t.y. diploma : sem v prelim question paper solution...

1013/TY/Pre_Pap/Comp/CT_Soln 159

Vidyalankar T.Y. Diploma : Sem V

[IF/MU] Communication Technique

Prelim Question Paper Solution

(i) Communication It is the process in which the information is transferred from one place called as source to other place called as destination. Telecommunication means communicating at a longer distance. The block diagram of basic communication system is as follows :

The basic elements of communication systems are Transmitter, a communication channel and the receiver. In addition to this every communication system requires input transducer and output transducer. Information or input signal : Communication system communicate information as messages. The two sources of information are, the ideas emanating from the human brain and the changes in the physical environment. Information can be in the form of sound signal like speech or music, it can be in the form of pictures (TV signal) or it can be data information coming from the computer. Input Transducer : The information in the form of sound, picture or data signals can be converted into suitable electrical signals by the input transducer. The input transducer commonly used in communication systems are microphone, TV, camera etc. Transmitter : The transmitter is a collection of electronic circuits designed to modify or convert the message signal into a suitable form for transmission over communication channel. It consists of amplifiers, mixers, oscillators etc. Communication Channel : It is the medium used for transmission of electrical signal from one place to others. It can be pair of conducting wire, coaxial cable, optical fibre, optical fibre cable or free space. Depending upon types of communication medium, two types of communication system will exist. (i) Wire or line Communication (ii) Wireless or Radio Communication.

(ii) The critical Frequency (fc)

"The highest frequency that will be returned back, to earth's surface by ionosphere when it is propagated directly upward called as critcal frequency fc."

1. (a)

1. (a)

Noise

Information (Destination)

Information (Source)

Input Transducer Transmitter

Transmission Channel Receiver

Output Transducer

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Vidyalankar : T.Y. Diploma CT


The critical frequency depends on the ionization density and varies with time of day and season. When angle of incidence (angle of elevation of beam also take off angle). varied keeping the frequency constant. It's result is shown below.

The angle at which wave will get reflected back by ionosphere called as critical angle c. If angle less than c the wave will reflected from lower region and if angle greater than c the wave will escape in space.

The critical frequency of any layer is given by

c maxf g N ... (1)

Where Nmax = Maximum electron density 1/m3 (electrons per meter cube)

Frequencies above UHF range are actually unaffected by ionosphere because their wavelength is extremely short. At these frequencies the distance between ion are considerably large and consequently the electromagnetic wave pass through them. So there must be an upper frequency limit for sky wave propogation i.e. called as critical frequency. Fading As an em wave propagates through Earth's atmosphere, the signal may

experience losses in signal strength beyond normal path poss. This valuation in signal loss is called fading and can be caused by

natural weather disturbances such as rainfall, snowfall, fog, hail and extremely cold over a warm Earth.

Fading can also be caused by man-made disturbances such as irrigation, etc.

To accommodate temporary fading, an additional fade loss is added to the normal path loss. This loss is called "fade margin".

Fm = 30 log D + 10 log (6 ABf) 10 log (1 R) 70

where, Fm = fade margin (dB) D = dist (km) f = frequency (GHz) R = reliability (0.9999 99.99%) A = roughness factor B = factor relating to climate cendns. 4 over water 1 over avg. terrain. 0.25 over rough mountainous teenain

Ray retunes to earth

c

Ionosphere

Ray penetrates ionosphere

Earth's surface

c : Critical angle

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D E M U X

M U X 1 + 2 + 3

1

2

3

1

2

3

(iii) Advantage of Digital Transmission: (Advantage of PCM) 1) Primary advantage of digital transmission over analog transmission is

noise immunity. With digital transmission it is not necessary to evaluate the amplitude, frequency and phase characteristics as precisely as it is with analog transmission. Instead, the received pulses are evaluated and a simple determination is made weather the pulse is above a below a certain threshold level. The exact amplitude frequency or phase of the received signal is not important. Therefore, it has better noise immunity.

2) Digital signals are better suited to processing and multiplexing than analog signal.

3) Regeneration of digital signal along the transmission path is possible. Therefore, the digital transmission systems are more noise resistant than their analog counterparts. Digital system use signal regeneration rather than signal amplification.

4) Communication can be kept private and secured through the use of encryption.

5) Digital signals are simpler to measure and evaluate. Therefore, it is easier to compare the performance of alternate digital systems with unequal signaling and information capacities than it is with comparable analog systems.

6) Digital systems are better suited to evaluate error performance. Transmission errors in digital signals can be detected and corrected more easily and more accurately than it is possible with analog systems.

Disadvantages of Digital Transmission 1) Transmission of digital signal required more bandwidth as compared to

analog transmission. 2) Analog signal must be converted to digital codes prior to transmission and

converted back to analog form at the receiver. Thus additional encoding and decoding circuitry needed.

3) Digital transmission requires precise time synchronization between transmit and receiver clocks. Therefore, digital systems require expensive clock recovery circuit in all receiver.

(iv) Wavelength Division Multiplexing (WDM)

WDM is an analog multiplexing technique to combine optical signals. In WDM, very narrow bands of light from different sources are combined

to make a wider band of light and at the receiver, the signals are separated by the demultiplexer.

WDM is conceptually same as FDM, except that multiplexing and demultiplexing involves optical signals.

The conceptual view of WDM is as shown below:

1. (a)

1. (a)

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Using this technique, a multiplexer can be made to combine several input beams of light, each containing narrow band of frequencies into one output beam of wider band of frequencies.

Thus, very narrow bands of light of differential wavelength are combined to make a wider band of frequencies, and all wavelengths travel through a signal cable.

Advantage i) In low bandwidth, large number of channels accomo dated.

ii) Time shaving not required.

iii) High speed.

Disadvantage i) Only possible with optical fiber cables which is expensive.

Applications i) Used in SONET (Synchronous optical network) where SONET is a high

data rate network for fiber optic networks.

Advantage of SONET i) Simplicity

ii) Easy generation and detection.

