CDMA Concepts 2003

8/9/2019 CDMA Concepts 2003

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CDMA CONCEPTS


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MULTIPLE ACCESS

SPREADING SPECTRUM CODES IN CDMA

CDMA CHANNEL STRUCTURE

POWER CONTROL

DIVERSITY

HAND OFFS


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Copper Local Loop

FSU BTS

ExchangeSubscriber

Wireless Local Loop

ExchangeSubscriber

BSC

TWO METHODS FOR SUBSCRIBER CONNECTION


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WHAT IS MULTIPLE ACCESS ?

NUMBER OF USERS ACCESS AND SHARE

TRANSMISSION MEDIUM

BANDWIDTH AVAILABLE

FOR COMMUNICATION AT THE SAME TIME.


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MULTIPLE ACCESS

MULTIPLE SIMULTANEOUS TRANSMISSIONS.

SHARING FINITE SPECTRUM AMONG LARGE

NO. OF SIMULTANEOUS USERS.

NO PRE-ASSIGNED CIRCUIT TO ANY USER.

ANY USER CAN ACCESS ANY CIRCUIT AND MAY

ACCESS DIFFERENT CIRCUITS FOR DIFFERENT

CALLS.

DEMAND ASSIGNED CIRCUITS ON FIRST COMEFIRST SERVE BASIS.

PRIVACY.

TRUNKING EFFICIENCY; THEREBY

IMPROVEMENT IN SYSTEM CAPACITY.


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MULTIPLE ACCESS METHODS

Frequency Division Multiple Access (FDMA)

FDMA is a multiple access method in which users are

assigned specific frequency bands. The user has sole

right of using the frequency band for the entire call

duration.

Time Division Multiple Access (TDMA)

In TDMA an assigned frequency band shared among

a few users. However, each user is allowed totransmit in predetermined time slots. Hence,

channelization of user is achieved through separation

in time.


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CODE DIVISION MULTIPLE ACCESS

(CDMA)

LARGE NUMBER OF TRANSMISSIONS ARE COMBINED

ON THE SAME RF CHANNEL AT THE SAME TIME BUT ARE

SEPERATED BY CODES.


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MULTIPLE ACCESS TECHNIQUES

F

R

E

Q

U

E

N

C

Y

TIME

FDMA

TDMA

CDMA

ALT CENTRE

ALTTC

RK/TR/ALTTC 7 WILL/ overview


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CDMA: Code Division Multiple Access

CDMA is a method in which users occupy the sametime and frequency allocations, and are channelized by

unique assigned codes. The signals are separated at the

receiver by using a correlator that accepts only signal

energy from the desired channel. Undesired signalscontribute only to the noise.

A CDMA system uses effective power control process.


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Advantages:-

The main advantages of this technology

are:

1. Fast Network deployment.

2. Reduced service interruptions.

3. Low Maintenance & operational cost.

4. Better system coverage flexibility

5. Higher capacity

6. Easy transition to mobile services.


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Salient Features of CDMA

It is an advanced comm. Technology.

It has Anti-jam and security features.

Large capacity as compared to other Technology

like FDMA and TDMA.

It uses spread spectrum technology


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SPREADING SPECTRUMSPREADING SPECTRUM

Shannons Equation

C= W Log (1+S/N)Where C=Capacity (bps)

W=Bandwidth

S=Signal Power

N=Noise Power

Shannons Capacity Equation is basis for spread spectrum.

System with large band width can operate at very low SNR level

& can provide acceptable data rate per user.

Therefore in CDMA- All users uses same 1.25 MHz spectrum.

- Each user has unique Digital code identifier.

- Digital codes separate users to avoid interference.


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Direct Sequence Spread using Walsh code

Consist of 64 orthogonal codes each 64 bits long

Spreads spectrum to 1.2288 M bps from 9.6 Kbps

Channel Capacity

C=W log (1+S/N)

Increasing BW improves Signal Transmission with lower S/N

Frequency

Power

Spectral

Density-------------------------------------------------------------------------------------- Noise Level

Narrow Band Waveform

Spread Waveform


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Spread spectrum principle:

Originally spread spectrum radio technology was

developed for military use to counter the interference

by hostile jamming. The broad spectrum of the

transmitted signal gives rise tospread spectrum. A

spectrum signal is generated by modulating the

radio frequency (RF) signal with a code consisting of

different pseudo random binary sequences, which is

inherently resistant to noisy signal environment.


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A number of spread spectrum RF signals thus

generated share the same frequency spectrum and

thus the entire bandwidth available in the band willbe used by each of the users using same frequency

at the same time.

On the received side only the signal energywith the selected binary sequence code is accepted

and information content is recovered. The other user

signals, whose codes do not match contribute only

the noise and are not de-spread back inbandwidth. This transmission and reception of

signals differentiated by codes using the same

frequency simultaneously by a number of users is

known as code Division Multiple Access (CDMA).


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Techniques as opposed to conventional method of

Frequency Division Multiple Access and Time

Division Multiple Access.

