Chapter 5 Revised p 52 Onwards

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EAB 4023Multimedia Network

Lecture 5

Signal Encoding Techniques

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Understand Coding Terminology & Design Issues

Acquire the knowledge of the Signal Encoding techniques. The encoding will include the following techniques.

DIGITAL DATA, DIGITAL SIGNALS : AMI, Manchester

DIGITAL DATA, ANALOG SIGNALS :ASK,FSK,PSK,QAM

ANALOG DATA, DIGITAL SIGNALS :PCM,Companding

ANALOG DATA, ANALOG SIGNALS :AM,FM

At the end of this lecture, you should be able to:At the end of this lecture, you should be able to:

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Digital Data, Digital Signals

Digital signalA sequence of discrete, discontinuous voltage

pulsesEach pulse is a signal elementBinary data are transmitted by encoding each

data bit into signal elements

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Coding Terminology Signal element

That part of a signal that occupies the shortest interval of a signaling code Digital (A voltage pulse of constant amplitude), Analog (A pulse of constant

frequency, phase, and amplitude)

Unipolar All signal elements have same algebraic sign All positive OR all negative voltage

Polar (Bipolar) Positive AND negative voltage (+ve & -ve)

Data rate / Bit rate (R) Rate of data transmission in bits per second (bps)

Duration or length of a bit (T = 1/R) Amount of time taken for transmitter to emit the bit

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Bit rate and bit interval

1 (s)

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Modulation rate (baud) Rate at which the signal level changes Expressed in baud (signal elements per second)

Mark and Space Binary 1 and Binary 0 respectively

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Modulation Rate

When signal-encoding techniques are used, a distinction need to be made between data rate (bps) and modulation rate (baud)

Data rate (bit rate) = 1/Tb, Tb = bit duration Modulation rate is the rate at which signal elements are generated.

Expressed in baud i.e. number of signal elements per second.

D = R/L = R / log2M ,whereD = modulation rate [baud]R = date rate [bps] = 1/Tb, Tb = bit durationM = # of different signal elements = 2L

L = # of bits per signal element

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Modulation Rate

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ExampleExample

An analog signal carries 4 bits in each signal unit. If 1000 signal units are sent per second, find the modulation rate(D) and the bit rate(R).

SolutionSolution

Modulation rate, D = 1000 baudModulation rate, D = 1000 baudBit rate, R = D L = 1000 x 4 = 4000 bpsBit rate, R = D L = 1000 x 4 = 4000 bpsNote: L= no. of bits per signal element = 4Note: L= no. of bits per signal element = 4

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Interpreting digital signals at the receiver Receiver need to

Know timing of each bit - when they start and endDetermine signal level for each bit position

Factors affecting successful interpreting of signalsData rate (R , BER )Signal to noise ratio (SNR , BER )Bandwidth (BW , R ) Encoding schemes-mapping from data bits to signal element

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Ways of evaluating/comparing various Encoding techniques

Signal Spectrum Lack of high frequencies components reduces required

bandwidth for transmission Lack of dc component reduce interference Good signal design concentrate power in the middle of the

transmission bandwidth Clocking

Need to determine beginning and end of each bit position Provide separate clock to synchronize transmitter and receiver Provide sync. mechanism based on signal by suitable encoding

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Error detectionCan be built into physical signaling encoding scheme

Signal interference and noise immunitySome codes perform better than others. Performance

expressed in bit error rate(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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Digital/digital EncodingDigital/digital Encoding

Unipolar Polar Bipolar

-Simple-Use only 1 levelof value (+ve or -ve)

-Use 2 voltage levels (+ve & -ve) for 2 binary digits-NRZ-RZ-BIPHASE

-Use 3 voltage levels(+ve,-ve,0)-AMI-B8ZS-HDB3

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Types of Polar EncodingPolar

NRZ RZ Biphase

NRZ-L NRZ-I Manchester DifferentialManchester

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Encoding Schemes

Nonreturn to Zero-Level (NRZ-L) Nonreturn to Zero Inverted (NRZI) Manchester Differential Manchester Pseudoternary Bipolar -AMI B8ZS HDB3

POLAR(NRZ)

POLAR(BIPHASE)

BIPOLAR

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Nonreturn to Zero(NRZ)

