U.S. patent application number 12/821042 was filed with the patent office on 2011-02-17 for scrambling code selection.
This patent application is currently assigned to UBIQUISYS LIMITED. Invention is credited to Alan James Auchmuty Carter, Andrea Giustina, Aminu Wada Maida, Simon Pearcey.
Application Number | 20110039539 12/821042 |
Document ID | / |
Family ID | 41129971 |
Filed Date | 2011-02-17 |
United States Patent
Application |
20110039539 |
Kind Code |
A1 |
Maida; Aminu Wada ; et
al. |
February 17, 2011 |
SCRAMBLING CODE SELECTION
Abstract
A basestation forms part of a group of basestations within a
cellular communications network, and selects an identifying code
for use in identifying transmissions from the basestation. The
basestation receives from a management node a first list of
identifying codes and a second list of identifying codes, wherein
the identifying codes of the first list can appear in neighbour
cell lists of basestations outside said group, and wherein the
identifying codes of the second list can not appear in neighbour
cell lists of basestations outside said group. The basestation
determines whether there is at least one identifying code either in
the first list of identifying codes or the second list of
identifying codes that is not used by any other basestation in said
group. If there is at least one identifying code in the first list
of identifying codes and at least one identifying code in the
second list of identifying codes that are not used by any other
basestation in said group, an identifying code from the first list
of identifying codes is selected in preference to an identifying
code from the second list of identifying codes.
Inventors: |
Maida; Aminu Wada; (Swindon,
GB) ; Carter; Alan James Auchmuty; (Swindon, GB)
; Pearcey; Simon; (Bath, GB) ; Giustina;
Andrea; (Milan, IT) |
Correspondence
Address: |
Weaver Austin Villeneuve & Sampson LLP
P.O. BOX 70250
OAKLAND
CA
94612-0250
US
|
Assignee: |
UBIQUISYS LIMITED
Wiltshire
GB
|
Family ID: |
41129971 |
Appl. No.: |
12/821042 |
Filed: |
June 22, 2010 |
Current U.S.
Class: |
455/422.1 |
Current CPC
Class: |
H04J 11/0056 20130101;
H04W 24/02 20130101; H04B 1/7083 20130101; H04B 2201/70702
20130101; H04W 84/045 20130101; H04W 8/26 20130101 |
Class at
Publication: |
455/422.1 |
International
Class: |
H04M 1/00 20060101
H04M001/00 |
Foreign Application Data
Date |
Code |
Application Number |
Aug 11, 2009 |
GB |
0914020.3 |
Claims
1. A method, for use in a basestation forming part of a group of
basestations within a cellular communications network, for
selecting an identifying code for use in identifying transmissions
from the basestation, the method comprising: receiving from a
management node a first list of identifying codes and a second list
of identifying codes, wherein the identifying codes of the first
list can appear in neighbour cell lists of basestations outside
said group, and wherein the identifying codes of the second list
can not appear in neighbour cell lists of basestations outside said
group; determining whether there is at least one identifying code
either in the first list of identifying codes or the second list of
identifying codes that is not used by any other basestation in said
group; and if there is at least one identifying code in the first
list of identifying codes and at least one identifying code in the
second list of identifying codes that are not used by any other
basestation in said group, selecting an identifying code from the
first list of identifying codes in preference to an identifying
code from the second list of identifying codes.
2. A method as claimed in claim 1, wherein the identifying codes
are scrambling codes.
3. A method as claimed in claim 1 or 2, wherein the basestation is
a femtocell access point, and the group of basestations is a group
of femtocell access points connected by means of a local area
network.
4. A method, for use in a basestation forming part of a group of
basestations within a cellular communications network, for
selecting an identifying code for use in identifying transmissions
from the basestation, the method comprising: receiving from a
management node a first list of identifying codes and a second list
of identifying codes, wherein the identifying codes of the first
list can appear in neighbour cell lists of basestations outside
said group, and wherein the identifying codes of the second list
can not appear in neighbour cell lists of basestations outside said
group; determining whether it is necessary to be able to perform
cell reselection between the basestation and basestations outside
said group; if it is determined that it is necessary to be able to
perform cell reselection between the basestation and basestations
outside said group, selecting an identifying code from the first
list of identifying codes; and if it is determined that it is not
necessary to be able to perform cell reselection between the
basestation and basestations outside said group, selecting an
identifying code from the first list of identifying codes or from
the second list of identifying codes.
5. A method as claimed in claim 4, further comprising: determining
whether it is necessary to be able to perform cell reselection
between the basestation and basestations outside said group, based
on a history of cell reselections involving the basestation.
6. A method as claimed in claim 4 or 5, wherein the basestation is
a femtocell access point, and the group of basestations is a group
of femtocell access points connected by means of a local area
network.
7. A method, for use in a management node of a cellular
communications network, wherein the network includes at least one
group of basestations and further includes other basestations not
within said group, the method comprising: dividing the available
identifying codes into a first list of identifying codes and a
second list of identifying codes, wherein the identifying codes of
the first list can appear in neighbour cell lists of basestations
outside said group, and wherein the identifying codes of the second
list can not appear in neighbour cell lists of basestations outside
said group; and notifying the basestations in said group of
basestations of the first list of identifying codes and the second
list of identifying codes.
8. A method as claimed in claim 7, wherein the identifying codes
are scrambling codes.
