I need to review something about HARQ.
What does the RV index represent?
I know the process, but I need to know and review what RV index represent.
- Rv0 is first Transmission
- Rv1 is second transmission for a HARQ Nack
- Rv3 is for fourth transmission
Currently max HARQ transmissions of 4 are deployed by mostly everyone in LTE.
Could you tell me in short sentence?
It changes the redundancy bits with every retransmission so with chase conbining is able to reconstitute a good transport block from 3 RV failures.
Basically, The parity bit stream generated is pushed into a circular buffer and is divided into four parts each identified by a RV.
UE is expected to decode the codeword with RV0.
If the CRC fails, network can transmit with the next RV (which is 2).
With RV0 and RV2, UE now has more parity bits and has higher probability of successful decode (soft combining).
If CRC still fails for RV2, network can continue to retransmit with RV3 and RV1 which will further increase the probability of decode.
The channel encoder in LTE is designed with a fixed code rate of 1/3 meaning for every 1 bit off data, 2 redundant bits are generated.
Based on UE channel condition, a different code rate may be optimal (good RF uses less redundant bits and vice versa)
Take an example of data block of 1000 bits so the encoder will generate 3000 bits (typically placed in a circular buffer)
If UE RF conditions are good and requires a code rate coresponding to 1400 bits for example so
1st TX (called RV0) of 1400 bits (mostly the original data +redundancy) is sent
If NACK is recieved, a ReTX is needed. In this case a different 1400 bits (from the same circular buffer) will transmitted known as RV1
Same of RV2 and RV3.
Note that some RVs could have some common bits but the 4 RVs are not identical