1 CS 6910 – Pervasive Computing Spring 2007 Section 7 (Ch.7): Multiple Access Techniques Prof. Leszek Lilien Department of Computer Science Western Michigan.

1

CS 6910 – Pervasive ComputingSpring 2007

Section 7 (Ch.7):

Multiple Access Techniques

Prof. Leszek LilienDepartment of Computer Science

Western Michigan University

Slides based on publisher’s slides for 1st and 2nd edition of: Introduction to Wireless and Mobile Systems by Agrawal & Zeng

© 2003, 2006, Dharma P. Agrawal and Qing-An Zeng. All rights reserved.

Some original slides were modified by L. Lilien, who strived to make such modifications clearly visible. Some slides were added by L. Lilien, and are © 2006-2007 by Leszek T. Lilien.

Requests to use L. Lilien’s slides for non-profit purposes will be gladly granted upon a written request.

Copyright © 2003, Dharma P. Agrawal and Qing-An Zeng. All rights reserved 2

Chapter 7 Multiple Division (Access)

Techniques


Outline 7.1. Introduction 7.2. Concept and Models for Multiple Access

7.2.1. Frequency Division Multiple Access (FDMA) 7.2.2. Time Division Multiple Access (TDMA) 7.2.3. Code Division Multiple Access (CDMA)

1) Introduction 2) Spread Spectrum 3) Direct Sequence Spread Spectrum (DSSS) 4) Frequency Hopping Spread Spectrum (HFSS) 5) Walsh Codes 6) Near-far Problem 7) Power Control

7.2.4. OFDM 7.2.5. SDMA 7.2.6. Comparison of FDMA, TDMA, and CDMA

7.3. Modulation Techniques 7.3.1. AM (Amplitude Modulation) 7.3.2. FM (Frequency Modulation) 7.3.3. FSK (Frequency Shift Keying) 7.3.4. PSK (Phase Shift Keying) 7.3.5. QPSK (Quadrature Phase Shift Keying) 7.3.6. /4 QPSK 7.3.7.QAM (Quadrature Amplitude Modulation) 7.3.8. 16QAM

(Modified by LTL)


7.1. Introduction

© 2007 by Leszek T. Lilien

Recall Large # of traffic

channels on each BS Bec. traffic channels used by

1 MS exclusively for call duration

Small # of controlchannels on each BS

Bec. control channelsshared by many MSsfor short periods

Too expensive/inefficient to assign control channnel for call duration

MS gets a traffic channel assigned for call duration Once assigned, no need to compete for access to traffic channels

As was the case for control channels

BS

Forward

(downlin

k) contro

l channel

MS

Reverse (

uplink) c

ontrol c

hannel

Forward

(downlin

k) traff

ic channel

Reverse (

uplink) tr

affic

channel


Introduction – cont.

For control channels — we used contention-based protocols Many MS’s competing for the same control channel

For traffic channels — we use now contention-free protocols Dedicated channel for each MS (not shared with other MSs)

Allocated by BS When “this” MS requestsOR: When another MS tries to reach “this” MS

Allocated thanks to exchange of control messages over control channels using contention-based protocols



7.2. Concept and Models for Multiple Access (Multiple Division)

Q: Why “multiple access”? A: Multiple MSs can share a radio channel (without

interference)

=> we can have multiple access to the same channel

Multiple traffic channels used simultaneously MS attached to a transmitter/receiver Transmission from any MS is received by all MSs

within its radio range

MS communicates with its BS via a traffic channel Dedicated traffic channel not shared by other MSs



7.2. Concept and Models for Multiple Division – cont. 1

MS can hear traffic on many channels From “foreign” BSs ‘ from other MSs

MS must distinguish its own traffic from any other traffic Ignore traffic from its own BSs to other MSs Ignore traffic from “foreign” BSs Ignore traffic from other MSs

Like a person picking up his conversation partner’s speech when many people have many independent conversations in a room

Also BS must distinguish traffic from different MSs

MS or BS can distinguish thanks to orthogonalization of signals on different traffic channels

OPTIONAL details – p. 144© 2007 by Leszek T. Lilien


7.2. Concept and Models for Multiple Division – cont. 2


Duplex communications = simultaneous 2-way communications Duplex communications requires

Forward (downlink) channel Reverse (uplink) channel

BS

Forward

(downlin

k) contro

l channel

MS

Reverse (

uplink) c

ontrol c

hannel

Forward

(downlin

k) traff

ic channel

Reverse (

uplink) tr

affic

channel


7.2.1. Frequency Division Multiple Access (FDMA)

User 1

User 2

User n

…

Time

Frequency

• Separate (unique) carrier frequency per user

• All 1G (first-generation) systems use FDMA


Basic Structure of a FDMA System

- 1 BS and n MSs - fi’ and fi – for MS #i

MS #1

MS #2

MS #n

BS

f1’

f2’

fn’

f1

f2

fn

…

……

Reverse channels(Uplink)

Forward channels(Downlink)


FDMA Channel Structure

1 2 3 … nFrequency

Total bandwidth W = N * Wc

(for reverse channels or for forward channels)

Guard Band Wg

4

Frequency

Protectingbandwidth

…

f1’ f2’ fn’

…

f1 f2 fn

Reverse channels Forward channels

Subband Wc

(Modified by LTL)


7.2.2. Time Division Multiple Access (TDMA)

Use

r 1

Use

r 2

Use

r n

…

Time

Frequency

• Separate (unique) time slot per user• The same carrier (frequency) split into time slots• Each frequency efficiently utilized by multiple users

• Most of 2G systems use TDMA


Basic Structure of TDMA

MS #1

MS #2

MS #n

BS

…

…



t

Frequency f ’#1 …#1 …

Frame

Slot

… #1 … #1

Frame

…t

Frequency f

Frame Frame

…t

#2 …#2 …

…t

#n … #n …

… #2 … #2…t

…#n …#n…t

Slot


Two Duplexing Modes for TDMA

• Two duplexing modes for TDMA

a) FDD = frequency division duplexing- Frequency for forward channels differs from

frequency for reverse channelse.g., next slide:

f used for all forward channels and f’ (not f) used for all reverse channel

#1

b) TDD = time division duplexing- same frequency for all forward and all reverse channels e.g., slide +2

f used for all forward channels and f (same) used for all reverse channels



a) Channel Structure in TDMA/FDD System

… t

f #1 #2 #n #1 #2 #n… …#1 #2 #n

Frame FrameFrame

(a) All forward channels on frequency f

… t

f ’

#1 #2 #n #1 #2 #n… …#1 #2 #n

Frame FrameFrame

(a) All reverse channels on different frequency f ’


b) Channel Structure in TDMA/TDD System

…

t

f

#1 #2 #n #1 #2 #n…

Forward channel

Reverse channel

…#1 #2 #n

Forward channel

Frame Frame

#1 #2 #n…

Reverse channel

(a) All forward and all reverse channels on the same frequency f

(1st half of each frame used for forward channels, 2nd half – for reverse channels)


TDMA Frame Structure

…Time

Frequency#1 #2 #n #1 #2 #n… …#1 #2 #n

Frame FrameFrame

Head Data

Guard time

Time slot

Notice Guard time between Time slots

- Minimize interference due to propagation delays


7.2.3. Code Division Multiple Access (CDMA)

Outline:

1) Introduction to CDMA2) Spread Spectrum3) Direct Sequence Spread Spectrum (DSSS)4) Frequency Hopping Spread Spectrum

(FHSS)5) Walsh Codes6) Near-far Problem7) Power Control



7.2.3. Code Division Multiple Access (CDMA) – cont.

1) Introduction

• Separate (unique) code per user• Code sequences are orthogonal => different users can use same frequency simultaneously (see Fig above)

• Some 2G systems use CDMA / Most of 3G systems use CDMA

Use

r 1

Time

Frequency

Use

r 2

Use

r n

Code

...

Do not confuse CDMA (conflict-

free) with CSMA (contention-based)

CSMA = carrier sense multiple access

(Modified by LTL)


Structure of a CDMA System (with FDD)

Notes:1) FDD (frequency division duplexing) since f for all forward channels,

and f’ for all reverse channels2) Ci = i-the code3) Ci’ x Cj’ = 0, i.e., Ci’ and Cj’ are orthogonal codes on f’

Ci x Cj = 0, i.e., Ci and Cj are orthogonal codes on f

MS #1

MS #2

MS #n

BS

C1’

C2’

Cn’

C1

C2

Cn

…

……



Frequency f ’ Frequency f


Two Implementation Methodologiesfor CDMA

• Two implementation methodologies for CDMA

a) DS = direct sequence- next slide

b) FH = frequency hopping- same frequency for all forward and all reverse channels e.g., slide +2

f used for all forward channels and f (same) used for all reverse channels



2) Spread Spectrum for CDMA Concept of spread spectrum:

Pseudorandom sequence c(t) phase-modulates data-modulated carrier of s(t), producing m(t)

m(t) occupies broader bandwidth and has lower peak power than s(t)

where: s(t) - original signal / m(t) – xmitted signal derived fr. s(t) by spreading c(t) – code signal (a parameter for spreading)

Results in better resistance to interference

Originaldigital

signal s(t)

Code c(t)

Xmittedspread signal

m(t)

Spreading

Frequency Frequency

Power Power

Transmitter

(Modified by LTL)


3) Direct Sequence Spread Spectrum Concept of DSSS for CDMA

Pseudorandom sequence c(t) phase-modulates data-modulated carrier of s(t), producing m(t)

m(t) occupies broader bandwidth & has lower peak power than s(t)

Originaldigital signal

s(t)

Code c(t)

Xmittedspread signal

m(t)

Code c(t)

Recreateddigital signal

s(t)

Spreading De-spreading

Frequency Frequency Frequency

Power Power Power

Transmitter Receiver

c(t) is Synchronized!

(Modified by LTL)


4) Frequency Hopping Spread Spectrum Concept of FHSS for CDMA

Pseudorand. hopping pattern sequence changes freq. of digital radio signal across broad freq. band in random way

Radio xmitter freq. hops fr. channel to channel in predetermined pseudorandom way (cf. next slide)

m(t) occupies broader bandwidth & has lower peak power than s(t)

Originaldigital signal

Hopping pattern

Xmitspread signal

Recreated(“dehopped”)digital signal

Spreading De-spreading

Frequency Frequency Frequency

Power Power PowerHopping pattern

Transmitter Receiver

Hopp. patt. Synchro-

nized!

(Modified by LTL)


Time

Frequency

An Example of Frequency Hopping Pattern


*** SKIP *** 5) Walsh Codes

Wal (0, t) t

Wal (1, t) t

Wal (2, t) t

Wal (3, t) t

Wal (4, t) t

Wal (5, t) t

Wal (6, t) t

Wal (7, t) t

Each user in CDMA assigned ≥ 1 orthogonal waveforms derived from 1 orthogonal code

Walsh Codesare an impor-tant set oforthogonalcodes


6) Near-far Problem

RSS = received signal strengthD = distance

MS1MS2 BS

D D0

d2 d1 MS1MS2 BS

RSS

Assume 1: xmission power of MS1 == xmission power of MS2=> RSS of MS1 at BS >> RSS of MS2 at BS

Assume 2: MS1 & MS2 use adjacent channels=>out-of-band radiation of MS1’s signal interferes with MS2’s signal in the adjacent channel (cf. next slide)

(Modified by LTL)


Adjacent Channel Interference in CDMA

f1 f2

MS1 MS2

Frequency

Power

Adjacent channel interference can be serious=> must keep out-of-band radiation small


Frequency

Baseband signal

Frequency

Interference baseband signals

Spread signal

Frequency

Despread signal

Interference signals

Adjacent Channel Interferencein Spread Spectrum System in CDMA

Adjacent channel interference can be especially serious when spread spectrum technique used

Cf. figure

Simple solution: power control (next slide)


7) Power Control in CDMA

Two alternatives:a) Control transmit power Pt of MS2 => received power Pr of adjacent channel interference from MS2 is controlled => CCIR is controlled (CCIR = cochannel interference ratio)

OR:

b) Control transmit power Pt of MS1 => received power Pr of MS1 is controlled (kept strong enough)



** SKIP ** 7) Power Control in CDMA –cont.

Pr Pt

= 1

4df

c

Pt = Transmit powerPr = Received power in free spaced = Distance between receiver and transmitterf = Frequency of transmissionc = Speed of light = Attenuation constant (2 to 4)


** SKIP** 7.2.4. OFDM

** SKIP** 7.2.5. SDMA


** SKIP ** 7.2.6. Comparisons of FDMA, TDMA, and CDMA (Example)

Operation FDMA TDMA CDMA

Allocated Bandwidth 12.5 MHz 12.5 MHz 12.5 MHz

Frequency reuse 7 7 1

Required channel BW 0.03 MHz 0.03 MHz 1.25 MHz

No. of RF channels 12.5/0.03=416 12.5/0.03=416 12.5/1.25=10

Channels/cell 416/7=59 416/7=59 12.5/1.25=10

Control channels/cell 2 2 2

Usable channels/cell 57 57 8

Calls per RF channel 1 4* 40**

Voice channels/cell 57x1= 57 57x4= 228 8x40= 320

Sectors/cell 3 3 3

Voice calls/sector 57/3=19 228/3=76 320

Capacity vs. FDMA 1 4 16.8

* Depends on the number of slots ** Depends on the number of codesDelay ? ? ?


7.3. Modulation Techniques

Why need modulation? “Transferring” from signal freq. to carrier frequency allows

to use small antenna size Antenna size is inversely proportional to frequency

E.g., 3 kHz 50 km antenna 3 GHz 5 cm antenna

Limits noise and interference E.g., FM (Frequency Modulation)

Multiplexing techniques (efficient use of spectrum) E.g., FDM, TDM, CDMA

(Modified by LTL)


Analog and Digital Signals

Analog signal (continuous signal)

Digital signal (discrete signal)

Time

Amplitude

1 1 1 1 0 0

Bit

+

_0

Time

Amplitude

0

S(t)


Hearing, Speech, and Voice-band Channels

Voice-grade Telephone channel

Human hearing

Human speech

Frequency (Hz)

Frequency (Hz)

Pass band

Frequency cutoff point

Guard band Guard band

100

0 200 3,500 4,000

10,000..


7.3.1. AM (Amplitude Modulation)

- Amplitude of carrier signal is varied as the message signal to be transmitted

- Frequency of carrier signal is kept constant

Message signalx(t)

Carrier signal

AM signals(t)

Time

Time

Time


7.3.2. FM (Frequency Modulation)

FM integrates message signal with carrier signal by varying the instantaneous frequency

Amplitude of carrier signal is kept constant

Carrier signal

Message signalx(t)

FM signal s(t)

Time

Time

Time


7.3.3. FSK (Frequency Shift Keying)

• 1/0 represented by two different frequencies slightly offset from carrier frequency

Message signalx(t)

Carrier signal 2 for message signal ‘0’

Carrier signal 1 for message signal ‘1’

FSK signals(t)

1 0 1 1 0 1

Time

Time

Time

Time


7.3.4. PSK (Phase Shift Keying)

• Use alternative sine wave phase to encode bits

Carrier signal 1

Carrier signal 2

)2sin( tfc

Message signalx(t)

)2sin( tfc

1 0 1 1 0 1

PSK signals(t)

Time

Time

Time

Time


** SKIP ** 7.3.5. QPSK(Quadrature Phase Shift Keying)

Q

I0,01,1

0,1

1,0

Q

I01

(a) BPSK (b) QPSK

Signal constellation

BPSK = binary phase shift keying (p. 161/-1)


** SKIP ** 7.3.6. /4 QPSK

All possible state transitions in /4 QPSK


** SKIP ** 7.3.7. QAM(Quadrature Amplitude Modulation)

A combination of AM and PSK

Two carriers out of phase by 90 deg are amplitude modulated


7.3.8. 16QAM

Splitting signal into 12 different phases and 3 different amplitudes – the total of 16 different possible values

Rectangular constellation of 16QAM

I

Q

0000010011001000

0001010111011001

0011011111111011

0010011011101010

Rectangular constellation of 16QAM

45

The End of Section 7 (Ch. 7)

1 CS 6910 – Pervasive Computing Spring 2007 Section 7 (Ch.7): Multiple Access Techniques Prof. Leszek Lilien Department of Computer Science Western Michigan.

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