Simulink Implementation Simulink Implementation of a Cable Modemof a Cable Modem

Gina ColangeloGina ColangeloNathan EganNathan EganEE194-SDREE194-SDR05/02/200605/02/2006

Topics CoveredTopics Covered

Cable Data Network OverviewCable Data Network Overview

Cable Modem ArchitectureCable Modem Architecture

Downstream/Upstream SpecificationsDownstream/Upstream Specifications

Simulink ImplementationSimulink Implementation

Implementation ResultsImplementation Results

Cable Modem Network OverviewCable Modem Network OverviewHeadend: DOCSIS-certified CMTS (Cable Modem Termination System)Headend: DOCSIS-certified CMTS (Cable Modem Termination System)

One Headend supports ~ 2000 Cable Modem Users on a single TV ChannelOne Headend supports ~ 2000 Cable Modem Users on a single TV Channel CMTS interfaces the CATV network to the InternetCMTS interfaces the CATV network to the Internet

CMTS output channel combined with TV video signalsCMTS output channel combined with TV video signalsCATV Network to Subscriber via coaxial cableCATV Network to Subscriber via coaxial cableOne-to-Two splitter: One signal to Set Top Box (STB), other to Cable ModemOne-to-Two splitter: One signal to Set Top Box (STB), other to Cable ModemCable ModemCable Modem

One Modem can support up to 16 users in a local-area networkOne Modem can support up to 16 users in a local-area networkPC/Ethernet CardPC/Ethernet Card

Cable Modem connected to PC via Ethernet, USB, PCI Bus, etcCable Modem connected to PC via Ethernet, USB, PCI Bus, etc

Cable Modem ArchitectureCable Modem ArchitectureTransmit/UpstreamTransmit/Upstream

QPSK/QAM Modulator performs:QPSK/QAM Modulator performs:QPSK/QAM-16 modulation QPSK/QAM-16 modulation Reed-Solomon EncodingReed-Solomon EncodingD/A ConversionD/A ConversionUp-conversion to the selected Up-conversion to the selected frequency/channelfrequency/channel

Receive/DownstreamReceive/Downstream RF TunerRF Tuner

Converts TV Channel to a fixed Converts TV Channel to a fixed lower frequency (6-40MHz)lower frequency (6-40MHz)

QAM Demodulator performs:QAM Demodulator performs: A/D conversionA/D conversionQAM-64/256 demodulationQAM-64/256 demodulationMPEG frame synchronizationMPEG frame synchronizationError Correction (Reed-Error Correction (Reed-Solomon)Solomon)

MAC - Media Access Control MAC - Media Access Control Implemented partially in Implemented partially in

hardware and softwarehardware and softwareData and Control LogicData and Control Logic

Receive PathReceive PathDiplex Filter – splits/combines bands for 2-way capability on CATV systemsDiplex Filter – splits/combines bands for 2-way capability on CATV systemsTuner – isolates TV channel and mixes it down to Analog IF (6-40MHz)Tuner – isolates TV channel and mixes it down to Analog IF (6-40MHz)Analog to Digital ConversionAnalog to Digital ConversionDecimation Filters to down-sample to the symbol rate (e.g. CIC)Decimation Filters to down-sample to the symbol rate (e.g. CIC)QAM DemodulatorQAM Demodulator

MPEG Frame synchronizationMPEG Frame synchronization Automatic Gain Control (AGC)Automatic Gain Control (AGC) Equalizer – removes distortions, and cancels echoes or multi-path conditionsEqualizer – removes distortions, and cancels echoes or multi-path conditions Carrier RemovalCarrier Removal Automatic Frequency Control (AFC) Automatic Frequency Control (AFC)

QAM Demodulator

Generic Receive Path of a Cable Modem

DiplexFilter

(Internal orExternal to

Modem)

Analog RF

TUNERAnalog IF

ADC

FixedFreq

Decimate to Symbol

Rate

QAMDemodulation

Digital Ctrl Logic/Signals

LNA TunableBPF

Digital Ctrl Logic/Signals

VGA LPF

VariableFreq

Dig BB

Downstream SpecificationsDownstream SpecificationsFrequency Range: 65-850MHzFrequency Range: 65-850MHzBandwidth: 6MHz (USA) or 8MHz (EU)Bandwidth: 6MHz (USA) or 8MHz (EU)Modulation: 64-QAM (6 bits/symbol) or 256-QAM (8 bits/symbol)Modulation: 64-QAM (6 bits/symbol) or 256-QAM (8 bits/symbol)Data rate: 27-56 Mbps (depends on modulation and bandwidth)Data rate: 27-56 Mbps (depends on modulation and bandwidth)

