CDMA & IS-95

Pulsetone Industries 1

A MaheswaranPulsetone Industries

[email protected]

CDMA Basics & IS-95 System


CDMA Basics


What is CDMA?

CDMA is Code Division Multiple Access Also called Direct Sequence Spread Spectrum

(DS-SS) Belongs to broader communication systems

called Spread Spectrum This is a wideband system having many

advantages Immune to narrow band interferences Exploits multipath propagation Better handover due to soft handover Increased capacity (users / sq km / MHz)


Understanding Spread Spectrum

Normally RF bandwidth is conserved In spread spectrum communication bandwidth is

deliberately increased Evolved out of military communication

• In avoiding detection and jamming

• For preventing eavesdropping There are 2 types of Spread Spectrum

Freq. Hopped SS (FH-SS) – Carrier frequency is changed periodically Bandwidth remains the same

Direct Sequence Spread Spectrum (DS-SS) – Suitable for data transmission & original data spread manifold (say 8 to 1024 times) - This will be our focus


Direct Sequence Spreading

Here, the binary message bit sequence is multiplied (Ex-Ored) with a bipolar (binary) code (chip) sequence

If L chips multiply every bit, then the BW of the message seq. increases L times

Usually we choose L of the form L = 2k

The Coding (or Spreading) Gain of the system is 3k dB

Tc

Tb


DS-SS Transmitter

Spread Spectrum Transmitter

RR

Information

Transmitted Signal

freq.WW

Processing Gain = W/R >> 1Processing Gain = W/R >> 1

Spreading Code

freq.


DS-SS Receiver

Information

freq.

Spread Spectrum Receiver

RR

Received Signal

Despreading Code

freq.WW


Interference Rejection In SS

Frequency Frequency

SpectralDensity

SpectralDensity Interfering

signal

Desiredsignal

Desiredsignal

Interferingsignal

a) At the SS receiver input b) Correlator output after despreading

Proc.Gain


DS-SS techniques useful for multi access purpose Same wide band used by different users Spread sequences (or codes) of users to be different

and mutually orthogonal This is called Code Division Multi Access

• Orthogonal codes ensure signals do not interfere with each other

Code used for despreading at the receiver has to be exactly the same

• Otherwise decorrelation occurs• Orthogonal codes need to generated by simple

process

Using DS-SS For Multiple Access


Spreading gain allows weaker signals to be

received without errors

Being a wide band signal, this is immune to

multipath fading and narrow band interference

Many orthogonal codes are available

Walsh Codes

PN Sequence – Long codes

PN Sequence – Short codes

Orthogonal codes allow efficient means of

sharing a given RF spectrum by different users

Using DS-SS For Multiple Access


Properties Of Orthogonal Codes

Orthogonal property Two codes C1 & C2 having a periodicity over T are

orthogonal, if and only if

C1 * C2 = 0 (over the period T) Walsh codes derived from Hadamard matrixes have good

properties for use as orthogonal codes Hadamard matrices are square matrices with n x n binary

elements All rows of this matrixes are mutually orthogonal, (if we

consider an agreement as having a weight of +1 and disagreement as having a weight of –1)


Hadamard Matrices

0 0

0 1H2 =0H1 =

A 2n x 2n Hadamard Matrix can be generated by following the recursive procedure

H4 =

0 0 0 0

0 1 0 1

0 0 1 1

0 1 1 0

H2N =HN

HN

HN

HN

All 2N rows of matrix are mutually orthogonal

H2N


Walsh Codes

The rows of Hadamard matrix are used as code words and these are called Walsh codes

Walsh codes are extensively used in CDMA systems both for spreading and modulation In IS-95 system, 64 x 64 Walsh codes are used for

spreading on forward link and for modulation on reverse link

• When it is used for spreading, the spreading factor is 64

• When it is used for modulation, the spreading factor is (64 / 6) since 6 bits will be represented

by 64 chips


Using Walsh Code For Spreading

Walsh Code Generator (Wi )

1.2288 Mcps

+

Traffic data of ith user (data rate of 19.2 kbps)

Output after spreading

In every symbol time (1 bit), we have 64 Walsh chips Wi

or inversion of Wi being sent depending upon whether the bit is 0 or 1


Using Walsh Code For Spreading (Contd.)

