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Claims  |
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What is claimed is:
1. A method of determining a position of a mobile object, comprising the
steps of:
transmitting a radio wave of a constant frequency from said mobile object;
receiving the radio wave transmitted from said mobile object and detecting
the frequency of the received radio wave at each of a plurality of spaced
positions in an area where said mobile object is movable;
determining a plurality of Doppler shift signals observed at the respective
spaced positions, each Doppler shift signal being indicative of a Doppler
shift value dependent on a movement of the mobile object, on the basis of
frequencies of the received radio wave detected at the respective
positions, respectively;
selecting at least two Doppler shift signals which satisfy predetermined
two different conditions, respectively, among the plurality of Doppler
shift signals;
obtaining the position of said mobile object from position data indicating
the positions corresponding to the selected Doppler shift signals; and
wherein the selected Doppler shift signals comprise a first Doppler shift
signal which provides a maximum value of upward Doppler shift values, a
second Doppler shift signal which provides a maximum value of downward
Doppler shift values and a third Doppler shift signal which provides a
minimum value of the upward and downward Doppler shift values which are
represented by the respective Doppler shift signals.
2. A method according to claim 1, wherein the position of said mobile
object is determined by a point where a first straight line and a second
straight line intersect, the first straight line connecting a first
position corresponding to the first Doppler shift signal and a second
position corresponding to the second Doppler shift signal, and the second
straight line passing through a third position corresponding to the third
Doppler shift signal and being perpendicular to the first straight line.
3. A method of determining a position of a mobile object, comprising the
steps of:
transmitting a radio wave of a constant frequency from said mobile object;
receiving the radio wave transmitted from said mobile object and detecting
the frequency of the received radio wave at each of a plurality of spaced
positions in an area where said mobile object is movable;
determining a plurality of Doppler shift signals observed at the respective
spaced positions, each Doppler shift signal being indicative of a Doppler
shift value dependent on a movement of the mobile object, on the basis of
frequencies of the received radio wave detected at the positions,
respectively;
selecting at least two Doppler shift signals which satisfy predetermined
two different conditions, respectively, among the plurality of Doppler
shift signals;
obtaining the position of said mobile object from position data indicating
the positions corresponding to the selected Doppler shift signals; and
wherein the selected Doppler shift signals comprise a first Doppler shift
signal which provides a maximum value of upward Doppler shift values, a
second Doppler shift signal which provides a maximum value of downward
Doppler shift values and a third and a fourth Doppler shift signals which
provide substantially equal upward and downward shift values,
respectively, among the upward and downward shift values represented by
the respective Doppler shift signals.
4. A method according to claim 3 wherein the position of said mobile object
is determined by a point where a first straight line and a second straight
line intersect, the first straight line connecting a first position
corresponding to the first Doppler shift signal and a second position
corresponding to the second Doppler shift signal, and the second straight
line connecting a third position corresponding to the third Doppler shift
signal and a fourth position corresponding to the fourth Doppler shift
signal.
5. An apparatus for determining a position of a mobile object, comprising:
means provided at each of a plurality of spaced positions for receiving a
radio wave of a constant frequency transmitted from the mobile object and
detecting a frequency of the radio wave;
means for generating a plurality of Doppler shift signals observed at the
respective spaced positions, each Doppler shift signal being indicative of
a Doppler shift value dependent on a movement of the mobile object, on the
basis of frequencies of the received radio wave detected at the respective
positions;
means for selecting at least two Doppler shift signals which satisfy
predetermined two different conditions among the plurality of Doppler
shift signals;
means for determining the position of the mobile object from position data
indicating the positions corresponding to the selected Doppler shift
signals; and
wherein the selected Doppler shift signals comprise a first Doppler shift
signal which provides a maximum value of upward Doppler shift values, a
second Doppler shift signal which provides a maximum value of downward
Doppler shift values and a third Doppler shift signal which provides a
minimum value of the upward and downward Doppler shift values which are
represented by the plurality of Doppler shift signals, respectively.
6. An apparatus according to claim 5, wherein the position of said mobile
object is determined by a point where a first straight line and a second
straight line intersect, the first straight line connecting a first
position corresponding to the first Doppler shift signal and a second
position corresponding to the second Doppler shift signal, and the second
straight line passing through a third position corresponding to the third
Doppler shift signal and being perpendicular to the first straight line.
