1. Signal Encoding Techniques
Chapter 5
Signal Encoding Techniques
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2. Encoding and Modulation
Two techniques for data transmission
Encoding
A data source is encoded into a digital signal
Encoding technique is chosen to optimize transmission medium
Modulation
The process of encoding source data onto a carrier signal with frequency fc
All modulation techniques involve operation on three signal parameters
Amplitude
Frequency
Phase
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3. Encoding Techniques Digital data, digital signal
Less complex
Less expensive than digital to analog modulation
Analog data, digital signal
Sometimes, it is more advantageous to shift to digital domain
Digital data, analog signal
Some transmission media (unguided media, optical fiber) only propagate analog signals
Analog data, analog signal
Analog data transmitted as analog signal easily and cheaply: voice transmission over voice grade lines
Shift the bw of baseband signal to another portion of the spectrum
Multiple signals can share the transmission medium
Frequency Division Multiplexing
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4. Digital Data, Digital Signal
Discrete, discontinuous voltage pulses
Each pulse is a signal element
Binary data encoded into signal elements
In the simplest case, one-to-one correspondence between data and signal elements
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5. Terms (1) Unipolar Polar Data rate Duration or length of a bit
All signal elements have same sign
Polar
One logic state represented by positive voltage the other by negative voltage
Data rate
Rate of data transmission in bits per second
Duration or length of a bit
Time taken for transmitter to emit the bit
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6. Terms (2) Modulation rate Mark and Space
Rate at which the signal level changes
Measured in baud = signal elements per second
Mark and Space
Binary 1 and Binary 0 respectively
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7. Interpreting Signals Need to know
Timing of bits - when they start and end
Signal levels
Factors affecting successful interpreting of signals
Signal to noise ratio
Data rate
Bandwidth
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8. Facts 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
An improvement in perfomance can also be obtained by encoding scheme
Encoding scheme is simply mapping from data bits to signal elements
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9. Comparison of Encoding Schemes (1)
Signal Spectrum
Lack of high frequencies reduces required bandwidth
Lack of dc component allows ac coupling via transformer, providing isolation, reducing interference
Concentrate power in the middle of the bandwidth
Clocking
Synchronizing transmitter and receiver
External clock: an expensive solution
Sync mechanism based on signal : can be achieved with suitable encoding
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10. Comparison of Encoding Schemes (2)
Error detection
Can be built into signal encoding
This permits errors to be detected more quickly
Signal interference and noise immunity
Some codes are better than others
Performance is expressed in BER
Cost and complexity
Higher signal rate (& thus data rate) lead to higher costs
Some codes require signal rate greater than data rate
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11. Encoding Schemes Nonreturn to Zero-Level (NRZ-L)
Nonreturn to Zero Inverted (NRZI)
Bipolar -AMI
Pseudoternary
Manchester
Differential Manchester
B8ZS
HDB3
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12. Nonreturn to Zero-Level (NRZ-L)
Two different voltages for 0 and 1 bits
Voltage constant during bit interval
no transition I.e. no return to zero voltage
e.g. Absence of voltage for zero, constant positive voltage for one
More often, negative voltage for one value and positive for the other
This is NRZ-L
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13. Nonreturn to Zero Inverted
NRZI: Nonreturn to zero invert on ones
Constant voltage pulse for duration of bit
Data encoded as presence or absence of signal transition at beginning of bit time
Transition (low to high or high to low) denotes a binary 1
No transition denotes binary 0
An example of differential encoding
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14. NRZ
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15. Differential Encoding
Data represented by changes rather than levels
More reliable detection of transition rather than level
In complex transmission layouts it is easy to lose sense of polarity
E.g. if the leads from an attached device to the twisted pair accidentally inverted, all NRZ-L bits will be inverted
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16. NRZ pros and cons Pros Cons Used for magnetic recording
Easy to engineer
Make good use of bandwidth
Cons
dc component
Lack of synchronization capability
Used for magnetic recording
Not often used for signal transmission
