How do multiple users share the airwaves

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How do multiple users share the airwaves?

• Multiple Access (MA) techniques
• Time Division MA (TDMA)
• Frequency Division MA (FDMA)
• Code Division MA (CDMA)
• Space Division MA (SDMA)
• Analogies with simultaneous conversations in a
  classroom

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Time Division Multiple Access

• One voice speaks at a time

3
Frequency Division Multiple Access

• Voices with different pitch speak at the same
  time

4
Code Division Multiple Access

• Voices speak different languages at the same time
  and same pitch

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Space Division Multiple Access

• Analogy with conversationsDirect voice to
  location of desired person
• Space - the final frontier, exploited by smart
  antennas in new wireless systems.

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Wireless Networks

• We are looking at physical layertransmission
  of bits
• Network issues
• Coding to correct errors
• Handoff from cell to cell
• Routing calls to mobile users
• Applications (voice, data/internet, audio,
  pictures (maps), video, etc.)

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A Closer Look at CDMA

• Developed commercially by Qualcomm
• ClaimCDMA allowsgreater capacity(more
  users)in a wirelesssystem

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CDMA Codes

• Consider 4 users
• Each user wants to transmit a bitFor user 1,
  for a 0, for a 1
• Codes for 4 users, with 4 chips per bit

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• Each user multiplies its code and bit
• All users transmit their 4 chips at the same time
• Base station antenna receives the sum of the
  transmissions (the waves add in space!)

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• User signals arrive at base station with
  different amplitudes.Why? ______________________
  ____
• Signal amplitudes at base station
• Model for four chips received at base station

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Example 1

• CodesAmplitudes and bits