Radio Transmission Aspects of UMTSRadio Transmission Aspects of UMTS

TELECOM ITALIA LABTELECOM ITALIA LAB

Sergio BARBERISSergio BARBERIS

sergio.barberis@tilab.comTel +39 011 228 7309

Wireless Techniques and Methodologies

Radio transmission Aspects of UMTSRadio transmission Aspects of UMTSOUTLINEOUTLINE

• Principles of spread spectrum communications (DS-CDMA,hybrid TD-CDMA)

• UTRA physical layer specification• System aspects (power control, soft handover, capacity)

Principles of spread spectrumPrinciples of spread spectrumcommunicationscommunications

ACCESS TECHNIQUES FORACCESS TECHNIQUES FORMOBILE COMMUNICATIONSMOBILE COMMUNICATIONS

P - PowerT - TimeF - Frequency

P

T

P

T

F

P

T

F

FDMA (TACS)

TDMA (GSM, DECT)

CDMA (UMTS)

F

Spread spectrum systemsSpread spectrum systems

• Transmission systems where the bandwidth of thetransmitted signal >> bandwidth of the information signal

• The bandwidth spreading is performed exploiting theproperties of PN sequences (codes) that must be knownat the receiving side

• Transmission systems where the bandwidth of thetransmitted signal >> bandwidth of the information signal

• The bandwidth spreading is performed exploiting theproperties of PN sequences (codes) that must be knownat the receiving side

AWGN bandlimited channel

C=B log2 (1 + S/N) [C]=bit/s [B]=Hz S/N=signal to noiseratio

How to obtain adesired bit rate R

Narrowband B and high S/N

Wideband B and low S/N

General characteristics of spreadGeneral characteristics of spreadspectrum systemsspectrum systems

• Robustness against jammers and fading

• Low interception probability

• Multiple access capability

DSDS--CDMACDMA

• Users sharing the same band are transmittedsimultaneously on the same carrier

• Users are distinguished each other by means of a“code” and, the mutual interference is reduced duringthe “decoding” process

• In the DS-CDMA technique, the spreading is obtainedmultiplying the user signal by the signal associatedto a “code” (a PN sequence or an orthogonalsequence)

DSDS--CDMACDMA

b(t): information signal (Rate Rb)c(t): PN sequence (Rate Rc)Rc>>Rb

The bandwidth spreading is obtained multiplying theinformation signal by the PN sequence

BPSKMOD.

BPSKDEMOD.

b(t)

c(t)

x(t)

j(t)

z(t)

c(t)~~

fo

DSDS--CDMA (II)CDMA (II)

c(t)1

-1

t

b(t)1 t

-1

c(t)b(t)1

-1

t

Gc(f)

f1/Tc

f1/Tb

Gb(f)

Gc*Gb

f1/Tc

Spectrum of the productsignal

DS-CDMA (III)

The spread signal b(t)c(t) is then modulated,transmitted, interfered by a narrowband signal j(t) anddemodulated at the receiveng end obtaining:

The information signal b(t) is recovered multiplyingz(t) by c(t):

z(t)= b(t)c(t) + j(t)~

Spread interferer

z(t)c(t)= b(t)c2(t) + j(t)c(t) = b(t) + j(t)c(t)~ ~

DSDS--CDMA (IV)CDMA (IV)

Spectrum of z(t)c(t)

f

f

Gz(f)

f

Spread interferer

Information signal

Information signal

jammer

After the low pass filtering, wedetect only a fraction of theoriginal interfering power(reduction factor=Rc/Rb)

Coding and bandwidth spreadingCoding and bandwidth spreading

• The bandwidth spreading can also be obtained bymeans of repetition codes or error correcting codes.

