U.S. patent application number 13/084913 was filed with the patent office on 2011-08-04 for handover in a wireless local area network (wlan).
This patent application is currently assigned to INTERDIGITAL TECHNOLOGY CORPORATION. Invention is credited to Maged Zaki.
Application Number | 20110188477 13/084913 |
Document ID | / |
Family ID | 34681628 |
Filed Date | 2011-08-04 |
United States Patent
Application |
20110188477 |
Kind Code |
A1 |
Zaki; Maged |
August 4, 2011 |
HANDOVER IN A WIRELESS LOCAL AREA NETWORK (WLAN)
Abstract
In triggering a handoff by a wireless transmit/receive unit
(WTRU) from a current basic service set (BSS) in a wireless local
area network (WLAN), the following are performed. A highest class
of traffic service and quality of service (QoS) is determined for
the highest class from a basic service set (BSS) beacon. Handoff is
terminated and communication is retained with a current BSS when
the signal to noise ratio (SNR) or received signal strength (RSS)
is greater than a high threshold of the highest class. Other
criteria are evaluated to determine whether a handoff is desired
when the SNR or RSS is less than the high threshold.
Inventors: |
Zaki; Maged; (San Diego,
CA) |
Assignee: |
INTERDIGITAL TECHNOLOGY
CORPORATION
Wilmington
DE
|
Family ID: |
34681628 |
Appl. No.: |
13/084913 |
Filed: |
April 12, 2011 |
Related U.S. Patent Documents
|
|
|
|
|
|
Application
Number |
Filing Date |
Patent Number |
|
|
11652271 |
Jan 11, 2007 |
7925263 |
|
|
13084913 |
|
|
|
|
10931112 |
Aug 31, 2004 |
7164915 |
|
|
11652271 |
|
|
|
|
60531513 |
Dec 19, 2003 |
|
|
|
Current U.S.
Class: |
370/333 ;
370/332 |
Current CPC
Class: |
H04W 36/26 20130101 |
Class at
Publication: |
370/333 ;
370/332 |
International
Class: |
H04W 36/30 20090101
H04W036/30 |
Claims
1. A method of performing handover in wireless communications
operating in a plurality of basic service sets (BSSs), the method
comprising: establishing a high and a low threshold for at least
one of either a signal to noise ratio (SNR) or a received signal
strength (RSS) for a BSS currently serving a wireless
transmit/receive unit (WTRU); initiating a handover procedure when
the at least one of SNR or RSS of the serving BSS falls below the
high threshold; determining a quality of service (QoS) index for
each of a plurality of neighboring BSSs and the serving BSS;
establishing a link between the WTRU and a BSS associated with a
largest of the determined QoS indexes.
2. The method of claim 1, further comprising: establishing a link
between the WTRU and a different BSS than the serving BSS when the
at least one of SNR and RSS of the serving BSS falls below the low
threshold without determining the QoS index for each neighboring
BSS.
3. The method of claim 1, further comprising: halting the handover
procedure when the at least one of SNR and RSS increases to greater
than the high threshold and maintaining a link between the WTRU and
the serving BSS.
4. The method of claim 1, wherein determining a QoS index for each
of a plurality of neighboring BSSs further comprises: determining a
list of neighboring BSSs; for each of a plurality of traffic
classes (TCs), establishing a list of QoS parameters; for each QoS
parameter for each TC, defining a high QoS threshold and a low QoS
threshold; for each TC for each neighboring BSS: initializing a QoS
index to zero; determining a value of each QoS parameter; comparing
the value of each QoS parameter to the high QoS threshold and low
QoS threshold associated with the QoS parameter; and incrementing
the QoS index by one when the value of the QoS parameter is higher
than the high QoS threshold, do not change the QoS index when the
value of QoS parameter is between the high QoS threshold and the
low QoS threshold, and subtracting (n-1) from the QoS index when
the value of the QoS parameter is less than the low QoS threshold,
where n is the number of QoS parameters in the list.
5. The method of claim 4, wherein the list of QoS parameters
includes SNR.
