Chapter 7
Multiple Division Techniques
Outline


Introduction
Concepts and Models for Multiple Divisions
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Frequency Division Multiple Access (FDMA)
Time Division Multiple Access (TDMA)
Code Division Multiple Access (CDMA)
Orthogonal Frequency Division Multiplexing (OFDM)
Space Division Multiple Access (SDMA)
Comparison of FDMA, TDMA, and CDMA
Modulation Techniques
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Amplitude Modulation (AM)
Frequency Modulation (FM)
Frequency Shift Keying (FSK)
Phase Shift Keying (PSK)
Quadrature Phase Shift Keying (QPSK)
p/4QPSK
Quadrature Amplitude Modulation (QAM)
16QAM
2
Concepts and Models for
Multiple Divisions


Multiple access techniques are based on
orthogonalization of signals
A radio signal is a function of frequency, time and
code as;
s(f, t, c) = s(f, t) c(t)
where s(f, t) is the function of frequency and time and c(t) is
the function of code
 Use of different frequencies to transmit a signal: FDMA
 Distinct time slot: TDMA
 Different codes CDMA
 Multiple simultaneous channels: OFDM
 Specially separable sectors: SDMA
3
Frequency Division Multiple Access
(FDMA)
Orthogonality conditions of two signals in FDMA:
User n
…
Frequency
1 i j
F si ( f , t ) s j ( f , t )df 0 i  j , i, j  1, 2,...,k
User 2
User 1
Time
• Single channel per carrier
• All first generation systems use FDMA
4
Basic Structure of FDMA
f1 ’
MS #1
f2’
f2
…
…
…
MS #2
f1
f n’
MS #n
Reverse channels
(Uplink)
fn
Forward channels BS
(Downlink)
5
Forward and Reverse channels in
FDMA and Guard Band
f1’
f2’
f n’
f1
f2
…
…
Reverse channels
Protecting
bandwidth
Guard Band
Wg
1
fn
2
Forward channels
Sub Band
Wc
3
4
…
N
Frequency
Total Bandwidth W = NWc
6
Time Division Multiple Access
(TDMA)
Orthogonality conditions of two signals in TDMA:
1 i j
i
j
User 2
User 1
Frequency
T
…
i j
, i, j  1,2,...,k
User n
 s ( f , t ) s ( f , t )dt   0
Time
• Multiple channels per carrier
• Most of second generation systems use TDMA
7
The Concept of TDMA
Slot
…
…
t
…
MS #n
#n
…
#n
…
MS #2
Frame
Reverse channels
(Uplink)
…
#1
…
t
…
…
t
Frame
t
…
…
#2
#2
#2
…
…
…
#n
t
MS #1
…
#1
…
#2
…
#1
#1
…
Frequency f
#n
Frequency f ’
…
t
Frame
Frame
BS
Forward channels
(Downlink)
8
TDMA: Channel Structure
#n
…
t
#n
#2
#1
…
Frame
#n
#1
…
Frame
#n
#2
#1
Frame
#2
f
t
(a) Forward channel
f’
#2
#1
…
Frame
#n
#2
#1
…
Frame
#n
#2
#1
Frame
…
(b) Reverse channel
Channels in TDMA/FDD (Frequency Division Duplexing )
9
Forward and Reverse
Channels in TDMA
Forward
channel
Reverse
channel
Forward
channel
…
Reverse
channel
#n
#2
#1
…
#n
#2
#1
…
#n
#2
Frame
#1
…
#n
#2
#1
Frequency Frame
f=f’
Time
Channels in TDMA/TDD
10
…
#n
#2
#1
…
Frame
#n
…
#1
#2
Frame
#n
Frame
#2
#1
Frequency
Frame Structure of TDMA
Time
Guard
time
Head
Data
11
Code Division Multiple Access
(CDMA)
Orthogonality conditions of two signals in CDMA:
1 i j
 s (t ) s (t )dt   0
i
j
C
i j
, i, j  1,2,...,k
.. .
User 2
User 1
User n
Frequency
Time
Code
• Users share bandwidth by using code sequences that are orthogonal to each other
• Some second generation systems use CDMA
• Most of third generation systems use CDMA
12
CDMA Encode/Decode
Channel output Zi,m
Data
bits
Code
d0 = 1
-1 -1 -1
1 1 1
1
1 1 1
-1 -1 -1
Slot 1
-1
slot 1
Channel
output
1
-1
1 1 1 1 1 1
1
d1 = -1
-1
Sender
Zi,m= di.cm
-1 -1 -1
1
-1
-1 -1 -1
slot 0
Channel
output
Slot 0
di = Zi,m.cm
Received
input
Receiver
1 1 1 1 1 1
1
Code
-1 -1 -1
-1
1 1 1
1
-1
1
-1
1 1 1
-1 -1 -1
Slot 1
d0 = 1
-1 -1 -1
1
-1
-1 -1 -1
Slot 0
d1 = -1
Slot 1
channel
output
Slot 0
channel
output
13
CDMA: Two-Sender Interference
14
Structure of a CDMA System
Frequency f ’
Frequency f
C1
MS #2
C2’
C2
Cn’
Cn
…
…
C1’
…
MS #1
MS #n
Reverse channels
(Uplink)
Forward channels
(Downlink)
BS
Ci’ x Cj’ = 0, i.e., Ci’ and Cj’ are orthogonal codes,
Ci x Cj = 0, i.e., Ci and Cj are orthogonal codes
15
Spread Spectrum
Spreading of data signal s(t) by the code signal c(t)
to result in message signal m(t) as:
m(t )  s(t )  c(t )
Power
Frequency
Digital
signal
s(t)
Spreading Spreading
signal m(t)
Code
c(t)
Power
Frequency
16
Direct Sequence Spread Spectrum
(DSSS)
Transmitter
Receiver
Spreading
Despread
Digital
signal s(t)
Power
Frequency
Digital
signal s(t)
Spreading
signal m(t)
Code
c(t)
Power
Frequency
Code
c(t)
Power
Frequency
17
Orthogonal Codes
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Orthogonal codes
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All pairwise cross correlations are zero
Fixed- and variable-length codes used in CDMA systems
For CDMA application, each mobile user uses one
sequence in the set as a spreading code
 Provides zero cross correlation among all users
Types


