Data encoding and modulation

Data Encoding and Modulation

Chapter 5

Introduction to Communication

System

• Communication- Basic Process of exchanging

information from one location (Source) to

destination (receiving end).

• System refers to process of sending, receiving

and processing of information/ signal/input

from one point to another point.

• Electronic communication system- defined as the wholemechanism of sending and receiving as well asprocessing of information electronically from source todestination.

• The main objective of communication system is toproduce an accurate replica of the transmittedinformation that is to transfer information between twoor more points (destinations) through a communicationchannel, with minimum error.

• E.g. Radiotelephony, broadcasting, point- to- point,mobile communication, radar and satellite systems.

Terms Related to Communications

Basic Elements of Communication

System

Block Diagram of a Transmitter

Block Diagram of Receiver

Baseband Communication• Baseband refers to the original frequency range of a

transmission signal before it is converted, ormodulated, to a different frequency range. E.g. an audiosignal may have baseband range from 20Hz to 20KHz.

• When it is transmitted on a radio frequency (RF), it ismodulated to a much higher, inaudible frequency range.

• Signal modulation is used for radio broadcast as well asseveral types of telecommunications including cellphones conversations and satellite transmission.

• Therefore, most telecommunication protocols requireoriginal baseband signals to be modulated to a higherfrequency before they are transmitted.

• These signals are then demodulated at the destination,so the recipient receives the original baseband signal.E.g.. MODEMs they modulate and demodulate signalswhen they are transmitted and received.

• Baseband signals cannot be transmitted over a radiolink. (free space)

• Baseband signals are suitable for transmission overcopper (pair of wires, coaxial cable) or glass (fiber)

• No shift in the range of frequencies of the signal.

Modulation• Modulation is the process of varying one or more properties of

a high frequency signal called carrier signal according with a

modulating signal which typically contains information to be

transmitted.

• The technique of superimposing the message signal on the

carrier is know as modulation.

• The three key parameter's are: Amplitude (volume) ,phase

(Phase) and frequency (Pitch).

• Modulation of a sine waveform is used to transform a

baseband message into a pass band signal, for example low

frequency audio signal into a radio-frequency signal (RF-

Signal)

Need for ModulationShort operating Range- when a wave has a large frequency, the

energy associated with it will also be large. Thus low frequency

signals have less power that does not enable them to travel over

long distances.

Poor Radiation Efficiency- The radiation becomes very poor for

low frequency signals.

Need for modulation cond..• Mutual Interference- If all audio frequencies are send

continuously from different sources, they would all get mixed

up and cause erroneous interference air. If modulation is done,

each signal will occupy different frequency levels and can be

transmitted simultaneously without any error.

• Huge Antenna Requirement- For a effective signal

transmission, the sending and receiving antenna should be at

least 1/4th of the wave length of the signal. Thus, for small

frequencies, the antenna will have kilometers of length. But if

the signal has the range of MHz frequency, then the antenna

size would be less.

• Requirements of multiple signal transmission- Modulation

allows us to send a signal over a band pass frequency range. If

every signal gets its own frequency range, then we can

transmit multiple signals simultaneously over a single channel,

all using different frequency ranges ( Multiplexing)

Encoding

• Encoding is the process of converting data into a format required for a number of information processing needs, including:– Program compiling and execution.

– Data transmission, storage and compression/decompression.

– Application data processing, such as file conversion.

– Encoding is also used to reduce the size of audio and video files.

– E.g. ASCII (American standard code for Information Interchange), MIME ( Multipurpose Internet Mail Extensions).

Amplitude Modulation• a type of modulation where the amplitude of

the carrier signal is modulated (changed) inproportion to the message signal while thefrequency and phase are kept constant.

