Note: Descriptions are shown in the official language in which they were submitted.
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METHOD AND APPARATUS FOR PERFORMING SOFT HAND-OFF IN A
WIRELESS COMMUNICATION SYSTEM
Divisional Application
This application is a divisional application of Canadian Patent
Application No. 2,279,314, which was filed on January 27, 1998.
I. Field of the Invention
The present invention relates to communication systems. More
particularly, the present invention relates to a novel and improved method and
system for performing hand-off in a wireless communication system.
11. Description of the Related Art
The use of code division multiple access (CDMA) modulation
techniques is but one of several techniques for facilitating communications in
which a
large number of system users are present. Although other techniques, such as
time
division multiple access (TDMA), frequency division multiple access (FDMA) and
AM
modulation schemes such as amplitude companded single sideband (ACSSB) are
known, CDMA has significant advantages over these other modulation techniques.
The use of CDMA techniques in a multiple access communication system is
disclosed in U.S. Patent No. 4,901,307, entitled "SPREAD SPECTRUM MULTIPLE
ACCESS COMMUNICATION SYSTEM USING SATELLITE OR TERRESTRIAL
REPEATERS" and U.S. Patent No. 5,103,459, entitled "SYSTEM AND METHOD
FOR GENERATING SIGNAL WAVEFORMS IN A CDMA CELLULAR TELEPHONE
SYSTEM", both of which are assigned to the assignee of the present invention.
The
method for providing CDMA mobile communications was standardized by the
Telecommunications Industry Association in TIA/EIA/IS-95-A entitled "Mobile
=
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la
Station-Base Station Compatibility Standard for Dual-Mode
Wideband Spread Spectrum Cellular System".
In the just mentioned patents, a multiple access
technique is disclosed in which a large number of mobile
telephone users, each having a transceiver, communicate
through satellite repeaters or terrestrial base stations
(also known as cell base stations or cell-sites) using code
division multiple access (CDMA) spread spectrum
communication signals. In using CDMA communications, the
frequency spectrum can be reused multiple times thus
permitting an increase in system user capacity. The use of
CDMA techniques results in much higher spectral efficiency
than can be achieved using other multiple access techniques.
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A method for simultaneously demodulating data that has traveled
along different propagation paths from one base station and for
simultaneously demodulating data redundantly provided from more than
one base station is disclosed in U.S. Patent No. 5,109,390 (the '390 patent),
entitled "DIVERSITY RECEIVER IN A CDMA CELLULAR
COMMUNICATION SYSTEM", assigned to the assignee of the present
invention. In the '390 patent, the
separately demodulated signals are combined to provide an estimate of the
transmitted data which has higher reliability than the data demodulated by
any one path or from any one base station.
Handoffs can generally be divided into two categories- hard handoffs
and soft handoffs. In a hard handoff, when a mobile station leaves and
origination cell and enters a destination cell, the mobile station breaks its
communication link with the origination cell and thereafter establishes a
new communication link with the destination cell. In soft handoff, the
mobile station completes a communication link with the destination cell
prior to breaking its communication link with the origination cell. Thus, in
soft handoff, the mobile station is redundantly in communication with both
the origination cell and the destination cell for some period of time.
Soft handoffs are far less likely to drop calls than hard handoffs. In
addition, when a mobile station travels near a cell boundary, it may make
repeated handoff requests in response to small changes in the environment.
This problem, referred to as ping-ponging, is also greatly lessened by soft
handoff. The process for performing soft handoff is described in detail in
U.S. Pat. No. 5,101,501, entitled "METHOD AND SYSTEM FOR PROVIDING
A SOFT HANDOFF IN COMMUNICATIONS IN A CDMA CELLULAR
TELEPHONE SYSTEM" assigned to the assignee of the present invention.
An improved soft handoff technique is disclosed in U.S. Pat.
No. 5,267,261, entitled "MOBILE STATION ASSISTED SOFT HANDOFF IN
A CDMA CELLULAR COMMUNICATIONS SYSTEM", which is assigned to
the assignee of the present invention.
In the system of the '261 patent, the soft handoff process is improved by
measuring the strength of "pilot" signals transmitted by each base station
within the system at the mobile station. These pilot strength measurements
are of assistance in the soft handoff process by facilitating identification
of
viable base station handoff candidates.
The viable base station candidates can be divided into four sets. The
first set, referred to as the Active Set, comprises base stations which are
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currently in communication with the mobile station. The second set,
referred to as the Candidate Set, comprises base stations which have been
determined to be of sufficient strength to be of use to the mobile station.
Base stations are added to the candidate set when their measured pilot
energy exceeds a predetermined threshold TADD. The third set is the set of
base stations which are in the vicinity of the mobile station ( and which are
not included in the Active Set or the Candidate Set). And the fourth set is
the Remaining Set which consists of all other base stations.
