網路多媒體研究所 1 WCDMA Technology Past, Present and Future Part II: CDMA Technology...

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WCDMA TechnologyWCDMA TechnologyPast, Present and FuturePast, Present and Future

Part II:CDMA Technology Overview

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Part II: CDMA Technology Overview

• Introduction to Wireless Communication– Radio Channel Characteristic– Modulation– Basic Concepts to Mobile Communication

• CDMA Technology Overview– History of CDMA– Introduction to Spread Spectrum– Direct Sequence CDMA (DS-CDMA)– CDMA System Characteristic– Challenges in CDMA System Design

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Typical Communication System

SourceSource

&ChannelEncoder

DigitalModulator

RF

RFDemodulator Equalization

Synchronization:Carrier and Timing

Source&ChannelDecoder

Sink

Antenna

Antenna

Radio Channel

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Radio Channel Features

• Path Loss• Shadowing • Multipath Fading• Noise

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What is Fading?

• Signals from different paths are received with different delays – due to reflections of buildings, trees, etc.

• Each path has different attenuation and time delay (phase delay)

• Sometimes the relative phase shifts align, so the signal received from different add constructively, but at other time they cancel each other.

• This is called Fading

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Features of the Fading Component

• Path loss (Area-mean):– Hundreds or thousands of meters

– Reflection, diffraction, and scattering • Shadowing (Local-mean):

– A few tens or hundreds of meters– Caused by obstruction and motion

• Multipath fading (Short-term fast fading or small-scale fast fading):– Variation of the signal strength over a short distance on the

order of a few wavelengths or over short term duration on the order of seconds

– Due to multipath reflections of transmitted wave by local scatters, such as houses, buildings, etc, surrounding a MS

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Received Signal Characteristics

• Received signal consists of many multipath components

• Amplitudes change slowly• Phases change rapidly

– Constructive and destructive addition of signal components

– Amplitude fading of received signal (both wideband and narrowband signals)

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Typical Ways to Overcome Fading

• Diversity • Equalization• Forward error correcting codes and

interleaving• Increasing power• RAKE Receiver

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Impact of Radio Channel on System Design (1/2)

Parameters System Design Considerations Path Loss System coverage

System capacity (propagation exponent)

Shadowing System coverage (need more power margin in the link budget)

System capacity

Delay Spread

(Coherent Bandwidth)

Number of resolvable paths (path diversity)

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Impact of Radio Channel on System Design (2/2)

Parameters System Design Considerations

Doppler Spread

(Coherent Time)

Channel coding and interleaving Power control scheme Channel estimation

Fading Distribution Required Eb/Io System capacity and coverage

Power Delay Profile Path diversity Channel estimation Power control (SIR measurement)

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Modulation

• Why Modulation ?– Different frequency band has different

radio characteristic, like fading, interference, thermal noise, etc…

– Convert information bits to waveform of digital signal with a carrier belong to certain frequency band

– Modulated signal could be easily transmitted and received than signal without modulating

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Digital Modulation Techniques

• ASK (Amplitude Shift Keying)• PSK (Phase Shift Keying)• FSK (Frequency Shift Keying)• QAM (Quadrature Amplitude Modulatio

n)• Etc…

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BPSK and QPSK

10

BPSKSignal Space

Diagram

11

10

QPSKSignal Space

Diagram

00

01

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16-QAM and 64-QAM

16-QAMSignal Space

Diagram64-QAM

Signal SpaceDiagram

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Modulation/Demodulation Block Diagram

Modulator

Demodulator

BasebandSignal

Antennacos(2π fct)Serial to

Parallel

sin(2π fct)

BasebandSignal

Antennacos(2π fct)

Parallel toSerial

sin(2π fct)

Integrationand dump

Integrationand dump

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Cellular Concept

• Cell– Virtual Boundary of Radio

Coverage– Possible Issues

• Handover• Interference• Capacity

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Multiple Access Concept

• Multiple Access is used to distinguish each user while using Radio Access Service

• Multiple Access– FDMA (Frequency Division Multiple

Access)– TDMA (Time Division Multiple Access)– CDMA (Code Division Multiple Access)

