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Copyright © 2011, Dr. Dharma P. Agrawal and Dr. Qing-An Zeng. All rights reserved. 1
Chapter 7
Multiple Division Techniques
Copyright © 2010, Dr. Dharma P. Agrawal and Dr. Qing-An Zeng. All rights reserved. 2
Outline
Introduction
Concepts and Models for Multiple Divisions 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 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
Copyright © 2010, Dr. Dharma P. Agrawal and Dr. Qing-An Zeng. All rights reserved. 3
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
Concepts and Models for Multiple Divisions
Copyright © 2010, Dr. Dharma P. Agrawal and Dr. Qing-An Zeng. All rights reserved. 4
Frequency Division Multiple Access (FDMA)
Fre
qu
ency
User 1
User 2
User n …
Time • Single channel per carrier
• All first generation systems use FDMA
kjiji
jidftfstfs j
F
i ,...,2 ,1,,0
1),(),(
Orthogonality conditions of two signals in FDMA:
Copyright © 2011, Dr. Dharma P. Agrawal and Dr. Qing-An Zeng. All rights reserved. 5
f1’
f2’
fn’
…
Reverse channels
(Uplink)
BS
f1
f2
fn
…
Forward channels
(Downlink)
MS #1
MS #2
MS #n
…
Basic Structure of FDMA
Copyright © 2011, Dr. Dharma P. Agrawal and Dr. Qing-An Zeng. All rights reserved. 6
Forward and Reverse channels in FDMA and Guard Band
Guard
Band Wg
1 2 3 …
N
Frequency Total Bandwidth W =
NWc
4
Sub Band
Wc
…
f1’ f2’ fn’
…
f1 f2 fn
Reverse channels Forward channels
Protecting bandwidth
Copyright © 2010, Dr. Dharma P. Agrawal and Dr. Qing-An Zeng. All rights reserved. 7
Time Division Multiple Access (TDMA)
Fre
qu
ency
Use
r 1
Use
r 2
Use
r n
…
Time
• Multiple channels per carrier
• Most of second generation systems use TDMA
Orthogonality conditions of two signals in TDMA:
kjiji
jidttfstfs j
T
i ,...,2,1,,0
1),(),(
Copyright © 2010, Dr. Dharma P. Agrawal and Dr. Qing-An Zeng. All rights reserved. 8
MS #1
MS #2
MS #n
…
…
Reverse channels
(Uplink)
t
Frequency f ’ #1
…
#1
…
Frame
Slot
Frame
…
t
#2
…
#2
…
…
t
#n
… #n
…
The Concept of TDMA
BS
Forward channels
(Downlink)
…
#1
…
Frame
…
t
Frequency f
Frame
…
#2
…
#2
…
t
…
#n
…
#n
…
t
#1
Copyright © 2011, Dr. Dharma P. Agrawal and Dr. Qing-An Zeng. All rights reserved. 9
TDMA: Channel Structure
f
(a). Forward channel
… t #1
#2
#n
#1
#2
#n
… … #1
#2
#n
Frame Frame Frame
… t
f ’
#1
#2
#n
#1
#2
#n
…
(b). Reverse channel
… #1
#2
#n
Frame Frame Frame
Channels in TDMA/FDD
Copyright © 2011, Dr. Dharma P. Agrawal and Dr. Qing-An Zeng. All rights reserved. 10
Forward and Reverse Channels in TDMA
Frequency f = f ’
Frame Frame
Time
…
#1
#2
#n
#1
#2
#n
…
Forward
channel
Reverse
channel
…
#1
#2
#n
Forward
channel
#1
#2
#n
…
Reverse
channel
Channels in TDMA/TDD
Copyright © 2011, Dr. Dharma P. Agrawal and Dr. Qing-An Zeng. All rights reserved. 11
Frame Structure of TDMA
…
Time
Fre
qu
ency
#1
#2
#n
#2
#n
… … #1
#2
#n
Frame Frame Frame
Head Data Guard
time
#1
Copyright © 2011, Dr. Dharma P. Agrawal and Dr. Qing-An Zeng. All rights reserved. 12
Code Division Multiple Access (CDMA)
Frequency
• 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 U
ser
1
Time
Use
r 2
Use
r n
Code
. . .
