CDMA Spread Spectrum in Communications Systems...EE, MAJU CDMA Spread Spectrum EE-5713 Advanced Digital Communications 17 Week 11-12; Spring - 2013 K SS Signals M 1 ( k& ) B m & P

Muhammad Ali Jinnah University, Islamabad Campus, Pakistan

© Dr. Noor M Khan

EE, MAJU

CDMA Spread

Spectrum

1 EE-5713 Advanced Digital Communications

Week 11-12; Spring - 2013

CDMA Spread Spectrum in Communications Systems

ORIGINAL

SIGNAL

SIGNAL

NBI

MAI AWGN

SPECTRUM

SPREADING

MAI

NBI

SIGNAL

AWGN

SPECTRUM

DESPREADING

Lecturer: Assoc. Prof. Dr Noor M Khan

Department of Electronic Engineering,

Muhammad Ali Jinnah University,

Islamabad Campus, Islamabad, PAKISTAN

Ph: +92 (51) 111-878787, Ext. 147

Fax: +92 (51) 2822743

email: [email protected], [email protected]

mailto:[email protected]

mailto:[email protected]


© Dr. Noor M Khan

EE, MAJU

CDMA Spread

Spectrum



Spread Spectrum Modulation

• Spread spectrum modulation (SSM) uses signals

which have a transmission bandwidth that is

several orders of magnitude greater than the

minimum required RF bandwidth.

• A pseudo-noise (PN) sequence converts a

narrowband signal to a wideband noise-like signal

before transmission.

• SSM provides robust multiple access capability

and immunity against multi-path interference .


© Dr. Noor M Khan

EE, MAJU

CDMA Spread

Spectrum



SS Modulation Bandwidth

Efficiency

• SS modulation is not very bandwidth efficient when used by a single user only.

• When many users can share the same spread spectrum bandwidth without interfering with one another, spread spectrum systems become bandwidth efficient

– in a multiple user environment

– in a multiple cell environment.


© Dr. Noor M Khan

EE, MAJU

CDMA Spread

Spectrum



Types of Spread Spectrum

• Direct Sequence Spread Spectrum (DSSS)

– The carrier is modulated by a digital code

– Digital code chip rate is much larger then the information signal bit rate

• Frequency Hopping Spread Spectrum (FHSS)

– The carrier frequency is shifted in increments in a random pattern generated by code sequence

– The FHSS could be fast FHSS or slow FHSS

• Time Hopping Spread Spectrum (THSS)

– Transmission time is divided into frames and frames are divided in time slots.

– Signal transmission takes place in a random time slot controlled by a random code (THSS has limited application, so far )


© Dr. Noor M Khan

EE, MAJU

CDMA Spread

Spectrum



Frequency Hopped Multiple Access(FHMA)

• In FHMA the carrier frequencies of the individual users are varied in a pseudorandom fashion within a wideband channel.

• The instantaneous bandwidth of any one transmission burst is much smaller than the total spread bandwidth.

• At any given point in time, a frequency hopped signal only occupies a single, relatively narrow channel.

m(t)F) ω M( p(t)F) ω P(

mB

ω

aB

ω


© Dr. Noor M Khan

EE, MAJU

CDMA Spread

Spectrum



FH Classification

• Fast frequency hopping system

– If the rate of change of the carrier frequency is greater than the symbol rate.

• Slow frequency hopping

– If the channel changes at a rate less than or equal to the symbol rate..

• A frequency hopped system provides a certain level of security.

– An intercepting receiver not knowing the PN sequence of frequency slots must chase rapidly the signal it wishes to intercept.


© Dr. Noor M Khan

EE, MAJU

CDMA Spread

Spectrum



DS SS System Model

Transmitter

Receiver

Channel

r(t)

a(t)

p(t)

n(t)

d(t)

m(t)

a(t)

m(t) – Message

a(t) – Spreading Code

p(t) – Spreaded Signal

n(t) – AWGN Noise

r(t) – Received Signal

d(t) – Despread Signal

n(t)a(t)m(t)r(t)

a(t)r(t)d(t)


© Dr. Noor M Khan

EE, MAJU

CDMA Spread

Spectrum



DS SS Modulation in

Time and Frequency

1 -1

1 1 -1

Time Frequency


© Dr. Noor M Khan

EE, MAJU

CDMA Spread

Spectrum



Spreading Code, a(t)