Carrier Signal: or

Modulating Message Signal: or

The AM Signal:

cos(2 ) cos( )

( ) : cos(2 ) cos( )

( ) [ ( )]cos(2 )

c c

m m

AM c c

f t t

m t f t t

s t A m t f t

• Mathematical expression for AM: time domain

• expanding this produces:

• In the frequency domain this gives:

( ) (1 cos )cosAM m cS t k t t

( ) cos cos cosc cAM mS t t k t t

• In frequency domain:

frequencyk/2

k/2

Carrier, A=1.

upper sidebandlower

sideband

Amplitude

fcfc-fm fc+fm

)cos()cos(coscos :using 21 BABABA

2 2( ) cos cos( ) cos( )c c c

k kAM m mS t t t t

AM Power Frequency Spectrum

• AM Power frequency spectrum obtained by squaring the amplitude:

• Total power for AM:

.

2 22

2

4 4

12

k kA

k

freq

k2/4k2/4

Carrier, A2=12 = 1Power

fcfc-fm fc+fm

Modulation Index of AM Signal

m

c

Ak

A

)2cos()( tfAtm mm

Carrier Signal: cos(2 ) DC: c Cf t A

For a sinusoidal message signal

Modulation Index is defined as:

Modulated Signal:

( ) [ cos(2 )]cos(2 )

[1 cos(2 )]cos(2 )

AM c m m c

c m c

S t A A f t f t

A k f t f t

Modulation index k is a measure of the extent to which a carrier voltage is varied by the modulating signal. When k=0 no modulation, when k=1 100% modulation, when k>1 over modulation.

CSULB May 22, 2006


• The carrier frequency is the frequency to which the radio receiver is tuned for station selection.

• For e.g. the AM radio band (broadcast band) is legally designed from 535 KHz to 1605KHz.

• If your favorite local radio station broadcasts on 830KHz, this means that the carrier frequency being used for transmission is 830KHz.

• Amplitude Modulation Applications:– AM radio Broadcasting– TV picture – Two way radio– Aircraft– Amateur radio (SSB)– Military Communication– Digital data communication– Computer Modems

Different versions of AM

Frequency Modulationa type of modulation where the Frequency of the

carrier signal is modulated (changed) in proportion

to the message signal while the amplitude and phase

are kept constant.

• FM modulation Index:

• Ratio of the frequency deviation to the modulating frequency.

β =Frequency Deivation

Modulating frequency

• The total bandwidth required for FM can be determined from

the bandwidth

of the audio signal: BFM = 2(1 + β)B. Where is usually 4.

• Narrowband FM: B is small enough that the terms in the

Bessel expansion. Modulation index must be less than 0.5,

used for short distance and data bandwidth is small. E.g. short

distance communications using vehicle mount radios.

• Wideband FM: modulation index is above 0.5, wider

bandwidth, high quality signals. E.g. broadcast FM stations .

• The amount by which the signal frequency varies is very important. This is known as the deviation and is normally quoted as the number of KiloHertz deviation.

• E.g. the signal may have a deviation of ±3KHz. In this case the carrier is made to move up and down by 3KHz.

• FM is used worldwide to provide high fidelity sound over broadcast radio.

• FM broadcasting is capable of better sound quality than AM broadcasting.

• FM broadcast band falls within the VHF part of the radio spectrum usually 88 to 108 MHz is used.

Phase Modulation

• a type of modulation where the phase of the

carrier signal is modulated (changed) in

proportion to the message signal while the

amplitude and frequency are kept constant.

• Phase modulation is widely used for transmitting radio waves and is an integral part of many digital transmission coding schemes that underlines a wide range of technologies like Wi-Fi, GSM and satellite television.

Phase modulated wave• The effect of variation in amount of phase shift is proportional

to change in the carrier frequency. So called indirect form of

frequency modulation.

• Advantage: increased immunity to noise

• Disadvantage: More complex hardware at receiver.

Assignments

• Define modulation. Why modulation isneeded?

• Explain the general block diagram ofcommunication system.

• Differentiate among AM, FM and PM.