Disadvantages of SONET i) It is used at very low bit rates.

ii) It is very noise sensitive.

(i) Pulse Amplitude Modulation

In PAM, the amplitude of the carrier pulses are varied in accordance with the instantaneous amplitude of modulating signal. In this system signal is sampled at regular interval & each sample is made proportional to amplitude of signal at the instant of sampling.

1. (b)

0 v

Signal

Signal

R1

R2

R3

R4

RE C0

V0

Multiplier

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It is an emitterfollower circuit. Here emitter follower act as a multiplier. The modulating signal and carrier signal are applied to the base of transistor as shown in figure. The amplitude of carrier is selected such that high level is at ground level and low level, at some negative voltage which is sufficient to keep transistor off. So when carrier pulse is at high level, the circuit behave as emitter follower and output follow the modulating signal during high pulse interval. When the carrier signal is at low level transistor is cutoff and output is zero. Thus we get the required PAM at the output of emitter follower.

PAM Demodulator : PAM signal can be detected by passing it through a low pass filter. Low pass filter is adjusted to fm so that all high frequency ripple i.e. sudden changes in the pulse is removed and allow to pass only the constant part of the pulse i.e. low frequency components. Hence output is approximately analog signal. If the sampling rate is fast, then output is very closed to analog signal.

Disadvantages i) Since noise affect the amplitude of waveform and in PAM information

contain in amplitude variation, like AM, PAM is less immune to noise. ii) Due to changes in amplitude of PAM pulses, the transmitted power is not

constant. iii) Large bandwidth required to transmit PAM.

t

t

t

t

0

0

0

0

Carrier Pulses

Modulating Signal

Natural Sampling

Flat-top Sampling

gnd level

ve level

Low Pass Filter Analog

Signal

PAM

Analog Signal

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Pulse Width Modulation In this system, the width of carrier pulses are varied in accordance with the instantaneous amplitude of modulating signal. The amplitude of pulses are fixed. From diagram it is seen that the width of the pulses goes on increasing as modulating signal goes on increasing and decreases for decreasing modulating signal. PWM Modulator : The block diagram of PWM modulator is as shown below:

Pulse

Generator

Ramp

Generator

PAM

Modulator

Summing

Amplifier

Comparator Modulating

Signal

O/P of summing Amplifier

Reference

Voltage Comparator

PWM O/P

PWM output

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The given circuit provides PWM output with trailing edges of the pulses remaining fixed and with varying leading edges to change pulse width.

The pulse generator generates the train of pulses of constant amplitude and width as shown in diagrama. These pulses are given to ramp generator as well as PAM modulator. The positive going edge of the output of pulse generator triggers the rump generator and it produces positive going ramp signal of equal slope, duration and height as shown diagram (b).

The modulating signal is applied to PAM modulator which is sampling network. Its output is flattop sampled PAM signal as shown in diagram c. The output of ramp generator and PAM modulator are added in the summing amplifier to produce ramp of varying height as shown in diagram (d). The output of summing amplifier is given to non-inverting input of comparator and fixed reference voltage is given to its inverting terminal. Therefore, whenever the output of summing amplifier becomes higher than reference voltage, the output of comparator goes high. Hence, it generates the pulses of varying width i.e. PWM signal as shown in diagram (e).

Advantages

i) Less effect of noise i.e. very good noise immunity. ii) Synchronization between the transmitter and receiver is not essential.

t

t

t

t

t

dia - a

dia - b

dia - c

dia - d

dia - e

Comparator level

Pulse Generator

O/P

Ramp Generator

O/P

PAM O/P with

Modulator signal

Summing Amplifier Output

PWM Output

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Disadvantages i) Due to the variable pulse width, the pulses have variable power contents

so transmitter must be powerful enough to handle maximum width pulse. ii) Required large bandwidth.

(ii) Polar Encoding

Polar encoding uses two voltage levels of amplihede one positive and one negative.

Note : In Manchester and Differential Manchester encoding, each bit consists of both positive and negative voltages. So the DC component is totally eliminated.

1) Non-return to Zero (NRZ) In NRZ encoding, the level of the signal is always either positive or

negative. There are two types of NRZ transmission. (i) NRZL encoding In NRZL encoding, the level of the signal depends on the type

of bit it represents. A positive voltage usually means bit is a O, and a negative

voltage means bit is a 1. In NRZ-L, the level of the signal depends upon the state of

the bit. A problem of synchronisation occurs here, when a receiver a

long stream of O's or 1's and sender or receiver clock are not synchronized properly

(ii) NRZ – I encoding In NRZ – I encoding, an inversion of voltage level represents

a 1-bit. It is the transition between a positive and negative voltage,

not the voltages themselves, that represent a 1-bit. A bit 0 is represented by no change.

NRZ – I is superior to NRZ – L due to the synchronisation provided by the signal change each time a 1-bit is encountered. The existence of 1's in the datastream allows the receiver to resynchronise its timer to the actual arrival of the transmission.

In NRZ-I the signal is inverted if a 1 is encountered.

The following diagram shows the representation of NRZ-L and NRZ – I of same series of bits.

1. (b)

Polar encoding

NRZ RZ Biphase

NRZL NRZI Manchester Differential Manchester

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Disadvantage : Long consecutive uninterrupted strings of O's or 1's present. 2) Return-to-zero (RZ) encoding At any time, if the original data contain strings of consecutive 1's or

0's, the receiver can lose its place. In RZ encoding we use the values :- Positive, negative or zero. In RZ encoding 1 is represented by a positive and then signal

redeems to zero ie, 1 bit is represented by positive-to-zero. 0 is represented by a negative half bit or then signal retunes to zero.

ie, 0 bit is represented by negative to zero. In RZ encoding, the ministerial transition allows for synchronisation. Thus, RZ encoding requires two signal changes to encode 1 bit, thus

occupies more bandwidth. 3) Biphase Encoding It is the best solution to the problem of synchronisation. In this method, the signal changes at the middle of bit interval, but it

does not retun to zero. Instead, it continues to the opposite polarity. Two types of Biphase encoding are used : (i) Manchester encoding

(ii) Differential Manchester encoding.

