U.S. patent application number 11/372762 was filed with the patent office on 2006-10-12 for method and system for station location based neighbor determination and handover probability estimation.
This patent application is currently assigned to InterDigital Technology Corporation. Invention is credited to Alan Gerald Carlton, Ulises Olvera-Hernandez.
Application Number | 20060227745 11/372762 |
Document ID | / |
Family ID | 37083062 |
Filed Date | 2006-10-12 |
United States Patent
Application |
20060227745 |
Kind Code |
A1 |
Olvera-Hernandez; Ulises ;
et al. |
October 12, 2006 |
Method and system for station location based neighbor determination
and handover probability estimation
Abstract
The present invention is a method and system for station
location based neighbor determination and handover probability
estimation. A serving access point determines the location and
movement measurements of a station (STA). Neighboring access points
report coordinates of cell fringes of the neighboring access
points. The STA sends a request for system information to the
current access point along with current location of the STA. The
current access point provides the system information, and the STA
utilizes the system information to determine best handover
candidate from among neighboring access points and an estimate of
the time for handover.
Inventors: |
Olvera-Hernandez; Ulises;
(Kirkland, CA) ; Carlton; Alan Gerald; (Mineola,
NY) |
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
19801
|
Family ID: |
37083062 |
Appl. No.: |
11/372762 |
Filed: |
March 10, 2006 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
|
|
60660963 |
Mar 11, 2005 |
|
|
|
Current U.S.
Class: |
370/331 |
Current CPC
Class: |
H04W 36/08 20130101;
H04W 64/003 20130101; H04W 4/029 20180201; H04W 36/00835 20180801;
H04W 4/02 20130101; H04W 36/32 20130101; H04W 36/36 20130101 |
Class at
Publication: |
370/331 |
International
Class: |
H04Q 7/00 20060101
H04Q007/00 |
Claims
1. A method for dynamic estimation of probability and initiation
time for handover of a station (STA) in a wireless communication
system having a serving access point (AP) and a plurality of
neighboring APs, the method comprising: determining the location of
the STA and speed and direction of movement; reporting coordinates
of cell boundaries of neighboring APs; the STA sending a request
for system information to the serving AP; the serving AP providing
the system information; and determining probability of handover to
each neighboring AP and an estimate of the initiation time for
handover based on the STA location and speed and direction of
movement and the cell boundary coordinates.
2. The method of claim 1, wherein the neighboring APs are
associated with heterogeneous networks.
3. The method of claim 1 wherein the request also provides
measurements on other APs and base stations.
4. The method of claim 3, further comprising: associating weighting
factors to each neighboring AP according to metrics related to the
probability of handover of the STA from the serving AP, whereby the
serving AP determines a neighboring AP with highest probability for
handover.
5. The method of claim 6 wherein the weighting factor is based on
received signal quality taken from neighboring APs as reported by
the STA.
6. The method of claim 1, wherein the serving AP provides network
topology information and the STA uses the information to estimate
distance to the cell boundary.
7. The method of claim 1 wherein the system information is
requested at system access by the STA.
8. The method of claim 1 wherein the STA requests update of the
system information based on the speed and direction of the STA's
movement.
9. The method of claim 8 wherein the update is triggered when a
signal quality becomes below a predetermined threshold.
10. The method of claim 1 wherein the measurements on APs and base
stations other than the serving AP and base station are performed
by other STAs within the network.
11. A method for dynamic estimation of probability and initiation
time for handover of a station (STA) in a wireless communication
network having an access controller (AC), a serving access point
(AP) and a plurality of neighboring APs, the method comprising:
determining the location and direction of movement of the STA by
the AC; reporting coordinates of cell boundaries of neighboring
APs; the STA sending a request for system information to the AC;
the serving AC providing the system information; and the STA
utilizing the system information to determine probability of
handover to each neighboring AP and an estimate of the initiation
time for handover.
12. The method of claim 11, wherein the request is made at system
access or as required by the STA.
13. The method of claim 11, further comprising the STA providing
its current location coordinates based on GPS or a network map
provided by the network, or both.
14. In a wireless network having a plurality of access points, a
mobile wireless transmit/receive unit (WTRU), comprising: a
processor configured to determine geographical coordinates of the
WTRU, determine signal strengths of signals received from
neighboring access points, and calculate a probability of handover
for at least one neighboring access point.
