U.S. patent application number 10/114588 was filed with the patent office on 2002-11-14 for soft handoff algorithm and wireless communication system for third generation cdma systems.
Invention is credited to Chen, Qingxin, Sorokine, Vladislav.
Application Number | 20020168982 10/114588 |
Document ID | / |
Family ID | 23883586 |
Filed Date | 2002-11-14 |
United States Patent
Application |
20020168982 |
Kind Code |
A1 |
Sorokine, Vladislav ; et
al. |
November 14, 2002 |
Soft handoff algorithm and wireless communication system for third
generation CDMA systems
Abstract
A method and apparatus for providing an improved soft handoff
algorithm in a wireless communication system for third generation
code division multiple access ("CDMA") systems. The method
comprises establishing communication between the wireless
communication device and the at least one serving base station, the
at least one serving base station having a list of at least one
neighboring base station which neighbors the respective serving
base station, monitoring the reverse channel signal strength at the
wireless communication device from at least one neighboring base
station, transmitting to a base station controller the wireless
communication device signal strength detected at each neighboring
base station, compiling at the base station controller a list of
effective neighboring base stations from the at least one
neighboring base station based on the monitored wireless
communication device signal strength, transmitting the list of
effective neighboring base stations to each at least one serving
base station, periodically transmitting a neighboring base station
list update message to the wireless communication device, the
neighboring base station list update message including the
effective neighboring base station list, storing the effective
neighboring base station list as a neighbor set in the wireless
communication device, performing forward channel signal strength
searching of the neighbor set in the wireless communication device
after storing the effective neighbor list as the neighbor set, and
monitoring the signals from the effective neighboring base stations
to accomplish a handoff between the at least one service base
station and the receiving neighboring base station.
Inventors: |
Sorokine, Vladislav; (San
Diego, CA) ; Chen, Qingxin; (Del Mar, CA) |
Correspondence
Address: |
QUALCOMM Incorporated
5775 Morehouse Drive
San Diego
CA
92121-1714
US
|
Family ID: |
23883586 |
Appl. No.: |
10/114588 |
Filed: |
April 1, 2002 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
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10114588 |
Apr 1, 2002 |
|
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|
09474450 |
Dec 29, 1999 |
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6430414 |
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Current U.S.
Class: |
455/442 ;
455/525 |
Current CPC
Class: |
H04W 36/18 20130101 |
Class at
Publication: |
455/442 ;
455/525; 455/67.1 |
International
Class: |
H04Q 007/20 |
Claims
1. A method of handing off a wireless communication device between
at least one serving base station and a receiving base station in a
wireless communication system comprising: establishing a call
between the wireless communication device and the at least one
serving base station; monitoring a signal from the wireless
communication device from at least one base station neighboring the
serving base station; compiling a list of effective neighboring
base stations from the at least one neighboring base station based
on the monitored wireless communication device signal; and
monitoring the signals from the effective neighboring base stations
to accomplish a handoff between the at least one serving base
station and the receiving base station.
2. The method of claim 1, wherein monitoring the signal from the
wireless communication device from the at least one neighboring
base station includes monitoring the reverse pilot channel signal
strength from the wireless communication device.
3. The method of claim 2, wherein compiling the list of effective
neighboring base stations is further based on the monitored
wireless communication device reverse pilot channel signal
strength.
4. The method of claim 3, wherein the list of effective neighboring
base stations includes base stations neighboring the at least one
serving base station which receive the wireless communication
device signal at a predetermined strength.
5. The method of claim 4, further comprising transmitting an
extended handoff direction message from the base station controller
to the wireless communication device autonomously.
6. The method of claim 5, wherein transmitting the extended handoff
direction message is based on the base station controller analysis
of the wireless communication device signal strength detected by
each neighboring base station and independent of the pilot strength
measurement message generated by the wireless communication
device.
7. The method of claim 1 wherein each neighboring base station to
the at least one serving base station constantly monitors the
reverse channel signal strength.
8. The method of claim 1, further comprising transmitting an
extended handoff direction message independent of a pilot strength
measurement message generated by the wireless communication
device.
9. A method of optimizing a neighbor list comprising: establishing
a call between a wireless communication device and at least one
serving base station; storing at each at least one serving base
station a list of neighboring base stations; monitoring a reverse
channel signal strength at each neighboring base station and
reporting the respective reverse channel signal strength to a base
station controller; compiling a list of effective neighbor base
stations based on the reported reverse channel signal strength;
transmitting the list of effective neighbors to each at least one
serving base station; transmitting a neighbor list update message
containing the list of effective neighbors to the wireless
communication device; and storing the effective neighbor list as a
neighbor set in the wireless communication device.
10. The method of claim 9, wherein the base station controller
directs each neighboring base station to monitor the reverse
channel signal strength of the wireless communication device.
11. The method of claim 10, wherein compiling the effective
neighbor list comprises processing the reverse channel signal
strength from the neighboring base stations and comparing the
respective reverse channel signal strength with a predetermined
threshold.
12. The method of claim 11, wherein the effective neighbor list
comprises neighboring base stations that have a reverse channel
signal strength of at least a predetermined value.
13. The method of claim 9,wherein the neighbor list contains at
most six neighboring base stations.
