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SIMULATION OF HYBRID AUTOMATIC REPEAT REQUEST USING VHDL
1 POONAM J. THANKI , 2 ANURADHA P. GHARGE
1 M.E. Student, PIET Waghodia, Anuradha P. Gharge
2 Asst. Professor E.C. Dept. PIET Waghodia
,
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ABSTRACT : Hybrid ARQ is a combination of Forward Error Correction (FEC) using Automatic Repeat reQuest in an optimal manner. Hybrid ARQ schemes are commonly used to facilitate reliable communication over noisy wireless channels. HARQ is able to compensate for link adaptation errors and provides a finer granularity of coding rate, resulting in better throughput performance than other FEC schemes. In this paper we focus on basic concept of HARQ, categories and schemes of HARQ.
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1. INTRODUCTION
A major concern in data communication is how to control transmission errors caused by the channel noise so that error free data can be delivered to the user. A solution to this problem is the use of Automatic Repeat request (ARQ) and Forward Error Correction (FEC) schemes.
The advantage of obtaining high reliability in ARQ systems can be coupled with the advantage of FEC systems to provide a good throughput even in poor channel conditions. Such a system, which is a combination of the two basic error control schemes: FEC and ARQ, is called Hybrid ARQ system.
Hybrid ARQ can be classified into three types:
i) Type I
The simplest method: Hybrid ARQ Type I use the CRC to detect whether or not an error in transmission has occurred. If a packet is found to be in error a retransmission request will be sent to the transmitter and the erroneous packet will be discarded. The transmitter will then retransmit the same packet until the packet is successfully decoded by the receiver or the maximum retransmission limit is reached.
Hybrid ARQ Type I can be extended to include packet combining, this is known as Hybrid ARQ Type I with Packet Combing or Chase Combining. After each failed retransmission the erroneous packets are stored in a buffer. The receiver then uses maximum ratio combining to combine each received channel bit with any previous transmissions of the same bit and the combined signal is fed to the decoder. Chase combining does not give any additional coding gain, it only increases the accumulated received signal to noise ratio (Eb/N0) for each retransmission.
ii) Type II
In Hybrid ARQ Type II, also known as full Incremental Redundancy (IR), each retransmission is not necessarily identical to the original transmission. Instead, multiple sets of coded bits are generated and whenever a retransmission is required the retransmitted data represents a different set of coded bits than the previous transmission. The receiver combines the retransmission with previous transmission attempts of the same packet. As the retransmission contains additional parity bits, not included in the previous transmission attempts, the resulting code rate is generally lowered by subsequent retransmissions. Each transmission contains a different set of parity bits resulting in a higher coding gain when compared to chase combining.
iii) Type III
The final method Hybrid ARQ Type III, also known as partial IR, decreases the coding rate
by sending additional redundancy bits in each retransmission. It does however ensure that retransmissions are able to self-decode. This means the retransmitted packet can be chase combined with previous packets to increase the diversity gain.
2. REVIEW OF PREVAILING TECHNIQUE
Automatic Repeat reQuest(ARQ) is an error control method for data transmission that makes use of error-detection codes, acknowledgment and/or negative acknowledgment messages, andtimeouts to achieve reliable data transmission. Anacknowledgmentis a message sent by the receiver to indicate that it has correctly received adata frame.Usually, when the transmitter does not receive the acknowledgment before the timeout occurs (i.e., within a reasonable amount of time after sending the data frame), it retransmits the frame until it is either correctly received or the error persists beyond a predetermined number of retransmissions .Three types of ARQ protocols areStop & wait ARQ,Go-Back-N ARQ, andSelective Repeat ARQ.
2.2.1 Stop and Wait ARQ
The basic idea of stop and wait ARQ is that the sender A transmits a frame to the receiver B and waits to receive either (1) a (positive) acknowledgment (ACK) of correct reception, at which point A will transmit a new frame corresponding to the next packet, or (2) a negative acknowledgement (NAK) of incorrect reception, at which point A will re-transmit a frame with the same packet data as the incorrectly received frame. Since it is also possible that frames are lost between A and B (e.g. due to framing errors), it may also be necessary for A and B to periodically re-transmit or request re-transmission, respectively, referred to as a timeout.
2.2.2 Go Back n ARQ
To remedy the poor utilization of the channel under stop and wait ARQ protocols, we investigate ARQ
protocols which allow the sender A to send multiple frames in sequence without waiting for
corresponding acknowledgments. The first type of generalized ARQ protocols is go back n ARQ in which the sender is allowed to transmit n frames before stopping to wait for acknowledgments. The receiver's algorithms under go back n is the same as in stop and wait ARQ.
2.2.3 Selective Repeat ARQ
Go back n ARQ is effective in improving the poor utilization of stop and wait ARQ. However, go back n ARQ still suffers from poor efficiency as a large number (as high as n) of frames are retransmitted each time there is an error in the forward direction, resulting in a low packet rate. This can be remedied, however, by allowing the receiver to buffer out-of order packets and request retransmission of specific packets, instead of the entire window of n packets.
Following is the waveform showing the process of Automatic Repeat Request using Go back-n ARQ.
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3. FORWARD ERROR CORRECTION
CRC is used for FEC..Following is the theory of CRC calculation and CRC checksum. The width (position of the highest 1 bit) of the poly is very important as it dominates the whole calculation. Typically, widths of 16 or 32 are chosen so as to simplify implementation on modern computers. The width of a poly is the actual bit position of the highest bit. For example, the width of 10011 is 4, not 5. For the purposes of example, we will chose a poly of 10011 (of width W of 4).
