300952
Wireless and Mobile Networks
Lecture 5. Signal Encoding Techniques
Lecture objectives
• Understand the reasons for using encoding techniques
• Compare and select encoding techniques
• Understand the three main ways digital data can be encoded in analog signals
• Understand methods to modulate analog data in analog and digital signals
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INTRODUCTION
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Review from Lecture 2
• Analogsignal:signalintensityvariesinasmoothfashionovertime
– No breaks or discontinuities in the signal
• Digital signal: signal intensity maintains a constant level for some period of time and then changes to another constant level
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Encoding and Modulation Techniques
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Reasons for Choosing Encoding Techniques
• Digitaldata,digitalsignal
– Equipment less complex and expensive than digital-to-analog modulation
equipment
• Analogdata,digitalsignal
– Permits use of modern digital transmission and switching equipment
• Digitaldata,analogsignal
– Some transmission media will only propagate analog signals – E.g., optical fiber and unguided media
• Analogdata,analogsignal
– Analog data in electrical form can be transmitted easily and cheaply – Done with voice transmission over voice-grade lines
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Signal Encoding Criteria
• What determines how successful a receiver will be in interpreting an incoming signal?
– Signal-to-noise ratio (SNR) – Data rate
– Bandwidth
• An increase in data rate increases bit error rate
• An increase in SNR decreases bit error rate
• Anincreaseinbandwidthallowsanincreaseindatarate
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Factors Used to Compare Encoding Schemes
• Signalspectrum
– With lack of high-frequency components, less bandwidth required – With no DC component, AC coupling via transformer is possible
– Transfer function of a channel is worse near band edges
• Clocking
– Ease of determining beginning and end of each bit position
• 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
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DIGITAL DATA, ANALOG SIGNALS
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Basic Encoding Techniques
• Digital data to analog signal – Amplitude-shift keying (ASK)
• Amplitudedifferenceofcarrier frequency
– Frequency-shift keying (FSK)
• Frequencydifferencenearcarrier
frequency
– Phase-shift keying (PSK)
• Phaseofcarriersignalshifted
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Amplitude-Shift Keying (1)
• One binary digit represented by presence of carrier, at constant amplitude
• Other binary digit represented by absence of carrier
• where the carrier signal is Acos(2πfct)
• Susceptible to sudden gain changes
s ( t ) =
(c) 0
binary1 binary 0
• Inefficientmodulationtechnique
• Usedtotransmitdigitaldataoveropticalfiber
Acos2 ft
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p
Digitizing Analog Data
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Binary Frequency-Shift Keying (BFSK)
• Two binary digits represented by two different frequencies near the carrier frequency
binary1 binary0
• where f1 and f2 are offset from carrier frequency fc by equal but opposite amounts fd
• LesssusceptibletoerrorthanASK
• Used for high-frequency (3 to 30 MHz) radio transmission
• CanbeusedathigherfrequenciesonLANsthatusecoaxialcable
Acos2 ft
(1) Acos2 ft
s ( t ) =
(2)
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Full-Duplex FSK Transmission on a Voice Grade Channel
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Multiple Frequency-Shift Keying (MFSK) (1)
• More than two frequencies are used
• More bandwidth efficient but more susceptible to error
s t =Acos2 ft i() i
• fi =fc +(2i–1–M)fd
• fc=thecarrierfrequency
• f d = the difference frequency
• M = number of different signal elements = 2L
• L=numberofbitspersignalelement
1£i£M
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Multiple Frequency-Shift Keying (MFSK) (2)
• 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,onesignalelementencodesLbits
• Total bandwidth required
2Mfd
• Minimumfrequencyseparationrequired
• 2fd=1/Ts
• Therefore, modulator requires a bandwidth of Wd=2L/LT=M/Ts
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Phase-Shift Keying (PSK) (1)
• Two-levelPSK(BPSK)
– Uses two phases to represent binary digits
– Used in IEEE 802.11a
=
()
Acos2 ft ()
binary1 binary 0
s ( t ) = c
A c o s 2 f t +
(c)
Acos2 ft
binary1
-Acos2 ft (c)
binary 0
c
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p p
p
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Phase-Shift Keying (PSK) (2)
• DifferentialPSK(DPSK)
– Phase shift with reference to previous bit
• Binary0–signalburstofsamephaseasprevioussignalburst
• Binary 1 – signal burst of opposite phase to previous signal burst
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Quadrature Phase-Shift Keying (PSK)
• Four-levelPSK(QPSK)
– Each element represents more than one bit
– Used in IEEE 802.11a
ì çè c 4÷ø
Acosæ2 ft+ ö ïAcosæ2 ft+3 ö
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01
00 10
s(t)= çè c 4÷ø íAcosæ2 ft-3 ö
ïçèc 4÷ø
îAcosæ2 ft- ö çè c 4÷ø
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p
p
p
p
p
p
p
p
QPSK Constellation Diagram
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Multilevel PSK
• Using multiple phase angles with each angle having more than one amplitude, multiple signal elements can be achieved
D=R= R
L log2M
– D = modulation rate, baud or symbols/sec
– R = data rate, bps
– M = number of different signal elements = 2L – L = number of bits per signal element
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Performance (1)
• Bandwidth of modulated signal (BT) – ASK, PSK BT = (1+r)R
– FSK BT = 2Δf+(1+r)R
• R=bitrate
• 0 < r < 1; related to how signal is filtered • Δf=f2–fc=fc-f1
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Performance (2)
• Bandwidth of modulated signal (BT)
– M P S K – MFSK
• L = number of bits encoded per signal element
• M = number of different signal elements
1+r 1+r B T = L R = l o g M R
2 BT = log M R
(1 + r )M
2
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Quadrature Amplitude Modulation
• QAM is a combination of ASK and PSK
