U.S. patent application number 09/988612 was filed with the patent office on 2002-03-07 for cellular radio system allowing mobile station to perform communication through base station to which mobile station is connected over cdma radio channel, and base station apparatus and mobile station apparatus which are used for cellular radio system.
This patent application is currently assigned to Kabushiki Kaisha Toshiba. Invention is credited to Mimura, Masahiko.
Application Number | 20020027891 09/988612 |
Document ID | / |
Family ID | 15048003 |
Filed Date | 2002-03-07 |
United States Patent
Application |
20020027891 |
Kind Code |
A1 |
Mimura, Masahiko |
March 7, 2002 |
Cellular radio system allowing mobile station to perform
communication through base station to which mobile station is
connected over CDMA radio channel, and base station apparatus and
mobile station apparatus which are used for cellular radio
system
Abstract
A CPU (42a) in each base station (BS) monitors the channel
occupancy of each of a plurality of radio frequencies allocated to
the base station (BS). When the difference between the channel
occupancies of a plurality of radio frequencies allocated to a
single base station (BS) becomes a predetermined state, the CPU
(42a) switches the radio frequency as a candidate to be used by a
predetermined base station (BS) of mobile stations (BS) that are
present in the cell formed by the base station (BS) and set in the
standby state to a radio frequency whose channel occupancy is in a
predetermined state.
Inventors: |
Mimura, Masahiko; (Tokyo,
JP) |
Correspondence
Address: |
FINNEGAN, HENDERSON, FARABOW, GARRETT &
DUNNER LLP
1300 I STREET, NW
WASHINGTON
DC
20005
US
|
Assignee: |
Kabushiki Kaisha Toshiba
|
Family ID: |
15048003 |
Appl. No.: |
09/988612 |
Filed: |
November 20, 2001 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
|
|
09988612 |
Nov 20, 2001 |
|
|
|
09230094 |
Jan 20, 1999 |
|
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Current U.S.
Class: |
370/331 ;
455/436 |
Current CPC
Class: |
H04W 36/18 20130101;
H04W 72/0486 20130101; H04W 36/22 20130101; H04W 36/06
20130101 |
Class at
Publication: |
370/331 ;
455/436 |
International
Class: |
H04Q 007/00; H04Q
007/20 |
Foreign Application Data
Date |
Code |
Application Number |
May 21, 1997 |
JP |
9-131015 |
Claims
1. A cellular radio system including a plurality of base stations
respectively belonging to a plurality of base station groups and
forming cells having a predetermined diameter, and mobile stations
connected to said base stations over CDMA radio channels, each of
said base stations using CDMA radio channels of a plurality of
radio frequencies allocated to said base station group to which
said base station belongs, characterized by comprising: channel
occupancy monitoring means for monitoring channel occupancies of a
plurality of radio frequencies allocated to each of said plurality
of base stations; idle hard handoff controlling means for, when a
difference between channel occupancies determined by said channel
occupancy monitoring means with respect to a plurality of radio
frequencies allocated to a single base station becomes a
predetermined state, giving a predetermined idle hard handoff
instruction for controlling a radio frequency whose channel
occupancy determined by said channel occupancy monitoring means is
a predetermined state to a predetermined mobile station of mobile
stations that are present in a cell formed by said base station and
in a standby state; and candidate radio frequency setting means,
arranged in said mobile station, for, when the idle hard handoff
instruction is received, setting a candidate radio frequency to be
used at the time of generating of a next call to the radio
frequency designated by the received idle hard handoff
instruction.
2. A cellular radio system according to claim 1, characterized in
that said base station transmits the same spreading code with the
same phase through each of pilot channels of a plurality of radio
frequencies allocated to said base station group to which said base
station belongs.
3. A cellular radio system according to claim 1, characterized in
that said candidate radio frequency setting means sets a candidate
radio frequency upon power-off as a candidate radio frequency to be
used immediately after power-on.
4. A cellular radio system including a plurality of base stations
respectively belonging to a plurality of base station groups and
forming cells having a predetermined diameter, and mobile stations
connected to said base stations over CDMA radio channels, each of
said base stations using CDMA radio channels of a plurality of
radio frequencies allocated to said base station group to which
said base station belongs, characterized by comprising: channel
occupancy monitoring means for monitoring channel occupancies of a
plurality of radio frequencies allocated to each of said plurality
of base stations; frequency use state notifying means for
generating frequency use state information for notifying said
mobile station of a use state of each radio frequency on the basis
of the channel occupancies determined by said channel occupancy
monitoring means; radio frequency list forming means for receiving
the frequency use state information transmitted from said base
station under the control of said frequency use state notifying
means and forming a radio frequency list in which priority levels
are assigned to a plurality of radio frequencies on the basis of
the frequency use state information; storage means, arranged in
said mobile station, for storing the radio frequency list formed by
said radio frequency list forming means even in a power-off state;
and initial candidate frequency setting means for performing a
search immediately after power-on to check in the order of priority
levels whether the radio frequencies indicated by the radio
frequency list stored in said storage means can be used, and
setting the first detected radio frequency that can be used as a
candidate radio frequency to be used.
5. A cellular radio system including a plurality of base stations
respectively belonging to a plurality of base station groups and
forming cells having a predetermined diameter, and mobile stations
connected to said base stations over CDMA radio channels, each of
said base stations using CDMA radio channels of a plurality of
radio frequencies allocated to said base station group to which
said base station belongs, characterized by comprising: channel
occupancy monitoring means for monitoring channel occupancies of a
plurality of radio frequencies allocated to each of said plurality
of base stations; handoff method determining means for, when said
mobile station moves to a new cell formed by another base station
belonging to the same base station group to which said base station
forming an old cell in which said mobile station has been present
belongs, checking, on the basis of the channel occupancy determined
by said channel occupancy monitoring means with respect to said
base station forming the new cell, whether the channel occupancy of
the radio frequency in the new cell, which has been used by said
mobile station in the old cell, is not less than a predetermined
value, and determining soft handoff if the channel occupancy of the
radio frequency in the new cell, which has been used by said mobile
station in the old cell, is less than predetermined value, and hard
handoff if the channel occupancy of the radio frequency in the new
cell, which has been used by said mobile station in the old cell,
is not less than predetermined value; network-side handoff control
means for performing predetermined handoff control associated with
said mobile station by the method determined by said handoff method
determining means, and notifying said mobile station of a radio
frequency whose channel occupancy in the new cell is not more than
a predetermined value, when hard handoff is to be performed; and
mobile-station-side handoff control means, arranged in said mobile
station, for performing predetermined handoff processing under the
control of said network-side handoff control means, when said
mobile station moves to a cell formed by another base station
belonging to the same base station group to which said base station
forming a cell in which said mobile station has been present
belongs, and performing hard handoff to switch to a radio frequency
notified by said network-side handoff control means when hard
handoff is designated.
