1

CDMA OVERVIEW

• MULTIPLE ACCESS

• SPREADING SPECTRUM

• CODES IN CDMA

• CDMA CHANNEL STRUCTURE

• POWER CONTROL

• DIVERSITY

• HAND OFFS

Copper Local Loop

FSU BTS

ExchangeSubscriber

Wireless Local Loop

ExchangeSubscriber

BSC

TWO METHODS FOR SUBSCRIBER CONNECTIONTWO METHODS FOR SUBSCRIBER CONNECTIONTWO METHODS FOR SUBSCRIBER CONNECTIONTWO METHODS FOR SUBSCRIBER CONNECTION

WHAT IS MULTIPLE ACCESS ?

NUMBER OF USERS ACCESS AND SHARE

• TRANSMISSION MEDIUM

• BANDWIDTH AVAILABLE

FOR COMMUNICATION AT THE SAME TIME.

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 COME FIRST SERVE

BASIS.

� PRIVACY.

�TRUNKING EFFICIENCY; THEREBY IMPROVEMENT IN SYSTEM

CAPACITY.

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 to transmit 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 CHANNELAT THE SAME TIME BUT ARE

SEPERATED BY “CODES”.

MULTIPLE ACCESS TECHNIQUES

F

R

E

Q

U

E

N

C

Y

TIME

FDMA

TDMA

CDMA

RK/TR/ALTTC 7 WILL/ overview

CDMA: Code Division Multiple CDMA is a method

in which users occupy the same time and frequency

allocations, and are channelized by unique assigned

codes. The signals are separated at the receiver by using

a correlator that Access

accepts only signal energy from the desired channel.

Undesired signals contribute only to the noise.

A CDMA system uses effective power control process.

Advantages:-

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.

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.

• Better use of the multipath.

• Frequency Reuse.

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.

3

C

D

B

AC

F

G

EB

A

E

GB

D

C

FE

G

F

CE

B

D

C

FD

B

A

Frequency Reuse pattern of 7 (FDMA)

A

A

A

AA

A

A

AA

A

A

AA

A

A

AA

A

A

AA

A

A

A

AA

A

A

CDMA Frequency Reuse

Frequency Bands

CDMA 824- 849 MHz

869- 894 MHz

PCS 1850- 1910 MHz

1930- 1990 MHz

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

SPREADING SPECTRUMSPREADING SPECTRUM

Shannon’s Equation

C= W Log (1+S/N)

Where C=Capacity (bps)

W=Bandwidth

S=Signal Power

N=Noise Power

• Shannon’s 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.

SPREAD SPECTRUM TECHNIQUES

1. Frequency Hopped Spread Spectrum:

Spreading can also be achieved by hopping the narrowband

information signal over a set of frequencies. The type of

spreading can be classified as fast or slow depending upon

the rate of hopping to the rate of information.

2. Direct Sequence Spread Spectrum:

The information signal is inherently narrowband, on the

order of less than 10KHz. The energy from this narrowband

signal is spread over a much larger bandwidth by

multiplying the information signal by a wideband spreading

code. DSS technique is used in the IS-95 CDMA cellular

system.

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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

PROCESSING GAIN

One of the major advantages with an SS system is its

robustness to interference. The system processing

gain Gp quantifies the degree of interference

rejection. The system processing gain is the ratio of

RF bandwidth to the information rate:

Gp =W/R

=1.2288x 106 /9.6 x 103

=128

dBgain =10log10 128

=21

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

to”spread spectrum”.

A spectrum signal is generated by modulating the radio

frequency (RF) signal with a code consisting of differentpseudo random binary sequences, which is inherently

resistant to noisy signal environment.

A number of spread spectrum RF signals thus

generated share the same frequency spectrum and thus the

entire bandwidth available in the band will be used by each

of the users using same frequency at the same time.

On the receiver side only the signal energy with the

selected binary sequence code is accepted and information

content is recovered. The other user signals, whose codesdo not match contribute only the noise and are not “de-

spread” back in bandwidth.