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17. Multilevel Binary Use more than two levels Bipolar-AMI
zero represented by no line signal
one represented by positive or negative pulse
one pulses alternate in polarity
No loss of sync if a long string of 1s (zeros still a problem)
No net dc component
Lower bandwidth
Easy error detection
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18. Pseudoternary One represented by absence of line signal
Zero represented by alternating positive and negative
No advantage or disadvantage over bipolar-AMI
AMI: string of 0s, pseudoternary: string of 1s
insert additional bits to force transitions
Expensive at high data rates
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19. Bipolar-AMI and Pseudoternary
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20. Trade Off for Multilevel Binary
With suitable modification, multilevel binary schemes overcome the problems of NRZ
Not as efficient as NRZ
Each signal element only represents one bit
In a 3 level system could represent log23 = 1.58 bits
Receiver must distinguish between three levels (+A, -A, 0)
Requires approx. 3dB more signal power than a two-valued signal for same probability of bit error
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21. Bit Error Rate for Various Encoding Schemes
Fig 5.4
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22. Biphase Manchester Differential Manchester
Transition in middle of each bit period
Transition serves as clock and data
Low to high represents one
High to low represents zero
Used by IEEE 802.3
Differential Manchester
Midbit transition is clocking only
Transition at start of a bit period represents zero
No transition at start of a bit period represents one
Note: this is a differential encoding scheme
Used by IEEE 802.5
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23. Manchester Encoding
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24. Differential Manchester Encoding
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25. Biphase Pros and Cons Con Pros
At least one transition per bit time and possibly two
Maximum modulation rate is twice NRZ
Requires more bandwidth
Pros
Synchronization on mid bit transition (self clocking)
No dc component
Error detection
Absence of expected transition
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26. Modulation Rate
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27. Remark
Data rate and modulation rate can differ. Recall that D=R/b=where
D = modulation rate (baud)
R = data rate (bps)
b = #bits per signal element
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28. Scrambling
Use scrambling to replace sequences that would produce constant voltage
Filling sequence
Must produce enough transitions to sync
Must be recognized by receiver and replace with original
Same length as original
No dc component
No long sequences of zero level line signal
No reduction in data rate
Error detection capability
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29. Remark
Note that biphase techniques are mostly used in LANs. Since high signaling rate is required relative to data rate, these techniques are not suitable for long distance transmission. Scrambling techniques improve this rate by replacing long sequences of zeros by a fixed pattern.
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30. B8ZS Bipolar With 8 Zeros Substitution Based on bipolar-AMI
If octet of all zeros and last voltage pulse preceding was positive encode as
If octet of all zeros and last voltage pulse preceding was negative encode as
Unlikely to occur as a result of noise
Receiver detects and interprets as octet of all zeros
Commonly used in North America
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31. Number of Bipolar Pulses (ones) since Last Substitution
HDB3
High Density Bipolar 3 Zeros
Based on bipolar-AMI
String of four zeros replaced with sequences containing one or two pulses
Number of Bipolar Pulses (ones) since Last Substitution
Polarity of Preceeding Pulse
Odd
Even -
000-
+00+ +
000+
-00-
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32. B8ZS and HDB3
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33. Digital Data, Analog Signal
Public telephone system
300Hz to 3400Hz – voice frequency range
Use modem (modulator-demodulator)
Amplitude shift keying (ASK)
Frequency shift keying (FSK)
Phase shift keying (PK)
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34. Modulation Techniques
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35. Amplitude Shift Keying (1)
Two binary values represented by different amplitudes of carrier frequency
Usually, one amplitude is zero
i.e. presence and absence of carrier is used
Susceptible to sudden gain changes
Inefficient
Up to 1200bps on voice grade lines
Used over optical fiber
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36. Amplitude Shift Keying (2)
 Acos (2fct) if 1
s(t) =
 if 0
For LED this operation is valid.
For laser transmitters normally have a fixed current that causes the device emit low level light.
