“However difficult life may seem, there is always something you can do and succeed at.”
— Stephen Hawking
Universidade de Aveiro Departamento deElectr´onica, Telecomunica¸c˜oes e Inform´atica, 2016
Christophe Daniel
Da Silva Ferven¸
ca
Multiple System Configuration for Next Generation
Optical Access Networks
M´
ultipla Configura¸
c˜
ao de Sistemas para Redes de
Acesso ´
Optico de Pr´
oxima Gera¸
c˜
ao
Universidade de Aveiro Departamento deElectr´onica, Telecomunica¸c˜oes e Inform´atica, 2016
Christophe Daniel
Da Silva Ferven¸
ca
Multiple System Configuration for Next Generation
Optical Access Networks
M´
ultipla Configura¸
c˜
ao de Sistemas para Redes de
Acesso ´
Optico de Pr´
oxima Gera¸
c˜
ao
Dissertation presented to the Univerty of Aveiro for fulfillment of the nec-essary requirements to the attainment of the degree of Master in Elec-tronic and Telecommunication Engineering, accomplished under the scien-tific orientation of Prof. Dr. Ant´onio Lu´ıs Jesus Teixeira, Dr. Ali Shahpari and Prof. Dr. M´ario Lima, of the Institute of Telecommunication (IT) of Aveiro’s University.
o j´uri / the jury
presidente / president Professor Doutor Jos´e Rodrigues Ferreira da Rocha
Professor Catedr´atico da Universidade de Aveiro (por delega¸c˜ao do(a) Reitor(a) da Universidade de Aveiro)
vogais / examiners committee Professor Doutor Ant´onio Lu´ıs de Jesus Teixeira
Professor Associado C/ Agrega¸c˜ao, Universidade de Aveiro (orientador)
Doutor Paulo S´ergio de Brito Andr´e
agradecimentos / acknowledgements
First of all, as an humble person, I am grateful for little things of life. I never understood why some people have so much and why some others are not able to see how much they have. So many people suffer from illness disabling them to fulfill their dreams. For all this people I got strengths to keep going on this journey.
To my Mother who is my source of strengths. She has been there every time, even far away I felt that she prayed for me. Without her I would not reach this accomplishment. With her patience, simplicity, advices, her way of fighting for life believing always in better days.
I want to thank all my friends, colleagues and family as well. I have made many friends those years, that I cannot count them all. I wish them the best and even if I will not see them again, I want them to know that they will be in my thoughts. I want to give a special thank to someone that truly fulfills my heart and understands me. She appeared in my life when I less expected and now part of this success is due to her. I have to thank even those who are not among us anymore. Those who crossed my life and let good memories.
I want to thank all the people from IT department who have been kind and patient with me. I was able to met some different culture from researchers and people working there. I got all the support I needed. A special thank to my co-supervisor who were always available for me and helped me producing UDWDM signal and Ph.D ´Alvaro Jos´e Caseiro de Almeida who helped me producing 4-PAM. Without them I would not be able to learn so much and make this step.
Keywords 4-Pulse Amplitude Modulation (4-PAM),Ultra Dense Wavelength Division Multiplexing (UDWDM), Video Overlay, Coexistence, Power Spectral Den-sity (PSD), Stimulated Raman Scattering (SRS), Cross-Phase Modulation (XPM), Passive Optical Network (PON)
Abstract Raman Crosstalk on Video Overlay is in focus. Coexistence is simulated in laboratory and analysed with 4-Pulse Amplitude Modulation (4-PAM) and Ultra Dense Wavelength Division Multiplexing (UDWDM) with Quaternary-Phase-Shift Keying (QPSK) and Dual Polarization - Quaternary-Quaternary-Phase-Shift Keying (DP-QPSK) signal. Rectangular (Or also known as Non-Return-to-Zero (NRZ)) pulse shaping and Nyquist pulse shaping. Power Spectral Density (PSD) is analysed as a noticeable element affecting crosstalk.
Resumo O Crosstalk de Raman na camada de v´ıdeo est´a em focus. A coexistˆencia ´
e simulada em laborat´orio e analisada com sinais 4-PAM e UDWDM com modula¸c˜ao QPSK e DP-QPSK. Pulso rectangular (mais conhecido como NRZ) e pulso de Nyquist. A densidade espectral de potˆencia ´e analisada como um elemento not´avel que afecta o Crosstalk.
Contents
Contents I List of Figures V List of Tables IX List of Acronyms XI 1 Introduction 1 1.1 Motivation . . . 1 1.2 Objectives . . . 3 1.3 Structure . . . 3 1.4 Main contributions . . . 4 2 State of Art 5 2.1 Introduction . . . 52.2 Passive Optical Networks . . . 7
2.3 History of PON Standards . . . 12
2.4 Next Generation PON 2 (NGPON2) . . . 13
2.4.1 Introduction . . . 13 2.4.2 Features . . . 14 2.4.3 Limitation . . . 15 2.4.4 Architecture . . . 15 2.4.5 Power budget . . . 17 2.4.6 Wavelength Range . . . 17
2.5 Ultra Dense Wavelength Division Multiplexing (UDWDM) . . . 17
2.5.1 Introduction . . . 17
2.5.2 Features . . . 18
2.5.3 Wavelength Range . . . 20
2.6 Video Overlay Systems . . . 20
2.6.1 Introduction . . . 20
2.6.2 Video Overlay in the 1550 nm . . . 21
2.6.3 Identification and Classification . . . 21
2.6.3.1 CATV . . . 22
2.6.3.2 DVB-S . . . 22
2.6.3.3 DVB-C . . . 22
2.6.3.5 Video Overlay Description . . . 22
2.6.4 Architecture . . . 24
2.6.5 RF-Video Channels Performance and Measurements . . . 25
3 Fiber Impairments 29 3.1 Introduction . . . 29 3.2 Linear Impairments . . . 29 3.2.1 Attenuation . . . 29 3.2.1.1 Material Absorption . . . 30 3.2.1.2 Rayleigh Scattering . . . 30 3.2.1.3 Waveguide Imperfections . . . 31 3.2.2 Chromatic Dispersion . . . 31 3.2.2.1 Material Dispersion . . . 31 3.2.2.2 Waveguide Dispersion . . . 31
3.2.2.3 Combined total Dispersion . . . 32
3.2.3 Linear Fiber Losses . . . 32
3.3 Non-Linear Impairments . . . 33
3.3.1 Kerr Nonlinearity . . . 33
3.3.1.1 Self-Phase Modulation (SPM) . . . 33
3.3.1.2 Cross-Phase Modulation (XPM) . . . 35
3.3.1.3 Four-Wave Mixing (FWM) . . . 36
3.3.2 Stimulated Light Scattering . . . 38
3.3.2.1 Stimulated Brillouin Scattering (SBS) . . . 38
3.3.2.2 Stimulated Raman Scattering (SRS) . . . 39
3.3.2.3 Raman gain . . . 47
4 Simulation and test Results 51 4.1 Power Spectral Density of baseband signal . . . 51
4.2 Raman Crosstalk . . . 54
4.3 Cross-Phase Modulation (XPM) . . . 56
5 Laboratory Results 61 5.1 Introduction . . . 61
5.2 Coexistence between NRZ and Video Overlay . . . 61
5.2.1 PSD of NRZ baseband signal . . . 61
5.2.2 Real Coexistence test between NRZ and Video Overlay . . . 62
5.3 Coexistence between UDWDM and Video Overlay . . . 68
5.3.1 Setup . . . 68
5.3.2 Real Coexistence test between UDWDM and Video Overlay . . . 70
5.4 Coexistence between PAM-4 and Video Overlay . . . 78
5.4.1 Setup . . . 78
5.4.2 PSD of 4-PAM baseband signal . . . 82
List of Figures
1.1 Revenue scenarios for GPON, XGPON and TWDM-PON for North America.[1] 1 1.2 Revenue scenarios for GPON, XGPON and TWDM-PON for Western Europe.[1] 2
1.3 Household Television evolution.[2] . . . 2
1.4 Revenue scenarios for GPON, XGPON and TWDM-PON with Fronthaul for North America.[1] . . . 3
