WCDMA - A Brief Overview

Stefan Parkvall

Ericsson Research

Outline

• Session 1

- Requirements, historical background, standardization process, 3GPP - CDMA basics

• Session 2 - Layer 1: coding, spreading, scrambling, slot structure, power control,

• Session 3 - Layer 2 and 3: MAC, RLC, PDCP, RRC

• Session 4 - Performance evaluation

Background to UTRA and WCDMA

What is Third Generation Radio Access?

TDMA EDGE CDPD

GSM GPRS

WCDMA (FDD/TDD)

PDC / PDC-P

cdma2000 1xEV cdmaOne c cdma2000 1X

2G First step into 3G 3G Evolved 3G

≤ 28.8 kbps 64 - 144 kbps 384 kbps - 2 Mbps > 2 Mbps

Time

Spectrum allocation

MSS IMT-2000 MSS ITU IMT-2000

Europe GSM 1800 MSS UMTS MSS UMTS

Japan PHS MSS MSS IMT-2000 IMT-2000

USA MSS MSS PCS

1800 1850 1900 1950 2000 2050 2100 2150 2200 2250

Frequency in MHz

UMTS Requirements

• Multimedia Service Requirements - High data rates

• At least 384 kbps wide-area coverage

• Up to 2 Mbps indoor and low-range outdoor coverage

- High service flexibility

• Packet- and circuit-oriented services • Wide range of bit rates with high granularity • Multiple services on one connection

• Additional requirements

- Improved capacity/coverage compared to GSM - Easy deployment, e.g. no frequency planning - Dual-mode/coexistence with GSM

• Terminal implementation/Harmonized parameters • Handoff between UMTS and GSM

History of UTRA and WCDMA Paris decision

Europe ETSI UTRA

FRAMES (FMA1, FMA2)

RACE II (CODIT, ATDMA)

RACE 1

1989 1990 1991 1992 1993 1994 1995 1996 1997 1998

Japan ARIB WCDMA

DoCoMo, NEC, etc.

1989 1990 1991 1992 1993 1994 1995 1996 1997 1998

European UTRA proposals

α β γ ε δ WCDMA OFDMA W-TDMA TD-CDMA ODMA

Wideband CDMA Orthogonal FDMA Wideband TDMA TDMA & CDMA Opportunity Driven Multiple Access

CDMA FDMA & TDMA TDMA TDMA & CDMA MS- 4.096 Mchip/s W: 100 kHz W: 1.6 MHz 2.167 Mchip/s relay system; W: 4.4 - 5.2 MHz 4 / 8 / 16 TS 16 / 64 TS W: 1.6 MHz enhancement f-reuse: 1/1 carrier & TS TS combining 8 TS for α- & δ- SF = 4 - 256 combining f-reuse: 1/3 concept

SF = 16

• Decision in January 1998

- UTRA/FDD: WCDMA was chosen for the paired FDD band - UTRA/TDD: based on TD/CDMA for unpaired bands

3GPP (3rd Generation Partnership Project) • Joint standardization of UTRA between:

- ETSI (Europe) ETSI ARIB

- ARIB (Japan) UTRA WCDMA

- Korea (TTA) 3GPP - T1P1/TR46.1 (US) UTRA TTA

T1P1 / TR46.1 - CATT (China) [TDD] Global CDMA II WP-CDMA

CATT TD-SCDMA

•3rd generation radio access based on WCDMA [FDD] • Evolved GSM Core Network

• First release of specification: End of 1999

UTRAN Architecture

Core Network UTRAN Architecture

RNC

Node B

UE

Protocol Architecture Control plane User plane

RRC Layer 3

Signaling Radio access channels bearers

RNC LC LC LC LC Layer 2 RLC

LC LC RLC RLC

Logical channels

MAC Layer 2 MAC

Transport channels

Physical Layer Layer 1 Node B

CDMA Basics

FDMA (Frequency-Division Multiple Access)

time • Users separated in frequency • Only possibility for analog systems • Used for NMT, AMPS, TACS

frequency 25 kHz (NMT) 30 kHz (AMPS)

TDMA (Time-Division Multiple Access)

time

• Users separated in time • Requires digital transmission • Normally wider bandwidth compared to pure FDMA

• Used for GSM, IS-136, PDC • In practice always combined

with FDMA frequency 200 kHz (GSM) 30 kHz (IS-136)

CDMA (Code-Division Multiple Access)

• Users separated by codes time code

• Requires digital transmission • Normally wider bandwidth compared to TDMA

• Used for IS-95 and 3rd generation WCDMA

• May be combined with FDMA (few carriers)

frequency • DS (Direct Sequence) CDMA most 1.25 MHz (IS-95) common 5 MHz (WCDMA)

CDMA Principle

rate = Rc User #N (rate = RN)

User #1, #2, …, #N cN

User #1 rate = Rc Σ User #2

(rate = R2)

c2 rate = Rc User #1

(rate = R1) c1

Ri: symbol rate for user #i ci: code for user #i

Rc: chip rate (same for all users)

DS-CDMA Spreading

A Channel B D Modulator

coding

C

Code generator “bit”

+1 B

-1

+1 C -1

+1 D -1

“chip”

Spreading Terminology

A Channel B D Modulator

coding

C

Code generator

• Spreading factor (SF) = Rspreading_output/Rspreading_input = = RD/RB = RC/RB

• Processing gain (PG) = Rchip/Rbit = = RD/RA = RD/RB × RB/RA = SF × 1/Rcoding

DS-CDMA Receiver

• Spreading code {ci}, ci ∈[+1, -1] Averaging = LP filter

• ci × ci = 1 !

Despread Σ signal

Code Code generator generator

Spreading and Despreading

User signal -1 +1 +1 +1

Spreading code

Spread signal

Despreading code (correct)

Despread signal

(sum over SF chips)/SF +1 -1 +1 +1

Despreading code (incorrect)

Despread signal

(sum over SF chips)/SF ≈0 ≈0 ≈0 ≈0

DS-CDMA Interference Suppression Transmitted

P P signal

f f

MOD

Narrowband interference

Wideband interference

DEMOD LP

P P P

f f f

Frequency Diversity

• Radio-channels suffer from frequency-selective fading

• Narrow-band carriers: A few users may suffer severely • Wideband carrier: All users suffer a small amount

Channel quality Channel quality

f f

Multi-path Diversity - the RAKE Receiver

T1

T2

•∆T = T2 - T1 > Tchip ⇒ Multi-path diversity

• RAKE receiver combines multi-path diversity components •4 Mcps ⇒ Tchip = 250 ns, corresponding to 75 m distance difference

RAKE Receiver Multi-path channel RAKE receiver

h1 h(t) h2

t τ τ+T h1 h1*

T LP h2 h2* Σ

T LP

Spreading code

• One RAKE finger for each channel path

• Each RAKE finger weighted with channel-path amplitude (maximum-ratio combining)

RAKE Receiver

Desired path for ray 1

T LP

Σ T LP

Undesired path for ray 1 “self interference”

The RAKE and Time Dispersion

• Time dispersion is good - Diversity between multi-path components

• Time dispersion is bad - Interference between multi-path components

• A RAKE receiver utilizes good side and suppresses bad side - A RAKE finger picks up one multi-path component, suppressing

the other by processing gain

- Several RAKE fingers for diversity

- Multi-path that is not picked up is suppressed by processing gain

RAKE Receiver

• More channels paths more RAKE fingers • Position (delay) and gain for each finger required

- Searcher: find new channel paths, assign finger to path - Tracker: track small changes in the finger positions

energy captured by the RAKE

energy not captured

RAKE searcher window

Spreading Sequences - Desired Properties

• Autocorrelation E{c1(t)c1(t+τ)}

- suppression of self interference (non-zero time shifts of the same code)

τ - ideally a delta pulse

- in practice close to zero at τ≠0

• Cross-correlation E{c1(t)c2(t+τ)}

- suppression of inter-user interference

- ideally zero τ - in practice close to zero

Different types of codes

• Random codes - Interference suppressed by processing gain

- Typically implemented as long pseudo-noise (PN) sequences - Almost infinitely many codes, long period

- Synchronization between user signals not needed

• Orthogonal codes

- Good correlation properties at lag=0, removes all inter-user interference - Relatively short period (period=n), typically equals bit duration - Limited number of codes (n codes)

- Poor correlation properties at lags≠0 synchronization between users required

• PN sequences and orthogonal codes are often combined

Example: Orthogonal code set (Walsh codes)

8 chips

C1 +1 +1 +1 +1 +1 +1 +1 +1 C5 +1 +1 +1 +1 -1 -1 -1 -1

C2 +1 -1 +1 -1 +1 -1 +1 -1 C6 +1 -1 +1 -1 -1 +1 -1 +1

C3 +1 +1 -1 -1 +1 +1 -1 -1 C7 +1 +1 -1 -1 -1 -1 +1 +1

C4 +1 -1 -1 +1 +1 -1 -1 +1 C8 +1 -1 -1 +1 -1 +1 +1 -1

• Multiplying any code with any other code yields zero • Multiplying a code with a shift of another yields non-zero ⇒ synchronization required

Example: Pseudo-random Codes

• Scrambling sequences in WCDMA - Two gold sequences, Clong,1,n, Clong,2,n

clong,1,n

m-sequence MSB LSB

m-sequence clong,2,n

CDMA in Cellular Systems

Frequency Reuse

• CDMA uses one-cell frequency reuse all cells use the same carrier frequency

FDMA/TDMA (reuse > 1) CDMA (reuse = 1)

Frequency planning needed No frequency planning needed

Soft Handover

• Soft handover: A mobile station communicates with two base stations simultaneously

• Soft handover possible because of one-cell reuse • Soft handover necessary because of one-cell reuse

RNC Radio Network

Controller

Soft Handover, uplink

• Two or more base stations receive the mobile’s signal, which is then combined in the network

• Selection combining normally used

RNC Radio Network Controller

h1, τ1 h2, τ2

c1

Soft Handover, downlink

• Mobile receives signal from two or more base stations, signal combined in mobile’s RAKE

• Maximum ratio combining normally used

RNC Radio Network Controller

h1, τ1 h2, τ2

c1 c2

Softer Handover

• Soft handover between cells (sectors) at same base station • In uplink, combining can be done in base station’s RAKE instead of in the network

• Less signaling in network

• Better combining possible, e.g. maximum ratio combining

Active Set Management

• Active set: the set of cells the mobile is engaged in soft/softer handover with

Handover cell SIR window

A

B

C

time A A A A A A Active

B B B B B set C C C C

Why Power Control?

• Several mobile terminals transmit on the same frequency • Same transmit power ⇒ large variations in received power • Mobiles with low path loss will cause large interference

PRX,1

L1 PRX,2

L2 PTX,2

PTX,1

L1 >> L2 ⇒ PRX,2 >> PRX,1 if PTX,2 ≈ PTX,1

Power Control

PRX,1

L1 PRX,2

L2 PTX,2

PTX,1

• Goal: Adjust transmit power so that all mobile terminals are received with approximately the same power

• Set PTX,1 and PTX,2 so that PRX,1 ≈ PRX,2 • Open-loop and closed-loop power control

Open-Loop Power Control

LDL PRX,MS

LUL PRX,BS PTX,MS

• Measure PRX,MS and estimate downlink path loss LDL • Assume uplink path loss LUL = LDL

• Determine PTX,MS from estimate of LUL and required PRX,BS • Compensates for distance and shadowing

• Does not compensate for frequency-selective fading

Closed-Loop Power Control

PRX,BS

Command: UP / DOWN

• Compare received PRX,BS with required PRX,BS • Send up/down power-control command • Power control parameters:

- Rate: ≈ 0.5 - 2 kHz

- Step size: ≈ 0.5 - 1 dB

Power Control “Removes” Fading

Without power control With power control

TX power TX power

t t

RX power RX power

t t

Capacity

• FDMA capacity

- Limited by number of available frequencies

• TDMA capacity

- Limited by number of available time slots

• CDMA capacity

- Limited by

• the amount of interference that can be tolerated P • the amount of interference generated by each user

f

Interference averaging

• The total interference is the sum of all interference • Σinstantaneous power ≤Σpeak power

• Average transmit power per user is the important factor

Power Levels from MS Received Power Levels at BTS

CA

CB

CC

Summary

• Codes used to increase bandwidth • RAKE receiver normally used • Reuse one possible

• Soft handover to increase coverage and capacity • Power control necessary

• Capacity normally interference limited