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WCDMA Physical Layer (Chapter 6)
Peter Chong, Ph.D. (UBC, Canada) Research Engineer Nokia Research Center, Helsinki, Finland

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WCDMA Phys ical Layer

Introduction
• This lecture presents a general WCDMA or UTRA (Universal Terrestrial Radio Access) FDD (Frequency Division Duplex) physical layer issues.

• Spreading and Scrambling • Transport Channels • Physical Channels• Signaling • Physical Layer Procedures

Mapping to

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WCDMA Phys ical Layer

Some Parameters of WCDMA Physical Layer
Carrier Spacing Chip Rate Frame Length No. of slots/frame No. of chips/slot Uplink SF Downlink SF Channel Rate
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5 MHz (nominal) 3.84 Mcps 10 ms (38400 chips) 15 2560 chips (Max. 2560 bits) 4 to 256 4 to 512 7.5 Kbps to 960 Kbps
WCDMA Physical Layer

Spreading and Scrambling

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WCDMA Phys ical Layer

Spreading Operation
• Spreading means increasing the signal bandwidth • Strickly speaking, spreading includes two operations:

• Channelisation (increases signal bandwidth) - using orthogonal
codes

• Scrambling (does not affect the signal bandwidth) - using pseudonoise codes
channelization codes (SF)scrambling codes

Data bit rate
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chip rate

chip rate
WCDMA Phys ical Layer

Channelisation (1/3)
• Channelisation codes are orthogonal codes, based on Orthogonal Variable Spreading Factor (OVSF) technique • The codes are fully orthogonal, i.e., they do not interfere with each other, only if the codes are time synchronized • Thus, channelisation codes can separate thetransmissions from a single source • In the downlink, it can separate different users within one cell/sector • Limited orthogonal codes must be reused in every cell • Problem: Interference if two cells use the same code • Solution: Scrambling codes to reduce inter-base-station interference

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WCDMA Phys ical Layer

Channelisation (2/3)
• In the uplink, it can only separate thephysical channels/services of one user because the mobiles are not synchronised in time. • It is possible that two mobiles are using the same codes. • In order to separate different users in the uplink, scrambling codes are used. • The channelisation codes are picked from the code tree as shown in next slide. • One code tree is used with one scrambling code on top of the tree. • If c4,4 is used, nocodes from its subtree can be used (c8,7 , c8,8 , …).

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WCDMA Phys ical Layer

Channelisation (3/3)
Code tree
c4,1=(1,1,1,1) c2,1=(1,1) c1,1=(1) (c) (c,c) c4,2=(1,1,-1,-1) c4,3=(1,-1,1,-1) c4,4=(1,-1,-1,1) c8,1 c8,2 c8,3 c8,4 c8,5 c8,6 c8,7 c8,8

...

(c,-c)

c2,2=(1,-1)

S F =1

S F =2

S F =4

S F =8

...

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WCDMA Phys ical Layer Scrambling
• In the scrambling process the code sequence is multiplied with a pseudorandom scrambling code. • The scrambling code can be a long code (a Gold code with 10 ms period) or a short code (S(2) code). • In the downlink scrambling codes are used to reduce the inter-basestation interference. Typically, each Node B has only one scrambling code for UEs to separate base stations. Since a code treeunder one scrambling code is used by all users in its cell, proper code management is needed. • In the uplink scrambling codes are used to separate the terminals.

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WCDMA Phys ical Layer

Summary
Channelisation code Scrambling code Usage UL: Separation of physcial data and control channels from same UE DL: Seperation of different users within one cell UL:4 – 256 chips sameas SF DL:4 – 512 chips same as SF No. of codes under one scrambling code = SF Orthogonal Variable Spreading Factor Yes, increase transmission bandwidth UL: Separation of terminals DL: Separation of cells/sectors

Length

Limited codes in each cell for DL. UL: 10ms=38400 chips 38400 DL: 10ms=38499 chips
UL: Several millions DL: 512 Long 10ms code: Gold code Short code: Extended S(2) code...
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