Note: Descriptions are shown in the official language in which they were submitted.
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METHOD OF HANDOVER AND ROUTE DIVERSITY
IN MOBILE RADIO COMMUNICATION
BACKGROUND OF THE INVENTION
Field of the Invention
The present invention relates to a method of handover
and route diversity operations in a mobile radio
communication system such as an automobile telephone
system.
Description of the Background Art
In a mobile radio communication system, the efficient
utilization of limited frequency spectrum is one of the
major technical problems. For this reason, a so called
cellular system is adopted in general. In this cellular
system, the efficient frequency spectrum utilization is
achieved by providing a plurality of mutually distanced
base stations within a service area of the systen so as to
enable a so called co-channel reuse which is a repetitive
use of the same frequency at different base stations, where
the base stations are sufficiently distanced from each
other to avoid a co-channel interference. In such a
cellular system, an area covered by each base station is
called a cell.
In this cellular system, a higher frequency spectrum
utilization efficiency can be achieved by making a radius
of each cell smaller, because it becomes possible for the
same frequency to be repetitively used at shorter
distances. Such a use of smaller size cells is effective in
increasing a system capacity regarding a number of
subscribers that can be handled by the system.
However, as a consequence of an increased number of
cells required for covering the entire service area,
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such a use of smaller size cells also necessitates an
increased number of so called handover operations required
to be performed when a mobile station moves from one cell
to another during a single continuous communication. As a
result, the capacities and sizes of each base station and a
central control station controlling the base stations will
be increased considerably and configurations of the base
stations and the central control station will inevitably
become quite complicated.
Namely, in the cellular system, as a mobile station
which is communicating through one particular base station
of one particular cell moves from that particular cell to a
neighboring cell, the receiving level at that particular
base station will gradually decreases, so that in order to
continue this communication it is necessary to switch the
base station through which the mobile station communicates
from that particular base station of that particular cell
to the base station of the neighboring cell. Here, in
switching the base station, an active land transmission
line connecting the central control station with the base
station needs to be switched, while the frequency used for
the communication by that mobile station needs to be
switched from the traffic channel used in an original cell
from which the mobile station is moving out to that of an
idle traffic channel available in the base station of a
destination cell to which the mobile station is moving in.
Such an operation is known as the handover operation
(which is sometimes also referred to as a handoff
operation).
More specifically, as shown in Fig. 1, a conventional
cellular system comprises a central control station 1, and
a plurality (four in Fig. 1) of base stations 2 to 5
controlled by the central control station 1 through land
transmission lines 11 to 14, respectively, where a mobile
station 10 moves through a plurality (four in Fig. 1) of
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cells 6 to 9 associated with the base stations 2 to 5,
respectively. In this cellular system, the central control
station 1 is connected to an ordinary telephone network
(not shown), such that a communication to and from the
mobile station 10 can be transmitted through one of the
base stations 2 to 5, the central control station 1, and
the ordinary telephone network.
Now, assuming that the mobile station 10 is located in
a cell 8 and communicating through the base station 4, as
the mobile station 10 moves to the neighboring cell 7, the
handover operation will proceed as follows.
(1) As a distance between the base station 4 and the
mobile station 10 becomes greater than the cell radius, the
receiving level at the base station 4 decreases below the
specific level. When this decrease of the receiving level
is detected by the base station 4, the base station 4
requests the handover of the communication of the mobile
station 10 to the central control station 1 through the
land transmission line 13.
(2) In response to this request for the handover from.
the base station 4, the central control station 1 commands
the neighboring base stations 2, 3, and 5 to monitor the
transmitted radio waves from the mobile station 10 through
the land transmission lines 11, 12, and 14, respectively.
(3) In response to this command from the central
control station 1, each of the base stations 2, 3, and 5
receives the transmitted radio waves of the traffic channel
used for the communication of the mobile station 10
specified by the command from the central control station
1, measures the receiving level for this traffic channel,
and reports the measured receiving level to the central
control station 1.
(4) Then, the central control station 10 selects the
base station for which the reported receiving level is the
highest among the base stations 2, 3, and 5. In this
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exemplary case, the base station 3 will have the highest
receiving level as the mobile station 10 is moving into the
cell 7.
(5) Next, the central control station 10 commands the
selected base station 3 to report its idle traffic channel
xN which is available for the handover of the communication
of the mobile station 10.
(6) In response to this command from the central
control station 1, the base station 3 reports the idle
traffic channel #N to the central control station 1.
(7) Then, the central control station 1 commands the
base station 4 to send a command signal for the handover to
the traffic channel nN to the mobile station 10. Meanwhile,
the central control station 1 also .commands the base
station 3 to activate the transmitter-receiver for the
traffic channel nN, while switching the active land
transmission line from that connected to the base station 4
to that connected to the base station 3.
(8) In response to the command from the central
control station 1, the base station 4 sends the command
signal for the handover to the mobile station 10.
(9) In response to the command signal from the base
station 4, the mobile station 10 switches its traffic
channel to the traffic channel nN specified by the command
signal, so as to establish the communication through the
base station 3.
Now, in addition to the increase of capacities and
sizes of each base station and the central control station
and the complication of configurations of the base stations
and the central control station already mentioned above,
such a conventional method of handover in the cellular
system has a drawback that it is quite time consuming as it
requires several signal exchanges between the central
control station 1 and the base stations after the lowering
of the receiving level is detected at the base station 4 in
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order to complete the handwer operation.
As a consequence, in a case the cell radius is made
smaller, it becomes possible for the mobile station to move
further onto the next neighboring cell before the handover
operation from one cell to its neighboring cell is
completed; such that the highly undesirable failure of the
handover of the communication could occur in the cellular
system. To eliminate such a possibility for the failure of
the handover, the cell radius cannot be made smaller than a
certain limit value, and this limitation on the cell radius
in turn creates an upper limit to the efficient frequency
spectrum utilization.
Moreover, when the mobile stations with higher moving
capability such automobile telephones and the mobile
stations with lower moving capability such as portable
telephones are involved in such a conventional cellular
system together, the cell radius must be determined in
accordance with the automobile telephones which can move
faster, in order to prevent the occurrence of the failure
of the handover, even though the cell radius so determined
is not capable of achieving the highest frequency spectrum
utilization efficiency for the portable telephones.
Consequently, the base station such as that located in a
heavily populated area where a number of portable telephone
subscribers is large needs to have a large number of
traffic channels assigned, but this in turn lowers the
frequency spectrum utilization efficiency further.
On the other hand, when the cell radius is determined
in accordance with the portable telephones, it becomes'
difficult for the system to handle the frequent handover
operations required by the communication of the portable
telephone used on a fast moving object such as an
automobile.
In addition, as the cell radius becomes smaller, an
area between neighboring cells which is not well served by
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any cell may appear, and also the quality of service will
be lowered drastically as soon as the mobile station moves
out of the area covered by the cells, such that there is a
great possibility for the subscribed portable telephone
user to suffer from the poor quality of service.
Now, in a mobile radio communication, the radio
transmission characteristic is quite adverse because of~the
large receiving level fluctuation and other causes, such
that the occurrence of errors in the transmission signals
is unavoidable. For this reason, the mobile radio
communication system utilizes various error correction
mechanisms. However, these error correction mechanisms are
effective only with respect to the instantaneous
fluctuation of the receiving level, and not very effective
with respect to the relatively gradual variation of the
receiving level caused by the shadowing of the transmission
path due to the terrain and environmental features.
To such a relatively gradual variation of the
receiving level, a so called route diversity reception in .
which the transmitted radio waves received by a plurality
of base stations are utilized is known to be effective. For
instance, the automobile telephone of NTT (Nippon Telegraph
and Telephone) Corporation utilizes the base station route
diversity reception for the control channel.
More specifically, a conventional route diversity
reception is achieved as follows.
Namely, a conventional mobile radio communication
system utilizing the route diversity reception has a
configuration shown in Fig. 2, where the system includes a
central control station 21 and a plurality of base stations
22 to 28 each of which is connected with the central
control station 21 through land transmission lines 29 to
35, respectively.
In this mobile radio communication system, a signal
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transmitted from each of the base stations 22 to 28 to the
central control station 21 has a format shown in Fig. 3
which includes a transmission signal 36 from a mobile
station and an encoded receiving level information 37
indicating the receiving level at each base station. In
other words, each base station relays the transmission
signal 36 from the mobile station to the central control
station 21 by attaching the receiving level information 3?
to the transmission signal 36.
The central control station 21 then selects the base
station for which the receiving level is the highest among
all the base stations 22 to 28, and uses the transmission
signal 36 transmitted from this selected base station as
the transmission signal 36 received, from the mobile
station.
