SCTE Reverse Path Overview Carrier to Noise and Reverse Carrier loading

1
SCTE Reverse Path OverviewCarrier to Noiseand
Reverse Carrier loading

• Steve DuChene
• Bright House Networks
• steve.duchene_at_mybrighthouse.com
• 1/26/2006

2
What are we going to cover today

• Carrier to Noise elements in the reverse path and
  how they interact with each other
• Laser set up and drive parameters
• Noise Power Ratio

3
Different Services require Different CNR

• HSD
• 16 QAM
• STB (VOD)
• QPSK
• Telemetry
• FSK
• Business Services
• QPSK to 16 QAM
• Modulation Type Required CNR
• Required CNR for various modulation schemes to
  achieve 10E-7 BER
• BPSK 12dB
• QPSK 15dB
• 16QAM 22dB
• 64QAM 28 dB

• Multiple services on the return path with
  different types of modulation schemes will
  require allocation of bandwidth and amplitudes.
• Can be engineered.
• Requires differential padding in Headend

4
Four Main Noise Areas
Drop System
Optical Plant
Coaxial Plant
Headend
5
Typical Two-Way HFC CATV System?
6
Carrier to Noise
7
Four Main Sources of Reverse Path Noise

• Home or Business drop plant (Ingress)
• RF Ingress should be over 60 dB down but many
  times it is not
• Coaxal RF plant
• About 45 to 55 dB C/N-3.2 MHz BW
• (Including Amplifier Noise and Drop Ingress)
• Thermal noise only will be 54 to 58 dB the rest
  is RF ingress
• Optical link from the node to Headend or Hub
• About 43 to 48 dB C/N -3.2 MHz BW (For FP Laser)
• Node combining in Headend and Hub
• (Two or more nodes combined to make service
  groups)
• All these sources add together to make your final
  C/N at the given receive system
• About 35 to 40 db C/N at 3.2 MHZ BW per carrier
  with up to 4 carriers

8
Carrier to Noise in Coaxial Plant
9
Reverse Path Unity Gain

• Unity gain in the upstream path exists when the
  amplifiers station gain equals the loss of the
  cable and passives upstream from that location.
  In this example, the gain of each reverse
  amplifier is 15 dB. The 30 MHz losses following
  each amplifier are 15 dB as well.
• For example, the 4 dB loss between the first and
  second amplifier is all due to the cable itself,
  so the second amplifier has an 11 dB output
  attenuator. The amplifier input is 20 dBmV,
  making the reverse amplifier module output 35
  dBmV. In order to obtain unity gain and the
  correct input at the first upstream amplifier
  location, an 11 dB output attenuator is required
  at the second amplifiers reverse output so that
  the total loss equals the gain of the amplifier.
• The third amplifier (far right) feeds a span that
  has 3 dB of loss in the cable and another 2 dB of
  passive loss in the directional coupler, for a
  total loss of 5 dB. In order for the total loss
  to equal the amplifiers 15 dB of gain, it is
  necessary to install a 10 dB output attenuator at
  the third amplifier.
• In the upstream plant, the unity gain reference
  point is the amplifier input.

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CATV Return Distribution Network Design
Level Values shown are at 30 MHz with a 2 port
drop splitter
Amplifier upstream input 18 dBmV
0.6 dB
0.8 dB
1.2 dB
1.3 dB
1.9 dB
125 ft
125 ft
125 ft
125 ft
125 ft
26
23
20
17
14
8
0.5 dB
0.5 dB
0.5 dB
0.5 dB
0.5 dB
125 ft splitter
125 ft splitter
125 ft splitter
125 ft splitter
125 ft splitter
125 ft splitter
5 dB
5 dB
5 dB
5 dB
5 dB
5 dB
Modem TX
49 dBmV
47 dBmV
45 dBmV
44 dBmV
43 dBmV
39 dBmV
Feeder cable 0.500 PIII, 0.4 dB/100 ft Drop
cable 6-series, 1.22 dB/100 ft
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CATV Return Distribution Network Design
Level Values shown are at 30 MHz with a 4 port
drop splitter
Amplifier upstream input 18 dBmV
0.6 dB
0.8 dB
1.2 dB
1.3 dB
1.9 dB
125 ft
125 ft
125 ft
125 ft
125 ft
26
23
20
17
14
8
0.5 dB
0.5 dB
0.5 dB
0.5 dB
0.5 dB
125 ft splitter
125 ft splitter
125 ft splitter
125 ft splitter
125 ft splitter
125 ft splitter
8.5 dB
8.5 dB
8.5 dB
8.5 dB
8.5 dB
8.5 dB
Modem TX
52.5 dBmV
50.5 dBmV
48.5 dBmV
47.5 dBmV
46.5 dBmV
42.5 dBmV
Feeder cable 0.500 PIII, 0.4 dB/100 ft Drop
cable 6-series, 1.22 dB/100 ft
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Coax Distribution Design Assumptions

