DOCSIS 3.1: Plan its deployment using the data in your current DOCSIS 3.0 OSS tools

A Technical Paper prepared for the Society of Cable Telecommunications Engineers
By
Patricio S. Latini
SVP Broadband Communications
Intraway Corporation
Campillo 2541 – Buenos Aires – Argentina
+54 (911) 6040-4023
patricio.latini@intraway.com

Contents

Introduction.. 3
Current Situation.. 4
DOCSIS 3.1 Technology. 8
OFDM… 8
LDPC.. 10
Modulations. 11
Spectrum… 12
Multiple Modulation Profiles / Variable Bit Loading. 12
Using Current OSS Data. 15
Spectrum Management 15
Phase I – Use DOCSIS 3.1 in the current HFC network to take advantage of new technologies. (5-42 MHz Upstream / 54-1008 MHz Downstream). 15
Phase II – Use DOCSIS 3.1 in the HFC network to take advantage of the new split 20
Phase III – Use DOCSIS 3.1 in the HFC networks with digital optics to take advantage of the new optional higher order modulations. 21
Capacity Planning. 21
Calculating the Channel Capacity of DOCSIS 3.1 Channels. 21
Conclusion.. 26
Bibliography. 27
Abbreviations & Acronyms. 29

Introduction

Most of the cable industry is now focused on planning the deployment of the new DOCSIS 3.1 standard. Cable operators will obtain a lot of technical advantages by using the new tools provided by the specification such as OFDM, LDPC, Variable Bit-loading, etc. However, the transition to implement this new standard will be phased and very gradual. The proper planning for this process will be key to its success.

This paper analyzes some of the new technologies provided by DOCSIS 3.1 and proposes the usage of data gathered by currently deployed DOCSIS 3.0 Provisioning and Monitoring Systems from field Cable Modems (CMs), and Cable Modem Termination Systems (CMTSs), such as Signal-to-Noise Ratio (SNR), Codeword Error Rate (CER) and throughput.  This analysis is done in order to have a much more accurate forecast of future network capacity and device compatibility.

The conclusions drawn from this white paper will allow cable operators to simulate different DOCSIS 3.1 implementation scenarios related to frequency allocations and modulations to be supported.  This will enable cable operators to make better decisions on what processes to follow in order to maximize the network capacity based on data that they have in their OSS (Operations Support System) data-warehouses.

Current Situation

In general, most cable operators are now actively using DOCSIS 3.0 technology, which has constantly evolved from previous specifications.
DOCSIS 3.0 has provided a very good toolset to support throughputs in excess of 300 Mbps in the downstream path as well as over 100 Mbps aggregated in the upstream side.  This was accomplished all over HFC networks using RF signals.
In general, we can mention several variables that have been used by cable operators over time to adapt and improve operating conditions of the DOCSIS Networks. These variables are listed in Table 1.

Downstream

Upstream

Channel Frequency

Modulation Order

Interleaving Depth

Might include: Channel Bonding, Ingress Cancellation, Load balancing, Equalization

Channel Frequency

Modulation Technique (TDMA/ATDMA/SCDMA)

Modulation Order

Forward Error Correction – Reed Solomon

Pre-Equalization

Interleaving Depth

Preamble Length

Might include: Load balancing, Channel Bonding, Ingress Cancellation, Frequency Hopping, Dynamic Modulation, Dynamic Interleaving

Table 1 – DOCSIS Network Operation Variables

The use of the previously listed variables has evolved over the years depending on network conditions, economic factors and new technologies. In order to be able to make wise decisions on which variable to adapt and how to change it, cable operators started to use their OSSs to monitor key indicators from manageable devices in order to have appropriate inputs.
The quantity of these key indicators has been growing over time.  Table 2 provides a list of the most common ones used by some cable companies that are typically using SNMP.

Per Modem Parameters

Downstream

Upstream

CM

CMTS

Rx Power

docsIfDownChannelPower docsIf3CmtsCmUsStatusRxPower

Modulation

docsIfDownChannelModulation docsIfUpChannelModulationProfile
docsIfCmtsModType.x.advPhyShortData
docsIfCmtsModType.x.advPhyLongData
Frequency

docsIfDownChannelFrequency

docsIfUpChannelFrequency

Channel Width docsIfDownChannelWidth

docsIfUpChannelWidth

SNR (RxMER) docsIfSigQSignalNoise

docsIf3CmtsCmUsStatusSignalNoise

Microreflections

docsIfSigQMicroreflections docsIf3CmtsCmUsStatusMicroreflections

Unerrored CW

docsIfSigQExtUnerroreds

docsIf3CmtsCmUsStatusUnerroreds

Uncorrectable CW

docsIfSigQExtUncorrectables

docsIf3CmtsCmUsStatusUncorrectables

Corrected CW

docsIfSigQExtCorrecteds

docsIf3CmtsCmUsStatusCorrecteds

Post EQ In-Data

docsIfSigQEqualizationData

docsIf3CmtsCmUsStatusEqData

Spectrum Analyzer

docsIf3CmSpectrumAnalysisMeasAmplitudeData**

docsIf3CmtsSpectrumAnalysisMeasAmplitudeData*

CMTS

CM

Tx Power

docsIfDownChannelPower

docsIf3CmStatusUsTxPower

Pre EQ Data

docsIf3CmtsCmUsStatusEqData

CMTS

CMTS

Traffic

docsQosServiceFlowOctets docsQosServiceFlowOctets
Traffic Packets docsQosServiceFlowPkts

docsQosServiceFlowPkts

Path Loss

Tx Power – Rx Power

Codeword Error Ratio (CER)

Uncorrectable CW / Total CW

Correctable Codeword Error Ratio (CCER)

Corrected CW / Total CW

* Only supported on some CMTS
** Only supported on newer cable modems with Full Bandwidth Capture Tuner

Table 2 – Key Variables Accessible via SNMP per Modem

CMTS Port Parameters  

Upstream

SNR (RxMER) docsIfSigQSignalNoise
Microreflections docsIfSigQMicroreflections
Unerrored CW docsIfSigQExtUnerroreds
Uncorrectable CW docsIfSigQExtUncorrectables
Corrected CW docsIfSigQExtCorrecteds
All Interfaces
Traffic ifHCInOctets
ifHCOutOctets

Table 3 – Key Variables Accessible via SNMP per CMTS Interface

It is not the objective of this paper to explain how to define operative thresholds for the indicators mentioned above. However, we are going to mention a few of them as they will be very important for the analysis presented in the rest of this document.

  1. SNR (RxMER)

One of the most-used indicators since the beginning of DOCSIS systems has been the SNR, more correctly called RxMER. The SNR has a direct and deterministic relationship with the Bit Error Rate (BER) for a given Modulation (as seen in Figures 1 and 2). SNR has also been the most used indicator to determine the quality of an HFC Network

Usually the minimum acceptable SNR for error-free operation for a specific modulation is defined where the waterfall curve intersects the 10-8 BER line. The waterfall curves above are very useful to see how forward error correction improves the bit error rates. An example of this is the following: In Figure 1, the red family of curves shows the relation for the QAM256 modulation, where the solid line showing the uncoded transmission intersects the 10-8 BER line at 35 dB while the dashed line showing the Reed Solomon coded transmission intersects the same line at 29 dB, clearly illustrating the 6 dB gain of the coding.
Another way to understand the curves is to define a SNR value and compare the performance of different modulations. For a SNR of 29 dB, uncoded QAM256 shows a BER or 5×10-2 while Reed Solomon coded QAM256 has a significantly better value of 10-8 BER.
Digital video and DOCSIS services use the ITU-T J.83 specification in the downstream path, where Annex B of the specification is used in North America and Annex A of the specification is used in Europe. The main difference between the two Annexes is the channel bandwidth (6 MHz vs. 8 MHz) and an extra layer of Trellis Coded Modulation channel coding in Annex B allows 2 dB of extra gain for the same modulation. Typically a margin of safety of 2 dB is used for standard operation.

Uncoded After RS FEC After TCM Operation Margin (2 dB)

Annex B QAM256

35 dB 29 dB 27 dB

29 dB

Annex A/C QAM256 35 dB 29 dB

31 dB

Table 4 – Downstream SNR Requirements

DOCSIS Services use ATDMA/SCDMA in the upstream path. Table 5 shows the SNR requirements for its operation.

Uncoded

After RS FEC

Operation Margin (6 dB)

ATDMA QAM64

29 dB 23 dB

30 dB

ATDMA QAM16

23 dB 17 dB

23 dB

Table 5 – Upstream SNR Requirements

DOCSIS 3.1 Technology

The new DOCSIS 3.1 specification adds a significant set of new technologies with the goal of allowing higher connection throughputs, up to 10 Gbps in the downstream path and 1 Gbps in the upstream. On the other hand, this standard is backwards compatible with previous DOCSIS versions and allows and even eases the coexistence with networks operating in DOCSIS 3.0.
Below is a description of some of the new technologies that will provide the most significant advantages:

OFDM

One of the most significant additions to the specification is the utilization of a new modulation scheme for DOCSIS called OFDM (Orthogonal Frequency Division Multiplexing). This technology has been used for years in applications such as Cellular, Wi-Fi, PLC and others. The main difference between OFDM and the modulation formats used before DOCSIS 3.0 is that the latter were Single Carrier QAM (SC-QAM), while OFDM provides a multicarrier transmission.

DOCSIS 3.1 exhibits a 25 or 50 kHz. subcarrier spacing. Initially, this is a much more granular approach than the current 6 MHz. channels and will provide a significant advantage in terms of noise immunity for both impulse and ingress types of impairments.
Ingress noise is mitigated by the ability to lower the modulation, stop transmitting on individual subcarriers where the ingress may be present, or avoid data transmission on that part of the RF spectrum. Impulse noise is greatly mitigated by the significantly longer symbol duration of each subcarrier of the multicarrier modulation [7] (240 times longer than a 6 MHz channel symbol), and diluting the effects of noise impulses in the receiver.

Given the multicarrier nature of OFDM and bandwidth needs, it is no longer necessary to maintain a 6 or 8 MHz. channelization. Due to this, the new specification offers channel bandwidths of 24, 48, 96 and 192 MHz in the downstream that are equivalent to 4 to 32 bonded 6 Mhz, QAM channels. This is aligned with current deployments as most cable operators are using DOCSIS 3.0 channel bonding of at least eight 6 MHz QAM channels in the Downstream and a range of two to four 6.4 MHz channels in the upstream.

LDPC

All previous DOCSIS versions have been using an Error Correction technique called Reed Solomon. While this technology provides a decent coding gain in dB compared to an uncoded data transmission, other technologies exist that have even greater coding gains and hence higher efficiencies in bits/Hz.  Some of these technologies are being explored as viable in the last few years since they take advantage of the increased processing capacity of new generation silicon.

One of these technologies is called Low Density Parity Check (LDPC). While it is not something new –– it was invented by Gallager in 1963 –– it was forgotten for many years because the processing requirements were so high that there was no silicon available to implement it.

Now the processor and FPGA capabilities enable the implementation of LDCP codes and take advantage of its capabilities.  While it is outside the scope of this this paper to examine LDPC in detail, LDPC can provide around an extra 6 dB coding gain compared to Reed Solomon error correction bringing the efficiency to less than 1 dB from the Shannon Theoretical Limit.

Modulations

Up to now, the maximum modulation allowed on DOCSIS was QAM256 for the downstream and QAM64 for an ATDMA upstream. Now, with the significant coding gains brought by LDPC, in an Annex B QAM256 scenario a 27 dB SNR is required to operate properly. With LDPC encoding, the equivalent QAM256 only requires a 22.5 dB SNR. The door is now open to add some higher order modulations. Figure 6 shows that for LDCP codes with the same SNR as QAM256, QAM1024 can be run with an advantage of almost 2 bps/Hz more. In order to understand how big of an impact this has in the real world, it means that a 6 MHz downstream modulated using QAM1024 would be able to carry 53 mbps instead of the 42 mbps that can be transported using QAM256 modulation. DOCSIS 3.1 adds support in the downstream for 512QAM, 1024QAM, 2048QAM and 4096QAM with optional/future 8192QAM and 16384QAM.

