R. K. Electronics: Communication

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Friday, September 28, 2018

Reflector Antenna

Er. Ramesh Kr. Majhi September 28, 2018 0

Reflector antenna:

-reflectors are widely used to modify the radiation patterns of a radiating element.

Microwave antenna:

UHF and SHF bands are resp. 300-3000MHz, and 3000-30000MHz but the microwave region extends from 1GHz to 100 GHz. The transmitting and receiving antennas in microwave frequencies are directive with high gain and narrow BW in both horizontal and vertical planes.

-As the frequencies increases, the wavelength decreases and thus it becomes easier to construct and antenna system that are large in terms of wavelength and which therefore can be made to have greater directivity.

Types of reflectors:

(i). Rod reflector:

-mainly used in yagi-antenna

-placed behind the driven elements

- L = λ/2

- it offers inductive reactance and contributes in increasing the gain

-main disadvantage is: it alters impedance of driven element

(ii). phase reflector:

-simplest reflector to direct electromagnetic energy in desired direction

-But it is difficult to collimate the energy in forward direction

Fig: phase reflector

-In fact polarization of the primary antenna and its position with respect to the reflecting surface is used to control the pattern characteristics, impedances, power gain, and directivity of complete system

(iii). Corner reflector:

-is a reflecting object, which consists of two or three, mutually intersecting, conducting flatter surfaces

-Dihedral forms of corner reflector are frequently used in antenna

-Trihedral forms with mutually perpendicular surfaces are used as radar targets

Where,

d = aperture size, l = length

d1 = spacing between vertex and feed point location

α – angle (inclined angle)

-A corner reflector is designed to improve the collimation of electromagnetic energy in the forward direction and to eliminate radiation in back and side direction

Features:

(a). most preferred value of α = 90°

(b). d1 is increased if α is decreased and vice-versa in order to improve efficiency

(c). when α is small, gain is increased by increasing the length of sides of reflector

(d). used as passive target for radar and communication application to return the signal exactly in same direction by choosing α = 90°

-due to this unique feature, most of defense-ships and vehicles are designed with minimum sharp corners to reduce the chances of their detection by enemies’ radars.

-also used in home television antennas

# Parabolic Reflector:

-It is the reflector antenna, which has the shape of paraboloid and employs the properties of parabola

Fig: Different types of parabolic feeder

-parabolic reflectors are based on the geometric optical principles

-A parabola may be defined as the locus of a point, which moves in such a way that it’s distance from the fixed point called focus plus its distance from a straight line called directrix is constant

Fig: Geometry of parabolic reflector

The parabola is a two-direction plane curve

OF=Focal length = f

O = Vertex

F = Focus

OO´ = Axis of parabola

AB = directrix

By the definition of parabola, we have

FP + PP´ = FQ + QQ´ = FS + SS´ = constant (K)

Where K = a constant depends on the shape of parabola curve

The eqⁿ of parabola is

y²= 4 fx

-the open mouth (D) of parabola is known as Aperture

-the ration of focal length to aperture size

i.e. . (f/D) known as ‘f over D ratio’ is an important characteristic of parabolic reflector and its value usually varies between 0.25 to 0.50

Sources of Errors in Digital communication systems

sources of errors on digital communication system

Er. Ramesh Kr. Majhi September 08, 2018 0

Sources of Errors in Digital communication systems:

Noise: (I) External Noise (ii) Internal Noise
Interference: (I) Inter-image impedance (ii) Inter-flag obstruction
Distortion

1). Noise:
Boss confinement in data transmission starting with one point then onto the next is forced by, different sorts of undesirable signs, which are normally lumped together and are called commotion.

Commotions are arbitrary and eccentric flag caused by outer or inner sources.

Classification of noise

(I). Outer Noise: This commotion is produced outside the gadget or circuit. The three essential wellspring of outer commotion are:

i) Atmospheric

ii) Extraterrestrial

iii) Industrial (Man-made)

(i). Atmospheric Noise:

It is normally happening electrical aggravation that begins inside earth's environment. It is usually called electricity produced via friction and is the recognizable sputtering, snapping et cetera, regularly got notification from speaker when there is no flag display. The wellspring of most static electrically is normally happening electrical conditions, for example, helping, or tempest. Usually as motivations that spread vitality all through extensive variety of frequencies. The size of this vitality is conversely corresponding to its recurrence. At recurrence over 30MHz., in this way, air clamor is moderately inconsequential i.e. vast commotion is delivered in low and medium recurrence groups and little in VHF and UHF groups.