Though Walsh codes have excellent properties by way of orthogonality, the transmitter and receiver need to have a perfectly synchronised copies – otherwise orthogonality is not guaranteed

If {0 0 1 1} is used as code word, then this can result in {0 1 1 0}. Hence Walsh code is used for spreading on forward link where perfect synchronisation can be assured

0 0 0 0

0 1 0 1

0 0 1 1

0 1 1 0


Walsh Code

Chip # 1

Chip # 2

Chip # 4

Chip # 5

Chip # 6

W01 1 1 1 1 1 1 1

W11 -1 1 -1 1 -1 1 -1

W21 1 -1 -1 1 1 -1 -1

W31 -1 -1 1 1 -1 -1 1

W41 1 1 1 -1 -1 -1 -1

W51 -1 1 -1 -1 1 -1 1

W61 1 -1 -1 -1 -1 1 1

W71 -1 -1 1 -1 1 1 -1

Chip # 3

Chip # 7

Chip # 8


Using Walsh Code For Spreading & Multiaccess

For demonstrating CDMA (using DS-SS techniques for multi access purpose) we will do the following:

Spreading (User1)

Spreading (User2)

Despreading (User2)

Despreading (User1)

User1 Data (0101)

User2 Data (0011)

W1

W5

W1

W5

Decoded User1 Data

(0101)

Decoded User2 Data

(0011)Code Division Multiplexed

Output


User1 Data 1 1 1 1 1 1 1 1 -1 -1 -1 -1 -1 -1 -1 -1

W1 1 -1 1 -1 1 -1 1 -1 1 -1 1 -1 1 -1 1 -1

Output1 1 -1 1 -1 1 -1 1 -1 -1 1 -1 1 -1 1 -1 1

User2 Data 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1

W5 1 -1 1 -1 -1 1 -1 1 1 -1 1 -1 -1 1 -1 1

Output2 1 -1 1 -1 -1 1 -1 1 1 -1 1 -1 -1 1 -1 1

Sum 2 -2 2 -2 0 0 0 0 0 0 0 0 -2 2 -2 2

W1 1 -1 1 -1 1 -1 1 -1 1 -1 1 -1 1 -1 1 -1

Product 2 2 2 2 0 0 0 0 0 0 0 0 -2 -2 -2 -2

Total = +8 = -8

User1 Data (+8/8 = +1 >0) 0 (-8/8 = -1 <0) 1

W5 1 -1 1 -1 -1 1 -1 1 1 -1 1 -1 -1 1 -1 1

Product 2 2 2 2 0 0 0 0 0 0 0 0 2 2 2 2

Total = +8 = +8

User2 Data (+8/8 = +1 >0) 0 (+8/8 = +1 >0) 0


User1 Data 1 1 1 1 1 1 1 1 -1 -1 -1 -1 -1 -1 -1 -1

W1 1 -1 1 -1 1 -1 1 -1 1 -1 1 -1 1 -1 1 -1

Output1 1 -1 1 -1 1 -1 1 -1 -1 1 -1 1 -1 1 -1 1

User2 Data -1 -1 -1 -1 -1 -1 -1 -1 -1 -1 -1 -1 -1 -1 -1 -1

W5 1 -1 1 -1 -1 1 -1 1 1 -1 1 -1 -1 1 -1 1

Output2 -1 1 -1 1 1 -1 1 -1 -1 1 -1 1 1 -1 1 -1

Sum 0 0 0 0 2 -2 2 -2 -2 2 -2 2 0 0 0 0

W1 1 -1 1 -1 1 -1 1 -1 1 -1 1 -1 1 -1 1 -1

Product 0 0 0 0 2 2 2 2 -2 -2 -2 -2 0 0 0 0

Total = +8 = -8

User1 Data (+8/8 = +1 >0) 0 (-8/8 = -1 <0) 1

W5 1 -1 1 -1 -1 1 -1 1 1 -1 1 -1 -1 1 -1 1

Product 0 0 0 0 -2 -2 -2 -2 -2 -2 -2 -2 0 0 0 0

Total = -8 = -8

User2 Data (-8/8 = -1 <0) 1 (-8/8 = -1 <0) 1


Using Walsh Code For Modulation

Walsh code is used in reverse traffic channel of IS-95 for modulating user traffic data

In everys symbol time (6 bits), we send chips of a particular Walsh code depending upon the 6 bit combination

Modulator using

Walsh Code

Traffic data of ith user (data rate of 28.8 kbps)

Output after Modulation

(307.2 kcps)

SymbolGenerator

4.8 ksps (6 bits / symbol)


Using Walsh Code For Modulation (Contd.)

This improves reception at the base station – detecting forward link is more difficult due to Near-Far problem and non-coherent detection

Modulation using orthogonal Walsh codes enhances the decision making algorithm at the receiver and is computationally efficient

We can view this Walsh modulation as a form of block error correcting code with (n,k) = (64,6)

with dmin = 32 (in fact the distance between any

code word is 32)


Using Walsh Code For Modulation

Data rate k bits per sec

Rate k/3 symbols per sec (3 bits / Symbol)

ModulationUsing Walsh

Codes

Serial to Parallel

Converter

Modulated output (k*8/3 chips per sec)

{W0, W1, W2, W3,W4, W5 , W6, W7}


Modulation Scheme

Symbol Bits Walsh code chosen for modulation

D2 D1 D0

0 0 0 W0

0 0 1 W1

0 1 0 W2

0 1 1 W3

1 0 0 W4

1 0 1 W5

1 1 0 W6

1 1 1 W7


Illustrating Modulation Using Walsh Code

Bit Sequence to be transmitted (101011001110) Converting to symbols with 3 bits / symbol

[101 011 001 110] Modulating with Walsh codes as per table yields

{W5 W3 W1 W6}

Corresponding chips are • {1 -1 1 -1 -1 1 -1 1

1 -1 -1 1 1 -1 -1 1 1 -1 1 -1 1 -1 1 -1 1 1 -1 -1 -1 -1 1 1}


Illustrating Modulation Using Walsh Code (Contd.)

