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Claims  |
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What is claimed is:
1. A mobile communication system comprising:
a receiving section which includes first and second local oscillating means
each for generating first and second local oscillation signals,
respectively, first frequency converting means for converting a received
signal into a first intermediate frequency signal by mixing with said
first local oscillation signal, second frequency converting means for
converting the received signal converted into said first intermediate
frequency signal into a second intermediate frequency signal by mixing
with said second local oscillation signal, reference frequency oscillating
means for generating a reference signal having a predetermined reference
frequency, calibration signal generating means for generating a
calibration signal based on said reference signal generated from said
reference frequency oscillating means, and a demodulator for demodulating
the received signal converted into said second intermediate frequency
signal;
a transmission section for modulating a transmission signal; and
an input/output section for receiving the demodulated signal and outputting
the transmission signal,
wherein said receiving section has frequency control means for controlling
an oscillation frequency of said reference frequency of said reference
frequency oscillating means based on the received signal converted into
said second intermediate frequency signal, and correcting means for
correcting said second local oscillating means based on a calibration
signal converted into said second intermediate frequency signal in which
said calibration signal from said calibration signal generating means is
inputted to an input of said second frequency converting means;
said first local oscillating means generates said first local oscillation
signal based on the controlled reference signal of said reference
frequency oscillating means; and
said calibration signal generating means generates said calibration signal
which is controlled based on said reference signal generated by the
controlled reference frequency oscillating means.
2. A mobile communication system as defined in claim 1,
wherein said calibration signal generating means further generates a signal
for modulating the transmission signal at said transmission section and
further includes switching means for switching the generation of said
calibration signal and said signal at said calibration signal generating
means.
3. A mobile communication system as defined in claim 1, wherein:
said receiving section further includes counter means for counting either
the received signal converted into said second intermediate frequency
signal by said second frequency converting means or the calibration signal
converted into said second intermediate frequency signal;
said frequency controlling means generates a frequency controlling signal
for controlling said oscillation frequency of said reference signal of
said reference frequency oscillating means based on frequency error
between a counted value of the received signal converted into said second
intermediate frequency signal counted by said counter means and a
predetermined intermediate frequency reference value; and
said reference frequency oscillating means is controlled with respect to
said oscillation frequency of said reference signal based on said
frequency controlling signal.
4. A mobile communication system as defined in claim 1, further comprising:
controlling means for instructing said calibration signal generating means
to activate generation of said calibration signal.
5. A mobile communication system as defined in claim 4, wherein said
controlling means instructs said calibration signal generating means to
activate generation of said calibration signal at a regular period of
time.
6. A mobile communication system as defined in claim 4, wherein said
controlling means monitors whether or not communication is being carried
out in said communication system and issues a command for generating said
calibration signal to said calibration signal generating means when
communication is not being carried out.
7. A mobile communication system as defined in claim 1, further comprising:
temperature sensoring means for instructing said calibration signal
generating means to activate generation of said calibration signal in a
case where a temperature reaches a predetermined temperature.
8. A mobile communication system as defined in claim 1, wherein said first
frequency converting means and said second frequency converting means are
provided in a double structure, and said mobile communication system
further comprising:
calibration signal switching means for inputting the generated calibration
signal at said calibration signal generating means into said second
frequency converting means which has received no received signal.
