1. Multiple Access Techniques
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 © by Leszek T. Lilien. Requests to use L. Lilien’s slides for non-profit purposes will be gladly granted upon a written request.

2. Multiple Division (Access) Techniques
Chapter 7
Multiple Division (Access) Techniques

3. Outline 7.1. Introduction 7.2. Concept and Models for Multiple Access
Frequency Division Multiple Access (FDMA)
Time Division Multiple Access (TDMA)
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
OFDM
SDMA
Comparison of FDMA, TDMA, and CDMA
7.3. Modulation Techniques
AM (Amplitude Modulation)
FM (Frequency Modulation)
FSK (Frequency Shift Keying)
PSK (Phase Shift Keying)
QPSK (Quadrature Phase Shift Keying)
/4 QPSK
7.3.7.QAM (Quadrature Amplitude Modulation)
QAM
(Modified by LTL)

4. 7.1. Introduction Recall Large # of traffic channels on each BS
Forward (downlink) control channel MS
Reverse (uplink) control channel
Forward (downlink) traffic channel
Reverse (uplink) traffic channel
Recall
Large # of traffic
channels on each BS
Bec. traffic channels used by
1 MS exclusively for call duration
Small # of control
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
© 2007 by Leszek T. Lilien

5. For control channels — we used contention-based protocols
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
© 2007 by Leszek T. Lilien

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

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

8. 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 (downlink) control channel MS
Reverse (uplink) control channel
Forward (downlink) traffic channel
Reverse (uplink) traffic channel
© 2007 by Leszek T. Lilien

9. 7.2.1. Frequency Division Multiple Access (FDMA)
User n …
User 2
User 1
Time
Separate (unique) carrier frequency per user
All 1G (first-generation) systems use FDMA

10. Basic Structure of a FDMA System - 1 BS and n MSs
Basic Structure of a FDMA System - 1 BS and n MSs fi’ and fi – for MS #i
f1’ f1
MS #1
f2’ f2
MS #2 … … …
fn’ fn
MS #n BS
Reverse channels
(Uplink)
Forward channels
(Downlink)

11. FDMA Channel Structure
Reverse channels
Protecting
bandwidth
Forward channels
f1’
f2’
fn’ f1 f2 fn … …
Frequency
Guard Band Wg
Subband Wc … 1 2 3 4 n
Frequency
Total bandwidth W = N * Wc
(for reverse channels or for forward channels)
(Modified by LTL)

12. 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

13. Basic Structure of TDMA
MS #1
MS #2
MS #n BS …
Reverse channels
(Uplink)
Forward channels
(Downlink) t
Frequency f ’ #1
Frame
Slot
Frequency f #2 #n

14. 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
© 2007 by Leszek T. Lilien

15. a) Channel Structure in TDMA/FDD System… t f #1 #2 #n
Frame
(a) All forward channels on frequency f … t
f ’ #1 #2 #n
Frame
(a) All reverse channels on different frequency f ’

16. b) Channel Structure in TDMA/TDD Systemf
Frame
Frame #1 #2 … #n #1 #2 … #n #1 #2 … #n #1 #2 … #n t
Forward channel
Reverse channel
Forward channel
Reverse channel
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)

17. TDMA Frame Structure Frequency Frame Frame Frame Time#1 #2 … #n #1 #2 … #n #1 #2 … #n
Time
Notice Guard time between Time slots
- Minimize interference due to propagation delays
Head
Data
Time slot
Guard time

18. 7.2.3. Code Division Multiple Access (CDMA)
Outline:
Introduction to CDMA
Spread Spectrum
Direct Sequence Spread Spectrum (DSSS)
Frequency Hopping Spread Spectrum (FHSS)
Walsh Codes
Near-far Problem
Power Control
© 2007 by Leszek T. Lilien

19. 7.2.3. Code Division Multiple Access (CDMA) – cont. 1) Introduction
Do not confuse CDMA (conflict-free) with
CSMA (contention-based)
CSMA = carrier sense multiple access
User 1
Time
Frequency
User 2
User n
Code .
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
(Modified by LTL)

20. Structure of a CDMA System (with FDD)
MS #1
MS #2
MS #n BS
C1’
C2’
Cn’ C1 C2 Cn …
Reverse channels
(Uplink)
Forward channels
(Downlink)
Frequency f ’
Frequency f
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

21. 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

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
Original
digital signal s(t)
Code c(t)
Xmitted
spread signal m(t)
Spreading
Frequency
Power
Transmitter
(Modified by LTL)

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)
Original
digital signal s(t)
Code c(t)
Xmitted
spread signal m(t)
Recreated
Spreading
De-spreading
Frequency
Power
Transmitter
Receiver
c(t) is Synchronized!
(Modified by LTL)

