U.S. patent application number 11/417638 was filed with the patent office on 2006-09-07 for wireless communication method and system for minimizing interference by determining mobile station zone locations and potential conflicts between cell zones.
This patent application is currently assigned to InterDigital Technology Corporation. Invention is credited to Paul Marinier, Vincent Roy.
Application Number | 20060198339 11/417638 |
Document ID | / |
Family ID | 31720289 |
Filed Date | 2006-09-07 |
United States Patent
Application |
20060198339 |
Kind Code |
A1 |
Marinier; Paul ; et
al. |
September 7, 2006 |
Wireless communication method and system for minimizing
interference by determining mobile station zone locations and
potential conflicts between cell zones
Abstract
A wireless communication method and system for minimizing severe
interference to one or more wireless transmit/receive unit (WTRUs).
A plurality of non-overlapping zones (e.g., an inner zone and an
outer zone) is defined for each cell of a wireless multi-cell
communication system, such as a time division duplex (TDD) system.
Information regarding potential conflicts between the
non-overlapping cell zones is obtained. Each potential conflict
indicates that there is a high likelihood of one of the WTRUs using
a specific time slot for transmitting in a zone of one cell causing
severe interference to a second one of the WTRUs using the specific
time slot for receiving in a zone of another (i.e., a different)
cell. Time slot usage (e.g., uplink, downlink or none) is then
determined on a zone-by-zone basis using the obtained information
to minimize interference caused by a conflicting uplink assignment
or a conflicting downlink assignment.
Inventors: |
Marinier; Paul; (Brossard,
CA) ; Roy; Vincent; (Montreal, CA) |
Correspondence
Address: |
VOLPE AND KOENIG, P.C.;DEPT. ICC
UNITED PLAZA, SUITE 1600
30 SOUTH 17TH STREET
PHILADELPHIA
PA
19103
US
|
Assignee: |
InterDigital Technology
Corporation
Wilmington
DE
|
Family ID: |
31720289 |
Appl. No.: |
11/417638 |
Filed: |
May 4, 2006 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
|
|
10334459 |
Dec 30, 2002 |
7046655 |
|
|
11417638 |
May 4, 2006 |
|
|
|
60403685 |
Aug 15, 2002 |
|
|
|
Current U.S.
Class: |
370/329 ;
370/347; 455/452.1 |
Current CPC
Class: |
H04W 16/00 20130101;
H04W 16/12 20130101; H04W 16/02 20130101; H04W 28/16 20130101; H04W
92/20 20130101; H04W 16/28 20130101; H04W 16/14 20130101; H04W
24/00 20130101 |
Class at
Publication: |
370/329 ;
370/347; 455/452.1 |
International
Class: |
H04Q 7/00 20060101
H04Q007/00; H04B 7/212 20060101 H04B007/212; H04Q 7/20 20060101
H04Q007/20 |
Claims
1. A method of minimizing interference in a wireless multi-cell
communication system that serves a plurality of wireless
transmit/receive units (WTRUs), the method comprising: (a) defining
a plurality of non-overlapping zones in a conflicting zone lookup
table for each cell in the system; and (b) determining time slot
usage on a zone-by-zone basis using information obtained from the
conflicting zone lookup table to minimize interference caused by a
conflicting uplink assignment or a conflicting downlink
assignment.
2. The method of claim 1 wherein the information obtained from the
conflicting zone lookup table indicates potential conflicts between
the non-overlapping cell zones, the conflicting zone lookup table
specifically identifies each of the zones of the cells, and each
potential conflict indicates that there is a high likelihood of one
of the WTRUs using a specific time slot for transmitting in one of
the specifically identified zones of one cell causing severe
interference to a second one of the WTRUs using the specific time
slot for receiving in one of the specifically identified zones of
another cell.
3. The method of claim 2 wherein the wireless multi-cell
communication system assigns channels to the WTRUs.
4. The method of claim 2 wherein a current zone location is
determined for each of the WTRUs based on a signal delay
measurement.
