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Simulink 4.1

CDMA Reference Blockset 1.1

Introductionto CDMA

Description ofSome Blocks

Description of aComplete Demo

Contents

Organization of Contents

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Introduction to CDMA (1)

• History of CDMA• General Concepts of CDMA• CDMA Transmission• Different CDMA methods• CDMA Capacity & Processing Gain• Suitable PN Codes• Intrinsic CDMA Advantages• CDMA Disadvantages

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Introduction to CDMA (2)• Power Control Methods• Cellular Systems Introduction• Cluster• Frequency Reuse• Handoff• CDMA Advantages in Cellular Systems• Generations of Cellular Systems• CDMA in Different Standards

History of CDMA (1)

• A method for multiple access {single channel ; multiple users}

• The conventional method was FDMA• Then , after developing

synchronization methods, TDMA was introduced {T1,E1}

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History of CDMA (2)

• In early 1980s CDMA was introduced • First for military usage.• In early 1990s used in Cellular

Communication Systems {IS-95}

• First commercial large scale usage in Seul.

General Concepts of CDMA

• Transmitted signal occupies a bandwidth much larger than the original signal !

• The bandwidth is spread by means of a PN code independent of the data

• The receiver synchronizes to the code to recover the data

• Many signals share the same times and frequencies but independent codes

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Spreading a Bit by means of many Chips

Relative Bandwidth of coded and original signal

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Steps of signal Transmission• A pseudo-random code is generated,

different for each channel• The Information data modulates the

pseudo-random code ( spreading step).

• The resulting signal modulates a carrier.

• The modulated carrier is amplified and broadcast.

Block Diagram of CDMA Transmitter

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Different CDMA methods

• CDMA is in fact a Spread Spectrum method

• Direct Sequence• Frequency Hopping• Time Hopping• Hybrid Modulation

Classification of CDMA methodsCDMA

PURE Hybrid

DS FHh

DS/FHDS/THFH/TH

DS/FH/TH

TDMA/CDMA

MC-CDMAMT-CDMA

THjj

WB/DS FAST

NB/DS SLOW

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Gp:Processing Gain

• Processing Gain (Gp): Gp = fc/fi

• Gp reflects the relative advantage that frequency spreading provides

• Usually Gp is 128 or even more

Importance of High Gp

There are two major benefits from high processing gain:

• Interference rejection: the ability of the system to reject interference is directly proportional to Gp.

• System capacity: the capacity of the system is directly proportional to Gp.the higher the PN code bit ratewider the CDMA bandwidththe better the system performance

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Suitable PN Codes

• Codes should be almost uncorelated.

• Each code should have small auto-corelation

System Capacity• The capacity of a system is approximated

by:Cmax=Gp/(Eb/No)* (1/(1+B))

• Cmax is the maximum number of simultaneous calls

• Gp is the processing gain• Eb/No is the total signal to noise ratio per

bit• B is the inter-cell interference factor

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Intrinsic CDMA Advantages

• Multipath access capability• Multipath interference rejection• Interference rejection• Anti-jamming {esp. NB jamming}

capability• Low prabablity of interception {LPI}

{in fact extreme Privacy}

CDMA Disadvantages

• Highly Accurate Synchronization {esp. for W-CDMA}: 0.2*Tc Synchronization is needed

• Accurate Power Control is needed {esp. in Reverse Channel} Near-End/Far-End Problem

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Power Control Methods

Power Control compensates Shadowing & FastFading effects also

• Open-Loop Power Control {by means of a Pilot signal only}

• Closed-Loop Power Control• Voice Activity Consideration

Generally Power Control increases the system capacity

Cellular Systems Introduction

• Low Power • Limited Frequency Band• Point to Point ; Not Broadcast

The Only solution is: Frequency Reuse!

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Covering an area by many Cells

A Cluster

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Frequency Reuse

Handoff

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CDMA Advantages in Cellular Systems

• No need to Frequency Managements {Number of Cells in a Cluster is (N)=1}

• Soft Capacity Limit• Prabablity of Force Termination is

negligible• Soft Handoff {No Ping-Pong effect!}

Generations of Cellular Systems• G1 : in 1980s , Analog , Vehicle Sets ,

Voice only , Uncompatible standards• G2 : in 1990s , Digital , Hand Sets ,

Voice+Low Data Rates , Few Global standards {Current Sys}

• G3 : in 2000s , High Data Rates , 3 Global standards , {Designed but not Commercial Implemented}

• G4: is being Designed !

