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

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

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

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

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

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 64-QAM 256-QAM 6 MHz 31.2 Mbps 41.6 Mbps 8 MHz 41.4 Mbps 55.2 Mbps

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

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

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

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

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

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

Simulink Block Diagram

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

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 - 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

Websites for More Info: http://www.ciscopress.com/articles/article.asp?p=31289&seqNum=2&rl=1 http://www.iec.org/online/tutorials/acrobat/cable_mod.pdf http://www.cable-modems.org/tutorial/index.htm#TOC http://www.broadcom.com/collateral/pb/3349-PB01-R.pdf http://www.analog.com/UploadedFiles/Data_Sheets/13374136AD9853_c.pdf http://www.wideband.com/products/Modems/QAM20Mod/QAMmod.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/descriptions.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