Chapter 7 Signal Encoding Techniques PDF

CHAPTER 7 SIGNAL ENCODING TECHNIQUES These slides are made available to faculty in PowerPoint form. Slides can be freely added, modified, and deleted to suit student needs. They represent substantial work on the part of the authors; Wireless Communication the...

CHAPTER 7 SIGNAL ENCODING TECHNIQUES These slides are made available to faculty in PowerPoint form. Slides can be freely added, modified, and deleted to suit student needs. They represent substantial work on the part of the authors; Wireless Communication therefore, we request the following. Networks and Systems If these slides are used in a class setting or posted on an internal or 1st edition, Global edition external www site, please mention the source textbook and note our copyright of this material. Cory Beard, William Stallings All material copyright 2016 © 2016 Pearson Education, Ltd. Cory Beard and William Stallings, All Rights Reserved Signal Encoding Techniques 7-1 Signal encoding is the conversion from analog/digital data to analog / digital signal. The encoding scheme is simply the mapping from data bits to signal elements 7.1 ENCODING AND MODULATION TECHNIQUES Signal Encoding Techniques 7-2  REASONS FOR CHOOSING ENCODING TECHNIQUES Digital data, digital signal – Equipment less complex and expensive than digital-to-analog modulation equipment Analog data, digital signal – Permits use of modern digital transmission and switching equipment Signal Encoding Techniques 7-3  REASONS FOR CHOOSING ENCODING TECHNIQUES Digital data, analog signal – Some transmission media will only propagate analog signals – E.g., optical fiber and unguided media Analog data, analog signal – Analog data in electrical form can be transmitted easily and cheaply – Done with voice transmission over voice-grade lines Signal Encoding Techniques 7-4  SIGNAL ENCODING CRITERIA What determines how successful a receiver will be in interpreting an incoming signal? – Signal-to-noise ratio – Data rate – Bandwidth An increase in data rate increases bit error rate An increase in SNR decreases bit error rate An increase in bandwidth allows an increase in data rate Signal Encoding Techniques 7-5  FACTORS USED TO COMPARE ENCODING SCHEMES Signal spectrum – With lack of high-frequency components, less bandwidth required – With no DC component, AC coupling via transformer is possible – Transfer function ( a measure of the signal output versus the signal input of a system) of a channel is worse near band edges Clocking – Ease of determining beginning and end of each bit position Signal Encoding Techniques 7-6  FACTORS USED TO COMPARE ENCODING SCHEMES Signal interference and noise immunity – Performance in the presence of noise Cost and complexity – The higher the signal rate to achieve a given data rate, the greater the cost Signal Encoding Techniques 7-8 BASIC ENCODING TECHNIQUES Digital data to analog signal – Amplitude-shift keying (ASK) is a digital modulation technique. It transmits the digital information by varying the amplitude of a carrier signal. In ASK, a high-amplitude carrier signal is used to represent a binary '1,' and a low-amplitude carrier signal represents a binary '0.' Amplitude difference of carrier frequency – Frequency-shift keying (FSK) is a frequency modulation scheme in which digital information is encoded on a carrier signal by periodically shifting the frequency of the carrier between several discrete frequencies. Frequency difference near carrier frequency – Phase-shift keying (PSK) is a modulation technique by which the data bits are expressed as phases in the signal wave. Phase of carrier signal shifted Signal Encoding Techniques 7-9 7.2 MODULATION OF ANALOG SIGNALS FOR DIGITAL DATA(BINARY) Signal Encoding Techniques 7-10  AMPLITUDE-SHIFT KEYING One binary digit represented by presence of carrier, at constant amplitude Other binary digit represented by absence of carrier   binary1 s t    0 binary 0  where the carrier signal is Acos(2πfct) Signal Encoding Techniques 7-11  AMPLITUDE-SHIFT KEYING Susceptible to sudden gain changes Inefficient modulation technique Used to transmit digital data over optical fiber Signal Encoding Techniques 7-12 BINARY FREQUENCY-SHIFT KEYING (BFSK) Two binary digits represented by two different frequencies near the carrier frequency   binary1 s t    binary 0  where f1 and f2 are offset from carrier frequency fc by equal but opposite amounts fd Signal Encoding Techniques 7-13 BINARY FREQUENCY-SHIFT KEYING (BFSK) Less susceptible to error than ASK Used for high-frequency (3 to 30 MHz) radio transmission Can be used at higher frequencies on LANs that use coaxial cable Signal Encoding Techniques 7-14 7.3 FULL-DUPLEX FSK TRANSMISSION ON A VOICE GRADE CHANNEL Signal Encoding Techniques 7-15  MULTIPLE FREQUENCY-SHIFT KEYING (MFSK) More than two frequencies are used More bandwidth efficient but more susceptible to error f i = f c + (2i – 1 – M)f d f c = the carrier frequency f d = the difference frequency M = number of different signal elements = 2L L = number of bits per signal element Signal Encoding Techniques 7-16  MULTIPLE FREQUENCY-SHIFT KEYING (MFSK) To match data rate of input bit stream, each output signal element is held for: Ts=LT seconds where T is the bit period (data rate = 1/T) So, one signal element encodes L bits Signal Encoding Techniques 7-17  MULTIPLE FREQUENCY-SHIFT KEYING (MFSK) Total bandwidth required 2Mfd Minimum frequency separation required 2fd=1/Ts Therefore, modulator requires a bandwidth of Signal Encoding Techniques 7-18 MFSK FREQUENCY USE (M = 4) f i = f c + (2i – 1 – M)f d  2fd=1/Ts Ts=LT seconds f i = f c + (2i – 1 – M)f d  PHASE-SHIFT KEYING (PSK) Two-level PSK (BPSK) – Uses two phases to represent binary digits   binary1 s t     binary 0   binary1    binary 0 Signal Encoding Techniques 7-21  PHASE-SHIFT KEYING (PSK) Differential PSK (DPSK) – Phase shift with reference to previous bit Binary 0 – signal burst of same phase as previous signal burst Binary 1 – signal burst of opposite phase to previous signal burst Signal Encoding Techniques 7-23 7.5 DIFFERENTIAL PHASE-SHIFT KEYING Signal Encoding Techniques 7-24