Hybrid automatic repeat request

Hybrid automatic repeat request (hybrid ARQ or HARQ) is a combination of high-rate forward error correction (FEC) and automatic repeat request (ARQ) error-control.

[2] To understand the difference between Type I and Type II Hybrid ARQ, consider the size of ED and FEC added information: error detection typically only adds a couple of 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.

Only Type I Hybrid ARQ suffers capacity loss in strong signal conditions.

Type II Hybrid ARQ does not because FEC bits are only transmitted on subsequent re-transmissions as needed.

In poor signal conditions, Type II Hybrid ARQ performs with as good sensitivity as standard FEC.

For example, in partial Chase combining only a subset of the bits in the original transmission are re-transmitted.

In partial incremental redundancy, the systematic bits are always included so that each re-transmission is self-decodable.

Type I Hybrid ARQ is used in ITU-T G.hn, a high-speed Local area network standard that can operate at data rates up to 1 Gbit/s over existing home wiring (power lines, phone lines and coaxial cables).