Properties

m(t)F) ω M(

a(t)F) ω A(

mB

aB

ω

ω

1a(t)a(t) 1)

a(t)r(t)d(t)

ma BB 2)

n(t)a(t)a(t)a(t)m(t)d(t)

(t)nm(t)d(t) a


© Dr. Noor M Khan

EE, MAJU

CDMA Spread

Spectrum



Spectrum of SS Signal

m(t)F) ω M(

a(t)F) ω A(

mB

ω

aB

ω ma BB since

a(t)m(t)p(t)

) ω A() ω M() ω P(

p(t)F) ω P(

aB

ωmB

ap BB

m

a

B

B=G


© Dr. Noor M Khan

EE, MAJU

CDMA Spread

Spectrum



Spread Spectrum System

m0

2 BNσ

a

m

BG =

B

power signal -P

TkN 0

) ω M(

ω

) ω P(

aB

mBP

) ω M(

mB

1a(t)2

2SpreadGσ

PSNR

Before Spreading

After Spreading

After Despreading ω

ω

mB

P

2oσ

PSNR


© Dr. Noor M Khan

EE, MAJU

CDMA Spread

Spectrum



Hiding Signal in Noise

mB

P

AWGN TkN 0

Before despreading

LPI – Low Probability of Interception

After despreading

) ω P(

) ω D(

AWGN

P


© Dr. Noor M Khan

EE, MAJU

CDMA Spread

Spectrum



Two Signals in Noise

n(t)(t)(t)am(t)(t)amr(t) 2211

(t)n(t)a(t)(t)ap(t)a(t)(t)am(t)d 1121111

(t)r(t)a(t)d 11

(t)n(t)a(t)(t)ap(t)m(t)d 11211

We want to find output SNRo to evaluate the Performance,

i.e. the BER

Signal Interference Noise


© Dr. Noor M Khan

EE, MAJU

CDMA Spread

Spectrum



Two SS Signals

)(M1

mB

P

)(M~

1

)(P1

aB

TkN 0

mB

P

)(M2

mB

P

)(P2

aB a2B

Interference

AWGN

Signal TkN 0


© Dr. Noor M Khan

EE, MAJU

CDMA Spread

Spectrum



Output Signal to Noise Ratio

P - power Signal

G

SNR1

SNR

G

Pσ

PSNR

m

m

2o

2σ - power Noise

G

P - power ceinterferen Effective

G

Pσ - power noise Total 2


© Dr. Noor M Khan

EE, MAJU

CDMA Spread

Spectrum



k Signals in Noise

n(t)(t)(t)am(t)(t)am(t)(t)amr(t) kk2211

(t)n(t)a(t)(t)ap(t)(t)ap(t)a(t)(t)am(t)d 11k121111

(t)r(t)a(t)d 11

(t)n(t)a(t)(t)ap(t)(t)ap(t)m(t)d 11k1211

Again we want to find output SNRo; -> BER

Signal Interference Noise


© Dr. Noor M Khan

EE, MAJU

CDMA Spread

Spectrum



K SS Signals

) ω (M1

mB

ω

P

) ω (M1

~

)ω(P1

aB

ωTkN 0

mB

ω

P

ω

P

aB a2B

Interference AWGN

Signal TkN 0

ω

P

) ω (M2 ) ω (Mk

)ω(P2 )ω(Pk

mB mB


© Dr. Noor M Khan

EE, MAJU

CDMA Spread

Spectrum



Output Signal to Noise Ratio

P - power Signal

m

m

2o

SNRG

1)-(k1

SNR

G

1)P-(kσ

PSNR

2σ - power Noise

G

1)P-(k - power ceinterferen Effective

G

1)P-(kσ - power noise Total 2


© Dr. Noor M Khan

EE, MAJU

CDMA Spread

Spectrum



High SNRm

m

m

2o

SNRG

1)-(k1

SNR

G

1)P-(kσ

PSNR

1-k

GSNRo

oSNR

G1-kk


© Dr. Noor M Khan

EE, MAJU

CDMA Spread

Spectrum



CDMA Frequency Reuse

A G

F E

C

D

B

G

F E

C

D A

B

G

F E

C

D A

B

CDMA

Reuse Factor =1/7 Reuse Factor =1

FDMA/TDMA


© Dr. Noor M Khan

EE, MAJU

CDMA Spread

Spectrum



Overview of Adaptive

CDMA Multi-user Detection

BS

ORIGINAL

SIGNAL

SIGNAL

NBI

MAI AWGN

SPECTRUM

SPREADING

MAI

NBI

SIGNAL

AWGN

SPECTRUM

DESPREADING