In the fig. It has been tried to explain that how

the base band signal of 9.6 kbps is spread using a

long pseudo-random Noise(PN) source to occupy

entire bandwidth of1.25 Mhz. At the receiving end

this signal will have interference from signals of

other users of the same cell, user different cells and

interference from other noise sources. All thesesignals get combined with the desired signal but

using a correlator and correct PN code, the original

data can be reproduced back.


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CellCell

TXRX

USER INPUT

1 0 0 1 1

10011001100110011001

CODE

USER OUTPUT

1 0 0 1 1

10011001100110011001

CODE

01101001100101100110

User Input 1 0 0 1 1

Spreading

Sequence1001 1001 1001 1001 1001

TX Data 0110 1001 1001 0110 0110

ORTHOGONAL SPREADING:-


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Decoding using correct Orthogonal Function

0110 1001 1001 0110 0110

1001 1001 1001 1001 1001

1111 0000 0000 1111 1111

Rx Data

Correct

Function

1 0 0 1 1


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Decoding using incorrect Orthogonal Function

0110 1001 1001 0110 0110

0101 0101 0101 0101 0101

0011 1100 1100 0011 0011

Rx Data

Incorrect

Function

? ? ? ? ?


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Frequency Reuse

In CDMA reuse patterns are not required.

Subscriber in every cell can use the same frequency

at the same time. Subscriber is discriminated from

another by the assignment of a unique code to every

conversation.

In GSM freq. Reuse pattern of 7 is used.


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C

D

B

A

C

F

G

E B

A

E

G

B

D

C

F E

G

F

C

E

B

D

C

FD

B

A

Frequency Reuse pattern of 7 (FDMA)


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A

AA

A

A

A

A

AA

AA

A

A

A

A

AA

AA

A

A

A

AA

AA

A

A

CDMA Frequency Reuse


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CDMA Channels

Forward Link Channels

Pilot Channel

Sync Channel

Paging Channels

Traffic Channels

Reverse Link Channels

Access Channels

Traffic Channels


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Forward CDMA Channel

(1.23 MHz Channel)

Transmitted by base station)

Pilot

Chan

Sync

Chan

Paging

Ch .1

Paging

Ch .7Traffic

Ch 1

Traffic

Ch N

Traffic

Ch 24

Traffic

Ch 25

Traffic

Ch 55

Traffic

data

PersonalStationPowerContractSub- channel

Example of a forward CDMA channelTransmitted by a base station

W= code channel

CDMA PCS Forward Channels


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Reverse CDMA Channel

(1.23 MHz Channel)

received by base station)

Access

Ch 1

Example of a Logical Reverse CDMA

channels Received at a base station

Addressed by Long code PNs

Access

Ch n

Traffic

Ch 1

Traffic

Ch m

CDMA PCS Reverse Channels


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45 MHz

1.25 MHz 1.25 MHz

80 MHz

1.25 MHz 1.25 MHz

CDMA Channel in PCS band

CDMA channel is 1.25 MHz wide and there is a separation

of 45 MHz in the forward and the reverse channel in case

of cellular band & and 80 MHz in case of PCS band.

CDMA Channel in 800 MHz band


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Frequency Bands

CDMA 82

4-

849 MHz869- 894 MHz

PCS 1850- 1910 MHz

1930- 1990 MHz


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Spreading Codes

cdmaOne systems use two types of code sequences:

Orthogonal sequences (Walsh codes).

Pseudorandom Noise (PN) sequences.

Long codes (242 =4400 Billion)

Short codes (215 =32768)


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Walsh Codes:-

In CDMA the traffic channels are separated byUnique Walsh code. These are

(a) 64 codes of 64 Bit Length.

(b) Forwarded traffic channel Codes.

(c) All codes are orthogonal to each other.

(d) These codes provide Isolation between

multiple signals transmitted by base stations


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The basic concept behind creation of the code is

as follows:

(a)Repeat the function right

(b)Repeat the function below

(c)Invert function (diagonally)

Fig: Seed 0 0 0 0 0 0 0

0 1 0 1 0 1

0 0 1 1

0 1 1 0


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By using this technique we create a set of 64

such codes of 64 bit length which is known as

Walsh codes. All such codes are orthogonal toeach other. The individual subscriber now can

start communication using one of these codes.

These codes are traffic channel codes and are

used for orthogonal spreading of the information in

the entire bandwidth. Orthogonality provides

nearly perfect isolation between the multiple

signals transmitted by the base station.


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LONG CODE

(a) 242

Bits polynomials.(b) Forwarded channel Data (traffic and paging chls)

scrambled.

(c) Provides channelizations for the reverse chls.

(d) This code is unique for every subscriber.

(e) It is known as user address mask or user

identification.

(f) Subscriber are differentiated as no two samecodes are used.


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SHORT CODE

(a) This PN sequence is based on 215

characteristics polynomial.

(b) Differentiates cells and sectors.

(c) Identification cells and sectors.

(d) Consist of codes for I to Q chls.

(e) Each cell uses different codes.


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CDMA CHL ARCHITECTURE

Pilot channel (W0)

The pilot is used by the subs unit to obtain initial

system synchronization and to distinguish cell

sites. Every sector of every cell site has a unique

pilot channel. Transmitted constantly.