In an IS-95-A communication system, the mobile station sends a Pilot
Strength Measurement Message when it finds a pilot of sufficient strength
that is not associated with any the of the Forward Traffic Channels currently
being demodulated or when the strength of a pilot that is associated with
one of the Forward Traffic Channels being demodulated drops below a
threshold for a predetermined period of time. The mobile station sends a
Pilot Strength Measurement Message following the detection of a change in
the strength of a pilot under the following three conditions:
1. The strength of a Neighbor Set or Remaining Set pilot
is found above the threshold TADD.
2. The strength of a Candidate Set pilot exceeds the
strength of an Active Set pilot by more that a threshold
(TcomP).
3. The strength of a pilot in the Active Set of Candidate
Set has fallen below a threshold (TDRoP) for greater than
a predetermined time period.
The Pilot Strength Measurement Message identifies the base station and the
measured pilot energy in decibels.
A negative aspect of soft handoff is that because it involves
redundantly transmitting information it consumes the available
communication resource. However, soft handoff can provide great
improvement in the quality of communication. Therefore, there is a need
felt in the art for a method of minimizing the number of base stations
transmitting redundant data to a mobile station user which provides
sufficient transmission quality.
SUMMARY OF THE INVENTION
The present invention is a novel and improved method and
apparatus for providing soft handoff in a mobile communication system. It
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should be noted at the outset, that one of the biggest problems with current
systems is that the members of active set are determined in accordance with
comparisons of measured pilot energy with fixed thresholds. However, the
value of providing a redundant communication link to a mobile station
depends strongly on the energy of other signals being provided to the mobile
station. For example, the value of redundantly transmitting to a mobile
station a signal with received energy of -15 dB,õ will not be of much value,
if
the mobile station is already receiving a transmission with signal energy of -
5dB,11. However, redundantly transmitting to a mobile station a signal of
received energy of -15 dBõ, may be of substantial value, if the mobile station
is
receiving transmissions with signal energy of only -13dB,11.
In a first embodiment of the present invention, the mobile station
under the conditions discussed above transmits a Pilot Strength
Measurement Message, which identifies each base station in the active and
candidate sets and their corresponding measured pilot energy. The Pilot
Strength Measurement Message is received by the base stations in
communication with the mobile station. The base stations provide this
information to a central control center, referred to as the base station
controller.
At the base station controller, the active set is determined in
accordance with the combined strength of other pilots in the active set. The
base station controller sorts the pilots of the Pilot Strength Measurement
Message according to their pilot strength measured at the mobile station.
Thus, after sorting the list of base stations consists of P1, P2¨PN, where Pi
is
the strongest pilot signal and PN is the weakest. An iterative process is then
undertaken to determine which of pilots Pi, P2...PN should be part of the
revised active set.
Initially, the revised active set comprises only the strongest pilots Pi
and P2. When determining whether a pilot Pi should be made part of the
active set, a COMBINED_PILOT value is computed. The
COMBINED_PILOT value consists of the sum of the energies of the pilots
currently in the revised active set (P1, P2...Pi4). A threshold is then
generated
in accordance with the COMBINED_PILOT. In the exemplary, embodiment
the threshold is generated by performing a linear operation on the value of
COMBINED_PILOT. If the pilot energy value, Pi, exceeds the threshold, it is
added to the revised active set and the process is repeated for the next pilot
Pi+1. If the pilot energy value, Pi, does not exceed the threshold, the
revised
active set comprises P1, P2¨Pi-1= The revised active list is transmitted to
the
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mobile station and the base station controller then sets up communications
with the mobile station in accordance with the revised active set.
In an alternative embodiment, the revised active set is generated in
the mobile station. The mobile station continuously measures received
5 pilot strengths of base stations. In determining whether to send a message
indicating that a pilot from the candidate set should be moved to the active
set, the measured pilot energy of a pilot in the candidate set is compared
against a threshold generated in accordance with the COMBINED_PILOT as
described above. If the strongest pilot in the candidate set satisfies the
rule,
then a message containing all active and candidate set pilots will be sent.
Following the iterative process performed on the members of the
candidate set, a second iterative process is performed to determine whether a
pilot should be deleted from the revised active set. In this operation, pilots
are tested from the weakest member of revised active set to the strongest. A
COMBINED_PILOT energy value is computed that is the sum of the
energies of all pilots belonging to the active set. A threshold value is
generated in accordance with the COMBINED_PILOT value as described
above and the pilot signal being tested is compared with the threshold. If a
pilot has been below the threshold value for a predetermined period of time,
a message would be sent to the base station indicating that such a pilot
should be dropped.
The revised active list is transmitted to the base station controller
through the base stations with which the mobile station is in
communication. The base station sets up the communication links with the
base stations in the mobile generated revised active list and transmits an
acknowledgment to the mobile station when the links are set up. The
mobile station then conduct communications through the base stations of
the revised active set.
