Chapter 4 4 1 Digital Modulation 4 2

Chapter 4 4. 1 : Digital Modulation 4. 2 : Digital Transmission 4. 3 : Multiple Access Methods 1

4. 1 Digital Modulation Outlines a. b. c. Introduction Information capacity, Bits, Bit Rate, Baud, M-ary Encoding Digital Modulation Techniques - ASK, FSK, PSK, QAM EKT 231 : COMMUNICATION SYSTEM CHAPTER 4 : DIGITAL MODULATION 2

Introduction : Analog modulation and digital modulation n n Both analog and digital modulation systems use analog carriers to transport the information signal. In analog modulation, the information is also analog, whereas with digital modulation, the information is digital which could be computer generated data or digitally encoded analog signals. EKT 231 : COMMUNICATION SYSTEM CHAPTER 4 : DIGITAL MODULATION 3

Digital modulation • Is the transmittal of digitally modulated analog signals between two or more points in a communications system. • Can be propagated through Earth’s atmosphere and used in wireless communication system - digital radio. • Offer several outstanding advantages over traditional analog system. • Ease of processing • Ease of multiplexing • Noise immunity EKT 231 : COMMUNICATION SYSTEM CHAPTER 4 : DIGITAL MODULATION 4

Cont’d. . . n Applications: n n Low speed voice band data comm. modems High speed data transmission systems Digital microwave & satellite comm. systems PCS (personal communication systems) telephone EKT 231 : COMMUNICATION SYSTEM CHAPTER 4 : DIGITAL MODULATION 5

Why digital modulation? n The modulation of digital signals with analogue carriers allows an improvement in signal to noise ratio as compared to analogue modulating schemes. EKT 231 : COMMUNICATION SYSTEM CHAPTER 4 : DIGITAL MODULATION 6

Important Criteria 1. 2. 3. 4. 5. 6. High spectral efficiency High power efficiency Robust to multipath Low cost and ease of implementation Low carrier-to-co channel interference ratio Low out-of-band radiation EKT 231 : COMMUNICATION SYSTEM CHAPTER 4 : DIGITAL MODULATION 7

Cont’d… 7. 8. Constant or near constant envelop Bandwidth Efficiency n n 9. Ability to accommodate data within a limited bandwidth Tradeoff between data rate and pulse width Power Efficiency n n To preserve the fidelity of the digital message at low power levels. Can increase noise immunity by increasing signal power EKT 231 : COMMUNICATION SYSTEM CHAPTER 4 : DIGITAL MODULATION 8

Forms of Digital Modulation FSK ASK PSK QAM EKT 231 : COMMUNICATION SYSTEM CHAPTER 4 : DIGITAL MODULATION 9

Forms of Digital Modulation • If the amplitude, V of the carrier is varied proportional to the information signal, a digital modulated signal is called Amplitude Shift Keying (ASK) • If the frequency, f of the carrier is varied proportional to the information signal, a digital modulated signal is called Frequency Shift Keying (FSK) EKT 231 : COMMUNICATION SYSTEM CHAPTER 4 : DIGITAL MODULATION 10

Cont’d… n If the phase, θ of the carrier is varied proportional to the information signal, a digital modulated signal is called Phase Shift Keying (PSK) n If both the amplitude and the phase, θ of the carrier are varied proportional to the information signal, a digital modulated signal is called Quadrature Amplitude Modulation (QAM) EKT 231 : COMMUNICATION SYSTEM CHAPTER 4 : DIGITAL MODULATION 11

Cont’d. . . EKT 231 : COMMUNICATION SYSTEM CHAPTER 4 : DIGITAL MODULATION 12

Example 4. 1 For the digital message 1101 1100 1010, sketch the waveform for the following: a. ASK b. FSK c. PSK d. QAM EKT 231 : COMMUNICATION SYSTEM CHAPTER 4 : DIGITAL MODULATION 13