Two different voltages levels for two binary digits (0 and 1) Voltage level is constant during a bit interval

no transition i.e. no return to zero voltage level e.g. Absence of voltage for 0, constant positive voltage for 1

Nonreturn to Zero-Level (NRZ-L) More often, negative voltage for 1 and positive voltage for 0 NRZ-L Refer Slide #18 Typically use code NRZ-L to generate or interpret digital data by

terminals and other devices

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Nonreturn to Zero Inverted (NRZI)

Nonreturn to zero inverted on ones (The signal is inverted if a 1 is encountered)

Maintains a constant voltage pulse for the duration of a bit time Data encoded as presence or absence of signal transition at

beginning of bit time A transition (low to high or high to low) denotes a binary 1 No transition denotes binary 0 Refer Slide #18 An example of differential encoding

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NRZ

Transition because next bit is 1

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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

Ex: NRZ-I

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NRZ pros and cons

ProsEasy to engineerMake good use of bandwidth

Consdc componentLack of synchronization capability

Used for magnetic recording Not often used for signal transmission

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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 ones (zeros still a problem) No net dc component Lower bandwidth Easy error detection

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Pseudoternary

A variation of Bipolar AMI (binary 0 alternates between +ve and –ve voltages)

One represented by absence of line signal Zero represented by alternating positive

and negative No advantage or disadvantage over

bipolar-AMI

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Bipolar-AMI and Pseudoternary0 1 0 0 1 1 0 0 0 1 1

The 1s are positive and negative alternately

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Trade Off for Multilevel Binary

Not as efficient as NRZEach signal element only represents one bit In a 3 level system could represent log23 = 1.58

bitsReceiver must distinguish between three levels

(+A, -A, 0)Requires approx. 3dB more signal power for

same probability of bit error

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Biphase

Manchester Transition in middle of each bit period Transition serves as clock (synchronization) and data (bit

representation) Low to high represents one High to low represents zero Used by IEEE 802.3

Differential Manchester Midbit transition is clocking (synchronization) 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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Manchester Encoding

Zero is One is

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Differential Manchester Encoding

Presence of transition at the beginningof bit time means zero

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Biphase Pros and Cons

Pros Synchronization on mid bit transition (self clocking) No dc component Error detection

Absence of expected transition Cons

At least one transition per bit time and possibly two Maximum modulation rate is twice NRZ Requires more bandwidth

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Scrambling

Biphase-widespread in LAN but not long-distance appl. 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

Design Goals No dc component No long sequences of zero level line signal No reduction in data rate Error detection capability

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B8ZS

Bipolar With 8 Zeros Substitution (in USA) Based on bipolar-AMI 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 of AMI code Unlikely to occur as a result of noise Receiver detects and interprets as octet of all zeros

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HDB3

High Density Bipolar 3 Zeros (Europe & Japan) Based on bipolar-AMI String of four zeros replaced with one or two pulses If four 0s come after another, we change the pattern in

one of four ways based on the polarity of the previous 1 and the number of 1s since the last substitution

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HDB3 Substitution Rules

Number of Bipolar Pulses (ones) since Last Substitution

Polarity of Preceding Pulse

Odd Even

- 000- +00+

+ 000+ -00-

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B8ZS and HDB3

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DIGITAL DATA, ANALOG SIGNALS

AGENDAAGENDA

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Digital Data, Analog Signal

Public telephone system300Hz to 3400HzUse modem (modulator-demodulator)

Amplitude shift keying (ASK aka OOK=On-off Keying)

Frequency shift keying (FSK) Phase shift keying (PSK)

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Modulation Techniques

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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 Up to 1200bps on voice grade lines Used over optical fiber

s(t) = Acos ωct, binary 10, binary 0

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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 Up to 1200bps on voice grade lines High frequency radio Even higher frequency on LANs using co-ax

s(t) = Acos ω1t, binary 1 Acos ω2t, binary 0

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Multiple FSK

More than two frequencies used More bandwidth efficient More prone to error Each signalling element represents more than one bit

si(t) = Acos ωit, 1<= i <= M

M = # of different signal elements = 2L

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FSK on Voice Grade Line

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Phase Shift Keying (PSK)

