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'Hybrid ARQ' (HARQ) is a variation of the ARQ error control method, which performs better than ordinary ARQ, particularly over wireless channels, at the cost of increased implementation complexity.
The simplest version of HARQ, 'Type I HARQ', simply combines forward error correction (FER) and ARQ by encoding the data block plus error-detection information (such as cyclic redundancy check, CRC) with an error-correction code (such as Reed-Solomon code or Turbo code) prior to transmission. When the coded data block is received, the receiver first decodes the error-correction code. If the channel quality is good enough, all transmission errors should be correctable, and the receiver can obtain the correct data block. If the channel quality is bad and not all transmission errors can be corrected, the receiver will detect this situation using the error-detection code, then the received coded data block is discarded and a retransmission is requested by the receiver, similar to ARQ.
In practice, incorrectly received coded data blocks are often stored at the receiver rather than discarded, and when the retransmitted block is received, the two blocks are combined ('chase combining') before being fed to the decoder of the error-correction code. This can increase the probability of successful decoding.
A further proposed improvement is 'Type II/III HARQ', or 'incremental redundancy HARQ', where different (re)transmissions are coded differently rather than repeating the same coded bits as in Chase combining. Performance is even better, since coding is effectively done across retransmissions. The difference between type III HARQ and type II HARQ is that the retransmission packets in Type III HARQ can be decoded by themselves.
An example of incremental redundancy HARQ is HSDPA: the data block is first coded with a punctured 1/3 Turbo code, then during each (re)transmission the coded block is usually punctured further (i.e. only a fraction of the coded bits are chosen) and sent. The punctuation pattern used during each (re)transmission is different, so different coded bits are sent at each time.
HARQ can be used in stop-and-wait mode or in selective repeat mode. Stop-and-wait is simpler, but waiting for the receiver's acknowledgment reduces efficiency. Thus multiple stop-and-wait HARQ processes are often done in parallel in practice: when one HARQ process is waiting for an acknowledgment, another process can use the channel to send some more data.

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Applications

References

Applications

HARQ is used in HSDPA and HSUPA, which provides high speed data transmission on downlink and uplink respectively for mobile phone networks such as UMTS, and in the IEEE 802.16-2005 standard for mobile broadband wireless access, also known as "mobile WiMAX".
HARQ is generally implemented by Hardware

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.