CorrelationReceiver

CorrelationReceiver

CorrelationReceiver

CorrelationReceiver

CorrelationReceiver

CorrelationReceiver

CorrelationReceiver

CorrelationReceiver

Threshold Decision

Maker

W0

W2

W4

W6

W7

W1

W3

W5

Received Chips

Decoded Symbol



Demodulation is done by correlating the received chips with various Walsh chips and finding the match with maximum likelihood decoding

Taking the chips in the first symbol {1-11-1-11-11} and doing this for W0

1 -1 1 -1 -1 1 -1 1

1 1 1 1 1 1 1 1

1 -1 1 -1 -1 1 -1 1

= 0 (Minimum matching)

• For W11 -1 1 -1 -1 1 -1 1

1 -1 1 -1 1 -1 1 -1

1 1 1 1 -1 -1 -1 -1




For W2 1 -1 1 -1 -1 1 -1 1

1 1 -1 -1 1 1 -1 -1

1 -1 -1 1 -1 1 1 -1


1 -1 1 -1 -1 1 -1 1

1 -1 -1 1 1 -1 -1 1

1 1 -1 -1 -1 -1 1 1


1 -1 1 -1 -1 1 -1 1

1 1 1 1 -1 -1 -1 -1

1 -1 1 -1 1 -1 1 -1


•For W3

•For W4



For W5 1 -1 1 -1 -1 1 -1 1

1 -1 1 -1 -1 1 -1 1

1 1 1 1 1 1 1 1

= 8 (Maximum matching)

1 -1 1 -1 -1 1 -1 1

1 1 -1 -1 -1 -1 1 1

1 -1 -1 1 1 -1 -1 1


1 -1 1 -1 -1 1 -1 1

1 -1 -1 1 -1 1 1 -1

1 1 -1 -1 1 1 -1 -1


•For W6

•For W7


Other Spreading Codes

CDMA systems use multiple spreading, each spreading serving a different purpose

So we need many classes of spreading codes User specific codes: A large number of mobiles

need to be allotted spreading codes and these have to be administered & synchronised

Station Specific codes: On the forward link we need to spread the combined signal to have a station specific spreading to provide isolation between transmissions of different base stations

Scrambling codes: Not all Walsh codes generate wide band signals (like W0) we need to scramble the data so that the resulting signal is truly wideband


Properties Of Spreading Code Desired Randomness Properties

P1: Balance Property• Relative frequencies of occurrence of 1’s and

0’s should be 1/2

P2: Run Length Property• Run lengths of 1’s and 0’s are as expected in

a coin-flipping experiment• 1/2 of all run lengths are unity,• 1/4 of the run lengths are 2, • 1/8 of the run lengths are 3, and so on

P3: Delay and Add Property• Equal number of agreements and

disagreements between a sequence and its shifted version


PN Sequences

Psuedo-random Noise (PN) Sequences A deterministically generated sequence

that `nearly’ satisfies properties P1 to P3 , within extremely small discrepancies

Maximum Length Shift Register (MLSR) generated sequences Are PN sequences which nearly satisfy P1 to

P3 Also called as m-sequence, where m is the

number of shift registers used to generate the sequence

Period of an m-sequence P, is given by

12 mP


MLSR Sequence Generator

Generating Function, G(D), is given by[G(D) is the generated sequence]

Characteristic polynomial, f(D), is given by[ƒ(D) gives the tap connections of Seq Gen]

D: Delay

operator

r

i

iiDcDf

1

1)(

0

)(n

nnDaDG

r

iinin aca

1

1c 2c rc

D D D rna 2na1na

No connection

Connection

,1

,0ic


3-stage MLSR Sequence Generator

+

D D D

Sequence No X0 X1 X2

0 0 0 11 1 0 02 0 1 03 1 0 14 1 1 05 1 1 16 0 1 17 0 0 1

PN Sequence Output

Clock


Generating PN Sequence With Offset

Initial State Output Sequence

0 0 1 1 0 0 1 0 1 1

1 0 0 0 0 1 0 1 1 1

0 1 0 0 1 0 1 1 1 0

1 0 1 1 0 1 1 1 0 0

1 1 0 0 1 1 1 0 0 1

1 1 1 1 1 1 0 0 1 0

0 1 1 1 1 0 0 1 0 1

PN Sequences with known offset can be generated by starting with an Initial State (seed)PN codes are generated unique to mobiles by using ESN as seed at both MS and BS


Autocorrelation Function Of PN Sequence

0 0 1 1 1 1 0 0 1 1 1 0 1 0 0 1 1 1 0 1 0 0 10

1 PN Period = N Tc

BinaryPN Seq.

