UMTS Scrambling Code Planning

UNCLASSIFIED

Author: Mattia Quinzio (Nokia) Owner: Paul Sheehan

Confidence:

Date: 12/07/02 Status: Issue Version: I1.0

Title: Scrambling code allocation strategy

Summary:

A strategy and methodology for DL primary scrambling code allocation in H3G network, for launch plus 6 months.

Document no.

RD&T\UM&S\363

Owner: Paul SheehanStatus: IssueDate : 24/09/02 Unclassified

Printed On: 17/04/23 of 18

Scrambling code allocation strategy

Hutchison 3G Engineering DocumentationRadio Design Group

Unclassified Radio Design Group

Owner: Paul SheehanStatus: IssueDate : 24/09/02

Unclassified Printed On: 17/04/23 of 18


1.1 Document Version History

Version Date Author Reason for Change

D1.0 12/07/02 Mattia Quinzio 1st version

D1.1 23/07/02 Mattia Quinzio Code naming added, code group strategy revised for carriers and layers

I1.0 24/09/02 Paul Sheehan Revised formatting

1.2 Distribution List

Name Organisation/Department

Kassir Hussain RF Strategy & Optimisation

David Hennessy National Rollout




1.3 Contents Page

1.1 Document Version History..............................................................................................21.2 Distribution List...............................................................................................................21.3 Contents Page.................................................................................................................3

2 Document Scope............................................................................................42.1 Purpose............................................................................................................................42.2 Intended Audience..........................................................................................................42.3 Synopsis..........................................................................................................................4

3 References.....................................................................................................4

4 Introduction...................................................................................................5

5 Scrambling codes..........................................................................................55.1 Uplink...............................................................................................................................55.2 Downlink..........................................................................................................................55.2.1 Code naming...............................................................................................................6

6 UE synchronization process...........................................................................76.1 Cell selection...................................................................................................................76.2 Neighbours measurements.............................................................................................7

7 Requirements for code allocation...................................................................8

8 Tools.............................................................................................................. 98.1 Enterprise v 3.4...............................................................................................................98.2 Enterprise v 4.0...............................................................................................................98.3 Enterprise v 4.1...............................................................................................................9

9 Code allocation............................................................................................109.1 Process overview...........................................................................................................109.2 Cell octets......................................................................................................................119.3 Group reuse distance....................................................................................................11

10 Code groups strategy...................................................................................1210.1 Carriers.....................................................................................................................1210.2 Layers.......................................................................................................................1210.2.1 Pico Layer.................................................................................................................1310.2.2 Micro layer................................................................................................................1310.2.3 Macro layer...............................................................................................................1310.2.4 Reserved code groups..............................................................................................1410.3 Restraints on code use, national borders................................................................14

11 Appendix.....................................................................................................1511.1 GIS tool.....................................................................................................................15




2 Document Scope

2.1 PurposeThis document proposes a strategy to accomplish the task of scrambling code allocation for H3G UMTS network.

2.2 Intended AudienceThe RF Design & Terminals and the National Network Rollout & Integration teams.

2.3 Synopsis

3 References

[1] 3GPP TS 25.214 v3.10.0 (2001-03); Technical Specification Group Radio Access Network; Physical Layer Procedures (FDD) Release 1999.

[2] Kourtis S, "Code Planning Strategy for UMTS-FDD networks"

[3] Memorandum of Understanding concluded between France and the United Kingdom on coordination in the 1900-1980 MHz, 2010-2025 MHz and 2110-2170 MHz frequency bands; Version Dec 2001; Doc (01) 05 E rev 1

[4] Memorandum of Understanding concluded between the administration of the United Kingdom and Ireland on coordination in the 1900-1980 MHz, 2010-2025 MHz and 2110-2170 MHz frequency bands; Version 06/1-/01

[5] Memorandum of Understanding concluded between the Isle of Man and the United Kingdom on coordination in the 1914.9-1919.9 MHz, 1920.3-1930.3 MHz and 2110.3-2120.3 MHz frequency bands; Version 27/03/01




4 IntroductionThere is the need to define a scrambling code allocation strategy before H3G network launch.

This documents first describes the basic concept of scrambling codes (chapter 5) and their impact on UE performances (chapter 6). Downlink scrambling code allocation is not crucial on the overall system performances. It has been proven though how the UE performances can be improved when scrambling codes are defined with certain constraints deriving from the neighbour lists (chapter 7). Unfortunately the Aircom planning tool currently available cannot support this process (chapter 8).