Walsh codes
Variable-length Orthogonal codes
18
Walsh Codes

Set of Walsh codes of length n consists of the n rows
of an n x n Walsh matrix:
 Wn Wn 

W2 n  
 W1 = (0)
 Wn Wn 
where n = dimension of the matrix.
 Every row is orthogonal to every other row
 Requires tight synchronization

Cross correlation between different shifts of Walsh
sequences is not zero
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Example:
Frequency Hoping Spread
Spectrum (FHSS)
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A number of channels are allocated for the FH signal
Width of each channel corresponds to bandwidth of
input signal
Signal hops from frequency to frequency at fixed
intervals
At each successive interval, a new carrier frequency
is selected
Frequency Hoping Spread
Spectrum
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Channel sequence dictated by spreading code
Receiver, hopping between frequencies in
synchronization with transmitter, picks up message
Advantages
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Eavesdroppers hear only unintelligible blips
Attempts to jam signal on one frequency succeed only at
knocking out a few bits
An Example of
Frequency Hopping Pattern
Frequency
Time
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Frequency Hopping Spread
Spectrum (FHSS)
Transmitter
Receiver
Spreading
Despread
Digital
signal s(t)
Spreading
signal
Digital
signal
Hopping
pattern
Power
Hopping
pattern
Power
Power
Frequency
Frequency
Frequency
24
Near-far Problem
MS2
BS
MS1
Received signal strength
Distance
0
MS2
d2
Distance
BS d1 MS1
25
Adjacent Channel Interference
Power
MS1
MS2
f1
f2
Frequency
26
Interference in Spread Spectrum
Interference
baseband signals
Spectrum
spreading signal
Baseband
signal
Despread signal
Interference
signals
0
Frequency
f
Frequency
f
Frequency
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Power Control
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Design issues making it desirable to include
dynamic power control in a cellular system
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Received power must be sufficiently above the
background noise for effective communication
Desirable to minimize power in the transmitted signal
from the mobile
 Reduce cochannel interference, alleviate health
concerns, save battery power
In SS systems using CDMA, it’s desirable to equalize the
received power level from all mobile units at the BS
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Types of Power Control
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Open-loop power control
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Depends solely on mobile unit
No feedback from BS
Not as accurate as closed-loop, but can react quicker to
fluctuations in signal strength
Closed-loop power control


