Lecture intro to_wcdma

Lecture 3Introduction to WCDMA

Outline

• What is spread spectrum.

• Spreading.

• Correlation and RAKE receiver.

• Uplink and Downlink Diversity.

• WCDMA Power control.– Closed loop.

– Open loop.

• WCDMA handovers.– Soft handover.

– Softer handover.

Properties of the Spread Spectrum

• Transmission bandwidth is much larger than information bandwidth.

• Bandwidth does not depend on the informational signal.

• Processing gain = Transmitted bandwidth/ Information bandwidth.

• Classification:

– Direct sequence: Data is scrambled by user specific pseudo noise code at thetransmitter side.

– Frequency Hopping: The signal is spread by changing the frequency over thetransmitted time of the signal:

• Fast frequency hopping.

• Slow frequency hopping.

– Time Hopping: The data is divided into frames, that itself are divided into timeintervals. The data is burst is hopped over the frames by utilising code sequences.

Background of SS

• First publications late 40s.– Patent proposal in 1941.

• 1949 C. Shannon and R. Pierce develop basic ideas of CDMA.

• First applications 50s.– Military with very low C/I, Anti-jam.

• RAKE receiver patent 1956.

• Cellular applications proposed late 70s.

• Investigations for cellular use 80s.

• IS-95 standard 1993.– Commercial introduction in 1995.

• 1997/1998 3G technology choice in ETSI/ARIBA/TTA … .

TDMA based system

• Frequency reuse >1.

• Frequency divided by time slots.

f1

f3

f1

f1

f2

f3

f1

f2

f3

f1

f3

f2

f2 f2

f1

f3

f2

f1

f2

f1

t

200 kHz

WCDMA based system

• All users share the same frequencytime domain.

• Users separated by the codes.

• Codes are orthogonal:

• FDD frequency division duplex.– Uplink, downlink in separate

frequency bands

• TDD time division duplex.– Uplink, downlink in the same

frequency band and separated intime.

f1

f1

f1

f1

f1

f1

f1

f1

f1

f1

f1

f1

f1 f1

f1

f1

f1

f1

f1f1

t5000 kHz( )1 2( ) 0

b

a

c t c t dt =∫

Processing Gain and Spreading• A narrowband signal is spread to a

wideband signal.

• Information rate at the input of theencoder is

• Available bandwidth is

• In order to utilize the entire availablebandwidth the phase of the modulatoris shifted pseudo randomly, according

R

W

Unspread narrowband signal

Spread wideband signal

Pow

er d

ensi

ty W

/Hz

Frequency

to the pattern from the PN generator at

a rate

• Chip is the rectangular pulse which occupiesthe whole bandwidth

• The duration of is called chip interval

• High bit rate means less processing gain andhigher transmit power or smaller coverage.

bitssR

W Hz

timessW

1cT

W=

cT

PNgenerator

Mod-2adder

modulator

PNgenerator

Mod-2adder

modulator

PNgenerator

Localoscillator

Adder

I

Q

Symbol

Chip

Despreading

Spreading

Data

SpreadingCode

Data x Code

Data

SpreadingCode

Despreading

Detection own signal

Own Data

Own SpreadingCode

Own Data x Code

Data aftermultiplication

SpreadingCode

Data afterintegration

+4

-4

+1

-1

+1

-1

+1

+1

-1

-1

Despreading

Detection other signal

Other Data

Other SpreadingCode

Other Data x Code


Own SpreadingCode


+4

-4

+1

-1

+1

-1

+1

+1

-1

-1

Codes (1)• Requirements for the spreading codes:

– Good auto-correlation properties. For separating different paths.

– Good cross-correlation properties. For separating different channels.

Channelisation codes used for channel separation from the same source.• Same codes from all the cells.

• Short codes: used for channel separation in Uplink and Downlink.– Othogonality property, reduce interference.

– Different spreading factors, different symbol rates.

– Limited resource, must be managed.

– Do not have good correlation properties, need for additional long code.

Scrambling codes.• Long Codes:

– Good correlation properties.

– Uplink: different users.

