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Annual 2015

Q.1. (a) Define communication. What are the basic elements of communication system?

COMMUNICATION:

The word communication has been derived from a Latin word “communis”

which means to share or exchange of information, thoughts or views through

some medium. So, communication is the process of passing information from a

source to a receiver.

Nowadays communication is done through Email, blogs, chatting, text

messaging, podcasting, website etc.

BASIC ELEMENTS OF COMMUNICATION SYSTEM:

There are three basic elements to every communication system namely:

transmitter, channel and receiver as shown in figure.

Transmitter:

The transmitter's function is to process the message signal into a form suitable

for transmission over the communication channel. This is called modulation.

Channel:

Its function is to provide a pathway between the transmitter's output and the

receiver's input. When the transmitted signal propagates along the channel, it is

distorted due to channel imperfection. Moreover, noise and interfering signal

(originated from other sources) are added to the channel output, with the result

that the received signal is a corrupted version of the transmitted signal.

Receiver:

The job of the receiver is to process the received signal to recover the

appropriate message signal. If the different elements do their jobs accordingly,

then the output signal should equal to the input message signal.

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(b) What is transmission media. Describes its types with examples.

TRANSMISSION MEDIA:

Sending of data from one device to another is called transmission of data.

Medium used to transmit the data is called media.

Transmission of data through medium is called transmission media. So, it is a

pathway that carries the information from sender to receiver.

We use different types of cables or waves to transmit data.

Data is transmitted normally in electrical or electromagnetic signals.

Transmission media are located below the physical layer.

Computers use signals to represent data.

Signals are transmitted in from of electromagnetic energy.

TYPES OF TRANSMISSION MEDIA:

Transmission media is broadly classified into two groups.

1. Wired or Guided Media or Bound Transmission Media:

Guided transmission media are the cables

that are tangible or have physical existence

and are limited by the physical geography.

Examples of guided transmission media are

twisted pair cable, power line, co-axial cable

and fiber optical cable. Each of them has its

own characteristics like transmission speed,

effect of noise, physical appearance, cost

etc.

2. Wireless or Unguided Media or Unbound Transmission Media:

Unguided transmission media are the ways of transmitting data without using any cables.

These media are not bounded by physical geography. This type of transmission is called

Wireless communication. Nowadays wireless communication is becoming popular. Wireless

LANs are being installed in office and college campuses. Microwave, Radio wave, Infra-red

are some popular examples of unguided transmission media.

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Q.2. (a) Describe parameters of analog and digital signals.

PARAMETERS OF ANALOG SIGNAL:

(i) Peak Amplitude (ii) Frequency (iii) Phase

(i) Peak Amplitude: The peak amplitude of a signal is the absolute value of its

highest intensity, proportional to the energy it carries. For electric signals, peak

amplitude is normally measured in volts. Given figure shows two signals and

their peak amplitude.

(i) Frequency: Frequency is defined as the number of periods in one second.

The period is the duration of time of one cycle in a repeating event, so the period

is the reciprocal of the frequency.

Mathematically: f = 1

𝑇 and T=

1

𝑓

The frequency is expressed Hertz (Hz) and period in seconds.

(ii) Phase: The term phase describes the position of the waveform relative to

time zero. It is measured in degrees or radians (360⁰= 2π rad). A phase shift of

360⁰ correspond to a shift of a complete period.

PARAMETERS OF DIGITAL SIGNAL:

(i) Bit rate (ii) Bit length

(i) Bit rate: The term bit rate (instead of frequency) is used to described digital

signals. Bit rate is defined as the number of bits sent in one second and is

expressed in bit per second (bps).

(ii) Bit length: The term bit length (instead of wavelength) is defined as the

distance one bit occupies on the transmission medium.

Bit length = propagation speed x bit duration

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(b) What is the point of difference between periodic and aperiodic signal.

The point of difference between periodic and aperiodic signal is given below:

PERIODIC SIGNAL APERIODIC SIGNAL A signal which repeats itself after a specific interval of time is called periodic signal.

