Multiple Access Techniques - ARWiC Ali Jinnah University, Islamabad Campus, Pakistan Multiple Access Techniques K Instructor: Prof. Dr. Noor M. Khan Department of Electrical Engineering,Authors:

Muhammad Ali Jinnah University, Islamabad Campus, Pakistan

Multiple Access Multiple Access Multiple Access Multiple Access Techniques Techniques Techniques Techniques

K

Instructor: Prof. Dr. Noor M. KhanDepartment of Electrical Engineering,Faculty of Engineering,Mohammad Ali Jinnah University,

Dr. Dr. Dr. Dr. NoorNoorNoorNoor M KhanM KhanM KhanM Khan

EE, MAJUEE, MAJUEE, MAJUEE, MAJUECE6473 Cellular Mobile Communications

Week 3Week 3Week 3Week 3----4444----5; Summer 5; Summer 5; Summer 5; Summer ---- 2015201520152015Multiple Access Techniques 1

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Mohammad Ali Jinnah University,Islamabad Campus, Islamabad, PAKISTANPh: +92 (51) 111-878787Ext. 129 (Office), 186 (ARWiC Lab) Fax: +92 (51) 2822743 emails: [email protected],

[email protected]


Cellular System




PSTNMSC


Frequency Reuse Pattern

BB

f

Bandwidt

h AG

FE

C

D

B




AG

FE

C

D

B

A

G

F

E

C

D

BE

AG

FE

C

D

B


BS3

Handoff (Handover)




BS1

BS2


Single Cell Multiple Access

MNUplink

Downlink




BS

M1

M2

MK


Multiple Access System

f

Bandwidth

‘A’ cell‘A’ cell‘A’ cell‘A’ cell

M1 M2 MK




BS

M1

M2

MK


Multiple Access Techniques

• Many mobile users to share the available radio spectrum simultaneously– Multiple Access

• Frequency division multiple access - FDMA




• Frequency division multiple access - FDMA

• Time division multiple access - TDMA

• Code division multiple access - CDMA

• Space division multiple access - SDMA

• Packet Radio Access


Duplex System

Uplink

Downlink




BS

Downlink = Forward Link

Uplink = Reverse Link


Duplexing

• Need to talk and listen simultaneously– Duplexing (uplink and downlink for the same user)

• Two types of duplexing schemes– Frequency division duplexing - FDD




– Frequency division duplexing - FDD

– Time division duplexing – TDD (3G, 4G)

• For example: there could be – FDMA/FDD (AMPS)or

– TDMA/FDD (GSM, IS-54) etc.


Channel Capacity C

• Amount of information (number of bits) transmitted/received C, with vanishingly small probability of error (Shannon 1948)

C




RECEIVERTRANSMITTER CHANNEL

+=NP

1logBC 2

C –Capacity

B –Bandwidth

P –Signal Power

N –Noise Power

CB

P

N


Communication Channel Capacity

• Communication Channel Resources are:• T - Time

• B - Bandwidth

• - Signal to Noise Ratio

+= 22

P1logS

σ

P




• - Signal to Noise Ratio

B

T

I

S

+= 22 1logSσ2

σ

I = T B S

+= 22σ

P1logBC

•Amount of information (number of bits) transmitted/received, with arbitrarily small probability of error (Shannon 1948)


FDMA

• Each user is allocated a unique frequency band or channel

• The allocation is done on demand




FDMA

AMPS

B

S T

B

T

I

S


FDMA Features

• One channel carries one phone circuit

• Idle speech periods results in wasted bandwidth




bandwidth

• Bandwidth of the channels are relatively narrow (for example: 30 kHz)

• Synchronous transmission with low overheads


Example: Start of AMPS• During this time the American cellular radio

system, known as the advanced mobile phone system, or AMPS, was developed primarily by AT&T and Motorola, Inc.

• AMPS was based on 666 paired voice channels,




• AMPS was based on 666 paired voice channels, spaced every 30 kilohertz in the 800-megahertz region.

• AMPS system employed an analog-frequency modulation, and was designed to support both mobile and portable subscriber units.


