Telecom Fundamentals1

Pseuodorandum codes

P K Divakaran

Copyright 2003 P K Divakaran

Pseudorandom codes (PN codes)  PN

code

– Random numbers generated by a deterministic

process – Generated by Shift registers and XOR gates – Length of the code = 2 n-1 where n= number

of shift registers

Copyright 2003 P K Divakaran

Autocorrelation 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 . Copyright 2003 P K Divakaran

Masking  Time

shift of PN codes are carried out by “Masking”.

 Different

shifts

“Masks” result in different Time

 Electronic

Serial Number (ESN) of a mobile ( like IMEI in GSM) is used for ‘Masking” Copyright 2003 P K Divakaran

PN Codes used in CDMA One (IS 95 )  One

42

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 Mobiles with time shifted PN (Reverse channels)

Copyright 2003 P K Divakaran

PN Codes used in CDMA One (IS 95 )  Two

15

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

Copyright 2003 P K Divakaran

CDMA One Channel Concepts

P K Divakaran

Copyright 2003 P K Divakaran

CDMA Channels  Physical

Channels

– Defined in terms of a Radio Frequency Carrier

& a Code sequence  Logical

Channels (mapped onto Physical channels) – Control Channels – Traffic Channels

Copyright 2003 P K Divakaran

Control Channels Control ch.

Forward link.

pilot

Sync

Reverse link.

Paging Copyright 2003 P K Divakaran

Access

Traffic Channels Rate 1 Rate 2 Rate 3 Rate 4

Traffic ch.

9.6kbps 4.8 kbps 2.4 kbps 1.2kbps Speech or Data.

1

1/2

Rate Depending on voice

1/4 1/8

Copyright 2003 P K Divakaran

Associated Signalling.

Forward Link Channels (Base Station to Mobile)  Control

Channels

– Pilot Channel  A 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 comparison between Base stations for “handoff” between Cells  Always assigned Walsh code W0 ( 000000…….) Copyright 2003 P K Divakaran

Pilot Channel Generation 1.2288 Mcps I Pilot PN

0000

1.2288 Mchips/s

+ To QPSK Modulator

+ Walsh code W0

+ Q Pilot PN 1.2288 Mcps

Pilot signal from all Base stations use the same PN sequence but are identified by a time offset of 64 chips Copyright 2003 P K Divakaran

Sync Channel   

 

Sync channel is used (by Mobiles) with Pilot channel to acquire the initial time synchronisation Always assigned Walsh code W32 Contains info such as, Network identifier, Pilot short PN offset (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 positioning System) Copyright 2003 P K Divakaran

Sync Channel Generation Sync Ch. msg

2.4 kbps

Walsh W32

4.8 kbps Symbol repetition

Tp QPSK Mod.

+

1.2kbps Convolution Coder 1/2

+

I Pilot PN 1.2288 mcps

Block interleaver

+ 1.2288 Mcps

Copyright 2003 P K Divakaran

Q Pilot PN 1.2288 Mcps

Paging Channel  To

alert the mobile  To respond to an “Access request “ message from mobile  To 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 ) Copyright 2003 P K Divakaran

Reverse Control channelAccess Channel  Used

by Mobile to Transmit control info to Base station  For Location Registration of the mobile with the Network  Also used to respond to a Paging message  To originate a call from mobile  For Authentication Copyright 2003 P K Divakaran

Access Channel Generation 14.4kbps

4.8kbps Convl. coder

Interleaver 64 ary Ortho modulator

Sym repetiton

+ 28.8ksps

Access Channel Mask

1.2288 mcps

+

I Ch PN sequence(No + offset)

Q channel Long code generator PN sequenc 1.2288 Mcps (No For every SIX bit output of interleaver, a Walsh code offset) 64 bit long is output by 64 ary Ortho Modulator. Copyright 2003 P K Divakaran

Reverse Traffic Channel  Supports

4 different data rates 9.6 kbps, 4.8 kbps, 2.4 kbps, 1.2 kbps of user data depending on voice activity (variable rate vocoder)  Convolution coding for Forward Error correction  Interleaving for spreading consecutive bits into non 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 mobile for unique identification Copyright 2003do P K not Divakaran  Short PN codes (I &Q) have any offset

Reverse Traffic Channel  Carries

traffic as well as some Signalling

info – Pilot strength measurement when mobile is in

Traffic channel state (conversation state) to assist in hand off – Hand off completion – Etc.

Copyright 2003 P K Divakaran

Forward Traffic Channel  Supports

4 data rates as in the case of Reverse Traffic channel based on Voice activity with Variable rate vocoder  Convolution coding done for FEC  Interleaving done as in the case of Reverse traffic channel  Long PN code generator uses ESN mask of mobile to identify the forward traffic channel to a particular mobile  Short PN codes (I & Q) has PN offset identifying the Base station Copyright 2003 P K Divakaran

Forward Traffic Channel (FTC)  Power

control subchannel is included in the FTC for mobile power control

Copyright 2003 P K Divakaran

Forward Traffic Channel  Carries

traffic as well as some signalling

info – Handoff direction – Power control

etc

Copyright 2003 P K Divakaran

Mobile Identity Numbers  IMSI

(15 digits) --International Mobile Subscriber Identity – Consists of MCC (3 digits), MNC, MSIN

 TMSI

assigned by VLR to protect confidentiality of IMSI  ESN- Electronic Serial Number (32 bit binary number) uniquely identifying the mobile Copyright 2003 P K Divakaran

ESN  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

Copyright 2003 P K Divakaran

CDMA IS 95 Traffic cases

P K Divakaran

Copyright 2003 P K Divakaran

Traffic States of a Mobile On Power up….  1-System initialisation state – Acquires pilot channel tuning to strongest

signal 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 Copyright 2003 P K Divakaran

Traffic States of a Mobile  3-

Access State

– To initiate a call mobile accesses the Access

channel and sends info to the System and receives 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) Copyright 2003 P K Divakaran