9. A communication system comprising a receiving section which includes
first frequency converting means for converting a received signal into a
first intermediate frequency signal by mixing with a first local
oscillation signal, second frequency converting means for converting the
received signal converted into said first intermediate frequency signal
into a second intermediate frequency signal by mixing the received signal
converted into the first intermediate frequency signal with a second local
oscillation signal, a demodulator for demodulating the received signal
converted into the second intermediate frequency signal, a transmission
section for modulating a transmission signal and an input/output section
for receiving the demodulated signal from the receiving section and
outputting it externally and receiving the transmission signal externally
and outputting it to the transmission section,
said receiving section including:
reference frequency generating means for generating a reference signal;
calibration signal generating means for generating a calibration signal of
said first intermediate frequency based on said reference signal; and
frequency estimating and correcting means for detecting a frequency error
of said reference frequency generated by said reference frequency
generating means and outputting an oscillation frequency control signal to
said reference frequency generating means based on the detected frequency
error,
said reference frequency generating means being controlled based on said
oscillation frequency control signal, with respect to an oscillation
frequency of said reference signal,
said second frequency converting means converting, in addition to the
received signal converted into said first intermediate frequency signal,
said calibration signal of said first intermediate frequency signal into a
second intermediate calibration frequency signal by mixing said
calibration signal of said first intermediate frequency signal with said
second local oscillation signal; and
said frequency estimating and correcting means detecting a frequency error
A between said second intermediate frequency calibration signal converted
by said second frequency converting means and a predetermined reference
intermediate frequency, detecting a frequency error C which is determined
by subtracting the frequency error A from a frequency error B which is a
frequency difference between the received signal converted into said
second intermediate frequency signal and said predetermined reference
intermediate frequency and outputting said oscillation frequency control
signal based on the detected frequency error C.
10. A communication system as defined in claim 9, wherein said frequency
estimating and correcting means includes:
a first frequency estimator for detecting the frequency error A;
a phase compensator for correcting a phase of the received signal converted
into said second intermediate frequency signal converted by the second
frequency converting means based on said frequency error A detected by
said first frequency estimator;
a second frequency estimator for detecting said frequency error C and
outputting a frequency error signal based on said frequency error C; and
frequency correcting means for outputting the oscillation frequency control
signal to said reference frequency generating means based on said
frequency error signal outputted from said second frequency estimator,
said second frequency estimator corrects a phase of the received signal
converted into said second intermediate frequency signal based on said
frequency error A so as to detect said frequency error C from the received
signal converted into said second intermediate frequency signal having a
corrected phase and the predetermined reference intermediate frequency.
11. A communication system as defined in claim 9, wherein said frequency
estimating and correcting means includes:
a first frequency estimator for detecting said frequency error A to be
outputted;
a third frequency estimator for detecting said frequency error B; and
frequency correcting means for detecting said frequency error C to output
said oscillation frequency control signal to said reference frequency
generating means based on the detected frequency error C, said frequency
correcting means detecting said frequency error C based on said frequency
error A outputted from said first frequency estimator and said frequency
error B outputted from said third frequency estimator.
12. A communication system as defined in claim 9, wherein said frequency
estimating and correcting means includes:
a first frequency estimator for detecting said frequency error A;
a phase compensator for correcting a phase of the received signal converted
into said second intermediate frequency based on said frequency error A
detected by said first frequency estimator;
a second frequency estimator for detecting said frequency error C to be
outputted; and
frequency correcting means for outputting said oscillation frequency
control signal to said reference frequency generating means based on said
frequency error C, said phase compensator further including a first mode
in which a phase of the received signal converted into said second
intermediate frequency is corrected and a second mode in which the
received signal converted into said second intermediate frequency signal
is outputted without being corrected, and selector means for selecting
either one of said modes according to a predetermined condition, said
frequency correcting means correcting a phase of the received signal based
on said frequency error A so as to detect said frequency error C from the
received signal converted into said second intermediate frequency signal
having a corrected phase and the predetermined reference intermediate
frequency and outputting said oscillation frequency control signal to said
reference frequency generating means.
13. A communication system as defined in claim 12, wherein said selector
means selects, according to said predetermined condition said first mode
in a case where said frequency error A is larger than a predetermined
value, and said second mode in the other cases.
14. A communication system as defined in claim 9, wherein said frequency
estimating and correcting means includes:
a frequency estimator for detecting the frequency error A;
a frequency memory for storing the frequency error A; and
frequency correcting means for outputting the oscillation frequency control
signal to the reference frequency generating means, said frequency
estimator detecting the frequency error B between the received signal
converted into the second intermediate frequency and the reference
intermediate frequency to output a frequency error signal, said frequency
correcting means detecting the frequency error C on the basis of the
frequency error A stored in the frequency memory and the frequency error B
detected by the frequency estimator, to output the oscillation frequency
control signal.
15. A communication system as defined in claim 9, wherein said frequency
estimating and correcting means includes:
a frequency estimator for detecting the frequency error A;
a frequency memory for storing the frequency error A;
a phase compensator for compensating a phase of the received signal
converted into said second intermediate frequency, on the basis of the
frequency error A stored in said frequency memory; and
frequency correcting means for outputting the oscillation frequency control
signal to the reference frequency generating means, said frequency
estimator detecting the frequency error C on the basis of the received
signal corrected with said phase compensator and the reference
intermediate frequency to output a frequency error signal, said frequency
correcting means outputting the oscillation frequency control signal to
the reference frequency generating means on the basis of said frequency
error signal.
16. A communication system as defined in claim 9, wherein
said frequency estimating and correcting means includes;
a frequency estimator for detecting said frequency error A;
a frequency memory for storing said frequency error A;
a phase compensator for compensating a phase of the received signal
converted into said second intermediate frequency based on said frequency
error A stored in said frequency memory; and
frequency correcting means for outputting said oscillation frequency
control signal to said reference frequency generating means, said phase
compensator further including a first mode in which a phase of the
received signal converted into said second intermediate frequency signal
is corrected and a second mode in which the received signal converted into
said second intermediate frequency signal is outputted without being
corrected, and selector means for selecting either one of said modes, said
frequency correcting means correcting the phase of the received signal
converted into said second intermediate frequency signal on the basis of
said frequency error A so as to detect said frequency error C from the
corrected received signal converted into said second intermediate
frequency signal and the predetermined intermediate frequency and
outputting the oscillation frequency control signal to the reference
frequency generating means.
17. A communication system as defined in claim 9, wherein said frequency
estimating and correcting means detects the frequency error C such that a
phase of the received signal converted into the second intermediate
frequency is corrected on the basis of said frequency error A, and the
frequency error C is detected on the basis of the so corrected received
signal and the predetermined reference intermediate frequency. |
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Claims |
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Description |
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BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a receiving system or a mobile
communications system which has a frequency stabilizing function to
control a frequency stability to a predetermined value. More particularly,
this invention relates to a communication system using a heterodyne
receiving system, i.e., a communication system having a function of
calibrating the frequency of a reference signal which is to be a reference
of transmitting/receiving frequencies, which is applicable to a digital
automatic telephone.
2. Description of Related Art
Prior techniques of improving the accuracy of the transmitting/receiving
frequencies in a mobile communications system are disclosed in Japanese
Patent Laid-Open Publication (KOKAI) Nos. 63-26020 and 63-28156. In these
prior techniques, the frequency of a transmission signal is automatically
settled within a predetermined range by converting a stable frequency of a
received signal transmitted from a base station into a second intermediate
frequency signal and then counting the frequency of the second
intermediate frequency signal by means of a counter device so as to obtain
a difference between the counted value and a predetermined second
intermediate frequency reference value. According to the difference, an
oscillation frequency control signal is issued by an operation controlling
device to a VC-TCXO (Voltage Controlled Temperature Compensated Crystal
Oscillator).
To obtain the frequency more accurately, the frequency of a second local
oscillator is also counted by the counter device, and a frequency error of
the second local oscillator is made to be corrected based upon the counted
value.
In the above mentioned automatic frequency control, the relationship
between the estimate accuracy of the frequency error based on the counted
value and time required for counting is reciprocal. If the accuracy of an
oscillator which is a time base of the counter device is sufficiently
high, in principal, an error of nearly 10 Hz is estimated in a 100 ms
count period.
On the other hand, in the digital automatic telephone system, microwave
digital communication by time division multiplex communication has been in
practical use in recent years. As a prior technique relating to a
demodulator suitable for time-division multiplex communication, it is
commonly known to use a demodulator with quasi-coherent demodulator which
vector-demodulates with respect to a continuous phase modulating wave
having a constant amplitude by use of a fixed reference carrier so as to
obtain a quasi-coherent demodulation signal, as disclosed in, for example,
Japanese Patent Application Laid-Open is (KOKAI) 2-46044. In this prior
demodulator, a phase rotating signal caused by carrier drift owing to
fading and relative deviation between the oscillation frequency of a base
station and the fixed reference carrier. The frequency error of the fixed
reference carrier wave is estimated by an operation method in which
primary approximation is performed with respect to the accumulation of the
phase rotation. This estimate processing is performed at one time with
respect to burst data, and hence it is completed at high speed within the
time required for operational processing started from the reception of the
burst data.