15. The WTRU of claim 14, wherein the processor is further
configured to calculate a time until an expected handover for at
least one neighboring access point.
16. The WTRU of claim 14, wherein the processor is further
configured to receive measurements of the WTRU's movement
determined by a serving access point according to the geographical
coordinates of the WTRU.
17. The WTRU of claim 14, wherein the processor performs multiple
determinations of geographical position of the WTRU.
18. The WTRU of claim 14, wherein the processor is further
configured to receive network topology that includes cell boundary
definitions of at least one neighboring access point.
19. The WTRU of claim 14, wherein multiple handover probabilities
are determined and ranked by the processor to permit selection of a
best handover candidate from among the neighboring access
points.
20. A wireless network having a plurality of access points (AP),
comprising: a mobile station (STA) configured to determine its
geographical coordinates and its speed and direction of movement;
an access controller configured to report network topology
information to the STA including cell boundaries of the APs; and a
serving access point (AP) configured to handover the STA to the
target AP according to an estimated handover initiation time;
wherein the STA estimates the handover initiation time based on the
network topology information and the determination of speed and
direction of movement.
Description
CROSS REFERENCE TO RELATED APPLICATION
[0001] This application claims the benefit of U.S. provisional
application No. 60/660,963, filed on Mar. 11, 2005, which is/are
incorporated by reference as if fully set forth.
FIELD OF INVENTION
[0002] The present invention is related to wireless communication
system. More particularly, the present invention is a method and
system for station location based neighbor determination and
handover probability estimation.
BACKGROUND
[0003] Integration of wireless local area networks (WLANs) with
other wireless access technologies has become increasingly
important among the various standards that develop wireless access
technologies. Handover mechanisms and procedures that might allow
seamless delivery of services across heterogeneous networks are of
prime importance towards multi-technology integration.
[0004] In particular within IEEE standards, 802.21 has been tasked
with providing mechanisms and notification that enable other
technologies to react quickly to changing conditions within the
WLAN network itself. These notifications provide details with
regard to the current state of the WLAN or access technology
supplying a service.
[0005] Link quality, (e.g., received signal strength indicator
(RSSI), bit error rate (BER), packet error rate (PER)), link
capabilities to support a particular service, (e.g., voice or high
speed data), and details on the service provider that delivers the
service are examples of the information that 802.21 can provide.
This information allows existing mobility management mechanisms to
set up alternate access resources in order to continue the delivery
of services with the same or similar level of quality as originally
requested by the user.
[0006] It is important to note that it is up to an access
technology to determine the course of action to follow, at the
upper layer, upon receipt of the notification and triggers.
However, in order to avoid the so-called "ping-pong" effect,
notifications hinting the need for a handover need to provide a
validity period and action time that will allow the upper layer
function to determine when the handover needs to occur.
[0007] In general handovers are triggered using two main criteria.
Either the current link is no longer suitable or a better candidate
has been found. Handover across heterogeneous networks entails
determining the best candidate across all available access
technologies within a region.
[0008] There are current proposals addressing selection and
discovery of handover candidates that base their criteria on
measurements and information centrally stored in the access point.
Information is stored in the form of neighboring maps that provide
a snap shot of the network and associate this information to
different zones. This poses a problem as measurements are taken
from the access point's perspective and not from the client
station's perspective. This is particularly problematic when the
base station or access points are moved frequently, as maps are not
updated that frequently.
[0009] Although measurements and settings are kept centrally at the
access point, the handover decision might be made at the client
station (STA). However, said current proposals do not take into
consideration measurements taken at the STA, even thought the STA
itself is in the best position to measure on prospective candidates
for handover.
[0010] Therefore, it would be desirable to provide a method and
system for station location based neighbor determination and
handover probability without the limitations of the prior art.
SUMMARY
[0011] The present invention provides a method and system to
estimate the best possible neighbor to handover to when the current
link is no longer suitable to provide a service. The method and
system also estimates the expected time at which a handover is to
be executed within a particular cell and determines the degree of
certainty that handover will occur towards a particular cell.
