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• 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
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
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
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)
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
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