Tc

111

1/N

Tc Tc

ACF

cT

For large N, 01

N


Near-Far Problem

CDMA base station receives signals from all mobile stations All of them transmit on the same wideband

channel with only channelisation codes / PN long codes to discriminate the signals

If we assume that all of them transmit with same power

• The signal from far off mobile will be comparatively weaker than the signal from a near by mobile

• This has to be detected in the presence of a stronger signal from a nearby mobile


Near-Far Problem (Contd.)

This problem is called “Near-Far” Problem and this problem gets further compounded by fading and other short term variations that take place due to the channel

Solution is to tightly control the power transmitted by all mobiles so that they are seen with equal power at the base station

• This is done by closed loop power control whereby the mobiles are asked to vary power continuously to meet the target


Rake Receiver Principles

Multipath creates ISI (inter symbol interference) problem in conventional data transmission However, in the case of CDMA the chip rate

being much is handled differently by constructively combining the multipath signals to improve signal to noise ratio

This is done in RAKE receiver where a separate correlator (called RAKE finger) is assigned for each multipath signal

Typically MS has 4 RAKE fingers and out of these 3 are used for combining signals and 1 is used as a searcher


Rake Receiver Principles (Contd.)

DEMOD

Code Generator

Input Data

MOD 2

3

1 a1

a2

a`3

a3

a`2

a´1

+

+

+

+ Output data

c(t-1)

c(t-3)

c(t-2)

RAKE receiver

Multipath channel


Rake Receiver Principles (Contd.) RAKE fingers are used in the Mobile Rx for

combining multipath components 3 fingers for tracking and demodulating

upto 3 different multipath signals of Forward channel

1 searcher for searching and estimating signal strength on different pilots, coarse timing of different multipaths• to select desired (strongest) base station in

idle mode• to provide hypothesis testing and coarse

timing estimation• to generate pilot strength information

messages during traffic mode to enable Handoff


Power Control On Reverse Link

Transmitter

Receiver

Gain

Duplexer

Open Loop Power Control

Transmitter

Receiver

Gain

Duplexer

Transmitter

Receiver

Duplexer

Base StationMobile Station

Closed Loop Power Control

Control path for closed loop power control


Power Control

To combat the effect of fading, shadowing and distance losses

Transmit only the minimum required power to achieve a target link performance (e.g., FER) Minimizes interference Increases battery life

Forward Link Power Control To send enough power to reach users at cell

edge Reverse Link Power Control is very critical

To overcome “near-far” problem in DS-CDMA


Handoffs In IS-95 CDMA

Types of Handoff Soft Handoff

• Mobile commences communication with a new BS without interrupting communication with old BS

• same frequency assignment between old and new BS

• provides different site selection diversity Softer handoff

• Handoffs between sectors in a cell CDMA-to-CDMA Hard Handoff

• Mobile transmits between two base stations with different frequency assignment


Micro Vs Macro Diversity

Diversity principles are used to improve RF signal quality This is based on the fact that statistical properties

of two or more paths will be uncorrelated • When one path is experiencing deep fade, the

other one is unlikely to experience deep fade In space diversity we may receive through 2

antennas separated in space and combine the signals to improve the performance

We call this Micro Diversity • RAKE receiver combining mulipaths from the

same BS can also be called some form of micro diversity


Micro Vs Macro Diversity (Contd.)

What we do during soft hand off is Macro Diversity, where 2 or more BSs are beaming the same signal toward the same MS

The received signals are combined in RAKE receiver

Resulting signal will be much better than any individual one

Transmission by MS is received by all BSs and the best signal received is selected at the BSC / MSC

Macro diversity is a boon in CDMA system


Handoffs In IS-95 CDMA (Contd.)

Basis for Handoff MS does measurements on Pilot channels on

serving base station + other base stations• Search finger in RAKE receiver helps Ms• Information about neighbouring PN offsets

provided by BS in System_Parameter helps MS classifies the Pilots into 4 categories

• Active Set• Candidate Set • Neighbour Set• Remaining Set

These sets are dynamically updated based upon the measurements


Handoffs In IS-95 CDMA (Contd.)

Pilot Sets Active Set

• Pilots associated with Forward Link traffic channels assigned to the mobile in soft handoff

Candidate Set• Pilots that are not in Active Set but are

received by the mobile with sufficient strength

Neighbour Set• Pilots not in Active or Candidate Set but are

likely candidates for handoff Remaining Set

• Set in the current system on current freq assignment, excluding the above 3 sets


Soft Handoff Architecture

Mobile

BSC BSC

BTS BTSBTS

New LinkOld Link

MSCTo other switch

Energy measurements are made at the mobile

Frame Selection:MSC selects the bit stream with lower error rate

BTS


Micro Vs Macro Diversity (Contd.)