In a receiver or a mobile communication system in which receiving frequency
stability is required, it is essential to maintain a reference frequency
in a predetermined range without controlling the system for a long time
period, i.e., for five or ten years. Further, in a mobile communication,
it is essential to improve the accuracy of the transmitting frequency, as
further even narrower frequency bands come into use.
In the prior art techniques, there is provided a VC-OCXO to be controlled
based on the receiving frequency, and the oscillation frequency of the
VC-TCXO is fine-controlled on the basis of the received signal so as to
obtain the desired frequency stability. Also, the frequency of a second
local oscillator can be counted by switching the mode, and the oscillation
frequency of the second oscillator is corrected using the counted value,
which makes it possible to control the frequencies to a high accuracy of
within 1 ppm.
On the other hand, however, there is a need for low power consumption in
the receiver of the mobile communication system.
In the prior techniques, low power consumption cannot be achieved owing to
a frequency estimator and an amplifier. The former counts high frequency
which is the output from the second local oscillator, and the latter
amplifies the second local oscillation frequency signal so that it can be
counted by the counter.
For example, it is assumed that a first intermediate frequency is 90 MHz,
and the second intermediate frequency is 455 kHz. The oscillation
frequency of the second local oscillator therefore becomes 89.545 MHz,
which is the threshold frequency in a CMOSIC. Taking into consideration
the need for lower power consumption, this frequency exceeds the
operational threshold frequency, which cannot be adequately dealt with in
the CMOSIC.
Further, in the time division communication in which the operation should
be performed in the burst mode, high-speed processing is required in an
automatic frequency control system. The counter requires a relatively long
time for counting, which makes it unsuitable for the automatic frequency
control system. Therefore, it has been suggested to provide an automatic
frequency control in which the estimated result is fedback to the VC-TCXO
in the demodulation system having the quasi-coherent demodulator and
operational device for estimating the frequency error. However, the
estimate of the frequency error is executed with respect to the output of
the quasi-coherent demodulator. In the mobile communication system
employing the double super heterodyne system, the frequency error is
estimated by the value accumulated with deviation factors such as the
deviation of VC-TCXO, the frequency variations of the second local
oscillator and the fixed reference carrier generator, carrier drift caused
by high-speed fading, etc., and hence, it is difficult to obtain only the
variation of the VC-TCXO accurately. In particular, a relatively cheap
oscillator is used as the second local oscillator in the mobile telephone.
When the estimated result estimated from the accumulated value is fedback
directly, the accuracy of the transmission frequency relative to the base
station is deteriorated. For example, when it is assumed that the first
intermediate frequency is 90 MHz in the mobile telephone system in the 900
MHz band, and the frequency stability of the second local oscillator is 10
ppm, 900 Hz frequency error is obtained. When this is converted as the
frequency error of 900 MHz, the deterioration is generated by 1 ppm.
Taking into consideration the high-accurate of frequencies in recent
years, this deteriorated value is not negligible.
To improve the accuracy of the transmission frequency, there may be
provided an oscillator having a high performance as the second local
oscillator, which is however not preferable in cost.
Further, it is difficult to make the counter into the LSI circuit owing to
the high frequency to be counted, which reduces the level of minimization
of the system. This is a serious problem particularly to a portable
communication system required to be reduced in size, because it needs to
be operated with a low voltage in order to reduce the number of cells.
Further, the second local oscillator has been included in the LSI circuit
in recent years. To pick up the output of the second local oscillator
securely, it is necessary to provide an amplifying circuit which gives no
influence to the operation of the LSI. However, this leads to an increase
in the number of circuits in the system, and is therefore not suitable for
the portable type.
SUMMARY OF THE INVENTION
To solve the above mentioned problem, the present invention provides a
communication system which can follow the frequency of the base station
with high accuracy, taking into consideration the frequency error of the
oscillator.
