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Testing Upstream and Downstream DOCSIS 3.1 Devices April 2015
Steve Hall
DOCSIS 3.1 Business Development Manager
Page
© Keysight
Technologies 2015
Agenda
1. Decoding and demodulating a real downstream DOCSIS 3.1 signal and reporting key metrics
such as MER and BER.
2. Combining powerful trigger and recording techniques to detect, identify and repair issues that
cause BER.
3. Understanding the complexities of demodulating and measuring low frequency, small grant
upstream signals.
4. Generating in-channel and out-of-channel upstream and downstream DOCSIS 3.1 signals.
2
Page
© Keysight
Technologies 2015
1. Decoding and demodulating a real downstream DOCSIS 3.1 signal and reporting key
metrics such as MER and BER.
2. Combining powerful trigger and recording techniques to detect, identify and repair issues that
cause BER.
3. Understanding the complexities of demodulating and measuring low frequency, small grant
upstream signals.
4. Generating in-channel and out-of-channel upstream and downstream DOCSIS 3.1 signals.
3
Agenda
Page
© Keysight
Technologies 2015
Physical Layer Link Channel (PLC) OFDM Channel Descriptor (OCD)
Downstream Profile Descriptor (DPD) messages
DOCSIS 3.1 Signal Format
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© Keysight
Technologies 2015
NCP
Fields
Data
Fields
DOCSIS 3.1 Signal Format
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Automatic Demodulation of Downstream signals in VSA
- New DOCSIS3.1 Downstream measurement option BHM for 89601B VSA
(BHM)Key features:
- Automatic setup for downstream configuration by demodulating and
decoding:
Physical Layer Link Channel (PLC)
OFDM Channel Descriptor (OCD)
Downstream Profile Descriptor (DPD) messages
- Decode the NCP and locate the codewords for each profile
- Demod the data for each profile - MER
- Post-process MER & BER/PER measurements to accumulate over
successive acquisition.
© Keysight
Technologies 2015 6
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Result Traces – DOCSIS3.1 Downstream
Step 1 – detect and decode PLC Physical Layer Link Channel
Step 2 – detect and decode OCD OFDM Channel Descriptor
(not necessarily sent every frame as per standard)
Step 3 – detect and decode DPD Downstream Profile Descriptor
© Keysight
Technologies 2015 7
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Result Traces – DOCSIS3.1 Downstream
Initial performance results…
© Keysight
Technologies 2015 8
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Result Traces – DOCSIS3.1 Downstream
…
…
Report Profile Results
Report NCP Results
De
sc
rip
tio
n o
f R
es
ult
s
© Keysight
Technologies 2015 9
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Result Traces – DOCSIS3.1 Downstream
© Keysight
Technologies 2015 10
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Result Traces – DOCSIS3.1 Downstream
© Keysight
Technologies 2015 11
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MER Measurement
Hardware Positioning
$
Max 160MHz
Demod BW + RF Measurements
Max 750 MHz
Demod BW
Max 510 MHz
Demod BW + RF Measurements
MER
12
© Keysight
Technologies 2015
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Introducing the N9040B UXA Signal Analyzer
8.4/13.6/26.5
GHz
Streamlined
touch driven
interface
Full BW RTSA
Up to 510
MHz analysis
BW
89600 VSA &
N9068C
Phase Noise
App
Industry
Leading
Phase noise © Keysight
Technologies 2015 13
Page
– U5303A PCIe High-Speed
Digitizer
• PCIe rack mount expansion
chassis
• Desktop PC
– UXA N9040B Signal Analyzer
• Install 89600 VSA software on
Windows 7 OS or on an
external PC
© Keysight
Technologies 2015
89601B Vector Signal Analyzer
OR
Hardware Platforms: U5303A digitizer or X-Series Spectrum Analyzer
Page
Hardware Platforms: U5303A digitizer and X-Series Spectrum Analyzer
Preserve your X-Series investment!
Combine the power of the PCIe Digitizer Modulation Measurements
with the power of the PXA/UXA RF Measurememts.
Personal Cpmputer
© Keysight
Technologies 2015 15
Page
© Keysight
Technologies 2015
Agenda
1. Decoding and demodulating a real downstream DOCSIS 3.1 signal and reporting key metrics
such as MER and BER.
2. Combining powerful trigger and recording techniques to detect, identify and repair issues
that cause BER.
3. Understanding the complexities of demodulating and measuring low frequency, small grant
upstream signals.
4. Generating in-channel and out-of-channel upstream and downstream DOCSIS 3.1 signals.
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Page
Device Under
Test
DOCSIS
Frame
Generator
M8190A
To BER or not to BER:
BER says everything is “working nicely” – or that it is NOT!
Hours of testing complete… …yet BER fails!
How do you determine WHY your Bit Error Rate is failing?
Golden Receiver Cable Modem
• CMTS (single ended)
• Optical Transport
• Amplifier
BER
What causes bit errors?
• Clipping?
• Noise?
• Compression?
• Interference?
How and where do they show up other than in an
accumulated over-time BER test?
• Rapid fire in-channel and adjacent-channel energy
spikes?