6. A base station apparatus belonging to any one of a plurality of
base station groups, forming a cell having a predetermined
diameter, and connected to a mobile station over CDMA radio
channels of a plurality of radio frequencies allocated to said base
station group, characterized by comprising: channel occupancy
monitoring means for monitoring a channel occupancy of each of a
plurality of radio frequencies allocated to said self-station; and
idle hard handoff controlling means for, when a difference between
channel occupancies determined by said channel occupancy monitoring
means becomes a predetermined state, giving a predetermined idle
hard handoff instruction for controlling a radio frequency whose
channel occupancy determined by said channel occupancy monitoring
means is in a predetermined state to a predetermined mobile station
of mobile stations that are present in a cell formed by said
self-station and set in a standby state.
7. A base station apparatus belonging to any one of a plurality of
base station groups, forming a cell having a predetermined
diameter, and connected to a mobile station over CDMA radio
channels of a plurality of radio frequencies allocated to said base
station group, characterized by comprising: channel occupancy
monitoring means for monitoring a channel occupancy of each of a
plurality of radio frequencies allocated to said self-station; and
frequency use state notifying means for generating and transmitting
frequency use state information for notifying said mobile station
of a use state of each radio frequency on the basis of the channel
occupancies determined by said channel occupancy monitoring
means.
8. A base station apparatus belonging to any one of a plurality of
base station groups, forming a cell having a predetermined
diameter, and connected to a mobile station over CDMA radio
channels of a plurality of radio frequencies allocated to said base
station group, characterized by comprising: channel occupancy
monitoring means for monitoring a channel occupancy of each of a
plurality of radio frequencies allocated to said self-station; and
handoff method determining means for, when said mobile station that
has been located in the cell of said self-station moves to a new
cell formed by another base station belonging to the same base
station group to which said-self station belongs, checking, on the
basis of the channel occupancy determined with respect to said base
station forming the new cell, whether the channel occupancy of the
radio frequency used by said mobile station in the new cell is not
less than a predetermined value, and determining soft handoff if
the channel occupancy of the radio frequency used by said mobile
station in the new cell is less than the predetermined value, and
hard handoff if the channel occupancy of the radio frequency used
by said mobile station in the new cell is not less than the
predetermined value; and network-side handoff control means for
performing predetermined handoff control associated with said
mobile station by the method determined by said handoff method
determining means, and notifying said mobile station of a radio
frequency whose channel occupancy in the new cell is not more than
a predetermined value, when hard handoff is to be performed.
9. A mobile station apparatus used as a mobile station in a
cellular radio system including a plurality of base stations
respectively belonging to a plurality of base station groups and
forming cells having a predetermined diameter, and mobile stations
connected to said base stations over CDMA radio channels, each of
said base stations using CDMA radio channels of a plurality of
radio frequencies allocated to said base station group to which
said base station belongs, said cellular radio system further
including: channel occupancy monitoring means for monitoring
channel occupancies of a plurality of radio frequencies allocated
to each of said plurality of base stations; and idle hard handoff
controlling means for, when a difference between channel
occupancies determined by said channel occupancy monitoring means
with respect to a plurality of radio frequencies allocated to a
single base station becomes a predetermined state, giving a
predetermined idle hard handoff instruction for controlling a radio
frequency whose channel occupancy determined by said channel
occupancy monitoring means is in a predetermined state to a
predetermined mobile station of mobile stations that are present in
a cell formed by said base station and in a standby state,
characterized by comprising: candidate radio frequency setting
means for, when the idle hard handoff instruction is received,
setting a candidate radio frequency to be used at the time of
generating of a next call to the radio frequency designated by the
received idle hard handoff instruction.
10. A mobile station apparatus used as a mobile station in a
cellular radio system including a plurality of base stations
respectively belonging to a plurality of base station groups and
forming cells having a predetermined diameter, and mobile stations
connected to said base stations over CDMA radio channels, each of
said base stations using CDMA radio channels of a plurality of
radio frequencies allocated to said base station group to which
said base station belongs, said cellular radio system further
including: channel occupancy monitoring means for monitoring
channel occupancies of a plurality of radio frequencies allocated
to each of said plurality of base stations; and frequency use state
notifying means for generating frequency use state information for
notifying said mobile station of a use state of each radio
frequency on the basis of the channel occupancies determined by
said channel occupancy monitoring means, and causing each of said
plurality of base stations to transmit the information,
characterized by comprising: radio frequency list forming means for
receiving the frequency use state information transmitted from said
base station under the control of said frequency use state
notifying means and forming a radio frequency list in which
priority levels are assigned to a plurality of radio frequencies on
the basis of the frequency use state information; storage means for
storing the radio frequency list formed by said radio frequency
list forming means even in a power-off state; and initial candidate
frequency setting means for performing a search immediately after
power-on to check in the order of priority levels whether the radio
frequencies indicated by the radio frequency list stored in said
storage means can be used, and setting the first detected radio
frequency that can be used as a candidate radio frequency to be
used.
11. A mobile station apparatus used as a mobile station in a
cellular radio system including a plurality of base stations
respectively belonging to a plurality of base station groups and
forming cells having a predetermined diameter, and mobile stations
connected to said base stations over CDMA radio channels, each of
said base stations using CDMA radio channels of a plurality of
radio frequencies allocated to said base station group to which
said base station belongs, said cellular radio system further
including: channel occupancy monitoring means for monitoring
channel occupancies of a plurality of radio frequencies allocated
to each of said plurality of base stations; handoff method
determining means for, when said mobile station moves to a new cell
formed by another base station belonging to the same base station
group to which said base station forming an old cell in which said
mobile station has been present belongs, checking, on the basis of
the channel occupancy determined by said channel occupancy
monitoring means with respect to said base station forming the new
cell, whether the channel occupancy of the radio frequency in the
new cell, which has been used by said mobile station in the old
cell, is not less than a predetermined value, and determining soft
handoff if the channel occupancy of the radio frequency in the new
cell, which has been used by said mobile station in the old cell,
is less than predetermined value, and hard handoff if the channel
occupancy of the radio frequency in the new cell, which has been
used by said mobile station in the old cell, is not less than
predetermined value; and network-side handoff control means for
performing predetermined handoff control associated with said
mobile station by the method determined by said handoff method
determining means, and notifying said mobile station of a radio
frequency whose channel occupancy in the new cell is not more than
a predetermined value, when hard handoff is to be performed,
characterized by comprising: mobile-station-side handoff control
means, arranged in said mobile station, for performing
predetermined handoff processing under the control of said
network-side handoff control means, when said mobile station moves
to a cell formed by another base station belonging to the same base
station group to which said base station forming a cell in which
said mobile station has been present belongs, and performing hard
handoff to switch to a radio frequency notified by said
network-side handoff control means when hard handoff is designated.
Description
TECHNICAL FIELD
[0001] The present invention relates to a cellular radio system
such as a car/portable telephone system and a cordless telephone
system and, more particularly, to a cellular radio system using a
code division multiple access (CDMA) scheme as a radio access
scheme between a base station and a mobile station, and a base
station apparatus and mobile station apparatus which are used for
the cellular radio system.
BACKGROUND ART
[0002] A spread spectrum communication scheme, which is robust
against interference and disturbance, has gained a great deal of
attention as one of communication schemes used for a mobile
communication system. The spread spectrum communication scheme is
mainly used to implement a cellular radio system using CDMA
scheme.