In the preferred embodiment, the mobile station monitors the pilot
signals and in response to the monitored pilot signals the mobile station
compiles members of the candidate set. Moreover, mobile station
determines whether a change to the current active set is desirable in view of
the criteria discussed above. Upon detecting any change in the desired
membership of the active set, the mobile station generates a pilot strength
measurement message that as described above includes the identities of all
pilots in the candidate and active sets corresponding measured energy
values and a corresponding indication whether the pilot should remain in
the sets or be dropped into the neighbor set (which is indicated by setting of
the KEEP variable described earlier). In the exemplary embodiment, the base
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station determines the members of the revised active set in accordance with
methods
described below.
According to one aspect of the present invention, there is provided a
method for determining the base stations to transmit to a remote station at
said
remote station in which a plurality of base stations comprising an Active Set
of base
stations transmit to said remote station and wherein a second set of base
stations
which are not members of said Active Set comprise a Candidate Set of base
stations,
said method comprising the steps of: aggregating received signal energies of
said
Active Set of base stations; generating a threshold value in accordance with
said
aggregated received signal energies of said base station members of said
Active Set;
comparing received signal energy of a base station of said Candidate Set of
base
stations with said threshold; and adding said base station of said Candidate
Set of
base stations to said Active Set of base stations when said received signal
energy of
a base station of said Candidate Set of base stations exceeds said threshold.
According to another aspect of the present invention, there is provided
an apparatus for determining base stations to transmit to a remote station at
said
remote station in which a plurality of base stations comprising an Active Set
of base
stations transmit to said remote station and wherein a second set of base
stations
which are not members of said Active Set comprise a Candidate Set of base
stations,
said apparatus comprising: means for aggregating received signal energies of
said
Active Set of base stations; means for generating a threshold value in
accordance
with said aggregated received signal energies of said base station members of
said
Active Set; means for comparing received signal energy of a base station of
said
Candidate Set of base stations with said threshold; and means for adding said
base
station of said Candidate Set of base stations to said Active Set of base
stations
when said received signal energy of a base station of said Candidate Set of
base
stations exceeds said threshold.
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According to still another aspect of the present invention, there is
provided an apparatus for revising a base station set, comprising: a processor
for
determining whether to revise the base station set with a base station by
comparing a selected pilot energy value associated with the base station to a
dynamic threshold, wherein the dynamic threshold is based on combined energy
values of at least two members of the base station set.
According to yet another aspect of the present invention, there is
provided a method for revising a base station set, comprising: combining
energy
values of at least two strong members of the base station set to form a
dynamic
threshold; comparing a selected pilot energy value to the dynamic threshold to
determine whether to revise the base station set with a base station
associated
with the selected pilot energy value; and if the selected pilot energy value
exceeds
the dynamic threshold, then incorporating the base station associated with the
selected pilot energy value into a revised base station set.
According to a further aspect of the present invention, there is
provided a method for generating a dynamic threshold used for revising a base
station set, comprising: (a) forming a combined energy value using energy
values
associated with members of the base station set; (b) using the combined energy
value to produce the dynamic threshold; (c) comparing a received pilot energy
value to the dynamic threshold to determine whether to update the base station
set
with a base station associated with the received pilot energy value; (d) if
the
received pilot energy value exceeds the dynamic threshold, then incorporating
the
received pilot energy value into the combined energy value, thereby producing
a
new combined energy value, whereupon the new combined energy value is used
to produce a new dynamic threshold; and (e) repeating steps (c)-(d) for each
received pilot energy value.
According to yet a further aspect of the present invention, there is
provided an apparatus for generating a dynamic threshold used for revising a
base
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station set, comprising: means for forming a combined energy value using
energy
values associated with members of the base station set; means for using the
combined energy value to produce the dynamic threshold; means for comparing a
received pilot energy value to the dynamic threshold to determine whether to
update the base station set with a base station associated with the received
pilot
energy value; and means for incorporating the received pilot energy value into
the
combined energy value if the received pilot energy value exceeds the dynamic
threshold, whereupon the new combined energy value is used to produce a new
dynamic threshold.
According to still a further aspect of the present invention, there is
provided an apparatus for shortening a search through adjacent pilot sets that
are
used for facilitating handoff of a mobile station within a wireless
communication
system, comprising: a processor configured to execute a set of instructions
for:
sorting a first pilot set from a strongest first pilot energy value to a
weakest first
pilot energy value; selecting a first pilot energy value from the sorted first
pilot set;
comparing the selected first pilot energy value to a first threshold value; if
the
selected first pilot energy value exceeds the first threshold value, then
reallocating
the selected first pilot energy value to a second pilot set and selecting the
next first
pilot energy value from the sorted first pilot set, wherein the above steps
repeat
until the selected first pilot energy value does not exceed the first
threshold value;
if the selected first pilot energy value does not exceed the first threshold
value,
then: sorting a second pilot set from a weakest second pilot energy value to a
strongest second pilot energy value; selecting a second pilot energy value
from the
sorted second pilot set; and comparing the selected second pilot energy value
to a
second threshold value, if the selected second pilot energy value exceeds the
second threshold value, then resetting a drop timer and finishing the search;
and if
the selected second pilot energy value does not exceed the second threshold
value, then advancing a drop timer, selecting a next second pilot energy value
from
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the sorted second pilot set, and comparing the next selected second pilot
energy to
the second threshold value, wherein the above steps repeat until the drop
timer
expires.