7. An apparatus for determining a position of a mobile object, comprising:
means provided at each of a plurality of spaced positions for receiving a
radio wave of a constant frequency transmitted from the mobile object and
detecting a frequency of the radio wave;
means for generating a plurality of Doppler shift signals observed at the
respective spaced positions, each Doppler shift signal being indicative of
a Doppler shift value dependent on a movement of the mobile object, on the
basis of frequencies of the received radio wave detected at the respective
positions;
means for selecting at least two Doppler shift signals which satisfy
predetermined two different conditions among the plurality of Doppler
shift signals;
means for determining the position of the mobile object from position data
indicating the positions corresponding to the selected Doppler shift
signals; and
wherein the selected Doppler shift signals comprise a first Doppler shift
signal which provides a maximum value of upward Doppler shift values, a
second Doppler shift signal which provides a maximum value of downward
Doppler shift values and third and fourth Doppler shift signals which
provide substantially equal upward and downward shift values,
respectively.
8. A method according to claim 7 wherein the position of said mobile object
is determined by a point where a first straight line and a second straight
line intersect, the first straight line connecting a first position
corresponding to the first Doppler shift signal and a second position
corresponding to the second Doppler shift signal, and the second straight
line connecting a third position corresponding to the third Doppler shift
signal and a fourth position corresponding to the fourth Doppler shift
signal.
9. A mobile communication system comprising:
a plurality of base stations distributed within a region where a mobile
object is movable for radio communication with the mobile object;
frequency detecting means provided in each of the base stations for
detecting a predetermined frequency of a radio wave transmitted from the
mobile object and received at the base station;
mobile object detecting means for detecting a position of the mobile object
on the basis of the respective positions of said base stations and
frequency differences between the detected frequencies detected by said
frequency detecting means provided in the respective base stations and the
predetermined frequency of the radio wave transmitted from the mobile
object; and
wherein said mobile object detecting means comprises means for determining
the position of said mobile object as a point where a first straight line
and a second straight line intersect with each other, the first straight
line passing through at least one of the base stations which detects a
substantially maximum upward Doppler shift value or its vicinity and at
least one of the base stations which detects a substantially maximum
downward Doppler shift value or its vicinity, the second straight line
intersecting with the first straight line substantially at a right angle
and passing through at least one of the base stations which detects a
substantially zero or minimum Doppler shift value or its vicinity.
10. A mobile communication system comprising:
a plurality of base stations distributed within a region where a mobile
object is movable for radio communication with the mobile object;
frequency detecting means provided in each of the base stations for
detecting a predetermined frequency of a radio wave transmitted from the
mobile object and received at the base station;
mobile object detecting means for detecting a position of the mobile object
on the basis of the respective positions of said base stations and
frequency differences between the detected frequencies detected by said
frequency detecting means provided in the respective base stations and the
predetermined frequency of the radio wave transmitted from the mobile
object; and
wherein said mobile object detecting means comprises means for determining
the position of said mobile object as a point where a first straight line
and a second straight line intersect with each other, the first straight
line passing through at least one of the base stations which detects a
substantially maximum upward Doppler shift value or its vicinity and at
least one of the base stations which detects a substantially maximum
downward Doppler shift value or its vicinity, the second straight line
passing through at least two of the base stations which detect
substantially equal up and down Doppler shift values or their vicinities.
11. An apparatus for determining a position of a mobile object, comprising:
a plurality of radio stations disposed in a region where a mobile object
which transmits a radio wave, is movable to make radio communication with
the mobile object;
frequency detecting means provided in each of said radio stations for
detecting a predetermined frequency of a radio wave transmitted from the
mobile object and received at the radio station;
mobile object detecting means for detecting the position of the mobile
object on the basis of respective positions of said radio stations and
frequency differences between the detected frequencies detected by said
frequency detecting means of the respective radio stations and the
predetermined frequency of the radio wave transmitted from the mobile
object; and
wherein said mobile object detecting means comprises means for determining
the position of the mobile object as a point where a first straight line
and a second straight line intersect with each other, the first straight
line passing through at least one of the radio stations which detects a
substantially maximum upward Doppler shift value or its vicinity and at
least one of the radio stations which detects a substantially maximum
downward Doppler shift value or its vicinity, the second straight line
intersecting with the first straight line substantially at a right angle
and passing through at least one of the radio stations which detects a
substantially zero or minimum Doppler shift value or its vicinity.
