# 05_Signal_Encoding_Techniques

Document Sample

```					William Stallings
Data and Computer
Communications

Chapter 5
Signal Encoding Techniques

١
Encoding Techniques
• Encoding is the conversion of streams of bits
into a signal (digital or analog)
• Digital data, digital signal: Digital transmission
—the simplest form is to assign two voltage level
• Digital data, analog signal: Analog transmission
• Analog data, digital signal: Digital transmission
—PCM, Delta Modulation
• Analog data, analog signal: Analog transmission
—AM, FM, Angle Modulation

٢
5.1 Digital Data, Digital Signal
• Digital signal
—Discrete, discontinuous voltage pulses
—Each pulse is a signal element
—Binary data are transmitted by encoding each data
bit into signal elements

٣
Terms (1)
• Unipolar
—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

٤
Terms (2)
• Modulation rate (baud)
—Rate at which the signal level changes
—Measured in baud = signal elements per second
• Mark and Space
—Binary 1 and Binary 0 respectively

٥
Signal Encoding Criteria
Term                        Units                        Definition

Data Element         Bits                            A single binary one or zero

Data Rate            Bits per second (bps)           The rate at which data
elements are transmitted

Signal Element       Digital: a voltage pulse of     That part of a signal that
constant amplitude.          occupies the shortest interval
Analogue: a pulse of constant   of a signalling code.
frequency, phase and
amplitude

Signalling Rate or   Signal elements per second      The rate at which signal
Modulation Rate         (bauds)                      elements are transmitted.

٦
• Need to know
—Timing of bits - when they start and end
—Signal levels
• Factors affecting successful interpreting of
—Signal to noise ratio
—Data rate (increase of DR increases bit error rate)
—Bandwidth (increase of BW allows an increase in DR)

٧
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
—Designing codes to concentrate power in the middle
of the bandwidth -> smaller distortion in the received
signal
• Clocking
—For determining the beginning and the end for each
bit position, Synchronizing transmitter and receiver
• External clock
• Sync mechanism based on signal

٨
Comparison of Encoding
Schemes (2)
• Error detection
—Can be built in to signal encoding
• Signal interference and noise immunity
—Some codes are better than others
• Cost and complexity
—Higher signal rate (& thus data rate) lead to higher
costs
—Some codes require signal rate greater than data rate

٩
Encoding Schemes
• 3 Broad Categories: Unipolar, Polar, and Bipolar
—Pseudoternary
—Manchester
LAN
—Differential Manchester
—Bipolar –AMI (Alternate Mark Inversion)
—B8ZS                                     WAN
—HDB3

١٠
• Two different voltages for 0 and 1 bits: Polar
Encoding
• Voltage constant during bit interval
—Such as 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
• 600/1200/2400,… bits/sec.

١١
NRZ

١٢
•   Polar
•   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
• example of differential encoding since have
— data represented by changes rather than levels (Better encoding
technique)
— more reliable detection of transition rather than level in noisy
channels
— In complex transmission layouts it is easy to lose sense of
polarity

١٣
NRZ pros and cons
• Pros
—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

١٤
Multilevel Binary: Bipolar-AMI
• Use more than two levels
• Bipolar-AMI (Alternate Mark Inversion)
—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 ones (zeros still a
problem)
—No net dc component
—Lower bandwidth
—Easy error detection

١٥
Multilevel Binary: Pseudoternary
• One represented by absence of line signal
• Zero represented by alternating positive and
negative
— each is the basis of some applications

١٦
Bipolar-AMI and Pseudoternary
0   1   0   0   1   1   0   0   0   1        1

١٧
• Synchronization with long runs of 0’s or 1’s
—can insert additional bits, cf ISDN
—scramble data (later)
• Not as efficient as NRZ
—long string of ones or zeros still a problem
—Receiver must distinguish between three levels
(+A, -A, 0)
• But in a 3 level system, each signal element could represent
log23 = 1.58 bits of information
—Requires approximately 3dB more signal power for
same probability of bit error

١٨
Biphase Encoding
1. Manchester Encoding
— 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 (Ethernet LAN)
2. Differential Manchester Encoding
— 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 (Token Ring LAN)

١٩
Differential Manchester &
Manchester Encoding

٢٠
Biphase Pros and Cons
• Con
—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)
—Has no dc component
—Has error detection
• Absence of expected transition can be used to detect errors

٢١
Modulation Rate (baud)
Is the rate in which the signal elements are generated.
in baud = signal elements per second
Ex. In NRZI:
Data rate
R= 1/bit duration=1 MHz
Modulation rate, D=R/b
=1 Mhz
In Manchester:
Data rate
R= 1/bit duration=1 MHz
Modulation rate, D=R/b
=1 Mhz/0.5=2MHz
b is the number of bits            b=0.5
per signal element                       ٢٢
Scrambling
• To be used in long-distance transmission (WAN)
• Use scrambling to replace sequences that would
produce constant voltage
• These filling sequence
— Must produce enough transitions to sync
— Must be recognized by receiver and replace with original
— Same length as original (so there is no data-rate increase)
• Design goals:
— Have no dc component
— Avoid long sequences of zero level line signal
— Have no reduction in data rate
— Give error detection capability
• B8ZS and HDB3

٢٣
B8ZS Encoding
• Bipolar With 8 Zeros Substitution
• Based on bipolar-AMI (with scrambling)
—If octet of all zeros and last voltage pulse preceding
was positive, encode as 000+-0-+
—If octet of all zeros and last voltage pulse preceding
was negative, encode as 000-+0+-
• Causes two violations (signal patterns not
allowed in AMI) of AMI code, an event unlikely
to occur as a result of noise
• Receiver detects and interprets as octet of all
zeros
٢٤
B8ZS
• In Bipolar-AMI: 1 pulses alternate in polarity
• Two violations in B8ZS

٢٥
HDB3 Encoding
•    High Density Bipolar 3 Zeros
•    Based on bipolar-AMI (with Scrambling)
•    String of four zeros replaced with one or two pulses
•    A rule is needed to ensure that the successive violation
are of alternate polarity

No. of Bipolar pulses (ones) since last substitution

Polarity of           Odd                      Even
preceding Pulse
-                    000-                     +00+
+                    000+                     -00-

٢٦
B8ZS and HDB3

if odd   if even

Violation within the substituted code

٢٧
Recap of Digital Signal Encoding
Formats
0                                    1

NRZL                          High level                           Low level

NRZI                          No transition at start of interval   transition

Bipolar-AMI                   No line signal                       +ve line signal

Manchester                    Transition from high to low in       Transition from low to high in
the middle of interval               the middle of interval

Diff Manchester               Tran at start of interval            No transition at start of interval
(always a Transition in the
middle of interval)

HDB3                          Same as bipolar-AMI, except that any string of four zeros is
replaced by a string with one code violation

B8ZS                          Same as bipolar-AMI, except that any string of eight zeros are
replaced by a string of two code violations

٢٨
5.2 Digital Data, Analog Signal
• Transmitting digital data using analog signal
• Public telephone system
—300Hz to 3400Hz
—Use modem (modulator-demodulator)
• Encoding Techniques:
—Frequency shift keying (FSK)
—Phase shift keying (PSK)

The resulting signal occupies a BW centered on the carrier
frequency
٢٩
Modulation Techniques

Modulation of analog
signals for digital data

٣٠
Amplitude Shift Keying
• Values represented by different amplitudes of carrier
• Usually, one amplitude is zero
— i.e. presence and absence of carrier is used
• Susceptible to sudden gain changes
• Inefficient
• Used for:
— Up to 1200bps on voice-grade lines
— Very high speeds over optical fiber (light on, off)

⎧ A cos(2π f c t ) binary 1
s (t ) = ⎨
⎩0                 binary 0

٣١
Binary Frequency Shift Keying
• Most common form is binary FSK (BFSK)
• Two binary values represented by two different
frequencies (near carrier)
• Less susceptible to error than ASK
• Used for:
—Up to 1200bps on voice-grade lines
—High frequency radio transmission (3-30 MHz)
—Even higher frequency on LANs using coaxial cables
⎧ A cos(2π f1t ) binary 1
s (t ) = ⎨
⎩ A cos(2π f 2t ) binary 0
f1 = f c + Δf
f 2 = f c − Δf                                 ٣٢
Multiple FSK
• More than 2 frequencies are used to represent
multiple levels of the signal
• Each signalling element represents more than
one bit (for ex. 2: 00, 01, 10, 11)
• Signal more bandwidth efficient, but more
susceptible to error
MFSK for 1 element:   MFSK            s (t ) = A ⋅ cos(2 ⋅ π ⋅ f i ⋅ t )   1≤ i ≤ M
f i = f c + (2 ⋅ i − 1 − M ) f d
f c = carrier frequency
f d = difference frequency
M = number of different signal elements = 2 L
L = number of bits per signal element
٣٣

٣٤
Phase Shift Keying
• Phase of carrier signal is shifted to represent data
• Binary PSK
— Two phases represent two binary digits
⎧ A cos(2π f c t + π ) binary 1
s (t ) = ⎨
⎩ A cos(2π f c t ) binary 0
• Differential PSK
— Phase shifted relative to previous transmission rather than some
reference signal
— A binary 0 is represented by sending a signal burst of the same
phase as the previous signal burst send.
— A binary 1 is represented by sending a signal burst of opposite
phase to the preceding one.
