31 Wireless Basics

8/9/2019 31 Wireless Basics

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Gemini Communication Ltd.Gemini Communication Ltd.

www.gcl.in Gemini Communication Company Confidential

Wireless BasicsWireless Basics


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Components of a point to pointconnectivity system

1.Access network also called last mile orlocal loop is the connection between thecustomer premises to the nearest


.

2.Backbone network the connectionbetween two exchanges which may be in

kms linked either by optic cable or satellitecommunication or underground orundersea cable links


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END TO ENDCONNECTIVITY

END TO ENDEND TO END

CONNECTIVITYCONNECTIVITY


5

X1LOC

X1

X2LOC

X2

1

2

3

4

5

6

ACCES

ACCESS

BACKBONE


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Access network

Wired networka. Dial up line b. leased line


Wireless network

a. WiLL wireless local loop


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Analog vs. digital Voicetransmission

Voice is nothing but vibration at differentfrequencies.

Audible range is 20 to 20,000 Hz.


Necessary or used frequency bandwidthis just 4kHz.

Analog transmission gives good voicequality as signal is continuous butsignal is different to handle and picks upnoise easily.


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When analog signal is amplified for longdistance transmission noise also getsamplified.

Digital signal is easy to process andtransmit. The signal is regenerated at


there is not much distortion. Qualitydepends on sampling frequency.

Sampling frequency should be at leasttwice the max. frequency to get goodregeneration.


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E1 signal

Analog signal of bandwidth 4kHz issampled at a frequency of 8 kHz. This is

then coded using 8 bits per signal. So theno. of bits per second is 64,000. This64kb s si nal channel is then clubbed


together, so that 32 channel is sent as oneE1 signal. One E1 signal contains 30channels of voice and 2 channels forsignaling and synchronization. One E1channel consists of 32X64kbps=2048kbps

data, which is nothing but 2mbps data.


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Transmission modesSimplex communication- where thecommunication is one way in a line. Example mike to loud speaker

Half duplex communication one line butcommunication in one direction at a time.Example walkie talkieFull duplex communicationTDD or FDD


TDD- one frequency is used but transmission istime multiplexedFDD- two different frequencies are used buttransmission takes place at the same timeWe use TDD communication which needssynchronization if multiple modems are used.TDD is slower in data transfer rate but is

cheaper than FDD.


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Units frequently used

Resistance --- Ohms.()


---Potential difference --- Volts (V)

Power --- Watts (W)

UPS power --- (VA)


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Logarithmic measurements

Log measurements are used as it is easy to use them whencalculations are to be done. By using log terms we just add or subtractinstead of multiplying or dividing.dB = log10 (P2 / P1)


If there is an input and output value involved as in the loss of an cablethen the gain in db is mentioned.

The gain of an transmitter is mentioned in dBm. When the gain of antransmitter is mentioned then the output is always measured againstinput power of 1mW. A dBm is a standard unit for measuring levels ofpower in relation to a 1 milliwatt reference signal

The gain of an antenna is always defined as dBi Which is the gain of andirectional antenna against the gain of an isotropic antenna used in thesame configuration. ( an isotropic antenna is one which radiates equallyin all directions). dBi the forward gain of an antenna compared to thehypothetical isotropic antenna which uniformly distributes energy in alldirections.


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Threshold (sensitivity)The minimum signalwhich can be received without signaldegradation

BER--- Bit error ratio

Fade margin =Threshold-RL (receive level)

Data transmission sync / async


and a stop bit sent with every transmission. Ex.Keyboard to system, RS232

In Synchronous data transfer the clocks of both

the transmitter and receiver are synchronisedusing a PLL (phase locked loop) and datatransfer is very fast. Ex. V.35, G703


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Elements of an RF link

InterfaceCABLE

CAT5/

COAXLOS

RF


Tx/Rx Tx/Rx

ODU

Data Data

Antenna

Tower

IF

Sync/AsyncInterfaceRS232V.35G.703ETHERNET


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Stable link

To determine if a link is stable RL of thelink is calculated.


.

The higher the fade margin the morestable the link stays.


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Link Budget

LOSTx PWR PT

Rx Cable

Loss LR Rx Level RL

Free S ace


Tx/Rx Tx/RxData Data

RS232V.35

G.703

ETHERNET

TX AntennaGain GT

Tx Cable Loss LTRx Antenna

Gain GR

Loss FP

RL=PT-LT+GT-FP+GR-LR

FP=36.56+20log f(MHz)+20log d(miles)


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Frequency ranges

0-3 KHz -VLF3-300KHz -LF

- -

RF wave as a band ofelectromagnetic wave


3-30 MHz -HF

30-300 MHz -VHF

300-3000 MHz -UHF3-30 GHz -SHF

30-300 GHz -EHF


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APPLICATION824 - 849 MHz: Cell Phones

900-MHz: Cordless phones

960 -1,215 MHz: Air traffic control radar


1,227 - 1,575 MHz: GPS2290 MHz - 2300 MHz: Deep space radiocommunications

2400 2483.5 MHz: ISM Band


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Line Of Sight (LOS)

Radio transmission requires a clear path between antennasknown as radio line of sight.

Line of sight is the direct free-space path that exists between twopoints. Using binoculars on a clear day, it is easy to determine ifvisual line of sight exists between two points that are miles apart.To have a clear line of si ht there must be no obstructions


between the two locations. Often this means that the observationpoints must be high enough to allow the viewer to see over anyground-based obstructions.

The following obstructions might obscure a visual link:

Topographic features, such as mountainsThe curvature of the Earth

Buildings and other man-made objects

Trees


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Obstructions that can interfere with visual line of sight

can also interfere with radio line of sight. But one mustalso consider the Fresnel zone.The Fresnel zone for a radio beam is an elliptical area


.thickness depending on the length of the signal pathand the frequency of the signal. The necessaryclearance for the Fresnel zone can be calculated, and

it must be taken into account when designing awireless links.


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Visual Line Of Sight- FresnelZone



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Visual LOS- Fresnel Zone



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Several options to establish or improve the line of

sight:

Raise the antenna mounting point on the existingstructure


, . . ,

enough to mount the antennaIncrease the height of an existing tower

Locate a different mounting point, i.e. building or

tower, for the antennaCut down problem trees


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Different types of connectivity

P2P - Point to point connectivityMMDS Multichannel multipoint


WiMAX Worldwide interoperability forMicrowave access

31 Wireless Basics

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