Hybrid Automatic Repeat Request - Hybrid ARQ With Soft Combining

Hybrid ARQ With Soft Combining

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. This is called Hybrid ARQ with soft combining (Dahlman et al., p. 120). While it is possible that two given transmissions cannot be independently decoded without error, it may happen that the combination of the previously erroneously received transmissions gives us enough information to correctly decode. There are two main soft combining methods in HARQ:

• Chase combining: every retransmission contains the same information (data and parity bits). The receiver uses maximum-ratio combining to combine the received bits with the same bits from previous transmissions. Because all transmissions are identical, Chase combining can be seen as additional repetition coding. One could think of every retransmission as adding extra energy to the received transmission through an increased Eb/N0.
• Incremental redundancy: every retransmission contains different information than the previous one. Multiple sets of coded bits are generated, each representing the same set of information bits. The retransmission typically uses a different set of coded bits than the previous transmission, with different redundancy versions generated by puncturing the decoder output. Thus, at every retransmission the receiver gains extra information.

Several variants of the two main methods exist. For example, in partial Chase combining only a subset of the bits in the original transmission are retransmitted. In partial incremental redundancy, the systematic bits are always included so that each retransmission is self-decodable.

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 puncturing pattern used during each (re)transmission is different, so different coded bits are sent at each time. Although the HSDPA standard supports both chase combining and incremental redundancy, it has been shown that incremental redundancy almost always performs better than chase combining, at the cost of increased complexity.

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 that can be used in an HARQ scheme besides Turbo codes, e.g. extended irregular repeat-accumulate (eIRA) code and Efficiently-Encodable Rate-Compatible (E2RC) code, both of which are Low Density Parity Check Code.