Lecturer: Tamanna Haque Nipa

Data Communicatio

n

Chapter 5: Analog Transmission

DIGITAL-TO-ANALOG CONVERSIONDIGITAL-TO-ANALOG CONVERSION

Digital-to-analog conversion is the process of changing one of Digital-to-analog conversion is the process of changing one of the characteristics of an analog signal based on the the characteristics of an analog signal based on the information in digital data (information in digital data (Public telephone system 300Hz to 3400Hz, modem (modulator-demodulator))

Types of digital-to-analog conversion

Bit rate is the number of bits per second. Baud rate is the number of signal elements per second. In the analog transmission of digital data, the baud rate is less than or equal to the bit rate.

Amplitude Shift Keying In amplitude shift keying, the amplitude of the

carrier signal is varied to create signal elements. Both frequency and phase remain constant while the amplitude changes. Values represented by different amplitudes of carrier

low bandwidth requirements very susceptible to interference Used over optical fiber

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Figure 5.3 Binary amplitude shift keying

BASK One binary digit

represented by presence of carrier, at constant amplitude

Other binary digit represented by absence of carrier where the carrier signal is

s(t)= Acos(2пfct) for 1 binary

0 for 0 binary

Frequency Shift Keying Values represented by different frequencies

(near carrier) Less susceptible to error than ASK change frequency with each symbol needs larger bandwidth Even higher frequency on LANs

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Binary Frequency-Shift Keying (BFSK) Two binary digits represented by two

different frequencies near the carrier frequency – where f1 and f2 are offset from carrier frequency fc

s(t)= Acos(2пf1t) for 1 binary Acos(2пf2t) for 0 binary

For MFSKS(t)=Acos(2пfit) , 1<=i<=M

Phase Shift Keying Phase of carrier signal is shifted to

represent data Change phase with each symbol More complex robust against interference

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Figure 5.9 Binary phase shift keying

Two-level PSK (BPSK) Uses two phases to represent binary digits

BPSK Constellation

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Quadrature PSK More efficient use by each signal

element representing more than one bitshifts of /2 (90o)Each element represents two bits

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4 PSK Characteristic

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4 PSK

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8 PSK Characteristic

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4-QAM and 8-QAM Constellations

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Time Domain for 8-QAM Signal

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ANALOG to ANALOGANALOG to ANALOGAnalog-to-analog conversion is the representation of analog Analog-to-analog conversion is the representation of analog information by an analog signal. One may ask why we need to information by an analog signal. One may ask why we need to modulate an analog signal; it is already analog. Modulation is modulate an analog signal; it is already analog. Modulation is needed if the medium is bandpass in nature or if only a needed if the medium is bandpass in nature or if only a

bandpass channel is available to us.bandpass channel is available to us.

Types of analog-to-analog modulation

Figure 5.16 Amplitude modulation

The total bandwidth required for AM can be determined from the bandwidth of the audio signal: BAM = 2B.

Standard Bandwidth Allocation for AM Radio The bandwidth of an audio signal (speech and music) is usually 5 kHz. Therefore, an AM radio station needs a bandwidth of 10 kHz. In fact, the Federal Communications Commission (FCC) allows 10 kHz for each AM station. AM stations are allowed carrier frequencies anywhere between 530 and 1700 kHz (1.7 MHz). However, each station's carrier frequency must be separated from those on either side of it by at least 10 kHz (one AM bandwidth) to avoid interference. If one station uses a carrier frequency of 1100 kHz, the next station's carder frequency cannot be lower than 1110 kHz (see Figure 5.17).

Figure 5.18 Frequency modulation

Figure 5.19 FM band allocation

The total bandwidth required for FM can be determined from the bandwidth of the audio signal: BFM = 2(1 + β)B.