U.S. patent application number 11/517399 was filed with the patent office on 2007-01-04 for handoff adaptation for cdma systems.
Invention is credited to Jason F. Hunzinger.
Application Number | 20070002799 11/517399 |
Document ID | / |
Family ID | 26915081 |
Filed Date | 2007-01-04 |
United States Patent
Application |
20070002799 |
Kind Code |
A1 |
Hunzinger; Jason F. |
January 4, 2007 |
Handoff adaptation for CDMA systems
Abstract
A method of determining handoff timing for a wireless device is
provided. The method includes selecting a call characteristic. The
call characteristic is evaluated to determine if it is relevant to
current conditions. If the call characteristic is relevant, an
adapted value for handoff timeout is determined based on the call
characteristic. The handoff timeout is set to the adapted
value.
Inventors: |
Hunzinger; Jason F.;
(Carlsbad, CA) |
Correspondence
Address: |
HARNESS, DICKEY & PIERCE, P.L.C.
P.O. BOX 828
BLOOMFIELD HILLS
MI
48303
US
|
Family ID: |
26915081 |
Appl. No.: |
11/517399 |
Filed: |
September 7, 2006 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
|
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09912795 |
Jul 24, 2001 |
7123916 |
|
|
11517399 |
Sep 7, 2006 |
|
|
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60220675 |
Jul 25, 2000 |
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Current U.S.
Class: |
370/331 ;
370/335 |
Current CPC
Class: |
H04W 36/24 20130101 |
Class at
Publication: |
370/331 ;
370/335 |
International
Class: |
H04Q 7/00 20060101
H04Q007/00; H04B 7/216 20060101 H04B007/216 |
Claims
1. A method for executing a handoff operation in a mobile station,
comprising: receiving from a base station at the mobile station at
least one parameter; determining a range of permissible values for
the handoff operation based on the parameter at the mobile station;
selecting a handoff timeout value within the range of permissible
values at the mobile station; assigning the handoff timeout value
for the handoff operation at the mobile station; scanning a pilot
signal of system frequencies; measuring an energy level of the
pilot signal; determining whether the energy level of the pilot
signal drops below a threshold level for a time period exceeding
the handoff timeout value; and executing the handoff operation
based on the determination when the energy level of the pilot
signal drops below the threshold level for the time period.
2. The method of claim 1, further comprising: setting at the mobile
station a minimum handoff timer value of the range of permissible
values and a maximum handoff timeout value of the range of
permissible values.
3. The method of claim 1, wherein the at least one parameter is
used to determine a minimum handoff timer value of the range of
permissible values and a maximum handoff timeout value of the range
of permissible values.
4. The method of claim 1, further comprising: storing the range of
permissible values for the handoff timeout value at the mobile
station.
Description
CROSS-REFERENCE TO RELATED APPLICATIONS
[0001] This application claims the benefit of U.S. Provisional
Application No. 60/220,675, filed Jul. 25, 2000, the entire
disclosure of which is incorporated herein by reference.
TECHNICAL FIELD
[0002] This invention relates to wireless communications devices,
and more particularly to handoff methods for wireless
communications based on CDMA.
BACKGROUND
[0003] Cellular telephones may operate under a variety of standards
including the code division multiple access (CDMA) cellular
telephone communication system as described in TIA/EIA, IS-95,
Mobile station-Base Station Compatibility Standard for Dual-Mode
Wideband Spread Spectrum Cellular System, published July 1993. CDMA
is a technique for spread-spectrum multiple-access digital
communications that creates channels through the use of unique code
sequences. In CDMA systems, signals can be and are received in the
presence of high levels of interference. The practical limit of
signal reception depends on the channel conditions, but CDMA
reception in the system described in the aforementioned IS-95
Standard can take place in the presence of interference that is 18
dB larger than the signal for a static channel. Typically, the
system operates with a lower level of interference and dynamic
channel conditions.
[0004] A mobile station using the CDMA standard constantly searches
a Pilot Channel of neighboring base stations for a pilot that is
sufficiently stronger than a pilot add threshold value T_ADD. As
the mobile station moves from the region covered by one base
station to another, the mobile station promotes certain pilots from
the Neighbor Set to the Candidate Set, and notifies the base
station or base stations of the promotion from the Neighbor Set to
the Candidate Set via a Pilot Strength Measurement Message. The
base station determines an Active Set according to the Pilot
Strength Measurement Message, and notifies the mobile station of
the new Active Set via a Handoff Direction Message. The mobile
station will maintain communication with both the old base station
and the new base station so long as the pilots for each base
station are stronger than a pilot drop threshold value T_DROP. When
one of the pilots weakens to less than a pilot drop threshold
value, the mobile station notifies the base station of the change.
