Nokia - Wcdma 4 Beginners Tutorial

1 © NOKIA COMPANY CONFIDENTIAL wcdma_basics.ppt / 27-Sep-2000 / KTuulos

WCDMA for beginners

WCDMA System Training27-Sep-2000


WCDMA history• Evolution towards third generation's global standard since 1985

• In April 1997 ITU asked for proposals for candidate technologies for IMT-2000• 16 results received, e.g. following:

• WCDMA - ETSI• WCDMA - ARIB• UWC-136 - UWCC• cdma2000 - CDG

• 3GPP was formed in December 1998 by five major standardization bodies

• First public network (2002 in Japan) will be 3GPP specific

• UWC-136 and cdma2000 are IMT-2000 but not 3GPP compliant


Evolution to 3G

GSM

2G 2.5G

HSCSD

GPRS

WCDMA (ARIB)

D-AMPS (IS-136) IS-136+

IS-136HS

cdmaOne (IS-95) IS-95B

PDC

WCDMA (ETSI)

UWC-136 (UWCC)

cdma2000 (CDG)

3Gproposals

IMT

-200

0

3G


Multiple access techniques• FDMA=Frequency division multiple access

• each user sends on his own his own frequency band

• TDMA=Time division multiple access• each user sends at his own time slot

• CDMA=Code division multiple access• each user sends a signal, which is multiplied by his own code

• TDMA is used e.g. in GSM• CDMA will be used in so called third generation or UMTS starting from 2002


Concepts• In e.g. GSM system each bit is represented by one pulse• In CDMA it takes several pulses to send one databit• Those pulses are called chips• The number of chips per symbol is called spreading factor• Because the duration of one chip is very short, CDMA system requires more

bandwidth• One the other hand, several users can use the same band simultaneously. Also

neighboring base stations can use the same band.

• Here bits and chips are represented by +1 and -1 (instead of 0 and 1)• In reality signals are not ideal

+1

- 1

- 0.5

0

+ 0.5

= Signal after sampling

= Original signal


Bits, symbols, chips• User bit rate e.g. 64 kbits/s

• Convolution coding (R=1/3) => 192 kbits/s ( = 3 x 64 )• Channel coding => 240 kbits/s

• Symbol = 2 bits (I bit & Q bit) sent simultaneously => Symbol rate120 ksym/s ( = 240 / 2 )

Q

+1

+1

- 1

- 1 I

• Chip rate = 3.84 Mchips per second• Spreading factor = Chip rate / Symbol rate = 32


From chips to slots to frames to channels

• Common Pilot Channel (CPICH),used in downlink

• Fixed spreading factor, SF = 256• 1 symbol represents 2 bits

• Dedicated Physical Data Channel& Dedicated Physical ControlChannel, used in uplink

• Variable spreading factor,SF = 4, 8, 16, 32, 64, 128 or 256

• 1 symbol represents 1 bit

PilotNpilot bits

TPCNTPC bits

DataNdata bits

Slot #0 Slot #1 Slot #i Slot #14

Tslot = 2560 chips, 10*2k bits (k=0..6)

1 radio frame: Tf = 10 ms

DPDCH

DPCCHFBI

NFBI bitsTFCI

NTFCI bits

Pre-defined symbol sequence

Slot #0 Slot #1 Slot #i Slot #14

Tslot = 2560 chips , 20 bits = 10 symbols

1 radio frame: Tf = 10 ms


Data xCode

Data

Code

Code(pseudonoise)

Data

+1

+1

+1

+1

+1

Symbol

-1

-1

-1

-1

-1

ChipChip

Spectrum

Spreading and despreadingSymbol


QPSK modulation

S

Im{S}

Re{S}

cos(ωt)

Complex-valuedchip sequencefrom spreadingoperations

-sin(ωt)

Splitreal &imag.parts

Pulse-shaping

Pulse-shaping


Detecting own signal with correlator (1/2)