9. A method of allocating identifying codes to basestations forming
part of a group of basestations within a cellular communications
network, the method comprising, in each basestation within the
group: selecting an identifying code in a manner that attempts
where possible to avoid selecting the same identifying code as a
neighbour, and the method further comprising, in at least one
basestation within the group: determining that a clash has occurred
if any neighbour basestation has selected the same identifying code
as said at least one basestation, and if a clash has occurred,
determining whether the clash can be resolved by said basestation
or by another basestation in said group.
10. A method as claimed in claim 9, further comprising, if a clash
has occurred: determining whether there are any identifying codes
that are not in use by neighbours of the basestation; and if so,
selecting an identifying code that is not in use by a neighbour of
the basestation.
11. A method as claimed in claim 10, further comprising: if there
are no identifying codes that are not in use by neighbours of the
basestation, identifying a neighbour that has a high freedom of
selection of an alternative identifying code; and selecting the
identifying code that is in use by that neighbour.
12. A method as claimed in claim 11, further comprising: for one or
more basestations, calculating a freedom of selection parameter,
based on a number of identifying codes potentially available for
use by that basestation and on a number of said available
identifying codes that are actually in use by neighbours of that
basestation.
13. A method as claimed in any of claims 9 to 12, wherein the
basestations are femtocell access points.
14. A method as claimed in any of claims 9 to 12, wherein the
identifying codes comprise scrambling codes.
15. A method as claimed in claim 14, wherein the identifying codes
further comprise MIB Value Tags.
16. A basestation, for use in a cellular communications network,
the basestation being configured to perform a method as claimed in
any of claim 1, 4, or 9.
Description
[0001] This invention relates to a mobile communication network,
and in particular to methods and systems whereby a cellular
basestation can select its own scrambling code.
[0002] It is known to establish femtocell access points in a
building, in order to provide improved coverage for users of a
cellular communication network, amongst other advantages. When a
registered user device is within the coverage area of a femtocell
access point, it can establish a connection with that access point,
with the connection from the access point into the core network of
the cellular network being established over a pre-existing
broadband internet connection, for example. When the user leaves
the coverage area of the femtocell access point, the connection can
be handed over to a macrocell base station of the cellular
network.
[0003] It is also known to establish a network of such femtocell
access points.
[0004] One issue that arises with cellular communications networks
is that there a limited number of scrambling codes, which must be
shared between the basestations of the network. In a conventional
network, the allocation of the scrambling codes to the basestations
is performed as part of a network planning operation, in order to
maximise the distance between basestations that share the same
scrambling code. In the case of femtocell access points, there is
no such network planning, and each femtocell access point is
responsible for selecting its own scrambling code, in a way that
similarly attempts to maximise the distance between basestations
that share the same scrambling code.
[0005] Where there is a network of femtocell access points, for
example within a single building or otherwise within a relatively
small area, the problem of scrambling code selection becomes more
complex.
[0006] In accordance with aspects of the invention, this problem is
solved by suitable selection of scrambling codes when there are
unused scrambling codes, and when there is a clash between two
femtocell access points using the same scrambling code.
[0007] According to a first aspect of the present invention, there
is provided a method, for use in a basestation forming part of a
group of basestations within a cellular communications network, for
selecting an identifying code for use in identifying transmissions
from the basestation, the method comprising: [0008] receiving from
a management node a first list of identifying codes and a second
list of identifying codes, wherein the identifying codes of the
first list can appear in neighbour cell lists of basestations
outside said group, and wherein the identifying codes of the second
list can not appear in neighbour cell lists of basestations outside
said group; [0009] determining whether there is at least one
identifying code either in the first list of identifying codes or
the second list of identifying codes that is not used by any other
basestation in said group; and [0010] if there is at least one
identifying code in the first list of identifying codes and at
least one identifying code in the second list of identifying codes
that are not used by any other basestation in said group, selecting
an identifying code from the first list of identifying codes in
preference to an identifying code from the second list of
identifying codes.
[0011] According to a second aspect of the present invention, there
is provided a method, for use in a basestation forming part of a
group of basestations within a cellular communications network, for
selecting an identifying code for use in identifying transmissions
from the basestation, the method comprising: [0012] receiving from
a management node a first list of identifying codes and a second
list of identifying codes, wherein the identifying codes of the
first list can appear in neighbour cell lists of basestations
outside said group, and wherein the identifying codes of the second
list can not appear in neighbour cell lists of basestations outside
said group; [0013] determining whether it is necessary to be able
to perform cell reselection between the basestation and
basestations outside said group; [0014] if it is determined that it
is necessary to be able to perform cell reselection between the
basestation and basestations outside said group, selecting an
identifying code from the first list of identifying codes; and
[0015] if it is determined that it is not necessary to be able to
perform cell reselection between the basestation and basestations
outside said group, selecting an identifying code from the first
list of identifying codes or from the second list of identifying
codes.
[0016] According to a third aspect of the present invention, there
is provided a method, for use in a management node of a cellular
communications network, wherein the network includes at least one
group of basestations and further includes other basestations not
within said group, the method comprising: [0017] dividing the
available identifying codes into a first list of identifying codes
and a second list of identifying codes, wherein the identifying
codes of the first list can appear in neighbour cell lists of
basestations outside said group, and wherein the identifying codes
of the second list can not appear in neighbour cell lists of
basestations outside said group; and [0018] notifying the
basestations in said group of basestations of the first list of
identifying codes and the second list of identifying codes.
