1Copyright © 2003, Dr. Dharma P. Agrawal and Dr. Qing-An Zeng. All rights reserved.

Chapter 7

Multiple Division Techniques

2Copyright © 2003, Dr. Dharma P. Agrawal and Dr. Qing-An Zeng. All rights reserved.

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)

3Copyright © 2003, Dr. Dharma P. Agrawal and Dr. Qing-An Zeng. All rights reserved.

Frequency Division Multiple Access (FDMA)

User 1

User 2

User n…

Time

Frequency

• Single channel per carrier

• All first generation systems use FDMA

4Copyright © 2003, Dr. Dharma P. Agrawal and Dr. Qing-An Zeng. All rights reserved.

Time Division Multiple Access (TDMA)

Use

r 1

Use

r 2

Use

r n…

Time

Frequency

• Multiple channels per carrier

• Most of second generation systems use TDMA

5Copyright © 2003, Dr. Dharma P. Agrawal and Dr. Qing-An Zeng. All rights reserved.

Code Division Multiple Access (CDMA)

Use

r 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 CDMAU

ser 2

Use

r nCode

...

6Copyright © 2003, Dr. Dharma P. Agrawal and Dr. Qing-An Zeng. All rights reserved.

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

7Copyright © 2003, Dr. Dharma P. Agrawal and Dr. Qing-An Zeng. All rights reserved.

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

8Copyright © 2003, Dr. Dharma P. Agrawal and Dr. Qing-An Zeng. All rights reserved.

FDMA

MS #1

MS #2

MS #n

BS

f1’

f2’

fn’

f1

f2

fn

…

……

Reverse channels

(Uplink)

Forward channels

(Downlink)

9Copyright © 2003, Dr. Dharma P. Agrawal and Dr. Qing-An Zeng. All rights reserved.

FDMA: Channel Structure

1 2 3 … NFrequency

Total Bandwidth W=NWc

Guard Band Wg

4

Sub Band Wc

Frequency

Protecting bandwidth

…

f1’ f2’ fn’

…

f1 f2 fn

Reverse channels Forward channels

10Copyright © 2003, Dr. Dharma P. Agrawal and Dr. Qing-An Zeng. All rights reserved.

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

11Copyright © 2003, Dr. Dharma P. Agrawal and Dr. Qing-An Zeng. All rights reserved.

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

12Copyright © 2003, Dr. Dharma P. Agrawal and Dr. Qing-An Zeng. All rights reserved.

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

Channels in Simplex Mode

13Copyright © 2003, Dr. Dharma P. Agrawal and Dr. Qing-An Zeng. All rights reserved.

TDMA: Frame Structure (Cont’d)

…Time

Frequency#1 #2 #n #1 #2 #n… …#1 #2 #n

Frame FrameFrame

Head DataGuard time

14Copyright © 2003, Dr. Dharma P. Agrawal and Dr. Qing-An Zeng. All rights reserved.

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

15Copyright © 2003, Dr. Dharma P. Agrawal and Dr. Qing-An Zeng. All rights reserved.

Comparisons of FDMA, TDMA, and CDMA (Example)

Operation FDMA TDMA CDMA

Allocated Bandwidth 12.5 MHz 12.5 MHz 12.5 MHz

Frequency reuse 7 7 1

Required channel BW 0.03 MHz 0.03 MHz 1.25 MHz

No. of RF channels 12.5/0.03=416 12.5/0.03=416 12.5/1.25=10

Channels/cell 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

Sectors/cell 3 3 3

Voice calls/sector 57/3=19 228/3=76 320

Capacity vs FDMA 1 4 16.8

* Depends on the number of slots ** Depends on the number of codesDelay ? ? ?

16Copyright © 2003, Dr. Dharma P. Agrawal and Dr. Qing-An Zeng. All rights reserved.

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

17Copyright © 2003, Dr. Dharma P. Agrawal and Dr. Qing-An Zeng. All rights reserved.

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

19Copyright © 2003, Dr. Dharma P. Agrawal and Dr. Qing-An Zeng. All rights reserved.

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

20Copyright © 2003, Dr. Dharma P. Agrawal and Dr. Qing-An Zeng. All rights reserved.

MS1MS2 BS

Distance Distance0

d2 d1

Received signal strength

MS1MS2 BS

Near-far Problem

21Copyright © 2003, Dr. Dharma P. Agrawal and Dr. Qing-An Zeng. All rights reserved.

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

22Copyright © 2003, Dr. Dharma P. Agrawal and Dr. Qing-An Zeng. All rights reserved.

Adjacent Channel Interference in CDMA

f1 f2

Channel1 Channel2

Frequency

Power

23Copyright © 2003, Dr. Dharma P. Agrawal and Dr. Qing-An Zeng. All rights reserved.

Power Control in CDMA

Pr Pt =

1

4df

c

Controlling transmitted power affects the CIR

Pt = Transmitted powerPr = Received power in free spaced = Distance between receiver and transmitterf = Frequency of transmissionc = Speed of light = Attenuation constant (2 to 4)

24Copyright © 2003, Dr. Dharma P. Agrawal and Dr. Qing-An Zeng. All rights reserved.

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

25Copyright © 2003, Dr. Dharma P. Agrawal and Dr. Qing-An Zeng. All rights reserved.

Analog and Digital Signals Analog Signal (Continuous signal)

Digital Signal (Discrete signal)

Time

Amplitude

Time

Amplitude

1 1 1 1 0 0

Bit

+

_0

0

S(t)

26Copyright © 2003, Dr. Dharma P. Agrawal and Dr. Qing-An Zeng. All rights reserved.

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..

27Copyright © 2003, Dr. Dharma P. Agrawal and Dr. Qing-An Zeng. All rights reserved.

Amplitude Modulation (AM)

Message signalx(t)

Carrier signal

AM signals(t)

Amplitude of carrier signal is varied as the message signal to be transmitted.

Frequency of carrier signal is kept constant.

Time

Time

Time

28Copyright © 2003, Dr. Dharma P. Agrawal and Dr. Qing-An Zeng. All rights reserved.

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 signalx(t)

FM signal s(t)

Time

Time

Time

29Copyright © 2003, Dr. Dharma P. Agrawal and Dr. Qing-An Zeng. All rights reserved.

Frequency Shift Keying (FSK)

• 1/0 represented by two different frequencies slightly offset from carrier frequency

Message signalx(t)

Carrier signal 2 for message signal ‘0’

Carrier signal 1 for message signal ‘1’

FSK signals(t)

1 0 1 1 0 1

Time

Time

Time

Time

30Copyright © 2003, Dr. Dharma P. Agrawal and Dr. Qing-An Zeng. All rights reserved.

Phase Shift Keying (PSK)• Use alternative sine wave phase to encode bits

Carrier signal

Carrier signal)2sin( tfc

Message signalx(t)

)2sin( tfc

1 0 1 1 0 1

PSK signals(t)

Time

Time

Time

Time

31Copyright © 2003, Dr. Dharma P. Agrawal and Dr. Qing-An Zeng. All rights reserved.

QPSK Signal Constellation

Q

I0,01,1

0,1

1,0

Q

I01

(a) BPSK (b) QPSK

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All Possible State Transitions in /4 QPSK

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Quadrature Amplitude Modulation (QAM)

Combination of AM and PSKTwo carriers out of phase by 90 deg are amplitude modulated

Rectangular constellation of 16QAM

I

Q

0000010011001000

0001010111011001

0011011111111011

0010011011101010