Chapter 5. Transmission System Engineering. Design the physical layer Allocate power margin for each impairment Make trade-off

Transcription

1 Chapter 5 Transmission System Engineering Design the physical layer Allocate power margin for each impairment Make trade-off 1

2 5.1 System Model Only digital systems are considered Using NRZ codes BER is the measurement factor 2

3 5.2 Power penalty Power penalty (1) The increase in Signal power required (db) to maintain the same BER in the presence of impairment. (2) The reduction in SNR due to a specific impairment (used in this course) Recall (p.260) I1 I 2 BER= Q = Q( r) (4.14) σ 0 + σ1 For PIN receiver with Gaussian noise I = RP, I = RP R( P1 P0 ) BER = Q σ 0 + σ 1 where the decision threshold is optimal I th = R ( σ P + σ P ) σ σ 0 1 (5.1) 3

4 When impairments appear, let P ', P ', σ ' andσ ' denote the received powers and noise standard deviations, at the same SNR, we have R( P1 ' P0 ') r ' = σ 0 ' + σ 1 ' The power penaltyindbis PP = 10log( r) 10log( r ') r ' = 10log r = R( P1 ' P0 ') σ ' + σ ' log (5.2) R( P1 P0 ) σ + σ 1 0 4

5 For the case of thermal noise dominant, 0 1 σ ' = σ ' = σ 0 1 th 2 2 shot m th ( PIN reveiver ) Thus noise is independent of the signal power P ' P ' 1 0 PPsig indep = 10 lo g P1 P0 For the APD receiver case. ( shot noise dominant ) Recall 1 σ = σ = σ σ = 2eG F = 10 log = 1 0 log RP 1 P 1 σ σ a : constant P P If the threshold = 0, ' ( G ) RPB (4.4 ) A m e Assume σ P σ = a P PP P σ σ RP1 ' P1 ' σ ' σ ' P ' P ' P =, ' a P ' a σ 1 σ P ' 1 PP = 5 lo g P1 Polarization plays an important role. When interfering signals havethe same state of polarization, the worst case occur. a P 5

6 6

7 5.3 Transmitter Design parameters 1. Output optical power (laser output, eye safety) 2. Rise/fall time 3. Extinction ratio (ER) 4. Modulation type 5. Side-mode suppression ratio 6. Relative intensity noise (RIN) 7. Wavelength stability and accuracy 7

8 In general the output power of a laser is about 1mw ~10mw Output power of optical amplifiers~50mw ER is defined as r = P P 1 0 P: powerof bit "1", P : powerof bit " 0 " 1 0 Foridealcase, P = 0, r = 1 average power P = P

9 Practical case Q r r = 1 0 or 2 0 or even lower 4 ~ 6 P P = P = = 0 ' P ' + P ' rp ' + P ' ' 2 2 2P 2rP P0 ' = P1 ' = r + 1 r + 1 The power penalty due to nonideal ER PP sig indep P ' P 1 0 = 1 0 lo g P1 P0 2 ( r 1) P ( r + 1) = 1 0 lo g P = 0 0 2P r 1 = 1 0 lo g r 1 + P1 ' Assume the same peak power, P1 ' = P1, P0 ' = r P ' P ' 1 0 r 1 PP = 1 0 lo g 1 0 sig indep = P1 r ' 9

10 5.4 Receiver Key system parameters: sensitivity and overload Dynamic range: P max -P sen Sensitivity is usually measured at BER = 10-12, and using a pseudo-random bit sequence. 10

11 5.5 Optical Amplifiers C-band and L-band EDFAs, Raman Amplifiers are available. EDFAs have BW=35nm at 1550nm and they can amplify multiple wavelength in a WDM system. Impairments of EDFA 1. Inducing noise 2. Nonlinear gain (depending on power) 3. Nonflat gain profile 11

12 5.5.1 Gain Saturation in EDFAs 12

13 The gain profileis approximately as G where G m ax sat m ax 1 ln (5.5 ) in : theunsaturated gain G : the saturated gain sat P : theinternal saturated gain P : theinput power in sat P : theoutput saturated power out = + P P G 1 = G m ax 3dB = G max 2 sat G max ( G 1) P = P ln in G sat 1 when G = Gout = G m ax G 2 P = GP P out in G = G P = P ln2 sat sat sat out out in 1 13

14 The saturation power (~10mw to 100mw) is proportional to pump and other parameters) Operating an EDFA in saturation has no fundamental problem Practically it is operated in saturation Gain Equalization in EDFAs The gain flatness becomes an important issue in WDM systems with cascaded amplifiers 1. Preequalization (preemphasis) 2. Equalization at each stage 14

15 15

16 5.5.3 Amplifier Cascades Let the loss between two stages = where α: attenuation coefficient l : amplifier spacing In general G e αl gain > loss sat Recall P G G = + l P e αl max 1 n (5.5) in G 16

17 Pin P out ASE At the first a few stages, the input power (signal + noise) to a stage increases as the number of stages increases, consequently, the amplifiers begin to saturate and gain drops. (Fig 5.3) 17

18 It reached steady-state condition where the amplifier output power, Pout, and gain G remain the same from stage to stage The total input power +ASE = the total output power ( αl) out n 0 P e G + 2 PB ( G 1) = Pout (5.6) αl wherepout e = thetotalinput powertothestage. Recall G G = + P P sat max 1 ln (5.5) = + n P sat 1 max ln αl out G (5.7) P e From (5.6) to (5.7) wecancomputegand P ( prob. 5.11) G G G out 18

19 αl αl Consider the case Ge = 1 G= e There are L l amplifiers (Fig 5.5) Using the equation (4.5), we have the total noise power at the output as total Pno ise = 2 PB n 0 ( G 1) L l 2 PB n 0 ( e α l = 1) L l Given a desired OSNR, OSNR P P total noise The launched power P must satisfy total P ( OSNR) Pnoise ( OSNR)2 PB n 0( e α l = 1) L l 19

20 ( P = n hf ) n sp c 20

21 5.5.4 Amplifier Spacing Penalty In a cascaded Amplifiers WDM system, G e α l If is small we may use a small gain amplifier. In this section, we will study the relation between penalty and spacing. The ASE noise power at the output of a cascade of amplifiers is L l l P tot noise αl = 2 PB n 0( e 1) L l (5.8) αl when G= e, l= lng P tot noise α = 2 PB n 0α( G 1) L (5.9) lng 21

22 Ideally when G=1 P = 0, tot noise the minimum noise power is achieved. (perfectly distributed gain) (N= NInG=αL) The power penalty for using lumped G amplifier is given PP = G > 1 lumped, 1 lng If G = 20 db, PPlumped = = = 21.5= 13.3dB ln If G = 10 db, PP = 5.9 db, lumped tot P isreducedby 7dB noise 22

23 For α = 0.25dB/km We reduce the spacing from 80km 40km However we have double the number of amplifiers Recall (4.11) page. 257 Noise figure F n =2n sp If an amplifier with F n =3.3dB is used It can be viewed as having an effective NF = 3.3dB dB = -10dB 23

24 5.5.5 Power Transients and Automatic Gain Control If some of channels fail, input power and the amplifier gain In Fig 5.7 λ 8 will be amplified unusually => receiver overloaded => We need an AGC. 24

25 (a) 25

26 (b) (c) monitoring wavelength 26

27 5.5.6 Lasing Loops In ring networks, if the amplifier gain is larger than the loss, the ring may lase. OXC ADD/Drop Lasing may occur even for a single wavelength Solutions: a. Gain is less than the loss being compensated for => degrade SNR a. No loop OXC 27

28 5.6 Crosstalk Filters, Mux/Demuxs, switches, optical amplifiers and fibers can induce crosstalk. Two kinds of crosstalk:(a) interchannel crosstalk, (b) intrachannel crosstalk (coherent crosstalk) Crosstalk results in a power penalty Intrachannel Crosstalk Causes:(a) reflection (b) leakage The penalty is high when the polarization is matched or out of phase. 28

29 In the worst case (polarization matched, out of phase) Let P be the average received signal power and P be the average crosstalk power from other signal channel. The electrical field at the receiver is E( t) = 2 Pd ( t)cos 2 πft+ ϕ ( t) [ ] [ π ϕ ] s c s + 2 Pdx( t)cos 2 ft c + x( t) whered ( t), d ( t) 0,1 s x { } ϕ ( t) and ϕ ( t) are random phases s x Assume the extinction ratio r = The received power is P = Pd ( t) + Pd ( t) + 2 Pd ( t) d ( t)cos( ϕ ( t) ϕ ( t)) + higherorderterm r s x s x s x Assume 1, andoutof phaseϕ ( t) ϕ ( t) = π cos( ϕ ( t) ϕ ( t)) = 1 s x Q 2cosα cosb = cos( α B) + cos( α + B) s x 29