© Dr. Noor M Khan

EE, MAJU

CDMA Spread

Spectrum



Technical challenges

• Is the mobile Internet possible in the same form as the wire-line Internet?

• Does the existing CDMA-based 3G mobile communication infrastructure support increased capacity demand of the future mobile Internet?

• If not, how it can be fixed?

• Does Adaptive Multi-user Detection help to solve any of the mentioned questions?


© Dr. Noor M Khan

EE, MAJU

CDMA Spread

Spectrum



Convergence of Applications

• Old but Still Existing Services: – Radio, TV, Telephone, VCR, CD, 1G/2G Cellular;

– Very LIMITED choice, Capacity Req: << 1 Mbps/user

– Dedicated Hardware.

• Current Services: – CD/DVD, WiFi, cable TV, Satellite Phone, 3G Cellular, Wireless

Broadband Internet Access, Personal Video;

– Large but still LIMITED choice, Capacity Req: = 1 Mbps/user

– Somewhat converged but still Dedicated Hardware.

• Future Services: – Wireless Broadband, Games, IP Library, IPTV , Personal Video, 4G

– Unlimited choice Capacity Req: >> 1 Mbps/user

– Internet software terminal, Software Defined Radio (SDR)


© Dr. Noor M Khan

EE, MAJU

CDMA Spread

Spectrum



Wireless Cellular Mobile

System

PSTN MSC


© Dr. Noor M Khan

EE, MAJU

CDMA Spread

Spectrum



Migration Path to 4G

Mobile Comm. Systems • First Major Migration Path (Europe…)

– I Gen, 80’s, ETACS (C-450,NMT-450..),(FDMA), Analog

– II Gen, 90’s, GSM, GPRS, EDGE, (TDMA) Digital

– III Gen, 00’s, W-CDMA , (CDMA), All Digital

• Second Major Migration Path (USA)

– I Gen, 80’s, AMPS, (FDMA), Analog

– II Gen, 90’s, IS-54 (TDMA), IS-95 (CDMA), Digital

– III Gen, 00’s, CDMA2000 (CDMA), All Digital

• Universal 3GPP Migration Path (USA, Europe and Asia)

– IV Gen, 10’s, HSPA+-A, EVDO-A, 1x-A (CDMA), All Digital

– IV Gen, 10’s, LTE-A, WiMAX-MAN (OFDM), All Digital


© Dr. Noor M Khan

EE, MAJU

CDMA Spread

Spectrum



Mobile Network Capacity

3G Cellular

Info

rma

tio

n B

it R

ate

, M

b/s

Office Building Stationary Walking Vehicle

Indoors Outdoors

Wir

ed

Te

rmin

al

Cordless

2G Cellular and PCS

WLAN &

WiMAX

MBS

60GHz 10

0.1

1.0

100 This is

where we

want to

be for 4G

MOBILITY


© Dr. Noor M Khan

EE, MAJU

CDMA Spread

Spectrum




© Dr. Noor M Khan

EE, MAJU

CDMA Spread

Spectrum



Communication

Channel Capacity

• Communication Channel Resources are:

• T - Time

• B - Bandwidth

• - Signal to Noise Ratio

B

T

I

S

22

P1logSσ2σ

P

I = T B S

22σ

P1logBC

•Amount of information (number of bits)

transmitted/received, with arbitrarily

small probability of error (Shannon 1948)


© Dr. Noor M Khan

EE, MAJU

CDMA Spread

Spectrum



Multiple Access

FDMA

AMPS

S

B

T

B

S T

B

S T

TDMA GSM

CDMA IS-95

B

T

I S


© Dr. Noor M Khan

EE, MAJU

CDMA Spread

Spectrum



CDMA a Case of PDMA

Multi-user channel

;

