Note: Descriptions are shown in the official language in which they were submitted.
2143539
METHOD FOR DETERMINING HANDOVER CANDIDATE IN A
MULTICELLULAR ENVIRONMENT
Field of the Invention
This invention relates in general to a method of determining
possible base sites as handover candidates in a multicellular
environment, and more particularly to determining handover
candidates in a multicellular environment based on a step change of
a parameter of an incoming signal from at least one of the
neighbouring cells or serving cell.
Background to the Invention
In a cellular environment, at any one time, there is usually
one serving cell defined as the cell with the base station that a
mobile unit is receiving service from so that the mobile unit may
receive and transmit communication and a number of surrounding
cells that are referred to as neighbouring cells. The serving cell may
also be referred to as the cell that the mobile unit is camped on to.
In a multicellular environment, there may be cells of different
sizes where a number of cells of the same size are located within
one larger cell (umbrella cell). The smaller cells within the umbrella
cell may be called microcells. Microcells are created in a dense
population of users to allow a greater capacity of users on the
cellular system. The microcells facilitate the reuse of frequencies
over a smaller distance. Thus, a mobile unit may be located within a
microcell as well as an umbrella cell.
Typically, rural areas that do not have a large number of users
or do not require a large capacity only need to be divided into larger
cells. As the areas grow or the cells get closer to densely populated
areas, the larger cells do not have the capacity to facilitate the
increased number of users. There are not enough frequencies
allocated. So microcells are created within the larger cells and the
larger cells become umbrella cells. This allows frequency reuse
among the microcells. Such microcellular techniques improve
spectral efficiency and increase the capacity of the cellular network.
21 43539 ~
Microcells have disadvantages. One disadvantage is that in
microcellular areas the number of handovers increases and the time
available to make handover decisions decreases. For example,
having too many smaller size microcells in an area where there is a
fast moving mobile, the fast moving mobile travels through a
number of microcells in a short amount of time causing a number of
handovers to be processed. Increasing the number of handovers in
a short amount of time decreases the call reliability and increases
the number of breaks in communication, thus, reducing the quality
of communication and in extreme cases, loses calls.
Thus, a fast and reliable method of determining when to
handover in a multicellular environment needs to be established.
One such method has been proposed in co-pending UK Patent
Application No. 9324428.3 entitled "Method for Determining
Handover in a Multicellular Environment" filed on November 27,
1993 by Motorola.
Digital cellular communications systems, such as the GSM
(Global System for Mobile Communications), integrate a large
number of cells in a microcellular environment. It is required in
GSM that a mobile station report a received signal level strength of
its six strongest neighbouring cells. Current handover techniques
choose a handover candidate from one of the six strongest
neighbouring cells. In a microcellular environment where the signal
strengths are varying rapidly, a cell may produce a strong signal
level strength in one measurement report and then a weak signal
level strength in a next measurement report. Thus, making a
handover decision based solely on a first report may result in
selecting a base site that would not be a reliable serving cell for the
mobile station.
It is desired to prevent a neighbouring cell whose signal
strength is varying rapidly from being considered as a handover
candidate. Thus, it is desired to have a method for determining a
handover candidate where a neighbouring cell is considered a
handover candidate only if it would truly be a reliable handover
3 5 candidate.
2143539 -~
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Summary of the Invention
According to the present invention, there is provided a
method for determining a handover candidate in a cellular
communication system having at least one mobile unit
communicating to a serving cell and a plurality of neighbouring cells
where the serving cell and the neighbouring cells comprise of at
least one umbrella cell and a plurality of microcells. Each cell has a
respective base station. The method for determining whether to
handover from the serving cell to a neighbouring cell includes
determining a step change of a parameter of an incoming signal to
the mobile from at least one cell base station and determining
handover in dependence upon the step change.
In a preferred embodiment of the invention, the parameter is
a rate of change of a received signal level.
In an alternate embodiment, the parameter is a received
signal strength level from neighbouring cells measured for a time
duration.
Brief Description of the Drawing
FIG. 1 illustrates a fast moving mobile unit in a multicellular
environment.
FIG. 2 illustrates a slow moving mobile unit in the
multicellular environment of FIG. 1.
FIG. 3 illustrates two buffer arrangements for a preferred
embodiment of the present invention.
FIG. 4 illustrates a typical multicellular environment.
FIG. 5 illustrates received signal levels plotted for the mobile
station of FIG. 4.
FIG. 6 illustrates a flow chart for a preferred embodiment of
the present invention.
FIG. 7 illustrates a decision chart for the preferred
embodiment of the present invention.