2.1 Encoded video bit rate forecasts. [3] . . . 6
2.2 Typical architecture of PON. [4] . . . 8
2.3 TDM-PON non-standard protocol. . . 9
2.4 WDM-PON non-standard protocol. . . 9
2.5 a) PON with passive optical splitter; b) PON with AWG; . . . 10
2.6 AON Network Architecture. . . 11
2.7 Wavelength range with Gigabit Passive Optical Network (GPON), Video Over-lay and 10 Gigabit-Capable Passive Optical Network (XGPON). . . 12
2.8 Bit rate and reach comparison of Next Generation Optical Network 2 (NG-PON2) and previous technologies to the premises.[5] . . . 15
2.9 Architecture of Time and Wavelength Division Multiplexing (TWDM)-Passive Optical Network (PON) . . . 16
2.10 Upgrading an existing GPON network - Modular TWDM line card approach. [1] . . . 16
2.11 Wavelength plan representation.[6] . . . 17
2.12 PSD of 1.25 and 2.5 Gbps NRZ and 1.25 Gbaud Nyquist DQPSK.[6] . . . 19
2.13 Crosstalk of Stimulated Raman Scattering (SRS) on Video Overlay.[Adapted from [6]] . . . 20
2.14 Typical Analog Quadrature Amplitude Modulation (QAM) scheme.[2] . . . . 23
2.15 General deployment Scenario.[2] . . . 25
2.16 Edge modulation Video Serving Office (VSO) PON.[2] . . . 25
2.17 Supertrunk PON.[2] . . . 25
2.18 RF Carrier-to-Noise Ratio (CNR) measurement.[7] . . . 26
2.19 RF CNR measurement for very low values.[7] . . . 26
3.1 Rayleigh Scattering combined with Material Absorption.[7] . . . 30
3.2 Combined total Dispersion.[8] . . . 32
3.3 Linear Fiber losses.[9] . . . 32
3.4 Effect of SPM on broadening signal.[8] . . . 34
3.6 VPI simulation of two simultaneous singals with different power. Blue line without Cross-Phase Modulation (XPM). Dark line with XPM. X axis in
Fre-quency and Y axis received Power.[Adapted from [8]] . . . 35
3.7 Four-Wave Mixing (FWM) with two starting frequency components (f1 and f2). [Adapted from [11]] . . . 36
3.8 a)FWM with three starting frequency components (f1, f2 and f3); b) FWM with different frequency spacing. [8] . . . 37
3.9 SRS effect [Adapted from [8]] . . . 39
3.10 Raman Gain with channel spacing.[12] . . . 41
3.11 6 channels SRS effect.[13] . . . 42
3.12 Modulated pump interaction at maximum Gain.[14] . . . 43
3.13 Modulated pump interaction at lower Gain.[14] . . . 43
3.14 Walk-Off phenomena at relative Low Frequency.[14] . . . 44
3.15 Walk-Off phenomena at relative Medium Frequency.[14] . . . 45
3.16 Walk-Off phenomena at relative High Frequency.[14] . . . 45
3.17 Triangle approximation of QPSK. [15] . . . 46
3.18 Raman gain.[14] . . . 48
3.19 Triangular approximation of Raman gain profile.[14] . . . 48
4.1 Several PSD comparison. . . 51
4.2 PSD of 1.25 Gbps NRZ signal with different Extinction Ratios. . . 52
4.3 PSD of 2.5 Gbps NRZ signal with different Extinction Ratios. . . 53
4.4 PSD of 10 Gbps NRZ signal with different Extinction Ratios. . . 53
4.5 SRS Crosstalk depending on wavelength spacing to 1556 nm with one channel of 7 dBm.[14] . . . 54
4.6 CNR depending on wavelength spacing at 50 MHz.[14] . . . 55
4.7 SRS Crosstalk depending on length (L) with one channel of 7 dBm.[14] . . . 55
4.8 SRS Crosstalk depending on power.[14] . . . 56
4.9 XPM and SRS crosstalk on a Continuous Wave (CW) probe at 1555 nm.[14] 57 4.10 XPM and SRS crosstalk on a CW probe at 1555 nm.[14] . . . 57
4.11 XPM and SRS crosstalk on a CW probe at 1555 nm.[14] . . . 58
4.12 XPM and SRS crosstalk on a CW probe at 1555 nm.[14] . . . 58
4.13 XPM and SRS crosstalk on a CW probe at 1555 nm.[14] . . . 59
4.14 XPM and SRS crosstalk on a CW probe at 1555 nm.[14] . . . 59
5.1 PSD of NRZ signal with different bitrates and Extinction Ratio (ER). . . 62
5.2 Video Overlay with 3.125 Gbps NRZ on the 1530.330 nm. . . 63
5.3 Video Overlay with 6.25 Gbps NRZ on the 1530.330 nm. . . 63
5.4 Video Overlay with 3.125 Gbps NRZ on the 1535.035 nm. . . 64
5.5 Video Overlay with 3.125 Gbps NRZ on the 1535.035 nm. . . 64
5.6 Video Overlay with 6.25 Gbps NRZ on the 1530.330 nm. . . 65
5.7 Video Overlay with 12.5 Gbps NRZ on the 1530.330 nm. . . 66
5.8 Video Overlay with 6.25 Gbps NRZ on the 1535.035 nm. . . 66
5.13 Scheme of the Dual polarization QPSK setup. . . 69
5.14 Optical spectrum of signals tested. . . 70
5.15 Four Channels with DP-QPSK. . . 71
5.16 Twenty Channels with DP-QPSK. . . 71
5.17 Twenty Channels with QPSK Single Polarization. . . 72
5.18 PSD of different baudrate and pulse shaping. . . 72
5.19 SRS Crosstalk with Nyquist pulse shaping for 0.625 and 1.25 Gbaud. . . 73
5.20 Closer look to Crosstalk with Nyquist pulse shaping. . . 73
5.21 SRS Crosstalk with 0.625 and 1.25 Gbaud DP-QPSK. . . 74
5.22 Closer look to Crosstalk with DP-QPSK. . . 74
5.23 SRS Crosstalk with Nyquist pulse shaping and DP-QPSK. . . 75
5.24 Closer look to Crosstalk with Nyquist pulse shaping and DP-QPSK. . . 75
5.25 SRS Crosstalk with 20 Channels of 0.625 Gbaud with Single polarization. . . 76
5.26 SRS Crosstalk with 20 Channels of 0.625 Gbaud with Dual polarization. . . . 77
5.27 SRS Crosstalk with 20 Channels of 1.25 Gbaud with Single polarization. . . . 78
5.28 Setup laboratory for 4-PAM . . . 79
5.29 Setup laboratory for 4-PAM . . . 79
5.30 Electric 4-PAM signal. . . 81
5.31 PSD of 4-PAM signal. . . 82
5.32 Coexistence setup of 4-PAM and Video Overlay. . . 83
5.33 Scheme of the setup. . . 83
5.34 SRS crosstalk of 4-PAM signal with a bitrate of 6.25 Gbps on the 1530.330 nm 84 5.35 Closer look from 10 MHz to 200 MHz . . . 84
5.36 Impact of Wavelength Spacing in SRS Crosstalk. . . 85
5.37 Impact of Bitrate of 4-PAM signal in SRS Crosstalk. . . 85
5.38 Impact of the fiber length in SRS Crosstalk. . . 86
7.1 a) SRS crosstalk of 4-PAM signal with a bitrate of 6.25 Gbps on the 1530.330 nm; b) Closer look from 10 MHz to 200 MHz . . . 89
7.2 a) SRS crosstalk of 4-PAM signal with a bitrate of 12.5 Gbps on the 1530.330 nm; b) Closer look from 10 MHz to 200 MHz . . . 90
7.3 a) SRS crosstalk of 4-PAM signal with a bitrate of 25 Gbps on the 1530.330 nm; b) Closer look from 10 MHz to 200 MHz . . . 90
7.4 a) SRS crosstalk of 4-PAM signal with a bitrate of 6.25 Gbps on the 1535.035 nm; b) Closer look from 10 MHz to 200 MHz . . . 91
7.5 a) SRS crosstalk of 4-PAM signal with a bitrate of 12.5 Gbps on the 1535.035 nm; b) Closer look from 10 MHz to 200 MHz . . . 91
7.6 a) SRS crosstalk of 4-PAM signal with a bitrate of 25 Gbps on the 1535.035 nm; b) Closer look from 10 MHz to 200 MHz . . . 92
7.7 a) SRS crosstalk of 4-PAM signal with a bitrate of 6.25 Gbps on the 1540.56 nm; b) Closer look from 10 MHz to 200 MHz . . . 92
7.8 a) SRS crosstalk of 4-PAM signal with a bitrate of 12.5 Gbps on the 1540.56 nm; b) Closer look from 10 MHz to 200 MHz . . . 93
7.9 a) SRS crosstalk of 4-PAM signal with a bitrate of 25 Gbps on the 1540.56 nm; b) Closer look from 10 MHz to 200 MHz . . . 93
7.10 a) SRS crosstalk of 4-PAM signal with a bitrate of 6.25 Gbps on the 1545.32 nm; b) Closer look from 10 MHz to 200 MHz . . . 94
7.11 a) SRS crosstalk of 4-PAM signal with a bitrate of 12.5 Gbps on the 1545.32 nm; b) Closer look from 10 MHz to 200 MHz . . . 94 7.12 a) SRS crosstalk of 4-PAM signal with a bitrate of 25 Gbps on the 1545.32 nm;