12. An apparatus for determining a position of a mobile object, comprising:
a plurality of radio stations disposed in a region where a mobile object
which transmits a radio wave, is movable to make radio communication with
the mobile object;
frequency detecting means provided in each of said radio stations for
detecting a predetermined frequency of the radio wave transmitted from the
mobile object and received at the radio station;
mobile object detecting means for detecting the position of the mobile
object on the basis of respective positions of said radio stations and
frequency differences between the detected frequencies detected by said
frequency detecting means of the respective radio stations and the
predetermined frequency of the radio wave transmitted from the mobile
object; and
wherein said mobile object detecting means comprises means for determining
the position of the mobile object as a point where a first straight line
and a second straight line intersect with each other, the first straight
line passing through at least one of the radio stations which detects a
substantially maximum upward Doppler shift values or its vicinity and at
least one of the radio stations which detects a substantially maximum
downward Doppler shift value or its vicinity, the second straight line
passing through at least two of the radio stations which obtained the
detected values including substantially equal up and down shift values or
their vicinities.
13. An apparatus for determining a position of a mobile object, comprising:
a plurality of spaced receiving means disposed in a space where a mobile
object which transmits a radio wave signal of a given frequency is movable
for receiving the radio wave signal from the mobile object;
frequency deviation measuring means for measuring a frequency deviation in
the radio wave signal received by each of said plurality of receiving
means;
position detecting means for detecting the position of the mobile object on
the basis of the frequency deviation associated with each of said
plurality of receiving means and the respective positions of said
receiving means; and
wherein said position detecting means comprises means for determining the
position of the mobile object as a point where a first straight line and a
second straight line intersect with each other, the first straight line
passing through at least one of the receiving means which detects a
substantially maximum upward Doppler shift value or its vicinity and at
least one of the receiving means which detects a substantially maximum
downward Doppler shift value or its vicinity, the second straight line
intersecting with the first straight line substantially at a right angle
and passing through at least one of the receiving means which detects a
substantially zero or minimum Doppler shift value or a vicinity.
14. An apparatus for determining a position of a mobile object, comprising:
a plurality of spaced receiving means disposed in a space where a mobile
object which transmits a radio wave signal of a given frequency is movable
for receiving the radio wave signal from the mobile object;
frequency deviation measuring means for measuring a frequency deviation in
the radio wave signal received by each of said plurality of receiving
means;
position detecting means for detecting the position of the mobile object on
the basis of the frequency deviation associated with each of said
plurality of receiving means and the respective positions of said
receiving means; and
wherein said position detecting means comprises means for determining the
position of the mobile object as a point where a first straight line and a
second straight line intersect with each other, the first straight line
passing through at least one of the receiving means which detects a
substantially maximum upward Doppler shift value or its vicinity and at
least one of the receiving means which detects a substantially maximum
downward Doppler shift value or its vicinity, the second straight line
passing through at least two of the receiving means which detect
substantially equal up and down Doppler shift values or their vicinities.
15. A method of determining a current position of a mobile object which has
moved from a known position, said method comprising the steps of:
transmitting a radio wave of a constant frequency from the mobile object;
receiving the radio wave transmitted from said mobile object and detecting
a frequency of the received radio wave at each of a plurality of spaced
fixed positions in an area where said mobile object is movable;
determining a plurality of Doppler shift signals observed at the respective
fixed positions, each Doppler shift signal being indicative of a Doppler
shift value dependent on a movement of the mobile object, on the basis of
frequencies of the received radio wave detected at the fixed positions,
respectively;
selecting at least two of said Doppler shift signals which satisfy a
predetermined condition among the plurality of Doppler shift signals;
determining the current position of said mobile object on the basis of
position data indicating the fixed positions corresponding to said
selected at least two Doppler signals; and
wherein said selected at least two Doppler signals include maximum and
minimum Doppler shift signals which provide maximum and minimum Doppler
shift values, respectively, among upward or downward Doppler shift values
given by said plurality of Doppler shift signals.
