Raptor Code

Raptor Code

In computer science, raptor codes (rapid tornado; see Tornado codes) are the first known class of fountain codes with linear time encoding and decoding. They were invented by Amin Shokrollahi in 2000/2001 and were first published in 2004 as an extended abstract. Raptor codes are a significant theoretical and practical improvement over LT codes, which were the first practical class of fountain codes.

Raptor codes, as with fountain codes in general, encode a given message consisting of a number of symbols, k, into a potentially limitless sequence of encoding symbols such that knowledge of any k or more encoding symbols allows the message to be recovered with some non-zero probability. The probability that the message can be recovered increases with the number of symbols received above k becoming very close to 1, once the number of received symbols is only very slightly larger than k. For example, with the latest generation of Raptor codes, the codes, the chance of decoding failure when k symbols have been received is less than 1%, and the chance of decoding failure when k+2 symbols have been received is less than one in a million. A symbol can be any size, from a single byte to hundreds or thousands of bytes.

Raptor codes may be systematic or non-systematic. In the systematic case, the symbols of the original message are included within the set of encoding symbols. An example of a systematic raptor code is the code defined by the 3rd Generation Partnership Project for use in mobile cellular wireless broadcast and multicast and also used by DVB-H standards for IP datacast to handheld devices (see external links). The Raptor codes in these standards is defined also in IETF RFC 5053. The most advanced version of a practical Raptor code is RaptorQ defined in IETF RFC 6330.

Online codes are another example of a non-systematic raptor code.