• Terminology:– Processing gain: ratio between chip rate and user net

bit rate– Spreading factor: number of chips representing a

user coded bit

CDMA systemCDMA system

DATA

0 f0 0

BACKGROUNDNOISE

f0

EXTERNALINTERFERENCE

f0

OTHER CELLINTERFERENCE

f0

OTHER USERINTERFERENCE

f0

ENCODING &INTERLEAVINGDATA

CARRIER

PN SOURCE

CARRIER

DIGITALFILTER

PN SOURCE

CORRELATOR

DEINTERLEAVING& DECODING DATA

WIDEBANDSPECTRUM

f0

• Short codes: they allow a better interference controlbut, a code management could be necessary

• Long codes: no code management is required but,the mutual interference cannot easily be controlled.

“Long codes” e “short codes”“Long codes” e “short codes”

PseudoPseudo--Noise sequencesNoise sequences

• They are obtained by means of linear shift registers (withfeedback defined by a characteristic polynomial)

• A N-cell LSR can provide a PN sequence with period 2N-1• PN sequences are suitable for multiple access because their

autocorrelation function decrease sharply after time shifts of fewchips (i.e., equal to zero everywhere but the origin)

out

0 0 1

WALSH CODES (I)WALSH CODES (I)

It is a set of orthogonal codes generated by the rowsof a Hadamard matrix

The Hadamard matrix of order two is defined as:

1 11 -1

H2 =

-1

1

1

Tc

Tc

t

t

Walsh functions

WALSH CODES (II)WALSH CODES (II)

The Hadamard matrix of order 2N (and then theWalsh codes corresponding to the matrix rows)is defined as

HN

HN -HNH2N = HN

Walsh functions are perfectly orthogonal (i.e., nomutual interference); actually, after transmission overa multipath channel, orthogonality is lost

Walsh codes:Walsh codes: exampexamp. of. of channelizchanneliz. (I). (I)

w1 = [-1, +1, -1, +1]w2 = [-1, -1, +1, +1]w3 = [-1, +1, +1, -1]

We assume for exampleRc = 4Rb i.e., SF = 4

Data stream to be transmitted

d1 = [1, -1, 1]d2 = [1, 1, -1]d3 = [-1, 1, 1]

Walsh codes:Walsh codes: exampexamp. of. of channelizchanneliz. (II). (II)

-1-13-1-13-1-13-1-1-1r(t) =d1w1+d2w2+d3w3

1-1-1

-1-11

111

-11

-1

-11

-1

111

-1-11

1-1-1

111

-11

-1

1-1-1

-1-11

d1w1d2w2d3w3

11

-1

-111

1-11

-1-1-1

11

-1

-111

1-11

-1-1-1

11

-1

-111

1-11

-1-1-1

w1w2w3

1-11

-111

11

-1

d1d2d3

Tc Tb

Walsh codes:Walsh codes: exampexamp. of. of channelizchanneliz. (III). (III)

To recover the information d1 multiply the compositesignal r(t) by the code w1 and then we sum up (integrate)over the bit time

4-44D1=ΣΣΣΣTb

-1131-1-3-1131-11r(t)w1

1-11-11-11-11-11-1w1 ∑Tb

D1>0 ⇒⇒⇒⇒ transmitted d1= 1D1<0 ⇒⇒⇒⇒ transmitted d1=-1

⇒⇒⇒⇒ d1=[1, -1, 1]

TimeTime--variantvariant multipathmultipath channel:channel:example of impulse responseexample of impulse response

t=t0

t=t0+a

t=t0+b

t=t1 t=t1+τ11 t=t1+τ12

t=t2 t=t2+τ21

t=t3+τ31t=t3 t=t3+τ32

Transmitted signal Received signal

t t

ττττn

xττττ1

f

t

f

t

+

αααα1ejφφφφ1

ααααnejφφφφn

x

PropagatPropagat. impairments due to. impairments due to multipathmultipath

Receiver: Rake receiverReceiver: Rake receiver

X

ττττi

αααα ϕϕϕϕi

je i

X

ττττn

αααα ϕϕϕϕn

je n

ΣΣΣΣ

Propagation channel

Spreadingsequence

ΣΣΣΣ

despreading

Channelestimat.

despreading

Channelestimat.

receiver

ττττi

ττττn

Phaserecovery

Phaserecovery

TDTD--CDMA Access TechniqueCDMA Access Technique

TDD (TDTDD (TD--CDMA) techniqueCDMA) technique

Frequen

cy

One Time Slot

3.84 Mch

ip/s

1 2 3 . . . 14 15

Codes

Energy

Time

frame with15 time slots

WB-TDMA/CDMA

TDTD--CDMA spreading codesCDMA spreading codes

• Within each 0.666 ms time slot, more channels can beallocated and separated each other by means ofspreading codes

• The codes can be allocated to different users or to asame user, according to the needs.

• The number of codes in a time slot is not fixed butdepends on the rate and spreading factor of eachphysical channel.

• SFMAX = 16• After spreading, data are scrambled with a cell

specific scrambling sequence

Resource allocationResource allocation

• In the TD-CDMA component a physical channel isidentified by a combination of carrier, time slot and code.

• Resources are allocated to cells by means of slow DCA:– slot clustering– each slot can be used in both uplink and downlink transmission,

according to the needs

• resources are allocated to bearers by means of fast DCA:– high bit rate services can be provided allocating to a same user several

codes in a same or different time slots.

Joint detectionJoint detection

• It reduces (ideally cancels completely) the mutualinterference among signals

• The receiver exploits the knowledge of all the spreadingsequences used by the other users on the same slot/carrierand perform the simultaneous demodulation of all signals.

• The output is a vector of information sequences (one for eachuser)

Joint detection basic principleJoint detection basic principle

CDMAcode 1 Midamble

channel 1

Traffic channel 1input data

Mobile 1

1

Channel estimationof K

radio channels

Joint Detection(JD) of K

traffic channels

1 K2

Traffic channel Koutput data

Traffic channel 1output data

Radiochannels Estimated radio

channels 1 - K

Base station

Midambleschannels 1 - K

CDMAcodes 1 - KK

CDMAcode K Midamble

channel K

Traffic channel Kinput data

Mobile K

X

X

Joint DetectionJoint Detection

If e = (d ·c)*h + n is the received vector

We have to estimate the transmitted vector dby means of the following equation:

d’ = M ·e

The matrix M has to be calculated so as tomaximize performance and minimize complexity

UTRA: physical layer specificationUTRA: physical layer specification

Technical Specification GroupTechnical Specification Groupresponsible for the L1 specificationresponsible for the L1 specification

• 3GPP/RAN-WG1 “Radio layer 1 specification”• Chairman: Antti TOSKALA (Nokia)

E-mail “antti.toskala@nokia.com”• Secretary: Shinobu IKEDA (ETSI)

E-mail “shinobu.ikeda@etsi.fr”• Meeting attendance: about 130 delegates• Documents available at the following address:

“ftp://ftp.3gpp.org”

UTRA L1 spec.UTRA L1 spec. organisationorganisation ((relrel.’99).’99)

The technical spec. is organised in 11 documents:

• A general overview of the specification (TS25.201)• Five specification documents on the FDD component (TS

25.211-TS 25.215)• Five specification documents on the TDD component (TS

25.221-TS 25.225)• Two technical reports (TR 25.833, TR 25.944)

Technical specification documents (I)Technical specification documents (I)

• TS 25.201: Physical layer - General descriptionIt describes the content of the TS 25.2xx documents andprovides an overview of the physical layer.

• TS 25.211: Physical channels and mapping of transportchannels onto physical channels (FDD); the correspondentdocument for the TDD component is TS 25.221

• TS 25.212: Multiplexing and channel coding (FDD); thecorrespondent document for the TDD component is TS 25.222

• TS 25.213: Spreading and modulation (FDD); the correspondentdocument for the TDD component is TS 25.223.

Technical specification documents (II)Technical specification documents (II)

• TS 25.214: Physical layer procedures (FDD); the correspondentdocument for the TDD component is TS 25.224.

