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Florida Institute of technologies

ECE 5221 Personal Communication SystemsPrepared by:

Dr. Ivica Kostanic

Lecture 23 Basics of 3G - UMTS

Spring 2011

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W-CDMA (UMTS-FDD)

3G and 2G completely different air interfaces

Advanced radio resource management required

by diverse 3G applications

Multi-rate spreading

W-CDMA is interference limited

Provides soft capacity and

Coverage, capacity, qualitytradeoffs

2

10 ms 10 ms 10 ms

Power spectrum

density

frequency

time

f1

f2

frame

W-CDMA

Channel

DL TX Diversity, DL and UL RAKE receiverreception, UL space diversity

Diversity support

Coherent on both UL and DLDetection

Open loop and closed loop with 1500commands/sec

Power control

10msFrame Length

15,30,60,120,240,480,960,1920 kb/sec

Up to 3 code aggregations

Single code user rates

DL(after coding)

15,30,60,120,240,480, 960 kb/sec

Up to 6 code aggregation

Single code user rates

UL (after coding)

Variable: UL 1-512 (power of 2), DL(1-256)Spreading

3.84 Mc/secondChip rate

5MHzBandwidth

DS-CDMA with FDD and asynchronous operationAccess scheme

ValueProperty

Summary of W-CDMA properties

UL uplink; DL - downlink

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DS SS Systems - basic principles

Three basic stages

Spreading

RF Modem

De-spreading

Page 3

xShaping

Filter xInput data at

user rate

Spreading

sequence at

the higher rate

than user data

Baseband Spreading RF Modulation

( )to

wcos

CDMA Transmitter

Digital processingRF (Analog)

processing

PAFront

End

Filterx

( )to

wcos

Base

Band

Filterx

RF Demodulation

Spreading

sequence at

the higher rate

than user data

Wireless

Channel

CDMA Receiver

"RF MODEM"

Integrate

and dump

Output data

at user rate

Baseband De-Spreading

Processing of the signal for a single CDMA user

RF modem part is independent of CDMA

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DS CDMA - multiple users

After spreading signals from multiple users are summed

Signals from multiple users co-exist in time and frequency

The spreading codes have to be orthogonal

x

X

X

S

X

X

X

S1

S2

Sn

S1

S2

Sn

C1

C1

Integrate

and Dump

Integrate

and Dump

Integrate

and Dump

C2

C2

Cn Cn

X= S1C

1+S

2C

2+S

3C

3

Ci

- Spreading code

Si

- User data

X - Spread spectrum signal

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Example of DS CDMA - two users same PG

Processing gain (PG) is the ratio of chip and bit rates

Page 5

x

X

S

X

X

S1

S2

C1C

1

Integrate

Integrate

C2

= 1

= -1

1 11 1

1 11 1

1 1-1 -1

0 0 2 2

X= S1C

1+S

2C

2

S1

C1

x

S2

C2

x

1 11 1

1 1 -1 -1

1 1 -1 -1

C2

0 0 2 2

0+0+2+2 = 4 > 0

0 0 - 2 -20+0-2-2 = -4 < 0

X C1

x

X C2

x

1 was

sent

-1 was

sent

Two signals coexist in

time and frequency

b

c

R

RPG

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UMTS Voice Example

Vocoder rate 12.2kbps

Chip rate 3.84Mbps

6

Processing gain

dB25102.121084.3log10dBPG

3

6

Required S/N ratio for voice after de-

spreading is around 5dB

Signal can have S/(N+I) of -20dB and still be

received successfully

DS CDMA allows demodulation of signals

that are below interference and/or noise floor

At RF (before de-spreading)

At the base-band (after de-spreading)

Note: processing gain is derived through

reshaping of the power spectrum density

in the frequency domain

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Orthogonal Variable Spreading Factor codes (OVSF)

UMTS-FDD uses OVSF codes

OVSF codes preserve

orthogonality even for different

code lengths

Codes are designated with 2numbers

first number is the length

second number is the

position in the code tree

OVSF codes are orthogonal ifthey are not on the same path

from the root of the code tree

Page 7

(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,-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,-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,-1,1)

C1,0

C2,0

C2,1

C4,0

C4,1

C4,2

C4,3

C8,0

C8,1

C8,2

C8,3

C8,4

C8,5

C8,6

C8,7

OVSF code generation

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OVSF code - orthogonality

OVSF codes are orthogonal if they are not on the same path Consequence: assignment of a given code eliminates all codes

down the path

Note: in CDMA unique code is the channel. In 3G one may havemany low rate or few high rate channels

