Sec.7--Multiple Access Techniques

1
CS 6910 – Pervasive Computing
Spring 2007
Section 7 (Ch.7):
Multiple Access Techniques
Prof. Leszek Lilien
Department 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 control
channels on each BS
 Bec. control channels
shared by many MSs
for 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
MS

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 requests
OR:
 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

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
MS

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 … n
Frequency
Total bandwidth W = N * Wc
(for reverse channels or for forward channels)
Guard Band Wg
4
Frequency
Protecting
bandwidth
…
f1’ f2’ fn’
…
f1 f2 fn
Reverse channels Forward channels
Subband Wc
(Modified by LTL)

7.2.2. Time Division Multiple Access (TDMA)
User
1
User
2
User
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
…
…
Reverse channels
(Uplink)
Forward channels
(Downlink)
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 channels
e.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 (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 Frame
Frame
(a) All forward channels on frequency f
… t
f ’
#1
#2
#n
#1
#2
#n
… …
#1
#2
#n
Frame Frame
Frame
(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 Frame
Frame
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 CDMA
2) Spread Spectrum
3) Direct Sequence Spread Spectrum (DSSS)
4) Frequency Hopping Spread Spectrum (FHSS)
5) Walsh Codes
6) Near-far Problem
7) 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
User
1
Time
Frequency
User
2
User
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 channels
2) Ci = i-the code
3) 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
…
…
…
Reverse channels
(Uplink)
Forward channels
(Downlink)
Frequency f ’ Frequency f

Two Implementation Methodologies
for 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 (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
Original
digital
signal s(t)
Code
c(t)
Xmitted
spread 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)
Original
digital signal
s(t)
Code
c(t)
Xmitted
spread signal
m(t)
Code
c(t)
Recreated
digital 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)
Original
digital signal
Hopping pattern
Xmit
spread signal
Recreated
(“dehopped”)
digital signal
Spreading De-
spreading
Frequency Frequency Frequency
Power Power Power
Hopping 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 Codes
are an impor-
tant set of
orthogonal
codes

6) Near-far Problem
RSS = received
signal strength
D = distance
MS1
MS2 BS
D D
0
d2 d1
MS1
MS2 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 Interference
in 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








4df
c

Pt = Transmit power
Pr = Received power in free space
d = Distance between receiver and transmitter
f = Frequency of transmission
c = 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 codes
Delay ? ? ?

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 signal
x(t)
Carrier signal
AM signal
s(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 signal
x(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 signal
x(t)
Carrier signal 2
for message signal ‘0’
Carrier signal 1
for message signal ‘1’
FSK signal
s(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
)
2
sin( 
 
t
fc
Message signal
x(t)
)
2
sin( t
fc

1 0 1 1 0 1
PSK signal
s(t)
Time
Time
Time
Time

** SKIP ** 7.3.5. QPSK
(Quadrature Phase Shift Keying)
Q
I
0,0
1,1
0,1
1,0
Q
I
0
1
(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
0000
0100
1100
1000
0001
0101
1101
1001
0011
0111
1111
1011
0010
0110
1110
1010
Rectangular constellation of 16QAM

45
The End of Section 7 (Ch. 7)

Sec.7--Multiple Access Techniques

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