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Unit- 3Modulation

In Binary Phase Shift Keying (BPSK) only one sinusoid is taken as basis function modulation.

Modulation is achieved by varying the phase of the basis function depending on the message

bits. The constellation diagram of BPSK will show the constellation points lying entirely on the x

axis. It has no projection on the y axis. This means that the BPSK modulated signal will have an

in-phase component (I) but no quadrature component (Q). This is because it has only one basis

function.

A BPSK modulator can be implemented by NRZcodingthe message bits (1 represented by +ve

voltage and 0 represented by -ve voltage) and multiplying the output by a reference oscillator

running at carrier frequency .

BPSK Modulator

BPSK Demodulation:

For BPSK demodulator, a coherent demodulator is taken as an example. In coherent detection

technique the knowledge of the carrier frequency and phase must be known to the receiver. This

can be achieved by using a Costas loop or a PLL (phase lock loop) at the receiver. A PLL

essentially locks to the incoming carrier frequency and tracks the variations in frequency and

phase. For the following simulation , neither a PLL nor a Costas loop is used but instead we

simple use the output of the PLL or Costas loop. In the demodulator the received signal is

multiplied by a reference frequency generator (assuming the PLL/Costas loop be present). The

multiplied output is integrated over one bit period using an integrator. A threshold detector

makes a decision on each integrated bit based on a threshold. Since an NRZ signaling format is

used with equal amplitudes in positive and negative direction, the threshold for this case would

be 0.


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BPSK Demodulator

MULTIPLE ACCESS TECHNIQUES

Multiple accesses is defined as the technique where in more than one pair of earth stations cansimultaneously use a satellite transponder. It is a technique used to explore the satellites geometricadvantages and is at the core of satellite networking.

These include the three familiar methods:

FDMA, TDMA, and

CDMA.Another multiple access system called space division multiple access (SDMA) has beensuggested in the past. In practice, SDMA is not really a multiple access method but rather a

technique to reuse frequency spectrum through multiple spot beams on the satellite.Because every satellite provides some form of frequency reuse (cross-polarization being

included), SDMA is an inherent feature in all applications.

FDMA:

The terminology multiple access indicates how the radio spectrum resource is intended to be used:by enabling more than one communications signal to pass within a particular band; and the

frequency division indicates how the sharing is accomplished: by allocating individual frequenciesfor each communications signal within the band.In an FDMA scheme, the given Radio Frequency (RF) bandwidth is divided into adjacent frequencysegments. Each segment is provided with bandwidth to enable an associated communications signalto pass through a transmission environment with an acceptable level of interference fromcommunications signals in adjacent frequency segments .FDMA is a channel access method used inmultiple-access protocols as a channelization protocol. FDMA gives users an individual allocation ofone or several frequency bands, or channels. Multiple Access systems coordinate access betweenmultiple users.


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The FDMA scheme allows the partitioning of a bandwidth-limited communication channel into a setof independent lower-speed channels, each of which uses its permanently assigned portion of thetotal frequency spectrum. Each frequency slot contains a unique pair of frequencies needed forSending its digital signals.

The FDMA scheme has some advantages and disadvantages. A major limitation arises from the

need for guard bands between adjacent channels in order to avert interference from adjacentchannels. These guard bands impose a practical limit on the efficiency of an FDMA system. Asecondary disadvantage is the need to control the transmitting power of earth stations in such a waythat the carrier powers at the satellite input are the same in order to avoid the capture effect. Despitethese disadvantages, FDMA is the oldest technique and would remain the most widely usedbecause of investments already made in it. Major advantages of FDMA are its simplicity andrelatively low cost in applications, particularly in multiplexing unclustered terminal groups whoseaggregate bit rate limit is not constrained.

Single Channel per CarrierAmong the various transmission schemes corresponding to different combinations of multiplexingand modulation is the single channel per carrier (SCPC) scheme. Traffic routing by this scheme isperformed according to the one-carrier-per-link principle [6]. For example, each voice (telephone)channel is independently modulated by a separate carrier and is transmitted to the satellitetransponder on an FDMA basis. A 36-MHz transponder can carry as many as 800 or more voicechannels.

