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Page 1: 1 WCDMA Fundamentals

1 © NSN Siemens Networks 3G Radio Planning Essentials / NPO Capability Development

WCDMA Fundamentals

Ashok Kumar Joshi

Page 2: 1 WCDMA Fundamentals

2 © NSN Siemens Networks 3G Radio Planning Essentials / NPO Capability Development

Downlink peak rate

Network Latency

Uplink peak rate

Bandwidth

Standard

3G Data Rate evolution

Spectral efficiency, DL

Spectral efficiency, UL

WCDMA R99

3GPP

5.0 MHz

100-200 ms

384 kbps

384 kbps

0.16 bps/Hz

0.16 bps/Hz

WCDMA HSPA

3GPP

5.0 MHz

<50 ms

1.8-14.4 Mbps

1-4 Mbps

0.2-0.8 bps/Hz

0.25 bps/Hz

3.9 G estim.

3GPP

1.25-20 MHz

<10 ms

Up to 100 Mbps

Up to 50 Mbps

1.6-2.5 bps/Hz

0.6-0.8 bps/Hz

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CN

circuitswitched

(cs) domain

packetswitched

(ps)domain

UTRAN

Radio Network Subsystem (RNS)

Radio Network Subsystem (RNS)

Iub

Iub

Iur

Iu-PS

Iu-CS

Uu

Uu

UE

UE

MSC/VLR

SGSN

RNC

RNC

UTRAN

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3G-MSC/VLR

3G-SGSN

UE Node BRNC

RNC

RNS

RNS

RRC

Iur: RNSAP

Iu-PS: RANAP

Iu-CS: RANAPIub: NBAP

UTRAN Specific Signalling and Control Protocols

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Module Contents

• Standardisation and frequency bands

• Main properties of UMTS Air Interface– UMTS Air interface technologies

– WCDMA – FDD

– WCDMA vs. GSM

– CDMA principle

– Processing gain

– WCDMA codes and bit rates

– Concepts of RSCP and Ec/No

WCDMA Handovers

• Overview of NSN Radio Resource Management (RRM)

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UMTS Air Interface technologies

• UMTS Air interface is built based on two technological solutions– WCDMA – FDD

– WCDMA – TDD

• WCDMA – FDD is the more widely used solution– FDD: Separate UL and DL frequency band

• WCDMA – TDD technology is currently used in limited number of networks

– TDD: UL and DL separated by time, utilizing same frequency

• Both technologies have own dedicated frequency bands

• This course concentrates on design principles of WCDMA – FDD solution, basic planning principles apply to both technologies

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WCDMA – FDD technology

• Multiple access technology is wideband CDMA (WCDMA)– All cells at same carrier frequency

– Spreading codes used to separate cells and users

– Signal bandwidth 3.84 MHz

• Multiple carriers can be used to increase capacity– Inter-Frequency functionality to support mobility between frequencies

• Compatibility with GSM technology– Inter-System functionality to support mobility between GSM and UMTS

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WCDMA Technology

5 M Hz

3.84 M Hz

f

5+5 MHz in FDD mode5 MHz in TDD mode

Freq

uenc

y

TimeDirect Sequence (DS) CDMA

WCDMA Carrier

WCDMAWCDMA5 MHz, 1 carrier5 MHz, 1 carrier

TDMA (GSM)TDMA (GSM)5 MHz, 25 carriers5 MHz, 25 carriers

Users share same time and frequency

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UMTS & GSM Network Planning

GSM900/1800: 3G (W CDM A):

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Differences between WCDMA & GSM

WCDMA GSM

Carrier spacing 5 MHz 200 kHz

Frequency reuse factor 1 1–18

Power controlfrequency

1500 Hz 2 Hz or lower

Quality control Radio resourcemanagement algorithms

Network planning(frequency planning)

Frequency diversity 5 MHz bandwidth givesmultipath diversity with

Rake receiver

Frequency hopping

Packet data Load-based packetscheduling

Timeslot basedscheduling with GPRS

Downlink transmitdiversity

Supported forimproving downlink

capacity

Not supported by thestandard, but can be

applied

High bit rates

Services withDifferent quality

requirements

Efficient packet data

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Multiple WCDMA carriers – Layered network