According to another aspect of the present invention, there is
provided in a wireless communication system wherein a remote station is in
communication with a plurality of base stations, a method for selecting an
additional base station for communication with the remote station comprising:
sorting a plurality of signal energies according to signal strength, wherein
each of
the plurality of signal energies correspond with one of the plurality of base
stations;
computing a threshold value from a subset of the plurality of signal energies,
wherein the subset of the plurality of signal energies is determined by the
step of
sorting the plurality of signal energies; comparing a signal energy
measurement of
the additional base station with said threshold value; and selecting the
additional
base station when the signal energy measurement of the additional base station
exceeds the threshold value.
According to yet another aspect of the present invention, there is
provided in a wireless communication system wherein a remote station is in
communication with at least one base station, an apparatus for maintaining
communications between the remote station and the at least one base station in
accordance with signal strength, comprising: means for receiving pilot signals
from
base stations; means for processing, which comprises means for: sorting a
plurality of signal energies according to signal strength, wherein each of the
plurality of signal energies correspond with one of the plurality of base
stations;
computing a threshold value from a subset of the plurality of signal energies,
wherein the subset of the plurality of signal energies is determined by the
step of
sorting the plurality of signal energies; comparing a signal energy
measurement of
the additional base station with said threshold value; and selecting the
additional
base station when the signal energy measurement of the additional base station
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exceeds the threshold value, wherein the means for processing is
communicatively
connected to the means for receiving.
According to still another aspect of the present invention, there is
provided a method for shortening a search through adjacent pilot sets that are
used for facilitating handoff of a mobile station within a wireless
communication
system, comprising: sorting a first pilot set from a strongest first pilot
energy value
to a weakest first pilot energy value; selecting a first pilot energy value
from the
sorted first pilot set; comparing the selected first pilot energy value to a
first
threshold value; and if the selected first pilot energy value exceeds the
first
threshold value, then reallocating the selected first pilot energy value to a
second
pilot set and selecting the next first pilot energy value from the sorted
first pilot set,
wherein the above steps repeat until the selected first pilot energy value
does not
exceed the first threshold value; or if the selected first pilot energy value
does not
exceed the first threshold value, then: sorting a second pilot set from a
weakest
second pilot energy value to a strongest second pilot energy value; selecting
a
second pilot energy value from the sorted second pilot set; and comparing the
selected second pilot energy value to a second threshold value, if the
selected
second pilot energy value exceeds the second threshold value, then resetting a
drop timer and finishing the search; and if the selected second pilot energy
value
does not exceed the second threshold value, then advancing a drop timer,
selecting a next second pilot energy value from the sorted second pilot set,
and
comparing the next selected second pilot energy to the second threshold value,
wherein the above steps repeat until the drop timer expires.
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BRIEF DESCRIPTION OF THE DRAWINGS
The features, objects, 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
identify correspondingly throughout and wherein:
FIG. 1 is an illustration of a cellular communication network;
FIG. 2 is an illustration of the cellular communication network of
FIG. which includes the base station controller;
FIG. 3 is a block diagram of the mobile station of the present
invention;
FIG. 4 is a block diagram of the base station of the present invention;
FIG. 5 is a flow diagram of the method for generating the revised
active set in the base station controller;
FIG. 6 is a flow diagram of the method for generating the revised
active set in the mobile station;
FIG. 7 is flow diagram illustrating the preferred method of generating
the candidate set in the mobile station; and
FIG. 8 is a flow diagram illustrating the preferred method of the
present invention wherein a change in the preferred members of active set
is detected and a pilot strength measurement message is transmitted to the
base station in response to the detected changed.
DETAILED DESCRIPTION OF THE PREFERRED
EMBODIMENTS
FIG. 1 illustrates wireless communication network in which the
geographical area has been divided up into coverage areas referred to as cells
and illustrated by a set of adjacent hexagons. Each cell is served by a
corresponding base station 4. Each base station transmits a pilot signal
which uniquely identifies that base station. In the exemplary embodiment,
the base stations 4 are CDMA base stations. A detail description of soft hand-
off in a wireless CDMA communication system is described in detail in the
aforementioned U.S. Patent Nos. 5,101,501 and 5,267,261.
Mobile station 2 is located within the cell served by base station 4A.
Since mobile station 2 is located near the cell boundary, it will likely be in
a
=
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soft hand-off condition, in which it is simultaneously in communication
with more than one base station. It may, for example be in communication
with base stations 4A and 4B. Thus, base stations 4A and 4B are said to make
up the active set. Moreover, it may be that mobile station 2 has determined
other base stations in its vicinity to have a measured pilot energy above a
predetermined threshold TADD, but that those base stations are not currently
in communication with the mobile station. Those pilots are said to make
up the candidate set. The candidate set could be made up of base stations 4C
and 4G.
Referring to FIG. 2, a typical communication network is illustrated.
