U.S. patent application number 10/526761 was filed with the patent office on 2005-12-15 for measurements in communications systems.
This patent application is currently assigned to Nokia Corporation. Invention is credited to Hakalin, Petteri, Hirvonen, Heikki, Schwarz, Uwe, Tolli, Antti.
Application Number | 20050277416 10/526761 |
Document ID | / |
Family ID | 31985219 |
Filed Date | 2005-12-15 |
United States Patent
Application |
20050277416 |
Kind Code |
A1 |
Tolli, Antti ; et
al. |
December 15, 2005 |
Measurements in communications systems
Abstract
A method of performing compressed mode measurements for
selecting communication means in a communication system. The
communication system comprises a network element and a plurality of
communication means for serving a mobile station. The method
comprises providing information associated with the plurality of
communication means to the network element, said information based
on a plurality of parameters associated with each of the plurality
of communication means; ordering the communication means based on
said information; and performing compressed mode measurements at
the mobile station based on said ordering.
Inventors: |
Tolli, Antti; (Oulu, FI)
; Schwarz, Uwe; (Veikkola, FI) ; Hakalin,
Petteri; (Malaga, ES) ; Hirvonen, Heikki;
(Tampere, FI) |
Correspondence
Address: |
SQUIRE, SANDERS & DEMPSEY L.L.P.
14TH FLOOR
8000 TOWERS CRESCENT
TYSONS CORNER
VA
22182
US
|
Assignee: |
Nokia Corporation
Keilalahdentie 4
Espoo
FI
FIN-02150
|
Family ID: |
31985219 |
Appl. No.: |
10/526761 |
Filed: |
March 7, 2005 |
PCT Filed: |
September 10, 2002 |
PCT NO: |
PCT/FI02/00728 |
Current U.S.
Class: |
455/436 ;
455/67.11 |
Current CPC
Class: |
H04B 17/318 20150115;
H04W 36/14 20130101; H04W 36/0094 20130101 |
Class at
Publication: |
455/436 ;
455/067.11 |
International
Class: |
H04B 017/00 |
Claims
1. A method of performing compressed mode measurements for
selecting communication means in a communication system, said
communication system comprising a network element and a plurality
of communication means for serving a mobile station and, said
method comprising: providing information associated with the
plurality of communication means to the network element, said
information based on a plurality of parameters associated with each
of the plurality of communication means; ordering the communication
means based on said information; and performing compressed mode
measurements at the mobile station based on said ordering.
2. A method as claimed in claim 1, wherein the selection is for
handover of the mobile station from a first communication means to
a second communication means.
3. A method as claimed in claim 2, wherein the first communication
means operates at a first frequency of a radio access technology
and the second communication means operates at a second frequency
of said radio access technology.
4. A method as claimed in claim 3, wherein the radio access
technology is code division multiple access.
5. A method as claimed in claim 3, wherein the radio access
technology is wideband code division multiple access.
6. A method as claimed in claim 2, wherein the first communication
means operates in accordance with a first radio access technology,
and the second communication means operates in accordance with a
second, different, radio access technology.
7. A method as claimed in claim 6, wherein the first radio access
technology is code division multiple access.
8. A method as claimed in claim 6, wherein the first radio access
technology is wideband code division multiple access.
9. A method as claimed in claim 2, wherein the second communication
means comprises a plurality of cells, and the compressed mode
measurements comprise signal strength measurements of at least one
of said plurality of cells.
10. A method as claimed in claim 6, wherein the second
communication means comprises a plurality of cells, and the
compressed mode measurements comprise signal strength measurements
of at least one of said plurality of cells, and wherein the
compressed mode measurements comprise decoding a parameter
associated with at least one of the plurality of cells.
11. A method as claimed in claim 10, wherein the parameter is the
base station identification code associated with one of the
plurality of cells.
12. A method as claimed in claim 1, wherein the plurality of
parameters comprises at least one of the following: a real time
load, a non real time load, a service priority weight or a signal
to interference ratio.
13. A method as claimed in claim 1, wherein the information
comprises a weighting value.
14. A method as claimed in claim 1, wherein the communication means
are ordered in a prioritised order.
15. A method as claimed in claim 1, wherein the network element is
a radio network controller.
16. A method as claimed in claim 1, wherein the information is
provided by a common resource radio management.
17. A method as claimed in claim 16, wherein the common resource
radio management is a common radio management server.
18. A communication system comprising: a network element; a mobile
station; a plurality of communication means, said communication
means being arranged to provide communication services to said
mobile station; means for providing information associated with the
plurality of communication means to the network element, said
information being based on a plurality of parameters associated
with each of the plurality of communication means; and means for
ordering the communication means being based on said information;
said mobile station being arranged to perform compressed mode
measurements based on said ordering for selecting one of the
plurality of communication means.
19. A method of determining a threshold for a cell in a
communication system, said communication system comprising said
cell and a plurality of other cells, said method comprising the
steps of: collecting statistics on the handovers from said cell to
said plurality of other cells; weighting the cell load of each cell
of said plurality of other cells by the percentage of handovers
from said cell to respective one of said plurality of other cells;
and determining the threshold based on said weighted cell
loads.
20. A method as claimed in claim 19, wherein said weighting
comprises multiplying said cell load by said percentage for each
cell.
21. A method as claimed in claim 20, wherein the threshold is
determined by adding together all said weighted cell loads.
Description
FIELD OF INVENTION
[0001] This invention relates to measurements in communications
systems, and especially to measurements for selection of
communication means.
BACKGROUND TO THE INVENTION
[0002] A diverse range of wireless communication systems are in use
today enabling communication between two or more entities, such
mobile stations or other user equipment. Examples of wireless
communications systems include, without limiting to these, GSM
(Global System for Mobile Communication), EDGE (Enhanced Data Rate
for GSM Evolution), GPRS (General Packet Radio Services), and so
called 3G (Third Generation) systems such as CDMA (Code Division
Multiple Access) and WCDMA (Wideband CDMA). These systems are
examples of radio access technologies (RAT).
[0003] Presently, GSM, a so called 2G (second generation) system,
is widely used by operators of wireless networks. However, wireless
systems have been developing at a rapid pace and more advanced 3G
systems, such as WCDMA, are predicted to supersede 2G systems in
the next few years. Network operators therefore have to consider
how to migrate from one system, such as GSM or EDGE, to another,
such as WCDMA, smoothly and cost effectively. One solution proposed
is for an operator to maintain their existing GSM/EDGE system
whilst introducing a WCDMA network that can run concurrently, and
that can ultimately be merged seamlessly together with it, forming
a `multiradio` network.
[0004] FIG. 1 shows generally the architecture for a network
operating under WCDMA. Such a network is sometimes referred to a
Universal Mobile Telecommunications System (UMTS). The network
comprises a number of base-stations (BSs) 101, 102 and 103. Each
base-station has a radio transceiver capable of transmitting radio
signals to and receiving radio signals from the area of cells 104,
105 and 106. By means of these signals the base-station can
communicate with a mobile station (MS) 107. Each base station is
linked to a single radio network controller (RNC) 108. An RNC can
be linked to one or more BSs. An RNC can be linked to another RNC
via an Iur interface 120. Each RNC is linked by an Iu interface 121
to a core network (CN) 109. The CN includes one or more serving
nodes that can provide communication services to a connected mobile
station, for example a mobile switching centre (MSC) 110 or a
serving GPRS (general packet radio service) support node (SGSN)
111. These units are connected by the Iu interface to the RNCs. The
CN is also connected to other telecommunications networks such as a
fixed line network, PSTN (public switched telephone network), 114,
the Internet 115 and another mobile network 116 to allow onward
connection of communications outside the UMTS network. The BSs and
the RNC and their interconnections constitute a UMTS terrestrial
radio access network (UTRAN).
[0005] When the mobile station (MS) moves between cells during a
communications connection there is a need to hand it over from
communication with the BS of the old cell to the BS of the new cell
without dropping the call due to a break in communications between
the mobile station and the network. This process is known as
handover. A need can also arise to hand over the MS even when it
does not move, for example when local conditions affect its
communications in the old cell and call quality can be improved by
handing over to another cell or if there is a need to free up
capacity in the old cell e.g. due to overloading.