Such a conventional mobile radio communication system
has drawbacks that the expensive land transmission line
must be provided between the central control station 21 and
each one of a plurality of the base stations and that the
control function of the central control station 21
inevitably becomes complicated as a consequence of
utilizing the route diversity reception.
SUMMARY OF THE INVENTION
It is therefore an object of the present invention to
provide a method of handover in a mobile radio
communication which is less time consuming such that the
smaller cell radius can be used so as to improve the '
frequency spectrum utilization efficiency, and capable of
securing the high quality of service regardless of the
moving speed of the mobile station.
It is also an object of the present invention to
provide a method of route diversity in a mobile radio
communication which can be realized less expensively,
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without requiring additional control function which
complicates the central control station.
According to one aspect of the present invention there
is provided a method of handover in a mobile radio
communication using a cellular system formed by a plurality
of base stations, the method comprising the steps of:
providing a loop transmission line by which each base
station is connected with neighboring base stations; I
transmitting a handover information through the loop
transmission line, where the handover information is
relayed by each base station from one of the neighboring
base stations to another one of the neighboring base
station; and carrying out the handover of a communication
of a mobile station from one traffic channel of one base
station to an idle traffic channel of another base station
by using the handover information transmitted through the
loop transmission line.
According to another aspect of the present invention
there is provided a method of handover in a mobile radio
communication using a cellular system formed by a plurality
of control base stations each of which is covering a cell
within a service area of the system, a plurality of base
stations each of which is covering a mini-cell within a
cell area of the cell covered by each of the control base
stations, the method comprising the steps of: providing a
loop transmission line by which a control base station and
each base station are connected with neighboring base
stations; transmitting a handover information through the
loop transmission line, where the handover information~is
relayed by each base station from one of the neighboring
base stations to another one of the neighboring base
station; and carrying out the handover of a communication
of a mobile station from one traffic channel of one base
station to an idle traffic channel of another base station
by using the handover information transmitted through the
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loop transmission line when a mobile station moves from one
mini-cell to another mini-cell.
According to another aspect of the present invention
there is provided a method of handover in a mobile radio
communication using a cellular system formed by a plurality
of base stations, the method comprising the steps of:
providing a loop transmission line by which each base
station is connected with neighboring base stations;
monitoring at a mobile station a receiving quality at each'
of the base stations in a control channel during idle
periods of a currently used traffic channel in order to
determine a destination base station for the handover;
transmitting a handover information through the loop
transmission line, where the handover information is
relayed by each base station from one of the neighboring
base stations to another one of the neighboring base
station; and carrying out the handover of a communication
of a mobile station from the currently used traffic channel
of a currently used base station to an idle traffic channel
of the destination base station determined by the mobile
station by using the handover information transmitted
through the loop transmission line.
According to another aspect of the present invention
there is provided a method of route diversity in a mobile
radio communication using a cellular system formed by a
central control station and a plurality of base stations,
the method comprising the steps of: providing an inter-
station transmission line by which each base station is
connected with neighboring base stations and to which the
central control station is connected at an end in a signal
transmission direction; receiving radio waves from a mobile
station and measuring a receiving quality for the radio
waves from the mobile station at each base station; and
transmitting a receiving quality information indicating a
currently highest receiving quality obtained by the base
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stations and a transmission signal representing the radio
waves from the mobile station received at the highest
receiving level, the receiving level information and the
transmission signal being updated at each base station such
that the receiving quality information received by the
central control station is the highest receiving quality
among the receiving qualities obtained by all the base
stations.
Other features and advantages of the present invention
will become apparent from the following description taken
in conjunction with the accompanying drawings.
BRIEF DESCRIPTION OF THE DRAWINGS
Fig. 1 is a schematic diagram of a conventional mobile
radio communication system in a form of a cellular system.
Fig. 2 is a schematic diagram of a conventional mobile
radio communication system utilizing a route diversity.
Fig. 3 is a diagram of a format for a signal
transmitted from each base station to a central control
station in the system of Fig. 2.
Fig. 4 is a schematic diagram of a first embodiment of
a mobile radio communication system in a form of a cellular
system using a method of handover according to the present
invention.
Fig. 5 is a diagram of a format for an original base
station signal transmitted from each base station through a
loop transmission line in the system of Fig. 4.
Fig. 6 is a schematic diagram of a configuration of
each base station in the system of Fig. 4.
Fig. 7 is a diagram of one format for a destination
base station signal transmitted from each base station
through the loop transmission line in the system of Fig. 4.
Fig. 8 is a diagram of another format for a
destination base station signal transmitted from each base
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station through the loop transmission line in the system of
Fig. 4.
Fig. 9 is a diagram of another format for an original
base station signal and a destination base station signal
transmitted from each base station to the loop transmission
line in the system of Fig. 4.
Fig. 10 is a schematic diagram of a second embodiment
of a mobile radio communication system in a form of a
cellular system using a method of handover according to the
present invention.
Fig. 11 is a diagram of a format for an original base
station signal transmitted from each base station through a
loop transmission line in the system of Fig. 10.
Fig. 12 is a schematic diagram of a configuration of
each base station in the system of Fig. 10.~
Fig. 13 is a diagram of a format for a receiving level
report signal transmitted from each base station through
the loop transmission line in the system of Fig. 10.
Fig. 14 is a diagram of a format for an idle traffic
channel signal transmitted from each base station through
the loop transmission line in a third embodiment of a
mobile radio communication system in a form of a cellular
system using a method of handover according to the present
invention.
Fig. 15 is a timing diagram for a case of relaying the
idle traffic channel signal of Fig. 14 at each base
station.
Fig. 16 is a schematic diagram of a fourth embodiment
of a mobile radio communication system in a form of a
cellular system using a method of handover according to the
present invention.
Fig. 17 is a schematic diagram of a part of the system
of Fig. 16 in one exemplary situation for explaining the
handover operation in the system.
Fig. 18 is a schematic diagram of a part of the system
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of Fig. 16 in another exemplary situation for explaining
the handover operation in the system.
Fig. 19 is a timing diagram for an operation of a
mobile station in a fifth embodiment of a mobile radio
communication system in a form of a cellular system using a
method of handover according to the present invention.
Fig. 20 is a diagram of a format for a control station
signal transmitted through the loop transmission line i.n
the system of the fifth embodiment.
Fig. 21 is a diagram of a format for a mobile station
signal transmitted through the loop transmission line in
the system of the fifth. embodiment.
Fig. 22 is a diagram of a format for a control channel
signal transmitted through the loop transmission line in
the system of the fifth embodiment.
Fig. 23 is a flow chart for a first procedure of the
handover operation in the system of the fifth embodiment.
Fig._24 is a flow chart for a second procedure of the
handover operation in the system of the fifth embodiment.
Fig. 25 is a flow chart for a third procedure of the
handover operation in the system of the fifth embodiment.
Fig. 26 is a flow chart for a fourth procedure of the
handover operation in the system of the fifth embodiment.
Fig. 2? is a flow chart for a fifth procedure of the
handover operation in the system of the fifth embodiment.
Fig. 28 is a schematic diagram of a sixth embodiment
of a mobile radio communication system in a form of a
cellular system using a method of route diversity according
to the present invention.
Fig. 29 is a schematic diagram of a configuration~of
each base station in the system of Fig. 28.
Fig. 30 is a timing diagram for an operation of a base
station to achieve the route diversity reception in the
system of Fig. 28.
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DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS
Referring now to Fig. 4, a first embodiment of a
mobile radio communication system using a method of
handover according to the present invention will be
described in detail.
In this first embodiment, the system comprises a
central control station 101 and a plurality (four in Fig.
4) of base stations 102 to 105, which are connected through
a loop transmission line formed by a transmission line
element 115 connecting between the central control station
101 and the base station 102, a transmission line element
116 connecting between the base stations 102 and 103, a
transmission line element 117 connecting between the base
stations 103 and 105, a transmission line element 118
connecting between the base stations 105 and 104, and a
transmission line element 119 connecting between the base
station 104 and the central control station 101.
In this system of the first embodiment; while the
mobile station 110 is communicating through one of the base
station, that base station outputs an original base station
signal to this loop transmission line of the system in a
format shown in Fig. 5, where the original base station
signal includes a signal identification 120 identifying the
type of the signal transmitted (i.e., that it is the
original base station signal), a base station
identification 121 identifying the base station from which
this original base station signal is transmitted (i.e.,
that base station itself), a traffic channel number 122
indicating the traffic channel currently used for the
communication of the mobile station 110 at that base
station, and a receiving level information 123 indicating
the receiving level of the transmitted radio waves from the
mobile station 110 at that base station.
