01 cdma concept

7/22/2019 01 CDMA Concept

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CDMA Concept

Dilshad SDE RTTC TVM

CDMA: Code Division Multiple Access


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Objectives

Evolution of CDMA

Duplex Radio channel

Multiple Access: CDMA, TDMA, FDMA

spread spectrum modulation

CDMA codes: Walsh code, short PN and long PN

Handoff


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2Mbps153.6kbps

CDMA

1x

CDMA

1xEV-DV

CDMA

IS-95

CDMA

1xEV-DO

CDMA

3x 5x

2G 2.5G 3G

9.6kbps

CDMA Technology Evolution

Code Division Multiple Access uses direct sequence spread-spectrum technology to

transmit a number of independent conversations across one or multiple 1.2288MHz

segments of the radio spectrum.


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Duplex Radio Channel

Duplex channel made of two 1.25MHz-wide bands1. BTS to MS ( DOWNLINK)

2. MS to BTS (UPLINK)

45 or 80 MHz

CDMA CHANNELCDMA

ReverseChannel 1.25 MHz

CDMA

ForwardChannel 1.25 MHz

Carrier Frequency

In 800 MHz Cellular these two duplex 1.25 MHz bands are 45 MHz apartIn 1900 MHz they are 80 MHz apart


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MS

Radio Channel Duplexing Technique

BTS

UPLINK

MOBILE

TRANSMIT

DOWNLINK

BTS TRANSMIT

1.25 MHz

1.25 MHz


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800MHz

Uplink: 824- 849 MHz

Downlink: 869- 894 MHz

824 824+25

824+45 824+45+25

1850 1850+60

1850+80 1850+80+60

CDMA Frequency Bands

1900MHz

Uplink: 1850- 1910 MHz

Downlink: 1930- 1990 MHz


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800MHz Uplink =825+0.03N

Downlink =870+0.03N

CDMA Frequency Calculation

N = FA (Frequency allocation)

1900MHz Uplink =1850+0.05N

Downlink =1930+0.05N


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Multiple Access

Each p air of users

enjoy s a dedicated,pr ivate circui t throug h

the transmiss ion

medium , unaware that

the other u sers exist .

Multiple Access: Simultaneous private use of a transmissionmedium by multiple, independent users.

Transmission

Medium


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Multiple Access

FDMA---Different user use different

frequency

TDMA---Different user use different

time slot of one frequency

CDMA---Different user use same

frequency at the same time,but with

different spre ding code


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CDMA Is a Spread-Spectrum System

Traditional technologies try to squeeze the signal into theminimum required bandwidth

Spread SpectrumTRADITIONAL COMMUNICATIONS SYSTEM

SlowInformation

Sent

TX

SlowInformationRecovered

RX

NarrowbandSignal

CDMASPREAD SPECTRUM SYSTEM

FastSpreadingSequence

SlowInformation

Sent

TX

SlowInformation

Recovered

RX

FastSpreadingSequence

Wideband Signal


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0 1 10 1Spreading

1 0

1 0 1 0

Spread Process

Digital Signal Spreading Signal

Spreading Code

1 0 0


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0 1 010 01 1 1 1 001 01 0De-spreading

1 0 1 0

Integrator1 0

De-spread Process

Out put data

De spreading Code

Input spreaded Signal


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Spread Spectrum PrinciplesSHANON Formula

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

Where,

C is capacity of channel, b/s

B is signal bandwidth, Hz

S is average power for signal

N is average power for noise


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Signal Spectrum Before SS

f

Sf

f0

Signal

ff0

Signal Spectrum after SS

Sf

Signal

Signal Spectrum After Decoding

f

Sf

f0

Signal

Noise

f

Signal Spectrum Before Decoding

Sf

f0

Signal

Noise

Spectrum Variation of Spread & De-spread


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SpreadingSequence

A

SpreadingSequence

B

SpreadingSequence

C

InputData

X

X+A X+A+B

ORIGINATING SITE

SpreadingSequence

C

SpreadingSequence

B

SpreadingSequence

A

RecoveredData

X

X+A+B X+A

DESTINATION

CDMA Spreading Principle

Using Multiple Codes

X+A+B+C

Spread-Spectrum Streams


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CDMA CODE CHANNEL


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CDMA Code Channel

All CDMA users separated based on their

digital code, rather than divide the

spectrum based on frequency or time.


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Walsh Code: Forward Code Channels

64 Forward code Channel 4 BTS to MS

traffic channels +overhead channels.