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)
Original
digital signal
Hopping pattern
Xmit
spread signal
Recreated
(“dehopped”)
Spreading
De-spreading
Frequency
Power
Transmitter
Receiver
Hopp. patt. Synchro-nized!
(Modified by LTL)

25. An Example of Frequency Hopping Pattern
Time

26. *** SKIP *** 5) Walsh Codes
Each user in CDMA assigned ≥ 1 orthogonal waveforms derived from 1 orthogonal code
Walsh Codes
are an impor-
tant set of
orthogonal
codes
Wal (0, t) t
Wal (1, t)
Wal (2, t)
Wal (3, t)
Wal (4, t)
Wal (5, t)
Wal (6, t)
Wal (7, t)

27. 6) Near-far Problem Assume 1: xmission power of MS1 =
=>
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)
MS2 BS
MS1
RSS
RSS = received
signal strength
D = distance D D d2 d1
MS2 BS
MS1
(Modified by LTL)

28. Adjacent Channel Interference in CDMA
MS1
MS2
Frequency
Power
Adjacent channel interference can be serious
=> must keep out-of-band radiation small

29. Adjacent Channel Interference in Spread Spectrum System in CDMA
Interference baseband signals
Despread signal
Baseband signal
Spread signal
Interference signals
Frequency
Frequency
Frequency
Adjacent channel interference can be especially serious when spread spectrum technique used
Cf. figure
Simple solution: power control (next slide)

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

31. Pr = Received power in free space
** SKIP ** 7) Power Control in CDMA –cont.
Pt = Transmit power
Pr = Received power in free space
d = Distance between receiver and transmitter
f = Frequency of transmission
c = Speed of light
a = Attenuation constant (2 to 4)

32. ** SKIP** 7.2.4. OFDM ** SKIP** 7.2.5. SDMA

33. ** SKIP ** 7.2.6. Comparisons of FDMA, TDMA, and CDMA (Example)
Operation
FDMA
TDMA
CDMA
Allocated Bandwidth
12.5 MHz
Frequency reuse 7 1
Required channel BW
0.03 MHz
1.25 MHz
No. of RF channels
12.5/0.03=416
12.5/1.25=10
Channels/cell
416/7=59
Control channels/cell 2
Usable channels/cell 57 8
Calls per RF channel 4*
40**
Voice channels/cell
57x1= 57
57x4= 228
8x40= 320
Sectors/cell 3
Voice calls/sector
57/3=19
228/3=76
320
Capacity vs. FDMA 4
16.8 ?
Delay
? ?
* Depends on the number of slots ** Depends on the number of codes

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)

35. Analog and Digital Signals
Analog signal (continuous signal)
Time
Amplitude
S(t)
Digital signal (discrete signal)
Time
Amplitude 1
Bit + _

36. Hearing, Speech, and Voice-band Channels
Voice-grade Telephone channel
Human hearing
Human speech
Frequency (Hz)
Pass band
Frequency cutoff point
Guard band
100
200
3,500
4,000
10,000 ..

37. 7.3.1. AM (Amplitude Modulation)
Message signal
x(t)
Carrier signal
AM signal
s(t)
Time
- Amplitude of carrier signal is varied as the message signal to be transmitted
- Frequency of carrier signal is kept constant

38. 7.3.2. FM (Frequency Modulation)
Message signal
x(t)
Time
Carrier signal
Time
FM signal
s(t)
Time
FM integrates message signal with carrier signal by varying the instantaneous frequency
Amplitude of carrier signal is kept constant

39. 7.3.3. FSK (Frequency Shift Keying)
1/0 represented by two different frequencies slightly offset from carrier frequency
Carrier signal 1
for message signal ‘1’
Time
Carrier signal 2
for message signal ‘0’
Time
Message signal
x(t)
Time
FSK signal
s(t)
Time

40. 7.3.4. PSK (Phase Shift Keying)
Use alternative sine wave phase to encode bits
Carrier signal 1
Time
Carrier signal 2
Time
Message signal
x(t)
Time
PSK signal
s(t)
Time

41. ** SKIP ** 7.3.5. QPSK (Quadrature Phase Shift Keying)
Signal constellation Q I
0,0
1,1
0,1
1,0 1
(a) BPSK
(b) QPSK
BPSK = binary phase shift keying (p. 161/-1)

42. ** SKIP ** /4 QPSK
All possible state transitions in /4 QPSK

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

44. Rectangular constellation of 16QAM
Splitting signal into 12 different phases and 3 different amplitudes – the total of 16 different possible values
Rectangular constellation of 16QAM

45. The End of Section 7 (Ch. 7)