Multiuser OFDM with Adaptive Subcarrier, Bit and Power Allocation

1
Multiuser OFDM with Adaptive Subcarrier, Bit and
Power Allocation
(IEEE JOURNAL ON SELECTED AREAS IN
COMMUNICATIONS, VOL. 17, NO. 10, OCTOBER 1999)
2
Outline

• Motivation for Multiuser OFDM
• Bit Allocation Algorithm for Single User
• Multiuser Subcarrier and Bit Allocation Algorithm
• Simulation Results
• Conclusions
• References

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Motivation

• Orthogonal Frequency Division Multiplexing (OFDM)
  provides a transmission scheme to avoid frequency
  selective fading in a wide band wireless
  communications.
• How to allocate different user to use different
  subcarriers with adaptive optimal bit
  (modulation) and power becomes an interesting
  problem in a wireless multiuser environment.

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Motivation
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Multiple Access Schemes
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System Model
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Problem Formulation

• -magnitude of the channel gain
• Assume 1 for all users
• required received power
• Then the transmitted power

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Problem Formulation
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Bit Allocation Algorithm for Single User

• Greedy Algorithm

back
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Multiuser Subcarrier and Bit Allocation Algorithm

• As users can not share the same subcarrier, this
  dependency makes the greedy algorithm a
  non-optimal solution.
• Some relaxations on previous problem have to be
  made.

to
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Relaxzation of the Constraints

• Let be a real number and
  introduce another variable

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Lagrangian Optimization

• Using the Lagrangian Multipliers to solve the
  problem

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Lagrangian Optimization
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Lagrangian Optimization
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Optimal Solution
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Optimal Solution (Cont)

• The nth subcarrier are selected by the kth user
  if
• To satisfy the bit rate requirement, increase
• gtH decrease gt will increase the pr
  to become 1 to transmit data
• However, optimal may be not an interger

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Multiuser Adaptive OFDM (MAO)

• Satisfy the constraint
• Using the optimal method to decide the proper
  carrier for a specific user.
• Then use the single user bits allocation
  algorithm to satisfy its data rate requiremet.
• Note that

to
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Simulation Results

• The required power for supporting c bits/symbol
  at a given BER Pe

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Simulation Results
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Simulation Results
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Simulation Results
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Simulation Results
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Simulation Results
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Conclusions

• The proposed MAO outperforms than the
  conventional MA techniques in an OFDM system.
• Quantify the performance improvement in terms of
  SNR, BER and Outage Pr.

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References

• Multiuser OFDM with Adaptive Subcarrier, Bit, and
  Power Allocation (IEEE JOURNAL ON SELECTED AREAS
  IN COMMUNICATIONS, VOL. 17, NO. 10, OCTOBER 1999)
• Digital Communication, Prokis, 4th edition
• Wireless Communications, Rappaport,2nd edition