hybrid arq
DESCRIPTION
A slide on Hybrid ARQ, variation of standard ARQ (error-control protocol)TRANSCRIPT
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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INTRODUCTION CONTINUED…
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INTRODUCTION CONTINUED…
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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INTRODUCTION CONTINUED…
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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TYPE-I HYBRID ARQ CONTINUED…
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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TYPE-I HYBRID ARQ CONTINUED…
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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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TYPE-II HYBRID ARQ CONTINUED…
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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TYPE-II HYBRID ARQ CONTINUED…
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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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