14. The method of claim 9, wherein the neighbor list contains at
most three neighboring base stations.
15. The method of claim 9, wherein transmitting the list of
effective neighboring base stations to each at least one serving
base station is performed by the base station controller which
updates the neighbor set using a neighbor list update message
including the list of effective neighboring base stations to each
at least one serving base station.
16. The method of claim 15, further comprising transmitting the
neighbor list update message from the at least one serving base
station to the wireless communication device.
17. A method of handing off a wireless communication device between
a plurality of serving base stations and at least one receiving
base station in a wireless communication system comprising:
establishing a call between the wireless communication device and
the plurality of serving base stations; monitoring the reverse
channel signal strength from at least one neighboring base station
to each of the plurality of serving base stations; compiling a list
of effective neighboring base stations corresponding to each of the
plurality of serving base stations; and monitoring the signals from
the effective neighboring base stations to accomplish a handoff
between the plurality of serving base stations and the at least one
receiving base station.
18. The method of claim 17, wherein the signal from the wireless
communication device is monitored from at least one neighboring
base station to each of the plurality of serving base stations, and
the lists of effective neighboring base stations includes
respective base stations neighboring each of the plurality of
serving base stations which receive the wireless communication
device signal at a predetermined strength.
19. A wireless communication system comprising: a wireless
communication device which communicates with a serving base
station; at least one base station neighboring the serving base
station which monitors a reverse channel signal strength from the
wireless communication device; and a base station controller which
compiles a list of effective neighboring base stations from the at
least one base station neighboring the serving base station based
on the monitored reverse channel signal strength from the wireless
communication device, the base station controller communicating the
list of effective neighboring base stations to the wireless
communication device.
20. The wireless communication system of claim 19, wherein the
serving base station periodically sends a neighbor list update
message containing the list of effective neighboring base stations
to the wireless communication device, and the wireless
communication device stores the list of effective neighboring base
stations as a neighbor set and performs forward channel searches
based on the updated neighbor set.
21. A wireless communication system comprising: a wireless
communication device which communicates with a serving base
station; a plurality of base stations neighboring the serving base
station, each of the plurality of base stations monitoring a
reverse channel signal strength from the wireless communication
device; a base station controller, the plurality of neighboring
base stations transmitting each of the monitored reverse channel
signals to the base station controller, which compiles a list of
effective neighboring base stations from the plurality of
neighboring base station based on the monitored reverse channel
signal strength from the wireless communication device; and the
base station controller transmitting the effective neighboring list
to the base station, and the base station transmitting a neighbor
list update message to the wireless communication device, the
neighbor list update message containing the effective neighboring
list.
22. The wireless communication system of claim 21, wherein the
serving base station periodically sends a neighbor list update
message containing the list of effective neighboring base stations
to the wireless communication device, and the wireless
communication device stores the list of effective neighboring base
stations as a neighbor set and performs forward channel searches
based on the updated neighbor set.
Description
RELATED APPLICATIONS
[0001] This Application is a continuation of U.S. patent
application Ser.No. 09/474,450, filed on Dec. 29, 1999, Attorney
Docket No. 990410.
BACKGROUND OF THE INVENTION
[0002] 1. Field of the Invention
[0003] The present invention relates to cellular telephone systems.
More specifically, the present invention relates to a novel and
improved system for providing more efficient soft handoffs in a
code division multiple access ("CDMA") cellular telephone system to
accommodate uninterrupted voice and high data rate
transmission.
[0004] 2. Description of Related Art
[0005] The next generation of wireless networks will provide
multiple services requiring high data rate transmission and
uninterrupted connections. This next generation is often referred
to as the "third generation" of CDMA wireless systems. The range of
services include text paging, two-way radio connections, internet
connectivity using microbrowsers, two-way wireless e-mail
capability and wireless modem functionality. The CDMA cellular
telephone system offers the capability to provide reliable radio
links between a wireless communications device such as a mobile
station ("MS") and a base station ("BS") with a much higher data
capacity than conventional networks that only support voice
service. As an example, in the third generation CDMA wireless
systems, radio links supporting high rate (up to 2 Mbps) data
transmissions will be established between the MS and the BS to
provide multimedia services such as Internet access.
[0006] One particularly important feature of CDMA systems for
effective third generation wireless communication is the soft
handoff, which allows the MS to move smoothly from the coverage of
one cell to another without interruption. The soft handoff is
accomplished by establishing simultaneous communications between
the MS and multiple base stations. A soft handoff is illustrated in
FIG. 1. A MS 10 passes to the edge of the coverage area 12a of a
serving BS 12. While the MS 10 is within a serving BS coverage area
12a and a receiving BS 14 coverage area 14a, both base stations 12,
14 simultaneously communicate with the MS 10. As the MS 10 passes
further into the coverage area 14a of the receiving BS 14, the
server BS 12 stops communicating with the MS 10. In this manner,
there is uninterrupted communication for the user of the MS 10 as
he or she passes from the serving cell to the receiving cell. An
efficient soft handoff algorithm plays an important role in
maintaining the link quality as well as conserving the
capacity-related network resources. As the demand to support high
rate data services increases, the need to improve the efficiency of
the handoff algorithm becomes more critical.