AMPS FrequenciesBand Frequency Range (MHz ) Use

A 824 to 835 and 845 to 846.5 Transmit from mobile

869 to 880 and 890 to 891.5 Receive at mobile




869 to 880 and 890 to 891.5 Receive at mobile

B 835 to 845 and 846.5 to 849 Transmit from mobile

880 to 890 and 891.5 to 894 Receive at mobile

416 channels of 30 kHz


FDMA Features• Higher cell site system costs than TDMA

– High cost of band-pass filters and other RF circuitry.

• Tight RF filtering – Mitigating adjacent channel interference

– Narrowband System; Very strong flat fading




– Narrowband System; Very strong flat fading

• The number of channels that can be simultaneous supported N= (Bt-2Bguard)/Bc

Where, Bt is total spectrum allocation; Bguard is guard band and Bc is channel bandwidth

• Synchronization not necessary


TDMA• Each user occupies a cyclically repeating time

slot

• Asynchronous transmission – digital modulation must be used & there are overheads

for synchronization.




for synchronization.

B

S T

TDMAGSM

B

T

I

S


TDMA Frame Structure





GSM Frame Structure

In GSM, a set of 8 TDMA slots is called a frame.

Mobile terminals transmit in bursts of 577 microseconds. One burst fits into one time slot.




There are two types of GSM frames:� Traffic Frames� Signaling Frames


GSM Burst Structure A normal GSM burst contains:� 3 tail bits allowing the transmitter to power up � 57 coded data bits � 1 bit stealing flag � 26 bits training sequence for synch and adaptive equalizer




� 26 bits training sequence for synch and adaptive equalizer� 1 bit stealing flag � 57 coded data bits � 3 tail bits � Guard time corresponding to 8.25 bits

In GSM, a terminal has 28 microseconds of guard time to power up or switch off the RF signal.


TDMA Features

• Shares single carrier frequency with several users• The number of time slots per frame depends on -

modulation technique, available bandwidth, etc.• Because of discontinuous transmission in TDMA,




• Because of discontinuous transmission in TDMA, the handover process is much simpler– A mobile can listen for other base stations during idle

time slots

• Possibilities for Bandwidth on Demand• Less sensitive to fading than FDMA


Efficiency• The frame efficiency ηf, is the percentage of bits

per frame which contain transmitted data

• Number of overhead bits per frame is:

grgtptrrOH bNbNbNbNb +++=




Nr - no of reference bursts per frame,

Nt - no of traffic bursts per frame,

bp - no of OH bits per preamble in each slot and

bg - no of equivalent bits each guard time

br - no of OH bits per reference burst

grgtptrrOH bNbNbNbNb +++=


Efficiency

• Total number information bits per frame

bT = Tf R

• T -frame duration and




• Tf -frame duration and

• R- channel bit rate

• Then

−=

T

OHf b

b1η


Spread Spectrum Multiple Access (CDMA)

• Uses signals that have BW >> minimum required RF bandwidth

• Pseudo-noise sequence converts a narrow band signal to a wideband noise like signal




signal to a wideband noise like signal

• Efficiency increases with the number of users

• Two main types of Spread Spectrum MAs– Frequency hopped (FH)

– Direct sequence (CDMA)


Frequency Hopping MA

• Carrier frequencies of individual users are varied in pseudo randomly (using PN)

• Data is broken into uniformed sized bursts and transmitted on different carrier frequencies




transmitted on different carrier frequencies• Instantaneous BW of a any one transmission is

much smaller than the total spread BW• Locally generated PN code is used to synchronize

the receivers instantaneous frequency with the transmitters


FHMA

• At any given time FH signal occupies a single relatively narrow channel

• If the rate of change of carrier is greater then the data symbol rate FHMA system is called fast frequency hopping




hopping• If the rate of change of carrier is smaller then the data

symbol rate FHMA system is called slow frequency hopping

• FHMA provides higher level of– Security– Protection against signal fading


Frequency Hopped Multiple Access(FHMA)

• In FHMA the carrier frequencies of the individual users arevaried in apseudorandom fashion within a wideband channel.