Orthogonality conditions of two signals in CDMA:
kjiji
jidttsts j
C
i ,...,2,1,,0
1)()(
Copyright © 2011, Dr. Dharma P. Agrawal and Dr. Qing-An Zeng. All rights reserved. 13
CDMA Encode/Decode
Slot 1 Slot 0
d1 = -1
1 1 1 1
1 - 1 - 1 - 1 -
Zi,m= di.cm d0 = 1
1 1 1 1
1 - 1 - 1 - 1 -
1 1 1 1
1 - 1 - 1 - 1 -
1 1 1 1
1 - 1 - 1 - 1 -
slot 0
Channel
output
slot 1
Channel
output
Channel output Zi,m
Render
Code
Data
bits
Slot 1 Slot 0
d1 = -1
d0 = 1
1 1 1 1
1 - 1 - 1 - 1 -
1 1 1 1
1 - 1 - 1 - 1 -
1 1 1 1
1 - 1 - 1 - 1 -
1 1 1 1
1 - 1 - 1 - 1 -
Slot 0
channel
output
Slot 1
channel
output Receiver
Code
Received
input
Di = S Zi,m.cm m=1
M
M
Copyright © 2011, Dr. Dharma P. Agrawal and Dr. Qing-An Zeng. All rights reserved. 14
CDMA: two-sender interference
Copyright © 2011, Dr. Dharma P. Agrawal and Dr. Qing-An Zeng. All rights reserved. 15
Structure of a CDMA System
MS #1
MS #2
MS #n
BS
C1’
C2’
Cn’
C1
C2
Cn
… …
…
Reverse channels (Uplink)
Forward channels (Downlink)
Frequency f ’
Ci’ x Cj’ = 0, i.e., Ci’ and Cj’ are orthogonal codes,
Ci x Cj = 0, i.e., Ci and Cj are orthogonal codes
Frequency f
Copyright © 2010, Dr. Dharma P. Agrawal and Dr. Qing-An Zeng. All rights reserved. 16
Digital
signal
s(t)
Code c(t)
Spreading
signal m(t) Spreading
Frequency
Power
Frequency
Power
Spread Spectrum
)()()( tctstm
Spreading of data signal s(t) by the code signal c(t)
to result in message signal m(t) as:
Copyright © 2010, Dr. Dharma P. Agrawal and Dr. Qing-An Zeng. All rights reserved. 17
Code
c(t)
Spreading
signal m(t)
Spreading
Transmitter
Code
c(t)
Digital
signal s(t)
Despread
Receiver
Direct Sequence Spread Spectrum (DSSS)
Power
Digital
signal s(t)
Frequency Frequency
Power
Frequency
Power
18
Orthogonal Codes
Orthogonal codes
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
19
Walsh Codes
Set of Walsh codes of length n consists of
the n rows of an n x n Walsh matrix:
W1 = (0)
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
nn
nn
nWW
WWW2
Example:
Copyright © 2010, Dr. Dharma P. Agrawal and Dr. Qing-An Zeng. All rights reserved. 21
Time
Frequency
An Example of Frequency Hopping Pattern
Copyright © 2010, Dr. Dharma P. Agrawal and Dr. Qing-An Zeng. All rights reserved. 22
Hopping
pattern
Spreading
signal
Spreading
Transmitter
Power
Digital
signal
Frequency Frequency
Power
Frequency Hopping Spread Spectrum (FHSS)
Digital
signal s(t)
Despread
Receiver
Hopping
pattern
Frequency
Power
Copyright © 2010, Dr. Dharma P. Agrawal and Dr. Qing-An Zeng. All rights reserved. 23
MS1 MS2 BS
Distance Distance 0
d2 d1
Received signal strength
MS1 MS2 BS
Near-far Problem
Copyright © 2011, Dr. Dharma P. Agrawal and Dr. Qing-An Zeng. All rights reserved. 24
Adjacent Channel Interference
Frequency f1 f2
MS1
MS2
Power
Copyright © 2010, Dr. Dharma P. Agrawal and Dr. Qing-An Zeng. All rights reserved. 25
Interference in Spread Spectrum
Frequency
Baseband
signal
0
Interference signals
Frequency
Despread signal
f
Frequency
Interference
baseband signals
Spectrum spreading signal
f
Copyright © 2010, Dr. Dharma P. Agrawal and Dr. Qing-An Zeng. All rights reserved. 26
Power Control in CDMA
Pr
Pt =
1
4pdf
c
Controlling transmitted power affects the CIR
Pr = Received power in free space
Pt = Transmitted power
d = Distance between receiver and transmitter
f = Frequency of transmission
c = Speed of light
= Attenuation constant (2 to 4)
Copyright © 2010, Dr. Dharma P. Agrawal and Dr. Qing-An Zeng. All rights reserved.