(ii). Extraterrestrial Noise:

It comprises of electrical signs that starts from outside earth's environment and is in this way, called profound space-clamor now and again. It begins fro, the milkyway, different universes, and the sun.

It is sub-isolated into two classifications: (a). solar (b). Cosmic

(a). solar

It is produced straightforwardly from sun's warmth. There are two sections of sun oriented commotion: a very condition when generally consistent radiation force exists and high power: sporadic unsettling influences caused by sun spot movement and sunlight based flare-ups. The greatness of the sporadic commotion caused by sun spot movement takes after a cyclic design that rehashes like clockwork.

(b). Cosmic:

Sources of this clamor are persistently dispersed all through the cosmic systems. Since sources are a long way from our sun, their commotion power is generally little. It is frequently called blackbody clamor and is dispersed genuinely even all through the sky.

(iii). Man-Made Noise:

It is just clamor delivered by humanity. Principle sources are start delivering component, for example, commutator in electrical engines, vehicle start framework, air conditioning power creating and abating gear and bright light. It is imprudent in nature and contains wide recurrence go. In more thickly populated metropolitan and mechanical territory, it is in some cases called modern commotion.

(II) Internal Noise:

It is electrical impedance created inside a gadget or circuit. There are three essential sorts of inside produced commotions:

i) shot noise

ii) Thermal noise

iii) Transit time noise

(I). Shot Noise:

- It is caused by irregular entry of transporter (gaps and electrons) at the yield component of an electronic gadget, for example, diode, FET or BJT. It was first seen in anode current of vacuum-tube intensifier and was depicted numerically by W. schottky in 1918. The present bearers (for both air conditioning and dc) are not moving in a persistent, consistent stream in light of the fact that the separation they make a trip differs because of arbitrary way of movement. Opening clamor is arbitrarily shifting and is superimposed onto any flag show when enhanced, it sounds like metal pellets falling on a tin rooftop.
- likewise called transistor commotion and is added substance with warm clamor (noise).

(ii) Thermal Noise:

It is related with fast and arbitrary movement of electrons inside a conductor because of warm disturbance.

The arbitrary development was first noted by English Botanist Robert Brown. Electron inside a conductor conveys a unit negative charge and the mean square speed of an electron is relative to the supreme temperature. Thus, each battle of an electron between impacts with atoms constitutes a short beat of current that grow little voltage over the resistive segment of the conductor. This sorts of electron development is absolutely irregular and toward all path, the normal voltage in substance because of this development is 0V dc. In any case, such a Radom development produces an air conditioner segment.

The air conditioner segment delivered from warm disturbance has a few names including warm commotion since it is temperature subordinate, Brownian clamor after its pioneer, Johnson clamor after the man who related Brownian molecule development of electron development and repetitive sound the irregular movement of frequencies.

Subsequently, warm commotion is arbitrary movement of free electron inside conductor caused by warm fomentation and temperature.

Scientifically,

Clamor control is:

N= KTB

Where, N = commotion control (watt)

B = Bandwidth

K = Boltzmann's consistent = 1.38*10-23J/K

T =Absolute temperature (K)

= Room temperature (27°c or 230K) (To change over °c to K include 273, i.e. T = °c + 273 K)

- Thermal commotion is irregular and consistent at all frequencies.

- Also warm commotion is unsurprising added substance and present in all gadgets. In this way, it is most noteworthy of all clamor.

(iii). Transit-Time Noise: (High frequency noise)

Any modification to a steam of carriers as they pass from input to output of a device (eg: emitter to collector of transistor) produces an irregular, random variation categorized as transit-time noise. When the time it takes for as carrier to propagate through a device is an appreciable part of the time of one cycle of signal, the noise becomes noticeable. In transistor, it is determined by carrier mobility, bias voltage, and transistor construction. Carrier travelling from emitter to collector suffers from emitter time delays, base transit-time delays and collector recombination-time and propagation –time delay. If transit delays are excessive at high frequencies, the device may add more noise than amplification to signal.

Other Internal Noises:

(i). Flicker noise (or low frequency noise):

A component of noise appears at frequency below few KHz known as flicker noise or low frequency noise. The spectrum density of this noise increases as the frequency decreases. In vacuum tubes, it occurs due to slow changes, which takes place in oxide structure of oxide coated cathodes and migration of impurity ions through the oxide and in semiconductor due to the fluctuation in carrier density around the junction. Fluctuation in carrier density causes fluctuation in conductivity of material, which produces a fluctuating voltage drop when dc flows. This fluctuating voltage is known as flicker noise voltage.

PSD = S(ω) ∝ 1/f

(ii). Partition noise:

- happens at whatever point current need to partition between at least two ways and there is irregular vacillations in this division. Along these lines diode are less loud than transistor.

- to stay away from parcel commotion the contributions of miniaturized scale wave collector are for the most part taken specifically to diode blends. The range of parcel clamor is level.

Distortion:

Nature of distortion in Audio and video signal

For distortion less transmission, transfer function required is

H(ω) = k e^(-jwtd)

i.e. |H(ω)| = k and

θn= -wtd

Human ear can promptly see adequacy contortion, in spite of the fact that it is heartless to stage bending. Along these lines, the producer of sound gear make accessible just |H(ω)|, the abundancy reaction normal for their framework.

→ The human eye is touchy to stage mutilation and relative harsh to abundancy twisting. Along these lines, stage twisting on TV-video flag results in a spread picture.

→ stage contortion is additionally essential in advanced correspondence framework on the grounds that the nonlinear stage qualities of a channel causes beat scattering (spread out), which thus makes beats meddle with neighboring heartbeats. The impedance can cause a mistake in beat sufficiency at recipient: a twofold I might be perused as 0, and the other way around.

→ A flag transmitted over a channel is contorted as a result of different channel blemish.

(I). Linear distortion :

Flag twisting can be caused in direct time invariant channels by no perfect attributes of either size, stage or both. This kind of bending is bothersome in TDM and FDM framework.

(ii). Non-Linear distortion:

- caused by channel non-linearity's (i.e. in non-straight channel)

- causes difficult issue in AM flag

- FM flag are not influenced

- causes genuine impedance issue in FDM frameworks (yet hot in TDM framework)

(iii). Distortion caused by multipath effects:

- A multipath transmission happens when a transmitted flag lands at the beneficiary by at least two ways of various postponements. The multipath transmission causes non-linearity in greatness and stage normal for channel and will cause direct twisting (stage scattering)

Digital Communication System

Er. Ramesh Kr. Majhi September 07, 2018 0

In simple correspondence framework, the message or data flag to be transmitted is simple in nature. Then again, in advanced correspondence framework, the message flag to be transmitted is computerized in nature.

In the present world, a noteworthy segment of correspondence is as simple framework yet these are quickly being supplanted by advanced framework. Inside the following couple of decades the greater part of the correspondence will be computerized.

Advanced correspondence framework can be utilized to exchange data from simple sources, anyway first it must be changed over to computerized shape by performing fundamental activities of inspecting, quantizing and encoding.

The utilitarian square outline of advanced correspondence framework comprises of data source, transmitter, channel, collector and goal. The transmitter and recipient are made out of practical squares combined as

1). Source encoder/decoder

2). Channel encoder/decoder

3). Modulator/demodulator

1). Data source:

Data source creates the message flag to be transmitted to the goal. For transmission of message through advanced correspondence framework, the flag must be as grouping of images happening at settled interim of time. i.e. computerized in nature . The yield of simple data sources must be changed over into advanced frame before it tends to be prepared further.

2). Source Encoder:

The source encoder expels excess data from the message flag and is in charge of the effective utilization of channel. It change over the grouping of images created by the data source into the twofold arrangement of 0s by allocating code words to every image at its information.

For the most part, source encoder utilizes FLC (Fixed Length Coding). FLC utilizes a settled number of bits to encode all the conceivable images. It is proficient just if the images happen with meet likelihood and are factually free. Case of FLC is the code word allocated to every image of print.

At the point when the images happen with unequal likelihood and are factually not free, FLC end up wasteful. Subsequently, FLC isn't utilized, rather VLC in which a square of at least two images are considered and VLC is allocated to each square of image.

The length of code word for each square of images relies upon the likelihood of event.

The yield of source encoder is known as source code word.

3). Channel Encoder:

The information stream yield by the source encoder is next prepared by the channel encoder. It adds controlled excess to the source code to deliver another steam of information bits longer than source code word. Channel coder upgrades the unwavering quality and productivity of the computerized flag transmission. The repetitive piece included by channel coder does not convey any data but rather assist the recipient with detecting and in the greater part of the cases, adjust the mistakes in the got message.

4). Modulator:

The modulator speaks to every arrangement of channel code word by a properly chose simple waveform appropriate for transmission over the correspondence channel. The modulator ought to adequately limit the impact of commotion, coordinate flag trademark with channel trademark and gives different information correspondence over the same physical channel.