Time or Distance

EC/It

Pilot 1Pilot 2

Active Set Total EC/It

Dynamic Soft Handoff Region


Handoff Example

Time

PilotStrength

(1)

T_ADD

T_DROP

(2) (3) (4) (5) (6) (7)

NeighbourSet

CandidateSet

Active Set

T_TDROP

NeighbourSet


Handoff Example (Contd.)

(1) Pilot strength exceeds T_ADD. Mobile sends a Pilot Strength Measurement Message (PSMM) to base station and transfers pilot to the Candidate Set

(2) Base station sends a Handoff Direction Message (HDM)

(3) Mobile transfers pilot to Active Set and sends Handoff Completion Message (HCM)

(4) Pilot strength drops below T_DROP. Mobile starts handoff drop timer

(5) Handoff drop timer expires. Mobile sends a PSMM

(6) Base station sends a HDM(7) Mobile moves pilot from Active Set to Neighbour

Set and sends a HCM


Recap Of CDMA

Being a wideband system, this is immune to multipath propagation

RAKE receiver exploits multipath to improve reception

No frequency planning is required since the same frequency can be used in nearby cell also

Offers advantage of soft capacity and soft handoff

Suffers form “Near-Far” problem

Tight closed loop power control is required to overcome this problem

Offers more capacity than FDMA or TDMA systems


Advantages of CDMA

In mobile environment multi path propagation is a serious issue resulting in ISI Multi path is not resolvable in narrow band

systems where symbol time is comparable to multi path delay• GSM has symbol time of 3.69 sec vis-à-vis

delay spreads of 3-8 sec CDMA is a wideband system (IS-95 symbol time is

about 0.8 sec, much smaller than multi path delay) Multi path can be resolved and constructively

combined using RAKE receivers Thus we are able to exploit the multi path

propagation for improving the signal to noise ratio


IS-95 System


Overview Of IS-95 System

IS-95 is a proprietary design by Qualcomm Developed to overcome capacity issues in AMPS

Network architecture greatly influenced by GSM

Meant for supporting circuit mode voice services Has been enhanced for supporting data services

DS-SS on both Forward and Reverse links

Universal frequency reuse requiring no frequency

planning

64 Walsh Codes used supporting a theoretical

maximum of 64 active users

Channel of 1.25 MHz wide (1.2288 Mcps chip rate)


Overview Of IS-95 System (Contd.)

Fast power control to combat Near-Far problem

RAKE receiver to take advantage of multipath

Soft handoff making use of macro diversity

Qualcomm 9600 bps Code Excited Linear Predictive

(QCELP) speech coder with a variable data rate from

9600 bps to 1200 bps is used

CDMA system is interference limited with soft capacity Interference reduction is a dominant theme

Variable rate coding for reducing the duty cycle

Closed loop power control


Is-95 Network Architecture

Um

MS

BTS BSCAbis MSCA

TR-45 / 46 Reference Model

HLR VLR

ACEIR

C B

H

F

DMSC

E

OtherVLR

G

PSPDN

PSTN

ISDN

PLMNMi

Di

Ai

Pi

IWFAUXOS

LXO

WPT2

TAP

WPT1

WPT0

TE2

TE2

TE1

Rm

Sm

Sm


Logical Channels In Is-95 System

Control Channels

Forward Reverse

Pilot Sync Paging Access

Dim & Burst

Traffic Channels

Speech or Data Associated Signaling

Full Rate 1/2 Rate 1/4 Rate 1/8 RateBlank & Burst

Power Control(Forward)


Forward And Reverse Channels

Forward Channels (making up a total of max 64) 1 Pilot Channel 1 Synchronisation Channel Upto 7 Paging Channels Traffic Channels

Reverse Channels Access Channels called Random Access

Channels Traffic Channels


Forward Link Channel Arrangement

...... ... .

Forward CDMA Link (1.2288 MHz channel transmitted by base station

Pilot

Sync

W0 W7W8

W63W32 W1

PCH#1 PCH#7

Code#1 Code#N

Code#P

Code#55

Code#M

FTCH FTCH with multiple code channel

FundamentalCode Channel Mobile Power

Control Subchannel


General Transmission Scheme - Forward

Data rate ranges from 1200 to 9600 bps

This rate is made upto 19200 bps by rate ½ coding

and repetition of 8 to 1

This data is spread to a channel chip rate of 1.2288

Mcps using combination of techniques

Each IS-95 channel occupies 1.25 MHz of spectrum

Channel chip rate is 1.2288 Mchips/s

64 orthogonal Walsh functions are used in forward

channel for spreading purpose


Pilot Channel

Pilot channel is the first channel mobile looks for Its power level is kept 4 to 6 dB above traffic

channels Provides phase reference for coherent

demodulation Allows pilot strength measurement for handoff

This is carried on Code Channel 0 (uses W0) Results in transmitting the station PN sequence

with a length of 32,768 {(215-1) PN Sequence plus a ‘0’}• Corresponds to a period of 26.667 msec