– Downlink: different BS.

Long and Short Codes

Short Code

Long Code

Combined Code+1

+1-1

-1

+1

-1

The tree of orthogonal codes

• Orthogonal short codes will only beuseful if channel can be synchronisedin the symbol level.

– Mainly used in DL.

• Orthogonal Variable Spreading Factortechnique.

• Orthogonality preserved across thedifferent symbol rates.

• Codes must be allocated in RNC.

• Code tree may become fragmentedcode reshuffling may be needed.

• Provision of multiple code treeswithin one sector by concatenationwith multiple sector specific longcodes.

C11=(1)

C21=(1,1)

C22=(1,-1)

C41=(1,1,1,1)

C43=(1,-1,1,-1)

C44=(1,-1,-1,1)

C42=(1,1,-1,-1)

SF1 SF2 SF3

Generation of a scrambling codes

• Spreading code is output of the binary shift register generator.

• Pseudo random codes are used: cyclic.

• Maximal length codes m-sequences: sequences that have maximal possiblesequence given the length of the shift registers.

• UL long scrambling code: complex scrambling codes, sum of two m-sequences (Gold sequence) generators:

– X25+X3+1.

– X25+X3+X2+X+1.

• UL short scrambling codes.– Used to supporting Multiuser detection.

– Sequence length around 255 chip.

• DL scrambling sequences:– Constructed by combining two real sequences with generator polynomials:

– 1+X7+X18

– 1+X5+X7+ X10+X18

clong,1,n

clong,2,n

MSB LSB

Configuration of uplink longscrambling sequence generator

Direct sequence (DS) Spread Spectrum

user n information signal.

user n spreading code.

user n transmit signal.

noise.

user n receive signal.

user n correlation signal.

user n output information signal.

symbol duration.

( )nx t( )nc t( )ns t

( )nr t

( )n t

( )nnz tˆ( )x t

( ) ( ) ( )nn n n

T

z t y t c u t du= +∫T

PSKmodulator

radiochannel

PSKdemodulator

decisioncircuit

correlatorX +( )nx t

( )nc t

( )ns t

( )n t

( )nr t ( )nnz t ˆ( )x t( )ny t

With ideal spreading codes andcorrect timing the cross correlationbetween different users is zero

Channel Repeating

• A multipath channel:

• Received signal is convolution of the received signal and the channel.

• Multipath will destroy the codes orthogonality:

0 2 4 6 8 10-1

0

1

amp

litud

e

chips

first tap

0 2 4 6 8 10-1

0

1

amp

litud

e

chips

second tap

0 2 4 6 8 10-1

0

1

amp

litud

echips

third tap

0 2 4 6 8 10-1

0

1

amp

litud

e

chips

received signal

( )1

2

0

( , ) k

Mj t

k kk

h t h e πνλ δ λ τ−

=

= −∑

– The codes are orthogonal if they aresynchronised, start at the samemoment

– If the codes are not synchronised thecross correlation is not zero.

– In multipath channel signalcomponents arrive at different timemoments.

– If the receiver is syncronised to atap. The integration covers part ofthe previous symbol and nextsymbol from an another tap.

( )1

2

0

( ) k

Lj t

n k n kl

y t h e s tπν τ−

=

= −∑

Maximal ratio “RAKE”combining of symbols

• Channel can rotate the signal to any phase and to any amplitude.

• QPSK symbols carry information in phase.

• Energy splitted to many finger -> combining.

• Maximal ratio combining corrects channel phase rotation and weightscomponents with channel amplitude estimate.