Page 8

Example: Illustration of the orthogonality

0,0

X

Code C8,1

1, 1, 1, 1, -1,-1,-1,-1

Code C4,3

1, -1, -1, 1, 1,-1,-1,1

C8,3

C4,3x

1, -1, -1, 1, -1,1,1,-1

S4

S8 0

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W-CDMA variable spreading - equal powers

User 1 and user 2 have different data rates

User 1 and user 2 use codes of different length

User 2 has a smaller processing gain

Decision making process is easier for user 1

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x

X

S

X

X

S8

S4

C8,0

C4,3 C

4,3

C8,0

(1,1,1,1,1,1,1,1)

(1,-1,-1,1)(-1,1,1,-1)

2,0,0,2,0,2,2,0

(1,1,1,1,1,1,1,1)

(1,-1,-1,1)

2+0+0+2+0+2+2+0=8

(0-2-2+0)= -4(2+0+0+2)= 4

Decision 1

Decision 1 -1

User 1

User 2

R1

R2

R2= 2R

1

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W-CDMA variable spreading - equal Eb/Nt

User 2 adjusts its power to compensate forsmaller processing gain

With power adjustments, both users havesame symbol energy after de-spreading

Page 10

X

S

X

X

S8

S4

C8,0

C4,3 C

4,3

C8,0

(1,1,1,1,1,1,1,1)

(2,-2,-2,2)(-2,2,2,-2)

3,-1,-1,3,-1,3,3,-1

(1,1,1,1,1,1,1,1)

(1,-1,-1,1)

3-1-1+3-1+3+3-1=8

(-1-3-3-1)= -8(3+1+1+3)= 8

Decision 1

User 1

User 2

R1

R2

R2= 2R

1

X

2

Decision 1 -1

(1,1,1,1,1,1,1,1)

(1,-1,-1,1)

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Multipath and Rake RX (1)

Terrestrial environment multipath

propagation of RF signal

Multipath propagation results: Signal dispersion: at the RX energy is

dispersed among multiple components

Signal fading: each component is subject to

fading

Signal dispersion energy reaches

received through resolvable multipath

components

Components are resolvable if their

relative delay is larger than a chip

interval

In W-CDMA one chip time interval

corresponds to 78m of path difference

Resolvable multipath components are

combined using maximum ratio

combining (MRC)

11

Power delay profile

example for 5MHz

channel

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Multipath and Rake RX (2)

Signal fading each delay position usually consists of

several components

The components have random phases causes fading

Fading occurs at the scale comparable to of a

wavelength (~ 7cm)

Fading may be as much as 30dB deep Fading is mitigated through interleaving and coding

12 Example signal variations due to fast fading

http://en.wikipedia.org/wiki/File:Rayleigh_fading_doppler_10Hz.svg

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Operation of Rake RX

Rake receiver consist of multiple

receiving fingers and searchers

Rake RX algorithm

Identify the time delay positions

with significant energy and assign

them to fingers

Demodulate resolvable multipathreceptions at each of the fingers

Combine demodulated and phase

adjusted symbols across all active

fingers and present them to the

demodulator

Typically phones rake receiver

has minimum of

Three fingers

One searcher

Delay resolution for a searcher is

typically 14/-1/2 chip interval

13

x

x

x

+

Correlator

Correlator

Correlator

t1

t2

t3

Phase and

amplitude

alignment

( )T

dt0. DECISION

Multipath

Response

t1

t2

t3

Finger 1

Finger 2

Finger 3

Simplified diagram of a rake receiver

Note: fingers may be used for multi-paths or

for different cells in soft handover

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Power control

WCDMA implements power control

on both uplink and downlink

On the UL - two loops for power

control Inner (fast loop)

Outer (slow loop)

Inner loop (uplink) Fast adjustments of MS TX power so that

target SIR at the base station is met

Rate: up to 1500 power adjustments/sec

Outer loop (uplink)

Adjustment of the SIR target at the basestation

On the DL Provide increase of power for edge of the

cell mobiles

Compensate for some fast fading effects

14

Note: SIR target usually changes as a

function of propagation environment and

mobiles speed

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Soft/softer handovers

Softer mobile is in communication

with sectors of the same cell

Soft mobile is in communication

with sectors of different cells

Essential interference mitigation tool Ensures that the mobile is power

controlled by all cells that cover

certain area

Prevents interference by reducing

near-far problem

Form mobile standpoint soft andsofter are essentially the same

From the system perspective soft

and softer differ in number of

allocated resources

15

Softer handover

Soft handoverNote: soft handover requires additional

resources between Node B and RNC