In this scenario, the proposed strategy comes as an interim solution for H3G network launch plus 6 months, described in chapter 9. In the long-term scenario more appropriate tools will be available, providing the flexibility to rewrite the strategy and methodology for scrambling code planning.

The last chapter deals with planning codes on different carriers, different layers and at national borders.

5 Scrambling codesIn the UMTS radio access network all cells can reuse the same frequency carrier for downlink transmission; the same happens for all the mobiles in the uplink. In both links the transmission is separated in reception through the use of scrambling codes.

5.1 Uplink

The allocation of scrambling codes on the uplink is performed by the RNC at any new connection and, depending on the vendor, little or no planning action is required.

5.2 Downlink

On the downlink, instead, scrambling codes are cell specific and must be planned on a per cell basis before network integration. There are 512 scrambling codes sets; each set includes one primary scrambling code and 15 secondary scrambling codes. The secondary scrambling codes will be used when intelligent antennas are deployed. As this is not going to happen in the first year of H3G network, a strategy for secondary scrambling codes is out of the scope of this document. The 512 primary scrambling codes are organised into 64 groups of 8 codes each. The definition of code groups is meant to increase the flexibility of the synchronisation process in terms of speed, reliability and processing requirement, as chapter 6 describes.




Each scrambling code should identify uniquely one cell in the area; this means that in any location all the cells that a UE can measure need to have different scrambling codes in order to be unambiguously detected.

5.2.1 Code naming

The RAN scrambling code parameter is simply numbered from 0 to 511, as in Table 1.

k\j 0 1 2 … 63

0 0 8 16 504

1 1 9 17 505

2 2 10 18 506

3 3 11 19 507

4 4 12 20 508

5 5 13 21 509

6 6 14 22 510

7 7 15 23 511

Table 1: Scrambling codes numbering

A further code-naming scheme can be defined for the allocation strategy described in this document. The proposed scheme is a three-character string: the first two digits indicate the code group (j, from 00 to 63) and the third letter is for the code inside the group (k, from A to H).

k\j 0 1 2 … 63

0 00A 01A 02A 63A

1 00B 01B 02B 63B

2 00C 01C 02C 63C

3 00D 01D 02D 63D

4 00E 01E 02E 63E

5 00F 01F 02F 63F

6 00G 01G 02G 63G

7 00H 01H 02H 63H

Table 2: Scrambling code-naming for code allocation

In this document codes will be referred to following Table 2 scheme. A simple look-up table can be used at any time to convert codes from one naming scheme to another.

1 0 1 2 … 509 510 511

2 00A 00B 00C 63F 63G 63H

Table 3: Scrambling codes look-up table




6 UE synchronization process

6.1 Cell selection

The UE follows a three-step synchronization process every time it has to synchronize to a new cell for cell selection (see Table 4). In this process the UE is blind, i.e. it has no knowledge of the scrambling code used by the detected cell.

Step

Cell selection

1 slot synchronization

2 Frame synch. & code group identification

3 scrambling code identification and CPICH measurement

Table 4 - Synchronization process

In the first step, slot synchronisation is achieved using the Primary Synchronisation Code on the P-SCH, common to all the cells in the network.

In the second step the frame synchronization is obtained and the scrambling code group is determined from the Secondary Synchronisation Code sequence; there is a one-to-one relationship between 64 different code sequences on the S-SCH and 64 scrambling code groups.

Once frame synchronization is achieved the primary scrambling code can be identified and the pilot channel measured [1]. The Ec/Io measurements on the CPICH are at the base of cell selection criteria.

6.2 Neighbours measurements

Even though the algorithm used by the UE to synchronize to a neighbour cell is vendor specific, it is reasonable to assume that the same synchronization process specified for cell selection will be used, as it has been designed to minimize time and complexity required to the UE.

While following the steps of the synchronization process for neighbour cell measurements, the UE knows in advance the scrambling code of the cell it has to synchronize to and measure. This knowledge might be used, while defining the code allocation strategy, to reduce complexity and to gain on performance [2].

