Telecom Fundamentals1

8/14/2019 Telecom Fundamentals1

1/27

Copyright 2003 P K Divakara

Pseuodorandum codesPseuodorandum codes

P K Divakaran


2/27

Copyright 2003 P K Divakaran

Pseudorandom codes (PNPseudorandom codes (PN

codes)codes)PN code

Random numbers generated by a deterministic

process

Generated by Shift registers and XOR gates

Length of the code = 2 -1 where n= numberof shift registers

n


3/27


Autocorrelation of PN codeAutocorrelation of PN code

Autocorrelation is the measure of correlation

between a code and the time shifted version of the

same code.Time shifted versions of the same PN code have

very little correlation

The separation of the users (mobiles) in the

Reverse link ( Uplink) is done by assigning time

shifted versions(based on ESN) of Long PN codes

.


4/27


MaskingMasking

Time shift of PN codes are carried out byMasking.

Different Masks result in different Timeshifts

Electronic Serial Number (ESN) of amobile ( like IMEI in GSM) is used forMasking


5/27


PN Codes used in CDMA OnePN Codes used in CDMA One

(IS 95 )(IS 95 )One Long PN code ( 2 - 1)

Repeats every 41 days at a clock rate of 1.2288

Mchips/s Used for spreading & scrambling of channels at

Base Station (Forward channels)

Used for separation of channels by Mobileswith time shifted PN (Reverse channels)

42


6/27


PN Codes used in CDMA OnePN Codes used in CDMA One

(IS 95 )(IS 95 )Two Short Codes (2 - 1)

Used for I & Q quadrature spreading

Unique offsets ( Time shifts) identify a Cell or

Sector ( The same Walsh codes can therefore

be used in ALL sectors or cells)

Clock rate 1.2288 Mchips/s

15


7/27


CDMA One Channel ConceptsCDMA One Channel Concepts

P K Divakaran


8/27


CDMA ChannelsCDMA Channels

Physical Channels

Defined in terms of a Radio Frequency Carrier

& a Code sequence

Logical Channels (mapped onto Physical

channels)

Control Channels Traffic Channels


9/27


Control ChannelsControl Channels

Control ch.

Forward link.

pilot Sync Paging

Reverse link.

Access


10/27


Traffic ChannelsTraffic Channels

Traffic ch.

Speech or Data.

1 1/2 1/4

Associated

Signalling.

1/8

Rate

epending on voice

Rate 1 9.6kbps

Rate 2 4.8 kbps

Rate 3 2.4 kbpsRate 4 1.2kbps


11/27


Forward Link Channels (BaseForward Link Channels (Base

Station to Mobile)Station to Mobile)Control Channels

Pilot ChannelA Reference for all Mobile stations in the Cell to be

used in the Demodulation process

Signal level higher than Traffic channels at

CONSTANT value

Pilot signal used for signal strength comparisonbetween Base stations for handoff between Cells

Always assigned Walsh code W0 ( 000000.)


12/27


Pilot Channel GenerationPilot Channel Generation

+0000

Walsh code W0

1.2288

Mchips/s

+

+

I Pilot PN

Q Pilot PN

1.2288 Mcps

1.2288 Mcps

To QPSK

Modulator

Pilot signal from all Base stations use the same PN sequence but

are identified by a time offset of 64 chips


13/27


Sync ChannelSync Channel

Sync channel is used (by Mobiles) with Pilot channel toacquire the initial time synchronisation

Always assigned Walsh code W32

Contains info such as, Network identifier, Pilot short PNoffset (64 chips offset) to identify Base station for the cell,System time, Long code synchronisation info etc

Long code sync info. ensures that PN long code generators

at both base station and Mobiles are aligned Chip rate on the Pilot Ch. & on all freq carriers is locked to

a precise system time by using GPS. (Global positioningSystem)


