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 two adjacent layers and its peer on the other side

• Each layer receives services from the layer below and provides services to the layer above

Page 2

• Intermediate communication nodes require layers 1 through 3

• Internal operation within each layer is independent of the internal operation in any other layer

A pplica tion Layer

P resenta tion Layer

S ession Layer

T ransport Layer

N etwork Layer

D ata L ink Layer

P hysica l Layer

A pp lica tion Layer

P resenta tion Layer

S ession Layer

T ransport Layer

N etwork Layer

D ata L ink Layer

P hysica l LayerP hysica l Layer

D ata L ink Layer

N etwork Layer

Physical M edium Physical M edium

Node A Node B Node C

Peer to peer protocols • WCDMA interfaces described using OSI model

• OSI = Open System Interconnect

• Developed by ISO as a general model for computer communication

• Used as a framework for development and presentation of most contemporary communication standards

Note: WCDMA covers Layers 1-3 of OSI Model

Page 3

UMTS Protocol stack

• UMTS offers new Access stratum protocol stack

• Non-Access Stratum is largely inherited from GSM

• First three layers of the protocol stack are part of UTRAN

Note: SMS exists on both circuit switched and packet switched side

4

UMTS CS protocols – control plane

• Control plane – carries signaling

• RNC terminates the Access Stratum (AS)

• RRC, RLC and MAC terminate at RNC

• PHY terminates at Node B except for outer loop power control

• RAN (access stratum) acts as transport for NAS

Note: UTRAN protocols are layered in an architecture that follows OSI model

5

UMTS CS protocols – user plane

• User plane – caries user data

• Application – end to end protocol

• Access stratum the same for both control plane and user plane

6

UMTS PS protocols – control plane

• Control plane for packet data

• Very similar to control plane for PS

• Identical access stratum

7

UMTS PS protocols – user plane

• Additional protocol PDCP

• PDCP – compression of IP headers

• PDCP may or may not be used

8

Layout of the Access Stratum

• Two planes– User plane - user data– Control plane – signaling

• User data enters access through radio bearers (RABs)

• Signaling is handled by RRC• Upper layer signaling –

encapsulated through RRC messages (direct transfer)

• RRC has a capability of reconfiguring all lower layers

9

ELEMENTS OF PHY LAYER PROCESSING

Part 6

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)

Page 10

F0 F1 F71

S0 S1 S14

S uperfram e = 72 F ram es

Fram e = 15 S lo ts

S lo t = 2560C hips

720 m s

10 m s

The num ber o f b its per s lo t varies

0 .667 m s

U ser D ata

C ontro lIn form ation

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

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

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S

S /P O V S F

X

X

X X

S /P O V S F

X

X

X X

S

X

X

M odulation

V ariab leS pread ing

S cram bling

G ainad justm ent

S ync add ition

C hanne l 1

C hanne l n

R b1

R bn

R b1 /2

R b1 /2

R bn /2

R bn /2

R c=3.84M c/sec

R c

R c

R c

R c

S C 1

S C n

G 1

G n

G p

G sR ea l S igna ls

C om plex S igna ls

P -S C H

S -S C H

I/QS epara tion

I

Q

I

Q

Note: Number of channels depends on number of active users. P-SCH and S-SCH are always transmitted

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

Page 12

W-CDMA Modulation

• UMTS-FDD uses root-raised cosine for the shaping filter

• The roll-off is = 0.22aPage 13

20

41

1cos41sin

CC

CCC

Tt

Tt

Tt

Tt

Tt

tRC

-1 -0.5 0 0.5 1-0.4

-0.2

0

0.2

0.4

0.6

0.8

1

1.2

time [microsec]

sha

pin

g fi

lter

imp

ulse

re

spo

nse

-5 0 5-60

-50

-40

-30

-20

-10

0

10

frequency [MHz]

ga

in [d

B]

5MHz

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

Analytical expression of the shaping filter impulse response

Note: only 30dBc on the sidebands – may cause interference to GSM in non 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

Page 14

Spreading Factor User data rateAfter coding[Kb/ sec]

Approximate ratebefore coding

[Kb/ sec]

512 15 1-3256 30 6-12128 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 provides high data rates through• variable spreading• code aggregation

User data rates assume 1/2 convolutional encoding

W-CDMA scrambling codes

• OVSF codes provide orthogonality between signals coming from the same BTS – form of channelization

• Scrambling codes allow mobile to distinguish signals coming from different base stations

• Scrambling codes do not change signal bandwidth

• Decoding a signal from a user is in 2 steps

– Descrambling the signal from the Node B

– De-spreading the signal from individual user

Page 15

Signal from BS2

BaseStation 1 Base

Station 2

S ignal from BS1

Frequancy

W -C D M Asigna ls

W-CDMA scrambling codes

• UMTS-FDD uses 8192 complex scrambling codes

• The codes are selected as parts of a 218 -1 long gold sequence (good correlation prperties)

• Each of the codes are associated with left and right alternative scrambling code

Page 16

8192 Scram bling codes

SC0

SC1

SC2

SC15

SC16

SC17

SC18

SC31

SC32

SC33

SC47

SC8176

SC8177

SC8178

SC8191

P rim ary C odes

S econdaryC odes

SC34

512

• Scrambling codes are 38400 chips long (10ms)

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

declared as the primary scrambling code (PrSC)

• PrSC are used for cell identification

Scrambling code 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

Page 17

• A cell is allocated one primary synchronization code

• The primary code is the same for all cells in the system

• Secondary code points to a group of primary scrambling codes

S ynchron iza tion C odes

P rim ary S econdary

P S C S S C 0

S S C 1

S S C 63

Note: PSC allows mobile to synchronize 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 provide orthogonality between different BS

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

Page 18

512 P rim ary S cram bling C odes

G roup 0 G roup 1 G roup 63

SC0

SC16

SC32

SC112

SC128

SC144

SC240

SC160

SC8064

SC8080

SC8096

SC8176

Note: after decoding SSC, the mobile needs to consider only 8 out of 512 PrSC

W-CDMA code assignment example

• Primary sync code is the same for all cells

• Secondary sync code number is the same as the group of the primary pSC

Page 19

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 is assigned automatically on the basis of PrSC assignment

W-CDMA UL processing - dedicated channels

• There are 5 steps in the UL DCHs processing

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

Page 20

S

S

X X

X X

X X

X X

X X

X X

X X

R /C X

D P D C H _1

D P D C H _3

D P D C H _5

D P D C H _2

D P D C H _4

D P D C H _6

D P C C H

Cd1 Gd

Gd

Gd

Gd

Gd

Gd

Gd

I

Q

SC

S pread ingG ain

A d justm ent S cram bling

M odu la tion

C om plexA dd ition

Cd2

Cd3

Cd5

Cd4

Cd6

Cc

DPDCH - Dedicated Physical Data ChannelDPCCH - Dedicated Physical Control Channel

Note: transmission from a single mobile can aggregate multiple codes to achieve higher 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 C256,0)

Page 21

Spreading Factor User data rate[Kb/ sec]

Approximate ratebefore coding

[Kb/ sec]

256 15 1-3128 30 6-1264 60 42-5232 120 ~ 4516 240 ~ 1058 480 ~ 2154 960 ~ 450

4, with 6 parallel codes 5740 ~ 2300

User data rates assume 1/2 convolutional encoding