[0007] For a third generation system based on CDMA technologies, a
highly efficient handoff algorithm is essential to successfully
provide the infrastructure to support the new range of services. A
conventional protocol for soft handoffs in a CDMA system has been
adopted by the Telecommunications Industry Association in the
industry standards IS-95, IS-95A or IS-95B (collectively "IS-95
A/B") for implementing a CDMA cellular system. Under the IS-95 A/B
standard, a MS communicates with one or more base stations
dispersed in a geographic region. Each BS continuously transmits a
pilot channel signal having the same spreading code but with a
different code phase offset. Phase offset allows the pilot signals
to be distinguished from one another, which in turn allows the base
stations to be distinguished. Hereinafter, a pilot signal of a BS
will be simply referred to as a pilot. The MS monitors the pilots
and measures the received energy of the pilots.
[0008] The IS-95 A/B standards define a number of states and
channels for communication between the MS and the BS. For example,
in the Mobile Station Control on the Traffic State, the BS
communicates with the MS over a Forward Traffic Channel, and the MS
communicates with the BS over a Reverse Traffic Channel. During a
call, the MS must constantly monitor and maintain four sets of
pilots collectively referred to as the Active Set, the Candidate
Set, the Neighbor Set, and the Remaining Set. The Active Set
comprises pilots associated with the Forward Traffic Channel
assigned to the MS. The Candidate Set comprises pilots that are not
currently in the Active Set but have been received by a particular
MS with sufficient strength to indicate that the associated Forward
Traffic Channel could be successfully demodulated. The Neighbor Set
comprises pilots that not currently in the Active Set or Candidate
Set but are likely candidates for handoff. The Remaining Set
comprises all possible pilots in the current system on the current
CDMA frequency assignment, excluding the pilots in the Neighbor
Set, the Candidate Set, and the Active Set.
[0009] The MS constantly searches the Pilot Channel of neighboring
base stations for a pilot that is sufficiently stronger than a
threshold value. As the MS moves from the region covered by one BS
to another, the MS promotes certain pilots from the Neighbor Set to
the Candidate Set, and notifies the BS or base stations of the
promotion via a Pilot Strength Measurement Message ("PSMM"). The BS
determines an Active Set according to the PSMM, and notifies the MS
of the new Active Set via a Handoff Direction Message. When the MS
commences communication with a new BS in the new Active Set before
terminating communications with the old BS, a "soft handoff" has
occurred.
[0010] In IS-95 A/B compliant CDMA systems, each BS is identified
by the pseudo-random ("PN") offset of its pilot channel signal. The
details of the PN offset identification procedures in IS-95 A/B are
well known to those of ordinary skill in the art and are therefore
not discussed further herein. The MS categorizes all pilots into
different sets based on the pilot's likelihood to be used as a
candidate for handoff.
[0011] The value T_ADD consists of the pilot strength threshold
specified by the BS (IS-95 A) or dynamically determined at the MS
(IS-95 B), above which the pilot is considered sufficiently strong
to be added to the Active Set. The value T_DROP reflects the pilot
strength threshold below which the pilot is considered sufficiently
weak to be removed from the Active Set. The PSMM is sent from the
MS to the BS to report the strength of all pilots in the Active and
Candidate Sets. In response to the PSMM, an Extended Handoff
Direction Message ("EHDM") which includes an updated Active Set is
sent from the BS to the MS.
[0012] The IS-95 A/B compliant MS typically has a searcher unit
that continuously measures the pilots in various sets and reports
to the BS the pilots that are sufficiently strong for an addition
to the Active Set, and the pilots that are relatively weak to be
removed from the Active Set. Pilots in the Neighbor Set are of
particular importance, and normally they are more frequently
measured than pilots in the Remaining Set.
[0013] The procedure of adding a pilot from the Neighbor Set to the
Active Set in the IS-95 A/B soft handoff algorithm is briefly
described as follows:
[0014] 1. Each BS has a stored Neighbor List ("NL") in terms of the
PN offsets and configuration information of the neighboring cells.
The MS receives a Neighbor List Update Message ("NLUM") containing
the NL from the BS and places the corresponding pilots into the
Neighbor Set.
[0015] 2. The MS is required to perform continuous measurement of
the pilot channel strength of every pilot in the Neighbor Set using
its searcher unit.
[0016] 3. The MS compares the measured pilot strength with the
T_ADD. Those neighbor pilots whose strengths are above T_ADD are
placed in the Candidate Set and the PSMM is sent to the BS.
[0017] 4. Based on the content of the PSMM and the availability of
the network resources, the BS sends an EHDM to the MS indicating a
new Active Set.
[0018] A similar reporting procedure is followed when the MS needs
to delete a pilot from its Active Set. In this case, the strength
of a pilot in the Active Set is compared with the threshold T_DROP
and a timer T_TDROP is activated whenever the pilot strength
decreases below T_DROP. Upon the expiration of T_TDROP, a PSMM is
sent to the BS and the BS usually responds with an EHDM indicating
a reduced Active Set.
[0019] According to the IS-95 A/B standard, when the MS receives a
NLUM, it increments a counter corresponding to each pilot in the
Neighbor Set and adds to the Neighbor Set each pilot named in the
NLUM, if such pilot is not already a pilot of the Candidate Set or
Neighbor Set. If the MS can store in the Neighbor Set only "k"
additional pilots and more than "k" new pilots were sent in the
NLUM, the MS 10 stores the first "k" new pilots listed in the
message. More details regarding the maintenance of the Neighbor Set
are found in the IS-95 A/B standards.
[0020] A more complete description of compatibility requirements
for handoffs is found in the IS-95 A/B standards, and such
information is incorporated herein by reference. Under the IS-95A
standard, the pilot strength threshold is specified by the BS as
part of an overhead information operation wherein the BS sends
system parameter data to the MS periodically. As part of the
overhead signal, a System Parameters Message from the BS to the MS
includes the pilot detection threshold T_ADD. More details
regarding the overhead information are found in Section 6.6.2.2
"Response to Overhead Information Operation" of IS-95A, which is
incorporated herein by reference. In the IS-95 B standard, the
pilot strength threshold is dynamically determined at the MS. The
relevant portions of IS-95 B which further discuss how the pilot
strength threshold is dynamically determined are incorporated
herein by reference.
[0021] The present soft hand-off algorithm does not provide
soft-handoffs sufficiently efficient for third generation wireless
services, however. Typically, the Neighbor List sent by the BS is a
static list that is determined at the time the network system is
deployed. It contains a list of the neighbor pilots that could be
possibly "seen" within the cell coverage. In the IS-95 A standard,
the minimum supported Neighbor Set size is 20 pilots, as
represented by the N.sub.8m constant in Appendix D of IS-95 A. In
the IS-95 B standard, the minimum supported size of the Neighbor
Set is 40. It is not uncommon for the BS to send a NL with the
maximum number of neighbor pilots just to be on the safe side,
especially in a poorly optimized network.
[0022] Since the Neighbor Set pilots are the most likely handoff
candidates, the frequency and the accuracy of the Neighbor Set
pilot measurements greatly affects the handoff performance.
However, the MS typically only has limited signal processing
capabilities due to its power, size and cost constraints. Passing a
large NL to the MS means that the MS has to distribute its limited
searcher power among many pilots which may (and typically does)
result in the poorer estimation of every pilot. A reduced sampling
rate for each pilot inhibits the MS's ability to estimate the
strength of each pilot accurately. Link failures occur more
frequently due to missed detections of fast time-varying
pilots.
[0023] In the current handoff procedure, the BS makes the handoff
decision based only on the MS's measurement reports of the forward
link pilot channel strength (F-PICH). A handoff procedure is
usually triggered by the PSMM sent from the MS when it sees a pilot
with sufficiently strong or weak strength. Although there exists a
mechanism by which the BS can autonomously order the MS to send a
PSMM, the BS solely relies on the MS's ability to estimate and
report the strength of its surrounding pilots to make handoff
decisions.
[0024] There are at least three factors that could lead to the
degradation in performance in the handoff algorithm. First,
compared with the BS, the MS's processing power is more restrictive
which limits its pilot searching ability, especially when it has to
search a large number of pilots as a result of un-optimized NL.
Second, the time spent in sending the PSMM and waiting for an EHDM
can sometimes be too long for the MS to react to rapid variations
of the radio link conditions. Third, the forward link quality only
approximately reflects the reverse link quality. Therefore, a
handoff decision based only on the F-PICH measurements may not
avoid failures caused by the reverse link degradation.
[0025] Attempts have been made to improve the soft-handoffs in a
CDMA system. For example, U.S. Pat. No. 5,920,550, to William D.
Willey, ("'550 patent"), assigned to the assignee of the present
invention and whose contents are incorporated herein by reference,
teaches providing at least one of the current measured pilot signal
strengths to the BS in each access probe. The system then specifies
the base stations for soft handoff according to the current
measured pilot signal strengths. The '550 patent, while improving a
soft-handoff operation by reporting the current pilot strength in
access probes subsequent to the System Access State, nevertheless
fails to further provide the necessary efficiency and uninterrupted
service that is necessary for third generation wireless
communications. The '550 patent teaches receiving a large-sized NL
from the BS as indicated in the IS-95 A/B standard. Although a
current pilot strength will be reported in subsequent access
probes, the '550 patent teaches distributing the MS limited
searcher power among many pilots which may (and typically does)
result in the poorer estimation of every pilot.
[0026] Another attempt to improve the soft handoff in a CDMA system
is found in U.S. Pat. No. 5,854,785, to William D. Willey ("'785
patent"), assigned to the assignee of the present invention and
which contents are incorporated herein by reference. The '785
patent teaches improving the soft handoff by measuring the neighbor
pilot strengths while in the System Access Mode and providing the
identities of the base stations corresponding to the measured pilot
strengths to the system in the initial access probe. The system
uses the neighboring BS identities and pilot signal strengths to
determine which neighboring BS has sufficient measured pilot
strength so that an associated Paging Channel may be successfully
demodulated. Thus, during a soft handoff, the MS demodulates the
paging channel from at least one neighboring pilot as well as the
MS's currently active pilot.
