Materi 10 - Repository UNIKOM

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KOMUNIKASI DATA

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Encoding and Modulation Techniques

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Digital Signaling Versus Analog Signaling



Digital signaling

 _{Digital or analog data}_{is encoded into a digital signal}  _{Encoding may be chosen to conserve bandwidth or to}

minimize error



Analog Signaling

 _{Digital or analog data}_{modulates analog carrier}

signal

 _{The frequency of the carrier fc is chosen to be}

compatible with the transmission medium used

 _{Modulation: the amplitude, frequency or phase of the}

carrier signal is varied in accordance with the modulating data signal

 _{by using different carrier frequencies, multiple data}

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Digital Signaling



Digital data, digital signal

 _{Simplest encoding scheme: assign one voltage level to}

binary one and another voltage level to binary zero

 _{More complex encoding schemes: are used to improve}

performance (reduce transmission bandwidth and minimize errors).

 _{Examples are NRZ-L, NRZI, Manchester, etc.}



Analog data, Digital signal

 _{Analog data, such as voice and video}

 _{Often digitized to be able to use digital transmission}

facility

 _{Example: Pulse Code Modulation (PCM), which involves}

sampling the analog data periodically and quantizing

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Analog Signaling

 _{Digital data, Analog Signal}

 _{A modem converts digital data to an analog signal so that it can}

be transmitted over an analog line

 _{The digital data modulates the amplitude, frequency, or phase of}

a carrier analog signal

 _{Examples: Amplitude Shift Keying (ASK), Frequency Shift Keying}

(FSK), Phase Shift Keying (PSK)

 _{Analog data, Analog Signal}

 _{Analog data, such as voice and video modulate the amplitude,}

frequency, or phase of a carrier signal to produce an analog signal in a different frequency band

 _{Examples: Amplitude Modulation (AM), Frequency Modulation}

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Modulation in wireless

communication

• Translate digital data to analog signal (baseband)

• Shifts center frequency of baseband signal up to the radio carrier

• example carrier frequency

• 802.11b/g: 2.4 GHz, 802.11a (5 GHz), GSM: 1.9 GHz

• Why?

• Antenna size: on the order of signal’s wavelength • More bandwidth available at higher carrier

frequency

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Modulation at

sender

modulation

digital data

modulation

radio carrier analog

baseband signal

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•

Pros: simple

•

Cons: susceptible to noise

•

Example: optical system, infra-red

1 0 1

t

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Describe Amplitude Shift

Keying

Amplitude-shift keying

(

ASK

) is a

form of

modulation

that represents

digital

data

as variations in the

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The simplest and most common form of ASK operates as a switch, using the presence of a carrier wave to indicate

a binary one and its absence to indicate a binary zero. This type of modulation is called on-off keying,

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Frequency Shift Keying

(FSK)

• Pros: less susceptible to noise

• Cons: requires larger bandwidth

1 0 1

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Describe Frequency Shift Keying

(FSK)

Frequency-shift keying (FSK) is a frequency modulation

scheme in which digital information is transmitted through discrete frequency changes of a carrier wave. The simplest FSK is binary

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Describe Frequency Shift Keying

(FSK)

• Two binary digits represented by two different frequencies near the carrier frequency

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Phase Shift Keying (PSK)

• Pros:



Less susceptible to noise



Bandwidth efficient

• Cons:



Receiver must synchronize in frequency

and phase w/ transmitter

t

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Describe Phase Shift Keying

(PSK)

Phase-shift keying (PSK)

is a digital modulation

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Describe Phase Shift Keying

(PSK)

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

BPSK (Binary Phase Shift

Keying):

 bit value 0: sine wave

 bit value 1: inverted sine wave  very simple PSK

 low spectral efficiency

 robust, used in satellite systems

Q

I 0

1

Variant of phase shift

keying

11 10 00 01 Q I 11 01 10 00 A t

 QPSK (Quadrature Phase Shift

Keying):

 2 bits coded as one symbol

 needs less bandwidth compared to

BPSK

 symbol determines shift of sine wave  Often also transmission of relative,

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 combines amplitude and phase shift keying  It is possible to code n bits using one symbol

 2n discrete levels

 bit error rate increases with n

0000 0001 0011 1000 Q I 0010 φ a

Quadrature Amplitude Modulation

(QAM)

• Example: 16-QAM (4 bits = 1 symbol)

• Symbols 0011 and 0001 have the same phase φ, but different

amplitude a. 0000 and 1000 have same amplitude but different phase

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What is transmitted in

air?

• radio wave (baseband modulated w/ carrier

radio)

• high-frequency, short wavelength

• wave length * frequency = speed of light



3x10

8

m/s

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Frequency range

(bandwidth)

• need a wide spectrum

• e.g., 802.11b bandwidth 20 MHz • w/o noise: Nyquist’s result

• w/ noise (e.g., thermal noise, background radiation)

• Shannon’s channel capacity theorem: the maximum number of bits that can be transmitted per second by a physical channel is:

)

1

(

log

₂ _NS

W



W: frequency range

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Frequencies for

communication

VLF = Very Low Frequency UHF = Ultra High Frequency LF = Low Frequency SHF = Super High Frequency

MF = Medium Frequency EHF = Extra High Frequency HF = High Frequency UV = Ultraviolet Light

VHF = Very High Frequency

1 Mm 300 Hz 10 km 30 kHz 100 m 3 MHz 1 m 300 MHz 10 mm 30 GHz 100 m 3 THz

1 m

300 THz

visible light

VLF LF MF HF VHF UHF SHF EHF infrared UV optical transmission coax cable

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 ITU-R holds auctions for new frequencies, manages frequency

bands worldwide (WRC, World Radio Conferences)