kjiji
jidttfstfs ji
F
,....,2,1,,,0
,1),(),( *
27
Orthogonal Frequency Division Multiplexing (OFDM)
Spectrunm of an
OFDM signal with
multiple subchannels
Orthogonality of two signals in OFDM can be given by
a complex congugate relation indicated by *:
Spectrum of a single
OFDM subchannel
Divide a channels into multiple sub-channels and do parallel transmission
Copyright © 2010, Dr. Dharma P. Agrawal and Dr. Qing-An Zeng. All rights reserved. 28
Modulation / Demodulation steps in OFDM
Modulation operation at the OFDM transmitter
High speed
Serial to parallel
conversion
IDFT Guard interval
insertion
data stream
Low speed bit stream
N2 ….
Nn
Transmission of
OFDM signal
Demodulation steps at the OFDM receiver
Guard interval
removal DFT
N1
N2
….
Nn
Parallel to serial
conversion
High speed
data stream
N1
Received
OFDM signal
Copyright © 2010, Dr. Dharma P. Agrawal and Dr. Qing-An Zeng. All rights reserved. 29
Space Division Multiple Access (SDMA)
The concept
of SDMA
Beam n Beam 1
Beam 2
Beam 3
Beam i
s(f,t,c) s(f,t,c)
s(f,t,c)
s(f,t,c)
s(f,t,c)
Space divided into spatially separate sectors
Omni-directional
transmission
Copyright © 2010, Dr. Dharma P. Agrawal and Dr. Qing-An Zeng. All rights reserved. 30
Transmission in SDMA
The basic structure of a SDMA system.
MS1
MS2 MS3 BS
Beam 1 Beam 2 Beam 3
Copyright © 2010, Dr. Dharma P. Agrawal and Dr. Qing-An Zeng. All rights reserved. 31
Comparison of Various Multiple Division Techniques
Technique FDMA TDMA CDMA SDMA
Concept
Divide the frequency
band into disjoint
subbands
Divide the time into
non-overlapping
time slots
Spread the signal
with orthogonal
codes
Divide the space in to
sectors
Active terminals
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
Signal separation
Filtering in
frequency
Synchronization in
time
Code separation Spatial separation
using smart antennas
Handoff Hard handoff Hard handoff Soft handoff Hard and soft
handoffs
Advantages Simple and robust Flexible Flexible Very simple, increases
system capacity
Disadvantages
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 intracell
handoffs
Current
applications
Radio, TV and
analog cellular
GSM and PDC 2.5G and 3G Satellite systems,
other being explored
Copyright © 2010, Dr. Dharma P. Agrawal and Dr. Qing-An Zeng. All rights reserved. 32
Modulation Techniques
Why need modulation?
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
Copyright © 2010, Dr. Dharma P. Agrawal and Dr. Qing-An Zeng. All rights reserved. 33
Analog and Digital Signals
Analog Signal (Continuous signal)
Time
Amplitude
Bit
Digital Signal (Discrete signal)
Tim
e
Amplitude
1 1 1 1 0 0 +
0
0
S(t)
Copyright © 2010, Dr. Dharma P. Agrawal and Dr. Qing-An Zeng. All rights reserved. 34
Amplitude Modulation (AM)
Message signal
x(t)
Carrier signal
AM signal
s(t)
The modulated carrier signal s(t) is:
Time
Time
Time
)2(cos)]([)( tftxAts cp Where fc is the carrier frequency and A its amplitude
Copyright © 2011, Dr. Dharma P. Agrawal and Dr. Qing-An Zeng. All rights reserved. 35
Frequency Modulation (FM)
Carrier signal
Message signal
x(t)
FM signal
s(t)
Time
Time
Time
The modulated carrier signal s(t) is:
t
t
c dxftfAts
0
0)(22(cos)( pp Where f is the peak frequency deviation from the original
frequency and f << fc
BW=2(b1)fm with b= f/fm; fm is the
maximum modulating frequency used
Copyright © 2011, Dr. Dharma P. Agrawal and Dr. Qing-An Zeng. All rights reserved. 36
Frequency Shift Keying (FSK)
1/0 represented by two different frequencies
Carrier signal ‘1’
for message signal ‘1’
1 0 1 1 0 1
Time
Time
Time
Time
Message signal x(t)
FSK signal s(t)
Carrier signal 2
for message signal ‘0’
Copyright © 2011, Dr. Dharma P. Agrawal and Dr. Qing-An Zeng. All rights reserved. 37
Phase Shift Keying (PSK)
• Use alternative sine wave phases to encode bits
Carrier signal
Carrier signal
)2sin( pp tfc
Message signal x(t)
)2sin( tfcp
1 0 1 1 0 1
PSK signal s(t)
Time
Time
Time
Time
Copyright © 2010, Dr. Dharma P. Agrawal and Dr. Qing-An Zeng. All rights reserved. 38
Quadrature Phase Shift Keying (QPSK)
2/3
2/
0
1,1
0,1
1,0
0,0
p
p
p
4/
4/3
4/3
4/
1,1
0,1
1,0
0,0
p
p
p
p
or
Four different phase shifts used are:
I (in-phase) and Q (quadrature) modulation used
Copyright © 2011, Dr. Dharma P. Agrawal and Dr. Qing-An Zeng. All rights reserved. 39
QPSK Signal Constellation
Q
I 0,0 1,1
0,1
1,0
Q
I 0 1
(a) BPSK
(Binary Phase Shift Keying)
(b) QPSK
(Quadrature Phase Shift Keying)
Copyright © 2010, Dr. Dharma P. Agrawal and Dr. Qing-An Zeng. All rights reserved. 40
p/4 QPSK
kkk 1
All possible states in p/4 QPSK
The phase of the carrier is: Where k is carrier phase shift corresponding to input bit pairs. If 0=0, input bit stream is [1011], then:
04/4/
4/
212
101
pp
p
Copyright © 2010, Dr. Dharma P. Agrawal and Dr. Qing-An Zeng. All rights reserved. 41
Quadrature Amplitude Modulation
(QAM) Combination of AM and PSK: modulate signals using two
measures of amplitude and four possible phase shifts
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
A representative QAM Table
Copyright © 2011, Dr. Dharma P. Agrawal and Dr. Qing-An Zeng. All rights reserved. 42
Quadrature Amplitude Modulation
(QAM)
Two carriers out of phase by 90 deg are amplitude modulated
Rectangular constellation of 16QAM
I
Q
0000 0100 1100 1000
0001 0101 1101 1001
0011 0111 1111 1011
0010 0110 1110 1010
Copyright © 2010, Dr. Dharma P. Agrawal and Dr. Qing-An Zeng. All rights reserved.
Homework
Exercises: 7.2, 7.17
Practice at home: 7.3, 7.6, 7.8
43
Quiz 2
What is the difference between collision detection and collision avoidance?
What are the differences between adjacent channel interference and co-channel interference?
44