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).

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 of

1.25 Mhz.

At the receiving end this signal will have interference from

signals of other users of the same cell, user different cellsand interference from other noise sources. All these signals

get combined with the desired signal but using a correlator

and correct PN code, the original data can be reproduced

back.

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

Decoding using incorrect Orthogonal Function

0110 1001 1001 0110 0110

0101 0101 0101 0101 0101

0011 1100 1100 0011 0011

Rx Data

Incorrect

Function

? ? ? ? ?

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)

Walsh Codes:-

In CDMA the traffic channels are separated by

Unique “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

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

By using this technique we create a set of 64such codes of 64 bit length which is known asWalsh 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 areused for orthogonal spreading of the information in

the entire bandwidth. Orthogonality providesnearly perfect isolation between the multiplesignals transmitted by the base station.

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PN Code Generation

- PN Codes are generated from prime polynomials using

modulo-2 arithmetic.

- State machines generating PN Codes consists of shift

registers & XOR gates.

- The length of the PN Code is equal to 2 -1 ( N= no. of shift

registers).

0out

0 1

Output will be a 7-digit sequence that

repeats continually 1001011

N

0 0 1 1

0 1 1 1

1 1 1 1

1 0 0 0

0 1 0 0

1 0 1 11 1 0 0

Sequence = 1001011 1001011

PN offset (Masking)

- Masking will cause the generator to produce the same

sequence but offset in time.

- Masking provides the shift in time for PN codes.

- Different masks corresponds to different time shifts.

- ESN are used as masks for users on the traffic channels.

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 same codes are used.

(g) Repeats every 41 days (at a clock rate of 1.2288

Mcps)

• SHORT CODES

(a) This PN sequence is based on 215

characteristics polynomial.

(b) Differentiates cells and sectors.

(c) Identifies cells and sectors.

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

(e) Each cell uses different codes.(f) Repeats every 26.67 msec (at a clock

rate of 1.2288 Mcps)

CDMA Channels

Forward Link Channels

Pilot Channel

Sync Channel

Paging Channels

Traffic Channels

Reverse Link Channels

Access Channels

Traffic Channels

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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 in the pilot.

• Has unique PN Offset(215 ) for each cell or sector.

Sync channel (W32)

Used during system Acquisition stage. Sync chl provides 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

PAGING CHLS (W1-W7)

On this chl base station can page the subs unit and it can send call set-up and traffic chl assignment 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

• 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

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 14.4kbps on rate set 2.

Access CHLS.

(a) Provides communication from Mobile to basestation when mobile is not using traffic Chl. The

access chl is used for call origination & forresponse to pages, orders & registrationrequests. It is paired with corresponding pagingchl.

(b) Each Access CHL use long PN code.

(c) Base station responds to transmission on aparticular Access CHL.

(d)Mobile responds to base station message byemitting on Access CHL.

(e) Tx at 4800bps

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cdmaOne Modulation

A/D Vocoder

FEC

Spreading code

generatorSpreader PSK

Information

Information Bits

Code symbols

Chips

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

User

Address

Mask

(ESN)

O PN

Power

Control

bit

19.2 ksps

1.2288

19.2

ksps

R=1/2

Mcps800bps

Rate set 2

14400 bps

7200 bps

3600 bps

1800 bps

R=3/4

Wt

1.2288

Mcps

64:1 24:1

Reverse Traffic Channel Generation

9600 bps

4800 bps

2400 bps

1200 bps

Rate set 1

I PN

Convolutional

Encoder & Repetition

Block

interleaver

Long

Code PN

User

Address

Mask

(ESN)

O PN

28.8 ksps

1.2288

R=1/3

Mcps

Rate set 2

14400 bps

7200 bps

3600 bps

1800 bps

R=1/21.2288

Mcps

Data

Burst

Rand.

Orthogonal

Modulation

307.2

KHz

Rake receiver

CDMA mobiles use rake receivers. The rakereceiver essentially a set of four or more receivers(or fingers). One of the receivers constantlysearches for different multipaths and helps to direct

the other three fingers to lock onto strong multipathsignals.