2

121

PN

P1logR rates bit with

N

P1logR 2

22

nxxy 21

PPP with 21

N

P1log

PN

P1logRR 2

2

2

1221

C.RR satisfies 21

N

P1logCcapacity user single and 2


© Dr. Noor M Khan

EE, MAJU

CDMA Spread

Spectrum



2 Users Multi-user Channel

N

P1

PN

P1logRR 2

2

1221

N

PPNlog 21

2

CN

Plog2

1

(ab)logblogalog 222

b

ab

b

a1

PPP 21

N

PN

PN

PPNlog 2

2

122

used identities


© Dr. Noor M Khan

EE, MAJU

CDMA Spread

Spectrum



N Users Multi-user Channel

The same applies for channel

With power limitation

And data rates

nxxxy m21 ...

m21 PPPP ...

;...

231m-m

121

PPPPN

P1logR

....

31m-m

222

PPPN

P1logR

;N

P1logR m

2m

.... m21 RRRC


© Dr. Noor M Khan

EE, MAJU

CDMA Spread

Spectrum



N Users Multi-user Channel

..........

;N

P1log

PPPN

P1log

PPPPN

P1logC m

2

31m-m

22

231m-m

12

.... m21 RRRC

N

PN

PN

PPN

PPPN

PPPPN

PPPPN

PPPPPNlogC m

m

1m-m

31m-m

231m-m

231m-m

1231m-m2 ...

...

...

...

...

N

PPPPPNlogC 1231m-m

2

...

N

PlogC 2 1

nxxxy m21 ...m21 PPPP ...

nxy

PP

N

PlogC 2 1


© Dr. Noor M Khan

EE, MAJU

CDMA Spread

Spectrum



Multi-user Channel

• The previous result requires involvement of powerful

coding techniques and interference cancellation

• In reality channel coding is part of every current digital

mobile communication standards

• Interference cancellation/suppression is still a hot research

topic

• In order to implement power division multiple access

(PDMA) concept in current practice we rely on the spread

spectrum technique such as code division multiple access

(CDMA)


© Dr. Noor M Khan

EE, MAJU

CDMA Spread

Spectrum



Why CDMA?

• Advantages of CDMA (IS-95, CDMA2000, WCDMA)

– Digital System (extensive use of digital algorithms)

– Voice activity factor (flexible use of channel resources)

– Frequency reuse factor (All cells on the same frequency)

– Fading resistant (wideband system, RAKE diversity)

• Limitations of CDMA (IS-95, CDMA2000, WCDMA)

– Near-Far effect

– Necessity of strict user power control


© Dr. Noor M Khan

EE, MAJU

CDMA Spread

Spectrum



CDMA System Model

+

Transmitter 1 x1

s1

Transmitter 2 x2

s2

Transmitter K xK

sK

Receiver 1 x1

s1

Receiver 2 x2

s2

Receiver K xK

sK

^

^

^

AWGN

S

y=Sx+n

y

y

y


© Dr. Noor M Khan

EE, MAJU

CDMA Spread

Spectrum



Multi-user Receivers Classification

Optimality

criterion

Multi-user Detectors Adaptive Multi-user Detectors

Source of

Sufficient

Statistics

Type of

detector

Short

Name

Ref. Source of

Sufficient

Statistics

Type of

detecto

r

Short

Name

Ref.

ML; BER MF Single user

limiter

MF Pursley77

Pursley82

ML; ZF MF Decorelator

R-1

MF-

DEC

Schneider80

Lupas89

MMSE MF Symbol

MMSE

(R+2I)-1

MF-

MMSE

Xie90

Madhow94

A-MMSE Single

user

limiter

MMSE Rapajiic 92-94

Miller95

ZF; MMSE MF DFE MF-

CDEC

Duel-Hallen

93-95

A-MMSE DFE MMSE-

DFE

Abdulrahman92-94 ;

DFE MMSE-

CDFE

Woodward2000 ;

Rapajiic 94

ML MF ML MF-

ML

Verdu86 A-MMSE ML MMSE-

ML

Rapajiic 99

Borah2001

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CDMA Spread Spectrum in Communications Systems...EE, MAJU CDMA Spread Spectrum EE-5713 Advanced Digital Communications 17 Week 11-12; Spring - 2013 K SS Signals M 1 ( k& ) B m & P