Data directed mobile station 2 is provided from a public switched telephone
network or other wireless system (not shown) to base station controller 6.
Base station controller 6 provides the data to the base stations in mobile
station 2's active list. In the example, base station controller 6 redundantly
provides data to and receives data from base stations 4A and 4B
The present invention is equally applicable to conditions where each
cell is divided into sectors. Communications to and from each sector can be
separately received and demodulated by mobile station 2. For simplicity, the
discussion will be described wherein in each base of base station 4 are
uniquely located base stations. However, it will be readily seen by one
skilled in the art that the present invention is equally applicable to
sectored
cells, simply by considering the possibility that the base stations can be
collocated and transmitting to separate sectors within a cell. The condition
where a mobile station is in simultaneous communication with more than
one sector of a cell is referred to as softer handoff. The method and
apparatus for performing softer hand-off are described in detail in copending
U.S. Patent Application No. 08/144,903, entitled "METHOD AND
APPARATUS FOR PERFORMING HANDOFF BETWEEN SECTORS OF A
COMMON BASE STATION", filed October 30, 1993, which is assigned to the
assignee of the present invention.
Within mobile station 2, each copy of the data packet is separately
received, demodulated and decoded. The decoded data is then combined to
give a estimate of the data of greater reliability than any one of demodulated
estimates of the data.
FIG. 3 illustrates mobile station 2 of the present invention. Mobile
station 2 continuously or at intermittent intervals measures the strength of
pilot signals of base stations 4. Signals received by antenna 50 of mobile
station 2 are provided through duplexer 52 to receiver (RCVR) 54 which
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amplifies down converts and filters the received signal and provides it to
pilot demodulator 58 of searcher subsystem 55.
In addition, the received signal is provided to traffic demodulators
64A-64N. Traffic demodulators 64A-64N, or a subset thereof, separately
demodulate signals received by to mobile station 2. The demodulated
signals from traffic demodulators 64A-64N are provided to combiner 66
which combines the demodulated data, which in turn provides an
improved estimate of the transmitted data.
Mobile station 2 measures the strength of pilot channels. Control
processor 62 provides acquisition parameters to search processor 56. In the
exemplary embodiment of a CDMA communication system, control
processor 62 provides a PN offset to search processor 56. Search processor 56
generates a PN sequence which is used by pilot demodulator 58 to
demodulate the received signal. The demodulated pilot signal is provided
to energy accumulator 60 which measures the energy of the demodulated
pilot signal, by accumulating the energy for predetermined lengths of time.
The measured pilot energy values are provided to control processor
62. In the exemplary embodiment, control processor 62 compares the energy
values to thresholds TADD and TDRop. TADD is threshold above which the
received signal is of sufficient strength to effectively provide
communications with mobile station 2. TDRop is a threshold value below
which the received signal energy is insufficient to effectively provide
communications with mobile station 2.
Mobile station 2 transmits a Pilot Strength Measurement Message
which includes all pilots with energy greater than TADD and all members of
the current active set whose measured pilot energy has not fallen below
TDROP for more than a predetermined time period. In the exemplary
embodiment, mobile station 2 generates and transmits a Pilot Strength
Measurement Message following the detection of a change in the strength of
a pilot under the following three conditions:
1. The strength of a neighbor Set or Remaining Set pilot
is found above the threshold TADD.
2. The strength of a Candidate Set pilot exceeds the
strength of an Active Set pilot by more that a threshold
(TcomP).
3. The strength of a pilot in the Active Set has fallen
below a threshold (TDRop) for greater than a
predetermined time period.
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In the exemplary embodiment, the Pilot Strength Measurement Message
identifies the pilot and provides a corresponding measured pilot energy. In
the exemplary embodiment, the base stations in the Pilot Strength
Measurement Message are identified by their pilot offsets and their
corresponding measured pilot energy is provided in units of decibels.
Control processor 62 provides the identities of the pilots and their
corresponding measured pilot energies to message generator 70. Message
generator 70 generates a Pilot Strength Measurement Message containing
the information. The Pilot Strength Measurement Message is provided to
transmitter (TMTR) 68, which encodes, modulates, upconverts and
amplifies the message. The message is then transmitted through duplexer
52 and anterma 50.
Referring to FIG. 4, the Pilot Strength Measurement Message is
received by antenna 30 of base station 4 and provided to receiver (RCVR) 28,
which amplifies, down converts, demodulates and decodes the received
signal and provides the message to base station controller (BSC) interface 26.
Base station controller (BSC) interface 26 sends the message to base station
controller (BSC) 6. The message is provided to selector 22, which may also
receive the message redundantly from other base stations which are in
communication with mobile station 2. Selector 22 combines message
estimates received from the base stations in communication with mobile
station 2 to provide an improved packet estimates.
Selector 22 provides the power strength measurement message to
hand-off control processor 20. In the first exemplary embodiment, hand-off
control processor 20 selects the base stations which will communicate with
mobile station 2, that is the members of the revised active set, in accordance
with the method provided in FIG. 5.