In the upstream 128QAM, 256 QAM, 512 QAM and 1024 QAM have been added with optional support for 2048QAM and 4096 QAM.

Modulation

RS SNR RS+Trellis SNR LDPC SNR

64QAM

23 21 16

128QAM

26 23 19

256QAM

29

27 22
512QAM

25

1024QAM

28

2048QAM

31
4096QAM

34

8192QAM

37

16384QAM

40

Table 6 – Modulation Operational Thresholds

 

Spectrum

In order to support the target throughputs for DOCSIS 3.1, new downstream and upstream bandwidths have been defined.

Mandatory Spectrum


In order to support a legacy mode with downstream channels below 258 Mhz, the equipment should support the optional mode described below.

Optional Spectrum

Multiple Modulation Profiles / Variable Bit Loading

As stated before, in earlier DOCSIS versions, the maximum supported modulation in the downstream has been 256QAM. However, another significant limitation has been that all the cable modems need to operate with the same modulation independently of how good the SNR is in that part of the plant.

Usually cable modem SNR follow a Gaussian distribution. Depending on how good the health of the network is, the SNR will have a different mean and deviation. In Figure 8, a typical HFC plant with a modem downstream SNR Gaussian distribution mean of 36 dB and a 2 dB deviation is shown. It operates in DOCSIS 3.0 with 100% of the modems in 256QAM showing a maximum capacity of 8 bits per second per hertz.

Figure 8 – Cable Modem Modulation Distribution vs. SNR for DOCSIS 3.0

 

Now, if Multiple Modulation Profiles are supported, each modem, depending on the network’s condition, may receive data with a different modulation. This means that modems with better SNRs can use higher order modulations.
Figure 9 shows the same HFC plant as in Figure 8 but with modems using the appropriate modulation profile for each SNR by using DOCSIS 3.1.  This is called variable bit loading.

 

Figure 9 – Cable Modem Modulation Distribution vs. SNR for DOCSIS 3.1

Table 7 shows that there was a jump from 8 to 10.87 bits/sec/Hz, which represents an increase of about 36% in network efficiency and capacity.

Modulation

% of Modems Weighted bps/Hz.

QAM256

0% 0.00
QAM512 2%

0.18

QAM1024

27% 2.71
QAM2048 54%

5.92

QAM4096

16% 1.97
QAM8192 1%

0.09

QAM16834

0% 0.00
Cumulative 100%

10.87

Table 7 – Modulation Distribution Example

Using Current OSS Data

Spectrum Management

As with the transition from DOCSIS 2.0 to 3.0, the adoption of DOCSIS 3.1 will be a gradual process and not a forklift upgrade. Planning the transition is a key factor for a smooth and cost-effective implementation.

The following data from the network devices will be used:

  1. SNR data from modems/CMTS
  2. Spectrum captures from modems/CMTS

Phase I – Use DOCSIS 3.1 in the current HFC network to take advantage of new technologies. (5-42 MHz Upstream / 54-1008 MHz Downstream)

Upstream

Option 1 – Use OFDMA in the higher and cleaner band of the upstream to increase spectral efficiency

From

asd

To

Step A – Retrieve the upstream SNR values of the modems in the channels that are being considered for the switch to OFDM

Channel 1 – 35.6 to 42 MHz.

Channel 2 – 29.2 to 35.6 MHz.

Step B – Plot a histogram of the SNR values with 0.1 dB steps.

Step C – Record the data in a table taking into consideration each modulation range and calculate the weighted bps/Hz. and the total average bps/Hz.

Modulation

SNR Min SNR Max bps/Hz. % Modems Weighted bps/Hz.
QAM32 17 20 5

QAM64

20 23 6
QAM128 23 26

7

QAM256

26 29 8
QAM512 29 32

9

QAM1024 32 10
Total

100%

Step D – Compare the estimated DOCSIS 3.1 capacity with the DOCSIS 3.0 capacity

Example from a Real Network

Figure 10 – Cable Operator Headend Upstream SNR Distribution

Modulation SNR Min SNR Max bps/Hz. % Modems Weighted bps/Hz.
QAM32 17 20 5 0.1% 0.005
QAM64 20 23 6 0.5% 0.0297
QAM128 23 26 7 2.7% 0.0178
QAM256 26 29 8 6.8% 0.542
QAM512 29 32 9 16.0% 1.432
QAM1024 32 10 73.9% 7.381
Total 100% 9.569

Table 8 – Cable Operator Upstream SNR Distribution

In this real example, the Operator, who is using QAM64 in the upper upstream spectrum, can get an extra 3.56 bps/Hz compared to a 12.8 MHz Spectrum, which means an extra 45.6 Mbps of raw capacity or around 38 Mbps of usable modulated capacity after error correction overheads.

