U.S. patent application number 09/745425 was filed with the patent office on 2001-05-24 for control channel placement method.
This patent application is currently assigned to MITSUBISHI DENKI KABUSHIKI KAISHA. Invention is credited to Kawabata, Takashi, Mikuni, Yuko, Toyota, Norihiko.
Application Number | 20010001609 09/745425 |
Document ID | / |
Family ID | 14725833 |
Filed Date | 2001-05-24 |
United States Patent
Application |
20010001609 |
Kind Code |
A1 |
Mikuni, Yuko ; et
al. |
May 24, 2001 |
Control channel placement method
Abstract
A first sector cell is set as the object for measurement (S101).
Reception levels in each time slot in the number of TDMA-TDD frames
corresponding to the number of sector cells within the base station
is measured for the object cell (S102). This measurement is
performed for all the cells (S103 and S105). The results of the
measurement of the reception levels of a specific sector cell are
added to the results of the measurement of the reception levels of
a sector cell opposite the specific sector cell, and time slots in
which control channels can be placed are extracted for each sector
cell (S106 to S108). TDMA-TDD frames are then allocated such that
the control channels of each sector cell coincide with time slots
in which control channels can be placed (S109). As a result, a
control channel placement method is provided in which a TDMA-TDD
mode is employed and in which control channels are placed in time
slots such that a base station having a sector cell structure
avoids interference from the same frequency.
Inventors: |
Mikuni, Yuko; (Tokyo,
JP) ; Kawabata, Takashi; (Tokyo, JP) ; Toyota,
Norihiko; (Tokyo, JP) |
Correspondence
Address: |
OBLON SPIVAK McCLELLAND MAIER & NEUSTADT PC
FOURTH FLOOR
1755 JEFFERSON DAVIS HIGHWAY
ARLINGTON
VA
22202
US
|
Assignee: |
MITSUBISHI DENKI KABUSHIKI
KAISHA
2-3, Marunouchi 2-chome, Chiyoda-ku,
Tokyo
JP
100-8310
|
Family ID: |
14725833 |
Appl. No.: |
09/745425 |
Filed: |
December 26, 2000 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
|
|
09745425 |
Dec 26, 2000 |
|
|
|
PCT/JP00/01145 |
Feb 28, 2000 |
|
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Current U.S.
Class: |
370/337 ;
370/522 |
Current CPC
Class: |
H04W 16/10 20130101;
H04W 16/24 20130101 |
Class at
Publication: |
370/337 ;
370/522 |
International
Class: |
H04J 003/12 |
Foreign Application Data
Date |
Code |
Application Number |
Apr 26, 1999 |
JP |
011-118010 |
Claims
1. A control channel placement method applied to a wireless
communication system, which wireless communication system having a
base station having directional antennas that match placement
angles of a plurality of sector cells performing communication in
the TDMA-TDD mode with a plurality of terminal stations, and in
which wireless communication system the up and down control
channels for establishing communication by the base station with
the terminal stations in each sector cell are placed in
predetermined time slot positions within TDMA-TDD frames, the
control channel placement method comprising: a reception level
measurement step in which a reception level of each time slot
extending over the TDMA-TDD frames corresponding to the number of
sector cells of the base station is measured in each of the sector
cells; a time slot extraction step in which time slots in which
control channels can be placed are extracted from among the time
slots extending over the TDMA-TDD frames corresponding to the
number of sector cells, using a result of the measurement of the
reception level in the reception level measurement step when a
first sector cell was measured, and a result of the measurement of
the reception level in the reception level measurement step when a
second sector cell in a position opposite to the first sector cell
was measured from among the sector cells; and a frame allocation
step in which the TDMA-TDD frames are allocated for each sector
cell such that those predetermined time slot positions in which
control channels are placed, from among the time slots within the
TDMA-TDD frames, are matched with time slots extracted in the time
slot extraction step.
2. The control channel placement method according to claim 1,
wherein, in the time slot extraction step, a time slot in which a
reception level measured in the first sector cell in the reception
level measurement step is equal to or less than a predetermined
value, and in which a reception level measured in the second sector
cell in the reception level measurement step is equal to or less
than a predetermined value, is determined to be a time slot in
which the control channel can be placed in the first sector
cell.
3. The control channel placement method according to claim 1,
wherein, in the time slot extraction step, time slots in which
control channels can be placed in the first sector cell are
determined in sequence starting from the time slot in which a
value, determined by combining a reception level obtained in the
reception level measurement step by measuring the first sector cell
with a reception level obtained in the reception level measurement
step by measuring the second sector cell, is at the minimum.
4. The control channel placement method according to claim 1,
wherein the TDMA-TDD frames are formed by securing a plurality of
time slot positions in which up control channels can be placed and
a plurality of time slot positions in which down control channels
can be placed and by placing in the plurality of time slot
positions control channels of sector cells that are different from
each other, and wherein, in the frame allocation step, within
TDMA-TDD frames extending for the number of the sector cells,
specific TDMA-TDD frames are allocated such that time slots
extracted in the time slot extraction step coincide with either one
of time slot positions for the placement of the control channels
within the specific TDMA-TDD frames determined for each of the
sector cells and time slot positions for the placement of the
control channels within other TDMA-TDD frames different to the
specific TDMA-TDD frames.
5. The control channel placement method according to claim 1,
wherein the TDMA-TDD frames are formed by securing a plurality of
time slot positions in which up control channels can be placed and
a plurality of time slot positions in which down control channels
can be placed and, based on a reception level measurement result
measured in the reception level measurement step, control channels
of sector cells that are different from each other are placed in a
plurality of time slot positions for the placement of the control
channels, and wherein, in the frame allocation step, within the
TDMA-TDD frames extending for the number of the sector cells,
specific TDMA-TDD frames are allocated such that time slots
extracted in the time slot extraction step coincide with either one
of time slot positions for the placement of the control channels
within the specific TDMA-TDD frames determined for each of the
sector cells and time slot positions for the placement of the
control channels within other TDMA-TDD frames different to the
specific TDMA-TDD frames.
6. The control channel placement method according to claim 1
further comprising: a reception level remeasurement step for
measuring, at regular intervals and for each of the sector cells,
reception levels of time slots in which control channels have been
placed in the frame allocation step; and a frame reallocation step
for performing the frame allocation step when a reception level
measured in the reception level remeasurement step is equal to or
greater than a predetermined threshold.
7. The control channel placement method according to claim 6,
wherein the frame allocation step performed in the frame
reallocation step allocates the TDMA-TDD frame to a position
shifted by one time slot towards the front or rear.
8. The control channel placement method according to claim 6,
wherein, in the reception level remeasurement step, the carrier to
interference wave strength ratio is measured from the reception
level and, in the frame reallocation step, the frame reallocation
step is performed when the carrier to interference wave strength
ratio measured in the reception level remeasurement step is equal
to or greater than a predetermined threshold.
9. The control channel placement method according to claim 6,
wherein, in the reception level remeasurement step, the carrier to
interference wave strength ratio is measured from the reception
level when the time slot being measured is in a blocked state, and,
in the frame reallocation step, the frame reallocation step is
performed when the carrier to interference wave strength ratio
measured in the reception level remeasurement step is equal to or
greater than a predetermined threshold.
10. The control channel placement method according to claim 8,
wherein, in the frame reallocation step, if the carrier to
interference wave strength ratio measured in the reception level
remeasurement step continues to be equal to or greater than the
predetermined threshold for a predetermined length of time, then it
is determined that the time slot being measured is receiving
interference obstruction and the frame allocation step is
performed.
11. The control channel placement method according to claim 9,
wherein, in the reception level remeasurement step, an error
detection ratio in an up control channel is measured as a reception
level and, in the frame reallocation step, if the error detection
ratio measured in the reception level remeasurement step is equal
to or greater than a predetermined threshold the frame allocation
step is performed.
Description
TECHNICAL FIELD
[0001] The present invention relates to a control channel placement
method in which control channels for establishing wireless
communication between a base station and a terminal station in a
wireless communication system are placed in optimal time slots
within a TDMA-TDD frame. The terminal station could be a vehicle
telephone, a portable telephone or the like.
BACKGROUND ART
[0002] A cellular system is generally used for providing a
communication service for vehicle telephones or portable
telephones. This cellular system comprises a plurality of base
stations and small zones (referred to hereafter as cells) around
each base station. The cells are so constructed that there is no
gap between then.
[0003] In the cellular system, same channel is repeatedly used in
zones under the condition that no mutual interference obstruction
is generated. Therefore, it is possible provides a service in a
very wide area with less number of channels. When the antenna of
the base station is non-directional within the horizontal plane,
and if the effects of topography and ground based obstructions are
ignored, then the shape of the area covered by each base station
will be circular. These circular cells are called omnicells.
Switching of channels is performed in the portions where adjacent
omnicells overlap.
[0004] On the other hand, sector cell structure zones are also
known. In case of sector cell structure zones, directional antennas
are provided in each base station. The beam width of each antenna
is set so as to divide a circular area into a plurality of
fan-shaped cells (referred to hereafter as sector cells) each
having the same size.
[0005] Since directional antennas are used, it is possible to
remove the effect of interference waves that arrive from directions
other than the direction to which the antenna is pointed. In
addition, there is no possibility of interfering waves broadcast
towards base stations using the same channel but situated in a
direction other than the one to which the antenna is pointed. As a
result, it is possible to decrease the distance between base
stations to which the same channel has been allocated and to
improve the spatial channel usage efficiency. Moreover, the overall
channel usage efficiency is improved in comparison with omnicells
using non-directional antennas.
[0006] Because of reasons such as this, in wireless communication
systems for performing mobile communication such as vehicle
telephones and portable telephones, as the number of users
increases, there is currently a transition underway from employing
a zone structure that uses omnicells having a small radius to
employing a zone structure that uses sector cells.
[0007] In the meanwhile, time division multiple access (TDMA) mode
is known as the representative mode among digital wireless access
modes employed in vehicle telephones and portable telephones. In
TDMA mode, signals having a predetermined frequency bandwidth are
divided along a time axis and communication is performed by
cyclically allocating the divided time band to each user as one
channel. Using this TDMA mode, a user can perform communication in
time slots allocated into short cycles in the same frequency having
a predetermined bandwidth.
[0008] The characteristics of this TDMA mode are described below.
Firstly, a plurality of users can perform communication
simultaneously using a single transreceiving device, therefore, the
base station can be constructed with a small size and at a low
cost. Secondly, open time is generated because the communication
time only needs to be the time of the allocated time slot, thus
allowing the state of peripheral bands to be observed at this time.
Thirdly, because the operating time during communication is
intermittent, the power consumption in the terminal station can be
reduced.
[0009] Furthermore, when the TDMA mode is employed as a wireless
access mode, it is possible to use the time division duplex (TDD)
mode in combination with the TDMA mode. The TDD mode uses the same
frequency for both, reception and transmission, between a terminal
station and a base station and performs the transmission and
reception by dividing the time. In the TDD mode, the
characteristics of the wave propagation are the same going "up"
(i.e. from the terminal station to the base station) as they are
going "down" (i.e. from the base station to the terminal station).
Therefore, it is possible to only have to carry out countermeasures
against variations in the wave propagation at the base station side
only.
[0010] The frame structure of the transmission and reception using
this TDMA-TDD may be one structured, for example, from four pairs
of time slots (one up and one down). In this case, the first pair
is used as a control channel with the remaining three pairs being
used as notification channels. As a result, in this example, three
terminal stations can perform communication with one base station
at the same time.
[0011] In this case, it is possible to divide the control channels
into "down control channels" going in the direction from the base
station to the terminal station and "up control channels" going in
the direction from the terminal station to the base station. The
down control channels are formed from a broadcast channel (BCCH)
for notifying a terminal station as to the ID of the host base
station and about information peculiar to the base station such as
the control channel frequencies of peripheral base stations and the
like, and from a paging channel (PCH) for transmitting incoming
message information, and the like.
[0012] The up control channels are formed from a random access type
of signal control channel (SCCH) for performing tasks such as
receiving transmissions and specifying wireless channels. FIG. 14
is an explanatory diagram showing the frame structure in a
conventional TDMA-TDD mode. In FIG. 14, in accordance with the
above example, four pairs of time slots comprising up and down are
set as one TDMA-TDD frame.
[0013] In particular, in this example, the up control channel is a
multiframe structure in which 20 frames are set as one new frame,
while the down control channel is a super frame structure in which
12 multiframes are set as one new frame.
[0014] The TDMA-TDD mode has commonly been applied as a wireless
access mode in portable telephones and vehicle telephones. Various
control channel setting methods which should be able to realize
higher quality communication are provided in a wireless
communication system that employs this TDMA-TDD mode.
[0015] For example, according to the "Control Channel Setting
Method" disclosed in Japanese Patent Application Laid-Open (JP-A)
No. 7-245780, in order to reduce interference in its own down
control channel, a parent station (a base station) measures the
reception levels of all channels of frequencies used for control
signals (control channels), and selects, for setting as control
channels, usable combinations from the reception levels of each
channel from among combinations of up control channels and down
control channels that form pairs in the TDMA-TDD mode, such that up
control channels and down control channels of peripheral base
stations do not overlap.
[0016] As another example, according to the "Radio Control System"
disclosed in U.S. Pat. No. 236,392, in the wireless communication
system of a sector cell zone structure that uses directional
antennas, when a particular base station performs communication,
interference is reduced by not using antennas transmitting in the
same direction as the antennas used for communication by base
stations adjacent to that particular base station, based on tables
that have been prepared in advance.
[0017] Moreover, as the channel allocation method, a fixed channel
allocation method may be used in which the channels to be used by
each cell are fixed in advance with consideration given to mutual
interference between zones. Alternatively, a dynamic channel
allocation method in which the channel allocation is changed
time-wise in accordance with the demands of the calls in each cell
may be used.
[0018] In the channel allocation method, in particular, a specific
channel that can be used by the base station is selected for each
communication from among all the channels. The carrier to
interference (CI) wave strength ratio is then measured for the
selected channel thereby allowing the reception levels of the
carrier and interference waves to be detected. A determination is
then made as to whether or not that channel can be allocated.
[0019] In the dynamic channel allocation method it is possible to
respond flexibly in the channel allocation to variations in the
voice level that occur overtime. Therefore, the efficiency of the
utilization of the frequencies is better in the dynamic channel
allocation method than in the fixed channel allocation method.
Accordingly, in a wireless communication system, by combining a
dynamic channel allocation method with the sector cell zone
structure, a remarkable improvement in the efficiency of the
utilization of the frequencies can be achieved.
[0020] However, the "Control Channel Setting Method" disclosed in
JP-A No. 7-245780 is intended to reduce interference in control
channels between a terminal station and a plurality of base
stations in an omnicell zone structure, and it is not able to
reduce interference in control channels allocated to each sector
for a base station having a sector cell structure.
[0021] Furthermore, although the "Radio Communication System"
disclosed in the above described U.S. Pat. No. 236,392 is designed
to reduce interference between adjacent base stations, in order to
achieve that, it does not alter the placement of the control
channels, and is not able to solve interference problems when
communication is performed simultaneously by a plurality of
terminal stations.
[0022] Moreover, when the dynamic channel allocation method is
used, if the carrier to interference ratio of a candidate
allocation channel is above a predetermined threshold value, that
channel will be allocated, however, the possibility will exist that
the allocation will generate interference obstruction in
surrounding terminal stations or base stations that are
communicating using the same channel. In addition, if the base
stations have a sector cell structure, because the amount of
interference received by each cell is changed by differences in the
timing of the channel allocation, it has not been possible to
obtain a constantly high quality communication state.
[0023] In particular, in a sector cell structure using directional
antennas having a directionality of 60.degree. within a regular
hexagonal shaped cell, namely, in a base station having 6 sector
cells, because the sector cells are opposite each other, cases
arise in which the direction of waves transmitted to a terminal
station by one sector cell match the direction of waves received
from the terminal station by the sector cell in the opposite
position, thereby generating interference between the two.
[0024] Moreover, when it is only possible to place one control
channel in one cycle (the number of TDMA-TDD frames extending for
the number of sectors), there are times when it is not possible to
place control channels at fixed time slot positions inside the
TDMA-TDD frame of one cycle. In this case, interference and the
like is generated after communication is established, and when
reception in the control channel becomes impossible, it is
necessary to reallocate the TDMA-TDD frames.
[0025] It is an object of the present invention to provide a method
for placing control channels which sets high quality wireless
channels in a wireless communication system employing the TDMA-TDD
mode by placing control channels in suitable time slots so that
interference from an identical frequency can be avoided.
DISCLOSURE OF THE INVENTION
[0026] According to the control channel placement method of the
present invention, in a wireless communication system in which a
base station having directional antennas that match placement
angles of a plurality of sector cells performs communication in a
TDMA-TDD mode with a plurality of terminal stations, up and down
control channels for establishing communication by the base station
with the terminal stations in each sector cell are placed in
predetermined time slot positions within TDMA-TDD frames,
comprising: a reception level measurement step in which a reception
level of each time slot extending over TDMA-TDD frames
corresponding to the number of sector cells of the base station is
measured in each of the sector cells; a time slot extraction step
in which time slots in which control channels can be placed are
extracted from among the time slots extending over the TDMA-TDD
frames corresponding to the number of sector cells, using a result
of the measurement of the reception level in the reception level
measurement step when a first sector cell was measured, and a
result of the measurement of the reception level in the reception
level measurement step when a second sector cell in a position
opposite the first sector cell was measured from among the sector
cells; and a frame allocation step in which the TDMA-TDD frames are
allocated for each sector cell such that those predetermined time
slot positions in which control channels are placed, from among the
time slots within the TDMA-TDD frames, are matched with time slots
extracted in the time slot extraction step.
[0027] According to the above invention, in a wireless
communication system employing the TDMA-TDD mode as a wireless
access mode and employing sector cells as the zone structure, when
a base station having a plurality of sector cells performs a
transmission with both up control channels and down control
channels placed in predetermined time slot positions within the
TDMA-TDD frames for each sector cell, the reception level in each
time slot extending across the TDMA-TDD frames corresponding to the
number of sector cells of the base station is measured in the
reception level measurement step. Next, in the time slot extraction
step, time slots in which it is possible for control channels to be
placed are extracted from among the time slots extending across the
TDMA-TDD frames corresponding to the number of sector cells, using
the result of the measurement of the reception level when the first
sector cell was measured, and the result of the measurement of the
reception level when the second sector cell at a position opposite
the first sector cell was measured from among all the sector cells
in the same base station. Finally, in the frame allocation step,
the TDMA-TDD frames are allocated for each sector cell such that
those predetermined time slot positions in which control channels
are placed, from among the time slots within the TDMA-TDD frames,
are matched with the time slots extracted in the time slot
extraction step. As a result, both the up control channels and the
down control channels can be placed simultaneously in optimum time
slot positions.