• Same method used also for antennae combining (BTS, MS), and softerhandover (BTS), and soft/softer handover (MS)

Transmittedsymbol

Receivedsymbol

Combinedsymbol

Modifiedwith channel

estiamate

Finger #1

Finger #3

Finger #2

RAKE diversity receiver

CorrelatorPhaserotator

DelayEqualizer

ChannelEstimator

I

Q

Input signal(from RF)

Timing (Finger Allocation)

CodeGenerators

Matchedfilter

Combiner

Finger #1Finger #2

Finger #3

Despreading

Detection own signal

Own Data

Own SpreadingCode

Own Data x CodePath 1


path 1

SpreadingCode


Path 1

+4

-4

+1

-1

+1

-1

+1

+1

-1

-1

Own Data x CodePath 2

+1

-1


path 2

-4


path 2-1

+1

+4

Ortogonality in multipath channel

Correlation in the receiver

-8 -6 -4 -2 0 2 4 6 8-0.4

-0.2

0

0.2

0.4

0.6

0.8

1correlation of the signal

Correlation functions are not deltafunctions. Correlation functions ofneighbouring paths could overlap.

If the channel taps are near enough thecorrelation functions overlap andcreate interference.

-2.5 -2 -1.5 -1 -0.5 0 0.5 1 1.5 2 2.50

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

0.9

1correlation of the received signal

0 1.5 2.5

Matched filter

• RAKE receiver needs data timing.

• When samples of incoming serial data bits are equal to bits of predefined data,there is a maximum at filter output.

tap 127 tap 126

tap 1. . .tap 126tap 127

tap 0. . .

XX

X

�

Predefined Parallel data

IncomingSerial data

Register 1

Register 2

Delay profile estimation

• Sum of the signals from different paths.

• Multipath propagation causes several peaks in matched filter output.

• Allocate RAKE fingers to these peaks.

• Later: track and monitor the peaks.

( ) ( ),lj

k l k k lL

P e m s t n tτ− Θ − +∑( )( ) ( )

( )( )( ) ( )

,

1

0

,

, , 1 ,

, ,0 ,

( )

( )

n l

l

l

n l

u

jk l k k k l k

T

k TL j

k l k k k l k

u

k

T

P e m s t u s t dt

z t

P e m s t u s t dt

n t s t dt

−

− Θ−

− Θ

−

=

+ −

+

∫∑

∫

∫

Signal in the channel Signal after correlation in receiver

The correlation generates multipath interference from other paths.

Performance of a DS-CDMA receiverSignal in the channel in a channel with multiple users:

Signal sample at the receiver:

( ) ( ),n l n nN L

P m s t n t+∑∑

( ) ( )( ) ( ) ( )( )

( )( ) ( ) ( )( )

( ) ( )

, 0

, ,

1 ,

, 0

, ,

1 ,

, , 1 , , ,0 ,

, , 1 , , ,0 ,

( )

( )

n l

k l k l

n l

n l

n l n l

n l

u Tj j

k l k k k l k k l k k k l kL T u

u Tj j

n l n n n l k n l n n n l kN L T un k

k

T

z t P e m s t u s t dt P e m s t u s t dt

P e m s t u s t dt P e m s t u s t dt

n t s t dt

−

−

− Θ − Θ−

− Θ − Θ−

≠

= − + −

+ − + −

+

∑ ∫ ∫

∑ ∑ ∫ ∫

∫is the received power of the signal for user

n.

is the transmitted symbol to user n.

is the spreading code of user n.

is the random noise after the carrierdemodulator.

Delay of the user n path compared to theuser k path.

,n lP

nm

( )ns t

( )n t

• The first term on the right side representsthe desired signal sample of the kth user.

• The second term represents the multipleaccess interference (MAI) and can bemodelled as Gaussian.

• The third term represents the randomnoise.

• Index k is used to select the parts fromthe equation with the user signal.

,n lu

• Receiver performance in a Gaussian channel is fully characterised by the firstand second moment of the received signal:

Example.• Assume:

– Single symbol transmission with single symbol transmission.

– Only one multipath component for each user ( L=1 ) and a real channel.

– Single cell network.

• The received signal can be simplified.

