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NED UNIVERSITY OF ENGINEERING AND TECHNOLOGY

Department of Telecommunications Engineering

SATELLITE COMMUNICATION (LAB ASSIGNMENT)

SATELLITE ACCESS TECHNIQUES

Group Members:

Syeda Faiza Warsi TC – 058 (GL)Syeda Faiza Warsi TC – 058 (GL)

Sanam Iftikhar TC - 305 Sanam Iftikhar TC - 305

Usama Aftab TC - 054Usama Aftab TC - 054

Sukena RizviSukena Rizvi TC - 033 TC - 033

Rabia JavedRabia Javed TC - 046 TC - 046

Farheen Khan TC - 303Farheen Khan TC - 303

Submitted To:

Ms. Farzeen IqbalMs. Farzeen Iqbal

Date of Submission:Date of Submission:

1111thth October 2011 October 2011

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SATELLITE ACCESS TECHNIQUES Multiple Access (MA) refers to the general process used in communications systems in Multiple Access (MA) refers to the general process used in communications systems in which system assets (circuits, channels, transponders, etc.) are allocated to users.which system assets (circuits, channels, transponders, etc.) are allocated to users.

Goal is to allow the communications network to respond to expected changes in user Goal is to allow the communications network to respond to expected changes in user demand and adapt resources to provide the desired level of performancedemand and adapt resources to provide the desired level of performance

Satellite systems often offer benefits over terrestrial transmission alternatives for Satellite systems often offer benefits over terrestrial transmission alternatives for implementation of efficient MA because the inherent ground/space link architecture allows implementation of efficient MA because the inherent ground/space link architecture allows network asset optimization without the need to add additional nodes or other components to the network asset optimization without the need to add additional nodes or other components to the system.system.

TIME DIVISION MULTIPLE ACCESS (TDMA)TIME DIVISION MULTIPLE ACCESS (TDMA) TDMA is a channel access method for shared medium networks TDMA is a channel access method for shared medium networks Shares single carrier frequency with multiple usersShares single carrier frequency with multiple users Non-continuous transmission makes handoff simplerNon-continuous transmission makes handoff simpler Slots can be assigned on demand in dynamic TDMASlots can be assigned on demand in dynamic TDMA Flexible bit rateFlexible bit rate No need for precise narrowband filtersNo need for precise narrowband filters Less stringent power control than CDMA due to reduced ICILess stringent power control than CDMA due to reduced ICI Requires network-wide timing synchronizationRequires network-wide timing synchronization Multi-path distortionMulti-path distortion

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TDMA FRAME STRUCTURETDMA FRAME STRUCTURECTBR:CTBR: Enables receiving ES to recover remainder of burst. Contains sequence of pre Enables receiving ES to recover remainder of burst. Contains sequence of pre determined signals used for synch of bit clockdetermined signals used for synch of bit clock

UW:UW: 20 - 48 bits; acts as station identifier / burst marker; Maintains Frame Synchronization 20 - 48 bits; acts as station identifier / burst marker; Maintains Frame Synchronization

SIC:SIC: identifies Tx Station identifies Tx Station

Preamble:Preamble: Allows the receiver to acquire on top of the coarse synchronization provided by the Allows the receiver to acquire on top of the coarse synchronization provided by the reference burst a fine estimate of timing and carrier phase.reference burst a fine estimate of timing and carrier phase.

BURST TRASMISSION PLANBURST TRASMISSION PLAN A master station with visibility of the whole network maintains access discipline and A master station with visibility of the whole network maintains access discipline and scheduling.scheduling. BTP specifies the frequency; polarization of the transmissions; position: duration of each burst BTP specifies the frequency; polarization of the transmissions; position: duration of each burst in the frame; name/identification of the originating station for every transponderin the frame; name/identification of the originating station for every transponder

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Guard Period:Guard Period: Used to avoid loss and to reduce interference to the following user, caused by Used to avoid loss and to reduce interference to the following user, caused by propagation delay. Thus a user's timeslot is protected from interference from the preceding user, propagation delay. Thus a user's timeslot is protected from interference from the preceding user, by the guard interval (guard period) at the end of that preceding user's timeslot. by the guard interval (guard period) at the end of that preceding user's timeslot.