A set of 64 mathematical codes to differentiate the 64

forward code channels. The codes are called Walsh Codes


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Walsh Codes

64 Sequences, each 64 bit long

Each Walsh Code is Orthogonalto all other

Walsh Codes

Two same-length binary strings are

orthogonal if the result of XORing them has

the same number of 0s as 1s


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Walsh function is formed by orthogonal square matrix.It is just

composed of 0 and 1.

creation of Walsh code

0110

1100

1010

0000

10

000

Hn Hn

H2n = ___

Hn Hn


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Orthogonal code

PARALLEL

XOR: all 0s

Correlation: 100%(100% match)

ORTHOGONAL

XOR: half 0s, half 1s

Correlation: 0%(50% match, 50% no-match)

ANTI-PARALLEL

XOR: all 1s

Correlation:100%(100% no-match)

#23

#23

(#23)

#23

#23

#59


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Discriminating Among BTS : short PN code

All BTS having same frequency.

A Mobile Station discriminate between different Sectors of different Base

Stations using short PN code.

These code can be used in 512 different ways. which can be used to identify a

particular sector.

215 bits long

A B

Short PN

Code 1

Short PN

Code 2


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CDMA Spread Code

Walsh Codes

Short PNSequences

Long PNSequences

Type ofSequence

User identity within cells signal- forward traffic

Distinguish Cells & Sectors

Distinguishusers & reverse traffic

Link Function


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Objectives

-- The forward channel

Pilot ;Sync ; Paging and Traffic

-- The reverse channelAccess; Traffic

-- CDMA Call Processing


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IS-95 CDMA Channels


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Pilot Channel

Function: obtains timing information

pilot signal strength used for perform soft handoffs

Characteristic:

transmitted constantly by the base station

use Walsh code 0, (W0)


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Sync Channel

Once a strong pilot channel is located,the mobile

station listen and acquire the corresponding syncchannel for system informat ion.


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Sync Channel

Carries a data stream of essential system identification and

parameter information used by mobiles during system

acquisition stage

Bit rate is 1200 bps

(Acquired Pilot)

Sync Channel


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Paging Channels

There is one paging channel per sector per CDMA carrier

The Paging Channel uses Walsh code 1 up to 7,unusedpaging channels can be used as forward traffic channel

Paging Channel

Used by the base station to

transmit system overhead informationand mobile station-specific messages.


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Reverse Traffic Channels

TO send:

Voice traffic from the subscriber

Response to commands/queries from the base station

Requests to the base station

Reverse Traffic Channel


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CDMA Handoffs

Duringa Call

Idle Handoff

Soft Handoff

Softer Handoff

Hard handoff

While in the

Idle State


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CDMA Idle Handoff

PN 104

PN 108

A

104 108

>3dB


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CDMA Soft Handoff

Soft Handoff:Make-before-break

Cell Site

B

Cell Site

BCell Site

A

CDMA

Cell Site

A


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Rake receiver

a set of four or more receivers (or fingers).

One of the receivers constantly searches for

different multipaths and helps to direct the other

three fingers to lock onto strong multipath signals.

Allows combined reception of up to three different

paths.

Provides both diversity.


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Correlator 1

Correlator 2

Correlator 3

Searcher

Correlator

c

o

m

b

i

n

e

r

CDMA mobile rake receiver

Input from

antenna

Output


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Speech Coding

Speech coding algorithms (digital compression) arenecessary to increase cellular system capacity.

Coding must also ensure reasonable fidelity,

A-to-D

C

O

N

V

E

R

TE

R

64 Kbps

V

O

C

O

D

E

R

Codebook

Instruction

8Kbps


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Variable Rate Vocoding

CDMA uses a superior Variable Rate Vocoder Full rate during speech

Low rates in speech pauses

Increased capacity

Voice, signaling, and user secondary data may be mixed inCDMA frames

VOCODER Internal structure.

Codebook

Pitch

Filter

Format

Filter

Coded Result Feed-back

20ms Sample


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Variable Rate Vocoder

BTS BSC MSC

Analog voice

Variable RatePCM


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Power control

Need effective power control, because of

near-far problem,

fading & varying path loss.

The system requires fast closed loop power control.


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O l l


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Open loop power control

in the reverse link (Mobile to Base)

When the mobile is turned on, it locks on tothe pilot, paging and synch channel.

MS adjust its transmit power according to the

changes in its received power from the BTS

Mobile BTS

Reverse Open Loop

Power Control


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Closed loop power control

1. Closed loop power control in reverse link (Mobile to Base)

After the traffic channel is assigned, the power control shifts to closed

loop control

BTS measure received power once every 1.25ms (800 Hz).

BTS send Power control bit in the next 1.25 ms cycle to increase or

decrease the power by 1db.

Mobile BTS

Signal StrengthMeasurement

Setpoint

orReverse Closed Loop

Power Control


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Questions ???

conclude


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Thank you

01 cdma concept

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