A signal which does not repeat itself after a specific interval of time is called aperiodic signal.

A signal that repeats its pattern over a period is called periodic signal

A signal that does not repeats its pattern over a period is called aperiodic signal or non-periodic.

They can be represented by a mathematical equation

They cannot be represented by any mathematical equation

Their value can be determined at any point of time

Their value cannot be determined with certainty at any given point of time

They are deterministic signals

They are random signals

Example: sine cosine square saw tooth etc. Example: sound signals from radio, all types of noise signals

Figure: Figure:

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(c) How baseband transmission is different from broadband.

BASEBAND TRANSMISSION BROADBAND TRANSMISSION Baseband transmission means sending a digital signal over a channel without changing the digital signal to an analog signal.

Broadband transmission means changing the digital signal to an analog signal for transmission.

It requires a low-pass channel, a channel with a bandwidth that starts from zero.

It uses a bandpass channel, a channel with a bandwidth that does not start from zero.

This can transmit only one signal at a time. This can transmit multiple signals simultaneously.

Baseband may use Time Division Multiplexing (TDM)

Broadband uses Frequency Division Multiplexing (FDM)

It is bidirectional but not at the same time It is unidirectional (two separate channels/frequencies to send/receive.

Signals can travel short distances Signals can travel over long distances

Example: Ethernet Example: Cable TV, Wi-Fi, Power line communications

Q.3. (a) Define noise and compare external and internal noise with its sources.

NOISE: Noise may be defined as any unwanted form of energy which tends to

interfere with proper reception and reproduction of wanted signal. OR

Noise is random, undesirable electrical energy that enters the communications system via the

communicating medium and interfere with the transmitted message. However, some noise is

also produced in the receiver.

It is a signal that does not convey any information.

1. EXTERNAL NOISE:

External Noise is the noise which is generated outside the device or circuit system.

External noises are somewhat uncontrollable and these are:

(i) Atmospheric Noise (ii) Extra-Terrestrial/ Space Noise (iii) Man-made or

Industrial Noise

(i) Atmospheric Noise: It is caused by lighting discharge in thunderstorm and other natural

disturbance in atmosphere. It spreads over the complete frequency spectrum which is used

for radio communication. The receiving antenna not only picks up the desired signal but also

the noise from thunderstorm and various disturbance causes at the output. Thus large

atmospheric noise is generated in low or medium frequency band (LF or MF) while very little

noise is generated in very high frequency (VHF) band.

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(ii) Extra-Terrestrial/ Space Noise: Space noise is divided into two categories:

(a) Solar noise (b) Cosmic noise

(a) Solar Noise: Solar noise is an electrical noise generated from the sun heat. This is

continuous radiation from sun. For example, the result from sun, a large body of very

high temperature (60000°C) will radiate electrical energy spectrum which is in the form

of noise which spread over all the spectrum used for radio communication.

(b) Cosmic noise: Cosmic noise is an electrical noise generated from the galaxies

such as star. The star and distant also like a sun which have high temperature.

Therefore, these stars radiate the noise in the same way as sun. The noise receives

from the distant, star is known as thermal noise and distributed almost uniformly over

the entire and almost effects on communication of radio waves.

(iii) Man-made or Industrial Noise: It is an electrical noise which produced by a source

like automobiles such as an aircraft ignition, electric motors, switch gear leakage from higher

voltage light, etc. Fluorescent light and many of man-made noise like electrical machine are

intensive in industrial area and populated urban area.

2. INTERNAL NOISE:

Internal Noise is the noise which is generated inside the communication system, within a

device or circuit. It is produced by properly design of receiver circuitry and these are:

(i) Thermal Noise / white noise / Johnson noise (ii) Shot Noise (iii)

Transit-time Noise

(i) Thermal Noise / white noise / Johnson noise: Thermal noise is produced by the

random motion of electrons in a conductor due to heat (thermal agitation). Each electron in a

conductor carry a unit negative charge and its velocity is proportional to the absolute

temperature. Because this type of electron movement is totally random and in all directions, it

is sometimes called random noise. Thermal noise is present in all electronic communications

system. It is a form of additive noise which meaning that it cannot be eliminated and it

increases in intensity with the number of devices and circuit length. Also known as Brownian

Noise, Johnson Noise, and White Noise (because the random movement of electrons is at all

frequencies).