F1

F2

F2

F3

F3

F3

Micro BTSMacro BTS

Pico BTSs

1 - 10 km

50 - 100 m200 - 500 m

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Spreading Code

Spread Signal

Data

Air Interface

Bits (In this drawing, 1 bit = 8 Chips SF=8)

Baseband Data

-1

+1

+1

+1

+1

+1

-1

-1

-1

-1

ChipChip

Despreading

Despreading

CDMA principle - Chips & Bits & Symbols

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Spreading and Despreading

SpreadingEach user data bit is multiplied with a sequence of 'x' code bits called CHIPS. This 'x' determines the SPREADING FACTOR!!!!The resulting spread data is at a rate of 'x' times R

DespreadingThe spread user data/chip sequence is multiplied with the same 'x' code chips to recover the original data.

Example:

Spreading code 1 = (1, -1)

Data to spread = (1,-1,1,1)

• Data after spreading = (1, -1).(1), (1,-1).(-1), (1,-1).1, (1,-1).1 = (1,-1, -1,1,1-1,1,-1)

• Despreading : Multiply the received signal with same spreading code

• ( 1, -1, -1, 1, 1, -1, 1, -1).(1,-1)– 1. Take first two chips = (1,-1).(1,-1) = 1+1 = 2 = +ve => 1

– 2. Take next two chips = (-1,1).(1,-1) = -1 -1 = -2 = -ve => 0

– 3. Take next two chips = (1,-1).(1,-1) = 1+1 = 2 = +ve => 1

– 4. Take next two chips = 1,-1).(1,-1) = 1+1 = 2 = +ve => 1

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Energy Box

Frequ

ency

Ban

d

Duration (t = 1/Rb)

Po

wer

/Hz

Originating Bit Received Bit

Higher spreading factor Wider frequency band Lower power spectral density

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FrequencyPow

er d

ensi

ty (

Wat

ts/H

z)

Unspread narrowband signal Spread wideband signal

Bandwidth W (3.84 Mchip/sec)

User bit rate R

sec84.3 MchipconstW

R

WdBGp Processing gain:

Spreading & Processing Gain

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Frequency (Hz)

Voice user (R=12,2 kbit/s)

Packet data user (R=384 kbit/s)

Pow

er

den

sity

(W

/Hz)

R

Frequency (Hz)

Gp=W/R=24.98 dB

Pow

er

den

sity

(W

/Hz)

R

Gp=W/R=10 dB

• Spreading sequences have a different length• Processing gain depends on the user data rate

Processing Gain Examples

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Transmission Power

Frequency

5MHz

Power density

Time

High bit rate user

Low bit rate user

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WCDMA Codes

• In WCDMA two separate codes are used in the spreading operation

– Channelisation code

– Scrambling code

• Channelisation code– DL: separates physical channels of different users and common channels,

defines physical channel bit rate

– UL: separates physical channels of one user, defines physical channel bit rate

• Scrambling code– DL: separates cells in same carrier frequency

– UL: separates users

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DL Spreading and Multiplexing in WCDMA

User 3

User 2

User 1

BCCH

Pilot X

CODE 1

X

CODE 2

X

CODE 3

X

CODE 4

X

CODE 5

+

X

SCRAMBLINGCODE

RF

SUM

User 2

User 1

BCCH

Pilot

Radio frame = 15 time slots

Time

User 3

3.84 MHzRF carrier

3.84 MHz bandwidth

CHANNELISATION codes:

P-CPICH

P-CCPCH

DPCH1

DPCH2

DPCH3

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DL & UL Channelisation Codes

• Walsh-Hadamard codes: orthogonal variable spreading factor codes (OVSF codes)

– SF for the DL transmission in FDD mode = {4, 8, 16, 32, 64, 128, 256, 512}

– SF for the UL transmission in FDD mode = {4, 8, 16, 32, 64, 128, 256}

• Good orthogonality properties: cross correlation value for each code pair in the code set equals 0