The main drawback of this option would be that only DOCSIS 3.1 modems would be using this part of the spectrum.

Option 2 – Use OFDMA in lower band 5-20 MHz to recover unused spectrum

From

To

Step A – Retrieve the upstream spectrum from the CMTSs ports under consideration.

Step B – Plot the upstream spectrum

Step C – Estimate the SNR of the lower spectrum based on the noise floor, considering different modulations depending on the SNR zone

Figure 11 – Live Spectrum of a CMTS Upstream Port using SNMP

Assigning modulations to the different parts of the spectrum

Freq

SNR SNR Max bps/Hz. Mhz of Spectrum

Weighted bps/Hz.

QPSK

8 14 2 5

5*2/16=0.625

QAM16

14 20 4 3

3*4/16=0.75

QAM64

20 23 6 4

4*6/16=1.5

QAM128

23 26 7 4

4*7/16=1.75

QAM256

26 29 8

QAM512

29 32 9

QAM1024

32

10

Total

16

4.645

Table 9 – Upstream Spectrum Efficiency Calculation

 In this example, the unused lower upstream spectrum can get 4.64 bps/Hz. compared to a 16 MHz spectrum, which means an extra 74.4 Mbps of raw capacity or around 63 Mbps of modulated capacity.

Downstream

Option 1 – Use OFDM in the higher band of the downstream to allocate a continuous OFDM block

From

To

Step A – Retrieve the downstream spectrum from a sample of full bandwidth capture modems

Step B – Plot the downstream spectrum

Step C – Analyze the spectrum (automated or manual) to look for interference on the desired spectrum (LTE, Cellular, etc.).

Step D – Discard any spectrum areas that may bring permanent problems.

Option 2 – Use OFDM in the downstream mixed with Single Carrier QAMs by using subcarrier muting

From

To

Step A – Retrieve the downstream SNR values of the modems in the channels that are being considered for the switch to OFDM or adjacent.

Step B – Plot a histogram of the SNR values with 0.1 dB steps.

Step C – Record the data in a table taking into consideration each modulation range and calculate the weighted bps/Hz. and the total average bps/Hz.

Modulation SNR Min SNR Max bps/Hz. % Modems Weighted bps/Hz.
QAM256 26 29 8
QAM512 29 32 9
QAM1024 32 35 10
QAM2048 35 38 11
QAM4096 38 41 12
QAM8192 41 13
Total 100%

Step D – Estimate the capacity by multiplying the non-muted subcarriers by the estimated efficiency.

 Figure 12 – Cable Operator Headend Downstream SNR Distribution

 

Modulation SNR Min SNR Max bps/Hz. % Modems Weighted bps/Hz.
QAM256 26 29 8 0.09 % 0.0074
QAM512 29 32 9 0.67 % 0.0602
QAM1024 32 35 10 7.41 % 0.7414
QAM2048 35 38 11 35.59 % 3.8934
QAM4096 38 41 12 54.10 % 6.4930
QAM8192 41 13 2.31 % 0.3007
Total 100% 11.4964

Table 10 – Downstream Spectrum Efficiency Calculation

In this real example, the operator is using QAM256 in the downstream.  Using OFDM the operator could get an estimated 11.50 bps/Hz in efficiency compared to the spectrum adjacent to the currently used QAM channels or in case that the operator decides to replace part or the currently used spectrum with OFDM channels.

Phase II – Use DOCSIS 3.1 in the HFC network to take advantage of the new split

5-85(204) MHz Upstream / 108(258)-1218 MHz Downstream

After having implemented DOCSIS 3.1 in the same HFC, some spectrum augmentation options can be considered. This may require a significant investment in outside plant network equipment such as changing the diplex filter band or moving the downstream upper limit to 1.2 GHz by changing the actives.

Some operators have already begun investing in expanding the upstream to 85 MHz.  This means using the 42-85 MHz band could easily be an intermediate step that brings an extra 350 Mbps of upstream capacity.

Phase III – Use DOCSIS 3.1 in the HFC networks with digital optics to take advantage of the new optional higher order modulations

A few years after having implemented DOCSIS 3.1 and having cable modems operating in high order modulations like QAM2048 and QAM4096, the next big limitation in performance is found in the C/N (carrier-to-noise) performance constraints of the analog links, particularly on the downstream side.

New proposals for using digital lasers for both downstream and upstream links may allow a new jump of two orders in both directions. This would enable a modulation of QAM16384 on the downstream and QAM4096 on the upstream, increasing the capacity again by 2 bps/Hz.

Capacity Planning

Capacity planning is going to be a key factor during the transition to DOCSIS 3.1. Currently in DOCSIS 3.0 all the modems use the same and unique modulation profile transmitted by the CMTS. With the implementation of multiple modulation profiles, two important factors need to be considered:

  1. Knowing the attainable modulation for each one of the cable modems will be key to knowing the total capacity of a network segment.
  2. Accounting for the total transferred data per cable modem with different modulation profiles will require the use a different number of timeslots for the same amount of data. That implies that heavy users using lower efficiency modulation profiles may have a larger impact on the total channel capacity.

Calculating the Channel Capacity of DOCSIS 3.1 Channels

Single Modulation Profile

This is the simplest scenario. It can be easily implemented on any OSS or capacity planning tool.

On the next page, there are two tables showing examples with the intermediates steps required for determining the available downstream and upstream bandwidth.

In particular for this example the following assumptions were made:

Downstream:

192 MHz. Channel

25 kHz. Subcarrier Spacing

192 Cyclic Prefix Samples

128 Roll off Period Samples

4096QAM Modulation

LDPC Code 8/9

Upstream:

96 Mhz. Channel

25 kHz. Subcarrier Spacing

192 Cyclic Prefix Samples

64 Roll off Period Samples

1024QAM Modulation

LDPC Code 8/9

OFDM Downstream OFDM Upstream
DOCSIS Clock 10.24 Mhz. DOCSIS Clock 10.24 Mhz.
Clock Multiplier 20 Clock Multiplier 10
FFT Sample Rate (fs) 204.8 Mhz. FFT Sample Rate 102.4 Mhz.
Elementary Period (Tsd) 4.8828125 ns. Elementary Period 9.765625 ns.
FFT Total Subcarriers 8192 FFT Total Subcarriers 4096
FFT Subcarrier spacing 25 Khz. FFT Subcarrier BW 25 Khz.
OFDM Total BW 192 Mhz. OFDM Total BW 96 Mhz.
OFDM Guard Band 2 Mhz. OFDM Guard Band 1 Mhz.
OFDM Usable BW 190 Mhz. OFDM Usable BW 95 Mhz.
OFDM Total Carriers 7600 OFDM Total Carriers 3800
FFT Symbol Duration (Tu) 40 us Symbol Duration 40 us
CPC Duration 192 CPC Duration 192
CPC Duration (Tcp = n * Tsd) 0.9375 us CPC Duration (Tcp = n * Tsd) 1.875 us
Window Duration 128 Window Duration 64
Window Duration  (Trp = n * Tsd) 0.625 us Window Duration  (Trp = n * Tsd) 0.625 us
Symbol Duration (Ts = Tu+Tcp) 40.9375 us Symbol Duration (Ts = Tu+Tcp) 41.875 us
Modulation QAM4096 Modulation QAM1024
SNR Mean 36 dBmV SNR Mean 28 dBmV
SNR Deviation 2 dB SNR Deviation 2 dB
Modulation Bit Load 12.00 bits/Hz. Modulation Bit Load 10.00
OFDM PHY Available BW 2227786260 bps OFDM PHY Available BW 907462687 bps
Continuous Pilot Interval 1/128 Continuous Pilot Interval 1/128
Continuous Pilots 59 Continuous Pilots 29
Scattered Pilot Interval 1/128 Scattered Pilot Interval 1/128
Scattered Pilot Stager 2 Scattered Pilot Stager 1
Scattered Pilots 59 Scattered Pilots 29
Total Pilots 118 Total Pilots 58
PLP Subcarriers 16
NCP Subcarriers 10
OFDM Available Subcarriers 7456 OFDM Available Carriers 3742
OFDM Data Available BW 2185575573 bps OFDM Data Available BW 893611940 bps
LDPC FEC Frame 16200 bits LDPC FEC Frame 16200 bits
LDPC Code 8/9 LDPC Code 8/9
LDPC Payload 14400 bits LDPC Payload 14400 bits
BCH FEC 168 bits
BCH Payload 14232 bits
Total FEC Efficiency 0.8785 Total FEC Efficiency 0.8889
Available BW 1920068614 bps Available BW 794321725 bps
Modulation Efficiency 10.1056 bits/Hz. Modulation Efficiency 8.3613 bits/Hz.