[0027] The '785 patent still fails to provide the necessary
capability for third generation wireless applications. Although a
paging channel message will be demodulated from a neighboring BS
with a sufficient pilot strength, the '785 patent teaches
distributing the MS limited searcher power among many pilots which
may (and typically does) result in the poorer estimation of every
pilot.
[0028] The cdma 2000 family of standards were established to
accommodate the third generation wireless communication systems.
The family of standards include: IS-2000-1; IS-2000-2; IS-2000-3;
IS-2000-4; IS-2000-5; and IS-2000-6. Each of these standards
specifies a portion of a spread spectrum radio interface that uses
CDMA technology and/or analog dual-mode technology for mobile
stations and base stations. The cdma 2000 standards are backward
compatible with IS-95 B.
[0029] Many new features have been introduced in the cdma2000
proposal in an effort to further increase the system capacity. One
of the features is the reverse link pilot channel transmitted by
each MS in the traffic state. The reverse pilot channel is an
unmodulated spread spectrum signal which is used to assist the BS
in detecting a MS transmission. When in the traffic state, the MS
communicates with the BS using the forward and reverse traffic
channels. Adding the reverse pilot channel enables coherent
detection of the mobile transmit signal at the BS and allows the
system to implement fast forward link power control. The fast
forward link power control is implemented by the MS inserting a
reverse power control sub-channel on the reverse pilot channel.
[0030] The IS-2000-2 portion of the cdma2000 family of standards
defines the physical layer standard for cdma2000 spread spectrum
systems. In this specification, the structure of the reverse pilot
channel includes a power control group consisting of the reverse
pilot channel signal contained in the first 1152.times.N PN chips,
and the reverse power control sub-channel in the following
384.times.N PN chips, where N is the spreading rate number. For
example, N=1 for spreading rate 1 and N=3 for spreading rate 3.
More details regarding the reverse power control sub-channel are
found in Section 2.1.3.1.10 (and subsections) of IS-2000-2 which is
incorporated herein by reference.
[0031] Although the cdma2000 family of standards provide some
benefits through increasing system capacity and by providing
reverse pilot strength measurements, the standards still fail to
provide sufficiently efficient soft handoffs with uninterrupted
data transmission. Therefore, the IS-95 A/B standards and the
cdma2000 family of standards do not adequately address providing
uninterrupted voice and data transmission during a soft handoff
between a MS and a BS.
SUMMARY OF THE INVENTION
[0032] What is needed in the art is a CDMA system which improves
the efficiency and uninterrupted connection between a MS and a BS
during a soft handoff. The invention disclosed and claimed herein
improves the existing soft handoff algorithm by using the reverse
pilot strength measurements at the BS as defined in the cdma2000
standard IS-2000-2 to optimize the neighbor list. The optimization
leads to enhanced handoff efficiency measured by the MS's speed to
handoff and its usage of network resources.
[0033] The field data collected during field trials for CDMA
markets indicates that in a properly optimized network that the
number of pilots with sufficient strength (Ec/Io>-14 dB in most
cases) "seen" by a particular MS should be no more than 3. Even in
a poorly optimized network subject to pilot pollution, the number
of competing pilots at any given time and location is at most 6.
Thus, the present algorithm unnecessarily requires the MS to
frequently monitor more neighboring base stations than is necessary
or efficient.
[0034] To address the deficiencies described above, the present
invention comprises a method of handing off a wireless
communication device between at least one serving cell and a
receiving neighboring cell in a wireless communication system. The
method comprises:
[0035] (1) establishing a call between the wireless communication
device and the at least one serving cell, the at least one serving
cell having a list of at least one neighboring cell which neighbors
the respective serving cell;
[0036] (2) monitoring the reverse channel signal strength received
from the wireless communication device from at least one
neighboring cell;
[0037] (3) transmitting to a BS controller the wireless
communication device signal strength detected at each neighboring
cell;
[0038] (4) compiling at the BS controller a list of effective
neighboring cells from the at least one neighboring cell based on
the monitored wireless communication device signal strength;
[0039] (5) transmitting the list of effective neighboring cells to
each at least one serving cell; (6) periodically sending a
neighboring cell list update message to the wireless communication
device, the neighboring cell list update message including the
effective neighboring cell list;
[0040] (7) storing the effective neighboring cell list as a
neighbor set in the wireless communication device;
[0041] (8) performing forward channel signal strength searching of
the neighbor set in the wireless communication device after storing
the effective neighbor list as the neighbor set; and
[0042] (9) monitoring the signals from the effective neighboring
cells to accomplish a handoff between the at least one service cell
and the receiving neighboring cell.
[0043] Although the above steps are numbered, they do not need to
be practiced in the above order. The invention also includes a
wireless communication system comprising:
[0044] a wireless communication device which communicates with a
serving cell;
[0045] at least one cell neighboring the serving cell which
monitors a reverse channel signal strength from the wireless
communication device; and
[0046] a BS controller which compiles a list of effective
neighboring cells from the at least one cell neighboring the
serving cell based on the monitored wireless communication device
signal, the BS controller communicating the list of effective
neighboring cells to the serving cell, wherein the serving cell
periodically sends a neighbor list update message containing the
list of effective neighboring cells to the wireless communication
device and the wireless communication device stores the list of
effective neighboring cells as the neighbor set and performs
forward channel searches on the updated neighbor set.
