In practice, incorrectly received data is stored at receiver side rather than discard . when second time data received then the erroneously received previous data and this newly received data is combined to get correct data. This is called HQRQ with soft combining.
There are two main soft combining method are defined.
1. 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.
- 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 encoder output.
Thus, at every retransmission the receiver gains extra information.