[0019] According to a fourth aspect of the present invention, there
is provided a method of allocating identifying codes to
basestations forming part of a group of basestations within a
cellular communications network, the method comprising, in each
basestation within the group: [0020] selecting an identifying code
in a manner that attempts where possible to avoid selecting the
same identifying code as a neighbour, [0021] and the method further
comprising, in at least one basestation within the group: [0022]
determining that a clash has occurred if any neighbour basestation
has selected the same identifying code as said at least one
basestation, and [0023] if a clash has occurred, determining
whether the clash can be resolved by said basestation or by another
basestation in said group.
[0024] According to other aspects of the invention, there are
provided basestations and management nodes operating in accordance
with these methods.
[0025] Thus, in an embodiment of the invention, at power up each
access point within the group will automatically select a primary
scrambling code/MIB Value Tag (PSC/MVT) combination based on one or
more lists of allowed PSCs. Based on the list of scrambling codes
provided by the ZMS, the PSCs already selected and communicated via
the MRT and the results of its own monitoring of the radio
environment, the femtocell access point would attempt to select a
unique PSC/MVT combination. If it is not possible to select a
unique PSC/MVT combination then one of the PSC/MVT combinations
already in use by another access point can be reused. When an
access point reuses a PSC it should try to minimise the possibility
that its coverage area will overlap with the coverage area of
another access point using the same PSC/MVT combination. When
making the selection, the femtocell access point should preferably
take into account all PSCs that are being used within the group,
and in other groups located nearby (multiple groups may be
designated in the same space), and by other femtocell access points
even if they are not part of any group.
[0026] For a better understanding of the present invention, and to
show how it may be put into effect, reference will now be made, by
way of example, to the accompanying drawings, in which:--
[0027] FIG. 1 shows a building in a coverage area of a cellular
communications network.
[0028] FIG. 2 shows the deployment of multiple femtocell access
points in the building.
[0029] FIG. 3 is a schematic illustration showing the presence of
femtocell access points in a wider communications network.
[0030] FIG. 4 is a flow chart illustrating a first process in
accordance with the present invention.
[0031] FIG. 5 is a schematic illustration of a situation in which
the present invention may be used.
[0032] FIG. 6 is a flow chart illustrating a second process in
accordance with the present invention.
[0033] FIG. 7 is a flow chart illustrating in more detail a part of
the process shown in FIG. 6.
[0034] FIG. 8 is a flow chart illustrating in more detail a further
part of the process shown in FIG. 6.
[0035] FIG. 9 is a schematic illustration of the use of the present
invention may be used.
[0036] FIG. 1 shows a building 10, which is located within the
coverage area of a macrocell base station 12 of a cellular
communications network. Thus, user devices, such as mobile phones
14, laptop computers and the like, that are in the vicinity of the
building 10 can obtain a cellular service by establishing a
connection into the cellular network through the macrocell base
station 12.
[0037] However, it is known that cellular coverage within buildings
can be poor, leading to unavailability of service, or forcing user
devices to transmit signals at high transmit powers, leading to
shorter battery life.
[0038] Femtocell access points are therefore deployed within the
building 10, with the intention that user devices located within
the building at least should be able to obtain a cellular service
by establishing a connection into the cellular network through one
of the femtocell access points.
[0039] Although the invention is described herein with reference to
the deployment of femtocell access points within a building, within
which users are expected to circulate, such as an office building,
an educational establishment, or a shopping mall, it will be
apparent that the invention is applicable to other situations. For
example, the invention is equally applicable to outdoor deployment
of femtocell access points, especially but not exclusively in
locations where there is common ownership and/or management of an
area in which users are expected to circulate.
[0040] FIG. 2 is a schematic representation of one level 16 within
the interior of the building 10. In this example, the building 10
is an office building, and the whole of the level 16 is occupied by
a single corporate entity. Based on the number of expected users
within the level 16 at any one time, a suitable number of femtocell
access points 18 are deployed. The eight femtocell access points
shown in FIG. 2 are indicated as AP1-AP8.
[0041] The femtocell access points 18 are located in suitable
positions. For example, it may be appropriate to provide a
femtocell access point close to the or each entrance/exit point, so
that users entering or leaving the building can spend as long as
possible connected to one of the femtocell access points. One or
more of the femtocell access points in the entrance/exit points of
the building, such as the access point AP5, can be designated as a
`gateway` cell, in that it provide the gateway to the femtocell
network from the surrounding macro layer. In addition, the
femtocell access points should be distributed throughout the space,
so that any user within the space will be able to establish a
connection with one of the femtocell access points.
[0042] FIG. 3 is a schematic diagram, illustrating network
connections of the femtocell access points. Specifically, the
femtocell access points 18 in a group are all connected to a local
area network (LAN) having a LAN server 20, which also has a
connection to a wide area network 22, in particular a public wide
area network such as the internet. The femtocell access points 18
are able to connect over the wide area network 22 to a core network
24 of the cellular communications network. The core network 24
includes a management node 26, which monitors and controls where
necessary the operation of the femtocell access points 18.
