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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