TDMA:-In TDMA many earth stations in the satellite communications network use a single carrier for

transmission via the satellite transponder on a time division basis. The earth stations transmit trafficbursts in a periodic time frame which is termed as TDMA frame. The earth stations during their traffictransmission have the access to the entire bandwidth of the transmission.

Time division multiple access (TDMA) is a channel access method for shared medium networks. It

allows several users to share the same frequency channel by dividing the signal into different timeslots. The users transmit in rapid succession, one after the other, each using his own time slot. Thisallows multiple stations to share the same transmission medium (e.g. Radio frequency channel)while using only a part of its channel capacity.

TDMA is a type of Time-division multiplexing, with the special point that instead of having onetransmitter connected to one receiver, there are multiple transmitters. In the case of the uplink from amobile phone to a base station this becomes particularly difficult because the mobile phone canmove around and vary the timing advance required to make its transmission match the gap intransmission from its peers.

Features of TDMA

Shares single carrier frequency with multiple users Non-continuous transmission makes handoffsimpler Slots can be assigned on demand in dynamic TDMA Less stringent power control thanCDMA due to reduced intra cell interference.

Higher synchronization overhead than CDMA Advanced equalization may be necessary for highdata rates if the channel is "frequency selective" and creates inter-symbol interference Cell breathing(borrowing resources from adjacent cells) is more Complicated than in CDMA

Frequency/slot allocation complexityPulsating power envelop: Interference with other devices


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For satellite communication TDMA works in the following manner.TDMA systems are used incommercial satellite applications. The first system type is the classic TDMA implementationemploying a single modulated carrier occupying the full transponders bandwidth.

This system is most common for TDMA networks and is also most efficient from a capacity

standards point.Each user is allocated a specific time slot for transmission due to which overlapping is avoided.System capacity is increased as only a single carrier is present at any given time.Disadvantage is that the messages need to be stored, compressed and transmitted during one ormore specific timeslots.

At network level, all transmissions must be synchronized to avoid collision between the bursts.

CDMACDMA is a form of multiplexing and a method of multiple access to a physical medium such as aradio channel, where different users use the medium at the same time thanks to using different codesequences. In CDMA the whole bandwidth of the transponder is used all the time and signals from

the users are encoded so that information from an individual transmitter can be detected andrecovered only by properly synchronized receiving station that knows the code being used.CDMA uses a modulation technique called spread spectrum. Here all the users transmit signalssimultaneously on the multiple access schemes.(Spread Spectrum: It refers to a modulationtechnique that converts the baseband signal to a modulated signal with a spectrum bandwidth thatcovers or is spread over the band orders of magnitude larger than that normally necessary totransmit the baseband signal itself.)It could be used as a multiple access system by giving each user a unique pseudo random coderather than a unique carrier frequency or time slot.

All the users contribute to the noise back ground. To detect the desired signal in the presence of allthe interferences, the composite signal is cross-correlated with the known pseudo random numberspreading sequence.

Direct Sequence Spread Spectrum (DSSS)

Direct sequence spread spectrum is a modulation technique where the transmitted signal takes upmore bandwidth then the information signal that is being modulated.Direct sequence spread spectrum transmissions multiply the data being transmitted by a noisesignal. The noise signal is the pseudo random sequence and has a frequency much higher than thatof the original signal. It thus spreads the energy of the original signal into a much wider bandThe resultant signal appears like noise which could be reconstructed to the original signal at thereceiving end by multiplying it by the same pseudo random sequence. This process is known as de-spreading. For de-spreading to work correctly, the transmitter and receiver must be synchronized.Sometimes while sending the signal from the transmitters end, other noises like inter modulationnoise and thermal noise are transmitted to the receiver. This is also called as narrow-band

interference.

