Simulink Implementation of a Cable Modem

1
Simulink Implementation of a Cable Modem

• Gina Colangelo
• Nathan Egan
• EE194-SDR
• 05/02/2006

2
Topics Covered

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

3
Cable Modem Network Overview

• Headend DOCSIS-certified CMTS (Cable Modem
  Termination System)
• 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-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
• PC/Ethernet Card
• Cable Modem connected to PC via Ethernet, USB,
  PCI Bus, etc

4
Cable Modem Architecture

• Transmit/Upstream
• QPSK/QAM Modulator performs
• QPSK/QAM-16 modulation
• Reed-Solomon Encoding
• D/A Conversion
• Up-conversion to the selected frequency/channel
• Receive/Downstream
• RF Tuner
• Converts TV Channel to a fixed lower frequency
  (6-40MHz)
• QAM Demodulator performs
• A/D conversion
• QAM-64/256 demodulation
• MPEG frame synchronization
• Error Correction (Reed-Solomon)
• MAC - Media Access Control
• Implemented partially in hardware and software
• Data and Control Logic

5
Receive Path

• Diplex Filter splits/combines bands for 2-way
  capability on CATV systems
• Tuner isolates TV channel and mixes it down to
  Analog IF (6-40MHz)
• Analog to Digital Conversion
• Decimation Filters to down-sample to the symbol
  rate (e.g. CIC)
• QAM Demodulator
• MPEG Frame synchronization
• Automatic Gain Control (AGC)
• Equalizer removes distortions, and cancels
  echoes or multi-path conditions
• Carrier Removal
• Automatic Frequency Control (AFC)

QAM Demodulator
Generic Receive Path of a Cable Modem
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Downstream Specifications

• Frequency Range 65-850MHz
• Bandwidth 6MHz (USA) or 8MHz (EU)
• Modulation 64-QAM (6 bits/symbol) or 256-QAM (8
  bits/symbol)
• Data rate 27-56 Mbps (depends on modulation and
  bandwidth)
• Continuous stream of data
• Framing MPEG-2 (based on DOCSIS spec)
• Encryption DES (Data Encryption Standard)
• Reed-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

7
Transmit Path

• QAM (Burst) Modulator
• Reed-Solomon Encoder (Forward Error Correction)
• Randomizer/Scrambler whitens the data
• Helps to avoid discrete spurs in output spectrum
• Improves synchronization at the receiver since
  data is more equiprobable
• Preamble Insertion
• Inserts a training sequence into the system
• Transmitted without R-S Encoding or Scrambling
• Modulation Encoder (QPSK/16-QAM)
• 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

QAM Modulator
To Diplexer
Preamble Insertion
Programmable Interpolation Filter
Digital Data In
Program- mable NCO
Pulse-Shaping FIR Filter
Reed- Solomon Encoder
Modulation Encoder (QPSK/16-QAM))
Randomizer
MUX
DAC
Pulse-Shaping FIR Filter
Programmable Interpolation Filter
Variable Gain CATV Line Driver
Fixed Freq
Digital Control Logic/Signals
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Upstream Specifications

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

9
TX Implementation

• 16-QAM Modulation 320ksps
• Raised Root Cosine Filter
• Upsamples by 8, data rate at output 2.56Msps
• Roll-off factor 0.7
• FIR Interpolation Filter
• Polyphase Implementation
• Interpolates by 32, data rate at output
  81.92Msps
• Order FIR filter 60, Cutoff Frequency 320kHz
• NCO - Mixes signal up to 30MHz
• Gain factor of 30 to compensate for losses
  throughout path

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TX Spectrum
Power
Output from the RRC Filter
MHz
-1.28
1.28
0
Power
Sampled signal after interpolation
FIR Interpolation Filter Response
MHz
0
-40.96
40.96
Output signal from up-conversion
Power
MHz
40.96
-40.96
30
-30
0
Interpolation Images will occur every 2.56MHz
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RX Implementation
I Rx FIR 32x
I
Raised Root Cosine Rx Filter
RF Input
QAM Demodulator
Baseband Signal
Q Rx FIR 32x
Q
COS
SIN
NCO

• NCO - Mixes signal down to Baseband
• FIR Decimation Filter
• Polyphase Implementation
• Decimates by 32, data rate at output 2.56Msps
• Order FIR filter 10, Cutoff Frequency 1.28MHz
• Raised Root Cosine Filter
• Downsamples by 8, data rate at output 320ksps
• Roll-off factor 1
• 16-QAM Demodulation

12
RX Spectrum
Power
Input signal to demodulator
MHz
40.96
-40.96
30
-30
0
Power
Signal after the down-conversion
Input Signal
-60
MHz
60
0
Power
Sampled signal after decimation
FIR Decimation Filter Response
MHz
0
-1.28
1.28
Power
Input to the QAM demodulator
RRC Filter Response
MHz
0.32
0
-0.32
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Simulink Block Diagram
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Simulink Results TX Path
TX Signal After Pulse Shaping
TX Signal After 32X Interpolation
TX Signal after Up-Conversion
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Simulink Results RX Path
Rx Signal After Down-Conversion
Rx Signal After 32X Decimation
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System Results Time Domain
Delay Through Data Path
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System Results 2
RX Constellation
TX Constellation
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Simulink Results - BER

• BER with current implementation 0.9741
• What effected BER?
• Design of the FIR
• SNR of datapath (AWGN Channel)
• Design of Raised Root Cosine Filter
• Main Tradeoff
• Dynamic Range/Spurious Content vs EVM/BER
• Better Implementation
• Compensation Filter after Interpolator and
  Decimator

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Websites for More Info

• http//www.ciscopress.com/articles/article.asp?p3
  1289seqNum2rl1
• http//www.iec.org/online/tutorials/acrobat/cable_
  mod.pdf
• http//www.cable-modems.org/tutorial/index.htmTOC
  
• http//www.broadcom.com/collateral/pb/3349-PB01-R.
  pdf
• http//www.analog.com/UploadedFiles/Data_Sheets/13
  374136AD9853_c.pdf
• http//www.wideband.com/products/Modems/QAM20Mod/Q
  AMmod.html
• http//focus.ti.com/lit/an/slwa023/slwa023.pdf
• http//www.broadcom.com/collateral/pb/3419-PB02-R.
  pdf
• http//www.cableaml.com/website3/wireless_catv/des
  criptions.htm
• http//www.fcc.gov/mb/engineering/cablemqa.html
• http//www.cablemodem.com/
• http//www.nextgendc.com/index.htm
• http//computer.howstuffworks.com/cable-modem.htm
• http//compnetworking.about.com/od/dslvscablemodem
  /a/dslcablecompare.htm