Hybrid automatic repeat-request

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Hybrid ARQ (HARQ) is a variation of the ARQ error control method. In standard ARQ, error-detection information (ED) bits are added to data to be transmitted (such as cyclic redundancy check, CRC). In Hybrid ARQ, forward error correction (FEC) bits are also added to the existing Error Detection (ED) bits (such as Reed-Solomon code or Turbo code). As a result Hybrid ARQ performs better than ordinary ARQ in poor signal conditions, but in its simplest form this comes at the expense of significantly lower throughput in good signal conditions. There is typically a signal quality cross-over point below which simple Hybrid ARQ is better, and above which basic ARQ is better.

The simplest version of HARQ, Type I HARQ, adds both ED and FEC information to each message 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 a more sophisticated form, Type II HARQ, transmits only ED bits or only FEC information and ED bits on a given transmission, typically alternating on successive transmissions.

To understand the difference between Type I and Type II Hybrid ARQ, consider the size of ED and FEC added information: detection typically only adds a couple bytes to a message, which is only an incremental increase in length. FEC, on the other hand, can often double or triple the message length with error correction parities. In terms of throughput, standard ARQ typically expends a few percent of channel capacity for reliable protection against error, while FEC ordinarily expends half or more of all channel capacity for channel improvement.

In standard ARQ a transmission must be received error free on any given transmission for the error detection to pass. In Type II Hybrid ARQ, the first transmission contains only data and error detection (no different than standard ARQ). If received error free, it's done. If data is received in error, the second transmission will contain FEC parities and error detection. If received error free, it's done. If received in error, error correction can be attempted by combining the information received from both transmissions.

Only Type I Hybrid ARQ suffers the capacity loss in strong signal condition. Type II Hybrid does not, because FEC bits are only transmitted on subsequent retransmissions as needed. In strong signal Type II Hybrid ARQ performs with as good capacity as standard ARQ. In poor signal conditions Type II Hybrid ARQ performs with as good sensitivity as standard 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"). This can increase the probability of successful decoding.

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.

There are other forward error correction codes can be used in HARQ scheme besides Turbo code,eg. extended irregular repeat-accumulate (eIRA) code and Efficiently-Encodable Rate-Compatible (E2RC) code,both of which are Low Density Parity Check Code.

[edit] 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".It also has been used in 3GPP Long Term Evolution.

HARQ is generally implemented in hardware, rather than in software.^{[citation needed]}

[edit] 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.