Having chosen a poly, we can proceed with the calculation. This is simply a division (in CRC arithmetic) of the message by the poly. The only trick is that W zero bits are appended to the message before the CRC is calculated. Thus we have:
Original message : 1101011011
Poly : 10011
Message after appending W zeros : 11010110110000
Now we simply divide the augmented message by the poly using CRC arithmetic.
1100001010 = Quotient (nobody cares about the quotient)
______
10011 ) 11010110110000 = Augmented message (1101011011 + 0000)
=Poly 10011,,.,,....
-----,,.,,....
10011,.,,....
10011,.,,....
-----,.,,....
00001.,,....
00000.,,....
-----.,,....
00010,,....
00000,,....
-----,,....
00101,....
00000,....
-----,....
01011....
00000....
-----....
10110...
10011...
-----...
01010..
00000..
-----..
10100.
10011.
-----.
01110
00000
-----
1110 = Remainder = THE CHECKSUM!!!!
The division yields a quotient, which we throw away, and a remainder, which is the calculated checksum. This ends the calculation.
Usually, the checksum is then appended to the message and the result transmitted. In this case the transmission would be: 11010110111110.
At the other end, the receiver can do one of two things:
a).Separate the message and checksum. Calculate the checksum for the message (after appending W zeros) and compare the two checksums.
b).Checksum the whole lot (without appending zeros) and see if it comes out as zero!
Following is the result of combination of CRC encoder and decoder using VHDL
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ISSN: 0975 –6779| NOV 11 TO OCT 12 | VOLUME – 02, ISSUE - 01 Page 260
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ISSN: 0975 –6779| NOV 11 TO OCT 12 | VOLUME – 02, ISSUE - 01 Page 260
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ISSN: 0975 –6779| NOV 11 TO OCT 12 | VOLUME – 02, ISSUE - 01 Page 260
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3. HARQ
HARQ is combination of ARQ+FEC(CRC).By combining ARQ and CRC and simulate it using VHDL we can get the output of HARQ.
ARQ and FEC may be combined, such that minor errors are corrected without retransmission, and major errors are corrected via a request for retransmission: this is calledHybrid Automatic Repeat Request (HARQ).
3.1 Category Of HARQ
In general, HARQ schemes can be categorized as either synchronous or asynchronous, with the retransmissions in each case being either adaptive or non-adaptive.
i) Synchronous HARQ
In a synchronous HARQ scheme, the retransmission(s) for each process occur at predefined times relative to the initial transmission. In this way, there is no need to signal information such as HARQ process number, as this can be inferred from the transmission timing. By contrast, in an asynchronous HARQ scheme, the retransmissions can occur at any time relative to the initial transmission, so additional explicit signalling is required to indicate the HARQ process number to the receiver, so that the receiver can correctly associate each retransmission with the corresponding initial transmission. In summary, synchronous HARQ schemes reduce the signalling overhead while asynchronous HARQ schemes allow more flexibility in scheduling.
In an adaptive HARQ scheme, transmission attributes such as the modulation and coding scheme, and transmission resource allocation in the frequency domain, can be changed at each retransmission in response to variations in the radio channel conditions.
In a non adaptive HARQ scheme, the retransmissions are performed without explicit signalling of new transmission attributes – either by using the same transmission attributes as those of the previous transmission, or by changing the attributes according to a predefined rule. Accordingly, adaptive schemes bring more scheduling gain at the expense of increased signalling overhead.
ii) Asynchronous HARQ
In LTE, asynchronous adaptive HARQ is used for the downlink, and synchronous HARQ for the uplink. In the uplink, the retransmissions may be either adaptive or non-adaptive, depending on whether new signalling of the transmission attributes is provided.
4. CONCLUSION
From this paper we can conclude that HARQ is very useful to increase speed by decreasing number of retransmission as there is no time out constraint which is there in case of ARQ. Also it decreases latency.HARQ provides higher data rates than all other previous generations .It provides higher spectral efficiency.LTE development goals include higher quality-of-service, always-on experience, and lower latency; and a simpler and more efficient network architecture.
These technologies further enable simpler end user receiver devices, as well improvements in system capacity and user experience. In addition to being well designed to carry high speed data services, LTE is also well designed for low bit-rate real-time streaming services such as VoIP.
5. BIBLIOGRAPHY
[1] Josep Colom Ikuno, Martin Wrulich, Markus Rupp’PERFORMANCE AND MODELING OF LTE H-ARQ’ Institute of Communications and Radio-Frequency Engineering Vienna University of Technology, Austria Gusshausstrasse 25/389, A-1040 Vienna, Austria,published at Berlin in 2009
[2] M.Wissem El Bahri,Hatem Boujemaa,Mohamed Siala”Performance Analysis of Type I,II and III Hybrid ARQ schemes over AWGN channels” IEEE 0-7803-8662 International Conference on Industrial Technology(ICIT),2004
[3] Antonio Maria Cipriano,Paul Gagneur,Guillaume Vivier,SerdarnSezginer “Overview of ARQ and HARQ in Beyond 3G systems”IEEE 2010
[4] 3GPP TS 36.211 Release 8 V8.9.0 (2009-12)
[5] 3GPP TS 36.201 – v1.0.0, LTE Physical Layer – General Description, http://www.3gpp.org/ftp/Specs/archive/36%5Fseries/36.201/
[6] Kian Chung Beh,Angela Doufexi,Simon Armour,University of Bristol“Performance Evaluation Of Hybrid ARQ Schemes of 3GPP LTE OFDMA system”IEEE 2007
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