Orthogonal Frequency Division Multiple Access (OFDMA)

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Orthogonal Frequency Division Multiple
Access(OFDMA)

• Khaja Mohammad Shazzad

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Outline

• Background
• Multiple Access (MA) Methods
• Orthogonal Frequency Division Multiplexing (OFDM)
  Based Multiple Access (OFDMA)
• Orthogonality Principle
• OFDM
• OFDM-FDMA
• Advantages and Disadvantages of OFDMA
• Conclusion

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Multiple Access (MA)

• General wireless cellular systems are multi-users
  systems
• Radio resource are limited
• Limited Bandwidth
• Limited number of channels
• The radio resource must be shared among multiple
  users
• Multiple Access Control (MAC) needed
• Contention-based
• Non-contention-based

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Contention-basedMultiple Access(MA)

• Contention-based
• Each terminal transmits in a decentralized way
• No central controller (Base stations or access
  points)
• Example
• ALOHA
• Carrier Sensing Multiple Access (CSMA)
• Standard
• GSM l uses the slotted ALOHA in the terminals
  initial access process
• IEEE 802.11 uses CSMA/CA based contention access
  scheme

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Non-contention-based Multiple Access (MA)

• A logic controller (BS or AP) is needed to
  coordinate the transmissions of all the terminals
• The controller informs each device when and on
  which channel it can transmit
• Collisions can be avoided entirely
• Two Subdivisions
• Non-channelization
• Channelization

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Non-channelizationNon-contention-based MA

• Terminals transmit sequentially using the same
  channel
• Example
• Polling based medium access
• Standard
• IEEE 802.15(WPAN)
• IEEE 802.11(WLAN)

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ChannelizationNon-contention-based MA

• Terminals transmit simultaneously using different
  channels
• Most commonly used protocols in cellular systems
• Example
• 1. Time Division Multiple Access (TDMA)
• 2. Code Division Multiple Access( CDMA)
• 1. Frequency Division Multiple Access (FDMA)
• Standard
• 1. GSM (TDMA)
• 2. IS-95 (CDMA)
• 3. American Mobile Phone System, AMPS (FDMA)

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Time Division Multiple Access (TDMA)

• GSM
• Time slot 0.577 ms
• Frame 4.6 ms
• 8 time slots per frame
• Frequency band 20 KHz

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Code Division Multiple Access (CDMA)

• IS-95
• Orthogonal Walsh codes
• 64 codes (channels)
• One pilot channel
• Seven paging channels
• 55 traffic channels
• Each carrier 1.25 MHz

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Frequency Division Multiple Access (FDMA)

• American Mobile Phone System (AMPS)
• Total Bandwidth 25 MHz
• Each Channel 30 KHz

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Orthogonal Frequency Division Multiplexing (OFDM)
Based Multiple Access (OFDMA)

• Orthogonality Principle
• OFDM
• OFDM-FDMA

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Orthogonality Principle

• Vector space
• A, B and C vectors in space are orthogonal to
  each other
• A.BB.CC.A0
• (ABC).A(mod A)2
• (ABC).B(mod B)2
• (ABC).C(mod C)2

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Orthogonality Principle cont..

• Real Function space

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Orthogonality Principle cont..
Here mw and nw are called m-th and n-th
harmonics of w respectively
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Orthogonal Frequency Division Multiplexing(OFDM)

• It is a special kind of FDM
• The spacing between carriers are such that they
  are orthogonal to one another
• Therefore no need of guard band between carriers.
• One example makes the thing clear

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Example of OFDM

• Lets we have following information bits
• 1, 1, -1, -1, 1, 1, 1, -1, 1, -1, -1, -1, -1, 1,
  -1, -1,
• Just converts the serials bits to parallel bits

C1 C2 C3 C4
1 1 -1 -1
1 1 1 -1
1 -1 -1 -1
-1 1 -1 -1
-1 1 1 -1
-1 -1 1 1
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Example of OFDM cont..
Modulate each column with corresponding
sub-carrier using BPSK
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Example of OFDM cont..

• Final OFDM Signal Sum of all signal

V(t)
Generated OFDM signal, V(t)
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OFDM-FDMA (OFDMA)

• Each terminal occupies a subset of sub-carriers
• Subset is called an OFDMA traffic channel
• Each traffic channel is assigned exclusively to
  one user at any time

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OFDM-FDMA (OFDMA)

• The IEEE 802.16e/ WiMax use OFDMA as Multiple
  access technique
• Bandwidth options 1.25, 5, 10, or 20 MHz
• Entire bandwidth divided into 128, 512, 1024 or
  2048 sub carriers
• 20 MHz bandwidth with 2048 sub carriers has 9.8
  KHz spacing between sub carriers

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OFDM-FDMA (System View)
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Advantages of OFDMA

• Multi-user Diversity
• broadband signals experience frequency selective
  fading
• OFDMA allows different users to transmit over
  different portions of the broadband spectrum
  (traffic channel)
• Different users perceive different channel
  qualities, a deep faded channel for one user may
  still be favorable to others

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Advantages of OFDMA cont..
Multi-user Diversity
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Advantages of OFDMA cont..

• Efficient use of Spectrum

4/3 Hz per symbol
6/5 Hz per symbol
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Advantages of OFDMA cont..

• Receiver Simplicity
• It eliminates the intra-cell interference
  avoiding CDMA type of multi-user detection
• Orthogonality of code destroyed by selective
  fading
• Only FFT processor is required
• Bit Error Rate performance is better only in
  Fading environment

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Disadvantages of OFDMA

• Peak to average power ratio (PAPR)

The large amplitude variation increases in-band
noise and increases the BER when the signal has
to go through amplifier nonlinearities.
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Disadvantages of OFDMA cont..

• Synchronization
• Tight Synchronization between users are required
  for FFT in receiver
• Pilot signals are used for synchronizations
• Co-channel interference
• Dealing with this is more complex in OFDM than in
  CDMA
• Dynamic channel allocation with advanced
  coordination among adjacent base stations

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Research issues and Conclusion

• Future works
• Peak-to-average power reduction in OFDM
• Timing and Frequency Synchronization
• Efficient digital signal processing
  Implementation of OFDM
• Multiple input/Multiple output (MIMO) OFDM
• Conclusion
• Different variations of OFDMA are proposed and
  have different pros and cons

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Thank You

• Questions
• or
• Comments?