spread spectrum ssssss 20040707-inoue

7/30/2019 Spread Spectrum SSSSSS 20040707-Inoue

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CDMA Technologies

for Cellular Phone System

July 7th, 2004

Takashi INOUE

KDDI R&D Laboratories Inc.


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Contents

Introduction

Spread Spectrum Technology

DS-CDMA

Spreading Codes Features of CDMA

RAKE Receiver

Power Control

Frequency Allocation

Soft Handoff Conclusion


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Introduction:

Overview of Cellular systems


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Evolution of Cellular Systems

1st.Generation

(1980s)Analog

NMT CT0

TACS CT1AMPS

3rd. Generation

(2000s)

2nd. Generation

(1990s)

Digital

GSM DECT

DCS1800 CT2

PDC PHS

IS-54

IS-95

IS-136UP-PCS

IMT-2000

CDMA2000

W-CDMA


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Major Operators of Cellular Phone Services

in Japan

Operator2G 3G

FrequencyRemarks for 3G handset

KDDI/au800 MHz(1.5GHz

For Tu-Ka)

800MHz

2GHz

backward compatibility

with 2G (cdmaOne)

NTT DoCoMo800 MHz1.5GHz

2GHzW-CDMA singleW-CDMA/PDC Dual

J-Phone(vodaphone)

1.5 GHz 2GHzW-CDMA singleW-CDMA/GSM Dual


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Japans Cellular Market Growth History

0

10,000,000

20,000,000

30,000,000

40,000,000

50,000,000

60,000,000

70,000,000

80,000,000

1988 1989 1990 1991 1992 1993 1994 1995 1996 1997 1998 1999 2000 2001 2002

CDMA2000 1x ,W-CDMA /6,093KcdmaOne /7,757KPDC/60,517K(NTT DoCoMo,KDDI, Tu-Ka,J-phone)Analog / 0(endof Srv.)

end of Mar. 2003

Total No . of Sub scrib ers: 74,368K

(end of each fiscal year)


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Growth of 3G Mobile Subscribers

in Japan

0

1,000

2,000

3,000

4,000

5,000

6,000

7,000

8,000

J-phone

FOMA

CDMA2000 1x

The number of subscribers

surpassed 7 million

in March, 2003

Oct-01 Nov-01 Dec-01 Jan-02 Feb-02 Mar-02 Apr-02 May-02 Jun-02 Jul-02 Aug-02 Sep-02 Oct-02 Nov-02 Dec-02 Jan-03 Feb-03 Mar-03

334 696 1151 1644 2142 2652 3293 3897 4673 5312 5891 6805

11 14 27 43 56 89 106 112 115 127 134 136 142 149 152 154 191 330

1 4 9 25

No.ofSubs

.(x1,

000)

CDMA2000 1x

FOMA

J-phone


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Requirements for 3G mobile systems

High Capacity

Tolerance for interference

Privacy Tolerance for fading

Ability to various data rate transmission

Flexible QoS


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IMT-2000 systems approved by ITU-R

Popular

name

Access

method

Body of

Technical

Spec

production

IMT-DS IMT-MC IMT-TC IMT-SC IMT-FT

(Direct Sequence) (Multi Carrier) (Time Code) (Single Carrier) (Frequency Time)

W-CDMA CDMA2000

UTRA-TDD

UWC-136 DECT

CDMA-FDD CDMA-TDDCDMA-FDD TDMA-TDDTDMA-FDD

3GPP(FDD 3GPP2 3GPP(TDD)CWTS

IS-136 DECT

ESTIESTI TIA

TTA

T1

CWTS

ARIB/TTC ARIB/TTC

CWTS

TTA

ESTI

TTA

T1

CWTS

TIA

Approved in 2000 as ITU-R M.1457

TD-CDMA

TD-SCDMA

Organization

Partners


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Duplex & Multiple Access Methods


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Duplex Methods of Radio Links

Mobile Station

Base Station

Forward link

Reverse link


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Frequency Division Duplex (FDD)

Forward link frequency and reverse link frequency isdifferent

In each link, signals are continuously transmitted in

parallel.

Mobile Station

Base Station

Forward link (F1)

Reverse link (F2)


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Time Division Duplex (TDD)

Forward link frequency and reverse link frequency isthe same.

In each link, signals are incontinuously transmitted

by turns just like a ping-pong.

Mobile Station

Base Station

Forward link (F1)

Reverse link (F1)


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Example of FDD systems

Transmitter

Receiver

BPF: Band Pass Filter

BPF

BPF

Transmitter

Receiver

BPF

BPF

F1

F2 F1

F2

Mobile Station Base Station


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Example of TDD Systems

Transmitter

Receiver

BPF: Band Pass Filter

BPF

Transmitter

Receiver

BPF

F1 F1

Mobile Station Base Station

Synchronous Switches


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Multiple Access Methods

Mobile Station

Base Station

Mobile StationMobile Station

Mobile Station

Forward link

Reverse link


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FDMA Overview

A A

B B

C C

Time

f2

f1

f0


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TDMA Overview

C B A C B A C B A C B A

C

A

B

Time

f0


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What is CDMA ?

Sender Receiver

Code A

A

Code B

B

AB

AB

CB

C

A

Code A

AB

C

Time

BC

B

A

Base-band Spectrum Radio Spectrum

spread spectrum


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Summary of Multiple Access

FDMA

TDMA

CDMA

power

power

pow

er


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Spread Spectrum Technology


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How to spread spectrum...

Direct Sequence (DS)

Modulation

(primary modulation)

user data

Sp

reading

(seconda

rymodulation)

Tx

Base-band

Frequency

Power

Density

Radio

Frequency

Power

Density

TIME

data rate

10110100

spreading sequence

(spreading code)


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Demodulating DS Signals (1/2)

If you know the correct spreading sequence (code) ,

Radio

Frequency

Power

Density

received signal

spreading sequence

(spreading code)

you can find thespreading timingwhich gives themaximum detectedpower, and

Accumulate forone bit duration

Demodulated data

Base-band

Frequency

gathering energy !

10110100

1011010010110100 10110100

TIME

0100101110110100 10110100

0 01

1111111100000000 00000000


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Demodulating DS Signals (2/2)

If you dont know the correct spreading sequence (code)

Base-band

Frequency

received signal

spreading sequence

(spreading code)

you cannot findthe spreadingtimingwithout correctspreading code,and

Accumulate for

one bit duration

Demodulated data

RadioFrequency

Power

Density

01010101 01010101 01010101

10101010 10101010 10101010

TIME

0100101110110100 10110100

No data can be detected

- --

1011010010110100 10110100


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Feature of SS

Privacy, Security

RadioFrequency

Power

Density

Power density of SS-signals could be lower than the noise density.

transmitted SS-signal

Noise

Power

Density

RadioFrequency

Noise

received signal de-

modulator

Base-band

Frequency

Power

DensityWith incorrect code

(or carrier frequency),SS-signal itselfcannot be detected.

They cannot perceive the existence of communication,because of signal behind the noise.

With correct code(and carrier frequency),data can be detected.

Base-bandFrequency

Power

Density


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DS-CDMA


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Freq.Freq.

BPFDespreader

Code B

Freq.Freq.

BPFDespreader

Code A

DS-CDMA System Overview

(Forward link)

CDMA is a multiple spread spectrum.

Difference between each communication path is only the spreading code

Data B

Code B

BPF

Freq.Freq.

Data A

Code A

BPF

Freq.Freq.

MS-A

MS-B

BS

Data A

Data B


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Freq.Freq.

BPFDespreader

Code B

Freq.Freq.

BPFDespreader

Code A

DS-CDMA System Overview

(Reverse Link)

CDMA is a multiple spread spectrum.

Difference between each communication path is only the spreading code

Data B

Code B

BPF

Freq.Freq.

Data A

Code A

BPF

Freq.Freq.

MS-B

MS-A

BS

Data A

Data B


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Spreading Code


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Cross-Correlation

Cross-Correlation

between Code A and Code B = 6/16

Self-Correlation

for each code is 1.

one data bit duration

Spreading Code A

1 0 10 1 1 0 0 10 10 1 0 0 1

one data bit duration

Spreading Code A

1 0 01 1 1 0 0 10 10 1 0 0 1

Spreading Code A

1 0 01 1 1 0 0 10 10 1 0 0 1

0 0 00 0 0 0 0 00 00 0 0 0 0

Spreading Code B

1 0 0

1 1 0 0 1 11 00 1 0 1 1

0 0 00 0 1 0 1 01 10 0 0 1 0


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Preferable Codes

In order to minimize mutual interference in DS-CDMA ,

the spreading codes

with less cross-correlation should be chosen.

Synchronous DS-CDMA :Orthogonal Codes are appropriate. (Walsh code etc.)

Asynchronous DS-CDMA : Pseudo-random Noise (PN) codes / Maximum sequence

Gold codes


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Multiplexing using Walsh Code

Co

d

e

fo

r0

0

Codefor01

Codefor10

Codefor11

Data

Modulator

Codefor01

Codefor10

Codefor11

0dtT

Select

maximumvalue

Codefor00

0dtT

0dtT

0dtT

Demodulator


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Synchronous DS-CDMA

Forward Link(Down Link)

Synchronous Chip Timing

A

A

Signal for B Station(after re-spreading)

Less Interference for A station

Synchronous CDMA Systems realized in Point to Multi-point System.

e.g., Forward Link (Base Station to Mobile Station) in Mobile Phone.


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Asynchronous DS-CDMA

In asynchronous CDMA system, orthogonal codes have bad cross-correlation.

Reverse Link(Up Link)

BA

Signal for B Station(after re-spreading)

Big Interference

from A station

Asynchronous ChipTiming

Signals from A and B areinterfering each other.