30 During 1 bit d ( t ) = d ( t ) = 1 ( ) P (1) = P 1 2, P << 2 P r During 0 bit P r ( 0 ) = 0 sig indep s For the case of thermal noise dominant the power penalty is PP P ' P ' = P1 P l o g ( 5. 3 ) 1 = 1 0 lo g P1 x P ' = 1 0 lo g (1 2 ) ( 5.1 1) In amlified systems or in systems with APD the dominant noise is signal dependent PP σ P, σ σ sig dep P ' = P1 1 5 lo g ( 5. 4 ) = 5 lo g (1 2 ) ( ) If there are N channels. = N i= 1 i 30

31 31

32 5.6.2 Interchannel Crosstalk Source:leakage of filter, switches, Mux/Demux 32

33 Worst case analysis P( t) = Pd ( t) + Pd ( t) r s x Intheworstcase P (1) = P ford ( t) = 1 d ( t) = 0 P ' = P (1) r s x 1 r and P (0) = P ford ( t) = 0 d ( t) = 1 P ' = P (0) r s x 0 r PP 1 sig indep P ' P 1 0 = 10log P1 P0 ( ) ' = 10 log 1 (5.13) For optical amplified or APD systems, the effective power P ' P(1) P(0) 1 PP = =( ) sig indep P ' 1 = 5log P1 = 5log 1 ( ) P 1 33

34 5.6.3 Crosstalk in Networks Crosstalk may accumulate. 34

35 5.6.4 Bidirectional Systems The near end crosstalk is more severe than the far end crosstalk. 35

36 5.6.5 Crosstalk reduction A. For switches 1. better switch device 2. spatial dilation 3. wavelength dilation 36

37 37

38 B. For Mux/Demux add a filter between the demux and the mux 38

39 5.6.6 Cascaded Filters Required 1. wavelength stability 2. wavelength accuracy 39

40 40

41 5.7 Dispersion 1. Intermode dispersion (multimode fibers) 2. Polarization mode dispersion (imperfect core) 3. Chromatic dispersion (different wavelengths) Chromatic Dispersion Limits: NRZ Modulation Let the pulse spreading due to chromatic dispersion be a fraction of the bit period. is specified by ITU(G.957) and Telcordia(GR- 253) for 1dB and 2dB penalty 0.309and

42 42

43 Narrow Source Spectral Width For SLM DFB lasers, the unmodulated lasers λ 50MHz Ideally a directly modulated laser, λ bit rate e.g 2.5GHz for 2.5Gb/s ook (B=1/2 B e ) When chirping occurs. λ 10GHz Reducing reflection, Isolator or reducing extinction ratio can reduces λ For external modulated lasers λ 2.5 bit rate 43

44 eg., at 1.55 µ m, B= 10Gb λ= 2.5B= 0.2nm s c Recall c = λf, λ= f 2 λ= c 2 f = λ f f c λ= DLB DLB ( 2 λ ) ( λ) 2 c 2 λ 2.5B < < 0.4 c or Bλ DL < 0.4 c (5.15) ForD= 17 ps nm km 2 2 B L< 8327( Gb ) km s narow spectral sources is widely used 44

45 45

46 5.7.3 Dispersion Compensation Methods to reducing the impact of dispersion 1. External modulation (reduce chirping) 2. Small dispersion fiber 3. Dispersion compensation fiber 46

47 If 80km fiber with 17 ps/nm-km dispersion is used, we have 1360 ps/nm dispersion. Then 13.5km DCF fiber with 100 ps/nm-km can compensate the dispersion to zero as shown in Fig However DCF fiber has high loss about 0.5dB/km 0.5dB/km 13.6km=7dB Figure of merit (MOF) for DCF fiber is MOF absolute amount of dispersion per unit wavelength = loss If the DCF fiber has -100 ps/nm-km dispersion and loss = 0.5dB/km then FOM = 100 ps/nm-km 0.5dB/km = 200 ps/nm-db Larger FOM is desirable. 47

48 Chirped Fiber Bragg Gratings In a regular fiber, chromatic dispersion introduces larger delays for the lower frequency components in a pulse, we can design a chirped grating fiber with larger delays for the higher frequency components to compress the pulse. 48

