K.S.O
SISTEM TRANSMISI DIGITAL
[email protected]
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
SIMPLE POINT-TO-POINT LINK
This p-p link forms the basis for examining more complex systems
Persyaratan utama sistem link 1.
Jarak transmisi yang diinginkan
2.
Laju data atau lebar bandwidth
3.
BER
PEMILIHAN PERANGKAT LINK OPTIK DIGITAL POINT-TO-POINT No
Komponen
Jenis
Keterangan
1
Serat Optik
Single Mode (SM)
Ukuran core Profile indeks bias Bandwidth atau dispersi Redaman NA atau Mode-field diameter
Multi Mode (MM)
2
Sumber Optik
LED Laser
3
Detektor Optik
PIN Avalanched Photo Diode (APD)
Panjang gelombang emisi Lebar spektral keluaran Daya keluaran Daerah radiasi efektif Pola emisi Jumlah mode emisi Responsivitas Panjang gelombang operasi Kecepatan respons Sensitivitas
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: 150 (Mb/s).km @ 800-900 nm and larger than 1.5 (Gb/s).km @ 1330 nm • InGaAsP lasers: 25 (Gb/s).km @ 1330 nm and ideally around 500 (Gb/s).km @ 1550 nm. 10-15 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
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
SELECTING THE OPTICAL SOURCE •Emission wavelength
•Spectral line width (FWHM) and number of modes •Output power •Stability •Emission pattern
•Effective radiating area
LED
LASER
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)
TYPICAL BIT RATES AT DIFFERENT WAVELENGTHS Wavelength
LED Systems
LASER Systems
800-900 nm 150 Mb/s.km (Typically Multimode Fiber)
2500 Mb/s.km
1300 nm (Lowest dispersion)
1500 Mb/s.km
25 Gb/s.km (InGaAsP Laser)
1550 nm (Lowest Attenuation)
1200 Mb/s.km
Up to 500 Gb/s.km (Best demo)
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
RECEIVER SENSITIVITIES VS BIT RATE
Sensitivitas penerima sebagai fungsi laju bit
OPTICAL POWER-LOSS MODEL
Keterangan: 𝛼f lc lsp
: Konstanta redaman fiber : Loss konektor : Loss splice
[dB/Km] [dB] [dB]
LINK POWER BUDGET Loss daya total:
PT Ps PR mlc nlsp f L System Margin Dimana: PT : Loss daya total PS : Daya optik dipancarkan dari sumber ujung fiber PR : Sensitivitas detektor m : (Jumlah) konektor n : (Jumlah) splicer L : Panjang link System margin
[dBm] [dBm] [dBm]
[Km] [dB]
EXAMPLE LINK-LOSS BUDGET
LINK POWER BUDGET TABLE Example: [SONET OC-48 (2.5 Gb/s) link] Transmitter: 3dBm @ 1550 nm; Receiver: InGaAs APD with -32 dBm sensitivity @ 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
Output/sensitivi ty/loss
Power margin (dB)
Laser output
3 dBm
APD Sensitivity @ 2.5 Gb/s
-32 dBm
Allowed loss
3-(-32) dBm
35
Source connector loss
1 dB
34
Jumper+Connecto r loss
3+1 dB
30
Cable attenuation
18 dB
12
Jumper+Connecto r loss
3+1 dB
8
Receiver Connector loss
1 dB
7(final margin)
RISE TIME BUDGET Untuk menentukan pembatasan dispersi link fiber optic. Total rise time depends (tsys) on: Transmitter rise time (ttx) Group Velocity Dispersion (tGVD) Modal dispersion rise time (tmod) Receiver rise time (trx) Rise time contributor (ti)
t sys
1/ 2
2 ti i 1 n
Total rise time of a digital link should not exceed 70% for a NRZ bit period, and 35% of a RZ bit period
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 u(t)
: lebar pita elektrik 3 dB dr penerima : 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
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)
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.
KODE NRZ
Contoh pola data NRZ-level
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
KODE RZ
Contoh data format RZ
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
PERBANDINGAN BEBERAPA KODE MBNB
W
: Pesentase n-bit word yg tidak digunakan
Nmax : Jumlah simbol identik berurutan terpanjang D
: Batas disparitas terakumulasi
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 ?
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 ?
RISE TIME BUDGET trx 350 /Brx ns; where Brx is receiver bandwidth in MHz Similarly
ttx 350 / Btx ns Assuming both transmitter and receiver as first order low pass filters
MODAL DISPERSION RISE TIME Bandwidth BM(L) due to modal dispersion of a link length L is empirically given by,
BM ( L) Bo / Lq B0 is the BW per km (MHz-km product) and q ~0.5-1 is the modal equilibrium factor
tmod 0.44 / BM 440L / B0 (ns) q
GROUP VELOCITY DISPERSION tGVD | D | L Where, D is the dispersion parameter (ns/km/nm) given by eq. (3.57) tGVD | D | L
σλ is the half power spectral width of the source (nm) L is the distance in km
TOTAL RISE-TIME t sys [ttx t mod tGVD t rx ] 2
2
2
2 1/ 2
2 440 Lq 350 2 2 2 D L ttx B0 Brx 2
t tx [ ns] : transmitter rise time
t rx [ ns] : receiver rise time
Brx [ MHz ]:3dBElectrical BW L[ km ]:Length of the fiber q 0.7 D[ ns /( km.nm)]:Dispersion
2 1/ 2
t mod [ n ] : modal dispersion B0 [ MHz ]:BW of the1 km of the fiber;
tGVD [ns]: rise - time due to group velocitydispersion
[nm]: Spectral width of the source
EXAMPLE: TRANSMISSION DISTANCE FOR MMFIBER NRZ signaling, source/detector: 800-900 nm LED/pin or AlGaAs laser/APD combinations. BER 109 ; 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;
PARAMETERS FOR FIG 8-5 Power coupled from LED : -13 dBm
Fiber loss 3.5 dB/km
System Margin 6 dB, couplers 1dB (LED-PIN)
Dmat = 0.07 ns/(nm.km)
LED 50 nm LASER 1 nm
Bo=800 MHz-km q = 0.7 (modal)
Power coupled Material from LASER = 0 dispersion limit dBm with LASER is off the graph
System Margin 8 dB (Laser-APD)
EXAMPLE:TRANSMISSION DISTANCE FOR A SM FIBER Communication at 1550 nm, no modal dispersion, Source:Laser; Receiver:InGaAs-APD (11.5 log B -71.0 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;
TWO-LEVEL BINARY CHANNEL CODES
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
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).
ANALOG COMMUNICATION LINKS
Analog (RF) links are used in Analog TV and audio services (Legacy) Cable modem services Satellite base stations
MULTI CHANNEL SYSTEMS
Number of RF carriers can be summed and directly modulate the laser
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