The base station may then determine a new Active Set, and notifies
the mobile station of that new Active Set. Upon notification by the
base station, the mobile station then demotes the weakened pilot to
the Neighbor Set.
[0005] Switching pilots into and out of the Active Set of a mobile
station causes several problems with the handoff. First, the mobile
station may have several pilots active at the same time. Second,
the system capacity is decreased. Third, excessive signaling
between the mobile station and base stations results from the
increased number of handoff messages. It is desired to provide a
method and system that reduces the switching of pilots into and out
of the Active Set of a mobile station.
SUMMARY
[0006] A method of determining handoff timing for a wireless device
is provided. The method includes selecting a call characteristic.
The call characteristic is evaluated to determine if it is relevant
to current conditions. If the call characteristic is relevant, an
adapted value for handoff timeout is determined based on the call
characteristic. The handoff timeout is set to the adapted
value.
[0007] The details of one or more embodiments of the invention are
set forth in the accompanying drawings and the description below.
Other features, objects, and advantages of the invention will be
apparent from the description and drawings, and from the
claims.
DESCRIPTION OF DRAWINGS
[0008] FIG. 1 is an illustration of a wireless communication
system.
[0009] FIG. 2 is an illustration of a portion of a wireless
communication system.
[0010] FIG. 3 is an illustration of a CDMA Handoff technique in
accordance with the principles of the invention.
[0011] FIG. 4 is an illustration of a timeout adaptation method in
accordance with the principles of the invention.
[0012] FIG. 5 is an illustration of an embodiment of a timeout
adaptation method in accordance with the principles of the
invention.
[0013] FIG. 6 is an illustration of waveforms depicting the
operation of an embodiment of the invention.
[0014] Like reference symbols in the various drawings indicate like
elements.
DETAILED DESCRIPTION
[0015] FIG. 1 illustrates components of an exemplary wireless
communication system. A mobile switching center 102 communicates
with base stations 104a-104k (only one connection shown). The base
stations 104a-104k (generally 104) transmits data to and receives
data from mobile stations 106 within cells 108a-108k (generally
108). A cell 108, corresponding to a geographic region, is served
by abased station. Practically, said geographic regions often
overlap to a limited extent.
[0016] A mobile station 106 is capable of receiving data from and
transmitting data to a base station 104. In one embodiment, the
mobile station 106 receives and transmits data according to a Code
Division Multiple Access (CDMA) technique. CDMA is a communication
technique that permits mobile users of wireless communication
devices to exchange data over a telephone system, wherein radio
signals carry data to and from the wireless devices. A set of
standards that define a version of CDMA that is particularly
suitable for use with the invention include IS-95, IS-95A, and
IS-95B, Mobile Station-Base Station Compatibility Standard for
Dual-Mode Spread Spectrum Systems; TIA/EIA/IS-2000-2, Physical
Layer Standard for cdma2000 Spread Spectrum Systems; and
TIA/EIA/IS-2000-5 Upper Layer (Layer 3) Signaling Standard for
cdma2000 Spread Spectrum Systems, all of which are herein
incorporated by reference in their entirety.
[0017] Under the CDMA standard, additional cells 108a, 108c, 108d,
and 108e adjacent to the cell 108b permit mobile stations 106 to
cross cell boundaries without interrupting communications. This is
so because base stations 104a, 104c, 104d, and 104e in adjacent
cells assume the task of transmitting and receiving data for the
mobile stations 106. The mobile switching center 102 coordinates
all communication to and from mobile stations 106 in a multi-cell
region. Thus, the mobile switching center 102 may communicate with
many base stations 104.
[0018] Mobile stations 106 may move about freely within the cell
108 while communicating either voice or data. Mobile stations 106
not in active communication with other telephone system users may,
nevertheless, continue to scan for transmissions from the base
stations 104 in the cells 108 to detect pilots that are
sufficiently strong with which to establish a communications link.
In addition, mobile stations 106 may drop base stations 104 in
which the energy level of the pilot is not sufficiently strong.