49 For WDM systems, we need to use a different grating for each wavelength as shown in Fig

50 5.7.4 Polarization-Mode Dispersion (PMD) Because of the ellipticity of the fiber core, different polarizations travel with different group velocities. Polarization changes with time. So PMD varies with time. The time-averaged differential time delay is given by τ = τ D PMD : differential group delay ( DGD) L : length PMD = D L PMD parameter 0.5 ~ 2 ps km 50

51 τ is a random variable (Appendix H) For the outrage probability (PMD 1dB) is less than (accumulative outage of about 20 minutes/yer) τ = D L < 0.1T PMD wheret : bit duration Fornewlinks, D 0.1 PMD ps km 51

52 Limitations for various dispersions (PMD limitations is not significant) 52

53 5.8 Fiber nonlinearities A. Scattering a. stimulated Brillouin scattering (SBS) b. stimulated Raman scattering (SRS) B. Refractive index change a. four-wave mixing (FWM) b. self-phase modulation (SPM) (page 83) c. cross-phase modulation (CPM) (page 89) SBS, SRS, FWM transfer energy from one channel to the others SPM and CPM affect the phase of signals and cause spectral broadening => dispersion 53

54 5.8.1 Effective Length in Amplified Systems on page 79 For a link without amplifies l e = 1 e α l α In a link of length L km with amplifies and spaced l km apart l l l l The total effective length is L L e = αl 1 e L α l (5.24) 54

55 55

56 ( P = n hf ) n sp c 56

57 57

58 5.8.3 Stimulated Raman Scattering (SRS) 58

59 To calculated the effect of SRS in WDM systems. We approximate the Raman gain shape as a triangle, where the Raman gain coefficient is given by g g( ) = R 0 λ λ c if λ m c = µ w isthe peak Raman gaincoefficient. 0 otherwise λ λ nm, gr 6 10 ( at 1.55 m) λ s c λ λ 0 1 λw 1 the system bandwidth Λ = ( W 1) λs λc λ 59

60 Channel 0 is affected the most. Consider the worst case, all channels are transmitting 1 bit with the same power and no interaction between the other channels. The fraction of the power coupled from channel 0 to channel i is given by ( ) i λs PL P0 i = gr λ 2A A : effective area P : initial power c e e 60

61 The fraction of the power coupled out from channel 0 is P W 1 gr λsplew ( W 1) = P ( i) = (5.25) 2 λ A i= 1 c e The power penalty is 0.1> PP ForPP = 10log(1 P ) 0.5dB WehaveP < 0.1 g λplw ( W 1) R s e 4 λ A λ c e 0 14 WW ( 1) P sle < nmAe 6 10 m w where A e 0 40,000mw nm km = 50µ m 2 61

62 The total system bandwidth Λ = ( W 1) λs The total input power tot P = WP tot P Λ L < e 40,000mw nm km If chromatic dispersion appears, Raman scattering decreases, we have P Λ L < tot e 80,000mw nm km 62

63 To alleviate the effects of SRS, we can (1) have small λ s (2) lower P (reduce the amplifier spacing) 63

64 5.9 Wavelength Stability In general, Mux/Demux made of Silica/Silicon have coefficients of 0.01nm/ A thermistor and a thermo-electric cooler can be used to control DFB laser temperature. In addition aging may change around ±0.1nm. If high wavelength stability is needed, OPLL can be employed. 64

65 5.12 Overall Design Considerations 1. Fiber type a. Single channel high speed systems use dispersion shifted fibers which is hard to use for WDM for upgrading the link capacity in the future due to four-wave mixing. b. WDM systems use standard single-mode fibers or NZ/DSF. 2. Transmit Power and Amplifier Spacing P Λ L < tot e 80,000 mw nm km Old systems have spacing about 80 km, New systems don t have this restriction. 65

66 3. Chromatic Dispersion Compensation 4. Modulation Most systems use NRZ Ultra-long-haul systems use chirped RZ modulation 5. Nonlinearities Reducing power or having larger effective area can reduce the effect of nonlinearities. 66

67 6. Interchannel spacing and number of wavelength a. 100 GHz is common b. For loop application coarse WDM is used c. We have to consider the bandwidth of amplifiers d. Because the output power of amplifiers is limited to 20~25dBm, when the number wavelength increases, the input power per channel decreases, such that the total system span is reduced. 7. All-optical Networks 67

68 8. Wavelength Planning Typical spacing 0.8nm=100 GHz at 1.55µm 9. Transparent Bit rate, protocols, modulation formats 68