All base stations use the same PN sequence, with unique offsets in increments of 64 chips

Base stations are identified by their unique offset


Modulation Structure Of Pilot Channel

1.2288 Mcps

I-Chl Pilot PN Seq

Q-Chl Pilot PN Seq

BalancedQPSK

Pilot takes 15-20% of total Tx power

Walsh Function 0(all zeros)

all zeros


Station Specific PN Short Codes & Offsets

32,768 Chips

Short code 0

Short code i

Short code I+1

Short code k Short

code 0

64 Chips

64 Chips

Short code 511

64 Chips

…………………………………………………………………….

1 Chip

Short code 1


Synchronisation Channel

This always operates at a fixed data rate of 1200 bps

After rate ½ convolutional coding and repetition (of 2) the date rate becomes 4800 bps

This gets spread by Code Channel 32 (W32) operating

at 1.2288 Mcps

Sync Channel message has the following information

System Identification (SID), Network Identification (NID), Pilot short PN sequence offset index (PILOT_PN), Long Code State, System time, Paging channel data rate (4.8 or 9.6 kbps), etc.


Modulation Structure Of Synchronisation Channel

Convolutional Encoder

and Repetitionr =1/2, K = 9

Sync.Data

Block Inter-leave

1.2288 Mcps4.8 Kbps

Walsh Code 32

I-Chl Pilot PN Seq

Q-Chl Pilot PN Seq

1.2Kbps BalancedQPSK

4.8 Kbps

Sync. Channel typically has about 10% of the Tx power of the Pilot


Paging Channel

Paging channel is used to transmit control information at either 4.8 or 9.6 kbps The format is similar to that of Sync channel

message 8 bit length indicator, 2-1146 bits long data, 30

bit CRC Paging message can use synchronised capsules

that end on a half frame boundary or unsynchronised capsules that can end anywhere• Synchronised capsules use padding bits to

reach the nearest half frame boundary• Synchronised paging messages carry

multiple of 48 bits (4.8 kbps) and 96 bits (9.6 kbps)


Paging Channel (Contd.)

Eight paging channel half-frames are combined to forma a paging channel slot of length 80 msec (384 bits at 4800 bps and 768 bits at 9600 bps)

Type of messages carried by paging channel include System parameter message Access parameter message Neighbour list message CDMA carrier list message Slotted page / Page message Order messages Channel assignment messages Authentication challenge message


Modulation Structure Of Paging Channel


(and Repetitionfor 4.8 kbps)r =1/2, K = 9

PagingData

Block Inter-leave

1.2288 Mcps19.2Kbps

Walsh Code p (1-7)

I-Chl Pilot PN Seq

Q-Chl Pilot PN Seq

9.6Kbps4.8Kbps Balanced

QPSK

19.2 Kbps

Totally, the Paging Channels have about 75% of the Tx power of the Pilot

Long Code generator Decimator

Long code mask for pth paging

channel

1.2288 Mcps

ScramblingCode -- 19.2Kcps


Slotted Operation Of Paging Channel

I=0, T=2 =1; PGSLOT = Slot number 6 out of every 16 slots

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 162047 0

1 Slot Cycle 1.28 Seconds

80 ms

Mobile in Nonactive State

Mobile in Nonactive State

Paging Channel Slot

Reacquisition Time

8 Paging Channel Half- Frame


Forward Traffic Channel

This channel is used for carrying user traffic Data rates are 9600, 4800, 2400 and 1200 with

lower data rates being associated with low voice activity

Convolutional coding, symbol repetition and block interleaving make the data rate 19.2 kbps

Scrambling of user data is achieved using long code generated with mobile’s ESN as long code mask and used for scrambling user data after a decimator block

Power control subchannel @ 800 bps is added by stealing the scrambled bits

Walsh code Wi (for the ith channel) spreads this rate to 1.2288 Mcps


Modulation Structure Of Forward Traffic Channel



User data(Taffic)

Block Interleaver

(24x16)

Long Code generator Decimator Decimator

MUX

Long code for ith user

1.2288 Mcps

1.2288 Mcps19.2 kbps

Power Control

Bit

800 Hz

Walsh Code i

(use 1/64x24)Scrambling(use 1/64)

I-Chl Pilot PN Seq

Q-Chl Pilot PN Seq

9600 bps4800 bps2400 bps1200 bps

BalancedQPSK

(Short Code of BS;215-1=26.67msecs)


Access Channel

Access channel is used by the mobile to transmit control information like call origination and response to paging

Data rate is 4800 bps Each access channel is identified by a distinct long

code sequence having an access number, a paging channel number associated with the access channel and other system data

