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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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ERROR CORRECTION PRINCIPLES - 2
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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ERROR CORRECTION PRINCIPLES - 3
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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ERROR CORRECTION PRINCIPLES - 4
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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ERROR CORRECTION PRINCIPLES - 5
Trellis Diagram, (ENCODER)
Initialstate Data flow
: path for input data = 0
: path for input data = 1
Input : 1 0 1 1
Output : 11 01 00 10
Initialstate Data flow
: path for input data = 0
: path for input data = 1
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