• Coax Drop loss (_at_30MHz)
• Total Loss (Coax Combiner), typical max 10dB
• Distribution coax tap port loss
• Typical 21dB
• Maximum 26 dB
• Operating window is 15 to 20 dB
• Reference carrier input level
• 18dBmV

Used to calculate RF power headroom Derived from
system and industry plant design data
Taken from system plant design guide
Selected to insure a balance between operating
headroom and sufficient carrier to ingress
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Why do we run Plant and Drop RF levels as high
as possible?

• To Keep signal above plant noise ( Fairly easy)
• Thermal noise is not really a problem in reverse
  plant if it is balanced properly
• To keep signal above drop ingress
• (not as easy)
• The drop plant is much harder to keep a handle
  on. As a rule of thumb the higher the return RF
  level is the fewer ingress problems you will
  have.
• Coaxial plant C/N should be about 45 to 55 dB

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Thermal Noise

• Characteristic of all active components
• Optoelectronics
• Upstream amplifiers
• In-home devices
• Improper network alignment or defective equipment
  can cause high levels of thermal noiseas can
  improper upstream combiningwhich will degrade
  carrier-to-noise ratio

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Thermal Noise
Good carrier-to-noise ratio (50 dB)
Poor carrier-to-noise ratio (12 to 15 dB)
16
RF Ingress

• Upstream spectrum is shared with over-the-air
  users
• Short-wave broadcasts
• Citizens band (CB) radio
• Amateur (ham) radio
• Ship and aeronautical communications
• Government communications
• RF signals can enter network through cable
  shielding defect

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RF Ingress

• CB radio operator had installed his own cable
  outlets

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Upstream Over-The-Air Spectrum, 5-30 MHz
Source NTIA (http//www.ntia.doc.gov/osmhome/allo
chrt.pdf)
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Impulse Noise

• Most upstream data transmission errors caused by
  bursts of impulse noise
• Fast rise time, short duration
• Most less than 10 microseconds duration
• Significant energy content over most of upstream
  spectrum
• Common sources
• Vehicle ignitions, neon signs, lightning, power
  line switching transients, electric motors,
  electronic switches, household appliances

20
Impulse Noise

• Impulse noise from arc welder in machine shop

21
Reverse Laser drive and performance

• Using NPR Curves

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What is NPR?

• NPR Noise Power Ratio
• Is means of easily characterizing an optical
  links linearity and noise contribution
• NPR and CNR are related, but not the same

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DFB Laser NPR Response
NPR ?CNR Along Right-hand Side
NPR CNR Along Left-hand Side

• 7dB Optical Link
• 37MHz Loading

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FP NPR Curves

• The NPR curve changes over temperature
• FP NPR curves shown over temperature range of -40
  to 140 F

25
FP and DFB NPR Curves

• FP and DFB NPR curves at room temperature.

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Adding Carriers to the Return Path
HSD
Business Services
VOD
VOIP
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Per Carrier Power vs. Composite Power

• As you add more carriers to the return path the
  composite power to the laser increases.
• To maintain a specific amount of composite power
  into the transmitter the carrier power must be
  reduced.
• When modulation schemes are changed the composite
  power into the transmitter changes.
• The higher the order of modulation the more peak
  energy the channel contains.

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Per Carrier Power vs. Composite Power
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Per Carrier Power vs. Composite Power
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Changing Modulation Type
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Changing Modulation Type
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Determining Power Levels

• Power per Hz
• Power per Hz total power - 10log(total
  bandwidth in Hz)
• Channel power from power per Hz
• Channel power power per Hz 10log(channel
  bandwidth in Hz)

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Power Levels

• Example Calculate the power per Hz for a
  manufacturers 45 dBmV maximum laser input power
  specification in the 5-40 MHz reverse spectrum
  (35 MHz bandwidth)
• Power per Hz Total power - 10log(total
  bandwidth in Hz)
• Power per Hz 45 dBmV - 10log(35,000,000)
• Power per Hz -30.44 dBmV per Hz
• -30.44 dBmV per hertz represents the maximum
  power into the laser allocated over 35 MHz
• Now lets calcluate what a 2MHz wide QPSK carrier
  would need to be to equate to that level.