Code

+1

+1

+1

-1

-1

-1

Ownsignal

+8

-8

Data afterIntegration

Code

+1

+1

+1

-1

-1

-1

Othersignal

+8


-8


Detecting own signal with correlator (2/2)+1

+1-1

-1

Ownsignal

-2

+2

Code+1

-1

-2

Othersignal

+8

-8


+2

Compositesignal


Processing gain• Defined as

ratedataratechip=pG

• Describes the improvement in SNR from received signal to theoutput of the receiver

• Given in dB• For example if spreading factor is 32 then processing gain is

dB1532log10ksymbols/s120

Mchip/s84.3log10 ≈⋅=

⋅=pG


Different types of codes• There are two different types of codes, scrambling and channelization

code• Codes are combined by multiplication:

+1

-1

Channelizationcode

+1Scramblingcode -1

+1

-1

Combinedcode


Channelization codes• Walsh codes• Used in both uplink and downlink to separate different channels• Orthogonality properties => reduced interference• Limited resource• Repeats itself after each symbol• Different lengths => different spreading factors => different symbol rates• More than one simultaneous code channels between one UE and BS

=multichannel transmission• Don´t have good correlation properties => scrambling code is needed


Scrambling codes• Gold codes

• Uplink:• Each mobile has its own scrambling codes, long and short one• Short code repeats itself after one symbol, long code after one frame• There are 2^24 codes in both groups

• Downlink:• Each base station or sector has its own scrambling code• The code repeats itself after one frame• There are 2^18 codes, but only 24576 (= 3 * 8192) first ones are used

• Scrambling codes have good correlation properties


Use of orthogonal codes

Code

Data aftermultiplication

+1

+1

+1

-1

-1

-1

Ownsignal

+8

-8


OrthogonalCode


+1

+1

+1

-1

-1

-1

Signal

+8


-8


Time shifted signal

Code


+1

+1

+1

-1

-1

-1

Ownsignal

+8

-8


Code


+1

+1

+1

-1

-1

-1

Time-shiftedsignal

+8

-8



Matched Filter

ΣIncomingserial data

Predefined(parallel) data

Register 1

Register 2

When samples ofincoming serialdata are equal tobits of predefineddata, there is amaximum at filteroutput.

Tap 127 Tap 126 Tap 0

Sample127

Sample126

Sample0

To make a successful despreading, codeand data timing must be known. It canbe detected e.g. by a matched filter.

+1

-1


Delay profile (Channel impulse response)

Multipath propagationcauses several peaksin matched filter output


Rake Receiver

Finger 3

Finger 2

Inputsignal

(from RF)

Matchedfilter

Σ I

ΣQ

I

Q

Correlator

Channelestimator

Phaserotator

DelayEqualizer

Codegenerators

Timing (Finger allocation)

Finger 1

I

Q

Combiner


Diversity RX antenna in UE

Fading

Time

Amplitude

= Antenna 2= Antenna 1


Diversity TX antenna in BS

Fading

Time

Amplitude

= Antenna 2= Antenna 1


Problems with many cells


Power control: near-far problem

P1

P2

BS UE2

UE1

• In one cell, all mobiles are transmitting to same frequency.• In practice BS cannot separate different codes if the power

levels of the mobiles are very different• In CDMA systems accurate and wide dynamic range

power control is essential for the network capacity• Mobile starts transmission with open loop power control

where the mobile TX power is inversely proportional to thereceived signal strength

• Later in closed loop power control the BS adjustscontinuously mobile power by sending control bits


Power control: fast responses

BS1

• When UE is transmitting to BS1 with high power, it might interfere users in BS2because cells could be in same frequency.

• UE should be capable to find new cells fast and report changes to network, so it canorder change in power level or handover

BS2


Hard handover• Handover between base stations on different frequencies• Usually neighbouring base stations are on same frequency so there is no need for

frequent hard handovers• Transmission and reception will stop momentarily - just like in GSM

FixedNetwork

Basestationcontrol After hard handover

it operates on frequency 2

First UE operateson frequency 1


Soft handover• During soft handover, the same signal goes through two (or more) base stations or

base station sectors (=softer handover)

FixedNetwork

Both basestations are insame frequencyBase

stationcontrol


About channels• There are physical channels and transport channels, just like in GSM