Longer Guard periods allow more distant echoes to be tolerated. However, longer guard Longer Guard periods allow more distant echoes to be tolerated. However, longer guard intervals reduce the channel efficiency.intervals reduce the channel efficiency.

NETWORK SYNCHRONISATION Open window method; Wait for UWOpen window method; Wait for UW Once detected, close detection gate Once detected, close detection gate Open detection gate before end of frameOpen detection gate before end of frame Wait for UWWait for UW if 3 times in a row correct; start operationsif 3 times in a row correct; start operations

NON-DETECTION FALSE DETECTION Probability decreases, ifProbability decreases, if Probability decreases, whenProbability decreases, when– – bit error rate of link decreasesbit error rate of link decreases – length of UW increases– length of UW increases– – length of UW decreaseslength of UW decreases – correlation threshold increases– correlation threshold increases– – correlation threshold decreasescorrelation threshold decreases

Pre-assigned TDMAEvery earth station is allotted a specific time slot

Demand assigned TDMAThe time slot are allotted to the earth station on request from the control station

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Comparison of Uplink power requirements for FDMA and TDMAComparison of Uplink power requirements for FDMA and TDMA

In TDMA , all station transmit bursts In TDMA , all station transmit bursts in the uplink producing equal power as in the uplink producing equal power as received at the satellite, therefore received at the satellite, therefore requires a higher bit rate and higher requires a higher bit rate and higher EIRP to transmit.EIRP to transmit.

In FDMA , there are modulated In FDMA , there are modulated carriers from different earth station in carriers from different earth station in form of digital or analog, so there is form of digital or analog, so there is no need to maintain power levels to no need to maintain power levels to transmit, low bit rate and low EIRP is transmit, low bit rate and low EIRP is requiredrequired

For small number of satellite FDMA For small number of satellite FDMA should be the mode of operation, should be the mode of operation, where as TDMA permit more efficient where as TDMA permit more efficient use of the satellite transponder by use of the satellite transponder by eliminating the need of backoff. eliminating the need of backoff.

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Satellite Switched TDMASatellite Switched TDMA SS-TDMA employs sequenced beam switching to add SS-TDMA employs sequenced beam switching to add an additional level of multiple access in a frequency an additional level of multiple access in a frequency translation satellite. translation satellite.

The switching is accomplished at RF or at an The switching is accomplished at RF or at an intermediate frequency (IF) and is unique to satellite intermediate frequency (IF) and is unique to satellite based systems.based systems.

Adds antenna beam switching to provide additional Adds antenna beam switching to provide additional MA capability to adapt to changing demand MA capability to adapt to changing demand requirements.requirements.

Provides fixed switch positions that increase options Provides fixed switch positions that increase options and allow the network to support varied users.and allow the network to support varied users.

3 antenna beams are used (each serving 2 ES)3 antenna beams are used (each serving 2 ES)

3x3 satellite switch matrix is the key component that permits the antenna 3x3 satellite switch matrix is the key component that permits the antenna interconnections to be made on a switched basis. interconnections to be made on a switched basis.

With three beams, six modes would be required for full interconnectivity With three beams, six modes would be required for full interconnectivity (signals carried in each beam are transferred to each of the other beams at (signals carried in each beam are transferred to each of the other beams at some time in the switching sequence).some time in the switching sequence).

UL and DL microwave frequencies are different.UL and DL microwave frequencies are different.

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Pre-Assigned FDMAPre-Assigned FDMA Frequency slots pre assigned to analog and digital Frequency slots pre assigned to analog and digital signalssignals

Pre-assignment also may be made on the basis of a Pre-assignment also may be made on the basis of a single channel per carrier (SPSC). An important feature single channel per carrier (SPSC). An important feature of the SPSC system is that on a 2-way channel, only 1 of the SPSC system is that on a 2-way channel, only 1 carrier is operatable at a time. carrier is operatable at a time.