16. A method according to claim 15, wherein the current position of said
mobile object is determined by a position where first and second straight
lines intersect with each other, wherein said first straight line passes a
first one of the fixed positions corresponding to said maximum Doppler
shift signal and extending toward said known position, while said second
straight line passes a second one of the fixed positions corresponding to
said minimum Doppler shift signal to extend toward said known position and
intersects at a right angle with said first straight line. |
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Claims |
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Description |
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FIELD OF THE INVENTION
The present invention relates to a method and an apparatus for determining
a position of a mobile object, and a mobile communication system using the
same. More particularly, the present invention relates to a method and an
apparatus for detecting a position, moving speed and moving direction of a
mobile object with high accuracy and a mobile radio communication system
using the method.
BACKGROUND OF THE INVENTION
In a mobile communication system using a TDMA (Time Division Multiple
Access) cellular process, a microcell mobile communication system has been
examined to use frequencies efficiently. A basic zone arrangement used in
the microcell mobile communication system includes the arrangement of a
service zone by contiguous microcells each having a radius of about 50 to
several hundred meters. Another idea has been proposed to form a service
zone by very small cells called picocells which are smaller than the
microcell. This system includes contiguous very small cells, each having a
radius of about 10-50 meters set in offices, in an underground street or a
building where the propagation of radio waves is relatively difficult.
Further, an idea of a so-called third generation mobile communication
system has been proposed to unify both techniques for very small cells and
the macrocells realized in a conventional mobile telephone system.
One of the third generation systems includes a FPLMTS (Future Public Land
Mobile Telecommunication Systems), the study of which has started
internationally. The FPLMTS uses a variety of cell structures such as
macrocells, microcells, and picocells arranged in a multilayered (or
complex-cellular) structure in a service zone. Thus, high degree radio
channel mobile control is indispensable which includes a control of zone
switching between the respective cells, and allocation and switching of
frequency bands among the respective cells. As the size of the cells
becomes very small, problems occur which include an increase of the
positional variation of the level of the receiving signal, an increase of
the frequency in channel switching (frequency or time slot) during
communication, and an increase of the installation cost of the base
stations. Further problem is to deal with traffic concentration or jam and
provide flexible service for mobile objects moving at very high or very
low speeds.
Conventionally, in order to solve those problems, a study has been made of
the decentralized autonomous control technique where each base station
autonomously determines and uses an available channel, and the dynamic
channel allocation which dynamically uses a frequency in accordance with
time-dependent and positional variation of the traffic. In particular, in
the switching control of a radio channel under communication ("handover"),
studies have been made of a system for synchronizing various timing
operations between the base stations, high-speed switching frequency
synthesizer, etc., in order to cope with an increase in the switching
frequency. (See H. Furukawa and Y. Akaiwa "Self-Organized Reuse
Partitioning (SORP), A Distributed Dynamic Channel Assignment Method"
Technical Report of IEICE. A.-P92-116, RCS92-126 1993-01, pp. 61-66) In
those proposed measures, the detection of the moving speed and moving
direction of a mobile object is especially important. However, no
practical detecting means have been proposed.
A conventional method of detecting the moving speed of a mobile object in a
mobile object communication system includes presumption of a Doppler
frequency, using indirect measurement. For example, as described in papers
B-400 and B-401 published at the 1993-Spring Meeting of Institute of
Electronics, Information and Communication Engineers of Japan, the speed
of a mobile object is presumed on the basis of the number of times of
intersection of power level in an envelope under Rayleigh fading and the
measured values of the frequency of switching branches in the reception
diversity. This is based on the principle that the number of times of
intersection of the power level and the frequency of switching the
branches are found statistically to be proportional to the Doppler
frequency. Since the results of those measurements and presumptions are
indirectly obtained, however, they involve large errors and are not
sufficiently satisfactory. In addition, those are applied solely to the
detection of the speed of movement of a mobile object.
Another application of the Doppler effect to the mobile communication is
intended to compensate for an undesirable frequency shift of the
communication wave due to the movement of the mobile object to ensure
stabilized communication, as disclosed in Japanese Patents JP-A-58-225741,
63-199527, 5-22183 and 5-37438. Other applications are VICSs (Vehicle
Information and Communication Systems) where the communication system
receives information on the position and speed, etc., of a car, GPS
(Global Positioning System) which use satellites as a global position
measuring system, which are, however, large-scaled and complicated.
A location detection system for a mobile terminal which is considered an
application of a so-called enterprise cordless telephone locates the
mobile terminal on the basis of the intensity of radio wave which has been
transmitted through not only the same floor of a building but also a floor
or ceiling of the building. Thus, in order to presume a fixed radio base
station in the vicinity of which the mobile terminal under detection
exists, various devices such as provision of additional devices on
passageways and stairs are required in each of the service zones, as
described in a paper entitled "A Study of Location Detection System" by
Ito, RCS Research Association, Institute of Electronics, Information and
Communication Engineers of Japan, RCS 90-48.
The basic concept of the above conventional counter method is to flexibly
handle the move of a mobile object or the generation of a new call on the
network side or user side. However, this method has an essential weak
point. In order to appropriately handle the move of the mobile object, it
is indispensable to detect the position of the object and its moving speed
and direction substantially simultaneously like in the automatic control
of a robot. Nevertheless, this conventional method only presumes the
position of the mobile object by causing each of the base stations around
the mobile object to monitor and measure the intensity of radio wave on
the basis of communication between the base station and the mobile object.
Since the intensity of the radio wave changes depending on place, time and
space, it is difficult to improve the accuracy of detection even if a
statistical technique is used. This implies that no appropriate detection
method has been proposed so far. Thus, there is naturally a limitation in
the accuracy of detection, by measurements including flexible network
control which has been considered as a method of solving this problem. The
Doppler effect, mentioned above, is solely applied to indirect detection
of frequency shift and detection of the moving speed of the mobile object.
Simultaneous detection of the position of the mobile object and its moving
speed and direction and a mobile communication system using the
simultaneous detection have not been proposed.
SUMMARY OF THE INVENTION
It is an object of the present invention to provide a method and an
apparatus for simultaneously detecting the position of a mobile object and
its moving speed and direction with high accuracy, which are suitable for
use with a mobile communication system and applicable to car navigation
and control of the travel of a robot in a factory.
It is another object of the present invention to provide a mobile
communication system having the function of simultaneously detecting the
position of a mobile object and its moving speed and direction
simultaneously with high accuracy.
According to the present invention, a method of determining a position of a
mobile object comprises the steps of transmitting a radio wave of a
constant frequency from the mobile object; receiving the radio wave
transmitted from the mobile object and detecting a deviation of a
frequency of the received radio wave from the constant frequency at each
of a plurality of spaced positions disposed in an area where the mobile
object is movable; and determining a position of the mobile object on the
basis of position data indicating the plurality of spaced positions and a
deviation of a frequency of the received radio wave from the constant
frequency detected at each of the spaced positions.
According to the present invention, an apparatus for determining a position
of a mobile object comprises means for receiving a radio wave of a
constant frequency transmitted from the mobile object and detecting a
deviation of a frequency of the received radio wave from the constant
frequency at each of a plurality of spaced positions disposed in an area
where the mobile object is movable; and means for determining a position
of the mobile object on the basis of position data indicating the
plurality of spaced positions and the deviation of the frequency of the
received radio wave from the constant frequency detected at each of the
spaced positions.
According to the present invention, a mobile radio communication system
comprises a plurality of base stations provided at a plurality of spaced
positions disposed in an area where the mobile object is movable for
making radio communication with the mobile object; means provided to each
of the base stations for receiving a radio wave of a constant frequency
transmitted from the mobile object and detecting a deviation of a
frequency; of the received radio wave from the constant frequency and
means for determining a position of the mobile object on the basis of
positional data indicating the spaced positions and the deviation of the
frequency of the received radio wave from the constant frequency detected
by each of the base stations.
The radio wave transmitted from the mobile object is received by each of
the distributed base stations. The frequency of the received wave by each
base station includes a Doppler shift component derived from the movement
of the mobile object. The Doppler shift component detected by each of the
base stations changes depending on not only the moving speed of the mobile
object but also the relationship between the moving direction of the
mobile object and the position of the base station. Since the moving speed
of the mobile object relative to the base station which is positioned
along the passageway of movement of the mobile object is large, that base
station detects a large Doppler shift component. Among the base stations
which detect large Doppler shift component, those base stations to which
the mobile object is approaching detect an up-shift component indicating
the frequency shifts upward or increase, whereas those base stations from
which the mobile object is moving away detect a down-shift component
indicating the frequency shifts downward or decrease.
In contrast, since the moving speed of the mobile object relative to the
base station which is positioned in a direction perpendicular to the
direction of movement of the mobile object is small, that base station
detects a smaller Doppler shift component. The results of the detection
including the Doppler shift components detected by the respective base
stations are reported to the mobile object detection means provided
preferably to a mobile local switch through radio or cable transmission
passages. The mobile object detection means detects the position of the
mobile object and, as required, the moving direction and speed of the
mobile object from the respective results of detection of the base
stations and the position data indicating the known positions or the
spatial distribution of the respective Doppler shift components, which are
detected by the base stations on the presumption that the Doppler shift
components are included in results of detection reported by the base
stations.
For example, the mobile object detection means detects the direction of
movement of the mobile object by setting a first line segment which passes
through at least one of the base stations which has reported the result of
the detection including substantially a maximum up-shift component or the
vicinity of the at least one base station and at least one of the base
stations which has reported the result of the detection including
substantially a maximum down shift component or the vicinity of the
last-mentioned at least one base station. Furthermore, the mobile object
detection means detects as the position of the mobile object a point where
a second line segment, which passes through at least one of the base
stations which has reported the result of the detection including
substantially a zero or minimum shift component or the vicinity of the
just-mentioned at least one base station, intersects with the first line
segment substantially at a right angle.
The frequency stability of a signal generated by each base station becomes
an issue in the detection of the frequency of the received radio wave
including the Doppler shift component. Generally, in the mobile object
communication system, the frequency stability is set as a technical
standard to be less than 3.times.10.sup.-6 in absolute accuracy in a PHP
(Personal Handy Phone), the practical use of which is expected. This value
is an absolute stability which should not be exceeded in any case. The
frequency band used in the PHP is 1.9 GHz, so that the frequency stability
is .+-.5.7 KHz from the calculation of 1.9.times.10.sup.-9
.times.3.times.10.sup.-6, from which it is required that the sum of the
frequency errors of all local oscillators used in frequency converters for
a so-called up or down conversion in the transmitter and receiver is less
than .+-.5.7 KHz.
Usually, a crystal oscillator is used as the local oscillator. Thus, the
frequency stability of the local oscillator actually is several times
higher than the above value. In particular, the frequency stability for a
short time of several to several tens of seconds except for an instant
just after the power supply is turned on is very high, for example, of
10.sup.-8 -10.sup.-10 due to the recent technical progress. Thus, the
frequency deviation can be measured directly with high accuracy from the
intermediate frequency IF, by using a local oscillator of high frequency
stability.
As described above, in view of a short-time frequency stability, the
position of a mobile object is detected with relatively high accuracy. In
order to improve the accuracy of detection, the calibration of frequency
is carried out among the base stations as in the mobile communication
system in one embodiment of the present invention. More specifically,
according to the system of this embodiment, any particular base station
transmits a standard signal of a predetermined frequency to the other base
stations around the particular base station prior to detection of the
frequency of the received radio wave including the Doppler shift
component. Each of the base stations including the particular base station
detects and stores the frequency of the standard signal transmitted from
the particular base station, and corrects or calibrates the result of
detection of the Doppler shift component by the stored frequency.
BRIEF DESCRIPTION OF THE DRAWINGS
FIG. 1 is a flowchart indicating a method of determining the position of a
mobile object according to a first embodiment of the present invention.
FIG. 2 is a view indicating the relationship between the locations of the
respective service zones of the base stations and a method of detecting
the position and moving direction and speed of a mobile object in a mobile
communication system according to one embodiment of the present invention.
FIG. 3 illustrates the principle of detecting the position of the mobile
object and its moving direction and speed by each base station from the
frequency of a radio wave which the base station has received from the
mobile object in a mobile communication system according to one embodiment
of the present invention.
FIG. 4 shows the relationship between the moving speed of a mobile object
and a maximum Doppler shift component observed by a base station.
FIG. 5 is a block diagram indicating the circuit configuration of a main
portion of each of the base stations which constitute a part of the mobile
communication system according to one embodiment of the present invention.
FIG. 6 is a schematic structure of a microcell mobile communication system
as a typical example the mobile communication system according to one
embodiment of the present invention.
FIG. 7 illustrates a zone switching operation using road map information
according to one embodiment of the present invention.
FIG. 8 illustrates the principle of detecting a Doppler shift component.
FIG. 9 is a block diagram indicating the structure of a mobile local
switch.
FIG. 10 is a flowchart indicating the steps of a method of determining the
position of a mobile object according to a second embodiment of the
present invention.
FIG. 11 illustrates a modification of the second embodiment.
FIGS. 12A and 12B are a flowchart indicating the steps of the method
according to a third embodiment of the present invention.
FIG. 13 illustrates a method of measuring a deviation in the direction of
movement of a mobile object.
DESCRIPTION OF THE PREFERRED EMBODIMENTS
A first embodiment of a method of determining the position of a mobile
object according to the present invention applied to a mobile
communication system will be described with reference to the drawings.
FIG. 6 shows an illustrative structure of a microcell mobile communication
system having the function of detecting the position of the mobile object
and its moving speed and direction according to one embodiment of the
present invention. Briefly, this system is provided with a mobile local
switch (MLS) 50; a line 51 connecting it to a fixed telephone network;
microcell service zones A-G; base stations (BSs) 53-59 each constituting a
microcell; and mobile stations (MSs) 60-69. The base stations 53-59 are
connected through corresponding wires 52 to the mobile local switch 50,
which sets/releases a radio channel as a radio access node and controls
various subscriber's functions such as handoff.
FIG. 5 is a block diagram indicating a structure of each of the base
stations 53-59 of FIG. 6. In the transmission section, the input signals
100 received through a plurality of wire channels for every predetermined
time interval are multiplexed by a time division multiplexer 40 in a
time-division mode. The input signals are allotted to frames to be
transmitted through a predetermined channel and each frame including
control data added thereto is compressed so that the frames can be
transmitted in a predetermined short time interval allotted to that
channel. The multiplexed signal is modulated by a modulator 41 and
transmitted through a transmission unit 42 and an antenna duplexer 43 from
an antenna 44.
In the receiving section, the control signal received by an antenna 39 and
a communication signal received by the antenna 44 and having passed
through the antenna duplexer 43 are fed through a receiving unit 47 to a
demodulator 48. The demodulator 48 includes an equalizer having the
function of eliminating possible distortions which the received signals
have suffered in the transmission path and demodulates the communication
signal. The demodulated signal is fed to a signal demultiplexer 49 which
demultiplexes the signal in an operation reverse to that of the
multiplexer 40. Base station control equipment (BSE) 46 controls the
timing of the framing operation and transmission/reception, and
setting/switching of a frequency synthesizer 45 including a stable
frequency oscillator. A part of so-called CODEC circuit is not shown which
includes A/D and D/A converters which receive the input signals 100,
output signals 200 from the demultiplexer 49 and input/output control data
111, 112.
FIG. 3 illustrates the principle of detection of a Doppler shift frequency
used for detecting the position of a mobile object and its moving speed
and direction. Assume that the mobile object 10 which may be a car
transmits radio wave at a frequency f.sub.0 (wavelength .lambda..sub.0)
while traveling at a speed v in the direction of an arrow 11. The
frequency f of the radio wave received by each of the base stations 13, 14
includes a frequency shift on the basis of the principle of the Doppler
effect. The frequency shift component changes in proportion to the speed
of the mobile object relative to the base station. A base station which
the mobile object is approaching encounters an up shift of the frequency
in which the frequency of the received wave increases, while a base
station which the mobile object is leaving encounters a down shift of the
frequency in which the frequency of the received wave decreases. Now let
the propagation speed of the radio wave be C (=f.sub.0 .lambda..sub.0);
let the maximum up shift value of the frequency of the radio wave
occurring at a base station positioned in the direction of movement
(straight forward) of the mobile object be .DELTA.F=(v/C)f.sub.0
=v/.lambda..sub.0 (>0); and let the angle between the direction of
movement of the mobile object 10 and a line connecting the base station 13
and that mobile object be .theta.. The approaching speed of the mobile
object 10 relative to that base station is then regarded as v cos .theta..
