SIST-TP CLC/TR 50460:2006

SLOVENSKI SIST-TP CLC/TR 50460:2006

STANDARD
januar 2006
Kabelska omrežja za televizijske in zvokovne signale ter interaktivne storitve –
Sistemske smernice za analogne optične prenosne sisteme
(istoveten CLC/TR 50460:2005)
Cable networks for television signals, sound signals and interactive services –
System guidelines for analogue optical transmission systems
ICS 33.060.40 Referenčna številka
SIST-TP CLC/TR 50460:2006(en)
©  Standard je založil in izdal Slovenski inštitut za standardizacijo. Razmnoževanje ali kopiranje celote ali delov tega dokumenta ni dovoljeno

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TECHNICAL REPORT CLC/TR 50460
RAPPORT TECHNIQUE
TECHNISCHER BERICHT August 2005

ICS 33.060.40


English version


Cable networks for television signals,
sound signals and interactive services –
System guidelines for analogue optical transmission systems



Réseaux de distribution par câbles  Kabelnetze für Fernsehsignale, Tonsignale
pour signaux de télévision, signaux de und interaktive Dienste –
radiodiffusion sonore et services System-Leitfaden für analoge optische
interactifs – Übertragungssysteme
Lignes directrices pour les systèmes de
transmission optique analogiques






This Technical Report was approved by CENELEC on 2005-05-21.

CENELEC members are the national electrotechnical committees of Austria, Belgium, Cyprus, Czech
Republic, Denmark, Estonia, Finland, France, Germany, Greece, Hungary, Iceland, Ireland, Italy, Latvia,
Lithuania, Luxembourg, Malta, Netherlands, Norway, Poland, Portugal, Slovakia, Slovenia, Spain, Sweden,
Switzerland and United Kingdom.

CENELEC
European Committee for Electrotechnical Standardization
Comité Européen de Normalisation Electrotechnique
Europäisches Komitee für Elektrotechnische Normung

Central Secretariat: rue de Stassart 35, B - 1050 Brussels


© 2005 CENELEC - All rights of exploitation in any form and by any means reserved worldwide for CENELEC members.

Ref. No. CLC/TR 50460:2005 E

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CLC/TR 50460:2005 - 2 -
Foreword
Standards are very important sources of information on terms, definitions and minimum requirements
for technical products. EN 60728-6 is such a standard for optical equipment used in cable networks for
television signals, sound signals and interactive services. However, a lot of additional information and
knowledge is needed for designing and planning complete optical transmission systems. This kind of
information is not suitable for a standard because system engineers need freedom in their methods of
finding economical solutions for special needs. CENELEC TC 209 therefore decided to develop this
Technical Report which provides guidelines in how to use EN 60728-6 for planning analogue optical
transmission systems.
This Technical Report was prepared by the Technical Committee CENELEC TC 209, Cable networks for
television signals, sound signals and interactive services.
The text of the draft was submitted to the formal vote and was approved by CENELEC as
CLC/TR 50460 on 2005-05-21.

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Contents
1 Scope .5
1.1 General.5
1.2 Scope of this Technical Report .5
2 Normative references .5
3 Terms, definitions, symbols and abbreviations .6
3.1 Symbols .6
3.2 Abbreviations .6
4 Topologies used for optical transmission systems in cable networks.6
4.1 Point to point system.7
4.2 Point to multi-point system.7
4.3 Multi-point to point system .7
4.4 Real wavelength division multiplex system.8
4.5 Combinations .8
5 Influences of equipment and fibre parameters on the system performance.8
6 Optical modulation index .9
6.1 Single wavelength system .10
6.2 WDM systems.11
6.3 Choosing the right input level at the transmitter .11
7 Carrier to noise ratio.12
7.1 Short haul links with a single transmitter.12
7.2 Long haul point to point link .13
7.2.1 Long haul links with DFB lasers.13
7.2.2 Long haul links with externally modulated transmitters .13
7.3 Multiple transmitter systems (WDM).14
7.4 Transmission systems with optical fibre amplifier.14
8 Linearity.15
8.1 Composite Second Order (CSO) .15
8.1.1 CSO of 1 310 nm systems.16
8.1.2 CSO of 1 550 nm systems.17
8.2 Composite Triple Beat (CTB).17
9 Flatness.18
10 Output level .19
Annex A (informative) Brillouin scattering in optical fibres .20
Annex B (informative) Noise sources of optical transmission systems.22
B.1 Intensity noise .22
B.2 Shot noise of the receiver .25
B.3 Thermal noise .25
B.4 Carrier to noise ratio.25