Step

Speed Reliability Processing

1 Slot sync - - -

2 Frame sync Slow Low Low

3 CPICH Fast High High




meas.

Table 5 - Characteristics of sync steps for neighbour cells measurements

Any strategy on scrambling code allocation should aim to find the best trade-off between the speed for a fast handover and the processing power requirement (and hence battery consumption) on the second and third stages of the synchronization process done for neighbour cell measurement.

In the neighbours measurement process the main roles are played by the number of neighbours (N), the number of code groups (M) and the number of code per groups (L) used in the neighbour list.

It has been studied [2] that minimizing M and maximizing L leads to faster handovers at the expenses of higher processing power requirements for the UE. As well it is currently believed that this increase in UE power consumption can be ignored if compared to the total UE power consumption (there might be scope for investigating more on this subject).

In conclusion it is recommended to minimize the number of code groups and consequently to maximize the number of codes per group in all neighbour lists in the network.

7 Requirements for code allocationFrom what is described in the previous chapters and from other considerations related to neighbour cell planning, a set of requirements could be deducted to clearly derive the process of scrambling code allocation.

a) All cells that a mobile station is able to measure in any location of the network service area should have different scrambling codes.

b) No cell should have the same code as any of its neighbour cells

c) No two cells in one neighbour list should have the same code

d) When cell A and cell B both have cell C in their respective neighbour lists, cell A and cell B should have different scrambling codes

e) In a neighbour list the number of code groups used by the neighbour cells should be kept at minimum and consequently the number of codes per group should be maximised

Comments:

- The effect of all the requirements involving the neighbour lists clearly depend on the neighbour definition strategy, that is treated separately in another document. For the scope of this document we can reasonably assume that for network launch, neighbours would be defined based on propagation prediction, in a way that two cells are defined as neighbours if their service areas overlap. Under this assumption requirements b) would be equivalent to requirement a).




- Requirements c) and d) introduce more restrictive conditions. A further speculation can be done whereas neighbours relationship are always symmetrical for network launch; in this case requirement d) would be the most restrictive one, and somehow guarantee that a), b) and c) are satisfied.

- Requirement e) is to be treated separately from all the others; an optimisation algorithm would be needed to find the optimal solution to such complex problem.

8 ToolsAt the time of writing this document no tool is available to perform a scrambling code planning that fulfil all the requirements from chapter 7.

8.1 Enterprise v 3.4

The Enterprise planning tool version 3.4 has no facility for scrambling code planning.

The frequency-planning module could possibly be used for that purpose, allocating codes as if they were frequency channels. This type of allocation wouldn’t allow setting any constraint on groups and codes per group, as well as any constraint on the neighbour lists. Furthermore the frequency-planning tool is not part of the license currently held by H3G, which means it would have to be bought separately.

For all these reasons we obviously conclude that Enterprise v3.4 cannot be used for any scrambling code planning.


Enterprise 4.0 release provides a cell specific scrambling codes planning and assignment functionality. The tool assigns scrambling code to all the cells having the same carrier.

The scrambling code planner functionality, though, is quite basic, and is purely driven by the only constraint of interference; scrambling code are assigned to each cell making sure that the interfering cells don’t have the same scrambling code (as from the first requirement, chapter 7).

At the time of writing this document, this tool release has not been tested yet on H3G side, therefore no further comment on it can be made available.


The scrambling code planner tool for Enterprise version 4.1 will be significantly enhanced compared to release 4.0.

The tool will be based on an optimisation algorithm. Its current design includes allocation of codes per groups with reference both to neighbour lists and




interfering cells. As well distance constraints and barred codes can be taken into account.

From the design, this tool seems to give enough flexibility for the planner to control the allocation under all the mentioned requirements.

Unfortunately it’s not still clear yet when this software release will be available. For the purposes of this document, it is assumed that this would be too late for H3G network launch. Therefore alternative solutions must be found.

9 Code allocation The strategy hereafter is meant to be valid for the first year of H3G network life. Along or at the end of the year, as soon as a code-planning tool is available, a revision of the strategy is strongly recommended.

The process described includes a proposal for an internal developed tool to perform the code allocation in an automated, assisted way for the Macro layer.

The code allocation for Micro and Pico cells can be done manually, even though following the same guidelines as for the Macro layer.