In block 100, hand-off control processor 20 sorts pilots in the Pilot
Strength Measurement Message according to their strengths. So, for
example, P1 would be the strongest received pilot, P2 would be the second
strongest pilot and so on. In block 102, the revised active set (ACTIVE_SET)
is set to include Pi and P2. In block 104, the variable COMBINED_PILOT is
set to the sum of the energies of P1 and P2. In block 106, the loop variable i
is
set to 3.
In block 108, the energy of the pilot signal of the ith strongest received
signal (Pi) is compared against a threshold value to determine whether it
should be added to the revised active set. In the exemplary embodiment, the
threshold (T) is determined in accordance with equation (1) below:
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T=SOF'T_SLOPE*COMBINED_PILOT+SOFT_INTERCEPT (1)
In the exemplary embodiment, SOFT_SLOPE is set to 2.25 and
SOFT_INTERCEPT is set to 3Ø The values of SOFT_SLOPE and
5 SOFT_INTERCEPT can be parameters that are sent over the air to the
mobile station or selected values could be programmed into the mobile
station. The values of SOFT_SLOPE and SOFT_INTERCEPT can be
determined in accordance with factors such as the amount of soft handoff
which is acceptable to a network manager and empirical studies on the
10 quality of transmission links. If the energy value Pi is less than the
threshold value, then the flow proceeds to block 110 and the revised active
set includes the signals corresponding to the pilots {P1.- Pi..1}.
If the energy value Pt is greater than the threshold value in block 108,
then the flow proceeds to block 112. In block 112, a new COMBINED_PILOT
is computed by summing the value of the energy of the ith strongest signal
in the pilot strength measurement message (Pi) with the current value of
COMBINED_PILOT. Because in the exemplary embodiment, the energy of
the pilot signals is provided in decibels, the energies must be converted to
linear representations before being summed and put back into decibel form.
In block 114, Pi is added to the revised active set.
In block 116, the loop variable (i) is incremented. In block 118, hand
off control processor 20 checks to determine whether all base stations in the
pilot strength measurement message have been tested. If there are no
remaining pilots to test, then the flow proceeds to block 120 and the revised
active set comprises all the base stations in the pilot strength measurement
message. If, in block 118, there are base stations in the pilot strength
measurement message which remain to be tested, the flow returns to block
108 and proceeds as described above.
After generating the revised active set, base station controller 6
determines whether the base stations in the revised active list can
accommodate communications with mobile station 2. If any of the base
stations in the revised active set cannot accommodate communications with
mobile station 2, they are removed from the revised active set. After
generating the revised active set, hand-off control processor 20 provides the
information to selector 22 indicating the members in the revised active set.
In response to the revised active set provided by hand-off control processor
20, selector 22 allocates traffic channels for performing communications to
the mobile station using the base stations in the revised active set.
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Hand-off control processor 20 provides a message indicating the
revised active set to message generator 24. Message generator 24 generates a
message for transmission to mobile station 2, referred to as the handoff
direction message. The handoff direction message indicates the base stations
in the revised active set and corresponding channels those base stations will
use to communicate with mobile station 2. The message is provided
through selector 22 and provided to the base stations which were in
communication with mobile station 2 prior to the generation of the revised
active set. The base stations in communication with mobile station 2
transmit the handoff direction message to mobile station 2.
Referring back to FIG. 3, the handoff direction message is received by
antenna 50 of mobile station 2. It is provided to receiver 54, which
amplifies, downconverts, demodulates and decodes the message and
provides it to control processor 62. Control processor 62, then, configures
the traffic channel demodulators 64A-64N to demodulate traffic channels in
accordance with the revised active set specified in the handoff direction
message.
In an alternative embodiment of the present invention, the revised
active set is generated at mobile station 2. This alternative embodiment,
provides more timely generation of the revised active set. Because the Pilot
Strength Measurement Message is only transmitted under the three
conditions described above, update of the active set may be undesirably
delayed. However, the alternative embodiment results in transmission of
the pilot strength measurement message in a more timely fashion.
In the alternative embodiment, mobile station 2 measures received
pilot energy as described above. The pilot energy values are provided to
control process 62. In response, control processor 62 generates a revised
active set. If the revised active set differs from the current active set,
mobile
station 2 transmits a message indicating the members of the revised active
set to base station controller 6 through base stations 4. Base station
controller 6 sets up communications with mobile station 2. Mobile station 2
reconfigures traffic channel demodulators 64A-64N to demodulate received
signals in accordance with the mobile generated revised active set.
In the exemplary embodiment, control processor 62 in mobile station
2 generates the revised active set in accordance with the method shown in
FIG. 6. In block 200, pilots with measured energy in excess of threshold TADD
are added to the candidate list and pilots who's measured energy has fallen
below TDRop for more that a predetermined time period are removed from
the candidate list. In the exemplary embodiment, the time a pilot is below
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'rpm) is tracked by a timer within control processor 62 referred to herein as
the TDRop timer.
