Pulse or digital communications – Receivers
Reexamination Certificate
2000-02-18
2004-02-24
Ghebretinsae, Temesghen (Department: 2631)
Pulse or digital communications
Receivers
C375S324000
Reexamination Certificate
active
06697437
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Technical Field
The present invention relates to a receiving apparatus for receiving a digitally modulated signal and particularly to a correcting technology for a received frequency.
2. Description of the Related art
A receiving apparatus for receiving a digitally modulated signal shown in
FIG. 6. A
digitally modulated signal is received by an antenna
11
, and after the received signal is amplified by a high frequency amplifier
12
, it is mixed with an output of a frequency synthesizer
14
at a frequency converter
13
and requency-converted to a predetermined intermediate frequency signal. After the converted intermediate frequency signal is amplified by an intermediate frequency amplifier
15
and passed through a band limiting filter
16
, it is supplied to an orthogonal detector
17
for orthogonal detection and becomes a base band signal. The orthogonal detection is carried out in synchronization with a frequency of a carrier wave supplied from a frequency synthesizer
18
.
The base band signal converted by the orthogonal detector
17
is supplied to a demodulator
19
and the demodulated data is error-corrected by an error-correcting circuit
20
and supplied to a voice decoder
21
to be subjected to a voice decoding process, the decoded voice signal is supplied to a voice output circuit (not shown) at a later stage.
Here, the frequency which is frequency-converted by the frequency converter
13
and the frequency which is detected by the orthogonal detector
17
is determined by the frequency supplied from the frequency convertor
13
, but frequency correction needs to be carried out for the frequencies supplied from the frequency synthesizers
14
,
18
to synchronize with the frequency of the received signal.
FIG. 2
is a block diagram showing an arrangement for carrying out the conventional frequency correction. The frequency correction is carried out based on a received signal symbol demodulated by the demodulator
19
. That is, a frequency offset of the received signal symbol demodulated by the demodulator
19
is detected by a frequency offset detector
31
and the value of the detected frequency offset is averaged by being integrated by a digital integrator
32
. The averaged value integrated by the integrator
32
is converted to an analog voltage by a digital/analog convertor
33
and the converted voltage is supplied to a control voltage input of a voltage controlled oscillator
34
to control the output frequency of the voltage controlled oscillator
34
. An output of the voltage controlled oscillator
34
is supplied to the frequency synthesizers
14
,
18
to control their output frequencies.
By the control in this circuit, the correction of the received frequency can be carried out. First of all, in regard to the digital modulated-signal to be received, if a digital information signal of an in-phase component is assumed to be I(t) while a digital information signal of an orthogonal component is assumed to be Q(t), a signal X(t) of the information component is defined by a complex number such as X(t)=I(t)+jQ(t). The digitally modulated signal can be expressed in the following.
s
(
t
)=
Re
(
x
(
t
)·
e
j2&pgr;fct
) [Equation 1]
Here, fc denotes a carrier wave frequency and the carrier wave is expressed by a complex sinusoidal wave c(t) =e
j2&pgr;fct
.
The digitally modulated received signal is, by being subjected to an orthogonal demodulation on the receiving apparatus side, extracted as a signal of the information component as a base band signal. The orthogonal demodulation will be explained hereinafter. The digitally modulated received signal is demodulated in the following way by being multiplied by a received carrier wave with a reverse phase rotation relative to the carrier wave.
F
(
s
(
t
)
·
ⅇ
π
f
-
2
π
fct
)
=
1
2
F
[
(
x
(
t
)
·
ⅇ
j2π
fct
+
x
*
(
t
)
·
ⅇ
-
j2π
fct
)
·
ⅇ
-
j2π
fct
]
=
1
2
F
[
x
(
t
)
+
x
*
(
t
)
·
ⅇ
-
j
2
π
fct
=
1
2
(
X
(
f
)
+
x
*
(
-
f
-
2
fc
)
)
[Equation 2]
Here, X(f) denotes a frequency of x(t) and x*(t) and X*(f) express complex conjugates of x(t) and X(f). By taking out only a low-band signal from the demodulated received signal by a low-pass filter, the base band signal can be obtained.
When there exists a frequency offset f between the carrier frequency used for the digital modulation and the frequency of the complex sinusoidal wave used for the orthogonal demodulation, the reception signal converted by the orthogonal demodulation is expressed in the following equation and the base band signal obtained after the low-band signal is extracted by the low-pass filter is, as shown in
FIG. 3
, observed as time phase rotation of an angular velocity &ohgr;=2f on an IQ plane.
F
(
s
(
t
)
·
ⅇ
-
j
2
π
(
fc
+
Δ
f
)
t
)
=
1
2
F
[
(
x
(
t
)
·
ⅇ
j
2
π
fct
+
x
*
(
t
)
·
ⅇ
-
j2π
(
fc
+
Δ
f
)
t
)
·
ⅇ
-
j2π
(
fc
+
Δ
f
)
t
]
=
1
2
F
[
(
x
(
t
)
·
ⅇ
-
j2π
Δ
ft
+
x
*
(
t
)
ⅇ
-
j2π
2
(
fc
+
Δ
f
)
t
)
=
1
2
(
X
(
f
-
Δ
f
)
X
*
(
-
f
-
2
(
f
c
+
Δ
f
)
)
)
[Equation 3]
Since this causes a serious influence on a code judgement error of the received signal, it is necessary to measure transmission and received frequency offsets as well as correcting a received carrier wave frequency on the receiving apparatus side.
Next, a process for estimating the frequency offset will be explained with reference to the system shown in FIG.
2
. When a received signal symbol r
t
outputted by the demodulator
19
of
FIG. 1 and a
transmission signal symbol corresponding to a previously demodulated received signal symbol r
t-n
are identical, by carrying out an arctan operation to a value obtained by complex conjugate multiplication between r
t
and r
t-n
, a phase rotation angle &Dgr;&thgr; can be detected. By obtaining the phase rotation angle &Dgr;&thgr; which rotates on the IQ plane at some constant interval &Dgr;t, the frequency offset &Dgr;f can be obtained by the following.
Δ
f
=
Δ
θ
2
π
·
Δ
t
In the conventional process for estimating a frequency offset, the following problems arise. When a phase rotation angle between symbols is detected, an arctan operation is necessary to obtain a value which is obtained by complex conjugate multiplication of the two symbols, but since the amount of calculation of the arctan operation is comparatively large, when a real time process is needed, it can happen that the arctan operation process does not end within a required time. In order to avoid this, the following approximation has been carried out. Here, amplitude r
tk
is denoted as A
t
, a phase angle as &thgr;
t
, an in-phase signal as it and an orthogonal signal as q
t
.
r
t
″
r
t
-
n
*
=
A
t
·
ⅇ
j
θ
t
·
A
t
-
n
·
ⅇ
-
j
θ
t
-
n
=
A
t
·
A
t
-
n
·
ⅇ
j
(
θ
t
-
θ
t
-
n
)
=
A
t
·
A
t
-
n
·
ⅇ
jΔ
θ
=
A
t
·
A
t
-
n
·
(
cos
Δ
θ
+
j
Arai Daisuke
Uno Masahiro
Ghebretinsae Temesghen
Maioli Jay H.
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