[0028] Moreover, according to the control channel placement method
of the present invention, in the above control channel placement
method, in the time slot extraction step, a time slot in which a
reception level measured in the first sector cell in the reception
level measurement step is equal to or less than a predetermined
value, and in which a reception level measured in the second sector
cell in the reception level measurement step is equal to or less
than a predetermined value, is determined to be a time slot in
which the control channel can be placed in the first sector
cell.
[0029] According to the above invention, the time slot extraction
step specifies those time slots, from among the time slots within
TDMA-TDD frames extending for the number of sector cells, whose
reception level in a first sector cell measured in the reception
level measurement step is a predetermined threshold value or less.
The time slot extraction step then determines that, out of the
specified time slots, those time slots whose measurement result in
a second sector cell (i.e. the sector cell located opposite the
first sector cell) measured in the reception level measurement step
is a predetermined threshold value or less are time slots in which
control channels can be placed in the first sector cell. As a
result, it is possible to remove time slot positions that are in an
unstable state due to interference and the like from being
candidates for the placement of an up control channel or a down
control channel.
[0030] Moreover, according to the control channel placement method
of the present invention, in the above control channel placement
method, in the time slot extraction step, time slots in which
control channels can be placed in the first sector cell are
determined in sequence starting from the time slot in which a
value, determined by combining a reception level obtained in the
reception level measurement step by measuring the first sector cell
with a reception level obtained in the reception level measurement
step by measuring the second sector cell, is at the minimum.
[0031] According to the above invention, the time slot extraction
step adds the result of measuring the reception level in the second
sector cell (i.e. the sector cell located opposite the first sector
cell) to the result of measuring the reception level in the first
sector cell measured in the reception level measurement step, and
determines the time slots in which control channels can be placed
in the first sector cell in sequence starting from that time slot
in which the value obtained from the above addition is at the
minimum. Therefore, it is possible to allocate time slot positions
giving a more stable state of communication as the positions for
placing up control channels and down control channels.
[0032] Moreover, according to the control channel placement method
of the present invention, in the above control channel placement
method, the TDMA-TDD frames are formed by securing a plurality of
time slot positions in which up control channels can be placed and
a plurality of time slot positions in which down control channels
can be placed and by placing in the plurality of time slot
positions control channels of sector cells that are different from
each other, and wherein, in the frame allocation step, within
TDMA-TDD frames extending for the number of the sector cells,
specific TDMA-TDD frames are allocated such that time slots
extracted in the time slot extraction step coincide with either one
of time slot positions for the placement of the control channels
within the specific TDMA-TDD frames determined for each of the
sector cells and time slot positions for the placement of the
control channels within other TDMA-TDD frames different to the
specific TDMA-TDD frames.
[0033] According to the above invention, the TDMA-TDD frames are
formed by securing a plurality of time slots positions for the
respective placement of up control channels and down control
channels and by placing control channels for sector cells that are
different from each other in the plurality of time slot positions.
When the frame allocation step allocates specific TDMA-TDD frames
(namely, TDMA-TDD frames corresponding to specific sector cells),
there is a prerequisite that, within TDMA-TDD frames extending for
the number of the sector cells, time slots extracted in the time
slot extraction step coincide with one of either time slot
positions for the placement of the above control channels within
the specific TDMA-TDD frames and time slot positions for the
placement of the above control channels within other TDMA-TDD
frames different to the specific TDMA-TDD frames. Therefore, it is
possible to choose from a plurality of control channel placement
positions spread across the number of TDMA-TDD frames corresponding
to the number of sector cells in order to allocate TDMA-TDD frames
for one sector cell.
[0034] Moreover, according to the control channel placement method
of the present invention, in the above control channel placement
method, the TDMA-TDD frames are formed by securing a plurality of
time slot positions in which up control channels can be placed and
a plurality of time slot positions in which down control channels
can be placed and, based on a reception level measurement result
measured in the reception level measurement step, control channels
of sector cells that are different from each other are placed in a
plurality of time slot positions for the placement of the control
channels, and wherein, in the frame allocation step, within
TDMA-TDD frames extending for the number of the sector cells,
specific TDMA-TDD frames are allocated such that time slots
extracted in the time slot extraction step coincide with either one
of time slot positions for the placement of the control channels
within the specific TDMA-TDD frames determined for each of the
sector cells and time slot positions for the placement of the
control channels within other TDMA-TDD frames different to the
specific TDMA-TDD frames.
[0035] According to the above invention, the TDMA-TDD frames are
formed by securing a plurality of time slot positions for the
placement of up control channels and down control channels and,
based on a reception level measurement result measured in the
reception level measurement step, by placing control channels for
sector cells that are different from each other in each of the
plurality of time slot positions. When the frame allocation step
allocates specific TDMA-TDD frames (namely, TDMA-TDD frames
corresponding to specific sector cells), there is a prerequisite
that, within TDMA-TDD frames extending for the number of the sector
cells, time slots extracted in the time slot extraction step
coincide with one of either time slot positions for the placement
of the above control channels within the specific TDMA-TDD frames
and time slot positions for the placement of the above control
channels within other TDMA-TDD frames different to the specific
TDMA-TDD frames. Therefore, it is possible to choose from a
plurality of control channel placement positions spread across the
number of TDMA-TDD frames corresponding to the number of sector
cells in order to allocate TDMA-TDD frames for one sector cell.
[0036] Moreover, according to the control channel placement method
of the present invention, in the above control channel placement
method, there is included a reception level remeasurement step for
measuring, at regular intervals and for each of the sector cells,
reception levels of time slots in which control channels have been
placed in the frame allocation step; and a frame reallocation step
for performing the frame allocation step when a reception level
measured in the reception level remeasurement step is at a
predetermined threshold level or greater.
[0037] According to the above invention, in a wireless
communication system that employs a TDMA-TDD mode for the wireless
access mode and employs sector cells for the zone structure, when a
base station having a plurality of sector cells performs
transmission by placing both up control channels and down control
channels at predetermined time slot positions within the TDMA-TDD
frames in each sector cell, the reception level in each time slot
extending for the number of TDMA-TDD frames corresponding to the
number of sector cells of the base station is measured in the
reception level measurement step. Next, in the time slot extraction
step, time slots in which control channels can be placed are
extracted from among the time slots that extend across TDMA-TDD
frames of the number of sector cells using the result of the
measurement of the reception levels measured in a first sector cell
and the result of the measurement of the reception levels measured
in a second sector cell that is located opposite the first sector
cell from among all the sector cells in the same base station.
Next, with communication having been established as a result of
TDMA-TDD frames allocated in each sector cell in the frame
allocation step such that time slots extracted in the time slot
extraction step coincide with predetermined time slot positions for
the placement of control channels from among the time slots within
the TDMA-TDD frames, the reception level remeasurement step
measures at regular intervals the reception levels of time slots in
which control channels have been placed in each sector cell. If the
result of this reception level measurement is equal to or greater
than a predetermined threshold, the frame reallocation step
performs the frame allocation step once again. As a result of the
above, it is possible to avoid deterioration in the quality of
communication caused by interference obstruction generated while
communication is established.
[0038] Moreover, according to the control channel placement method
of the present invention, in the above control channel placement
method, the frame allocation step performed in the frame
reallocation step allocates the TDMA-TDD frame to a position
shifted by one time slot towards the front or rear.
[0039] According to the above invention, because the above frame
allocation step performed in the frame reallocation step allocates
the TDMA-TDD frame to a position shifted by one time slot towards
the front or rear, by attempting to establish communication each
time by shifting the reallocation of the TDMA-TDD frames by one
time slot forwards or backwards, it is possible to omit the
calculation to search for time slots in which control channels can
be placed. At the same time, it is possible to keep the time when
communication cannot be established to the minimum.
[0040] Moreover, according to the control channel placement method
of the present invention, in the above control channel placement
method, in the reception level remeasurement step, the carrier to
interference wave strength ratio is measured from the reception
levels and, in the frame reallocation step, the frame reallocation
step is performed when the carrier to interference wave strength
ratio measured in the reception level remeasurement step is equal
to or greater than a predetermined threshold.
[0041] According to the above invention, because the carrier to
interference wave strength ratio is acquired from the reception
levels measured in the reception level remeasurement step, and the
frame reallocation step is able to perform frame reallocation in
accordance with this carrier to interference wave strength ratio,
it is possible to accurately determine whether or not a control
channel is receiving interference obstruction.
[0042] Moreover, according to the control channel placement method
of the present invention, in the above control channel placement
method, in the reception level remeasurement step, the carrier to
interference wave strength ratio is measured from the reception
level when the time slot being measured is in a blocked state and,
in the frame reallocation step, the frame reallocation step is
performed when the carrier to interference wave strength ratio
measured in the reception level remeasurement step is equal to or
greater than a predetermined threshold.
[0043] According to the above invention, because the carrier to
interference wave strength ratio is acquired from the reception
levels measured in the reception level remeasurement step when the
time slots being measured are in a blocked state, and the frame
reallocation step is able to perform frame reallocation in
accordance with this carrier to interference wave strength ratio,
it is possible to accurately determine whether or not variations in
the reception level in a control channel are only caused by
interference obstruction.