[0003] In the cellular radio system using CDMA scheme, for example,
the transmitting apparatus modulates digital speech data or digital
image data by a digital modulation scheme such as PSK. This
modulated transmission data is then converted into a broadband
baseband signal by using a spreading code such as a pseudorandom
noise code (PN code). This baseband signal is up-converted into a
signal in the radio frequency band and is then transmitted. The
receiving apparatus down-converts the received signal in the radio
frequency band into a signal having an intermediate or baseband
frequency. The apparatus then de-spreads this down-converted signal
by using the same spreading code as that used by the transmitting
apparatus. Digital demodulation is then performed for the de-spread
signal by a digital demodulation scheme such as PSK, thereby
reconstructing data from the received data.
[0004] That is, in CDMA scheme, different spreading codes are
assigned to radio communications between a plurality of mobile
station apparatuses and base stations to ensure channel separation
between the respective radio communications.
[0005] FIG. 14 shows the schematic arrangement of a CDMA cellular
radio system. Referring to FIG. 14, a plurality of base stations
BS1 to BSn are distributed in a service area. These base stations
BS1 to BSn are respectively connected to a control station CS
through wire lines L1 to Ln. The base stations BS1 to BSn are
further connected to a wire network NW through the control station
CS. The base stations BS1 to BSn respectively form radio zones Z1
to Zn called cells. Mobile stations MS1 to MSm are respectively
connected to base stations BS in the cells where the respective
mobile stations are present by CDMA scheme over radio paths.
[0006] In a system of this type, when any one of the mobile
stations MS1 to MSm moves between cells during communication,
handoff, i.e., the switching of the base station to another to
which radio path is to be connected is performed. There are two
types of handoff: soft handoff and hard handoff.
[0007] Soft handoff is unique to a CDMA cellular radio system. The
perform handoff, a mobile station has two radio paths at the same
time. One of the radio paths connects the mobile station to the
source base station with which it has been communicating. The other
radio path connects the mobile station to the destination base
station with which it will communicate. The mobile station then
performs path diversity synthesis, by using the signals it receives
over these radio paths. Thereafter, of the paths under path
diversity synthesis, a path on which the reception electric field
strength of a pilot channel has dropped below a threshold for a
predetermined period of time or more is disconnected, thereby
switching the base stations to which the mobile station is
connected. As described above, in a soft handoff, one of two paths
is always connected to a base station at the time of handoff, and
hence no path disconnection occurs. An advantage of soft handoff is
that switching can be smoothly performed without any short break in
speech.
[0008] Soft handoff, however, requires both the source and
destination base stations to use the same radio frequency. For this
reason, for example, as shown in FIG. 14, in a system in which
different radio frequencies f1, f2, and f3 are allocated to a
plurality of base station groups BSa, BSb, and BSc, when a mobile
station MSi moves from a cell of the base station group Bsa to a
cell of another base station group BSb or BSc, soft handoff cannot
be performed.
[0009] In contrast to this, hard handoff is mainly performed when
the above source and destination base stations use different
frequencies. More specifically, when a mobile station must change
the radio frequency in use at the time of handoff, a message for
instructing handoff is sent from a base station to the mobile
station. Upon reception of this message, the mobile station
temporarily stops transmission/reception and forms a new radio path
allocated by the base station between itself and the base station.
After this radio path is formed, the mobile station resumes
transmission/reception by using the path. In a hard handoff, the
radio path is temporarily disconnected to switch the radio
frequencies, and a radio path must be formed again by using a new
radio frequency.
[0010] In such a system, a plurality of radio frequencies may be
allocated to the respective base station groups, and the respective
base stations may use CDMA scheme with the respective radio
frequencies, thereby increasing the number of traffic channels.
[0011] In this case, however, the occupancies of the traffic
channels having the respective radio frequencies in the respective
base stations may become uneven. Assume that the occupancies of the
traffic channels having the respective radio frequencies become
uneven. In this case, if an originating mobile station selects a
radio frequency with dense traffic, the base station may be found
busy in spite of the presence of an available channel, resulting in
blocked communication. Assume also that a mobile station selects a
radio frequency with dense traffic in a hard or soft handoff. In
this case, even if an available channel is present, it takes a long
period of time to form a radio path again. As a result, speech
communication may be interrupted or noise may be produced,
deteriorating the speech communication quality. Alternatively, the
call may be dropped due to a handoff failure.
DISCLOSURE OF INVENTION
[0012] It is an object of the present invention to provide a
cellular radio system which can average the traffic channel
occupancies of the respective radio frequencies, thereby allowing
effective use of traffic channels.
[0013] In order to achieve the above object, according to the
present invention, in a cellular radio system including a plurality
of base stations respectively belonging to a plurality of base
station groups and forming cells having a predetermined diameter,
and mobile stations connected to the base stations over CDMA radio
channels, each of the base stations using CDMA radio channels of a
plurality of radio frequencies allocated to the base station group
to which the base station belongs, the channel occupancies of a
plurality of radio frequencies allocated to each of the plurality
of base stations are monitored by a channel occupancy monitoring
means.
[0014] When the difference between the channel occupancies of the
respective radio frequencies allocated to the same base station,
determined by the channel occupancy monitoring means, becomes a
predetermined state, the radio frequency as a candidate to be used
by a predetermined mobile station of mobile stations that are
present in the cell formed by the base station and set in the
standby state is switched to the radio frequency whose channel
occupancy determined by the channel occupancy monitoring means is
in a predetermined state.
[0015] Consequently, as radio frequencies to be used by mobile
stations in the standby state to newly start communications, radio
frequencies whose channel occupancies are in a predetermined state
are properly distributed at random, thereby averaging the channel
occupancies of the respective radio frequencies.
[0016] In addition, according to the present invention, a frequency
use state notifying means generates frequency use state information
for notifying the mobile station of the use state of each radio
frequency on the basis of the channel occupancies determined by the
channel occupancy monitoring means, and transmits the information
to the mobile station. The mobile station forms a radio frequency
list in which priority levels are assigned to a plurality of radio
frequencies on the basis of the frequency use state information
before power-off, and stores it in a power-off state. Immediately
after power-on of the mobile station, a search is performed to
check in the order of priority levels whether the radio frequencies
indicated by the radio frequency list stored in the storage means
can be used, and the first detected radio frequency that can be
used is as a candidate radio frequency to be used.
[0017] Consequently, as radio frequencies to be used by mobile
stations in the standby state to newly start communications, radio
frequencies are properly distributed at random on the basis of the
use state of each radio frequency before power-off, thereby
averaging the channel occupancies of the respective radio
frequencies.
[0018] When the mobile station moves to a new cell formed by
another base station belonging to the same base station group to
which the base station forming an old cell in which the mobile
station has been present belongs, it is checked, on the basis of
the channel occupancy determined by the channel occupancy
monitoring means with respect to the base station forming the new
cell, whether the channel occupancy of the radio frequency in the
new cell, which has been used by the mobile station in the old
cell, is not less than a predetermined value. Soft handoff is
determined if the channel occupancy of the radio frequency in the
new cell, which has been used by the mobile station in the old
cell, is less than predetermined value. Hard handoff is determined
if the channel occupancy of the radio frequency in the new cell,
which has been used by the mobile station in the old cell, is not
less than predetermined value. When hard handoff is to be
performed, a radio frequency whose channel occupancy is not more
than a predetermined value in the new cell is used in the mobile
station.