(ii) Shot Noise: Shot noise is caused by the random arrival of current carriers (holes and

electrons) at the output element of an electronic device, such as a diode, field-effect transistor

(FET) or bipolar transistor (BJT). These random arrival of the carriers because of the random

paths and difference distance of travels. Shot noise is sometimes called transistor noise and

is additive with thermal noise.

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(iii) Transit-time Noise: Transit-time noise is any modification to a stream of carrier signals

as they pass from the input to the output of a device (such as from the emitter to the collector

of a transistor) produces an irregular, random variation. Transit-time noise in transistors is

determined by carrier mobility, bias voltage, and transistor construction.

Q.3. (b) What is SNR. Describe its role in communication.

SNR:

SNR is defined as, ratio of the signal power level to the noise power level is known as SNR.

NP

PSpower noise average

power signal averageSNR

Express in logarithmic function:

SNRdB = 10 log10

NP

PS or SNRdB = 10 log10 SNR

ROLE OF SNR IN COMMUNICATION:

The role of SNR in communication is given below:

It is a parameter used in measuring the effectiveness of a communication system or

network.

It is related to the quality of measurement where a high SNR guarantees clear gaining

with low distortions caused by noise.

In communication systems, the message signal should be dominant than the noise in

the received signal for the message to be detected and decoded. Hence a threshold

limit is established beyond which if the noise increases, the message would be

distorted. Thus for effective communication, the SNR should be as large as possible

and always be greater than "1".

The better is SNR, the better the signal stands out, the better the quality of signals, and

the better ability to get the results you desired.

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(c) Noise power at the output of the receiver’s IF stage is measured at 45 µW. With

receiver tuned to test signal, output power increases to 3.58 mW. Compute the SNR.

Given Data:

Noise power = 45 µW

Signal power = 3.58 mW

Required:

SNR =?

Solution:

As we know that

SNR = 𝐴𝑣𝑒𝑟𝑎𝑔𝑒 𝑆𝑖𝑔𝑛𝑎𝑙 𝑃𝑜𝑤𝑒𝑟

𝐴𝑣𝑒𝑟𝑎𝑔𝑒 𝑁𝑜𝑖𝑠𝑒 𝑃𝑜𝑤𝑒𝑟 putting the values in the given equation

= 3.58 mW / 45 10-6 W

= 0.08 x 10-3 x 106

= 0.08 x 103

= 80

SNR = 80

Q.6. (a) Why multiplexing is used.

USING OF MULTIPLEXING:

In communication, multiplexing is a term used to refer to a process where multiple analog

signals or digital data streams are combined into one single over a shared medium. The aim

is to share an expensive resource. For example, several phone calls may be transferred using

one wire.

In communication, the multiplexed signal is transmitted over a communication channel which

may be a physical transmission medium. The multiplexing divides the capacity of the low-level

communication channel into several higher-level logical channels, one for each message

signal or data stream to be transferred.

Multiplexing reduce cost because one channel can be used to send many information signals.

Thus that’s the reason of using multiplexing.