Detailed Description of the Preferred Embodiment
Referring to FIG. 1, a multicellular (or microcellular)
environment is shown comprising of at least one umbrella cell 1 and
214353~9
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a plurality of microcells 2, 3, 4, 5, 6. Each cell includes a base station
typically located in the geographic area covered by the cell. Not all
base stations are shown in FIG. 1. A base station typically
determines the size and capacity of the cell. A communication
5 system may include different sized cells as well as a mobile radio
unit 20 which may be receiving service from either a base station
23 of the umbrella cell 1 or a base station 25 of one of the microcells
3. Receiving service from a particular base station in terms of being
able to receive and transmit calls is also referred to as being camped
10 on that particular base station. When a mobile radio unit enters the
multicellular environment a decision should be made to determine
whether to remain being served by the current cell type or
handover to a new cell type. The decision may be dependent upon
the speed of the mobile unit.
Two cases may be defined when a mobile radio unit enters a
multicellular environment or coverage area. The mobile unit may
be moving at a high speed as shown by a car 20 in FIG. 1 or at a
slower speed as shown by a person 30 in FIG. 2. Both FIGS. 1 and 2
assume that the serving cell is the umbrella cell 1 and a plurality of
20 neighbouring cells are microcells 2, 3, 4, 5, 6.
When a fast moving mobile station 20 as shown in FIG. 1
enters a microcellular environment the present invention requires
that the mobile station rem~in on the current cell type (umbrella) to
alleviate the number of handovers that would be required in a short
25 amount of time. By measuring the received signal strength level
from a number of neighbouring cells for a period of time or
measuring the step changes in the received signal level none of the
neighbouring microcells will be reported by the mobile station as a
handover candidate. The microcells will be transparent to the base
30 station as handover candidates and any handovers required will be
to umbrella cells.
On the other hand, when a slow moving mobile station 30,
such as shown in FIG. 2, enters a microcellular environment the
present invention requires that the slow moving mobile station 30 is
35 handed over to a microcell. This ensures that the umbrella cell does
not become congested and the maximum traffic is handled by the
21~3539
s
microcell. Thus, according to one embodiment of the present
invention when the mobile station 30 enters a microcellular
environment the six strongest cells in terms of received signal
strength levels received at the mobile station are from surrounding
5 base stations of the neighbouring microcells. Since the mobile
station is slow moving some of the neighbouring cells will be
reported long enough, or for a predetermined time duration, to be
considered handover candidates. The present invention requires
handover to the most suitable cell.
According to one embodiment of the present invention a cell
does not become a handover candidate unless it has been one of the
strongest cells for a given time duration Tn. A timer is associated
with each neighbour and may be different for each neighbour. It
may be predetermined or adaptively defined by the user of the
1 5 database.
FIG. 3 shows one way in which the timer Tn may be used in a
base station handover method. In FIG. 3 the mobile station reports
the six strongest neighbour carriers in terms of received signal
levels to the base station. The system controller of the base station
20 puts them in a first buffer 35 and starts a timer Tn. If at the expiry
of the timer Tn the cell is still one of the six strongest carriers than
it is moved to a second buffer 37 as one of the handover candidates.
The handover candidate list should always contain at least one
cell which is a standby handover cell. Standby handover cell may
25 be defined as a cell that is cell barred for normal call origination but
is available for any emergency handovers. It will be one of the
umbrella cells (or certain timeslots in a given umbrella cell). This
will ensure that in the cases that where a handover is required but
there are no suitable microcells available that a call can be handed
30 over without being dropped.
Any of the following criteria may be used to checked if a cell
is suitable as a handover candidate:
A neighbouring cell has been continuously reported in the
mobile measurement reports (e.g. SACCH multiframes) over the time
35 Tn. Under this condition the neighbouring cell has the highest
probability to be a good handover candidate but it suffers from the
2143S39
disadvantage that it does not take into account fast changes in the
received signal level (RXLEV) due to fading, shadowing, etc.
The neighbouring cell has been reported at least n out of m
times in the mobile measurement report over time duration, Tn.
This method takes into account any fast changes in received signal
level in the neighbouring cell reporting and therefore has a high
probability of identifying a good handover candidate. The values of
n and m have to be optimised for each cell.
The average received signal level of the neighbouring cell
exceeds a threshold over the time Tn. This method averages out all
the peaks and troughs of received signal levels.
Signal quality and other criterion currently used in
determining handovers must also be satisfied if the neighbouring
cell is to be considered as a handover candidate.
The microcellular environment is characterised by the fast
rate of change of received signal levels (e.g. when turning corners,
going under bridges, etc.) which makes the current handover
methods sometimes inaccurate in the prediction of handover
candidates.
FIG. 4 shows a typical multicellular environment. Particularly,
when a mobile station 40 moves from point A to point B, the
received signal level of a first neighbouring base station 44
increases to a relatively high level at point X and according to the
present invention becomes one of the neighbouring cells being
reported by the mobile station 40 as a strong carrier while the
received signal level of the first neighbouring base station 44
rem~ins relatively constant.