[0019] One example of such a mobile station 106 is a cellular
telephone used by a vehicle driver who converses on the cellular
telephone while driving in a cell 108b. Referring to FIG. 2, a
portion of a wireless system is shown. The cellular telephone
synchronizes communication with the base station 104b by monitoring
a pilot that is generated by the base station 104b. While powered
on, the mobile station 106 continues to scan predetermined CDMA
system frequencies for pilots from other base stations 104 such as
the pilots from the base stations 104d and 104e as well as the
pilot corresponding to the base station 104b. Upon detecting a
pilot from another base station 104d, the mobile station 106
initiates a handoff sequence to add the pilot to the Active Set.
Likewise, upon determining that the energy level of an Active Set
pilot has weakened sufficiently and the handoff timeout value,
T_TDROP, has been exceeded, the mobile station 106 initiates a
handoff sequence to drop the pilot.
[0020] Wireless communications standards, require a mobile station
to use a static T_TDROP value that is generally provided by the
network. The pilot add and drop thresholds, T_ADD and T_DROP
respectively are similarly generally provided by the network as are
additional Pilot Set Maintenance parameters. The static value of
T_TDROP is used for all pilots. Using a static T_TDROP value may
lead to a pilot being thrashed into and out of the active set.
[0021] FIG. 3 shows a flow diagram of a Pilot Set Maintenance and
handoff process in accordance with the teachings of the invention.
Incorporated within the handoff process is a method of dynamically
selecting a value for the handoff timeout, T_TDROP, on a per pilot
basis. In one embodiment this value may be selected within a range
of permissible values where the network generally transmits such
parametric information through base stations 104 transmitting to
mobile stations 106. These parameters can be included in overhead
messages, parameter messages, traffic or idle messages, or handoff
messages. The network may specify such a range by either specifying
a nominal handoff timeout value and a range about that nominal
value or by specifying maximum and minimum values. The range may be
specified indirectly according to a formula or table lookup. In one
embodiment, the range may be specified by the network as an index
into the static T_TDROP table included in the IS-95-A standard. In
another embodiment, the range may be specified by a Handoff Timeout
Deviation about a nominal value such that the adapted handoff
timeout is limited to being between [Nominal Handoff
Timeout.times.(1-2.sup.8-Handoff Timeout Deviation)] and [Nominal
Handoff Timeout.times.(1+2.sup.8-Handoff Timeout Deviation)] In
this case, the network need only send the Nominal Handoff Timeout
and the Handoff Timeout Deviation parameters.
[0022] The network can be calibrated or configured with parameter
values as a result of network engineering or that are known to
result in compliance with minimum performance standards.
[0023] Dynamically selecting the T_TDROP value advantageously
improves the signal to noise ratio (SNR) of the system, increases
capacity, reduces the signaling overhead, and reduces the chances
of a call being inadvertently dropped. The described handoff
process is merely exemplary and is non-limiting. The scope of the
invention is not limited to the described handoff process. In
addition, although the process is described as being executed
within a mobile station, the scope of the invention includes
performing the process either entirely or partially in other system
components such as base stations and mobile switching centers.
[0024] At block 200, the mobile station 106 searches for pilots
from base stations 104 and measures their energies. When a pilot's
energy is measured, the process proceeds to block 202 and the pilot
energy is filtered such as using an infinite impulse response
filter, or a running average or mean. Proceeding to block 204 the
Active Set is checked to determine if the pilot is active, i.e.
included. If the pilot is not active, then at block 206 the energy
level of the pilot is tested against the default minimum required
energy level for adding a pilot, T_ADD. If the energy level of the
pilot does not exceed T_ADD, control returns to the start of the
process at block 200. If the energy level of the pilot does exceed
T_ADD, the mobile station 106 moves the pilot into the Candidate
Set and sends a Pilot Strength Measurement Message (PSMM) to the
current base stations to request that the pilot be added to the
Active Set, block 208. Proceeding to block 210, the mobile station
106 then determines an adapted value for the pilot drop handoff
timeout, T_TDROP. At a later step in the handoff process, the value
of T_TDROP is compared to the time duration that the pilot signal
energy is below a predetermined level to determine whether a
request to drop the pilot should be sent. The adapted value of
T_TDROP is determined on a per pilot basis.
[0025] In the present embodiment of the handoff process, the value
of T_TDROP is adapted for each specific pilot at the time of a
handoff request to add that pilot. The scope of the invention also
includes adapting the value of T_TDROP periodically, as well as at
other times within the handoff process such as a request to remove
a pilot, block 230. In other embodiments, the adaptation could
occur when a pilot energy crosses a threshold, parametric
information is updated from the network, or at the time of
measuring pilot energy. In addition, a single value of T_TDROP may
be dynamically determined and applied to all pilots or particular
sets of pilots. For example, these sets could be the Active Set,
Neighbor Set, Candidate Set, and Remaining Set.