All three earth stations (as shown) access a single All three earth stations (as shown) access a single satellite transponder channel simultaneously, and each satellite transponder channel simultaneously, and each communicates with both of the others. Thus it is communicates with both of the others. Thus it is assumed that the satellite receive and transmit antenna assumed that the satellite receive and transmit antenna beams are beams are globalglobal, encompassing all three earth , encompassing all three earth stations. Each earth station transmits one uplink carrier stations. Each earth station transmits one uplink carrier modulated with a 60-channel super group and receives modulated with a 60-channel super group and receives two similar downlink carriers.two similar downlink carriers.

Demand-Assigned FDMADemand-Assigned FDMATransponder frequency bandwidth is subdivided into a number of channelsTransponder frequency bandwidth is subdivided into a number of channels

a.a. Polling method :Polling method : A master earth station continuously polls all the earth A master earth station continuously polls all the earth stations in sequence, and if a stations in sequence, and if a call requestcall request is encountered, frequency is encountered, frequency slots are assigned from the pool of available frequencies. The polling slots are assigned from the pool of available frequencies. The polling delay with such a system tends to become excessive as the number of delay with such a system tends to become excessive as the number of participating earth stations increases.participating earth stations increases.

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b.b. Centrally controlled random access: Centrally controlled random access: earth stations may request calls through the master earth earth stations may request calls through the master earth station as the need arises. Frequencies are assigned, if available, by the master station, and when station as the need arises. Frequencies are assigned, if available, by the master station, and when the call is completed, the frequencies are returned to the pool. If no frequencies are available, the the call is completed, the frequencies are returned to the pool. If no frequencies are available, the blocked call requests may be placed in a queue, or a second call attempt may be initiated by the blocked call requests may be placed in a queue, or a second call attempt may be initiated by the requesting station.requesting station.

c. c. Distributed control Random Access:Distributed control Random Access: Control is assigned at each earth station. An example is the Control is assigned at each earth station. An example is the Spade system operated by INTELSATSpade system operated by INTELSAT

SPADE SYSTEMSPADE SYSTEM SpadeSpade is a loose acronym for SCPC pulse-code-modulated multiple-access demand-assignment is a loose acronym for SCPC pulse-code-modulated multiple-access demand-assignment equipment.equipment.

Spade was developed by Comsat for use on the INTELSAT satellites and is compatible with the Spade was developed by Comsat for use on the INTELSAT satellites and is compatible with the INTELSAT SCPC pre-assigned system INTELSAT SCPC pre-assigned system

All the earth stations are permanently connected through the CSC.All the earth stations are permanently connected through the CSC.

Each ES has a memory of the list of frequencies available and the list Each ES has a memory of the list of frequencies available and the list is constantly updated through the CSC. is constantly updated through the CSC.

The CSC also passes the signaling info from the calling to destination station. The CSC also passes the signaling info from the calling to destination station.

Each earth station has a DASS( demand assignment signaling and switching) Each earth station has a DASS( demand assignment signaling and switching) equipment for this function.equipment for this function.

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Bandwidth limited and power limited TWTA operation: Carriers can access transponder (BW=BTR) in FDMA modeCarriers can access transponder (BW=BTR) in FDMA mode If there are K carriers of band B, then best achievement would be K= BTR /B.If there are K carriers of band B, then best achievement would be K= BTR /B.

By increasing EIRP, this cant be improved, so it is band limited and the system is said to be By increasing EIRP, this cant be improved, so it is band limited and the system is said to be bandwidth-limitedbandwidth-limited. Likewise, for digital systems, the bit rate is determined by the bandwidth, which . Likewise, for digital systems, the bit rate is determined by the bandwidth, which again will be limited to some maximum value by again will be limited to some maximum value by BB TR TR.

The signal bandwidth will be approximately equal to the noise bandwidth, and if the EIRP is The signal bandwidth will be approximately equal to the noise bandwidth, and if the EIRP is below a certain level, the bandwidth will have to be correspondingly reduced to maintain the [ below a certain level, the bandwidth will have to be correspondingly reduced to maintain the [ CC/ / NN] ] at the required value. at the required value.

C/N=EIRP+(GR)-Losses-k-TS -BNC/N=EIRP+(GR)-Losses-k-TS -BN

FDMA Downlink AnalysisFDMA Downlink AnalysisConsider the overall carrier-to-noise ratio, in terms of noise power instead of noise power Consider the overall carrier-to-noise ratio, in terms of noise power instead of noise power density.density.