b) Closer look from 10 MHz to 200 MHz . . . 95 7.13 a) SRS crosstalk of 4-PAM signal with a bitrate of 6.25 Gbps on the 1530.330
nm; b) Closer look from 10 MHz to 200 MHz . . . 95 7.14 a) SRS crosstalk of 4-PAM signal with a bitrate of 12.5 Gbps on the 1530.330
nm; b) Closer look from 10 MHz to 200 MHz . . . 96 7.15 a) SRS crosstalk of 4-PAM signal with a bitrate of 25 Gbps on the 1530.330
nm; b) Closer look from 10 MHz to 200 MHz . . . 96 7.16 a) SRS crosstalk of 4-PAM signal with a bitrate of 6.25 Gbps on the 1535.035
nm; b) Closer look from 10 MHz to 200 MHz . . . 97 7.17 a) SRS crosstalk of 4-PAM signal with a bitrate of 12.5 Gbps on the 1535.035
nm; b) Closer look from 10 MHz to 200 MHz . . . 97 7.18 a) SRS crosstalk of 4-PAM signal with a bitrate of 25 Gbps on the 1535.035
nm; b) Closer look from 10 MHz to 200 MHz . . . 98 7.19 a) SRS crosstalk of 4-PAM signal with a bitrate of 6.25 Gbps on the 1540.56
nm; b) Closer look from 10 MHz to 200 MHz . . . 98 7.20 a) SRS crosstalk of 4-PAM signal with a bitrate of 12.5 Gbps on the 1540.56
nm; b) Closer look from 10 MHz to 200 MHz . . . 99 7.21 a) SRS crosstalk of 4-PAM signal with a bitrate of 25 Gbps on the 1540.56 nm;
b) Closer look from 10 MHz to 200 MHz . . . 99 7.22 a) SRS crosstalk of 4-PAM signal with a bitrate of 6.25 Gbps on the 1545.32
nm; b) Closer look from 10 MHz to 200 MHz . . . 100 7.23 a) SRS crosstalk of 4-PAM signal with a bitrate of 12.5 Gbps on the 1545.32
nm; b) Closer look from 10 MHz to 200 MHz . . . 100 7.24 a) SRS crosstalk of 4-PAM signal with a bitrate of 25 Gbps on the 1545.32 nm;
List of Tables
2.1 Multimedia applications and their bandwidth requirements.[Adapted from [16]] 5 2.2 Comparison of Bandwidth and Reach for Popular Access Technologies.[Adapted
from [16]] . . . 6 2.3 DVB data rate capacity of RF video overlay transmission technologies.[17] . . 23 2.4 Minimum required optical input power for video overlay Optical Network Unit
List of Acronyms
Symbols
3DTV 3D Television. 21
4-PAM 4-Pulse Amplitude Modulation. V, VI, 1, 3, 4, 78–101 A
AM Amplitude Modulation. 21, 22, 24
AM-VSB Amplitude Modulated Vestigial Sideband Channels. 23, 25, 39 AON Active Optical Network. 4, 10
APD Avalanche Photodiode. 17
APON Asynchronous Passive Optical Network. 12 ATM Asynchronous Transfer Mode. 12
AWG Array Waveguide Grating. 7, 9, 10, 15, 17, 68 B
BPON Broadband Passive Optical Network. 12, 20 C
CATV Community Antenna Television. 20–24 CE Coexisting Element. 17
CNR Carrier-to-Noise Ratio. III, IV, 25–27, 45, 47, 54–56, 58, 88 CO Central Office. 7, 14, 25
CW Continuous Wave. IV, 54, 56–59
CWDM Coarse Wave Division Multiplexing. 18 D
DP-QPSK Dual Polarization - Quaternary-Phase-Shift Keying. V, 1, 4, 70, 71, 73–75, 78 DS Downstream. 6, 12–15, 17, 18, 20, 21
DSP Digital Signal Processing. 20 DTV Digital Television. 20, 26
DVB Digital Video Broadcasting. 21, 23
DVB-C Digital Video Broadcasting - Cable. 21, 22, 24 DVB-S Digital Video Broadcasting - Satellite. 21–24 DVB-T Digital Video Broadcasting - Terrestrial. 21
DWDM Dense Wavelength Division Multiplexing. 14, 18, 24, 37 E
EDFA Erbium Doped Fiber Amplifier. 15, 21, 47, 70, 83, 86 EPON Ethernet Passive Optical Network. 12
ER Extinction Ratio. IV, 61, 62, 68 ESA Electrical Spectrum Analyser. 82 F
FM Frequency Modulation. 24
FPGA Field-Programmable Gate Array. 20 FSAN Full Service Access Network. 12–14 FTTB Fiber to the Building. 8
FTTC Fiber to the Curb. 8 FTTCab Fiber to the Cabinet. 8 FTTH Fiber to the Home. 8
FWM Four-Wave Mixing. IV, 33, 36–38 G
GPON Gigabit Passive Optical Network. III, 1–3, 12, 13, 17, 18, 21, 24 GVD Group-Velocity Dispersion. 31, 32, 37
IPTV Internet Protocal Television. 21 IQ Inphase Quadrature. 13, 18
ITU-T International Telecommunication Union - Telecommunication. 12, 14 M
MMF Multimode Fiber. 61
MZM MachZehnder Modulator. 81, 83 N
NGPON2 Next Generation Optical Network 2. III, 12, 13, 15, 17, 18 NRZ Non-Return-to-Zero. III, IV, 1, 3, 4, 19, 45, 46, 51–53, 61–67, 73, 87 NTSC National Television System Committee. 20, 22, 23
O
ODN Optical Distribution Network. 13–15, 17
OFDM Orthogonal Frequency Division Multiplexing. 22 OLT Optical Line Terminal. 7, 8, 10, 15–17, 21
ONT Optical Network Terminal. 7–10, 16
ONU Optical Network Unit. VII, 7–10, 13, 15–18, 21, 24, 83 OSA Optical Spectrum Amplifier. 70, 83, 86
P
P2MP Point to Multipoint. 8, 10 P2P Point-to-Point. 10, 14
PAL Phase Alternating Line. 20, 22, 23
PON Passive Optical Network. III, 1, 3, 4, 7–10, 12–14, 16, 17, 21, 25 PRBS Pseudo-Random Binary Sequence. 80
PSD Power Spectral Density. III–V, 1, 3, 4, 19, 45–47, 51–53, 61, 62, 72, 78, 82, 87 Q
QAM Quadrature Amplitude Modulation. III, 22, 23, 25, 39, 47
S
SBS Stimulated Brillouin Scatering. 38, 39 SDTV Standard-Definition Television. 23 SE Spectral efficiency. 22
SECAM Sequenciel Couleur Avec Memoire. 20, 22 SMF Single Mode Fiber. 7, 8, 29, 61
SNR Signal-to-Noise Ratio. 22, 24, 26 SOA Semiconductor Optical Amplifier. 15 SPM Self-Phase Modulation. III, 33–35
SRS Stimulated Raman Scattering. III–VI, 1, 4, 18, 20, 21, 39–43, 45–47, 51, 54–59, 73–78, 83–101
T
TDM Time Division Multiplexing. III, 8–10, 12 TTA Tunable Transmitter Assembly. 80, 81
TWDM Time and Wavelength Division Multiplexing. III, 14–18, 20, 47
TWDM-PON Time and Wavelength Division Multiplexing - Passive Optical Network. III, 1–3, 13, 18
U
UDWDM Ultra Dense Wavelength Division Multiplexing. IV, 1, 3, 4, 14, 17, 18, 20, 37, 47, 68, 87
US Upstream. 6, 12–15, 17, 18, 20 V
V-OLT Video-Optical Line Terminal. 24 VHO Video Hub Office. 24, 25
VOD Video-on-Demand. 21 VSO Video Serving Office. III, 25 W
WDM Wavelength Division Multiplexing. III, 9, 10, 12, 14, 36, 37, 46 X
Chapter 1
Introduction
1.1
Motivation
Humanity are living the era of network and communication to a level and an extend that nobody could predict some years ago. The Internet, as a necessary tool for any productivity in the modern world, has increased and required more and more speed. From previous electrical signal, an higher reach, speed and reliability was essential for a continuous evolution. The fascinating use of what we know as the fastest way of communication is being used by humans to share knowledge. Incredible improvements can be done and an exciting evolution is about to come. With this, cautions have to be in mind and some analysis is essential to prevent future rework. As it is known, the optical fiber is not so expensive, but the implementation and installation costs are something to care about. Even though, this technology has to have profit and the next figures 1.1 and 1.2 will demonstrates the increasing revenues from this evolution:
Figure 1.2: Revenue scenarios for GPON, XGPON and TWDM-PON for Western Europe.[1] In the above figures, from left to right, new technologies merged and it is possible to see how much profitable it is becoming. Enterprises and Businesses are adhering to it as the best communication to their premises.
Apart from network communications, there is an older technology that is still, for a huge group of the population, the best way to access information. This is obviously Television. As we expand our network, why not making it a complete integrated solution. The dream here is the use of one only physical connection to each premises. To accomplish this, communication have to abide improvements which can stimulate the growth toward any physical change. In next figure 1.3, the growth of Digital television is notorious, but overall Television is still a strong interest.
It is already easy to picture the benefits and contributions that optical fiber can provide for general user. The last figure 1.4 of this section include as well the fronthaul services, which are very important for mobile communications.
Figure 1.4: Revenue scenarios for GPON, XGPON and TWDM-PON with Fronthaul for North America.[1]
1.2
Objectives
The objectives of this thesis is the study of impairments in optical fiber communication. In particular, Raman Crosstalk is highlighted with the 4-PAM modulation format and UDWDM with NRZ and QPSK modulation formats. Following the knowing about Raman crosstalk effects, scenarios of test were made in laboratory. The output power of the laser, length of fiber, channel spacing and PSD are the key factors to this impairments. The most affected channel is the Video Overlay as it has very strict requirements to be clearly received.
1.3
Structure
• Introduction;
• State of the art and description of PON topology; • Multiplexing technologies in optical fiber;
• Video Overlay characterization and classification; • Fiber impairments. Linear and Non-Linear effects;
• Laboratory results of NRZ, UDWDM and 4-PAM coexisting with Video Overlay; • Conclusion and Future Work;
1.4
Main contributions
As main contribution it is possible to select the following thematics:
• Revision and description of PON topologies with focus on advantages compared to Active Optical Network (AON);
• Revision of UDWDM and its capabilities;
• Classification of Video Overlay and multiple possibilities for next improvements of the video channel;
• Revision of all fiber impairments. Linear and Non-Linear effects; • Description of SRS Crosstalk.
• Study of Raman Crosstalk and impairments introduced in Video Overlay channel; • Laboratory Coexistence results of NRZ signal with four and eight extinction ratio and
Video Overlay;
• Laboratory Coexistence results of UDWDM with QPSK and DP-QPSK modulation formats with Video Overlay;
• Description of 4-PAM signal creation in laboratory.
• Laboratory Coexistence results of 4-PAM signal with Video Overlay;
• Relation of SRS Crosstalk with PSD of QPSK Nyquist detection, DP-QPSK and 4-PAM.
Chapter 2
State of Art
2.1
Introduction
With the rapid development and globalization of the modern society, a large quantity of data needs to be transmitted, thus resulting in the explosive growth of information content. The explosive growth of information content enables people to places a higher demand on bandwidth, which is a symbol of communication content. New applications are about to come, if not already there, that can be analysed in the next table 2.2 to have a better point of view of our demand for bandwidth.
Application Bandwidth Latency Voice over IP (VoIP) 64kb/s 200 ms
Videoconferencing 2 Mb/s 200 ms File sharing 3 Mb/s 1 s Interactive gaming 5 Mb/s 200 ms Telemedicine 8 Mb/s 50 ms Real-time video 10 Mb/s 200 ms Video on demand 10 Mb/s/ch 10 s Network-hosted software 25 Mb/s 200 ms
Table 2.1: Multimedia applications and their bandwidth requirements.[Adapted from [16]]
With the growing resolution of televisions, it is evident that higher bandwidth demand for television will follow. For that reason it is shown in the next figure 2.1 a prediction of growing resolution with their bandwidth, alongside with improvement expected on new techniques of compression of video.
Figure 2.1: Encoded video bit rate forecasts. [3]
For ”8K” television it is expected a bandwidth need around 55 Mb/s while 6 years before less than 5 Mb/s was enough.
Table 2.2 refer some of the improvements made in wired communications system to satisfy our needs, linking the new applications with our evolution in communication to follow this demand.
Service Medium DS (Mb/s) US (Mb/s) Max Reach (km)
ADSL Twisted pair 8 0.896 5.5
ADSL2 Twisted pair 15 3.8 5.5
VDSL1 Twisted pair 50 30 1.5 VDSL2 Twisted pair 100 30 0.5 HFC Coax cable 40 9 25 BPON Fiber 622 155 20 EPON Fiber 1000 1000 20 GPON Fiber 2488 1244 20
Table 2.2: Comparison of Bandwidth and Reach for Popular Access Technologies.[Adapted from [16]]
2.2
Passive Optical Networks
PON arise from higher speed, bandwidth and lower power consumption demand from user. Emerging multimedia applications and service providers call for better solutions when copper wire technologies have reached bandwidth limits. The combination of ultrahigh band-width and low attenuation of optical fibers with low-cost photonics components and PON architecture have allowed the deployment for wide area network and metro area network. The architecture of a passive optical network only comprises active devices in the Central Of-fice (CO) and at user premises, which improve the power budget on optical networks. From CO to user premises a standard Single Mode Fiber (SMF) runs to a passive optical power splitter/Array Waveguide Grating (AWG).[16]
The network starts in a Optical Line Terminal (OLT) element from the CO, runs through the passive optical splitter/AWG and finishes in an outdoor cabinet in a collocation room, named ONU, or in the end subscriber’s premises, called Optical Network Terminal (ONT).[18] The network’s description setup is the followed:
• OLT: it is a device at the service providers central office, performing conversion between the electrical signals used by the service providers equipment and the fiber optic signals used by the passive optical network and coordinating the multiplexing between the conversion devices on the other end of that network. In other words, it adpts the network traffic of operator (data, voice and video) to the access network. [19] [4]
• ODN: it is used for distributing signals to users in a telecommunications network by optical fiber. ODN has been made up entirely of passive optical components particularly single mode optical fibers and optical splitters or AWGs. [4]
• ONU or ONT: they are devices near end users, delivering traffic-load information provided by OLTs to each end user. The ONU or ONT are the same device with the only difference that the first one is located close to the subsciber, outside the building for example, and the second is located in user premises.[19] [4]
Figure 2.2: Typical architecture of PON. [4]
PON comes in different flavours with Fiber to the Home (FTTH), Fiber to the Building (FTTB), Fiber to the Curb (FTTC) and Fiber to the Cabinet (FTTCab). After reaching premises, the signal flows commonly through twisted pair.
A PON has a structure Point to Multipoint (P2MP) that is seen as a tree, colloquially called a PON tree. A SMF make the link between the OLT and the passive optical splitter that further splits and share his content to, generally, 16, 32 or 64 customers without any switching or buffering. In other words, an OLT is connected and shared by ONUs via a passive optical splitter. Signals are sent from and to different ONUs with unique ONU identification in the frame header.[18] [19] To avoid collision between upstream and down-stream communications a multiplexing scheme is used, where each subscriber receives their own time slot to transmit and receive. For this purpose Time Division Multiplexing (TDM) allows a coordinated communication and ONTs/ONUs are responsible for the synchronization of the right user time slot. [18]
Figure 2.3 represents the time modulation procedure of a Time Division Multiplexing (TDM)-PON.
Figure 2.3: TDM-PON non-standard protocol.
Within the same approach it is possible to define Wavelength Division Multiplexing (WDM) technology, which aims to multiplex several optical signals through different wave-lengths. Figure 2.4 allows a better description.
Figure 2.4: WDM-PON non-standard protocol.