• TS 25.215: Physical layer - Measurements (FDD); thecorrespondent document for the TDD component is TS 25.225.

Technical ReportsTechnical Reports

• TR 25.833: Physical layer items not for inclusion in Release ‘99.

• TR 25.944: Channel coding and multiplexing examplesIt is a document “strongly supported” by NTT DoCoMocontaining several examples of channel coding andmultiplexing for some typical transport channels.

• A technical report is being produced for each Work Item to beincluded in Release 4 and 5. When the Work Item is completedthe text from the TR is moved in the relevant TS by a CRprocedure.

Release 4Release 4

• Same document structure as release 99 (the first digit of theversion is 4)

• Main new features with respect to release 99:– 1.28 Mchip/s TDD option– DSCH power control improvement in soft handover– TDD Node B synchronisation– UE positioning

SomeSome WorkWork/Study/Study ItemItems fors for RReleaseelease 55

• High Speed Downlink Packet Access (HSDPA)• Uplink Synchronous Transmission (USTS)• Radio Link Performance enhancements• Node B synchronisation for 1.28 Mchip/s TDD

UTRA/FDD UTRA/TDD

Access technique WCDMA Hybrid WCDMA+TDMA

Chip rate

Carrier spacing 4.4-5 MHz (200 kHz carrier raster)

3.84 Mcps (SF FDD:4-256, TDD 1-16)

Frame duration 10 ms

N. slot per frame 15

BTS synchronizationNot required Not required

(advisable)

Modulation DL: QPSKUL: Dual-channel QPSK UL: QPSK

Coherent receiver Uplink e downlink

Multi-rate Variabile SF + Multi-code + Multi-slot (TDD only)

Main parametersMain parameters

DL: QPSK

UTRA/N-TDD UTRA/W-TDD

Access technique Hybrid WCDMA+TDMA

Chip rate

Carrier spacing 4.4-5 MHz (200 kHz carrier raster)

Frame duration 10 ms

N. slot per frame 15

synchronization Both uplink and downlink DL: Not required (advisable)UL: not required

Modulation QPSK8 PSK optional

Coherent receiver Uplink e downlink

Multi-rate Variabile SF + Multi-code + Multi-slot

QPSK

Main parameters: Wideband TDDMain parameters: Wideband TDD vsvsNarrowband TDDNarrowband TDD

CDMA (Synch)+TDMA

1.28 Mcps (SF 1-16) 3.84 Mcps (SF 1-16)

1.6 MHz

10 ms (5 ms subframes)

Subframe: 7+3 minislots

Smart antennas Strongly advised Optional

Map. of transport channels onto physical channelsMap. of transport channels onto physical channelsT r a n s p o r t C h a n n e ls

D C H

R A C H

C P C H

B C H

F A C H

P C H

D S C H

P h y s ic a l C h a n n e l s

D e d ic a te d P h y s ic a l D a ta C h a n n e l (D P D C H )

D e d ic a te d P h y s ic a l C o n tro l C h a n n e l (D P C C H )

P h y s ic a l R a n d o m A c c e s s C h a n n e l (P R A C H )

P h y s ic a l C o m m o n P a c k e t C h a n n e l (P C P C H )

C o m m o n P ilo t C h a n n e l (C P IC H )

P r im a r y C o m m o n C o n tro l P h y s ic a l C h a n n e l (P -C C P C H )

S e c o n d a r y C o m m o n C o n tro l P h y s ic a l C h a n n e l (S -C C P C H )

S y n c h ro n is a t io n C h a n n e l (S C H )

P h y s ic a l D o w n lin k S h a re d C h a n n e l (P D S C H )

A c q u is i t io n In d ic a to r C h a n n e l ( A IC H )

A c c e s s P re a m b le A c q u is i t io n I n d ic a to r C h a n n e l (A P -A I C H )

P a g in g In d ic a to r C h a n n e l (P IC H )

C P C H S ta tu s I n d ic a to r C h a n n e l (C S IC H )

C o llis io n -D e te c t io n /C h a n n e l -A s s ig n m e n t In d ic a to r

C h a n n e l (C D /C A -IC H )

Physical channels ULPhysical channels UL

• Physical channels:– DPDCH (Dedicated Physical Data Channel): it is used to carry dedicated

data generated at OSI layer 2 and above (user data or associatedsignalling).