Each of the base stations 102 to 105 has a schematic
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configuration shown in Fig. 6, where the base station
comprises an antenna 124 for transmitting and receiving the
radio waves to and from the mobile station 110, a
transmitter-receiver 125 for supplying the transmission
signal to the antenna 124 and receiving the transmitted
signal from the antenna 124, a control circuit 126, a
transmission line element 128 for receiving signals from an
upper station through the loop transmission line, and a.
transmission line element 129 for transmitting signals to a
lower station through the loop transmission line. Here,
each of the base stations 102 to 105 has a predetermined
number of traffic channels assigned according to the
traffic demand of each cell area.
In this configuration, the control circuit 126 of each
base station generates the signal identification 120, the
base station identification 121, the traffic channel number
122, and the receiving level information 123 for one of the
traffic channel through which the mobile station 110 is
currently communicating, and outputs the original base
station signal in a format of Fig. 5 to the loop
transmission line of the system through the transmission
line element 129.
On the other hand, from the transmission line element
128, the similar original base station signal in a format
of Fig. 5 outputted by the upper station is transmitted to
each base station. Each base station relays such original
base station signals transmitted from the upper station to
the loop transmission line through the transmission line
element 129.
In addition, in this system, for each base station,
the base stations which are neighboring this base station
such that the handover from this base station may occur are
determined in advance, and the control circuit 126 of each
base station memorizes the base stations for which this
base station has such neighboring relationships.
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When the original base station signal in a format of
Fig. 5 transmitted through the loop transmission line is
received through the transmission line element 128, each
base station determines whether this original base station
signal is transmitted from one of the base stations with
which it is in the neighboring relationships. If so, the
base station starts receiving the traffic channel specified
by the traffic channel number 122 of that original base.
station signal'and measures the receiving level for that
traffic channel.
In a case the measured receiving level is higher than
the receiving level indicated by the receiving level
information 123 of that original base station signal, the
base station fudges that the mobile station 110 has moved
into the cell area of its own cell. In this. case, the base
station outputs a destination base station signal
indicating its own base station identification, its own
idle traffic channel number, and the base station
identification of the original base station from which the
mobile station 110 is moving out. Here, in a case there is
no idle traffic channel available in that base station, the
destination base station signal will not be generated.
This destination base station signal can be in a
format shown in Fig. 7. In this format shown in Fig. ?, the
destination base station signal includes a signal
identification 130 identifying the type of the signal
transmitted (i.e., that it is the destination base station
signal), a base station identification 131 identifying the
base station from which this destination base station
signal is transmitted (i.e., that base station itself), an
idle traffic channel number 132 indicating the idle traffic
channel available at that base station, an original base
station identification 133 identifying the original base
station from which the mobile station 110 is moving out,
and an original traffic channel number 134 indicating the
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traffic channel used for the communication of the mobile
station 110 at the original base station.
Alternatively, the destination base station signal may
be in a format shown in Fig. 8, in which the destination
base station signal in the first format of Fig. 7 is
attached behind the original base station signal in a
format of Fig, 5 such that the original base station signal
and the destination base station signal are transmitte d
together when the original base station signal is to be
relayed to the loop transmission line. Here, the signal
identification 130 may be omitted as it is meaningless in
this format,~and the original base station identification
133 and the original traffic channel number 134 may be
omitted as they overlap with the base station
identification 121 and the traffic channel number 122.
Alternatively, the destination base station signal may
be in a format shown in Fig. 9, where the destination base
station signal includes the signal identification 120
identifying the type of the signal transmitted, the base
station identification 121 identifying the base station
from which this destination base station signal is
transmitted, the traffic channel number 122 indicating the
traffic channel currently used for the communication of the
mobile station 110, a measuring base station identification
135 identifying the base station at which the highest
receiving level for the traffic channel specified by the
traffic channel number 122 is measured up to then, a
measured receiving level information 136 indicating the
receiving level of the transmitted radio waves from the
mobile station 110 at the base station identified by the
measuring base station identification 135, and the idle
traffic channel number 132 indicating the idle traffic
channel available at the base station identified by the
measuring base station identification 135.
When this format of Fig. 9 is adopted, the original
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base station outputs the original base station signal also
in this format of Fig. 9, where the measuring base station
identification 135 is identical to the base station
identification 121 initially, and the receiving level
information 123 in the format of Fig. 5 is replaced by the
measured receiving level information 136 in the format of
Fig. 9.
Then, at each base station which receives the original
base station signal in a format of Fig. 9, the receiving
level for the traffic channel specified by the traffic
channel number 122 is measured and compared with the
receiving level indicated by the measured receiving level
information 136. When the receiving level measured at that
base station is less than the receiving level indicated by
the measured receiving level information 136, the
transmitted original base station signal is relayed to the
loop transmission line without any change as the
destination base station signal at that base station. On
the other hand when the receiving level measured at that
base station is greater than the receiving level indicated
by the measured receiving level information 136, the
measuring base station identification is changed to the
base station identification of that base station, and the
measured receiving level information 136 is changed to the
receiving level measured at that base station, while the
idle traffic channel number 132 is registered, before being
transmitted as the destination base station signal to the
loop transmission line.
When such a destination base station signal returns
back to the original base station after circulating around
the loop transmission line, the original base station
determines the destination base station and the idle
traffic channel in this destination base station from the
destination base station signal, and transmits a handover
command signal to the mobile station 110 so as to command
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the mobile station 110 to carry out the handover of its
communication to the idle traffic channel of the
destination base station determined from the destination
base station signal.
Here, it is noted that in this first embodiment, each
base station does not assign the traffic channel which is
registered as the idle traffic channel number 132 in the
destination base station signal to any communication until
the final determination of the destination base station and
the handover traffic channel.
Now, assuming that the mobile station 110 is
communicating through the base station 104, and moving out
from the cell area of this base station 104, the base
station 104 generates and outputs the original base station
signal described above to the loop~transmission line. Then,
assuming that the base station 103 received the radio waves
from the mobile station 110 at the highest receiving level
and reported its idle traffic channel nN in the destination
base station signal which is subsequently transmitted to
the original base station 104, the handover operation will
proceed as follows.
(1) The base station 104 notifies the central control
station 101 through the loop transmission line that the
base station 103 is determined as the destination base
station.
(2) In response to this notification from the base
station 104, the central control station 101 notifies the
base station 103 through the loop transmission line that it
is determined as the destination base station for the
handover, while also notifying the other base stations
through the loop transmission line that they are not
determined as the destination base station for the
handover.
(3) The base station 104 transmits a handover command
signal to the mobile station 110 so as to command the
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mobile station 110 to carry out the handover of its
communication to the idle traffic channel #N of the
destination base station 103.
(4) Then, the base station 103 activates the
transmitter-receiver for the traffic channel #N, while
notifying the central control station 101 through the loop
transmission line that the handover of the communication of
the mobile station 110 from the base station 104 to the
traffic channel #N of the base station 103 has been
completed.
In this first embodiment, when the idle traffic
channel registered by a certain base station is rewritten
at another subsequent base station, that certain base
station cannot know that the idle traffic channel it
registered has been rewritten at a time of rewriting.
However, each base station can recognize whether the idle
traffic channel it registered is going to be the handover
traffic channel or not as the central control station 101
notifies each base station as to whether it is determined
as the destination base station for the handover or not.
Thus, according this first embodiment, it becomes
possible to establish a so called decentralized autonomous
control in the cellular system in which each base station
monitors the traffic channels of the other base stations,
fudges whether the mobile station is moving into its own
cell area, and transmits its idle traffic channel to the
original base station without any command from the central
control station, such that the handover operation can be
carried out less time consumingly compared with a
conventional cellular system without increasing the
capacities and sizes of each base station and the central
control station and complicating the configurations of the
base stations and the central control station. As a
consequence, it becomes possible to use the smaller cell
radius so as to improve the frequency spectrum utilization
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efficiency, and to secure the high quality of service
regardless of the moving speed of the mobile station.
Referring now to Fig. 10, a second embodiment of a
mobile radio communication system using a method of
handover according to the present invention will be
described in detail.
In this second embodiment, the system comprises a
central control station 201 and a plurality (four in Fig.
10) of base stations 202 to 205, which are connected
through a loop transmission line formed by a transmission
line element~215 connecting between the central control.
station 201 and the base station 202, a transmission line
element 216 connecting between the base stations 202 and
203, a transmission line element 217 connecting between the
base stations 203 and 205, a transmission line element 218
connecting between the base stations 205 and 204; and a
transmission line element 219 connecting between the base
station 204 and the central control station 201.
In addition, the base stations 202 to 205 are directly
connected with the central control station 201 through land
transmission lies 211 to 214, respectively, besides the
loop transmission line.
In this system of the second embodiment, while the
mobile station 210 is communicating through one of the base
station, that base station outputs an original base station
signal to this loop transmission line of the system in a
format shown in Fig. 11, where the original base station
signal includes a receiving level detection request 220
indicating the request for monitoring the receiving level,
a base station identification 221 identifying the base
station from which this original base station signal is
transmitted (i.e., that base station itself), and a traffic
channel number 222 indicating the traffic channel currently
used for the communication of the mobile station 210 at
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that base station.
Each of the base stations 202 to 205 has a schematic
configuration shown in Fig. 12, where the base station
comprises an antenna 224 for transmitting and receiving the
radio waves to and from the mobile station 210, a
transmitter-receiver 225 for supplying the transmission
signal to the antenna 224 and receiving the transmitted
signal from the antenna 224, a control circuit 226, a
transmission line element 227 for transmitting and
receiving signals to and from the central control station
201 through the land transmission line, a transmission line
element 228 for receiving signals from an upper station
through the loop transmission line, and a transmission line
element 229 for transmitting signals to a lower station
through the loop transmission line. Here, each of the base
stations 202 to 205 has a predetermined number of traffic
channels assigned according to the traffic demand of each
cell area.
In this configuration, the control circuit 226 of each
base station monitors the receiving level of the traffic
channel used for the communication of the mobile station
210 and recognize that the mobile station 210 is moving out
from its cell area so that the handover operation is
necessary when the receiving level decreases below the
specific level, in which case the handover operation is
requested to the control circuit 226. In response, the
control circuit 226 generates the receiving level detection
request 220, the base station identification 221, and the
traffic channel number 222 for one of the traffic channel
through which the mobile station 210 is currently
communicating, and outputs the original base station signal
in a format of Fig. 11 to the loop transmission line of the
system through the transmission line element 229.
On the other hand, from the transmission line element
228, the similar original base station signal in a format
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of Fig. 11 outputted by the upper station is transmitted to
each base station. Each base station relays such original
base station signals transmitted from the upper station to
the loop transmission line through the transmission line
element 229.
Thus, the original base station signal transmitted
from the original base station to the loop transmission
line will subsequently be relayed through the base stations
202 to 205 and the central control station 201 until it
returns back to the original base station.
In addition, in this system, for each base station,
the base stations which are neighboring this base station
such that the handover from this base station may occur are
determined in advance, and the control circuit 226 of each
base station memorizes the base stations for which this
base station has such neighboring relationships. For
instance, in Fig. 10, the base station 203 has the
neighboring relationships with the base station 202, 204,
and 205, while the base station 205 has the neighboring
relationships with the base station 203 and 204.
When the original base station signal in a format of
Fig. 11 transmitted through the loop transmission line is
received through the transmission line element 228, each
base station determines whether this original base station
signal is transmitted from one of the base stations with
which it is in the neighboring relationships. If so, the
base station starts receiving the traffic channel specified
by the traffic channel number 222 of that original base
station signal and measures the receiving level for that
traffic channel. For instance, in Fig. 10, if the original
base station is the base station 202, the base stations 203
and 204 will be fudged as the neighboring base stations of
the original base station 202 so that they will start
measuring the receiving level for that traffic channel as
soon as the original base station signal is relayed to
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them, whereas the base station 205 will be judged as not a
neighboring base station of the original base station 202,
so that it will only relay the original base station signal
to the loop transmission line.
The base station which is judged as the neighboring
base station of the original base station then outputs a
receiving level report signal in a format shown in Fig. 13,
where the receiving level report signal includes a signal
identification 229 identifying that it is the receiving
level report signal, a monitoring base station
identification 230 identifying the base station which
measured the receiving level (i.e., that base station
itself), a monitored traffic channel number 231 indicating
the traffic channel monitored by that base station, and a
receiving level code 232 indicating the receiving level
obtained for the monitored traffic channel in an encoded
form.
The central control station 201 compares the receiving
level indicated by the receiving level code 232 of the
receiving level report signals transmitted through the loop
transmission line from the base stations, and selects the
base station for which the receiving level indicated by the
receiving level code 232 in the receiving level report
signal is the highest as the destination base station for
the handover.
Hereafter, the central control station 201 controls
the cellular system substantially similarly to the case of
the handover operation in a conventional cellular system.
Thus, according this second embodiment, it becomes
possible to establish a partially decentralized autonomous
control in the cellular system in which each base station
monitors the traffic channels of the other base stations
without any command from the central control station, such
that the handover operation can be carried out less time
consumingly compared with a conventional cellular system
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without increasing the capacities and sizes of each base
station and the central control station and complicating
the configurations of the base stations and the central
control station. As a consequence, it becomes possible to
use the smaller cell radius so as to improve the frequency
spectrum utilization efficiency, and to secure the high
quality of service regardless of the moving speed of the
mobile station..
Referring now to Figs. 14 and 15, a third embodiment
of a mobile.radio communication system using a method of
handover according to the present invention will be
described in detail.
In this third embodiment, the.system configuration is
substantially the same as that of the second embodiment
shown in Fig. 10. Also, each of the base stations has a
configuration substantially the same as that of the second
embodiment shown in Fig. 12.
Now. in this third embodiment, each base station
outputs an idle traffic channel signal to the loop
transmission line of the system in a format shown in Fig.
14, where the idle traffic channel signal includes a base
station identification 320 identifying the base station
from which this idle traffic channel signal is transmitted
(i.e., that base station itself), an idle traffic channel
number 322 indicating the idle traffic channel available at
that base station, and an idle traffic channel reservation
321 indicating a presence or an absence of a reservation
for the idle traffic channel indicated by the idle traffic
channel number 322.
Thus, the control circuit 226 of each base station
regularly generates the base station identification 320,
the idle traffic channel reservation 321 which is initially
set to indicate the absence of the reservation, and the
idle traffic channel number 322 indicating the idle traffic
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channel available at that base station at a time of the
generation of the idle traffic channel signal, and outputs
the generated idle traffic channel signal in a format of
Fig. 14 to the loop transmission line of the system through
the transmission line element 229. In a case there is no
idle traffic channel available at that base station at a
time of generation of the idle traffic channel signal, this
absence of the available idle traffic channel is indicated
by the idle traffic channel number 322.
On the other hand, from the transmission line element
228, the similar idle traffic channel signal in a format of
Fig. 14 outputted by the upper station is transmitted to
each base station. Each base station relays such idle
traffic channel signals transmitted from the upper station
to the loop transmission line through the transmission line
element 229.
In relaying the idle traffic channel signal from the
other base station, each base station may change the idle
traffic channel reservation 321 to indicate the presence of
the reservation by inserting its own base station
identification whenever it is fudged that the idle traffic
channel indicated by the idle traffic channel number 322 is
needed for the handover of the communication of the mobile
station 210 which is currently communicating through that
base station.
Namely, as shown in Fig. 15, when the base station
receives the idle traffic channel signal with the base
station identification 3201, idle traffic channel
reservation 3211, and idle traffic channel number 322i.from
the upper station through the transmission line element
228, the control circuit 226 of the base station determines
whether it is necessary to read the base station
identification 3201 and the idle traffic channel
reservation 3211 of this idle traffic channel signal, i.e.,
whether the idle traffic channel indicated by this idle
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traffic channel signal is needed for the handover at that
base station. In a case it is determined to be unnecessary
to read them, the control circuit 226 transmits this idle
traffic channel signal unchanged, by turning the base
station identification 320, idle traffic channel
reservation 3211, and idle traffic channel number 3221 into
the base station identification 3202, idle traffic channel
reservation 3212, and idle traffic channel number 3222 for
the idle traffic channel signal to be transmitted to the
lower station through the transmission line element 229. On
the other hand, when it is determined to be necessary to
read them, the control circuit 226 transmits this idle
traffic channel signal by turning the base station
identification 3201 into the base station identification
3202, and idle traffic channel number 3221 into and idle
traffic channel number 3222 for the idle traffic channel
signal to be transmitted to the lower station through the
transmission line element 229, while replacing the idle
traffic channel reservation 3211 by the idle traffic
channel reservation 3212 in which the presence of the
reservation is indicated by the inserted base station
identification of that base station.
Thus, the idle traffic channel signal transmitted from
each base station to the loop transmission line will
subsequently be relayed through the base stations 202 to
.205 and the central control station 201 while registering
the reservation for the idle traffic channel whenever the
need arises until it returns back to the base station which
generated this idle traffic channel signal.
Here, each base station does not assign the traffic
channel which is registered as the idle traffic channel
number 322 in the idle traffic channel signal to any
communication until the idle traffic channel signal returns
back after circulating through the loop transmission line.
Now, assuming that the mobile station 210 is
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communicating through the base station 204, and moving out
from the cell area of this base station 204 such that a
need for the handover from the base station 204 arises, the
handover operation will proceed as follows.
(1) As a distance between the base station 204 and the
mobile station 210 becomes greater than the cell radius,
the receiving level at the base station 204 decreases.below
the specific level. When this decrease of the receiving
level is detected by the base station 204, the base station
204 requests the handover of the communication of the
mobile station 210 to the central control station 201
through the land transmission line 213.
(2) In response to this request for the handover from
the base station 204, the central control station 201
commands the neighboring base stations 202,.203, and 205 to
monitor the transmitted radio waves from the mobile station
210 through the land transmission lines 211, 212, and 214,
respectively.
(3) In response to this command from the central
control station 201, each of the base stations 202, 203,
and 205 receives the transmitted radio waves of the channel
used for the communication of the mobile station 210 which
is specified from the central control station 201, and
measures the receiving level which is then reported to the
central control station 201.
(4) Then, the central control station 201 selects the
base station for which the reported receiving level is the
highest among the base stations 202, 203, and 205. In this
exemplary case, the base station 203 will have the highest
receiving level.
Here, it is noted that these four steps (1) to (4)
are substantially the same as the conventional handover
operation procedure. In this third embodiment, however,
these steps (1) to (4) are followed by the following steps
(5) to (8).
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(5) The central control station 201 commands the base
station 204 to carry out the handover of the communication
of the mobile station 210 to the base station 203.
Meanwhile, the central control station 201 also switches
the active land transmission line from that connected to
the base station 204 to that connected to the base station
203, and notifies the base station 203 through the loop
transmission line that it is determined as the destination
base station for the handover.
(6) In response to the command for the handover from
the central control station 201, the base station 204
selects the idle traffic channel signal having the base
station identification 320 indicating the base station 203
among the idle traffic channel signals circulating in the
loop transmission line, and reads the idle traffic channel
number 322 of the selected idle traffic channel signal so
as to recognize the idle traffic channel nN for the
handover. Then, the base station 204 changes the idle
traffic channel reservation 321 of this idle traffic
channel signal to indicate the presence of the reservation
for the idle traffic channel #N by inserting its own base
station identification to the idle traffic channel
reservation 321, and then relays this idle traffic channel
signal to the lower station to the loop transmission line
through the transmission line element 229.
(7) The base station 204 transmits a handover command
signal to the mobile station 210 so as to command the
mobile station 210 to carry out the handover of its
communication to the idle traffic channel ~N of the base
station 203.
(8) When the idle traffic channel signal whose idle
traffic channel reservation 321 has been changed by the
base station 204 returns back to the base station 203 which
generated this idle traffic channel signal, the base
station 203 recognizes from the idle traffic channel
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reservation 321 of this idle traffic channel signal that
the base station 204 is going to use the idle traffic
channel #N for the handover, and activates the transmitter-
receiver for the traffic channel nN, while notifying the
central control station 201 through the loop transmission
line that the handover of the communication of the mobile
station 210 is going to be carried out from the base
station 204 to the traffic channel #N of the base station
203, so as to complete the handover operation.
The base station 203 subsequently generates and
outputs the idle traffic channel signal again, in which the
traffic channel other than nN will be registered as the
idle traffic channel.
In a case the idle traffic channel signal returns back
to the base station which generated this idle traffic
channel signal without any reservation for the idle traffic
channel registered and this idle traffic channel is still
available~at that base channel, that base station may
outputs the same idle traffic channel signal again to the
loop transmission line, or the new idle traffic channel
signal in which the idle traffic channel is replaced by
another idle traffic channel available.
In a case one traffic channel of one destination base
station happens to be potentially usable by two original
base stations, the original base station which receives the
idle traffic channel signal from the destination base
station first will make the reservation for the idle
traffic channel, so that by the time the other original
base station receives this idle traffic channel signal. from
the destination base station, the idle traffic channel is
no longer available and therefore this other original base
station have to wait for the other idle traffic channel
signal in which the idle traffic channel is not yet
reserved. Thus, in this third embodiment, the conflict
between two base stations over a single idle traffic
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channel of the other base station can be avoided.
Actually, the idle traffic channel reservation 321 is
not absolutely indispensable in this third embodiment. In a
case the idle traffic channel reservation 321 is not used
in the idle traffic channel signal, it becomes possible for
two original base stations requiring the handover at the
same time to assign the same traffic channel of one
destination base station to two different mobile stations.
However, even in such a case the conflict between two base
stations over a single idle traffic channel of the other
base station, can also be avoided by means of a loop check
operation as follows.
Namely, in general, the mobile station for which a new
traffic channel is assigned as a result of the handover
carries out an operation called a loop check in order to
confirm the correctness of the assignment of the new
traffic channel. This loop check operation is carried out
by exchanging a loop check signal including a mobile
station number between the destination base station and the
mobile station, and the communication through a traffic
channel assigned by the handover operation cannot be
started until this loop check operation is successfully
completed. Thus, even when more than one original base
stations assign the same traffic channel of one destination
base station to two different mobile stations at the same
time, only the mobile station which completed the loop
check with the destination base station first can actually
start communicating through the new traffic channel. and
the other mobile station which inevitably fails to complete
the loop check operation will have to wait for the handover
to another traffic channel of the destination base station,
so that the conflict between two base stations over a
single idle traffic channel of the other base station can
also be avoided.
Thus, according this third embodiment, it becomes
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possible to establish a partially decentralized autonomous
control in the cellular system in which each base station
monitors the idle traffic channels of the other base
stations without any command from the central control
station, such that the handover operation can be carried
out less time consumingly compared with a conventional
cellular system without increasing the capacities and sizes
of each base station and the central control station and
complicating the configurations of the base stations and
the central control station. As a consequence, it becomes
possible to use the smaller cell radius so as to improve
the frequency spectrum utilization efficiency, and to
secure the high quality of service regardless of the moving
speed of the mobile station.
Referring now to Fig. 16, a fourth embodiment of a
mobile radio communication system using a method of
handover according to the present invention will be
described in detail.
In this fourth embodiment, the entire service area of
the system is covered by a plurality of cells such as a
cell 416. Each cell 416 has a control base station 415
which covers the entire cell area of the cell 416, and a
plurality of base stations 417 to 420 covering mini-cells
421 to 424 formed within the cell area of the cell 416. The
control base station 415 and the base stations 417 to 420
are connected through a loop transmission line 425 such
that the mini-cells 421 to 424 are managed substantially as
in the first embodiment described above with the control
base station 415 playing a role of a central control
station. Regions within the cell 416 which are not covered
by the mini-cells 421 to 424 are covered by the control
base station 415 alone. Here, each of the base stations 417
to 420 has a predetermined number of traffic channels
assigned according to the traffic demand within each mini-
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cell, while the control base station 415 also has a
predetermined number of traffic channels assigned according
to the traffic demand within the cell 416.
In addition, the control base stations such as the
control base station 415 are connected with a system
control station 414 through land transmission lines such as
a land transmission line 426, such that the handover
between the control base stations are managed by the system
control station 414.
In this system, each of the base stations 417 to 420
has a limited functional capacity compared with the control
base station 415, so that when an excessively low receiving
level or an excessively large receiving level fluctuation
beyond the functional capacity of each base station is
detected for a communication of a mobile station at the
base station, the handover of the communication is carried
out from the base station to the idle traffic channel of
the control base station 415.
Namely, as shown in Fig. 17, when the mobile station
429 located within the mini-cell 421 and making a
communication 431 through the base station 417 is moving
out of the mini-cell 421 by moving along an arrow 433, the
receiving levels reported from the other base stations 418
to 420 through the loop transmission line 425 indicate that
the receiving levels are not high enough, so that the
control base station 415 commands the handover to its own
idle traffic channel to the base station 417 and the mobile
station 429 through the loop transmission line 425.
Similarly, when the mobile station moving within the
mini-cells 421 to 424 is moving very fast, the receiving
levels reported from the other base stations 418 to 420
through the loop transmission line 425 indicate that the
receiving levels are excessively fluctuating, so that the
control base station 415 commands the handover to its own
idle traffic channel to the base station and the mobile
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station through the loop transmission line 425.