Thus, the Doppler shift component .DELTA.f occurring in the radio wave
received at the base station 13 is given by
##EQU1##
Similarly, the Doppler shift component .DELTA.f' occurring in the radio
wave received at the base station 14 is given by
##EQU2##
The speed v of the mobile object detected by a base station is calculated
from the Doppler shift component .DELTA.f and .theta. or the Doppler shift
component .DELTA.f' and .theta.' as follows:
v=.DELTA.F.lambda..sub.0 cos .theta. (1)
FIG. 4 shows the relationship between a maximum frequency shift value
.DELTA.F and the speed v of the mobile object obtained from expression (1)
for each of 1.9 GHz (.lambda..sub.0 .apprxeq.15.79 cm) used in a PHP and
other near frequencies higher and lower than 1.9 GHz when .theta.=0 in
expression (1). It is seen from the relationships that the walking speed
of a human being .apprxeq.4 km/h, car's velocities.apprxeq.10-30 km/h and
30-100 km, and further higher velocities are measurable. In order to
detect the frequency shift value accurately, the stability of the method
used for detecting such shift value and that of the frequency of a signal
generated in each base station to be used for that detection are ensured.
The stability of a local oscillator of each base station used in the mobile
communication system of the present embodiment will be described next.
Generally, the local oscillator is used to convert the received frequency
to an intermediate frequency. The received radio wave and the output from
the local oscillator are mixed to output the intermediate frequency as the
difference in frequency between the radio wave and the oscillator output.
Thus, the frequency stability of the local oscillator of each base station
directly influences the accuracy of detection of the Doppler shift
component. The radio channel control of the current mobile communication
system is based on so-called multi-channel access. A frequency synthesizer
has been developed and put in practical use as means for easily realizing
the multi-channel access. The technical development of the frequency
synthesizer is now advancing toward an increasing speed switching
operation in accordance with a demand for handoff, etc., as mentioned
above.
When the high-speed switching frequency synthesizer is considered from a
standpoint of frequency stability, however, it will be seen that the
purpose of technical development of the frequency synthesizer is to reduce
the time taken for channel switching. For example, it is said that it
takes 0.5-2.0 ms from the setting of a command to switch a channel to the
settling of the output frequency of the frequency synthesizer within a
range of frequency error of .+-.300 Hz-.+-.1 KHz. A finally obtained
frequency stability and a time taken for obtaining that stability are
changeable. Since the factors of fluctuation are considered to exist in
the stability and phase resolution of a phase comparator which is one of
the constituents of the frequency synthesizer and the deviation of the
control voltage used in its voltage controlled oscillator (VCO), a mere
crystal oscillator is considered to be better in whole frequency stability
than the VCO. Under such situation, the recent technical advancement has
achieved 10.sup.-8 -10.sup.-10 of frequency stability for a short time of
several to several tens of seconds, as mentioned above.
Now, a detecting process in the present embodiment will be described which
takes into consideration the features of very small cells and the
realization of a short-time frequency stability in a microcell mobile
communication system. FIG. 2 is a systemic view for explaining the
function of detecting the position of a mobile object and its speed and
direction of movement according to the present invention. In FIG. 2, base
stations 20-35 positioned at substantially the centers of hexagonal cells
are also detecting points for the Doppler shift frequency occurring as the
mobile object 10 moves. The straight lines in FIG. 2 include a first
straight line 11 coincident with the direction of movement of the object
10 shown by an arrow and a second straight line 12 intersecting with the
first straight line 11 almost at a right angle.
A first embodiment of a method of detecting the position of the mobile
object and its speed and direction of movement to be executed by a base
station control equipment (BCE) 46 of FIG. 5 and a mobile local switch
(MLS) 50 of | | |