– DPCCH (Dedicated Physical Control Channel): it is used to carry controlinformation generated at OSI layer 1; the information include pilot bits forchannel estimation, Transmit Power Control bits (TPC), bits to indicate thebit rate (TFCI) and, in the UL only, Feedback Information bits required fortransmission diversity (FBI)

– DPDCH and DPCCH are transmitted on the I and Q branch respectively of aQPSK modulator; they are distinguished by means of different codes.

– PRACH (Physical Random Access Channel): it is used to carry the RACH,the transport channel used by the mobile to access the system

– PCPCH (Physical Common Packet Channel): it is used to carry the CPCH,the transport channel for packet transmission (contention access)

Frame structure for channelsFrame structure for channels DPxCHDPxCH (UL)(UL)

DataNdata bits

Tslot = 2560 chips , 10x2k bits (k=0..6)

Tf = 10 ms

DPDCH

DPCCH

Frame duration: 10 mseach frame is split into15 slot (0.667 ms)corresponding to onepower control period.

Spreading factor SF:4 ≤≤≤≤ SF ≤≤≤≤ 256DPDCH and DPCCHcan be characterisedby different values ofSF

Slot #iSlot #0 Slot #1 Slot #14

PilotN pilot bits

TPCNTPC bits

FBINFBIbits

TFCIN TFCI bits

Tslot = 2560 chips, 10 bits

0.667 ms

Spreading and modulationSpreading and modulation -- up linkup linkChannelizationcodes (OVSF)

cD, cC : channelization codesc’ scramb: scrambling code (short or long)p(t): pulse-shaping filter (root raised cosine, roll-off 0.22)

DPDCH and DPCCH are separate by means of different codes.During a call (circuit switched), at least the DPCCH is always activeA same code can be reused on the I and Q branch.The scrambling codes are complex sequencesQPSK modulation is used

DPDCH

cD

I

DPCCH

cC

Q ∗∗∗∗j

I+jQ

c’scramb

cos(ωωωωt)

p(t)

p(t)

Real

Imag

sin(ωωωωt)

Physical and logical channels:Physical and logical channels:downlinkdownlink

• Physical channels:– DPDCH and DPCCH are transmitted as in the uplink case but, here are time

multiplexed– The Downlink Shared Channel (DSCH) is used to transmit packet traffic

scheduled by the base station according to the traffic originated by theusers. A dedicated channel used to carry the physical layer controlinformation is always associated to DSCH.

– The Broadcast CHannel BCH is a downlink transport channel that is used tobroadcast system and cell specific information; the BCH is time multiplexedwith the SCH (Synchronisation Channel), the channel which allows themobile to acquire the synchronisation so as to demodulate the signalreceived from the base station. The resulting time multiplex is transmittedover the Primary CCPCH (Common Control Physical Control Channel). Acode multiplexed common pilot (CPICH) is transmitted too, separated fromthe Primary CCPCH and transmitted on a separate code.

– The secondary CCPCH is used to transmit the paging channel (PCH) and theForward Access Channel (FACH). They can carry also short user packets.