Here the concept is the use of different colours to carry different signals. To each
ONT/ONU only one respective wavelength is transmitted. Thus, in the same physical in-frastructure, it is possible to create a virtual PON. The AWG will determine each optical
signal to send to each port on the basis of the laser colour, allowing a bi-directional data flow through a single optical fiber. The bi-directional flow is achieved using one wavelength for downstream traffic and another for upstream traffic. Thereby, even being P2MP, it provides a virtual Point-to-Point (P2P) connection with benefits from both sides. [19]
Between TDM and WDM there is advantages and disadvantages. The first technology in the market was TDM, with his simplicity of implementation and the ability of easily supporting the desired data rates at the time of adoption. TDM scheme only needs one transceiver at the OLT, independent of number of ONUs/ONTs, which implies low cost implementation. However, the use of synchronization to avoid signals collision comes as higher complexity. WDM appears as a better solution decreasing the complexity in a way the Ethernet protocol is preserved from end-to-end. Which means that the use of connections through wavelength domain (color lasers) do not need the media access control or any security tools while TDM use a cumbersome conversion protocol to Ethernet (and back) at all end points. WDM allows longer reach and larger split ratios. Nevertheless, although having those advantages, the component costs are higher and wavelengths tend to drift with environment temperatures. [19] [20]
The alternative is an AON structure that is Point-to-Point (P2P). It is important to understand difference between PON and AON. There is a key technical difference that is in PON technology a splitter or AWG (depending on multiplexing protocol) distributes the signal to different optical lines without any power requirement.[18] Figure 2.5 represents the PON network architecture while figure 2.6 represents AON network architecture with use of an Ethernet switch to route the signals to premises.
a) b)
2.3
History of PON Standards
From all the technology of PON we can refer two entities: Full Service Access Network (FSAN) with International Telecommunication Union - Telecommunication (ITU-T) or Insti-tute of Electrical and Electronics Engineers (IEEE). The first one focused in Asynchronous Transfer Mode (ATM) protocols, while the secound focused on Ethernet protocols. PON started in 1995 with the first proposal of Asynchronous Passive Optical Network (APON), that later were approved by ITU-T in 1998 as G.983.1 recommendation with only two wave-length (downstream and upstream) and speeds of 622Mb/s Downstream (DS) and 155Mb/s Upstream (US).
At this point, merging RF video in this system would be very attractive. Broadband Passive Optical Network (BPON) came to answer to this implementation with the recom-mendation ITU-T G.983.3 that settle some changes to the wavelength to include video in 2001. It was decided a 1550 nm wavelength for Video Overlay due to different reasons that will be approached in following sections.
FSAN kept this evolution to attain 1 Gb/s of bandwidth. ITU-T approved GPON in 2003 with the G.984 recommendation matching 2.5 Gb/s of DS and 1.25 Gb/s of US. Alongside, IEEE developed Ethernet Passive Optical Network (EPON) with the standard IEEE 802.3ah with a symmetric 1.25 Gb/s for both ways. GPON and EPON replaced the wavelengths of BPON.
All those technologies use TDM scheme. BPON, GPON and EPON are also known as TDM-PON.
The continuous growth of our technology were requiring more bandwidth. A mid-term update had to be done. Thus, in 2010, arose the G.987.1 standard from ITU-T and FSAN studies. This new technology had to reach 10 Gb/s to satisfy demands. Therefore, this technology got the name of XGPON (also known as 10GPON or NGPON). XGPON was cost-effective to meet the needs, with complete coexistence and characterized for using WDM. However, in terms of split ratio and reach it did not presented significant differences. [19][8]
Figure 2.7 represent the wavelength occupation at this stage, before NGPON2.
2.4
Next Generation PON 2 (NGPON2)
2.4.1 Introduction
At this stage, keeping the usual trend of demands for new services, some progress in equipments of optical transmission allowed this new model for PON. The main motto was still the same, better transmission capacity, longer reach, compatibility with actual Optical Distribution Network (ODN) and lower cost. In further detail there is more requirements:
• Must be able to operate over the same passive infrastructure previously defined for GPON and XGPON;
• Keep no impact (or minimal) from linear/non-linear crosstalk and no increase to legacy optical budget;
• Use of low quantity of new equipment, that results in a colourless ONU;
• Lower the price of services due to a tendency of residential customers (Customers do not expect to pay more as broadband service rates increase);
• Mobile network operators look forward to introduce the so called mobile fronthaul (Mo-bile fronthaul needs high capacity links to carry Inphase Quadrature (IQ) samples from the air interface with strict latency requirements);
• Target a baseline DS of 40Gbit/s and 10Gbit/s of US; • 1:256 split ratio (minimum);
• Basic 40 km range;
• 60 km range, at least, with reach extended; • Increased security and data integrity;
Scientific community strove to find the best suitable candidate to this technology [19][8]. Among the candidates some deserve to be mentioned:
• OFDM - Orthogonal Frequency Division Multiplexing; • OCDM - Optical Code Division Multiplexing;
• WDM - Wavelength Division Multiplexing; • 40 Gbps TDM - Time Division Multiplexing;
• UDWDM - Ultra Dense Wavelength Division Multiplexing; • TWDM - Time Wavelength Division Multiplexing;
In April 2012, after a benchmarking exercise undertaken in FSAN, TWDM-PON was chosen as the architecture for NGPON2. Key factors were considered as follows:
• Loss budgets with passive ODNs;
• Compatibility with legacy PON and deployed ODNs; • Relative complexity;
• Potential for higher line-rates and 10Gbit/s services; • Efficient utilisation of system capacity;
• Ability to offer both residential and high bandwidth, low latency business/backhaul services on the same ODN;
• Timeframe for first system availability; • Relative maturity of components;
• Prospects for low cost residential equipment;
• Port density and fiber management complexity at the CO;
• Potential for incremental capacity growth or channels dedicated to different services; • Power consumption and applicability of power saving modes;
• And technology risk;
The decision made jointly by FSAN and ITU-T predicted an optional addition of coexisting P2P-WDM (or known as UDWDM) overlay channels to distinguish a dedicated service with high capacity, low latency links for some services on the same ODN. [21]
2.4.2 Features
TWDM, as primary solution, is a hybrid system combining time and wavelength division multiplexed. It can be interpreted as attachment of multiple XGPON, through pairs of wavelengths. The baseline is used with 4 pairs of wavelengths, Dense Wavelength Division Multiplexing (DWDM) spaced, bi-directional
λ-channels, each of 10Gbit/s DS and 2.5Gbit/s US line rate.[19][21]
To have a better vision of this improvement, a comparison between this technology and previous ones can be made. For instance, if 24 premises are connected to a PON and the available throughput is equally shared then for GPON each connection receives 100 Mbps downstream and 40 Mbps upstream over a maximum of 20 km of fiber. For 10G-PON, which was the second PON revision, each of the 24 connections would receive about 400 Mbps downstream and 100 Mbps upstream. The recently approved NG-PON2 will provide a total throughput of 40 Gbps downstream and 10 Gbps upstream over a maximum of 40 km of fiber so each of the 24 connections would receive about 1.6 Gbps downstream and 410 Mbps upstream. NG-PON2 provides a greater range of connection speed options including 10/2.5
Figure 2.8: Bit rate and reach comparison of NGPON2 and previous technologies to the premises.[5]
2.4.3 Limitation
Opt-electronic components available created a restriction in wavelength. The use of Semi-conductor Optical Amplifier (SOA) did not create any impairments since it can work in any pretended bands. However, Erbium Doped Fiber Amplifier (EDFA) are just able to work properly in C and L bands. But the key challenge was to realise the underlying tunable components at low cost. Tunable lasers and coherent receivers are able to deliver high level of power, which is required to achieve long reaches and splits. Moreover, existing standards, like video overlay, have guard bands restrictions that must be respected and the already crowded bandwidth did not allow much choice. NGPON2 should stand in L band or in some nm of the C band (1530-1539 nm).[19][21]
2.4.4 Architecture
TWDM technology enables, as said previously, 40Gbit/s DS and 10Gbit/s US using dif-ferent split ratios but commonly 1:64. This architecture can be considered for 4, 8 or even 16 wavelength channels. In the OLT there is an optical amplifier (EDFA) which aims to increase power budget, stimulating DS signals and pre-amplifying the US signals (This is an important effect on optical communication that will be approached in next sections). Also there is multiplexer and demultiplexer in order to multiplex signals to the ODN and isolate different wavelengths that comes from ONUs, respectively. This is implemented through an AWG. The following figure 2.9 represents the architecture of this system.
Figure 2.9: Architecture of TWDM-PON
In context of PON definition, this is still a Passive network even with amplifiers. It is noteworthy that all the active elements are only found in OLTs and ONUs/ONTs.[19][8]
A simple integration by modular card in the OLT has operational and economic advan-tages. Pay-as-you-grow and easy bit rate configuration for each wavelength are the strongest advantages. This approach is depicted in the next figure 2.10.
XGPON upgrades, since a Coexisting Element (CE), or technically an AWG, would also have to be introduced to enable the combination of GPON and XGPON wavelengths.
