Testing DOCSIS 3.1 Networks - Part 2A whitepaper that examines leakage detection within DOCSIS 3.1 systems with leakage equipment already deployed in many CATV systems

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1. Executive SummaryToday’s multiple system operators (MSOs) are being challenged by both in-creased competition from fellow service providers and continuing customer demands for higher speeds and more bandwidth. For the CATV industry, implementing the DOCSIS 3.1 specification is intended to level the playing field with competitors outside of the HFC world and provide customers with the speeds they demand.

To achieve this, most MSOs will start rolling out DOCSIS 3.1 as soon as they possibly can. With the continued success of DOCSIS 3.1 lab testing, it is very likely that the MSOs will start to roll out DOCSIS 3.1 in late 2015 to early 2016 timeframe. With all of the big changes associated with the deployment of the DOCSIS 3.1 specification, MSOs will still need to consider how test equip-ment that they already own can be used to ensure that their networks are ready for DOCSIS 3.1.

This aggressive deployment timeline places a tremendous amount of pres-sure on test & measurement suppliers to provide creative solutions for testing before true DOCSIS 3.1 chipsets are available. Until these chipsets are widely available in early 2016, it is necessary that the MSOs be able to continue using their existing test equipment. This will help to defer any major upfront costs for all new test equipment and reduce the impact on MSOs with already strained capital budgets.

About the Author: Gary Sinde is the Vice-President of Trilithic Engineering and has over 25 years’ experience in design and manufacturing of embedded electronics and firmware with 20 years specifically dedicated to design of communication test instrumentation. Mr. Sinde is a graduate of Purdue University with a Master of Science in Electrical Engineering with a focus on signal processing.

Gary Sinde VP Engineering

2 A Primer in DOCSIS 3.1 Test & Measurement

The power and adaptability of legacy and current-gen leakage detection equipment will provide you many valuable measurement tools for use during intial deployments of DOCSIS 3.1 systems until chipsets that are capable of DOCSIS 3.1 will be made widely available in early 2016.

2. IntroductionAlthough leakage detection equipment and its configuration programs do not yet understand the concept of DOCSIS 3.1, we hope to alleviate some of the anxiety that often comes with new technologies by showing you how these leakage detectors will still be very effective testing tools for years to come.

DOCSIS 3.1 introduces the concept of a subcarrier, which is a small slice of the complete channel to the CATV industry. The DOCSIS 3.1 channel can be anywhere from 24 MHz to 192 MHz in bandwidth and is allowed to operate in either 4K Mode (4096 subcarriers) or 8K Mode (8192 subcarriers). In the 4K Mode, the subcarriers are spaced 50 kHz apart, while with the 8K Mode the subcarriers are spaced 25 kHz apart.

Remember that the subcarriers overlap as shown in the image below. So, it is very difficult to see individual subcarriers on a spectrum analyzer.

25 kHz or 50 kHz

3A Primer in DOCSIS 3.1 Test & Measurement

3. MethodologyDuring our research, we used the Rohde & Schwarz CLGD Cable Load Gen-erator to create a mixed SC-QAM and DOCSIS 3.1 system combined with the CT-4 Digital Leakage Tagger. This type of system was chosen to represent typical plans within the CATV industry for existing systems to gradually roll-out DOCSIS 3.1 channels while using existing leakage detection equipment.

The CT-4 is combined with the Rohde & Schwarz CLGD Cable Load Generator using a splitter (-3.5 dB) and a Pad (-3.0 dB). The combined signal output of the splitter is connected to a Rohde & Schwarz FSW8 Spectrum Analyzer (with DOCSIS 3.1 demodulation option) using a Pad (-3.0 dB).

We inserted a single DOCSIS 3.1 OFDM channel centered at 603 MHz with the SC-QAM channels adjacent to each side of the OFDM channel. The following channel details were configured within the channel plan:

• CT-4 Channel Tag - 612 MHz center frequency at -30 dB from the total power of the adjacent 6 MHz SC-QAM Channel.

• SC-QAM Channels - 6 MHz wide, 256 QAM, symbol rate of 5.360537 MS/sec and a transmit level of +15 dBmV.

• DOCSIS 3.1 OFDM Channel - 96 MHz wide channel, 603 MHz center frequency (Channel 87), 50 kHz wide subcarriers, 4K FFT, 256 QAM and a transmit level of +29 dBmV (total power).

It is anticipated that DOCSIS 3.1 signals will be used at or above the lower downstream specification of 258 MHz. Leakage detection equipment used in the aeronautical band will operate as they do today. It is possible to move the DOCSIS 3.1 signal down to as low as 108 MHz and should that happen, a similar exclusion zone could be created for aeronautical band leakage detection equipment.

» Laboratory Setup Using Rohde & Schwarz CLGD DOCSIS Cable Load Generator as a source combined with the CT-4 Digital Leakage Channel Tagger

4 A Primer in DOCSIS 3.1 Test & Measurement

4. Test & MeasurementAll measurements shown within this document were performed by simply using the DOCSIS 3.1 Demodualtion Option of the Rohde & Schwarz FSW8 Spectrum Analyzer in a closed lab environment on a simulated network and repeated at the CableLabs DOCSIS 3.1 interoperability sessions.

4.1 Affect of Exclusion Zone SpansOnce we have defined which subcarriers to remove from the OFDM channel to create a five (5) subcarrier exclusion zone, we will start removing sub-carriers one-by-one while measuring the MER on the adjacent subcarriers. This will allow us to determine the minimum number of subcarriers to be excluded without affecting the adjacent subcarriers.