BRIEF DESCRIPTION OF THE DRAWINGS
[0012] A more detailed understanding of the invention may be had
from the following description of a preferred example, given by way
of example and to be understood in conjunction with the
accompanying drawing wherein:
[0013] FIG. 1 shows a scheme of station location-based neighbor
determination in accordance with the present invention; and
[0014] FIG. 2 shows a scheme of handover based on station location
information in accordance with the present invention.
[0015] FIG. 3 shows a signaling diagram for measurement message
exchanges between a serving access point and a station for handover
decision calculations.
DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS
[0016] Hereafter, a station (STA) includes but is not limited to a
user equipment, a wireless transmit/receive unit, a fixed or mobile
subscriber unit, a pager, or any other type of device capable of
operating in a wireless environment. When referred to hereafter, an
access point (AP) includes but is not limited to a Node-B, a site
controller, a base station or any other type of interfacing device
in a wireless environment.
[0017] FIG. 1 shows a wireless local area network (WLAN) 100 of
multiple access points AP1, AP2, AP3, AP4 and AP5, an access
controller AC, a base station BS and a target station STA1. The
handover method described herein is also applicable in
heterogeneous networks such that surrounding access points and/or
base stations in the vicinity of station STA1 belong to different
networks rather than a common network as with WLAN 100. In
accordance with the present invention, the handover decision is
preferably based on knowledge of the target station STA1 with
respect to the direction of its movement, the position of the
target station STA1 relative to its currently associated access
point AP1 and neighboring candidate access points AP2-AP5, and the
measurements reported from both the target station STA1 and by
historical measurements of other STAs that once were located at or
near the same coordinates.
[0018] A preferred method of STA location-based neighbor
determination in accordance with the present invention will be
described in reference the network 100. Initially, the mobile
station STA1 is associated with access point AP1. The direction of
motion 101 is away from the access point AP1 and at the edge of
AP1's cell. At this location, a handover of the station STA1 is
necessary so that its communication link to the network remains
unbroken. To facilitate handover, an access controller (AC) or any
other central entity controlling the APs in the WLAN assesses the
speed and direction of the station STA1's movement, determines the
locations of handover candidate APs or base stations, and
determines the probability that STA1 is approaching the boundary of
the cell, and therefore the need for handover.
[0019] The metrics regarding the speed and direction of the station
STA1's movement are preferably measured as a function of explicit
geographical location and signal strength measurements from
different APs and BSs. The metric regarding the location of the
candidate APs and BSs is measured as a function of the signal
strength received at both the target station STA1 and the current
access point AP1. The AC gathers information about the current
location of the APs within the WLAN. Each access point AP1-AP5
performs measurements of its current location for reporting to the
access controller AC or other central entity. Each access point
AP1-AP5 also performs measurements of transmitted signals and
received signals to determine its cell boundary. These measurements
may be performed with assistance from GPS. The access points
AP1-AP5 store the information for reference by other network
entities, such as the access controller AC and the target station
STA1. The location and cell boundary measurements by each access
point AP1-AP5 are preferably made on a periodic basis to account
for any repositioning of the access point that may occur either
deliberately or inadvertently. Based on the STA1 metrics and the
AP1-AP5/BS metrics, the access controller AC assists the station
STA1 in selection of the best candidate for handover from the
access point AP1.
[0020] Alternatively, the cell boundaries are determined with the
assistance of neighboring network stations. For simplicity, FIG. 1
shows only the target station STA1. However, the network may
include a plurality of additional stations located within one or
more of the cells of access points.
[0021] According to the present invention, any of these additional
stations within the network report their measurements on their
current connection, including received signal strength and link
quality (e.g., bit error rate (BER)). From these reported
measurements, the access controller AC determines an estimate of
the boundary locations of all the cells in the WLAN and the
distance from these cell boundaries and the current location of the
target station STA1 at any point within the network. In a mobile
environment where the stations and/or the access points are not
stationary, measurements may be taken as frequently as the movement
itself demands. This includes fixed periodic measurements, and
measurements triggered by the movement itself. With this assistance
of the neighboring stations, the target station STA1 now has
accurate location information available to it provided by the
access controller AC, so that the handover from access point AP1 to
the best candidate access point is improved. Furthermore, power
consumption is reduced thereby conserving battery resources. The
above measurements of access point AP1-AP5 location and their
respective cell boundaries can also be made for surrounding base
station BS and any other network entity.
[0022] The access controller AC communicates the network topology
information for WLAN 100, such as the location and the cell
boundary information of the access points AP1-AP5 and the base
station BS, to the target station STA1 and any other mobile or
stationary stations within the WLAN 100. To receive this
information, the target station STA1 requests the network topology
information from the access controller AC at system access or as
required by the station STA1. Along with the request, the station
STA1 may provide its current location coordinates. The station STA1
knows its own location based on both GPS and the network map
provided by the network. The network map provides detail on the
network topology that can help calculate its location by
triangulation, for example.
[0023] Alternatively, the serving access point AP1 may use an
estimate of the station STA1's current position using radio
propagation information. One example of a technique utilized by the
AP1 for this step involves retrieving GPS coordinates from the
station STA1 and tracking the change of these coordinates over time
to determine directional information and the position of the
station STA1. This is then used to estimate the expected time until
the station STA1 will reach the cell boundary and at which point
handover will be required to commence.
[0024] After the initial report of the WLAN topology information
provided by the access controller AC is delivered to the station
STA1, subsequent updates of the information are provided to
accommodate the mobility of the station STA1. These updates may be
periodic or upon request by the station STA1. The station STA1 may
request updates from the access controller AC as movement is
detected and signal quality thresholds are crossed. The frequency
of requests preferably depends upon the time the last topology
update was retrieved and the speed and direction of the movement.
The station might detect its own movement based on GPS and
measurements from other access points AP2-AP5 and current access
point AP1. The AC or other network entity might calculate the
station STA1's movement based on measurements coming from the
station STA1. The WLAN topology information received by the station
STA1 is based on the current position of the station STA1. As the
station STA1 moves toward the outside boundary of the current cell,
the station STA1 uses the topology information and knowledge of
current movement, including its speed and direction of its movement
to determine the probability to reach the boundary of a particular
candidate cell. The probability refers to how quickly the station
STA1 will reach the cell boundary for AP1 within a predefined time.
For example, it may be estimated that the station STA1 will reach
the cell boundary within 5 ms with 95% probability. This is
calculated by sampling speed and direction at specific intervals
along with knowledge of the distance to the cell boundary.
[0025] FIG. 2 shows a WLAN 200 comprising several access points
AP1, AP2, AP3, AP4 and AP5 with a mobile station STA1. In this
second embodiment, handover of the station STA1 is achieved without
the assistance of an access controller. The following example
describes a location based handover according to this second
embodiment. Initially, the station STA1 is associated with the
access point AP1. The geographical position of the station STA1
along with its direction of movement is determined by the access
point AP1 similar techniques as described above in reference to
FIG. 1.
[0026] Concurrently, the neighboring access points AP2, AP3, AP4
and AP5 report their estimates of the location of their cell
boundaries. Under this embodiment, the station STA1 is able to
detect and receive signals from the neighboring access points
AP2-AP5 as the station STA1 approaches their cell boundaries. The
station STA1 is able to read this information during so called
silent periods. Thus, the station STA1 obtains the cell boundary
information directly from the network without the central access
controller intervention. In addition, the access point AP1 can read
signals of the other access points AP2-AP5. While the access points
AP1-AP5 may not necessarily report to each other, the access point
AP1 can report to the station STA1 the cell boundary information
read by AP1 from the access points AP2-AP5. Preferably, four
geographical coordinates of each cell are provided (e.g., S, N, E,
W coordinates). Alternatively, additional coordinates may be
provided for better resolution of the cell boundary. If the access
points AP1-AP5 are mobile, the cell fringe estimates are updated on
a periodic basis. The access points AP1-AP5 may use the assistance
of GPS technology for deriving the coordinates.
[0027] The station STA1 requests WLAN 200 topology information from
the serving access point AP1 either at system access or as needed.
The request provides the station STA1's current location
coordinates and measurements of signal strength received from the
serving access point AP1. Optionally, the request provides
measurements made with respect to received signals from the access
points AP2-AP5, and/or measurements made by the access points
AP1-AP5 of each other.
[0028] The initially serving access point AP1 determines
probability for the station STA1 to reach the cell boundaries of
the neighboring access points AP2-AP5 for handover, using metrics
as described above in reference to FIG. 1. The serving access point
AP1 associates a weighting factor for the accessible neighboring
access points AP2-AP5 so that the station STA1 can select the best
handover candidate. Based on the weighting factor, the handover
candidates AP2-AP5 are ranked in order of probability of a
successful handover with unbroken communication link.
Alternatively, the access point AP1 provides WLAN 200 network
topology information, such as access point location and quality of
signal, and the STA1 performs the calculation necessary to estimate
the distances to the cell boundaries of the neighboring access
points AP2-AP5 and thereby select the best candidate for
handover.
[0029] In addition to determining the best handover candidate, the
time when a handover is likely to happen is also calculated by
either the access point AP1 or the station STA1. This handover
timing is estimated according to the proximity of the cell boundary
of each handover candidate access point as well as the direction
and speed of the mobile station STA1 as it approaches each
respective handover candidate cell boundary.
[0030] As the serving access point AP1 determines the weighting
factors, each factor may be adjusted and tuned based on the
received signal quality of neighboring access points AP2-AP5 as
measured by the station STA1 and as reported by the station STA1 to
the access point AP1. The station STA1 may take these measurements
of the access points AP2-AP5 during periods when the station STA1
is not transmitting to the access point AP1.
[0031] FIG. 3 shows a signaling diagram for measurement message
exchanges between station STA1 and serving access point AP1.
Station STA1 comprises a measurement processor 301 and access point
AP1 comprises a measurement processor 302 for making the following
measurements used for the handover probability and estimated time
until the expected handover. Upon determining its geographical
location coordinates using GPS, station STA1 reports the
coordinates to the access point AP1 at successive messages
303.sub.1 to 303n. Based on the multiple location coordinates, the
serving access point AP1 tracks the motion of the station STA1 to
calculate the speed and direction of the station STA1 and reports
the results back to the station STA1 at message 304. Station STA1
also reports received signal strength measurements with respect to
neighboring access points AP2-AP5 at message 305. The station STA1
requests the network topology information at message 306, which
includes cell boundary definitions for the neighboring access
points AP2-AP5. Based on the received signal strength measurements,
the serving access point AP1 updates the network topology
information, and reports it back to the station STA1 at message
307. Using the network topology information and the movement
measurements received by the station STA1, calculation 308 is
performed to determine the probability for handover to any
particular neighboring access point AP2-AP5 and the estimated time
that such a handover will occur.
[0032] The present invention is applicable to any wireless
communication system including, but not limited to, IEEE 802
technologies, cellular standards such as 3GPP or 3GPP2, and other
standardized or proprietary wireless technologies similar to IEEE
802 WLANs, including 802.15 Bluethooth, HIPERLAN/2, etc. More
particularly, the applicable IEEE 802 technologies include:
[0033] WLAN baseline air interface standards: [0034] 802.11
baseline [0035] 802.11a OFDM 5 GHz WLAN [0036] 802.11b HR-DSSS 2.4
GHz WLAN [0037] 802.11g OFDM 2.4 GHz WLAN [0038] 802.11j OFDM 10
MHz option WLAN [0039] 802.11n High-Throughput WLAN
[0040] WLAN standards supplements to extend operation for
particular scenarios: [0041] 802.11e QoS extensions (including WMM
and WMM/2 brands) [0042] 802.11s ESS Mesh [0043] 802.11k Radio
Resource Measurement [0044] 802.11v Wireless Network Management
[0045] 802.21 Media Independent Handover
[0046] The present invention may be implemented in any type of
wireless communication system, as desired. By way of example, the
present invention may be implemented in any type of 802 type
system, including but not limited to 802.11, 802.16 and 802.21 or
any other type of wireless communication system. The present
invention may also be implemented as software, middleware or
application-based. The invention is applicable to the data link
layer, the network layer and transport layer of a wireless
communication system or device.
[0047] Although the features and elements of the present invention
are described in the preferred embodiments in particular
combinations, each feature or element can be used alone without the
other features and elements of the preferred embodiments or in
various combinations with or without other features and elements of
the present invention.
* * * * *