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CLC/TR 50460:2005 - 4 -
Annex C (informative) Non-linear distortion in optical transmission systems .26
C.1 Non-linear products definition .26
C.2 Non-linear products weighting .27
C.3 Distortion generators in direct modulation cable network transmissions.28
C.4 Third order modelling .37
C.5 "Summation" of distortion products.38
Bibliography.40
Figure 1 - Point to point (PTP) system .7
Figure 2 - Point to multi-point (PMP) system.7
Figure 3 - Multipoint-to-point (WDM) system.7
Figure 4 - Real WDM system .8
Figure 5 - Definition of OMI for an optical transmitter .10
Figure C.1 - (a) second and (b) third order distortion of Volterra series expansion.27
Figure C.2 - Weighted second and third order distortions repartition for the 42 carriers frequency
allocation map defined in EN 50083-3 (software window capture).28
Figure C.3 - Polarisation-mode dispersion in single-mode fibre .29
Figure C.4 - static curve of a laser showing the clipping effect.30
Figure C.5 - Transmitted and backscattered power in the range of the Brillouin threshold .31
Figure C.6 - Cause of self phase modulation.32
Figure C.7 - CSO caused by laser chirping and chromatic dispersion .33
Figure C.8 - CSO degradation without PDL .34
Figure C.9 - CSO degradation with PDL = 0,5 dB [12].34
Figure C.10 - Simulated and measured intermodulation due to laser clipping .35
Figure C.11 - Zoom on a CTB .39
Table 1 - Interdependencies between equipment and system properties and the performance
parameters .9
Table C.1 - Weightings of HD2, HD3, IMD2 and IMD3 .27

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1 Scope
1.1 General
Standards of the EN 50083 and EN 60728 series deal with cable networks for television signals,
sound signals and interactive services including equipment, systems and installations
- for headend-reception, processing and distribution of sound and television signals and their
associated data signals and
- for processing, interfacing and transmitting all kinds of signals for interactive services using all
applicable transmission media.
All kinds of networks like
- CATV-networks
- MATV- and SMATV-networks
- individual receiving networks
and all kinds of equipment, systems and installations installed in such networks, are within this scope.
The extent of this standardisation work is from the antennas, special signal source inputs to the
headend or other interface points to the network up to the terminal input.
The standardisation of any user terminals (i.e. tuners, receivers, decoders, multimedia terminals etc.)
as well as of any coaxial and optical cables and accessories thereof is excluded.
1.2 Scope of this Technical Report
This Technical Report provides guidelines and procedures for determining the overall performance of
optical transmissions systems used in cable networks for television signals, sound signals and
interactive services. It is based on the requirements for optical equipment defined in the standard
EN 60728-6 (Cable networks for television signals, sound signals and interactive services – Part 6:
Optical equipment) and should be used together with this standard. The information provided is meant
to help field engineers and network planners (system designers) in planning and designing optical
systems. Though this content is less dense than in a standard, basic knowledge about system
parameters of cable networks is needed.
2 Normative references
The following referenced documents are indispensable for the application of this document. For dated
references, only the edition cited applies. For undated references, the latest edition of the referenced
document (including any amendments) applies.
EN 50083-3 2002 Cable networks for television signals, sound signals and
interactive services - Part 3: Active wideband equipment
for coaxial cable networks
EN 50083-7 1996 Cable networks for television signals, sound signals and
+ A1 2000 interactive services - Part 7, System performance
EN 60728-6 2003 Cable networks for television signals, sound signals and
interactive services - Part 6, Optical equipment
(IEC 60728-6:2003)
EN 60793-2 2004 Optical fibres - Part 2: Product specifications - General
(IEC 60793-2:2003)
IEC/TR 61931 1998 Fibre optics – Terminology