• Increased unexpected signal amplitude?
• Spurious emissions?
• Reduced Adjacent Channel Power performance? © Keysight
Technologies 2015 17
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How do DOWNSTREAM events manifest themselves?
These events show up at various times and places
and are distinguishable as perturbations in the
frequency domain!
By building a very flexible mask around the spectrum,
these perturbations can be trapped, recorded,
analyzed and subsequently – questions answered! © Keysight
Technologies 2015 18
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© Keysight
Technologies 2015
The test steps…
Monitor Transient Spectral Behavior
in Real Time
Trigger on Transient Spectral Behavior in
Real Time
Record Transient Spectral Behavior Including Behavior
Leading up to the event
Determine if the Transient Spectral
Behavior Caused Bit Errors
Determine Root Cause and Repair
Issue
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© Keysight
Technologies 2015
The Tools: How are transients found?
Real Time Spectrum Analysis
Watch for the transient event…
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© Keysight
Technologies 2015
The Tools: How are transients found?
Real Time Spectrum Analysis
Watch for the transient event…
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© Keysight
Technologies 2015
Frequency Mask trigger
Catches the transient…
The Tools: How are transients found?
Real Time Spectrum Analysis
Watch for the transient event…
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© Keysight
Technologies 2015
Set up frequency Mask Trgger in VSA…
The Tools: How are transients found?
Real Time Spectrum Analysis + Vector Signal Analysis
Watch for the transient event…
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© Keysight
Technologies 2015
Multiple Domain Insight
Watch for the transient event…
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© Keysight
Technologies 2015
Note spectral behavior of transient event…
Multiple Domain Insight
Watch for the transient event…
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© Keysight
Technologies 2015
Transient Event Impact on Constellation
Impact on MER
The Tools: How are transients found?
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DUT (Amplifier/Optical
Transport or CMTS)
M8190A (DOCSIS 3.1 Frame
Generator)
Real Time Spectrum
Analyzer uses
frequency Mask
Trigger to detect
unwanted energy in
and around the
desired signals.
Vector Signal Analyzer begins recording triggered by a violation detected by
the Real Time Spectrum Analyzer including a pre-trigger window
Monitor Transient Spectral Behavior
in Real Time
Trigger on Transient Spectral Behavior in
Real Time
Record Transient Spectral Behavior Including Behavior
Leading up to the event
The test steps…
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Study demodulation test results for the gap
free recording to answer questions like:
Was signal quality impacted buy the event
that caused the recording to be triggered?
Calculate and
analyze BER
results for the
entire gap-free
recorded frames in
order to answer the
question: Did THIS
event cause bit
errors?
Determine if the Transient Spectral
Behavior Caused Bit Errors
Determine Root Cause and Repair
Issue
© Keysight
Technologies 2015 28
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Automating the process…
Test Controller
Create scripts to automate and record
events.
Monitor Transient Spectral Behavior
in Real Time
Trigger on Transient Spectral Behavior in
Real Time
Record Transient Spectral Behavior Including Behavior
Leading up to the event
Determine if the Transient Spectral
Behavior Caused Bit Errors
Determine Root Cause and Repair
Issue
© Keysight
Technologies 2015 29
Page
Testing for the Real Upstream Environment…
CM 1
CM 2
CM 3
CM n
Node Node FIBER
CMTS
Environmental
Interference
How tolerant is this system of this signal?
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Technologies 2015 30
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Testing for the Real Upstream Environment…
Node Node FIBER
Environmental
Interference
How tolerant is this system of this signal?
Upstream
Generator
Plus
Impairments
Real Time
Spectrum
Analyzer
Plus
VSA
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Technologies 2015 31
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Real Time Spectrum Analysis and Event recording…
Step 2: Initiate Signal recording!
Step 3: Post-process using Multi-
measurements to show impact during
the event… Step 4: Post-process Multimeasurements
Show recovery after the event the event…
Step 1: RTSA Trigger Mask
© Keysight
Technologies 2015 32
Page
© Keysight
Technologies 2015
Agenda
1. Decoding and demodulating a real downstream DOCSIS 3.1 signal and reporting key metrics
such as MER and BER.
2. Combining powerful trigger and recording techniques to detect, identify and repair issues that
cause BER.
3. Understanding the complexities of demodulating and measuring low frequency, small
grant upstream signals.
4. Generating in-channel and out-of-channel upstream and downstream DOCSIS 3.1 signals.
33
Page
Hybrid Fiber
Coax (HFC)
Network
Stability of MER Measurements at Low Grant %
CMTS
1
96MHz Channel
Spectrum
3
4
5
2
OFDMA
96MHz Channel
Cable
Modems
OFDMA: Low-grant signals are those where less
than about half of the subcarriers are active, and the
remainder are transmitted as nulls.
1 2 3 4 5
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Technologies 2015 34
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Stability of MER Measurements at Low Grant %
Measurement Challenge: Vector Signal Analysis synchronization algorithms are
optimized for signals with a relatively high grant percentage.