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CDMA in Different Standards• No CDMA in G1 ! {Only FDMA}• G2: Only IS-95 System CDMA

Others {GSM,IS-54,PDC}: FDMA/TDMA

• G3: WCDMA & CDMA2000 are CDMA Only UWC-136 is FDMA/TDMA

• G4: Proposals for various Spread Spectrum methods , other than DS-CDMA !

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Description of Some Blocks of CDMA Blockset (1)

• IS-95 PN Codes• Fwd Link Logical Chs.• Rev Link Logical Chs.• Forward Channel Coding• Coherent Rake Receiver• Non-Coherent rake Receiver• Rates and Rate Sets

Description of Some Blocks of CDMA Blockset (2)

• CDMA Reference Blockset Library• IS-95 Base Station Transmitter• IS-95 Mobile Station Receiver• IS-95 Mobile Station Transmitter• IS-95 Base Station Receiver• IS-95 Common• IS-95 Forward Ch.• IS-95 Reverse Ch.

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IS-95 PN Codes (1)

Chip rate: 1.2288Mchips/s So,Spread Signal Bandwidth: 1.25MHz

• Walsh Codes: 64 chips , mutually Orthogonal Ensure Orthogonality between signals from a Base for different Mobiles. Also used for modulation for the Rev. Ch.

IS-95 PN Codes (2)• Short PN Codes:

A pair of periodic Binary sequences with period 2^15 chips.

used for spreading and despreadingsignals into in-phase and quadraturecomponents.

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IS-95 PN Codes (3)• Long PN Codes:

Period: 2^42-1.

Used for Data Scrambling. Used for Spreading on the Reverse link. Used for Power Control Burst randomization

Fwd Link Logical Chs.

• Pilot Ch: modulates a constant sumbol! used for ch. Estimation, Coherent demodulation of other channels.

• Sync Ch.• Paging Ch: for Controling , paging the

requested Mobile.• Traffic Ch: Speech or Data.

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Rev Link Logical Chs.

• Access Ch: for Controling , sending Requests , responding to pagings , etc.

• Traffic Ch: Speech or Data.

Forward Channel Coding

• Uses 20 ms frames for all channels• Only Sync channel uses 26.666 ms frames• Error protection : 1/2-rate convolutional

code• Viterbi algorithm for optimal decoding• For protection against bursts of errors,

Interleaving before modulation.

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Coherent Rake Receiver

• In Fwd Ch.• Several paths with different propagation

delays.• Detects the transmitted data from each

delayed-path component and combines the results.

• In fact we have a diversity gain !

Non-Coherent rake Receiver

• In Rev Ch.• No Pilot available !• Each of 64 possible walch Codes are

multiplied bye both In-phase and Qaudrature components and then are added.

• The biggest result is selected !

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Rates and Rate Sets

CDMA Reference Blockset Library

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IS-95 Base Station Transmitter

IS-95 Mobile Station Receiver

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IS-95 Mobile Station Transmitter

IS-95 Base Station Receiver

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IS-95 Common

IS-95 Forward Ch.

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IS-95 Reverse Ch.

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IS-95A Forward Traffic Channel Detection

Library Blocks in the Demo• IS-95A Fwd Ch Base Station Transmitter

Interface (inside Spreading and Modulation subsystem)

• IS-95A Fwd Ch Detector• IS-95A Long Code Generator (inside Data

Source subsystem)• IS-95A Short Code Generator (inside Data

Source subsystem)• IS-95A Walsh Code Generator (inside Data

Source subsystem)• IS-95A Fwd Ch Scrambler

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Data Source Subsystem

Components of Data Source

• Random number generator: generates the bipolar random data symbols for the Traffic, Sync and Paging channels.

• A signal that represents the data rate.• The IS-95A Long Code Generator library

block: used to scramble the data.• The IS-95A Walsh Code Generator and

IS-95A Short Code Generator library blocks: used for orthogonal spreading.

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Inside Spreading and ModulationSubsystem

Rayleigh Multipath and AWGN Channel subsystem

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Variable Parameters

• Simulation Duration• Data Rate• Rate Set• Doppler Frequency in Channel• S/N Ratio• Random Seed• Ch Paths & Rake Fingers