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CLC/TR 50460:2005 - 6 -
3 Terms, definitions, symbols and abbreviations
For the purposes of this technical report, the terms, definitions, symbols and abbreviations given in
EN 50083-7, EN 60728-6 and IEC/TR 61931 apply.
3.1 Symbols
Additionally to the symbols given in the above mentioned references, the following graphical symbol is
used in the figures of this technical report:
wavelength division multiplexer
WDM

3.2 Abbreviations
Additionally to the abbreviations given in the above mentioned references, the following abbreviations
are used in this technical report:
2HD second harmonic distortion
3HD third harmonic distortion
DFB distributed feedback
DWDM Dense wavelength division multiplex
IIN induced intensity noise
IMD2 second order intermodulation
IMD3 third order intermodulation
IM-DD intensity modulation – direct detection
I equivalent input noise current density of an optical receiver
r
OMI optical modulation index
MPI multi-path interference
PMD polarisation mode dispersion
PMP point to mulit-point
PTP point to point
RMS root mean square
SBS stimulated Brillouin scattering
WDM wavelength division multiplexer
4 Topologies used for optical transmission systems in cable networks
The overall performance of optical transmission systems depends on many parameters and
conditions. Separating the applications into different categories simplifies the step by step analysis
and leads to a better overview. A logical way to build up these categories is to distinguish different
network topologies because it can be assumed that the network architecture is always known in
advance. Starting from this point of view the following five topologies can be identified as relevant for
the user.

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4.1 Point to point system
Point-to-point (PTP) systems consist of a single optical transmitter and a single optical receiver
connected by a single line of fibre (Figure 6).
E O
O E

Figure 6 - Point to point (PTP) system
This configuration can typically be found in trunk lines feeding areas cabled with coax (HFC networks).
Both wavelengths, 1 310 nm and 1 550 nm, are used for these systems. Most of the optical budget is
consumed by the fibres attenuation (long distance system). At 1 550 nm, optical amplifiers can be
used to extend the range of this kind of system.
4.2 Point to multi-point system
In point to multi-point (PMP) systems a single optical transmitter feeds more than one optical receiver.
The receivers are connected to a main fibre via optical couplers and tap fibres as shown in Figure 7.
O
E
E O
O E
O
E

Figure 7 - Point to multi-point (PMP) system
An alternative configuration for feeding more than one receiver from a single transmitter is to use an
optical splitter at the transmitter node and individual fibres from the transmitter node to each receiver.
This leads to a star topology which should be treated as multiple PTP systems with a single
transmitter.
PMP systems are typically used when different coaxial parts of a network shall be supplied with the
same signal saving as much fibre as possible (optical distribution systems). Depending on the fibre
lengths, both wavelengths are used. At 1 550 nm, optical amplifiers can be used to compensate for
the fibre and splitting losses.
4.3 Multi-point to point system
Multi-point-to-point systems consist of at least two transmitters with different wavelengths sending their
signals to a common receiver. The transmitter signals may be combined by an optical coupler or if the
link loss is critical, by a wavelength multiplexer (Figure 8).
E
O
O
WDM
E
E
O

Figure 8 - Multipoint-to-point (WDM) system

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CLC/TR 50460:2005 - 8 -
Since optical receivers usually have a very broad input wavelength range, the central wavelengths of
the transmitters may be extremely different (e.g. 1 310 nm and 1 550 nm). In order to avoid signal
mixing in the receiver the optical spectrums of the transmitters have to differ at least by the upper limit
of the receivers electrical frequency range. If only signals in the 1 550 nm wavelength range are used,
optical amplifiers can be employed for extending the fibre length. Since all input signals of the system
are provided at the same system output, different frequency ranges have to be used for modulating
the transmitters.
This kind of topology is typically chosen if part of a network shall be provided with signals from
different locations.
4.4 Real wavelength division multiplex system
Real wavelength division multiplex systems consist of at least two PTP systems operating on the
same fibre. The transmitter signals are combined at the transmitter node with a wavelength multiplexer
or if the link loss is not critical, by an optical coupler. At the receiver node the different signals are
separated by another wavelength multiplexer and led to individual receivers (Figure 9).
E
O
O
E
WDM WDM
E O
O E