[0019] Consequently, as radio frequencies to be used after mobile
handoff, radio frequencies whose channel occupancies are not more
than the predetermined value in new cells are properly distributed
at random, thereby averaging the channel occupancies of the
respective radio frequencies.
BRIEF DESCRIPTION OF DRAWINGS
[0020] FIG. 1 is a view showing the schematic arrangement of a CDMA
cellular radio system according to an embodiment of the present
invention;
[0021] FIG. 2 is a block diagram showing the arrangement of a
mobile station MS used in the system in FIG. 1;
[0022] FIG. 3 is a block diagram showing the arrangement of a base
station BS used in the system in FIG. 1;
[0023] FIG. 4 is a flow chart showing a control procedure for idle
handoff control in a CPU 42a;
[0024] FIG. 5 is a view showing a first determination condition for
determining the necessity of idle handoff;
[0025] FIG. 6 is a view showing a second determination condition
for determining the necessity of idle handoff;
[0026] FIG. 7 is a view showing a third determination condition for
determining the necessity of idle handoff;
[0027] FIG. 8 is a flow chart showing a control procedure for idle
handoff control in a CPU 13a;
[0028] FIG. 9 is a flow chart showing a control procedure for
control at power-off in the CPU 13a;
[0029] FIG. 10 is a flow chart showing a control procedure for
start-up control in the CPU 13a;
[0030] FIG. 11 is a flow chart showing a control procedure for
mobile handoff control in the CPU 13a and the CPU 42a;
[0031] FIG. 12 is a view for explaining mobile handoff control;
[0032] FIGS. 13A and 13B are views each showing a modification of
the arrangement of the base station BS; and
[0033] FIG. 14 is a view showing the schematic arrangement of a
CDMA cellular radio system.
BEST MODE OF CARRYING OUT THE INVENTION
[0034] An embodiment of the present invention will be described
below with reference to the accompanying drawings.
[0035] FIG. 1 shows the schematic arrangement of a CDMA cellular
radio system according to this embodiment. The apparent arrangement
of this system is the same as that shown in FIG. 14. However, this
system differs from that shown in FIG. 14 in the allocation of
radio frequencies to base station groups BSa to BSc and the handoff
control function of the system as follows.
[0036] The system of this embodiment has a total of six radio
frequencies f1 to f6. The radio frequencies f1 and f2 are allocated
to the base station group BSa; the radio frequencies f3 and f4, to
the base station group BSb; and the radio frequencies f5 and f6, to
the base station group BSc. Note that the radio frequencies f1 to
f6 are respectively composed of upstream carriers fU1 to fU6 for
transmitting signals from mobile stations MS (MS1 to MSm) to base
stations BS (BS1 to BSm) and downstream carriers fD1 to fD6 for
transmitting signals from the base stations BS to the mobile
stations MS. An upstream carrier fUk (k is 1 to 6) and a downstream
carrier fDk satisfy:
fUk=FDk+[predetermined frequency offset]
[0037] Each base station BS transmits a pilot channel, a sync
channel, a paging channel, and a downstream traffic channel with
the above downstream carriers fD. The mobile station MS transmits
an access channel and an upstream traffic channel with the above
upstream carriers fU.
[0038] This system performs radio communication based on CDMA
scheme between the base station BS and the mobile station MS by
selectively using radio frequencies allocated to the base station
BS. Various control operations associated with this radio
communication include mobile handoff control to be performed when
the mobile station MS moves between the cells of the base stations
BS, and idle handoff control to be performed when the traffic of a
specific radio frequency considerably increases as compared with
the traffic of another radio frequency allocated to the same base
station BS.
[0039] FIG. 2 is a block diagram showing the arrangement of the
mobile station MS used in the above CDMA cellular radio system.
[0040] As shown in FIG. 2, the mobile station MS includes a
microphone 10a, a speaker 10b, an analog-digital converter (to be
referred to as an A-D converter hereinafter) 11a, a digital-analog
converter (to be referred to as a D-A converter hereinafter) 11b, a
voice coder-decoder (to be referred to as a vocoder hereinafter)
12, a mobile station control section 13, a data generating circuit
14, a convolutional encoder 15, an interleave circuit 16, a spread
spectrum unit 17, a digital filter 18, a digital-analog converter
(to be referred to as a D-A converter hereinafter) 19, an analog
front end 20, an antenna 21, an analog-digital converter (to be
referred to as an A-D converter hereafter) 22, a search receiver
(to be referred to as a search receiver hereafter) 23, an automatic
gain control (AGE) circuit 24, finger circuits 25, 26, and 27, a
symbol combiner 28, a de-interleave circuit 29, a Viterbi decoder
30, an error correction circuit 31, a keypad/display 32, and a
memory section 33.
[0041] A speaker's transmission speech signal output from the
microphone 10a is converted into a digital signal by the A-D
converter 11a and is coded by the vocoder 12. The mobile station
control section 13 adds a control signal and the like to the coded
transmission signal output from the vocoder 12 to generate
transmission data.
[0042] The data generating circuit 14 adds an error detection code
and an error correction code to this transmission data. The
transmission data output from the data generating circuit 14 is
coded by the convolutional encoder 15. The interleave circuit 16
performs interleave processing for the transmission data output
from the convolutional encoder 15. The transmission data output
from the interleave circuit 16 is spectrum-spread into a broadband
signal by the spectrum spreader 17 using PN and Walsh codes. The
digital filter 18 removes unnecessary frequency components from
this spectrum-spread transmission data. The transmission data
output from the digital filter is converted into an analog
transmission signal by the D-A converter 19. This analog
transmission signal is up-converted into a signal having a
predetermined radio channel frequency and power-amplified to a
predetermined transmission power level by the analog front end 20.
Thereafter, the signal is transmitted from the antenna 21 to the
base station BS.
[0043] On the other hand, a radio signal received by the antenna 21
is low-noise-amplified and down-converted into a signal having an
intermediate frequency or baseband frequency by the analog front
end 20. The received signal output from the analog front end 20 is
converted into a digital signal at a predetermined sampling period
by the A/D converter 22. The received transmission data output from
the A-D converter 22 is then input to the search receiver 23, the
automatic gain control circuit 24, and the three finger circuits
25, 26, and 27.
[0044] Each of the finger circuits 25, 26, and 27 includes an
initial capturing section, a clock tracking section, and a data
demodulation section. The data demodulation section de-spreads the
spectrum of the received transmission signal from the base station
BS, and integrates the resultant data through the integrating dump
filter for a one-symbol period. Note that the three finger circuits
are used to receive a multipath reception signal at a high S/N
ratio by using the path diversity effect, and to switch base
stations BS, to which the mobile station is connected, during
communication without disconnecting the radio path, i.e., to
perform a so-called soft handoff.
[0045] The respective symbols demodulated by the finger circuits
25, 26, and 27 are input to the symbol combiner 28, together with
synchronization information, to be synthesized. The synthesized
demodulated symbol is input to the de-interleave circuit 29,
together with timing information, to be subjected to de-interleave
processing in the de-interleave circuit 29. The demodulated symbol
after this de-interleave processing is Viterbi-decoded by the
Viterbi decoder 30. The demodulated symbol after this
Viterbi-decoding is subjected to error correction decoding
processing in the error correction circuit 31 to become received
data. The received data is input to the mobile station control
section 13. The mobile station control section 13 separates the
input received data into speech data and control data. The speech
data is speech-decoded by the vocoder 12 and converted into an
analog signal by the D/A converter 11b. The analog signal is then
output as speech from the speaker 10b.