• Physical channels are used to carry transport channels

• Transport channels carry data for higher layers

• Different channels for uplink and downlink

• Some channels are common for many users, some are dedicated for only one


Transport channelsDedicated transport channels

• DCH - Dedicated Channel• DL / UL

DL = DownlinkUL = Uplink

Common transport channels

• BCH - Broadcast Channel (DL)

• FACH - Forward Access Channel (DL)

• PCH - Paging Channel (DL)

• RACH - Random Access Channel (UL)

• CPCH - Common Packet Channel(UL)

• DSCH - Downlink Shared Channel(DL)


Downlink physical channels (1/2)

• DPCH - Downlink Dedicated PhysicalChannel

• Carries DCH

• CPICH - Common Pilot Channel• Carries pre-defined symbol

sequence• There are primary and secondary

channels for different purposes• Used as channel estimation

reference for all other channels

• P-CCPCH - Primary Common ControlPhysical Channel

• Carries BCH

• S-CCPCH - Secondary CommonControl Physical Channel

• Carries FACH and PCH

• PDSCH - Physical Downlink SharedChannel

• Carries Downlink Shared Channel

Channels, which carry data for higher layers:


Downlink physical channels (2/2)Channels for L1 internal use:

• AICH - Acquisition Indicator Channel• Carries Acquisition Indicator as

response to PRACH or PCPCH

• CSICH - CPCH Status IndicatorChannel

• Carries CPCH status information

• PICH - Page Indicator Channel• Carries Paging Indicator which

indicates if PCH for certain paginggroup is available

• CPICH - Common Pilot Channel• Carries pre-defined symbol

sequence• There are primary and secondary

channels for different purposes• Used as channel estimation

reference for all other channels

• SCH - Synchronization Channel• Used for cell search


Uplink physical channels• DPDCH - Dedicated Physical Data channel

• Carries DCH

• DPCCH - Dedicated Physical Control Channel• Carries control information generated in L1

• PRACH - Physical Random Access Channel• Carries RACH

• PCPCH - Physical Common Packet Channel• Carries CPCH


Channel mapping, overviewTransport Channels

DCH

RACH

CPCH

BCH

FACH

PCH

DSCH

Physical Channels

Dedicated Physical Data Channel (DPDCH)

Dedicated Physical Control Channel (DPCCH)

Physical Random Access Channel (PRACH)

Physical Common Packet Channel (PCPCH)

Common Pilot Channel (CPICH)

Primary Common Control Physical Channel (P-CCPCH)

Secondary Common Control Physical Channel (S-CCPCH)

Synchronisation Channel (SCH)

Physical Downlink Shared Channel (PDSCH)

Acquisition Indication Channel (AICH)

Page Indication Channel (PICH)

CPCH Status Indicator Channel (CSICH)


Capasity analysis• The restricting factor is the level of noise and interference caused by e.g. other users

=> impossible to tell exactly how many users there can be simultaneously• Soft blocking• Higher bitrate (smaller spreading factor) requires more transmitting power which

means more interference to others• Same frequency can be used in neighbour cells which increases capacity• Voice activity detection => less transmission => less interference


Benefits of WCDMA• Increased capasity due to new spectrum• Increasing quality• More tolerant to noise• More flexible bit rates• Imperceptile soft handovers• Easier frequency planning

• Increased spectrum efficiency ?


Drawbacks of WCDMA• High chip rate• Uplink power control• Needs more MIPSes in DSP than GSM• Power consumption


Printed literature• Wideband CDMA for Third Generation Mobile Communications

Tero Ojanperä & Ramjee Prasad, 1998

• WCDMA for UMTSWiley, this is quite new book (04/2000)

• ?? Third book


3GPP specifications• http://cst.ntc.nokia.com/ESSV/stand/DataBase/default.htm

• 4 main development groups (Techical Specification Groups):• TSG-SA: Services and System Aspects• TSG-RAN: Radio Access Network• TSG-CN: Core Network• TSG-T: Terminals

• Starting points:• 21.101: "3rd Generation Mobile System Release 1999 Specifications"• 25.201: "Physical layer - general description"• 25.401: "UTRAN Overall Description"

Nokia - Wcdma 4 Beginners Tutorial

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