EE 194 Software Defined Radio

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EE 194Software Defined Radio

• Cable Modem
• Greg Kastan

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• Source http//www.cable-modems.org/tutorial

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Some Key Terminology

• CMTS
• Cable Modem Termination System (Connects CATV to
  internet)
• Downstream Flow
• CMTS to modem
• Carrier frequencies 42/65-850 MHz
• Upstream Flow
• Modem to CMTS
• Carrier frequencies 5-42 MHz (US) and 5-65 MHz
  (Europe)
• Medium
• Coax cable with one physical channel
• Industry Standard for Modems
• DOCSIS Data Over Cable Service Interface
  Standard
• Define specs for both CMTS and modem

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Block Diagram of Modem Inside
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• Tuner
• Reception of modulated signal
• Pass to demodulator
• Demodulator
• Downstream flow
• 64-QAM or 256-QAM
• ADC
• Error correction and transmission in frames
• MAC (Media Access Control)
• Located between up and downstream flows
• Provides hardware/software interface
• Extracts and filters demodulated data frames
• Transmission timing for upstream bursts
• Modulator
• Upstream flow
• QPSK/16-QAM
• Reed Solomon encoding of each burst
• DAC
• Combined Modulator/Demodulator Chips

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Downstream

• Continuous stream received by modem
• Carrier BW is 6 or 8 MHz
• 64-QAM or 256-QAM
• Rate of transmission is 27-56 Mbps

• Source http//www.cable-modems.org/tutorial

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Upstream

• Data bursts transmitted by TDM
• Carrier BW is 2 MHz
• QPSK or 16-QAM
• Rate of transmission is 3 Mbps

• Source http//www.cable-modems.org/tutorial

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QAM Receiver
QAM Transmitter
Source http//en.wikipedia.org/wiki/Quadrature_am
plitude_modulation
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Performance Metrics
Raw Data Rates

• Effective data rate is reduced
• Error correction (Reed Solomon), framing,
  overhead

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Applications

• Internet service over existing CATV lines
• WAN and LAN high speed

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References

• http//www.cable-modems.org/tutorial
• http//www.howstuffworks.com/cable-modem.htm
• http//en.wikipedia.org/wiki/Quadrature_amplitude_
  modulation