0 1 0 0 1 1 1 0

t

t NRZ-I

NRZ-L

amplitude

Transition because next bit is 1.

NRZ – L or NRZ I Encoding.

0 1 0 0 1 1 0

t

These transitions can be used for synchronisation.

amplitede

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(i) Manchester Encoding In Manchester encoding, negative-to-positive transition represents

1 bit and a positive-to-negative transition represents 0 bit. Unlike RZ encoding, it uses only two levels of amplihede.

(ii) Differential Manchester In differential manchester encoding, the inversion at the middle

of bit interval is used for synchronisation, but the presence or absence of an additional transition at beginning of interval is used to identify the bit.

Here transition means bit 0 and no transition means bit 1. Thus, Differential Manchester requires two signal changes to

represent bit 0 but only one signal change to represent bit 1. Factors affecting the loop condition for subscribers are as follows i) Attenuation Attenuation is signal loss or it is a function of frequency.

1

df

..... as frequency increases, distance the signal can travels

decreases by square root of frequency

ii) Resistance As signals are transmitted through wires at very high frequencies, a

phenomenon called "skin effect occurs. As electricity migrates on the surface of wires, resistance increase or this

weakens the signal. Because of skin effect, services above 1GHz over wired media are not

available .

iii) Crosstalk When two adjacent wires carry signals, the signals from one wire may

enter the other wire as a result of electromagnetic radiation called "crosstalk".

Crosstalk increases as frequency increases or this is the main cause of signal degradation.

0 1 0 0 1 1 1

t

amplitude

0

t

Manchester

Differential Manchester

Manchester or Differential Manchester Encoding

2. (a)

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iv) Phase Error Phase error introduces bit errors where modulation techniques depend

on phase modulation.

v) Loaded Pairs Loaded pairs means that loading coils are placed on twisted pair for

improving performance over 5.5 km. A coil is an inductor that acts like a low pass fitter, filtering high

frequencies. Loaded pairs cannot pass frequencies above 3500 Hz and hence cannot

be used for DSL.

vi) Taps Taps are the one points on a cable bundle that are left so that technicians

will be able to easily splice off a pair to bring additional service to a home or bring service to a new home.

Taps generally cause too much distortion to be used with DSL.

Sequence of operations for Mobile originated call i) The user places the called number into an originating register of mobile unit,

checks to see that the number is correct and pushes the ‘send’ button. ii) A call initiation request is sent to the base station on the reverse control

channel. With this request the mobile unit transmits the following information. Its own telephone number (MlN) Electronic Serial Number (ESN) The telephone number of the called party and

A Station Class Mark (SCM) which indicates what the maximum power level is for the particular user.

iii) The base station receives this request and forwards it to MTSO using high-speed data link.

iv) The MTSO analyses the identification to ensure that it is proper and then analyses the called number to see if it refers to a mobile or fixed subscriber. It also instructs the Forward Control Channel of the originating base station to select the appropriate unused forward and reverse voice channels for the caller's cell-phone.

A) Sequence of operations for Call to Mobile

i) The MTSO sends a paging message containing the called mobile; telephone number [MIN] to all base stations in the cellular system.

ii) The base stations transmit this paging message on their forward set up channels.

Paging Call Accepted

2. (b)

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iii) The mobile unit recognizes its identification number and responds to the corresponding base station by identifying itself on the Reverse Control Channel.

iv) The base station relays the acknowledgement sent by the mobile to MTSO.

v) The MTSO instructs the base station to move the call to an unused voice channel pair within the cell.

vi) The base station signals the mobile to change frequencies to an unused forward and reverse voice channel pair on the FCC and switches to the best directive antenna to link to the mobile unit.

vii) A data message (called an alert) is transmitted over the forward (voice) channel to instruct the mobile telephone to ring, thereby instructing the mobile user to answer the phone.

B) Sequence of operations for Call to Fixed Subscriber

i) The MTSO routes the call to the telephone company zone office from where it is forwarded to its destination through the PSTN.

Ground Wave Propagation

The electromagnetic wave which travels along the surface of Earth called as ground wave, sometimes it is also called as surface waves. Ground waves must vertically polarized. If electric field is horizontally polarized, wave will be parallel to Earth's surface and such wave would be short circuited by the conductivity of Earth. While passing over the Earth's surface, the ground wave induces some current into it. Thus they loss some energy due to absorption (i.e. they get attenuated as they propagate). Ground waves propagates best over a surface that is good conductor, such as sea water (salt water), and poorly over dry-desert areas. Ground waves attenuated rapidly with increase in frequency. So ground wave propogation is limited to the frequency below 2MHz. Following figure shows the ground wave propagation.

Wavefront perpendicular

to Earth's surface

Excessive tilt, wavefront dies

Increasing angle of tilt

Atmos

phere

Earth

'ssu

rface

More

dens

e

Less dense

Wavefront propagation.

(30 kHz to 3 MHz)

2. (c)

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The Earth's atmosphere has gradient density i.e. it decreases gradually with distance from Earth's surface, which causes the wavefront to tilt progressively in forward direction. Therefore, the ground waves propagate around Earth, remaining close to Earth's surface and if enough power is transmitted, the wavefront could propagate beyond horizon or even around the entire circumference of Earth. The tilt angle increases with increase in frequency (>3 MHz). Increase in power does not help in increasing range at these frequencies (>3MHz).

Advantage i) With enough transmitting power ground waves can be used to communicate

between any two location in the world. ii) Ground wave are relatively unaffected by changing atmospheric condition.

Disadvantages i) It requires relatively high transmission power (>1MW). ii) Ground waves are limited to very low, low & medium frequencies requiring large antenna. iii) Ground losses vary considerably with surface material.

Applications i) In the AM radio broadcasting operating in MW band. ii) Ship to ship & ship to shore communication for radio navigation, and iii) For maritime (near the sea or of the sea) mobile communication.

Frequency Division Duplexing (FDD)

Frequency Division Duplexing (FDD) provides two simultaneous, but separate radio transmission channels for the subscriber and the base station, so that both may be able to transmit to and receive signals from each other simultaneously. A pair of simplex channels with a fixed and known frequency separation (about 5 % of the nominal RF frequency) is used to define a specific radio channel in the system. The channel used to convey traffic from the base station to the mobile user is called the forward channel. The channel used to carry traffic from the mobile user to a base station is called the reverse channel. At the base station separate transmit and receive antennas are used to handle the two separate channels. At the subscriber unit a single antenna is used for both transmission and reception from the base station. A device called the duplexer is used inside the subscriber unit to enable the same antenna to be used for simultaneous transmission and reception.

Uses : FDD is used exclusively in analog mobile radio systems.

(i) Maximum usable frequency (MUF) The maximum usable frequency is defined for a certain value of incidence angle s rather than defining it at normal as in case of critical frequency fc. The MUF is the highest frequency that can be used for sky-wave propagation between two specific points on Earth's surface.

2. (d)

2. (e)

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Maximum U.F. for a specific angle of incidence (angle between incident wave and normal) is given by

MUF = s

Critical frequencycos

MUF = fC sec s This is called as secant law

Due to instability of the ionosphere, the highest frequency used between two points is often selected lower than the MUF. It is proven that operating at a frequency 85% of MUF provides more reliable communication. This frequency is called as optimum working frequency (OWF) Following figure shows what happens when angle of incidence is kept constant and frequency is varied.

S angle of incidence

When f < MUF Wave reflected from lower point in the layer. f > MUF Wave escapes through the layer.

(ii) Skip Distance The skip distance (ds) is the minimum distance from a transmit antenna that a sky wave of given frequency (which must be less than the MUF) will be returned to Earth. Figure shows several rays with different elevation angles being radiated from the same point on Earth. It can be seen that the point where the wave is returned to Earth moves closer to the transmitter as the elevation angle () is increased. Eventually however, the angle of elevation is sufficiently high that the wave penetrates through the ionosphere and totally escapes Earth atmosphere.

c

Ionized layer

s

f < MUF

f > MUF

Earth's atmosphere

Skip distance

Refracted rays

Earth's surface

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Comparison of FDM and TDM

FDM TDM 1) Multiplexing is done in frequency

domain. Multiplexing is done in time domain.

2) Circuitry required is complex because different carrier modulator mixers and demodulators and filters of different frequencies are required.

Circuitry in TDM is not as complex as that of FDM system. Moreover, the blocks (circuits) used in TDM will be identical in design.

3) Cross-Talk problems are significant because of improper filtering

Cross-Talk problems are not significant as that of in FDM.

4) Synchronization between the transmitter and receiver is not required

Synchronization between transmitter and receiver is must.

5) Analog signals are not sampled. The analog signals are sampled 6) The linear summing amplifier is

used to obtain FDM The time division multiplexer is used to obtain TDM.

7) No synchronizing signals. Synchronization bits and frame bits are added with PCM samples

8) It uses analog modulation system. It uses digital modulation system. Block diagram of satellite communication system

A satellite system represents a microwave repeater. It receives energy beamed up at it by an earth station, amplifies it or return it

back to the earth.

The following figure shows the functional block diagram of satellite. It includes power subsystem (figure a), telemetry and control subsystem,

main or armillary propulsion subsystem (figure. b), communication channel subsystem antennas.

2. (f)

Transponder Main downlink antenna

Mainuplink

antenna

Power (Several voltages)

Figure (a)

Main rockets

Auxiliary rockets fuel fuel

Control

Sense

Control

Sense

Figure (b)

3. (a)

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Power Subsystem It is responsible for providing the dc power or regulated secondary dc voltage

for satellite circuits. (Power may be from several hundred to several thousand watts.)

Primary power originates from solar cells which converts sunlight into electricity.

The solar cell power is not used directly, instead in storage batteries that must be able to provide power for more than 12 hours of operation without recharging.

Telemetry or Control Subsystem It gives information about satellites internal circuitry, current, temperature, orbit position etc. to the ground station.

Main and Auxiliary propulsion subsystem The satellite is launched in the desired orbit by a large rocket which may be

carrying several satellites at the same time. The satellite then uses its own propulsion system to go to its final orbit,

correct for any unavoidable errors in the initial launch. Communication Channel Subsystem A transponder takes the received signal from uplink antennas, amplifies it,

down converts frequency or re-transmits to ground station receiver. Digital Subscriber Line (DSL) DSL is an entire family of technologies, which provides digital transmission

over copper wires used in local loop. The XDSL family of standards include a large variety of speed or distance

specification. High bit rate digital subscriber line (HDSL) Symmetrical or single line digital subscriber line (SDSL) Symmetrical high speed digital subscriber line (G.SHDL)

Processor Transmitter Receiver

Receiver

Control signal Sensing signal

Telemetryuplink

antenna

Telemetry downlink antenna

Solar cells

Telemetry power converter

Storage batteries

dcac Transformer dcac regulators

Voltages to all functions

Sunlight main power converter

3. (b) Vidy

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Asymmetrical Digital subscriber line (ADSL) Rate-adaptive digital subscriber line (RADSL) Very – high – bit rate Digital subscriber line (VDSL). Working principle DSL is a technique to filter out the background noise on copper wires and to

allow clear corrections through the use of DSL modems. DSL uses basic telephone line to carry digital data without interfering with

traditional voice services. DSL provides high speed internet access to residential users. DSL implementation includes two main components. (i) DSL modem It is located at the customer premises and performs the necessary

conversion between digital signals from computer or voltage signals required by the network.

The subscriber can simply play DSL filter into each telephone outlet which acts as a "splitter" whose purpose is to separate voice or data channels.

