Hi All.
Very nice blog post on: 5G NR Downlink HARQ
While RLC layer uses Automatic Repeat Request (ARQ) retransmission protocol for error detection and correction, MAC layer uses Hybrid ARQ (HARQ) protocol for that purpose.
Source → (Continue reading on LinkedIn): 5G NR Downlink HARQ Demystified
Because in 5G, it’s not just about retransmitting—it’s about smartly managing what to retransmit, when, and how.
Mastering HARQ in 5G NR: Retransmissions, Redundancy Versions & Real-Time Scheduling 
Understanding HARQ (Hybrid Automatic Repeat reQuest) is critical for grasping 5G reliability and throughput enhancements—but how does it actually work under the hood?
To help decode the complexities, I’m sharing a detailed walkthrough document that explores every corner of 5G HARQ operation—from ACK/NACK messaging to dynamic retransmissions, Redundancy Versions (RV), and how NR differs from LTE.
What’s Inside the Document?
Transmission-to-Acknowledgement Flow:
Breakdown of HARQ processes through multiple cases—from successful first-pass decoding to multi-step retransmissions using RV = 0, 2, 3, 1.
Asynchronous Uplink & Downlink in NR:
Unlike LTE where uplink HARQ was synchronous, NR brings flexibility—DCI now carries HARQ process numbers for both UL and DL, enabling independent, out-of-order HARQ operations.
Flexible HARQ Timing with K1 Parameter:
No more rigid 4ms ACK delays! Timing is now dynamically configurable via RRC tables and signaled through DCI, enhancing spectral efficiency in TDD slots.
Redundancy Versions Explained:
Detailed explanation of how RVs are constructed from circular buffers and how each transmission selects a new RV, improving decoding probability with each attempt.
Codebook-Based HARQ Bit Construction:
Insights into how HARQ-ACK bits are generated using NR’s more complex, codebook-driven approach—key to interpreting test logs and DCI traces.
DCI Sample Analysis:
Includes real-world examples of DCI messages showing HARQ process ID, RV index, feedback timing, and more—ideal for anyone analyzing protocol logs or RAN behavior.
HARQ Process Configuration via RRC IEs:
NR allows RRC signaling to define the number of HARQ processors, dynamically tuned to network needs (2, 4, 8, 16). The document includes the exact ASN.1 IE format.
Bonus: Trace Log Sample
A sample Amarisoft trace helps visualize the exact retransmission flow with real-time parameter values.
This resource is curated from public domain content, repackaged for clarity and learning. It bridges theory and implementation, helping telecom engineers, testers, and learners decode HARQ in practical contexts.
PDF: 
5G NR Downlink HARQ Demystified (By Mandeep Jha).pdf (995.5 KB)
LinkedIn:
