WCDMA - A brief overview by Ericsson

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Slide 1: 

WCDMA - A Brief Overview Stefan Parkvall Ericsson Research

Slide 2: 

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

Slide 3: 

Background to UTRA and WCDMA

Slide 4: 

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

Slide 5: 

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

Slide 6: 

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

Slide 7: 

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

Slide 8: 

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

Slide 9: 

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

Slide 10: 

UTRAN Architecture

Slide 11: 

Core Network UTRAN Architecture RNC Node B UE

Slide 12: 

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

Slide 13: 

CDMA Basics

Slide 14: 

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)

Slide 15: 

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)

Slide 16: 

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)

Slide 17: 

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)

Slide 18: 

DS-CDMA Spreading A Channel B D Modulator coding C Code generator “bit” +1 B -1 +1 C -1 +1 D -1 “chip”

Slide 19: 

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

Slide 20: 

DS-CDMA Receiver • Spreading code {ci}, ci ∈[+1, -1] Averaging = LP filter • ci × ci = 1 ! Despread Σ signal Code Code generator generator

Slide 21: 

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

Slide 22: 

DS-CDMA Interference Suppression Transmitted P P signal f f MOD Narrowband interference Wideband interference DEMOD LP P P P f f f

Slide 23: 

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

Slide 24: 

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

Slide 25: 

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)

Slide 26: 

RAKE Receiver Desired path for ray 1 T LP Σ T LP Undesired path for ray 1 “self interference”

Slide 27: 

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

Slide 28: 

RAKE Receiver • More channels paths Æ 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

Slide 29: 

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

Slide 30: 

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≠0Æ synchronization between users required • PN sequences and orthogonal codes are often combined

Slide 31: 

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

Slide 32: 

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

Slide 33: 

CDMA in Cellular Systems

Slide 34: 

Frequency Reuse • CDMA uses one-cell frequency reuse Æ all cells use the same carrier frequency FDMA/TDMA (reuse > 1) CDMA (reuse = 1) Frequency planning needed No frequency planning needed

Slide 35: 

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

Slide 36: 

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

Slide 37: 

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

Slide 38: 

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

Slide 39: 

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

Slide 40: 

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

Slide 41: 

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

Slide 42: 

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

Slide 43: 

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

Slide 44: 

Power Control “Removes” Fading Without power control With power control TX power TX power t t RX power RX power t t

Slide 45: 

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

Slide 46: 

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

Slide 47: 

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

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