» This is an example of removing a single subcarrier

» This is an example of removing three subcarriers

» This is an example of removing five subcarriers

612 MHz

612 MHz

612 MHz

5A Primer in DOCSIS 3.1 Test & Measurement

The subcarriers of our OFDM channel use a QAM 256 modulation profile. During testing there was no adjacent subcarrier MER degredation with five (5) subcarriers (250 kHz) removed while only measuring a 1 dB MER degre-dation with three (3) subcarriers (150 kHz) removed.

On a 190 MHz maximum bandwidth DOCSIS 3.1 channel in 4K Mode with a five (5) subcarrier exclusion zone, there would only be a 0.13% reduction in data capacity. On the same type of channel with a three (3) subcarrier exclusion zone, there will be a reduction in capacity of 0.08%.

To further reduce data capacity losses, the OFDM channel could use the 8K Mode and profile management to reduce the QAM modulation order on adjacent subcarriers.

4.1 Defining the Exclusion ZoneTo define the exclusion zone of the CT-4 channel tag, we need to know the following information:

• What is the number of the subcarrier that resides at the location of the CT-4 channel tag (612 MHz) carrier? The answer is 2228, see calculations below:

Constants

Total number of subcarriers (TSC) = 4096

Frequency span of subcarriers (SCFS) = 50 kHz (0.05 MHz)

OFDM channel center frequency (OFDMCF) = 603 MHz

Channel tag center frequency (CTCF) = 612 MHz

Calculations

OFDM channel center subcarrier (OFDMCSC) = TSC / 2

OFDMCSC = TSC / 2

OFDMCSC = 4096 / 2

OFDMCSC = 2048

OFDM channel and tag center frequency difference (DCF)

DCF = OFDMCF - CTCF

DCF = 612 MHz - 603 MHz

DCF = 9 MHz

6 A Primer in DOCSIS 3.1 Test & Measurement

Number of subcarriers above center of OFDM channel (NSC)

NSC = DCF / SCFS

NSC = 9 MHz / 0.05 MHz

NSC = 180

Number of subcarrier at center of channel tag (NCT)

NCT = OFDMCSC + NSC

NCT = 2048 + 180

NCT = 2228

• Which subcarriers do we need to remove to create an exclusion zone of five subcarriers at 612 MHz? The answer is numbers 2226 through 2230, see calculations below:

Constants

Number of subcarrier at center of channel tag (NCT) = 2228

Number of subcarriers to remove (NSCR) = 5

Calculations

Number of lowest subcarrier (NSCL) = NCT - [(NSCR - 1)/2]

NSCL = 2228 - [(5 - 1)/2]

NSCL = 2228 - 2

NSCL = 2226

Number of highest subcarrier (NSCH) = NCT + [(NSCR - 1)/2]

NSCH = 2228 + [(5 - 1)/2]

NSCH = 2228 + 2

NSCH = 2230

7A Primer in DOCSIS 3.1 Test & Measurement

» This is a zoomed view of the five subcarrier exclusion zone created for inserting the CT-4 tag at 612 MHz, note each of the pilot carriers on either side of the exclusion zone

» This is a view of the full span of the 96 MHz OFDM channel with pilot carriers, PLC pilots and a five sub-carrier exclusion zone around the CT-4 channel tag at 612 MHz

» This is the same zoomed view shown above but with a CT-4 channel tag inserted into the five subcarrier exclusion zone at 612 MHz

8 A Primer in DOCSIS 3.1 Test & Measurement

5. ConclusionThe most important item to remember with all of the changes within the industry regarding DOCSIS 3.1 is that your existing leakage detection equip-ment from Trilithic will continue to work in a DOCSIS 3.1 environment with minimal impact to data capacity. Furthermore, leakage detection equipment will continue to evolve with the ability to identify DOCSIS 3.1 OFDM & pilot carrier signatures emitting from the cable plant, thus providing even more flexibility in leakage detection.

Trilithic will be providing periodic updates over the next year to keep you informed of the latest and greatest DOCSIS 3.1 test and measurement tech-niques. Our next part will show you even more detailed information on our suite of measurements for DOCSIS 3.1 that will be helpful in assisting the technician with troubleshooting of new DOCSIS 3.1 implementations.

9A Primer in DOCSIS 3.1 Test & Measurement

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Telephone: 1 (317) 895-3600Toll Free in North America: 1 (888) 895-7630

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References1 CableLabs. “Data-Over-Cable Service Interface Specifications DOCSIS® 3.1 Physical Layer Specification CM-SP-PHYv3.1- I04-141218”, http://http://www.cablelabs.com/wp-content/uploads/specdocs/CM-SP-PHYv3.1-I04-141218.pdf

Product innovation at Trilithic has always been characterized by one thing: it’s practical. It makes life easier for customers. It’s the natural result of listening to them. That philosophy has been the driving force behind the company’s growth from its beginnings as a two-man engineering team in 1986 to its current position as a global manufacturer with more than 130 employees. As new communications applications continue to emerge, part of Trilithic’s business has evolved into managing change—helping customers respond quickly to market opportunities with innovative technology and individualized solutions. But the core value of Trilithic’s business approach—listening to customers—hasn’t changed. Keeping that focus intact will help provide better products in the long run and ensure continued growth for decades to come.

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10 A Primer in DOCSIS 3.1 Test & Measurement