5). Channel:

This is the medium through which information are transmitted starting with one then onto the next place(i.e. transmitter to collector). The station might be wired like optical fiber, co-pivotal link, phone wire and so on or might be remote. The clamor is no doubt influence the flag at the channel.

6). Demodulator:

It changes over got electrical simple waveform into succession of bits with least mistake.

7). Channel decoder:

It recoups data bearing piece succession from the bit grouping recuperated by the demodulator, by evacuating the excess piece.

8). Source decoder:

It changes over the yield of channel into the arrangement of images produced by the data source. The recouped (remade) message signals (images) are conveyed to the client of data at goal.

The plan of computerized correspondence framework is significantly impacted by the channel qualities. It endeavors to locate a limited arrangement of waveform firmly coordinated to the channel qualities and which are consequently more tolerant to the channel impedances. Once the suitable arrangement of waveform is chosen, the source data can be encoded into channel waveforms and productive transmission of data from the source the client is guaranteed. The advanced correspondence framework henceforth guarantees effective correspondence framework, which rolls out moderately shallow improvements in the message flag.

Advantages and Disadvantages of Digital Communication System

Advanced frameworks are picking up ubiquity in light of their focal points over simple framework.

The benefits of computerized correspondence frameworks are:

i) Digital correspondence framework are easier and less expensive contrasted with simple correspondence framework in view of preferences made in the IC innovation.

ii) Privacy can be protected i.e. utilizing information encryption , just allowed beneficiary might be permitted to distinguish transmitted information (generally utilized in military application)

iii) since channel encoding is utilized, the commotion does not amass from repeater to repeater in long separation correspondence

iv) By utilizing channel coding, mistake might be distinguished and remedied at the beneficiary.

v) Since transmitted flag is computerized, a lot of commotion obstruction might be endured, i.e. mistake in got and distinguished information is little, notwithstanding when there is substantial measure of commotion.

vi) In advanced correspondence, the discourse, video and other information might be consolidated and transmitted over a typical channel utilizing multiplexing (- less demanding and more effective to multiplex a few computerized signals).

vii) Digital correspondence is versatile to other propelled branches of information handling, for example, DSP , picture preparing and information handling and so on.

viii) Digital flag stockpiling is moderately simple and cheap. It likewise has capacity to seek and select data from separate electronic storage facility.

ix) Reproduction of computerized messages is to a great degree dependable without weakening.

Weaknesses:

It has a few disadvantages. Be that as it may, the focal points out weight drawbacks.

(I). Computerized correspondence requires synchronization between the transmitting and accepting framework (i.e. in synchronous tweak)

(ii). Because of simple to advanced transformation, the information rate is high. Along these lines, more/expansive transmission data transfer capacity is required.

Cellular System in mobile communication

frequency reuse

Er. Ramesh Kr. Majhi September 07, 2018 0

Concept of Cellular System

Supplanting a solitary high power transmitter (expansive cell), with many low power transmitters (little cells), each giving scope to just a little segment of the administration region each base station is allotted a part of the aggregate number of channels accessible to the whole framework, and adjacent base stations are relegated distinctive gatherings of channels so all the accessible channels are appointed to a moderately modest number of neighboring base stations. Neighboring base stations are doled out various gatherings of channels with the goal that the obstruction between base stations (and the versatile clients under their control) is limited.

By deliberately dispersing base stations and their channel bunches all through a market, the accessible channels are circulated all through the geographic district and might be reused the same number of times as essential inasmuch as the impedance between co-channel stations is kept beneath worthy levels.

As the interest for benefit increments (i.e., as more channels are required inside a specific market), the quantity of base stations might be expanded (alongside a comparing diminish in transmitter capacity to evade included obstruction), in this manner giving extra radio limit no extra increment in radio range

Concepts of Cell

A cell is the essential geographic unit of a cell framework. The term cell originates from the honeycomb state of the region into which a scope district is separated. Cells are base stations transmitting over little geographic zones that are spoken to as hexagons. Cell measure differs relying upon the land space (Cell sizes from nearly 100 m in urban areas to, e.g., 35 km for GSM framework).

Cell estimate decides number of cells accessible to cover geographic region and (with recurrence reuse) the aggregate limit accessible to all clients and the Capacity inside cell constrained by accessible transfer speed and operational prerequisites. Each system administrator needs to measure cells to deal with expected activity request.

Advantages of cell structures:

– Higher limit, higher number of clients

– Less transmission control required.