Table 11 – OFDM Capacity Calculation

 

 

Multiple Modulation Profiles

In this scenario the existing historical data from an OSS is a very important asset, as it is used to extrapolate the total capacity of those modems when they operate in a DOCSIS 3.1 OFDM channel.

This is the process for performing the average bit-loading calculation that will later be used as an input for the capacity calculation:

  1. Get the SNR values for all the modems on a certain port.
  2. Based on the SNR value, map each modem to a certain modulation profile based on the modulation SNR operation ranges.
  3. Perform a summation of all the upstream or downstream traffic during a 24 hour period of all the modems mapped to a modulation profile.
  4. Do the same for all the modulation profiles.
  5. Calculate the percent of traffic for each modulation profile.
  6. Weight the bit-loading of each modulation profile based on its percent of traffic.
  7. Calculate the average bit-loading.

An example for downstream fiber node is shown below:

Modulation SNR Min SNR Max bps/Hz. # Modems Total Bytes Traffic Percentage Weighted bps/Hz.
QAM256 26 29 8 50 3.23326E+11 4% 0.304881
QAM512 29 32 9 113 7.56326E+11 9% 0.802328
QAM1024 32 35 10 245 1.87408E+12 22% 2.208965
QAM2048 35 38 11 360 3.76585E+12 44% 4.882648
QAM4096 38 41 12 162 1.65983E+12 20% 2.34772
QAM8192 41 13 15 1.04569E+11 1% 0.160231
Total 8.48398E+12 100% 10.70677
OFDM Downstream OFDM Upstream
DOCSIS Clock 10.24 Mhz. DOCSIS Clock 10.24 Mhz.
Clock Multiplier 20 Clock Multiplier 10
FFT Sample Rate (fs) 204.8 Mhz. FFT Sample Rate 102.4 Mhz.
Elementary Period (Tsd) 4.8828125 ns. Elementary Period 9.765625 ns.
FFT Total Subcarriers 8192 FFT Total Subcarriers 4096
FFT Subcarrier spacing 25 Khz. FFT Subcarrier BW 25 Khz.
OFDM Total BW 192 Mhz. OFDM Total BW 96 Mhz.
OFDM Guard Band 2 Mhz. OFDM Guard Band 1 Mhz.
OFDM Usable BW 190 Mhz. OFDM Usable BW 95 Mhz.
OFDM Total Carriers 7600 OFDM Total Carriers 3800
FFT Symbol Duration (Tu) 40 us Symbol Duration 40 us
CPC Duration 192 CPC Duration 192
CPC Duration (Tcp = n * Tsd) 0.9375 us CPC Duration (Tcp = n * Tsd) 1.875 us
Window Duration 128 Window Duration 64
Window Duration  (Trp = n * Tsd) 0.625 us Window Duration  (Trp = n * Tsd) 0.625 us
Symbol Duration (Ts = Tu+Tcp) 40.9375 us Symbol Duration (Ts = Tu+Tcp) 41.875 us
Modulation MULTIPLE Modulation MULTIPLE
SNR Mean dBmV SNR Mean dBmV
SNR Deviation dB SNR Deviation dB
Modulation Bit Load 10.60 bits/Hz. Modulation Bit Load 8.70
OFDM PHY Available BW 1967877863 bps OFDM PHY Available BW 789492537 bps
Continuous Pilot Interval 1/128 Continuous Pilot Interval 1/128
Continuous Pilots 59 Continuous Pilots 29
Scattered Pilot Interval 1/128 Scattered Pilot Interval 1/128
Scattered Pilot Stager 2 Scattered Pilot Stager 1
Scattered Pilots 59 Scattered Pilots 29
Total Pilots 118 Total Pilots 58
PLP Subcarriers 16
NCP Subcarriers 10
OFDM Available Subcarriers 7456 OFDM Available Carriers 3742
OFDM Data Available BW 1930591756 bps OFDM Data Available BW 777442388 bps
LDPC FEC Frame 16200 bits LDPC FEC Frame 16200 bits
LDPC Code 8/9 LDPC Code 8/9
LDPC Payload 14400 bits LDPC Payload 14400 bits
BCH FEC 168 bits
BCH Payload 14232 bits
Total FEC Efficiency 0.8785 Total FEC Efficiency 0.8889
Available BW 1696060609 bps Available BW 691059900 bps
Modulation Efficiency 8.9266 bits/Hz. Modulation Efficiency 7.2743 bits/Hz.

