sistem transmisi digital - ernasugesti's...
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SISTEM TRANSMISI DIGITAL
Ref : Keiser
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• Persyaratan utama sistem link :– Jarak transmisi yg diinginkan– Laju data atau lebar pita kanal– BER
Link Optik Dijital point to point
Link point to point simpleks
DETEKTOR
OPTIKSUMBER
OPTIK
SINYAL OPTIK
USER USER
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Pemilihan Perangkat Link OptikDijital point to point
Avalanched Photo Diode (APD)
ResponsivitasPanjang gelombang operasi
Kecepatan responSensitivitas
pinDetektor optik3
LASER
Panjang gelombang emisiLebar spektral keluaran
Daya keluaranDaerah radiasi efektif
Pola emisiJumlah mode emisi
LED
Sumber optik2
Multi Mode (MM)
Ukuran coreProfil indeks bias core
Bandwidth atau DispersiRedaman
NA atau Mode-field Diameter
Single Mode(SM)
Serat Optik1
KarakteristikJenisKomponenNo
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Link Power Budget
Loss daya optis pd point-to point link
αf : konstanta redaman fiber
αc: loss konektor
αsp : loss splice
αsp αsp αspαcαcαc
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Link power budget :
PR = PS – A TA T = 2 Ac + n Asp + αfL + MS
PS : daya optis dipancarkan dr sumber ujung fiber [dBm]
PR : daya diterima detektor [dBm]
AT : redaman total [dB]
A c : loss konektor [dB/bh]
A sp : loss splice [dB/bh]
αf : konstanta redaman fiber [dB/Km]
L : panjang link [Km]
MS : margin sistem [dB]
Daya diterima detektor :
Anggaran daya terpenuhi jika :
daya diterima didetektor >= sensitifitas penerima
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Sensitifitas penerima sbg fungsi laju bit.
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Link loss budget utk sistem 800 nm LED/pin pd 20 Mb/s
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Rise Time Budget
Utk menentukan pembatasan dispersi link fiber optik.
Rise time sistem keseluruhan :
∑=
=N
iisys tt
1
2
ti : rise time kontributor.
ttx : rise time sumber optik/pemancar
tmat : rise time dispersi material fiber
tmod : rise time dispersi modus fiber
twg : rise time dispersi pandu gelombang
trx : rise time detektor optik/penerima
Rise time dispersi = pelebaran pulsa krn dispersi σ
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Umumnya degradasi transition time link digital :
- NRZ ≤ 70 % perioda bit
- RZ ≤ 35 % perioda bit
Anggaran rise time dipenuhi jika :
Rise time sistem < degradasi transition time
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Respon front end penerima dpt dimodelkan sbg LPF ordepertama :
[ ] ( )tuetg tBrxπ21)( −−=
Brx : lebar pita elektrik 3 dB dr penerima
u(t) : fungsi tangga berharga 1 utk t ≥ 0 dan 0 utk t < 0
Rise time penerima (10 % - 90 %)g(t) :
rxrx B
t 350=
trx : dlm ns
Brx : dlm MHz
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Pd fiber MM rise time tergantung pd dispersi antarmodus dan material (krn dispersi pandu gel << dispersi material).
Dampak dispersi material diabaikan pd sumberlaser baik pd panj gel panjang maupun pendek, sedangkan pd LED hanya pd panj gel panjang.
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Laser/APD limit
Attenuation limit with LED/pin
Batas jarak transmisi sbg fungsi laju data 800 MHz-Km fiber, sumberLED dng Si pin dan 800 nm dioda laser dgn Si APD
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Batas jarak transmisi sbg fungsi laju data utk dioda laser DFB 1550-nm, APD InGaAs, fiber SI dgn D = 2,5 ps/(nm-Km) dan redaman 0,3 dB/Km
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Pengkodean saluran
Format sinyal optis transmisi penting utk dipertimbangkan krnkepraktisan, sirkit decision hrs dpt memisahkan secaratepat informasi timing.
Maksud timing :
(a) Memungkinkan sinyal disampling pd S/N maks
(b) Menjaga spasi pulsa
(c) Menunjukan interval start dan stop/end
Pengkodean sinyal adalah sejumlah aturan utk mengurutkansimbol sinyal dgn pola tertentu.
Jenis dasar kode saluran biner dua-level pd trans optik :
(a) NRZ
(b) RZ
(c) Phase Encoded (PE)
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Kode NRZ
- Mudah dibangkitkan/dikodekan
- Mudah di-dekodekan
- Tdk memiliki error monitoring atau kemampuan koreksi
- Ttdk memiliki self-clocking (timing)
- Lebar pita minimal
- Daya rata masukan penerima tergantung pd pola data base line wander
- String 1 atau 0 panjang tidak terdapat informasi timing krntidak ada transisi level.
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Contoh pola data NRZ-Level
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Baseline wander di penerima mrpk hasil transmisi string panjang NRZ bit 1.
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Kode RZ
- Tiap data bit dikodekan dgn dua bit kode saluran
- Unipolar string 0 panjang akan kehilangansinkronisasi timing
- Biphase timing dpt diatasi
- Manchester mudah mengkodekan dan dekodekan
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Contoh data format RZ.
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Kode Blok
-Kode blok mBnB (n > m) : tiap m bit biner dikodekandgn n bit biner.
- Peningkatan lebar pita sebesar n/m
- Timing cukup
- Terdpt informasi error minitoring
- Tidak ada string 1 atau 0 panjang tak terjadi base line wander
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Perbandingan beberapa kode mBnB
W : pesentase n-bit word yg tidak digunakan
Nmax : jumlah simbol identik berurutan terpanjang
D : batas disparitas terakumulasi
sederhana
Reduksi BW
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ContohRancangan siskom optik laju data 60 Mb/s sbb :
Jarak 60 Km
Fiber SM konstanta redaman 0,2 dB/Km, pelebaran pulsadispersi material 2 ps/Km, panjang kabel 2 Km/haspel.
Redaman splice 0,2 dB/bh
Redaman konektor 0,5 dB/bh
Sumber : daya 1 mW, rise time 5 ns
Detektor : sensitifitas – 40 dBm (BER 10-9), rise time 2 ns
Margin sistem = 6 dB
Selidiki apakah sistem tsb memenuhi anggaran daya ?
Selidiki apakah sistem tsb memenuhi anggaran rise time transmisi NRZ dan RZ ?
Kesimpulan ?
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LATIHANSuatu siskom optik memiliki spesifikasi :
λ = 1,3 μm trx = 0,25 ns
B = 1 Gb/s Dmat = 2 ps/(Km-nm)
Fiber SM panjang kabel 2 Km/haspel
αf = 0,4 dB/Km αsp = 0,1 dB/bh
σλ = 3 nm αc = 1 dB/bh
ttx = 0,2 ns Ms = 6 dB
Ps = 1 mW Prmin = - 42 dBm (BER 10-9)
L = 60 Km twg diabaikan
Selidiki apakah sistem tsb dpt digunakan utk transmisi dgnline coding RZ dan NRZ ?
Agar bisa digunakan untuk RZ berapa L maks (B = 1 Gb/s) ?
Agar bisa digunakan untuk RZ berapa B maks (L = 60 Km) ?