CDMA Technologies for Cellular Phone System, Dec. 22, 2005 1 CDMA Technologies for Cellular Phone...

CDMA Technologies for Cellular Phone System , Dec. 22, 2005 1

CDMA Technologies for Cellular Phone System

CDMA Technologies for Cellular Phone System


ContentsContents

1. Introduction2. Spread Spectrum Technology3. DS-CDMA4. Spreading Codes5. Features of CDMA

– RAKE Receiver– Power Control– Frequency Allocation– Soft Handoff

6. Conclusion


Introduction: Overview of Cellular systems

Introduction: Overview of Cellular systems


Evolution of Cellular SystemsEvolution of Cellular Systems

1st.Generation(1980s)

Analog

NMT CT0TACS CT1AMPS

3rd. Generation(2000s)

2nd. Generation(1990s)

Digital

GSM DECTDCS1800 CT2PDC PHSIS-54IS-95IS-136UP-PCS

IMT-2000 CDMA2000 W-CDMA


Japan’s Cellular Subscriber Growth Record

Japan’s Cellular Subscriber Growth Record

0

20

40

60

80

100

95 96 97 98 99 00 01 02 03 04 05

No.

of

Su

bscri

bers

(M

illion

)

end December of Year

end Nov ．2005

89,679K

cdmaOne/CDMA2000 1x/

EV-DO21 ， 222K

PDC (TDMA)47,787K

W-CDMA20,670K

PDC

AnalogcdmaOne/CDMA2000 1x/EV-DO

W-CDMA

/Nov.


Requirements for 3G mobile systemsRequirements for 3G mobile systems

• High Capacity

• Tolerance for interference

• Privacy

• Tolerance for fading

• Ability to various data rate transmission

• Flexible QoS


Duplex & Multiple Access MethodsDuplex & Multiple Access Methods


Duplex Methods of Radio LinksDuplex Methods of Radio Links

Mobile Station

Base Station

Forward link

Reverse link


Frequency Division Duplex (FDD)Frequency Division Duplex (FDD)

• Forward link frequency and reverse link frequency is different

• In each link, signals are continuously transmitted in parallel.

Mobile Station

Base Station

Forward link (F1)

Reverse link (F2)


Time Division Duplex (TDD)Time Division Duplex (TDD)

• Forward link frequency and reverse link frequency is the same.

• In each link, signals are in continuously transmitted by turns just like a ping-pong.

Mobile Station

Base Station

Forward link (F1)

Reverse link (F1)


Example of FDD systemsExample of FDD systems

Transmitter

Receiver

BPF: Band Pass Filter

BPF

BPF

Transmitter

Receiver

BPF

BPF

F1

F2 F1

F2

Mobile Station Base Station


Example of TDD SystemsExample of TDD Systems

Transmitter

Receiver

BPF: Band Pass Filter

BPF

Transmitter

Receiver

BPF

F1 F1

Mobile Station Base Station

Synchronous Switches


Multiple Access MethodsMultiple Access Methods

Mobile Station

Base Station

Mobile StationMobile Station

Mobile Station

Forward link

Reverse link


FDMA OverviewFDMA Overview

A A

B B

C C

Freq

uenc

y

Time

f2

f1

f0


TDMA OverviewTDMA Overview

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

C

A

B

Time

f0

Freq

uenc

y


What is CDMA ?What is CDMA ?

Sender Receiver

Code A

A

Code B

B

AB

AB

CBC

A

Code A

AB

C

Time

Freq

uenc

y

BC

B

A

Base-band Spectrum Radio Spectrum

spread spectrum


Summary of Multiple AccessSummary of Multiple Access

FDMA

TDMA

CDMA

time

time

time

pow

er

pow

er

pow

er

frequency

frequency

frequency


Spread Spectrum TechnologySpread Spectrum Technology


How to spread spectrum...How to spread spectrum...

Direct Sequence (DS)

Modulation(primary modulation)

Modulation(primary modulation)

user data

Sp

read

ing

(sec

on

dar

y m

od

ula

tio

n)

Sp

read

ing

(sec

on

dar

y m

od

ula

tio

n)

Tx

Base-bandFrequency

Pow

erD

ensi

ty

RadioFrequency

Pow

erD

ensi

ty

TIME

data rate

10110100

spreading sequence(spreading code)


Demodulating DS Signals (1/2)Demodulating DS Signals (1/2)If you know the correct spreading sequence (code) ,

RadioFrequency

Pow

erD

ensi

ty

Accumulate for one bit duration


Demodulated data

Base-bandFrequency

gathering energy !


1011010010110100 10110100

received signal

TIME

0100101110110100 10110100

0 01

1111111100000000 00000000

0+0=01+0=10+1=11+1=0


Demodulating DS Signals (2/2)Demodulating DS Signals (2/2)

If you don’t know the correct spreading sequence (code) •••

Base-bandFrequency

received signal




Demodulated data

RadioFrequency

Pow

erD

ensi

ty

1010101010101010 10101010

TIME0100101110110100 10110100

No data can be detected

- --

1011010010110100 10110100


Feature of SSFeature of SS

Privacy, Security

RadioFrequency

Po

we

rD

en

sity

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

transmitted SS-signal

••••

••

Noise

Po

we

rD

en

sity

RadioFrequency

Noise

••••

••received signal de-

modulator

de-modulator

Base-bandFrequency

Po

we

rD

en

sityWith incorrect code

(or carrier frequency),SS-signal itself cannot be detected.

Other system cannot recognize the existence of communication, because of signal behind the noise.

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

Base-bandFrequency

Po

we

rD

en

sity


DS-CDMADS-CDMA


Freq.Freq.