(ii) DSL Access Multiplier (DSLAM) DSLAM terminates or aggregates a large number of DSL subscriber

lines, separating voice or data traffic handling it off to proper network. Proper network refers to i) PSTN for relational circuit switched voice calls. OR, ii) Packet network for data or multimedia traffic.

Properties of DSL The performance of DSL depends on loop length as well as loop conditions. DSL modems range upto a maximum of about 3.5 miters (5.5 km). In DSL, as laitance increases, the quality of performance degrades. In DSL, if distance is shorter, the data rate is greater. Various data rate supported by various DSL standards all as summarized

below: Original ADSL 7Mbps (Over 2 km) Least ADSL 24 Mbps (Over 1.5 km) VDSL2 100 Mbps (Over 0.5 km)

Another characteristic of DSL is that it is a point-to-point connection that is always on. So when you have access to your ISP through DSL line or your computer is powered on, the connection is "ON' throughout the day. Hence, for security purposes, it is necessary to incorporate some form of firewall or security software to prevent attackers.

DSL uses line coding technique, echo cancellation, frequency splitting (to separate voice or data channels form one wire), thus providing full duplex TX over a single electrical path.

DSL also retains power in the event of power failure, if the electricity goes out, you loose high speed data services, but retain your voice services

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Troposperic Scatter Propogation It is also called as tropo scatter or forward scatter propogation, which means

beyond the horizon propogation for UHF signals. Troposphere is the nearest portion of atmosphere within about 15 km of

ground.

From the above figure, (i) Consider two directional antennas pointed such that their beams intersect

midway between them. (ii) One of these is a UHF transmitting antenna and other is a UHF Receiving

antenna. (iii) Scattering takes place due to the signal incident on a dust particle. (iv) The energy content of forward scatter is one millionth of the incident power.

Hence, a very high transmitting power is required. Time Division Duplexing (TDD)

In Time Division Duplexing (TDD) radio transmission and reception between a subscriber and a base station is established by providing adjacent time slots on a single radio channel for communication to and from the user.

If the data to transmission rate in the channel is much greater than the end user's data rate, it is possible to store information bursts and provide the appearance of full duplex operation to the user, even though they are not two simultaneous radio transmissions at any instant.

TDD is possible only with digital transmission formats and digital modulation and is very sensitive timing. Uses : TDD is suitable only for indoor and small area wireless applications where physical coverage distances (and thus radio propagation delay) arc relatively smaller.

Digital carrier Systems

Telephone companies began to provide digital services to their subscribers (users).

Scatter volume

Shortest path

Longest path

Lost scatter No scattering

Back Tropospheric Scatter Propogation

Forward scatter

Transmitter Receiver

3. (c)

3. (d)

3. (e)

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Digital services are preferred because of the advantages, that, digital services are less sensitive than analog services to noise or other form of interferences.

(i) Switched /56 Service This is the digital version of an analog switched line. It is a switched digital service that allows data rates upto 56 kbps. Between two subscribers, one needs to use another device called DSU

(Digital service unit) to use this service. Because line in switched/56 service is already digital, subscribers do not

need to use modems, to transmit digital data. DSU changes rate of digital data created by subscribers device to 56

kbps or encodes it in the format used by service provider. DSU is more expensive than a modem, The is more expensive than a modem, better quality or less susceptibility

to noise than an equivalent analog line. (ii) Digital Data Service (DDS) It is the digital version of an analog leased line, it is a digital leased line

with a maximum data rate of 64 kbps. Like switched/56, DDS also requires the use of DSU.

Digital services

Digital data service (DDS) Switehed/56

Digital signal service (DDS)

Switehed/56 Service

Telephone network

DSU

DSU

DDS Service

Telephone network

DSU

DSU Vidyala

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(iii) Digital Signal (DS) Service / Digital Hierarchy Digital signals (DS) service is a hierarchy of original signals. As shown in diagram below, DS-O service resembles DDS. It is a single

digital channel of 64 kbps. DS-1 is a 1.544 Mbps service, obtained by multiplexing 24 DS-O channel.

DS 2 is a 6.312 Mbps service, used to multiplex 4 DS1 channels. DS3 is a 44.376 Mbps service used to multiplex 7 DS2 channel. DS – 4 is a 274.176 Mbps service used to multiplex 6 DS 3 channels. (i) FM Transmitter

In direct methods of FM generation, LC oscillators are used. The LC oscillators are not stable enough for broadcast purposes, hence indirect method called “Armstrong method” is used for FM generation.

The crystal oscillator generates the carrier at low frequency upto 1 MHz. The buffer is a unity gain amplifier which presents a constant signal at the

input of combining network. The AF signal (Modulating signal) is passed through audio equaliser to

boost the low modulating frequencies. 90 phase shifted carrier and equalized audio signal is applied to

balanced modulator. It produces two sidebands such that their resultant is 90 phase shifted with respect to unmodulated carriers.

The unmodulated carrier and 90 phase shifted sidebands are added in the combining network.

The output of combining network is FM wave with low carrier frequency ‘fc’ and low value of M.I. ‘mf’.

The fc and mf is increased using the 1st group of multipliers. The carrier frequency is again raided by mixer. The fc and mf both are raised to required high values using the 2nd group

of multipliers. This signal is then given to a classC power amplifier which increases

the power level of FM signal, which is finally transmitted on air using the transmitting antenna.

4. (a)

T D M

DS3

T D M

DS1

T D M

DS2

T D M

64 kbps

DSO

24

DS4

274.176 Mbps 6DS3 or 42 DS2

44.376 Mbps 7DS2 or 28DS1 or 672 DSO

6.312 Mbps 4DS1 or 96 DSO

1.544 Mbps 24 DSO

DS Hierarchy

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The block diagram of Armstrong FM system is as shown below : (ii) Comparision of Digital pulse Modulation System

Parameter PCM DM ADM 1) No of bits per

sample N bit / sample 1 bit / sample 1 bit / sample

2) Step size Depends on the no of Q levels

Step size is fixed Step size is variable

3) Distortion / error Quantization error

Slope overload and granular noise

Granular noise

4) Signalling rate and Bandwidth

Highest Low if the input is slow varying

Lowest

5) System complexity

Complex Simple Simple

6) Feedback from output

No feedback Feedback is present

Feedback is present.

