Originally published on 4GNation, October 2015 — republished here as part of the Telco & Beyond Engineering Archive.
HARQ is one of those mechanisms that quietly does an enormous amount of work to keep an LTE link efficient — without it, every dropped block would mean a full TCP-layer retransmission, with all the latency that implies. This short piece captures the core idea in a way I still find useful when explaining radio efficiency to non-RF audiences.
The radio retransmission mechanism in LTE is called Hybrid Automatic Repeat Request (H-ARQ). H-ARQ allows erroneous blocks to be retransmitted quickly between the eNodeB and the UE, avoiding the much longer retransmission delay that would otherwise happen at the TCP layer.
The H-ARQ process runs in both the eNodeB and the UE, based on ACK/NACK messages carried over PUCCH or PUSCH.
A typical hard-HARQ implementation works like this:
- It reserves the same resource blocks and MCS used in the initial transmission.
- It reuses information from previous transmissions of the same block to increase the probability of successful decoding.
- If a data block isn’t received correctly, soft values are stored so they can be reused after retransmission.
- On retransmission, a different puncturing scheme is used, so the retransmitted bits don’t carry exactly the same information as the first attempt.
- If the puncturing schemes across transmissions are disjoint, the number of coded bits transmitted after the second attempt effectively doubles — meaning the coding rate is halved. After a third transmission, the coding rate is divided by three, and so on. Each retransmission increases the probability of successful decoding.
It’s a good example of engineering pragmatism: rather than treating every failed block as a full loss, the network extracts maximum value from every transmission attempt, incrementally improving the odds of success rather than starting over each time.