Monday, June 4, 2012
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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
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
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.
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This post was written by: Alex Wanda