Chapt-07.pdf

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Copyright © 2002, Dr. Dharma P. Agrawal and Dr. Qing-An Zeng. All rights reserved.
Chapter 7
Multiple Division Techniques

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Outline
„ Frequency Division Multiple Access (FDMA)
„ Time Division Multiple Access (TDMA)
„ Code Division Multiple Access (CDMA)
„ Comparison of FDMA, TDMA, and CDMA
„ Walsh Codes
„ Near-far Problem
„ Types of Interferences
„ Analog and Digital Signals
„ Basic Modulation Techniques
„ Amplitude Modulation (AM)
„ Frequency Modulation (FM)
„ Frequency Shift Keying (FSK)
„ Phase Shift Keying (PSK)
„ Quadrature Phase Shift Keying (QPSK)
„ Quadrature Amplitude Modulation (QAM)

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Frequency Division Multiple Access (FDMA)
User 1
User 2
User n
…
Time
Frequency
• Single channel per carrier
• All first generation systems use FDMA

4
Time Division Multiple Access (TDMA)
User
1
User
2
User
n
…
Time
Frequency
• Multiple channels per carrier
• Most of second generation systems use TDMA

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Code Division Multiple Access (CDMA)
User
1
Time
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
User
2
User
n
Code
.
.
.

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Types of Channels
„ Control channel
„ Forward (Downlink) control channel
„ Reverse (Uplink) control channel
„ Traffic channel
„ Forward traffic (information) channel
„ Reverse traffic (information) channel

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Types of Channels (Cont’d)
MS BS
f1’
f2’
fn’
f ’
f
…
Reverse channel (Uplink)
Forward channels
(Downlink)
f1
f2
fn
…
Control channels
Traffic channels

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FDMA
MS #1
MS #2
MS #n
BS
f1’
f2’
fn’
f1
f2
fn
…
…
…
Reverse channels
(Uplink)
Forward channels
(Downlink)

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FDMA: Channel Structure
1 2 3 … N
Frequency
Total Bandwidth W=NWc
Guard Band Wg
4
Sub Band Wc
Frequency
Protecting bandwidth
…
f1’ f2’ fn’
…
f1 f2 fn
Reverse channels Forward channels

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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
TDMA

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TDMA: Channel Structure
… t
f
#1
#2
#n
#1
#2
#n
…
(a). Forward channel
…
#1
#2
#n
Frame Frame
Frame
… t
f ’
#1
#2
#n
#1
#2
#n
…
(b). Reverse channel
…
#1
#2
#n
Frame Frame
Frame

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TDMA: Frame Structure (Cont’d)
…
Time
Frequency
f = f ’
#1
#2
#n
#1
#2
#n
…
Forward
channel
Reverse
channel
…
#1
#2
#n
Forward
channel
Frame Frame
#1
#2
#n
…
Reverse
channel

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TDMA: Frame Structure (Cont’d)
…
Time
Frequency
#1
#2
#n
#1
#2
#n
… …
#1
#2
#n
Frame Frame
Frame
Head Data
Guard
time

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Code Division Multiple Access (CDMA)
MS #1
MS #2
MS #n
BS
C1’
C2’
Cn’
C1
C2
Cn
…
…
…
Reverse channels
(Uplink)
Forward channels
(Downlink)
Frequency f ’
Note: 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

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

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Digital signal
s(t)
Code
c(t)
Spreading signal
m(t)
Code
c(t)
Digital signal
s(t)
Spreading De spread
Frequency Frequency Frequency
Power Power Power
Transmitter Receiver
Concept of Direct Sequence Spread Spectrum

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Digital signal
Hopping Pattern
Spreading signal Digital signal
Spreading Despread
Frequency Frequency Frequency
Power Power Power
Hopping Pattern
Transmitter Receiver
Concept of Frequency Hopping Spread Spectrum

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Time
Frequency
An Example of Frequency Hopping Pattern

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Walsh Codes (Orthogonal 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

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MS1
MS2 BS
Distance Distance
0
d2 d1
Received signal strength
MS1
MS2 BS
Near-far Problem

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Frequency
Baseband signal
Frequency
Interference baseband signals
Spreading signal
Frequency
Despread signal
Interference
signals
Interference in spread spectrum system
Types of Interference

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Adjacent Channel Interference
f1 f2
MS1 MS2
Frequency
Power

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Power Control
Pr
Pt
=
1








4πdf
c
α
Controlling transmitted power affects the CIR
Pt = Transmitted power
Pr = Received power in free space
d = Distance between receiver and transmitter
f = Frequency of transmission
c = Speed of light
α = Attenuation constant

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Modulation
„ Why need modulation?
„ Small antenna size
Antenna size is inversely proportional to frequency
e.g., 3 kHz Æ 50 km antenna
3 GHz Æ 5 cm antenna
„ Limit noise and interference,
e.g., FM (Frequency Modulation)
„ Multiplexing techniques,
e.g., FDM, TDM, CDMA

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Analog and Digital Signals
„ Analog Signal (Continuous signal)
ƒ Digital Signal (Discrete signal)
Time
Amplitude
Time
Amplitude
1 1 1 1
0 0
Bit
+
_
0
0
S(t)

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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
..

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

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Frequency Modulation (FM)
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

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

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Phase Shift Keying (PSK)
• Use alternative sine wave phase to encode bits
Carrier signal
Carrier signal
)
2
sin( π
π +
t
fc
Message signal
x(t)
)
2
sin( t
fc
π
1 0 1 1 0 1
PSK signal
s(t)
Time
Time
Time
Time

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QPSK Signal Constellation
Q
I
0,0
1,1
0,1
1,0
Q
I
0
1
(a) BPSK (b) QPSK

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All Possible State Transitions in π/4 QPSK

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Quadrature Amplitude Modulation (QAM)
Combination of AM and PSK
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

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