Block Diagram Simplified block diagram of a digital modulation system Transmitter EKT 231 : COMMUNICATION SYSTEM CHAPTER 4 : DIGITAL MODULATION Receiver 14

Cont’d… n Precoder performs level conversion & encodes incoming data into group of bits that modulate an analog carrier. n Modulated carrier filtered, amplified & transmitted through transmission medium to Rx. n In Rx, the incoming signals filtered, amplified & applied to the demodulator and decoder circuits which extracts the original source information from modulated carrier. EKT 231 : COMMUNICATION SYSTEM CHAPTER 4 : DIGITAL MODULATION 15

Information Capacity, Bits, Bit Rate, Baud, M-ary Encoding n Information capacity, Bits & Bit Rate ¡ ¡ Information capacity is a measure of how much information can be propagated through a communication system and is a function of bandwidth and transmission time. Represents the number of independent symbols that can be carried through a system in a given unit of time. Basic digital symbol is the binary digit or bit. Express the information capacity as a bit rate – the number of bits transmitted during one second (bps). EKT 231 : COMMUNICATION SYSTEM CHAPTER 4 : DIGITAL MODULATION 16

Hartley’s Law Where I = information capacity (bps) B = bandwidth (Hz) t = transmission time (s) From the equation, Information capacity is a linear function of bandwidth and transmission time and directly proportional to both. EKT 231 : COMMUNICATION SYSTEM CHAPTER 4 : DIGITAL MODULATION 17

Shannon’s Formula Where I = information capacity (bps) B = bandwidth (Hz) = signal to noise power ratio (unitless) The higher S/N the better the performance and the higher the information capacity EKT 231 : COMMUNICATION SYSTEM CHAPTER 4 : DIGITAL MODULATION 18

Example 4. 2 By using the Shannon’s Formula, calculate the information capacity if S/N = 30 d. B and B = 2. 7 k. Hz. EKT 231 : COMMUNICATION SYSTEM CHAPTER 4 : DIGITAL MODULATION 19

Nyquist Sampling Rate n fs is equal or greater than 2 fm fs >= 2 fm fs = minimum Nyquist sample rate (Hz) fm = maximum analog input frequency (Hz) EKT 231 : COMMUNICATION SYSTEM CHAPTER 4 : DIGITAL MODULATION 20

Example 4. 3 Determine the Nyquist sample rate for a maximum analog input frequency 7. 5 k. Hz. EKT 231 : COMMUNICATION SYSTEM CHAPTER 4 : DIGITAL MODULATION 21

Example 4. 1 For the digital message 1101 1100 1010, sketch the waveform for the following: a. ASK b. FSK c. PSK d. QAM EKT 231 : COMMUNICATION SYSTEM CHAPTER 4 : DIGITAL MODULATION 22

Answer EKT 231 : COMMUNICATION SYSTEM CHAPTER 4 : DIGITAL MODULATION 23

Example 4. 2 By using the Shannon’s Formula, calculate the information capacity if S/N = 30 d. B and B = 2. 7 k. Hz. EKT 231 : COMMUNICATION SYSTEM CHAPTER 4 : DIGITAL MODULATION 24

Example 4. 3 Determine the Nyquist sample rate for a maximum analog input frequency 7. 5 k. Hz. fs >= 2 fm fs >= 2(7. 5 k. Hz) = 15 k. Hz EKT 231 : COMMUNICATION SYSTEM CHAPTER 4 : DIGITAL MODULATION 25

M-ary Encoding n n It is often advantageous to encode at a level higher than binary where there are more then two conditions possible. The number of bits necessary to produce a given number of conditions is expressed mathematically as OR Where N = number of bits necessary M = number of conditions, level or combinations possible with N bits. EKT 231 : COMMUNICATION SYSTEM CHAPTER 4 : DIGITAL MODULATION 26

Cont’d… n n Each symbol represents n bits, and has M signal states, where M = 2 N. Example; A digital signal with four possible conditions (voltage levels, frequencies, etc) is an M-ary system with number of possible conditions, M=4. EKT 231 : COMMUNICATION SYSTEM CHAPTER 4 : DIGITAL MODULATION 27