A

B


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Features of CDMA


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Mobile Propagation Environment

Multi-path Fading

The peaks and bottoms of received power appear,in proportion to Doppler frequency.

Base Station (BS)Mobile Station (MS)

multi-path propagation

Path Delay

Power

path-2

path-2

path-3

path-3

path-1

path-1

Time

Power


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Fading in non-CDMA System

Path Delay

Power path-1

path-2

path-3

With low time-resolution,

different signal paths cannot be discriminated.

These signals sometimes strengthen,

and sometimes cancel out each other,

depending on their phase relation.

This is fading.

In this case, signal quality is damaged

when signals cancel out each other.

In other words, signal quality is dominated

by the probability for detected power

to be weaker than minimum required level.

This probability exists with less than two paths.

Time

Power

Detected Power

In non-CDMA system, fading damages signal quality.


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Fading in CDMA System ...

Because CDMA has high time-resolution,

different path delay of CDMA signalscan be discriminated.

Therefore, energy from all paths can be summed

by adjusting their phases and path delays.

This is a principle of RAKE receiver.

Path Delay

Power path-1

path-2

path-3

CDMA

Receiver

CDMA

Receiver

Synchro

nization

Adder

Path Delay

Power

CODE A

with timing ofpath-1

path-1

P

ower

path-1

path-2

path-3

Path Delay

Power

CODE A

with timing ofpath-2

path-2

interference from path-2 and path-3


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Fading in CDMA System (continued)

In CDMA system, multi-path propagation improves

the signal quality by use of RAKE receiver.

Time

Power

Detected Power

RAKEreceiver

Less fluctuation of detected power,because of adding all energy .

Power

path-1

path-2

path-3


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Near-Far Problem

CODE B

CDMA

Transmitter

DATA B

CODE A

CDMA

ReceiverCODE A

CDMA

Transmitter

DATA A

P

Desired Signal Power = P/Lp-a

Interfered Signal Power =

P/Lp-b/(processing gain )

Demodulated DATA

P

Lp-a

Lp-b

When user B is close to the receiver and

user A is far from the receiver,

Lp-a could be much bigger than Lp-b.

In this case, desired signal power is

smaller than the interfered power.


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Power Control...

Time

DetectedPower

from A

from B

When all mobile stations transmit the signals at the same power (MS),

the received levels at the base station are different from each other,

which depend on the distances between BS and MSs.

Moreover, the received level fluctuates quickly due to fading.

In order to maintain the received level at BS, power control technique must be

employed in CDMA systems.


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Power Control (continued)

Open Loop Power Control Closed Loop Power Control

estimating pathloss

calculatingtransmission

power

transmitmeasuring

received power

transmit receive

decidetransmission

power

transmit measuringreceived power

power controlcommand

about 1000 timesper second


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Effect of Power Control

Time

De

tectedPower

from MS Bfrom MS A

Effect of Power Control

Power control is capable of compensating the fading fluctuation.

Received power from all MS are controlled to be equal.

... Near-Far problem is mitigated by the power control.


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Frequency Allocation (1/2)

In FDMA or TDMA,

radio resource is allocated not to interfere among neighbor cells.

f1f2

f3f4

f5f6

f7

cell :

a cell means covered area by one base station.

Neighbor cells cannot use the

same (identical) frequency

band (or time slot).

The left figure shows the

simple cell allocation with

seven bands of frequency.

In actual situation, because of

complicated radio

propagation and irregular cell

allocation, it is not easy to

allocate frequency (or timeslot) appropriately.


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Frequency Allocation (2/2)

In CDMA,identical radio resource can be used among all cells,

because CDMA channels use same frequency simultaneously.

Frequency allocation in CDMA

is not necessary.

In this sense, CDMA cellular

system is easy to be designed.


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Soft Handoff (1/2)

Handoff :

Cellular system tracks mobile stations in order to maintain their communication links.

When mobile station goes to neighbor cell, communication link switches from current cell

to the neighbor cell.

Hard Handoff :

In FDMA or TDMA cellular system, new communication establishes after breaking current

communication at the moment doing handoff. Communication between MS and BS

breaks at the moment switching frequency or time slot.

Hard handoff : connect (new cell B) after break (old cell A)

switching

Cell BCell A


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Soft Handoff (2/2)

Cell

BCell A

Soft handoff : break (old cell A) after connect (new cell B)

transmitting same signal from both BS A and

BS B simultaneously to the MS

Soft Handoff :

In CDMA cellular system, communication does not break even at the moment doing

handoff, because switching frequency or time slot is not required.


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Conclusion

CDMA is based on the spread spectrum

technique which has been used at military

field.

CDMA cellular system is deemed superior tothe FDMA and TDMA cellular systems for the

time being.

Therefore, CDMA technique becomes more

important in radio communication systems.

spread spectrum ssssss 20040707-inoue

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