• Allows combined reception of up to three differentpaths.

• Provides searcher receiver to identify changes inpath characteristics/new cells.

• Provides both path diversity and frequency diversity.

Correlator 1

Correlator 2

Correlator 3

Searcher

Correlator

c

o

m

b

i

n

e

r

CDMA mobile rake receiver

Rake receiver

T0 T1 T2 T3

W3W2W1W0

Summing

Circuit

OUTPUT

ANTENNA

DELAY

TAPS

TAP

WEIGHTS

9

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

Hand offs

Softer handoff

Multi 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

CDMA Capacity

W/R 1 1N= -------*-----*--------*n *g

Eb/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

Power control

MS1

MS2

d

12

1

21

==

=

R

R

RR

P

P

I

C

PP

P

f1.25 MHz

1RP

2RP

1TP

2TP

Ideal CDMA System

MS1

MS2

d

( )

16

1

16

1

2/

1

2

1

21

142

==

⋅≈

⋅=

R

R

RR

RR

P

P

I

C

PP

Pd

P

P

f1.25 MHz

12 16RR

PP =

1TP

2TP

d/2

Near-Far Effect PC Strategies• UPLINK (Reverse Link) PC

Controls the mobile’s transmit power

– Open-Loop PC

Mobile measures received power and adjusts its transmit power

accordingly. (Note: 85dB dynamic range, rapid response is in

µsec)

– Closed-Loop PC

BS measures received power from all mobiles and

simultaneously commands the individual mobiles to raise or

lower transmit uplink power so that the received SNR from all

MSs at the BS is the same (800 commands/sec, + 1dB)

• Closed Outer-loop PC

• Closed Inner-loop PC

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• DOWNLINK (Forward Link) PC

Base station periodically reduces transmit power

(±0.5 dB every 15-20 ms) to mobile until mobile

senses increasing error rate. (Note: Dynamic range is

about ±6 dB around nominal transmit power)

PC Strategies (cont.) PC Strategies (contd)

• Intracell Interference

– Undesired CDMA signals appear as noise at the receiver

output

– With PC, the equivalent Eb/N0 for CDMA receiver is

given by

c

b

eq

b

EMN

E

N

E

⋅−+=

)1(0,0

c

b

c

b

eq

b

RPM

RP

EM

E

N

E

/)1(

/

)1(,0 −=

−≈

Processing Gainb

c

eq

b

R

R

MN

E⋅

−≈

)1(

1

,0

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.

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.

=== 0 ===

Power control in CDMA

• There are basically two methods of power control in CDMA

– Open loop power control: This is purely a mobile unit function. It gives open estimation. This is done only during the initial stage as soon as the mobile is turned on.

– Closed loop power control:This involves both, the Base station and the mobile unit and gives the closed loop power correction.

Open loop power control

• When the mobile is turned on, it locks on to

the pilot, paging and synch channel.

• There is no traffic channel assigned to the

mobile and hence no closed loop.

• The mobile Tx power will be inversely

proportional to the pilot strength received.

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Open loop power control

• The first access probe is sent at a nominal power given by:

– (Pt)mobile=-(Prx)-73+ Nom_pwr +Init_pwr+ correction

– Init_pwr= its range is –16 to +15 db with 0db nominal value

– Nom_pwr= correction required(range –8 to +7db with 0db nom value)

– For example,if the nominal rec. level is –90dbm, then the mobile Tx power is +17dbm without any correction.

– If BTS does not responds,then pwr_step=0-7db

Closed loop power control

• After the traffic channel is assigned, the power

control shifts to closed loop control

• Reverse channel has got 16 power control

groups of 1.25ms in one 20ms frame.

• BTS receives the mobile receive power once

every 1.25ms and BTS send Power control bit

in the 2nd next 1.25 ms cycle to increase(0) or

decrease(1) the power by 1db.

•Thanks