Multiple Division Techniques - faculty.csie.ntust.edu.tw

1Copyright © 2002, Dr. Dharma P. Agrawal and Dr. Qing-An Zeng. All rights reserved.

Chapter 7

Multiple Division Techniques


OutlineFrequency Division Multiple Access (FDMA)Time Division Multiple Access (TDMA)Code Division Multiple Access (CDMA)Comparison of FDMA, TDMA, and CDMAWalsh CodesNear-far ProblemTypes of InterferencesAnalog and Digital SignalsBasic 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)


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)

Use

r 1

Use

r 2

Use

r n…

Time

Frequency

• Multiple channels per carrier

• Most of second generation systems use TDMA


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

...


Types of Channels

Control channel Forward (Downlink) control channelReverse (Uplink) control channel

Traffic channelForward traffic (information) channel Reverse traffic (information) channel


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


FDMA

MS #1

MS #2

MS #n

BS

f1’

f2’

fn’

f1

f2

fn

…

……

Reverse channels

(Uplink)

Forward channels

(Downlink)


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


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


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 (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


TDMA: Frame Structure (Cont’d)

…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


Comparisons of FDMA, TDMA, and CDMA (Example)

222Control channels/cell

16.841Capacity vs FDMA320228/3=7657/3=19Voice calls/sector

333Sectors/cell8x40=32057x4=22857x1=57Voice channels/cell

40**4*1Calls per RF channel85757Usable channels/cell

12.5/1.25=10416/7=59416/7=59Channels/cell12.5/1.25=1012.5/0.03=41612.5/0.03=416No. of RF channels

1.25 MHz0.03 MHz0.03 MHzRequired channel BW177Frequency reuse

12.5 MHz12.5 MHz12.5 MHzAllocated BandwidthCDMATDMAFDMAOperation

* Depends on the number of slots ** Depends on the number of codes


Digital signal s(t)

Code c(t)

Spreading signal m(t)

Code c(t)

Digital signal s(t)

Spreading De spread

Frequency Frequency Frequency

Power Power Power

Transmitter Receiver

Concept of Direct Sequence Spread Spectrum


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


Time

Frequency

An Example of Frequency Hopping Pattern


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


Frequency

Baseband signal

Frequency

Interference baseband signals

Spreading signal

Frequency

Despread signal

Interference signals

Interference in spread spectrum system

Types of Interference


Adjacent Channel Interference

f1 f2

MS1 MS2

Frequency

Power


Power Control

Pr Pt =

1

4πdf

cα

Controlling transmitted power affects the CIR

Pt = Transmitted powerPr = Received power in free spaced = Distance between receiver and transmitterf = Frequency of transmissionc = Speed of lightα = Attenuation constant


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 antennaLimit noise and interference, e.g., FM (Frequency Modulation)Multiplexing techniques,

e.g., FDM, TDM, CDMA


Analog and Digital SignalsAnalog 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..


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


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


Frequency Shift Keying (FSK)

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

Message signalx(t)

Carrier signal 2for binary ‘0’

Carrier signal 1for binary ‘1’

FSK signals(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( ππ +tfc

Message signalx(t)

)2sin( tfcπ

1 0 1 1 0 1

PSK signals(t)

Time

Time

Time

Time


QPSK Signal Constellation

Q

I0,01,1

0,1

1,0

Q

I01

(a) BPSK (b) QPSK


All Possible State Transitions in π/4 QPSK


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

Multiple Division Techniques - faculty.csie.ntust.edu.tw

Documents

Transcript of Multiple Division Techniques - faculty.csie.ntust.edu.tw