(iii) 1) Data rate : It means how fast we can send data, in bits/sec, over a

channel. The data rate depends on 3 factor i) Bandwidth available ii) Levels of signal iii) Quality of channel. 2) Bit rate : It is the number of bits/sec. 3) Baud rate It is the number of signal units per second. Note Baud rate is less than or equal to bit rate. Baud rate determines the bandwidth required to send the signal.

Crystal oscillator

Buffer Combining

network 1st group of multipliers Mixer

Crystal oscillator Balanced

modulato

Audio equaliser

90 phase shifter carrier

at 90

AF in

medium fc low mf

FM wave (very low fc, mf)

carrier only

sidebands only

equalized

2nd group of monitor

Class C power

amplifier

High fc and mf Transmitting

antenna

High fc and mf

4. (a)

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(iv) Telephone Lines Telephone carrier system is a digital carrier system used for transfer of

voice, data and video signals, having high bandwidth. T 1 lines can connect offices to internet, at speeds ranging from 1.5

mbps upto 44 Mbps. Using T1 version of T carrier, 24 calls can be carried on two pairs of

inexpensive copper wires. Transmit line Speed

Now, 24 channels 8 bits

frame channel = 192 bits/frame

Due to 1 framing bit, three will be 193 bits/frame. Transmit line speed

= 193bitframe

8000 frames/sec

= 1.544 Mbps. T – carrier system uses a process called alternate mark inversion or

bipolar encoding. T1 carrier is only 4 to 8 times more expensive than DDS but Bandwidth

is 24 times that of DDS. (i) Telephone Carrier System

A carrier system is a multi-channel telecommunication systems in which a number of individual circuits eg.: data, voice or combination of both are multiplexed for transmission between nodes of a network.

Multiplexing may be TDM or FDM. The purpose of carrier system is to save money and bandwidth. Telephone companies provide their subscribers with analog or digital

services.

(a) Analog Scotched Services It is a dial-up service used in telephone. Twisted pair cables are used to connect the subscribers handset to

network via an exchange. This connection is called "local loop". The signal on a local loop is analog or B.W = 0 to 4KHz. With scoitehed lines, the caller dials a number, the call is conveyed

to a switch at exchange. Then the appropriate switch is activated to link the caller's line to called person.

4. (a)

Telephone Services

Analog services Digital services

Switch analog services

Leased analog services

4. (b)

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(b) Analog Leased Services An analog leased service offers customers the opportunity to lease a

line, called "dedicated line", that is permanently connected to another customer.

Here no dialing is needed

Analog Hierarchy

To maximize efficiency of their infrastrchce, telephone companies have multiplexed signals from lower bandwidth lines on higher bandwidth lines.

The Analog hierarchical system is as shown below :

Calling party

Calling party

Analog switched service

Telephone network

Analog leased service

Telephone network

F D

Master group 16.984 MHz 3600

voice channels

F D

4KHz

4KHz

4KHz

Group

F D

5 gr

oups

Super group

10 S

uper

gro

ups

6 m

aste

r gr

oups

12 v

oice

cha

nnel

s

Jumbo Group

48 KHz 12 voice channels

240 KHz 60 voice channels

2.52 MHz 600 vice channels

F D

Analog Hierarchy

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In this analog hierarchy, 12 voice channels are multiplexed onto a higher bandwidth line to create a group.

At the next level, five groups are multiplexed to create a composite signal called a super group.

At the next level, 10 super groups are multiplexed to create a master group.

Finally 6 master groups can be multiplexed into a jumbo group. (ii) Polarisation

2

1

EAR

E 2

1

EAR 0

E

The polarisation of a plane em wave is simply the orientation of the electric field vector with respect to the surface of the earth.

If the polarisation remains constant, it is described as linear polarisation. Linear polarisation is divided into horizontal and vertical polarisation. If the electric field is propagating parallel to the earth's surface, the wave

is said to be horizontally polarised. If the electric field is propagating perpendicular to the earth's surface, the

wave is said to be vertically polarised. If the polarisation vector rotates 360 as the wave moves one

wavelength through space and field strength is equal to all angles of polarisation, the wave is described as having circular polarisation.

Right handed E2 = E1 Left handed Circular Polarisation AR = 1 Circular Polarisation

360 360

4. (b)

E2

z

x

y

E1 = 0

y

z

x

E2 = 0 E1

Vertical polerisation Horizontal polerisation

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With the field strength varies with changes in polarisation, this is described as elliptical polarisation.

Shadow Zone

Since space waves travels very close to ground (height of antenna is limited), any tall or massive (large sheavy) objects like hills, buildings, trees etc will block the space wave. The objects will absorb as well as scatter the energy. Therefore shadow zone [The zone or area in which space wave can not reach directly] will form as shown below.

The signal strength is very low in shadow zones. Therefore, receiving antennas needs to be taller to receive adequate amount of signal. On the other hand due to reflection from the object like hills or building "ghost" images are seen on TV. It is because of difference in the path length so in phase between direct and reflected waves. The ghosting is worse near a transmitter than at a distance because reflected waves are stronger nearby.

Problems with Delta Modulation

There are two error associated with DM 1) Slope overload error 2) Granular error

y

E2

x

E1 z

E2

x

Right handed elliptical

Polarisation

Left handed elliptical

Polarisation E2 > E1

E1

®

T

Ghost interference

Scattering

shadow zone

Refracted

Rays

5. (a)

5. (b)

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1) Slope Overload Error

As shown in figure m (t) has large slope extending over more time as compared to slope of m̂ (t). Therefore, the initial difference between m (t) and m̂ (t) goes on increasing i.e. the error m (t) m̂ (t) goes on increasing. This error is called slope overload error. To avoid slope overload error adaptive delta modulation is used in which step size gradually increases, wherever slope overload occurs.

