HARQ — Hybrid ARQ — the standing bet, settled every slot

5G Systems Notes · Concept map · HARQ Route · Hierarchy · Index · All concepts · Hub

Procedures over time

Where it sits

Sits atLevel 10 of the hierarchy · Procedures over time · explained
Also writtenhybrid ARQ · HARQ-ACK
Learn firstTransport block · CRC · Rate matching — 10 concepts in the full chain, see the paths
UnlocksPUCCH · Link adaptation · URLLC and eMBB
Primary clauseTS 38.300 §6.2.4, TS 38.321 §5.3.2
Used innr-pdsch 26 · index 4 · ref-stack 4 · ref-channels 1 · ref-core 1 · ref-session 1
Scanned from the notes at page load and joined with terms.json; nothing on this card is typed by hand.

What it is

HARQ is the standing bet, settled every slot. A transmission is provisional; the device says whether it decoded, and a failure is repaired with fresh parity rather than a repeat.

Up to 16 processes per cell, or 32 by capability, with 8 the default if unconfigured (TS 38.214 clause 5.1). The DCI carries a 4-bit process number — exactly $\log_2 16$. Several run in parallel so the link is not idle for a whole round trip after every block.

Asynchronous, unlike LTE's downlink. A retransmission arrives whenever the scheduler chooses, identified by its process number on an ordinary grant — which is why NR could delete LTE's PHICH.

The New Data Indicator is the whole protocol: toggled means new block, discard the buffer; not toggled means combine.

Read on

The full treatment is in nr-pdsch — this page is the definition and the map reference; that note is where the subject is actually developed.

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Before this concept, the hierarchy says to learn the following — the full chain, in order:

To understand HARQ (level 10) you first need 10 other concepts. Read them in this order — everything on one line can be read in any order, but no line before the one above it:
Level Learn these
0 Subcarrier
1 OFDM
2 OFDM symbol
3 Resource element
4 Physical layer
5 Transport block
6 CRC
7 Code block
8 LDPC
9 Rate matching
Immediately before HARQ: CRC, Rate matching, Transport block.
Keep going — where this sits on the route
◀MCSstep 99 · level 10
Step 100 of 118level 10 · Procedures over time
Schedulingstep 101 · level 10▶
Level 10 → 11 · this unlocks
PUCCH · URLLC and eMBB level 12 · Link adaptation level 12
↳ all 6 concepts at level 11 · level 10 in full · the whole route
The route is every concept in the folder ordered by level, so nothing here needs anything after it. Computed at page load from terms.json; the same numbering as the route page.
5G Systems Notes · Concept map · HARQ Top · Concept map · Hub