- The radio retransmission mechanism is called Hybrid Automatic Request (H-ARQ).
- H-ARQ allows to retransmit fastly erroneous blocks between the eNodeB and the UE.
- It avoids long retransmission between 2 TCP layers.
The H-ARQ process runs in the eNodeB and in the UE.
The H-ARQ is based on ACK/NACK messages carried by PUCCH or PUSCH.
The LA1.X implementation is a hard HARQ technique:
- It reserves the same RB resources & MCS used for the initial transmission.
- Using information that was sent in the previous transmissions of the same block to increase the probability of decoding.
- If data block not received correctly, soft values are stored in order to reuse them after the retransmission of the block.
- When data is retransmitted, a different puncturing scheme is used so that the transmitted bit do not carry the same information as the first time.
- If puncturing schemes are disjoint between two transmission, number of coded bits transmitted after the second transmission is twice as high, thus coding rate has been divided by 2. After the third transmission, coding rate has been divided by 3. Probability to decode the block is increased after each transmission.
- If puncturing schemes are not disjoint, soft values corresponding to the samed bits are added. Decisions’ average reliability increases.