[0006] Handovers may also occur in other systems such as GSM or
EDGE. The reasons for handover may also be similar, though it will
be appreciated by one skilled in the art that the elements of the
network may be different to those shown in FIG. 1 if the system
differs.
[0007] In both proposed WCDMA systems and existing GSM/EDGE
systems, the coverage area of the network may be made up various
cell types such as macrocell, microcell and picocell. Macrocells
may be defined as cells having the largest coverage area, followed
by microcells, with picocells defined as cells having the smallest
coverage area. It should be appreciated that areas containing a
large number of MSs, such as in cities, would generally be served
by microcells or picocells, as a collection of several microcells
or picocells would be able to handle more MSs and traffic than a
macrocell with the same coverage area. Conversely, areas with lower
MS numbers would generally be served by macrocells. However, the
types of cells used in any given area are not mutually exclusive,
and typically a layered approach may be adopted. This is where
macrocells, microcells and picocells are all used within the same
area in an overlapping/layered manner. The result is a more robust
network that is better equipped to handle traffic fluctuations as
handovers can be more effectively used to share traffic load across
the overlapping layers of cells and not just to extend coverage
across adjacent cells.
[0008] Such a layered approach is particularly important in a
multiradio network, which may incorporate, for example, both
GSM/EDGE and WCDMA. FIG. 2 shows a layered structured of cells in
an example of part of a multiradio network cell structure 400. The
network comprises a macrocell layer 250 of a macrocell 201
operating under GSM/EGDE and a macrocell 202 operating under WCDMA
at frequency f1. With substantially the same coverage area as the
macrocell layer is a microcell layer 260. The microcell layer 260
comprises microcells 203, 204 and 205 operating under GSM, and
microcells 206, 207 and 208 operating under WCDMA at frequency f2.
Another layer, a picocell layer 270, operates below the microcell
layer. The picocell layer 270 comprises picocells 209 and 210
operating under WCDMA at frequency f3, 211 and 212 operating under
TDD (Time Division Duplex access mode), and 213, 214, 215 and 216
operating under a WLAN (wireless local area network) system. This
layered architecture is particularly suited to traffic load sharing
described earlier.
[0009] It should be appreciated that the number and coverage area
of each type of cell may vary, and is not limited to those depicted
in FIG. 2. For example, the macrocells may have the same coverage
area as 4 or 5 microcells instead of the 3 depicted.
[0010] In a multiradio network, it is important to utilise all the
systems (e.g. GSM, WCDMA) or carriers (e.g. different layers of
WCDMA operating at different frequencies) in the most efficient way
possible. For example, the coverage area of the network can be
increased by moving users from one cell to an adjacent cell, such
as between GSM microcells 203 and 204, or WCDMA picocells 209 and
210 by utilising handover techniques.
[0011] The capacity of the network can also be increased by moving
users from a highly loaded cell to one with a low load by utilising
traffic reason handover techniques.
[0012] Service changes may also trigger a handover. Typical
services may include real time services such as conversational
communications (at various data rates), data streaming (at various
data rates), and non real time services such as interactive web
browsing and emailing. A handover may take place when a
neighbouring cell/system is better suited to provide the service
requested.
[0013] Traffic load handover may be triggered when the load of a
cell increases beyond a certain threshold, and the overloaded cell
instructs one or more of the MSs in the cell to perform
handovers.
[0014] In general, these handovers fall into two types:
inter-frequency/carrier (IF) and inter-system (IS) or inter-RAT
(Radio Access Technology). IF handover takes place when the systems
of the cells stay the same, but the frequency of the systems
change. This can occur for example between WCDMA systems operating
in different frequency bands. For example, a handover from WCDMA
macrocell 202 to WCDMA microcell 207 would constitute an IF
handover. IS handover is handover across systems. For example, a
handover from WCDMA microcell 207 to GSM macrocell 201, or from
WLAN picocell 213 to WCDMA picocell 209 would constitute IS
handovers.
