Analog Transmission 01204325 Data Communications and Computer Networks

Chaiporn Jaikaeo Department of Computer Engineering Kasetsart University

Based on lecture materials from Data Communications and Networking, 5th ed., Behrouz A. Forouzan, McGraw Hill, 2012.

Revised 2020-08-06 Outline • Aspects of Digital-to-Analog Conversion • Amplitude Shift Keying • Frequency Shift Keying • Phase Shift Keying • Quadrature Amplitude Modulation • Analog-to-Analog Conversion

2 Digital-to-Analog Conversion • Required to send digital data over a band-pass channel ◦ Also known as modulation

(band-pass channel)

https://en.wikipedia.org/wiki/Modem 3 Carrier Signals • Carrier signal is a high-frequency signal acting as a base for information signal • Also known as Carrier Frequency

4 Sine Waves: Revisited

• Simplest form of periodic signal signal strength

period T = 1/f

peak amplitude

time

• General form: �(�) = �×sin(2�� + �)

phase / phase shift

5 Conversion Techniques

6 Bit Rate vs. Baud Rate • Bit rate ® the number of bits per second • Baud rate ® the number of signal elements (symbols) per second

• In the analog transmission of digital data, the baud rate is less than or equal to the bit rate

7 Binary Amplitude Shift Keying

• Or Binary ASK • Represent bits by amplitudes; fix frequency and phase • Simplest form ® On-Off Keying (OOK)

fc : Carrier frequency 0 < d < 1

8 Implementation of Binary ASK

• An everyday example: IR remote

9 Frequency Spectrum of ASK (1) • Consider a digital signal of the form $

� � = �! + [�" sin 2��!� + �"cos 2��!�] "#!

• After modulation with the carrier frequency fc

� � = sin 2��%� ⋅ �(�) %

� � = �!sin 2��"� + [�# sin 2��"� ⋅ sin 2��!� + �# sin 2��"� ⋅ cos 2��!�] #$!

10 Frequency Spectrum of ASK (2) • Using trigonometric identities %

� � = �!sin 2��"� + [ �#sin 2��"� ⋅ sin 2��!� + �# sin 2��"� ⋅ cos 2��!�] #$! 1 � [cos 2�� − 2�� − cos(2�� + 2�� )] 2 # " ! " ! 1 � [sin 2�� − 2�� + sin(2�� + 2�� )] 2 # " ! " !

Bandband signal Modulated signal

f f fn fc Demo on Desmos fc– fn fc+fn

11 Example – ASK Bit Rate • Given bandwidth of 100 kHz, spanning from 200 kHz to 300 kHz • Modulate data using ASK with communication circuitry that requires bandwidth of 2 Hz/baud • What is the carrier frequency and the bit rate? Solution: 300 �� + 200 �� = = 250 �� % 2 ��ℎ = 100 �� = 2×�� = 2×��×1 �� = 50 ��

12 Binary Frequency Shift Keying • Or Binary FSK • Represent bits by frequencies; fix amplitude and phase

13 Implementation of Binary FSK

14 Multilevel FSK • Use one frequency to send more than one bit at a time • E.g., the whole bandwidth divided into 8 frequency ranges ◦ Each frequency can represent 3 bits

000 001 010 011 100 101 110 111

15 Binary Phase Shift Keying • Or Binary PSK • Represent bits by phase shifts; fix amplitude and frequency

16 Implementation of Binary PSK

17 Quadrature PSK • Each signal element carries 2 bits

18 Example – QPSK Bandwidth • Find the bandwidth for a signal transmitting at 12 Mbps for QPSK • Assume the communication circuitry requires 1 Hz/baud Solution • For QPSK, two bits is carried by one signal element. This means 1 baud = 2 bits per second

12 �� ℎ = 1×�� = 1× = 6 �� 2

19 Implementation of QPSK

Data Bits 00 01 10 11 Phase -135 135 -45 45

20 Constellation Diagrams • A constellation diagram helps define the amplitude and phase of a signal element

Phasor Visualization

21 Example – Constellation Diagram • Show the constellation diagrams for OOK, BPSK, and QPSK modulations

22 Constellation Diagram for 8-PSK

23 Quadrature Amplitude Modulation • QAM – Quadrature Amplitude Modulation • A combination of ASK and PSK

24 QAM Modulation Process

25 QAM Demodulation Process

Basic of Signal Demodulation

26 Telephone Line Bandwidth

27 V.32 and V.32bis Modem Standards

28 Analog-to-Analog Conversion • Process of transmitting analog information by an analog signal • Although the signal is already analog, modulation is needed if a band-pass channel is available to us.

29 Types of Analog-to-Analog Modulations

30 Amplitude Modulation (AM)

31 AM Band Allocation

32 Frequency Modulation (FM)

33 FM Band Allocation

34 Phase Modulation (PM) • Considered a variation of FM

35 Summary • Band-pass channels require signals modulated into specific frequency range • Three properties of sine wave can be altered to represent the original data bits ◦ Amplitude-shift keying (ASK) ◦ Frequency-shift keying (FSK) ◦ Phase-shift keying (PSK) • Quadrature Amplitude Modulation (QAM) combines both ASK and PSK • Analog-to-analog conversion: AM, FM, PM