Electrical Communications Systems 0909.331.01 Spring 2005

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Electrical Communications Systems0909.331.01
Spring 2005
Lecture 1bJanuary 19, 2005

• Shreekanth Mandayam
• ECE Department
• Rowan University
• http//engineering.rowan.edu/shreek/spring05/ecom
  ms/

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ECOMMS Topics
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ECOMMS Topics
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Plan

• Baseband and Bandpass Signals
• Recall Comm. Sys. Block diagram
• Aside Why go to higher frequencies?
• International US Frequency Allocations
• Intoduction to Information Theory
• Recall List of topics
• Probability
• Information
• Entropy
• Signals and Noise

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Comm. Sys. Bock Diagram
Noise
Channel
Rx
m(t)
Tx
r(t)
s(t)

• Low Frequencies
• lt20 kHz
• Original data rate

• High Frequencies
• gt300 kHz
• Transmission data rate

Formal definitions will be provided later
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Aside Why go to higher frequencies?
Half-wave dipole antenna
c f l c 3E08 ms-1 Calculate l for f 5
kHz f 300 kHz
Tx
l/2
There are also other reasons for going from
baseband to bandpass
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Information

• Recall
• Information Source a system that produces
  messages (waveforms or signals)
• Digital/Discrete Information Source Produces a
  finite set of possible messages
• Digital/Discrete Waveform A function of time
  that can only have discrete values
• Digital Communication System Transfers
  information from a digital source to a digital
  sink

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Another Classification of Signals (Waveforms)

• Deterministic Signals Can be modeled as a
  completely specified function of time
• Random or Stochastic Signals Cannot be
  completely specified as a function of time must
  be modeled probabilistically

• What type of signals are information bearing?

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Signals and Noise
Lab 1
Comm. Waveform
Noise (undesired)
Signal (desired)

• Strictly, both signals and noise are stochastic
  and must be modeled as such
• We will make these approximations, initially
• Noise is ignored
• Signals are deterministic

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Measures of Information

• Definitions
• Probability
• Information
• Entropy
• Source Rate
• Recall Shannons Theorem
• If R lt C B log2(1 S/N), then we can have
  error-free transmission in the presence of noise

MATLAB DEMO entropy.m
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Summary