[0046] The keypad/display 32 is used by the user to input dial
data, control data, and the like, and serves to display various
information associated with the operation state of the mobile
station MS. The operation of the keypad/display 32 is controlled by
the mobile station control section 13.
[0047] The memory section 33 is used to store various data required
when the mobile station control section 13 performs various
operations, and has a nonvolatile storage medium such as an EEPROM.
This memory section 33 stores a radio frequency list (to be
described later) in a storage area formed from the nonvolatile
storage medium.
[0048] The search receiver 23 basically has the same arrangement as
that of each of the finger circuits 25, 26, and 27. The search
receiver 23 searches the PN codes of pilot signals broadcasted from
the base stations BS in units of radio frequencies to capture the
offsets of the PN codes. The power control data obtained by this PN
code searching operation is loaded into the mobile station control
section 13.
[0049] The mobile station control section 13 has a CPU 13a, a ROM
13b, a RAM 13c, and an I/O port 13d connected to each other through
a system bus 13e.
[0050] The CPU 13a operates on the basis of the programs stored in
the ROM 13b, and collectively controls the respective sections of
this mobile station MS, thereby implementing the operation of the
mobile station MS.
[0051] The ROM 13b stores the operation programs and the like for
the CPU 13a.
[0052] The RAM 13c temporarily stores data required for the CPU 13a
to perform various operations.
[0053] The functions implemented by the CPU 13a by means of
software processing include a candidate frequency setting function,
a radio frequency list forming function, a candidate frequency
initializing function, and a mobile-station-side handoff processing
function, as well as the known general control function of the
mobile station MS.
[0054] The candidate frequency setting function serves to set a
radio frequency to be used when communication is started next,
i.e., a candidate frequency to be used as a radio frequency in the
standby state. This candidate frequency setting function includes
the function of changing candidate frequencies in accordance with
an idle handoff instruction from the base station BS, i.e., the
function of performing idle handoff.
[0055] The radio frequency list forming function serves to receive
frequency use state information from the nearest base station BS,
which indicates the use state of radio frequencies in the cell
formed by the base station BS and the use state of radio
frequencies in a predetermined neighboring cell, and form a radio
frequency list indicating the priority levels of the respective
radio frequencies on the basis of the frequency use state
information. The radio frequency list forming function serves to
store this formed radio frequency list in the memory section
33.
[0056] The candidate frequency initializing function serves to
initialize candidate frequencies by referring to the radio
frequency list stored in the memory section 33 when the power to
the mobile station MS is turned on.
[0057] When the mobile station MS moves from the cell of a given
base station BS to the cell of another neighboring base station BS,
the mobile-station-side handoff control function serves to perform
control, together with these base stations BS, to switch a first
radio path connecting the mobile station MS to the source base
station BS to a second radio path connecting the mobile station MS
to the destination base station BS. At this time, the
mobile-station-side handoff control function performs soft handoff
control when the radio frequency of the first radio path is equal
to that of the second radio path. Assume that in hard handoff
control, allocated radio frequency notification information for
notifying the radio frequency to be used to connect the second
radio path is supplied from the source base station BS. In this
case, the mobile-station-side handoff control function serves to
connect the second radio path by using this radio frequency.
[0058] FIG. 3 is a block diagram showing the arrangement of the
base station BS.
[0059] As shown in FIG. 3, the base station BS includes a control
station interface section 41, a base station control section 42, p
(p is the number of traffic channels for one radio frequency) CDMA
modulation sections 43 (43-1 to 43-p) for the first radio
frequency, p CDMA modulation sections 44 (44-1 to 44-p) for the
second radio frequency, a pilot signal generating section 45,
synthesizers 46 and 47, an analog front end 48, an antenna 49, p
CDMA demodulation sections 50 (50-1 to 50-p) for the first radio
frequency, and p CDMA demodulation sections 51 (51-1 to 51-p) for
the second radio frequency.
[0060] The control station interface section 41 transmits/receives
speech data and control data to/from the control station CS. For
example, speech data sent in a time-divisionally multiplexed state
from the control station CS is demultiplexed by the control station
interface section 41. Each of the demultiplexed speech data is
converted into data in a data form for transmission on a radio path
by the control station interface section 41.
[0061] The speech data having undergone data conversion are
parallelly supplied to the base station control section 42. The
base station control section 42 adds control signals and the like
to the respective speech data to generate transmission data.
[0062] These transmission data are input to CDMA modulation
sections 34 and 44 for the corresponding traffic channels. Each of
CDMA modulation sections 34 and 44 has circuits similar to the data
generating circuit 14, the convolutional encoder 15, the interleave
circuit 16, the spread spectrum unit 17, the digital filter 18, and
the D-A converter 19 in the mobile station MS. The transmission
data are therefore subjected to addition of error detection codes
and error correction codes, convolutional coding, interleave
processing, spread spectrum processing, and conversion to analog
signals in CDMA modulation sections 43 and 44. As a result, analog
transmission signals are obtained. Note that CDMA modulation
sections 43-1 to 43-p use different Walsh codes corresponding to
the respective traffic channels in spreading the signal spectrum.
In addition, CDMA modulation sections 44-1 to 44-p use different
Walsh codes corresponding to the respective traffic channels in
spreading the signal spectrum.
[0063] The analog transmission signals obtained by the respective
CDMA modulation sections 43 are synthesized with each other by the
synthesizer 46. At this time, the synthesizer 46 also synthesizes a
pilot channel signal generated by the pilot signal generating
section 45 with the above signals. The analog transmission signals
obtained by the respective CDMA modulation sections 44 are
synthesized with each other by the synthesizer 47. At this time,
the synthesizer 47 also synthesizes a pilot channel signal
generated by the pilot signal generating section 45 with the above
signals. Each pilot channel signal contains a PN code (common to
CDMA modulation sections 43 and 44) used by CDMA modulation
sections 43 and 44 for spread spectrum processing.
[0064] Both the output signals from the synthesizers 46 and 47 are
input to the analog front end 48. The output signal from the
synthesizer 46 is up-converted into the first radio frequency and
power-amplified to a predetermined transmission power level by the
analog front end 48. The output signal from the synthesizer 47 is
up-converted into the second radio frequency and power-amplified to
a predetermined transmission power level by the analog front end
48. The output signals from the analog front end 48 are transmitted
from the antenna 49 to the mobile station MS by radio. Note that
the first radio frequency is one of the two radio frequencies
allocated to the base station, and the second radio frequency is
the other of the two radio frequencies allocated to the base
station. That is, for example, in the base station BS belonging to
the base station group BSa, the first radio frequency is f1, and
the second radio frequency is f2.
[0065] An RF signal obtained when a radio signal is received
through the antenna 49 is supplied to the analog front end 48. This
RF signal is low-noise-amplified by the analog front end 48.
