6-WCDMA Scrambling Code Planning 28

WCDMA Scrambling code Planning

ZTE University

TD&W&PCS BSS Course Team

Objectives

At the end of this course, you will be able to:

Ø Grasp WCDMA SC principle

Ø Grasp WCDMA SC planning method

Content

Introduction to WCDMA cell scramblingScrambling planning principle

Cluster-Based scrambling multiplexing methodScrambling setting principle

Introduction to WCDMA Cell Scrambling

n The WCDMA system adopts the CDMA technology, and

different BSs or different sectors of a BS are differentiated

by scrambling.

n The downlink scrambling code is the PN sequence

generated by an 18-bit shift register, with a total of 218 – 1

= 262,143 scrambling sequences. However, the system

only uses these scrambling sequences numbered from 0

to 24,575.

Introduction to WCDMA cell scrambling


n The 24,576 scrambling sequences are divided into three parts:

� Common scrambling codes: k = 0,1,2,×××8,191, corresponding to

8,192 common scrambling codes, and used in normal mode.

� Left secondary scrambling code: k + 8,192 and k = 0,1,2,×××8,191,

is the replaceable scrambling code used in compression mode in

the case of n < SF/2, with a total number of 8,192.

� Right secondary scrambling code: k + 16,384 and k =

0,1,2,×××8,191, is the replaceable scrambling code used in

compression mode in the case of n ³ SF/2, with a total number of

8,192.

� -n is the corresponding n value in the channelization code Cch,SF,n.



n Common scrambling codes frequently used are 8,192,

which are subdivided into primary scrambling codes (red,

512) and secondary scrambling codes (yellow, 8,192 –

512 = 7,680).


The 8,192 scrambling sequences that are commonly used fall into 512 sets, with each set

including one primary scrambling sequence and 15 secondary scrambling sequences

following the primary one:

Primary scrambling sequence No.: n=16*i i=0, ××× ,511

Scrambling No. of the corresponding secondary scrambling group: n=16*i+k k=1, ××× ,15

The primary scrambling code and 15 secondary scrambling codes of each set correspond to

each other.

One set




n The WCDMA downlink has a total of 512 primary

scrambling codes .

n Totally 512 codes can be divided into 64 groups, each

group contain 8 primary codes.

n P-CCPCH，P-CPICH，PICH，AICH，S_CCPCH

（carring PCH）in one cell should use the same primary

scrambling code.

n The planning of the SC will affect the cell searching

performance.


Cell search procedure (1)

n The cell search is typically carried out in three steps:

n Step1:Slotsynchronization

During the first step of the cell search procedure the UE

uses the SCH's primary synchronization code to acquire

slot synchronization to a cell.

n This is typically done with a single matched filter (or any

similar device) matched to the primary synchronization

code which is common to all cells. The slot timing of the

cell can be obtained by detecting peaks in the matched

filter output.



n Step2: Frame synchronization and code-groupidentificationDuring the second step of the cell search procedure, theUE uses the SCH's secondary synchronization code tofind frame synchronization and identify the code group ofthe cell found in the first step.

n This is done by correlating the received signal with allpossible secondary synchronization code sequences, andidentifying the maximum correlation value. Since thecyclic shifts of the sequences are unique the code groupas well as the frame synchronization is determined.



n Step3: Scrambling-code identificationDuring the third and last step of the cell search procedure,the UE determines the exact primary scrambling codeused by the found cell.

n The primary scrambling code is typically identifiedthrough symbol-by-symbol correlation over the CPICHwith all codes within the code group identified in thesecond step.

n After the primary scrambling code has been identified, thePrimary CCPCH can be detected and the system- andcell specific BCH information can be read.


Cell search procedure


Content


Scrambling planning principleCluster-Based scrambling multiplexing method

Scrambling setting principle

Scrambling Planning Principle

n From the viewpoint of network planning, WCDMA scrambling

planning is to distribute the primary scrambling code to the cell.

n The WCDMA downlink has a total of 512 primary scrambling

codes and one primary scrambling code is distributed to one

cell, as an identification parameter of the cell.

n When the number of cells is larger than 512, you can

repeatedly distribute a primary scrambling code to different

cells, as long as the distance between cells using the same

primary scrambling code is long enough to make the coverage

of received signals in another cell with the same primary

scrambling code lower than the threshold level.

Scrambling planning principle

BS i BS jLij

Ri Rj

Cell i

Cell j


n To determine the minimum distance between BSs withthe same scrambling code is a basis and key issue ofcode planning. Suppose the cell i and cell j use the samescrambling code, the distance between two BSs is Lij,and the coverage radiuses of the two cells arerespectively Ri and Rj (see the following figure).