Types of access channel messages are Registration message, Order message, Origination

messages, Page response message, Authentication challenge response message and so on


Modulation Structure of Access Channel



Block Interleaver

32x18

Long Code generator

Long code Mask of Access Channel

1.2288 Mcps

Pilot PN Seq I Chl

4800 bps

28.8 Kbps

64-aryOrthogonalModulator

Pilot PN Seq Q ChlData

burstrandomizer

Code symbol

Walshchip

307.2 Kcps{ 28.8 (64/6) } D

1/2 chip delay

Modulator

OffsetQPSK


Reverse Traffic Channel

This channel is used for carrying user traffic Data rates are 9600, 4800, 2400 and 1200 with

lower data rates being associated with low voice activity

Convolutional coding (rate 1/3), symbol repetition and block interleaving make the data rate 28.8 kbps

Interleaved data at 28.8 kbps is modulated using Walsh code with 6 bits being used as symbol to select one of 64 Walsh codes to get modulated data at 307.2 ksps

PN long code generated using user’s ESN as seed is used for taking out the repeated data and for spreading this 4-fold to 1.2288 msps

Finally the Pilot PN code is used for scrambling


Modulation Structure of Reverse Traffic Channel



Block Interleaver

32x18

Long Code generator

Long code Mask for user i

1.2288 Mcps

Pilot PN Seq I Chl

9600 bps4800 bps2400 bps1200 bps

28.8 Kbps

64-aryOrthogonalModulator

Pilot PN Seq Q ChlData

burstrandomizer

Code symbol

Walshchip

307.2 Kcps{ 28.8 (64/6) } D

1/2 chip delay

Modulator

OffsetQPSK


Power Control

Types of Power Control Open Loop Power Control Closed Loop Power Control – based on feedback

Open Loop Power Control (on forward link) Channel state on the forward link is estimated by

mobile Reverse link transmit power made proportional to

forward link channel loss Works well if forward link and reverse link are

highly correlated• which is generally true for slowly varying

distance and shadow losses• but not true with fast multipath Rayleigh

fading


Closed Loop Power Control in Reverse Link Reverse link subjected to

Inner Loop Power control – for overcoming Near-Far problem

• Control bits are punctured into the traffic data stream

• Closed loop power control step size is +/- 1 dB

• Errors on account of power control bits recovered by error decoding

Outer Loop Power control – for maintaining performance of individual links

• Done by means of messages

• Takes somewhat longer time to effect changes Both open (outer) and closed (inner) loops drive the

transmit power to ensure a target FER of 1-2 %


Base station measures the received Eb/Io Compares it with the `Target Eb/Io’ and generates

power UP/DOWN command Sends UP/DOWN command to mobile asking it to

increase or decrease the transmit power PC rate must be fast enough (approx 10 times the

max Doppler BW) to track multipath fading At 900 MHz Carrier frequency and 120 km/h

mobile speed, Doppler = 100 Hz In IS-95A, closed loop power control is operated

at 800 Hz update rate Propagation and processing delays are critical to

loop performance

Closed Loop Power Control in Reverse Link (Contd.)


Position Of Power Control Bits

20 21 22 23 0 1 2 3 4 5 6 7 8 9 10 1112 131415 16 17 18 19 2021 22 23 0 1 2 3

0 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15

1 Frame (20 ms) consists of 16 Power Control Groups

1.25 msec

1.25 msec = 24 scrambled traffic data bits

Power Control Bit repeated twice

20212223

0 0 1 1

The values of these 4 bits determine the location of power control bit in the subsequent frame (1100 equals 12 and hence this starts

at bit position 12)


Generating Long Code

110001111(9 bits)

Access Channel Number

Paging Channel Number (3 bits)

Base Station Identification (16 bits)

Pilot Offset of Forward Channel

(9 bits)

(5 bits) (16 bits) (9 bits)

Access Channel

Reverse Traffic Channel

1100011000(10 bits)

Permuted ESN (32 bits)

Permuted ESN = (E0, E31, E22, E13, E4, E26, E17, E8, E30, E21, E12, E3, E25, E16, E7, E29, E20, E11, E2, E24, E15, E6, E28, E19, E10, E1, E23, E14, E5, E27, E18, E9)

(3 bits)


Operation Of Data Burst Randomizer

Data Burst Randomiser

0 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15

0 1 2 3 4 5 6 7 8 9 10 11 12 13 14 1514

Operating at 2.4 kbps

PCG Bits transmitted

1.25 msec (1 PCG)

20 msec (16 PCGs)


Forward And Reverse – Key Differences

Forward Reverse

Synchronous CDM – Walsh codes provide channelisation

Asynchronous CDMA – Long codes provide channelisation

Short code provides scrambling code, helps in identifying a BS and also provides pilot channel for timing recovery

Short codes provide scrambling code and helps in identifying BS

Data rate made up to 19.2 kbps by repeating

Data rate made up to 28.8 kbps by repeating and the extra bits are removed by randomiser

Walsh codes provide orthogonal spreading – unique for each channel

Walsh codes enablle 64-ary orthogonal modulation


Forward And Reverse– Key Differences (Contd.)