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FDMA and TDMA

FDMAFDMA TDMATDMA

N

2

1

W

●●

●

Ts Time

Frequency

Ws

Ws

Ws

W ●●●

NTs Time

Frequency

N

Ts 2Ts

11 2 ●●●

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CDMA

CDMACDMA

●●●

NCode N

Code 2

1Code 1

2

FrequencyTs

Code

●●●

N

1

2

Time

W

●●●

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Duplex Concept

• Duplex is provided a two way Radio Access Service for each user– Uplink (reverse link)– Downlink (forward link)

• Duplex– TDD (Time Division Duplex)– FDD (Frequency Division Duplex)

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FDD and TDD

FDDFDD TDDTDD

UPLINK

DOWNLINK

Ts Time

Frequency

Wdl

Wul

W ●●●

NTs Time

Frequency

Ts 2Ts

UPLINK

●●●

UPLINK

DOWNLINK

DOWNLINK

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Handover

• When mobiles travel from cell to cell, changes of channels may occur and this action of changing channels is known as hand-over or hand-off.

• It is required that the call already established (or in progress) must not be interrupted

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Hard Handover

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Soft Handover

RNC

Cell 1

Cell 2

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Softer Handover

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Interference

• Inter Symbol Interference• Co-Channel Interference• Adjacent Channel Interference

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Inter-Symbol Interference (ISI)

symbol

Inter-symbolinterference

Time

Amplitude

Amplitude

Time

Tx Signal

Rx Signal

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Co-channel Interference (CCI) &

Adjacent Channel Interference (ACI)

fc fc‘

Channel Bandwidth Channel Bandwidth

fc = fc‘ fc fc

‘

Co-channelInterference

Adjacent ChannelInterference

Power

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Illustration of Cellular Frequency Reuse Concept

• Reuse Efficiency = i2+ij+j2

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Interference Contributions from Other Cells

• IS-95 Air Interface Standards

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Part I: CDMA Technology Overview

• Introduction to Wireless Communication– Radio Channel Characteristic– Modulation– Basic Concepts to Mobile Communication

• CDMA Technology Overview– What is “CDMA”– History of CDMA– Introduction to Spread Spectrum– Direct Sequence CDMA (DS-CDMA)– Frequency Hopping CDMA (FH-CDMA)– CDMA System Characteristic– Challenges in CDMA System Design

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CDMA Concept

CDMACDMA

●●●

NCode N

Code 2

1Code 1

2

FrequencyTs

Code

●●●

N

1

2

Time

W

●●●

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What is CDMA?

• A multiple access technique using pseudo-random (PN) codes to spread the spectrum of each user signal.

• Signals are shared a common wide-band channel at the same time.

• Signals are distinguished from each other by using different PN codes.

• Receiver processes only the desired signal. All other user signals appear as interference (multiple-access interference) to the desired signal.

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Pioneer CDMA Era

• 1949, Claude Shannon & Robert Pierce: basic idea of CDMA

• 1950, De Rosa-Rogoff: DSSS• 1956, Pierce & Green: RAKE receiver• 1961, Magnuski: Near-far problem• 1970s, Development for military & navig

ation systems

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Narrowband CDMA Era

• 1978, Cooper & Nettleton: Cellular application of SS

• 1980s, Investigation of NB-CDMA for cellular application

• 1984, DS-CDMA & hybrid CDMA/FDMA proposal for GSM

• 1986, Verdu: Formulation of optimal multiuser detection

• 1993, IS-95 standard

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Wideband CDMA Era

• 1995, WCDMA, CDMA2000• 2000, TD-SCDMA

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I ntegrate and DumpUser 1 Data Detected Output

Spreading Code(User 1)

Despreading Code(User 1)

f

f

f

NarrowbandInterference

Other userInterference

Concept of Spread Spectrum

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Definition of Spread Spectrum

• Spread spectrum is a mean of transmission in which the signal occupies a bandwidth in excess of the minimum necessary to send the information.

• Band spread is accomplished by means a code which is independent of the data, and a synchronized reception with the code at the receiver is used for despreading and subsequent data recovery.

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Spread/Depread Spectrum• Spectrum After Spreading/Before Despreading

• Spectrum After Despreading/Before Spreading

frequency

… …BSS

BSS

BT frequency… …

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Pseudorandom Codes

• The ideal spreading code would be an infinite sequence of equally likely random binary digits.

• In practice, periodic pseudorandom codes are used instead (PN codes).

• Specific PN codes include:– maximal-length code– Gold codes

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Principle of S.S. (1/4)

1 -1 1 1 ...