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Q.8. Write short notes on the following:

(i) Communication through ionosphere

(ii) DSB-SC and VSB Modulation

(i) COMMUNICATION THROUGH IONOSPHERE:

Ionosphere is the layer of the Earth's atmosphere that is ionized by solar and cosmic

radiation. It lies 75-1000 km (46-621 miles) above the Earth. This region consists of layers of

free electrically charged particles that transmit, refract, and reflect radio waves, allowing those

waves to be transmitted great distances around the earth. The interaction of the ionosphere

on impinging electromagnetic radiation depends on the properties of the ionospheric layer,

the geometry of transmission, and the frequency of the radiation. For any given signal path

through the atmosphere, a range of workable frequency bands exists. This range, between

the maximum usable frequency (MUF) and the lowest usable frequency (LUF), is where radio

waves are reflected and refracted by the ionosphere much as a partial mirror may reflect or

refract visible light. The reflective and refractive properties of the ionosphere provide a means

to transmit radio signals beyond direct "line-of-sight" transmission between a transmitter and

receiver. Ionospheric reflection and refraction has therefore been used almost exclusively for

long-range HF (from 3 to 30 MHz) communications. Radio waves with frequencies ranging

from above 30 MHz to 300 GHz are usually used for communications requiring line-of-sight

transmissions, such as satellite communications. At these higher frequencies, radio waves

propagate through the ionosphere with only a small fraction of the wave scattering back in a

pattern analogous to a sky wave.

(ii) DSB-SC AND VSB MODULATION:

DSB-SC Modulation: Double-sideband suppressed-carrier modulation (DSB-SC) is a

modulation in which frequencies produced by amplitude modulation (AM) are symmetrically

spaced above and below the carrier frequency and the carrier level is reduced to the lowest

practical level, ideally being completely suppressed.

There are two methods used for DSB-SC Modulation

1. Multiplier modulation: DSBSC = the message x the carrier

2. Balance modulator: A balance modulator consists of two standards amplitude

modulations arranged in a balanced configuration, so as to suppers the carrier wave, as

shown in block.

S₁(t)=Ac[1+Kₐm(t)] cos(2πfᴄt)

S₂(t)=Ac[1-Kₐm(t)] cos(2πfᴄt)

S(t)= S₁(t)- S₂(t)=2KₐAc cos(2πfᴄt) m(t), which is equal to the produced message signal

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VSB MODULATION:

Vestigial sideband (VSB) is a type of amplitude modulation ( AM ) technique (sometimes called VSB-AM ) that encodes data by varying the amplitude of a single carrier frequency . Portions of one of the redundant sidebands are removed to form a vestigial sideband signal - so-called because a vestige of the sideband remains.

In AM, the carrier itself does not fluctuate in amplitude. Instead, the modulating data appears in the form of signal components at frequencies slightly higher and lower than that of the carrier. These components are called sidebands. The lower sideband (LSB) appears at frequencies below the carrier frequency; the upper sideband (USB) appears at frequencies above the carrier frequency. The actual information is transmitted in the sidebands, rather than the carrier; both sidebands carry the same information. Because LSB and USB are essentially mirror images of each other, one can be discarded or used for a second channel or for diagnostic purposes.

VSB transmission is similar to single-sideband (SSB) transmission, in which one of the sidebands is completely removed. In VSB transmission, however, the second sideband is not completely removed, but is filtered to remove all but the desired range of frequencies .

Eight-level VSB ( 8-VSB ) was developed by Zenith for inclusion in the Advanced Television Systems Committee ( ATSC set of digital television ( DTV ) standards.

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Annual 2014

Q.1. (a) Explain Time and Frequency Domain with examples. Also explain composite

signal.

TIME DOMAIN AND FREQUENCY DOMAIN:

A sine wave is comprehensively defined by its amplitude, frequency and phase. We have

been showing a sine wave by using what is called time-domain plot. The time domain plot

shows changes in signal amplitude with respect to time (it is amplitude versus time plot).

Phase is not explicitly shown on a time-domain plot.

To show the relationship between amplitude and frequency, we can use what is called

a frequency-domain plot. A frequency-domain plot is concerned with only the peak value and

the frequency. Changes of the amplitude during one period are not shown.

Figure shows a signal in both the time and frequency domain.

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COMPOSITE SIGNAL:

A signal-frequency sine wave is not useful in data communications, we need to send a

composite signal, a composite signal is one which is made of many simple sine waves.

