K.S.O SISTEM TRANSMISI DIGITAL.

Transkripsi

1 K.S.O SISTEM TRANSMISI DIGITAL

2 OVERVEIEW In this section we develop a simple point-to-point digital transmission link design considering (Ch8-Keiser): Link power budget calculations and Link rise time calculations A link should satisfy both these budgets

3 SIMPLE POINT-TO-POINT LINK Persyaratan utama sistem link 1. Jarak transmisi yang diinginkan 2. Laju data atau lebar bandwidth 3. BER This p-p link forms the basis for examining more complex systems

4 PEMILIHAN PERANGKAT LINK OPTIK DIGITAL POINT-TO-POINT No Komponen Jenis Keterangan 1 Serat Optik Single Mode (SM) Ukuran core Multi Mode (MM) Profile indeks bias Bandwidth atau dispersi Redaman NA atau Mode-field diameter 2 Sumber Optik LED Panjang gelombang emisi Laser Lebar spektral keluaran Daya keluaran Daerah radiasi efektif Pola emisi Jumlah mode emisi 3 Detektor Optik PIN Responsivitas Avalanched Photo Diode (APD) Panjang gelombang operasi Kecepatan respons Sensitivitas

5 SYSTEM DESIGN CHOICES: PHOTODETECTOR, OPTICAL SOURCE, FIBER Photodetectors: Compared to APD, PINs are less expensive and more stable with temperature. However PINs have lower sensitivity. Optical Sources: LEDs: nm and larger than nm InGaAsP lasers: nm and ideally around nm db more power. However more costly and more complex circuitry. Fiber: Single-mode fibers are often used with lasers or edge-emitting LEDs. Multi-mode fibers are normally used with LEDs. NA and particular application. should be optimized for any

6 SELECTING THE FIBER Bit rate and distance are the major factors Other factors to consider: attenuation and distance-bandwidth product cost of the connectors, splicing etc. Then decide Multimode or single mode Step or graded index fiber

7 SELECTING THE OPTICAL SOURCE LED LASER Emission wavelength Spectral line width (FWHM) and number of modes Output power Stability Emission pattern Effective radiating area

8 SELECTING THE DETECTOR Type of detector APD: High sensitivity but complex, high bias voltage (40V or more) and expensive PIN: Simpler, thermally stable, low bias voltage (5V or less) and less expensive Responsivity (that depends on the avalanche gain & quantum efficiency) Operating wavelength and spectral selectivity Speed (capacitance) and photosensitive area Sensitivity (depends on noise and gain)

9 TYPICAL BIT RATES AT DIFFERENT WAVELENGTHS Wavelength LED Systems LASER Systems nm (Typically Multimode Fiber) 1300 nm (Lowest dispersion) 1550 nm (Lowest Attenuation) 150 Mb/s.km 2500 Mb/s.km 1500 Mb/s.km 25 Gb/s.km (InGaAsP Laser) 1200 Mb/s.km Up to 500 Gb/s.km (Best demo)

10 DESIGN CONSIDERATIONS Link Power Budget There is enough power margin in the system to meet the given BER Rise Time Budget Each element of the link is fast enough to meet the given bit rate These two budgets give necessary conditions for satisfactory operation

11 RECEIVER SENSITIVITIES VS BIT RATE Sensitivitas penerima sebagai fungsi laju bit

12 OPTICAL POWER-LOSS MODEL Keterangan: αf : Konstanta redaman fiber [db/km] lc : Loss konektor [db] lsp : Loss splice [db]

13 LINK POWER BUDGET Loss daya total: Dimana: P P P ml nl L System Margin T s R c sp f PT : Loss daya total [dbm] PS : Daya optik dipancarkan dari sumber ujung fiber [dbm] PR : Sensitivitas detektor [dbm] m : (Jumlah) konektor n : (Jumlah) splicer L : Panjang link [Km] System margin [db]

14 EXAMPLE LINK-LOSS BUDGET

15 LINK POWER BUDGET TABLE Example: [SONET OC-48 (2.5 Gb/s) link] Transmitter: 1550 nm; Receiver: InGaAs APD with -32 dbm 2.5 Gb/s; Fiber: 60 km long with 0.3 db/km attenuation; jumper cable loss 3 db each, connector loss of 1 db each. Component/loss parameter Laser output APD 2.5 Gb/s Output/sensitivi ty/loss 3 dbm -32 dbm Allowed loss 3-(-32) dbm 35 Source connector loss Jumper+Connecto r loss 1 db db 30 Cable attenuation 18 db 12 Jumper+Connecto r loss Receiver Connector loss 3+1 db 8 Power margin (db) 1 db 7(final margin)

16 RISE TIME BUDGET Untuk menentukan pembatasan dispersi link fiber optic. Total rise time depends (tsys) on: Transmitter rise time (t tx ) Group Velocity Dispersion (t GVD ) Modal dispersion rise time (t mod ) Receiver rise time (t rx ) Rise time contributor (ti) t sys n i1 Total rise time of a digital link should not exceed 70% for a NRZ bit period, and 35% of a RZ bit period t 2 i 1/ 2

17 RISE TIME BUDGET Umumnya degradasi transition time link digital: NRZ 70% perioda bit RZ 35% perioda bit Respon front end penerima dpt dimodelkan sbg LPF orde pertama: 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) : trx Brx : dlm ns : dlm MHz

18 PENGKODEAN SALURAN Format sinyal optis transmisi penting utk dipertimbangkan krn kepraktisan, sirkit decision hrs dpt memisahkan secara tepat 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 menggunakan sejumlah aturan utk mengurutkan simbol sinyal dgn pola tertentu. Jenis dasar kode saluran biner dua-level pd trans optik : (a) NRZ (b) RZ (c) Phase Encoded (PE)

19 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 krn tidak ada transisi level.

20 KODE NRZ Contoh pola data NRZ-level

21 KODE RZ - Tiap data bit dikodekan dgn dua bit kode saluran - Unipolar string 0 panjang akan kehilangan sinkronisasi timing - Biphase timing dpt diatasi - Manchester mudah mengkodekan dan dekodekan

22 KODE RZ Contoh data format RZ

23 KODE BLOK - Kode blok mbnb (n > m) : tiap m bit biner dikodekan dgn 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

24 PERBANDINGAN BEBERAPA KODE MBNB W : Pesentase n-bit word yg tidak digunakan Nmax : Jumlah simbol identik berurutan terpanjang D : Batas disparitas terakumulasi

25 LATIHAN Rancangan siskom optik laju data 60 Mb/s sbb : Jarak 60 Km Fiber SM konstanta redaman 0,2 db/km, pelebaran pulsa dispersi 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?

26 LATIHAN Suatu siskom optik memiliki spesifikasi : λ = 1,3 μm trx = 0,35 ns B = 1 Gb/s Dmat = 2 ps/(km-nm) Fiber SM panjang kabel 2 Km/haspel αf = 0,4 db/km lsp = 0,1 db/bh σλ = 3 nm lc = 1 db/bh ttx = 0,25 ns Ms = 6 db Ps = 1 mw Pr = - 42 dbm (BER 10-9) L = 60 Km twg diabaikan Selidiki apakah sistem tsb dpt digunakan utk transmisi dgn line coding RZ dan NRZ?

27 RISE TIME BUDGET t rx B rx Similarly 350/B rx ns; where is receiver bandwidth in MHz t 350 / B tx tx ns Assuming both transmitter and receiver as first order low pass filters

28 MODAL DISPERSION RISE TIME Bandwidth B M (L) due to modal dispersion of a link length L is empirically given by, B 0 B ( L) B / M is the BW per km (MHz-km product) and q ~0.5-1 is the modal equilibrium factor o L q t mod q 0.44/ BM 440L / B0 (ns)

29 GROUP VELOCITY DISPERSION t D GVD L Where, D is the dispersion parameter (ns/km/nm) given by eq. (3.57) D L σ t GVD λ is the half power spectral width of the source (nm) L is the distance in km

30 TOTAL RISE-TIME 2 1/ / mod ] [ rx q tx rx GVD tx sys B L D B L t t t t t t thesource of width [nm]:spectral )]:Dispersion. /( [ dispersion [ns]:rise-time dueto groupvelocity 0.7 thefiber; the1kmof of ]: [ thefiber of ]:Length [ BW ]:3dBElectrical [ : modaldispersion ] [ time : receiver rise ] [ time transmitter rise : ] [ 0 mod km nm ns D t q BW MHz B km L MHz B n t ns t ns tx t GVD rx rx

31 EXAMPLE: TRANSMISSION DISTANCE FOR MM- FIBER NRZ signaling, source/detector: nm LED/pin or AlGaAs laser/apd 9 combinations. BER 10 ; LED output=-13 dbm;fiber loss=3.5 db/km;fiber bandwidth 800 MHz.km; q=0.7; 1-dB connector/coupling loss at each end; 6 db system margin, material dispersion ins 0.07 ns/(km.nm); spectral width for LED=50 nm. Laser ar 850 nm spectral width=1 nm; laser ouput=0 dbm, Laser system margin=8 db;

32 PARAMETERS FOR FIG 8-5 Power coupled from LED : -13 dbm D mat = 0.07 ns/(nm.km) Power coupled from LASER = 0 dbm Fiber loss 3.5 db/km LED 50 nm LASER 1 nm Material dispersion limit with LASER is off the graph System Margin 6 db, couplers 1dB (LED-PIN) Bo=800 MHz-km q = 0.7 (modal) System Margin 8 db (Laser-APD)

33 EXAMPLE:TRANSMISSION DISTANCE FOR A SM FIBER Communication at 1550 nm, no modal dispersion, Source:Laser; Receiver:InGaAs-APD (11.5 log B dbm) and PIN (11.5log B-60.5 dbm); Fiber loss =0.3 db/km; D=2.5 ps/(km.nm): laser spectral width 1 and 3.5 nm; laser output 0 dbm,laser system margin=8 db;

34 TWO-LEVEL BINARY CHANNEL CODES

35 SYSTEM RISE-TIME & INFORMATION RATE In digital transmission system, the system rise-time limits the bit rate of the system according to the following criteria: t sys 70% of NRZ bit period t sys 35% of RZ bit period

36 EXAMPLE Laser Tx has a rise-time of 25 ps at 1550 nm and spectral width of 0.1 nm. Length of fiber is 60 km with dispersion 2 ps/(nm.km). The InGaAs APD has a 2.5 GHz BW. The rise-time budget (required) of the system for NRZ signaling is 0.28 ns whereas the total rise-time due to components is 0.14 ns. (The system is designed for 20 Mb/s).

37 ANALOG COMMUNICATION LINKS Analog (RF) links are used in Analog TV and audio services (Legacy) Cable modem services Satellite base stations

38 MULTI CHANNEL SYSTEMS Number of RF carriers can be summed and directly modulate the laser

39 MULTI CHANNEL SYSTEMS These have the capability to multiplex several RF channels Each RF channel is independent, it may carry different type of data (analog video, digital video, digital audio etc.) The data could be modulated onto the RF carrier using different techniques (AM, FM, QAM etc.) Nonlinearity is the major concern