9.1 Process overview

1. The following figure resumes the basic steps of scrambling code allocation

Scrambling codes allocation

Neighbour lists definition

Are requirements all met

YES

NO

Implement codes and neighbours

Scrambling codes allocation

Neighbour lists definition

Are requirements all met

YES

NO

Implement codes and neighbours

Figure 1: Process overview

Ideally the code allocation should be performed on the input of the neighbour lists but:

No tool is available for that purpose

Any optimal code allocation done for a certain neighbour list input might not be optimal when the input changes; the neighbour lists of




a non-mature network are subject to continuous changes, which would imply frequent re-run of the code allocation.

The proposed process hence can be a preferable one if it’s possible to find a set of conditions for the scrambling code allocation strategy to guarantee that all or most of the requirements are satisfied with virtually any possible neighbour lists set. No new code allocation would be needed when neighbour lists changes.

The two next subchapters describe how scrambling code assignment can be performed. To effectively allocate codes on the entire network, it is suggested to develop a tool based on GIS application (i.e. MapInfo) to support the process (see 11 Appendix).

9.2 Cell octets

Figure 2 suggests how cells can be grouped together to form cell octets.

Figure 2: Cell octets

The way the groups are formed is purely based on cell location and azimuth, with no involvement of propagation predictions; therefore no planning tool is required. A cell octet is formed by 8 contiguous cells and is limited by a maximum distance, i.e. the ‘radius’.

When a mobile runs through a network like in Figure 2 it is expected to be either:

Inside a cell octet area; in this case it is likely that all cells belonging to the octet are neighbours of the cell the mobile is connected to (adjacent cells neighbours cells)

At the border of a cell octet area; in this case the neighbour list currently held by the mobile could include cells from 3-4 different cell octets

The idea is to create cell octets and assign eight codes from one code group to the eight cells inside the octet. Doing this way the number of different code groups used by the adjacent cells is kept at minimum and, intuitively, so it



cell octet

radius


could be for the neighbour cells. The octet approach is a simple, geometrical solution to the problem expressed with requirements e).

9.3 Group reuse distance

Once cell octets are defined, different code groups will be assigned to adjacent octets. The code group assignment must be done maximizing the distance between octets using the same code group.

With 50 code groups available, each one of 8 codes, there is a potential reuse pattern of 50 x 8 = 400 cells. The distance between cells using the same scrambling code can be maximised so to guarantee that all the requirements from a) to d) can be satisfied, even in the densest part of the network.




10 Code groups strategyFigure 3 resumes the code group allocation strategy described in the next subchapters.

Code groups

00 01 … 09 10 … 19 20 … 31 32 … 41 42 … 51 52 … 59 60 … 63

Test Reserved (9) Macro Layer (50) Pico layer (4)

Isle of Man border (32)

France border (32) Upon conditions, see chapter

Ireland border (32)

Figure 3: Scrambling code groups allocation

10.1 Carriers

H3G will operate on three carriers: F1, F2 and F3. Only F1 will be used initially, while at the time of writing there’s still no certainty about when F2 and F3 will be utilized.

F1 will be used by both Macro and Pico layer. For code allocation on F1 we refer to the layer section in this chapter

F2 will most certainly be used by the Micro layer, but not in the first year. A strategy and methodology for code allocation on F2 is therefore out of the scope of this document.

F3 might be used both for special sites (H3G shops locations) or to deploy more capacity to Macro layer urban sites. In the first case a manual code allocation is suggested, using codes from the 9 reserved code groups. In the second case, the same code allocation pattern done for F1 can be reused for F3 (50 code groups available). To reduce at minimum the effect of interference between two collocated transmitters using the same codes on different carriers a simple shift in code groups can be introduced, as from next table.

F1 00 01 … 31 32 … 63

F3 31 32 … 63 00 … 31

Table 6: Code groups shift between carriers

As an example, a sector who has been assigned scrambling code 01D on F1 could use code 32D on F3.

10.2 Layers

Code coordination is needed when different layers share the same carrier; this is the case of the Macro-cell layer and Pico-cell layer sharing F1 carrier. As they are likely to grow independently, a suggested strategy is to reserve a certain number of code groups to each layer.




