Note: Descriptions are shown in the official language in which they were submitted.
2195039
SPECIFICATION
The present invention relates to frequency stabilization for
radio communication sy~tem. In particular,it relates to mobile
communication system for decreasing Doppler effect caused by
relative motion between transceivers and for decreasing ~he
influence from frequency-shift of the local oscillation signal
of the transceivers.
Early radio communication has mainly consisted of fixed point
to point communication. Mobile communications are being used
increasingly. This creates a number of problems.First,there is
Doppler frequency-drift in the signal received by both of the
fixed and mobile stations that will make the received signal
frequencies exceed channel bandwidth range so as to fall into
adjacent channels to generate substantial interferences when
the transceivers are in relative motions. This either causes
the receivers detune into the received signal frequencies or
place the received signal into the adjacent channels of the
receivers.Second, previously known super heterodyne receivers
have been mainly used in fixed-point reception mode and their
operating frequencies have been low enough for the receiver's
performances to be satisfactory.But this situation has changed
significantly with the appearance of mobile communication and
much higher operating frequencies. A signal frequency-shift is
difficult to control because:
l.The stability of the local oscillation generators of the
transceivers are limited. The higher the frequency,the greater
is the drift.The local oscillation frequency of the receiver
increases with higher its receiving signal frequency, which is
not necessary to signal selection but is necessary to signal
frequency conversion; so, the stability of the intermediate
signal frequency, which depends upon the balance between the
local oscillation stability of two communicating transceivers,
become worse and worse.
2.The higher the frequencies, the worse the Doppler effects
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become.This causes the $tability of the intermediate-frequency
signal of the receiver to decrease.
3.The power consumption of the local oscillation generator
increase heavily as the loca' oscillation frequency raises.
In order to solve the above adjacent interference, a means has
been previously employed to set a guard band on a channel;
however, this occupies more bandwidth and therefore, reduces
the frequency utilization ratio.Especially,in the cases of SSB
communications,this will become a big concern since the guard
bandwidth will occupy considerable part of the each channe " s
bandwidth.
In order to solve the above frequency-drift problem, some
methods have been invented,including one of inbound pilot tone
, etc..Nevertheless,the common ground of those methods are the
device performance is restricted by its frequency response and
the local oscillation frequency is still related to operating
band frequency.For these reasons, it is limited to the applied
range and condition of those methods.
In order to solve the above power consumption problem, an
intermittent duty mode of receiver local oscillation has been
employed but it may slow down the response of the system and
makes no difference once in normal operation.
Therefore, the above problems have not been solved or entirely
solved by known methods and much more effective communication
appl_ances are required to respond to the needs of the
developing communication industry and market. Existing radio
appliances and the single carrier way of radio communication
have exposed obvious defects and problems, which have impeded
the development of the industry.
It is the object of the invention to provide improved methods,
appliances and communication systems. It provide an improved
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communication system which can maintain the received signal
carrier-frequency of the base station and get the receiver of
the mobile station into the tuning state regardless of the
movement of the mobile station. It also provides an improved
receiver which is not influenced by the movement and the
variance of operating frequency and limits the effect of
Doppler frequency-shift and local oscillation frequency-shift
without depending upon t~e improvement of device performance.
Furthermore, it enables the power consumption of the receiver'
s local oscillation signal generator not to change as the
frequency band of the received signal changes.
In a first aspect it provides a frequency stabilized communi-
cation system which has, in combination,some receivers and /or
transmitters of base station and mobile stations whose charac-
teristic is that the frequency of reception and transmission
assigned by the base station can be real-time modified ~y
mobile station itself or according to the received signaling
from base station as responsing to Doppler-shift caused by the
movement of the mobile one so that the received carrier fre-
quency of base that maintain always in a permissible range;and
the receiver of mobile that is in accord with its signal
throughout.
The radio receiver may be a dual carrier type and select the
~I the frequency of reception of the dual-carrier signals by mix-
ing the signals with a local oscillation signal and frequency
conversion of the dual-carrier signals is finished by the dual
carrier signal mixed itself.
The dual-carrier receiver may have the feature that frequency
difference between signal frequencies of the first local osci-
lation and the dual-carrier frequencies are neither equal to
the first intermediate frequency.
The dual-carrier may has the feature that said frequency-shift
characteristics of the two carriers corresponds, but said
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carriers are different in frequency and modulated content.
The transceiver of mobile station and dual-carrier signals may
have the feature that the unmodulated carrier signal, which is
one of the dual-carrier,can be employed as part of the carrier
signal of the transmitter.