Allows the mobile to acquire the system.

Provides mobile with signal strength comparison.

Approximately 20% of the radiated power is inthe pilot.

Has unique PN Offset(215

) for each cell or sector.


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Sync channel (W32)

Used during system Acquisition stage. Sync chlprovides the subs unit with network information

related to cell site identification, pilot transmit

power & cell site PN offset.

Used by mobile to synchronize with the system

Transmits sync message with

- Pilot PN offset - System time

- Long PN code - System ID

- Network ID - Paging chl data rate

Tx at 1200 bps


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PAGING CHLS (W1-W7)

On this chl base station can page the subs unit

and it can send call set-up and traffic chlassignment information.

Means of communication between base to

mobile station.

Paging CHL data Rates can be 2.4,4.8 or 9.6

Kbps.

CDMA assignment has 7 paging CHLs.

Each paging CHL supports 180 pages per set.

Total pages/ CDMA RF chl = 1260


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Provides mobile with

- System Parameter message - Neighbour list

- Access Parameter list - CDMA Channel list

Used by base station to :

- Page mobile - Transmit overhead information

- Assign mobile to traffic channel


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Traffic Channels ( W8-W31 & W33-W63)

The traffic chl carries the actual call. That is, the

voice and control information between the subs unit

& base station.

TX upto 9.6kbps on rate set 1 and upto 14.4kbpson rate set 2.


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Access CHLS.

(a) Provides communication from Mobile to base

station when mobile is not using traffic Chl. The

access chl is used for call origination & for

response to pages, orders & registration

requests. It is paired with corresponding paging

chl.

(b) Each Access CHL use long PN code.

(c) Base station responds to transmission on a

particular Access CHL.(d)Mobile responds to base station message by

emitting on Access CHL.

(e) Tx at 4800bps


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Forward Traffic Channel Generation

9600 bps

4800 bps

2400 bps

1200 bps

Rate set 1

I PN

Convolutional

Encoder & Repetition

Block

interleaver

Long

Code PN decimator decimator

UserAddress

Mask

(ESN)

O PN

Power

Control

bit

19.2 ksps

1.2288

19.2

ksps

R=1/2

Mcps800Hz

Rate set 2

14400 bps

7200 bps

3600 bps

1800 bps

R=3/4

Wt

1.2288

Mcps


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Rake receiver

CDMA mobiles use rake receivers. The rake

receiver essentially 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 multipathsignals.

Allows combined reception of up to three different

paths.

Provides searcher receiver to identify changes in

path characteristics/new cells.

Provides both path diversity and frequency diversity.


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Rake receiver

T0 T1 T2 T3

W3W2W1W0

Summing

Circuit

OUTPUT

ANTENNA

DELAY

TAPS

TAP

WEIGHTS


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A/DSpeech

Encoder

CDMA

ModemRF

Mobile

RF

Channel

RFCDMA

Modem

Speech

DecoderD/A

S(t) S(n) P(n)

S(t) S(n) P(n)

Speaker

Microphone

VOCODER

Disadvantage of Digital Comm. System

Bandwidth expansion of digitally sampled speech

Solution :Variable rate vocoder

S(t): Input analog Speech

S(n): Input digitized speech blocked into 20 msec frames

P(n): Encoded packets every 20 msec representing parameters of speech

such as spectral envelope, pitch, energy and phases

S(n): Reconstructed digital speech

S(t): Reconstructed analog speech


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Hand offs

Softer handoffMulti sector hand off (Intra BTS)

Can have upto 3or 6 sectors involved (same cell)

Voice data is combined at cell and passed as one cell to BSC

Make before break

Soft handoff

Multi-cell Handoff (Inter BTS)

Can have upto 3 cells involved (same FA) Each cell provides voice data to BSC

Voice data is selected at SVC to vocoder in BSC

Make before break


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CDMA Capacity

W/R 1 1N= -------*-----*--------*n *gEb/Io d 1+f

Where

N= calls per sector

W= Spread spectrum Bandwidth (1.25 MHz)R= data rate (9.6 kbps or 14.4 kbps)

Eb/Io= Bit energy/ other user interference density (7dB)

d= Voice activity factor (0.4)

f= other interference/ same interference (0.6)

n= loading factor (0.8)

g= reduction for variable power (0.85)

N= 27 users per sector for R=9.6Kbps

18 users per sector for R=14.4Kbps


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Power control

CDMA will not work without an effective power control,

because of the near-far problem, fading & varying path

loss.

The system requires fast closed loop power control for

Raleigh fading.

Requires wide dynamic range open loop power control to

handle variations in path losses in different locations.

Open loop power control:

An original estimate is made by the mobile. Mobile

adjusts its trans power according to changes in its received

power from the base station.


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Closed loop power control:

Base station provides rapid correction to the mobiles.

Compares with the threshold value.

Takes decision for increasing or decreasing the power.

Commands the mobile to adjust the output power

accordingly.

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Documents

CDMA Concepts 2003