Spread spectrum systems II: WCDMA

• WCDMA basic properties

• Channel mapping

• Chip sequence processing

• Soft handover

• Power control

1. WCDMA basic properties

Issues / important concepts:

• Two duplex alternatives: UTRA FDD vs. UTRA TDD

• Spectrum allocation (UTRA FDD)

• Spreading in WCDMA

• DPDCH/DPCCH/DPCH channel bit rates

Two duplex alternatives: FDD vs. TDD

In UTRA FDD (Frequency Division Duplex), uplink and downlink are separated in frequency domain:

UL UL DLDL

frequency

In UTRA TDD (Time Division Duplex), uplink and downlink are separated in time domain:

time

UL UL DLDL UL UL DLDL ......

Two duplex alternatives: FDD vs. TDD

UTRA FDD will be more widely used in the near future, since UTRA TDD technology is more complex.

However, UTRA TDD offers some benefits:

More flexible UL/DL capacity allocation (in non-voice applications, DL usually demands more capacity than UL) Channel reciprocity (channel estimation in one direction could be used in the other direction)

No need for duplex filter.

1.

2.

3.

Spectrum allocation for UTRA FDD

1920 - 1980 MHz1920 - 1980 MHz 2110 - 2170 MHz2110 - 2170 MHz

Uplink Downlink

60 MHz

5 MHz

Chip sequnces are multiplexed in code domain and transmitted within a 5 MHz frequency slot.

The chip rate is always 3.84 Mchips/s.

Spectrum is allocated to operators at

this level

(Europe & part of Asia)

Spreading in WCDMA

Channel data

Channel data

Channelization code

Channelization code

Scrambling code

Scrambling code

Channel bit rate

Chip rate Chip rate

Usage of codeUsage of code UplinkUplink DownlinkDownlink

Channelization code

Scrambling code

User separation

User separation Cell separation

(always 3.84 Mchips/s)

QPSK

Spreading in WCDMA

Chip rate = SF x channel bit rateChip rate = SF x channel bit rate

Uplink: DPCCH SF = 256, DPDCH SF = 4 - 256Uplink: DPCCH SF = 256, DPDCH SF = 4 - 256

Downlink: DPCH SF = 4 - 256 (512)Downlink: DPCH SF = 4 - 256 (512)

SF = Spreading factor

One bit consists of 256 chips

One bit consists of 4 chips

Uplink DPDCH bit rates

256

128

64

32

16

8

4

SF Channel bit rate (kb/s)

User data rate (kb/s)

15

30

60

120

240

480

approx. 7.5

approx. 15

approx. 30

approx. 60

approx. 120

approx. 240

960 approx. 480

Uplink DPCCH bit rate

256

SF Channel bit rate

15 kb/s

How many control channel bits does one

time slot contain?

Each 10 ms radio frame (38400 chips long) is divided into 15 time slots (2560 chips long).

Since SF = 256, each time slot contains 10 control channel bits that can be used, for example, like this:

FBI TPCTFCIPilot

3GPP TS 25.211 Slot format 3

Downlink DPCH bit rate

256

128

64

32

16

8

4

SF Channel bit rate (kb/s)

User data rate (kb/s)

15

30

60

120

240

480

approx. 1-3

approx. 6-12

approx. 20-24

approx. 45

approx. 105

approx. 215

960 approx. 456

512

1920 approx. 936

User data rate vs. channel bit rate

Channel bit rate (kb/s)Channel bit rate (kb/s)

User data rate (kb/s)User data rate (kb/s)

Channel codingChannel coding

InterleavingInterleaving

Bit rate matchingBit rate matching

Interesting for user

Interesting for user

Important for system

Important for system

2. Channel mapping

Issues / important concepts:

• Physical channels

• Transport channels

• Logical channels

• DPDCH/DPCCH multiplexing in uplink

• DPCH user/control data multiplexing in downlink

Phy Phy Lower layers

Lower layers

Logical / transport / physical channels

RLCRLC RLCRLC

MACMAC

UE Base station RNC

MACMAC

Lower layers

Lower layers Phy Phy

Logical channels

Physical channels

Transport channels

: :

WCDMA

Logical / transport channel mapping

CCCHCCCH DCCHDCCH

PCHPCH DCHDCHDSCHDSCHFACHFACHBCHBCHDCHDCHCPCHCPCHRACHRACH

CTCHCTCHBCCHBCCHPCCHPCCH

Uplink Downlink

DTCHDTCH

Logical channels

Transport channels

Note the different possibilities for transmitting user data over transport channels

DCCHDCCH

DTCHDTCH

CCCHCCCH

Transport / physical channel mapping

PCHPCH DCHDCHDSCHDSCHFACHFACH BCHBCHCPCHCPCHRACHRACH

PRACHPRACH PCPCHPCPCH SCCPCHSCCPCH PCCPCHPCCPCHDPDCHDPDCH

DPCCHDPCCH

SCHSCHCPICHCPICH

AICHAICH

PICHPICH

CSICHCSICHPhysical channels

Transport channels

DPCHDPCH

CD/CA-ICH

CD/CA-ICH

Uplink Downlink

PDSCHPDSCH

DCHDCH

These channels are only for transport of information in the physical layer at the air (radio) interface

DPDCH / DPCCH structure in uplink

DataData

PilotPilot TFCITFCI FBIFBI TPCTPC

DPDCH (I-branch)DPDCH (I-branch)

10 ms radio frame (38400 chips)

0 1 2 14

Time slot containing 2560 chips

DPCCH (Q-branch)DPCCH (Q-branch)

Dual-channel QPSK modulation:

DPCH structure in downlink

QPSK modulation, time multiplexed data and control information:

TFCITFCI DataData TPCTPC DataData

0 1 2 14

PilotPilot

Time slot containing 2560 chips

10 ms radio frame (38400 chips)

3. Chip sequence processing

Issues / important concepts:

• Spreading, scrambling, multiplexing and modulation

• Uplink and downlink processing somewhat different

• Channelization codes vs. spreading codes

Uplink spreading

DPDCH

DPCCH

OVSF Code 1

The DPCCH is spread on the Q-branch using SF = 256.

In case of very high user bit rates, up to six DPDCH channels can be used in parallel by distributing the signals to the I and Q branches using additional OVSF codes.

OVSF Code 2

In the UE, the user data (DPDCH) and control data (DPCCH) signals are spread to the chip frequency of 3.84 Mchips/s using different channelisation codes, also called OVSF (Orthogonal Variable Spreading Factor) codes.

I branch

Q branch

Uplink multiplexing

++

DPDCH

DPCCH

I branch

Q branch

Weight 1

jWeight 2

The DPCCH signal and DPDCH signal (or up to 6 DPDCH signals) are synchronously combined, i.e. ”multiplexed in code domain”, to form the complex signal I+jQ.

Complex-valued signal I + jQ

Uplink scrambling

++

I + jQ

After scrambling, signals from different UEs can be separated at the base station, since each UE uses a different scrambling code.

Scrambling codes must have good correlation properties even when not synchronized (=> m-sequence or Gold codes).

Scrambling codeRe{S}

Im{S}

Complex-valued signal S

The complex signal I+jQ is multiplied by the complex-valued, UE specific scrambling code.

Uplink modulation

++

Re{S}

Im{S}

To RF part and UE antenna

The real and imaginary parts of the scrambled signal S are fed to the I and Q branches of the modulator and are modulated by sinusoids with a 90-degree phase shift to achieve the desired QPSK modulation.

The QPSK signal is transmitted from the UE antenna.

Pulse shaping

Pulse shaping

Pulse shaping

Pulse shaping

cos(t)

-sin(t)

At the receiver side

At the transmitter side, signal formats and processing details are standardised (see 3GPP TS 25.213).