According to the first aspect of the present invention, there is provided a
communication system which comprises a receiving section which includes at
least one frequency converting device for converting a received signal
into an intermediate frequency signal by mixing with a local oscillation
signal, and a demodulator for demodulating the received signal converted
into the intermediate frequency signal, a transmission section for
modulating a transmission signal received thereby to transmit the so
modulated signal, and an input/output section for receiving the
demodulated signal from the receiving section and outputting it externally
and receiving the transmission signal externally and outputting it to the
transmission section. The receiving section includes at least one local
oscillating device for generating the local oscillation signal, reference
frequency generating device for generating a reference signal, calibration
signal generating device for generating a calibration signal having a
predetermined frequency on the basis of the reference signal, to input the
calibration signal to the frequency converting device, counter device for
counting the frequency of the intermediate frequency signal, and
correcting device for detecting a frequency error on the basis of the
counted value of the intermediate frequency signal. The frequency
converting device converts the calibration signal, in addition to the
received signal, into the intermediate frequency signal by mixing with the
local oscillation frequency, and the counter device counts the frequency
of the calibration signal converted by the frequency converting device as
the intermediate frequency signal. The correcting device detects the
frequency error from a frequency difference between the counted value of
the calibration signal counted by the counter device, and a predetermined
intermediate frequency reference value, thereby generating a signal for
frequency correction on the basis of the thus detected frequency error.
The signal for frequency correction generated by the correcting device can
be an oscillation frequency control signal, which is to be inputted to the
local oscillating device, and the local oscillating device is controlled
with respect to a frequency of the local oscillation signal on the basis
of the oscillation frequency control signal. Further, the local
oscillating device can include first local oscillating device for
oscillating a frequency on the basis of the reference oscillation
frequency generated from the reference frequency generating device and
second local oscillating device for generating the local oscillation
signal. The frequency converting device may include first and second
frequency converting device, which correspond to the first and second
local oscillating device, respectively, for mixing the received signal
with the respective local oscillation signals to convert them into first
and second intermediate frequency signals, respectively. The second
frequency converting device may convert, in addition to the received
signal, the calibration signal which is converted into the first
intermediate frequency signal or corresponds to the first intermediate
frequency, into a second intermediate frequency signal by mixing with the
oscillation signal oscillated by the second oscillating device. The
counter device may count the calibration signal converted into the second
intermediate frequency signal by the second frequency converting device,
and the correcting device may detect a frequency error between the counted
value of the calibration signal converted into the second intermediate
frequency signal which is counted by the counter device, and the
predetermined intermediate frequency reference value, to thereby generate
a signal for frequency correction on the basis of the thus detected
frequency error.
In the communication system as described, the calibration signal generating
device can generate the calibration signal having a predetermined
frequency to output it before the first frequency converting device. The
first frequency converting device converts the calibration signal into the
first intermediate frequency signal by mixing with the local oscillation
signal oscillated by the first local oscillating device, and the second
frequency converting device converts the calibration signal converted into
the first intermediate frequency signal into the second intermediate
frequency signal by mixing with the local oscillation signal oscillated by
the second local oscillating device.
The communication system may further include frequency controlling device
for outputting the oscillation frequency control signal to the reference
frequency generating device on the basis of the frequency error between
the counted value of the calibration signal converted into the second
intermediate frequency signal and the intermediate frequency reference
value, which is detected by the counter device. The reference frequency
generating device is controlled with respect to the frequency of the
reference oscillation signal on the basis of the oscillation frequency
control signal.
The communication system may further include controlling device for
instructing the calibration signal generating device to activate
generation of the calibration signal, and a temperature sensoring device
for instructing the calibration signal generating device to activate
generation of the calibration signal. The controlling device instructs the
calibration signal generating device to activate generation of the
calibration signal at a regular period of time. The controlling device
monitors whether or not communication is being carried out in the
communication system and issues a command for generating the calibration
signal to the calibration signal generating device when the communication
is not being carried out.