5. The method of claim 4 wherein the determination of the current
zone location for each of the WTRUs is further based on a received
power measurement.
6. The method of claim 4 wherein the current zone location
identifies a specific cell of the system in which the WTRU is
located.
7. The method of claim 4 wherein the non-overlapping zones include
an inner zone and an outer zone, and the current zone location
indicates whether the WTRU is located in the inner zone or the
outer zone.
8. The method of claim 2 wherein the wireless multi-cell
communication system is a time division duplex (TDD) system.
9. The method of claim 2 wherein the specific time slot can be
simultaneously used by WTRUs in zones of different cells if the
conflicting zone lookup table indicates that the different cell
zones do not conflict.
10. The method of claim 2 wherein a slow dynamic channel allocation
(SDCA) process is used to determine the best use of each time slot
in each cell zone based on constraints imposed by conflicting zones
indicated by the conflicting zone lookup table, and average traffic
characteristics.
11. A wireless multi-cell communication system comprising: (a) a
plurality of wireless transmit/receive units (WTRUs); (b) a
plurality of cells, each of the cells having a plurality of
non-overlapping zones; (c) means for defining a plurality of
non-overlapping zones in a conflicting zone lookup table for of the
cells in the system; and (d) means for determining time slot usage
on a zone-by-zone basis using information obtained from the
conflicting zone lookup table to minimize interference caused by a
conflicting uplink assignment or a conflicting downlink
assignment.
12. The system of claim 11 wherein the information obtained from
the conflicting zone lookup table indicates potential conflicts
between the non-overlapping cell zones, the conflicting zone lookup
table specifically identifies each of the zones of the cells, and
each potential conflict indicates that there is a high likelihood
of one of the WTRUs using a specific time slot for transmitting in
one of the specifically identified zones of one cell causing severe
interference to a second one of the WTRUs using the specific time
slot for receiving in one of the specifically identified zones of
another cell.
13. The system of claim 12 wherein the wireless multi-cell
communication system assigns channels to the WTRUs.
14. The system of claim 12 wherein a current zone location is
determined for each of the WTRUs based on a signal delay
measurement.
15. The system of claim 14 wherein the determination of the current
zone location for each of the WTRUs is further based on a received
power measurement.
16. The system of claim 14 wherein the current zone location
identifies a specific cell of the system in which the WTRU is
located.
17. The system of claim 14 wherein the non-overlapping zones
include an inner zone and an outer zone, and the current zone
location indicates whether the WTRU is located in the inner zone or
the outer zone.
18. The system of claim 12 wherein the wireless multi-cell
communication system is a time division duplex (TDD) system.
19. The system of claim 12 wherein the specific time slot can be
simultaneously used by WTRUs in zones of different cells if the
conflicting zone lookup table indicates that the different cell
zones do not conflict.
20. The system of claim 12 wherein a slow dynamic channel
allocation (SDCA) process is used to determine the best use of each
time slot in each cell zone based on constraints imposed by
conflicting zones indicated by the conflicting zone lookup table,
and average traffic characteristics.
Description
CROSS REFERENCE TO RELATED APPLICATIONS
[0001] This application is a continuation of U.S. patent
application Ser. No. 10/334,459 filed Dec. 30,2002, which issued as
U.S. Pat. No. 7,046,655 on May 16, 2006, which in turn claims
priority from Provisional application No. 60/403,685 filed Aug. 15,
2002, which are incorporated by reference as if fully set
forth.
BACKGROUND OF THE INVENTION
[0002] The present invention relates generally to radio
communication systems using the time division duplexing (TDD) mode.
More particularly, the invention relates to assignment of slots and
slot-to-cell assignments in radio communication systems using
TDD.