According to Fourier analysis, any composite signal is a combination of simple sine waves

with different frequencies, amplitudes and phases.

A composite signal can be periodic or nonperiodic. A periodic composite signal can be

decomposed into a series of simple sine waves with discrete frequencies, frequencies that

have integer values (1,2,3, and so on). A nonperiodic composite signal can be decomposed

into a combination of an infinite number of simple sine waves with continuous frequencies,

frequencies that have real values.

(b) (I) BANDWIDTH: Bandwidth is the difference between the upper and lower

frequencies in a continuous set of frequencies. It is typically measured in hertz, and may

sometimes refer to passband bandwidth, sometimes to baseband bandwidth, depending on

context. Passband bandwidth is the difference between the upper and lower cutoff

frequencies of, for example, a band-pass filter, a communication channel, or a signal

spectrum. In the case of a low-pass filter or baseband signal, the bandwidth is equal to its

upper cutoff frequency.

(II) BAUD RATE: Baud is defined as the number of signal elements per second in

communication channel. In analog transmission of digital data, the baud rate is less than or

equal to the bit rate.

(III) BIT INTERVAL: The bit interval is the time required to send one single bit.

(IV) BIT RATE: Bit rate is defined as the number of data bits (i.e. ‘0’ or ‘1’)

transferred per second in a communication channel. It is expressed in bits per second (bps).

(V) PROPAGATION TIME: The time required for a bit to travel from the source to

the destination.

Propagation time = Distance/Propagation speed

Q.2. (a) What is PCM? Draw block diagram and explain in detail.

PULSE CODE MODULATION (PCM): The most common technique to change an analog

signal to digital data (digitization) is called pulse code modulation (PCM).

With PCM, the amplitude of the sound wave is sampled at regular intervals and translated into

a binary number.

The difference between the original analog signal and the translated digital signal is called

quantizing error.

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BLOCK DIAGRAM:

EXPLANATION: PCM encoder has the following three processes.

(i) The analog signal is sampled.

(ii) The sampled signal is quantized.

(iii) The quantized values are enclosed as stream of bits.

Sampling:

The process of generating pulses of zero width and of amplitude equal to the instantaneous

amplitude of the analog signal. The no. of pulses per second is called “sampling rate”.

In sampling:

Analog signal is sampled every TS sec.

Ts is referred to as the sampling interval.

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

There are 3 sampling methods:

o Ideal - an impulse at each sampling instant

o Natural - a pulse of short width with varying amplitude

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

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Different types of sampling methods:

The sampling process is sometimes referred to as pulse amplitude modulation (PAM),

however that the result is still an analog signal with nonintegral values.

Quantization:

The instantaneous amplitude of the analog signal at each sampling is rounded off to the

nearest of several specific, predetermined levels. This process is called Quantization.

The followings are the steps in quantization:

1. We assume that the original analog signal has instantaneous amplitudes between

Vmin and Vmax.

2. We divide the range into L zones, each of height ∆ (delta).

∆ = Vmin − Vmax

𝐿

3. We assign quantized values of 0 to L -1 to the midpoint of each zone.

4. We approximate the value of the sample amplitude to the quantized values.

Encoding:

The last step in PCM is encoding. The number of levels is always a power of 2 -- for example,

8, 16, 32, or 64. These numbers can be represented by three, four, five, or six binary digits

(bits) respectively. The output of a pulse code modulator is thus a series of binary numbers,

each represented by some power of 2 bits.

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What is multiplexing? Explain interleaving process with proper and clean diagram.

MULTIPLEXING:

Multiplexing is the set of techniques that allows the simultaneous transmission of multiple

signals across a single data link.

There are three common types of multiplexing i.e.

Frequency division multiplexing (FDM)

Wavelength division multiplexing and

Time division multiplexing (TDM).