Continuous stream of dataContinuous stream of dataFraming: MPEG-2 (based on DOCSIS spec)Framing: MPEG-2 (based on DOCSIS spec)Encryption: DES (Data Encryption Standard)Encryption: DES (Data Encryption Standard)Reed-Solomon Forward Error CorrectionReed-Solomon Forward Error Correction

* Symbol rate listed under Modulation does not compensate for error correction and other overhead* Symbol rate listed under Modulation does not compensate for error correction and other overhead** Data rates listed in the table use a symbol rate of 6.9 Msym/s for 8MHz BW and 5.2Msym/s for 6MHz BW** Data rates listed in the table use a symbol rate of 6.9 Msym/s for 8MHz BW and 5.2Msym/s for 6MHz BW

64-QAM64-QAM 256-QAM256-QAM

6 MHz6 MHz 31.2 31.2 MbpsMbps

41.6 Mbps41.6 Mbps

8 MHz8 MHz 41.4 41.4 MbpsMbps

55.2 Mbps55.2 Mbps

Transmit PathTransmit PathQAM (Burst) ModulatorQAM (Burst) Modulator

Reed-Solomon Encoder (Forward Error Correction)Reed-Solomon Encoder (Forward Error Correction) Randomizer/Scrambler – whitens the dataRandomizer/Scrambler – whitens the data

Helps to avoid discrete spurs in output spectrumHelps to avoid discrete spurs in output spectrumImproves synchronization at the receiver since data is more equiprobableImproves synchronization at the receiver since data is more equiprobable

Preamble InsertionPreamble InsertionInserts a training sequence into the systemInserts a training sequence into the systemTransmitted without R-S Encoding or ScramblingTransmitted without R-S Encoding or Scrambling

Modulation Encoder (QPSK/16-QAM)Modulation Encoder (QPSK/16-QAM) Pulse Shaping (e.g. Raised Root Cosine)Pulse Shaping (e.g. Raised Root Cosine) Programmable Interpolation Filters (e.g. CIC)Programmable Interpolation Filters (e.g. CIC) Programmable NCO, digitally modulates carrier anywhere in the Nyquist BandwidthProgrammable NCO, digitally modulates carrier anywhere in the Nyquist Bandwidth Digital to Analog ConversionDigital to Analog Conversion

Variable Gain CATV Line DriverVariable Gain CATV Line Driver

Variable GainCATV Line

Driver

DigitalData In

QAM Modulator

Reed-SolomonEncoder

Randomizer

PreambleInsertion

MUX

Pulse-ShapingFIR Filter

Pulse-ShapingFIR Filter

Modulation Encoder

(QPSK/16-QAM))

ProgrammableInterpolation

Filter

ProgrammableInterpolation

Filter

Digital Control Logic/Signals

Program-mableNCO

DAC

FixedFreq

To Diplexer

Upstream SpecificationsUpstream Specifications

Frequency Range: 5-65 MHzFrequency Range: 5-65 MHz

Variable Bandwidth: 200 kHz to 3.2 MHz (2 MHz Variable Bandwidth: 200 kHz to 3.2 MHz (2 MHz typically)typically)

Modulation: QPSK (2 bits/symbol) or 16-QAM (4 Modulation: QPSK (2 bits/symbol) or 16-QAM (4 bits/symbol)bits/symbol)

Data rate: Variable 320 Kbps to 10 MbpsData rate: Variable 320 Kbps to 10 Mbps

Transmit bursts of data in timeslots (TDM)Transmit bursts of data in timeslots (TDM)

Encryption: DESEncryption: DES

Reed Solomon Forward Error CorrectionReed Solomon Forward Error Correction

TX ImplementationTX Implementation

QAM

Modulator

Raised Root Cosine Xmit

Filter

I Xmit FIR

32x

Q Xmit FIR

32x

NCO

I

Q

COSSIN

GAIN

Input Bit Stream

RF OutputSignal

16-QAM Modulation – 320ksps16-QAM Modulation – 320kspsRaised Root Cosine Filter:Raised Root Cosine Filter:

Upsamples by 8, data rate at output = 2.56MspsUpsamples by 8, data rate at output = 2.56Msps Roll-off factor = 0.7Roll-off factor = 0.7

FIR Interpolation FilterFIR Interpolation Filter Polyphase ImplementationPolyphase Implementation Interpolates by 32, data rate at output = 81.92MspsInterpolates by 32, data rate at output = 81.92Msps Order FIR filter = 60, Cutoff Frequency = 320kHzOrder FIR filter = 60, Cutoff Frequency = 320kHz

NCO - Mixes signal up to 30MHzNCO - Mixes signal up to 30MHzGain factor of 30 to compensate for losses throughout pathGain factor of 30 to compensate for losses throughout path

TX SpectrumTX Spectrum

Output signal from up-conversion

1.280

PowerOutput from the RRC Filter:

FIR Interpolation Filter Response

0

PowerSampled signal after interpolation*

40.96-40.96

MHz-1.28

MHz

30-30 0

Power

MHz40.96-40.96

*Interpolation Images will occur every 2.56MHz*Interpolation Images will occur every 2.56MHz

RX ImplementationRX Implementation

QAM

Demodulator

Raised Root Cosine Rx

Filter

I Rx FIR

32x

Q Rx FIR

32x

NCO

I

Q

COSSIN

RF Input

Baseband

Signal

NCO - Mixes signal down to BasebandNCO - Mixes signal down to BasebandFIR Decimation FilterFIR Decimation Filter

Polyphase ImplementationPolyphase Implementation Decimates by 32, data rate at output = 2.56MspsDecimates by 32, data rate at output = 2.56Msps Order FIR filter = 10, Cutoff Frequency = 1.28MHzOrder FIR filter = 10, Cutoff Frequency = 1.28MHz

Raised Root Cosine Filter:Raised Root Cosine Filter: Downsamples by 8, data rate at output = 320kspsDownsamples by 8, data rate at output = 320ksps Roll-off factor = 1 Roll-off factor = 1

16-QAM Demodulation16-QAM Demodulation

RX SpectrumRX SpectrumInput signal to demodulator

0

PowerSampled signal after decimation

600

PowerSignal after the down-conversion Input Signal

0.320-0.32

PowerInput to the QAM demodulator

RRC Filter Response

1.28-1.28

-60

30-30 0

Power

MHz40.96-40.96

MHz

MHz

FIR Decimation Filter Response

MHz

Simulink Block DiagramSimulink Block Diagram

Simulink Results – TX PathSimulink Results – TX Path

TX Signal After Pulse ShapingTX Signal After Pulse Shaping TX Signal After 32X InterpolationTX Signal After 32X Interpolation

TX Signal after Up-ConversionTX Signal after Up-Conversion

Simulink Results – RX PathSimulink Results – RX Path

Rx Signal After Down-Conversion Rx Signal After 32X Decimation

System Results – Time DomainSystem Results – Time Domain

Delay Through Data Path

System Results 2System Results 2

TX Constellation RX Constellation

Simulink Results - BERSimulink Results - BER

BER with current implementation = 0.9741BER with current implementation = 0.9741

What effected BER?What effected BER? Design of the FIRDesign of the FIR SNR of datapath (AWGN Channel)SNR of datapath (AWGN Channel) Design of Raised Root Cosine FilterDesign of Raised Root Cosine Filter

Main TradeoffMain Tradeoff Dynamic Range/Spurious Content vs EVM/BERDynamic Range/Spurious Content vs EVM/BER

Better ImplementationBetter Implementation Compensation Filter after Interpolator and DecimatorCompensation Filter after Interpolator and Decimator

Websites for More Info:Websites for More Info:http://www.ciscopress.com/articles/article.asp?p=31289&seqNum=2&rl=1http://www.ciscopress.com/articles/article.asp?p=31289&seqNum=2&rl=1http://www.iec.org/online/tutorials/acrobat/cable_mod.pdfhttp://www.iec.org/online/tutorials/acrobat/cable_mod.pdfhttp://www.cable-modems.org/tutorial/index.htm#TOChttp://www.cable-modems.org/tutorial/index.htm#TOChttp://www.broadcom.com/collateral/pb/3349-PB01-R.pdfhttp://www.broadcom.com/collateral/pb/3349-PB01-R.pdfhttp://www.analog.com/UploadedFiles/Data_Sheets/13374136AD9853_c.pdhttp://www.analog.com/UploadedFiles/Data_Sheets/13374136AD9853_c.pdffhttp://www.wideband.com/products/Modems/QAM20Mod/QAMmod.htmlhttp://www.wideband.com/products/Modems/QAM20Mod/QAMmod.htmlhttp://focus.ti.com/lit/an/slwa023/slwa023.pdfhttp://focus.ti.com/lit/an/slwa023/slwa023.pdfhttp://www.broadcom.com/collateral/pb/3419-PB02-R.pdfhttp://www.broadcom.com/collateral/pb/3419-PB02-R.pdfhttp://www.cableaml.com/website3/wireless_catv/descriptions.htmhttp://www.cableaml.com/website3/wireless_catv/descriptions.htmhttp://www.fcc.gov/mb/engineering/cablemqa.htmlhttp://www.fcc.gov/mb/engineering/cablemqa.htmlhttp://www.cablemodem.com/http://www.cablemodem.com/http://www.nextgendc.com/index.htmhttp://www.nextgendc.com/index.htmhttp://computer.howstuffworks.com/cable-modem.htmhttp://computer.howstuffworks.com/cable-modem.htmhttp://compnetworking.about.com/od/dslvscablemodem/a/http://compnetworking.about.com/od/dslvscablemodem/a/dslcablecompare.htmdslcablecompare.htm