Methods for Compression of Feedback in Adaptive Multi-carrier 4G Schemes

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Methods for Compression of Feedback in Adaptive
Multi-carrier 4G Schemes

• Víctor P. Gil Jiménez
• Ana García Armada
• M. Julia Fernández-Getino García
• University Calos III de Madrid
• Spain

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Outline

• OFDMA for Downlink in 4G.
• Feedback information.
• Opportunistic Feedback.
• System Description.
• Algorithms for compression
• Time correlation
• Frequency correlation
• Time-Frequency correlation
• Results.
• Conclusions.

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OFDMA for downlink in 4G

• Several techniques for Downlink.
• OFDMA (Orthogonal Frequency Division Multiple
  Access) is one of the candidates.
• The BS (Base Station) allocates the different
  users transmission across the orthogonal
  frequencies.

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Feedback Information

• The Adaptive Modulation selects the adequate
  Modulation and Coding Scheme (MCS) according to
  instantaneous channel conditions for each
  terminal.
• Channel is usually known (estimated/predicted) at
  the receiver.
• The receiver should feedback this information.
• It may be highly rate-demanding.

Reduction Compression
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Opportunistic Feedback

• The scheduler usually selects the best user(s)
  for transmitting. Maximum Throughput criterion.
• It is a waste of resources if a terminal with a
  bad channel feeds back its data (it will never be
  served except for fairness policies).
• The BS broadcasts a minimum quality for feeding
  back.

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Opportunistic FeedbackSome Results
Prob of finding a sub-carrier able to tx gt 4 bits
(Load factor subcarr occupied when user
arrives)
UMTS Vehicular A Channel
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Opportunistic FeedbackSome Results
Load Factor 0
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System Description
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System DescriptionParameters

• Downlink OFDMA.
• 512 sub-carriers (416 useful for data).
• Sub-carriers divided into groups of 8
  sub-carriers during 12 OFDM symbols a chunk.
• 52 chunks in total.
• The same MCS in the same chunk but different
  (possibly) among them. Adaptive Modulation at
  chunk level.
• Available modulations BPSK 256 QAM.
• UMTS channel models vehicular A, pedestrian A.

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System DescriptionA Chunk
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Algorithms for compression

• Huffman coding for compression. Needs well
  conditioned data.
• Using Time correlation.
• Using Frequency correlation.
• Using Time-Frequency correlation.

(BPSK, QPSK, 64QAM)
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Algorithms for CompressionTime and Frequency
Correlation
Feed-back difference between actual and former
chunk
Time
Frequency
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Algorithms for CompressionIterative Time -
Frequency

Design a Huffman code for each combination bit-1,
bi-1t 64 codes
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Algorithms for CompressionBlock Time - Frequency

Time
Design a Huffman code for each combination
bi-1t-1, bit-1 , bi1t-1 512 codes
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ResultsComparison to other algorithms

• 30 users. UMTS Vehicular A channel. 50 km/h and
  SNR 20 dB

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ResultsUMTS Vehicular A Channel

• 30 users

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ResultsUMTS Pedestrian A Channel

• 30 users

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ResultsRobustness

• 100 km/h. AWGN Feedback Channel. Different
  Refresh Rate

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Conclusions

• Opportunistic feedback allows 50 of reduction
  in feedback data.
• The compression of feedback data by using time,
  frequency or both correlation is feasible and
  offers compression in the range of 50 more.
• Time-Frequency techniques exhibit better
  performance than the others (in general).
• Compression algorithms jointly with Opportunistic
  feedback allow reductions of more than six times.
• Once the codes are designed and stored,
  complexity is almost negligible.
• Time Frequency algorithms need refresh for
  robustness.
• Future OFDMA systems are closer to be implemented.

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Thank you very much