– In theoretical environment users of one cell do not interfere each other in DL

– In practical multipath environment orthogonality is partly lost Interference between users of same cell

• Orthogonal codes are suited for channel separation, where synchronisation between different channels can be guaranteed

– Downlink channels under one cell

– Uplink channels from a single user

• Orthogonal codes have bad auto correlation properties and thus not suited in an asynchronous environment

– Scrambling code required to separate signals between cells in DL and users in UL

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Channelisation Code Tree

C0(0)=[1]

C2(1)=[1-1]

C2(0)=[11]

C4(0)=[1111]

C4(1)=[11-1-1]

C4(2)=[1-11-1]

C4(3)=[1-1-11]

C8(0)=[11111111]

C8(1)=[1111-1-1-1-1]

C8(2)=[11-1-111-1-1]

C8(3)=[11-1-1-1-111]

C8(0)=[1-11-11-11-1]

C8(5)=[1-11-1-11-11]

C8(6)=[1-1-111-1-11]

C8(7)=[1-1-11-111-1]

C16(0)=[............]C16(1)=[............]

C16(15)=[...........]

C16(14)=[...........]

C16(13=[...........]

C16(12)=[...........]

C16(11)=[...........]

C16(10)=[...........]

C16(9)=[............]

C16(8)=[............]

C16(7)=[............]

C16(6)=[............]

C16(5)=[............]

C16(4)=[............]

C16(3)=[............]

C16(2)=[............]

SF=1

SF=2

SF=4

SF=8

SF=16

SF=256

SF=512

...

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Physical Layer Bit Rates (DL) - HSDPA

• 3GPP Release 5 standards introduced enhanced DL bit rates with High Speed Downlink Packet Access (HSDPA) technology

– Shared high bit rate channel between users – High peak bit rates

– Simultaneous usage of up to 15 DL channelisation codes (In HSDPA SF=16)

– Higher order modulation scheme (16-QAM) Higher bit rate in same band▪ 16-QAM provides 4 bits per symbol 960 kbit/s / code physical channel peak

rate

Coding rateCoding rate

QPSKQPSK

Coding rateCoding rate

1/41/4

2/42/4

3/43/4

5 codes5 codes 10 codes10 codes 15 codes15 codes

600 kbps600 kbps 1.2 Mbps1.2 Mbps 1.8 Mbps1.8 Mbps

1.2 Mbps1.2 Mbps 2.4 Mbps2.4 Mbps 3.6 Mbps3.6 Mbps

1.8 Mbps1.8 Mbps 3.6 Mbps3.6 Mbps 5.4 Mbps5.4 Mbps

16QAM16QAM

2/42/4

3/43/4

4/44/4

2.4 Mbps2.4 Mbps 4.8 Mbps4.8 Mbps 7.2 Mbps7.2 Mbps

3.6 Mbps3.6 Mbps 7.2 Mbps7.2 Mbps 10.7 Mbps10.7 Mbps

4.8 Mbps4.8 Mbps 9.6 Mbps9.6 Mbps 14.4 Mbps14.4 Mbps

HSDPA

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DL & UL Scrambling Codes

DL Scrambling Codes

• Pseudo noise codes used for cell separation– 512 Primary Scrambling Codes

UL Scrambling Codes

• Two different types of UL scrambling codes are generated– Long scrambling codes of length of 38 400 chips = 10 ms radio frame

– Short scrambling codes of length of 256 chips are periodically repeated to get the scrambling code of the frame length

▪ Short codes enable advanced receiver structures in future

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Scrambling Codes & Multipath Propagation

Scrambling code C1

Scrambling code C2

C 1+ 3

C1+2C1+

1

C2

UE has simultaneous connection to two cells (soft handover)

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RAKE Receiver

• Combination or multipath components and in DL also signals from different cells

Del

ay

1Code used

for theconnection

Rx

Output

Finger

t

Cell-1

Cell-1

Cell-1

Cell-2

Rx

Rx

Rx

Finger

Finger

Finger

Del

ay

2

Del

ay

3

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Channelisation and Scrambling Codes

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Concepts of RSCP and Ec/No

Two Important Terms

• RSCP

• Ec/No, Ec/Io

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Scrambling Codes & CPICH

The Common Pilot Channel (CPICH) is broadcast from every cell

It carries no information and can be thought of as a “beacon” constantly transmitting the Scrambling Code of the cellWCDMA cells are identified by their SC.