In block 202, the pilots in the candidate list are sorted from strongest
to weakest. Thus, Po is stronger than Pc2, and so on. In block 204, the
variable COMBINED_PILOT is set equal to the energy of all pilots in the
active set. Also, in block 204, loop variable (i) is initialized to the value
1. In
block 206, the candidate set member Po is tested to determine whether it
should be made part of the revised active set. Pci is compared against a
threshold generated in accordance with the current value of
COMBINED_PILOT. In the exemplary embodiment, the threshold (T) is
generated in accordance with equation (1) above.
If the pilot energy of Pa exceeds threshold T, then the flow moves to
block 208. In block 208, pilot Pc, is added to the revised active set. In
block
210, a new value of COMBINED_PILOT is computed which is equal to the
old value of COMBINED_PILOT plus the energy of pilot Pc,. In block 212,
the loop variable (i) is incremented.
In block 213, it is determined whether all pilots in the candidate set
have been tested. If all pilots in the candidate set have not been tested,
then
the flow moves to block 200 and proceeds as described above. If all pilots in
the candidate set have been tested or if, back in block 206, the pilot energy
of
Pci did not exceed threshold T, then the flow moves to block 214. In block
214, the revised active set is sorted from lowest energy to highest energy.
Thus, 1'A1 has the minimum measured energy in the revised active set, PA2
has the second lowest and so on up to the last member of the revised active
set PAN.
In block 216, it is determined whether 1'A1 is a member of the
candidate set. If Pm is a member of the candidate set then the flow moves to
block 34 and the revision of the active set is complete. In block 218, loop
variable i is set to 1. In block 220, COMBINED PILOT for testing Pm is
computed. The value of COMBINED_PILOT is set equal to the sum of the
measured energy of all pilots having energy greater than the pilot currently
being tested. Thus, COMBINED_PILOT is determined by the equation:
COMBINED_PILOT= Pk (2)
1=i+i
In block 222, the current pilot being tested is compared against a
threshold (T) determined in accordance with the computed value of
COMBINED_PILOT. In the exemplary embodiment, threshold T is
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determined in accordance with equation (1) above. If the measured pilot
energy PAi exceeds threshold T, then the flow moves to block 224 and the
drop timers for pilots PAi to PAN are reset to zero and determination of the
revised active set ends in block 234.
If the measured pilot energy PAi does not exceed threshold T, then the
flow moves to block 226. In block 226, it is determined whether the TDRop
timer for Pim has expired. If the 'rpm> timer has expired, then, in block 228,
the pilot PAi is removed from the revised active set and put in the candidate
set and the flow proceeds to block 230. If in block 226, it is determined that
the TDRop timer for PAi has not expired, then the flow proceeds directly to
block 230. In block 230, the loop variable (i) is incremented. Then, in block
232, it is determined whether all the pilots in the revised active set PAi
have
been tested. If all the pilots in the revised active set have been tested,
then
the flow proceeds to block 234 and generation of the revised active set is
complete. If all the pilots in the revised active set have not been tested,
then
the flow proceeds to block 220 and proceeds as described above.
Referring now to FIGS. 7 and 8, a preferred method for implementing
the present invention is illustrated. In the preferred embodiment, the
mobile station monitors the pilot signals and in response to the monitored
pilot signals the mobile station compiles members of the candidate set.
Moreover, mobile station determines whether a change to the current active
set i desirable in view of the criteria discussed above. Upon detecting any
change in the desired membership of the active set, the mobile station
generates a pilot strength measurement message that as described above
includes the identities of all pilots in the candidate and active sets
corresponding measured energy values and a corresponding indication
whether the pilot should remain in the sets or be dropped into the neighbor
set (which is indicated by setting of the KEEP variable described earlier). In
the exemplary embodiment, the base station determines the members of the
revised active set in accordance with the method described with respect to
FIG. 5.
The preferred embodiment provides for timely modification to the
members of the active set and provides for determination of the members of
the revised active set at the base station, which reduces computations at the
mobile station and allows for the selection process to include capacity
constraints of the base stations. Capacity constraints of the base stations
can
be taken into account by the base station controller simply by removing or
weighting pilot signals which are transmitted by base stations under high
capacity load conditions.
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FIG. 7 is a flowchart illustrating the method for updating the
candidate set, which in the exemplary embodiment is performed within the
mobile station. In block 300, the loop variable (i) is initialized to the
value 1.
In block 302, the pilots of the neighbor set (PN) are sorted such that PNi >
PN2
> PN3, and so on. In block 306, the neighbor set pilot currently being tested
(PNi) is compared with the threshold TADD.
If the pilot signal energy (PNi) exceeds the threshold then in
block 31.0 the pilot signal is added to the candidate set and the flow
proceeds
to block qn8. If the pilot signal energy (PNi) does not exceed the threshold,
then, from block 306, the flow proceeds directly to block 312.
In block 308, the index number of the neighbor set pilot being tested is
incremented. Then, in block 304, it is determined whether all members of
the neighbor set have been tested. If all members of the neighbor set have
not been tested, then the flow moves to block 306 and proceeds as described
before. If all members of the neighbor set have been tested, then the flow
moves to block 312.