Example 4. 4 Find the number of voltage levels which can represent an analog signal with a. 3 Bits b. 8 bits c. 12 bits Ans: M=8, 256, 4096 EKT 231 : COMMUNICATION SYSTEM CHAPTER 4 : DIGITAL MODULATION 28

Concept of Signal states 8 possible states 3 Bit ADC EKT 231 : COMMUNICATION SYSTEM CHAPTER 4 : DIGITAL MODULATION 29

Baud n Baud refers to the rate of change of a signal on the transmission medium after encoding and modulation have occurred. ts Where baud = symbol rate (symbol per second) ts = time of one signaling element or symbol (seconds) EKT 231 : COMMUNICATION SYSTEM CHAPTER 4 : DIGITAL MODULATION 30

Bit rate and baud n n Bit rate refers to the rate of change of a digital information signal, which is usually binary. (bps or b/s ) Baud, like bit rate, also a rate of change but it refers to the rate of change of a signal on transmission medium after encoding and modulation process. EKT 231 : COMMUNICATION SYSTEM CHAPTER 4 : DIGITAL MODULATION 31

Minimum BW n Minimum Bandwidth refers to the minimum bandwidth necessary to pass M-ary digitally modulated carriers. ¡ From the Nyquist formulation for channel capacity, fb Then, Where fb= channel capacity (bps) B = minimum Nyquist bandwidth (Hz) M = number of discrete signal or voltage levels Where N is the number of bits encoded into each signaling element. EKT 231 : COMMUNICATION SYSTEM CHAPTER 4 : DIGITAL MODULATION 32

Example 4. 5 n. Determine the minimum bandwidth and baud necessary to pass a 10 kbps binary signal using amplitude shift keying. Solution ASK : N=1, and the minimum bandwidth are EKT 231 : COMMUNICATION SYSTEM CHAPTER 4 : DIGITAL MODULATION 33

Additional note n N=1 1 bit is represented for a signaling element or symbol. ts N=1, gives the following equation becomes = fb , in binary system baud = bit per second are equal. In higher system, bps always greater than baud. 34

Digital Modulation Techniques n n Amplitude Shift Keying (ASK) Frequency Shift Keying (FSK) Phase Shift Keying (PSK) Quadrature Amplitude Modulation (QAM) EKT 231 : COMMUNICATION SYSTEM CHAPTER 4 : DIGITAL MODULATION 35

Amplitude Shift Keying (ASK) The simplest digital modulation technique A binary information signal is directly modulates the amplitude of an analog carrier. Similar to standard AM except there are only two output amplitudes possible. EKT 231 : COMMUNICATION SYSTEM CHAPTER 4 : DIGITAL MODULATION 36

Changes in Amplitude Shift Keying (ASK) amplitude of the carrier signal n n A binary information signal directly modulates the amplitude of an analog carrier. Sometimes called Digital Amplitude Modulation (DAM) Where vask (t) = amplitude shift keying wave vm(t) = digital information signal (volt) A/2 = unmodulated carrier amplitude (volt) ωc = analog carrier radian frequency (rad/s) EKT 231 : COMMUNICATION SYSTEM CHAPTER 4 : DIGITAL MODULATION The modulating signal is the normalized binary waveform 37

Cont’d. . . Digital Amplitude Modulation EKT 231 : COMMUNICATION SYSTEM CHAPTER 4 : DIGITAL MODULATION 38

Frequency Shift Keying (FSK) Also the relatively simple digital modulation technique Similar to standard FM except the modulating signal is the binary signal that varies between 2 discrete voltage levels rather than a continuously changing analog waveform. Sometimes called as Binary Frequency Shift Keying (BFSK) EKT 231 : COMMUNICATION SYSTEM CHAPTER 4 : DIGITAL MODULATION 39

