Pulse or digital communications – Systems using alternating or pulsating current – Plural channels for transmission of a single pulse train
Reexamination Certificate
1999-03-05
2003-01-28
Deppe, Betsy L. (Department: 2634)
Pulse or digital communications
Systems using alternating or pulsating current
Plural channels for transmission of a single pulse train
C375S229000, C375S232000, C375S298000
Reexamination Certificate
active
06512798
ABSTRACT:
BACKGROUND OF THE INVENTION
The present invention relates to a system for transmitting two types of signal by orthogonal modulation, or more in particular to a digital signal transmission system comprising an orthogonal modulation transmission unit and an orthogonal demodulation receiving unit having a digital configuration.
In recent years, a signal transmission system of orthogonal modulation type such as QPSK or QAM scheme has been employed for improving the transmission rate in the digital radio communication for mobile units and terrestrial communication. The orthogonal modulation scheme is for transmitting two types of signals by orthogonal modulation using two types of mutually orthogonal carriers expressed as
cos(2
&pgr;×f′c×t
)
and
sin(2
&pgr;×f′c×t
)
where t is the time and f′c the carrier frequency.
The conventional digital signal transmission system of orthogonal modulation type has employed an orthogonal modulator/demodulator circuit mainly using a mixer of analog configuration for the apparent reason of its low cost and small circuit size.
The conventional signal transmission system using this orthogonal modulator/demodulator circuit of analog type will be explained with reference to
FIGS. 19 and 20
.
First,
FIG. 19
shows an example of a conventional transmission circuit of orthogonal modulation type. The information code input from an input terminal
1
is converted into two types of baseband digital signals Id(n), Qd(n) by a transmission signal processing circuit
2
and then applied to an orthogonal modulator
3
, where n is an integer not less than 1 indicating the order of the clock signal pulses.
The signals Id(n), Qd(n) input to the orthogonal modulator
3
are converted into analog signals by D/A converters
4
i
,
4
q
, respectively. Then, the transmission bandwidth of signals is limited to a predetermined value B by LPFs (low-pass filters)
5
i
,
5
q
of analog configuration.
The signals I(t), Q(t) output from the LPFs
5
i
,
5
q
are input to a mixer
6
where they are orthogonally modulated in analog fashion by the operation according to equation (1) below.
D
(
t
)=
I
(
t
)×cos(2
&pgr;×fc×t
)+
Q
(
t
)×sin(2
&pgr;×fc×t
) (1)
The signal D(t) thus orthogonally modulated is input to a BPF (bandpass filter)
7
where the unrequited component generated in the mixer
6
is removed. The output signal of the BPF
7
is supplied to an up converter
8
where it is converted to a carrier signal of a still higher frequency f′c. The high-frequency transmission signal is transmitted from an antenna
9
.
FIG. 20
shows an example of a conventional receiving circuit of orthogonal demodulation type. The signal received by an antenna
10
is restored to the original orthogonal modulation signal D(t) by a down converter
11
.
The orthogonal modulation signal D(t) supplied to an orthogonal demodulator
13
is input to a mixer
14
and converted into a signal of a carrier frequency fc. The mixer
14
processes the signal D(t) in two ways by equations (2) and (3) shown below, and orthogonally demodulates the two types of signals I(t), Q(t) in analog fashion using the orthogonality of the trigonometric function.
I
(
t
)=
D
(
t
)×cos(2
&pgr;×fc×t
) (2)
Q
(
t
)=
D
(
t
)×sin(2
&pgr;×fc×t
) (3)
The two signals I(t), Q(t) orthogonally demodulated in this way have the unrequited components thereof removed by the analog LPFs
15
i
,
15
q
, respectively. The signals I(t), Q(t) are converted into digital signals Id(n), Qd(n) by the A/D converters
16
i
,
16
q
, respectively, and supplied to a receiving signal processing circuit
17
.
The receiving signal processing circuit
17
demodulates the two digital input signals Id(n), Iq(n), and outputs the resulting information code from an output terminal
18
.