[0043] In one embodiment of the invention, the management node 26
distributes to all femtocell access points 18 in the group the
relevant information about the group, including: the IDs of all
femtocell access points in the group; and their main RF parameters,
such as the UTRA Absolute RF Channel Number (UARFCN) and scrambling
code (SC), the Location Area Code (LAC) and Cell-ID, and the
initial power levels.
[0044] The femtocell access point can enter the downlink monitor
mode, in which it can detect signals transmitted by other femtocell
access points, to capture the identities of the neighbouring
femtocell access points. Thus, by matching the detected UARFCN/SC
and LAC/Cell-ID transmitted by each femtocell access point with the
information received from the management node 26, the femtocell
access point 18 is able to populate automatically the neighbour
table. This can then be used in the case of handovers for local
mobility. Thus, mobility within the group is fully supported.
Cell-reselection with other femtocell access points is achieved by
each broadcasting the relevant carrier and scrambling code
information. Handover from one femtocell access point to another
can be achieved because each femtocell access point has a full map
of its neighbour femtocell access points, including their IDs, and
so it can send a handover command that is unequivocally pointing to
a specific femtocell access point. Full support is provided for
circuit-switched (CS), packet-switched (PS) and multiple Radio
Access Bearer (Multi-RAB) call mobility, and for intra-frequency
and inter-frequency handovers between femtocell access points.
[0045] In addition, each femtocell access point receives periodic
measurement reports from its connected user equipments, with these
reports indicating the signal strengths of intra-frequency
neighbouring femtocell access points. Further, each femtocell
access point sends measurement control messages to its connected
user equipments that are operating in compressed mode, requiring
them to provide periodic measurements of their inter-frequency
neighbouring femtocell access points.
[0046] Further, each femtocell access point is able to communicate
with the other femtocell access points by means of the local area
network to which they are connected.
[0047] FIG. 4 is a flow chart illustrating in general terms the
procedure that is followed in a femtocell access point when
selecting a primary scrambling code. This procedure is preferably
performed whenever the femtocell access point is powered up. The
procedure can then be performed again whenever it appears that it
would produce different results. For example, when the femtocell
access point detects signals from a new nearby femtocell access
point, the procedure can be performed again in order to check that
the selected scrambling code remains optimal.
[0048] In step 40, the femtocell access point notes the data that
it has received in its downlink monitor mode (DLMM). As mentioned
above, this includes the identity of each cell from which it is
able to detect signals, and also includes the scrambling codes used
by such cells.
[0049] In addition, the femtocell access point notes the data
contained in the current Master Relationship Table (MRT).
[0050] The Master Relationship Table includes the following
information about each femtocell access point in the group, namely:
the unique Cell ID of the femtocell access point; the Group ID of
the femtocell access point; the frequency and Primary Scrambling
Code selected by the femtocell access point; the Cell ID, Primary
Scrambling Code, UARFCN, CPICH Tx power adjustment and CPICH Tx
power of other femtocell access points and Macro Layer nodeBs
detected by that femtocell access point; and strongest detected
cell information.
[0051] Whenever a femtocell access point powers up for the first
time it broadcasts a message to indicate that it now part of the
network. A random femtocell access point then sends it a copy of
the MRT so that it can start its automatic configuration.
[0052] New femtocell access points are always added into the MRT
with a particular time stamp (known as the creation time stamp).
The priority of the femtocell access point is sometimes determined
by the value of the time stamp, as described below.
[0053] Whenever a femtocell access point changes its configuration
(either chooses a new frequency and/or scrambling code, or updates
the Mobility Table) it will rebroadcast the MRT over the local area
network with these changes. In addition, the management system may
remove femtocell access points from the MRT if they appear to be
inactive.
[0054] Whenever a femtocell access point receives an updated master
relationship table, it will check if it has been added as a
neighbour to any other femtocell access point, and is so will
reciprocate the listing.
[0055] In addition, the femtocell access point will check for and
resolve any primary scrambling code (PSC) conflicts, by following
the procedure for setting the scrambling code, as described
below.
[0056] Based on the information received in step 40, the femtocell
access point is able to divide the other femtocell access points in
the group into tiers. The tier of a neighbour femtocell access
point (or Macro Layer Neighbour) indicates the number of steps
through which the femtocell access point has become aware of the
neighbour. Thus, a Tier 1 neighbour may be one which the femtocell
access point has itself detected in its Downlink Monitor Mode.
Alternatively, the neighbour may have detected the femtocell access
point in its own Downlink Monitor Mode, and the femtocell access
point may have become aware of this through the Master Relationship
Table and reciprocated the relationship. A Tier 2 neighbour is one
which the femtocell access point has become aware of through a Tier
1 neighbour. Knowledge of the Tier 2 neighbour may be obtained from
SIB (System Information Block) 11 of a Tier 1 femtocell access
point or Macro Layer Neighbour. Alternatively, knowledge of the
Tier 2 neighbour may be obtained by looking up the Master
Relationship Table entry of a Tier 1 neighbour. A Tier 3 neighbour
is one which the femtocell access point has become aware of by
looking up the Master Relationship Table entry of a Tier 2
neighbour. Depending on the size of the network, lower Tier
neighbours might also exist, with the femtocell access point
becoming aware of them through looking up the Master Relationship
Table entry of a neighbour in the previous tier.
[0057] In step 42 of the procedure shown in FIG. 4, the femtocell
access point receives information about the primary scrambling code
(PSC) selection pool, i.e. the list of PSCs from which it can
select its own PSC.
[0058] In one embodiment of the invention, the primary scrambling
codes that are available in the cellular network are divided into
an External Scrambling Code List and an Internal Scrambling Code
List. The primary scrambling codes in the External Scrambling Code
List are the preferred scrambling codes. These scrambling codes
would appear in the neighbour cell lists of the Macro Layer and
would typically be used by those femtocell access points that can
see the Macro Layer. The primary scrambling codes in the Internal
Scrambling Code List are the non-preferred scrambling codes. These
scrambling codes would not appear in the neighbour cell lists of
the Macro Layer and would therefore typically be used, if at all,
by those femtocell access points that can not see the Macro
Layer.
[0059] Thus, in step 42, the femtocell access point receives, for
example from the management node 26, information as to whether its
selection pool contains only the primary scrambling codes in the
External Scrambling Code List, or whether it also contains the
scrambling codes in the Internal Scrambling Code List. This
mechanism is used to explicitly provision a femtocell as a
`gateway` cell, i.e. one in the entrance/exit point of the
femtocell coverage.
[0060] In addition the femtocell can also receive information from
the management node 26 that causes it to determine automatically
whether it should configure itself with this `gateway` status. For
example, the femtocell access point can be configured such that it
gathers statistics based on the history of user equipments
reselecting from the femtocell access point to the macro network,
or to the femtocell access point from the macro network. Based on
the recent history, the femtocell access point can determine
whether it should act as a `gateway` cell. This status can change.
For example, a femtocell access point might configure itself as a
`gateway` cell, restricting itself to a selection of primary
scrambling codes from the External Scrambling Code List. However,
if a new femtocell access point is placed in the group closer to
the exit/entry point of the overall coverage area, there would
thereafter be far fewer reselections between the first femtocell
access point and the macro layer, and so the first femtocell access
point could then, on this more recent history, determine that it
should no longer act as a `gateway` cell. It could then select a
primary scrambling codes either from the External Scrambling Code
List or from the Internal Scrambling Code List.
[0061] In step 44, the femtocell access point sets its selection
score, the use of which will be described later. The selection
score is equal to [the number of PSCs in the selection pool]-[the
number of unique PSCs that are in the selection pool and are used
by one of the Tier 1 neighbours].
[0062] In step 46, the femtocell access point determines whether
there is any primary scrambling code in its selection pool that is
not already in use in one of the femtocell access points listed in
the Master Relationship Table or in any other femtocell access
point that it may be able to detect. (For example, the femtocell
access point may be able to detect femtocell access points in other
enterprises, or in nearby residential properties.)
[0063] If there is an unused PSC in the selection pool, the
procedure passes to step 48, in which the femtocell access point
selects the unused PSC. If there is more than one such PSC, the
femtocell access point may select one at random. However, priority
should preferably be given to primary scrambling codes in the
External Scrambling Code List over scrambling codes in the Internal
Scrambling Code List, if the selection pool contains both. In step
50, the femtocell access point then selects a Master Information
Block (MIB) value tag (MVT). For example, it may select the MVT
value randomly.
[0064] If it is determined at step 46 that there is no completely
unused scrambling code, the process passes to step 52, in which it
is determined whether there is any primary scrambling code in its
selection pool that is not already in use in one of its Tier 1 or
Tier 2 neighbours. It should be noted that the PSC selection
algorithm only considers those neighbours that are on the same
UARFCN, although in most cases this will include all of the
femtocell access points within a group. Thus, if there are no
unused PSCs in the selection pool, the algorithm attempts if
possible to find a PSC that is in use only by a Tier 3 (or higher)
neighbour.
[0065] If it is determined at step 52 that there is one or more
primary scrambling code in its selection pool that is not already
in use in one of its Tier 1 or Tier 2 neighbours, the process
passes to step 54. In step 54, for each of these PSCs, the
femtocell access point forms a count of the number of times each
PSC/MVT combination appears in the Detected neighbour lists of all
femtocell access points. It would be expected that if a PSC/MVT
combination appears often as a Detected neighbour in the MRT then
this combination is not in use by an isolated femtocell access
point, and hence it would be preferable to try to avoid this
PSC/MVT combination. In step 54, therefore, the femtocell access
point selects the PSC/MVT combination that has the smallest count
value. The femtocell access point may be able at this step to
select a PSC/MVT combination that is not yet in use, even though
all of the PSCs in the selection pool are in use. This use of
different MVT values forces a UE to perform a Location Area Update
when moving into the coverage area of another femtocell access
point with the same PSC.
[0066] If it is determined at step 52 that there is no primary
scrambling code in its selection pool that is not already in use in
one of its Tier 1 or Tier 2 neighbours, the process passes to step
56. In step 56, it is determined whether there is any primary
scrambling code in its selection pool that is not already in use in
one of its Tier 1 neighbours (i.e. whether there is any primary
scrambling code in its selection pool that is only in use in one of
its Tier 2 neighbours).
[0067] If it is determined in step 56 that there is one or more PSC
that is only in use in a Tier 2 neighbour, the process passes to
step 58. In step 58, the femtocell access point selects a PSC/MVT
combination. Specifically, as in step 54 above, the femtocell
access point selects the PSC/MVT combination that has the smallest
count value.
[0068] If it is determined in step 56 that all of the primary
scrambling codes in the selection pool are already in use in Tier 1
neighbours, the process passes to step 60 in which one of the
primary scrambling codes is selected. In this example, the
selection is made in a way that attempts to minimise the risk of
interference with the other devices using that primary scrambling
code. In this example, the femtocell access point selects a
scrambling code from amongst the primary scrambling codes in the
External Scrambling Code List only.
[0069] In order to make the selection, the femtocell access point
forms for each of these primary scrambling codes a value
representing the combination of the count value and a pathloss
weighting. More specifically, the combination is formed by adding a
normalized version of the count value and a normalized pathloss
value.
[0070] The normalized count value (norm_occ_t1) can be obtained
from the count value (occ_t1) by first determining the maximum
count value at Tier 1 for any PSC (max_occ_t1). Then for each PSC,
the normalized count value is given by:
norm.sub.--occ.sub.--t1=occ.sub.--t1/max.sub.--occ.sub.--t1.
[0071] The normalized pathloss value for each PSC is determined by
finding the pathloss (Path_Loss) between the femtocell access point
performing the procedure and each of its Detected Neighbours. The
pathloss can be calculated because each cell broadcasts its CPICH
Tx power (which is also communicated via the MRT), and the
femtocell access point is able to determine the pathloss from this
Tx power and the RSCP that it detects in its Downlink Monitor Mode.
That is:
Path_Loss(in dB)=CPICHTx power[Detected Neighbour]-RSCP[Detected
Neighbour]
[0072] Having calculated these pathloss values, the femtocell
access point finds the smallest and largest path losses between the
femtocell and its detected Neighbours.
[0073] That is:
Min Path Loss=Smallest path loss between femtocell access point and
any Detected Neighbour
Max Path Loss=Largest path loss between femtocell access point and
any Detected Neighbour
[0074] Then an offset is applied, using a parameter,
Path_Loss_Weight_Offset, that is provided by the management
node.
Max Path Loss=Max Path Loss-Max Path
Loss*Path_Loss_Weight_Offset
[0075] Then, for all Detected neighbours, the normalized pathloss
value (Norm_Path_Loss_Weight) can be calculated as following:
Norm_Path _Loss _Weight = 1 - ( Path_Loss - Min_Path _Loss ) (
Max_Path _Loss - Min_Path _Loss ) ##EQU00001##
[0076] As mentioned above, the femtocell access point then
calculates a combination of the count value and the pathloss
weighting, by adding the normalized version of the count value
(norm_occ_t1) and the normalized pathloss value
(Norm_Path_Loss_Weight). Based on these calculations, the femtocell
access point selects the PSC that has the smallest combined
value.
[0077] The process then passes to step 62, in which it is
determined whether, for that selected PSC, there is any MVT that is
unused. If so, the process passes to step 64, in which an unused
MVT is selected.
[0078] If it is determined in step 62 that there is no unused MVT,
the process passes to step 66. In step 66, the femtocell access
point considers separate values of the combination of the count
value and the pathloss weighting for each MVT associated with the
selected PSC, and that MVT is then selected.
[0079] FIG. 4 therefore shows the procedure that is performed in
each access point in the group, on startup.
[0080] It will be apparent that the method attempts where possible
to avoid selecting Primary Scrambling Codes that are in use by
other femtocell access points, or at least by Tier 1 neighbour
femtocell access points. If the same Primary Scrambling Code is
used by two nearby femtocell access points, then a Location Area
Update between the two may fail, or a UE receiver combining signals
from the two femtocell access points could cause a call to
drop.
[0081] The process supports two possible ranges of scrambling codes
(External and Internal). The operator has the option of biasing the
selection towards Internal PSCs to minimize the number of External
PSCs, and hence minimize the size of the neighbour cell lists
maintained by cells in the Macro Layer. However, unless specified
by the operator through the management system, the process tends to
select an External Scrambling Code in preference to an Internal
Scrambling Code.
[0082] The MRT received by the femtocell access point selecting its
PSC will contain information about all femtocell access points in
the enterprise, even if they are allocated to different groups.
Therefore, the algorithm can take into account PSC/MVT combinations
that are in other enterprise groups, even if they cannot be
directly detected by the femtocell access point selecting its PSC.
Other femtocell access points will be taken into consideration only
if they can be detected by the femtocell access point selecting its
PSC, or if they appear in the neighbour cell list of a cell that
can be detected.
[0083] In general, the process selects unused Scrambling Code (PSC)
and MVT combinations before reusing a PSC/MVT combination. For
example, when it is necessary to reuse a PSC, the process will try
to select a non used PSC/MVT combination for that PSC that is least
often used.
[0084] The process will try to avoid the selection of a PSC/MVT
combination in use by a Tier 2 neighbour, and will aim to select a
PSC/MVT is use by a Tier 3 neighbour in preference. When it is
necessary to use a PSC/MVT combination in use by a Tier 2
neighbour, a normalized path loss weight is formed for use in the
selection. This is a combination of both path loss weight and
occurrence weight.
[0085] The process is self healing in that, if a femtocell access
point is removed from the MRT, then the PSC/MVT combination that it
was using becomes available for reuse. Moreover, this change to the
MRT will trigger at least one femtocell access point to restart the
procedure to select a PSC.
[0086] In one further embodiment, the management node can enable
one or more of the femtocell access points to determine whether it
should be allowed to determine whether it should act as a gateway
cell. If this is allowed, the femtocell access point will be
allowed to select an Internal Scrambling Code/MVT combination, even
if there are External PSC/MVT combinations (for example less than a
predetermined percentage of such combinations) still available. For
example, the decision to select an Internal PSC might be made if
all of the following conditions are met:
the femtocell access point is allowed by the management node to
activate this feature; the femtocell access point has detected
femtocell neighbours; other femtocells can detect the femtocell
access point; and there has been no idle mode reselection from the
macro layer to the femtocell access point, or handout to a macro
layer cell.
[0087] There is provided a mechanism whereby, in the event a
conflict is detected, an attempt can be made to resolve the
conflict. Otherwise an alarm can be raised at the management
system.
[0088] The various femtocell access points in an enterprise group
will go through the startup procedure in an unpredictable order. It
is therefore quite possible that the procedure shown in FIG. 4 will
result in a femtocell access point making a decision about
selection of a scrambling code that, after several other access
points have powered up, will no longer appear optimal. For example,
these decisions may result in two Tier 1 neighbours being forced to
select the same scrambling code.
[0089] FIG. 5 shows such a situation, in the context of environment
illustrated in FIG. 2, in a case where there are four available
primary scrambling codes PSC:1-PSC:4. It can be seen that, in this
illustrated situation, AP5 has five Tier 1 neighbours, namely AP1,
AP2, AP4, AP6 and AP7. In addition, AP5 is the last of these access
point to power up, and at the time that it powers up, AP1 has
selected PSC:4; AP2 has selected PSC:1; AP4 has selected PSC:3; and
AP6 has selected PSC:2. As a result, AP5 must select a primary
scrambling code that clashes with one of its Tier 1 neighbours. As
shown in FIG. 5, it selects PSC:4.
[0090] The clash resolution part of the algorithm tries to correct
this sort of occurrence without causing a ripple effect through the
group of access points, and without causing oscillations in PSC
selections.
[0091] The procedure consists of a set of rules which look at the
MRT relationships as well as the PSC choices and then decides
whether a change can be made to resolve the clash. A suitable
change may involve either one of the clashing access points
selecting another PSC-MVT that is free at its Tier 1, or one of the
clashing access points swapping PSCs with one of its tier 1
neighbours that it is not clashing with. If resolution is not
possible, no changes are made, and the procedure relies on being
able to differentiate between two access points using the same PSC
by means of their MVT.
[0092] FIG. 6 is a flow chart, illustrating a method by which
scrambling code clashes are detected and, if possible resolved.
[0093] In step 80, the femtocell access point completes its initial
primary scrambling code selection, as shown in FIG. 4. In step 82,
a clash detection timer is set, defining a time interval at which
the femtocell access point attempts to detect and resolve any
scrambling code clashes. In step 84, this timer elapses, and the
process passes to step 86, in which it is detected whether the
femtocell access point is involved in a scrambling code clash.
[0094] FIG. 7 is a flow chart, illustrating in more detail step 86
of the process. Specifically, it is determined in step 88 whether
the primary scrambling code selected by the femtocell access point
is also in use by any Tier 1 neighbour of the femtocell access
point. If so, the process passes to step 90, indicating that a Tier
1 clash has been detected.
[0095] If there is no Tier 1 clash, the process passes to step 92,
in which it is determined whether the primary scrambling code
selected by the femtocell access point is also in use by any Tier 2
neighbour of the femtocell access point. If so, the process passes
to step 94, indicating that a Tier 2 clash has been detected. If
there is no Tier 2 clash either, it is determined in step 96 that
there is no clash that needs to be resolved, and the process of
FIG. 6 passes to step 98, where it awaits new information, for
example received in its Downlink Monitor Mode or in an updated
Master Relationship Table, that might indicate that a new check
needs to be made.
[0096] If it is determined in step 86 that there is a clash, the
process passes to step 100, in which it is determined whether the
femtocell access point has a lower priority than the other
femtocell access point with which the clash has been detected. The
priority of the femtocell access point is determined first by the
PSC Selection Score 44, with priority given to those access points
with a lower score. If the PSC Selection Scores are equal for the
access points then the value of the creation time stamp indicating
when the femtocell access point was added into the MRT is used,
with priority given to the earlier creation time stamp. If the
creation time stamps are both equal then the cell identifiers of
the femtocells are used, with priority given to the lesser value.
If the femtocell access point has a higher priority than the other
femtocell access point with which the clash has been detected, the
process passes to step 98 as described above.
[0097] If the femtocell access point has a higher priority than the
other femtocell access point with which the clash has been
detected, the process passes to step 102, in which it is determined
whether the clash can be resolved and, if so, how.
[0098] FIG. 8 is a flow chart, illustrating in more detail step 102
of the process. Firstly, it is determined in step 104 whether the
selection score of the femtocell access point, previously
calculated in step 44 of the process shown in FIG. 4, is greater
than zero. If the score is equal to zero (i.e. if all of the PSCs
in the selection pool are used by Tier 1 neighbours of the
femtocell access point), the process passes to step 106.
[0099] In step 106, it is determined whether the femtocell access
point, with which the clash has been detected, has a selection
score greater than zero. If so, the process passes to step 108,
indicating that the clash can best be resolved by taking action in
the other femtocell access point, and so the process of step 102
comes to an end.