2) Granular Error

This is also called hunting error. When the input signal m (t) is relatively constant in amplified then approximated signal m̂ (t) will hunt above or below m (t). Thus the error produce during hunting. This is called granular noise. It is similar to quantization noise in the PCM system. To reduce granular noise, step size should be as small as possible. However, this will increase the slope overload error.

Disadvantages of Unipolar encoding 1) DC component 2) Synchronisation

1) DC Component The average amplihede of a unipolar encoded signal is nonzero This creates a direct current (DC) component ie, a component with zero

frequency. When a signal contains a DC component, it cannot travel through media

that cannot handle DC components.

2) Synchronisation Since, in unipolar encoding, long uninterrupted series of 1s and 0s are

present, the receiver cannot determine the beginning and end of each bit. eg.: Series of seven 1s indicate one positive voltage level.

m (t)

m̂(t)

Granular noise

Slope Overload

Error

5. (c)

Received m (t)

Transmitted m (t)

m (t)

m t

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Whenever there is no signal change to indicate start of next bit in sequence, the receiver has to reply on a timer.

Lack of synchronisation between sender and receiver's clock distorts timing of the signal.

A solution to control synchronisation to use a separate, parallel line that carouses a clock pulse and allows the receiving device to resynchronise its time to that of signal.

But doubling the number of lines used for transmission increases the cost and so it is uneconomical.

Concept of Antenna

An antenna is generally a metallic object used to convert HF current flowing through it into electromagnetic waves or vice versa.

Functions of antenna i) It couples the transmitter output to free space or received input to the

receiver. ii) It must be capable of radiating or receiving em waves. iii) It converts high frequency current into electro magnetic waves. Microwave antennas Transmitting or receiving antennas designed for use in UHF (0.3 3 GHz)

and microwave (1 – 100 GHz) all known as microwave antennas. There are several microwave applications such as radar, all used in direction

finding or measuring field, microwave communication links, point –topoint services.

Two types of microwave antennas are : i) Dish antenna ii) Horn antenna.

i) Dish antenna

A parabolic dish antenna is a high gain reflector antenna used for radio, television and data communication.

It uses UHF or SHF band of the electromagnetic spectrum. ii) Horn antenna

Horn antenna is basically an open ended waveguide, of increasing cross-sectional area, teminated by horn.

It racliates directly in a desired direction. There are 3 configurations.

5. (d)

(a) Sectorial (b) Pyramidal (c) Conical

Configurations of HORN Antenna

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The sartorial horn flares out only in one direction. The pyramidal horn flares out in both directions or has shape of

truncated pyramid. The conical horn is the termination of a circular waveguide. There are two types of horn antenna. i) Cass horn antenna

It is a low noise antenna used in satellite tracking or communication stations.

As shown above, the radio waves are collected by large curved button surface.

These waves are reflected updated at angle of 45. After hitting the top hyperbolic surface they are reflected downward to

the focal point. ii) Hog horn antenna As shown below, hog horn antenna consists of a parabolic cylincler

joined by pyramidal horn with waves transmitted or received at the apex of horn.

It is used in satellite tracking and communication stations. Application of Horn antenna

i) Horn antennas are used at microwave frequencies at modulate power gain

ii) Horn antennas are used is satellite tracking.

primary antenna

Typical ray

Top hyperbolic surface

Bottom parabolic surface Cass-Horn Antenna

pytmidal hom

Parabolic reflector

Aperature

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A wide Area Paging System

i) Base Station : It is a fixed station in a mobile radio system, used for radio communication with mobile stations. Base stations are located at the centre or the edge of a coverage region and consist of transmitter and receiver antennas mounted on a tower.

ii) Mobile Station : It is a unit in the cellular radio service intended for use while in motion at unspecified locations. Mobile stations may be hand held personal units (portables) or installed in vehicles (mobile).

iii) Page : It represents a brief message which is broadcast over the entire service area usually in a simulcast (simultaneous broadcast) fashion by many base stations at the same time.

Paging Systems

Paging systems are communication systems that send brief messages to a subscriber. Depending on the type of service, the message may be either a numeric message, or an alpha numeric message, or a voice message. Uses Paging systems are typically used to notify a subscriber of the need to call a

particular telephone number or to travel to a known location to receive further instructions.

In modern paging systems, news headlines, stock quotations and faxes may be sent.

Wide Area Paging The figure above shows a Wide Area Paging System. Wide Area Paging systems consists of a network of telephone lines, many base station transmitters and large radio towers that simultaneously broadcast a page from each base station (simulcasting).

5. (e)

5. (f)

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Basic operation i) The page initiator uses a normal telephone set or a computer with a modem

to make the call for establishing contact with the pager user. The call is directed to the telephone exchange over the local telephone network from where it is directed to the Paging Control Centre.

ii) The Paging Control Centre dispatches pages received from the PSTN, to Paging Terminals throughout several cities at the same time.

iii) All transmitters (Paging Terminals) operate on the same frequency and all pages are sent by all transmitters in the system. Each Pager has a unique address called a capcode, which is sent every time it is paged. Transmissions addressed to other pagers are simply ignored.

iv) Pager Receiver : The receiving unit in the paging system is basically an FM radio receiver with built-in decoder and display function. The receivers are tuned to the same frequency as the transmitter. A built in decode permits the pager to get activated when its unique code is detected.

If there is a message for certain pager within the receive area, the pager gives beeping sound or it vibrates making the user alert. The User then operates the control switch to read the message, which is displayed on a small LCD screen.

Binary Phase Shift Keying In BPSK, the phase of courier is varied to represent binary one and zero. Both amplifier and frequency remains constant as phase changes.

Block Diagram

NRZ encoder converts binary data s/g (zero and one) into NRZ bipolar signal.