Similarly when the mobile station 40 moves from point B to C,
the received signal level of a serving cell base station 42 suddenly
decreases at point Y to a low level while the received signal level of
the neighbouring base station 44 remains relatively constant. Due
to the nature of microcells the change in the received signal level of
the serving cell 42 and the neighbouring cell 44 will be a step
function as shown in FIG. 5 and such characteristics may be used to
predict handover candidates according to the present invention.
`~ 2143539
FIG. 5 shows the received signal level received by the mobile
station 40 as it travels from point A to B to C in FIG. 4. From point A
to B the received signal level of neighbouring cell 44 suddenly
increases at point X resulting in a step change function then rem~ins
5 relatively constant. The received signal level of serving cell 42
rem~in~s constant.
Similarly as the mobile station travels from point B to C the
received signal level of the serving cell suddenly decreases at point
Y resulting in a step change function. The received signal level of
10 neighbouring cell 44 remains constant. Points X and Y represent
edges of cell coverage.
Step changes in the received signal level of neighbouring cells
signify that the mobile station is on a corner or on a junction of a
street. Step changes in the serving cell received signal level
15 signifies that the mobile station has turned a corner and it is likely
that a handover is required. When these step changes are
recognised by the handover method of the present invention the
system may move into different modes of operation accordingly.
For, example the timer Tn associated with a particular neighbouring
20 cell may be reduced so that a fast handover decision may be made.
It is necessary to define an averaging method which is capable of
detecting step changes in the received signal level.
FIG. 6 shows an embodiment of the present invention that
brings two parameters together whereby the timer Tn associated
25 with each neighbouring cell is changed interactively when a step
change is recognised. This makes a given neighbouring cell more
likely to become a handover candidate when there are step changes
detected in the received signal (e.g. when a mobile is turning a
corner). Furthermore, the recognition of a step change also enables
30 the method to re-evaluate a priority of current handover
candidates.
According to FIG. 6, the mobile station or the serving cell base
station monitors the neighbouring cells or surrounding carriers
received at the mobile station as in step 50. If no step change is
35 determined in step 52 then it is determined if a respective timer Tn
has expired and the handover criteria is met as in step 54. If so, the
` ` 2143539
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carrier or neighbouring cell is added to the handover candidate list
in step 56 and then the process begins again. If not, then it must be
determined as in step 58 whether a new carrier has been
identified, if not, the process starts over. If so, the respective timer
5 Tn is started for the new carrier as in step 60 and the process starts
over.
If in step 52 a step change is identified then the step change is
evaluated as in step 62. It is determined if there is a handover
required as in step 64. If a handover is required then it is
10 determined whether there is a microcell as a handover candidate as
in step 66. If yes, then a handover is performed to the highest
priority microcell as in step 70. If there is no microcell as a
handover candidate as determined in step 66 then a handover is
performed to a standby cell as in step 68.
If no handover is required in step 64 then it is determined
whether a timer change is required as in step 72. If a timer change
is required as determined by step 72 then the respective timer is
changed as in step 74.
If a no timer change is required as determined by step 72
20 then it is determined whether a handover list evaluation is required
as in step 76, if no, then the process begins again, if yes, then the
handover list priority is re-evaluated as in step 78 and then the
process begins again.
Depending upon step changes in neighbouring cells and
25 serving cells certain steps may be taken which are sllmm~rised in
the table shown as FIG. 7. The top row describes possible states for
a serving cell while the first column describes possible states for a
neighbouring cell. If the mobile station receives a constant receive
signal level from the serving cell and the mobile station receives a
30 step change increase from a neighbouring cell then the mobile
station is possibly on a junction of n cells as described in state 80. If
the neighbouring cell is already a member of the second buffer then
the priority is re-evaluated. If the neighbour is not a member of
the second buffer then its associated timer Tn is changed in the first
35 buffer.
21~3539
,
The receive signal level of the serving cell shows a step change
increase and the receive signal level of the neighbouring cell shows
a step change increase then the mobile station is possibly on a
junction of n cells if neighbouring cell level is comparable to the
serving cell as described in state 82. If the neighbouring cell is
already a member of the second buffer then the priority is re-
evaluated. If the neighbour is not a member of the second buffer
then its associated timer Tn is changed in the first buffer.
If the receive signal level from the serving cell shows a step
change decrease and the receive signal level of the neighbouring cell
shows a step change increase then the mobile station has probably
turned a corner and a handover is possibly required as described in
state 84. If the neighbouring cell is already a member of the second
buffer then the priority is re-evaluated. If the neighbour is not a
member of the second buffer then its associated timer Tn is changed
in the first buffer. If no microcells are members of the second
buffer then handover to standby handover cell.