14/27


Sync Channel GenerationSync Channel GenerationSync

Ch. msg

1.2kbps

ConvolutionCoder 1/2

2.4

kbps

Symbolrepetition

4.8

kbps

Blockinterleaver

+

Walsh

W32

+

+

1.2288

Mcps

I Pilot P

1.2288

mcps

Q Pilot

PN1.2288

Mcps

Tp

QPS

Mod


15/27


Paging ChannelPaging Channel

To alert the mobile

To respond to an Access request message from

mobileTo convey system info and instruction to mobile

Walsh codes W1 to W7 used

For SMS to mobile

Info content Base station identifier

Neighbouring Base station parameters (For handoff)

CDMA channel assignment message (Freq. & Code )


16/27


17/27


Access Channel GenerationAccess Channel Generation

Long code generator

Access

Channel

Mask

+

+

+

Q channel

PN sequen

I Ch PN

sequence(N

offset)

1.2288Mcps (No

offset)

1.2288mcps

4.8kbpsConvl.

coder

Sym

repetiton

Interleaver14.4kbps

28.8ksps

64 ary Orthomodulator

For every SIX bit output of interleaver, a Walsh code

64 bit long is output by 64 ary Ortho Modulator.


18/27


Reverse Traffic ChannelReverse Traffic Channel

Supports 4 different data rates 9.6 kbps, 4.8 kbps,2.4 kbps, 1.2 kbps of user data depending on voiceactivity (variable rate vocoder)

Convolution coding for Forward Error correction

Interleaving for spreading consecutive bits intonon consecutive bits before transmission to

counteract error bursts in the air interface 64 ary Ortho Modulation as in Access channel

Long code PN generator uses ESN mask of mobilefor unique identification

Short PN codes (I &Q) do not have any offset


19/27


Reverse Traffic ChannelReverse Traffic Channel

Carries traffic as well as some Signalling

info

Pilot strength measurement when mobile is inTraffic channel state (conversation state) to

assist in hand off

Hand off completion

Etc.


20/27


Forward Traffic ChannelForward Traffic Channel

Supports 4 data rates as in the case of ReverseTraffic channel based on Voice activity withVariable rate vocoder

Convolution coding done for FEC

Interleaving done as in the case of Reverse trafficchannel

Long PN code generator uses ESN mask of mobileto identify the forward traffic channel to a

particular mobile

Short PN codes (I & Q) has PN offset identifying

the Base station


21/27



(FTC)(FTC)Power control subchannel is included in the

FTC for mobile power control


22/27



Carries traffic as well as some signalling

info

Handoff direction

Power control

etc


23/27


Mobile Identity NumbersMobile Identity Numbers

IMSI (15 digits) --International MobileSubscriber Identity

Consists of MCC (3 digits), MNC, MSINTMSI assigned by VLR to protect

confidentiality of IMSI

ESN- Electronic Serial Number (32 bitbinary number) uniquely identifying themobile


24/27


ESNESN

Set in the factory by mobile manufacturer

Not alterable in the field

Very important since used for masking

Forward & Return Traffic channel Long

PN code for a mobile for unique

identification


25/27


CDMA IS 95 Traffic casesCDMA IS 95 Traffic cases

P K Divakaran


26/27


Traffic States of a MobileTraffic States of a Mobile

On Power up.

1-System initialisation state

Acquires pilot channel tuning to strongestsignal of PN I & PN Q possibilities

Sync channel acquired. Gets all system info

2-Idle state Acquires Paging channel and monitors it Mobile receives info to initiate or receive a call


27/27


Traffic States of a MobileTraffic States of a Mobile

3- Access State To initiate a call mobile accesses the Access

channel and sends info to the System andreceives response on the Paging channel

4- Traffic channel state Traffic channels assigned in both directions &

communications on. Associated signalling messages also sent on

Traffic channel (by stealing speech packet)

Telecom Fundamentals1

Documents

Transcript of Telecom Fundamentals1