The courier oscillator generates sinewave carrier signal. The product modulator multiplies input data and courier which results BPSK

signal. The BPF limits the frequency band of BPSK.

Advantages i) Bandwidth less than FSK. ii) Good noise immunity. iii) Used for high bit rate than 1800 bits/sec. Disadvantage : Generation and detection of BPSK is not easy.

Application : Due to low bandwidth requirement used for high speed modern.

NRZ Encoder

Binary Frequency

BPF Product

Modulator

BPSK Digital Input

Band limited BPSK

Data Input

Carrier Oscillator

BPSK Generation

6. (a)

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(i) Elevation angle It is the angle between the horizontal plane and the pointing direction of antenna. i.e., Elevation is the angle at which the satellite is viewed from a site on earth.

Elevation angle between a straight line from satellite to an earth site and a line tangent to the earth at that site.

It is represented by E.

e

e 0

Rcos cos

R RtanE

1 cos2 cos2

Where, E = Elevation angle in degrees = S N S = Satellite long hide in degrees. N = Earth site long hide in degrees. = Earth site long hide in degrees. Re = Earth radius = 6378 km. R0 = Distance of satellite from earth = 35786 km

(ii) Azimuth Angle It is defined as angle by which the angle pointing at horizon must be

rotated clockwise around its vertical axis with respect to north. Azimuth angle refers to direction where north is equal to 0 Azimuth angle is given as,

1 tanA 180 tan

tan

where, A = Azimuth angle in degrees = S N S = Satellite longihede in degrees. N = Earth site longihede in degrees. = Earth site latihede in degrees.

E E

earth station antenna

Satellite

E = angle of elevation

O N

270

180

90

elevation angle

direction of antenna

Azimuth angle

6. (b)

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Modulation It is the process in which amplitude, frequency or phase of carrier signal is varied with respect to instantaneous amplitude of the modulating signal. In this process, the high frequency signal is called carrier signal and it will carry the modulating signal to the destination. Due to modulation the low frequency baseband signal is translated i.e. shifted into a high frequency carrier signal. Need of modulation The modulation process has the following advantages : i) Reduction in the height of antenna. ii) Avoid mixing of signals. iii) Increases the range of communication. iv) Allows multiplexing of the signals. v) Improves quality of reception. Hexagonal pattern theme The actual radio coverage of a cell is known as footprint and is determined from field measurements. Although the real footprint is amorphous in nature, a regular cell shape is needed for systematic design.

Radio coverage is ideally radial in nature; hence it would seem natural to choose the circular cell shape to represent the coverage area of a base station. However adjacent circles cannot be overlaid upon a map without leaving gaps or creating overlapping regions as illustrated in the figure alongside.

Thus, when considering geometric shapes, which cover an entire region without overlapping or leaving gaps and with equal area, there are three sensible choices, namely i) Equilateral Triangle ii) Square and iii) Hexagon

The figure below illustrates the cell patterns for each of the geometric shapes by considering : i) Base station at the centre of each cell. ii) Minimum required radiation reaches the farthest point on the perimeter of the

cell. This is taken to be the cell radius r.

6. (c)

Overlapping of Adjacent Cells

Interleaving Gaps between Adjacent Cells

6. (d)

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In general, by using Hexagon geometry, i) The fewest number of cells can cover a given geographic region and ii) We can closely approximate a circular radiation pattern which would occur

for an omni directional base station antenna and free space propagation.

Thus we conclude that hexagonal honeycomb structure [Hexagonal Cell Geometry] is the best way of achieving ideal radio coverage. Hence this shape is widely used.

Mobile phone system The three major limitations of conventional mobile systems are i) Limited service capability ii) Poor service performance and iii) Inefficient frequency spectrum utilization

i) Limited Service Capability : A conventional mobile telephone system is designed to achieve a large coverage area, by using a single high powered transmitter with an antenna mounted on a tall tower. While this approach achieved a very good coverage it still suffered form the following drawbacks: It was impossible to reuse the same frequencies throughout the system,

since any attempt to use frequency reuse would result in interference. Thus only a limited number of assigned channels were possible which allowed only a limited number of active users at any given time.

6. (e)

Block Diagram of Basic Cellular System

/PSTNLand telephone network/PSTN

Voice circuits

Switches and

processor Mobile telephone switching office

Dedicated voice grade circuits

Cell sites (Radio base station sites)

Cell #2 Cell #1

Voi

ce li

nk V

oice

link

Dat

a lin

k

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A user who initiates a call in the current coverage area has to reinitiate the call when moving outside the coverage area because the call will be dropped. In the new coverage area the user will be serviced by another high powered transmitter operating on a different set assigned channels.

Thus conventional mobile telephone system provides limited service capability in terms of

Limited number of users Limited coverage, i.e. connectivity only in the large service area and not

outside it.

ii) Poor Service Performance : In conventional mobile telephone systems, only a limited number of assigned channels were possible which allowed only limited number of active users at any given time. As the demand for mobile telephone systems grew, the number of users grew manifold. The large number of subscribers could not be supported by the limited number of channels available. This led to high blocking probability during busy hours. Hence the number of blocked calls increased giving very poor service performance to the subscribers.

iii) Inefficient frequency spectrum utilisation : In any mobile telephone

system, the frequency spectrum allocation allowed by the country's regulating body is a limited resource. Hence it should he used efficiently.

In a conventional mobile telephone system, the frequency utilisation measurement M0 is defined as the maximum number of customers that could be served by one channel at busy hour.

M0 =Number of customersNumber of channels

i.e., Number of customers = M0 Number of channels

Also, the offered load (A) by the system (in Erlangs) can be calculated as :

A = Average calling time (minutes) total customers

60 min

[Erlangs]

A = 0Average calling time (minutes) M Number of channels

60min

… (1)

Once A is calculated then from the data sheets the Blocking Probability (B) can be found for this offered load A. with N number of channels used. Higher the offered load, greater is the Blocking Probability.

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t.y. diploma : sem v prelim question paper solution...

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