Scrambling code planning is usually associated with assigning a downlink
primary scrambling code to each cell. It can also be associated with assigning
groups of uplink scrambling codes to each RNC. The resultant uplink and
downlink scrambling code plans form part of the RNC databuild.
3GPP TS 25.213 specifies 512 downlink primary scrambling codes. Each
primary scrambling code has 15 secondary scrambling codes. Each primary and
each secondary scrambling code has a left alternative scrambling code and a
right alternative scrambling code. Secondary scrambling codes may be used for
beamforming whereas left and right alternative scrambling codes may be used by
SF/2 compressed mode.
Each cell belonging to the radio network plan must be assigned one primary
scrambling code. This defines the set of 15 secondary scrambling codes as well
as the set of left and right alternative scrambling codes. The fundamental
requirement for scrambling code planning is that the isolation between cells
which are assigned the same scrambling code should be sufficiently great to
ensure that a UE never simultaneously receives the same scrambling code from
more than a single cell.
Scrambling code planning should be completed in combination with neighbour
list planning to ensure that neighbour lists never include duplicate scrambling
codes. This is important because RRC signalling procedures can use the
scrambling code as a way to reference each neighbour. The RNC will face an ambiguity if the UE reports
measurements from a neighbour which has a duplicate scrambling code within the
neighbour list, i.e. the RNC will be unable to deduce from which neighbour the
measurements were recorded. The illustration below shows the scenario where
multiple neighbours have been allocated the same scrambling code.
The RNC implementation may be designed to combine neighbour lists when UE
are in soft handover. Neighbour list combining helps to reduce the potential
for missing neighbours and so helps to improve network performance. The illustration
here shows a UE which is in soft handover with cells A and B. If neighbour list
combining has been implemented the UE will be provided with a neighbour list
generated from the combination of the neighbour lists belonging to cells A and
B.
If both cells have neighbours with the same scrambling code, the RNC will
be unable to deduce from which cell UE measurements have been recorded. This
scenario requires that when cells A and B are neighboured there should not be
any duplicate scrambling codes within the neighbour lists belonging to cells A
and B. A second example scenario is presented below.
In this case, cell A is neighboured with cells B and C while cells B and C
are not necessarily
neighboured with one another. The UE could trigger an active set update
which results in the active set
including cells B and C. The neighbour lists belonging to cells B and C
would then be combined and a
duplicate scrambling code introduced. In general, neighbour list auditing
should be completed after
scrambling code planning to exclude the possibility of neighbour lists
including duplicate scrambling
codes.
Scrambling code planning can also have an impact upon the cell synchronisation
procedure. This procedure is used whenever a UE needs to access a cell or
measure the quality of a cell, e.g. neighbour cell measurements. 3GPP TS 25.213
specifies that the 512 primary scrambling codes are organised into 64 groups of
8. The cell synchronisation procedure is based upon this grouping and the
following three
steps:
1. The P-SCH is used to achieve slot synchronisation.
2. The S-SCH is used to achieve frame synchronisation and identify the
primary scrambling code group.
3. The CPICH is used to identify the primary scrambling code.
The first step is relatively independent of the scrambling code plan
although there is some potential to improve performance if the scrambling codes
assigned to each cell of a Node B are from the same scrambling code group. If
all cells belong to the same scrambling code group then both the P-SCH and
S-SCH will be identical for each cell. If all cells are configured with a Tcell
of 0 chips then each cell will transmit the P-SCH and S-SCH with the same
timing. This helps to improve the signal quality of the P-SCH and S-SCH in the
softer handover regions and so improves the reliability of the cell
synchronisation procedure. The effectiveness of this approach depends upon the
UE implementation.
There is a danger that after receiving a single P-SCH from a Node B the UE
will assume there is only a single cell. In this case, the UE would identify
only one scrambling code during step 3 instead of potentially two or more
scrambling codes.
Step 2 of the synchronisation procedure involves selecting 1 scrambling
code group out of 64, whereas step 3 involves selecting 1 scrambling code out
of 8. Step 3 is likely to be more reliable because there are fewer
alternatives. Step 3 is also likely to require greater UE processing and so
have a greater impact upon UE battery life. Both of these factors are UE
implementation dependent and the impact upon UE battery life needs to be kept
in perspective relative to other procedures. Nevertheless, it is possible to
adopt a scrambling code planning strategy which places the emphasis upon either
step 2 or step 3. Placing the emphasis upon step 2 has the potential to reduce
UE power consumption whereas placing the emphasis upon step 3 has the potential
to improve cell synchronisation reliability. Placing the emphasis upon step 2
can be achieved by planning the scrambling codes such that each neighbour
belongs to a different scrambling code group. In this case, the UE has to check
for a relatively large number of different S-SCH during step 2, but once the
scrambling code group has been identified the scrambling code is also known.
Step 3 would serve as a check to ensure that the cell being measured is
actually the cell within the neighbour list (rather than a missing neighbour
belonging to the same scrambling code group). Placing the emphasis upon step 3
can be achieved by planning the scrambling codes such that neighbours tend to
belong to the same scrambling code group. In this case, the UE has to check for
a relatively small number of different S-SCH during step 2, but a relatively
large number of scrambling codes within each group during step 3.
The illustration below shows the concept of planning scrambling codes to minimise
the number of neighbours belonging to different scrambling code groups, i.e.
placing the emphasis upon step 3 of the cell synchronisation procedure.
Clusters of cells are assigned scrambling codes belonging to the same group.
However, it is not possible to generate a scrambling code plan in which all
neighbours belong to the same scrambling code group. In practice, it is likely
that neighbour lists would include cells belonging to three or four different
scrambling code groups.
The illusttration below presents a simpler scrambling code planning
strategy based upon assigning a different scrambling code group to each Node B.
This approach avoids the requirement to plan clusters of cells and reduces the
scrambling code planning process to the allocation of 1 out of 64 scrambling
code groups, i.e. a scrambling code re-use pattern of 64. It is possible that
each scrambling code group is divided into two for the purposes of scrambling
code planning. This would generate 128 scrambling code subgroups and a
corresponding re-use pattern of 128. Adjacent Node B could be assigned
subgroups belonging to the same group to approximate clusters of cells
belonging to the same code group.
The scrambling code planning strategy should also account for future network
expansion. Future network expansion could mean the inclusion of additional Node
B or increased sectorisation of existing Node B. Scrambling codes should be
excluded from the original plan so they can be assigned when additional cells
are introduced.
Additional rules for scrambling code planning are required at locations
close to international borders where there may be another 3G operator using the
same RF carrier. These rules are often specified by regulatory organisations.
For example, in Europe the Electronic Communications Committee (ECC) within the
European Conference of Postal and Telecommunications Administrations (CEPT) has
specified ERC Recommendation 01-01, Border Coordination of UMTS. This
recommendation prioritises the use of specific scrambling code groups on either
side of an international border. It also recommends maximum allowed signal
strengths for transmissions which cross international borders.
Scrambling code planning can be completed independently for different RF
carriers. If a radio network includes Node B which are configured with two or
three RF carriers the same scrambling code plan can be assigned to each
carrier. This approach helps to reduce the quantity of work associated with
scrambling code planning and reduces system complexity.