In a second aspect the invention provides a frequency stabili-
zed communication system with mobile stations and base station
A Doppler frequency detective and corrective signal generation
unit may be employed to the transceiver either of the mobile
station or the base station. In the first case, the mobile
station first receive the carrier signal from the base station
and then, detect the amount of Doppler-shift in the received
carrier signal. A transmitting and receiving frequency correc-
ting signal will be generated by the unit then according to
the detected amount of Doppler-shift and be used for real-time
adjusting the transmitting and receiving frequencies of mobile
transceiver so that the received carrier frequency of the base
station that maintain always in a permissible range; and the
receiver of the mobile that is in accord with its signal
throughout. In the second case, instead of applying the unit
to each mobile station, the unit is only employed to the bese
station for real-time adjusting of the transmitting and recie-
ving frequencies of all mobile transceivers which belong to
~I the same base station according to their states of movement
. The unit applied to the transceiver of base station works in
the same way as it is in the mobile transceiver and for the
same purpose and effect besides that the Doppler-shift correc-
ting signal is transmitted to each mobile transceiver by radio
as command in signaling form. After ~he signaling is received,
the mobile transceiver will make corresponding adjustments to
their transmitting and receiving frequencies in the same way
as if communication channel changes.
For a better understanding of the present invention and to
more clearly how it may be carried into effect, reference will
219503g
now be made by way of example to the accompanying drawings,
which show the preferred embodiments of the prèsent invention
and in which:
~igure 1 is a block diagram of mobile station transceiver
with a Doppler shift detecting & correcting signal
generative unit for automatic real-time control of
transmitting and receiving frequencies.
~igure 2 is a block diagram of base station transceiver
with a Doppler shift detecting & correcting signal
generative unit for remote real-time control of
transmitting and receiving frequencies of mobile
transceivers.
~igure 3 is a block diagram of a dual-carrer receiver.
~igure 4 is an illustration of frequency stabilized mobile
radio~communication system in which Doppler shift
lS automatically corrected by mobile station.
~igure 5 is an illustration of frequency stabilized mobile
radio communication system in wh~ch Doppler shift
is remotely real-t~me corrected by base station.
Referring to FIG.1, a transceiver of mobile station works at
frequencies set by base station initially. The mobile station
receiver receives the signal from the base station and passes
it to the transmitting carrier generator and to the Doppler
shift detecting & correcting signal generative unit then. In
the transmitting carrier generator, transmitting frequency is
set initially according to the signaling from base station
and a transmitting carrier signal is generated and provided
to the modulator. Meanwhile, in the Doppler shift detecting &
correcting signal unit, carrier frequency of received signal
is compared with a preset receiving frequency and if there is
any difference between them, a frequency offset signal ~ F1
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will be generated and passed to both the frequency selective
unit of receiver and the modulation unit of transmitter to
adjust their presetting frequencies and makes the frequency
value deviate from the original value so that carrier signal
received by base station is still equal to the presetting
frequency value and meanwhile,the mobile station reception is
always in signal-tuning condition despite the exist of rela-
tive movement between mobile station and base station.
Referring to FIG.2, a transceiver of base station works at
frequencies set by itself. A Doppler shift detecting & corre-
cting signal generative unit in the transceiver works in the
same way as it is in the mobile transceiver but the frequency
offset signal ~ F2 is transferred by radio wave to the mobile
stations as signaling form to remotely real-time control and
correct the transmitting and receiving frequency of mobile
transceiver. The effect of such an arrangement will be the
same as that of FIG.1 but it prevents the Doppler-shift unit
from being installed in every mobile transceivers which be-
long to the same base station.
Referring to FIG.4, since the Doppler-shift unit is installed
in mobile transceiver, supposing the mobile station is appro-
aching the base station, the frequency received by the mobile
station will be increased by Doppler-shift ~F.Thus,the mobile
receiver need to adjust its receiving frequency by adding an
offset value of ~F so that the signal-tuning condition can be
maintained and meanwhile,to adjust its transmitting frequency
by subtracting an offset value of ~ F so that the frequency of
signal received by the base station will still be equal to
its presetting value. So, the transmitting and receiving fre-
quencies will always be as what are preset initially.
Referring to FIG.5, since the Doppler-shift unit is installed
only in base station, signaling ~F2 is added as information
signal by the base station and transferred to mobile stations
. At mobile station,~F~2 is interpreted and transferred to a
corresponding frequency offset value to correct transmitting
.
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and receiving frequencies. Supposing the mobile station is
moving away from the base station, the actual signal frequency
of received signal will be decreased by ~F owing to the exist
of Doppler-shift.Therefore, the receiving frequency of mobile
receiver need to be subtracted by an offset value ~F and mean-
while, the transmitting frequency of the mobile transmitter
need to be added by an offset value ~ F1 to get the equal
affect as that of FIG.4
Referring to FIG.3, the receiver is a dual-carrier type. It
receives two adjacent carrier signals which come from the same
source when working.The carrier signals have different carrier
frequency. One is a modulated carrier signal for the transfer
of information; the other is an unmodulated signal for freque-
ncy conversion and signal processing.