• What are the advantages and disadvantages ofFM over AM?

Pulse Modulation System

• The process of transmitting signals in the form ofpulses (discontinuous signals) by using specialtechniques.

• Two Types of Pulse Modulation • Analog Pulse Modulation

– Pulse Amplitude Modulation (PAM)– Pulse Width Modulation (PWM)– Pulse Position Modulation (PPM)

• Digital Pulse Modulation– Pulse Code Modulation (PCM)– Delta Modulation (DM)

Pulse Amplitude Modulation (PAM) • The signal is sampled at regular intervals such that each

sample is proportional to the amplitude of the signal at that

sampling instant. This technique is called sampling.

• For minimum distortion, the sampling rate should be more

than twice the signal frequency.

Pulse Width Modulation (PWM/PLM/PDM)

• In this type, the amplitude is maintained constant but the

duration or length or width of each pulse is varied in

accordance with instantaneous value of the analog signal.

• The negative side of the signal is brought to the positive side

by adding a fixed D.C voltage.

Pulse Position Modulation (PPM)• In this type, the sampled waveform has fixed amplitude and

width whereas the position of each pulse is varied as per

instantaneous value of the analog signal.

• PPM signal is further modification of PWM signal. It has

positive thin pulses (Zero time or width) corresponding to the

starting edge of a PWM pulse and negative thin pulses

corresponding to the ending edge of a pulse.

PAM, PWM and PPM at a glance

Pulse Code Modulation (PCM)• PCM is a digital scheme for transmitting analog data. The

signals in PCM are binary; that is, there are only two possible

state, 1 and 0.

• Using PCM, it is possible to digitize all forms of analog data,

including full-motion video, voices, music, telemetry.

• Analog signal is converted into digital signal by using a digital

code.

• PCM involves three steps:

– Sampling

– Quantization

– Encoding

Basic Block diagram of PCM1.Sampling: The process of generating pulses of zero width and of

amplitude equal to the instantaneous amplitude of the analog signal. The

number of pulses per second is called sampling rate.

2. Quantization: The process of dividing the maximum value of the analog

signal into a fixed number of levels in order to convert the PAM into a

Binary code. The levels obtained are called Quantization levels.

3. Encoding/Coding: The process of assigning digital signals to the

quantized levels.

Sampling

• Analog signal is sampled every TS secs.

• Ts is referred to as the sampling interval.

• fs = 1/Ts is called the sampling rate or sampling frequency.

• There are 3 sampling methods:– Ideal - an impulse at each sampling instant

– Natural - a pulse of short width with varying amplitude

– Flattop - sample and hold, like natural but with single amplitude value

• The process is referred to as pulse amplitude modulation PAM and the outcome is a signal with analog (non integer) values

Three different methods of sampling

According to the Nyquist theorem, the sampling rate must be

at least 2 times the highest frequency contained in the signal.

Quantization Zones

• Assume we have a voltage signal with amplitutes Vmin=-20V and Vmax=+20V.

• We want to use L=8 quantization levels.

• Zone width = (20 - -20)/8 = 5

• The 8 zones are: -20 to -15, -15 to -10, -10 to -5, -5 to 0, 0 to +5, +5 to +10, +10 to +15, +15 to +20

• The midpoints are: -17.5, -12.5, -7.5, -2.5, 2.5, 7.5, 12.5, 17.5

Assigning Codes to Zones• Each zone is then assigned a binary code.

• The number of bits required to encode the zones, or the number of bits per sample as it is commonly referred to, is obtained as follows:

nb = log2 L

• Given our example, nb = 3

• The 8 zone (or level) codes are therefore: 000, 001, 010, 011, 100, 101, 110, and 111

• Assigning codes to zones:– 000 will refer to zone -20 to -15

– 001 to zone -15 to -10, etc.