[0015] Present radio access networks have consisted of just a
single radio access network, such as GSM, or several independent
systems. Some functionality is available for load sharing and
interference distribution within GSM networks, but the efficiency
of such methods are limited to the area controlled by the
associated radio resource controller, such as or a Base Station
Controller (BSC) in GSM system or an RNC in the WCDMA system
illustrated in FIG. 1. As such, selection of the new cell/system in
handovers in a multiradio system has been less than adequate. This
is partly due to the limitations in the signalling between the
different controllers. Thus, load information sharing is not
generally available between cells, and selection of a new
cell/system is based only on received signal strength or quality
and some predefined offsets and parameters. These may include, for
example, minimum signal level thresholds for target system/cell to
be selected or some offset for target system/cell's signal strength
over the current level in the source cell.
[0016] To this end, Common Resource Radio Management (CRRM) and
Common Radio Management Server (CRMS) have been introduced to help
manage some of these issues that have arisen in multiradio systems.
Their roles include overall resource management of controller and
system borders to provide load sharing for efficient use of
resources, interference distribution to provide higher spectral
efficiency and improved QoS (quality of service) management. CRRM
can be implemented in both a centralised in a CRMS and distributed
manner across other elements such as RNCs.
[0017] FIG. 3 illustrates the structure of a typical distributed
CRRM arrangement. Here the CRRM entities, 304, 306 and 308, are
located at each of the RNCs/BSCs 301, 302 and 303. Radio resource
management within each controller is managed by the Radio Resource
Management entity (RRM), 305, 307 and 309. The CRRMs are
responsible for control between the RRM entities, and communication
between the controllers is done via the CRRMs.
[0018] FIG. 4 illustrates the structure of a centralised CRRM
arrangement utilising a CRMS. The arrangement comprises RNC/BSC
403, 405 and 406, associated with RRMs 404, 406 and 408
respectively. Control of these RRMs is done by a centralised CRRM
entity, the CRMS 401.
[0019] When a MS is connected to a GSM cell, the transmission gaps
in GSM communications can be used for continuous measurements of
other systems or layers to assist in handovers. These measurements
may include determining cell ID (identity) information required for
connecting to a cell of new system/frequency. However, when a MS is
in connected to a WCDMA cell its scope for making measurements at
other frequencies or of other systems is significantly reduced
because of the continuous transmission nature of communications in
WCDMA. Therefore, any measurements in WCDMA have to be typically
performed in compressed mode.
[0020] In compressed mode, reception of a signal is stopped for a
certain period of time to enable the MS to measure at another
frequency. To achieve this, the data has to be compressed before
sending it to the MS. This data compression is controlled by the
RNC.
[0021] However, in compressed mode, data is transmitted at a higher
power, generating more interference, which affects cell capacity
further. Therefore, measurements in compressed mode are generally
kept to a minimum. Compressed mode measurements take time, which
delays the handover procedure. Simultaneous IF and IS compressed
mode measurements might not be allowed due to practical limitations
such as time limitations on measurement of multiple neighbour
cells. Furthermore the number of GSM BSICs (base station
identification codes) that can be decoded from neighbouring cells
is limited. All these factors impose practical limitations on the
number of target cells that can be measured before handover takes
place.
[0022] As both IF and IS compressed mode measurements made
simultaneously might not be possible, the selection between IF and
IS compressed mode measurements must be made after handover is
triggered. If the selection is based on only signal strength and/or
service priorities then the undesirable selection of a highly
loaded cell is possible. Furthermore, as CM measurements are time
consuming, a reduction in such measurements is desirable to reduce
handover delays. Selecting the correct system is also important.
For example, if the call is a circuit switched speech service then
preference may be for the cell to be a GSM microcell. Conversely,
if the call is packet switched conversational service (e.g. video
telephony) then a WCDMA cell may be preferred.
[0023] Another problem arises when a MS is connected to a WCDMA
cell and is making IS compressed mode measurements to GSM cells. IS
compressed mode measurements generally take two parts: a received
signal strength indicator (RSSI) measurement from all neighbouring
GSM cells; and BSIC decoding for all (RSSI) measured neighbouring
GSM cells. BSIC decoding is very time consuming, and typically, the
BSIC of the measured GSM neighbour with the highest RSSI is decoded
and chosen as the target cell.