Forward Reverse

Rate ½ Convolutional coding is used for error control

More robust rate 1/3 rate Convolutional coding is used for error control

Simple open loop power control used

Open loop power control as well as closed inner loop and closed outer loop power controls are used

QPSK modulation is used Balanced OQPSK is used to get a tighter control of spectrum


Forward / Reverse Traffic Channel Payload

171 bits0 F=12 T=8

80 bits F=8 T=8

40 bits

T=8

T=8

16 bits

48 bits (20 msec)

96 bits (20 msec)

24 bits (20 msec)

192 bits (20 msec)

F – Frame Quality Indicator bits (CRC bits)

T – Tail bits (all zeros)


Blank And Burst & Dim And Burst

Signaling / Secondary Traffic Bits (168)1

Primary traffic bits (80)

Primary trafficBits (40)

Primary traffic (16 bits)

172 bits0/1

1

1 0/1

0/1

11

00

01

1 0/1 10

Signaling / SecondaryTraffic Bits (88)

Signaling / Secondary Traffic Bits (128)

Signaling / Secondary Traffic Bits (152)

- MM bit (1) - TT bit (1) - TM bits (2)BB – Blank & Burst Format

DB – Dim & Burst Formats

MM – Mixed Mode TT – Traffic Type TM – Traffic Mode


Transmission Formats (Full Rate)

MM Bit

TT Bit

TM Bits Voice Bits (Primary)

Data Bits (Secondary)

Signaling Bits

0 - - - 171 - -

1 0 1 1 - - 168

1 1 1 1 - 168 -

1 0 0 0 80 - 88

1 1 0 0 80 88 -

1 0 0 1 40 - 128

1 1 0 1 40 128 -

1 0 1 0 16 - 152

1 1 1 0 16 152 -


General Transmission Scheme - Reverse Data rate ranges from 1200 to 9600 bps

This rate is made upto 19200 bps by rate 1/3 coding and repetition of 8 to 1

64 orthogonal Walsh codes are used in modulation

This data is spread to a channel chip rate of 1.2288 Mcps using user specific long code PN-sequence and (BS specific) Pilot PN short code sequence

Each IS-95 channel occupies 1.25 MHz of spectrum Channel chip rate is 1.2288 Mchips/s Traffic channels power control is as per base

station command


Reverse Link Channel Arrangement

...... .. .

Reverse CDMA Link (1.23 MHz channel received by base station

. .AccessChannel(PCH1)

AccessChannel(PCH1)

AccessChannel(PCHN)

TrafficChannel# 1

TrafficChannel# T

Addressed by Long Code PN

FundamentalCode Channel


IS-95 – Key Facts

CDMA/FDD based technology No need for frequency planning Walsh codes used in forward link for channelisation

and in reverse for modulation Supporting voice coding at 9.6 (4.8, 2.4, 1.2) kbps Channel spacing of 1.25 MHz Per user gross rate of 19.2 kbps Power control in reverse and forward link for interfere

reduction Soft handoff improves handoff efficiency

Hard handoff (make before break) not supported Offered at least three fold spectral efficiency


IS-95 Evolution To IS-95B

Voice quality in IS-95 was not adequate Data rates in IS-95A was enhanced to 14.4, 7.2, 3.6 &

1.8 kbps (called Rate Set 1 or RS1) in addition to IS-95 rates of 9.6, 4.8, 2.4 & 1.2 kbps (Rate Set 2 or RS2) Main idea was to support QCELP-13

IS-95B defined forward and reverse traffic channels having 1 fundamental code channel and up to 7 supplementary channels Data rates of up to 76.8 kbps in RS1 (8 x14.4) or

115.2 kbps in RS2 (8 x 14.4) can be supported Data Inter Working Function (IWF) was defined for

supporting packet data using these traffic channels Inter frequency hard handoff supported in addition to

soft handoff leading to better interworking


Forward Traffic Channel Arrangement In IS-95B

...... ... .

Forward CDMA Link (1.2288 MHz channel transmitted by base station

Pilot

Sync

W0 W7W8

W63W32 W1

PCH#1 PCH#7

Code#1 Code#N

Code#P

Code#55

Code#M

FTCH FTCH with multiple code channel

FundamentalCode Channel Mobile Power

Control Subchannel

SupplementaryCode Channel


Reverse Traffic Channel Arrangement In IS-95B

...... .. .

Reverse CDMA Link (1.23 MHz channel received by base station

. .AccessChannel(PCH1)

AccessChannel(PCH1)

AccessChannel(PCHN)

TrafficChannel# 1

TrafficChannel# T

Addressed by Long Code PN

FundamentalCode Channel



.. ..

Documents

CDMA & IS-95