[0044] Moreover, according to the control channel placement method
of the present invention, in the above control channel placement
method, in the frame reallocation step, if the carrier to
interference wave strength ratio measured in the reception level
remeasurement step continues to be equal to or greater than a
predetermined threshold for a predetermined length of time, then a
determination is made that the time slot being measured is
receiving interference obstruction and the frame allocation step is
performed.
[0045] According to the above invention, because the frame
reallocation step is able to determine that the time slot being
measured is receiving interference obstruction if the measured
carrier to interference wave strength ratio continues to be equal
to or greater than a predetermined threshold for a predetermined
length of time, and then perform the frame allocation step, it is
possible to accurately determine whether variations in the
reception level are caused by collisions in the up control channels
or by interference obstruction.
[0046] Moreover, according to the control channel placement method
of the present invention, in the above control channel placement
method, in the reception level remeasurement step, an error
detection ratio in an up control channel is measured as a reception
level and, in the frame reallocation step, if the error detection
ratio measured in the reception level remeasurement step is equal
to or greater than a predetermined threshold, the frame allocation
step is performed.
[0047] According to the above invention, because the error
detection ratio in the up control channel can be measured in the
reception level remeasurement step and the frame reallocation step
can perform the frame allocation step in accordance with this error
detection ratio, it is possible to determine whether the usage of
the time slot of a control channel has become difficult or
impossible.
BRIEF DESCRIPTION OF THE DRAWINGS
[0048] FIG. 1 is an explanatory diagram showing the sector cell
structure of a base station in which the control channel placement
method according to the first embodiment is applied; FIG. 2 is a
flow chart showing the in the control channel placement method
according to the first embodiment; FIG. 3 is an explanatory diagram
showing the reciprocal relationship between each sector cell and
it's opposite sector cell in the control channel placement method
according the first embodiment; FIG. 4 is an explanatory diagram
which shows an example of the structure of the TDMA-TDD frames in
the control channel placement method according to the first
embodiment; FIG. 5 is an explanatory diagram which shows the result
when time slots each having a high reception level in both a
specific sector cell and in the opposite sector cell are registered
in each sector cell in the control channel placement method
according to the first embodiment; FIG. 6 is an explanatory diagram
which explains the allocation of TDMA-TDD frames in the control
channel placement method according to the first embodiment; FIG. 7
is an explanatory diagram which shows an example of a different
allocation of TDMA-TDD frames in the control channel placement
method according to the first embodiment; FIG. 8 is a flow chart
showing the sequence of steps in the control channel placement
method according to the second embodiment; FIG. 9 is an explanatory
view which shows an example of the structure of the TDMA-TDD frames
in the control channel placement method according to the second
embodiment; FIG. 10 is an explanatory diagram which explains an
example of the allocation of TDMA-TDD frames in the control channel
placement method according to the second embodiment; FIG. 11 is an
explanatory diagram which shows an example of the structure of the
TDMA-TDD frames in the control channel placement method according
to the third embodiment; FIG. 12 is an explanatory diagram which
shows an example of the allocation of TDMA-TDD frames in the
control channel placement method according to the third embodiment;
FIG. 13 is an explanatory diagram which shows an example of the
allocation of TDMA-TDD frames in the control channel placement
method according to the fourth embodiment; FIG. 14 is an
explanatory diagram which shows the frame structure in a
conventional TDMA-TDD mode.
BEST MODE FOR CARRYING OUT THE INVENTION
[0049] A detailed description will now be given of the embodiments
of the control channel placement method according to the present
invention while referring to the drawings. However, the present
invention is not limited by these embodiments.
[0050] FIG. 1 shows the sector cell structure of a base station in
which the control channel placement method according to the first
embodiment is applied. A base station 10 is provided with
directional antennas a1 to a6 having a directionality of 60.degree.
within each sector cell so as to divide a circular zone centered on
the base station 10 into 6 fan-shaped sector cells C1 to C6 having
same size. Here, it will be assumed that the same frequency has
been allocated in each sector cell.
[0051] Moreover, the wireless access mode employed in the wireless
communication system provided by the base station 10 is the above
described TDMA-TDD mode. By combining the TDMA-TDD mode and the
sector cell structure, it is possible to achieve highly efficient
usage of a limited number of frequencies.
[0052] As is shown in FIG. 1, when a terminal station TA1 is
located within the sector cell C1, the base station 10 is able to
communicate with the terminal TA1 using the directional antenna a1.
In addition, because the TDMA-TDD mode is employed, even if a
plurality of terminal stations are located within the sector cell
C1, simultaneous communication is possible in the number of time
slots allowed in the TDMA-TDD frame.
[0053] The sequence of steps involved in the control channel
placement method according to the first embodiment will now be
explained. FIG. 2 is a flow chart showing these sequence of steps.
In an initialization state in which communication for starting up
and the like can be established (established in synchronization),
the base station 10 selects one sector cell out of the sector cells
C1 to C6. Let us assume that the sector cell C1 is set as the
sector cell whose reception level is to be measured (step
S101).
[0054] At this point, the reciprocal relationship between the
respective sector cells and each sector cell in a position opposite
the respective sector cells is determined. FIG. 3 shows the
reciprocal relationship between each sector cell and it's opposite
sector cell. As can be confirmed from FIG. 1, because each sector
cell is allocated as one of six equilateral triangles forming a
hexagon, the sector cells are placed opposite each other at
symmetrical positions centered on the base station 10. Accordingly,
as shown in FIG. 3, the sector cell C1 has an opposite relationship
to the sector cell C4; the sector cell C2 has an opposite
relationship to the sector cell C5; and the sector cell C3 has an
opposite relationship to the sector cell C6.
[0055] After the processing of step S101, the reception levels in
each time slot within the TDMA-TDD frames corresponding to the
total number of sector cells are measured (step S102). FIG. 4 shows
an example of the structure of the TDMA-TDD frames. The TDMA-TDD
frames shown in FIG. 4 are formed from 5 time slots. In this case,
in the processing in step S102, the number of time slots within the
TDMA-TDD frames corresponding to the number of sector cells is 6
(the total number of sector cells) times 5 (time slots per TDMA-TDD
frame), in other words 30.
[0056] In this way, the level of the wave band received in the
sector cell C1 selected in step S101, namely, by the directional
antenna a1 shown in FIG. 1, is measured for each of the 30
consecutive time slots, and the results of the measurement are
stored. Variations in the reception level in each time slot are
mainly caused by incoming interference waves from sector cells of
other base stations provided peripheral to the base station 10.
[0057] Here, it is possible to calculate the carrier to
interference ratio from the measurement of the reception levels. In
this case, time slots in which the measured carrier to interference
ratio is equal to or greater than a predetermined threshold value
can be seen as being time slots that are receiving interference
obstruction. As is shown in FIG. 4, these time slots are registered
as not being suitable time slots (i.e. as time slots having a high
level of reception) for receiving a control channel.
[0058] In FIG. 4, out of the time slots of 6 TDMA-TDD frames in the
sector cell C1, the time slots T.sub.1, T.sub.8, T.sub.11,
T.sub.21, and T.sub.28 are each represented by an X mark. This
means that these time slots are not suitable for receiving a
control channel.
[0059] It is then determined whether or not the processing of step
S102 has been performed for all sector cells (step S103). If the
processing of step S102 has not been performed for all the sector
cells, the next sector cell is selected as the sector cell to be
measured (step S105). Since the sector cells are in the order C1,
C2, C3, C4, C5, and C6 the sector cell C2 will be selected
next.
[0060] If the processing of step S102 has been performed for all
sector cells, namely, if measurement results such as those shown in
FIG. 4 have been obtained for all sector cells, then the sector
cell to be processed in the time slot extraction processing
described below is set in the same way as in step S101 (step S104).
Accordingly, the sector cell C1 is set as the sector cell to be
processed.
[0061] Next, the result of the measurement of the reception level
corresponding to the opposite sector cell as stipulated by the
reciprocal relationship shown in FIG. 3 is added to the result of
the measurement of the reception level corresponding to the sector
cell set in step S104, and the time slot is registered again as
being unsuitable for receiving a control channel.
[0062] FIG. 5 shows the result when time slots each having a high
reception level in both a specific sector cell and in the opposite
sector cell are registered in each sector cell. In FIG. 5, as
regards the results of the registration of the cell C1, for
example, in addition to those positions in the time slots of the
sector cell C1 itself that show a high reception level being
registered as an X mark (see the sector cell C1 in FIG. 4), the
positions T.sub.6 and T.sub.2, showing a high reception level in
the time slots of the sector cell C4 itself that is positioned
opposite the sector cell C1 are registered as a .DELTA. mark.
[0063] The reason for adding the results of the measurement of the
reception level of sector cells in opposite positions in the same
base station, in this way, is explained below. As is shown in FIG.
1, for example, in a state in which an interfering station U4 is
located in the direction of the sector cell C4 and a terminal
station TA1 is located within the sector cell C1, if the base
station 10 receives a signal from the interfering station U4 via
the directional antenna a4, then if the base station 10 transmits a
signal towards the terminal TA1 via the directional antenna a1,
because the sector cells C1 and C4 are located opposite to each
other, the directions in which the signals are transmitted end up
approximately matching each other. The possibility thus exists that
the terminal station TA1 will receive not only waves from the base
station 10, but also from the interfering station U4.
[0064] Accordingly, in the time slot time series of the sector cell
C1 shown in FIG. 5, those time slots registered with an X mark can
be regarded as not suited for performing reception because the
reception level in their host sector cell C1 is high. Moreover,
time slots registered with a .DELTA. mark can be regarded as not
suited for performing transmission from the sector cell C1 because
the reception level in the sector cell C4 located opposite the
sector cell C1 is high.
