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Chapter 7
Multiple Division Techniques
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
Time Division Multiple Access (TDMA)
User1
User2
Usern
Time
Frequency
Multiple channels per carrier
Most of second generation systems use TDMA
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Code Division Multiple Access (CDMA)
User1
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
User2
Usern
Code
...
Types of Channels
Control channel
Forward (Downlink) control channel
Reverse (Uplink) control channel
Traffic channel Forward traffic (traffic or information) channel
Reverse traffic (traffic or information) channel
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Types of Channels (Contd)
MS BS
f1
f2
fn
f
f
Reverse channel (Uplink)
Forward channels
(Downlink)
f1
f2
fn
Control channels
Traffic channels
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
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 #2t
#n #nt
TDMA
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TDMA: Channel Structure
t
f
#1
#2
#
n
#
1
#
2
#
n
(a). Forward channel
#1
#2
#
n
Frame FrameFrame
t
f
#1
#2
#n
#1
#2
#n
(b). Reverse channel
#1
#2
#n
Frame FrameFrame
TDMA: Frame Structure (Contd)
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
Channels in Simplex Mode
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TDMA: Frame Structure (Contd)
Time
Frequency
#1
#2
#n
#1
#2
#n #
1#2
#n
Frame FrameFrame
Head DataGuard
time
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)
222Control channels/cell
16.841Capacity vs FDMA
320228/3=7657/3=19Voice calls/sector
333Sectors/cell
8x40=32057x4=22857x1=57Voice channels/cell
40**4*1Calls per RF channel
85757Usable channels/cell
12.5/1.25=10416/7=59416/7=59Channels/cell
12.5/1.25=1012.5/0.03=41612.5/0.03=416No. of RF channels1.25 MHz0.03 MHz0.03 MHzRequired channel BW
177Frequency reuse
12.5 MHz12.5 MHz12.5 MHzAllocated Bandwidth
CDMATDMAFDMAOperation
* Depends on the number of slots ** Depends on the number of codes
Delay ? ? ?
Digital signal
s(t)
Code
c(t)
Spreading signal
m(t)
Code
c(t)
Digital signal
s(t)
Spreading Despread
Frequency Frequency Frequency
Power Power Power
Transmitter Receiver
Direct Sequence Spread Spectrum for CDMA
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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
MS1MS2 BS
Distance Distance0
d2 d1
Received signal strength
MS1MS2 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 in CDMA
Types of Interference in CDMA
Adjacent Channel Interference in CDMA
f1 f2
Channel1 Channel2
Frequency
Power
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Power Control in CDMA
Pr
Pt=
1
4df
c
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
= Attenuation constant (2 to 4)
Modulation
Why need modulation?
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,
e.g., FDM, TDM, CDMA
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Analog and Digital Signals
Analog Signal (Continuous signal)
Digital Signal (Discrete signal)
Time
Amplitude
Time
Amplitude
1 1 1 10 0
Bit
+
_0
0
S(t)
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
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 message signal 0
Carrier signal 1for message signal 1
FSK signal
s(t)
1 0 1 1 0 1
Time
Time
Time
Time
Phase Shift Keying (PSK)
Use alternative sine wave phase to encode bits
Carrier signal
Carrier signal
)2sin( +tfc
Message signal
x(t)
)2sin( tfc
1 0 1 1 0 1
PSK signal
s(t)
Time
Time
Time
Time
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QPSK Signal Constellation
Q
I0,01,1
0,1
1,0
Q
I01
(a) BPSK (b) QPSK
All Possible State Transitions in /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 constel lation of 16QAM
I
Q
0000010011001000
0001010111011001
0011011111111011
0010011011101010