Personal Communication Systems

8/11/2019 Personal Communication Systems

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ECE 5221 Personal Communication Systems Prepared by:

Dr. Ivica Kostanic

Lecture 24 – Basics of 3G – UMTS (3)

Spring 2011



OSI Communication model

• Each layer communicates only with twoadjacent layers and its peer on the other side

• Each layer receives services from the layerbelow and provides services to the layerabove

• Intermediate communication nodesrequire layers 1 through 3

• Internal operation within each layer isindependent of the internal operationin any other layer

Application Layer

Presentation Layer

Session Layer

Transport Layer

Network Layer

Data Link Layer

Physical Layer

Application Layer

Presentation Layer

Session Layer

Transport Layer

Network Layer

Data Link Layer

Physical Layer Physical Layer

Data Link Layer

Network Layer

Physical Medium Physical Medium

Node A Node B Node C

Peer to peer protocols • WCDMA interfacesdescribed using OSI model

• OSI = Open SystemInterconnect

• Developed by ISO as a

general model for computercommunication

• Used as a framework fordevelopment andpresentation of mostcontemporary

communication standards

Note: WCDMA covers Layers 1-3 of OSI Model



UMTS Protocol stack

• UMTS offers new Accessstratum protocol stack

• Non-Access Stratum islargely inherited fromGSM

• First three layers of theprotocol stack are part ofUTRAN

Note: SMS exists on

both circuit switchedand packet switchedside



UMTS CS protocols – control plane• Control plane –

carries signaling• RNC terminates

the AccessStratum (AS)

• RRC, RLC andMAC terminate atRNC

• PHY terminates atNode B except forouter loop powercontrol

• RAN (accessstratum) acts astransport for NAS

4

Note: UTRAN protocols are layered in an architecture that follows OSImodel



UMTS CS protocols – user plane• User plane –

caries userdata

• Application – end to end

protocol• Access stratum

the same forboth controlplane and userplane

5



UMTS PS protocols – control plane• Control plane

for packet data• Very similar to

control planefor PS

• Identicalaccess stratum

6



UMTS PS protocols – user plane• Additional protocol

PDCP• PDCP –

compression of IPheaders

• PDCP may or maynot be used

7



Layout of the Access Stratum• Two planes

– User plane - user data – Control plane – signaling

• User data enters accessthrough radio bearers(RABs)

• Signaling is handled by RRC• Upper layer signaling –

encapsulated through RRCmessages (direct transfer)

• RRC has a capability ofreconfiguring all lower layers

8



ELEMENTS OF PHY LAYERPROCESSING

Part 6

9



UMTS-FDD PHY frame structure• UMTS-FDD PHY frame structure is based on 10ms frames

• Frames are broken in 15 time slots

• The number of bits/slot is variable

• Chip rate is always the same (3.84 Mchips/sec)

F0 F1 F71

S0 S1 S14

Superframe = 72 Frames

Frame = 15 Slots

Slot = 2560Chips

720 ms

10 ms

The number of bits per slot varies

0.667 ms

User Data

ControlInformation



UMTS-FDD DL processing• There are 6 steps in DL

PHY processing – I/Q separation – Variable spreading – Scrambling – Gain adjustment – Sync addition – Modulation

S

S/P OVSF

X

X

X X

S/P OVSF

X

X

X X

S

X

X

Modulation

VariableSpreading Scrambling

Gainadjustment

Sync addition

Channel 1

Channel n

R b1

R bn

Rb1 /2

Rb1 /2

Rbn /2

Rbn /2

R c=3.84Mc/sec

R c

R c

R c

R c

SC1

SCn

G1

Gn

Gp

GsReal Signals

Complex Signals

P-SCH

S-SCH

I/QSeparation

I

Q

I

Q

Note: Number of channelsdepends on number ofactive users. P-SCH andS-SCH are alwaystransmitted



W-CDMA DL Modulation

• UMTS-FDD uses simple QPSK modulation scheme• Complex code sequence is split into real and imaginary part and modulated

using carriers in quadrature



W-CDMA Modulation

• UMTS-FDD uses root-raisedcosine for the shaping filter

• The roll-off is a = 0.22

=20

41

1cos41sin

C C

C C C

T

t

T

t

T

t

T

t

T

t

t RC

a

a a a

-1 -0 .5 0 0 .5 1-0.4

-0.2

0

0. 2

0. 4

0. 6

0. 8

1

1. 2

i i

i

i l

i

l

-5 0 5-6 0

-5 0

-4 0

-3 0

-2 0

-1 0

0

10

i

5MHz

Impulse response of the shaping filter Frequency response of the shaping filter

Analytical expression of the shaping filterimpulse response

Note: only 30dBc on the sidebands –

may cause interference to GSM innon 1-1 overlay scenarios



W-CDMA DL variable spreading• Different data channels have different rates• The chip rate is always the same• W-CDMA supports variable spreading on the DL• Variable spreading is accomplished through use of orthogonal codes of

different length

Spreading Factor User data rateAfter coding

[Kb/ sec]

Approximate ratebefore coding

[Kb/ sec]512 15 1-3256 30 6-12

128 60 42-5264 120 ~ 4532 240 ~ 10516 480 ~ 2158 960 ~ 4504 1920 ~ 930

4, with 3 parallel codes 5760 ~ 2300

UMTS-FDD available DL data rates

UMTS-FDD provideshigh data rates through• variable spreading• code aggregation

User data rates assume 1/2 convolutional encoding



W-CDMA scrambling codes

• OVSF codes provideorthogonality between signalscoming from the same BTS – form of channelization

• Scrambling codes allow mobile

to distinguish signals comingfrom different base stations• Scrambling codes do not change

signal bandwidth• Decoding a signal from a user is

in 2 steps –

Descrambling the signal from theNode B – De-spreading the signal from

individual user

Signal from BS2

BaseStation 1 Base

Station 2

Signal from BS1

Frequancy

W-CDMAsignals



W-CDMA scrambling codes• UMTS-FDD uses 8192 complex

scrambling codes• The codes are selected as parts of a

218 -1 long gold sequence (goodcorrelation prperties)

•

Each of the codes are associated withleft and right alternative scramblingcode

8192 Scrambling codes

SC0

SC1

SC2

SC15

SC16

SC17

SC18

SC31

SC32

SC33

SC47

SC8176

SC8177

SC8178

SC8191

Primary Codes

SecondaryCodes

SC34

512

• Scrambling codes are 38400 chipslong (10ms)

• Scrambling code repeats every frame• Organized in 512 groups of 16 codes• The first code in each group is

declared as the primary scramblingcode (PrSC)

• PrSC are used for cell identification

Scramblingcode tree



W-CDMA synchronization codes• Synchronization codes are

used for system detection• They are 256 chips long

complex codes• One primary and 64

secondary codes• Secondary codes consist of

15 code words• Secondary codes remain

unique under cyclic shifts

smaller than 15

•

A cell is allocated one primarysynchronization code• The primary code is the same for all

cells in the system• Secondary code points to a group of

primary scrambling codes

Synchronization Codes

Primary Secondary

PSC SSC0

SSC1

SSC63

Note: PSC allows mobile tosynchronize to the time slots.SSC allows mobile to synchronize

with the beginning of frame.



W-CDMA primary scrambling codes

• There are 512 primary scrambling codes• They are divided in 64 groups of 8 codes• Each cell is assigned one primary code

• Primary scrambling code is used to provideorthogonality between different BS

• Primary scrambling code is broadcast onthe Common Pilot Channel (CPICH)

512 Primary Scrambling Codes

Group 0 Group 1 Group 63

SC0

SC16

SC32

SC112

SC128

SC144

SC240

SC160

SC8064

SC8080

SC8096

SC8176

Note: after decodingSSC, the mobile needsto consider only 8 outof 512 PrSC



W-CDMA code assignment example

• Primary sync codeis the same for allcells

•

Secondary synccode number isthe same as thegroup of theprimary pSC

pSC: SC16(1)SSC: 0

pSC: SC128(8)SSC: 1

pSC: SC256(16)SSC: 2

pSC: SC32(2)SSC: 0

pSC: SC64(4)SSC: 0

pSC: SC80(5)SSC: 0

pSC: SC8064(504)SSC: 63

pSC: SC5760(360)SSC: 45

pSC: SC4096(256)SSC: 32

A

B

C

pSC - Primary Scrambling CodeSSC - Secondary Sync Code

Task: use previous two slides to verify code assignments for the above cells

Note: in practice network operator assigns only PrSC. SSC isassigned automatically on the basis of PrSC assignment



W-CDMA UL processing - dedicatedchannels

• There are 5 steps in the ULDCHs processing

– Spreading – Gain adjustment – Complex addition – Scrambling – Modulation

S

S

X X

X X

X X

X X

X X

X X

X X

R/C X

DPDCH_1

DPDCH_3

DPDCH_5

DPDCH_2

DPDCH_4

DPDCH_6

DPCCH

Cd1

Gd

Gd

Gd

Gd

Gd

Gd

Gd

I

Q

SC

SpreadingGain

Adjustment Scrambling

Modulation

Complex Addition

Cd2

Cd3

Cd5

Cd4

Cd6

Cc

DPDCH - Dedicated Physical Data ChannelDPCCH - Dedicated Physical Control Channel

Note: transmission from asingle mobile can aggregatemultiple codes to achievehigher data rate



W-CDMA UL variable spreading• Variable data rates are allowed on U DPDCH• Variable data rate achieved through

– variable spreading 4 to 256 – code aggregation - up to 6 parallel codes

• if code aggregation is used, spreading for all DPDCH is 4•

UL DPCCH is a constant rate channel ~ 15kb/sec (assigned code C 256,0 )

Spreading Factor User data rate[Kb/ sec]

Approximate ratebefore coding

[Kb/ sec]256 15 1-3128 30 6-1264 60 42-52

32 120 ~ 4516 240 ~ 1058 480 ~ 2154 960 ~ 450

4, with 6 parallel codes 5740 ~ 2300

User data rates assume 1/2 convolutional encoding

Personal Communication Systems

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