6. The method of claim 5, wherein the list of QoS parameters
includes at least one of frame error rate, data rate, and delay
bounds.
7. The method of claim 4, wherein the value of the QoS parameter is
read from a BSS service load element.
Description
CROSS REFERENCE TO RELATED APPLICATION(S)
[0001] This application is a continuation of U.S. patent
application Ser. No. 11/652,271, filed on Jan. 11, 2007, which is a
continuation of U.S. patent application Ser. No. 10/931,112, filed
on Aug. 31, 2004, which issued as U.S. Pat. No. 7,164,915 on Jan.
16, 2007, which claims the benefit of U.S. provisional application
No. 60/531,513, filed Dec. 19, 2003, all of which are incorporated
herein by reference as if fully set forth.
FIELD OF INVENTION
[0002] The present invention relates wireless communication
systems. In particular, the invention relates to handover in such
systems.
BACKGROUND
[0003] FIG. 1 is a simplified illustration of a wireless
transmit/receive unit (WTRU) 14.sub.1N potentially handing over
between two basic service sets (BSSs), BSS.sub.1 12.sub.1 and
BSS.sub.2 12.sub.2, in a wireless local area network (WLAN).
Originally, BSS.sub.1 12.sub.1 has an access point (AP) 10.sub.1
and a plurality of WTRUs 14.sub.11 to 14.sub.1N and BSS.sub.2
12.sub.2 has an access point (AP) 10.sub.2 and a plurality of WTRUs
14.sub.21 to 14.sub.23. The WTRU 14.sub.1N is in wireless
communication with AP 10.sub.1. As illustrated in FIG. 1, both APs
10.sub.2, 10.sub.1 are connected to a distribution system 16. To
decide whether to handover between BSSs 12, such as BSS.sub.1
12.sub.1 and BSS.sub.2 12.sub.2, the WTRU 14.sub.1N measures the
received signal strength (RSS) or signal to noise ratio (SNR) for
each BSS 12.sub.1, 12.sub.2. The BSS 12 having the better RSS or
SNR is selected for further communication. If BSS.sub.1 12.sub.1 is
selected, the current communication links are maintained, as
illustrated as a solid line. If BSS.sub.2 12.sub.2 is selected, a
new link is established with BSS.sub.2, as illustrated as a dashed
line.
[0004] Although this approach most likely provides the WTRU
14.sub.1N with the strongest link, other criteria may make such a
connection undesirable. To illustrate, the BSS having the strongest
link may be overloaded and can not meet some quality of service
(QoS) requirements of the WTRU 14.sub.1N. Accordingly, it is
desirable to have alternate handover schemes.
SUMMARY
[0005] In triggering a handoff by a wireless transmit/receive unit
(WTRU) from a current basic service set (BSS) in a wireless local
area network (WLAN), the following are performed. A highest class
of traffic service and quality of service (QoS) is determined for
the highest class from a basic service set (BSS) beacon. Handoff is
terminated and communication is retained with a current BSS when
the signal to noise ratio (SNR) or received signal strength (RSS)
is greater than a high threshold of the highest class. Other
criteria is evaluated to determine whether a handoff is desired
when the SNR or RSS is less than the high threshold.
BRIEF DESCRIPTION OF THE DRAWING(S)
[0006] The present invention will be understood from consideration
of the accompanying figures, wherein like elements are designated
by like numerals, and wherein:
[0007] FIG. 1 is an illustration of a WTRU in potential
handover.
[0008] FIG. 2 is a flow chart of an embodiment of a RSS/SNR and
other system statistic handover algorithm.
[0009] FIG. 3 is a simplified diagram of an embodiment of a WTRU
capable of RSS/SNR and other system statistic handover.
[0010] FIG. 4 is a flow chart of a RSS/SNR and other system
statistic handover algorithm embodiment.
[0011] FIG. 5 is a flow chart of an embodiment of an algorithm for
calculation of a QoS index, which may be employed by FIG. 4.
DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT(S)
[0012] 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.
[0013] Hereafter, a wireless transmit/receive unit (WTRU) includes
but is not limited to a user equipment, station, mobile station,
fixed or mobile subscriber unit, pager, or any other type of device
capable of operating in a wireless environment. When referred to
hereafter, an access point includes but is not limited to a base
station, Node-B, site controller, or any other type of interfacing
device in a wireless environment. Although the following is
discussed with respect to WLANs, the invention can be applied to
other wireless networks.
[0014] FIG. 2 is an embodiment of a RSS/SNR and other system
statistic handover. A WTRU, such as WTRU 14.sub.1N of FIG. 1,
initiates the handover algorithm to determine whether handing over
between BSSs 12 is desirable, such as from BSS.sub.1 12.sub.1 to
BSS.sub.2 12.sub.2, step S30. The RSS and/or SNR is measured for
each BSS 12, including the current BSS and any potential handover
BSSs, step S32. Other system statistics are measured for each BSS
12, step S34. The other system statistics may relate to the quality
of service, such as delay bounds, bandwidth requirements (i.e. data
rate), and frame error rate. Based on the RSS/SNR and other system
statistics, a handover decision is made, step S36. Typically, the
other system statistics are based on the traffic class of the
WTRU's services.
[0015] FIG. 3 is an embodiment of a WTRU 18 capable of such a
handover. The components of FIG. 3 may be implemented on a single
integrated circuit (IC), such as an application specific integrated
circuit (ASIC), on multiple ICs, by discrete components or a
combination of IC(s) and discrete component(s). Wireless signals
are received and transmitted over an antenna 20 or antenna array
and a transceiver (Xceiver) 22 of the WTRU 18. A RSS/SNR measuring
device 24 measures the RSS and/or SNR of each BSS 12. A handover
controller 26 receives the RSS/SNR measurements and other system
statistics and determines whether a handover to another BSS 12 is
desired. The other system statistics may be recovered from received
communications, as shown in FIG. 3 or by other means.
[0016] FIG. 4 is an illustration of a preferred embodiment for
RSS/SNR and other system statistic handover. For each traffic
channel, QoS characters are defined, such as delay bounds,
bandwidth requirements (data rate), and frame error rate. Minimum
and maximum values for each parameter are defined for each traffic
class. A minimum and maximum value of SNR is also defined for each
traffic class. Table 1 illustrates an example of QoS
characteristics and SNR values for different traffic classes.
TABLE-US-00001 TABLE 1 QoS Characteristics and SNR definition for
different traffic classes Data Rate Frame Error Rate Delay (D) (BW)
(FER) SNR Traffic Class_1 Dmin Dmax BWmin BWmax FERmin FERmax
SNRmin SNRmax - - - - - - - - - - - - - - - - - - - - - - - -
Traffic Class_n Dmin Dmax BWmin BWmax FERmin FERmax SNRmin
SNRmax
[0017] The handover algorithm is triggered when the SNR value drops
below a high SNR threshold, i.e., SNR max, for the given traffic
class (TC) associated with the entity seeking a handover, step S40.
The TC may be one of those shown in Table 1. The algorithm compares
the SNR value with a low SNR threshold and depending on the result
acts, generally, as follows.
[0018] If the SNR value is between the low and high SNR thresholds,
the algorithm checks the QoS index for this traffic class. The QoS
index may be derived from any or all the criteria in Table 1 or,
alternately, other criteria may be used. If the QoS index is below
the QoS index threshold, the WTRU starts scanning neighboring cells
to trigger a handover. If the SNR value is higher than the high SNR
threshold, the algorithm terminates since link quality is good and
there is no need for handover. For SNR values below the low
threshold, the WTRU starts scanning neighboring BSSs without
comparing the QoS index with the QoS index threshold. Although the
above refers to SNR; RSS or a combination of RSS and SNR may be
used instead.