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Determining Digital Power Levels

• Example Calculate allocated channel power for a
  2 MHz wide QPSK digitally modulated signal
  carried in the reverse path of the previous
  example.
• Channel power power per Hz 10log(channel
  bandwidth in Hz)
• Channel power -30.44 10log(2,000,000)
• Channel power 32.57 dBmV

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Setting the Transmitter Window

• RF input levels into a return laser determine the
  CNR of the return path.
• Higher input better CNR
• Lower input worse CNR
• Too much level and the laser clips.
• Too little level and the noise performance is
  inadequate
• Must find a balance, or, set the window the
  return laser must operate in
• Not only with one carrier but all the energy that
  in in the return path.
• The return laser does not see only one or two
  carriers it sees the all of the energy
  (carriers) that in on the return path that is
  sent to it.

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NPR Response Corrected for Optical Budget
Left side moves down 1dB

• Moving from 7 to 8dB Optical Link
• 37MHz Loading

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NPR Response Corrected for Bandwidth
Right side moves out 6.2dB for 9MHz Noise
Bandwidth

• 8dB Optical Link
• Change from 37 to 9MHz Noise Power Loading

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Signal Clipping

• RF ingress and impulse noise may cause signal
  clipping
• Can effect Composite power into return laser
• Excessive signals from in-home devices such as
  pay-per-view converters also may cause signal
  clipping
• Clipping occurs in upstream amplifiers and fiber
  optics equipment
• FP Upstream lasers most susceptible
• Energy that can cause clipping found mostly from
  5 MHz to 15 MHz range
• Signals at all other frequencies are affected by
  cross-compression
• Cross-compression affects all upstream
  frequencies
• Can reduce data throughput (TCP/IP controlled
  resend)

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Signal Clipping

• To avoid clipping setup return laser operational
  window to recommended level
• Do not adjust levels at the node once setup is
  accomplished
• Dont change the pad to get more RF level in the
  Headend.
• Dont change the pad to get better CNR at the
  return RX.
• Set it and Leave it.

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Optical Receivers

• Receiver input padding
• To ease the use of a wide verity of optical
  receivers each with unique input power limits it
  is common practice to pad the receiver input
  power levels to a common sweet-spot range,
  typically -7 to -8 dBm.
• Receiver output level
• The receiver RF gain is adjusted to yield a
  reference output level per carrier (e.g. 38 dBmV)
  

41
Return RX Setup

• Rule of Thumb
• Do not optically attenuate the return path so all
  the optical inputs are the same as the lowest.
• The lower the optical input power the lower the
  CNR of the receiver.
• Attenuate RF internally or externally of the
  device
• Must have enough level so that the CMTS or other
  devices receiving the signals from the return
  path operate acceptably.
• There can be excessive passive loss from the
  output of the optical receiver to the terminating
  device.
• 8-way splitter/combiner 10.2 dB typical
• 4-way splitter/combiner 6.8 dB typical
• Typical input into terminating device.
• CMTS 0 dBmV
• DNCS - -3 to 27 dBmV

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Return RX Setup

• On analog returns from the node the less optical
  power into a receiver the less RF you will have
  on the output.
• The RF levels on the output of the return
  receivers should be set with internal or external
  RF attenuation such that with the X level that is
  placed into the forward test point on the node X
  level will exist on the output of all receivers.
• To much optical power can cause intermodulation
  in the receiver
• Typical maximum input -3 dBm newer receivers are
  good to 0 dBm
• Use optical attenuators on extremely short paths
  or where to much optical power exists into a
  receiver. Most new reverse transmitters have
  higher optical output than older models
• To little optical power can cause CNR problems
  with that return path.
• If combined with other node return receiver
  outputs noise issues due to more paths can occur.

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Final C/N numbers and how they change
44
Zero Point cross over Optical and Coaxial plant
are at same C/N
45
Zero Point cross over Optical and Coaxial plant
are at same C/N
46
C/N power adds (gets worse) faster as Coaxial C/N
and Optical C/N approach each other The zero
point is when both fiber and coaxial C/N are the
same valueDo you see the 3 db point?
Zero Point cross over Optical and Coaxial plant
are at same C/N
10 dB change here degrades C/N by 3 db
10 dB change here degrades C/N by 7 db
47
Headend Combining and Other Issues
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Headend Combining

• Combining ratios (node to application Rx)
• 41, required a 6dB increase in minimum CNR and
  NPR
• 21, requires a 3dB increase in minimum CNR and
  NPR
• 11, no minimum CNR change required (this one for
  16 QAM use)
• Ratio planning
• 41, desirable for applications with low
  subscriber densities or take-rates (e.g. VoIP,
  commercial services), FPs are challenged
• 21, suitable for maturing applications with
  higher subscriber take-rates (e.g. HSD), FPs are
  OK for lower bandwidths
• 11, suitable for mature high subscriber density
  and take-rate applications (e.g. HSD), FPs can
  work if plant clean
• Combining may vary by application!