Synchronous TDM

Statistical / Asynchronous TDM

INTERLEAVING PROCESS:

Time-division multiplexing can be visualized as two fast rotating switches on the multiplexing

and demultiplexing side. At the same speed these switches rotate and synchronize, but in

opposite directions. When the switch opens at the multiplexer side in front of a connection, it

has the opportunity to send a unit into the path. In the same way, when the switch opens on

the demultiplexer side in front of a connection that has the opportunity to receive a unit from

the path. This process is called interleaving. OR

The process of taking a group of bits from each input line for multiplexing is called

interleaving.

The given below Figure (a) shows the interleaving process for the connection shown in the

above Figure (b). In this figure, we assume that no switching is involved and that the data

from the first connection at the multiplexer side go to the first connection at the

demultiplexing.

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EXPLANATION OF DIGITAL SIGNAL (DS) SERVICE:

Telephone companies implement TDM through a hierarchy of digital signals, called digital

signal (DS) service or digital hierarchy. Figure (A) shows the data rates supported by each

level. The commercial implementations of these services are referred to as T lines.

DS-0 service is a single digital channel of 64 kbps.

DS-1 is a 1.544-Mbps service.

DS-2 is a 6.312-Mbps service.

DS-3 is a 44.376-Mbps service.

DS-4 is a 274.176-Mbps service.

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Explain Natural and Flat-Top sampling with input and output wave forms.

NATURAL SAMPLING:

Natural Sampling is a practical method of sampling in which pulse

have finite width equal to τ. Sampling is

done in accordance with the carrier signal

which is digital in nature.

With the help of functional diagram of a

Natural sampler, a sampled signal g(t) is

obtained by multiplication of sampling

function c(t) and the input signal x(t).

Spectrum of Natural Sampled Signal is given by:

G(f) = Aτ/ Ts .[ Σ sin c(n fs.τ) X(f-n fs)]

FLAT-TOP SAMPLING:

Flat top sampling is like natural sampling i.e. practical in nature. In comparison to natural

sampling flat top sampling can be easily obtained. In this sampling techniques, the top of the

samples remains constant and is equal to the instantaneous value of the message signal x(t)

at the start of sampling process. Sample and hold circuit are used in this type of sampling.

Figure(a), shows functional diagram of a sample hold circuit which is used to generate fat top

samples.

Figure(b), shows the general waveform of the flat top samples. It can be observed that only

starting edge of the pulse represent the instantaneous value of the message signal x(t). Spectrum of Flat top Sampled Signal is given by: G(f) = fs .[ Σ X(f-n fs). H(f)]

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MAHO:

A mobile assisted handoff (MAHO) is a process used in GSM cellular networks where a

mobile phone assists/helps the cellular base station to transfer a call to another base station.

It is a technique used in mobile telecom to transfer a mobile phone to a new radio channel

with stronger signal strength and improved channel quality.

Mobile assisted handoff can also be referred to as mobile assisted handover.

MAHO is based on a mobile phone’s capabilities in detecting and identifying better radio

channels to be used within a call. MAHO works when a mobile phone can scan, review and

monitor nearby radio channels. The mobile collects the measurements, usually in the form of

RF signal quality, received signal strength indication (RSSI), bit error rate and similar results

from other available channels. These measurements are then sent to the base station, which

evaluates them and transfers the call to the best available channel.

Difference between cellular and non-cellular (radio) communication.

CELLULAR COMMUNICATION:

Cellular communication is designed to provide communications between two moving units, or

between one mobile unit and one stationary phone or land unit (PSTN). A service provider

must be able to locate and track a caller, assign a channel to the call.

Cellular communication services: Voice communication, SMS, MMS, GPS, WAP-to access

internet, security (PIN).

NON-CELLULAR (RADIO) COMMUNICATION:

A radio communication system is composed of several communications subsystems that give

exterior communications capabilities. A radio communication system comprises a transmitting

conductor in which electrical oscillations or currents are produced and which is arranged to

cause such currents or oscillations to be propagated through the free space medium from one

point to another remote therefrom and a receiving conductor at such distant point adapted to

be excited by the oscillations or currents propagated from the transmitter.