In present day cell communication, rustic and urban locales are separated into regions as indicated by particular provisioning rules. Organization parameters, for example, measure of cell-part and cell sizes, are dictated by engineers experienced in cell framework design.

Cell Geometry

Thoughtfully, an Omni-directional base station transmitter has a round scope region. a substantial topographical territory can be separated into covering round regions. On the off chance that the circles totally cover the territory (there are no "openings" in scope) and are all a similar size, they bolster the idea of hexagonal "cells, as recommended in Figure.

In real practice, be that as it may, the scope territory for a specific base station isn't round in light of the fact that the spread misfortune is influenced by common and synthetic landscape. There are three sensible decisions - a square, a symmetrical triangle, and a hexagon.

By utilizing the hexagon geometry, the least number of cells can cover a geographic district, and the hexagon nearly approximates a roundabout radiation design which would happen for an Omni-directional base station recieving wire and free space proliferation.

Base station transmitters are portrayed as either being in the focal point of the cell (focus energized cells) or on three of the six cell vertices (edge-energized cells). Regularly, Omni-directional reception apparatuses are utilized in focus energized cells and sectored directional radio wires are utilized in corner-energized cells.

Fig.: Concept of hexagonal Cells

Clusters

A bunch is a gathering of cells and no channels are reused inside a group. Accept a bunch measure k is 7, this implies the aggregate 395 voice channels are isolated into gatherings of seven.

Frequency Reuse

Each phone base station is assigned a gathering of radio channels to be utilized inside a little geographic territory called a phone.

Base stations in contiguous cells are alloted channel bunches which contain totally unexpected diverts in comparison to neighboring cells .We go for constraining the scope territory to inside the limits of a phone. The outline procedure of choosing and distributing channel bunches for the greater part of the phone base stations inside a framework is called recurrence reuse or recurrence arranging.

The genuine radio scope of a cell is known as the impression and is resolved from field estimations or engendering forecast models.

Fig.: Method of locating co-channel cells in a cellular system. In this example, N = 19 (i.e., i = 3, j = 2)

To comprehend the recurrence reuse idea, consider a cell framework which has an aggregate of S duplex channels accessible for utilize. In the event that every cell is dispensed a gathering of k channels (k < S), and if the S channels are separated among N cells into one of a kind and disjoint channel bunches which each have a similar number of channels, the aggregate number of accessible radio channels can be communicated as

S= kN

The N cells which altogether utilize the entire arrangement of accessible frequencies is known as a bunch.

In the event that a bunch is repeated M times inside the framework, the aggregate number of duplex channels, C, can be utilized as a proportion of limit and is given by

C= MkN = MS

The factor N is known as the bunch estimate and is regularly equivalent to 4, 7, or 12.

From an outline perspective, the littlest conceivable estimation of N is alluring keeping in mind the end goal to augment limit over a given scope region (i.e., to boost C).

The recurrence reuse factor of a cell framework is given by 1/N, since every cell inside a group is just doled out 1/N of the aggregate accessible diverts in the framework.

Impact of Cluster Size in Capacity:

• A portable or base station can just endure such a great amount of impedance from different cells utilizing a similar recurrence and keep up adequate quality.

 If N is lessened while the cell estimate is kept consistent, more groups are required, thus greater limit is accomplished.

 A huge group measure shows proportion between the cell range and the separation between co-channel cells is little.

 The esteem for N is an element of how much obstruction a versatile or base station can endure while keeping up an adequate nature of correspondence.

 From configuration perspective, the littlest estimation of N is alluring so as to boost limit over a given scope territory.

Advantages and disadvantages of Cellular Network

Focal points of Cellular Networks:

1-More limit because of otherworldly reuse.

2-Lower transmission control because of littler transmitter/recipient separations.

3-More vigorous framework as Base Station issue just impacts the quick cell.

4-More unsurprising proliferation condition because of shorter separations.

Drawbacks of Cellular Networks:

1-Need for more foundations.

2-Need for settled system to associate Base Stations.

3-Some leftover impedance from co-channel cells.

4-Handover strategy required.

5-High many-sided quality of the framework.

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Friday, September 28, 2018

Reflector Antenna

Saturday, September 8, 2018

Sources of Errors in Digital communication systems

Other Internal Noises:

Distortion:

Friday, September 7, 2018

Digital Communication System

Cellular System in mobile communication

Concept of Cellular System

Concepts of Cell

Advantages and disadvantages of Cellular Network

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