In block 312, the index variable (i) is reset to 1. Then, in block 314, the
pilots in the candidate set (Pc) are sorted from weakest to strongest, such
that
< PC.2 < Pc3, and so on. In block 318, the energy of the candidate list being
tested (Po) is compared to the drop threshold TDRop. If the energy is below
the drop threshold, then the flow proceeds to block 324. If the energy is
above the drop threshold, then the flow proceeds to block 320. Since the list
of pilots is sorted, all the remaining members to be tested are necessarily
greater than TDRop. So, in block 320, the Timm timers for Po and all pilots
stronger than (Pc) are reset and the update of the candidate set is complete.
As described above the TDRop timer is a timer that keeps track of the
time that a pilot has been below the drop threshold. The purpose of the
TDRop timer is to avoid mistakenly dropping a strong pilot which may have a
weak measured energy due to short duration change in the propagation
environment, such as a fast fade. In block 324, the TDRop timer is started if
the timer for Po is not already running or advanced if it is.
In block 326, a test made to determined whether the TDRop timer for
the pilot (Pc) has expired. If the timer has expired, then the flow moves to
block 328 and the pilot (Pa) is removed from the candidate set. Then the
flow moves to block 322. Also, if the timer had not expired in block 326, the
flow moves directly to block 322. In block 322, the candidate set index
variable (i) is incremented. Then, in block, 316, it is determined whether all
pilots in the candidate set have been tested. If all members of the candidate
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set have been tested the candidate set update is complete. If less than all
members of the candidate set have been tested, the flow moves to block 31.4
and proceeds as described above.
In the preferred embodiment, the selection of the candidate set
5 members is performed in the mobile station. This is because selection of the
candidate set, typically, does not require knowledge of capacity constraints
of
the base stations in the network. However, in an alternative embodiment,
the method for dropping candidate set members to the neighbor set may be
performed in the base station controller. Moreover, addition of members to
10 the candidate set could be performed in the base station controller
provided
the base station controller has knowledge of or is provided with knowledge
of the members of the mobile station's neighbor set.
FIG. 8 illustrates the method for detecting the need to revise the active
set, which in the preferred embodiment is performed in the mobile station.
15 In block 400, the strongest pilot in the candidate set (P'0) is selected.
Note
the prime is to differentiate the pilot from Po referred to in FIG. 7 which
represented the weakest candidate set pilot. In block 402, the energy of (Pci)
is compared to a threshold (T) which is based on the cumulative energy of
the pilots in the active set, as shown in Equation 3 below.
T = f(EPAi). SOFT_SLOPE* EP Ai +SOFT_ADD_INTERCEPT (3)
If (Ivo) exceeds the threshold (T), then the mobile station transmits the
pilot
strength measurement message to the base station, in block 404.
If (P'0) does not exceed the threshold (T), then the flow proceeds to
block 406. In block 406, the active set is sorted from weakest pilot to
strongest
pilot. In block 408, the active set index variable (i) is set to 1. Then in
block
410, the active set pilot (Pai), which is being tested to determine whether it
should remain in the active set, is tested against a threshold (T) generated
in
accordance with a sum of energies of all stronger pilots as shown in equation
(4) below:
T = f(EPAi)= SOFT_SLOPE* p,õ +SOFT_DROP_INTERCEPT (4)
J>i J>i
If the pilot being tested (PAO exceeds the threshold (T), then it and all
pilots of
strength greater than it should remain in the active set. Thus, in block 412
the TDRop timers for all pilots with strength greater than PAi are reset and
the
current search for a revision of the active set is complete, with no need for
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revision detected by the mobile station. In the preferred embodiment, the
intercept value (SOFT_ADD_INTERCEPT) used to generate the add
threshold is permitted to be of a value different from the intercept value
SOFT_DROP_INTERCEPT used to generate the drop threshold. This
provides for greater flexibility and allows the network to introduce
additional hysterisis into the signal levels.
If the pilot (PAO is less than the threshold (T), then the flow proceeds
to block 422. In block 422, the TDRop timer for pilot (Pm) is started if not
running and advanced if already running. In block 424, whether the TDROP
timer for pilot (Pm) has expired is tested. If the TDRop timer has expired,
then
the mobile station transmits a pilot strength measurement message to the
base station in block 430. If the Timor, timer has not expired, then the flow
moves to block 426 where the active set pilot index (i) is advanced. Then,
the flow moves to block 420, where it is determined whether all active set
members have been tested. If all active set members have been tested, then
the search ceases with no need to revise the active set detected. If less than
all of the members of the active set have been tested, the flow moves to
block 410 and proceeds as described previously.
The previous description of the preferred embodiments is provided to
enable any person skilled in the art to make or use the present invention.
The various modifications to these embodiments will be readily apparent to
those skilled in the art.
Thus, the present invention is not intended to be limited to the
embodiments shown herein but is to be accorded the widest scope consistent
with the claims disclosed herein.