• Variance of the interference is:

Performance of a DS-CDMA receiver (2)

2be

EP Q

σ

=

( )

( )

{ } ( ) ( )

2

2,

0 0

2

0 0 0

( )

( )

MAI n i n k iNn k

N N

i j i j ik i jk ji ji k j k

N N N

i j i j ik i jk j i ik ii j ii k j k i k

E P m R u

E PP m m R u R u

PP E m m R u R u PR u

σ≠

= =≠ ≠

= = =≠ ≠ ≠

= =

= =

∑

∑∑

∑∑ ∑

{ }

20

0 0

0

20 0

( ) ( )

( ) ( ) ( ) ( )

( ) ( ) ( )2

( ) (0)2 2

n k

T

k k

T T

k k

T T

k kk

T

E n t s t

E n t n u s t s u dtdu

Nt u s t s u dtdu

N Ns t dt R

σ

δ

=

=

= −

= =

∫

∫ ∫

∫ ∫

∫

By using definition of the autocorrelation:

Performance of a DS-CDMA receiver (3)

( ) (0) ( ) ( )k k kk n n nkNn k

z t P m R P m R u n T

≠

= + +∑is the code autocorrelation function of user k.

is the codes crosscorrelation function between spreading codes of user n anduser k.

is the cross correlation function between the random noise and the spreadingcode of user k.

(0)kkR

( )nkR u

( )n T

The performance of the receiver is expressed in terms of the Q function:

0

2

22

(0)

( ) (0)k kk

be Nn nk n kk

Nn k

P REP Q Q

I P R u Rη≠

= = + + ∑

In the asynchronous case when the delay u is uniformly distributed over thesymbol interval, the expected value of the correlation function ratio is about:

2

2

( ) 1

(0) 3nk n

kk c

R uE

R G

≈

where c

cs

R chip rateG N processing gain

R symbol rate= = = =

Performance of a DS-CDMA receiver (4)The average bit error probability can be calculated as a function of number of users:

Assume:00

2

222

( )(0)

3(0)

k k

NsNnk nn sn kk

NN kkn kn k

P PERR uI P RP RWR

η

≠≠

= ≈+ ++ ∑∑

k nP P=

If the target SIR ratio given we can estimate the average capacity in the cell.

Assumptions made:

• Powers have the same level:– Near far effect.

– power control suitable for uplink.

• No intracell interference:– can be considered by the intracell interference factor.

– Other cells change the transmission power in the same way than the users cell.

• Orthogonality:– In downlink all the codes from one BS synchronous - codes orthogonal - no

interference.

– Multipath channel ruins orthogonality.

– Can be considered in downlink as orthogonality factor.

CDMA capacity an another approach

• Same assumptions as before. We attempt directly evaluate the equation

0

I Total interferenceI the noise density in demodulator =

W entire spread bandwidth= =

The total interference power is: where N is number of users.

Total number of users in the system is:

Compared to analyse in previous slides we assume here that Coding Gain (G) is equalto . Before we assumed it to be . In practice both of these values are onlyassumptions and the real coding gain depends on the particular codes and multipathdelays in the system.

=nb

n

P received signal powerE received energy per bit

R data rate= =

E

I η+

( )1 nI N P= −

0

1n b

I W RN

P E I− = =

n

WR

3n

WR

Capacity in multicell environmentProblems:

• We assume that all the powers are the same (suitable only for uplink).

• No other cell interference:Other cell interference can be considered by the interference factor f. Assume that

other cells generate that is added to the own cell interference. Thus capacity in thewhole system is reduced.

The new capacity is:

• Codes that are synchronised are orthogonal:– In downlink all the signals are emitted from the same source and propagate along

the same path. The spreading codes that are synchronised are orthogonal.

– Can be considered by the orthogonality factor . That is a term that describes howmuch the interference is reduced due to the codes orthogonality.

1 1interference from other cell

finterference from own cell

+ = +

0

1 11

1 1n b

I W RN

P f E I f− = =

+ +

α

(1 )k

nNn k

PW WSIR CIR

R R Pα η≠

= =− +∑

Simple equation describing quality ofCDMA system

where

signal power for user k in cell I

noise power

0

0,0

11

,0 ,1 1 1

jKK N

k k jk j k

PCIR

P P η−−

= = =

=+ +∑ ∑∑ ,k iP

η

0 ,0

0

1,0

0

,00

1

1,0 ,0 ,1 1

1

0,0 ,0 ,1 1

1

0 ,0 1,0 ,1 1

... 0

... 0

... 0

j

j

jK

KNP

K k jCIRj k

KNP

K k jCIRj k

KNP

k jCIRj k

P P P

P P P

P P P

η

η

η

−

= =

−

= =

−

= =

− + + + + + =

− + + + + =

+ + − + + =

∑ ∑

∑ ∑

∑ ∑

�

Equations for all users

Near far effect

Uplink: Because of different attenuation signals to/from users nearer to BS arestronger than signals to/from further located users.