[0047] One of ordinary skill in the art will understand that the
communication system requires other infrastructure equipment which
is not shown, such as equipment for switching, call routing, and
the like.
BRIEF DESCRIPTION OF THE DRAWINGS
[0048] The features and advantages of the present invention will
become more apparent from the detailed description set forth below
when taken in conjunction with the drawings in which like reference
characters correspond throughout and wherein:
[0049] FIG. 1 illustrates generally a soft handoff procedure;
[0050] FIG. 2 illustrates the development of the effective neighbor
set as a MS travels through a serving cell;
[0051] FIG. 3 illustrates the formation and communication of a
neighbor list update message from a BS controller to a MS; and
[0052] FIG. 4 illustrates the present invention wherein two base
stations are serving the MS simultaneously.
DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS
[0053] In the cdma2000 family of standards, each MS sends
reverse-link channel strength signal on the reverse pilot channel
("R-PICH") in the Traffic State to the BS which provides the BS
with the opportunity to collect more information on the MS's RF
environment. Each MS supports both an inner power control loop and
an outer power control loop for forward traffic channel power
control. The outer power control loop estimates a setpoint value
based on Eb/Nt to achieve a target frame error rate ("FER") on each
assigned forward traffic channel. These setpoints are communicated
to the BS either implicitely or through signal messages. The
differences between these set points helps the BS derive the
appropriate transmit levels for the forward traffic channels that
do not have inner loops.
[0054] The inner power control loop compares the E.sub.b/N.sub.t of
the received forward traffic channel with the corresponding output
power control loop setpoint to determine the value of the power
control bit to be sent to the BS on the forward power control
subchannel. The present invention involves improving the existing
algorithm by dynamically optimizing the NL, providing an autonomous
EHDM for a faster handoff, and allowing the BS to direct the
forward channel pilot strength (F-PICH) search by the MS.
[0055] Variations of the received signal at the MS result from the
multi-path propagation, shadow fading and the path loss. While
multi-path fading exhibits short-term effects and is mostly
un-correlated between the forward link and the reverse link, shadow
fading and path loss incur longer-term variations in the received
signal strength and are generally considered highly correlated
between the forward and reverse links. An IS-95 A/B CDMA system
uses the RAKE receiver to tackle short-term channel variations
caused by the multi-path fading. The soft handoff algorithm is
designed to overcome long-term channel variations caused by shadow
fading and path loss.
[0056] As shown in FIG. 2, as the MS 10 moves from a position A to
position B within the cell 20a served by BS 20, the "effective set"
of neighbors changes. Cells 21a, 22a, and 23a may be the correct
candidates for handoff at location A but cells 24a, 25a, and 26a
are better candidates when the MS 10 moves to location B. With the
cdma2000 and IS/95 A/B algorithms, the MS 10 most likely will
receive a NL from the serving BS 20 consisting of pilots for BSs
21, 22, 23, 24, 25 and 26 plus other pilots in the neighborhood
regardless of its location with respect to neighboring cells.
[0057] A preferred embodiment of the present invention is
illustrated in FIG. 3. A method for optimizing the NL comprises
establishing a call between a MS 10 and serving BS 20, which has a
stored NL of its neighboring cells 21a, 22a, 23a, 24a, 25a and 26a.
The Base Station Controller ("BSC") 31 informs all the cells in NL
to monitor the R-PICH signal strength of the MS 10 and report it to
the BSC 31. Accordingly, each neighboring BS 21, 22, 23, 24, 25 and
26 monitors the R-PICH signal from the wireless communication
device or MS 10 and transmits the R-PICH data 20c, 21c, 22c, 23c,
24c, 25c, and 26c to the BSC 31. The BSC 31 processes the R-PICH
measurements from the NL members 21a, 22a, 23a, 24a, 25a and 26a
and compares each respective R-PICH data with a pre-determined
threshold, which preferrably is lower than T_ADD. It is understood
that the pre-determined threshold may also be some other value
unrelated to T_ADD.
[0058] Based on the analysis of the R-PICH data, the BSC 31
compiles an effective neighbor list ("ENL") consisting of those
base stations that report sufficiently strong R-PICH measurements
from the wireless communication device or MS 10. The threshold
levels for what is determined to be "sufficiently strong" are based
on a predetermined strength arrived at using various factors known
to those of ordinary skill in the art. The BSC 31 transmits the ENL
data 30 to BS 20 which is serving the MS 10. Periodically, BS 20
sends a Neighbor List Update Message (NLUM) 32 to the MS 10 which
contains the ENL. The MS 10 stores the pilots received from the ENL
as the Neighbor Set and afterwards performs the forward pilot
channel F-PICH search on the Neighbor Set. The Neighbor Set in the
wireless communication device may comprise at most six neighboring
base stations, or at most three neighboring base stations. Other
maximum numbers of base stations are also contemplated.