Frequency Hopping Spread Spectrum (FHSS)It is a method of transmitting radio signals by rapidly switching a carrier among many frequencychannels, using the pseudo random sequences (which are known to both transmitter and receiver).Frequency hopping spread spectrum offers three main advantages over fixed frequencytransmission techniques:


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i) Spread spectrum signals are highly resistant to narrow band interferences. The process of re-collecting a spread signal spreads out the interfering signal, causing t to recede into the background.ii) Spread spectrum signals are difficult to intercept. Frequency hopping spread spectrum signalssimply appears as an increase in the background noise to a narrow band receiver.iii) Spread spectrum transmission can share a frequency band with many types of conventionaltransmissions with minimal interference.

Interference in frequency hopping spread spectrum is caused at instants when an unwanted signalappears within the pass band of the desired signal.It can occur under following conditions:i) Transmission of other users of multiple-access channel falls

Within the range of receivers pass band.ii) Inter modulation noise can be generated due to non- linearitys of receivers channels.Interference is noise like when hopping rate is much higher than the information rate. Interference iscoherent when hopping rate is smaller than the information rate.

Data:

Datais

transmitted

in

digital

form

through

router

that

determines

the

proper

path

for

data

to

travelbetweenthenetworks,andforwardsdatapacketstothemodemalongthispath.Modem

convertsthisdigitalformofdataintoanalogform.Thefrequencyofthissignalisthenincreased

withthehelpofupconverter.Thepowerlevelofthesignalisthenamplifiedbythehighpower

amplifier[HPA]andthensenttotheantennaforthetransmission.

Computer Router MODEM UP HPA


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DataReception

The data is received by the antenna and then passes through the low noise amplifier That

amplifies the weak signal received by the antenna. This amplifies signal is then passed throughthe down converter that decreases the frequency of the signal.Now this analog signal is thenconverted to digital signal by the modem. This signal is then routed to the destination computer

by the router.

VoiceExchange:

VoiceTransmission:

LNA Down MODEM Router Computer


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Voiceistransmittedthroughrouterthatdeterminestheproperpathfordatatotravelbetweenthe

networks,and forwardsdatapacketstothemodemalongthispath.Modemconvertsthisdigital

formofdata intoanalogform.Thefrequencyofthissignal isthen increasedwiththehelpofup

converter.Thepower levelofthesignal isthenamplifiedbythehighpoweramplifier [HPA]and

thensenttotheantennaforthetransmission.

VoiceReception

Thevoicesignal isreceivedbytheantennaandthenpassesthroughthe lownoiseamplifierthat

amplifiestheweaksignalreceivedbytheantenna.Thisamplifiessignalisthenpassedthroughthe

downconverterthatdecreasesthefrequencyofthesignal.

Nowthisanalogsignalisthenconvertedtodigitalsignalbythemodem.Thissignalisthenrouted

tothedestinationbytheexchange

UpConverter

Phone Exchange MUX UP HPA

Modem

LNA Down MODEM PhoneExchange


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Theup converter contains frequency, translating circuits,which convert70MHz input signal to

signalinthefrequency,rangeof5.85GHzto6.425GHz.Theupconverterhasnominalgainof15

dB,withthenominalpowerbeing0dBm.Theupconvertercontainsfiltersforsuppressionof

localoscillator

leak

and

spurious

products.

Equalizers

compensate

for

group

delay

is

reduced

by

thefiltersandkeepamplituderesponsewithinspecifications.

HighPowerAmplifier(HPA):

ThehighpoweramplifieramplifiestheRFoutputsignalfromtheupconvertertotherequired

power level for transmission to the satellite. Amplifiers for satellite video applications are

typicallysizedintherangefrom1wattto3watt.Amplifiersinthe1to10wattrangesavailable

aresolid

state

configuration.

Traveling

wave

tube

(TWT)

amplifiers

are

available

in

configurationuptoapproximately750watt.Forpower levelsabove750Wattsklystron tube

amplifiers are used. TheHPA usually contains BPF to reject harmonics and power sampling

circuitsformonitoringtheoutputtransmitpowerandthereflectedpowerfromtheantenna.