Physical channelsPhysical channels -- down linkdown link

DPCCH and DPDCH aredefined as the ULbut, they are timemultiplexed

4 ≤≤≤≤ SF ≤≤≤≤ 512Slot #0 Slot #1 Slot #i

Tf = 10 ms

Tslot = 0.666 ms (2560 chips), 10x2k bits (k=0..7)

Slot #14

TPCNTPC bits

Data2N data2 bits

DPDCH

TFCIN TFCIbits

PilotN

pilotbitsData1

Ndata1 bits

DPDCH DPCCH DPCCH

Spreading and modulationSpreading and modulation -- down linkdown link

A same channelisation codes is used on the I and Q branch.The scrambling code is a real sequence that is used onboth the I and Q branch.QPSK modulation

cch: channelization codesc’ scramb: scrambling codep(t): pulse-shaping filter (root raised cosine, roll-off 0.22)

cos(ωωωωt)

DPDCH/DPCCH

I

Q

p(t)

p(t)

c scrambS P cch sin(ωωωωt)

Spreading codesSpreading codes

SF = 1 SF = 2 SF = 4

C2,1 = (1,1)

Two kind of codes are used:-Orthogonal Variable SpreadingFactor (OVSF) codes are usedas channelisation codes; OVSFare defined by means of abinary tree-scrambling codes are used soas to guarantee goodautocorrelation properties andin order to distinguishdifferent cells (in the downlink)and to distinguish mobile users(in the uplink)

C2,2 = (1,-1)

C1,1 = (1)

C4,1 = (1,1,1,1)

C4,2 = (1,1,-1,-1)

C4,3 = (1,-1,1,-1)

C4,4 = (1,-1,-1,1)

Uplink Variable Rate (No DTX)Uplink Variable Rate (No DTX)

1-rate

10 ms

Variablerate

1/2-rate

1/4-rate

0-rate

: DPCCH (Pilot+TPC+TFCI)

: DPDCH (Data)

R = 1 R = 1/2 R = 0 R = 0 R = 1/2

Downlink Variable Rate (DTX based)Downlink Variable Rate (DTX based)

1-rate

1/2-rate

1/4-rate

0-rate

0.666 ms

: DPCCH-part (Pilot+TPC+TFCI)

: DPDCH-part (Data)

Transport channelTransport channelmultiplexingmultiplexing

Rate-Matching

Multiplexing

2nd Interleaving

Mapping toPhysical channels

Add CRC perTr. block

Add CRC perTr. block

Channelcoding

Channelcoding

TrCH TrCH

1st Interleaving 1st Interleaving

Packet AccessPacket Access

• There are three possible cases– Short and infrequent packets are transmitted on common

control channels (FACH, RACH)– Big size packets or scheduled packets can be transmitted over

a dedicated channel– In the downlink case it is possible to use a shared channel

(DSCH) where the access of the different user packets isscheduled by the BS; DSCH is suitable for medium and largeamount of data

– On the uplink it is possible to use the CPCH (contentionchannel); CPCH is suitable for medium and large amount ofdata

Common Channel Packet AccessCommon Channel Packet Access

• No link maintenance when no packet to transmit• Limited to small packets and medium data rates• No fast power control• No soft handover

Accessrequest

Userpacket

Accessrequest

Userpacket

Arbitrary time

Common Channel (RACH/FACH)

Dedicated Channel MultiDedicated Channel Multi--PacketPacketTransmissionTransmission

• Scheduled and non-scheduled packet access• Closed-loop power control and soft handover• Link released after time-out period has expired

Userpacket

Userpacket

Dedicated Channel (DCH)

Accessrequest

Userpacket

Accessrequest

Link maintenance (pilot, TPC, TFCI)

Scheduled packets

Non-scheduledpacket

DSCH (Downlink Shared Channel)DSCH (Downlink Shared Channel)

• The DSCH is a downlink channel,shared in timeamong all users (orthogonal code shared betweenusers)

• DSCH is used in parallel with a low bit ratededicated channel

• Closed loop power control allowed; no softhandover

Common Packet Channel (UplinkCommon Packet Channel (Uplink

• the CPCH is an uplink channel: users can access CPCH by meansof a contention mechanism

• Channels used to optimise the radio resources in case of packettransmission

• Fixed code per cell• closed loop power control allowed; no soft handover