2.4.5 Power budget
Power budget is an important parameter, used to evaluate the network performance. Features like split ratios and maximum transmission distance, which characterize the coverage of a PON technology, are limited by the power budget of the US and DS ways. This parameter is obtained through measurements of the transmitting power and received signal sensitivity. Regarding to the power launched into the transmitting fiber, it is necessary to be careful to avoid fiber non-linearities. Besides, the ODN must remain passive so it is not possible to add an amplifier in the transmission link between the OLT and ONU. Thus, the power budget parameter can just be optimized by higher signal sensitivity, which can be achieved through Avalanche Photodiode (APD) and pre-amplification employed at the receiver. The research about this topic has an interesting relevance. NGPON2 operators are determined to extend PON reach and split, in order to decrease the number of central offices - concentrate the OLTs - and server a larger geographic area from each OLT. This way, the objective of reduce costs can be achieved, since the monetary value can be split by a larger number of subscribers.[19]
2.4.6 Wavelength Range
To define the wavelength range of TWDM, several options were studied. Some of them proposed even the use of XGPON bandwidth, which implied his total removal from OLT rack and thus, upgrading to NGPON2 would never be coexist with XGPON. Fortunately a solution were found where GPON, XGPON, and Video Overlay could co-exist. Figure 2.11 represent the best scenario of wavelength, with NGPON2 and UDWDM extension.
Figure 2.11: Wavelength plan representation.[6]
Wavelength allocations include 1524 nm to 1544 nm in the upstream direction and 1596 nm to 1602 nm in the downstream direction.
2.5
Ultra Dense Wavelength
Division Multiplexing (UDWDM)
2.5.1 Introduction
UDWDM comes to meet demanding operator requirement for business and backhaul ser-vices. Improvements in coherent detection allowed longer reach. As an extension of NGPON2, it became scalable and flexible in terms of bandwidth spectrum.
2.5.2 Features
UDWDM enables a dedicated λ-channel to be provided to each ONU. In the baseline configuration, 8 channels of UDWDM are considered to allow full co-existence with legacy systems. Depending on the particular deployment scenario, a network operator may dedicate unused spectrum to adicional wavelength channels in a flexible way.
As requirement, ONUs have similar low cost Tx and Rx elements as for Time and Wave-length Division Multiplexing - Passive Optical Network (TWDM-PON) with the main differ-ence being the continuous mode operation for UDWDM (c.f. burst mode for TWDM-PON). Advanced IQ modulator, coherent detection, Silicon Photonics (SiP) and paired-Channel technology were essential to increase selectivity and sensitivity to perform a high division in narrow bandwidth of spectrum.[19][8][21]
DWDM systems, similarly to UDWDM were developed previously to deal with the rising bandwidth needs of backbone optical networks. It uses a wider spacing (usually 0.8 nm or 100 GHz) between wavelength bands.
Coarse Wave Division Multiplexing (CWDM) was another alternative and a low-cost ver-sion of DWDM. Generally these systems were not amplified and therefore had limited range. They typically used less expensive light sources that were not temperaturestabilized. Larger gaps between wavelengths were necessary, usually 20 nm (2.5 THz). Of course, this reduced the number of wavelengths that could be used and thus also reduced the total available band-width. This fact hindered a development toward a long-term solution.[22]
It was already proven a UDWDM communication in a scenario of complete coexistence, with Video Overlay and TWDM US and DS. This experiment achieved 80 km of UDWDM communication in a bidirectional 2.5 Gbps, with a Nyquist DQPSK detection. The results were satisfactory due to a low impact of SRS and XPM [23].
The experiment were as well tested in a Field-Trial, with a length of fiber of 8.4 km. In this simulation, UDWDM was coexisting with GPON, NGPON2 and Video Overlay in a bidirectional communication.[6]
The results of the experiment related with SRS are shown in the next figures 2.12 and 2.13.
Figure 2.13: Crosstalk of SRS on Video Overlay.[Adapted from [6]]
The previous figure 2.13 shows the Raman effects that will be approached in following chapters.
All this experiment used Nyquist shaping, which uses a raised-cosine filter and is used to the limit, assuming a minimum bandwidth, which allows very narrow channel spacing. The use of Field-Programmable Gate Array (FPGA) is necessary to make a real-time Digital Signal Processing (DSP). [24]
2.5.3 Wavelength Range
The wavelength reserved for UDWDM is the 1540 to 1550 nm between Video Overlay and TWDM US. The other solution available is the wavelength between OTDR and TWDM DS. Those two possibilities are depicted in the previous figure 2.11.
2.6
Video Overlay Systems
2.6.1 Introduction
Tele-triggered the migration to Digital transmission domain. Nowadays, all around the globe, Television is Digital and known as Digital Video Broadcasting (DVB).
In fact, broadcasting video has different advantages. One of them, is the fact that video over Internet is made through unicast and multicast IP video transmission which is used for Video-on-Demand (VOD) or Internet Protocal Television (IPTV) and the positive effect of video overlay comes to reduce the requirements on the performance and capacity of the IP backbone network. Furthermore, with the increase of video services even more demand-ing with 3D Television (3DTV), 4K TV and even 8K TV, providdemand-ing broadcast via a video overlay is a serious option. This solution is very interesting for TV operators, using optical communication and transport for broadcasting. In terms of commodity this implementation is interesting too. Using a coax cable, antenna or satellite dish will no longer be necessary. Every communication can be integrated in optical fiber.
2.6.2 Video Overlay in the 1550 nm
A study about the impact of video on data and vice versa had to take place before merging this service into the optical fiber. Therefore, the window wavelength were chosen to be in the 1550 nm to 1560 nm due to the following reasons:
• Relatively cheap amplification through EDFA.
• Use of common lasers and filters because of the relative high guard band between DS of GPON before the introduction of XGPON.
• Optimal range window with the minimal attenuation (further details in next sections), which improves the range of the signal.
Preventing linear crosstalk to or from data signals is, in general, achieved easily using adequate quality optical diplexers in the OLT and the ONU. The challenging problem evolves from non-linear crosstalk due to the very high optical power levels on the PON. The resulting impairments are only on the RF video overlay signals and not on the IP signals. The non-linear crosstalk is caused by SRS and visible on analogue transmitted video signals with Amplitude Modulation (AM). Digital modulated signals are, in general, sufficiently robust not to suffer any impairment. [17]
2.6.3 Identification and Classification
DVB has three standards:
• Digital Video Broadcasting - Satellite (DVB-S): a wireless communication between satel-lites and Satellite dishes located generally on the top of roofs;
• Digital Video Broadcasting - Cable (DVB-C): a physical communication over cable (usually Coax);
• Digital Video Broadcasting - Terrestrial (DVB-T): wireless terrestrial communication, from antenna to antenna;
• NTSC: Used in the majority of the countries of America.
• PAL: Used in Central and Northern Europe countries, African countries, South Asia and Australia.
• SECAM: Used in French speaking countries and Estern Europe countries.
2.6.3.1 CATV
CATV uses AM channels. RF channels are from around 50 MHz to 800 MHz. 2.6.3.2 DVB-S
A satellite channel is limited by his downlink power and large available bandwidth. It requires a robust codification and modulation format with good immunity to noise and non-linear distortion to avoid high error bit rates. The main source of distortion are the amplifiers used in the reception. That is why AM is not a good choice. For this transmission QPSK format is used because of his 3dB gain on Signal-to-Noise Ratio (SNR) in comparison with binary system and Spectral efficiency (SE).
DVB-S uses channels with 36 MHz of bandwidth and a baud rate of 27.5 Msymbols/s (55 Mb/s) which is very satisfying for a single channel. His frequency range is from 950 MHz to 2000MHz
2.6.3.3 DVB-C
Transmission by cable offers better SNR than DVB-S and a much shorter channel band-width of typically 7-8 MHz. This allows the use of a modulation format with more SE, like QAM. Very similar to CATV, DVB-C has a range of 45 MHz to 1000 MHz.
2.6.3.4 DVB-T
Due to the specific terrestrial transmission requirements, Orthogonal Frequency Division Multiplexing (OFDM) format were the most appropriated to code channels. It uses a band-width of 6, 7 or 8 MHz. RF range goes from 200 MHz to 800 MHz.
2.6.3.5 Video Overlay Description
The next figure 2.14 demonstrates the characteristics of a typical analog QAM scheme at the input of an optical link.
