CD Ma Technology

8/6/2019 CD Ma Technology

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CDMA Basics 1

CDMA Technologies

for Cellular Phone System

CDMA Technologies

for Cellular Phone System


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CDMA Basics 2

Contents

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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CDMA Basics 3

Introduction

Introduction

Now when I say mobile technology every one here wouldthought in the back of your mind of GSM and CDMA

technology.

Gsm stands for global system of mobile communication and is

very much older than the cdma technology it uses TDMA (time

division multiplexing) technique. and is famous throughoutEurope, while the CDMA (Code division multiplexing) uses

spread spectrum technology it is famous in American continent

and south Asia specially in South Korea.

In India most of us uses GSM technology and cdma is not

much popular but the cellular company which provide Cdmanetwork in India are Reliance,tataindicom.

Now we discuss the various transmission techniques like

Tdma,Fdma etc.


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CDMA Basics 4

Duplex & Multiple Access Methods

Duplex & Multiple Access Methods


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CDMA Basics 5

Duplex Methods of Radio Links

Duplex Methods of Radio Links

Mobile Station

Base Station

Forward link

Reverse link


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CDMA Basics 6

Frequency Division Duplex (FDD)

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)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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Multiple Access MethodsMultiple Access Methods

Mobile Station

Base Station

Mobile StationMobile Station

Mobile Station

Forward link

Reverse link


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FDMA OverviewFDMA Overview

A A

B B

C CFreq

uency

Time

f2

f1

f0

Now in FDMA system signal from various source are transmitted atdifferent frequency, and at the receiver side the demodulation are done

by separating the signal from different frequency.


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TDMA OverviewTDMA Overview

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

C

A

B

Time

f0

Fre

quency

While in case of TDMA system the transmission is done by accessof time.As the name says time division multiplexing.


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What is CDMA ?What is CDMA ?

Sender Receiver

Code A

A

Code B

B

AB ABC BC

A

Code A

ABC

Time

Fre

quency

BC

B

A

Base-band Spectrum Radio Spectrum

While cdma uses the spread spectrum technology.


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Summary of Multiple AccessSummary of Multiple Access

FDMA

TDMA

CDMA

time

time

time

power

power

pow

er

frequ

ency

frequ

ency

frequency


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Spread Spectrum TechnologySpread Spectrum Technology


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

Direct Sequence (DS)

Modulation

(primary modulation)

Modulation

(primary modulation)

user

data

Spr

eading

(secondarymodulati

on)

Sp

reading

(seconda

r ymodulation)

Tx

Base-band

Frequency

Power

Density

Radio

Frequency

Power

Density

TIME

data rate

10110100

spreading sequence

(spreading code)

Random signal


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Demodulating DS Signals (1/2)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 for

one bit duration

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)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

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 SSFeature 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

de-

modulator

Base-band

Frequency

Power

DensityWith incorrect code

(or carrierfrequency),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-CDMADS-CDMA


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CDMA Basics 19

Freq.Freq.

BPFDespreader

Code B

Freq.Freq.

BPFDespreader

Code A

DS-CDMA System Overview

(Forward link)


(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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CDMA Basics 20

Freq.Freq.

BPFDespreader

Code B

Freq.Freq.

BPFDespreader

Code A


(Reverse Link)


(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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CDMA Basics 21

Spreading CodeSpreading Code


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CDMA Basics 22

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


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CDMA Basics 23

Synchronous DS-CDMASynchronous 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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CDMA Basics 24

Asynchronous DS-CDMAAsynchronous 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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CDMA Basics 25

Features of CDMAFeatures of CDMA


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CDMA Basics 26

Mobile Propagation Environment

Multi-path Fading

Mobile Propagation Environment

Multi-path Fading

The peaks and bottoms of received powerappear, 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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CDMA Basics 27

Fading in non-CDMA SystemFading 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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CDMA Basics 28

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

Because CDMA has high time-resolution,

different path delay of CDMA signals

can 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-1path-2

path-3

CDMA

Receiv

er

CDMA

Receiv

er

Sy

nc

h

roniz

atio

n

A

dde

r

Path Delay

Power

CODE A

with timing ofpath-1

path-1

Power

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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CDMA Basics 29

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

In CDMA system, multi-path propagation improves

the signal quality by use of RAKE receiver.

Time

Power

Detected Power

RAKEreceiv

erLess fluctuation of detectedpower, because of adding all

energy .

Power

path-1

path-2

path-3


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CDMA Basics 30

Near-Far ProblemNear-Far Problem

CODE B

CDMA

Transmitt

er

DATA B

CODE A

CDMA

Receiv

erCODE A

CDMA

Transmitt

er

DATA A

P

Desired Signal Power = P/Lp-a

Interfered Signal Power =

P/Lp-b/(processing gain )

Demodulated

DATAP

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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CDMA Basics 31

Power Control...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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CDMA Basics 32

Power Control (continued)Power Control (continued)

Open Loop Power Control Closed Loop Power Control

estimating pathloss

calculatingtransmission

power

transmitmeasuringreceived power

transmit receive

decidetransmission

power

transmit measuringreceived power

power controlcommand

about 1000 timesper second


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CDMA Basics 33

Effect of Power ControlEffect of Power Control

Time

De

tectedPower

from MS Bfrom MS A

closedl

ooppow

er

control

forMSB

.

forM

SA.

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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CDMA Basics 34

Frequency Allocation (1/2)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 thesimple 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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CDMA Basics 35

Frequency Allocation (2/2)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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CDMA Basics 36

Soft Handoff (1/2)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 B Cell A


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CDMA Basics 37

Soft Handoff (2/2)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 doinghandoff, because switching frequency or time slot is not required.


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ConclusionConclusion

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.

Date post:	07-Apr-2018
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