According to the second aspect of the present invention, there is provided
a mobile communications system which comprises a receiving section which
includes first and second local oscillating device each for generating
local oscillation signal, first and second frequency converting device
each for converting a received signal into an intermediate frequency
signal by mixing with the corresponding local oscillation signal,
reference frequency oscillating device for generating a reference signal
having a predetermined reference frequency, calibration signal generating
device for generating a calibration signal on the basis of the reference
signal generated from the reference frequency oscillating device, and a
demodulator for demodulating the received signal converted into the
intermediate frequency signals, a transmission section for modulating a
transmission signal received thereby to transmit the so modulated signal,
and an input/output section for receiving the demodulated signal and
outputting the transmission signal. The reference oscillating device is
controlled on the basis of the received signal, and the first local
oscillating device oscillates on the basis of the so controlled reference
frequency oscillating device. The calibration signal generating device
generates the calibration signal which is controlled on the basis of the
reference signal generated by the controlled reference frequency
oscillating device, and the second oscillating device is corrected on the
basis of an oscillation frequency control signal which is determined on
the basis of the frequency difference between the intermediate frequency
reference value and the counted value of the controlled calibration
signal, which is generated by the calibration signal generating device and
inputted to the second frequency converting device to be converted into a
second intermediate frequency signal.
The mobile communication system may include selector device for selecting
either one of the output of the received signal and the output of the
calibration signal, or vice versa.
According to the third aspect of the present invention, there is provided a
calibration method comprising the steps of oscillating a reference signal
having a predetermined reference frequency, controlling the reference
frequency of the reference signal on the basis of a received signal
externally, generating a calibration signal on the basis of the so
controlled reference signal, oscillating a local oscillation signal having
a predetermined local oscillating frequency, converting the calibration
signal into an intermediate frequency signal by mixing with the local
oscillation signal, counting the calibration signal converted into the
intermediate frequency signal, and detecting a frequency error between an
intermediate frequency reference value and the counted value of the
calibration signal converted into the intermediate frequency signal,
thereby correcting a local oscillation frequency of the local oscillation
signal on the basis of the frequency error.
According to the fourth aspect of the present invention, there is provided
a communication system comprising a receiving section which includes first
frequency converting device for converting a received signal into a first
intermediate frequency signal by mixing with a first local oscillation
signal, second frequency converting device for converting the received
signal converted into the first intermediate frequency signal into a
second intermediate frequency signal by mixing with a second local
oscillation signal, and a demodulator for demodulating the received signal
converted into the intermediate frequency signal; a transmission section
for modulating a transmission signal received thereby, to transmit the so
modulated signal, and an input/output section for receiving the
demodulated signal from the receiving section and outputting it externally
and receiving the transmission signal externally and outputting it to the
transmission section. The receiving section include a reference frequency
generating device for generating a reference signal, a calibration signal
generating device for outputting a calibration signal of the first
intermediate frequency on the basis of the reference signal, and a
frequency estimating and correcting device for detecting a frequency error
of the reference frequency generating device and outputting an oscillation
frequency control signal to the reference frequency generating device on
the basis of the so detected frequency error. The reference frequency
generating device is controlled on the basis of the oscillation frequency
control signal, with respect to an oscillation frequency of the reference
signal. The second frequency converting device converts, in addition to
the received signal converted into the first intermediate frequency
signal, and the calibration signal converted into the first intermediate
frequency into a second intermediate frequency signal by mixing with the
second local oscillation signal. The frequency estimating and correcting
device detecting a frequency error A between the calibrating signal
converted into the second intermediate frequency and a predetermined
reference intermediate frequency, further detecting a frequency error C
which is determined by eliminating the frequency error A from a frequency
error B which is a frequency difference between the received signal
converted into the second intermediate frequency and the intermediate
frequency, to thereby output an oscillation frequency control signal on
the basis of the thus detected frequency error C.
The frequency estimating and correcting device may include a first
frequency estimator for detecting the frequency error A between to be
outputted, a phase compensator for correcting an phase of the received
signal converted into the second intermediate frequency on the basis of
the frequency error A detected by the first frequency estimator, a second
frequency estimator for detecting the frequency error C and outputting a
frequency error signal on the basis of the frequency error C, and a
frequency correcting device for outputting the oscillation frequency
control signal to the reference frequency generating device on the basis
of the frequency error signal outputted from the second frequency
estimator. The second frequency estimator detecting the frequency error C
on the basis of the received signal corrected by the phase compensator and
the intermediate frequency.