Signals in the bands of the two radio frequencies allocated to the
base station BS are extracted from this high-frequency signal.
These extracted signals are down-converted into intermediate
frequencies or baseband frequencies by the analog front end 48. The
resultant signals are reception signals.
[0066] Of these reception signals, the signal extracted from the
first radio frequency band is branched/input to CDMA demodulation
sections 50-1 to 50-p. Of the reception signals, the signal
extracted from the second radio frequency band is branched/input to
CDMA demodulation sections 51-1 to 51-p.
[0067] Each of CDMA demodulation sections 50 and 51 includes
circuits similar to the A-D converter 22, the search receiver 23,
the automatic gain control circuit 24, the finger circuits 25, 26,
and 27, the symbol combiner 28, the de-interleave circuit 29, the
Viterbi decoder 30, and the error correction circuit 31 in the
mobile station MS. Reception signals are therefore subjected to
conversion to digital signals, spread spectrum processing,
integration for a one-symbol period, symbol synthesis,
de-interleave processing, Viterbi decoding, and error correction
decoding processing in CDMA demodulation sections 50 and 51. As a
result, reception data are obtained and parallelly input to the
base station control section 42. In this case, CDMA demodulation
sections 50-1 to 50-p use different Walsh codes corresponding to
the respective traffic channels in spreading the signal spectrum.
CDMA demodulation sections 51-1 to 51-p use different Walsh codes
corresponding to the respective traffic channels in de-spreading
the signal spectrum. As a result, in CDMA demodulation sections 50
and 51, reception data received through the corresponding to
traffic channels are extracted.
[0068] The base station control section 42 separates the respective
reception data into speech data and control data. Of these data,
the speech data are converted into data in a data form for
transmission through a transmission path between itself and the
control station by the control station interface section 41. The
respective speech data having undergone data conversion are
transmitted in, for example, a time-divisional multiplexed state to
the control station CS.
[0069] The base station control section 42 has a CPU 42a, a ROM
42b, a RAM 42c, and an I/O port 42d connected to each other through
a system bus 42e.
[0070] The CPU 42a operates on the basis of the programs stored in
the ROM 42b, and collectively controls the respective sections of
this base station BS, thereby implementing the operation of the
base station BS.
[0071] The ROM 42b stores operation programs and the like for the
CPU 42a.
[0072] The RAM 42c temporarily stores data required for the CPU 42a
to perform various operations.
[0073] Note that the functions implemented by the CPU 42a by means
of software processing include a channel occupancy monitoring
function, an idle handoff controlling function, a frequency use
state notifying function, a handoff method determining function,
and a network-side handoff control function, as well as a known
general control function in the base station BS.
[0074] In this case, the channel occupancy monitoring function
serves to monitor the channel occupancies of radio frequencies
allocated to the base station BS.
[0075] When the difference between the channel occupancies of the
respective radio frequencies allocated to the base station BS
becomes a predetermined state, the idle handoff controlling
function serves to output an idle handoff instruction to the mobile
station MS in the standby state in the cell formed by the base
station BS to set the radio frequency with the lower channel
occupancy as a candidate frequency.
[0076] The frequency use station notifying function serves to
generate frequency use state information age indicating the use
state of the radio frequencies in the cell of the base station BS
and the use state of the radio frequencies in a predetermined
neighboring Ace cell, and perform processing for transmission of
the information to the mobile station MS.
[0077] The handoff method determining function serves to determine
whether to perform soft handoff or hard handoff for the mobile
station MS, when the mobile station moves from the cell of the base
station BS to the cell of another base station BS belonging to the
same base station group.
[0078] The network-side handoff control function serves to perform
handoff control to perform handoff for the mobile station MS by the
method determined by the handoff method determining function when
the mobile station MS moves from the cell of the base station BS to
the cell of another base station BS belonging to the same base
station group. The network-side handoff control function includes
the function of notifying the mobile station MS of the radio
frequency with a channel occupancy lower than that of the radio
frequency that has been used by the mobile station MS in the cell
it is entering, when the method determined by the handoff method
determining function is hard handoff.
[0079] The operation of the system having the above arrangement
will be described next.
[0080] Idle handoff control will be described first.
[0081] FIG. 4 is a flow chart showing a control procedure for idle
handoff control in the CPU 42a.
[0082] The CPU 42a in each base station BS performs idle handoff
control at a predetermined timing, e.g., at predetermined periods.
In this idle handoff control, first of all, the CPU 42a monitors
the occupancies (channel occupancies) of the downstream traffic
channels of the respective radio frequencies allocated to the
self-station during operation (step ST1). The base station control
section 42 checks, on the basis of the monitoring result on the
channel occupancies, whether the current state corresponds to any
one of the following three conditions (steps ST2 to ST5).
[0083] (1) For example, as shown in FIG. 5,
[0084] the difference between the channel use ratios of the two
radio frequencies exceeds 20%, and
[0085] neither of the channel occupancies of the two radio
frequencies exceeds 95%.
[0086] (2) For example, as shown in FIG. 6,
[0087] one of the channel occupancies of the two radio frequencies
exceeds 95%,
[0088] the other of the channel occupancies of the two radio
frequencies is equal to or lower than 94%, and
[0089] the difference between the channel use ratios of the two
radio frequencies does not exceed 20%.
[0090] (3) For example, as shown in FIG. 7,
[0091] one of the channel occupancies of the two radio frequencies
exceeds 95%,
[0092] the other of the channel occupancies of the two radio
frequencies is equal to or lower than 94%, and
[0093] the difference between the channel use ratios of the two
radio frequencies exceeds 20%.
[0094] These three conditions are conditions for determining
whether idle handoff is necessary. When the current state
corresponds to any one of them, it indicates that idle handoff is
necessary.
[0095] If the CPU 42a determines that idle handoff is necessary,
and the determination is based on condition (1) or (2), the number
of idle mobile stations is set to "1" (step ST6). If the
determination is based on condition (3), the number of idle mobile
stations is set to "2" (step ST7).
[0096] Subsequently, the CPU 42a checks whether the count value of
an idle handoff counter for counting the number of times idle
handoff is performed corresponds to any one of the following three
conditions (steps ST8 and ST9):
[0097] (1) smaller than "20",
[0098] (2) equal to or larger than "2" and smaller than "25",
and
[0099] (3) equal to or larger than "25".
[0100] If the count value of the idle handoff counter corresponds
to condition (2), the CPU 42a changes the number of idle mobile
stations to a number twice the number set in step ST6 or ST7 (step
ST10). If the counter value of the idle handoff counter corresponds
to condition (3), the CPU 42a changes the number of idle mobile
stations to a number four times the number set in step ST6 or ST7
(step ST11). If the count value of the idle handoff counter
corresponds to condition (2), the CPU 42a keeps the number of idle
mobile stations equal to the number set in step ST6 or ST7.
[0101] Subsequently, the CPU 42a randomly selects the mobile
stations MS equal in number to the number set at this time point
from the mobile stations MS that are located in the cell of the
self-base station BS and in the standby state. The CPU 42a sends an
idle handoff instruction controlling a radio frequency with a lower
channel occupancy to each of the selected mobile stations MS (step
ST12). When this idle handoff instruction sending operation is
complete, the CPU 42a increments the count value of the idle
handoff counter by one (step ST13). Thereafter, idle handoff
control is terminated.