[0026] Returning to block 204, if the pilot that was detected is an
active pilot, control continues on to block 212 to timer operations
and determining whether to remove the pilot. Block 214 checks if
the timer corresponding to the pilot is running or stopped. If the
timer is not running, then the process continues to block 216. If
Ec/Io is equal to or exceeds T_DROP, the pilot strength is
considered sufficient to retain the pilot in the Active Set and
control returns to the start of the process. Otherwise, if Ec/Io is
less than T_DROP for the Active Set pilot, then instead of
immediately removing the pilot from the Active Set, the timer
begins a timeout sequence, block 218. If a later measurement of
Ec/Io for the pilot exceeds T_DROP before the timer times out by
reaching T_TDROP, the timer is reset, blocks 220 and 222.
[0027] On the other hand, if a later measurement of Ec/Io does not
exceed T_DROP, then continuing to block 224, the timer is
incremented. At block 226, the timer is compared to the adapted
value of T_TDROP(i) corresponding to the pilot to determine whether
the timer has timed out. If the timer has not timed out, control
returns to the start of the process at block 200. If the timer has
timed out, indicating that the pilot strength has been below T_DROP
for an interval exceeding T_TDROP, the process proceeds to block
228 and the mobile station 106 sends a Pilot Strength Measurement
Message (PSMM) to the current Base Stations 104 requesting a
Handoff Direction Message (HDM) to remove the pilot from the Active
Set.
[0028] Referring to FIG. 4, a block diagram of an adaptation
process for dynamically determining the handoff timeout value,
T_TDROP, is shown. Initially, at block 240 a call characteristic
that can be statistically analyzed for predicted performance is
selected. In one embodiment, the handoff frequency is selected.
However, the scope of the invention includes other call
characteristics such as motion of the mobile station, signal
quality (as indicated by but not limited to combined Eb/Nt, Frame
Error Rate, Symbol Error Rate, Bit Error Rate, number of
retransmissions), energy level crossing, and energy variance. A
history or statistics are collected for the selected call
characteristic, block 241. History may include the recent timing of
handoffs adding or deleting pilots, the recent signal quality
changes over time, position and motion of the wireless mobile
station, or the past adapted handoff timeout values that were in
use. Data relevant to the call characteristic is evaluated to
determine if that characteristic is relevant to the current
conditions, block 242. The call characteristic data includes
historical or statistical information of the call characteristic.
For example, if handoff frequency is the selected call
characteristic, then the history of additions and drops of the
pilot is evaluated to determine if the time since the last drop is
greater than a predetermined minimum time; if so, then handoff
frequency is considered not currently relevant. If the call
characteristic is considered not currently relevant, the process
continues on to block 244 and the value of T_TDROP is set to a
default timeout value. The default timeout value is preferably a
minimum time, however the default timeout value may be other
measures of time such as a maximum or a mid-point. If the call
characteristic is relevant, the process proceeds to block 246 and
the predicted handoff performance is determined. Then at block 248
of the process, a new adapted value for the handoff timeout is
determined based upon the predicted handoff performance and the
current handoff timeout value. A linear function is applied to the
current handoff timeout value to determine the adapted handoff
timeout value. The scope of the invention also includes employing
other functions for determining the adapted handoff value such as
applying a multi-order function, incrementing/decrementing by a
fixed quantity, and selecting a value from a lookup table that is
constructed based on past decisions. T_TDROP is then set to the
adapted value at block 250 of the process.
[0029] Referring to FIG. 5, a detailed flow diagram of an
embodiment of the handoff adaptation is illustrated. The flow
diagram illustrates a method of determining predicted handoff
performance in order to effectively adapt the value of handoff
timeout. The handoff adaptation uses the past performance of the
handoff frequency to compute an adapted value. The time since the
pilot was last removed from the Active Set is computed so that the
relevance of using the handoff frequency can be evaluated, block
260. Proceeding to process block 262, if the time since the last
drop is greater than Tmin, the process continues on to block 264
where the value of T_TDROPs(i) is set equal to the default value
T_TDROPmin. If the time since the last drop is less than Tmin, then
the process proceeds to block 266 and statistics on the periods of
time that the pilot has remained out of the active set are
collected. The historical data is used to determine whether the
total estimated time that the pilot could be removed from the
Active Set is less than the maximum drop timer value, block 268.
