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Simulink Simulink Implementation of Implementation of a Cable Modem a Cable Modem Gina Colangelo Gina Colangelo Nathan Egan Nathan Egan EE194-SDR EE194-SDR 05/02/2006 05/02/2006

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Simulink Implementation of a Cable ModemGina Colangelo Nathan Egan EE194EE194-SDR 05/02/2006

Topics CoveredCable Data Network Overview Cable Modem Architecture Downstream/Upstream Specifications Simulink Implementation Implementation Results

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

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

CMTS output channel combined with TV video signals CATV Network to Subscriber via coaxial cable One-toOne-to-Two splitter: One signal to Set Top Box (STB), other to Cable Modem Cable Modem

One Modem can support up to 16 users in a local-area network localCable Modem connected to PC via Ethernet, USB, PCI Bus, etc

PC/Ethernet Card

Cable Modem ArchitectureTransmit/Upstream

QPSK/QAM Modulator performs:QPSK/QAMQPSK/QAM-16 modulation ReedReed-Solomon Encoding D/A Conversion UpUp-conversion to the selected frequency/channel


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

QAM Demodulator performs:A/D conversion QAMQAM-64/256 demodulation MPEG frame synchronization Error Correction (Reed(ReedSolomon)

MAC - Media Access Control

Implemented partially in hardware and software

Data and Control Logic

Receive PathDiplex Filter splits/combines bands for 2-way capability on CATV systems 2Tuner isolates TV channel and mixes it down to Analog IF (6-40MHz) (6Analog to Digital Conversion Decimation Filters to down-sample to the symbol rate (e.g. CIC) downQAM Demodulator

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

TUNERDiplex Filter (Internal or External to Modem)Analog RF Analog IF

QAM DemodulatorVGA


Tunable BPFVariable Freq


ADCFixed Freq

Decimate to Symbol Rate

QAM Demodulation

Dig BB

Digital Ctrl Logic/Signals

Digital Ctrl Logic/Signals

Generic Receive Path of a Cable Modem

Downstream SpecificationsFrequency Range: 65-850MHz 65Bandwidth: 6MHz (USA) or 8MHz (EU) Modulation: 64-QAM (6 bits/symbol) or 256-QAM (8 bits/symbol) 64256Data rate: 27-56 Mbps (depends on modulation and bandwidth) 27-


256-QAM 256-

6 MHz 31.2 Mbps 41.6 Mbps 8 MHz 41.4 Mbps 55.2 MbpsContinuous stream of data Framing: MPEG-2 (based on DOCSIS spec) MPEGEncryption: DES (Data Encryption Standard) ReedReed-Solomon Forward Error Correction* 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

Transmit PathQAM (Burst) Modulator ReedReed-Solomon Encoder (Forward Error Correction) Randomizer/Scrambler whitens the dataHelps to avoid discrete spurs in output spectrum Improves synchronization at the receiver since data is more equiprobable

Preamble InsertionInserts a training sequence into the system Transmitted without R-S Encoding or Scrambling R-

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

To Diplexer Programmable Interpolation Filter Programmable Interpolation Filter

QAM ModulatorPreamble Insertion Pulse-Shaping FIR Filter Pulse-Shaping FIR Filter Modulation Encoder (QPSK/16-QAM)) Randomizer ReedSolomon Encoder Digital Data In


Programmable NCO


Variable Gain CATV Line Driver

Fixed Freq

Digital Control Logic/Signals

Upstream SpecificationsFrequency Range: 5-65 MHz 5Variable Bandwidth: 200 kHz to 3.2 MHz (2 MHz typically) Modulation: QPSK (2 bits/symbol) or 16-QAM (4 16bits/symbol) Data rate: Variable 320 Kbps to 10 Mbps Transmit bursts of data in timeslots (TDM) Encryption: DES Reed Solomon Forward Error Correction

TX ImplementationI Input Bit Stream

I Xmit I

QAM Modulator

ai ed oot o ine Xmit ilterQ


Q Xmit I

Output Signal




1616-QAM Modulation 0k p ai ed oot o ine ilter:

Up ample by 8, data rate at output = .56M p olloll-off factor = 0.7 Polypha e Implementation Interpolate by , data rate at output = 81.9 M p Order I filter = 60, utoff requency = 0kHz

I Interpolation ilter

N O - Mi e ignal up to 0MHz Gain factor of 0 to compen ate for lo

e throughout path

TX SpectrumOutput from the RRC Filter: Power -1.28 Sampled signal after interpolation* 0 Power 1.28 MHz FIR Interpolation Filter Response

-40.96 Output signal from up-conversion

0 Power









*Interpolation Images will occur every 2.56MHz

RX ImplementationI R Input


IR Rai ed Root o ine R ilter QAM DemodulatorBa eband Signal






N O - Mi e ignal down to Ba eband IR Decimation ilter

Polypha e Implementation Decimate by , data rate at output = .56M p Order IR filter = 10, utoff requency = 1. 8MHz Down ample by 8, data rate at output = RollRoll-off factor = 1 0k p

Rai ed Root o ine ilter:

1616-QAM Demodulation

RX SpectrumInput signal to demodulator Power -40.96 Signal after the down-conversion -30 0 Power Input Signal 30 40.96 MHz

-60 Sampled signal after decimation

0 Power



FIR Decimation Filter Response

-1.28 Input to the QAM demodulator

0 Power



RRC Filter Response





Simulink Block Diagram

Simulink Results TX Path

TX Signal After Pulse Shaping

TX Signal After 32X Interpolation

TX Signal after Up-Conversion Up-

Simulink Results RX Path

Rx Signal After Down-Conversion

Rx Signal After 32X Decimation

System Results Time Domain

Delay Through Data Path

System Results 2

TX Constellation

RX Constellation

Simulink Results - BERBER with current implementation = 0.9741 What effected BER?

Design of the FIR SNR of datapath (AWGN Channel) Design of Raised Root Cosine Filter Dynamic Range/Spurious Content vs EVM/BER Compensation Filter after Interpolator and Decimator

Main Tradeoff

Better Implementation

Websites for More Info: http://www.cable f htm