[0059] The above description and illustration according to FIG. 3
assumes that the MS 10 has only one serving BS 20. However, it is
understood that the MS 10 could be in soft handoff with multiple
cells. If the MS 10 is in a soft handoff where multiple cells are
simultaneously communicating with the MS 10, the BSC 31 provides a
copy of the NLUM to each BS simultaneously communicating with the
MS 10. This will be discussed in more detail below relative to FIG.
4.
[0060] To update the ENL for the MS 10 effectively, each BS 21-26
in the NL must constantly measure the R-PICH of every MS 10 in its
respective neighborhood. However, the additional signal processing
required at the BS is less critical when compared with the MS
because the BS is not as restricted in its power consumption and
size. The extra cost for the BS to perform R-PICH estimation is
converted into better F-PICH estimation at the MS 10. Using this
method, the Neighbor Set size is reduced from 20 to 6 or less,
which results in about a 3-fold increase in the search frequency
for each pilot. The increase in search frequency for each pilot
provides a significant improvement towards early detection of fast
rising pilots.
[0061] With the signal processing power that the BS 20 can afford
to have, the R-PICH measurements arrive at the BSC 31 with much
higher frequency and accuracy than what could possibly be achieved
by the F-PICH measurement at the MS 10. It is therefore possible
for the BSC 31 to issue an EHDM 20b, 21b, 22b, 23b, 24b, 25b, 26b
autonomously based on its analysis of the R-PICH data without
having to wait for the PSMM from the MS 10. This will effectively
shorten the turn around time of the soft handoff procedure reducing
the chance of link failures due to the system's delayed reaction to
the channel variation.
[0062] The R-PICH-measurement-triggered soft handoff mechanism can
be used in combination with the existing F-PICH
measurement-triggered soft handoff to ensure the quality of both
forward and reverse links.
[0063] In the existing algorithm, after the MS 10 receives the NL
from the BS 20, it follows a certain schedule in performing the
F-PICH search without much intervention from the BS 20. The MS 10
only sends the PSMM when a certain pilot passes the threshold test,
which may be too late in some cases given that the MS 10 has to
track more than one pilot. Having a much higher signal processing
power, the BSC 31 is capable of applying more sophisticated channel
estimation and prediction techniques on the R-PICH data, and thus
provide guidance for the MS 10 to search F-PICH intelligently
according to the method and apparatus of the present invention.
[0064] The structural embodiment of the invention is also
illustrated by FIG. 3. A wireless communication network 34
comprises at least one BS 20 acting as a serving cell 20a for a MS
10, or wireless communication device. The serving BS 20 stores a
list of neighboring cells 21a, 22a, 23a, 24a, 25a and 26a which
comprise base stations 21, 22, 23, 24, 25 and 26 which are adjacent
to or near the serving BS 20. A BS controller 31 compiles an ENL
from the neighboring cells 21a, 22a, 23a, 24a, 25a and 26a to the
serving cell 20a based on the monitored reverse channel signal
strength from the MS 10. The BS controller 31 transmits the
effective neighbor list 30 to the serving BS 20. The serving BS 20,
or base stations, will periodically transmit a Neighbor List Update
Message 32 to the MS 10, which will then update its Neighbor Set
based on the effective Neighbor List.
[0065] FIG. 4 illustrates the preferred embodiment of the invention
during a soft handoff operation where two base stations 20, 23 are
simultaneously communicating with the MS 10. For simplicity, some
of the communication lines shown in FIG. 3 are omitted from FIG. 4,
such as the lines representing the communication between the MS 10
and the neighboring base stations and the line representing the
communication from the base stations to the BSC 31. Similarly, only
one reference number, i.e., 21, will refer to the BS, the cell
containing that particular BS, and any communication to or from
that BS.
[0066] In FIG. 4, the MS 10 is simultaneously communicating with BS
20 and BS 23. The method for optimizing the NL comprises
establishing a call between a MS 10 and BS 20 and BS 23, which each
have a stored NL of its respective neighboring cells. For BS 20,
the neighoring list of cells comprises cells 21, 22, 23, 24, 25 and
26. For BS 23, the neighboring list of cells comprises 20, 22, 27,
28, 29, and 24. The BSC 31 separately informs all the cells in the
respective NL to monitor the R-PICH signal strength of the MS 10
and report it to the BSC 31. Accordingly, each cell in the NL of BS
20 transmits the R-PICH data to the BSC 31 and each cell in the NL
of BS 23 transmits the R-PICH data to BSC 31. The BSC 31 processes
the R-PICH measurements from the respective NL members and compares
the respective R-PICH data with a pre-determined threshold, which
preferrably is lower than T_ADD. The predetermined threshold may
also be some other pre-determined value unrelated to T_ADD.
[0067] Based on the analysis of the R-PICH data, the BSC 31
compiles a separate ENL for each BS 20, 23 consisting of those base
stations that report sufficiently strong R-PICH measurements from
the MS 10. The threshold levels for what is determined to be
"sufficiently strong" are determined based on various factors known
to those of ordinary skill in the art. The BSC 31 transmits the BS
20 ENL data to BS 20 and the BS 23 ENL data to BS 23, each of which
is serving the MS 10. Periodically, BS 20 transmits to the MS 10 a
NLUM 32 which contains the corresponding ENL. Also periodically, BS
23 transmits to the MS 10 its respective NLUM 32. The MS 10 stores
the pilots received from the respective ENLs in the Neighbor Set
and performs the F-PICH search accordingly using the Neighbor Set
after storing the ENL as the Neighbor Set.
[0068] To update the respective ENLs for the MS 10 effectively,
each BS in the NL of BS 20 and BS 23 must constantly measure the
R-PICH of every MS 10 in the respective neighborhood. Using this
method, the Neighbor Set within the MS may be reduced from 20 but
may also be modified to accommodate two sets of ENLs in its
Neighbor Set. For example, if BS 20 and BS 23 each transmit an NLUM
containing their respective ENLs, the MS 10 may have a Neighbor Set
defined to receive 12 pilots.
[0069] As a variation of this method, the BSC 31 may further refine
each respective ENL based on data compiled from each set of
neighboring base stations to either the BS 20 or the BS 23. In this
scenario, the BSC 31 will transmit a combined ENL to each of BS 20
and BS 23. Therefore, when the NLUM is transmitted to the MS 10
from each of BS 20 and BS 23, the contained ENL will be refined and
optimized for the MS 10. The Neighbor Set may still be reduced from
20 to 6 or less, which results in about a 3-fold increase in the
search frequency for each pilot.
[0070] Similar to the embodiment shown in FIG. 3, with the signal
processing power that the BS 20 can afford to have, the R-PICH
measurements arrive at the BSC 31 with much higher frequency and
accuracy than what could possibly be achieved by the F-PICH
measurement at the MS 10. It is therefore possible for the BSC 31
to issue an EHDM to a respective BS autonomously based on its
analysis of the R-PICH data without having to wait for the PSMM
from the MS 10.
[0071] The R-PICH-measurement-triggered soft handoff mechanism can
be used in combination with the existing F-PICH
measurement-triggered soft handoff to ensure the quality of both
forward and reverse links.
[0072] Returning to FIG. 3, in the IS-95 A/B and cdma2000
algorithms, after the MS 10 receives the NL from the BS 20, it
follows a certain schedule in performing the F-PICH search without
much intervention from the BS 20. The MS 10 only sends the PSMM
when a certain pilot passes the threshold test, which may be too
late in some cases given that the MS 10 has to track more than one
pilot. Having a much higher signal processing power, the BSC 31 is
capable of applying more sophisticated channel estimation and
prediction techniques on the R-PICH data, and thus provide guidance
for the MS 10 to search F-PICH intelligently according to the
method and apparatus of the present invention.
[0073] While the structure necessary to practice the invention is
disclosed herein, one of ordinary skill would readily understand
what other structures and components would be used to practice the
invention, such as equipment for switching, call routing, and so
forth. Furthermore, the terms "cell" and "base station," while not
the same thing, are often interchangeable in the above description.
A BS is the transmitting/receiving unit and its effect range or
capability to communicate with a MS defines the "cell." Therefore,
a neighboring "cell" to a serving "cell" will contain a
corresponding neighboring BS to a serving BS. When referring to a
neighboring cell, such a term may also mean the neighboring BS, or
a pilot signal from the neighboring BS contained in the respective
neighboring cell.
[0074] In another embodiment of the invention, the BSC 31
prioritizes the ENL using the results of a channel prediction
process so that the MS 10 can concentrate its searcher power on
pilots with a higher likelihood to be the handoff candidate.
[0075] In yet another embodiment of the present invention in which
the BS 20 directs the F-PICH search involves the BS 20 sending a
message ordering the MS 10 to report the F-PICH strength of a
certain set of pilots that are under consideration for handoff.
With both the F-PICH and R-PICH data available, the BSC 31 can
direct the MS 10 to make a better handoff decision. The present
inventors contemplate that within the scope of the concepts
disclosed above, one of ordinary skill in the art would understand
that there may be variations of the above embodiments which involve
optimizing the Neighbor Set to increase the efficiency of the soft
handoff.
[0076] By combing the information from the F-PICH and R-PICH
measurements and taking advantage of the signal processing power at
the BS, the proposed handoff algorithm promises to improve
efficiency of existing handoff algorithms. One measure of the
efficiency improvement is the ability to perform an early and quick
handoff as a benefit from the dynamically optimized NL and the
inclusion of autonomous EHDM. Another measure of the efficiency is
the ability to make better handoff decisions as a result of using
more sophisticated signal processing techniques at the BS and the
possibility of BS-directed intelligent search at the MS 10. Such
improved handoff efficiency is particularly crucial to the third
generation system, where a link failure may cause the loss of
multiple applications and each bad handoff decision will incur a
large cost in terms of network resources.
[0077] The present invention provides numerous benefits over the
prior art. The time which it takes to accomplish a handoff is
reduced, which is a measure of the reaction speed to fast rising
pilots. The average size of the Active Set can be reduced, which is
a measure of the network resources usage in terms of physical
channels. The frequency of handoff can be reduced, which is a
measure of the handoff decision validity and the network resource
usage in terms of signaling overhead. Finally, the combined Active
Set pilot strength is increased, which is a measure of radio link
quality.
[0078] Although the description above contains many details, these
should not be construed as limiting the scope of the invention but
as merely providing illustrations of the presently preferred
embodiment.
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