Figure 9 - Real WDM system
For only two different wavelengths this configuration can be built up easily combining a 1 310 nm
system and a 1 550 nm system. If wavelength dependent fibre losses can’t be tolerated or more than
two PTP systems have to be combined, a closer spacing of the wavelengths has to be chosen
(DWDM = dense WDM). This is usually done in the 1 550 nm wavelength range. Optical amplifiers can
be used to achieve longer link lengths in this case. Care has to be taken to avoid overlapping of the
transmitters spectrums. A narrow wavelength spacing means high efforts to control the transmitter
wavelengths and high costs for the wavelength division multiplexers.
The main reason for using this configuration is to save fibres. This approach allows to transmit digitally
and analogue modulated signals over the same fibre.
4.5 Combinations
The basic configurations described above can be combined to more complex architectures. The best
way dealing with such complex structures is to split them up to their basic parts which could be treated
separately.
5 Influences of equipment and fibre parameters on the system performance
The performance of analogue optical transmission systems depends not only on various equipment
parameters but also on the properties of the fibre installation. Some of these parameters and
properties interact in a way making it necessary to look at the transmission system at a whole. The
interdependencies between the equipment and system properties and the performance parameters
are shown in Table 2. The numbers in the table refer to the clauses of this report containing the
relevant information:

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Table 2 - Interdependencies between equipment and system properties
and the performance parameters
System performance parameters
Equipment properties C/N CSO CTB Flatness Output level
and effects
TX OMI 6, 7 6, 8.2 6, 8.1 (9) 6, 10
CSO 8.1
CTB  8.2
linewidth B.1.2
chirping 7.2 C.3.7.1, C.3.7.2C.4.1
RIN 7
power 7  10
(7.3)
λ
flatness  9
RX I 7
r
CSO 8.1
CTB  8.2
flatness  9
AGC range  9 10
F
OFA 7.4
power 7.4  10
gain 7.4 (8.1.2) (8.2)
7.4
λ
Fibre dispersion C.3.7.1, C.3.7.2(8.2)
SBS (7.2) (8.1.2) (C.4.3)
SPM C.3.10
PMD (8.1.1)
loss 7  10
Passive return loss (B.1.3)
PDL (C.3.7.2)
loss 7  10
X: relevant (X): can be relevant

This table can be used as an entry point and quick reference to the contents of this report.
6 Optical modulation index
The optical modulation index (OMI) is one of the most important parameters of analogue optical links.
It has to be chosen very carefully in order to obtain the best carrier to noise ratio without getting too
much distortion due to clipping effects (see C.3.3).
Working with OMIs two cases have to be considered: single wavelength systems and wavelength
division multiplex (WDM) systems.

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CLC/TR 50460:2005 - 10 -
6.1 Single wavelength system
The definition of the OMI is very similar to the definition of the modulation index in ordinary AM-
modulation. An illustrative explanation of this definition is shown in Figure 10.
P / mW
P
max
P
avg
P
min
P − P
max min
m =
2P
avg
I / mA
I I
th bias

Figure 10 - Definition of OMI for an optical transmitter
The OMI is defined as
(1)
P − P P − P
max min max min
m = =
P + P 2⋅ P
max min avg
where
m is the optical modulation index
P is the peak optical output power
max
P is the minimum optical output power
min
 is the mean optical output power
P
avg
Laser currents below I lead to clipping, the waveform of the optical output power becomes distorted.
th
The OMI is more than 1 then.
This definition relates to a single channel and a sinusoidal signal. The same definition can also be
used with QAM signals if the equivalent power is used to calculate a new peak value of the modulating
current. However, the signals transmitted in cable networks are a mixture of a whole bunch of
channels containing carriers with various modulation schemes. For each channel an individual OMI
can be determined.