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37. Binary Frequency Shift Keying (1)
Most common form is binary FSK (BFSK)
Two binary values represented by two different frequencies (near carrier)
Less susceptible to error than ASK
Up to 1200bps on voice grade lines
Commonly used for high frequency radio (3 to 30 MHz)
Used at even higher frequency on LANs using co-ax
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38. Binary Frequency Shift Keying (2)
 Acos (2f1t) if 1
s(t) =
 Acos (2f2t) if 0
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39. Multiple FSK More than two frequencies used More bandwidth efficient
More prone to error
Each signalling element represents more than one bit
Each signal element is held for a period of LT seconds, L: number of bits per signal element, T: bit period
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40. FSK on Voice Grade Line
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41. Phase Shift Keying
Phase of carrier signal is shifted to represent data
Differential PSK
Phase shifted relative to previous transmission rather than some reference signal
 Acos (2fct + ) if 1
s(t) =
 Acos (2fct) if 0
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42. Differential PSK
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43. Quadrature PSK (1)
More efficient use of bw by each signal element representing more than one bit
e.g. shifts of /2 (90o)
Each element represents two bits
Can use 8 phase angles and have more than one amplitude
9600bps modem use 12 angles , four of which have two amplitudes
Offset QPSK (orthogonal QPSK)
Delay in Q stream
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44. Quadrature PSK (2) Example:shifts of /2 (90o)
 Acos (2fct + /4) if 11
 Acos (2fct)+ 3/4) if 10
s(t) =  Acos (2fct + 5/4) if 00
 Acos (2fct + 7/4) if 01
In general, D=R/b = R/log2L where
D is the modulation rate, R is the data rate, L is the number of different signal elements and b is the number of bits per signal element
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45. QPSK and OQPSK Modulators
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46. Examples of QPSF and OQPSK Waveforms
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47. Bandwidth For ASK and PSK the bandwidth is given as
BT = (1 + r) R, where
R is the bit rate and r is a constant between 0 and 1.
For multilevel FSK, the bandwidth is given as
BT = ((1 + r)M/ log2 M)R., where
For multilevel PSK, bandwidth can be given as
BT = ((1 + r)/ log2 M)R.
L: number of bits encoded per signal element
M: number of different signal elements
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48. Performance of Digital to Analog Modulation Schemes
Bandwidth
ASK and PSK bandwidth directly related to bit rate
In the presence of noise, bit error rate of DPSK and BPSK are about 3dB superior to ASK and BFSK
R/BT: bandwidth efficiency
For MFSK, error probability decreases as M increases, while opposite is true for MPSK
Bandwidth efficiency of MFSK decreases as M increases, while opposite is true of MPSK
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49. Example
What is the bandwidth efficiency for FSK, ASK, PSK and QPSK for a bit error rate of on a channel with an SNR of 12 dB?
Solution:
Recall that Eb/N0 = S/(N0R).
Since the noise in a signal with bandwidth BT is N = N0 BT , we have
Eb/N0 = (S/N)(BT/R). Or in dB,
(Eb/N0)dB = (S/N)dB + (BT/R)dB or (Eb/N0)dB = (S/N)dB - (R/BT)dB
 From Figure 5.4, for FSK and ASK, Eb/N0 = 14.2 dB
 And substitution to above formula yields
 14.2 dB = 12 dB – (R/BT)dB yielding
 R/BT = 0.6.
 For PSK, from Fig. 5.4, Eb/N0 = 11.2 dB. Repeating the above computation yields R/BT = 1.2.
 In QPSK, since baud rate is D = R/2, we have R/BT = 2.4.
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50. Quadrature Amplitude Modulation
QAM used on asymmetric digital subscriber line (ADSL) and some wireless
Combination of ASK and PSK
Logical extension of QPSK
Send two different signals simultaneously on same carrier frequency
Use two copies of carrier, one shifted 90°
Each carrier is ASK modulated
Two independent signals over same medium
Demodulate and combine for original binary output
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51. QAM Modulator
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52. QAM Levels Two level ASK
Each of two streams in one of two states
Four state system
Essentially QPSK
Four level ASK (four different amplitude levels)
Combined stream in one of 16 states
64 and 256 state systems have been implemented
The greater the number of states, the higher the data rate
Increased potential error rate
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53. Analog Data, Digital Signal
Digitization
Conversion of analog data into digital signals
After analog data is converted into digital data:
Digital data can then be transmitted using NRZ-L
Digital data can then be transmitted using code other than NRZ-L (thus an extra step is required)
Digital data can then be converted to analog signal
Analog to digital conversion done using a codec
Pulse code modulation
Delta modulation
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54. Digitizing Analog Data
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55. Pulse Code Modulation(PCM) (1)
Sample Theorem: If a signal is sampled at regular intervals at a rate higher than twice the highest signal frequency, the samples contain all the information of the original signal
Voice data limited to below 4000Hz
Require 8000 sample per second
Analog samples (Pulse Amplitude Modulation, PAM)
Each sample assigned digital value
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56. Pulse Code Modulation(PCM) (2)
4 bit system gives 16 levels
Quantized
Quantizing error or noise
Approximations mean it is impossible to recover original exactly
8 bit sample gives 256 levels
Quality comparable with analog transmission
8000 samples per second of 8 bits each gives 64kbps
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57. PCM Example
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58. PCM Block Diagram
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59. Nonlinear Encoding Quantization levels not evenly spaced
Reduces overall signal distortion
Can also be done by companding
SNR for quantizing noise
SNR = 6.02 n dB, where n is the number of bits representing each voltage level
For voice signals, with nonlinear encoding, an improvement of 24 dB to 30 dB have been achieved.