10.2.1 Pico Layer

Figure 4: UK and London town - Pico cells location

Figure 4 shows the distribution of indoor sites for the first year of H3G network in all the country and, with higher zoom, in London town, the densest area.

Each Pico site configuration might vary depending on the building layout and on the capacity need. We assume that one site can have 4 cells maximum, i.e. it would need 4 scrambling codes.

The Pico cell design, in general, is such that the overlap between the Pico layer and the macro layer is kept at its minimum; furthermore two Pico sites would be hardly close enough to produce any mutual interference at all. This means that in principle reserving 4 scrambling codes for the whole Pico layer could possibly be enough.

Nevertheless, given the high number of code groups available in total (64), the possibility of expanding the Pico layer network and the chance of having unexpected interference between Pico sites, 4 code groups (16 codes) should be reserved for indoor cells transmitting on F1, from group 60 to 63 (codes 60A to 63H).

10.2.2 Micro layer

The Micro cell layer is not going to be deployed in the first year. Furthermore it is expected to be deployed using F2 carrier; in this case all the 512 scrambling codes could be made available and planning should be done with the same requirements as for the Macro layer (see chapters 7).

Constraints on code groups usage might come for Micro cells located close to the borders with Ireland, France or the Isle of Man, for which we refer to chapter 10.4.

10.2.3 Macro layer

50 code groups are made available for code allocation on Macro layer, for a total of 50x8=400 scrambling codes, from group 10 (code 10A) to 59 (code 59H).




Figure 3 shows how 50 groups could be reduced to 32 close to national border areas, as specified in the chapter 10.4.

10.3 Reserved code groups

10.3.1 Test

Code group 00 (codes 00A to 00H) are reserved for test purposes.

10.3.2 Exceptional cases

It is recommendable reserving a few code groups for cell splitting / new sites integration. In fact, even if the code planning is meant for network launch + 1 year so that all sites integrated during the first year should have their codes already planned, these reserved groups would cope with the possibility that after network launch some area might be redesigned and new sites could be ‘urgently added’ to the plan.

Code groups from 01 to 09 are reserved for that purpose.

10.4 Restraints on code use, national borders

According to the current Memorandum of Understanding with France [3], Ireland [4], and The Isle of Man [5] there may be some coordination required between operators of these countries on the usage of scrambling codes on the same carriers within UMTS FDD DL bandwidth.

The conditions that determine whether coordination is required are based on the mean field strength of each carrier as predicted at a height of 3 m above ground at all points on the border or the territory of the neighbouring countries.

Signal threshold Power (@2 GHz)

Codes allowed

Coordination required

<= 21 dBµV/m/5MHz

<= 122 dBm All No

<= 45 dBµV/m/5MHz

<= 98 dBm Preferential No

> 45 dBµV/m/5MHz > 98 dBm Preferential Yes

Table 7: Condition for coordination on scrambling codes

Enterprise planning tool is to be used to generate coverage predictions for all sites potentially interfering the neighbour countries. Prediction should be calculated with PA transmitting at full power and with the correct antenna gains and cable losses. The coverage array can be displayed with the power thresholds defined in Table 7 to evaluate the interference on the neighbour land.

The preferential code groups are set differently for the neighbouring countries and are reported in the next table.

Neighbour country Preferential code groups for




H3G

France 10 … 41

Ireland 20 … 51

Isle of Man 00 … 31

Table 8: Preferential code groups per country border




11 Appendix

11.1 GIS tool

The problem described in chapters 9.2 and 9.3 can be solved with the use of a GIS application like MapInfo.

Using Map Basic programming language a tool can be developed to perform the following operations:

1. Import a network text file from Enterprise

2. Draw sites and sectors on a map according to the imported locations and azimuth

3. Generate cell octets all over the imported network, grouping contiguous cells

4. Assign one code group to each octet, maximising the reuse distance

5. Assign one code to each cell inside the octets, maximizing the distance between cells using the same code

6. Import neighbours lists from text files generated from Enterprise

7. Provide check functionalities based on allocated codes and imported neighbour lists as from requirements b) to d)

8. Produce statistical results on number of code groups and number of codes per group from the neighbour list (requirement e)

9. Export the results of scrambling code allocation in a format suitable for import in Enterprise planning tool and/or NPPS



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