[0019] Referring to FIG. 4, the highest class of service for
traffic at the monitoring WTRU 18 and the QoS requirements of the
WTRU 18 are examined, step S42. If the SNR is at or above the low
threshold, step S44, the channel utilization and the frame loss
rate from the QBSS load element is determined, step S48. The QoS
parameter set element is checked, step S50, and the QoS index is
calculated, step S52. If the QoS index is greater than a QoS index
threshold, the handover algorithm is ended, steps S54, S86. If the
QoS index is less than or equal to the threshold, the algorithm
proceeds to determining a list of neighboring BSSs 12 to scan as
described subsequently, for steps S62 to S84.
[0020] If the SNR is below the low threshold, step S44, the channel
utilization is determined and frame loss rate derived from the QBSS
load element, step S56. The QoS parameter set element is checked,
step S58, and the QoS index is calculated, step 60.
[0021] A list of neighbor BSSs 12 is determined, step S62, and a
scan neighbor routine is initiated, step S64. The first BSS 12 of
the list is scanned, step S66. The probe response is obtained from
the first BSS 12 and the frame loss rate, channel utilization and
QoS parameters are obtained from the probe response, step S68. The
SNR and QoS parameter elements are checked, step S70. A QoS index
calculation for the first BSS 12 of the neighbors to be scanned is
performed, step S72.
[0022] In the event that there are more BSSs 12 in the list, step
S74, the next BSS 12 is picked, step S76. Steps S68 through S74 are
repeated for the next BSS 12.
[0023] When there are no more BSSs 12 to be scanned, the BSS 12
with the highest QoS index is picked, at step S78. A difference is
taken between the QoS index of the selected BSS 12 and the QoS
index of the current BSS 12. To keep the WTRU 18 from frequently
handing over between BSSs 12, the QoS index difference value is
compared with a hysteresis to determine if it is bigger than the
hysteresis, step S80. The hysteresis is preferably a function of
the traffic class (TC), although it may be derived by other
techniques. If the calculated difference is greater than the last
stored hysteresis, the handover to the new cell is initiated and
the hysteresis value is reset to its original value, step S82. The
handoff algorithm terminates, step S86. If the difference between
the current and target cell QoS indexes is smaller than the
hysteresis, the hysteresis value is updated, step S84. Preferably,
the hysteresis value is decreased in order to enable the WTRU 18
utilizing the handover algorithm to have a better chance to obtain
a handover to a new cell in the event that the WTRU 18 continues to
experience poor service.
[0024] An embodiment of a QoS index calculation algorithm is shown
in FIG. 5. Although the algorithm can be used in other
applications, it is preferably used with steps S52 and S72 of FIG.
4. The QoS index is initially set to zero, steps S88, S90, and a
list of available QoS parameters is created, step S92. The first
QoS parameter in the list is selected, step S94. The selected QoS
parameter is compared with the high threshold taken from the
associated traffic class (TC), step S96. If the selected parameter
is greater than the high threshold, the QoS index is incremented,
step S98. Alternatively, if the QoS parameter is less than the high
threshold and less than the low threshold, step S100, the QoS index
remains unchanged. If the QoS parameter is less than both the high
and low threshold, the present QoS index is decreased by n+1, where
n is the total number of BSSs being examined, step S102. After one
of these three (3) steps, S90, S100, S102 has been performed, it is
determined if there are any more QoS parameters to be examined,
step S104. In the event that there are more QoS parameters, the
next QoS parameter is selected, step S106. Steps S96 to S104 are
repeated until all of the QoS parameters have been examined. After
all of the QoS parameters have been evaluated, the QoS index is
produced, step S108.
[0025] Although FIG. 5 is one embodiment for producing a QoS index,
others may be used. For example, the QoS index may be produced by
weighting QoS parameters.
[0026] One application of the algorithms in FIGS. 4 and 5 can be
with an 802.11e compliant AP and WTRU. Additionally, another
application is with an 802.11b AP and WTRU with the needed
parameters for the algorithm added to the 802.11 beacon and probe
response frames or through proprietary signaling. These algorithms
can be also applied to other wireless environments.
* * * * *