[0065] In particular, those time slots indicated by the X mark and
the A mark in FIG. 5 are not suitable for receiving an up control
channel and a down control channel necessary for performing the
establishment of synchronized communication, and are an obstacle to
high quality communication.
[0066] Therefore, based on the registration contents shown in FIG.
5, the extraction of time slots from which those time slots having
a high level of reception have been removed, namely, the extraction
of time slots that are suitable for having control channels
arranged therein, is performed in the host sector cell and in the
sector located opposite the host sector cell (step S106).
[0067] Subsequently, it is determined whether or not the processing
of step S106 has been performed for all the sector cells (step
S107). If the processing of step S106 has not been performed for
all the sector cells, the sector cell that should be selected next
is set for processing (step S108). In this case, the sector cells
are set in the same order as in the processing in step S105.
[0068] If the processing of step S106 has been performed for all
the sector cells, namely, if registration results such as those
shown in FIG. 5 have been obtained for all sector cells, then
TDMA-TDD frames are allocated to the optimum time slots in each
sector cell, based on these registration results (step S109).
[0069] FIG. 6 explains in detail the allocation of TDMA-TDD frames
performed in step S109. In the TDMA-TDD frame structure shown in
FIG. 6, in one TDMA-TDD frame formed from 5 time slots, a down
control channel B (broadcast) is placed in the first time slot,
while an up control channel R (random) is placed in the second time
slot. The remaining 3 time slots are used as communication channels
so that, in this example, simultaneous communication with three
terminal stations is possible.
[0070] Further, in the control channel method according to the
present embodiment, by referring to those time slots in which
control channels can be placed extracted in step S106 in FIG. 2, it
is possible to perform the optimum TDMA-TDD frame allocation.
[0071] Specifically, the TDMA-TDD frames are allocated such that
the down control channel B and the up control channel R shown in
FIG. 6 are placed in positions where there is no X mark or .DELTA.
mark which indicate time slots incapable of placement of control
channels. In this case, as a result of the TDMA-TDD frame
allocation in the sector cell C1, it is necessary to select the
optimum TDMA-TDD frame allocation positions such that the positions
where the down control channels B and the up control channels R in
the TDMA-TDD frames in each of the other sector cells C2 to C6 are
placed do not overlap with either X marks or .DELTA. marks.
[0072] Moreover, as described above, because the X marks shown in
FIG. 5 and FIG. 6 can be regarded as time slots in which reception
cannot be performed, it is preferable that the up control channels
R be placed as far as possible from time slots with this X mark. In
contrast, because the .DELTA. marks can be regarded as time slots
in which transmission cannot be performed, it is preferable that
the down control channels B be placed as far as possible from time
slots with this .DELTA. mark.
[0073] In consideration of the above, in FIG. 6, the TDMA-TDD frame
21 in which the sector cell C1 control channel is transmitted is
allocated from the time slot T.sub.2, while the TDMA-TDD frame 22
in which the sector cell C2 control channel is transmitted is
allocated from the time slot T.sub.7. In the same way, for the
sector cell C3 the TDMA-TDD frame 23 is allocated from the time
slot T.sub.12, for the sector cell c4 the TDMA-TDD frame 24 is
allocated from the time slot T.sub.17, for the sector cell c5 the
TDMA-TDD frame 25 is allocated from the time slot T.sub.22, and for
the sector cell c6 the TDMA-TDD frame 26 is allocated from the time
slot T.sub.27, respectively.
[0074] Note that, in FIG. 6, in order to simplify the description,
only the time slots T.sub.1 to T.sub.30 of the control channel
transmission cycle, namely, of the 6 TDMA-TDD frames are shown,
and, in particular, for the sector cell C6, the last time slot in
the TDMA-TDD frame 26, that is the time slot T.sub.1 looks
separated from the other time slots. However, in reality, the last
time slot is in continuation with the time slot T.sub.30 because it
is a time series. Therefore, in the present description, the
symbols T.sub.1 to T.sub.30 are simply used to specify illustrated
time slots for description. Moreover, in each figure, only those
TDMA-TDD frames in which control channels are arranged are shown
with a bold outline. However, in reality, TDMA-TDD frames of the
same size are connected to these. The same applies to the
description given below.
[0075] In the above description, both the down control channel B
and the up control channel R were placed within the same TDMA-TDD
frame. However, the present invention can also be applied even when
the down control channel B and the up control channel R are placed
within different TDMA-TDD frames.
[0076] FIG. 7 shows the TDMA-TDD frame allocation when the down
control channel B and the up control channel R are placed within
different TDMA-TDD frames. As shows in FIG. 7, the up control
channel R is placed in the first TDMA-TDD frame while the down
control channel B is placed in the second TDMA-TDD frame from among
the six TDMA-TDD frames of the control channel transmission
cycle.
[0077] In particular, the up control channel R is placed in the
third time slot of the of a TDMA-TDD frame, while the down control
channel B is placed in the first time slot of a TDMA-TDD frame. In
addition, in the same way as in the above described frame
allocation processing, frame allocation for the two TDMA-TDD frames
in which the control channels have thus been placed is also
performed such that the up control channel R and the down control
channel B are placed in locations other than those having the X
mark or the .DELTA. mark indicating time slots in which control
channels cannot be placed.
[0078] The TDMA-TDD frame in which the up control channel R of the
sector cell C1 is transmitted is allocated from the time slot
T.sub.2, while the TDMA-TDD frame in which the down control channel
B of the sector cell C1 is transmitted is allocated from the time
slot T.sub.7. Moreover, the TDMA-TDD frame in which the up control
channel R of the sector cell C2 is transmitted is allocated from
the time slot T.sub.7 within the TDMA-TDD frame that has been
shifted along by one frame from the TDMA-TDD frame in which the up
control channel R of the sector cell C1 was transmitted.
Subsequently, the TDMA-TDD frame in which the down control channel
B of the sector cell C2 is transmitted is allocated. Thereafter,
the TDMA-TDD frames are allocated in the same way for the sector
cells C2 to C6.
[0079] As described above, according to the control channel
placement method of the first embodiment, when transmission is
performed by a base station having a plurality of sector cells with
both up control channels and down control channels placed in
predetermined time slot positions within TDMA-TDD frames for each
sector cell, the reception level of each time slot is measured for
each sector cell across TDMA-TDD frames corresponding to the number
of sector cells of the base station. Consequently, using the result
of the measurement of the reception level measured in the first
sector cell and the result of the measurement of the reception
level measured for the second sector cell at a position opposite to
the first sector cell from among all the sector cells within the
same base station, time slots in which control channels can be
placed are extracted from among the time slots extending for the
TDMA-TDD frames corresponding to the number of sector cells.
Subsequently, the TDMA-TDD frames are allocated for each sector
cell such that the positions of the above extracted time slots and
the predetermined time slot positions in which control channels are
placed (in the above described example, the first and second time
slots within a TDMA-TDD frame) out of the time slots within the
TDMA-TDD frames coincide with each other. Because of such
arrangement, the up control channel and the down control channel
can be placed simultaneously in optimum time slot positions. As a
result, it is possible, in a wireless communication system, to
establish high quality and stable wireless channel
communication.
[0080] In the above-explained first embodiment, the reception level
of each time slot within the TDMA-TDD frames corresponding to the
entire number of sector cells was measured and stored, and time
slots whose reception level was over a predetermined threshold
value out of the measured reception levels were considered to be
unsuitable for the placement of control channels. However, after
the reception level of each time slot within the TDMA-TDD frames
corresponding to the entire number of sector cells has been
measured and stored, it is also possible to acquire the reception
level distribution from the reception level results from each
sector cell and the reception level results from the sector cell
opposite each sector cell, and to specify time slots suitable for
the placement of control channels for each sector cell from the
overall reception level distribution.
[0081] When such a method is employed, it is possible, for example,
to allocate TDMA-TDD frames such that control channels are placed
in sequence beginning with the time slot having the lowest
reception level (or carrier to interference ratio calculated from
the reception levels) from among the acquired reception level
distribution. When this TDMA-TDD frame allocation is performed as
well, as described above, because the allocation of TDMA-TDD frames
for the other sector cells is automatically determined by the
allocation of TDMA-TDD frames for one sector cell, it is necessary
to find and fix the position where the total reception level of the
time slots in which each control channel is going to be placed in
all the sector cells will be the lowest.
[0082] Next, the control channel placement method according to the
second embodiment will be described. In the control channel
placement method according to the second embodiment, the placement
of control channels is carried out using the above control channel
placement method according to the first embodiment. When wireless
communication has been established, the reception level of the
control channels is measured. The difference is that, based on the
results of the measurement, the control channels are placed once
again by performing again the above time slot extraction process
and TDMA-TDD frame allocation process.
[0083] Because the sector cell structure of the base station and
the wireless access mode are the same as in the first embodiment, a
description thereof is omitted here. The processing after the
establishment of wireless communication by the placement of control
channels using the control channel placement method according to
the first embodiment will only be described here.
[0084] FIG. 8 shows the sequence of steps in the control channel
placement method according to the second embodiment. The base
station 10 measures the reception levels in the time slots of
control channels placed so as to avoid those time slots (having the
X mark or the .DELTA. mark) in which control channels could be
placed in step S106 of FIG. 2 in each sector cell (step S201).