(C/N)(C/N)REQREQ<(C/N)<(C/N)DD

Ignoring the inter-modulation noise since it is reduced by FDMA back offIgnoring the inter-modulation noise since it is reduced by FDMA back off

Consider the effect of power limitation; the output power for each FDMA carrier is (if there Consider the effect of power limitation; the output power for each FDMA carrier is (if there are K carriers): are K carriers): EIRPEIRPDD=EIRP=EIRPSS-BO-BO00-K-K

The back off is needed here to be equal to the following to achieve the best (C/N):-The back off is needed here to be equal to the following to achieve the best (C/N):-BOBO0 0 =K(B/BTR)=K(B/BTR)

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CODE DIVISION MULTIPLE ACCESS (CDMA)CODE DIVISION MULTIPLE ACCESS (CDMA) Channel access method used by various radio communication technologiesChannel access method used by various radio communication technologies Allows several transmitters to send information over a single communication channelAllows several transmitters to send information over a single communication channel CDMA also known as Spread Spectrum Multiple Access (SSMA)CDMA also known as Spread Spectrum Multiple Access (SSMA)

StepsSteps Spreads the bandwidth of the data uniformly for Spreads the bandwidth of the data uniformly for the same transmitted powerthe same transmitted power Spreading Code is a pseudo-random codeSpreading Code is a pseudo-random code Locally generated code runs at a much higher rate Locally generated code runs at a much higher rate than the data to be transmitted i.e Rthan the data to be transmitted i.e Rchch>>R>>Rbb

FH-CDMAFH-CDMA It is the repeated switching of frequencies during radio transmission, often to minimize the It is the repeated switching of frequencies during radio transmission, often to minimize the effectiveness of the unauthorized interception or jamming of telecommunications.effectiveness of the unauthorized interception or jamming of telecommunications.

Transmits radio signals by rapidly switching a carrier among many frequency channels, using a Transmits radio signals by rapidly switching a carrier among many frequency channels, using a pseudorandom sequence known to both transmitter and receiver. pseudorandom sequence known to both transmitter and receiver.

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DS-CDMADS-CDMA DSSS phase-modulates a sine wave pseudo-randomly with a continuous string of pseudo-DSSS phase-modulates a sine wave pseudo-randomly with a continuous string of pseudo-noise (PN) code symbols called "chips", each of which has a much shorter duration than an noise (PN) code symbols called "chips", each of which has a much shorter duration than an information bit. information bit.

DSSS uses a signal structure in which the sequence of chips produced by the transmitter is DSSS uses a signal structure in which the sequence of chips produced by the transmitter is known a priori by the receiver. The receiver can then use the same PN sequence to counteract known a priori by the receiver. The receiver can then use the same PN sequence to counteract the effect of the PN sequence on the received signal in order to reconstruct the information the effect of the PN sequence on the received signal in order to reconstruct the information signal.signal.

DS-CDMA Modulation

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Spreading and De-SpreadingSpreading and De-Spreading Multiply the data being transmitted by a "noise" signal at a frequency much higher than that of Multiply the data being transmitted by a "noise" signal at a frequency much higher than that of the original signal, thereby spreading the energy of the original signal into a much wider band.the original signal, thereby spreading the energy of the original signal into a much wider band.

For de-spreading to work correctly, the transmit and receive sequences must be synchronized. For de-spreading to work correctly, the transmit and receive sequences must be synchronized.

The resulting effect of enhancing signal to noise ratio on the channel is called process gain. This The resulting effect of enhancing signal to noise ratio on the channel is called process gain. This effect can be made larger by employing a longer PN sequence and more chips per bit, but physical effect can be made larger by employing a longer PN sequence and more chips per bit, but physical devices used to generate the PN sequence impose practical limits on attainable processing gain.devices used to generate the PN sequence impose practical limits on attainable processing gain.

DSSS allows multiple transmitters to share the same channel within the limits of the cross-DSSS allows multiple transmitters to share the same channel within the limits of the cross-correlation properties of their PN sequences.correlation properties of their PN sequences.

DS-CDMA

De-Modulation