• The instantaneous bandwidth of any one transmission burstis muchsmaller than the total spread bandwidth.

• At anygivenpoint in time, a frequencyhoppedsignalonly occupiesa




• At anygivenpoint in time, a frequencyhoppedsignalonly occupiesasingle, relatively narrow channel.

{ }m(t)F) ω M( = { }p(t)F) ω P( =

mB

ω

aB

ω


Code Division MA (CDMA)• The signal is multiplied by a very large bandwidth

spreading signal• Spreading signal is a PN code sequence that has a chip rate

much larger then the data rate of the message• All users use the same carrier frequency




B

T

I

S S

B

T

CDMAIS-95


Direct Sequence Spread Spectrum System Model

TransmitterTransmitterTransmitterTransmitter

ChannelChannelChannelChannel

p(t)p(t)p(t)p(t)

n(t)n(t)n(t)n(t)

m(t)m(t)m(t)m(t)

ssss(t(t(t(t))))

m(t) m(t) m(t) m(t) –––– MessageMessageMessageMessage

s(ts(ts(ts(t) ) ) ) –––– Spreading CodeSpreading CodeSpreading CodeSpreading Code

p(t) p(t) p(t) p(t) –––– Spread SignalSpread SignalSpread SignalSpread Signal

n(t) n(t) n(t) n(t) –––– AWGN NoiseAWGN NoiseAWGN NoiseAWGN Noise




ReceiverReceiverReceiverReceiverr(t)r(t)r(t)r(t)

s(ts(ts(ts(t))))

d(t)d(t)d(t)d(t)

n(t) n(t) n(t) n(t) –––– AWGN NoiseAWGN NoiseAWGN NoiseAWGN Noise

r(t) r(t) r(t) r(t) –––– Received SignalReceived SignalReceived SignalReceived Signal

d(t) d(t) d(t) d(t) –––– Despread SignalDespread SignalDespread SignalDespread Signal

n(t)m(t)s(t)r(t) +=r(t)s(t)d(t) =


DS SS Modulation in Time and Frequency

1111----1111

1111 1111----1111

m(t) m(t) m(t) m(t) –––– MessageMessageMessageMessage


Frequency DomainFrequency DomainFrequency DomainFrequency DomainTIME DomainTIME DomainTIME DomainTIME Domain




TimeTimeTimeTime FrequencyFrequencyFrequencyFrequency


p(t) p(t) p(t) p(t) –––– Spread SignalSpread SignalSpread SignalSpread Signal

BandBandBandBand----limited Spread limited Spread limited Spread limited Spread

Spectrum SignalSpectrum SignalSpectrum SignalSpectrum Signal


Properties of the Spreading Code, s(t)

{ }m(t)F) ω M( =

ω

1s(t)s(t) 1) =⋅

mS BB 2) >>




{ }s(t)F) ω S( =

mB

SB

ω

s(t)r(t)d(t) ⋅=n(t)s(t)s(t)s(t)m(t)d(t) +=

(t)nm(t)d(t) a+=


Spectrum of SS Signal{ }m(t)F) ω M( =

{ }s(t)F) ω S( =mB

ω s(t)m(t)p(t) =

) ω S() ω M() ω P( ⊗=




{ }s(t)F) ω S( =

sB

ω mS BB since >>

{ }p(t)F) ω P( =

sB

ω

mB

Sp BB ≈

m

s

BB

=G


Spread Spectrum System

m02 BNσ =

sB=G

power signal -P) ω M(

ω

) ω P( mBP

1 s(t) 2 =

Before Spreading

After

mB




m

s

BB

=G

TkN 0 =

) ω P(

sB

mB

) ω M(

mB

2b GσP

SNR =

2oσ

PSNR =

After Spreading

After Despreading ω

ω

P


Hiding Signal in Noise

ωAWGN TkN

0=

Before despreading

) ω P(

) ω D(

P



Week 3Week 3Week 3Week 3----4444----5; Summer 5; Summer 5; Summer 5; Summer ---- 2015201520152015CDMA Spread Spectrum