Radio tower

Received pow

er at BS

MS1

MS2

MS3

Without Power Control

Received pow

er at BS

MS1

MS2

MS3

With Power Control

Downlink: Beacause of the nature of attenuation at the cell border the usersexperience higher interference that near to the BS. They have high level ofinterfering signals from own BS and from other BS.

• UplinkPower control in uplink must make signal powers from different users nearly

equal in order to maximise the total capacity in the cell.

• DownlinkIn downlink the power control must keep the signal at minimal required level in

order to decrease the interference to users in other cells.

• Removes near far effect.

• Mitigates fading.

• Compensates changes in propagation conditions.

• In the system level– decrease interference from other users

– increase capacity of the system

Amplitude

Time

Purpose of Power Control in WCDMA

Power Control types in WCDMA

• Open Loop power control: for initial power setting of MS

Across the air interface

• Fast closed loop power control:– Mitigates fast fading rate 1.5 kbps.

– On UL and DL.

– Uses a fixed quality target set in MS/BS.

• Outer loop power control:– Compensates changes in environment.

– Adjust the SIR target to achieve the required FER/BER/BLER.

– Depends on: MS speed available, multipath diversity.

– In the soft handover comes after frame selection.

Modulation

DemodulationChanneldecoder

ChannelEncoder

SourceEncoder

SourceDecoder

Channel

Multiple access interference

Noise

Transmitter

Receiver

Powercontrol

C/I target

FER BER C/I

Open Loop PC

What is initial transmission power?

The BS transmits in BCCH • power of the PRACH.• power step P.

- MS sets the initial transmission power Ptr in RACH/CPCH and waits for ack.

- if no ack during TCPCH

Ptr(i+1) = Ptr(i) + P

BCCH RACH/CPCH

Closed Loop Fast PC

• Uses channel in other direction for transmitting the order for power change.

• Applied only to dedicated channels.

• Makes Eb/No requirements lower.

• Introduces peaks into the transmit power.

• PC speed 0.666 ms, compensates the fading for slow and medium speed.

• PC step– uplink 1, 2, 3 dB

– downlink 0.5, 1 dB

• Control range– uplink 80 dB

– downlink 30 dB

coder decodermodulator demodulatorchannel

decoder coderchannel modulatordemodulator

Fast PC

Uplink:

Behaviour precisely standardised.• Equalizes received powers at BS

• BS measures the received CIRand compares to the target CIRvalue

• BS transmits the TPC commandin downlink and orders the MS toincrease/decrease thetransmission power

• MS change the transmitted poweraccordingly to the TPC command

Downlink:

Precise algorithm not standardised• MS estimates the received SIR and

compares it with required SIR target

• MS transmits the TPC command in firstavailable TPC field

• In soft handover (diversity transmission)– two downlink PC modes

• MS sends unique PC command in eachslot

• MS repeats the same PC command over3 slots

• Changes of power are multiplies of theminimum step size

– it is mandatory for BS to support 0.5 and1 dB step size

Outer loop PC• Used for long term quality control

• Controls of the channel by setting target Eb/No

– the quality requirement is given as long term average of FER/BER

• Uplink– SRNC sets the target value

– Control is located in the Node B for FDD

– for TDD function is performed by UTRAN but target quality value is sent to theMS

• Downlink– located in UE, the initial control parameters are set by UTRAN

– receives inputs of the quality estimates of the transport channel

FER and FEP are calculated upon number of framesExample function for outer loop PC

EbNoTarget(t+1) = EbNoTarget(t) + [dB]where

=fs-Fsf is 1 if frame has error accordingly CRC and 0 if notF is the wanted FERs is the step size.