[0085] FIG. 9 shows an example of the structure of the TDMA-TDD
frames. In the same way as in the first embodiment, the TDMA-TDD
frames shown in FIG. 9 are formed from five time slots with six
TDMA-TDD frames set as one cycle. Transmission and reception are
performed with the down control channel B and the up control
channel R placed in the first and second time slots respectively
within the leading TDMA-TDD frame.
[0086] In FIG. 9, before the control channel placement method
according to the present embodiment is employed, namely, in a state
before the TDMA-TDD frames are reallocated, the TDMA-TDD frame in
which the above control channels have been placed is allocated from
the time slot T.sub.11. In this state, the processing of step S201
is performed.
[0087] Next, based on the reception levels measured in the
processing of step S201, it is determined whether or not the time
slot measured was receiving interference obstruction (step S202).
Note that, in the processing of step S201, it is possible to
calculate the carrier to interference ratio from the measurement of
the reception levels and, in this case, in the determination of
step S202, the time slots of the control channels are regarded as
receiving interference obstruction when the measured carrier to
interference ratio is above a predetermined threshold value.
[0088] At this time, the control channels are temporarily freed
from the control channel time slots and, in a state in which the
establishment of communication is shut, namely, in a blocked state,
it is possible to perform the above carrier to interference ratio
measurement. As a result, it is possible to accurately determine
whether or not the sole cause of variations in the reception level
is the interference.
[0089] Moreover, in the measurement of the carrier to interference
ratio, there is a possibility of collisions occurring because the
up control channel is a random access type of channel. In this
case, it is unclear whether the variations in the reception level
in particular are due to collision or due to interference
obstruction. Therefore, when a state in which the carrier to
interference ratio is equal to or greater than the predetermined
threshold for a predetermined length of time, it is also possible
to determine that this is due to interference obstruction.
[0090] Apart from using the carrier to interference ratio, it is
also possible to calculate the error detection rate in the
communication channel and up control channel received by the base
station 10 from a terminal station, and to determine from the
result thereof whether or not the control channel time slot is
receiving interference obstruction.
[0091] When it is not determined in step S202 that interference
obstruction is being received, this processing is ended and the
processing of step S201 is performed again after the lapse of a
predetermined length of time. If, however, it is determined in step
S202 that interference obstruction is being received, in the same
way as in the processing of step S109 of FIG. 2, the allocation of
the TDMA-TDD frames is performed once again while avoiding time
slots in which control channels cannot be placed, and the up
control channel and the down control channel are set (step S203).
Namely, the reallocation of the TDMA-TDD frames is achieved by the
processing of step S203.
[0092] In FIG. 9, if it is determined by the processing of the
above step S202 that the time slot T.sub.12 of the up control
channel R is receiving interference obstruction, the reallocation
of the TDMA-TDD frames is performed such that the TDMA-TDD frame in
which the control channels have been placed is allocated from the
time slot T.sub.9.
[0093] Note that, in this reallocation of the TDMA-TDD frames, the
allocation position is decided using time slots newly receiving
interference obstruction (the time slot T.sub.12 in the above
example) and the result of the time slot extraction of the
processing of step S106 of FIG. 2.
[0094] Moreover, in addition to the registering of the time slots
newly receiving interference obstruction as time slots having a
high reception level, in the same way as in the processing of step
S106 of FIG. 2, measurement of the reception level is performed
once again for each time slot extending over the TDMA-TDD frames of
the entire number of sector cells (including the time slots of the
sector cells positioned opposite). After those time slots in which
control channel placement is possible have then been extracted, the
allocation positions of the TDMA-TDD frames may then be decided
using the extraction results.
[0095] In the above description of the second embodiment, the
TDMA-TDD frame reallocation positions were decided such that the
control channel placement positions coincided with time slots in
which control channel can be placed, by referring to all of the
time slots extending over the TDMA-TDD frames corresponding to the
number of sector cells, however, if it is determined that the
control channels are receiving interference obstruction, it is also
possible to try and reestablish communication by shifting the
TDMA-TDD frames in which control channels are placed by one time
slot either forwards or backwards.
[0096] In this case, after a predetermined time has lapsed, the
reception level is measured again in the same way as in step S201
of FIG. 8 and, in accordance with the result of the measurement,
this movement of the TDMA-TDD frame placement by one time slot is
repeated until finally the control channels are placed in time
slots not receiving interference.
[0097] FIG. 10 is an explanatory diagram for describing an example
of the TDMA-TDD frame allocation in this case. In FIG. 10, looking
at the sector cell C1, before the control channel placement method
of the present embodiment is implemented, namely, in a state before
the TDMA-TDD frame reallocation, a state is shown in which the time
slot T.sub.12 of the up control channel R and the time slot
T.sub.13 of the communication channel within the TDMA-TDD frame 31
allocated from the time slot T.sub.11 are receiving
interference.
[0098] In this case, using the same processing as in the above is
steps S201 and S202, when it is determined that the time slot
T.sub.12 of the up control channel R is receiving interference,
reallocation is performed by shifting the TDMA-TDD frame 31 by one
time slot in the forward direction (after stage 1).
[0099] Due to this reallocation after stage 1, the down control
channel B of the TDMA-TDD frame 31 is placed in the time slot
T.sub.10, however, because this time slot T.sub.10 in the opposite
sector cell C4 was registered as a high reception level time slot
in step S106 (indicated in the figure by the mark .DELTA.), it
becomes necessary to reallocate the TDMA-TDD frame 31 by measuring
the reception levels once again, in the same way as in step S201 in
FIG. 8, after the lapse of a predetermined time.
[0100] Therefore, the TDMA-TDD frame 31 is shifted by one time slot
again in the forward direction and reallocated (after stage 2). As
a result, the down control channel B within the TDMA-TDD frame 31
is placed in the time slot T.sub.9 so as to escape from the
interference in the time slot T.sub.10, however, the up control
channel R then receives the interference in the time slot T.sub.10.
Accordingly, in this state as well, it again becomes necessary to
reallocate the TDMA-TDD frame 31 by measuring the reception levels
once again, in the same way as in step S201 in FIG. 8, after the
lapse of a predetermined time (after stage 3).
[0101] Because both the down control channel B and the up control
channel R are free from interference as a result of the
reallocation after stage 3, it is possible to perform high quality
communication in this final control channel placement position.
Note that, at this time, because the TDMA-TDD frame allocation is
also altered for the other sector cells, it is actually necessary
to decide the final placement position using the state of the
reception levels of control channels for all the sector cells after
the reallocation.
[0102] It is also possible to perform reallocation by shifting the
TDMA-TDD frame 31 by one time slot towards the rear, however, in
the above example, because the time slot T.sub.12 of the up control
channel R and the time slot behind that T.sub.13 are receiving
interference in the state before reallocation, it would be
necessary to repeat the reallocation by the number of stages that
take this into account.
[0103] In this way, by attempting to establish communication each
time by shifting the reallocation of the TDMA-TDD frames by one
time slot forwards or backwards, it is possible to omit the
calculation to search for time slots capable of receiving control
channels. Furthermore, it is possible to keep the time when
communication cannot be established to the minimum, thereby
enabling communication to be rapidly restored.
[0104] As explained above, in the control channel placement method
according to the second embodiment, when transmission is performed
by a base station having a plurality of sector cells with both up
control channels and down control channels placed in predetermined
time slot positions within TDMA-TDD frames for each sector cell,
the reception level of each time slot is measured for each sector
cell across TDMA-TDD frames corresponding to the number of sector
cells of the base station. Consequently, using the result of the
measurement of the reception level measured in the first sector
cell and the result of the measurement of the reception level
measured for the second sector cell at a position opposite to the
first sector cell from among all the sector cells within the same
base station, time slots in which control channels can be placed
are extracted from among the time slots that extending across the
same number of TDMA-TDD frames as there are sector cells.
Subsequently, in a state in which as a result of the TDMA-TDD
frames being allocated for each sector cell such that the positions
of the above extracted time slots and predetermined time slot
positions in which control channels are placed, from among the time
slots within the TDMA-TDD frames, coincide with each other, the
reception levels of time slots in which the control channels are
placed are measured for each sector cell each time a predetermined
length of time has elapsed. When the result of the measurement of
the reception level is equal to or greater than a predetermined
threshold, the TDMA-TDD frame allocation is performed once again.
Therefore, it is possible to avoid deterioration in the quality of
communication caused by interference obstruction generated while
communication is established, and it is possible, in a wireless
communication system, to establish high quality and stable wireless
channel communication.
[0105] Next, the control channel placement method according to the
third embodiment will be described. The control channel placement
method according to the third embodiment is characterized in that,
in contrast to the respective placement one by one of the up
control channel R and the down control channel B within the
TDMA-TDD frames of the control channel placement method according
to the first embodiment, a plurality of up control channels R and
down control channels B are each placed within the same TDMA-TDD
frame and each sector cell uses one of this plurality of control
channels as the control channel.
[0106] The sector cell structure and the wireless access method of
the base station are the same as in the first embodiment.
Therefore, an explanation of these is omitted here. Moreover, the
processing using the control channel placement method according to
the third embodiment is the same as that shown in FIG. 2, except
for the processing at step S109. Therefore, the TDMA-TDD frame
allocation processing that corresponds to the processing at step
S109 will only be explained here.