[0003] Cellular systems generally divide the time axis into
intervals of equal durations called frames. Cellular systems
employing the TDD scheme divide frames into a finite number of
intervals of equal duration, called slots, and allow a cell to use
some or all of the slots for uplink transmissions (mobile-to-base)
or downlink (base-to-mobile) transmissions. The slot assignment of
a cell defines how each slot is used by this cell. There are three
possible ways for a cell to use a slot: 1) uplink transmissions; 2)
downlink transmissions; or 3) the slot is not used.
[0004] The slot assignment of a cell can be varied by the system in
order to adapt to the long-term variations of the traffic load. For
example, the system may modify the assignment of one slot from
uplink to downlink if the intensity of downlink traffic increases
while the uplink traffic decreases. In addition, different cells of
a system do not generally need to have the same slot assignment.
Accordingly, if traffic characteristics in one geographical area
are different from another area, the cells covering those areas may
have different slot assignments to best adapt to local traffic
conditions.
[0005] In the prior art, a simple approach in order to avoid
base-to-base and mobile-to-mobile interference is to use the same
slot assignment for all cells in the same geographic area; only
allowing different assignments between cells that are clearly
isolated from each other. The obvious disadvantage of this approach
is when cells are deployed in a way to provide continuous coverage,
as is often the case, it is difficult to segregate one subset of
cells from another, unless the use of certain slots are completely
disallowed in some cells. This ultimately results in a capacity
loss of the system.
[0006] Therefore, both mobile-to-mobile and base-to-base
interference restrict the use of independent slot assignments
between cells in the same geographical area. What is needed is a
system which avoids the loss of capacity as the traffic asymmetry
metric varies over a coverage area.
SUMMARY
[0007] The present invention is related to a wireless communication
method and system for minimizing severe interference to one or more
wireless transmit/receive unit (WTRUs). A plurality of
non-overlapping zones are defined for each cell of a wireless
multi-cell communication system, such as a TDD system. Information
regarding potential conflicts between the non-overlapping cell
zones is obtained. Each potential conflict indicates that there is
a high likelihood of one of the WTRUs using a specific time slot
for transmitting in a zone of one cell causing severe interference
to a second one of the WTRUs using the specific time slot for
receiving in a zone of another (i.e., a different) cell. Time slot
usage (e.g., uplink, downlink or none) is then determined on a
zone-by-zone basis using the obtained information to minimize
interference caused by a conflicting uplink assignment or a
conflicting downlink assignment. In one embodiment, the location of
a WTRU is tracked by determining whether the WTRU is located in an
inner or outer zone of a particular cell.
BRIEF DESCRIPTION OF THE DRAWINGS
[0008] A more detailed understanding of the invention may be had
from the following description of a preferred embodiment, given by
way of example and to be understood in conjunction with the
accompanying drawings wherein:
[0009] FIG. 1 shows an example of division of cells into zones;
[0010] FIG. 2 illustrates how a cell can be segmented into two
simple zones;
[0011] FIG. 3 is a look-up table showing zones that are conflicting
with each other; and
[0012] FIG. 4 illustrates the border of two neighboring cells and a
WTRU in close proximity to the border.
DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS
[0013] The present invention will be described with reference to
the drawing figures wherein like numerals represent like elements
throughout.
[0014] Although the present invention will be described with
reference to the example shown in FIG. 1 for the case of a
hexagonal deployment, the present invention is readily extensible
to other types of deployment. It is up to the system administrators
and operators to define the zones according to their specific
situations. The cell division shown in FIG. 1 is only an example of
one type of cell division, it should be noted that in an actual
deployment that there are other ways to divide cells into
zones.
[0015] As illustrated in FIG. 1, cell A 100 has been divided into
six (6) zones A1-A6 and likewise, the neighboring cells B 110 and C
120 have each been divided into six (6) zones B1-B6 and C1-C6,
respectively. Several WTRUs 130, 140 are also randomly located. For
the first embodiment it is assumed that the WTRUs are not equipped
with adaptive antennas. The first WTRU 130 is located in sector A6
and a second WTRU 140 is located in sector B3.