In the conventional analog modulation system described above, the two types of carrier signal used for orthogonal modulation and demodulation, i.e. the two carrier signals expressed by “cos” and “sin” in
FIGS. 19 and 20
, if insufficient in orthogonality (accuracy of 2&pgr; in phase difference), develop inter-code interference between the two types of components and increases the error rate of the demodulated code, resulting in a deteriorated communication quality.
In view of this, in the prior art, a mixer of analog configuration regulated to high accuracy is used so that the orthogonal (phase difference) error between the two reference carrier waves assumes a sufficiently small value of 1 degree or less.
The conventional system described above fails to take digitization into consideration and poses the problem of difficulty of improving the performance thereof.
Specifically, in recent years, schemes such as 64QAM or OFDM with a large number of points more than the conventional BPSK and QPSK schemes have come to be employed. These schemes require a still higher accuracy of orthogonality. The analog technique, however, has its own limit of improving the orthogonality accuracy and therefore is difficult to improve the performance of the transmission system.
In view of this, the inventors have studied a digital orthogonal modulator and a digital orthogonal demodulator in which orthogonal modulation and orthogonal demodulation are performed by digital signal processing in order to secure a sufficiently high accuracy required for orthogonality. An example will be explained with reference to
FIGS. 21 and 22
.
First, the digital orthogonal modulator of
FIG. 21
is a digital version of the orthogonal modulator
3
of analog configuration shown in FIG.
19
. In similar fashion, the digital orthogonal demodulator of
FIG. 22
is a digital version of the orthogonal demodulator
13
of analog configuration shown in FIG.
20
. The circuits other than the orthogonal modulator and the orthogonal demodulator are the same as those of
FIGS. 19 and 20
. Therefore, the configuration and operation of the orthogonal modulator and the orthogonal demodulator will be mainly described below.
First, typical signal waveforms of the first digital signal Id(n) and the second digital signal Qd(n) of a sampling frequency fd supplied from the transmission signal processing circuit
2
are illustrated in FIGS.
23
(
a
), (
b
).
In FIGS.
23
(
a
), (
b
), the solid curve represents the signal waveform of the I signal before sampling; and the dashed curve represents the signal waveform of the Q signal before sampling. The corresponding sampling signals are also indicated by solid and dashed arrows, respectively.
Of all the signals applied to the digital orthogonal modulator shown in
FIG. 21
, the first digital signal Id(n) is converted to a signal I′d
4
(
m
) of a quadruple sampling frequency 4×fd by a first quadruple sampling converter
19
i
in such a manner that three zeros are inserted between the nth sampling value Id(n) and the (n+1)th sampling value Id(n+1) as shown in FIG.
23
(
c
), where m is an integer representing the order of the clock signal for the quadruple sampling operation.
The signal I′d
4
(
m
) thus converted, as shown typically in
FIG. 24
, contains the unrequited harmonic components
20
. In
FIG. 24
, the hatched portions show the bands of the required signal information components.
The digital LPF
21
i
, like the conventional LPF
5
i
shown in
FIG. 19
, limits the signal I′d
4
(
m
) to the bandwidth B while at the same time removing the unrequited harmonic components
20
.
As a result, the signal waveform of the output signal Id
4
(
m
) of the digital LPF
21
i
assumes a sampling waveform with zeros filled between the signals as shown in FIG.
23
(
e
).
This is also the case with the second signal Qd(n). As shown in FIG.
23
(
d
), the second signal Qd(n) is converted into a signal Q′d
4
(
m
) of a quadruple frequency 4×fd by a second quadruple sample converter
19
q
, limited to the bandwidth B by a digital LPF
21
q
, and as shown in FIG.
23
(
f
), con
Akiyama Toshiyuki
Miyashita Atsushi
Tsukamoto Nobuo
Antonelli Terry Stout & Kraus LLP
Deppe Betsy L.
Hitachi Denshi Kabushiki Kaisha
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