The first local oscillation signal of the ~eceiver is only
employed to select frequency,and the unmodulated carrier is
employed to change frequency. The receiver mixs a local osci-
llation signal,whose frequency is much lower than the dual
carrier frequency with the dual-carrier signal, then the mixed
signal frequency is inserted into the range of two different
parallel bandpass filters.The signals are separated from pass-
ing through the respective filter to obtain the aim of freque-
ncy selection and signal separation.
~I
The separated signal is transferred to another mixer.The mixed
signal is an intermediate-frequency signal to implement the
frequency conversion of modulated carrier signal. The interme-
diate-frequency signal has subtracted the local oscillation
drift component of receiver because of frequency difference.
Similarly, Doppler frequency-drift in the modulated carrier
and the local oscillation signal drift of transmitter are
mostly subtracted by the unmodulated carrier. Moreover, the
local oscillation frequency range depends upon the received
passband width as local oscillation signal is used for freque-
ncy selection.Thus the frequency of local oscillation can be
chosen much lower than the dual-carrier signal frequency, .he
2195 039
change of receiving signal frequency band has no influence on
the local oscillation frequency. The power consumption of the
local oscillation generator dose not change as the ~receiving
frequency-band changes.
The receiver resolves some technical key problems of the pre-
sent appliances entirely with conventional devices. The single
sideband communication now is possible to be applied at any
radio frequency band in mobile communication. Mobile communi-
cation of stabilized frequency is provided for preferred embo-
diment oï the receiver.
In FIG.3,the first local oscillation signal VO(t) is only used
for selective frequency,the second local oscillation signal
VO'(t) is applied to synchronous demodulation. Vl(t) is an un-
modulated carrier signal. V2(t) is a modulated carrier signal.
Assuming that the frequency difference between two carrier
frequencies is a defined value which has the same frequency as
VO'(t),and narrow-band filter A and B has its fixed frequency
and bandwidth.
_,
signal Vl(t) and V2(t) are amplified by a pre-amplifier. The
amplified signal transfers to mixer O to mix with the first
local oscillation signal VO(t) so that dual-carrier signal out
put from the mixer match respectively with the passbands of
narrow-band filter A and B. Wlth this manner, the signal that
passes through filter A and B is chosen and separated.There is
only V2(t) component,in filter A output. There is only Vl(t)
component in filter B output. The frequency selection process
is finished until now.
Then, the output signal Vll(t) of filter B is transferred to a
wa~-eform shaper B to eliminate an amplitude modulation genera-
ted from Vl(t) because of radio transmission.V12(t) from wave-
form shaper B are transferred respecti~ely to mixer A and C.
The frequency conversion is implemented in mixer A where the
carrier signal V21(t) and V12(t),which are separated by narrow
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band filter A and B, are mixed to generate an IF signal V22(t)
whose frequency is equal to the frequency difference between
V12(t) and V21(t),which has no effect of frequency-drift VO(t)
Most of Doppler frequency-shift in V21(t) and frequency-drift
transmitter local oscillation are also offset by the same
signal at Vll(t). The frequency-conversion process finished.
Signal V22(t) is conveyed to an IF amplifier for amplifying,
thereafter mixed with the second local oscillation signal
VO'(t) at mixer B to implement sync-demodulation of V22(t). At
the same time,signal Vll(t) at mixer C mixes with V12(t) to
demodulate amplitude modulation signal due to radio transmi-
ssion. An output signal V13(t) of mixer C,which is filtered by
lowpass filter C and amplified by amplifier B, has employed to
control the amplification times of amplifier; A real-time to
eliminate the amplitude modulation component in V2(t) because
of radio transmission. Finally,the amplifier A output is fed
to a synchronization unit whose output signal can enable
VOl'(t) to maintain synchronization with signal V22(t) strict-
ly for implementing to syn-demodulation of V23(t).
By applying a Doppler frequency detective & corrective signal
generation unit to the transceiver equipment of either the
base station or mobile station, a frequency stabilized commu-
nication system can monitor the frequency-shift quantity of
received signal,and give a corrective signal that enables the
receiving and transmitting frequencies of mobile station to be
changed as the mobile station state changes. So the received
signal of base station maintains always in the signal channel
width,the mobile receiver is in tuning condition.
By providing an improved receiver, a dual-carrier receiver,
Doppler-shift and transceiver local oscillation signal drift
can be extremely decreased without depending upon the device
performance and the power consumption of mobile transceiver
local oscillation signal generators will not change as the
the operating band changes.
By applying the frequency stabilized communication system in
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mobile SSB communication, SSB mobile communication may be
applicable at any radio frequency band and therefore,the each
channel's bandwidth may be narrowed to 5KHz from 25KHz now,
and power consumption of mobile transceiver may be reduced by
80%.
It will be understood that this description is made with
reference to the prefer.red embodiments of the invention.
However, it is possible to make other embodiments that employ
the principles of the invention and that fall within its
spirit and scope as defined by the following claims.
.