[0016] The method and system of the present invention permit two
neighboring cells to use conflicting slot assignments (i.e., a slot
is used for the uplink in one cell but for the downlink in the
neighboring cell) based on the concept of "zones." A zone is a
subdivision of a cell defined by the system operator. Cells are
divided in a certain number of (non-overlapping) zones. Two zones
(belonging to different cells) are defined to be conflicting with
each other if there is a high likelihood that a first WTRU
transmitting in one zone would cause severe interference to a
second WTRU receiving in the second zone. The determination of
whether two zones are conflicting with each other can be made by
analysis of the cell layout. Alternatively, one could envision a
more sophisticated scheme based on collecting measurements made by
WTRUs.
[0017] Although the method of determining conflicting zones will be
described with reference to FIG. 3, this embodiment is illustrative
and should not be construed as the only procedure for determining
such conflicting zones. The conflicting zone look-up table in FIG.
3 facilitates determining the conflicting zones. For example, the
first WTRU 130 is located in zone A6. Zone A6 is first located in
the vertical column of the conflict lookup table. An X in its
corresponding row indicates that zone B3 is a potential conflicting
zone and a "" in the table indicates an entry corresponding to a
zone of the same cell (e.g. A1, A2, A3, A4 and A5). Because a slot
cannot be used for both uplink and downlink simultaneously in the
same cell, zones belonging to the same cell cannot be allowed to
have opposing directions. This means that the transmissions from a
first WTRU 130 in zone A6 have the potential to cause interference
to a second WTRU 140 in zone B3. Interference would occur if both
WTRUs 130 and 140 use the same slot for transmission and reception,
respectively.
[0018] Having determined the conflicting zones, the physical
resources can now be used more flexibly and efficiently. For
example, suppose that it is desired that a given slot (S) be used
for the uplink in cell A and in the downlink in cell B. Such a
situation can happen if, for example, cell B has more downlink
traffic than cell A. Without using the concept of zones, it would
be difficult for cell A and cell B to use slot S in different
directions. This is because a WTRU of cell A transmitting in slot S
near the border of cell B would create too much interference to
WTRUs receiving in slot S in cell B. However, when the concept of
zones is used this problem is surmounted in the following way. Slot
S can be used by some WTRUs of both cells A and B, provided that it
is not used simultaneously by two WTRUs in conflicting zones. For
example, suppose that zone A3 on FIG. 1 is not conflicting with any
zone of cell B. Suppose also that slot S is used for the uplink in
cell A and for the downlink in cell B. A WTRU in zone A3 would be
allowed to use slot S (for the uplink) even though that slot may be
used for the downlink in cell B. This represents a flexibility
advantage and ultimately a capacity advantage, since otherwise slot
S would have been completely unavailable for uplink in cell A.
[0019] In the description of FIG. 3, it is assumed that the
location of the WTRU is known. However, as the WTRU traverses
through the coverage area, the system must keep track of the zone
it is currently located in. For example, if the WTRU starts out in
zone C3 and travels across the coverage area to zone B4, the WTRU
would travel through zones C3, C4, B3 and B4 on its route. Based on
the knowledge of the lookup table showing conflicting zones, the
system can determine the best use of each slot (uplink, downlink or
none) in every zone, taking into account the constraints imposed by
the conflicts between zones and the average traffic
characteristics. This is known as the slow dynamic channel
allocation (SDCA) process, which occurs over a relatively large
time scale (hours, days, weeks or months). The SDCA uses the
conflict zone lookup table in this determination of slot usage.
[0020] Given the framework of slot usage defined by the SDCA
process, when a WTRU connects to the system it is allocated at
least one downlink channel in one or more of the slots which can be
used for the downlink in the zone where it is located. It is also
allocated at least one uplink channel in one or more of the slots
that can be used for the uplink. Whenever a WTRU moves to another
zone, the system checks if the channel allocation needs to be
changed, a problem situation which can occur if the slot(s) used by
this WTRU are no longer allowed in a given direction in the new
zone it is moving in. This can be achieved by the fast dynamic
channel allocation (FDCA) process. Therefore, there is no need to
re-consult the conflicting zone lookup table every time a WTRU
moves to a new zone. Rather, each zone is associated with a
possible usage, uplink, downlink or none for every slot which is
determined by the slow DCA based on the lookup table. The system
uses this slot usage information when it is time to assign channels
to a WTRU. The system then makes the appropriate non-interfering
uplink and downlink slot assignments.
