Telecommunications – Receiver or analog modulated signal frequency converter – Signal selection based on frequency
Reexamination Certificate
2000-10-30
2004-04-27
Maung, Nay (Department: 2684)
Telecommunications
Receiver or analog modulated signal frequency converter
Signal selection based on frequency
C455S192200, C455S196100, C455S075000
Reexamination Certificate
active
06728523
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a method and an apparatus for stabilizing frequency of a signal outputted from a relay station in a communication system, an more particularly to a method and an apparatus for stabilizing frequency of a signal outputted from a relay station in a telecommunication system and in a broadcasting system by use of a satellite.
2. Description of the Related Art
A telecommunication system that performs digital communication for mobile terminal devices and a broadcasting system that broadcasts programs for digital television by use of a relay station located at a high altitude such as a communication satellite or a balloon in the stratosphere have been suggested recently.
FIG. 1
is a block diagram showing a radio communication system using a communication satellite for digital communication between a base station and a radio terminal device. The radio communication system shown in
FIG. 1
includes a base station
11
, a relay station
12
, a radio terminal device
13
, and data-relay apparatuses
14
and
15
.
The base station initially
11
modulates communication information, and transmits a code-division-multiplexed signal having a frequency f
1
to the relay station
12
such as a communication satellite. The relay station
12
coverts the signal having the frequency f
1
received from the base station
11
to a signal having a frequency f
2
and a signal having a frequency f
3
, and transmits those signals in a code-division-multiplex format to earth stations.
The radio terminal device
13
receives the signal having the frequency f
3
from the relay station
12
, and demodulates the communication information included therein. The data-relay apparatus
14
receives the signal having the frequency f
2
from the relay station
12
, and converts the received signal to a code-division-multiplexed signal having the frequency f
3
, then transmitting the converted signal to other devices such as the radio terminal device
13
. The data-relay apparatus
14
amplifies the received signal if necessary. The data-relay apparatus
15
receives the signal having the frequency f
3
from the relay station
12
, and amplifies the received signal in addition to attending to other signal processing, followed by transmitting the processed signal to other devices without converting the frequency of the received signal.
Accordingly, the radio terminal device
13
can receive the signal having the frequency f
3
from the relay station
12
through the apparatuses
14
and
15
even if the radio terminal device
13
is located in a blind zone. The above-described system may also be applied to digital-television broadcasting. In this case, the system substitutes a broadcasting station for the base station
11
. Additionally, a broadcasting satellite is used as the relay station
12
.
FIG. 2
is a block diagram showing a conventional data-relay apparatus
14
that converts frequency of a signal to another frequency, and transmits the signal to other devices. It should be noted that the above-described conventional data-relay apparatus
14
is, hereinafter, referred to as a conventional frequency-converting-data-relay apparatus
14
.
The conventional frequency-converting-data-relay apparatus
14
includes a data-receiving antenna
21
, an amplifier
22
, a mixer
23
, a generator
24
, a band-pass filter
25
, an amplifier
26
and a data-transmitting antenna
27
. In
FIG. 2
, frequency f
2
′ is the frequency f
2
with an error &Dgr;f
2
. Similarly, frequency f
3
′ is the frequency f
3
with an error &Dgr;f
3
.
The conventional frequency-converting-data-relay apparatus
14
initially receives a signal having the frequency f
2
′ from the data-receiving antenna
21
, and amplifies the signal by use of the amplifier
22
. Subsequently, the conventional frequency-converting-data-relay apparatus
14
converts the frequency f
2
′ of the signal amplified by the amplifier
22
to the frequency f
3
′ by mixing the signal amplified by the amplifier
22
with a signal generated by the generator
24
by use of the mixer
23
. The frequency f
3
′ includes the error &Dgr;f
3
of the frequency f
3
that is caused by the error &Dgr;f
2
of the frequency f
2
and performance of the generator
24
.
The conventional frequency-converting-data-relay apparatus
14
then filters the frequency f
3
′ of the signal by use of the band-pass filter
25
. Additionally, the conventional frequency-converting-data-relay apparatus
14
amplifies the signal by use of the amplifier
26
, and then outputs the signal having the frequency f
3
′ from the data-transmitting antenna
27
.
FIG. 3
is a block diagram showing a conventional data-relay apparatus
15
that transmits the signal to other devices without converting frequency of a signal to another frequency. The above-described conventional data-relay apparatus
15
includes a data-receiving antenna
31
, an amplifier
32
, a band-pass filter
33
, an amplifier
34
and a data-transmitting antenna
35
.
The conventional data-relay apparatus
15
initially receives a signal having the frequency f
3
′ from the data-receiving antenna
31
, and amplifies the signal by use of the amplifier
32
. The conventional data-relay apparatus
15
then filters the frequency f
3
′ of the signal by use of the band-pass filter
33
. Additionally, The conventional data-relay apparatus
15
amplifies the signal by use of the amplifier
34
, and then outputs the signal having the frequency f
3
′ from the data-transmitting antenna
35
.
In the communication system described with reference to
FIG. 1
, frequency deviation occurs in a signal received by the radio terminal device
13
according to performance of generators provided in the base station
11
, the relay station
12
such as a satellite, and the data-relay apparatuses
14
and
15
. Additionally, the frequency of the signal received by the radio terminal device
13
is shifted by the Doppler shift that is caused by movement of the radio terminal device
13
and speed of the satellite relative to the earth.
Additionally, when the radio terminal device
13
simultaneously receives a code-division-multiplexed signal having the frequency f
3
from the relay station
12
and the signal having the frequency f
3
′ from the data-relay apparatuses
14
and
15
, the radio terminal device
13
cannot receive the signal effectively if the frequency deviation of the signals outputted from the data-relay apparatuses
14
and
15
is large. Consequently, quality of the signal received by the radio terminal device
13
decreases.
In order to solve the above-described problems, the radio terminal device
13
may include functions to search frequency of the received signal and execute an automatic frequency control (AFC) on the frequency, thereby reducing the frequency deviation of the received signal before demodulating the received signal. However, the number of the radio terminal devices
13
in the above-described communication system is very large, especially in the broadcasting system. Thus, it is necessary to minimize production cost of the radio terminal device
13
. If the radio terminal device
13
is to execute a frequency search and the AFC, a circuit arrangement of the device
13
becomes complex, and the circuit scale becomes large. As a result, the production cost of the radio terminal device
13
increases.
SUMMARY OF THE INVENTION
Accordingly, it is a general object of the present invention to provide a method and an apparatus for stabilizing frequency of an output signal of a data-relay apparatus by minimizing frequency deviation of the output signal, thereby minimizing the size of a radio terminal device that receives the output signal from the data-relay apparatus.
The above-described objects of the present invention is achieved by a method of stabilizing frequency of an output signal of a data-relay apparatus provided in a radio communication system that includes a relay station and a rad
Ohba Takeshi
Yamashita Atsushi
Katten Muchin Zavis & Rosenman
Lee John J
Maung Nay
LandOfFree
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