LTE HARQ is an enhancement to ARQ to include an additional Forward Error Correction
(FEC) technique. This correction avoids retransmission of complete packets which
is an advantage over ARQ.
We have included more details on HARQ, ARQ and FEC in this section.
Few words on ARQ - Traditionally ARQ only supports a Error Detection (ED) technique.
ED technique uses CRC code and a sliding window to detect errors in receiving packets.
In case of errors ARQ discards complete packets and requests complete retransmission.
Can you say which one is better ARQ or HARQ?
No, instead its a mathematical tradeoff.
ARQ has one overhead - CRC bits
HARQ has two overheads - CRC bits and FEC bits.
Crossover is calculated based on following know how.
In-case of good radio channel conditions ARQ will perform better with a higher throughput.
And in-case of poor radio channel conditions HARQ will perform better. So an optimal
point is calculated in software to switch the packet transmissions to HARQ
Different techniques of HARQ.
Type -1 or Chase Combination : Its a simple addition of CRC and FEC bits to data
Type -2 or Incremental Redundancy: The data, CRC and FEC is partitioned in multiple
blocks and transmitted separately.
So in Type-1 (chase combination) retransmission is an copy of original transmission.
This technique provides minimal gain in power/throughput savings.
But in case of Type 2 (incremental redundancy) only a block of complete packet is
retransmitted. This technique provides maximum gain in power/throughput savings.
In case of Type 2 the packet needs to be partitioned in blocks in following order
- Data, Redundancy, Redundancy.