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WCDMApo

wer tim

e

frequency

~5 MHz

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Outline

• IMT-2000 Requirements• WCDMA system• Multiservice concepts

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IMT-2000 requirements• Full coverage and mobility for 144 kbps,

and preferably for 384 kbps• Limited coverage and mobility for 2 Mbps• High spectral efficiency• Flexibility to introduce new services

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IMT-2000 user rate vs coverage and mobility

Fixed area / low mobility Wide area / high mobility

User bit rate

2 Mbps

384 kbps

144 kbps

10 kbpsBasic 2G

Evolved 2G

GSM EDGE (Enhanced Data rates using optimised modulation)

IMT-2000

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JargonFest

IMT-2000 = International Mobile Telephony 2000– ITU (International Telecommunications Union) terminology for 3G

UMTS = Universal Mobile Telecommunication System– “The UMTS Forum is an international and independent body, uniquely

committed through the building of cross-industry consensus to the successful introduction and development of UMTS/IMT-2000 ’third generation’ mobile communications systems.”

UTRA = Universal Terrestrial Radio Access3GPP = 3rd Generation Partnership Project

– joint standardisation group– WCDMA is known within 3GPP as UTRA FDD + UTRA TDD

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WCDMA Concepts

• WCDMA system specifications• Logical Channels• Physical Channels• Packet Access• Multiservice support• TDD mode

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WCDMA key characteristics• Multiple access scheme DS-CDMA• Duplex scheme FDD/TDD• Chip rate 3.84 Mcps• Carrier Spacing Flexible 4.4-5.0 MHz (3.84 Mcps)• Frame length 10 ms• Multi-rate/Variable rate Variable spreading factor (4 to 256)

+ Multi-code• Channel coding Convolutional coding (rate 1/2 or 1/3)

Optional outer Reed-Solomon coding (rate 4/5)

• Packet access Dual mode (common channel or dedicated channel)

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Common Control Channels• Broadcast control channel (BCCH)• Forward Access Channel (FACH)• Paging Channel (PCH)• Random Access Channel (RACH)

Dedicated Channels• Dedicated Control Channel (DCCH)• Dedicated Traffic Channel (DTCH)

Downlink

Uplink

Bidirectional

Uplink and/or Downlink

WCDMA Logical Channel structure

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• Primary Common Control Physical Channel (Primary CCPCH) (full cell coverage)

• Secondary Common Control Physical Channel (Secondary CCPCH)(may be transmitted over only part of a cell, e.g. a lobe)

• Physical Random Access Channel (PRACH)

• Dedicated Physical Data Channel (DPDCH)• Dedicated Physical Control Channel (DPCCH)

The logical channels are mapped into the above physical channels, (conceptually similarly to GSM).

Com

mon

Dedi

cate

dWCDMA Physical Channel Structure

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Broadcast Control Channel BCCH (DL)• Downlink point to multipoint channel• Broadcasts system and cell specific information

(including info on available codes at the cell)• BCCH is transmitted over entire cell• Mapped to Primary CCPCH

Forward Access Channel (DL)• Carries control information to mobile• FACH may also carry short user packets• FACH may be transmitted over only part of a cell

(e.g. smart antennas)• Mapped to Secondary CCPCH

Common Control Channels

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Paging Channel PCH (DL)• Carries control information to mobile when mobile location is

unknown• Transmitted over the entire cell• Mapped to Secondary CCPCH

Random Access Channel (UL)• Carries control information from mobile station• RACH may carry short user packets• Received from entire cell• Mapped to PRACH

Common Control Channels cont’d

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Dedicated Control Channel DCCH (UL and DL)• Bidirectional channel used to carry control information• Mapped to DPDCH (together with DTCHs)

Dedicated Traffic Channel DTCH (DL and/or UL)• Bidirectional or unidirectional channel• Used to carry user information• Mapped to DPDCH

(together with DCCH and other DTCHs)

Dedicated Control Channels

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Common Channels

BCCH Mapped to Primary CCPCHFACH Mapped to Secondary CCPCHPCH Mapped to Secondary CCPCHRACH Mapped to PRACH

Dedicated Channels

DCCH Mapped to DPDCHDTCH Mapped to DPDCH

Summary of Logical Channels

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Dedicated Physical Data Channel DPDCH•carries dedicated data, generated at level 2 and above

Dedicated Physical Control Channel DPCCH•carries control information generated at level 1, i.e.

– pilot signals to assist in coherent detection– transmit power control signals– rate information

Dedicated Physical Channels

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Superframe, 72 frames, 720 ms

S#1 S#2 S#3 S#4 S#5 S#6 S#7 S#8 S#9 S#10 S#11 S#12 S#13 S#14 S#15 S#16

Frame, 16 slots, 10 ms

RITPCPilot

Slot, 0.625 ms, 2560 chips

Data

Rate InformationTransmit Power Control

Pilot signal

Data Bits

DPCCH DPDCH

WCDMA Frame Structure:Downlink Dedicated Physical Channels

Slot length is 0.625 mswith 20××××2k bits , k = 0,1,...,6SF=256/2k

=>SF from 4 to 256

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Spreading and Modulation forDownlink Dedicated Physical Channels

cch is the channelisation codecscramb is the scrambling code (cell-specific)p(t) is the pulse shaping filter

For multicode transmission, each DPDCH/DPCCH should be assigned a distinct channelisation code

SerialtoParallel

chc scrambc )sin( tω

)cos( tω

DPDCH/DPCCH

I

Q

)(tp

)(tp

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Channelisation CodesOrthogonal Variable Spreading Factor (OVSF) codes• defined by code tree:

)1(1,1 =C

)1,1(1,2 =C

)1,1(2,2 −=C

)1,1,1,1(1,4 =C

)1,1,1,1(2,4 −−=C

)1,1,1,1(3,4 −−=C

)1,1,1,1(4,4 −−=C

SF = 1 SF = 2 SF = 4

• a code can only be used iff no other code is used between that code and the root of the code tree

... SF = 256

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Superframe, 72 frames, 720 ms

S#1 S#2 S#3 S#4 S#5 S#6 S#7 S#8 S#9 S#10 S#11 S#12 S#13 S#14 S#15 S#16

Frame, 16 slots, 10 ms

WCDMA Frame Structure:Uplink Dedicated Physical Channels

Slot length is 0.625 mswith 10 ×××× 2k bits , k = 0,1,...,6

Pilot TPC RI

Slot, 0.625 ms, 2560 chipsData

DPCCH

DPDCH

Rate InformationTransmit Power Control

Pilot signal

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Spreading and Modulation forUplink Dedicated Physical Channels

cC, cD - channelisation codesc’scramb - primary scrambling code

- a complex code cI+jcQc’’scramb - secondary scrambling code (optional)p(t) is the pulse shaping filter

scrambc'

)sin( tω

)cos( tωDPDCH I

Q

)(tp

)(tpDPCCH

Cc

Dc

j*

jQI +(optional)

'' scrambc Real

Imag

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For multicode transmission, each additional DPDCH may be transmitted on either the I or Q branch, with a distinct channelisation code

Uplink Dedicated Physical Channels….

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Superframe, 72 frames, 720 ms

S#1 S#2 S#3 S#4 S#5 S#6 S#7 S#8 S#9 S#10 S#11 S#12 S#13 S#14 S#15 S#16

Frame, 16 slots, 10 ms

Pilot

Slot, 0.625 ms, 2560 chips

Data

Pilot signalData Bits

Slot length is 0.625 mswith 20 × 2k bits , k = 0,1,...,6

CCPCH has•no power control•constant rate

WCDMA Frame Structure:Common Control Physical Channels

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CCPCH is modulated and spread as for the Downlink Dedicated Physical Channel

Primary CCPCH•has fixed predefined rate of 32 kbps•is transmitted over an entire cellSecondary CCPCH•has constant rate, which may be different for different cells,

depending on capacity needed for FACH and PCH•only transmitted when data is available, e.g. in a narrow lobe•has the FACH and PCH time multiplexed frame-by-frame.The set of allocated frames is broadcast on the BCCH.

Common Control Physical Channels (contd)

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SCH is used for cell searchSCH consists of 2 sub-channels,

the Primary SCH and Secondary SCHSCH is transmitted one codeword per slot

Primary SCH is used to acquire slot synchronisation to the strongest BSSecondary SCH is used to obtain frame synchronisationand identify the code group of the BS.→then the mobile can determine the scrambling code, → then detect the Primary CCPCH, → then acquire superframe synchronisation etc

Synchronisation Channel SCH (DL)

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Primary SCH is• an unmodulated Gold code of length 256 chips, • transmitted once per slot, aligned with slot boundary• same for every BS

Secondary SCH is• a modulated Gold code of length 256 chips• transmitted in parallel with the Primary SCH• chosen from a set of 16 different codes,

to match the BS downlink scrambling code

Synchronisation Channel SCH (DL) cont’d

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Physical Random Access Channel PRACH

Random access burst contains• a preamble of 16*256 chips (1ms)• a variable length data part

Preamble Data

• The preamble consists of 16 symbols spread by the preamble codeof length 256 chips (find these from BCCH)

• Each symbol is randomly chosen from a set of 16 orthogonal code words each of length 16 bits

• Neighbouring BSs use different preamble codes

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CRC

PRACH: Data Part

Data part contains• Mobile station ID (16 bits)• Required service (3 bits) (e.g short packet, dedicated channel setup)• Optional user packet (variable length)• CRC (8 bits)

Spreading and modulation as for uplink dedicated physical channels

Preamble DataPreamble Req

SerMS ID

User Packet

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Scrambling code for data partis based on:• the BS specific preamble code• plus the randomly chosen preamble sequence• plus a randomly chosen time offset

This ensures that random access attempts using different preamble codes/sequences won’t collide

PRACH: Data Part...

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First:• obtain chip and frame synchronisation• obtain information on available preamble codes• determine transmit power, estimated to achieve target SIR

using open loop power control

Then:transmit the burst with a randomly chosen 2n ms (n = 0,1,2,3,4)time offset relative to the frame boundary

A BS may the receive up to 80 random access attempts within one 10 ms frame(80 = 16 preamble sequences, with 5 time offsets)

Random Access procedure

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Power Control

SIR-based power control, using both open loop andclosed loop power control

Operates similarly on both uplink and downlink

Target SIR is independently adjusted for each connection, based on the estimated quality of the connection.Quality estimate is obtained using a combination of BER and FER estimates

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WCDMA Concepts

• WCDMA system specifications• Logical Channels• Physical Channels• Packet Access• Multiservice support• TDD mode

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Outer interleaving

Inner interleaving

Inner interleaving

Channel coding/interleaving for QoS

Inner coding

Inner coding

Outer coding

BER=10-3

BER=10-6

Service Specific Coding

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Innercoding/ interleaving

Outercoding/ interleaving

Time Mux

Multiple services belonging to the same connection are normally time-multiplexed, then mapped to one or more DPDCHs, as necessary

Time Mux

Time Mux

.

.

.

.

.

.

.

.

.

DPDCH #1

DPDCH #2

DPDCH #NPar

alle

l Ser

vice

s

Service Multiplexing

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Service Multiplexing (contd)Multiple services belonging to the same connection may alternatively be treated completely separately, in multicode fashionThis allows QoS for separate services to be individually controlled,but MS complexity is greater.

Coding/ interleaving

DPDCH #1

Coding/ interleaving

DPDCH #2

Coding/ interleaving

DPDCH #N

.

.

.

Parallel Services

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Rate MatchingMultiplexed rates can produce almost arbitrary total bit rates

There are a limited set of rates available on a DPDCH

To match the rates: use rate matching • repetition coding or • code puncturing

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Example 8 kbps bearer service

Tail (8 bits)CRC (8 bits)Data (80 bits)

Data (96 x 3 = 288 bits)

Data (288 x 10/9 = 320 bits)

Rate 1/3 convolutional coding

9->10 unequal repetition

DPDCH32 kbps

Bearer8 kbps

Coded channel28.8 kbps

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Example 144 kbps bearer service

Tail (8 bits)

Data (1440 bits)

Data (1440 x 225/180 = 1800 bits)

Data (1808 x 3= 5424 bits)

Rate 180/225 RS coding

Rate 1/3 convolutional code

DPDCH512 kbps

Bearer144 kbps

Data (5424x 320/339 = 5120 bits)

339->320 code puncturing

Coded channel542.4 kbps

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Example 384 kbps bearer service

Tail (24 bits)

Data (3840 bits)

Data (3840 x 240/192 = 4800 bits)

Data (4824 x 2 = 9648bits)

Rate 192/240 RS coding

Rate 1/2 convolutional code

DPDCH1024 kbps

Bearer384 kbps

Data (9648 x 640/603 = 10240bits)

603->640 unequal repetition

Coded channel964.8 kbps

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HandoverSoft Handover• Active MS receives a priority list from the network• MS searches priority list for new BSs

Softer Handover• Soft handover between sectors of the same BS• Operation as for soft handover• Differences only at network implementation level

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Handover (contd)Interfrequency HandoverNeeded:• When handover occurs between cells where

a different # of carriers have been allocated• For handover between cell layers using different

carrier frequency (e.g hierarchichal cells)• For interoperator handover• For handover to GSM

