Fall 2012Afshin Hemmatyar

Sharif University of TechnologyInternational Campus – Kish Island

Data CommunicationsIntroduction

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Contents

• Background and Preview• Random Process• Continuous-Wave Modulation• Pulse Modulation• Base-band Pulse Transmission• Signal-Space Analysis• Pass-band Digital Transmission• Spread-Spectrum Modulation

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References

• Simon Haykin, “Communication Systems,” 4th Edition, 2001.

• B.P. Lathi, “Modern Analog and Digital Communication Systems,” 3rd Edition, 1998.

• John J. Proakis, “Digital Communications,”

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Background and Preview

• Digital Communication System• Communication Channels• Mathematical Models for Communication Channels • Primary Communication Resources• Basic Modes of Communication• Channel Sharing• Information Capacity of Channel

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Digital Communication System (1)

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Digital Communication System (2)

• Information Source Generates message signals: Voice, Music, Picture, or Data.

• Source Encoder Removes redundant information from message signal and is responsible for efficient use of channel.

• Channel Encoder Adds controlled redundancy and produces sequence of symbols regarding channel specifications.

• Modulator Represents symbols in waveforms suitable for transmission over the physical media of interest.

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Communication Channels (1)

• Wire-line (Twisted Pair, Coaxial Cable, . . . )

• Fiber Optic• Wireless (Electromagnetics) (Broadcast, Mobile, Satellite, . . . )

• Underwater Acoustic• Storage

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Communication Channels (2)

• Linear or Nonlinear

• Time Invariant or Time Varying

• Frequency Dependent or Independent

• Bandwidth Limited or Power Limited

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Communication Channels (3)

•

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Mathematical Models for Communication Channels

• Additive Noise R(t) = C.S(t) + n(t)

• LTI Filter R(t) = C(t)*S(t) + n(t)

• LTV Filter R(t) = C(t,τ)*S(t) + n(t)

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Primary Communication Resources

• Transmitter PowerAverage power of transmitted signal

• Channel BandwidthBand of frequencies allocated for Transmission

Usually one is more important: • Power Limited Channel• Band Limited Channel

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Basic Modes of Communications

• Point-to-Point• Broadcasting

• Unidirectional (Simplex)• Bidirectional (Half and Full Duplex)

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Channel Sharing

• Multiplexing• FDM: Frequency-Division Multiplexing (WDM: Wavelength-Division Multiplexing)• TDM: Time-Division Multiplexing• CDM: Code-Division Multiplexing

• Multiple Access• FDMA: Frequency-Division Multiple Access (WDMA: Wavelength-Division Multiple Access)• TDMA: Time-Division Multiple Access• CDMA: Code-Division Multiple Access

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Information Capacity of Channel

• Shannon’s Theorem

C = B log2(1+SNR)

Channel Capacity in bits/Sec Signal to Noise Ratio (no units)

Channel Bandwidth in Hz