Hybrid Automatic Repeat Request

8/3/2019 Hybrid Automatic Repeat Request

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Hybrid automatic repeat request

The supervision of Dr / Ghassan Dahman

Prepare students :

1. Abdullah Al-Shamrani

2. Nayef Omar Yamani

3. Mohammed Salman

4. Ibrahim Al Harazi


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Introduction1

Simple Hybrid2

Hybrid ARQ with soft combining3

Using Turbo Code4

5

OverviewOverview

Application

References6


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Introduction

Hybrid Automatic Repeat re Quest (HARQ or Hybrid

ARQ) is a sheme wherein information blocks are

encoded for partial error correction at receiver and

additional, uncorrected errors are retransmitted.


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Simple Hybrid

FEC, ARQ, hybrid ARQ and retransmission

strategies.

FEC Forward Error Correction Channel code used to only correct errors.


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Simple Hybrid

ARQ Automatic Repeat Request

Channel code used to detect errors.

A feedback channel is present

If no detected errors, an acknowledgement (ACK) is sent backto transmitter.

If there are detected errors, a negative acknowledgement

(NACK) is sent back.


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Simple Hybrid

Retransmission if NACK or no ACK.

Several retransmission strategies: Stop and wait, go-back-N, selective repeat, etc.

Selective repeat has better throughout performancethan the others in the presence of propagationdelays.

However, throughput of stop and wait and selective

repeat protocols are the same if no transmissiondelay is assumed.


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Simple Hybrid

Two generic types of hybrid FEC/ARQ.

Type I hybrid ARQ:

Discard erroneous received code word.

Retransmit until packet correctly received or until pre-setnumber of retransmissions is achieved.

Small buffer size required but an inefficient scheme.


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Simple Hybrid

Type II hybrid ARQ:

Store erroneous received code word.

Optimally combine with retransmitted code word. Exploit incremental redundancy concept

Effective Code rate is gradually lowereduntil packetis decoded correctly.

System adapts to varying channel conditions.

Larger buffer size required than Type-I but is a veryefficient scheme.


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Simple Hybrid


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Hybrid ARQ with soft

combining

Is a data reception received by mistake, then correct

them in a certain way .

How it works:

1. Received coded data stored incorrectly.

2. The two blocks are combined .


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combining

o There are two main soft combining methods in HARQ:

1. Chase combining .

A. Retransmission contains.

B. The receiver.

2. Incremental redundancy.

Incremental redundancy HARQ is HSDPA


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combining

Symbols that can be used :

1. extended irregular repeat-accumulate (EIRA) code .

2. Efficiently-E ncodable Rate-Compatible (E2RC) code


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combining


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Using Turbo Code

A. Turbo codes :

Imposing a structure on the transmitted bit sequence.If

the received bit sequence does not match this known

structure ,the receiver knows an error has occurred. Ifthe number of error is low enough and receiver can

determine reconstruct the correct sequence .


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Using Turbo Code

B. Turbo Encoder :

Two Recursive Systematic Convolutional (RSC) encoders are arranged

in parallel, to construct a turbo encoder. The encoders are combined

with a random interleaver, together with a puncturing block, as seen

in Fig. 2


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Using Turbo Code

C. Turbo Decoder :

The Turbo decoder is built in the similar way as the turbo

encoder. Two elementary decoders are interconnected

to each other, but in serial way, not parallel. An iterativedecoding is proposed in which is basically a modification

of the algorithm


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Using Turbo Code

D. Decoding Algorithms :

The decoding algorithms are divided into two

different families. The family is categorizedbased on the trellis-based detection algorithm.


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Using Turbo Code

E. ARQ Protocols :The most popular ARQ protocols are:

1. stop-and wait.

2. go-back-N.

3. selective repeat .

we present the performance of hybrid ARQ which implies

stop and wait protocols .


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Using Turbo Code

F. HARQ type I using Turbo code :

The performance of hybrid ARQ based on Turbo-codes has

been evaluated. Fig.2


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Application


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Application


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Application

HARQ In LTE (Long Term Evolution)

The LTE downlink HARQ is asynchronous and adaptive. every

downlink transmission is accompanied by an explicit signaling

of control information.

The uplink HARQ process is synchronous, and it can be non

adaptive or adaptive.

Under a non adaptive HARQ process, because of the absence

of the explicit signaling, the RV is a predefined value, whichdepends on the retransmission instance.


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Using Turbo Code

HARQ In Local Area Network

Hybrid ARQ is used in a high-speed Local area

network standard that can operate at data rates up to 1

G bit/s over existing home wiring (power lines, phonelines and coaxial cables). high-speed Local area

uses CRC-32C for Error Detection, LDPC for Forward

Error Correction and Selective Repeat for ARQ.


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Hybrid Automatic Repeat Request

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