Figure Quantization and encoding of a sampled signal

Advantages and Disadvantages of PCM

• advantages– Robustness to noise and interference– Efficient regeneration– Uniform format– Easily multiplexed– Signals may be stored.– Enables encryption– Easy storage

• DisadvantagesRequires larger bandwidth.Need synchronizationNot compatible to analog system.

Encoding Digital Data as Digital signals

The technique used in a number of LANs.

Digital signal- is a sequence of discrete discontinuous voltage pulses.

Bit duration- the time it takes for the transmitter to emit the bit.

Issues:

Bit timing

Recovery from signal

Noise Immunity

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

Terms (2)

• Modulation rate

– Rate at which the signal level changes

– Measured in baud = signal elements per second

• Mark and Space

– Binary 1 and Binary 0 respectively

Line Coding

• In telecommunication, a line code is a code chosen foruse within a communications system for transmitting adigital signal down a transmission line. Line coding isoften used for digital data transport

• The waveform pattern of voltage or current used torepresent the 1s and 0s of a digital signal on atransmission link is called line encoding. The commontypes of line encoding are unipolar, polar, bipolar andManchester encoding. Line codes are used commonlyin computer communication networks over shortdistances.

Encoding Schemes

• Nonreturn to Zero-Level (NRZ-L)

• Nonreturn to Zero Inverted (NRZ-I)

• Bipolar -AMI

• Pseudoternary

• Manchester

• Differential Manchester

• B8ZS

• HDB3

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

Nonreturn to Zero Inverted

• Nonreturn to zero inverted 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

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

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

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

Pseudoternary

• One represented by absence of line signal

• Zero represented by alternating positive and negative

• No advantage or disadvantage over bipolar-AMI

Bipolar-AMI and Pseudoternary

Biphase

• 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 (Ethernet)

• 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 (token ring)

Manchester Encoding

Differential Manchester Encoding

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

B8ZS

• Bipolar With 8 Zeros Substitution• 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

HDB3

• High Density Bipolar 3 Zeros

• Based on bipolar-AMI

• String of four zeros replaced with one or twopulses

B8ZS and HDB3

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)2cos( tfA c

Digital Data, Analog Signal

Keying is a family of modulation forms where the modulating

signal takes one of a specific (predetermined) number of values at

all times. The goal of keying is to transmit a digital signal over an

analog channel. The name derives from the Morse code key used

for telegraph signaling.

Modulation is the general technique of shaping a signal to convey

information. When a digital message has to be represented as an

analog waveform, the technique and term keying.

Several keying techniques exist, including phase-shift keying,

frequency-shift keying and amplitude-shift keying. Bluetooth, for

example, uses phase-shift keying to exchange information between

devices.

Keying

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Differential PSKPhase shifted relative to previous transmission rather than some

reference signal

Advantage:

• Very good noise immunity.

• For the same bit error rate, the bandwidth required by

QPSK is reduced to half as compared to BPSK.

• Because of reduced bandwidth, the information

transmission rate of QPSK is higher.

• Low error probability.

Disadvantages:

• Inter-channel interference is significantly large in

QPSK.

• QPSK relative to BPSK is that it is more sensitive to

phase variations.

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• 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

Quadrature Amplitude Modulation

4.80

Quadrature Amplitude Modulation, QAM is a signal in which two

carriers shifted in phase by 90 degrees are modulated and the

resultant output consists of both amplitude and phase variations.

In view of the fact that both amplitude and phase variations are

present it may also be considered as a mixture of amplitude and

phase modulation. QAM is extensively used as modulation

scheme for digital telecommunication system such as 802.11 Wi-

Fi standards. QAM is being used in optical fiber systems as bit

rates increases 16QAM and 64QAM.

4.81

Signal Constellation

Advantage:

increase the efficiency of transmission for radio

communications systems by utilizing both amplitude and

phase variations

Disadvantage: it is more susceptible to noise because the

states are closer together.

Data encoding and modulation

Engineering

Transcript of Data encoding and modulation