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Interfrequency HandoverIdle period is created for measurements of other frequencies, either by reducing the spreading factor by 2, or by code puncturing

Frame

Idle period available for interfrequency measurements

Rate is variable, ~100 ms intervals

Handover (contd)

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Interfrequency HandoverWhen service allows interleaving over several frames, multiple frames can be compressed to create a 5 ms measurement slot

Frame

Idle period available for interfrequency measurements

Compressed transmission during one interleaver span

Handover (contd)

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WCDMA Packet AccessWCDMA packet access can take place• on a common fixed-rate channel• on a dedicated channel

Common channel packet transmission• Uplink packet is appended directly to a random access burst• Limited to short packets that use only a

limited amount of capacity

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WCDMA Packet AccessDedicated channel packet transmission• Single-packet transmission mode, or• Multiple-packet transmission mode

Single packet transmission modeSend a random access request, indicating amount of data

to be sentNetwork responds:• with an immediate scheduling message • OR with a short ACK, followed by a scheduling message Scheduling message indicates when transmission can begin, the bit rate, etc

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WCDMA Packet Access (contd)

Randomaccessburst

Userpacket

Randomaccessburst

Userpacket

ArbitraryTime

Packet transmission on common channel

Userpacket

Userpacket

Single packet transmission on dedicated channel

Randomaccessburst

Randomaccessburst

ArbitraryTime

Common Channel

Dedicated Channel

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WCDMA Packet Access (contd)

Scheduled user packet

Multi- packet transmission on dedicated channel

Dedicated Channel

Accessrequest

Randomaccessburst Common Channel

Scheduled user packet

Unscheduled user packet

Accessrequest

Link maintenance

Multi-packet transmission• Random access request is used to set up a dedicated packet channel• Short packets may be sent on dedicated channel without scheduling• Long packets require an access request

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TDD Operation• TDD mode is based on the same frame structure as FDD mode,

i.e. 10 ms frame split into 16 x 0.625 ms slots• Multiplexing and spreading as for FDD mode• Each TDD slot can be used either for uplink or downlink

TDD Alternating mode (e.g. outdoor suburban environment)

Rx Rx Rx Rx Rx Rx Rx Rx

Tx Tx Tx Tx Tx Tx Tx Tx

0.625 ms

1.25 ms

10 ms

Coded Data Guard BandPilot

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TDD Operation (contd)

TDD Asymmetric mode (e.g. indoor/low speed outdoor)

0.625 ms10 ms

ReceivePi G

TransmitPi G

Receive block is a multiple of 0.625 msi.e. allows asymmetry of up to 15:1

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UE UTRAN

UMTS High Level System Architecture

UE:User Equipment

CN

UTRAN:UMTS Terrestrial Radio Access Network

CN:Core Network

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UEME+USIM

UTRANNode B + RNC

UTRA High Level System Architecture

ME: Mobile EquipmentUSIM: UMTSSubscriber Identity Module

CNGMSC+

MSC/VLR+HLR

Node B: Base StationRNC: Radio Network Controller

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RNC

Node BMSC/VLR

HLR

MSCN

UTRAN

Uu Interface Iub Interface Iu interface

UTRA: Network Elements

GMSC

UE

USIM

Cu Interface

ExternalNetwork

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UMTS Terrestrial Radio Access Network(UTRAN)• Support of soft handover• Support of the WCDMA-specific radio resource

management functions• Maximisation of commonalities in handling packet-

switched and circuit-switched data• Maximisation of commonalities with GSM• Use ATM transport as the main transport mechanism

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Radio Network Controller (RNC)

• Each BS has a Controlling RNC (CRNC)– load and congestion control– admission control– code allocation

• Since more than one RNC may be involved (e.g. soft handoff), each connection may involve:– Serving RNC (SRNC) controls outer loop power control, handoff

decisions. The SRNC may be the same as the CRNC used by some Node B used by the mobile.

– Drift RNC (DRNC) controls any other cells used by the mobile. One UE may have multiple DRNCs

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ETSI,Wideband Direct Sequence CDMA (WCDMA) Part 1: System Description and Performance Evaluation”,ETSI Tdoc SMG2 359/97, December 1997

H. Holma and A. Toskala (eds), WCDMA for UMTS: Radio Access for Third Generation Mobile Communications,Wiley, 2000

R Prasad, T Ojanperä, “An Overview of CDMA Evolution toward Wideband CDMA”,IEEE Communication Surveys (http://www.comsoc.org/pubs/surveys), Vol 1, No 1, pp 1-29, Fourth Quarter 1998

References