Phase of carrier signal is shifted to represent data Binary PSK

Two phases represent two binary digits

• Differential PSK (DPSK) 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 burst sent A binary 1 is represented by sending a signal burst of opposite

phase to the preceding one

s(t) = Acos ωct, binary 1 -Acos ωct, binary 0

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Differential PSK

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Quadrature PSK (QPSK /4-PSK)

More efficient use by each signal element representing more than one bit e.g. phase shifts of multiples /2 (90o) Each element represents two bits (dibit) 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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QPSK and OQPSK Modulators

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Examples of QPSK and OQPSK Waveforms

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Performance of Digital to Analog Modulation Schemes ASK and PSK

ASK and PSK bandwidth directly related to bit rate BT = (1 + r)R; R=bit rate, r=filter variable, 0<r<1

FSK FSK bandwidth related to data rate for lower frequencies, but to

offset of modulated frequency from carrier at high frequencies BT = 2ΔF + (1 + r)R, ΔF = f2 – fc = fc – f1

In the presence of noise, bit error rate of PSK and QPSK are about 3dB superior to ASK and FSK

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R/BT for various D/A Encoding Schemes

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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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QAM Modulator

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QAM Levels

Two level ASK Each of two streams in one of two states Four state system Essentially QPSK

Four level ASK Combined stream in one of 16 states

64 and 256 state systems have been implemented Improved data rate for given bandwidth

Increased potential error rate

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ANALOG DATA, DIGITAL SIGNALS

AGENDAAGENDA

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Analog Data, Digital Signal

Transform analog data into digital signals To convert analog data into digital form for transmission

and subsequently recover the original analog data from digital – use codec (coder-decoder)

2 principal techniques used in codec:

• Pulse code modulation (PCM)

• Delta modulation (DM)

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Pulse Code Modulation(PCM)

Three essential processes of PCM are: • sampling : continuous-time signal is sampled by measuring its amplitude at discrete instants • quantizing : representing the sampled values of the amplitude by a finite set of levels • encoding : designating each quantized level by a code

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Pulse Code Modulation(PCM)

SAMPLING Based on the sampling theorem Sampling theorem: If a signal f(t) is sampled at regular intervals

at a rate higher than twice the highest signal frequency, then the samples contain all the information of the original signal

Example: If voice data are limited to frequencies below 4000Hz (i.e.highest signal freq. is 4000Hz) then we require 8000 samples per second to characterize the voice signal completely

Analog samples called Pulse Amplitude Modulation(PAM) samples

To convert to digital, each analog sample must be assigned a binary code – need quantizing and encoding

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QUANTIZING

Assume the amplitude of a signal f(t) is confined to the

range (-fp,fp).

This range is divided in L zones, each of step size ∆, given by

∆ =2 fp

L

A sample amplitude value is approximated by the midpoint of

the interval in which it lies.

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Put fig 5.5 a p.94

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Quantizing error (noise) is the difference between the input and output signals of the quantizer

The PCM scheme can be refined using 2 techniques:

• non-linear encoding – the quantization level are not equally spaced

• companding (compressing-expanding) the input analog signals – process that compresses the intensity range of a signal by imparting more gain to weak signals than to strong signals on input – uniform quantizing

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ENCODING

• An encoder in PCM translates the quantized sample into a code number.• The code number is converted to its representation in binary sequence• The binary sequence is converted to a sequential string of pulses for transmission

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Example 1 p.97 fig 5.8

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Example 2

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Delta Modulation (DM)

A method for converting analog signals to a string of binary digits

Analog input is approximated by a staircase function that moves up or down one quantization level () at each sampling interval (Ts)

Important characteristic of staircase function -binary behavior At each sampling time, the function moves up or down a

constant amount The output of the DM process can be represented as a single

binary digit for each sample Binary 1 – if staircase function go up during the next interval Binary 0 - if staircase function go down during the next interval

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Delta Modulation - example

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ANALOG DATA, ANALOG SIGNALS

AGENDAAGENDA

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Analog Data, Analog Signals

Modulation – the process of combining an input signal m(t) and a carrier at frequency fc to produce a signal s(t) whose bandwidth centered on fc

Why do we need analog modulation of analog signals? Higher frequency can give more efficient transmission Modulation permits frequency division multiplexing

(chapter 8) 3 principal techniques for modulation using analog data

Amplitude modulation (AM) Frequency modulation(FM) Phase modulation(PM)

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Analog Modulation