[0024] This can significantly reduce CRRM performance since a
highly loaded GSM cell could be selected as a target cell even if
there were lower loaded cells with adequate signal levels
available.
[0025] When setting the threshold over which handover should be
triggered in a source cell, factors such as potential overlap of
the neighbouring cells as well as the neighbouring cells' loads
should be considered. Handover should only be triggered if there
are neighbouring cells available to handover to with a lower load,
hence any threshold that is set for a source cell must take this
into account. Neighbouring cells that are not overlapping or at
adjacent to the source cell should not influence the setting of the
threshold as much. However, when handover is to be made
intersystem, then information relating to the precise coverage area
of target cells may not be readily available, thus making
calculation of a handover threshold for the source cell difficult
when presented only with a list of neighbouring cells and their
associated loads. The problem is not just limited to intersystem
handovers, and similar problems arise with inter-frequency
handovers and trying to estimate cell loads and handover
thresholds.
SUMMARY OF THE INVENTION
[0026] Embodiments of the present invention aim to overcome one or
several of the above problems.
[0027] According to one aspect of the present invention, there is
provided a method of performing compressed mode measurements for
selecting communication means in a communication system, said
communication system comprising a network element and a plurality
of communication means for serving a mobile station and, said
method comprising: providing information associated with the
plurality of communication means to the network element, said
information based on a plurality of parameters associated with each
of the plurality of communication means; ordering the communication
means based on said information; performing compressed mode
measurements at the mobile station based on said ordering.
[0028] The selection may be for handover of the mobile station from
a first communication means to a second communication means.
Furthermore, the first communication means may operate at a first
frequency of a radio access technology and the second communication
means may operate at a second frequency of said radio access
technology.
[0029] Preferably the radio access technology is code division
multiple access or wideband code division multiple access.
[0030] The first communication means may operate in accordance with
a first radio access technology, and the second communication means
may operate in accordance with a second, different, radio access
technology. Furthermore, the first radio access technology may be
code division multiple access or wideband code division multiple
access.
[0031] Preferably, the communication means comprises a plurality of
cells, and the compressed mode measurements comprise signal
strength measurements of at least one of said plurality of
cells.
[0032] Preferably, the compressed mode measurements comprise
decoding a parameter associated with at least one of the plurality
of cells. The parameter may be the base station identification code
associated with one of the plurality of cells.
[0033] The plurality of parameters may comprise at least one of the
following: a real time load, a non real time load, a service
priority weight or a signal to interference ratio.
[0034] Preferably, the information comprises a weighting value. The
communication means may be ordered in a prioritised order.
[0035] The network element may be a radio network controller, and
the information may be provided by a common resource radio
management. The common resource radio management may be a common
radio management server.
[0036] According to a second aspect of the present invention, there
is provided a communication system comprising: a network element, a
mobile station, a plurality of communication means, said
communication means arranged to serve said mobile station; means
for providing information associated with the plurality of
communication means to the network element, said information based
on a plurality of parameters associated with each of the plurality
of communication means; and means for ordering the communication
means based on said information; said mobile station arranged to
perform compressed mode measurements based on said ordering for
selecting one of the plurality of communication means.
[0037] According to a third aspect of the present invention there
is provided a method of determining a threshold for a cell in a
communication system, said communication system comprising said
cell and a plurality of other cells, said method comprising the
steps of: collecting statistics on the handovers from said cell to
said plurality of other cells; weighting the cell load of each cell
of said plurality of other cells by the percentage of handovers
from said cell to respective one of said plurality of other cells;
and determining the threshold based on said weighted cell
loads.
[0038] Preferably the weighting comprises multiplying said cell
load by said percentage for each cell, and the threshold is
determined by adding together all the weighted cell loads.
BRIEF DESCRIPTION OF THE DRAWINGS
[0039] Embodiments of the present invention will now be described
by way of example with reference to the accompanying drawings, in
which:
[0040] FIG. 1 illustrates the general architecture of a
telecommunications system.
[0041] FIG. 2 illustrates the layered cell structure of a
multiradio network.
[0042] FIG. 3 illustrates the structure of a distributed CRRM
arrangement.
[0043] FIG. 4 illustrates the structure of a centralised CRRM
arrangement utilising a CRMS.
[0044] FIG. 5 illustrates measurement table according to one
embodiment of the present invention.
[0045] FIG. 6 illustrates the determining and prioritising of cells
according to load.
[0046] FIG. 7 is a flow diagram for one embodiment of the present
invention.
[0047] FIG. 8 is a flow diagram for another embodiment of the
present invention.
[0048] FIG. 9 is a flow diagram for another embodiment of the
present invention.
[0049] FIG. 10 is a table showing the typical HO and neighbouring
cell load statistics for a cell.
[0050] FIG. 11 is a graph showing how threshold values may be
set.
DETAILED DESCRIPTION OF EMBODIMENTS
[0051] Handover in a cell can be triggered by various factors such
a mobile station moving out of the coverage area of a cell, in an
attempt to spread traffic load from a highly loaded cell to less
loaded cells, or when a request for a different service is
made.
[0052] In a preferred embodiment of the present invention, a
prioritised measurement list or table for system or frequency (in a
system) in the network is generated by the CRRM, and maintained at
either a CRMS (centralised CRRM arrangement), at all the RNCs in
network (distributed CRRM arrangement), or any other suitable
network element(s). FIG. 5 illustrates a typical prioritised
measurement table, 500.
[0053] The first column of the measurement table 500 lists the
target system/frequency, each of which may include a plurality of
cells operating under the given system at the given frequency. The
list shown here has a single entry for a GSM/EDGE system. The other
columns provide a value to each system/frequency for various
services, such as conversational circuit switched (CS) speech in
the column 2, and streaming packet switched (PS) data in the column
7. The conversational and streaming services are classed as real
time (RT) services, and the interactive and background service in
FIG. 5 are classed as non real time (nRT) services.
[0054] The values in the measurement table provide a prioritised
indication of the order that any compressed mode (CM) measurements
should be performed in by a mobile station that wishes to handover
to another cell of a system/frequency. For example, the
system/frequency best suited to receiving a handover may be
allocated the highest priority value (or first measurement order)
of 1, and CM measurements should then be made with system/frequency
first in an attempt to handover to it.
[0055] FIG. 6 illustrates how the values in the measurement table,
priority indicators, may be calculated. An algorithm calculates a
weighting value, Wn, for all systems/frequencies, including that of
the present cell, from information such as average RT or nRT load,
service priority weight (some systems will be better equipped to
handle certain services), average cell quality, and other
parameters. A fuzzy logic approach may be used in the algorithm.
The weighting values, Wn, are ordered to create a list indicating
each system/frequency's relative ability to handle the handover of
a mobile station for a given service, with those
systems/frequencies with the highest weighting allocated the
highest priority indicator, or measurement order. Here,
system/frequency(4) has generated a weighting value of 25, the
lowest, and has therefore been assigned the lowest priority
indicator (or last measurement order), 4. The priority indicators
in FIG. 6 are calculated for each service and used to create the
measurement table 500 providing a complete list of system/frequency
priorities for all services.
[0056] The measurement table may be updated periodically or may be
event driven, for example when the loading in a cell or system
changes by a certain amount. Other events occurring in a cell or to
system could also trigger a recalculation of the table, as could
manual changes to the algorithm used.
[0057] The different systems/frequencies may be different
communication means operating in accordance with different radio
access technologies, such as GSM or WCDMA. The radio access
technologies may further operate under different frequencies such
as WCDMA(frequency1) and WCDMA(frequency2).
[0058] FIG. 7 shows a flowchart illustrating how a preferred method
of the invention can utilise the measurement table 500.
[0059] FIG. 7 describes the following:
[0060] 1. Inter-cell handover is triggered for a MS in Cell1 for a
reason such as change in service or to spread cell load, step
701.
[0061] 2. In step 702, if the RNC does not have the measurement
table stored locally, requests it form the CRRM. The requested
table may be a cut down version of the one shown in FIG. 5, which
includes only entries for the service requested by the MS.
[0062] 3. The RNC checks the measurement table to find the
system/frequency with the highest priority, and selects this as the
target handover system/frequency, step 703.
[0063] 4. The RNC instructs the MS to perform CM measurements to
find target handover cell, step 704. The RNC does this by providing
the MS with a list of all cells in the target system/priority to
perform CM measurements on.
[0064] 5. Handover is then triggered to the cell with the highest
measured signal level, step 705. This helps filter out those cells
in a system/frequency that may be out of the general coverage area
of the MS.
[0065] 6. If CM measurement is not successful or possible with the
cells in the target handover system/frequency, the RNC selects the
system/frequency with the next highest priority as the new target
handover system/frequency, step 706, and the process of steps 704
and 705 are repeated.
[0066] The measurement table may typically have one general entry
covering all cells of a different system such as a GSM/EDGE system.
If IS handover is selected, then the CM measurements made by the MS
may also include RSSI measurements and BSIC decoding, which is
required to select and connect to the optimum GSM/EDGE cell.
[0067] One advantage of the above method is that all
systems/frequencies in the area have been optimally sorted
according to their capability for receiving a MS in a handover. The
result is that CM measurements are kept to a minimum, and only
performed according to the prioritised measurement table.
Furthermore, the prioritisation can also take into account
different systems, such as GSM/EDGE. This has the advantage of
overcoming the problem of previously being limited by the number of
IF or IS CM measurements than can practically be done, as now a
decision has already been made as to whether to perform IF or IS
prior to starting any measurements.
[0068] Cell load may be better shared in the embodiment described
above as those residing in a lightly loaded system/frequency may be
given a higher priority, so that handover is more likely to take
place to such cells. As a result, calls may be better handled, with
fewer drop-outs for users during a call due to excessive loading.
The capacity of the network as a whole may also be optimised, with
the total number of handovers reduced, as a result of this
optimised method.
[0069] As fewer CM measurements may result in preferred embodiments
of the invention, the actual time taken to perform a hand over may
be reduced. As pointed out above, if the target cell is actually
another system, then IS handover must take place. In another
embodiment of the present invention, a prioritised neighbour cell
list is generated by the CRRM. The list contains an indicator of
the attractiveness for handovers of each cell in a given system.
The indicator may be represented as a numerical value, and the list
may be prioritised according to this indicator. The list can either
be generated centrally at the CRMS or at the relevant RNC under the
control of the CRRM. In both cases, the list is available at the
RNC, which uses it to determine a prioritised order in which the
BSIC of the neighbouring cells in the system should be decoded
in.
[0070] The indicator may be generated based on information relating
to the neighbouring cells such as cell load, signal strength, QoS,
cell capacity, service priorities, and cell quality indicators.
[0071] FIG. 8 shows a flowchart illustrating one preferred method
utilising the prioritised neighbour cell list to determine a
prioritised order for BSIC decoding.
[0072] FIG. 8 describes the following:
[0073] 1. IS handover is triggered for MS in a cell, step 801, to a
GSM system.
[0074] 2. RNC sends inter system neighbour list to CRRM, step
802.
[0075] 3. RNC commands MS to begin CM measurements of the RSSI of
neighbouring GSM cells, step 803.
[0076] 4. RNC receives prioritised neighbour cell list from CRRM,
step 804.
[0077] 5. MS reports the results of the RSSI CM measurements to
RNC, step 805.
[0078] 6. RNC weights received RSSI values by the indicators in the
prioritised list, step 806, then reorders the list accordingly. One
way of weighting the RSSI values by multiplying the RSSI values by
the associated indicators in prioritised list. The list can then be
reordered according to the result of this weighting to give a
reordered prioritised list that takes into account the measured
RSSI value.
[0079] 7. MS then decodes BSIC according to the reordered
prioritised list, step 807.
[0080] 8. RNC instructs MS to stop BSIC decoding once a suitable
cell, i.e. one that has been successfully decoded, is found.
[0081] It should be noted that the method described above is based
on a centralised CRRM arrangement, where the CRRM provides the
prioritised list. The same method may be employed in a distributed
CRRM arrangement, where the CRRM instructs the RNC to generate the
prioritised list, and providing the RNC with the relevant inputs
such as cell loads, service priorities etc.
[0082] An alternative method of generating and utilising a
prioritised neighbour cell list is shown in the flow diagram in
FIG. 9.
[0083] FIG. 9 describes the following:
[0084] 1. IS handover is triggered for a MS in a cell, step 901, to
a GSM system.
[0085] 2. RNC instructs MS to begin CM measurements of the RSSI of
neighbouring GSM cells, step 902.
[0086] 3. RNC maps cell identifiers to the measured RSSI cells and
sends to CRMS with RSSI measurements for prioritisation, step
903.
[0087] 4. RNC commands MS to perform BSIC decoding for the N
highest RSSI measured cells, step 904. If N is set to 1, then only
the cell with the highest RSSI is measured and BSIC decoded.
[0088] 5. RNC receives a prioritised neighbour cell list from the
CRMS, step 905. If the N highest measured RSSI cells is within the
top M prioritised neighbour cell list, then the RNC commands
handover of the MS to one of the N cells (typically the top
prioritised cell). If none of the N cells fall within the top M
prioritised cells, then BSIC decoding of the highest prioritised
cell is commanded by the RNC, and handover to that cell is
performed.
[0089] The values of N and M may be set to various values, and may
depend on the capacity of the system.
[0090] As already highlighted, when IS handover takes place, BSIC
decoding is required, which is computationally very time consuming.
In the past, this has meant that typically only BSIC decoding of
the cell with the highest measured RSSI is performed. The result is
that a highly loaded cell could be selected even if there were
lower loaded cells available. The above methods provide for
optimised handover that takes into account neighbour cell load,
which helps avoid handovers to highly loaded cells. Optimised
handover is also achieved using minimum CM measurements.
[0091] In the above embodiments, load based handovers may be
triggered when a predetermined threshold in the source cell is
reached. This threshold may be set in various ways by the CRRM or
any other appropriate network element such as a RNC.
[0092] In another embodiment of the present invention, statistics
are collected on which of the neighbouring cells of a source cell
have been the subject of handovers. Each neighbour cell load value
is weighted by the percentage of handovers performed to the cell
from a source cell. The resulting average weighted load is used to
determine the load threshold for the source cell. Similarly,
statistics may be gathered for handovers to the source cell from
other cells, thus providing greater statistical accuracy for
weighting each neighbour cell load of the source cell.
[0093] The same averaging method may also used in the target
system/frequency selection algorithm described above, and not just
to determine the load threshold. Furthermore, the calculation of
the average weighted load is not limited to just neighbouring
cells, but also between systems to neighbouring systems.
[0094] FIG. 10 shows a table listing handover statistics for a
source cell. The IF/IS load average for this source cell is
calculated by multiplying each neighbour cell load by its
corresponding handover percentage as follows:
0.76*84%+0.15*80%+0.04*75%+0.03*76%+ . . . =81.12%
[0095] The new threshold may then be set according the graph shown
in FIG. 11.
[0096] FIG. 11 shows a graph illustrating a hysteresis method for
determining the new threshold by using the average load calculated
above. The new threshold set using this method is done at discrete
levels in accordance with the average calculated load. For example,
with reference to the graph, if the average neighbour load is
calculated to be 82% and the current threshold is at 80%, then the
new threshold will be set to 84%. However, if the average neighbour
load is calculated to be 84% and the current load threshold is at
84%, then the threshold will remain the same. It will not be reset
until average load goes over 84%, when it will be reset to 88%.
[0097] The hysteresis in this method also means that thresholds are
not reset too frequently due to minor fluctuations in the average
neighbour load, which may result in handover to be triggered when
the average load of the neighbouring cells differ only marginally
to that in the source cell.
[0098] Instead of, or in addition to, the load percentage, the
available capacity (e.g. time slots, available transmission power)
or any other measurement associated with the neighbouring cells
could also be used in the calculation of the weighted average.
[0099] It is also noted herein that while the above describes
exemplifying embodiments of the invention, there are several
variations and modifications which may be made to the disclosed
solution without departing from the scope of the present invention
as defined in the appended claims.
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