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60. Effect of Non-Linear Coding
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61. Typical Companding Functions
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62. Delta Modulation One of the most popular alternatives to PCM
Analog input is approximated by a staircase function
Move up or down one level () at each sample interval
Binary behavior
Function moves up or down at each sample interval
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63. Delta Modulation - example
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64. Delta Modulation - Operation
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65. Performance Good voice reproduction
PCM levels (7 bit)
Voice bandwidth 4khz
Should be 8000 x 7 = 56kbps for PCM
Digital techniques continue to grow in popularity
Repeaters instead of amplifiers
TDM instead of FDM (intermodulation noise)
Efficient digital switching techniques
Data compression can improve codecs
e.g. Interframe coding techniques for video
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66. Performance
Use of a telecommunication system will result in both digital-to analog and analog-to-digital processing
Local terminals into comm. network is analog and network uses a mixture of analog and digital techniques
Analog signals represents voice and digital data
Voice signals tend to be skewed lower portion of the bandwidth
Because of the presence of higher frequencies, PCM-related techniques is preferable to DM
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67. Analog Data, Analog Signals
Why modulate analog signals?
Higher frequency may be needed for efficient transmission
Permits frequency division multiplexing (chapter 8)
Types of modulation
Amplitude
Frequency
Phase
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68. Amplitude Modulation s(t) = [1 + na x(t)] cos 2fct where,
  2fct is the carrier,
x(t) is the input signal,
na is the modulating index
This is Double Side Band Transmitted Carrier , DSBTC
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69. AM
Fig 5.15
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70. Amplitude Modulation, Example
Derive an expression for s(t) if x(t) = cos(2fmt)
Solution: We have
s(t) = [1 + na cos(2fmt)] cos 2fct.
Since cos(a)cos(b) = (1/2)cos(a+b) + (1/2)cos(a-b)
s(t) = cos 2fct + (na/2) cos 2(fc – fm) t + (na/2) cos 2(fc + fm) t.
If Pt is the total power transmitted in s(t) and Pc is the power transmitted in the carrier, we have
Pt = Pc (1 + na2 / 2)
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71. Spectrum of AM
Fig 5-16
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72. Single Side Band (SSB)
Each sideband of AM contains complete spectrum of the modulating signal
In fact, only one sideband is enough to retrieve the complete signal
SSB contains only one sideband
Bandwidth is halved
Less power is required
Since carrier is lost, synchronization at receiver might be difficult
Vestigial sideband (VSB): uses one sideband and reduced power carrier
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73. Angle Modulation s(t) = Ac cos [2fct + (t)]
PM and FM are special cases of angle modulation. For PM
(t) = np m(t)
and for FM
(t) = nf m(t) (frequency is the rate of change of phase).
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74. Analog Modulation
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75. Bandwidth
For AM the bandwidth is given as BT = 2B (where B is the bandwidth of the modulating signal),
For FM, the bandwidth is given as BT = 2B + 2F, where F is the peak frequency deviation.
Both FM and PM require higher bandwidth than AM.
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76. Power and Bandwidth In Amplitude Modulation In Frequency Modulation
Increase in m(t) amplitude increases power but does not change bandwidth
In Frequency Modulation
Increase in m(t) amplitude increases F and hence the bandwidth but does not change the power
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77. Required Reading
Stallings chapter 5
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