K.S.OSISTEM TRANSMISI DIGITAL

ekofajarcahyadi@st3telkom.ac.id

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

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

PEMILIHAN PERANGKAT LINK OPTIK DIGITAL POINT-TO-POINTNo 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 Panjang gelombang emisi

Lebar spektral keluaran

Daya keluaran

Daerah radiasi efektif

Pola emisi

Jumlah mode emisi

Laser

3 Detektor Optik PIN Responsivitas

Panjang gelombang operasi

Kecepatan respons

Sensitivitas

Avalanched Photo Diode (APD)

SYSTEM DESIGN CHOICES: PHOTODETECTOR, OPTICAL SOURCE, FIBERPhotodetectors:

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 should be optimized for any particular application.

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

(Typically Multimode

Fiber)

150 Mb/s.km 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 : Konstanta redaman fiber [dB/Km]

lc : Loss konektor [dB]

lsp : Loss splice [dB]

LINK POWER BUDGET

Loss daya total:

Dimana:

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]

ystem MarginT s R c sp fP P P ml nl L S

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)

1/ 2

2

1

n

sys i

i

t t

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 bitRespon 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 : dlm ns Brx : 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

MHzin bandwidth receiver is

wherens; 350

rxB

/Bt rxrx

Similarly

txtx Bt /350 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,

B0 is the BW per km (MHz-km product) and q ~0.5-1 is the modal equilibrium factor

q

oM LBLB /)(

(ns) /440/44.0 0mod BLBt q

M

GROUP VELOCITY DISPERSION

LDtGVD ||

Where,

D is the dispersion parameter (ns/km/nm) given by eq. (3.57)

σλ is the half power spectral width of the source (nm)

L is the distance in km

LDtGVD ||

TOTAL RISE-TIME

2/12

222

2

0

2

2/1222

mod

2

350440

][

rx

q

tx

rxGVDtxsys

BLD

B

Lt

ttttt

source theof width Spectral :[nm] Dispersion:)]./([

dispersion velocity group todue time-rise :[ns] 7.0

fiber; theof km 1 theof :][fiber theofLength :][BW Electrical 3dB:][

dispersion modal :][ timerisereceiver :][ timeriseer transmitt:][

0

mod

nmkmnsD

tq

BWMHzBkmLMHzB

ntnstnstxt

GVD

rx

rx

EXAMPLE: TRANSMISSION DISTANCE FOR MM-FIBER

NRZ signaling, source/detector: 800-900 nm LED/pin or AlGaAs laser/APD combinations. ; 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;

910BER

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

from LASER = 0

dBm

Material

dispersion limit

with LASER is

off the graph

System Margin 8

dB (Laser-APD)

EXAMPLE:TRANSMISSION DISTANCE FOR A SM FIBERCommunication 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:

periodbit RZ of %35

periodbit NRZ of %70

sys

sys

t

t

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