Effectiveness of PC (1)

• The figure of fading from the file.

• Uplink:– In uplink an effective power control follows fading as good as possible.

– In the “own” BS received powers are equal. In other BS high variations.

• Downlink:– The power control attempts to estimate the overall interference level in the

cell (system).

– The PC attempts to provide good CIR to the as many users as possible.

WCDMA handover types.• Intra-system handovers:

– Intra-frequency handovers.• MS handover within one cell between different sectors: softer

• MS handover between different BS:– Soft.

– Hard.

– Inter-frequency handovers.• Hard

• Inter-system handovers:– Handover between WCDMA <--> GSM900/1800: Hard

– Handower between WCDMA/FDD <--> TDD: Hard

WCDMA handovers

• Avoidance of near far situation for circuit switched connections– for high mobility users shadow fading + (slow) hard handovers would create near

far situations.

• Soft/Softer handovers will improve cell capacity (around 40-60 %)

• Soft/Softer provide macrodiversity gain: compared the hard handover largercell range.

– Gain against shadow fading ( 1 -3 dB).

– Gains against fast fading, typically 0.5 - 2 dB assumed.

• Soft/Softer essential interference mitigating tool.

Softer handover

• MS in overlapping cell coverage area oftwo adjacent sectors of a BS.

• Communication between MS and BS isvia two air interface channels (one foreach separate sector).

• Different sectors have differentscrambling codes.

• UL: MS tunes the RAKE fingers todifferent sectors and combines theoutputs.

• DL: BS receives signals with differentantennas and decodes and combinesthem.

Radio tower

Soft handover• User has at the same time connection to more than one BS.

• Except PC bits exactly the same information is sent via air interface.

• Soft handover probability 20-40 %.

• UL/DL processing different.– MS: At Rake Maximal Ratio Combining of signals from different BS.

– BS: Frame selection. Extra transmission across Iub.

Radio tower

Radio tower

RNC

CNFrame reliability

info

Frame reliability info

Handover impact to capacityAttenuation in the channels

P[d b ]

H andoff w indow

P[d b ]

increase o f tran sm itted pow er

P[d b ]

C e ll s ite 2C e ll s ite 1C e ll

b ou nd a ry

Transmitted power in UL

Transmitted power in DL

Handover procedure• Strength of the A becomes equal to

defined lower threshold. Theneighbouring signal has adequatestrength. B is added to active set.

• Quality of signal B starts to becomebetter than signal A. The RNC keepsthat point as starting point forhandover margin calculation.

• The strength of signal B becomesequal or better than the defined lowerthreshold. Thus its strength isadequate to satisfy the required QoSof the connection. The strength of thesummed signal exceeds thepredefined upper threshold, causingadditional interference to the system.As a result, RNC deletes signal Afrom the Active Set.

Time

Sig

nal S

tren

gth

Summed Signal

Handover margin

Signal A Signal B

Radio tower Radio tower

Cell A Cell B

Parameter in the handover algorithm

• Upper threshold: the level at which the signal strength of the connection is atthe maximum acceptable level in respect with the requested QoS.

• Lower threshold: is the level at which the signal strength of the connection isat the minimum acceptable level to satisfied the required QoS. Thus the signalstrength of the connection should not fall below it.

• Handover margin: is a predefined parameter, which is set at the point wherethe signal strength of the neighbouring cell (B) has started to exceed the signalstrength of current cell (A) by a certain amount and/or for a certain time.

• Active Set: is a set of signal branches (Cells) through which the MS hassimultaneously connection to the UTRAN.

• Candidate Set: is a list of cells that are not presently used in the soft handoverconnection, but whose pilot E/I are strong enough to be added to the active set.

– Candidate set is not used in WCDMA handover algorithm.

• Neighbour Set: The neighbour set or monitored set is the list of cells that themobile station continuously measures, but whose pilot E/I are not stron enoughto be added to the active set.

Lecture intro to_wcdma

Engineering

Transcript of Lecture intro to_wcdma