Figure 2.14: Typical Analog QAM scheme.[2]
The scheme is made up of Amplitude Modulated Vestigial Sideband Channels (AM-VSB), commonly carrying analog video at different frequencies separated by 6 MHz for NTSC or 8 MHz for PAL. QAM channels follow the same frequency separation, but are generally 6 to 10 dB down in composite channel power in comparison to peak value of the AM-VSB channels. QAM can have different symbol schemes, but the most common is 256 symbols modulation. This Video Overlay scheme achieves 6.6 Gb/s, which can support up to 918 HD channels or 3978 Standard-Definition Television (SDTV) channels.[2] Video is typically High power (around 17 dBm) which generates significant scattering in fibers. The analog part of the signal is very sensitive to distortions and crosstalk of any kind as it will be discussed in the following sections.
More recent studies related with different scenarios on video overlay are presented. Table 2.3 shows some of the video overlay options and their DVB data rate capacity.
Table 2.3: DVB data rate capacity of RF video overlay transmission technologies.[17] Some solutions presented refer to extended frequency which can offer both CATV and DVB-S in scenarios where the focus can be more in DVB-S channels and less in CATV.
Frequency stacking is another solution, by stacking two or more L-Bands. With the current modulation/codding scheme, the data capacity of an RF video overlay system offers between 3 and 10 Gb/s of video broadcast transmission capacity. This capacity can be greatly increased using DWDM within the available 1550 nm wavelength window. Solutions with up to 8 DWDM channels were proposed, increasing the total video data rate capacity by a factor of 8.[17]
Table 2.4: Minimum required optical input power for video overlay ONU.[17]
From this table, some caution is required. For the widely used CATV RF video overlay, about -10 dBm of optical input power, is sufficient to achieve satisfactory video picture quality. With a maximum launch power of +19 dBm from the Video-Optical Line Terminal (V-OLT), this provides a maximum 29 dBm optical budget, exceeding the requirements of the GPON standards.
If the AM TV as well as Frequency Modulation (FM) radio channels signals are replaced with DVB-C channels, a 3 dBm improvement can be achieved, which can be used to halve the V-OLT related cost per subscriber.
The transmission of DVB-S signals increases the power budget dramatically due to the very robust nature of DVB-S transmission which requires a SNR of only about 11 dB. Con-sequently, optical budgets between about 42 dB (single L-Band) and 35.5 dB (Quad L-Band) transmission can be achieved. Therefore, DVB-S has the better solution. Thousands of sub-scribers can be connected to just one +19 dBm port of a V-OLT.[17]
to analog AM-VSB or 256 QAM services, via an edge modulation platform, for transport over an analog 1550 nm video link to the PON. The VHO then serve the VSO which is inside the CO building. The next figure 2.15 depics this scenario.
Figure 2.15: General deployment Scenario.[2]
The transmission between the VHO and VSO can be deployed in two different Edge Modulation solutions. The first one in figure 2.16.
Figure 2.16: Edge modulation VSO PON.[2]
The second one, where the distance from VHO and VSO is ¡ 50km. An analog Supertrunk can be implemented between the VHO and VSO before PON deliveries as seen in figure 2.17.
Figure 2.17: Supertrunk PON.[2]
2.6.5 RF-Video Channels Performance and Measurements
To qualify a Video channel performance CNR. CNR is a pre-detection measurement per-formed on RF signals, with a relation between raw carrier power to raw noise power in the
RF transport. SNR is a pre-modulation or Post-detection measurement performed on base-band signals. One of the first difference and advantage of DTV is the lower level of CNR required to the receiver, comparing to SNR of analog signal. Figure 2.18 demonstrate the CNR measurement on a spectrum analyzer.
Figure 2.18: RF CNR measurement.[7]
When the CNR value is very low (single-digit decibel (dB) values) some issues have to be taken under consideration as seen in the next figure 2.19.
In this figure the red line represents the S signal without noise (N). The blue haystack (Signal with noise) above the red haystack represents the measurement error. Through ma-nipulation of equation we can derive the true CNR:
(S + N ) S = 1 + 1 S+N N − 1 ⇔ S N = S + N N − 1 (2.1)
The calculation of the true CNR in dB is as follow:
CN R(dB) = 10haystack height (dB)10 − 1 (2.2)
Chapter 3
Fiber Impairments
3.1
Introduction
In this section crosstalk will be introduced and explained. All optical communication suffer from this effect and it cannot be ignored. Discarding this consideration would result in errors on the reception or even no communication at all. In this document only SMF is treated, which allows the longer reach for our communication.
3.2
Linear Impairments
Linear Crosstalk refers to two main characteristics: Attenuation and Dispersion. The dependency is the wavelength of the signal. It restrains the maximum reach and the maximum transmission rate. Those Fiber losses represent a limiting factor because they reduce the signal power reaching the receiver. As optical receivers need a certain minimum amount of power for recovering the signal accurately, the transmission distance is inherently limited by fiber losses. Optical fiber producers are mitigating this impairment, with improved fibers. [9]
3.2.1 Attenuation
Under quite general conditions, changes in the average optical power P of a bit stream propagating inside an optical fiber are governed by Beer’s law:
dP
dz = −αP (3.1)
Defining z as an axis parallel to the direction of the beam and α the attenuation coefficient in m−1 and P is the optical power.
Pout= Pin· e−αL (3.2)
The above equation 3.2 describes the output power that decays exponentially relatively to the input power. L refers to the length of the optical fiber.
The coefficient α is generally used in units of dB/km, which can be derived from the following equation 3.3: αdB/km= − 10 L · log10 Pin Pout ≈ 4.343αkm−1 (3.3)
Finally, the total attenuation is due to three components: Material Absorption, Rayleigh Scattering and Waveguide Imperfections.
3.2.1.1 Material Absorption
Material absorption can be divided into two categories. Intrinsic absorption losses corre-spond to absorption by fused silica (material used to make fibers) whereas extrinsic absorption is related to losses caused by impurities within silica. Any material absorbs at certain wave-lengths corresponding to the electronic and vibrational resonances associated with specific molecules. For silica molecules, electronic resonances occur in the ultraviolet region, whereas vibrational resonances occur in the infrared region.[9] Additional isolated absorption peaks can result from certain impurities. For example, silica fibers exhibit increased absorption losses around 1390 nm and 1240 nm if the core material is not water-free as it will be visible in following figures.[10]
3.2.1.2 Rayleigh Scattering
Rayleigh scattering is a fundamental loss mechanism arising from local microscopic fluc-tuations in density. Silica molecules move randomly in the molten state and freeze in place during fiber fabrication. Density fluctuations lead to random fluctuations of the refractive index on a scale smaller than the optical wavelength. Light scattering in such a medium is known as Rayleigh scattering.[9]
As a result, the intrinsic loss of silica fibers from Rayleigh scattering can be written as: αR=
C
λ4 (3.4)
C is the constant depending on the constituents of the fiber core.[9]
In the next figure 3.1 it is possible to observe combination of Rayleigh Scattering loss and IR absorption loss.
distance one still has 1% of the original optical power. That is often sufficient for reliable detection of data signals, even at very high bit rates.[10]
3.2.1.3 Waveguide Imperfections
An ideal single-mode fiber with a perfect cylindrical geometry guides the optical mode without energy leakage into the cladding layer. In practice, imperfections at the core cladding interface (e.g., random core-radius variations) can lead to additional losses which contribute to the net fiber loss. Bends in the fiber constitute another source of scattering loss.[9]
3.2.2 Chromatic Dispersion
Different spectral components of the pulse travel at slightly different group velocities, a phenomenon referred to as Group-Velocity Dispersion (GVD), intramodal dispersion, or sim-ply fiber dispersion. The frequency dependence of the group velocity leads to pulse broadening simply because different spectral components of the pulse disperse during propagation and do not arrive simultaneously at the fiber output. Intramodal dispersion has two contributions, material dispersion and waveguide dispersion.[9]
3.2.2.1 Material Dispersion
Material dispersion occurs because the refractive index of silica, the material used for fiber fabrication, changes with the optical frequency. On a fundamental level, the origin of material dispersion is related to the characteristic resonance frequencies at which the material absorbs the electromagnetic radiation.[9]
3.2.2.2 Waveguide Dispersion
This dispersion is related with the power distribution between the core and cladding. 80% of the optical power is inside the core while 20% remaining power is in the cladding at a su-perior speed. The main effect of waveguide dispersion is to shift λ0 by an amount around 30-40 nm. It is possible to design the fiber such that is shifted into the vicinity of 1550.[9][8]
3.2.2.3 Combined total Dispersion
Figure 3.2: Combined total Dispersion.[8]
Figure 3.2 shows the wavelength shift due to the negative Waveguide Dispersion and the resulting total Chromatic Dispersion. An important parameter used to classify the total Chromatic Dispersion is the following equation 3.5:
D = −2πc
λ2 β2 (3.5)
The parameter β2 = d2β/dω2 is known as the GVD parameter. It determines how much an optical pulse would broaden on propagation inside fiber. D is called the dispersion parameter and is expressed in units of ps/(km-nm) and c is the speed of light.