They have also the provision for increase of power from minimum to maximum value.

Conventional tube fails tooperate satisfactorilyabove300MHzmainlydue to transmit time

effect.UHFtubestrytoovercomethetransmittimeeffectbyreducingthetubedimensions.

DownConverter:

ThedownconvertercontainsfrequencytranslatingcircuitwhichconvertsfcMHzinputsignalto

70MHz signal.Thedown converter contains compensate for groupdelay introducedby the

filtersandkeepamplituderesponsewithinspecifications.

LowNoiseAmplifier(LNA):

Thelow

noise

amplifier

provides

high

gain

and

low

noise

to

establish

high

system

G/Te.

G/Te

ratioisafigureofmeritusedtorepresentthequalityofasatellitereceiveroranearthstation.

TotalgainGbecomes the sumofantennagainGaandLNAgainGlna.Te isaneffectivenoise

temperatureattheinputofLNA.


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Atransponder(alsoTPDR,TR,XPNDR,XPDR)isanelectronicdeviceusedinwireless

communications,theworditselfisshorthandfortransmitterresponder.

Thisdeviceisprimarilyusedasaretransmitterduetothefactthatitreceivesaparticular

signalfromaparticularsource,thenitamplifies(strengthens)thesignalbeforesendingittoa

predefinedlocation.Transpondershaveanabnormallylargenumberofapplicationsinourdaily

lives.Someofthemostcommonusesare:satellitetelevision,satellitetelephony,airtraffic

controlandinautomobiles.Theyarealsoembeddedincarstoopengatesautomatically.We

shalllookatsomeoftheseapplicationslater.Firstofallitisimportanttomentionthat

transpondersareoftwogeneralvarietieswhichareactivetranspondersandpassive

transponders.

Activetransponder:Thesedevicesasthenameimplies,continuallyemitradiosignalswhichare

trackedandmonitored.Thesecanalsobeautomaticdeviceswhichstrengthenthereceived

signalsand

relay

them

to

another

location.

Thesedevicesaresofrequentlyusedthatweoftenfailtorecognizethem.Forexample,howdo

youthinklaptimesofNASCARandformulaonecarsaremonitoredsoaccurately?Wellthe

answerliesinthetransponderswhichcarshaveembeddedinthem.EachcarhasauniqueID

codewhichistransmittedasthecarmoves.Aspecialcableloopisdugintothegroundatthe

startfinishlines.Sowhenthecarszoombythefinishline,theirIDsarerecordedalongwith

theirlaptimes.Thesetimesareautomaticallydisplayedonthepositionboardalongwithsplit

times,lapsremainingandsoon.

Anotherimportantuseofactivetranspondersisinsatellitecommunications.Normallythere

arehundredsofthousandsoftinytranspondersembeddedinonesatellite.Thesereceivean

incomingsignaloverarangeoffrequencies(band),measuredinhertzandmegahertzand

retransmitthesesignalsonadifferentbandsimultaneously.Theincomingsignaloriginating

fromapointontheearth(e.g.Abroadcaster),iscalledtheuplinkandtheoutgoingsignalback

totheearthiscalledthedownlink.Thelogicbehindusingsatellitesforthispurposeissimple

asradiosignalscannotcurvealongthecurvatureoftheearth,theyaresentinastraightlineup

andreceiveddowninastraightline.Thisreducestimeofsignaldeliveryandincreasesrange.

Nowwecometothepassivetransponderwhichalthoughnotasactiveastheircounterparts

stillplayaveryimportantrole.Thesetransponderscontaininformationwhichisusedto

identifyparticularobjects.Forexamplepassivetranspondersaresometimesembeddedinour

creditcardsandonmagneticlabelsinlargestores.Thesearepairedwithactivetransponders

whichamplifyandtranscribetheinformation.


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