Frequency Division Multiple Access (FDMA). User 1. User 2. User n … Time.
Frequency. • Single channel per carrier. • All first generation systems use FDMA ...
Chapter 7
Multiple Division Techniques
Copyright © 2002, Dr. Dharma P. Agrawal and Dr. Qing-An Zeng. All rights reserved.
1
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)
Copyright © 2002, Dr. Dharma P. Agrawal and Dr. Qing-An Zeng. All rights reserved.
2
Frequency Division Multiple Access (FDMA) Frequency User n … User 2 User 1
Time
• Single channel per carrier • All first generation systems use FDMA
Copyright © 2002, Dr. Dharma P. Agrawal and Dr. Qing-An Zeng. All rights reserved.
3
Time Division Multiple Access (TDMA)
…
User n
User 2
User 1
Frequency
Time • Multiple channels per carrier • Most of second generation systems use TDMA
Copyright © 2002, Dr. Dharma P. Agrawal and Dr. Qing-An Zeng. All rights reserved.
4
Code Division Multiple Access (CDMA)
...
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 Copyright © 2002, Dr. Dharma P. Agrawal and Dr. Qing-An Zeng. All rights reserved.
5
Types of Channels
Control channel
Forward (Downlink) control channel Reverse (Uplink) control channel
Traffic channel
Forward traffic (information) channel Reverse traffic (information) channel
Copyright © 2002, Dr. Dharma P. Agrawal and Dr. Qing-An Zeng. All rights reserved.
6
Types of Channels (Cont’d) Reverse channel (Uplink)
Control channels
f’ f
f2 ’ fn ’
…
f1 ’
…
f1
MS
Traffic channels
Forward channels (Downlink)
Copyright © 2002, Dr. Dharma P. Agrawal and Dr. Qing-An Zeng. All rights reserved.
f2 fn
BS 7
FDMA f1
f2 ’
f2
fn ’
fn
…
…
MS #2
f1 ’
MS #n
…
MS #1
Reverse channels
Forward channels
(Uplink)
(Downlink)
Copyright © 2002, Dr. Dharma P. Agrawal and Dr. Qing-An Zeng. All rights reserved.
BS
8
FDMA: Channel Structure Sub Band Wc
Guard Band Wg
1
2
3
4
…
N
Frequency
Total Bandwidth W=NWc
f1 ’
f2 ’
fn ’
f1
f2
…
fn
… Frequency
Reverse channels
Protecting bandwidth
Forward channels
Copyright © 2002, Dr. Dharma P. Agrawal and Dr. Qing-An Zeng. All rights reserved.
9
TDMA Slot
MS #n
Frame
Reverse channels
… t
#1
t
…
…
…
#2
t
…
#1
#1
Frame
…
… #2
…
…
(Uplink)
t
… t
…
…
#n
…
…
#n
…
MS #2
…
#2
#2
MS #1
…
#n
#1
…
Frequency f
#n
Frequency f ’
…
t Frame
Frame
BS
Forward channels (Downlink)
Copyright © 2002, Dr. Dharma P. Agrawal and Dr. Qing-An Zeng. All rights reserved.
10
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 Copyright © 2002, Dr. Dharma P. Agrawal and Dr. Qing-An Zeng. All rights reserved.
11
TDMA: Frame Structure (Cont’d)
…
#n
#2
#1
…
#n
#2
#1
…
#n
#2
…
Frame #1
Frame #n
#2
#1
Frequency f=f’
Time Forward channel
Reverse channel
Forward channel
Copyright © 2002, Dr. Dharma P. Agrawal and Dr. Qing-An Zeng. All rights reserved.
Reverse channel
12
TDMA: Frame Structure (Cont’d)
Guard time
Head
…
#n
#2
#1
…
Frame #n
#2
#1
…
Frame #n
#2
#1
Frequency Frame
Time
Data
Copyright © 2002, Dr. Dharma P. Agrawal and Dr. Qing-An Zeng. All rights reserved.
13
Code Division Multiple Access (CDMA) Frequency f ’
Frequency f C1
MS #2
C2’
C2
Cn’
Cn
… MS #n
Note:
…
C1’
…
MS #1
Reverse channels
Forward channels
(Uplink)
(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
Copyright © 2002, Dr. Dharma P. Agrawal and Dr. Qing-An Zeng. All rights reserved.
14
Comparisons of FDMA, TDMA, and CDMA (Example) Operation
FDMA
TDMA
CDMA
Allocated Bandwidth
12.5 MHz
12.5 MHz
12.5 MHz
7
7
1
0.03 MHz
0.03 MHz
1.25 MHz
12.5/0.03=416
12.5/0.03=416
12.5/1.25=10
416/7=59
416/7=59
12.5/1.25=10
Control channels/cell
2
2
2
Usable channels/cell
57
57
8
Calls per RF channel
1
4*
40**
Voice channels/cell
57x1=57
57x4=228
8x40=320
3
3
3
57/3=19
228/3=76
320
4
16.8
Frequency reuse Required channel BW No. of RF channels Channels/cell
Sectors/cell Voice calls/sector
Capacity vs FDMA 1 * Depends on the number of slots ** Depends on the number of codes
Copyright © 2002, Dr. Dharma P. Agrawal and Dr. Qing-An Zeng. All rights reserved.
15
Concept of Direct Sequence Spread Spectrum Transmitter
Receiver
Spreading
De spread
Digital signal s(t)
Power
Frequency
Digital signal s(t)
Spreading signal m(t) Code c(t)
Power
Code c(t)
Frequency
Copyright © 2002, Dr. Dharma P. Agrawal and Dr. Qing-An Zeng. All rights reserved.
Power
Frequency 16
Concept of Frequency Hopping Spread Spectrum Transmitter
Receiver
Spreading
Digital signal
Despread
Spreading signal
Hopping Pattern Power
Frequency
Digital signal
Hopping Pattern Power
Power
Frequency
Copyright © 2002, Dr. Dharma P. Agrawal and Dr. Qing-An Zeng. All rights reserved.
Frequency
17
An Example of Frequency Hopping Pattern Frequency
Time
Copyright © 2002, Dr. Dharma P. Agrawal and Dr. Qing-An Zeng. All rights reserved.
18
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
Copyright © 2002, Dr. Dharma P. Agrawal and Dr. Qing-An Zeng. All rights reserved.
19
Near-far Problem
MS2
BS
MS1
Received signal strength
Distance
Distance
0
MS2
d2
BS
d1
MS1
Copyright © 2002, Dr. Dharma P. Agrawal and Dr. Qing-An Zeng. All rights reserved.
20
Types of Interference Interference baseband signals
Baseband signal
Spreading signal
Despread signal Interference signals
Frequency
Frequency
Frequency
Interference in spread spectrum system Copyright © 2002, Dr. Dharma P. Agrawal and Dr. Qing-An Zeng. All rights reserved.
21
Adjacent Channel Interference MS1
MS2 Power
f1
f2
Copyright © 2002, Dr. Dharma P. Agrawal and Dr. Qing-An Zeng. All rights reserved.
Frequency
22
Power Control Controlling transmitted power affects the CIR
1 Pr Pt = 4πdfα c Pt = Pr = d = f = c = α=
Transmitted power Received power in free space Distance between receiver and transmitter Frequency of transmission Speed of light Attenuation constant
Copyright © 2002, Dr. Dharma P. Agrawal and Dr. Qing-An Zeng. All rights reserved.
23
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
Copyright © 2002, Dr. Dharma P. Agrawal and Dr. Qing-An Zeng. All rights reserved.
24
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 Copyright © 2002, Dr. Dharma P. Agrawal and Dr. Qing-An Zeng. All rights reserved.
25
Hearing, Speech, and Voice-band Channels Human hearing Human speech
Voice-grade Telephone channel
..
100
10,000
Frequency (Hz)
Pass band Guard band
Guard band Frequency cutoff point
0
200
3,500
Copyright © 2002, Dr. Dharma P. Agrawal and Dr. Qing-An Zeng. All rights reserved.
4,000
Frequency (Hz)
26
Amplitude Modulation (AM) Message signal x(t)
Time
Carrier signal
Time
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. Copyright © 2002, Dr. Dharma P. Agrawal and Dr. Qing-An Zeng. All rights reserved.
27
Frequency Modulation (FM) 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. Copyright © 2002, Dr. Dharma P. Agrawal and Dr. Qing-An Zeng. All rights reserved.
28
Frequency Shift Keying (FSK) • 1/0 represented by two different frequencies slightly offset from carrier frequency Carrier signal 1 for binary ‘1’
Time
Carrier signal 2 for binary ‘0’
Time 1
0
1
1
0
1
Message signal x(t)
Time
FSK signal s(t)
Time
Copyright © 2002, Dr. Dharma P. Agrawal and Dr. Qing-An Zeng. All rights reserved.
29
Phase Shift Keying (PSK) • Use alternative sine wave phase to encode bits Carrier signal
Time
sin( 2πf c t )
Carrier signal
Time
sin( 2πf c t + π ) 1
0
1
1
0
Message signal x(t) PSK signal s(t)
Copyright © 2002, Dr. Dharma P. Agrawal and Dr. Qing-An Zeng. All rights reserved.
1 Time Time
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QPSK Signal Constellation Q
Q
0,1
1
0
I
1,1
0,0
I
1,0
(a) BPSK
(b) QPSK
Copyright © 2002, Dr. Dharma P. Agrawal and Dr. Qing-An Zeng. All rights reserved.
31
All Possible State Transitions in π/4 QPSK
Copyright © 2002, Dr. Dharma P. Agrawal and Dr. Qing-An Zeng. All rights reserved.
32
Quadrature Amplitude Modulation (QAM) Combination of AM and PSK Two carriers out of phase by 90 deg are amplitude modulated Q
1000
1100
0100
0000
1001
1101
0101
0001
I
1011
1010
1111
1110
0111
0011
0110
0010
R ectangular constellation of 16Q A M
Copyright © 2002, Dr. Dharma P. Agrawal and Dr. Qing-An Zeng. All rights reserved.
33