n) by an amount of delay, Z
−1
. Next, the multipliers M
0
to M
L−1
multiply signals at the respective output nodes (referred to as “taps”) of the shift registers SR
0
to SR
L−1
and the coefficients C
0
to C
L−1
. Next, the adders A
1
to A
L−1
add the multiplied results obtained by the multipliers M
0
to M
L−1
. The sum of the multiplied results from the multipliers M
0
to M
L−1
corresponds to the output signal y(n).
The distortion due to frequency-selective fading may not be completely removed by the discrete filter
6
, but may remain in the output signal y(n). The distortion due to frequency-selective fading remaining unremoved in the output signal y(n) is referred to as a distortion error. When the distortion error is large, it causes intersymbol interference
Mizutani Hiroyuki
Murakami Shuji
Yamanaka Kazuya
Burns Doane , Swecker, Mathis LLP
Liu Shuwang
Mitsubishi Denki & Kabushiki Kaisha
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