[0102] If the CPU 42a determines in steps ST2 to ST6 that none of
conditions (1) to (3) is satisfied, the CPU 42a terminates this
idle handoff control without sending any idle handoff instruction.
If, however, there is no radio frequency whose channel occupancy
exceeds 95%, and the difference between the two radio frequencies
does not exceed 20%, the CPU 42a terminates the processing after
resetting the idle handoff counter (step ST14).
[0103] Upon reception of the idle handoff instruction transmitted
from the base station BS in the above manner, the CPU 13a of the
mobile station MS starts idle handoff control as shown in FIG.
8.
[0104] Upon reception of the idle handoff instruction, the CPU 13a
recognizes the contents of the idle handoff instruction first, and
then determines the designated radio frequency (step ST21). The CPU
13a checks whether the designated radio frequency differs from the
current candidate frequency (step ST22).
[0105] If the designated radio frequency differs from the current
candidate frequency, the CPU 13a captures the pilot channel of the
designated radio frequency (step ST23). This pilot channel
capturing operation is performed by acquiring the phase and PN
timing through the search receiver 23. Upon completion of the pilot
channel capturing operation, the CPU 13a receives a sync channel to
acquire information indicating the system configuration and the
system timing, and also captures a paging channel (step ST24).
[0106] Upon completion of the above processing, the CPU 13a returns
to the standby state. If the designated radio frequency is equal to
the current candidate frequency, the CPU 13a maintains the same
standby state as that has been set until now without performing the
processing in steps ST23 and ST24. With this operation, the mobile
station MS is set in the standby state in which the radio frequency
designated by the idle handoff instruction is set as a candidate
frequency.
[0107] This operation raises the possibility that a radio frequency
with a lower channel occupancy will be used for communication from
now on, although the channel occupancies of the respective radio
frequencies do not change instantly. Eventually, therefore, the
channel occupancies of the respective radio frequencies are
averaged. When the channel occupancies of the respective radio
frequencies are averaged, available traffic channels can be ensured
in the respective radio frequencies in many cases, thereby reliably
handling new calls and mobile handoffs.
[0108] Control at power-off and start-up control at power-on in the
mobile station MS will be described next.
[0109] When the user designates power-off in the mobile station MS,
the CPU 13a of the mobile station MS starts control at power-off as
shown in FIG. 9.
[0110] Upon reception of a power-off instruction, the CPU 13a
performs reception processing for frequency use state information
first (step ST31).
[0111] Note that the CPU 42a of each base station BS performs use
state notification control as shown in FIG. 9 at a predetermined
timing, e.g., at predetermined periods or the time required by the
mobile station. In this use state notification control, first of
all, the CPU 42a checks the use states of radio frequencies in the
self-station and a predetermined neighboring base station (step
ST41). The CPU 42a then generates frequency use state information
for notifying the use state of radio frequencies in each base
station, and transmits the information to the mobile station MS
(step ST42).
[0112] In the mobile station MS, the CPU 13a outputs a request to
the base station BS or waits for frequency use state information to
receive the frequency use state information transmitted from the
base station BS in the above manner in step ST31. Upon completion
of the reception of the frequency use state information, the CPU
13a generates a radio frequency list in consideration of the use
states of radio frequencies in the respective base stations,
indicated by the frequency use state information, and stores the
list in the memory section 33 (step ST32).
[0113] In this radio frequency list, priority levels are assigned
to the respective radio frequencies indicated by the frequency use
state information in accordance with a predetermined rule. For
example, the highest priority level ("1") is assigned to a radio
frequency set as a candidate frequency, and lower priories are
respectively assigned to another frequency in the cell where the
mobile station is currently located and the radio frequencies in
neighboring cells (in the order of occupancies).
[0114] Upon completion of the generation and storage of a radio
frequency list, the CPU 13a performs known processing to turn off
the power (step ST33), and terminates control at power-off.
[0115] When the user designates power-on in the mobile station MS,
the CPU 13a of the mobile station MS performs start-up control as
shown in FIG. 10.
[0116] Upon reception of the power-on instruction, the CPU 13a
initializes a variable X to "1" (step ST42).
[0117] The CPU 13a then searches the radio frequency list stored in
the memory section 33 for a radio frequency whose priority level is
"X" (step ST52). The CPU 13a checks the searched-out radio
frequency (step ST53), and determines whether the radio frequency
can be used (step ST54). If the radio frequency cannot be used, the
CPU 13a updates the variable X to "X+1" (step ST55), and repeats
the processing in steps ST52 to ST54.
[0118] If a radio frequency that can be used is found, the CPU 13a
captures the pilot channel of the radio frequency (step ST56). This
pilot channel capturing operation is performed by acquiring the
phase and PN timing through the search receiver 23. Upon completion
of pilot channel capturing, the CPU 13a receives a sync channel to
receive information indicating the system configuration and the
system timing, and also captures a paging channel (step ST57).
[0119] Upon completion of the above processing, the CPU 13a enters
the standby state.
[0120] With this operation, at the time of start-up operation by
power-on, the CPU 13a enters the standby state while a radio
frequency with a higher priority level, which is set at the time of
power-off in accordance with the use state of each radio frequency,
is set as a candidate frequency. If, therefore, the use state of
each radio frequency at the time of power-on has not greatly
changed from that at the time of power-off, the CPU 13a is set in
the standby state with a radio frequency with a low occupancy. This
raises the possibility that a radio frequency with a lower channel
occupancy will be used for communication from now on. Eventually,
therefore, the channel occupancies of the respective radio
frequencies are averaged.
[0121] Even if the use state of each radio frequency at the time of
power-on has changed from that at the time of power-off, since many
mobile stations MS power off at various timings, radio frequencies
are randomly selected by the respective mobile stations MS after
power-off. As a result, the channel occupancies of the respective
radio frequencies will be averaged.
[0122] When the channel occupancies of the respective radio
frequencies are averaged in this manner, available traffic channels
can be ensured in the respective radio frequencies in many cases,
thereby reliably handling new calls and mobile handoffs.
[0123] Mobile handoff control will be described next. A mobile
station MSj located in a cell Z4 of the base station BS4 will be
exemplified.
[0124] The CPU 13a in the mobile station MSj performs mobile
handoff control as shown in FIG. 11 at a predetermined timing,
e.g., at predetermined periods, during radio communication.
[0125] At the predetermined timing, the CPU 13a detects the
reception power level of the pilot channel that corresponds to the
currently used radio frequency and is transmitted from the base
station BS other than the base station BS4 forming the cell Z4 in
which the mobile station is present (step ST61). The CPU 13a
compares the detected value with a predetermined threshold (step
ST62). If the reception power level of the pilot channel from the
base station BS other than the base station BS4 is equal to or
lower than the threshold, the mobile station MSj is not located at
the boundary between the cell Z4 and another cell. In this case,
since no mobile handoff is required, the CPU 13a terminates this
mobile handoff control.
[0126] Assume that the mobile station MSj moves from a position I
in the cell Z4 of the base station BS4 to a boundary position II
between the cell Z4 of the base station BS4 and a cell Z3 of the
base station BS4, as shown in FIG. 12. In this case, the reception
level of the pilot channel from the base station BS3 increases in
the mobile station MSj. When the reception power of the pilot
channel from the base station BS other than the base station BS4
exceeds the threshold, the CPU 13a transmits a message indicating
the phase of a PN code sent over the pilot channel and the
reception power level of the pilot channel to the base station BS4
(to be referred to as a source base station hereinafter) forming
the cell Z4 in which the mobile station has been located (step
ST63). Thereafter, the CPU 13a checks whether a handoff instruction
is sent from the source base station BS4 within a predetermined
period of time (step ST64).
[0127] In the base station BS4, the CPU 42a performs mobile handoff
control as shown in FIG. 11 at a predetermined timing, e.g.,
predetermined periods.
[0128] At the predetermined timing, the CPU 42a checks whether the
message transmitted from the base station MS is received (step
ST71). If the message transmitted from the mobile station MSj is
received as described above, the CPU 42a analyzes the message to
check whether handoff is required (step ST72).
[0129] If it is determined that handoff is not required, or it is
determined in step ST71 that no message is received, the CPU 42a
terminates mobile handoff control. If, however, a message is
received from the mobile station MSj that has moved to a boundary
position II between the cell Z4 of the base station BS4 and the
cell Z3 of the base station BS3 as shown in FIG. 12, handoff is
required. In such as case, therefore, the CPU 42a determines the
cell Z3 as a cell the mobile station MSj is entering from the
message, and transfers a handoff request to the base station (to be
referred to as a destination base station) forming the cell Z3
through the control station CS (step ST73). The CPU 42a of the
destination base station BS4 checks the channel occupancy measured
by the channel occupancy monitoring means 42a in the new control
station BS3 (step ST74). The CPU 42a of the source base station BS4
then checks whether the channel occupancy of the radio frequency
currently used by the mobile station MSj in the destination base
station BS3 exceeds a predetermined threshold (step ST75).
[0130] If the channel occupancy of the radio frequency currently
used by the mobile station MSj in the destination base station BS3
is equal to or lower than the predetermined threshold, the radio
frequency currently used by the mobile station MSj can also be used
at the new. The CPU 42a of the source base station BS4 therefore
determines soft handoff as a handoff method to be used, and
transmits a handoff instruction for controlling soft handoff to the
mobile station MSj (step ST76). Thereafter, the CPU 42a of the
source base station BS4 performs soft handoff by a known procedure
in cooperation with the mobile station MSj and the destination base
station BS3 (step ST77).
[0131] If the channel occupancy of the radio frequency currently
used by the mobile station MSj in the destination base station BS3
exceeds the predetermined threshold, continuous use of the radio
frequency currently used by the mobile station MSj in the cell it
is entering is not preferable. The CPU 42a of the source base
station BS4 determines hard handoff as a handoff method to be used,
and transmits a handoff instruction for controlling hard handoff to
the mobile station MSj (step ST78). The CPU 42a of the source base
station BS4 determines a radio frequency whose channel occupancy in
the destination base station BS3 does not exceed the predetermined
threshold and is lowest as a radio frequency to be used, and
notifies the mobile station MSj of this radio frequency (step
ST79). The CPU 42a of the source base station BS4 performs hard
handoff by a known procedure in cooperation with the mobile station
MSj and the destination base station BS3 (step ST80).
[0132] If a handoff instruction transmitted from the source base
station BS4 in the above manner is received, the CPU 13a of the
mobile station MSj determines in step ST64 that handoff is
instructed. In this case, the CPU 13a checks whether this handoff
instruction designates hard handoff (step ST65).
[0133] If the handoff instruction does not designate hard handoff,
the CPU 13a performs soft handoff by a known procedure in
cooperation with the source base station BS4 and the destination
base station BS3 (step ST66).
[0134] If the handoff instruction designates hard handoff, the CPU
13a receives a radio frequency notification following the handoff
instruction (step ST67). The CPU 13a then performs hard handoff to
a state in which the radio frequency designated by the radio
frequency notification is used, by a known procedure, in
cooperation with the source base station BS4 and the destination
base station BS3 (step ST68).
[0135] With this operation, when soft handoff cannot be performed
because there is no available traffic channel in the new cell with
respect to the radio frequency currently used by the mobile station
MSj, another radio frequency that can be used is selected, and hard
handoff is performed. In this case, although a slight deterioration
in speech communication quality cannot be avoided, at lest the
worst case, e.g., a handoff failure, can be prevented. In addition,
when one of the two radio frequencies allocated to a given base
station BS exhibits dense traffic, the radio frequency used by the
mobile station that moves while using the radio frequency with
dense traffic is switched to another radio frequency. Therefore,
the channel occupancies of the two radio frequencies can be
averaged.
[0136] As described above, according to this embodiment, measures
are taken to make the use of radio frequencies random with respect
to a radio frequency as a candidate to be used by each mobile
station MS in the standby state, a radio frequency as a candidate
to be used by each mobile station MS at power-on, and a radio
frequency to be used in a new cell when each mobile station MS
under radio communication moves between cells within the same base
station group. With this, the channel occupancies of the respective
radio frequencies in each base station BS can be averaged. By
averaging the channel occupancies of the respective radio
frequencies in each base station BS, traffic channels can be
efficiently used.
[0137] The present invention is not limited to the above
embodiment. For example, the above embodiment uses an arrangement
like the one shown in FIG. 13(a), in which one base station BS
handles both radio frequencies (e.g., f1 and f2). However, as shown
in FIG. 13(b), one base station BS may be made up of a base station
apparatus 61 for handling only one (e.g., f1) of the two radio
frequencies, a base station apparatus 62 for handling only the
other (e.g., f2) of the two radio frequencies, and a base station
control apparatus 63 for controlling the two base station
apparatuses 61 and 62.
[0138] In each embodiment described above, the base stations forms
cells having the same diameter. In this case, however, the same
diameter includes differences within a predetermined range. The
maximum difference is set, for example, such that the diameter of a
cell having the maximum diameter is less than twice that of a cell
having the minimum diameter.
[0139] In addition, the conditions for determining the necessity of
idle handoff are not limited to those in the above embodiment. For
example, an arbitrary threshold may be set.
[0140] The conditions for setting the number of mobile stations to
which an idle handoff instruction is to be output are not limited
those in the above embodiment, and may be arbitrary set. In
addition, the number of mobile stations to which an idle handoff
instruction is to be output need not always be variable, and may be
constant.
[0141] Furthermore, the number of mobile stations to which an idle
handoff instruction is to be output may be set in accordance with
the channel occupancy of a carrier. In this case, it is preferable
that a threshold for determining a channel occupancy be arbitrarily
changed by manual operation by a person in charge of maintenance
and management in a base station.
[0142] The channel occupancy monitoring means 42a, the idle handoff
controlling means 42b, the frequency use state notifying means 42c,
the handoff method determining means 42d, and the network-side
handoff control means 42e, which are mounted in the base station BS
in the above embodiment, may be mounted in the control station
CS.
[0143] Furthermore, the circuit arrangements of each mobile station
and each base station, the control procedures for mobile handoff
and idle hard handoff, the contents of the control, and the like
can be variously modified within the spirit and scope of the
invention.
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