Access Method on Vsat

8/8/2019 Access Method on Vsat

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ACCESS METHOD on VSAT

Bogi WitjaksonoPT. Telekomunikasi Indonesia


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CODING & MODULATION


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TRANSMISSION TECHNIQUES

SourceCoding

ChannelCoding

ModulationTechniques

10111 01001110

00

01

10

11

PCMADPCMCELPVSELP

BCHREED SOLOMONVITERBITURBO CODE

BPSKQPSK8 PSK16 QAM

A/D ConversionCompressionQuality/MOS

CompensateChannel non linearity

Error detection Error Correction BW efficiency

Latency

Spectral efficiencyPower efficiency


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CHANNEL CODING

Fungsi dasar :

Error Detection, Mendeteksi kasalahan untuk selanjutnya minta kirim ulang

informasi. Digunakan pada sistem Non Latency Sensitive sistem

komunikasi terresterial.

Error Correction, Mendeteksi kesalahan dan selanjutnya langsung

membetulkan kesalahan. Digunakan pada sistem Latency Sensitive sistem

komunikasi satelit, sistem komuniaksi data yang kompleks.


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ERROR DETECTION PRINCIPLES - 1

Data = d = { d1 d2 d3 d4 }

Coded = C = { d1 d2 d3 d4 P1 P2 P3 }

Rule of coding,P1 = d1 + d2 + d3

P2 = d2 + d3 + d4

P3 = d1 + d2 + d4

No error State :

Parity Bit :

P1 + d1 + d2 + d3 = Q1 = 0

P2 + d2 + d3 + d4 = Q2 = 0

P3 + d1 + d2 + d4 = Q3 = 0

Even rule(0 for no error)


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ERROR DETECTION PRINCIPLES - 3

Combinational Circuit,

d1 d2 d3 d4 P1 P2 P3

Q1

Q2

Q3


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ERROR CORRECTION PRINCIPLES - 1

Forward Error Correction / FEC, Memberikan tambahan bit sebelum proses transmisi,

Jika terjadi error, decoder akan mendeteksi dan selanjutnyaakan membetulkan,

Macam-macam FEC, Block Code - Reed Solomon, BCH, Hamming

Digunakan untuk membetulkan burst error

Konvolusional Viterbi

Digunakan untuk membetulkan random error

Turbo Code

Digunakan untuk membetulkan random & burst error.

Masih memiliki kendala dalam hal Latency


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Konvolusi-Viterbi Coding, Proses encoding & decoding merupakan suatu rangkaian

proses yang tidak terpisah.

encoder circuit,

SR1 SR1input output

0

1 1 01

0 11

01

0

1

1

1

0

0

0

0 0 11 0


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1 10 10 11 0S3: 1 1S3: 1 1

1 00 00 01 1S2: 0 1S2: 0 11 11 00 10 1S1: 1 0S1: 1 0

1 01 10 00 0S0: 0 0S0: 0 0

TransitionState

OutputTransitionState

Output

1100

INPUTINPUT

PresentPresent

SateSate

State Truth Table,


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State Diagram,

-S0-00

-S3-11

-S1-10

-S2-01

0/00

0/11 1/11

0/01

1/00

1/10

1/01

0/10


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Trellis Diagram, (ENCODER)

Initialstate Data flow

: path for input data = 0


Input : 1 0 1 1

Output : 11 01 00 10

Initialstate Data flow



Input : 1 0 1 1

Output : 11 01 00 10


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MODULATION TECHNIQUE (1)

BPSK

( )P QE

Nb

b

o

=

2.

QPSK

( )P QE

Nb

b

o

=

2.

( )P Q

E

NSS

o

=

2.


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MODULATION TECHNIQUE (2)

16 QAM

I

Q

P C II QE

NQ

E

N

s s=

1 2 51

50 0

..

..

P C I Q EN

s=

1 2 5

0

2

..

2

0.51

=

N

EQIIICP s

P P C I P C II P C III s( ) . . . = + +

1

4

16

8

16

4

16

I II

III

CORRECT RECEPTION PROBABILITY

ERROR RECEPTION PROBABILITY


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MULTIPLE ACCESS METHOD


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Common Multiple Access Tech.

Digunakan untuk memaksimalkan penggunaan darisatellite dan elemen2 komunikasi satelit lainnya.

Metoda yang umum digunakan : FDMA = Frequency Division Multiple Access

Demand Assigned Multiple Access (DAMA) Pre-Assigned Multiple Access (PAMA)

TDMA = Time Division Multiple Access

Fixed Assigned TDMA

ALOHA & Slotted ALOHA

Dynamic Reservation

CDMA = Code Division Multiple Access


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FDMA Frequency Division Multiple Access

Sharing pada domain frequency untuk seluruh earthstation dalam penggunaan transponder. Narrow bandwidth receiver yang berdampak pada less

noise power.

Membutuhkan power transmit yang lebih kecil. Dioperasikan pada topologi jaringan STAR dan MESH. Membutuhkan backoff untuk pengendalian odd order

intermod.

Membutuhkan frequency guardbands yang dapatmenurunkan throughput Dapat mengurangi size dari earth station.


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F D M A Frequency Domain

Bandwidth dibagi menjadi beberapa channel dan digunakan

secara bersamaan.

Frequency

Tim

e

1 2 3 4 5


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FDMA Implementation Options

PAMA (Pre Assigned Multiple Access) - implies that the VSATs are pre-allocated a designated frequency. Equivalent of the terrestrial leased line

solutions, PAMA solutions use the satellite resources constantly. Consequentlythere is no call setup delay which makes them most suited for interactive dataapplications or high traffic volumes . As such PAMA is used typically to connecthigh data traffic sites within an organization. SCPC (Single Channel Per Carrier)refers to the usage of a single satellite carrier for carrying a single channel ofuser traffic. The frequency is allocated on a pre-assigned basis in case ofSCPC VSAT's. The term SCPC VSAT is often used interchangeably with PAMAVSAT.

DAMA (Demand Assigned Multiple Access) - network uses a pool of satellitechannels, which are available for use by any station in that network. Ondemand a pair of available channels are assigned so that a call can beestablished. Once the call is completed, the channels are returned to the pool

for an assignment to another call. Since the satellite resource is used only inproportion to the active circuits and their holding times, this is ideally suited forvoice traffic and data traffic in batch mode. DAMA offers point to point voice,fax, and data requirements and supports video conferencing.


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Multiple Carrier Intermodulation Products

(2f1-f2) f1 f2 (2f2-f1)

3rd order intermodulation productsfrom f1 and f2

f

(3f1-2f2) Fifth Order (3f2-2f1)


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Amplifier Transfer Characteristic

-30 -20 -10 0

Power Input Relative to Saturation (dB)

0

-10

-20

-30

-40

Pow

erOutputRelativ

etoSaturation(d

B)

Input Backoff

12 dBOnput Backoff

Saturation

1 dB Compression Point

Linear Region


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Kapasitas Transponder Pada Multiple CarrierTransmission

Intermodulasi dapat dihindari dengan back offpada TWTA / SSPA.

Frequency guard-bands yang dibutuhkan

mengurangi kapasitas dan throughput. Typical backoff adalah 3 dB untuk 2 carriers

dan hingga 10 dB untuk > 10 carriers.

Ada beberapa kendala pada Power Control.


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TDM/TDMA Time Division Multiple Access

Membutuhkan proses synchronization dan timing beacon

Membutuhkan bit Preambles untuk synchronization dandestination information.

Can require larger earth stations since each earth station mustreceive and transmit the full bandwidth of the transponder.

Pembagian waktu transmit dengan paket secara individual.

Each packet must be acquired thus requiring a large number ofpreamble bits.


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T D M A Time Domain

Frequency

Tim

e

1

2

3

1e

tc.

Beberapa channel menggunakan seluruh bandwidth ada dan

pembagian penggunaannya didasarka atas waktu penggunaan( Predictable Time Assignments )..


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TDMA Packet Structure

Slot1

Slot2

Slot3

Slot

SlotN

preamble Data Bit( information & error control)

Timing &acquisition

Destination & Source Control

time


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Contoh : Inbound TDMA -TDM

A B C D E

In-bound, downlink TDM stream to the hub

Hub

In-bound, uplink MF-TDMA VSAT bursts


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CDMA Code Division Multiple Access

Frequency

time

Channels use different codes.

Other channels cause

noise-like interference.

Time

Beberapa channels yang berbeda ditransmisikan pada

bandwidth dan waktu yang bersamaan.


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DSSS - Direct Sequence Spread Spectrum

Modulator

Transmitter

Channel

Spreading(PN) Code

Tc


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DSSS-Transmit Side

+1

-1

time

time

time+1

-1-1-1

+1 +1 +1

Traffic

(9 Kbps)

Spreading

Signal

27 Kcps

Transmitted

Signal

27 Kcps

+1 +1+1

-1 -1


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DSSS-Receiver Side

time

time+1

-1-1-1

+1 +1 +1Despreading

Signal

27 Kcps

ReceivedSignal

27 Kcps

+1 +1+1

-1 -1

+1

-1

timeRecovered

Traffic

9 Kbps

Access Method on Vsat

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