Hybrid ARQ

HYBRID ARQ

Presented by:Ajna Aslam

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CONTENTS

i. Introduction About basic ARQ Introducing Hybrid ARQ

ii. Types of Hybrid ARQ Type-I Hybrid ARQ Type-II Hybrid ARQ

iii. Comparison of Type-I & Type-II schemes

iv. Applicationsv. Conclusionvi. References

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INTRODUCTION

About Basic ARQ

An error control method for data transmission

Uses acknowledgments and timeouts

Acknowledgment: Message sent by the receiver to the transmitter to indicate that it has correctly received a data frame or packet

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INTRODUCTION CONTINUED…

Timeout: A reasonable point in time after the sender sends the frame/packet

Frame/packet re-transmitted if

acknowledgement not received before timeout

Error-Detection(ED) bits are added to the data to be transmitted

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Introducing Hybrid ARQ

Variation of the ARQ error control method

Forward-Error-Correction(FEC) bits added to the existing Error-Detection (ED) bits

Performs better than ordinary ARQ in poor signal conditions

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Lower throughput in good signal conditions

Works better below certain cross-points

Above these cross-points, basic ARQ is better

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TYPES OF HYBRID ARQThere are mainly two types of Hybrid ARQ

viz.,

i. Type-I Hybrid ARQii. Type-II Hybrid ARQ

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TYPE-I HYBRID ARQ

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FEC scheme used is capable of simultaneously detecting and correcting the error

TYPE-I HYBRID ARQ CONTINUED…

Decoder detects the error, if any

The detected error is corrected by the decoder

Packets re-transmitted if uncorrectable errors occur

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Re-transmitted code corrected for errors by the receiver

Packets re-transmitted if further uncorrectable errors occur

Process continues till the codeword is successfully decoded

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ADVANTAGES OF TYPE-I SCHEME

Advantageous at high channel error rates

Provides higher throughput than basic ARQ scheme

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DISADVANTAGES OF TYPE-I SCHEME Lower throughput at low channel error

rates

Receiver must be able to detect and correct the received erroneous packets

More robust coding than simple ARQ

Increases the overhead for each transmission

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TYPE-II HYBRID ARQ First transmitted packet need not contain

parity bits for error correction

Subsequent re-transmissions contains the parity for error correction

Referred to as Incremental Redundancy

Only parity bits for error detection appended to the message at first

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TYPE-II HYBRID ARQ CONTINUED…

Detected error in the received words if any, is stored in receiver buffer

Re-transmission request is sent to the transmitter

Re-transmission is not the original codeword but only a block of parity bits

Re-transmission with parity bits used to correct the previous erroneously received codeword 16

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Re-transmission request sent if transmission fails

Transmitter re-transmits the original codeword or another parity block

Process continues until original codeword is successfully decoded

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COMPARISON OF TYPE-I & TYPE-II SCHEMES

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Discards erroneously received code word

Re-transmits until packet is correctly received or until the pre-set number of re-transmissions is achieved

Small buffer size required, but an inefficient scheme

Stores erroneously received code word

Optimally combine with re-transmitted code word

Exploits Incremental Redundancy concept

System adapts to varying channel conditions

Larger buffer size required than Type-I, but is very efficient

Type-I scheme Type-II scheme

APPLICATIONS

Used in HSDPA and HSUPA they provide high speed data transmission

on downlink and uplink for mobile phone networks

Used in mobile WiMAX

Used in 3GPP Long Term Evolution

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CONCLUSION

Hybrid ARQ schemes can adapt to channel changing conditions and thus avoid the need for very complex coding techniques

They can achieve very high efficiency as they use redundancy bits only when needed

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REFERENCES

Soljanin E., Hybrid ARQ in Wireless Networks, presented at Wireless System Lab Seminar, Texas A&M University, April 2003, and DIMACS Workshop on Network Information Theory, March 2003.

R.A.Comroe and D.J.Costello. "ARQ schemes for data transmission in mobile radio systems". IEEE J. Select. Areas Commun., 2:472-481, July 1984.

G. Davida and S. Reddy. "Forward Error Correction with Decision Feedback", Information and Control, Vol 21, 1972.

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THANK YOU…

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Documents

Hybrid ARQ