– Two different signals sent simultaneously on the same carrier frequency
st =Itcos2 ft+Qtsin2 ft ()()c()c
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p
p
16QAM Constellation Diagram
Used in IEEE 802.11a
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ANALOG DATA, ANALOG SIGNALS
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Reasons for Analog Modulation
• Modulation of digital signals
– When only analog transmission facilities are available, digital to analog
conversion required
• Modulation of analog signals
– A higher frequency may be needed for effective transmission – Modulation permits frequency division multiplexing
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Basic Encoding Techniques
• Analogdatatoanalogsignal
– Amplitude modulation (AM)
– Angle modulation
• Frequency modulation (FM) • Phasemodulation(PM)
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Amplitude Modulation (1)
• Amplitude Modulation
st =é1+nxtùcos2 ft
() ()
• cos2πfct=carrier
• x(t)=inputsignal
• na = modulation index
– Ratio of amplitude of input signal to carrier
– a.k.a double sideband transmitted carrier (DSBTC)
ëaûc
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p
Amplitude Modulation (2)
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Amplitude Modulation (3)
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Single Sideband (SSB)
• VariantofAMissinglesideband(SSB) – Sends only one sideband
– Eliminates other sideband and carrier
• Advantages
– Only half the bandwidth is required – Less power is required
• Disadvantages
– Suppressed carrier cannot be used for synchronization purposes
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• Angle modulation
Angle Modulation (1)
st=Acosé2ft+ tù () ()
cëcû – Phase is proportional to modulating signal
• Phasemodulation
(t)=npm(t) • np = phase modulation index
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j p
Angle Modulation (2)
• Frequencymodulation
– Derivative of the phase is proportional to the modulating signal
't=nmt ()f()
• nf = frequency modulation index
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j
Amplitude, Phase, and Frequency Modulation of a Sine-Wave Carrier by a Sine-Wave Signal
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Angle Modulation (3)
• ComparedtoAM,FMandPMresultinasignalwhosebandwidth:
– is also centered at fc
– but has a magnitude that is much different
• Angle modulation includes cos( (t)) which produces a wide range of frequencies
• Thus, FM and PM require greater bandwidth than AM
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j
ANALOG DATA, DIGITAL SIGNALS
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Basic Encoding Techniques
• Analogdatatodigitalsignal – Pulse code modulation (PCM) – Delta modulation (DM)
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Pulse Code Modulation
• Basedonthesamplingtheorem
• Each analog sample is assigned a binary code
– Analog samples are referred to as pulse amplitude modulation (PAM) samples
• The digital signal consists of block of n bits, where each n-bit number is
the amplitude of a PCM pulse
• By quantizing the PAM pulse, original signal is only approximated
• Leads to quantizing noise
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Pulse Code Modulation Example
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Analog Data to Digital Signal
• Once analog data have been converted to digital signals, the digital data: – can be transmitted using NRZ-L
– can be encoded as a digital signal using a code other than NRZ-L
– can be converted to an analog signal, using previously discussed techniques
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Delta Modulation (1)
• Analog input is approximated by staircase function
– Moves up or down by one quantization level (d in the figure on next slide) at
each sampling interval
• The bit stream approximates derivative of analog signal (rather than amplitude)
– 1 is generated if function goes up – 0 otherwise
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Example of Delta Modulation
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Delta Modulation (2)
• Twoimportantparameters
– Size of step assigned to each binary digit – Sampling rate
• Accuracy improved by increasing sampling rate – However, this increases the data rate
• AdvantageofDMoverPCMisthesimplicityofitsimplementation
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Reasons for Growth of Digital Techniques
• Growthinpopularityofdigitaltechniquesforsendinganalogdata – Repeaters are used instead of amplifiers
• Noadditivenoise
– TDM is used instead of FDM
• Nointermodulationnoise
– Conversion to digital signaling allows use of more efficient digital switching techniques
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Model-based encoding and Vocoders
• PCM and DM are types of waveform encoders – Seek to reproduce the sampled waveform
• The nature of the signal can also be encoded – For example, voice has particular characteristics – Pitch
– Voiced sounds – m, n, b, etc.
– Unvoiced sounds – c, k, t, etc.
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Model-based encoding and Vocoders
• Linear Prediction Coding (LPC)
– Estimate how the voice is producing the sound
• Code-excitedLinearPrediction(CELP)
– Create a codebook of typical LPC results
– Transmit a digital signal of codebook indices
• If the codebook is small, the data rate of the signal can be quite small
• But larger codebooks produce better quality results since they match the voice more precisely
• LPC and CELP coders can produce rates of 1.2 kbps to 13 kbps
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VIDEO CODING
• Videocompression
– Pictures may not change much frame-to-frame
• So, just mainly encode the differences instead of the entire images – There may be redundancy inside each image as well
• These redundant elements can be encoded as repetitions
• Video can be compressed down to 1-3 Mbps, even down to 64 kbps
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Sources for this lecture
Cory Beard, William Stallings. Wireless Communication Networks and Systems, 1st edition. Pearson Higher Education, 2016
(Chapter 7)
All material copyright 2016
Cory Beard and William Stallings, All rights reserved
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