— DPSK avoids the requirement for an accurate local oscillator
phase at the receiver that is matched with the transmitter

٣٥
Differential PSK

٣٦
• More efficient use by each signal element representing
more than one bit
— e.g. shifts of π/2 (90o)
— Each element represents two bits (00, 01, 10, 11)
— split input data stream in two & modulate onto carrier & phase
shifted carrier
⎧ A ⋅ cos(2 ⋅ π ⋅ f c ⋅ t + π 4)    11
⎪ A ⋅ cos(2 ⋅ π ⋅ f ⋅ t + 3π 4)
⎪                                   01
s(t ) = ⎨                   c

⎪ A ⋅ cos(2 ⋅ π ⋅ f c ⋅ t − 3π 4)   00
⎪ A ⋅ cos(2 ⋅ π ⋅ f c ⋅ t − π 4)
⎩                                   10
• Can use 8 phase angles & have more than one
amplitude
— A standard 9600bps modem uses 12 angles, four of which have
two amplitudes
٣٧
Performance of Digital to
Analog Modulation Schemes
• Bandwidth
—ASK and PSK bandwidth directly related to bit rate
—multilevel PSK gives significant improvements in BW
• In the presence of noise,
—bit error rate of PSK and QPSK are about 3dB
—for MFSK & MPSK have tradeoff between bandwidth
efficiency and error performance
• an increase in bandwidth efficiency results in an increase in
error probability

٣٨
Modulation
• QAM used on asymmetric digital subscriber line
• 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°
—Two independent signals over same medium
—Demodulate and combine for original binary output

٣٩
QAM Modulator

٤٠
5.3 Analog Data, Digital Signal
• Digitization is conversion of analog data into
digital data which then:
—be transmitted using NRZ-L
—be transmitted using code other than NRZ-L
—be converted to analog signal
• Analog to digital conversion done using a codec
—Pulse code modulation
—Delta modulation

٤١
Digitizing Analog Data

٤٢
Pulse Code Modulation(PCM) (1)
• Sampling 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
—Eg. 4000Hz voice data, requires 8000 sample per
second
• These are analog samples (called: Pulse
Amplitude Modulation, PAM)
• Convert to digital: Each sample assigned digital
value
٤٣
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

٤٤
PCM Example

sec.
PAM samples are taken at a rate of 2B, or once every Ts = 1/2B ٤٥
PCM Block Diagram

٤٦
Nonlinear Encoding Technique
• To refine The PCM schema: In Nonlinear Encoding the
quantization levels not equally spaced
— The problem with equal spacing is that the mean absolute error
for each sample is the same
• Lower amplitude values are relatively more distorted
• Nonlinear Encoding reduces overall signal distortion
— Nonlinear encoding can significantly improve the PCM SNR ratio
• The same effect can be achieved by using uniform
quantizing but companding (compressing-expanding)
the input analog signal

٤٧
Effect of Non-Linear Coding

٤٨
Typical Companding Functions

Companding is a
process that compresses
the intensity range of a
signal by imparting more
gain to weak signals
than to strong signals on
input.
At output, the reverse
operation is performed.

٤٩
Delta Modulation
• To improve the performance of PCM or to
reduce its complexity
function
—Can move up or down one level (δ) at each sample
interval
• Has binary behavior
—Since function only moves up or down at each
sample interval
—hence can encode each sample as single bit
—1 for up or 0 for down

٥٠
Delta Modulation - example
δ must be chosen to produce a balance between two
types of errors or noise.                    When the analog
waveform is changing
very slowly, there will
be quantizing noise.
This noise increases
as δ is increased.
When the analog
waveform is changing
more rapidly than the
staircase can follow,
there is slope
noise increases as δ is
decreased         ٥١
Delta Modulation - Operation
For transmission: At each sampling
to the most recent value of the
approximating staircase function. If
the value of the sampled waveform
exceeds that of the staircase
function, a 1 is generated; otherwise,
a 0 is generated.

٥٢
Delta Modulation - Performance
• DM has simplicity compared to PCM
• But has worse SNR
• Issue of bandwidth used
—Eg. Good voice reproduction via PCM
• want 128 levels (7 bit) & Voice bandwidth 4khz
• need 8000 x 7 = 56kbps (Data rate)
• Data compression can improve on this
—e.g. Interframe coding techniques for video

٥٣
5.4 Analog Data, Analog Signals
• Modulate carrier frequency with analog data
• Why modulate analog signals?
—Higher frequency can give more efficient transmission
• since for unguided transmission, it is virtually impossible to
transmit baseband signals
—Permits frequency division multiplexing (chapter 8)
• Types of modulation
—Amplitude
—Frequency
—Phase

٥٤
Analog
Modulation

• Amplitude
Modulation
• Frequency
Modulation
• Phase Modulation

٥٥

```
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