Adjusts signal strength in reverse channel based on metric
of performance
BS makes power adjustment decision and communicates to
mobile on control channel
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Orthogonal Frequency Division
Multiplexing (OFDM)
 Divide a channels into multiple sub-channels and do parallel
transmission
 Orthogonality of two signals in OFDM can be given by:
1, i  j
F si( f , t )s ( f , t )dt  0, i  j, i, j  1,2,....,k
*
j
Spectrum of a single
OFDM subchannel
Spectrunm of an OFDM
signal with
multiple subchannels
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Modulation/Demodulation Steps in
OFDM
Low speed bit
stream
N1
High speed Serial to parallel
N2
IDFT
….
conversion
data stream
Nn
Guard interval
insertion
Transmission of
OFDM signal
Modulation operation at the OFDM transmitter
N1
Received
OFDM signal
Guard interval
removal
DFT
N2
….
Parallel to serial
conversion
High speed
data stream
Nn
Demodulation steps at the OFDM receiver
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Space Division Multiple Access
(SDMA)
Space divided into spatially separate sectors
Omni-directional
transmission
s(f,t,c)
Beam i
s(f,t,c)
Beam 3
s(f,t,c)
s(f,t,c) Beam 2
The concept
of SDMA
s(f,t,c)
Beam n Beam 1
33
Transmission in SDMA
 Noise and
interference for
each MS and BS is
minimized
 Enhance the quality
of communication
link and increase
overall system
Beam 1
capacity
 Intra-cell channel
reuse can be easily
exploited
MS1
Beam 2
MS2
Beam 3
BS
MS3
The basic structure of a SDMA system
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Comparison of Various Multiple
Division Techniques
Technique
Concept
Active terminals
Signal
separation
Handoff
Advantages
Disadvantages
Current
FDMA
TDMA
CDMA
SDMA
Divide the frequency
band into disjoint
sub-bands
Divide the time into
non-overlapping
time slots
Spread the signal
with orthogonal
codes
Divide the space in to
sectors
All terminals active
on their specified
frequencies
Terminals are active
in their specified slot
on same frequency
All terminals active
on same frequency
Number of terminals
per beam depends on
FDMA/
TDMA/CDMA
Filtering in
frequency
Synchronization in
time
Code separation
Spatial separation
using smart antennas
Hard handoff
Hard handoff
Soft handoff
Hard and soft
handoffs
Simple and robust
Flexible
Flexible
Very simple, increases
system capacity
Inflexible, available
frequencies are fixed,
requires guard
bands
Requires guard
space,
synchronization
problem
Complex receivers,
requires power
control to avoid
near-far problem
Inflexible, requires
network monitoring to
avoid intra cell
handoffs
Radio, TV and
GSM and PDC
2.5G and 3G
Satellite systems, LTE
35
Modulation Techniques

Why need modulation?
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Small antenna size
Antenna size is inversely proportional to frequency
(wavelength)
e.g., 3 kHz  50 km antenna
3 GHz  5 cm antenna
Limits noise and interference,
e.g., FM (Frequency Modulation)
Multiplexing techniques,
e.g., FDM, TDM, CDMA
36
Concepts Related to Channel
Capacity
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
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Data rate - rate at which data can be
communicated (bps)
Bandwidth - the bandwidth of the transmitted
signal as constrained by the transmitter and the
nature of the transmission medium (Hertz)
Noise - average level of noise over the
communications path
Error rate - rate at which errors occur

Error = transmit 1 and receive 0; transmit 0 and
receive 1
Frequency-Domain Concepts
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Fundamental frequency - when all frequency
components of a signal are integer multiples of
one frequency, it’s referred to as the fundamental
frequency
Spectrum - range of frequencies that a signal
contains
Absolute bandwidth - width of the spectrum of a
signal
Effective bandwidth (or just bandwidth) - narrow
band of frequencies that most of the signal’s
energy is contained in
Transmission Rate Constraint

Nyquist’s Theorem



Given a BW B, the highest signal rate that can be
carried is 2B
With multilevel signaling C = 2B log2 L, bit/sec
where L = number of discrete signal or voltage levels
Shannon’s Theorem: theoretical maximum that
can be achieved C  B log2 1  S/N  bits/sec

Where S is the signal power and N is noise power
41
Modulation Techniques

Analog Modulation: used for transmitting analog
data
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

Amplitude Modulation (AM)
Frequency Modulation (FM)
Digital Modulation: used for transmitting digital
data



Amplitude Shift Keying (ASK)
Frequency Shift Keying (FSK)
Phase Shift Keying (PSK)
42
Analog and Digital Signals