3.2.3 Linear Fiber Losses
The water peak is a wavelength band where the attenuation reaches high levels. The water peak have been improved through different solutions in optical fiber material to achieve a complete dry fiber. Once more it is visible why the 1550 nm wavelength were chosen to implement video overlay.
3.3
Non-Linear Impairments
The response of any dielectric to light becomes nonlinear for intense electromagnetic fields, and optical fibers are no exception. In this section it will be presented and discussed non-linearities of optical fiber and their effect on new technologies. Two main categories can be set for this type of crosstalk: Kerr effect and Stimulated Light Scattering.
3.3.1 Kerr Nonlinearity
The refractive index of silica is not power independent. In reality, all materials behave nonlinearly at high intensities and their refractive index increases with intensity. Kerr effect represents this nonlinearity where the refractive index depends on the intensity of the light | E |2. Essentially, this means that the phase delay in the fiber gets larger if the optical intensity increases.[10][9]
Equation 3.6 shows that refraction index depends on intensity of light:
n = n0+ n2|E|2 (3.6)
With a linear index refraction of n0 ≈ 1.5 and n2 ≈ 2.35 × 10−20 m2/W the nonlinear refraction index.
The resulting effects are SPM, XPM and FWM. Those effect will be presented in next subsection.[8]
3.3.1.1 Self-Phase Modulation (SPM)
As the name suggests, the phase of the signal is affected by himself. Next equation 3.7 explains better the evolution of the phase of the signal all along with the fiber length (L).
φ = nk0L (3.7)
With the parameter k0 = 2π/λ.
Combining equation 3.6 and 3.7 we derive equation 3.8:
φ = (n0+ n2|E|2)k0L (3.8)
The corresponding introduced phase by SPM is φSP M = n2k0|E|2.[8]
The SPM-induced spectral broadening is a consequence of the time dependence of non-linear phase shift. This can be understood by noting that a temporally varying phase implies that the instantaneous optical frequency differs across the pulse from its central frequency. The time dependence of 4ω is referred to as frequency chirping.[25]
Figure 3.4: Effect of SPM on broadening signal.[8]
In figure 3.4 it is represented the negative shift of phase by the red line and positive shift of phase by blue line. This lead to new frequencies in spectrum of the signal received, which enlarge the bandwidth of the signa visible in the greed dotted line.[25][8]
An overlap of the images can give a better understanding of the resulting effect in figure 3.5:
3.3.1.2 Cross-Phase Modulation (XPM)
XPM is a result of co-existing signals in the same fiber. So naturally XPM is followed by SPM. Supposing the existence of two different optical signals, one with power |E1|2 and the other one with |E2|2, the next equation 3.9 will describe the total influence in the phase variation provided to the first signal (|E1|2) due to the second one.
φSP M +XP M = n2k0 |E1|2+ 2|E2|2
(3.9) Analysing this equation there is some remarks. First of all, the XPM (n2k02|E2|2) is affected by a factor of two. Which means that, if both signals have the same power, the contribution of the second signal will be two times higher than the SPM (n2k0|E2|2). And second remark is that, if XPM is too high compared to SPM, SPM can be completely discarded from the total phase variation.[8]
A simulation in software has been already done to confirm this impact. The results are shown in figure 3.6:
Figure 3.6: VPI simulation of two simultaneous singals with different power. Blue line without XPM. Dark line with XPM. X axis in Frequency and Y axis received Power.[Adapted from [8]]
The simulation were tested on a system with two signals of different power. It is depicted in blue color the weaker signal without XPM evaluation and in dark color the XPM contri-bution and effect from the stronger signal to the weaker. Moreover, there was no remark on any effect from the weaker signal to the stronger one.
Generally it is possible to describe the frequency shift of one channel by its N-1 neighbors channels with equation 3.10:
φ(XF M )j= n2k0L 2 N X (k=1,k6=j) |Ek|2 (3.10)
It is obvious the impact on the broadening of the signals and consequently interference in received data may occur if there is not enough guardband between those signals.
This crosstalk affects mainly WDM and regions with high power channels in the spectrum, as example, video overlay that requires a high power for transmission.
The following equation 3.11 have been used to study the crosstalk created from this phenomena. [14] Crosstalk (XP M ) = 10 log10 4πn2βΩ 2P CHiρXP M λAef f 2 · 1 + e2αL− 2eαL(1 − αL) cos (d iΩL) − 2L α + diΩe−αLsin (diΩL) + α2+ (diΩ)2 L2 α + (diΩ)2 2 (3.11) In this equation, the parameter n2 is the non-linear index, β is the dispersion for phase, Ω the frequency, α is the attenuation, L is the length and di the group velocity mismatch. 3.3.1.3 Four-Wave Mixing (FWM)
Four-wave mixing is a nonlinear effect arising from a third-order optical nonlinearity, as it is described with a x3 coefficient. It occurs when at least two different frequency components propagate together in a nonlinear optical fiber.[10]
With two frequency components of f1 and f2 (with f2> f1), an refractive index modula-tion at the difference frequency occurs, which creates two addimodula-tional frequency components. The components can be derived by equations 3.12 and the following figure 3.7 represents this effect:
Note that f1 and f2 do not get affected by the two new other components created. In further detail, it is possible to evaluate the interference when three starting frequencies components are mixed. The frequencies fi, fj and fk(i, j 6= k) will mix and produce a number of new components that can assume values in a range of 1 to 3 in their i, j and k indexes. Next equation 3.13 and figure 3.8 a) determine the new component frequencies created and the resulting components from the three starting ones.
fijk= fi+ fj − fk (3.13)
Figure 3.8: a)FWM with three starting frequency components (f1, f2 and f3); b) FWM with different frequency spacing. [8]
Comparing figure 3.8 a) and figure 3.7 the original frequency components are getting overlapped by new frequencies created, while in a scenario with only two starting frequency components the new frequencies are not interffering directly with the desired signal.
This is particularly nagging in WDM, DWDM and UDWDM when channels have equal spacing between them. As it is shown in figure 3.8 b), the use of different channel spacing between frequency components (4f16= 4f2) avoids the overlapping on original frequency of signals.
New WDM systems avoid FWM using techniques of dispersion management in which GVD is kept locally high in each fiber section. This technique uses fibers designed with a dispersion of about 4 ps/(km-nm), which is enough to suppress FWM.[8]
Generally, the M number of extra frequency components created from N original frequen-cies can be calculated through equation 3.14:
M = N 2
In conclusion, the worst case scenario for distortion by FWM is the narrow channel and symmetric spacing between multiple channels, combined with zero dispersion from fiber and high power optical input.
3.3.2 Stimulated Light Scattering
Rayleigh scattering in previous section is an example of elastic scattering for which the frequency (or the photon energy) of scattered light remains unchanged. By contrast, the frequency of scattered light is shifted downward during inelastic scattering. Both scattering processes result in a loss of power at the incident frequency. However, their scattering cross sections are sufficiently small that loss is negligible at low power levels. The intensity of the scattered light in both cases grows exponentially once the incident power exceeds a specific value.[9]
3.3.2.1 Stimulated Brillouin Scattering (SBS)
Stimulated Brillouin Scatering (SBS) is an acoustic phenomenon in which the optical signal transmitted is converted into a backward scattered signal. The high optical power is absorbed by phonon causing a vibration that affects the refractive index by generating an acoustical fiber Bragg grating. The vibration produced is at a frequency (around 11.6 GHz) lower than the optical frequency and, in result, a backscattered light beam is created. The SBS can only be observed in backward direction, since Brillouin shift in forward direction is zero. The frequency shift is given by 3.15:
f = 2n
λvB (3.15)
The n parameter is still the refraction index of 1.5 and vB the velocity of sound waves. This interference is determined by a threshold power which can be written accordingly to the Smith condition 3.16:
PCW = 21 AeK gBLe 4vP × 4vB 4vB (3.16)
In this equation 3.16, CW stands for Continuous Wave and ⊗ denotes convolution. Ae is the effective core area of the fiber, vP is the pump linewidth and K is the polarization factor (1 ≤ K ≤ 2).
Le is the effective interaction length given by 3.17. Le=
1 − e−αL