At the receiver side, base station manufacturers are free to implement any receiver structure they wish.

In general terms, the code processing is in the reverse order (demodulation, despreading, demultiplexing ...) and makes use of a Rake receiver able to resolve and despread separate multipath replicas of the transmitted signal.

Channel estimation and phase synchronisation is based on pilot bits transmitted in the DPCCH signal.

Downlink spreading

Serial/parallel conversion is applied to two consecutive channel bits. The bits in the I and Q branches are then spread using the same OVSF (channelisation) code.

++S/PAny downlink physical channelexcept SCH

OVSF Code n

OVSF Code n j

Complex-valued signal I + jQ

Downlink scrambling

Scrambling code

++

The spreaded, complex-valued signal is chipwise multiplied with a complex-valued scrambling code.

Scrambling codes are selected from a base station specific code set. A scrambling code can be shared among several physical channels.

Adjacent base stations use different (sets of) scrambling codes.

Downlink multiplexing

Weight n

Other downlink physical channels

SCH signal(s)

Before the spreaded and scrambled physical channels are ”multiplexed in code domain”, signal powers are adjusted to the appropriate levels determined by the downlink closed loop power control (on a channel-by-channel basis).

Note that all channels are multiplexed synchronously.

Re{S}

Im{S}

Complex-valued signal S

Downlink multiplexing

Weight n

Other downlink physical channels

SCH signal(s)

Synchronisation channels (SCH) are spread using special code sequences (i.e. no OVSF codes are involved).

SCH is first multiplexed with CCPCH in time domain. The composite signal is then added to the other channels in code domain (see 3GPP TS 25.211).

Re{S}

Im{S}

Downlink modulation

++

To RF part and

base station antenna

Pulse shaping

Pulse shaping

Pulse shaping

Pulse shaping

cos(t)

-sin(t)

Re{S}

Im{S}

The real and imaginary parts of the multiplex signal S are fed to the QPSK modulator like in uplink.

Note that this signal contains information for many UEs.

Synchronous / non-s. chip sequences

Sequences start here

Sequences end here

One sequence starts here

Another sequence starts here

Two synchronous chip sequences

Two non-synchronous chip sequences

Chips

Chip Sequence = encoded bit/symbol

Synchronous / non-s. chip sequences

Synchronous chip sequences

Non-synchronous chip

sequences

Channelization (Hadamard-Walsh)

codes

Scrambling (m-sequence, Gold)

codes

No interference (sequences are all

orthogonal)

Little interference (sequences are

near orthogonal)

Little interference (sequences are

near orthogonal)

Large interference

Different effect on different

types of codes:

4. Soft handover

Issues / important concepts:

• Serving RNC, Drift RNC, SRNS Relocation

• Micro/macrodiversity combining

• Soft handover in uplink

• Soft handover in downlink

Serving RNC and Drift RNC in UTRAN

Core network

Iu

Iur

Iub

Iub

DRNC

SRNC

UEUE

BSBS

BSBS

RNCRNC

RNCRNC

Concept needed for:Soft handover between base stations belonging to different RNCs

SRNS Relocation

Core network

Iu

Iur

Iub

Iub

DRNC

SRNC

UEUE

BSBS

BSBS

RNCRNC

RNCRNC Iu

SRNC

SRNC provides: 1) connection to core network 2) macrodiversity combining point

Micro- / macrodiversity combining

Iu

Iur

Iub

Iub

DRNC

SRNC

UEUE

BSBS

BSBS

RNCRNC

RNCRNC Macrodiversity combining point in

SRNC

Core network

Rake receiver

Multipath propagation

Microdiversity combining point in base station

(uplink)

Diversity combining, soft handover

Microdiversity combining: multipath signal components are processed in Rake branches and combined (MRC = Maximum Ratio Combining)

Macrodiversity combining: bit sequences carrying the same signal (but with different bit error positions) are either combined at SRNC (bit-by-bit majority voting), or best quality signal is selected.