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Since the peak-to-average ratio of combined signals decreases with the number of channels, the
individual OMIs don’t add up linearly for the total OMI. The total OMI for several channels is instead
calculated by summing the powers of individual carriers:
(2)
2 2 2
m = m + m +K+ m
T 1 2 N
Provided that all channels have equal OMI the formula simplifies to
(3)
m = m N
T
The total OMI is a practical value for making estimations of the maximum channel counts or for C/N
calculations with a certain number of channels [6]. But the peak-to-average ratio for relatively small
numbers of channels can be surprisingly high and clipping may occur at smaller total OMI values than
in case of more than 10 channels. For analogue carriers m = 0,3 is a typical value for 1 310 nm
T
directly modulated transmitter and m = 0,25 to 0,28 for externally modulated 1 550 nm transmitter.
T
6.2 WDM systems
In WDM systems outputs of optical transmitters are combined using wavelength division multiplexers
or optical couplers. Thereby the average optical output powers add up resulting in a reduced OMI for
the individual channels. For the combination of signals from two transmitters the resulting OMI of a
channel can be calculated by:
(4)
m ⋅ P
1 1
m =
P + P
1 2
where
m is the optical modulation index (OMI) of the channel to be considered, transmitted by the first
1
transmitter
P is the optical power of the first transmitter at the output of the combining device (optical
1
coupler or WDM)
P is the optical power of the second transmitter at the output of the combining device (optical
2
coupler or WDM)
For more than two transmitters the denominator has to be replaced by the sum of all powers at the
output of the combining device. If the combined signal is fed to an optical receiver, the carrier to noise
ratio at its output is lower than with a single optical signal due to the reduced OMIs. Therefore multi-
point to point systems are not very popular and true WDM systems with separate receivers for each
wavelength are preferred in cable networks. Nevertheless this configuration can be useful for adding
narrowcast signals to broadcast signals in existing networks. As equation (4) shows, great care has to
be taken by adjusting the power levels of the input signals.
6.3 Choosing the right input level at the transmitter
The OMI is directly related to the driving current of the laser hence to the input level of the transmitter.
Therefore choosing the right transmitter input level is crucial for any optical transmission link. Some
manufacturers solved this problem by developing special driving amplifier with an automatic gain
control (AGC) for their transmitters. This results in a broader input level range for achieving the
optimum OMI. Nevertheless even with this kind of solution, the input level should be chosen carefully
in order to save the AGC range for unwanted changes in the level or the channel load.
EN 60728-6 requires manufacturers to publish the required input level at which the required
performance can be met (see 6.1.1 of EN 60728-6). Starting from this level the optimum input level for
a given channel load can be calculated, using the following procedure.

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CLC/TR 50460:2005 - 12 -
1) Check the count of channels related to the given reference input level stated in the data sheet of
the optical transmitter. Since according to EN 60728-6 all transmitters shall be designed for the
frequency range 47 MHz to 862 MHz and all measurements shall be carried through using the
channel allocation specified in EN 50083-3, this count will usually be n = 42.
0
2) Determine the effective channel load for the considered system. For the effective channel load
the different levels of the channels to be transmitted have to be taken into account:
ΔU
n
10
n = 10     (5)
∑
e
n
where ΔU is the deviation from the level of an analogue channel in dB.
n
3) With this effective channel load the target deviation ΔU from the reference input level can be
calculated easily with:
 
n
e
U 10 lg  in dB    (6)
Δ = ⋅
 
n
 0 
4) If the OMI is given for the reference input level (m ), the OMI for the new channel load can be
ref
calculated with:
n
0
m = m     (7)
e ref
n
e
This procedure should be used only when the number of channels (n and n ) is higher than 10 (see
e 0
6.1). For lesser channel loads significant deviations from the optimum OMI can occur.
7 Carrier to noise ratio
Noise in optical links can be divided into three different components: intensity noise, shot noise and
thermal noise (Annex B). Intensity noise is noise associated with the generation of light in transmitters
and optical amplifiers, shot noise appears in the receiver and thermal noise is a noise mechanism of
the electrical amplifiers. The parameters needed for calculating the carrier to noise ratio of an optical
link are either well known or shall be given in the data sheets of the equipment as required by
EN 60728-6.
7.1 Short haul links with a single transmitter
The carrier to noise ratio for a single channel of an optical point to point link up to lengths of about
L = 30 km can be calculated using equation (25) in 4.19 of EN 60728-6:
(8)
2
 
1  
2e
I
− RIN r
 
 
C/N = 20lgm −10lg(2B) −10lg + +
10 10
 2 2 
 
rPopt,RX r Popt,RX
 
 
where
-1
RIN is the relative intensity noise in dB/(Hz) . This value has to be published in the data sheet
of the optical transmitter.
m is the optical modulation index (OMI) of the channel to be considered. For choosing the
right OMI see Clause 6.
P is the optical power incident on the photodiode in W;
opt,RX
r is the responsivity of the photodiode in A/W;
B is the band
...