mB

ω

P

LPI – Low Probability of Interception

After despreading

) ω D(

AWGN


Multiple Access

S TS TS

T




FDMA

AMPS

BBB

TDMA

GSM

CDMA

IS-95


Two CDMA Signals)(M 1 ω

mB

ωP

)(P1 ω

)(M 2 ω

mB

ωP

)(P2 ω




)(M~

1 ω

)(P1 ω

sB

ωTkN 0 =

mB

ωP

)(P2 ω

sB s2B

InterferenceAWGN

Signal TkN 0 =


Two Signals in Noise

n(t)(t)(t)sm(t)(t)smr(t) 2211 ++=

(t)n(t)s(t)(t)s(t)sm(t)s(t)(t)sm(t)d ++=

(t)r(t)s(t)d 11 =




(t)n(t)s(t)(t)s(t)sm(t)s(t)(t)sm(t)d 11221111 ++=

(t)n(t)s(t)(t)sp(t)m(t)d 11211 ++=

We want to find output SNRo to evaluate BER

Signal Interference Noise


Output Signal to Noise Ratio

P - power Signal >2σ - power Noise >

GP

- power ceinterferen Effective >




GSNR

1

SNR

GP

σ

PSNR

m

m

2o

+=

+=

G - power ceinterferen Effective >

GP

σ - power noise Total 2 +>


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


K SS Signals ) ω (M1

mB

ω

P

)ω(P1

ω

P

ω

P•••

•••

) ω (M2 ) ω (Mk

)ω(P2 )ω(Pk

mB mB




) ω (M1

~aB

ωTkN 0 =

mB

ω

P

aB a2B

InterferenceAWGNSignal TkN 0 =

2 )ω(Pk


Output Signal to Noise Ratio

P - power Signal >

2σ - power Noise >

G1)P-(k

- power ceinterferen Effective >




m

m

2o

SNRG

1)-(k1

SNR

G1)P-(k

σ

PSNR

+=

+=

G - power ceinterferen Effective >

G1)P-(k

σ - power noise Total 2 +>


CDMA

• Each user has its own PN code which is orthogonal (near-orthogonal) to all other PN codes

• To receive the receiver needs to know the code used by the transmitter




used by the transmitter

• The power of other users at a receiver determines the noise floor after signal demodulation

• Therefore it is necessary to control the power of each user in order to avoid excessive interference


CDMA Features

• Many users share the same frequency

• Increasing the number of users does not raise the noise floor in a linear manner




raise the noise floor in a linear manner

• Multi-path fading may be substantially reduced

• Channel data (chip) rates are very high


Space Division MA (SDMA)

• Controls the radiated energy for each user in space

• Servers different users using directional




• Servers different users using directional antennas

• The different areas may be served by the same frequency (TDMA or CDMA) or different frequencies (FDMA)


Smart Antenna System (SDMA) (1)

MK

N

K




BS

M1

M2

1

2

K


Smart Antenna System (SDMA) (2)

N

K

T

I

S




BS2

K

1

S

B

T

I

•N times the original amount of information I

•Smart Antennas are reusing channel resources B,T,S

B

T

I

S

BI

B

T

I

S


Orthogonal Frequency Division Multiple Access (OFDMA)

• A method of encoding digital data on multiple carrier frequencies

• A method of digital modulation in which a




signal is split into several narrowband channels at different frequencies.

• A digital transmission technique that uses a large number of carriers spaced apart at slightly different frequencies


• Total bandwidth available within a communications system is divided into smaller non-overlapping frequency sub-bands

OFDM: Operation




smaller non-overlapping frequency sub-bands

• Usually a separate data signal is associated to each frequency sub-band

• Passband filter at receiver extracts requested frequency sub-band / data signal


• Each terminal occupies a subset of sub-carriers

• Subset is called an OFDMA traffic channel

OFDMA




• Subset is called an OFDMA traffic channel

• Each traffic channel is assigned exclusively to one user at any time


• It is a special kind of FDM

• The spacing between carriers are such that they are orthogonal to one another

OFDM System




they are orthogonal to one another

• Therefore, there is no need of guard band between carriers.


