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
Copyright © 2003, Dharma P. Agrawal and Qing-An Zeng. All rights reserved 3
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)
Copyright © 2003, Dharma P. Agrawal and Qing-An Zeng. All rights reserved 4
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
Copyright © 2003, Dharma P. Agrawal and Qing-An Zeng. All rights reserved 5
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
© 2007 by Leszek T. Lilien
Copyright © 2003, Dharma P. Agrawal and Qing-An Zeng. All rights reserved 6
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
© 2007 by Leszek T. Lilien
Copyright © 2003, Dharma P. Agrawal and Qing-An Zeng. All rights reserved 7
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
Copyright © 2003, Dharma P. Agrawal and Qing-An Zeng. All rights reserved 8
7.2. Concept and Models for Multiple Division – cont. 2
© 2007 by Leszek T. Lilien
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
Copyright © 2003, Dharma P. Agrawal and Qing-An Zeng. All rights reserved 9
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
Copyright © 2003, Dharma P. Agrawal and Qing-An Zeng. All rights reserved 10
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)
Copyright © 2003, Dharma P. Agrawal and Qing-An Zeng. All rights reserved 11
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)
Copyright © 2003, Dharma P. Agrawal and Qing-An Zeng. All rights reserved 12
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
Copyright © 2003, Dharma P. Agrawal and Qing-An Zeng. All rights reserved 13
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
Copyright © 2003, Dharma P. Agrawal and Qing-An Zeng. All rights reserved 14
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
© 2007 by Leszek T. Lilien
Copyright © 2003, Dharma P. Agrawal and Qing-An Zeng. All rights reserved 15
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 ’
Copyright © 2003, Dharma P. Agrawal and Qing-An Zeng. All rights reserved 16
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)
Copyright © 2003, Dharma P. Agrawal and Qing-An Zeng. All rights reserved 17
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
Copyright © 2003, Dharma P. Agrawal and Qing-An Zeng. All rights reserved 18
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
© 2007 by Leszek T. Lilien
Copyright © 2003, Dharma P. Agrawal and Qing-An Zeng. All rights reserved 19
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)
Copyright © 2003, Dharma P. Agrawal and Qing-An Zeng. All rights reserved 20
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
…
……
Reverse channels(Uplink)
Forward channels(Downlink)
Frequency f ’ Frequency f
Copyright © 2003, Dharma P. Agrawal and Qing-An Zeng. All rights reserved 21
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
© 2007 by Leszek T. Lilien
Copyright © 2003, Dharma P. Agrawal and Qing-An Zeng. All rights reserved 22
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)
Copyright © 2003, Dharma P. Agrawal and Qing-An Zeng. All rights reserved 23
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)
Copyright © 2003, Dharma P. Agrawal and Qing-An Zeng. All rights reserved 24
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)
Copyright © 2003, Dharma P. Agrawal and Qing-An Zeng. All rights reserved 25
Time
Frequency
An Example of Frequency Hopping Pattern
Copyright © 2003, Dharma P. Agrawal and Qing-An Zeng. All rights reserved 26
*** 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
Copyright © 2003, Dharma P. Agrawal and Qing-An Zeng. All rights reserved 27
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)
Copyright © 2003, Dharma P. Agrawal and Qing-An Zeng. All rights reserved 28
Adjacent Channel Interference in CDMA
f1 f2
MS1 MS2
Frequency
Power
Adjacent channel interference can be serious=> must keep out-of-band radiation small
Copyright © 2003, Dharma P. Agrawal and Qing-An Zeng. All rights reserved 29
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)
Copyright © 2003, Dharma P. Agrawal and Qing-An Zeng. All rights reserved 30
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)
© 2007 by Leszek T. Lilien
Copyright © 2003, Dharma P. Agrawal and Qing-An Zeng. All rights reserved 31
** 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)
Copyright © 2003, Dharma P. Agrawal and Qing-An Zeng. All rights reserved 32
** SKIP** 7.2.4. OFDM
** SKIP** 7.2.5. SDMA
Copyright © 2003, Dharma P. Agrawal and Qing-An Zeng. All rights reserved 33
** 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 ? ? ?
Copyright © 2003, Dharma P. Agrawal and Qing-An Zeng. All rights reserved 34
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)
Copyright © 2003, Dharma P. Agrawal and Qing-An Zeng. All rights reserved 35
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)
Copyright © 2003, Dharma P. Agrawal and Qing-An Zeng. All rights reserved 36
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..
Copyright © 2003, Dharma P. Agrawal and Qing-An Zeng. All rights reserved 37
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
Copyright © 2003, Dharma P. Agrawal and Qing-An Zeng. All rights reserved 38
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
Copyright © 2003, Dharma P. Agrawal and Qing-An Zeng. All rights reserved 39
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
Copyright © 2003, Dharma P. Agrawal and Qing-An Zeng. All rights reserved 40
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
Copyright © 2003, Dharma P. Agrawal and Qing-An Zeng. All rights reserved 41
** 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)
Copyright © 2003, Dharma P. Agrawal and Qing-An Zeng. All rights reserved 42
** SKIP ** 7.3.6. /4 QPSK
All possible state transitions in /4 QPSK
Copyright © 2003, Dharma P. Agrawal and Qing-An Zeng. All rights reserved 43
** 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
Copyright © 2003, Dharma P. Agrawal and Qing-An Zeng. All rights reserved 44
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)