Analog Signal (Continuous signal)
Amplitude
S(t)
Time
0

Digital Signal (Discrete signal)
Amplitude
1
+
0
1
1
0
1
Time
0
Bit
43
Amplitude Modulation

Amplitude Modulation
st   1 na xt cos2pf ct



cos 2pfct = carrier
x(t) = input signal
na = modulation index ≤ 1
 Ratio of amplitude of input signal to carrier
44
45
46
Amplitude Modulation (AM)
Message signal
x(t)
Carrier signal
AM signal
s(t)
Time
Time
Time
The modulated carrier signal s(t) is:
s(t )  [ A  x(t )] cos(2pf ct )
Where fc is the carrier frequency
and A its amplitude
47
Frequency Modulation (FM)
Message signal
x(t)
Time
Carrier signal
Time
FM signal
s(t)
Time
The modulated carrier signal s(t) is:
t



s(t )  A cos (2p f c t  2p f   x( )d   0 


t0


Where f∆ is the peak frequency
deviation from the original
frequency and f∆ << fc
48
Basic Digital Modulation

Digital data to analog signal

Amplitude shift keying (ASK)


Frequency shift keying (FSK)


Amplitude difference of carrier frequency
Frequency difference near carrier frequency
Phase shift keying (PSK)

Phase of carrier signal shifted
Basic Encoding Techniques
50
Amplitude Shift Keying


One binary digit represented by presence of
carrier, at constant amplitude
Other binary digit represented by absence of
carrier
A cos2pf ct 


s t   
0



binary1
binary 0
where the carrier signal is Acos(2πfct)
51
Binary Frequency-Shift Keying
(BFSK)

Two binary digits represented by two different
frequencies near the carrier frequency

 A cos2pf1t 
s t   

 A cos2pf 2t 
binary1
binary 0
where f1 and f2 are offset from carrier frequency fc by
equal but opposite amounts

52
Frequency Shift Keying (FSK)
1/0 represented by two different frequencies
Carrier signal
for message signal
‘1’
Carrier signal
for message signal ‘0’
Time
Time
1
0
1
1
0
1
Message signal x(t)
Time
FSK signal s(t)
Time
53
Phase Shift Keying (PSK)
• Use alternative sine wave phases to encode bits
Carrier signal
Time
sin(2pf ct )
Carrier signal
Time
sin(2pf ct  p )
1
0
1
1
0
1
Message signal x(t)
Time
PSK signal s(t)
Time
54
Quadrature Phase Shift Keying
(QPSK)
 Four different phase shifts used are:






0,0  0
0,1  p / 2
1,0  p
1,1  3p / 2
or
 0 , 0  p / 4
   3p / 4
 0,1

 1,0  3p / 4
 1,1  p / 4
I (in-phase) and Q (quadrature) modulation used
55
Phase-Shift Keying (PSK)

Four-level PSK (QPSK)

Each element represents more than one bit



s t   



p

A cos 2pf ct  
4

3p 

A cos 2pf ct 

4 

3p 

A cos 2pf ct 

4 

p

A cos 2pf c t  
4

11
01
00
10
QPSK Signal Constellation
Q
0,1
Q
1
0
I
1,1
0,0
I
1,0
(a) BPSK
(b) QPSK
(Binary Phase Shift Keying)
(Quadrature Phase Shift Keying)
57
p/4 QPSK
 The phase of the carrier is: k  k 1  k , where θk is
carrier phase shift corresponding to input bit pairs.
 If θ0=0, input bit stream is [1011], then:
1  0  1  p / 4
 2  1  2  p / 4  p / 4  0
All possible states in
p/4 QPSK
58
Quadrature Amplitude
Modulation (QAM)
Combination of AM and PSK: modulate signals using two
measures of amplitude and four possible phase shifts
A
representative
QAM Table
Bit sequence
represented
Amplitude
Phase shift
000
1
0
001
2
0
010
1
p/2
011
2
p/2
100
1
p
101
2
p
110
1
3p/2
111
2
3p/2
59
Quadrature Amplitude Modulation
(QAM)
 Two carriers out of phase by 90 degrees are
amplitude modulated
Q
1000
1100
0100
0000
1001
1101
0101
0001
1011
1111
1010
1110
0111
0011
0110
0010
I
Rectangular constellation of 16QAM
60
Homework


Exercises: 7.2, 7.10, 7.17 (Due: Nov. 11)
Practice at home: 7.3, 7.6, 7.8, 7.20
61