Also, when the mobile station 430 located outside the
mini-cells 421 to 424 and making a communication 432
through the control base station 415 is moving out of the
cell 416 to the neighboring cell by moving along an arrow
434, the handover is carried out between the control base
station 415 and the control base station of the neighboring
cell under the control of the system control station 414.
On the other hand, while the mobile station is
communicating through the control base station 415, the
base stations 417 to 420 monitor this communication and
report the receiving level to control base station 415,
such that when the receiving level at one of the base
stations 417 to 420 becomes sufficiently high for the
capacity of that base station, the .handover.of the
communication is carried out from the control base station
415 to that base station.
Namely, as shown in Fig. 18, when the mobile station
435 located outside the mini-cells 421 to 424 and making a
communication 438 through the control base station 415 is.
moving into the mini-cell 417 by moving along an arrow 441,
the handover is carried out from the control base station
415 to the idle traffic channel of the base station 417.
When the mobile station 436 located within the mini-
cell 421 and making a communication 439 through the base
station 41? is moving into the neighboring mini-cell 422 by
moving along an arrow 442, the control base station 415
commands the handover from the base station 417 to the base
station 418 as in the first embodiment described above,
Here, however, when the mobile station moving within
the mini-cells 421 to 424 is moving very fast, the
receiving levels reported from the other base stations 418
to 420 through the loop transmission line 425 indicate that
the receiving levels are excessively fluctuating, so that
the control base station 415 continues to maintain the
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communication of the mobile station without commanding the
handover to the base stations 417 to 420.
Thus, according to this fourth embodiment, the mobile
station communicates through the base station of a mini-
cell as much as possible, so that the improved frequency
spectrum utilization efficiency can be achieved. In
addition, the mini-cells are enclosed within a larger.size
cell covered by, a control base station, such that the
regions around the mini-cells and the fast moving mobile
station can be handled by the control base station having a
larger capacity. Therefore, it becomes possible in this
fourth embodiment to provide a method of handover in a
mobile radio communication capable of securing the high
quality of service regardless of the moving speed of the
mobile station.
Referring now to Fig. 19, a fifth embodiment of a
mobile radio communication system using a method of
handover according to the present invention will be
described in detail.
In this fifth embodiment, the system configuration is
substantially the same as that of the first embodiment
shown in Fig. 4. Also, each of the base stations has a -
configuration substantially the same as that of the first
embodiment shown in Fig. 6.
This fifth embodiment concerns with a case of
utilizing a 3-channel TDMA (time division multiple access)
method, in which a decentralized autonomous control is
realized in the cellular system by a mobile station .
monitoring the receiving levels of the base stations
without any command from the central control station in
order to carry out the handover operation.
Here, each of the base stations has a predetermined
number of traffic channels assigned according to the
traffic demand of each cell area, and a control channel
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which is either common to all the base stations or
different for each base station. The co-channel reuse of
these traffic channels and the control channel is possible
in this system.
In the control channel, the set up of calls to and
from the mobile station to assign the traffic channel to be
used is controlled, and each base station notifies the
control channel numbers of the neighboring base stations to
the mobile station. The mobile station selectively monitors
the control channel of the highest receiving level at any
given moment.
In this system, the mobile station determines a cell
in which it is located by monitoring the receiving levels
of the base stations, and notifies the determined located
cell information to the base stations in order to carry out
the handover operation, by operating in a manner shown in
Fig. 19.
Namely, in the traffic channel, the mobile station
operates in a sequence of the mobile station transmission
time slots 500, 502, and 504 in the traffic channel in
which the mobile station transmits the radio waves to the
currently used base station through which it is
communicating, and the mobile station reception time slots
501, 503, and 505 in the traffic channel in which the
mobile station receives the signals from that currently
used base station.
In addition, the mobile station switches to the
control channel at idle periods of the traffic channel such
as a period between the mobile station reception time slot
501 and the mobile station transmission time slot 502, and
operates in the neighboring base station receiving level
monitoring time slots 506 and 50? in the control channel in
which the receiving levels of the control channels of the
neighboring base stations are monitored. In a case the
control channel of each base station is different from that
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of the other base stations, the control channels of the
neighboring base stations specified in the control channel
of the currently used base station are sequentially
monitored in each neighboring base station receiving level
monitoring time slot. On the other hand, in a case the
common control channel is used for the time division
multiple access by all the base stations, the time slot is
sequentially switched after the mobile station switches~to
the control channel.
Then, the mobile station selects the neighboring base
station for which the monitored receiving level is the
highest among all the neighboring base stations, and
compares the receiving level of the currently used base
station with the receiving level of the selected
neighboring base station. When the .receiving level of the
neighboring base station is higher than that of the
currently used base station, the mobile station fudges that
it has moved to a cell of that selected neighboring base
station. When this moving into a cell of the neighboring
base station is detected, the mobile station switches to
the control channel of that selected neighboring base
station and transmits the destination base station
notification signal at the located cell information
notification time slot 508.
In this system, the central control station outputs
the control station signal in a format shown in Fig. 20 to
the loop transmission line, where the control station
signal includes an addressed base station number 509
indicating a base station from which this control station
signal is to be transmitted to the mobile station, a
control signal 510, and a transmission signal 511 to be
transmitted to the mobile station in the traffic channel.
This control station signal is sequentially relayed through
the loop transmission line by the base stations to the
addressed base station indicated by the addressed base
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station number 509 from which this control station signal
is transmitted to the mobile station.
On the other hand, when the base station receives the
signal from the mobile station, the base station outputs a
mobile station signal in a format shown in Fig. 21, where
the mobile station signal includes a relay base station
number 512 indicating a base station at which this mobile
station signal.is received from the mobile station, a
control signal 513, and a transmission signal 514 to be
transmitted to the central control station. This mobile
station signal is sequentially relayed through the loop
transmission line by the base stations to the central
control station.
Also, the control channel signal in a format shown in
Fig. 22 is transmitted through the loop transmission line,
where the control channel signal includes a generating
station number 515 indicating a station from which this
control channel signal is generated, an addressed station
number 516 indicating a station to which this control
channel signal is to be transmitted, and a control signal.
517. Here, the generating station and the addressed station
can includes any of the base stations as well as the
central control station and the mobile station.
In this fifth embodiment, the handover operation can
be carried out by any one of the following five procedures
according to the need.
The first procedure shown in Fig. 23 is for a case in
which the control channels of the base stations are
different each other.
In this first procedure, first at the step 519, the
mobile station measures the current receiving level of the
currently used base station through which it is
communicating at the mobile station reception time slot,
while measuring the receiving levels of the control
channels of the neighboring base stations by switching to
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the control channel during the idle periods in the traffic
channel and determining the selected neighboring base
station which has the highest neighboring base station
receiving level.
Then, at the step 520, the mobile station judges
whether it has moves into a cell of the neighboring base
station or not by comparing the highest neighboring base
station receiving level and the current receiving level.
When the highest neighboring base station receiving level
is higher than the current receiving level, it is judged
that the mobile station has moved to a cell of the selected
neighboring base station at the step 520, and next at the
step 521 the mobile station switches to the control channel
of the selected neighboring base station during the idle
period in the traffic channel and transmits~the destination
base station notification signal at the located cell
information notification time slot, whereas otherwise the
step 519 is repeated.
When the destination base station receives this
destination base station notification signal in the control
channel at the step 522, next at the step 523, the
destination base station selects an idle traffic channel
available, and outputs the control channel signal of Fig.
22 to the loop transmission line, with itself as the
generating station, the original base station as the
addressed base station, and the control signal indicating
the handover to the selected idle traffic channel.
When the original base station receives this control
channel signal through the loop transmission line at the
step 524, the original base station transmits the handover
command signal indicating the idle traffic channel of the
destination base station specified in the control channel
signal to the mobile station in the currently used traffic
channel at the step 525, and then releases the currently
used traffic channel at the step 526 so as to stop relaying
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the communication of the mobile station.
When the mobile station receives this handover command
signal from the original base station at the step 527, the
mobile station switches its traffic channel to the idle
traffic channel of the destination station at the step 528,
while the destination base station starts relaying the
communication of the mobile station by assigning the idle
traffic channel at the step 529.
The second procedure shown in Fig. 24 is also for a
case in which the control channels of the base stations are
different each other.
In this second procedure, first at the step 530, the
mobile station measures the current receiving level of the
currently used base station through which it is
communicating at the mobile station reception time slot,
while measuring the receiving levels of the control
channels of the neighboring base stations by switching to
the control channel during the idle periods in the traffic
channel and determining the selected neighboring base
station which has the highest neighboring base station
receiving level.
Then, at the step 531, the mobile station fudges
whether it has moves into a cell of the neighboring base
station or not by comparing the highest neighboring base
station receiving level and the current receiving level.
When the highest neighboring base station receiving level
is higher than the current receiving level, it is fudged
that the mobile station has moved to a cell of the selected
neighboring base station at the step 531, and next at the
step 532 the mobile station switches to the control channel
of the selected neighboring base station during the idle
period in the traffic channel and transmits the destination
base station notification signal at the located cell
information notification time slot, whereas otherwise the
step 530 is repeated.
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When the destination base station receives this
destination base station notification signal in the control
channel at the step 533, next at the step 534, the
destination base station outputs the control channel signal
of Fig. 22 to the loop transmission line, with itself as
the generating station, the original base station as the
addressed base station, and the control signal indicating
the occurrence,of the handover and commanding the release
of the currently used traffic channel.
When the original base station receives this control
channel signal through the loop transmission line at the
step 535, the' original base station releases the currently
used traffic channel at the step 536 so as to stop relaying
the communication of the mobile station.
Then, at the step 537, the destination~base station
selects an idle traffic channel available, and transmits
the handover command signal indicating the idle traffic
channel for the handover to the mobile station by using the
currently used traffic channel.
When the mobile station receives this handover command
signal from the destination base station at the step 538,
the mobile station switches its traffic channel to the idle
traffic channel of the destination station at the step 539,
while the destination base station starts relaying the
communication of the mobile station by assigning the idle
traffic channel at the step 540.
The third procedure shown in Fig. 25 is for a case in
which a common control channel is shared by all the base
stations.
In this third procedure, first at the step 550, the
mobile station measures the current receiving level of the
currently used base station through which it is
communicating at the mobile station reception time slot,
while measuring the receiving levels of the control
channels of the neighboring base stations by switching to
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the control channel during the idle periods in the traffic
channel and determining the selected neighboring base
station which has the highest neighboring base station
receiving level.
Then, at the step 551, the mobile station fudges
whether it has moves into a cell of the neighboring base
station or not by comparing the highest neighboring base
station receiving level and the current receiving level..
When the highest neighboring base station receiving level
is higher than the current receiving level, it is fudged
that the mobile station has moved to a cell of the selected
neighboring base station at the step 551, and next at the
step 552 the mobile station switches to the common control
channel of the base stations during the idle period in the
traffic channel and transmits the destination base station
notification signal at the located cell information
notification time slot, whereas otherwise the step 550 is
repeated.-
When the original base station receives this
destination base station notification signal in the control
channel at the step 553, the original base station releases
the currently used traffic channel at the step 554 so as to
stop relaying the communication of the mobile station.
Meanwhile, when the destination base station receives
this destination base station notification signal in the
control channel at the step 555, next at the step 556, the
destination base station selects an idle traffic channel
available, and transmits the handover command signal
indicating the idle traffic channel for the handover to the
mobile station by using the currently used traffic channel.
When the mobile station receives this handover command
signal from the destination base station at the step 557,
the mobile station switches its traffic channel to the idle
traffic channel of the destination station at the step 558,
while the destination base station starts relaying the
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communication of the mobile station by assigning the idle
traffic channel at the step 559.
The fourth procedure shown in Fig. 26 is carried out
as follows.
First at the step 560, the mobile station measures the
current receiving level of the currently used base station
through which it is communicating at the mobile station
reception time slot, while measuring the receiving levels
of the control channels of the neighboring base stations by
switching to the control channel during the idle periods in
the traffic channel and determining the selected
neighboring base station which has the highest neighboring
base station receiving level.
Then, at the step 561, the mobile station fudges
whether it has moves into a cell of the neighboring base
station or not by comparing the highest neighboring base
station receiving level and the current receiving level.
When the highest neighboring base station receiving level
is higher than the current receiving level, it is fudged
that the mobile station has moved to a cell of the selected
neighboring base station at the step 561, and next at the
step 562 the mobile station transmits the destination base
station notification signal by using either the control
channel or the currently used traffic channel of the
original base station, whereas otherwise the step 560 is
repeated.
When the original base station receives this
destination base station notification signal in the control
channel or the currently used traffic channel at the step
563, the original base station outputs the mobile station
signal of Fig. 21 to the loop transmission line, with
itself as the relay base station, and the control signal
indicating the handover command at the step 564, and then
releases the currently used traffic channel at the step 565
so as to stop relaying the communication of the mobile
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station.
When the destination base station receives this mobile
station signal through the loop transmission line at the
step 566, next at the step 567, the destination base
station selects an idle traffic channel available, and
transmits the handover command signal indicating the idle
traffic channel for the handover to the mobile station by
using the currently used traffic channel.
When the mobile station receives this handover command
signal from the destination base station at the step 568,
the mobile station switches its traffic channel to the idle
traffic channel of the destination station at the step -569,
while the destination base station starts relaying the
communication of the mobile station by assigning the idle
traffic channel at the step 570.
The fifth procedure shown in Fig. 27 is carried out as
follows .
First at the step 580, the mobile station measures the
current receiving level of the currently used base station
through which it is communicating at the mobile station
reception time slot, while measuring the receiving levels
of the control channels of the neighboring base stations by
switching to the control channel during the idle periods in
the traffic channel and determining the selected
neighboring base station which has the highest neighboring
base station receiving level.
Then, at the step 581, the mobile station ,judges
whether it has moves into a cell of the neighboring base
station or not by comparing the highest neighboring base
station receiving level and the current receiving level.
When the highest neighboring base station receiving level
is higher than the current receiving level, it is ,judged
that the mobile station has moved to a cell of the selected
neighboring base station at the step 581, and next at the
step 582 the mobile station transmits the destination base
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station notification signal by using either the control
channel or the currently used traffic channel of the
original base station, whereas otherwise the step 580 is
repeated.
When the original base station receives this
destination base station notification signal in the control
channel or the currently used traffic channel at the step
583, the original base station outputs the idle traffic
channel report command signal to the loop transmission line
at the step 584.
When the destination base station receives this idle
traffic channel report command signal through the loop
transmission line at the step 585, next at the step 586,
the destination base station selects an idle traffic
channel available, and transmits the idle traffic channel
report signal indicating the idle traffic channel for the
handover to the loop transmission line.
When the original base station receives this idle
traffic channel report signal through the loop transmission
line at the step 587, the original base station transmits,
the handover command signal indicating the reported idle
traffic channel of the destination base station to the
mobile station at the step 588 by using the currently used
traffic channel at the step 588, and then releases the
currently used traffic channel at the step 589 so as to
stop relaying the communication of the mobile station.
When the mobile station receives this handover command
signal from the original base station at the step 590, the
mobile station switches its traffic channel to the idle
traffic channel of the destination station at the step 591,
while the destination base station starts relaying the
communication of the mobile station by assigning the idle
traffic channel at the step 592.
It is to be noted that this fifth embodiment is
equally applicable to a case using a method other than
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TDMA, such as FDMA (frequency division multiple access) and
CDMA (code division multiple access). In using FDMA, the
monitoring of the receiving levels of the neighboring base
stations can be achieved by using the method disclosed in
Japanese Patent Application No. 6I-192284. In using CDMA,
the channels in the above description for the case of using
TDMA should be replaced by codes.
Thus, according to the fifth embodiment, it becomes
possible to establish a so called decentralized autonomous
control in the cellular system in which a mobile station
monitors the,receiving levels of the base stations, fudges
whether it is moving into a cell of the neighboring base
station, and transmits the located cell information to the
base stations in order to initiate the handover operation,
without any command from the central control station, such
that the handover operation can be carried out less time
consumingly compared with a conventional cellular system
without increasing the capacities and sizes of each base
station and the central control station and complicating
the configurations of the base stations and the central
control station. As a consequence, it becomes possible to
use the smaller cell radius so as to improve the frequency
spectrum utilization efficiency, and to secure the high
quality of service regardless of the moving speed of the
mobile station.
Referring now to Fig. 28, a sixth embodiment of a
mobile radio communication system using a method of route
diversity according to the present invention will be .
described in detail.
In this sixth embodiment, the system comprises a
central control station 621 and a plurality (seven in Fig.
28) of base stations 622 to 628, which are connected
through a inter-station transmission line formed by a
transmission line element 629 connecting between the base
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stations 622 and 623, a transmission line element 630
connecting between the base stations 623 and 624, a
transmission line element 631 connecting between the base
stations 624 and 625, a transmission line element 632
connecting between the base stations 625 and 626, a
transmission line element 633 connecting between the base
stations 626 and 627, a transmission line element 634
connecting between the base stations 627 and 628, and a
transmission line element 635 connecting between the base
station 628 and the central control station 621.
Each of. the base stations 622 to 628 has a schematic
configuration shown in Fig. 29, where the base station
comprises an antenna 636 for receiving the radio waves from
a mobile station, a receiver 637 for receiving the
transmitted signal from the antenna 626, a control circuit
640 connected with the receiver 637 through a received
signal line 638 and a receiving level information line 639,
a transmission line element 641 for receiving signals from
an upper station through the inter-station transmission
line, and a transmission line element 642 for transmitting
signals to a lower station through the inter-station
transmission line.
This system of the sixth embodiment operates according
to a diagram shown in Fig. 30 as follows.
The base station 622 is located at a top position of
the inter-station transmission line, so that there is no
signals from the upper station for this base station 622.
Therefore, when this base station 622 receives a signal 643
from a mobile station as a received signal 644, the
receiver 637 of the base station 622 measures the receiving
level of this received signal 644 at a measurement timing
645, and the measured receiving level is transmitted to the
control circuit 640 through the receiving level information
line 639 while the received signal 644 is transmitted to
the control circuit 640 through the received signal line
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638. At the control circuit 640, a signal transmission
timing 650 for transmitting the signal to the lower station
is set to be equal to or somewhat longer than the
measurement timing 645, and the control circuit 640 outputs
an encoded receiving level information 648 indicating the
receiving level measured by the receiver 637 and a
transmission signal 649 identical to the received signal
644 together to the inter-station transmission line through
the transmission line element 642.
At each of the remaining base stations 623 to 628,
when the signal 643 from a mobile station is received as
the received signal 644, the receiver 637 of the base
station measures the receiving level of the received signal
644 at the measurement timing 645, and the measured
receiving level is transmitted to the control circuit 640
through the receiving level information line 639 while the
received signal 644 is transmitted to the control circuit
640 through the received signal line 638. Meanwhile, at the
control circuit 640, the signal including the receiving
level information 646 and the transmission signal 647 is
received from the upper station through the inter-station
transmission line and the transmission line element 641.
The control circuit 640 then compares the receiving level
measured by the receiver 637 and the receiving level
indicated by the receiving level information 646.
When the receiving level measured by the receiver 637
is higher than the receiving level indicated by the
receiving level information 646, the control circuit 640
outputs the encoded receiving level information 648
indicating the receiving level measured by the receiver 637
and the transmission signal 649 identical to the received
signal 644 received by the receiver 637 together to the
inter-station transmission line through the transmission
line element 642 at the signal transmission timing 650.
On the other hand, when the receiving level indicated
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by the receiving level information 646 is higher than the
receiving level measured by the receiver 637, the control
circuit 640 outputs the encoded receiving level information
648 identical to the receiving level information 646 and
the transmission signal 649 identical to the transmission
signal 647 together to the inter-station transmission line
through the transmission line element 642 at the signal
transmission timing 650.
Thus, the receiving level information 648 and the
transmission signal 649 to be transmitted to the lower
station are sequentially updated at the base stations 623
to 628, such that when the central control station 621
receives the receiving level information 646 and the
transmission signal 647 from the last base station 628
through the inter-station transmission line, the receiving
level indicated by the receiving level information 646 is
the highest receiving level obtained among the base
stations -622 to 628, and the transmission signal 64?
represents the signal received from the mobile station by
one of the base stations 622 to 628 at this highest
receiving level, so that the route diversity reception is
achieved by using the transmission signal 647 received by
the central control station 621 as the signal received from
the mobile station in the system.
Now, the signal transmission timing 650 will be
described in further detail.
Namely, when the base stations 622 to 628 are
sequentially numbered by orders of positions on the inter-
station transmission line in a direction of the signal.
transmission, the signal transmission timing T,,; for the
i-th base station is given by:
Te , ~ - t + Tn + ~Tr
where T" is a time required for receiving the receiving
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level information 646 transmitted from the upper station,
and ATE is a sum of a radio propagation delay difference
due to the difference of the distances between the mobile
station and the base stations 622 to 628 and a signal
transmission delay between the neighboring base stations on
the inter-station transmission line.
The radio propagation delay difference compensates the
variation of the receiving time of the radio waves from~the
mobile station at the base stations due to the difference
of the distances between the mobile station and the base
stations 622 to 628, and can be considered as a propagation
time for a distance between the neighboring base stations
on the inter-station transmission line, so that when the
distance between the neighboring base stations is 10 km for
example, the radio propagation delay difference is equal to
10 x 103/(3 x 108) - 3 x 10-5 sec - 0.03 ms.
As for the signal transmission delay between the
neighboring base stations on the inter-station transmission
line, considering the fact that the transmission line i.s
not straight, it is sufficient for this signal transmission
delay to take a value equal to several times the radio
propagation delay difference, so that OTC - 0.2 ms to 0.3
ms is sufficient for the above described exemplary case.
Now, the operation of the system of the sixth
embodiment described so far assumes that all the base
stations 622 to 628 can receive the signal from the mobile
station. However, in reality, only a part of the base
stations 622 to 628 will be able to receive the signal from
the mobile station in general.
In a case only a part of the base stations 622 to 628
can receive the signal from the mobile station, the system
of the sixth embodiment operates as follows.
Let the ~-th base station be the earliest base station
in the order of positions along the inter-station
transmission line which can receive the signal from the
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mobile station. When the signal from the mobile station is
received at this ,~-th base station, the control circuit 640
of this ,j-th base station will wait for the receiving level
information 646 and the transmission signal 647 from the
upper station. However, in this case, no upper station can
transmit the receiving level information 646 and the
transmission signal 647 to the inter-station transmission
line because no upper station is capable of receiving the
signal from the mobile station. Thus, after waiting for the
signal transmission timing 650, the control circuit 640 of
the ,j-th base station outputs the receiving level
information 648 indicating the receiving level measured by
the receiver 637 and the transmission signal 649 identical
to the received signal 644 received by the receiver 637 to
the inter-station transmission line through~the
transmission line element 642.
On the other hand, let the k-th base station be the
base station whose upper station is capable of receiving
the signal from the mobile station but which itself is.not
capable of receiving the signal from the mobile station. At
this k-th base station, the receiving level information 646
and the transmission signal 647 are received from the upper
station through the inter-station transmission line and the
transmission line element 641 while not receiving the
signal from the mobile station at the receiver 637. Thus,
after waiting for T" + AT~ since the reception of the
receiving level information 646 and the transmission signal
647 from the upper station, the control circuit 640 of this
k-th base station outputs the receiving level information
648 identical to the receiving level information 646 and
transmission signal 649 identical to the transmission
signal 647 to the inter-station transmission line and the
transmission line element 642.
In this sixth embodiment, the level fluctuation can be
averaged out by using the longer receiving level
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measurement timing 645 such that the accuracy of the
receiving level measurement can be improved. However, the
use of the longer receiving level measurement timing also
makes the transmission time on the transmission line longer
so that the transmission delay may present some problem in
such a case.
Thus, the receiving level measurement timing 645
should be determined by taking the maximum allowable
transmission delay of the system into account. In a case
the maximum allowable transmission delay is sufficiently
large, the receiving level measurement timing 645 can be
set equal to~the entire signal duration, whereas in a case
the maximum allowable transmission delay is not so large,
the receiving level measurement timing 645 should be set
equal to only a part of a top portion of the entire signal
duration.
Also, in this sixth embodiment, the receiving level
information 648 and the transmission signal 649 may be
transmitted through the inter-station transmission line
separately. In such a case, the receiving level
informations from a plurality of the base stations may be
multiplexed and transmitted by using a channel different
from a channel used for transmitting the transmission
signals.
Thus, according to the sixth embodiment, it is
possible to provide a method of route diversity in a mobile
radio communication which can be realized less expensively,
without requiring additional control function which
complicates the central control station.
It is to be noted that in the above embodiments, the
central control station and the base stations are provided
separately, but the present invention is equally applicable
to a system in which one of the base stations plays the
role of the central control station as well.
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Moreover, in the above embodiments, the receiving
level is utilized as the key for determining the
destination base station, but the other signal reception
characteristics such as a number of detected errors or an
eye opening in a case of digital signal with error
correction encoding, an S/N ratio in a case of analog
signal, and their combinations may also be utilized as the
key for determining the destination base station.
Besides these, many modifications and variations of
the above embodiments may be made without departing from
the novel and advantageous features of the present
invention. Accordingly, all such modifications and
variations are intended to be included within the scope of
the appended claims.
20
30
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