Further, the frequency estimating and correcting device may include a first
frequency estimator for detecting the frequency error A to be outputted, a
second frequency estimator for detecting the frequency error B and
outputting to be outputted, and a frequency correcting device for
detecting the frequency error C to output the oscillation frequency
control signal to the reference frequency generating device on the basis
of the detected frequency error C. The frequency correcting device can
detect the frequency error C on the basis of the frequency error A
outputted from the first frequency estimator and the frequency error B
outputted from the second frequency estimator.
The phase compensator further may include a mode in which a phase of the
received signal converted into the second intermediate frequency is
corrected and an another mode in which the received signal is outputted
without being corrected, and selector device for selecting either one of
the modes. In this case, the frequency correcting device detects the
frequency error C according to the selected mode and outputting the
oscillation frequency control signal to the reference frequency generating
device.
In the communication system as described above, the frequency estimating
and correcting device may includes a frequency estimator for detecting the
frequency error A a frequency memory for storing the frequency error A, a
phase compensator for compensating a phase of the received signal
converted into the second intermediate frequency, on the basis of the
frequency error A stored in the frequency memory, and a frequency
correcting device for outputting the oscillation frequency control signal
to the reference frequency generating device. The frequency estimator can
detect the frequency error C on the basis of the received signal corrected
with the phase compensator and the reference intermediate frequency to
output a frequency error signal, and the frequency correcting device
outputs the oscillation frequency control signal to the reference
frequency generating device on the basis of the frequency error signal.
In the above mentioned communication system, the frequency estimating and
correcting device can detect the frequency error C in such a manner that
the frequency error B is initially detected, and the frequency error C is
detected by subtracting the frequency error A from the frequency error B.
Or, the frequency estimating and correcting device can detect the
frequency error C such that a phase of the received signal is corrected on
the basis of the frequency error A, and the frequency error C is detected
on the basis of the so corrected received signal and the predetermined
reference intermediate frequency.
BRIEF DESCRIPTION OF THE DRAWINGS
FIG. 1 is a block diagram showing the structure of a communication system
according to the first embodiment of the present invention;
FIG. 2 is a block diagram showing the structure of a communication system
according to the second embodiment of the present invention;
FIG. 3 is a block diagram showing the structure of a communication system
according to the third embodiment of the present invention;
FIG. 4 is a block diagram showing an example of a calibration signal
generator;
FIG. 5 is a block diagram showing another example of the calibration signal
generator;
FIG. 6 is a block diagram showing the structure of a communication system
according to the fourth embodiment of the present invention;
FIG. 7 is a block diagram showing the structure of a communication system
according to the fifth embodiment of the present invention;
FIG. 8 is a block diagram showing the structure of a mobile communications
system according to the sixth embodiment of the present invention;
FIG. 9 is a block diagram showing the structure of a quasi-coherent
detector 308;
FIG. 10 is a block diagram showing the structure of a phase compensator
310;
FIG. 11 is a block diagram showing the structure of a phase compensator
312;
FIG. 12 is a block diagram showing the structure of a mobile communications
system according to the seventh embodiment;
FIG. 13 is a block diagram showing the structure of a mobile communications
system according to the eighth embodiment;
FIG. 14 is a flowchart of an automatic frequency control processing
according to the eighth embodiment;
FIG. 15 is a block diagram showing the structure of a mobile communications
system according to the ninth embodiment;
FIG. 16 is a block diagram showing the structure of a phase compensator
310A;
FIG. 17 is a timing chart showing an automatic frequency control processing
according to the ninth embodiment;
FIG. 18 is a flowchart of an automatic frequency control processing
according to the ninth embodiment;
FIG. 19 is a block diagram showing the structure of a mobile communications
system according to the tenth embodiment;
FIG. 20 is a block diagram showing the structure of a mobile communications
system according to the eleventh embodiment;
FIG. 21 is a block diagram showing the structure of a mobile communications
system according to the twelfth embodiment;
FIG. 22 is a timing chart of an automatic frequency control processing
according to the tenth embodiment; and
FIG. 23 is a timing chart of an automatic frequency control processing
according to the tenth embodiment.
DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS
FIG. 1 shows the structure of a communication system according to the first
embodiment of the present invention. The communication system shown in
FIG. 1 comprises a receiving section C for receiving a signal, a
transmission section B for modulating the received signa | | |