Its like a BCCH in GSM

It is this “beacon” that is used by the phone for its cell measurements for network acquisition and handover purposes (Ec, Ec/Io).

CPICH

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Total Received Power Io

In a WCDMA network the User Equipment (UE) receives signals from many cells

Io* = The sum total of all of these signals (dBm)

*Note: Sometimes Io is referred to as No, RSSI

Io

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Received Power of CPICH : RSCP

RSCP

Using the properties of SCs the UE is able to extract the respective CPICH levels from the sites received

RSCP = The Received Power of a Particular CPICH (dBm)

Ec = Energy per Chip

RSCP 1 RSCP 2

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CPICH Quality (Ec/Io)

IoRSCP

From the previous two measures we can calculate a signal quality for each CPICH (SC) received

Ec/Io = (Energy per chip / Noise spectral density) = RSCP/RSSI

*Note: Sometimes Ec/Io is referred to as Ec/No

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Relation between Ec/Io and Eb/No

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Handover types

Soft Handover

4

Hard/Inter-Frequency Handover

Inter-System Handover

Node B

Frequencyf1

Frequencyf1

Frequencyf1

Frequencyf2

UMTS GSM900/1800

Frequencyf1

Frequencyf1

RNC RNC

Iur

Iub Iub

Node B

Node B Node B

Node BNode B

Softer Handover

Sector 1f1

Sector 2f1

Sector 3f1

Sector 1

Sector 3

Node B

Node B BTS

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Power control (PC) in WCDMA

• Fast, accurate power control is of utmost importance – particularly in UL;

– UEs transmit continuously on same frequency Always interference between users

– Poor PC leads to increased interference reduced capacity

• Every UE accessing network increases interference– PC target to minimise the interference Minimize transmit power of each

link while still maintaining the link quality (BER)

• Mitigates 'near far effect‘ in UL by providing minimum required power for each connection

• Power control has to be fast enough to follow changes in propagation conditions (fading)

– Step up/down 1500 times/second

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Uplink power control target

Minimise required UL received power minimised UL transmit power and interference

UE1 UE2

min(Prx1)

min(Prx2)

&

About equal when

Rb1 = Rb2

Target:

Ptx1

Ptx1

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Power Control types

• Power control functionality can be divided to three main types

• Open loop power control– Initial power calculation based on DL pilot level/pathloss measurement by UE

• Outer (closed) loop power control– Connection quality measurement (BER, BLER) and comparison to QoS target

– RF quality target (SIR target) setting for fast closed loop PC based on connection quality

• Fast closed loop power control– Radio link RF quality (SIR) measurement and comparison to RF quality target

(SIR target)

– Power control command transmission based on RF quality evaluation

– Change of transmit power according to received power control command

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UL Outer LoopPower Control

Open Loop Power Control (Initial Access)

Closed Loop Power Control

RNC

BS

MS

DL Outer LoopPower Control

Power Control types

BLER target

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Power control in HSPA

• In HSDPA (DL) the transmit power from base station is kept constant and the signal modulation and coding is adapted according to the channel conditions

– 2 ms interval 500 Hz

• In HSUPA (UL)– The power control of HSUPA channels in UL utilises both

▪ Fast closed loop power control

▪ Outer loop power control

– Both work according to similar principles as the R99 power control

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Handover Control (HC)

• HC is responsible for:– Managing the mobility aspects of an RRC connection as UE moves around the

network coverage area

– Maintaining high capacity by ensuring UE is always served by strongest cell

• Soft handover– MS handover between different base stations

• Softer handover– MS handover within one base station but between different sectors

• Hard handover– MS handover between different frequencies or between WCDMA and GSM

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Thank you for your attention