BPFDespreader

Code B

Freq.Freq.

BPFDespreader

Code A

DS-CDMA System Overview (Forward link)

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


Freq.Freq.

BPFDespreader

Code B

Freq.Freq.

BPFDespreader

Code A

DS-CDMA System Overview (Reverse Link)

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


Spreading CodeSpreading Code


Cross-CorrelationCross-Correlation

Cross-Correlationbetween Code A and Code B = 6/16

Self-Correlationfor each code is 1.

one data bit duration

Spreading Code A

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

one data bit duration

Spreading Code A

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

Spreading Code A

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

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

Spreading Code B

1 0 01 1 0 0 1 11 0 0 1 0 1 1

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

0+0=01+0=10+1=11+1=0


Preferable CodesPreferable 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


Multiplexing using Walsh CodeMultiplexing using Walsh Code

Code for 00

Code for 01

Code for 10

Code for 11

Data

Modulator

Code for 01

Code for 10

Code for 11

0dtT

Select maximum

value

Code for 00

0dtT

0dtT

0dtT

Demodulator


Synchronous DS-CDMASynchronous DS-CDMA

Forward Link(Down Link)

Synchronous Chip Timing

Ａ

ＢA

A

Signal for B Station(after de-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.


Asynchronous DS-CDMAAsynchronous DS-CDMA

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

Reverse Link(Up Link)

BA

Signal for B Station(after re-spreading)

Big Interference from A station

Asynchronous Chip Timing

Signals from A and B are interfering each other.

A

B


Features of CDMAFeatures of CDMA


Mobile Propagation Environment ･･･ Multi-path Fading

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

Pow

er

path-2

path-2path-3

path-3

path-1

path-1

TimeP

ower


Fading in non-CDMA SystemFading in non-CDMA System

Path Delay

Pow

er

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.

Time

Pow

er

Detected Power

In non-CDMA system, “fading” damages signal quality.

Required signal level


Fading in CDMA System ...Fading in CDMA System ...

• As the CDMA system has high time-resolution, different path delay of CDMA signals can be discriminated.

• Energy from all paths can be summed by adjusting their phases and path delays.

• This is a principle of RAKE receiver.Path Delay

Po

we

r path-1

path-2

path-3

CDMAReceiver

CDMAReceiver

•••

Synchron

ization

Add

er

CODE Awith timing of path-1

Path Delay

Po

we

r path-1

Po

we

r

path-1

path-2

path-3

CODE Awith timing of path-2 Path Delay

Po

we

r

path-2

interference produced by path-2 and path-3

•••


Fading in CDMA System (continued)Fading in CDMA System (continued)

In CDMA system, multi-path propagation improves the signal quality by adopting RAKE receiver.

TimeP

ower

Detected Power

RAKEreceiver

Pow

er

path-1

path-2

path-3

Detected power of CDMA signal will be less fluctuated

by combining all energy


Near-Far ProblemNear-Far Problem

CODE B

CDMATransmitter

DATA B

CODE A

CDMAReceiverCODE A

CDMATransmitter

DATA A

P

• Desired Signal Power = P/Lp-a• Interfered Signal Power = P/Lp-b/G

G: 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.


Power Control...Power Control...

ＡＢ

TimeDe

tect

ed

Po

we

r

from A

from B

• As the propagation losses between BS and MSs are different according to individual communication distances, the received levels at the base station are different from each other when all mobile stations transmit their signals at the same power.

• Moreover, the received level fluctuates quickly due to fading.

• In order to maintain the strength of received signal level at BS, power control technique must be employed in CDMA systems.


Power Control (continued)Power Control (continued)

（（（

②

①

Open Loop Power Control Closed Loop Power Control

estimating path loss

calculating transmission

power

transmitmeasuring received power

transmit receive

decide transmission

power

transmit measuring received power

power control command

about 1000 times per second

①

②


Effect of Power ControlEffect of Power Control

ＡＢ

TimeDet

ect

ed P

ow

er

from MS B from MS A

closed loop

power control for

MS B.

for 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.


Frequency Allocation (1/2)Frequency Allocation (1/2)

In FDMA or TDMA, radio resource is allocated not to be interfered 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 frequency sub-bands.

• In actual situation, it is difficult to allocate the frequency (or time slot) appropriately because of complicated radio propagation and irregular cell allocation.


Frequency Allocation (2/2)Frequency Allocation (2/2)

In the CDMA system, identical radio resource can be allocated among all cells as explained in Introduction.

• Frequency allocation in CDMA is not necessary.

• In this sense, CDMA cellular system is easy to be designed.


Soft Handover (1/2)Soft Handover (1/2)

Handover :•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 Handover :• In FDMA or TDMA cellular system, a new communication link is established after breaking the current communication link by hard handover.

•Communication between MS and BS instantaneously breaks by switching a frequency or a time slot.

Hard handover: make connection (new cell B) after break (old cell A)

switching

Cell B Cell A


Soft Handover (2/2)Soft Handover (2/2)

Σ

Cell B Cell A

Soft handover : break (old cell A) after make connection (new cell B)

BS A and BS B transmit the same signal to the MS simultaneously

• In CDMA cellular system, communication link keeps a connection even in the handover procedure. Because the system does not require the frequency or time slot switching.


ConclusionConclusion

• CDMA is based on the spread spectrum technology which has been used in military field.

• CDMA cellular system has many advantages compared with the FDMA and TDMA cellular systems.

• CDMA system was adopted as the international standard for the 3rd generation mobile communications.

• The number of CDMA users will dramatically increase in the next five years all over the world.

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