002 CBB_T01_E2 Principle and Implementation of CDMA Spread Spectrum-27 [Compatibility Mode]

8/6/2019 002 CBB_T01_E2 Principle and Implementation of CDMA Spread Spectrum-27 [Compatibility Mode]

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CDMA PrincipleCBB_T01_E2

ZTE UniversityCDMA-BSS Team


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Objectives

In this section, the student will learn:

Definitions of CDMA

Spread spectrum modulation

Spreading codes used in CDMA

Vocoding in CDMA


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Multiple Access Technologies

Power

FDMA

Power

CDMATDMA Time Division

Multiple AccessEach user on adifferent windowperiod in time (timeslot)

CDMA Code DivisionMultiple AccessA channel is a uniquecode patternEach user uses the

same frequency allthe time, but mixedwith differentdistinguishing codepatterns

FDMA FrequencyDivision MultipleAccess

Each user on adifferent frequency

A channel is afrequency Power

TDMA

A channel is aspecific time slot ona specific frequency


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Defining Our Terms

CDMA Forward Channel

1.25 MHz Forward Link

CDMA Reverse Channel

1.25 MHz Reverse Link45 or 80 MHz

CDMA CHANNEL

CDMAReverseChannel1.25 MHz

CDMAForwardChannel1.25 MHz

CDMA Code Channel Code channels in the forward link: Pilot, Sync, Paging andForward Traffic channels

Code channels in the reverse link: Access and ReverseTraffic channels


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Main Content

Definitions of CDMA



Vocoding in CDMA


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ORIGINATING SITE DESTINATION

InputData

RecoveredData

Spread Data Stream

What is Spread Spectrum

SpreadingSequence

SpreadingSequence


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SHANON Formula

Where,

C=B*log2(1+S/N)

Spread Spectrum Principles

s capac y o c anne , sB is signal bandwidth, HzS is average power for signal, WN is average power for noise, W

It is the basic principle and theoryfor spread spectrum communications.It is the basic principle and theoryfor spread spectrum communications.


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CDMA Is a Spread-Spectrum System

Spread SpectrumTRADITIONAL COMMUNICATIONS SYSTEM

Slow

InformationSent

TX

Slow

InformationRecovered

RX

NarrowbandSignal

Spread Spectrum Payoff:

Processing Gain

-

FastSpreadingSequence

SlowInformation

Sent

TX

SlowInformationRecovered

RX

FastSpreadingSequence

Wideband Signal


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Power is Spread Over a Larger Bandwidth

1.25 MHz

30 KHz


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Many code channels are individuallyspreadand then added together tocreate a composite signal


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Spreading and De-spreading

ORIGINATING SITE DESTINATION

SpreadingSequence

SpreadingSequence

Input

Data

Recovered

Data

Spread Data Stream


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CDMA Spreading Principle

Using Multiple Codes

InputData

X

RecoveredData

X

X+A X+A+B X+A+B+C X+A+B X+A

Spread-Spectrum Chip StreamsORIGINATING SITE DESTINATION

Multiple spreading sequences can be applied in succession and then reapplied

in opposite order to recover the original data stream. The spreading sequences can have different desired properties.

All spreading sequences originally used must be available in proper

synchronization at the recovering destination.

SpreadingSequence

A

SpreadingSequence

B

SpreadingSequence

C

SpreadingSequence

C

SpreadingSequence

B

SpreadingSequence

A


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Main Content

Definitions of CDMA Spread spectrum modulation

Vocoding in CDMA


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Spreading Codes in CDMA

Spreading codes selection is the key of spreading

Spectrum modulation!

Spreading codes selection is the key of spreading

Spectrum modulation!

prea ng co e c p spee . c s

Spreading code forward linkWalsh code & Short PN

Reverse linkLong PN


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Walsh Code Definition

The Walsh function is named after Walsh, the mathematician who proved it an

orthogonal function in 1923. It is expressed as Walsh (n,t), n for the serial number.

The CDMA system of IS-95 is differentiated with the Walsh function.

Walsh code is an orthogonal square matrix. It is just composed of +1(0) and

1(1).

0110

1100

1010

0000

10

00

0

Hn Hn

H2n = ___Hn Hn

H1 H2 H4


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Walsh Code Function

in Forward Link

SyncPilot

FW Traffic FW Traffic

(for user #1)Paging

or user

FW Traffic(for user #3)


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Short PN: 4-bits register example

1 001

0 011

0 110

1 101

0 100p1 p2 p3 p4

p2 p3

p4

p4

The PN sequences are deterministic and periodic.

The length of the generated string is 2n-1, where n is thenumber of elements in the register

The number of zeroes in the sequence is equal to the number ofones minus 1

0 1011 011

0 111

1 111

1 110

10 0 0

0 010

0 100

1 100

1 000

p4 p5 p2 p3

= 1 4


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Short PN Function

in Forward Link

The short PN Sequences are 32,768 chips long. Short PN is used to distinguish

A B

Up to 64

Code Channels

Up to 64

Code Channels

.

Each Sector in each Base Station is transmitting a Forward Traffic Channelcontaining up to 64 forward code channels.

A Mobile Station must be able to discriminate between different Sectors ofdifferent Base Stations.

The Short PN Sequences are defined for the purpose of identifying sectors of

different base stations. The Short PN Sequences can be used in 512 different ways in a CDMA system.Each one of them constitutes a mathematical code which can be used to identify aparticular sector.


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Long PN:4-bits

shift register example

mask

XOROriginal PNsequence

1 001

0 011

0 110

1 101

0 100

XOR)

New PNsequence

AND AND AND AND

0 1011 011

0 111

1 111

1 110

10 0 0

0 010

0 100

1 100

1 000

Attention:different mask lead to different offset!Attention:different mask lead to different offset!


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The Long PN Sequence

Long Code Register(@ 1.2288 MCPS)

Public Long Code Mask(STATIC)

User Long Code

1 1 0 0 0 1 1 0 0 0 P E R M U T E D E S NAND

=S U M

Each mobile station uses a unique User Long Code Sequence

generated by applying a mask, based on its 32-bit ESN, to the 42-bit Long

Code Generator which was synchronized with the CDMA system during

the mobile station initialization.

(@1.2288 MCPS)


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Long PN Function

in Reverse Link The CDMA system must be able toidentify each Mobile Station that mayattempt to communicate with a Base

Station. A very large number of MobileStations will be in the market.

RV Trafficfrom M.S.

#1837732008RV Trafficfrom M.S.

#1997061104

RV Trafficfrom M.S.

#1994011508

System AccessAttempt by M.S.

#2000071301(on access channel #1)


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Main Content

Definitions of CDMA and How to realize



oco ng n


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Vocoding (A1 & A2 interface)

BTS BSCBSC MSCMSC

Analog voiceVariable Rate PCM


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TrFO (Transcoder Free Operation

AP interface)


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RTO (Remote Transcoder

Operation - AP interface)

002 CBB_T01_E2 Principle and Implementation of CDMA Spread Spectrum-27 [Compatibility Mode]

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