[0100] If the other femtocell access point, with which the clash
has been detected, does not have a selection score greater than
zero, the process passes to step 110. In step 110, the femtocell
access point considers its Tier 1 neighbours, excluding the
neighbour whose PSC clashes. Specifically, the femtocell access
point considers any of the non-clashing Tier 1 neighbours that have
selection scores greater than zero (i.e. that themselves have PSCs
in their selection pools that are not used by any of their Tier 1
neighbours). The femtocell access point then determines whether any
of the PSCs of those neighbours are in its own selection pool.
[0101] If not, the femtocell access point determines that the clash
cannot be resolved, and passes to step 112.
[0102] If the determination in step 110 is positive, the process
passes to step 114, in which the femtocell access point selects the
PSC-MVT combination of one of the neighbours identified in step
110. The intention here is to find the neighbour that has the
greatest freedom of selection of other PSC/MVT combinations, and
then to select the PSC/MVT combination presently in use by that
neighbour. More specifically, the femtocell access point examines
the selection scores of those non-clashing Tier 1 neighbours that
have selected PSCs within its own selection pool. Based on this,
the femtocell access point finds the highest selection score, and
it selects for its own use the PSC-MVT combination in use by the
neighbour with the highest selection score.
[0103] The process then passes to step 116, where it ends as far as
the first femtocell access point is concerned. The clash can then
be resolved by a change of PSC in the neighbour whose PSC-MVT
combination was selected in step 114.
[0104] If the process of FIG. 8 ended at step 112, this leads to
step 128 in FIG. 6, in which it is determined that the clash cannot
be resolved, and an alarm is raised. For example, the alarm may
indicate that the problem could be resolved if the femtocell access
point were physically relocated. Alternatively, the alarm may
indicate to the operator that it should allocate more scrambling
codes to the selection pool, for example by adding more Internal
Scrambling codes. The process then passes to step 98 as described
previously.
[0105] If it is determined in step 104 that the score is greater
than zero, the process passes to step 118, in which it is
determined whether the score is greater than or equal to the score
of the femtocell access point with which the clash has occurred. If
not, it is determined that the clash is best resolved by that other
femtocell access point, and the process passes to step 108. If it
is determined in step 118 that the score is greater than or equal
to the score of the femtocell access point with which the clash has
occurred, the process passes to step 120, in which it is determined
whether the score is equal to the score of the femtocell access
point with which the clash has occurred.
[0106] If it is determined in step 120 that the scores are equal,
the process passes to step 122, in which it is determined which of
the femtocell access points has the higher priority. If the
femtocell access point did not boot up after the clashing femtocell
access point, it is determined that the clash should be resolved by
that other femtocell access point, and the process passes to step
108.
[0107] If it is determined in step 120 that the femtocell access
point has a higher score than the other femtocell access point, or
if it is determined in step 122 that the femtocell access point did
boot up after the clashing femtocell access point, the process
passes to step 124.
[0108] In step 124, the femtocell access point selects a PSC-MVT
combination using a PSC that is not in use by any of its tier 1
neighbours. This can be achieved by rerunning the scrambling code
selection procedure of FIG. 4, and the process then ends at step
126.
[0109] If the process of FIG. 8 ended at step 116 or at step 126,
this leads to step 130 in FIG. 6, in which the femtocell access
point updates the MRT to indicate the PSC-MVT combination that it
has selected. Again, the process then passes to step 98 as
described previously.
[0110] If the process of FIG. 8 ended at step 108, no further
action is taken in the first femtocell access point, as the clash
resolution depends on action in another femtocell access point, and
the process passes directly to step 98 in FIG. 6.
[0111] When the process reaches step 98, if any new information is
received, the femtocell access point recalculates its selection
score based on the new information in step 132, sets the clash
detection timer in step 134, and proceeds to step 84 as described
previously.
[0112] FIG. 9 shows the situation previously illustrated in FIG. 5,
where the clash has been resolved. More specifically, AP5 was
forced on startup to select a primary scrambling code that clashed
with one of its Tier 1 neighbours, and selected PSC:4, which was
also in use by AP1.
[0113] The clash was able to be resolved by AP1 performing the
procedure shown in FIG. 6. It will be noted from an examination of
FIG. 5 that AP1 was unable to resolve the clash simply by selecting
a PSC that was not being used by any of its Tier 1 neighbours,
because PSC:1 had been selected by AP2; PSC:2 had been selected by
AP6; PSC:3 had been selected by AP4; and PSC:4 had been selected by
AP5.
[0114] However, AP1 was able to identify that one of its Tier 1
neighbours (AP6) did not have any Tier 1 neighbour using PSC:1.
Therefore, AP1 was able to select the PSC that had previously been
selected by AP6 (i.e. PSC:2), and AP6 was then able to go on to
select PSC:1 in order to resolve the clash.
[0115] There is thus described a mechanism by which each femtocell
access point in a network is able to select a primary scrambling
code in such a way that overall performance of the network is
improved.
[0116] Although the invention is described herein with reference to
a procedure for selecting scrambling codes used by femtocell access
points, it will be noted that, where different base stations of a
cellular network differentiate their transmissions by means of some
alternative form of identifier, the same process can be used in the
different base stations to select their identification codes.
* * * * *