Understanding HARQ Concepts in 5G

1. Introduction to HARQ

HARQ (Hybrid Automatic Repeat reQuest) is a critical error control method used in wireless systems like LTE and 5G to combine retransmissions and boost throughput while maintaining low latency.

Unlike ARQ, which retransmits entire packets on error, HARQ combines erroneous packets (soft combining) with retransmissions to reconstruct the correct message.

2. The HARQ Process

Basic Steps:

1. Transmission: The transmitter sends a data packet (Transport Block - TB).

2. Reception and Decoding: The receiver attempts to decode.

3. Acknowledgment:

   • If successful: sends ACK

   • If failed: sends NACK

4. Retransmission: On NACK, the transmitter sends a retransmission.

5. Combining: The receiver combines all versions (soft bits) using chase combining or incremental redundancy.

3. HARQ Algorithms and Concepts

Types of HARQ:

A. Chase Combining (CC-HARQ)

• Retransmits the same codeword bits.

• Combines soft bits (LLRs) from all transmissions.

• Simple but less spectrum efficient.

B. Incremental Redundancy (IR-HARQ)

• Transmits new parity bits on retransmissions.

• Multiple redundancy versions (RVs): improves coding gain.

• More complex, more efficient.

HARQ Redundancy Versions (RV)

In IR-HARQ, the full codeword (after channel encoding, e.g., turbo or LDPC) is not transmitted at once. Instead, it's stored in a circular buffer, and only selected parts are sent in each retransmission. This allows efficient use of radio resources and improved error correction by transmitting different parts of the codeword each time. Each retransmission carries a different RV (Redundancy Version).

 How the Process works:

Codeword Generation:

• Let's say the output of the channel encoder (e.g., LDPC) is N encoded bits.

Buffering:

• These N bits are loaded into a circular buffer (think of a ring of bits).

RV-Based Extraction:

• Depending on the RV index (0, 1, 2, 3), different start points and/or bit patterns are selected from the buffer.

• The physical layer extracts TBS (Transport Block Size) worth of bits for that transmission.

Transmit:

• Those extracted bits are mapped to physical resources and transmitted.

Retransmissions:

• On a NACK, the next RV is selected (usually in a predefined sequence), and another segment is extracted from the same circular buffer. 

 For example:

Codeword length = 1024 bits

TBS (transport block size) = 256 bits

RV index options = 0, 1, 2, 3

The extraction might look like:

RV	Start Offset	Extracted Bits (Segment)
0	0	Bits 0 – 255
1	256	Bits 256 – 511
2	512	Bits 512 – 767
3	768	Bits 768 – 1023

All are combined at the receiver using soft combining to recover original payload.

4. LTE HARQ vs 5G HARQ

Feature	LTE HARQ	5G HARQ
Max HARQ processes	8 (UL), 8 (DL)	Up to 16 or more
Duplexing	TDD/ FDD	TDD/FDD (dynamic slot assignment)
Feedback Timing	Fixed (e.g., 4ms)	Configurable (e.g., short/long TTI)
Transport block	1 TB per slot	Up to 2 TBs per slot (MIMO + slot agg)
HARQ Buffer Size	Fixed	Dynamically configurable
Scheduling Flexibility	Limited	Advanced (Mini-slot, URLLC optimized)
Support for Asynchronous HARQ	Optional	Fully supported

Asynchronous HARQ in 5G

• UL/DL feedback and retransmission are not fixed in time.

• Enables better use of resources and support for URLLC.

5. RLC ARQ vs MAC/PHY HARQ

A. HARQ (MAC/PHY Layer)

• Location: MAC/PHY layers

• Retransmission at link layer

• Fast response (sub-ms to a few ms)

• Uses soft combining

• Limited retry (e.g., 4 attempts)

B. ARQ (RLC Layer)

• Location: RLC Acknowledged Mode

• Retransmission at higher layer

• Retransmits only if HARQ fails and higher-layer detects missing/incomplete SDUs

• Uses sequence numbers and timers (e.g., poll and re-transmit timers)

6. How HARQ and ARQ Complement Each Other

Aspect	HARQ	ARQ (RLC)
Layer	MAC/PHY	RLC
Latency	Low (1-5 ms)	Higher (10s of ms)
Redundancy Strategy	Soft combining (same TB or IR)	Hard retransmission (entire PDU)
When Used	First line of defense	After HARQ exhaustion
Example Failure Cause	Interference, weak signal	HARQ failures, packet loss

Together, they ensure:

• HARQ handles short-term errors

• ARQ handles persistent or residual errors

7. Example: End-to-End Packet Journey

Let’s walk through an example:

Initial Transmission

• UE sends TB with RV=0

• eNB/gNB receives with low SINR → NACK

Retransmission 1

• RV=2 sent

• Receiver combines bits from RV=0 and RV=2 → still fails → NACK

Retransmission 2

• RV=3 sent

• Receiver combines RV=0,2,3 → decoding successful → ACK

HARQ Max Retries Reached

If still fails:

• HARQ gives up

• Packet is marked lost

• RLC layer detects missing sequence number → initiates ARQ retransmission

8. Summary

• HARQ is a smart hybrid of FEC and ARQ.

• In 5G, HARQ is asynchronous, flexible, and faster.

• Complemented by RLC ARQ for robust delivery, especially for non-URLLC services.

• Understanding RV, combining strategies, and buffer management is key for performance tuning.

  
  

9. References

• 3GPP TS 36.321 – LTE MAC Protocol Specification

• 3GPP TS 38.321 – 5G NR MAC Protocol Specification

• 3GPP TS 36.212 / 38.212 – Channel Coding