5% grant
96 subcarriers
96 MHz Acquisition BW
DC
Sample Freq. 102.4 MHz
Center Freq. 53 MHz
2048 point FFT
© Keysight
Technologies 2015 35
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Stability of MER Measurements at Low Grant %
5% grant
96 subcarriers
96 MHz Acquisition BW
DC
Sample Freq. 102.4 MHz
Center Freq. 53 MHz
2048 point FFT
Mathematically
equivalent to:
Sample Freq. 6.4 MHz
Center Freq. 8 MHz
128 point FFT
100% grant
96 subcarriers
~5 MHz Acquisition BW
Measurement Approach: Reconfigure existing measurements to zoom in on the active
subcarriers only. Thus, rather than analyzing a small percentage of a large FFT block,
the demodulator processes a smaller FFT block that is almost fully-populated!
Smaller acquisition BW improves SNR to the sync algorithms, stabilizes MER readings. Mathematically, the two
approaches are identical – there is no effect on the calculation of MER, carrier frequency error, symbol clock error,
or other measurement results.
Automatic Bandwidth Translation
Compliments of Optimizer Macro!
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Technologies 2015 36
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Stability of MER Measurements at Low Grant %
“Optimize” VSA Macro
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Results Comparison
Demod config: 2K FFT – 5% grant Demod config: 128 FFT – 100% grant
Avg. MER: ~37 dB
Variance: ±5 dB
Sync stability: 50%
Avg. MER: 53.8 dB
Variance: ±1 dB
Sync stability: 100%
Identical hardware, identical input signal:
© Keysight
Technologies 2015 38
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Stability of MER Measurements at Low Grant %
Small Grant 1 Small Grant 2
Setting the grants side by
side creates a challenging
measurement scenario.
Hybrid Fiber
Coax (HFC)
Network
CMTS
1
96MHz Channel
Spectrum
3
4
5
2
OFDMA
96MHz Channel
Cable
Modems
1 2 3 4 5
© Keysight
Technologies 2015 39
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Reconfiguring OFDM for Synchronized ACP
• The DOCSIS 3.1 PHY specification requires that Adjacent Channel
Power be measured in the spectrum regions directly adjacent to
the transmitted signal, starting at one subcarrier spacing (25 or 50
kHz) from the outermost active subcarriers.
• However, even well-windowed OFDM signals will have significant
sinX/X modulation sidebands that extend into this region which are
visible when viewed on a traditional swept spectrum analyzer.
© Keysight
Technologies 2015 40
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Reconfiguring OFDM for Synchronized ACP
• The ACP specification is not meant to include these sidebands, but
rather to measure only the unwanted noise and distortion that may
accompany them.
• In order to separate these components, the spec requires that ACP
measurements at offsets of 2 MHz or less be performed
synchronously - that is, sampled at the same frequency and in
phase with the underlying OFDM modulation clock.
• This suppresses all orthogonal (modulation-related) energy,
leaving only the non-linear distortion to be summed.
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Technologies 2015 41
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Reconfiguring OFDM for Synchronized ACP
Adds extra “null” subcarriers to the OFDM
demod configuration.
Configure analyzer to display the null
subcarriers.
Read ACP from VSA spectrum display
using band power markers.
Optimizer
VSA
© Keysight
Technologies 2015 42
Page
Agenda
1. Decoding and demodulating a real downstream DOCSIS 3.1 signal and reporting key metrics
such as MER and BER.
2. Combining powerful trigger and recording techniques to detect, identify and repair issues that
cause BER.
3. Understanding the complexities of demodulating and measuring low frequency, small grant
upstream signals.
4. Generating in-channel and out-of-channel upstream and downstream DOCSIS 3.1 signals.
© Keysight
Technologies 2015 43
Page
M8190A – Main Features
Breakthrough performance
Up to 90 dBc SFDR for
Reliable and repeatable measurements
14 bit 8 Gsa/s or 12 bit 12 Gsa/s
Variable sample rate from 125 Msa/s to 8/12 Gsa/s
2 GSa memory for
Long play time
5 GHz analog bandwidth
For today’s and tomorrows apps
Operation with leading
software platforms
© Keysight
Technologies 2015 44
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M9099T Waveform Creator - Downstream
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M9099T Waveform Creator - Upstream
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SystemVue Libraries
© Keysight
Technologies 2015 47
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ATP PHY 31 Spectrum Loading
Cable Load
Signals
Notch
Cable Load
Signals
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Technologies 2015 48
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ATP PHY 31 Interfering spectrum
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Technologies 2015 50
Output of M8190A as per settings described above
DOCSIS 3.0 = 158
channels, 6 MHz spacing
DOCSIS 3.1, 1901
active subcarriers
Page
© Keysight
Technologies 2015
Questions?
1. Decoding and demodulating a real downstream DOCSIS 3.1 signal and
reporting key metrics such as MER and BER.
2. Combining powerful trigger and recording techniques to detect, identify and
repair issues that cause BER.
3. Understanding the complexities of demodulating and measuring low frequency,
small grant upstream signals.
4. Generating in-channel and out-of-channel upstream and downstream DOCSIS
3.1 signals.
51
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