This determines whether the T_TDROP value can be adapted so that
the pilot is not thrashed in and out of the Active Set needlessly.
If the pilot is likely to be thrashed, then the process proceeds to
block 270 and the value of T_TDROP is incremented. Otherwise the
process continues on to block 272, and the value of T_TDROP is
decremented. The rate or amount of increment and decrement may be
based on adaptive functions, fixed functions, functions that are
dependent on the range, as well as other well known functions. The
rate or amount of increment and decrement may be transmitted by the
network. In one embodiment, the rate or amount of increment and
decrement may be computed by scaling a nominal step value by a
function of the current set of minimum time periods associated with
a set of pilots. In another embodiment, the function for the amount
of increment is a scaling by the ratio of the number of handoff
timeouts above the nominal timeout to the total handoff timeouts
and the function for the amount of decrease is a scaling by the
ratio of the number of handoff timeouts below the nominal timeout
to the total handoff timeouts.
[0030] A hysteresis factor, C_hyst_comp, block 268, may be used to
compensate for differences between the add and drop energy
thresholds. In one embodiment, the hysteresis factor is larger if
the difference between the pilot add and drop thresholds T_ADD and
T_DROP respectively is large and vice-versa. In the embodiment
shown in FIG. 5, the hysteresis factor is multiplied by the maximum
value of the range of the dynamic handoff timeout value and
compared to the sum of the time since the last handoff and the
current handoff timeout value. If the hysteresis factor term is
less than the sum, this indicates that even using the maximum
handoff timeout value may be unlikely to result in a pilot being
kept during a drop in energy below the T_DROP threshold and,
therefore, T_TDROP is decremented according to block 270. If the
hysteresis factor term is greater than the sum, this indicates that
using a larger handoff timeout value may enable the mobile station
to avoid dropping a pilot whose energy temporarily drops below the
T_DROP threshold and, therefore, T_TDROP is incremented according
to block 272.
[0031] Referring to FIG. 6, waveforms associated with a mobile
station 106 dropping and adding a pilot in accordance with the
teachings of the invention are shown. A first waveform 280 shows a
channel 1 test pilot in which the pilot energy-to-interference
ratio, Ec/Io, remains greater than the threshold value at which a
pilot is moved into the candidate set, T_ADD. A second waveform 282
shows a channel 2 test pilot in which Ec/Io fluctuates from above
T_ADD to below T_DROP. An active set maintenance profile 284 shows
the channel 2 pilot set maintenance of the mobile station 106 in
response to the fluctuations of the channel 2 test pilot. The
profile 284, being high or low, signifies that the channel 2 pilot
is in or out, respectively, of the active set.
[0032] For the initial couple of cycles of the active set waveform,
the channel 2 test pilot is added and dropped in conjunction with
changes in Ec/Io. Then, at point A 286 of the active set
maintenance profile 284, the adaptation of T_TDROP to a larger
value, causes the mobile station 106 to retain the channel 2 test
pilot in the active set despite Ec/Io of channel 2 being less than
T_DROP. The channel 2 test pilot is retained in the active set
until at point B 288 the adapted value of T_TDROP is exceeded and
the channel 2 test pilot is dropped from the active set.
[0033] A test procedure for testing the operation of the adaptation
technique may include Active Set Loss Detection tests included in
TIA/EIA/IS-98-C, Recommended Minimum Performance Standards for
Dual-Mode Spread Spectrum Mobile Stations, which is hereby
incorporated by reference in its entirety. In another embodiment
the test procedure may include testing for compliance with the
expected behavior similar to that shown in FIG. 6. That test
procedure includes varying the amplitudes and time durations
associated with the call characteristics to verify the operating
characteristics of the adaptation technique. For example, either
the amplitude of the pilot energy or the time duration that the
pilot energy is below T-DROP may be varied to test the adaptation
algorithm. Increasing the amplitude of the pilot energy during a
drop, or decreasing the time duration of a drop, both cause an
increase in the adaptation of T-TDROP, leading to an increased
likelihood of the pilot being retained in the Active Set.
[0034] A number of embodiments of the invention have been
described. Nevertheless, it will be understood that various
modifications may be made without departing from the spirit and
scope of the invention such as executing all or portions of the
adaptation method in a base station. Accordingly, other embodiments
are within the scope of the following claims.
* * * * *