Table 12 – OFDM Capacity Calculation

Conclusion

 This paper analyzed the current status of OSS tools that monitor DOCSIS 3.0 networks. It described the principal Network Operation variables as well as the Key Performance Indicators used to adjust them. Later, a detailed list of SNMP variables was provided together with an analysis of why SNR is still one of the key indicators.

A detailed explanation of the most important DOCSIS 3.1 technologies such as OFDM, LDPC, spectrum expansion and variable bit loading was presented in order to provide the theoretical background for the analysis that was presented in the chapter on “Using Current OSS Data.”

Finally, the last chapter presented an analysis of Spectrum Management and Capacity Planning for DOCSIS 3.1 deployments using data available from the OSS Systems that are currently deployed, including alternatives and phases. The paper makes it clearly evident that OFDM as a new modulation and LDPC as a new forward error correcting algorithm bring huge improvements. However, the path to implement all these new features will require careful planning.

Ultimately, having OSS tools that perform both continuous data mining and an intelligent analysis of the most important variables described in this paper will be a key factor to a successful DOCSIS 3.1 deployment.

Bibliography

[1] DOCSIS® 3.0 Operations Support System Interface Specification, Cablelabs
[2] DOCSIS® 3.1 Operations Support System Interface Specification, Cablelabs
[3] DOCSIS® 3.1 Physical Layer Interface Specification, Cablelabs
[4] DOCSIS® 3.1 MULPI Interface Specification, Cablelabs
[5] DVB Fact Sheet 2nd Generation Cable – July 2012, DVB.org
[6] An Evolutionary Approach to Gigabit-Class DOCSIS – Authors J. Chapman, M. Emmendorfer, R. Howald, & S. Shulman
[7] Multicarrier Modulation for Data Transmission: An Idea Whose Time Has Come. – IEEE Commun // Com, 1990. – P. 5–14.

Abbreviations & Acronyms

CCER                        Corrected Codeword Error Ratio
CER               Codeword Error Ratio
CM                  Cable Modem
CMTS             Cable Modem Termination System
CPE                Customer Premises Equipment
C/N                 Carrier to Noise
DN                  Downstream
US                  Upstream
DOCSIS         Data Over Cable Service Interface Specification
OSS                Operations Support System
SNMP                        Simple Network Management Protocol
OFDM                        Orthogonal Frequency Division Multiplexing
LDPC             Low Density Parity Check
ATDMA          Advanced Time Division Multiplexing Access
SCDMA         Synchronous Code Division Multiplexing Access
QAM               Quadrature Amplitude Modulation
SNR               Signal to Noise Ratio
LTE                 Long Term Evolution
HFC                Hybrid Fiber Coax
FPGA             Field Programmable Gate Array
MBPS             Megabits per Second
GBPS             Gigabits per Second
BPS                Bits per Second

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