[0021] Another advantage of the present invention is the use of
slot allocation to affect load balancing. Referring to FIG. 4, if
the traffic loads in each of two adjoining cells 300, 310 have
differing asymmetry use characteristics, (that is, the first cell
300 is mostly downlink traffic, while the second cell 310 is mostly
uplink traffic), without conflict zone management, the
transmissions of WTRU 32 would cause interference with the first
cell 310, if the cells are using the same slot for different
directions. A fast allocation protocol could attempt employ an
escape mechanism, which is a means for a channel allocation of a
specific WTRU to be changed due to excessive interference as
detected by a particular WTRU using this channel. The use of escape
mechanisms is unsatisfactory since it would result in effectively
forbidding the use of that slot in significant parts of the cell.
Conflict zoning allows a mobile to traverse through a coverage
area, (including zones), while having negligible impact on the
cell's usage characteristics.
[0022] In an alternative embodiment in accordance with the present
invention, a simple zone division scheme is used. A cell is
separated into two zones as shown in FIG. 2. The first zone is the
outer zone 21, which is defined by the coverage area 24 of the
cell, (exclusive of the inner zone 22); and the second zone is the
inner zone 22. The serving base station 20 is located at the center
of the inner zone 22.
[0023] In one embodiment, the system determines both the cell in
which the WTRU 28 is located and whether the WTRU is in an inner
zone 22 or the outer zone 21. First measurements of the signal
delay and the received signal power are performed. This allows the
determination of whether WTRU 28 is located in the inner or outer
zone. The example in FIG. 2, illustrates a WTRU 28 located in the
inner zone 22, and the WTRU 26 is located in the outer zone 21.
[0024] Another method to determine location utilizes additional
nearby base stations or other WTRU. However, because a WTRU
requires constant tracking as it traverses the coverage area, there
is a need for continuous system participation and coordination of
the other base stations and WTRUs causing this to be a very
resource intensive technique
[0025] An additional technique to determine the location utilizes
global positioning satellites of a Global Positioning System (GPS).
A GPS receiver is put in each WTRU to identify the location of the
WTRU. The coordinates are reported by the WTRU to the base station
and the system uses the coordinates as aforementioned. However,
there are some disadvantages with this option. The first is the
need for the WTRU to be in a favorable position allowing it to
properly receive the satellite signals (outdoors). Also, since the
measurement is performed by the WTRU, the WTRU needs to constantly
transmit location information to the system, which increases the
signaling burden over the air interface and utilizes precious
battery resources.
[0026] The consistency and accuracy of zone location is improved
with adaptive antennas. The location measurements including
inclination angle and signal level readings are employed to
determine the position the WTRU. An advantage to adaptive antennas
is that a position is obtained without the need of measurements
from any other base stations or WTRUs. Therefore, adaptive antennas
provide an efficient and independent means for tracking WTRUs.
[0027] The cellular system has two types of interference to cope
with, the first is base-to-base interference and can occur when a
first base station's downlink is another base station's uplink and
the uplink base station receives the other base station's downlink,
which impedes or degrades the intended uplink signal. The second
type of interference that can occur in the cellular system
(mobile-to-mobile interference) occurs when a first mobile's
reception is impeded or degraded by another mobile's
transmission.
[0028] Adaptive antennas can be placed at the base station, on the
WTRUs or at both the base station and the WTRUs. The performance of
the zone division scheme depends on basically two factors: 1) the
effectiveness (i.e. reliability and convenience) with which one can
determine the zone where a user is situated and the ability to
track a moving user; 2) the size of the neighborhoods of the zones.