[0107] FIG. 11 shows the TDMA-TDD frame allocation in the control
channel placement method according to the third embodiment. In the
control channel placement method according to the third embodiment,
down control channels B are placed in the first to third time slots
t.sub.1 to t.sub.3, while up control channels R are placed in the
fourth to sixth time slots t.sub.4 to t.sub.6 within a single
TDMA-TDD frame formed from a plurality of time slots. The remaining
time slots are used as communication channels.
[0108] Moreover, with the frame structure formed such that the
TDMA-TDD frames corresponding to the total number of sector cells,
namely, the six TDMA-TDD frames from 41 to 46 are set as one cycle,
in each sector cell, one of the plurality of time slots in which
the same type of control channel can be placed within the TDMA-TDD
frame is allocated in advance as the control channel placement
position.
[0109] In the example shown in FIG. 11, in the sector cell C1, the
first time slot t.sub.1 in the first TDMA-TDD frame 41, the third
TDMA-TDD frame 43, and the fifth TDMA-TDD frame 45 is provided as
the down control channel B. In the sector cell C2, the first time
slot t.sub.1 in the second TDMA-TDD frame 42, the fourth TDMA-TDD
frame 44, and the sixth TDMA-TDD frame 46 is provided as the down
control channel B.
[0110] In the sector cell C3, the second time slot t.sub.2 in the
first TDMA-TDD frame 41, the third TDMA-TDD frame 43, and the fifth
TDMA-TDD frame 45 is provided as the down control channel B. In the
sector cell C4, the second time slot t.sub.2 in the second TDMA-TDD
frame 42, the fourth TDMA-TDD frame 44, and the sixth TDMA-TDD
frame 46 is provided as the down control channel B.
[0111] Further, in the sector cell C5, the third time slot t.sub.3
in the first TDMA-TDD frame 41, the third TDMA-TDD frame 43, and
the fifth TDMA-TDD frame 45 is provided as the down control channel
B. In the sector cell C6, the third time slot t.sub.3 in each of
the second TDMA-TDD frame 42, the fourth TDMA-TDD frame 44, and the
sixth TDMA-TDD frame 46 is provided as the down control channel
B.
[0112] Thus, in each sector cell, hatched time slots can be used as
the down control channel B. Note that, in the same way as the down
control channel B, for the up control channel R, in each sector
cell, one of the time slots t.sub.4 to t.sub.6 within each TDMA-TDD
frame can be used as the down control channel B.
[0113] Accordingly, in each sector cell, it is possible to select
for use any one of the same type of control channels provided in
three TDMA-TDD frames from among the six TDMA-TDD frames
corresponding to one cycle.
[0114] The specific TDMA-TDD frame allocation method in this case
is described below. Firstly, as is shown in FIG. 11, control
channel placement positions allocated in each sector cell are
determined in advance in each of the six TDMA-TDD frames
corresponding to one cycle. Next, as explained in the first
embodiment, the reception level of each time slot within the six
TDMA-TDD frames is measured, and the reception level registration
result is acquired, as was shown in FIG. 5. Next, the placement of
the TDMA-TDD frames is decided such that the determined control
channel placement positions do not coincide with the positions of
time slots registered with the X marks or the .DELTA. marks in the
acquired registration result.
[0115] Note that, after the above TDMA-TDD frame allocation has
been completed, in a state in which wireless communication has been
established, the reception level in the control channels actually
placed in each of the sector cells is regularly measured. If the
result of the measurement exceeds a predetermined reception level,
then the TDMA-TDD frame allocation processing may be performed once
again.
[0116] In this case, as was described above, because it is possible
to select and use any one of the same type of control channels
placed in three TDMA-TDD frames from among the six TDMA-TDD frames
in each sector cell, if there are any time slots in which the
predetermined reception level has not exceeded from among those
remaining time slots that can be selected as the same type of
control channel, those time slots can be set and used as control
channels. Accordingly, in this case, there is no need to implement
the TDMA-TDD frame allocation processing.
[0117] In contrast, if all of the remaining time slots that can be
selected as the same type of control channel exceed the
predetermined reception level, it is necessary to again implement
the TDMA-TDD frame allocation processing. This new TDMA-TDD frame
allocation processing, namely, the TDMA-TDD frame reallocation
processing can be achieved, for example, by repeatedly shifting all
of the TDMA-TDD frames in which control channels are currently
placed either forwards or backwards by one time slot, and then
attempting to reestablish communication.
[0118] FIG. 12 shows the TDMA-TDD frame reallocation in this case.
Particularly, FIG. 12 shows a case in which reallocation is
performed with the entire TDMA-TDD frames shifted backwards by one
time slot. The reception levels of the control channels actually
placed in each sector cell are measured again, and a determination
is made as to whether or not the result of the measurement exceeds
the predetermined reception level. As a result of this, a decision
is made as to whether to shift the TDMA-TDD frames by a further
time slot and again perform the allocation, or whether to set this
as the control channel placement position.
[0119] As explained above, in the control channel placement method
according to the third embodiment, a plurality of time slots in
which the same type of control channel can be placed are prepared
within a single TDMA-TDD frame, and different sector cells use the
above control channels within the same TDMA-TDD frame. Moreover,
time slots capable of receiving control channels that can be used
by the same sector cell are fixed in advance across a plurality of
TDMA-TDD frames from among the TDMA-TDD frames corresponding to the
total number of sector cells corresponding to one cycle. Therefore,
in the TDMA-TDD frame allocation processing or in the TDMA-TDD
frame reallocation processing, even if one control channel becomes
temporarily unusable (when viewed from the terminal station side)
due to interference, other control channels that are capable of
reception can be selected and used, thereby enabling high quality,
stable communication to be provided in a wireless communication
system.
[0120] Next, the control channel placement method according to the
fourth embodiment will be described. The control channel placement
method according to the fourth embodiment is characterized in that,
in contrast to the securing of a plurality of time slots capable of
receiving control channels of the same type and the determining in
advance of the placement positions of time slots able to be used as
control channels by the same sector cell of the control channel
placement method according to the third embodiment, the placement
positions of those time slots able to be used as control channels
is decided in accordance with the result of the measurement of the
reception level of each time slot.
[0121] The wireless access mode and the sector cell structure of
the base station are the same as those of the first embodiment and
a description thereof is omitted here. Only the TDMA-TDD frame
allocation processing corresponding to the processing in step S109
shown in FIG. 2 will be explained here.
[0122] FIG. 13 shows the TDMA-TDD frame allocation in the control
channel allocation method according to the fourth embodiment. In
the control channel allocation method according to the fourth
embodiment, in the same way as in FIG. 12, down control channels B
are placed in the first to third time slots t.sub.1 to t.sub.3
within a single TDMA-TDD frame formed from a plurality of time
slots. Similarly, although not shown, up control channels R are
placed in the fourth to sixth time slots t.sub.4 to t.sub.6. The
remaining time slots are used as communication channels.
[0123] Namely, in a frame structure in which these six TDMA-TDD
frames from 41 to 46 form one cycle, it is possible to allocate a
plurality of time slots in which the same type of control channels
can be placed within each TDMA-TDD frame as control channel
positions for each sector cell.
[0124] Next, the TDMA-TDD frame allocation method in the control
channel placement method according to the fourth embodiment will be
described specifically below. Firstly, assuming the TDMA-TDD frame
structure is as described above, the reception levels in each time
slot within the six TDMA-TDD frames are measured, as described
above in the first embodiment, and reception level registration
results such as those shown in FIG. 5 are acquired.
[0125] Next, the placement of the TDMA-TDD frames and the placement
of the control channels in each sector cell are decided such that
time slots registered with X marks or .DELTA. marks are avoided in
the acquired registration results.
[0126] In the example shown in FIG. 13, based on the above
reception level measurement results, the sector cell C1 may use the
first time slot t.sub.1 of the first TDMA-TDD frame 41, the second
time slot t.sub.2 of the second TDMA-TDD frame 42, and the first
time slot t.sub.1 of the fifth TDMA-TDD frame 45 as down control
channels B.
[0127] The sector cell C2 may use the third time slot t.sub.3 of
the second TDMA-TDD frame 42, the second time slot t.sub.2 of the
third TDMA-TDD frame 43, and the first time slot t.sub.1 of the
sixth TDMA-TDD frame 46 as down control channels B.
[0128] The sector cell C3 may use the first time slot t.sub.1 of
the second TDMA-TDD frame 42, the first time slot t.sub.1 of the
fourth TDMA-TDD frame 44, and the third time slot t.sub.3 of the
fifth TDMA-TDD frame 45 as down control channels B.
[0129] The sector cell C4 may use the second time slot t.sub.2 of
the first TDMA-TDD frame 41, the second time slot t.sub.2 of the
fourth TDMA-TDD frame 44, and the second time slot t.sub.2 of the
fifth TDMA-TDD frame 45 as down control channels B.
[0130] The sector cell C5 may use the first time slot t.sub.3 of
the third TDMA-TDD frame 43, the third time slot t.sub.3 of the
fourth TDMA-TDD frame 44, and the second time slot t.sub.3 of the
sixth TDMA-TDD frame 46 as down control channels B.
[0131] The sector cell C6 may use the third time slot t.sub.3 of
the first TDMA-TDD frame 41, the third time slot t.sub.3 of the
third TDMA-TDD frame 43, and the third time slot t.sub.3 of the
sixth TDMA-TDD frame 46 as down control channels B.
[0132] In other words, each sector cell may use the hatched time
slots as-down control channels B. Note that, with regard to the up
channels R, in the same way as for the down control channels B,
each sector cell may use any one of the time slots t.sub.4 to
t.sub.6 within each of the TDMA-TDD frames as a down control
channel B based on the results of the reception measurement.
[0133] Note also that, after the above TDMA-TDD frame allocation
has been completed, in a state where wireless communication has
been established, the reception levels in the control channels
actually placed in each sector cell are measured at regular
intervals, and when the results of that measurement exceed a
predetermined reception level, the TDMA-TDD frame allocation
process is performed once again. Because this processing is the
same as in the third embodiment, a detailed description thereof is
omitted here.
[0134] As explained above, in the control channel placement method
according to the fourth embodiment, a plurality of time slots
capable of receiving the same type of control channel are prepared
within a single TDMA-TDD frame, and different sector cells use the
above control channels within the same TDMA-TDD frame. Moreover,
based on the result of the measurement of the reception level in
each time slot, time slots capable of receiving control channels
that can be used by the same sector cell are fixed in advance
across a plurality of TDMA-TDD frames from among the TDMA-TDD
frames of the total number of sector cells corresponding to one
cycle. Therefore, in the TDMA-TDD frame allocation processing or in
the TDMA-TDD frame reallocation processing, even if one control
channel becomes temporarily unusable (when viewed from the terminal
station side) due to interference, other control channels that are
capable of reception can be selected and used, thereby enabling
high quality, stable communication to be provided in a wireless
communication system.
[0135] Thus, according to the present invention, when a base
station having a plurality of sector cells performs a transmission
with both up control channels and down control channels placed in
predetermined time slot positions within the TDMA-TDD frames for
each sector cell, the reception level of each time slot extending
across the TDMA-TDD frames corresponding to the number of sector
cells of the base station is measured. Next, time slots in which it
is possible for control channels to be placed are extracted from
among the time slots extending across the TDMA-TDD frames
corresponding to the number of sector cells, using the result of
the measurement of the reception level when the first sector cell
was measured, and the result of the measurement of the reception
level when the second sector cell at a position opposite the first
sector cell was measured from among all the sector cells within the
same base station. Next, the TDMA-TDD frames are allocated for each
sector cell such that those predetermined time slot positions in
which control channels are placed, from among the time slots within
the TDMA-TDD frame, are matched with the extracted time slots. As a
result, both the up control channels and the down control channels
can be placed simultaneously in optimum time slot positions.
Consequently, in a wireless communication system, the effect is
achieved that it is possible to establish stable, high quality
wireless channel communication.
[0136] According to another aspect of invention, the time slot
extraction step specifies those time slots, from among the time
slots within the TDMA-TDD frames extending for the corresponding
number of sector cells, whose reception level in a first sector
cell measured in the reception level measurement step is equal to
or greater than a predetermined threshold. The time slot extraction
step then determines that, out of the specified time slots, those
time slots whose measurement result in a second sector cell (i.e.
the sector cell located opposite the first sector cell) measured in
the reception level measurement step is equal to or greater than a
predetermined threshold are time slots in which control channels
are able to be placed in the first sector cell. As a result, it is
possible to remove time slot positions that are in an unstable
state due to interference and the like from being candidates for
the placement of an up control channel or a down control
channel.
[0137] Accordingly, it becomes possible to establish stable and
high is quality wireless channel communication.
[0138] According to still another aspect of this invention, the
time slot extraction step adds the result of measuring the
reception level in the second sector cell (i.e. the sector cell
located opposite the first sector cell) to the result of measuring
the reception level in the first sector cell measured in the
reception level measurement step, and determines the time slots in
which control channels can be placed in the first sector cell in
sequence starting from that time slot in which the value obtained
from the above addition is at the minimum. Therefore, it is
possible to allocate time slot positions giving a more stable state
of communication as the positions for placing up control channels
and down control channels. Accordingly, it becomes possible to
establish stable and high quality wireless channel
communication.
[0139] According to still another aspect of this invention, the
TDMA-TDD frames are formed by securing a plurality of time slots
positions for the respective placement of up control channels and
down control channels and by placing control channels for sector
cells that are different from each other in the plurality of time
slot positions. When the frame allocation step allocates specific
TDMA-TDD frames (namely, TDMA-TDD frames corresponding to specific
sector cells), there is a prerequisite that, within TDMA-TDD frames
extending for the number of the sector cells, time slots extracted
in the time slot extraction step coincide with one of either time
slot positions for the placement of the above control channels
within the specific TDMA-TDD frames and time slot positions for the
placement of the above control channels within other TDMA-TDD
frames different to the specific TDMA-TDD frames. Therefore, it is
possible to choose from a plurality of control channel placement
positions spread across the number of TDMA-TDD frames corresponding
to the number of sector cells in order to allocate TDMA-TDD frames
for one sector cell, and it is also possible to select other
control channels capable of reception when one control channel
becomes unusable due to interference and the like (as seen from the
terminal station side). Consequently, that it becomes possible to
establish stable and high quality wireless channel
communication.
[0140] According to still another aspect of this invention, the
TDMA-TDD frames are formed by securing a plurality of time slot
positions for the placement of up control channels and down control
channels and, based on a reception level measurement result
measured in the reception level measurement step, by placing
control channels for sector cells that are different from each
other in each of the plurality of time slot positions. When the
frame allocation step allocates specific TDMA-TDD frames (namely,
TDMA-TDD frames corresponding to specific sector cells), there is a
prerequisite that, within TDMA-TDD frames extending for the number
of the sector cells, time slots extracted in the time slot
extraction step coincide with one of either time slot positions for
the placement of the above control channels within the specific
TDMA-TDD frames and time slot positions for the placement of the
above control channels within other TDMA-TDD frames different to
the specific TDMA-TDD frames. Therefore, it is possible to choose
from a plurality of control channel placement positions spread
across the number of TDMA-TDD frames corresponding to the number of
sector cells in order to allocate TDMA-TDD frames for one sector
cell, and it is also possible to select other control channels
capable of reception when one control channel becomes unusable due
to interference and the like (as seen from the terminal station
side). Consequently, it becomes possible to establish stable and
high quality wireless channel communication.
[0141] According to still another aspect of this invention, when a
base station having a plurality of sector cells performs
transmission by placing both up control channels and down control
channels at predetermined time slot positions within the TDMA-TDD
frames in each sector cell, the reception level in each time slot
extending for the number of TDMA-TDD frames corresponding to the
number of sector cells of the base station is measured. Next, time
slots in which control channels can be placed are extracted from
among the time slots that extend across TDMA-TDD frames of the
number of sector cells using the result of the measurement of the
reception levels measured in a first sector cell and the result of
the measurement of the reception levels measured in a second sector
cell that is located opposite to the first sector cell from among
all the sector cells in the same base station. Next, with
communication having been established as a result of TDMA-TDD
frames allocated in each sector cell such that time slots extracted
in the time slot extraction step coincide with predetermined time
slot positions for the placement of control channels from among the
time slots within the TDMA-TDD frames, the reception levels of time
slots in which control channels have been placed in each sector
cell are measured at regular intervals. If the result of this
reception level measurement is equal to or greater than a
predetermined threshold, the TDMA-TDD frame allocation step is
performed once again. Because of such an arrangement, it is
possible to avoid deterioration in the quality of communication
caused by interference obstruction generated while communication is
established. Consequently, it becomes possible to establish stable
and high quality wireless channel communication.
[0142] According to still another aspect of this invention, because
the frame allocation step is performed once again to allocate the
TDMA-TDD frame to a position shifted by one time slot towards the
front or rear, by attempting to establish communication each time
by shifting the reallocation of the TDMA-TDD frames by one time
slot forwards or backwards, it is possible to omit the calculation
processing to search for time slots in which control channels are
able to be placed. At the same time, the effect is achieved that it
is possible to keep the time when communication cannot be
established to the minimum.
[0143] According to still another aspect of this invention, the
carrier to interference wave strength ratio is acquired from the
reception levels measured for the time slots in which control
channels have been placed, and TDMA-TDD frame reallocation can be
performed in accordance with this carrier to interference wave
strength ratio. Accordingly, it becomes possible to accurately
determine whether or not a control channel is receiving
interference obstruction.
[0144] According to still another aspect of this invention, the
carrier to interference wave strength ratio is acquired from the
reception levels measured when the time slots being measured in
which the control channels have been placed are in a blocked state,
and TDMA-TDD frame reallocation can be performed in accordance with
this carrier to interference wave strength ratio. Accordingly, it
becomes possible to accurately determine whether or not variations
in the reception level in a control channel are only caused by
interference obstruction.
[0145] According to still another aspect of this invention, because
it is possible to determine that a time slot being measured in
which a control channel has been placed is receiving interference
obstruction if the measured carrier to interference wave strength
ratio continues to be a predetermined threshold or greater for a
predetermined length of time, and then perform the TDMA-TDD frame
reallocation step, the effect is achieved that it is possible to
accurately determine whether variations in the reception level are
caused by collisions in the up control channels or by interference
obstruction.
[0146] According to still another aspect of this invention, because
the error detection ratio in the up control channel can be measured
and the TDMA-TDD frame allocation can be performed in accordance
with this error detection ratio, the effect is achieved that it is
possible to determine whether the usage of the time slot of a
control channel has become difficult or impossible.
INDUSTRIAL APPLICABILITY
[0147] The control channel placement method according to the
present invention is applied to the establishment of wireless
communication between a base station and a wireless terminal
station in a wireless communication system having a sector zone
structure using wireless terminal stations such as a vehicle
telephone and a portable telephone and the like.
* * * * *