1 1 -1 -1 -1 1 -1

1 1 -1 -1 -1 1 -1-1 -1 1 1 1 -1 11 1 -1 -1 -1 1 -11 1 -1 -1 -1 1 -1

......

1 1 -1 -1 -1 1 -1

積 分 器 7 -7 7 7 ...U ser 1 D a ta D etec ted O u tp u t

S p read in g C o d e(U ser 1 )

D esp read in g C o d e(U ser 1 )

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-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 -11 -1 -1 -1 1 -1 1

......

1 1 -1 -1 -1 1 -1

積 分 器 1 1 1 -1 ...U ser 2 D ata D etec ted O u tp u t

S p read in g C o d e(U ser 2 )

D esp read in g C o d e(U ser 1 )

Principle of S.S. (2/4)

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1 -1 1 1 ...

1 1 -1 -1 -1 1 -1

0 2 0 0 -2 2 -2-2 0 2 2 0 0 00 2 0 0 -2 2 -22 0 -2 -2 0 0 0

......

1 1 -1 -1 -1 1 -1

積 分 器 8 -6 8 6 ...U ser 1 D ata D etec ted O u tp u t

S p read in g C o d e(U ser 1 )

D esp read in g C o d e(U ser 1 )

-1 -1 -1 1 ...

1 -1 -1 -1 1 -1 1S p read in g C o d e

(U ser 2 )

Principle of S.S. (3/4)

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

積 分 器U ser 1 D a ta D etec ted O u tp u t

S p read in g C o d e(U ser 1 )

D esp read in g C o d e(U ser 1 )

f

f

f

N arro w b an dIn te rfe ren ce

O th er u se rIn te rfe ren ce

Principle of S.S. (4/4)

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Spreading/Despreading

Data

Spreading code

Spread signal= Data x code

Spreading code

Data = Spreadsignal x code

1

-1

1

-1

1

-1

1

-1

1

-1

Symbol

Chip

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Block Diagram of a Typical S.S. System

Informationsource

Sourceencoder

{1,2,....,q}Encryptor

Channelencoder

Datamodulator

Spreadspectrummodulator

Poweramplification

(powerlimitation)

Informationsink

Sourcedecoder

DecryptorChanneldecoder

Datademodulator

Spreadspectrumdespreader

Receiverfrontend

Waveformchannel

(bandwidthlimitation)

Spreadingcode

generator

Timing andsynchronization

Discrete memoryless source

Noise

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Advantages of Spread Spectrum

• Multiple access capability– Low cross-correlation of the code

• Protection against multipath interference– Frequency diversity

• Privacy– Only the user knows the spreading code

• (NB) Interference rejection– The code will spread the received interference

• Anti-jamming capability– The code will spread the received jamming

• Low probability of interception (LPI)– Low power spectrum density

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Classification of CDMA

CDMA

Pure CDMA Hybrid CDMA

DS FH TH

Narrowband

Wideband

Fast FH

Slow FH

DS/FH DS/TH FH/TH DS/FH/TH TDMA/CDMA MC-CDMA

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Time/Freq. Occupancy of DS, FH, and TH Signals

Freq

uenc

y

Time

Freq

uenc

y

Freq

uenc

y

Time Time

x

x

x

x x

x

DS FH TH

x

x

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RAKE Receiver

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Maximal Ratio Combining (MRC) in RAKE

• Radio channel consist of many multipaths• Each multipath change the amplitude and the phase of the transmitted signa

l• The data in QPSK signal is in phase• Energy splitted to many fingers detected by the mached filter• MRC corrects the phase with channel amplitude estimate

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Challenges in CDMA System Design

• Soft Handover• Power Control

– Open loop– Closed loop (Inner and Outer loop)

• Code Design– Synchronization codes– Channelization codes– Scrambling codes

• Baseband Design– Synchronization (Frame/Slot/Symbol/Chip)– Searcher (find and maintain qualified paths)– AFC, AGC, and Clock Recovery– RAKE receiver (Channel estimation, RAKE fingers, and Maximal-ratio combini

ng)– Channel codec (convolutional codes, concatenated codes, turbo code

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Conclusion:Features of S.S and CDMA

• Features of CDMA– High spectrum efficiency– Release from frequency management– Low mobile station transmit power– Soft-handover– Path Diversity (RAKE Fingers)– Security– Anti-jamming capability

• Features of Wideband Spreading– High and wide variety of data rates– Improvement of multipath resolution– Statistical multiplexing efficiency