Frequency Division Multiple Access

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)


2

Frequency Division Multiple Access (FDMA) Frequency User n … User 2 User 1

Time

• Single channel per carrier • All first generation systems use FDMA


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


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


6

Types of Channels (Cont’d) Reverse channel (Uplink)

Control channels

f’ f

f2 ’ fn ’

…

f1 ’

…

f1

MS

Traffic channels

Forward channels (Downlink)


f2 fn

BS 7

FDMA f1

f2 ’

f2

fn ’

fn

…

…

MS #2

f1 ’

MS #n

…

MS #1

Reverse channels

Forward channels

(Uplink)

(Downlink)


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


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)


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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


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


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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


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


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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


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


Frequency

17

An Example of Frequency Hopping Pattern Frequency

Time


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


19

Near-far Problem

MS2

BS

MS1

Received signal strength

Distance

Distance

0

MS2

d2

BS

d1

MS1


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


Frequency

22

Power Control Controlling transmitted power affects the CIR

1 Pr Pt = 4πdfα  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


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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) 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.

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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


4,000

Frequency (Hz)

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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


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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)


1 Time Time

30

QPSK Signal Constellation Q

Q

0,1

1

0

I

1,1

0,0

I

1,0

(a) BPSK

(b) QPSK


31

All Possible State Transitions in π/4 QPSK


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


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