The neighborhood of zone Z is defined as the set of other zones in
other cells that conflict with zone Z. This means that the entries
corresponding to Z and any zone belonging to the neighborhood of Z
should be checked in the lookup table of FIG. 3.
[0029] The use of zones for slot allocation requires locating the
WTRU with reasonable certainty. As the accuracy of the WTRU
positioning is increased, it is possible to define a larger number
of zones per cell, resulting in higher flexibility and increased
efficiency of the system.
[0030] A small neighborhood is one defined as one with a small
number of zones. This means that for every zone, there is a
relatively small number of other zones that are conflicting with
it. Thus, there are fewer constraints, which allow for more
flexibility in the determination of the slot usage for every zone.
This allows for the greatest flexibility in terms of assigning
different proportions of uplink versus downlink traffic for
different cells. Therefore, any scheme that tends to restrict the
size of neighborhoods results in a gain of flexibility.
[0031] When two zones are conflicting with each other, this may be
due to one or both of the following: [0032] a) The probability of
base-to-base interference is high if the zones are using slots in
opposite directions [0033] b) The probability of mobile-to-mobile
interference is high if the zones are using slots in opposite
directions. When the mobiles are also equipped with adaptive
antennas there will be less pairs of zones that are conflicting
with each other because the overall probability of mobile-to-mobile
interference is decreased. This is because mobiles equipped
adaptive antennas tend to transmit and receive energy from specific
directions, which reduces the probability that one mobile
interferes with another. To say that there will be less pairs of
zones that are conflicting with each other is the same as saying
that the sizes of the neighborhoods of every zone is reduced under
the definition of neighborhood of a zone.
[0034] The size of neighborhood depends of several factors, such as
the specific geography of deployment or the propagation conditions
present. If the mobiles are equipped with adaptive antennas, the
neighborhood of a zone may be restricted to fewer zones. In the
best case scenarios, a neighborhood can even be limited to only one
or two zones, if the deployment is such that there are few
scatterers around the mobiles. This is because the probability of
mobile-to-mobile interference diminishes when mobiles are
transmitting using narrow beams. Similarly, when adaptive antennas
are used at the base station, the sizes of the neighborhoods should
be reduced for the same reason.
[0035] In another embodiment the transmission power of mobiles is
taken into account. The transmitted power is regarded as an
important factor and influences the size of a neighborhood. If a
zone Z is geographically defined as being close to the base
station, any WTRUs transmitting in that zone will tend to transmit
at a lower power level since they are closer to the base station
and will need less power than other WTRUs that are further away.
They per se generate less interference than other WTRUs and are
less susceptible to the affect of other mobiles that would be
receiving in the same timeslot. As a result, the number of zones
that are conflicting with this zone Z tends to be smaller. In other
words, a neighborhood of a zone close to the serving base station
will normally be smaller than the neighborhood of a zone closer to
the edge of the cell.
[0036] As stated above, mobiles normally transmit at a lower power
when they are closer to their serving base station. But, with the
advent of new high data rate cellular technology, an increase in
power is often necessary to facilitate high data rate exchange at
both the mobile and the base station. To accommodate the higher
data rates, the WTRUs and the serving base station will have to
increase transmission power, even if the WTRU is close to the
serving base station. Therefore, it may be necessary to define
service-dependent neighborhoods.
[0037] For example, it may be possible that two zones which are not
conflicting with each other if the WTRUs are using the voice
service or a low-rate service, but they would be conflicting if the
WTRUs were using a high-rate service, for example a data rate of
384 kbps. In that case, a WTRU could be allowed to use a certain
slot in a given zone only if it is using a low-rate service.
[0038] Although the present invention has been described in detail,
it is to be understood that the invention is not limited thereto,
and that various changes can be made therein without departing from
the spirit and scope of the invention, which is defined by the
attached claims.
* * * * *