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
Diplex
Filter
(Internal or
External to
Modem)
Analog RF
TUNER
Analog IF
ADC
Fixed
Freq
Decimate
to Symbol Rate
QAM
Demodulation
Digital Ctrl Logic/Signals
LNA
Tunable
BPF
VGA
LPF
Variable
Dig BB
Generic Receive Path of a Cable Modem

6. Downstream Specifications
Frequency Range: MHz
Bandwidth: 6MHz (USA) or 8MHz (EU)
Modulation: 64-QAM (6 bits/symbol) or 256-QAM (8 bits/symbol)
Data rate: 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
64-QAM
256-QAM
6 MHz
31.2 Mbps
41.6 Mbps
8 MHz
41.4 Mbps
55.2 Mbps

7. Transmit Path QAM Modulator 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
Program-
mable
NCO
Digital
Data In
Pulse-Shaping
FIR Filter
Modulation
Encoder
(QPSK/16-QAM))
Reed-
Solomon
Encoder
DAC
MUX
Randomizer
Programmable
Interpolation
Filter
Pulse-Shaping
FIR Filter
Variable Gain
CATV Line
Driver
Fixed
Freq
Digital Control Logic/Signals

8. 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. Raised Root Cosine Xmit Filter
TX Implementation
QAM
Modulator
Raised Root Cosine Xmit Filter
I Xmit FIR
32x
Q Xmit FIR
NCO I Q
COS
SIN
GAIN
Input Bit Stream
RF OutputSignal
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

10. FIR Interpolation Filter Response
TX Spectrum
Output from the RRC Filter:
Power
-1.28
1.28
MHz
Sampled signal after interpolation*
Power
FIR Interpolation Filter Response
-40.96
40.96
MHz
Output signal from up-conversion
Power
-40.96
-30 30
40.96
MHz
*Interpolation Images will occur every 2.56MHz

11. Raised Root Cosine Rx Filter
RX Implementation
I Rx FIR
32x I
RF Input
Raised Root Cosine Rx Filter
QAM
Demodulator
Baseband
Signal
Q Rx FIR
32x Q
SIN
COS
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. FIR Decimation Filter Response
RX Spectrum
Power
Input signal to demodulator
-40.96
-30 30
40.96
MHz
Signal after the down-conversion
Power
Input Signal
-60 60
MHz
Sampled signal after decimation
Power
FIR Decimation Filter Response
-1.28
MHz
1.28
Input to the QAM demodulator
Power
RRC Filter Response
MHz
-0.32
0.32

13. Simulink Block Diagram

14. Simulink Results – TX Path
TX Signal After Pulse Shaping
TX Signal After 32X Interpolation
TX Signal after Up-Conversion

15. Simulink Results – RX Path
Rx Signal After Down-Conversion
Rx Signal After 32X Decimation

16. System Results – Time Domain
Delay Through Data Path

17. System Results 2
TX Constellation
RX Constellation

18. 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

19. Websites for More Info: