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WCDMA Air Interface Training

Part 2CDMA Power Control,

RAKE Receiver,and Soft Handover

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CDMA Reception Issues

• Unequal received power levels degrade SSMA performance Near-Far Ratio, terrain, RF obstacles, “Turn-the-Corner” effects, ...

• Multipath fading cancellation

• Time of Arrival delay spread

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Effective Power Control Required

• Near-far ProblemNear-far Problem• Path LossPath Loss• FadingFading

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The Power Control Solution

• Power ControlPower Control• Open loopOpen loop• Fast closed loopFast closed loop

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CDMA Power ControlOpen-Loop Power Control

Compute Initial

Transmit Power

Measure received power

from BS

Read BS transmit power from

Broadcast Channel

Transmit Access Preamble

Access Acknowledged

?

Increase Transmit Power

by 1 dB

No

Yes

MS BeginsUplink TCH

Transmission

Outer-Loop (slow) Power Control Inner-Loop (fast) Power Control

FER Acceptable

?

Raise RxPower Target

Lower RxPower Target

No

Yes

Received power

> target?

Increase MS Transmit Power

by 1 dB

Decrease MS Transmit Power

by 1 dB

No

Yes

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CDMA Power Control

Inner-loop power Control(Initial Receive Power Target)

BS Receive Power Target

Open-loop Power ControlAccess Preambles

Inner-loop power Control(Updated Receive Power Target)

BS Receive Power

time800 updates/sec (IS-95, cdma2000)1500 updates/sec (WCDMA)

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Multipath Fading

• Fast (Rayleigh) Fading

time (mSec)

CompositeReceived

Signal Strength

Time between fades is related to

• RF frequency

• Geometry of multipath vectors

• Vehicle speed: Up to 2 fades/sec per kilometer/hour

Deep fade caused by destructive summation of two or more multipath reflections

msec

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Frequency selective fading - Frequency Diversity

Interference Averaging

Forward Error Correction

f

ChannelQuality

f

ChannelQuality

Why Spread Spectrum ?

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Time DispersionMultipath Propagation

10

2

3

h()

10 2 3

Radio Environment

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Narrow band

Broad band

Delay Profiles

b. Coherence Windowsa. Impulse Response

Time Delay

Pow

er

(dB

)

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The RAKE Receiver

• CDMA Mobile Station RAKE Receiver Architecture

Each finger tracks a single multipath reflection

Also be used to track other base station’s signal during soft handover

One finger used as a “Searcher” to identify other base stations

Finger #1

Finger #2

Finger #N

Searcher Finger

Combiner

Sum ofindividual multipath components

Power measurement of Neighboring Base Stations

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The RAKE Receiver

• The CDMA Pilot Channel

All CDMA standards include a full-time “Pilot” Channel

Broadcast by the Base Station

Unique to each cell (or sector)

Serves as precision coherent phase reference for Downlink channels

Equivalent to a continuous-loop transmission of the Cell’s PN code

Pilot Channel Output

FIR Filter

FIR Filter

I/Q

Modulator

‘ I’ PN Code

‘ Q’ PN

Code

Orthogonal Code 0

Data

All 0’s

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BPF LPF

“ I” PNCode

(+1/-1)

“ Q” PNCode

(+1/-1)

Orthogonal

Code(+1/-1)

Integrateover

‘SF’ chips

De-Interleave

Data

Viterbi/Turbo

Decoder

CRCVerification

DecodedOutputBits

ErrorIndication

cos(2fRFt)

PilotOrthogonal

Code(all zeros)

TimingAdj.

bit rate = chip rate / SF

cos(2fIFt)

CarrierFrequencyTrackingLoop

Other Rake Receiver Finger

Rake Receiver “Finger”

D

D

I/Q

Demod

Correlator

CDMA RAKE Receiver Architecture

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0 50 100 150 200 250 300 350 400-2

0

2

4

6

8

10

12

14

16

18

0 50 100 150 200 250 300 350 400-2

0

2

4

6

8

10

12

14

16

18

0 50 100 150 200 250 300 350 400-2

0

2

4

6

8

10

12

14

16

18

The RAKE Receiver

1/2-chip delay

To Viterbi Decoder

Composite Received Signal

PN, Walsh Codes

1/2-chip delay

1/2-chip delay

Ai

Ai

Ai

Correlator

Correlator

Correlator

Equal Combining, ML Combining,

or Select Strongest

time

0 50 100 150 200 250 300 350 400-2

0

2

4

6

8

10

12

14

16

18

1

23

1

23

1

23

1

23 1

2

3 + Interference

+ Interference

+ Interference

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Handover

• Inter-System Handover Handover from a CDMA system to an Analog or TDMA system

Traffic and Control Channels are Disconnected and must be Reconnected

• Hard Handover When the MS must change CDMA carrier frequency during the Handover

Traffic and Control Channels are Disconnected and must be Reconnected

• Soft Handover Unique to CDMA

During Handover, the MS has concurrent traffic connections with two BS’s

Handover should be less noticeable

• Softer Handover Similar to Soft Handover, but between two sectors of the same cell

Handover is simplified since timing sectors have identical timing

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Soft Handover

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CDMA Without Soft Handover

time

Trouble zone: Prior to Hard Handover, the MS causes excessive interference to BS2

BS2 Receive Power Target

MS responding to BS1power control bits

MS responding to BS2power control bits

time

BS1 Receive Power Target

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CDMA Soft Handover

• CDMA Soft Handover

One finger of the RAKE receiver is constantly scanning neighboring Pilot Channels.

When a neighboring Pilot Channel reaches the t_add threshold, the new BS is added to the active set

When the original Base Station reaches the t_drop threshold, originating Base Station is dropped from the active set

Monitor Neighbor BS Pilots Add Destination BS Drop Originating BS

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Soft Handover Add/Drop Thresholds

• Soft Handover Measurement and Decision

Cell 1 Connected

Add Cell 2 Drop Cell 1Add Cell 3

time

Drop Cell 3

Eb / N0

Cell 1

T_DROP

Cell 2

Cell 3

T_ADD T_ADD

t t t

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CDMA With Soft Handover

time

BS2 Receive Power Target

1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 2 2 2 2 2 2 1 1 1 12 2 2 2 2 2 2 2 2 2 2 2 2 2 2 2 2 2

MS responding to BS1power control commands

MS responding to BS2power control commands

time

BS1 Receive Power Target

1 1 11 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 12 2 2 2 2 2

1 1 12 2 2 2 2 2 2 2 2 2 2 2 2

2 2 2 2

BS1 BS2 Action0 0 Reduce power0 1 Reduce power1 0 Reduce power1 1 Increase power

MS responds to power control commandsfrom both BS1 and BS2

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CDMA Soft Handover

• Key points to remember about Soft Handover

SSMA used to distinguish all transmitters in a Cellular CDMA system

Fast power control is required to sustain SSMA performance

When fast power control is used, soft handover is essential

Allows MS to operate in most conservative power control mode

Soft handover provides performance benefits

“Seamless” coverage at cell fringes

Handover may be less noticeable to the user

Increases apparent system capacity when system is lightly loaded

Soft handover also degrades system capacity

Uses redundant physical layer resources from adjacent or overlapping cells