If the receive signal level from the serving cell is constant and
the receive signal level from the neighbouring cell shows a step
change decrease then continue to monitor receive signal levels as in
state 86. Similarly if the receive signal level from the serving cell
shows a step change increase or step change decrease and the
receive signal level from the neighbouring cell shows a step change
decrease then the receive signal levels will continue to be monitored
as in states 88 and 90.
If the receive signal level from serving cell is constant or
shows a step change increase and the received signal level from the
neighbouring cell is constant the receive signal levels will continued
to be monitored as is states 92 and 94.
If the received signal level of the serving cell shows a step
decrease and the receive signal level from the neighbouring cell is
constant then the mobile station has probably turned a corner and a
handover is possibly required as described in state 96. If the
neighbouring cell is already a member of the second buffer then the
priority is re-evaluated. If the neighbour is not a member of the
second buffer then its associated timer Tn is changed in the first
21g3539
buffer. If no microcells are members of the second buffer then
handover to standby handover cell.
Thus, the present invention provides a method that
determines handover candidates reliably lessening the possibility of
loss calls. There are four important scenarios that may be analysed
according to the method of the present invention.
First, a mobile station is camped on an umbrella cell and the
mobile station is moving at a high speed. According to the method
of the present invention as described in reference to FIG. 1 none of
the microcells will be reported by the mobile station for a long
enough time to be considered as handover candidates. Thus, any
handovers required will be to umbrella cells.
Second, a mobile station is camped on an umbrella cell and
moving at a slow speed. As described in reference to FIG. 2, the
mobile station will report the six strongest cells as neighbouring
cells and some of which will be reported for long enough to become
handover candidates. Thus, handover to a microcell will result.
Third, the mobile station is camped on a microcell and moving
at a fast speed. The mobile station will originate a call on a
microcell and the call will be handed over to an umbrella cell to
minimi~e the number of handovers required. With every handover
performed there is a probability that a call may be dropped. Thus,
minimi~ing the number of handovers reduces the probability of
dropped calls.
If the mobile station is moving fast through the microcellular
coverage area, the list of neighbouring microcells being reported by
the mobile station will keep changing and none of the microcells will
become a handover candidate. However one of the cells being
reported back by the mobile station will be an umbrella cell which
will be a handover candidate. This umbrella cell may be the
standby handover cell. If a handover is required and no suitable
microcells are available then the mobile station will be handed over
to the umbrella cell.
Fourth, if the mobile station is moving slow through the
microcellular coverage area, some microcells will be reported by the
mobile station for long enough time duration to be considered as
214353~
11
handover candidates. If handover is required the mobile station
will be handed over to the most suitable microcell. A measured (or
detected) step change in the neighbouring and serving cells receive
signal levels as described in FIG. 7 may be used to predict the cell to
5 which the mobile station will be handed over to.
Particularly, received signal levels have been described as
being used for determining step changes and handover candidates.
The received signal levels received from monitored cells are usually
kept constant by power control comm~nds.
An alternative embodiment of the present invention may use
a step change of the power control level as the determining
parameter. Thus, a decision to handover may be made by the base
station of the serving cell dependent upon a step change of the
power control signals received from the base stations of the cells
being monitored at the mobile unit.
Although the method has been described as being
implemented at the base station of the serving cell, the method
could actually be implemented in the mobile unit provided that the
required intelligence is built into the mobile unit. The method could
also be implemented at the base station of the neighbouring cell
provided the proper information was passed to the base station of
the neighbouring cell. As cellular systems expand, methods such as
the one of the present invention may be implemented elsewhere in
the infrastructure of the system.
The parameters defined in this method may be used to predict
the most suitable cell to which handover should be made. This
prediction is based on monitoring any step changes in the received
signal level of the serving and neighbouring cells or may be based
on the number of step changes detected in a particular period of
time. For example, if there are a number of step changes made in a
relatively short period of time it may make sense to handover to an
umbrella cell to avoid a high number of microcell handovers.
In conclusion, the present invention provides a method for a
microcellular communication system, including microcells and
umbrella cells, where slow moving mobile units utilise the
microcells and the faster moving mobile units utilise the umbrella
~ 2143~3-9
12
cells. Particularly, when a fast moving mobile unit enters a
microcell it is required that the mobile unit remains served by the
current cell type, umbrella cell, to minimi~se the number of
handovers required. Thus, the number of handovers is significantly
S reduced and the cellular environment is efficiently utilised. By
efficiently utilising the cellular environment, capacity of the cellular
communication system may be increased. The present invention
improves the reliability of handovers resulting in lower number of
dropped cells and reduced amount of processing done by the
10 network.