OFDM: Operation





OFDM: Operation





OFDM: System Model





OFDM: System Model





OFDM: System Model





OFDM: An ExampleLet’s we have following information bits

1, 1, -1, -1, 1, 1, 1, -1, 1, -1, -1, -1, -1, 1, -1, -1, …

Just convert the serial bits to parallel bits

C1 C2 C3 C4




1 1 -1 -1

1 1 1 -1

1 -1 -1 -1

-1 1 -1 -1

-1 1 1 -1

-1 -1 1 1


OFDM: An ExampleModulate each column with corresponding sub-carrier using BPSK




Modulated signal for C1 Modulated signal for C2

Modulated signal for C3 Modulated signal for C4


OFDM: An Example• Final OFDM Signal = Sum of all signal

)2sin()()(1

0nttItV

N

nn

π∑−

==




Generated OFDM signal, V(t)


OFDMA’s Encouraging Features• Can easily adapt to severe channel conditions without complex

time-domain equalization.

• Robust against narrow-band co-channel interference.

• Robust against ISI and fading caused by multipath propagation.

• High spectral efficiency as compared to conventional modulation




• High spectral efficiency as compared to conventional modulation schemes, spread spectrum, etc.

• Efficient implementation using Fast Fourier Transform (FFT).

• Low sensitivity to time synchronization errors.

• Tuned sub-channel receiver filters are not required (unlike conventional FDM).


OFDMA’s Discouraging Features

• Sensitive to Doppler shift.

• Sensitive to frequency synchronization problems.

• High peak-to-average-power ratio (PAPR), requiring linear transmitter circuitry, which




requiring linear transmitter circuitry, which suffers from poor power efficiency.

• Loss of efficiency caused by cyclic prefix/guard interval.


Packet Radio

• Many subscribers attempt to access a single channel in an uncoordinated or minimally coordinated manner




coordinated manner

• Transmission is done by using bursts of data

• Collisions from simultaneous transmission of multiple transmitters are detected by the BS - ACK or NACK


Packet Radio

• ACK/NACK provides perfect feedback, even though traffic delay due to collisions may be high




may be high

• Very easy to implement but has low spectral efficiency and may induce delays


Packet Radio

• Subscribers use a contention technique to transmit on a common channel – Eg. ALOHA




• The performance of packet radio schemes can be evaluated by – the throughput (T) - the average no of messages

successfully transmitted per unit time and

– the average message delay (D)


Packet Radio Protocols





Packet Radio Protocols• Assume that the system uses fixed packet sizes and

fixed channel data rates

• Packets are generated by users randomly and packet transmissions occur with a Poisoon distribution having a mean arrival rate of λ pps




having a mean arrival rate of λ pps

• If τ is the packet duration, in seconds, the traffic occupancy

R= λ τ• R - normalized channel traffic


Throughput

)Pr(eR

nVRn

V−

=




!)Pr(

neR

nV

V=

VVR λτ=


Throughput

!)Pr(

neR

nVRn

V−

=




!)Pr(

nn =

Ve λτ−=)0Pr(


Pure ALOHA

• User grabs a channel as soon as a message is ready for transmission

• After transmission, the user waits for an ACK/NACK (acknowledgement)




ACK/NACK (acknowledgement)

• In the case of collisions, the user waits for a random period of time and re-transmits

• The vulnerable period is double the packet length


Pure ALOHA

0at)2(

)Pr(2

==−

neR

nRn




0at!

)2()Pr( == n

n

eRn


Slotted ALOHA• Time is divided into equal time slots of length

greater than the packet duration τ• Subscribers transmit messages only at the beginning

of a new time slot

• The vulnerable period is only one packet duration




• The vulnerable period is only one packet duration

• The probability that no other packet will be generated during the vulnerable period is e-R.

• Then the throughput of the slotted ALOHA is:

T = Re-R


ALOHA Throughput




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Multiple Access Techniques - ARWiC Ali Jinnah University, Islamabad Campus, Pakistan Multiple Access Techniques K Instructor: Prof. Dr. Noor M. Khan Department of Electrical Engineering,Authors: