On the Effect of Cancellation Order in Successive Interference Cancellation for CDMA Systems

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On the Effect of Cancellation Order in Successive
Interference Cancellation for CDMA Systems

• Soshant Bali

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Introduction

• CDMA has gained popularity as multiple access
  scheme for cellular systems
• Conventional matched filter receiver treats
  Multiple Access Interference (MAI) as Gaussian
  Noise
• Multiuser Detection improves capacity of cellular
  CDMA systems by trying to reduce effect of MAI
• Successive Interference Cancellation (SIC) is a
  Multiuser Detection technique where MAI is
  cancelled in the order of perceived reliability
  of detected signals
• Investigate effect of cancellation order of SIC
  on BER performance

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System Model Conventional Receiver
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Successive Interference Cancellation

• Most reliable signal is detected first
• Signal is regenerated and cancelled from
  aggregate received signal
• Resulting signal is used to detect the next most
  reliable signal
• This process is continued until all signals have
  been detected

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Cancellation Order Based on Average Power

• Average received signal power is used as a
  measure of reliability
• Strongest received signal detected first,
  followed by next strongest and so on
• Cancel the strongest signal first
• It has the most negative effect (maximum benefit
  first)
• Best bit decision can be made on that signal
  (most reliable cancellation)
• But in perfect power control, long term average
  power of all users is equal
• In that case, ordering is fixed but arbitrary

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Reordering Every Symbol

• Order of detection is recomputed for every symbol
• Order based on decreasing matched filter outputs
• Once order is determined, signals are cancelled
  in that order
• Better performance than Average Power Ordering
• Increased computations compared to Average Power
  Ordering

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Reordering After Each Cancellation
Find Max(Abs(Z))
Find Max(Abs(Z))
Cancel MAI contribution, reconstruct r(t)
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BER v/s Eb/No for AWGN ChannelPerfect Power
Control, 10 Users, Processing Gain 31
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BER v/s Eb/No for AWGN ChannelPerfect Power
Control, 20 Users, Processing Gain 31
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Conclusion

• Reordering After Each Cancellation provides best
  performance
• But is computationally intensive
• Reordering Each Symbol provides better
  performance than Average Power ordering, with
  reduced computations compared to Reordering After
  Each Cancellation