- Hybrid automatic repeat request (Hybrid ARQ or 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 cost of much 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.
So here is how HARQ works,
HARQ adds both ED and FEC information to each message before 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 get 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.
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.