Frequency Shift Keying (FSK) Changes in the freq of the carrier signal n The phase shift in carrier frequency (∆f) is proportional to the amplitude of the binary input signal (vm(t)) and the direction of the shift is determined by the polarity Where vfsk(t) = binary FSK waveform The modulating signal is a normalized binary waveform Vc = peak analog carrier amplitude (volt) fc = analog carrier center frequency (Hz) ∆f = peak shift in analog carrier frequency (Hz) vm(t) = binary input signal (volt) EKT 231 : COMMUNICATION SYSTEM CHAPTER 4 : DIGITAL MODULATION 40

lower frequency Higher frequency or space frequency or mark frequency EKT 231 : COMMUNICATION SYSTEM CHAPTER 4 : DIGITAL MODULATION 41

Exercise Prove the following equations to represent binary 1 and 0 respectively. a) 5 marks b) 5 marks 42

EKT 231 : COMMUNICATION SYSTEM CHAPTER 4 : DIGITAL MODULATION 43

Cont’d. . . Binary Input Frequency Output 0 Space (fs) 1 Mark (fm) n. Baud for FSK determined by setting N=1 44

Example 4. 6 For an FSK signal, given a mark frequency = 49 k. Hz, a space frequency = 51 k. Hz and input bit rate =2 kbps. Determine (a) (b) (c) The peak frequency deviation Minimum bandwidth Baud for a binary FSK signal Ans: 1 k. HZ, 6 k. Hz, 2000 45

Solution The peak frequency deviation Minimum bandwidth Baud for a binary FSK signal, for FSK, N=1 46

Phase Shift Keying (PSK) n n Another form of angle-modulated, constant amplitude digital modulation. Binary digital signal input & limited number of output phases possible. M-ary digital modulation scheme with the number of output phases defined by M. The simplest PSK is Binary Phase-Shift Keying (BPSK) ¡ ¡ ¡ N= 1, M=2 Two phases possible for carrier with one phase for logic 1 and another phase for logic 0 The output carrier shifts between two angles separated by 180° 47

Cont’d. . . M=2, N=1 a) Truth Table b) Phasor Diagram c) Constellation Diagram 48

BPSK Transmitter Cont’d. . . • Balanced modulator – phase reversing switch 49

BPSK Receiver Cont’d. . . 50

CONSTELLATION DIAGRAM Definition : A graphical representation of the complex envelope of each possible symbol state. q The x-axis represents the in-phase component and the y-axis the quadrature component of the complex envelope q The distance between signals on a constellation diagram relates to how different the modulation waveforms are and how easily a receiver can differentiate between them. 51

Cont’d. . . 52

Cont’d. . . M=8 N=3 tribits 53

Quadrature Amplitude Modulation (QAM) n n Combine amplitude and phase-shift keying. Similar with PSK except that it is not a constant amplitude signal. Both amplitude and phase change. Method of voice band data transmission. QAM = 4 -PSK 54

Cont’d. . . quadbits 55

Cont’d. . . Both amplitude and phase vary Constant amplitude, phase varies 56

Cont’d. . . n Amplitude and phase shift keying can be combined to transmit several bits per symbol. ¡ Often referred to as linear as they require linear amplification. ¡ More bandwidth-efficient, but more susceptible to noise. n For M = 4, 16 QAM has the largest distance between points, but requires very linear amplification. 16 PSK has less stringent linearity requirements, but has less spacing between constellation points, and is therefore more affected by noise. n High level M-ary schemes (such as 64 -QAM) are very bandwidth-efficient but more susceptible to noise and require linear amplification 57

Bandwidth Efficiency ¡ Used to compare the performance of one digital modulation technique to another. Bη = Transmission bit rate (bps) Minimum bandwidth (Hz) 58

CONCLUSION n To decide which modulation method should be used , we need to make considerations of a) b) c) Bandwidth Speed of Modulation Complexity of Hardware 59

Assignment #1 n 379 60