Lij must be big enough, to make the signal power of a remote BS far less than that of the main BS. In addition,

the signal level of the remote BS should be lower than the noise level. To meet the above requirements, the

following inequation should be tenable:

Where, Lij is the minimum reuse distance, a is the path loss coefficient, and D passloss is the pass

loss difference. 10log(Lij-max(Ri,Rj))a indicates the minimum pass loss of the remote BS signal

and 10log(max(Ri,Rj))a indicates the maximum pass loss of the main BS signal.

passloss10log( max( , )) 10log(max( , ))ij i j i jL R R R Ra a- - > D



)101).(max( 10apathloss

jiij RRL ++=

Content

Introduction to WCDMA cell scramblingScrambling planning principle

Cluster-Based scrambling multiplexing method


Cluster-Based Scrambling Multiplexing Method

n Under the precondition that the scrambling reuse distance issatisfied, the same scrambling code can be reuse at differentBSs. Generally, a group of BSs using different scramblingcodes within the reuse distance is called a cluster, and thescrambling code is multiplexed in cluster mode, as shown in thefollowing figure:


75 kmRural area

18 kmSuburb

8.12 kmCommon urban area

4 kmDense urban area

Scrambling Multiplexing DistanceWireless Transmission Environment

Cluster-Based Scrambling Multiplexing Method

n According to scrambling planning principles, the following

table gives recommended scrambling multiplexing

distance values under four typical wireless transmission

environments:


Content


Scrambling planning principleCluster-Based scrambling multiplexing method


Scrambling Setting Principle

n Downlink scrambling planning is to distribute 512 groups ofprimary scrambling codes to sectors. Scrambling planningshould be built on the final result of network planning, andobserve the following principles:� To reuse the same scrambling code, the reuse BSs should have

an adequate physical distance between them.

� Scrambling planning should take the features of stepwise networkconstruction into full consideration, and reserve a certain numberof scrambling codes, to satisfy demands of indoor distributedsystems and network capacity expansion.

� It is necessary to reserve a certain number of scrambling codesfor border BS coordination.

� Scrambling codes of adjacent cells had better belong to differentscrambling sets.


Ø To multiplex the same scrambling code, the multiplexing BSs should have

an adequate physical distance.

Note: For a 3-sector BS, two sectors within the scrambling reuse distance may have no jointcoverage scope due to the sector coverage direction, so one scrambling code can be reuse.Therefore, in the case of three sectors, the actual number of distributed scrambling codes

within the scrambling reuse distance can be smaller than the total sector number.

225755 to 1075Rural area

225751.2 to 318Suburb

183610.6 to 1.28.12Common urbanarea

183610.3 to 0.64Dense urban area

Maximum Number ofScrambling Codes in Use (Takethe 3-Sector BS as an Example)

Maximum BSNumber in aCluster

BS Radius (km)ScramblingMultiplexingDistance (km)

Area




n Scrambling planning should take the features of stepwise networkconstruction into full consideration, and reserve a certain number ofscrambling codes, to satisfy demands of indoor distributed systemsand network capacity expansion.

n The maximum number of scrambling codes used in an area iscalculated according to the minimum radius of the macro cell BS in thearea. The actual number of distributed scrambling codes is calculatedaccording to the number of macro cell BSs in the planning area. Thescrambling code not in use can be reserved for follow-up capacityexpansion. For the micro cell and indoor distributed system, theisolation between BSs, especially between different indoor distributedsystems, is good. Therefore, the scrambling reuse rate is very high,and 100 scrambling codes can be reserved for distribution.



n It is necessary to reserve a certain number of scrambling codesfor border BS coordination.� Generally, a big WCDMA network is divided into several subnets,

which are planned by different planners. If you do not know thescrambling distribution schemes of other subnets, it is easy todistribute the one scrambling code to two subnet border BSs. Inaddition, the interference occurs because the distance betweentwo BSs with the same scrambling code cannot satisfy the reusedistance required for the same scrambling code. To avoid theabove case, a certain number of scrambling codes must bereserved for border BS coordination. The number of reservedscrambling codes is that distributable in one cluster (see thefollowing table). However, the actual number of scrambling codesin use can be smaller than the number of reserved scramblingcodes.


The recommended scrambling distribution scheme is as follows:

325 to 511225 to 3240 to 224Recommended value

Border BS

Scrambling

Distribution

Scrambling Distribution for Micro Cells and

Indoor Distributed Systems

Common

Scrambling

Distribution




n Scrambling codes of adjacent cells had better belong to

different scrambling sets.

� When two adjacent cells use scrambling codes in one

scrambling group, they has the same secondary

synchronous code word combination. If the signal strength

of the two cells is close to each other, it may be difficult for

the terminal to distinguish them. Therefore, the scrambling

codes of two adjacent cells had better belong to different

scrambling groups.


6-WCDMA Scrambling Code Planning 28

Documents

Transcript of 6-WCDMA Scrambling Code Planning 28