Combining ratio, QAM index bandwidth drives
laser type selection!
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Changes to the Return PathLong Loop AGC
50
2-Way Applications Design Assumptions

• Supported Applications
• Number of upstream carriers
• Noise bandwidth per carrier
• QAM index (type of QAM) per carrier
• Client Device (CPE)
• Transmitter maximum output (dBmV)
• Auxiliary return amplifier gain (if required)
• Terminal Server
• Target per-carrier input power level (dBmV)
• CPE power management enabled
• Absolute minimum required CNR

Used to calculate RF payload bandwidth
Used to establish minimum low and high power NPR
Used to calculate RF power headroom
Used in X-Factor setup process
Used to select NPR targets. CableLabs specs 25dB
for DOCSIS _at_ 16QAM
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Setting up the Return Path

• Finding the X Level
• Determining the Return Transmitter Window
• Padding the Transmitter
• Return Receiver Setup
• Distribution out of the Return Receiver
• Padding the inputs to the Headend Equipment

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Setting Upstream Signal Levels

• X level or Reference level
• The easiest way to set upstream signal levels is
  to establish what is called the X level.
• This is a headend upstream signal level that is
  the result of providing the proper level at the
  input to the last reverse amplifier (the first
  amplifier or node out of the headend).
• To establish the X level, go to the first
  downstream amplifier or node location out of the
  headend.
• Here you should inject a signal into that
  locations reverse amplifier module input at a
  level known to be correct.
• This will result in a signal at the headend that
  is measured and defined as the X level.
• Assuming your system was designed for unity gain
  operation, when you go to the next amplifier
  location and inject the proper amplitude test
  signal there, the resulting signal at the headend
  will be the same as the original X level.
• If it is not, you can make necessary adjustments
  and install the proper output attenuator and
  equalizer to achieve the correct upstream input
  level at the first amplifier location, which will
  give you the desired headend X level.

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X Level work Template
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Return Path at the Network Layer
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Changes to the Return Network

• ANY CHANGES TO THE RETURN PATH FROM THE SUBSRIBER
  TO THE HEADEND CAN EFFECT ITS PERFORMANCE
• Planned
• Segmentation of Return
• Changes in HE or Node
• Moving to higher order modulation such as 16 QAM
  from QPSK
• Un-Planned
• Bad tap
• Optronics Failure
• Ingress
• Technician Laser RF input level changes in the
  field

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Conclusions

• Return system is a loop
• Changes anywhere in the loop can effect the
  performance of the network
• Modem outputs can vary from manufacturer to
  manufacturer
• Once the return laser is setup DONT TOUCH IT
• Changing the drive levels can effect the window
  of operation of the laser
• Work as a team to diagnose system problems

57
Critical Factors Recap

• Key laser transmitter selection drivers
• QAM type establishes minimum CNR and NPR
• Combining adds to minimum CNR and NPR
• Bandwidth establishes laser response (NPR
  correction)
• A reference level tone (18dBmV) applied at the
  node must result in the application receivers
  target input level (padding of Rx inputs)
• Set the node up properly and do NOT change it!
• Avoid routinely solving modem output level
  problems by individually inserting return path
  loss at the modem (if your doing this something
  else is wrong!)
• CPE power management must be enabled on this
  application servers and modems
• Avoid performing node setups during extremes in
  outdoor temperature

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References
Steve DuChene Modeling for Advanced
Services, August 2003 Bright House Networks
Engineering Hranac, R. Linear Distortions,
Part 1 Communications Technology, July
2005 www.ct-magazine.com/archives/ct/0705/0705_li
neardistortions.htm Hranac, R. Linear
Distortions, Part 2 Communications Technology,
August 2005 www.ct-magazine.com/archives/ct/0805/
0805_lineardistortions.htm Farmer, J., D.
Large, W. Ciciora and M. Adams. Modern Cable
Television Technology Video, Voice and Data
Communications, 2nd Ed., Morgan Kaufmann
Publishers 2004 Scientific Atlanta - Reverse
path laser specification sheet 750874 Rev B,
November 2003 www.scientificatlanta.com Scient
ific Atlanta Engineering Customer Support
www.scientificatlanta.com Hranac, R. Two-way
(no impairments), October 2003 www.cisco.com