Hard handover: slow, complex signalling

Soft handover: fast selection in SRNC is possible due to macrodiversity combining

(uplink)

Microdiversity combining, soft handover

Soft handover: same signal is transmitted via several base stations

Advantage: number of multipath components is increased

Draw-back: in downlink, soft handover decreases capacity

(downlink)

BSBS

BSBS

Rake receiver

UEUE

Different code

sequences

5. Power control

Issues / important concepts:

• Near-far problem

• Uplink SIR expression; what means Target SIR?

• Open loop power control

• Inner loop (closed loop) power control

• Outer loop (closed loop) power control

Why is power control needed?

Strong signal dominates

Near-far problem arises in uplink when all UEs use the same transmit power:

BSBS

UEUE

UEUE

Weak signal will be drowned

Rather, UEs should adjust their transmit power levels so that the received power levels are approximately the same at the base station.

Uplink SIR expression

Ps . SF

(N-1).Ps + Pn

SIR

In uplink, in case of same received power levels (and ignoring interference from UEs located in other cells) the signal-to-interference ratio for the k:th user is

This simple rule-of-thumb expression

• is useful for estimating capacity in uplink

• is the basis of admission control in uplink

• explains “Target SIR” used in power control.

Analysis of uplink SIR expression

Ps . SF

(N-1).Ps + Pn

SIR

Signal-to-interference ratio (SIR) is a very important parameter in a DS-CDMA system.

SIR describes the situation after despreading in the CDMA receiver.

The corresponding ratio before despreading is called CIR (carrier-to-interference ratio).

Analysis of uplink SIR expression

Ps . SF

(N-1).Ps + Pn

SIR

SF = Spreading Factor.

We assume here that the power of the desired signal (of k:th user) after despreading is SF times the power of interfering signal (of another user) after despreading if the powers before despreading are the same.

In other words, this is a crude model for estimating the level of cross-correlation in the CDMA receiver.

Analysis of uplink SIR expression

Ps . SF

(N-1).Ps + Pn

SIR

It is assumed that the received signal power (before despreading) of all N active users in the cell is Ps.

Pn is the thermal noise power in the receiver.

(In case there are users with different bit rates - and thus different spreading factors - this expression must obviously be modified)

Analysis of uplink SIR expression

Ps . SF

(N-1).Ps + Pn

Target SIR

The SIR for the k:th user must be larger than a certain value, Target SIR. In other words, the total interference in the system must remain below a certain target level.

>

First, we see that the best case is when Ps >> Pn . In other words, CDMA systems are interference limited, not noise limited.

Second, the inequality above is valid for values of N up to a maximum value Nmax , the capacity of the cell.

Analysis of uplink SIR expression

The target SIR value depends on various issues, such as required Bit Error Ratio (BER) or Frame Error Ratio (FER), user/channel bit rate of k:th user, etc.

BER

SIR

High BER means low target SIR

Low BER means high target SIR

Open loop power control

UE estimates the average path loss in downlink ...

This simple and inaccurate power control scheme must be used during the random access process at the beginning of a connection until more accurate control information is available.

... and adjusts the uplink transmission power accordingly

BSBSUEUE

(Note: uplink / downlink fading in UTRA FDD is not the same)

Inner loop power control

UE transmits initial signal

Is measured SIR larger (smaller) than

Target SIR?

BSBSUEUE

Inner loop power control (also called fast power control) is used both in uplink (shown in this figure) and downlink.

If answer is yes: decrease (increase) power

This loop is performed 1500 times per second

UE decreases (increases) transmit power

Outer loop power control

Is signal quality (BER)

ok?

BSBSUEUE

Outer loop power control is used both in uplink (shown in this figure) and downlink.

Inner loop power control uses new Target SIR value

RNCRNC

If not, increase or decrease Target SIR