Hybrid Arq

HYBRID ARQ Presented by: Ajna Aslam 1

CONTENTS  

Types of Hybrid ARQ

iii.   iv.

v. vi. vii.

About basic ARQ Introducing Hybrid ARQ

Hybrid ARQ

Introduction

ii.

Type-I Hybrid ARQ Type-II Hybrid ARQ

Comparison of Type-I & Type-II schemes Applications Conclusion References

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INTRODUCTION About Basic ARQ An error control method for data transmission

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Uses acknowledgments and timeouts

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

Hybrid ARQ

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INTRODUCTION

CONTINUED…

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

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Frame/packet re-transmitted if acknowledgement not received before timeout

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

Hybrid ARQ

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INTRODUCTION

CONTINUED…

Hybrid ARQ

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INTRODUCTION

CONTINUED…

Introducing Hybrid ARQ Variation of the ARQ error control method

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

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Performs better than ordinary ARQ in poor signal conditions

Hybrid ARQ

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INTRODUCTION

CONTINUED…

Lower throughput in good signal conditions

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Works better below certain cross-points

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Above these cross-points, basic ARQ is better

Hybrid ARQ

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TYPES OF HYBRID ARQ There are mainly two types of Hybrid ARQ viz., Hybrid ARQ iv.Type-II Hybrid ARQ

Hybrid ARQ

iii.Type-I

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TYPE-I HYBRID ARQ 

Hybrid ARQ

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

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TYPE-I HYBRID ARQ CONTINUED… Decoder detects the error, if any

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The detected error is corrected by the decoder

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Packets re-transmitted if uncorrectable errors occur

Hybrid ARQ

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TYPE-I HYBRID ARQ CONTINUED… Re-transmitted code corrected for errors by the receiver

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Packets re-transmitted if further uncorrectable errors occur

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Process continues till the codeword is successfully decoded

Hybrid ARQ

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TYPE-I HYBRID ARQ

CONTINUED…

Hybrid ARQ

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ADVANTAGES OF TYPE-I SCHEME Advantageous at high channel error rates

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Provides higher throughput than basic ARQ scheme

Hybrid ARQ

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

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

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More robust coding than simple ARQ

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Increases the overhead for each transmission

Hybrid ARQ

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TYPE-II HYBRID ARQ First transmitted packet need not contain parity bits for error correction

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Subsequent re-transmissions contains the parity for error correction

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Referred to as Incremental Redundancy

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

Hybrid ARQ

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TYPE-II HYBRID ARQ CONTINUED… Detected error in the received words if any, is stored in receiver buffer

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Re-transmission request is sent to the transmitter

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

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

Hybrid ARQ

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TYPE-II HYBRID ARQ CONTINUED… Re-transmission request sent if transmission fails

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Transmitter re-transmits the original codeword or another parity block

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Process continues until original codeword is successfully decoded

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TYPE-II HYBRID ARQ CONTINUED…

Hybrid ARQ

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COMPARISON OF TYPE-I & TYPE-II SCHEMES Type-I scheme

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

Hybrid ARQ

Discards erroneously received code word

Type-II scheme

Exploits Incremental Redundancy concept System adapts to varying channel conditions Larger buffer size required than Type-I, but is very efficient

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APPLICATIONS 

 they

provide high speed data transmission on downlink and uplink for mobile phone networks

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Hybrid ARQ

Used in HSDPA and HSUPA

Used in mobile WiMAX Used in 3GPP Long Term Evolution 20

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

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

Hybrid ARQ

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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.

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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.

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

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Hybrid ARQ

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